miRNA

• ミクロRNA、マイクロRNA

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microRNA

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/09/10 12:02:42」(JST)

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miRNAの化学構造式。RNA構成単位のヌクレオチドが直鎖状にn個連結している。Baseは核酸塩基を示している。miRNAの平均分子量は330 (g mol^—1) × n (塩基数) の式によって簡易に算出できる。

miRNA (microRNA, マイクロRNA) は、ゲノム上にコードされ、多段階的な生成過程を経て最終的に20から25塩基長の微小RNAとなる機能性核酸である。

この鎖長の短いmiRNAは、機能性のncRNA (non-coding RNA, ノンコーディングRNA, 非コードRNA： タンパク質へ翻訳されないRNAの総称) に分類されており、ほかの遺伝子の発現を調節するという、生命現象において重要な役割を担っている。

miRNAの発見

1993年、C.elegans (Caenorhabditis elegans, 線虫) で、R. C. Leeらは最初のmiRNAを発見した。このときはmiRNAではなくstRNA (small temporal RNA, 小分子RNA) と呼ばれていた。 miRNAという学名は、2001年に発行されたScience誌の論文で提唱された。

2001年以降、miRNA研究は急速に進展した。年々新たな種類のmiRNAが発見され、miRNAは種別を問わず、さまざまな種類の生物 (植物： シロイヌナズナ, イネ; 動物： シー・エレガンス, ショウジョウバエ; 哺乳類： マウス, ヒト) に存在していることが明らかになった。

2014年時点で、223種類の生物において35,828種類のmiRNAが発見されている。

miRNAの命名法

新たに発見されたmiRNAの名称は、以下の(1)～(5)にしたがって決められる。

(1) 生物の種類を定義する。 ショウジョウバエ型miRNAの場合、dmeの略称を使用する。 マウス型miRNAの場合、mmuの略称を使用する。 ヒト型miRNAの場合、hsaの略称を使用する。 (2) 構造のタイプを定義する。 前駆体の場合、mirを使用する。 成熟体の場合、miRを使用する。 一部例外も存在する。 (3) 塩基配列の登録番号を定義する。 同定された順に番号を付ける。 類似した配列の場合、番号の後ろに小文字のアルファベットを付ける。 (4) 生成過程の由来を定義する。 異なる遺伝子座を由来とする場合、数字で区別する。 異なる前駆体を由来とする場合、5'末端側の鎖を5p、3'末端側の鎖を3pとして区別する。 定義しない場合もある。 (5) miRNAの名称を定義する。 (1)～(4)で定義した因子をハイフン (“‐”) でつなげる。 一例として、dme-let-7、mmu-mir-1-1およびhsa-miR-15a-5pなどが挙げられる。

miRNAの生合成

miRNAの生合成と制御機能。

ゲノムDNAには、miRNAの塩基配列とほぼ相補な配列が含まれている。一般に、この配列はmiRNA遺伝子と呼ばれている。

ここでいう相補とは、核酸塩基がワトソン・クリック型塩基対を形成していることを意味する。具体的には、A (adenine, アデニン), T (tymine, チミン), G (guanine, グアニン), C (cytosine, シトシン), U (uracil, ウラシル) の5種類の天然型塩基が特定のペアー同士で水素結合によって塩基対を形成する。AとT/Uは2本の水素結合により対をなす。GとCは3本の水素結合により対をなす。したがって水素結合の数が1本多いことから、G：Cの塩基対はA：T/Uの塩基対よりも安定性が高い。

pri-miRNAの生成

miRNA遺伝子がRNAポリメラーゼIIによって1本鎖RNAに転写されると、転写されたRNA配列内で相補な部分は、内在的に結合して2本鎖になる。そうして最終的な構造はヘアピンループ型の構造（tRNAに類似した形状）となる。この構造をとったmiRNAはpri-miRNA (primary miRNA, 初期転写産物) と呼ばれている。pri-miRNAは数百～数千塩基長の長鎖RNAであり、5’末端側にキャップ構造 (5'-cap: 7mGppp, 7-methyl guanosine) が形成されており、3'末端側にポリAテール (3'-poly-A tail: AA...A, polyadenosine) が付与されている。

通常、ヘアピンループ型の構造をとる部分以外については配列がミスマッチになっており、この構造はスターフォームと呼ばれている。

pre-miRNAの生成

核内に存在するRNaseIII様のDrosha (ドローシャ) と呼ばれる酵素がこのpri-miRNA分子の一部を切断して、約70塩基長のステムループ構造をもつpre-miRNA (precursor miRNA, 成熟したmiRNAの前駆体) を作る。次いで、pre-miRNA分子はExportin-5と呼ばれるキャリアタンパク質によって細胞核の外に輸送される。

mature-miRNAの生成

細胞質に放出された後、Dicer（ダイサー：RNAi (RNA interference, RNA干渉) を参照）と呼ばれる酵素のスプライシングによって、pre-miRNAは20から25塩基長の2本鎖miRNAとなる。

ただし植物の場合、2本鎖miRNAはpri-miRNAからDicerによって直接的に切り出される。

2本鎖miRNAは、Ago（Argonaute, アルゴノート）タンパク質からなるRISC (RNA-induced silencing complex, RNA誘導サイレンシング複合体) タンパク質群に取り込まれる。

RISCに取り込まれた2本鎖miRNAはRISC中で解かれ、2つの1本鎖miRNAとなる。そのうち、より不安定な1本鎖は分解される。

もう一方の安定な1本鎖miRNAはmRNA (messenger RNA, 伝令RNA) の3'-UTR (untranslated region, 非翻訳領域) と部分相補的な遺伝子配列をもっている。つまり、その1本鎖miRNAは自身と部分相補的な塩基配列をもつmRNAに結合することで、その遺伝子の翻訳反応を阻害する。このような機能をもつ1本鎖miRNAはmature-miRNA (成熟miRNA, 機能性miRNA) と呼ばれている。ただし植物の場合、siRNA (short-interfering RNA, 低分子干渉RNA) によるRNAiのように、miRNAは標的mRNAを分解させる。

mature-miRNAの塩基配列の5'-末端側より2番目～8番目までの7塩基は、seed配列と呼ばれている。この配列と相補的な配列をもつmRNAは、mature-miRNAの標的として強く認識される。

mature-miRNAのseed配列とmRNAの3'-UTRの相互作用について、熱力学的な手法を用いた解析が進められてきた。その結果、初期状態のギブズエネルギーは－4.09 (kcal mol^—1)であり、最終状態のギブズエネルギーは－14 (kcal mol^—1)であることが明らかにされた。したがって、mature-miRNAとmRNAの結合反応におけるギブズエネルギー変化は－10 (kcal mol^—1) であると考えられる。この値は水素結合の結合エネルギーとほぼ等しい。このことから、mature-miRNAとmRNAはワトソン・クリック型の塩基対を形成しているといえる。つまり、GC含量の高いseed配列をもつmature-miRNAは、mRNAの3'-UTRに対して安定に結合できる。

miRNAの制御機能

機能性のncRNAの中でも、短鎖RNAはsmall RNA (スモールRNA, 小さなRNA) と呼ばれている。 small RNAとして広く知られているものに、siRNAとmiRNAがある。

siRNAは、ウイルスRNAの転写を阻止する機能をもつ。また、細胞の遺伝子発現の調節に関わっている。siRNAは外来性の2本鎖RNAであり、ヘアピンループ構造を形成している。その作用機構はRNAi機構である。 siRNAは単一のmRNA配列に完全相補的に結合する。特定のmRNAを分解に導くことで遺伝子発現を強力に阻害する。

一方、miRNAはsiRNAと違い、複数のmRNA配列に部分相補的に結合する。miRNAは、mRNAの翻訳反応を物理的に阻害することで、さまざまな遺伝子発現を抑制する。

このことから、1種類のmiRNAは、多種類のmRNAの遺伝子発現調節に関わっているといえる。 逆に言い換えれば、1種類のmRNAの遺伝子発現は、多種類のmiRNAによって調節されている。

したがって、miRNAとmRNAは複雑な遺伝子発現制御ネットワークを形成していると考えられる。 つまり、miRNAは単一で機能しているのではなくて、ほかの種類のmiRNAと協力することで、複数のmRNAの遺伝子発現を抑制している。 このようにして、miRNAは制御機能のバランスを保ち、生物の恒常性を維持している。

miRNAの解析

miRNAは、細胞の発生、分化、増殖および細胞死などの基本的な生命現象の調節に関わっている。特に哺乳類の場合、2,500種類以上のmiRNAが遺伝子発現の30-90%を制御していることが知られている。これらの知見から、miRNAと生命現象の間には非常に密接な関係が成り立っているといえる。そのため、世界各国の研究者らによってmiRNAの解析が進められている。

miRNA mimicとmiRNA inhibitorの作用機構。

miRNAの解析には、大きく分けて2つのツールが広く使われている。

miRNA mimic (miRNAミミック、miRNA擬態者) とmiRNA inhibitor (miRNAインヒビター、miRNA阻害剤) である。

miRNA mimicは、miRNA分子を模倣した2本鎖RNAである。miRNAの機能活性を高めることでmiRNAの機能を解析する。

miRNA inhibitorは、miRNA分子に特異的に結合する1本鎖RNAである。miRNAの機能活性を低めることでmiRNAの機能を解析する。

miRNA inhibitorの原理には、アンチセンス核酸分子を用いたアンチセンス法が用いられている。アンチセンス法とは、ターゲットの核酸分子と対になるような塩基配列を有する核酸分子をターゲットの核酸分子に結合させることで、ターゲットの核酸分子の機能を阻害する手法である。つまり、ターゲットのmiRNAの機能のみを強力に阻害することで、そのmiRNAの細胞内での役割を明らかにできる。

実際、miRNAの機能を強力に阻害することを主な目的として、非天然型の人工合成核酸で配列選択的に修飾されたアンチセンス核酸分子の設計と開発の研究が盛んに行われている。一般に、このようなアンチセンス核酸分子はmiRNAの機能を阻害するアンチセンス核酸分子という意味をもつので、AMO (anti-miRNA oligonucleotide, 抗miRNAオリゴヌクレオチド) と呼ばれている。

AMO設計の研究について、有機化学的な合成手法と、分子生物学的および熱力学的な解析手法を用いることで、さまざまな種類のAMO分子が開発されている。

miRNAとがん

miRNAは、がん、心血管疾患、神経変性疾患、精神疾患、慢性炎症性疾患などの発症と進行に関わっている。特に、がんの原因因子についてはさまざまな議論がなされているが、その中でもmiRNAは、細胞のがん化に深く関与していることが多くの研究者らによって指摘されている。

miRNAとがん化の関わりについて、2002年、米国のG. M. CalinとC. M. Croceらによって最初の報告がなされた。

それ以降、がんに関わるmiRNAに、正の制御をする (がん化を促進する) ものと負の制御をする (がん化を抑制する) ものの2種類のタイプが存在することがわかった。

正の制御をするmiRNAはonco miRNA (oncogenic miRNA, がん促進型miRNA) 、負の制御をするmiRNAはTumor Suppressor miRNA (がん抑制型miRNA) と呼ばれている。 特に、onco miRNAに関して、その発現量の亢進が細胞のがん化を誘発していることが明らかにされている。つまり、miRNAの発現量異常は、生物の恒常性の破綻に直結する。

代表的なonco miRNAとtumor suppressor miRNAの一例

miRNAの種類	onco miRNA/tumor suppressor miRNA	がんの種類
miR-21	onco miRNA	乳がん、子宮頸がん、肺がん、大腸がんなど
miR-34	tumor suppressor miRNA	前立腺がん、肺がん、大腸がん、肝臓がんなど

2010年、米国の研究グループは初期のリンパ腫を人為的に発症させたトランスジェニックマウスを作製した。そのマウスにおいて、onco miRNAのひとつであるmiR-21 (miRNA-21) というmiRNAの発現量を抑制した場合、プレB細胞由来のリンパ腫の消滅が観察された。そのため、onco miRNAの発現量を低下させ、がん細胞を消滅させるという方法は、がんの新たな治療薬の開発手法のひとつとして注目されている。

事実、miRNAとがん発症に高い相関性が見られることから、がんを中心としたさまざまなヒト疾患に対する核酸医薬品の開発が盛んに行われている。

2013年、米国の研究グループによって、miRNA mimic (miR-34a mimic) をがん患者に投与した、最初の臨床研究がはじまった。以降、2015年までに米国では数十例の臨床研究が進められてきた。また、マイクロRNAについてはがん診断のためのバイオマーカーへの応用も期待されている。

外部リンク

• miRNAデータベース： miRBase
• ヒト疾患におけるmiRNA： miR2Disease

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Diagram of micro RNA (miRNA) action with mRNA

Examples of miRNA stem-loops, with the mature miRNAs shown in red

A microRNA (abbreviated miRNA) is a small non-coding RNA molecule (containing about 22 nucleotides) found in plants, animals and some viruses, that functions in RNA silencing and post-transcriptional regulation of gene expression. While the majority of miRNAs are located within the cell, some miRNAs, commonly known as circulating miRNAs or extracellular miRNAs, have also been found in extracellular environment, including various biological fluids and cell culture media.

Encoded by eukaryotic nuclear DNA in plants and animals and by viral DNA in certain viruses whose genome is based on DNA, miRNAs function via base-pairing with complementary sequences within mRNA molecules. As a result, these mRNA molecules are silenced, by one or more of the following processes:

• Cleavage of the mRNA strand into two pieces,
• Destabilization of the mRNA through shortening of its poly(A) tail, and
• Less efficient translation of the mRNA into proteins by ribosomes.

miRNAs resemble the small interfering RNAs (siRNAs) of the RNA interference (RNAi) pathway, except miRNAs derive from regions of RNA transcripts that fold back on themselves to form short hairpins, whereas siRNAs derive from longer regions of double-stranded RNA. The human genome may encode over 1000 miRNAs, which are abundant in many mammalian cell types and appear to target about 60% of the genes of humans and other mammals.

miRNAs are well conserved in both plants and animals, and are thought to be a vital and evolutionarily ancient component of gene regulation. While core components of the microRNA pathway are conserved between plants and animals, miRNA repertoires in the two kingdoms appear to have emerged independently with different primary modes of action.

Plant miRNAs usually have near-perfect pairing with their mRNA targets, which induces gene repression through cleavage of the target transcripts. In contrast, animal miRNAs are able to recognize their target mRNAs by using as little as 6–8 nucleotides (the seed region) at the 5' end of the miRNA, which is not enough pairing to induce cleavage of the target mRNAs. Combinatorial regulation is a feature of miRNA regulation in animals. A given miRNA may have hundreds of different mRNA targets, and a given target might be regulated by multiple miRNAs.

The first miRNA was discovered in the early 1990s. However, miRNAs were not recognized as a distinct class of biological regulators until the early 2000s. miRNA research revealed different sets of miRNAs expressed in different cell types and tissues and multiple roles for miRNAs in plant and animal development and in many other biological processes. Aberrant miRNA expression are implicated in disease states. MiRNA-based therapies are under investigation.

Estimates of the average number of unique messenger RNAs that are targets for repression by a typical miRNA vary, depending on the estimation method, but multiple approaches show that mammalian miRNAs can have many unique targets. For example, an analysis of the miRNAs highly conserved in vertebrates shows that each has, on average, roughly 400 conserved targets. Likewise, experiments show that a single miRNA species can reduce the stability of hundreds of unique messenger RNAs. Other experiments show that a single miRNA species may repress the production of hundreds of proteins, but that this repression often is relatively mild (much less than 2-fold). The first human disease associated with deregulation of miRNAs was chronic lymphocytic leukemia. Other B cell malignancies followed.

History

The first miRNA was discovered in 1993 by a group led by Ambros and including Lee and Feinbaum; but additional insight into its mode of action required simultaneously published work by Ruvkun's team, including Wightman and Ha. These groups published back-to-back papers on the lin-4 gene, which was known to control the timing of C. elegans larval development by repressing the lin-14 gene. When Lee et al. isolated the lin-4 gene, they found that instead of producing an mRNA encoding a protein, it produced short non-coding RNAs, one of which was a ~22-nucleotide RNA that contained sequences partially complementary to multiple sequences in the 3' UTR of the lin-14 mRNA. This complementarity was proposed to inhibit the translation of the lin-14 mRNA into the LIN-14 protein. At the time, the lin-4 small RNA was thought to be a nematode idiosyncrasy.

In 2000, a second small RNA was characterized: let-7 RNA, which represses lin-41 to promote a later developmental transition in C. elegans. The let-7 RNA was found to be conserved in many species, leading to the suggestion that let-7 RNA and additional "small temporal RNAs" might regulate the timing of development in diverse animals, including humans.

A year later, the lin-4 and let-7 RNAs were found to be part of a large class of small RNAs present in C. elegans, Drosophila and human cells. The many RNAs of this class resembled the lin-4 and let-7 RNAs, except their expression patterns were usually inconsistent with a role in regulating the timing of development. This suggested that most might function in other types of regulatory pathways. At this point, researchers started using the term "microRNA" to refer to this class of small regulatory RNAs.

The first human disease associated with deregulaton of miRNAs was chronic lymphocytic leukemia.

Nomenclature

Under a standard nomenclature system, names are assigned to experimentally confirmed miRNAs before publication. The prefix "miR" is followed by a dash and a number, the latter often indicating order of naming. For example, miR-124 was named and likely discovered prior to miR-456. A capitalized "miR-" refers to the mature form of the miRNA, while the uncapitalized "mir-" refers to the pre-miRNA and the pri-miRNA, and "MIR" refers to the gene that encodes them.

miRNAs with nearly identical sequences except for one or two nucleotides are annotated with an additional lower case letter. For example, miR-124a is closely related to miR-124b.

Pre-miRNAs, pri-miRNAs and genes that lead to 100% identical mature miRNAs but that are located at different places in the genome are indicated with an additional dash-number suffix. For example, the pre-miRNAs hsa-mir-194-1 and hsa-mir-194-2 lead to an identical mature miRNA (hsa-miR-194) but are from genes located in different genome regions.

Species of origin is designated with a three-letter prefix, e.g., hsa-miR-124 is a human (Homo sapiens) miRNA and oar-miR-124 is a sheep (Ovis aries) miRNA. Other common prefixes include 'v' for viral (miRNA encoded by a viral genome) and 'd' for Drosophila miRNA (a fruit fly commonly studied in genetic research).

When two mature microRNAs originate from opposite arms of the same pre-miRNA and are found in roughly similar amounts, they are denoted with a -3p or -5p suffix. (In the past, this distinction was also made with 's' (sense) and 'as' (antisense)). However, the mature microRNA found from one arm of the hairpin is usually much more abundant than that found from the other arm, in which case, an asterisk following the name indicates the mature species found at low levels from the opposite arm of a hairpin. For example, miR-124 and miR-124* share a pre-miRNA hairpin, but much more miR-124 is found in the cell.

Biogenesis

As many as 40% of miRNA genes may lie in the introns or even exons of other genes. These are usually, though not exclusively, found in a sense orientation, and thus usually are regulated together with their host genes.

The DNA template is not the final word on mature miRNA production: 6% of human miRNAs show RNA editing (IsomiRs), the site-specific modification of RNA sequences to yield products different from those encoded by their DNA. This increases the diversity and scope of miRNA action beyond that implicated from the genome alone.

Transcription

miRNA genes are usually transcribed by RNA polymerase II (Pol II). The polymerase often binds to a promoter found near the DNA sequence, encoding what will become the hairpin loop of the pre-miRNA. The resulting transcript is capped with a specially modified nucleotide at the 5' end, polyadenylated with multiple adenosines (a poly(A) tail), and spliced. Animal miRNAs are initially transcribed as part of one arm of an ∼80 nucleotide RNA stem-loop that in turn forms part of a several hundred nucleotide-long miRNA precursor termed a primary miRNA (pri-miRNA). When a stem-loop precursor is found in the 3' UTR, a transcript may serve as a pri-miRNA and a mRNA. RNA polymerase III (Pol III) transcribes some miRNAs, especially those with upstream Alu sequences, transfer RNAs (tRNAs), and mammalian wide interspersed repeat (MWIR) promoter units.

Nuclear processing

A crystal structure of the human Drosha protein in complex with the C-terminal helices of two DGCR8 molecules (green). Drosha consists of two ribonuclease III domains (blue and orange); a double-stranded RNA binding domain (yellow); and a connector/platform domain (gray) containing two bound zinc ion (spheres). From PDB: 5B16​.

A single pri-miRNA may contain from one to six miRNA precursors. These hairpin loop structures are composed of about 70 nucleotides each. Each hairpin is flanked by sequences necessary for efficient processing.

The double-stranded RNA (dsRNA) structure of the hairpins in a pri-miRNA is recognized by a nuclear protein known as DiGeorge Syndrome Critical Region 8 (DGCR8 or "Pasha" in invertebrates), named for its association with DiGeorge Syndrome. DGCR8 associates with the enzyme Drosha, a protein that cuts RNA, to form the Microprocessor complex. In this complex, DGCR8 orients the catalytic RNase III domain of Drosha to liberate hairpins from pri-miRNAs by cleaving RNA about eleven nucleotides from the hairpin base (one helical dsRNA turn into the stem). The product resulting has a two-nucleotide overhang at its 3' end; it has 3' hydroxyl and 5' phosphate groups. It is often termed as a pre-miRNA (precursor-miRNA). Sequence motifs downstream of the pre-miRNA that are important for efficient processing have been identified.

Pre-miRNAs that are spliced directly out of introns, bypassing the Microprocessor complex, are known as "Mirtrons." Originally thought to exist only in Drosophila and C. elegans, mirtrons have now been found in mammals.

As many as 16% of pre-miRNAs may be altered through nuclear RNA editing. Most commonly, enzymes known as adenosine deaminases acting on RNA (ADARs) catalyze adenosine to inosine (A to I) transitions. RNA editing can halt nuclear processing (for example, of pri-miR-142, leading to degradation by the ribonuclease Tudor-SN) and alter downstream processes including cytoplasmic miRNA processing and target specificity (e.g., by changing the seed region of miR-376 in the central nervous system).

Nuclear export

The human exportin-5 protein (red) in complex with Ran-GTP (yellow) and a pre-microRNA (green), showing two-nucleotide overhang recognition element (orange). From PDB: 3A6P​.

Pre-miRNA hairpins are exported from the nucleus in a process involving the nucleocytoplasmic shuttler Exportin-5. This protein, a member of the karyopherin family, recognizes a two-nucleotide overhang left by the RNase III enzyme Drosha at the 3' end of the pre-miRNA hairpin. Exportin-5-mediated transport to the cytoplasm is energy-dependent, using GTP bound to the Ran protein.

Cytoplasmic processing

In the cytoplasm, the pre-miRNA hairpin is cleaved by the RNase III enzyme Dicer. This endoribonuclease interacts with 5' and 3' ends of the hairpin and cuts away the loop joining the 3' and 5' arms, yielding an imperfect miRNA:miRNA* duplex about 22 nucleotides in length. Overall hairpin length and loop size influence the efficiency of Dicer processing. The imperfect nature of the miRNA:miRNA* pairing also affects cleavage. Some of the G-rich pre-miRNAs can potentially adopt the G-quadruplex structure as an alternative to the canonical stem-loop structure. For example, human pre-miRNA 92b adopts a G-quadruplex structure which is resistant to the Dicer mediated cleavage in the cytoplasm. Although either strand of the duplex may potentially act as a functional miRNA, only one strand is usually incorporated into the RNA-induced silencing complex (RISC) where the miRNA and its mRNA target interact.

Biogenesis in plants

miRNA biogenesis in plants differs from animal biogenesis mainly in the steps of nuclear processing and export. Instead of being cleaved by two different enzymes, once inside and once outside the nucleus, both cleavages of the plant miRNA are performed by a Dicer homolog, called Dicer-like1 (DL1). DL1 is expressed only in the nucleus of plant cells, which indicates that both reactions take place inside the nucleus. Before plant miRNA:miRNA* duplexes are transported out of the nucleus, its 3' overhangs are methylated by a RNA methyltransferaseprotein called Hua-Enhancer1 (HEN1). The duplex is then transported out of the nucleus to the cytoplasm by a protein called Hasty (HST), an Exportin 5 homolog, where they disassemble and the mature miRNA is incorporated into the RISC.

RNA-induced silencing complex

Main article: RNA-induced silencing complex

The mature miRNA is part of an active RNA-induced silencing complex (RISC) containing Dicer and many associated proteins. RISC is also known as a microRNA ribonucleoprotein complex (miRNP); A RISC with incorporated miRNA is sometimes referred to as a "miRISC."

Dicer processing of the pre-miRNA is thought to be coupled with unwinding of the duplex. Generally, only one strand is incorporated into the miRISC, selected on the basis of its thermodynamic instability and weaker base-pairing on the 5' end relative to the other strand. The position of the stem-loop may also influence strand choice. The other strand, called the passenger strand due to its lower levels in the steady state, is denoted with an asterisk (*) and is normally degraded. In some cases, both strands of the duplex are viable and become functional miRNA that target different mRNA populations.

Members of the Argonaute (Ago) protein family are central to RISC function. Argonautes are needed for miRNA-induced silencing and contain two conserved RNA binding domains: a PAZ domain that can bind the single stranded 3' end of the mature miRNA and a PIWI domain that structurally resembles ribonuclease-H and functions to interact with the 5' end of the guide strand. They bind the mature miRNA and orient it for interaction with a target mRNA. Some argonautes, for example human Ago2, cleave target transcripts directly; argonautes may also recruit additional proteins to achieve translational repression. The human genome encodes eight argonaute proteins divided by sequence similarities into two families: AGO (with four members present in all mammalian cells and called E1F2C/hAgo in humans), and PIWI (found in the germ line and hematopoietic stem cells).

Additional RISC components include TRBP [human immunodeficiency virus (HIV) transactivating response RNA (TAR) binding protein], PACT (protein activator of the interferon-induced protein kinase), the SMN complex, fragile X mental retardation protein (FMRP), Tudor staphylococcal nuclease-domain-containing protein (Tudor-SN), the putative DNA helicase MOV10, and the RNA recognition motif containing protein TNRC6B.

Mode of silencing and regulatory loops

Gene silencing may occur either via mRNA degradation or preventing mRNA from being translated. For example, miR16 contains a sequence complementary to the AU-rich element found in the 3'UTR of many unstable mRNAs, such as TNF alpha or GM-CSF. It has been demonstrated that given complete complementarity between the miRNA and target mRNA sequence, Ago2 can cleave the mRNA and lead to direct mRNA degradation. Absent complementarity, silencing is achieved by preventing translation. The relation of miRNA and its target mRNA(s) can be based on the simple negative regulation of a target mRNA, but it seems that a common scenario is the use of a "coherent feed-forward loop", "mutual negative feedback loop" (also termed double negative loop) and "positive feedback/feed-forward loop" Some miRNAs work as buffers of random gene expression changes arising due to stochastic events in transcription, translation and protein stability. Such regulation is typically achieved by the virtue of negative feedback loops or incoherent feed-forward loop uncoupling protein output from mRNA transcription.

Turnover

Turnover of mature miRNA is needed for rapid changes in miRNA expression profiles. During miRNA maturation in the cytoplasm, uptake by the Argonaute protein is thought to stabilize the guide strand, while the opposite (* or "passenger") strand is preferentially destroyed. In what has been called a "Use it or lose it" strategy, Argonaute may preferentially retain miRNAs with many targets over miRNAs with few or no targets, leading to degradation of the non-targeting molecules.

Decay of mature miRNAs in Caenorhabditis elegans is mediated by the 5´-to-3´ exoribonuclease XRN2, also known as Rat1p. In plants, SDN (small RNA degrading nuclease) family members degrade miRNAs in the opposite (3'-to-5') direction. Similar enzymes are encoded in animal genomes, but their roles have not been described.

Several miRNA modifications affect miRNA stability. As indicated by work in the model organism Arabidopsis thaliana (thale cress), mature plant miRNAs appear to be stabilized by the addition of methyl moieties at the 3' end. The 2'-O-conjugated methyl groups block the addition of uracil (U) residues by uridyltransferase enzymes, a modification that may be associated with miRNA degradation. However, uridylation may also protect some miRNAs; the consequences of this modification are incompletely understood. Uridylation of some animal miRNAs has been reported. Both plant and animal miRNAs may be altered by addition of adenine (A) residues to the 3' end of the miRNA. An extra A added to the end of mammalian miR-122, a liver-enriched miRNA important in hepatitis C, stabilizes the molecule and plant miRNAs ending with an adenine residue have slower decay rates.

Cellular functions

Interaction of microRNA with protein translation process. Several translation repression mechanisms are shown: M1) on the initiation process, preventing assembling of the initiation complex or recruiting the 40S ribosomal subunit; M2) on the ribosome assembly; M3) on the translation process; M7, M8) on the degradation of mRNA. 40S and 60S are light and heavy components of the ribosome, 80S is the assembled ribosome bound to mRNA, eIF4F is a translation initiation factor, PABC1 is the Poly-A binding protein, and "cap" is the mRNA cap structure needed for mRNA circularization (which can be the normal m7G-cap or modified A-cap). The initiation of mRNA can proceed in a cap-independent manner, through recruiting 40S to IRES (Internal Ribosome Entry Site) located in 5'UTR region. The actual work of RNA silencing is performed by RISC in which the main catalytic subunit is one of the Argonaute proteins (AGO), and miRNA serves as a template for recognizing specific mRNA sequences.

The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. Perfect or near perfect base pairing with the target RNA promotes cleavage of the RNA. This is the primary mode of plant miRNAs. In animals the match-ups are imperfect.

For partially complementary microRNAs to recognise their targets, nucleotides 2–7 of the miRNA (its 'seed region') must be perfectly complementary. Animal miRNAs inhibit protein translation of the target mRNA (this is present but less common in plants). Partially complementary microRNAs can also speed up deadenylation, causing mRNAs to be degraded sooner. While degradation of miRNA-targeted mRNA is well documented, whether or not translational repression is accomplished through mRNA degradation, translational inhibition, or a combination of the two is hotly debated. Recent work on miR-430 in zebrafish, as well as on bantam-miRNA and miR-9 in Drosophila cultured cells, shows that translational repression is caused by the disruption of translation initiation, independent of mRNA deadenylation.

miRNAs occasionally also cause histone modification and DNA methylation of promoter sites, which affects the expression of target genes.

Nine mechanisms of miRNA action are described and assembled in a unified mathematical model:

• Cap-40S initiation inhibition;
• 60S Ribosomal unit joining inhibition;
• Elongation inhibition;
• Ribosome drop-off (premature termination);
• Co-translational nascent protein degradation;
• Sequestration in P-bodies;
• mRNA decay (destabilisation);
• mRNA cleavage;
• Transcriptional inhibition through microRNA-mediated chromatin reorganization followed by gene silencing.

It is often impossible to discern these mechanisms using experimental data about stationary reaction rates. Nevertheless, they are differentiated in dynamics and have different kinetic signatures.

Unlike plant microRNAs, the animal microRNAs target diverse genes. However, genes involved in functions common to all cells, such as gene expression, have relatively fewer microRNA target sites and seem to be under selection to avoid targeting by microRNAs.

dsRNA can also activate gene expression, a mechanism that has been termed "small RNA-induced gene activation" or RNAa. dsRNAs targeting gene promoters can induce potent transcriptional activation of associated genes. This was demonstrated in human cells using synthetic dsRNAs termed small activating RNAs (saRNAs), but has also been demonstrated for endogenous microRNA.

Interactions between microRNAs and complementary sequences on genes and even pseudogenes that share sequence homology are thought to be a back channel of communication regulating expression levels between paralogous genes. Given the name "competing endogenous RNAs" (ceRNAs), these microRNAs bind to "microRNA response elements" on genes and pseudogenes and may provide another explanation for the persistence of non-coding DNA.

Evolution

microRNAs are useful phylogenetic markers because of their apparently low rate of evolution. microRNAs' origin as a regulatory mechanism developed from previous RNAi machinery that was initially used as a defense against exogenous genetic material such as viruses. Their origin may have permitted the development of morphological innovation, and by making gene expression more specific and 'fine-tunable', permitted the genesis of complex organs and perhaps, ultimately, complex life. Rapid bursts of morphological innovation are generally associated with a high rate of microRNA accumulation.

New microRNAs are created in multiple ways. Novel microRNAs can originate from the random formation of hairpins in "non-coding" sections of DNA (i.e. introns or intergene regions), but also by the duplication and modification of existing microRNAs. microRNAs can also form from inverted duplications of protein-coding sequences, which allows for the creation of a foldback hairpin structure. The rate of evolution (i.e. nucleotide substitution) in recently originated microRNAs is comparable to that elsewhere in the non-coding DNA, implying evolution by neutral drift; however, older microRNAs have a much lower rate of change (often less than one substitution per hundred million years), suggesting that once a microRNA gains a function, it undergoes purifying selection. Individual regions within an miRNA gene face different evolutionary pressures, where regions that are vital for processing and function have higher levels of conservation. At this point, a microRNA is rarely lost from an animal's genome, although newer microRNAs (thus presumably non-functional) are frequently lost. In Arabidopsis thaliana, the net flux of miRNA genes has been predicted to be between 1.2 and 3.3 genes per million years. This makes them a valuable phylogenetic marker, and they are being looked upon as a possible solution to outstanding phylogenetic problems such as the relationships of arthropods. On the other hand, in multiple cases microRNAs correlate poorly with phylogeny, and it is possible that their phylogenetic concordance largely reflects a limited sampling of microRNAs.

microRNAs feature in the genomes of most eukaryotic organisms, from the brown algae to the animals. However, the difference in how these microRNAs function and the way they are processed suggests that microRNAs arose independently in plants and animals.

Focusing on the animals, the genome of Mnemiopsis leidyi appears to lack recognizable microRNAs, as well as the nuclear proteins Drosha and Pasha, which are critical to canonical microRNA biogenesis. It is the only animal thus far reported to be missing Drosha. MicroRNAs play a vital role in the regulation of gene expression in all non-ctenophore animals investigated thus far except for Trichoplax adhaerens, the only known member of the phylum Placozoa.

Across all species, in excess of 5000 different miRNAs had been identified by March 2010. Whilst short RNA sequences (50 – hundreds of base pairs) of a broadly comparable function occur in bacteria, bacteria lack true microRNAs.

Experimental detection and manipulation

While researchers focused on miRNA expression in physiological and pathological processes, various technical variables related to microRNA isolation emerged. The stability of stored miRNA samples has been questioned. microRNAs degrade much more easily than mRNAs, partly due to their length, but also because of ubiquitously present RNases. This makes it necessary to cool samples on ice and use RNase-free equipment.

microRNA expression can be quantified in a two-step polymerase chain reaction process of modified RT-PCR followed by quantitative PCR. Variations of this method achieve absolute or relative quantification. miRNAs can also be hybridized to microarrays, slides or chips with probes to hundreds or thousands of miRNA targets, so that relative levels of miRNAs can be determined in different samples. microRNAs can be both discovered and profiled by high-throughput sequencing methods (microRNA sequencing). The activity of an miRNA can be experimentally inhibited using a locked nucleic acid (LNA) oligo, a Morpholino oligo or a 2'-O-methyl RNA oligo. A specific miRNA can be silenced by a complementary antagomir. microRNA maturation can be inhibited at several points by steric-blocking oligos. The miRNA target site of an mRNA transcript can also be blocked by a steric-blocking oligo. For the "in situ" detection of miRNA, LNA or Morpholino probes can be used. The locked conformation of LNA results in enhanced hybridization properties and increases sensitivity and selectivity, making it ideal for detection of short miRNA.

High-throughput quantification of miRNAs is error prone, for the larger variance (compared to mRNAs) that comes with methodological problems. mRNA-expression is therefore often analyzed to check for miRNA-effects in their levels (e.g. in). Databases can be used to pair mRNA- and miRNA-data that predict miRNA-targets based on their base sequence. While this is usually done after miRNAs of interest have been detected (e. g. because of high expression levels), ideas for analysis tools that integrate mRNA- and miRNA-expression information have been proposed.

Disease

Just as miRNA is involved in the normal functioning of eukaryotic cells, so has dysregulation of miRNA been associated with disease. A manually curated, publicly available database, miR2Disease, documents known relationships between miRNA dysregulation and human disease.

Inherited diseases

A mutation in the seed region of miR-96, causes hereditary progressive hearing loss.

A mutation in the seed region of miR-184, causes hereditary keratoconus with anterior polar cataract.

Deletion of the miR-17~92 cluster, causes skeletal and growth defects.

Cancer

Role of miRNA in a cancer cell

The first human disease known to be associated with miRNA deregulation was chronic lymphocytic leukemia. Many other miRNAs also have links with cancer and accordingly are sometimes referred to as "oncomirs". In malignant B cells miRNAs participate in pathways fundamental to B cell development like B-cell receptor (BCR) signalling, B-cell migration/adhesion, cell-cell interactions in immune niches and the production and class-switching of immunoglobulins. MiRNAs influence B cell maturation, generation of pre-, marginal zone, follicular, B1, plasma and memory B cells.

A study of mice altered to produce excess c-Myc — a protein with mutated forms implicated in several cancers — shows that miRNA affects the cancer development. Mice engineered to produce a surplus of types of miRNA found in lymphoma cells developed the disease within 50 days and died two weeks later. In contrast, mice without the surplus miRNA lived over 100 days. Leukemia can be caused by the insertion of a viral genome next to the 17-92 array of microRNAs, leading to increased expression of this microRNA.

Another study found that two types of miRNA inhibit the E2F1 protein, which regulates cell proliferation. miRNA appears to bind to messenger RNA before it can be translated to proteins that switch genes on and off.

By measuring activity among 217 genes encoding miRNAs, patterns of gene activity that can distinguish types of cancers were identified. miRNA profiling can determine whether patients with chronic lymphocytic leukemia had slow growing or aggressive forms of the cancer.

A novel miRNA-profiling-based screening assay for the detection of early-stage colorectal cancer is undergoing a clinical trial. Early results showed that blood plasma samples collected from patients with early, resectable (Stage II) colorectal cancer could be distinguished from those of sex-and age-matched healthy volunteers. Sufficient selectivity and specificity could be achieved using small (less than 1 mL) samples of blood.

Another role for miRNA in cancers is to use their expression level for prognosis. For example, one study on NSCLC samples found that low miR-324a levels could serve as an indicator of poor survival. Either high miR-185 or low miR-133b levels may correlate with metastasis and poor survival in colorectal cancer.

Furthermore, specific miRNAs may be associated with certain histological subtypes of colorectal cancer. For instance, expression levels of miR-205 and miR-373 have been shown to be increased in mucinous colorectal cancers and mucin-producing Ulcerative Colitis-associated colon cancers, but not in sporadic colonic adenocarcinoma that lack mucinous components. In-vitro studies suggested that miR-205 and miR-373 may functionally induce different features of mucinous-associated neoplastic progression in intestinal epithelial cells.

Hepatocellular carcinoma cell proliferation may arise from miR-21 interaction with MAP2K3, a tumor repressor gene. Optimal treatment for cancer involves accurately identifying patients for risk-stratified therapy. Those with a rapid response to initial treatment may benefit from truncated treatment regimens, showing the value of accurate disease response measures. Cell-free miRNA are highly stable in blood, are overexpressed in cancer and are quantifiable within the diagnostic laboratory. In classical Hodgkin lymphoma, plasma miR-21, miR-494, and miR-1973 are promising disease response biomarkers. Circulating miRNAs have the potential to assist clinical decision making and aid interpretation of positron emission tomography combined with computerized tomography. They can be performed at each consultation to assess disease response and detect relapse.

A 2009 study explored miR-205 targeted for inhibiting the metastatic nature of breast cancer. Five members of the microRNA-200 family (miR-200a, miR-200b, miR-200c, miR-141 and miR-429) are down-regulated in tumour progression of breast cancer.

The specific microRNA, miR-506 has been found to work as a tumor antagonist in several studies. In a 2014 study, a significant number of cervical cancer samples were found to have down-regulated expression of miR-506. Additionally, studies found that miR-506 works to promote apoptosis of cervical cancer cells, through its direct target hedgehog pathway transcription factor, Gli3.

A 2015 study used a triple helix of three miRNAs embedded in a dextran aldehyde/dendrimer gel in a mouse model of triple negative breast cancer. mir-205 and mir-212 targeted specific RNAs, while the other miRNA stabilized the others. The treatment reduced tumor sizes by 90% with survival times of 75 days.

MicroRNAs have the potential to be used as targets for treatment of different cancers. The specific microRNA, miR-506 has been found to work as a tumor antagonist in several studies. In a 2014 study, a significant number of cervical cancer samples were found to have downregulated expression of miR-506. Additionally, studies found that miR-506 works to promote apoptosis of cervical cancer cells, through its direct target hedgehog pathway transcription factor, Gli3.

DNA repair and cancer

DNA damage is considered to be the primary underlying cause of cancer. If DNA repair is deficient, damage can accumulate. Such damage can cause mutational errors during DNA replication due to error-prone translesion synthesis. Accumulated damage can also cause epigenetic alterations due to errors during DNA repair. Such mutations and epigenetic alterations can give rise to cancer (see malignant neoplasms).

Germ line mutations in DNA repair genes cause only 2–5% of colon cancer cases. However, altered expression of microRNAs, causing DNA repair deficiencies, are frequently associated with cancers and may be an important causal factor.

Among 68 sporadic colon cancers with reduced expression of the DNA mismatch repair protein MLH1, most were found to be deficient due to epigenetic methylation of the CpG island of the MLH1 gene. However, up to 15% of MLH1-deficiencies in sporadic colon cancers appeared to be due to over-expression of the microRNA miR-155, which represses MLH1 expression.

In 29–66% of glioblastomas, DNA repair is deficient due to epigenetic methylation of the MGMT gene, which reduces protein expression of MGMT. However, for 28% of glioblastomas, the MGMT protein is deficient, but the MGMT promoter is not methylated. In glioblastomas without methylated MGMT promoters, the level of microRNA miR-181d is inversely correlated with protein expression of MGMT and the direct target of miR-181d is the MGMT mRNA 3'UTR (the three prime untranslated region of MGMT mRNA). Thus, in 28% of glioblastomas, increased expression of miR-181d and reduced expression of DNA repair enzyme MGMT may be a causal factor.

HMGA proteins (HMGA1a, HMGA1b and HMGA2) are implicated in cancer, and expression of these proteins is regulated by microRNAs. HMGA expression is almost undetectable in differentiated adult tissues, but is elevated in many cancers. HMGA proteins are polypeptides of ~100 amino acid residues characterized by a modular sequence organization. These proteins have three highly positively charged regions, termed AT hooks, that bind the minor groove of AT-rich DNA stretches in specific regions of DNA. Human neoplasias, including thyroid, prostatic, cervical, colorectal, pancreatic and ovarian carcinomas, show a strong increase of HMGA1a and HMGA1b proteins. Transgenic mice with HMGA1 targeted to lymphoid cells develop aggressive lymphoma, showing that high HMGA1 expression is associated with cancers and that HMGA1 can act as an oncogene. A 2003 study showed that HMGA1 protein binds to the promoter region of DNA repair gene BRCA1 and inhibits BRCA1 promoter activity. They also showed that while only 11% of breast tumors had hypermethylation of the BRCA1 gene, 82% of aggressive breast cancers have low BRCA1 protein expression, and most of these reductions were due to chromatin remodeling by high levels of HMGA1 protein.

HMGA2 protein specifically targets the promoter of ERCC1, thus reducing expression of this DNA repair gene. ERCC1 protein expression was deficient in 100% of 47 evaluated colon cancers (though the extent to which HGMA2 was involved is not known). A 2012 study showed that in normal tissues, HGMA1 and HMGA2 genes are targeted (and thus strongly reduced in expression) by miR-15, miR-16, miR-26a, miR-196a2 and Let-7a. However, each of these HMGA-targeting miRNAs are drastically reduced in almost all human pituitary adenomas studied, when compared with the normal pituitary gland. Consistent with the down-regulation of these HMGA-targeting miRNAs, an increase in the HMGA1 and HMGA2-specific mRNAs was observed. Three of these microRNAs (miR-16, miR-196a and Let-7a) have methylated promoters and therefore low expression in colon cancer. For two of these, miR-15 and miR-16, the coding regions are epigenetically silenced in cancer due to histone deacetylase activity. When these microRNAs are expressed at a low level, then HMGA1 and HMGA2 proteins are expressed at a high level. HMGA1 and HMGA2 target (reduce expression of) BRCA1 and ERCC1 DNA repair genes. Thus DNA repair can be reduced, likely contributing to cancer progression.

In contrast to the previous example, where under-expression of miRNAs indirectly caused reduced expression of DNA repair genes, in some cases over-expression of certain miRNAs may directly reduce expression of specific DNA repair proteins. A 2011 study referred to 6 DNA repair genes that are directly targeted by the miRNAs indicated: ATM (miR-421), RAD52 (miR-210, miR-373), RAD23B (miR-373), MSH2 (miR-21), BRCA1 (miR-182) and P53 (miR-504, miR-125b). More recently, A 2014 study listed multiple DNA repair genes directly targeted by these additional miRNAs: ATM (miR-100, miR18a, miR-101), DNA-PK (miR-101), ATR (mir-185), Wip1 (miR-16), MLH1, MSH2, MSH6 (miR-155), ERCC3, ERCC4 (miR-192) and UNG2 (miR-16, miR-34c). Among these miRNAs, miR-16, miR-18a, miR-21, miR-34c, miR-101, miR-125b, miR-155, miR-182, miR-185, miR-192 and miR-373 were identified as over-expressed in colon cancer through epigenetic hypomethylation. Over expression of any one of these miRNAs can cause reduced expression of its target DNA repair gene.

Heart disease

The global role of miRNA function in the heart has been addressed by conditionally inhibiting miRNA maturation in the murine heart. This revealed that miRNAs play an essential role during its development. miRNA expression profiling studies demonstrate that expression levels of specific miRNAs change in diseased human hearts, pointing to their involvement in cardiomyopathies. Furthermore, animal studies on specific miRNAs identified distinct roles for miRNAs both during heart development and under pathological conditions, including the regulation of key factors important for cardiogenesis, the hypertrophic growth response and cardiac conductance. miRNA's in animal models have also been linked to cholesterol metabolism and regulation.

miRNA-712

Murine microRNA-712 is a potential biomarker (i.e. predictor) for atherosclerosis, a cardiovascular disease of the arterial wall associated with lipid retention and inflammation. Non-laminar blood flow also correlates with development of atherosclerosis as mechanosenors of endothelial cells respond to the shear force of disturbed flow (d-flow). A number of pro-atherogenic genes including matrix metalloproteinases (MMPs) are upregulated by d-flow , mediating pro-inflammatory and pro-angiogenic signals. These findings were observed in ligated carotid arteries of mice to mimic the effects of d-flow. Within 24 hours, pre-existing immature miR-712 formed mature miR-712 suggesting that miR-712 is flow-sensitive. Coinciding with these results, miR-712 is also upregulated in endothelial cells exposed to naturally occurring d-flow in the greater curvature of the aortic arch.

Gene origin

Pre-mRNA sequence of miR-712 is generated from the murine ribosomal RN45s gene at the internal transcribed spacer region 2 (ITS2). XRN1 is an exonuclease that degrades the ITS2 region during processing of RN45s. Reduction of XRN1 under d-flow conditions therefore leads to the accumulation of miR-712.

Mechanism

MiR-712 targets tissue inhibitor of metalloproteinases 3 (TIMP3). TIMPs normally regulate activity of matrix metalloproteinases (MMPs) which degrade the extracellular matrix (ECM). Arterial ECM is mainly composed of collagen and elastin fibers, providing the structural support and recoil properties of arteries. These fibers play a critical role in regulation of vascular inflammation and permeability, which are important in the development of atherosclerosis. Expressed by endothelial cells, TIMP3 is the only ECM-bound TIMP. A decrease in TIMP3 expression results in an increase of ECM degradation in the presence of d-flow. Consistent with these findings, inhibition of pre-miR712 increases expression of TIMP3 in cells, even when exposed to turbulent flow.

TIMP3 also decreases the expression of TNFα (a pro-inflammatory regulator) during turbulent flow.  Activity of TNFα in turbulent flow was measured by the expression of TNFα-converting enzyme (TACE) in blood. TNFα decreased if miR-712 was inhibited or TIMP3 overexpressed, suggesting that miR-712 and TIMP3 regulate TACE activity in turbulent flow conditions.

Anti-miR-712 effectively suppresses d-flow-induced miR-712 expression and increases TIMP3 expression. Anti-miR-712 also inhibits vascular hyperpermeability, thereby significantly reducing atherosclerosis lesion development and immune cell infiltration.

Human homolog microRNA-205

The human homolog of miR-712 was found on the RN45s homolog gene, which maintains similar miRNAs to mice. MiR-205 of humans sshare similar sequences with miR-712 of mice and is conserved across most vertebrates. MiR-205 and miR-712 also share more than 50% of the cell signaling targets, including TIMP3.

When tested, d-flow decreased the expression of XRN1 in humans as it did in mice endothelial cells, indicating a potentially common role of XRN1 in humans.

Kidney disease

Targeted deletion of Dicer in the FoxD1-derived renal progenitor cells in a murine model resulted in a complex renal phenotype including expansion of nephron progenitors, fewer renin cells, smooth muscle arterioles, progressive mesangial loss and glomerular aneurysms. High throughput whole transcriptome profiling of the FoxD1-Dicer knockout mouse model revealed ectopic upregulation of pro-apoptotic gene, Bcl2L11 (Bim) and dysregulation of the p53 pathway with increase in p53 effector genes including Bax, Trp53inp1, Jun, Cdkn1a, Mmp2, and Arid3a. p53 protein levels remained unchanged, suggesting that FoxD1 stromal miRNAs directly repress p53-effector genes. Using a lineage tracing approach followed by Fluorescent-activated cell sorting, miRNA profiling of the FoxD1-derived cells not only comprehensively defined the transcriptional landscape of miRNAs that are critical for vascular development, but also identified key miRNAs that are likely to modulate the renal phenotype in its absence. These miRNAs include miRs‐10a, 18a, 19b, 24, 30c, 92a, 106a, 130a, 152, 181a, 214, 222, 302a, 370, and 381 that regulate Bcl2L11 (Bim) and miRs‐15b, 18a, 21, 30c, 92a, 106a, 125b‐5p, 145, 214, 222, 296‐5p and 302a that regulate p53-effector genes. Consistent with the profiling results, ectopic apoptosis was observed in the cellular derivatives of the FoxD1 derived progenitor lineage and reiterates the importance of renal stromal miRNAs in cellular homeostasis.

Nervous system

miRNAs appear to regulate the development and function of the nervous system. Neural miRNAs are involved at various stages of synaptic development, including dendritogenesis (involving miR-132, miR-134 and miR-124), synapse formation and synapse maturation (where miR-134 and miR-138 are thought to be involved). Some studies find altered miRNA expression in schizophrenia, as well as bipolar disorder and major depression and anxiety disorders.

Alcoholism

The vital role of miRNAs in gene expression is significant to addiction, specifically alcoholism. Chronic alcohol abuse results in persistent changes in brain function mediated in part by alterations in gene expression. miRNA global regulation of many downstream genes deems significant regarding the reorganization or synaptic connections or long term neuroadaptions involving the behavioral change from alcohol consumption to withdrawal and/or dependence. Up to 35 different miRNAs have been found to be altered in the alcoholic post-mortem brain, all of which target genes that include the regulation of the cell cycle, apoptosis, cell adhesion, nervous system development and cell signaling. Altered miRNA levels were found in the medial prefrontal cortex of alcohol-dependent mice, suggesting the role of miRNA in orchestrating translational imbalances and the creation of differentially expressed proteins within an area of the brain where complex cognitive behavior and decision making likely originate.

miRNAs can be either upregulated or downregulated in response to chronic alcohol use. miR-206 expression increased in the prefrontal cortex of alcohol-dependent rats, targeting the transcription factor brain-derived neurotrophic factor (BDNF) and ultimately reducing its expression. BDNF plays a critical role in the formation and maturation of new neurons and synapses, suggesting a possible implication in synapse growth/synaptic plasticity in alcohol abusers. miR-155, important in regulating alcohol-induced neuroinflammation responses, was found to be upregulated, suggesting the role of microglia and inflammatory cytokines in alcohol pathophysiology. Downregulation of miR-382 was found in the nucleus accumbens, a structure in the basal forebrain significant in regulating feelings of reward that power motivational habits. miR-382 is the target for the dopamine receptor D1 (DRD1), and its overexpression results in the upregulation of DRD1 and delta fosB, a transcription factor that activates a series of transcription events in the nucleus accumbens that ultimately result in addictive behaviors. Alternatively, overexpressing miR-382 resulted in attenuated drinking and the inhibition of DRD1 and delta fosB upregulation in rat models of alcoholism, demonstrating the possibility of using miRNA-targeted pharmaceuticals in treatments.

Obesity

miRNAs play crucial roles in the regulation of stem cell progenitors differentiating into adipocytes. Studies to determine what role pluripotent stem cells play in adipogenesis, were examined in the immortalized human bone marrow-derived stromal cell line hMSC-Tert20. Decreased expression of miR-155, miR-221, and miR-222, have been found during the adipogenic programming of both immortalized and primary hMSCs, suggesting that they act as negative regulators of differentiation. Conversely, ectopic expression of the miRNAs 155,221, and 222 significantly inhibited adipogenesis and repressed induction of the master regulators PPARγ and CCAAT/enhancer-binding protein alpha (CEBPA). This paves the way for possible genetic obesity treatments.

Another class of miRNAs that regulate insulin resistance, obesity, and diabetes, is the let-7 family. Let-7 accumulates in human tissues during the course of aging. When let-7 was ectopically overexpressed to mimic accelerated aging, mice became insulin-resistant, and thus more prone to high fat diet-induced obesity and diabetes. In contrast when let-7 was inhibited by injections of let-7-specific antagomirs, mice become more insulin-sensitive and remarkably resistant to high fat diet-induced obesity and diabetes. Not only could let-7 inhibition prevent obesity and diabetes, it could also reverse and cure the condition. These experimental findings suggest that let-7 inhibition could represent a new therapy for obesity and type 2 diabetes.

Non-coding RNAs

When the human genome project mapped its first chromosome in 1999, it was predicted the genome would contain over 100,000 protein coding genes. However, only around 20,000 were eventually identified. Since then, the advent of bioinformatics approaches combined with genome tiling studies examining the transcriptome, systematic sequencing of full length cDNA libraries and experimental validation (including the creation of miRNA derived antisense oligonucleotides called antagomirs) have revealed that many transcripts are non protein-coding RNA, including several snoRNAs and miRNAs.

Viruses

Viral microRNAs play an important role in the regulation of gene expression of viral and/or host genes to benefit the virus. Hence, miRNAs play a key role in host–virus interactions and pathogenesis of viral diseases. The expression of transcription activators by human herpesvirus-6 DNA is believed to be regulated by viral miRNA.

Target prediction

See also: List of RNA structure prediction software § Inter molecular interactions: MicroRNA:UTR

miRNAs can bind to target messenger RNA (mRNA) transcripts of protein-coding genes and negatively control their translation or cause mRNA degradation. It is of key importance to identify the miRNA targets accurately. A comparison of the predictive performance of eighteen in silico algorithms is available.

See also

• Gene expression
• List of miRNA gene prediction tools
• List of miRNA target prediction tools
• RNAi
• siRNA
• Small nucleolar RNA derived microRNA
• Anti-miRNA oligonucleotides

Bibliography

• miRNA definition and classification: Ambros V, Bartel B, Bartel DP, Burge CB, Carrington JC, Chen X, Dreyfuss G, Eddy SR, Griffiths-Jones S, Marshall M, Matzke M, Ruvkun G, Tuschl T (2003). "A uniform system for microRNA annotation". RNA. 9 (3): 277–279. PMC 1370393 . PMID 12592000. doi:10.1261/rna.2183803. 
• Science review of small RNA: Baulcombe D (2002). "DNA events. An RNA microcosm". Science. 297 (5589): 2002–2003. PMID 12242426. doi:10.1126/science.1077906. 
• Discovery of lin-4, the first miRNA to be discovered: Lee RC, Feinbaum RL, Ambros V (1993). "The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14". Cell. 75 (5): 843–854. PMID 8252621. doi:10.1016/0092-8674(93)90529-Y. 

External links

• The miRBase database
• miR2Disease, a manually curated database documenting known relationships between miRNA dysregulation and human disease.
• semirna, Web application to search for microRNAs in a plant genome.
• ONCO.IO: Integrative resource for microRNA and transcription factors analysis in cancer.
• MirOB: MicroRNA targets database and data analysis and dataviz tool.
• ChIPBase database: An open access database for decoding the transcription factors that were involved in or affected the transcription of microRNAs from ChIP-seq data.
• An animated video of the microRNA biogenesis process.

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• 1. 臨床腫瘍学における遺伝子発現プロファイリング、プロテオミクス、およびマイクロRNAプロファイリングの概要 overview of gene expression profiling proteomics and microrna profiling in clinical oncology
• 2. 好酸球性食道炎（EoE）：遺伝学および免疫病原性 eosinophilic esophagitis eoe genetics and immunopathogenesis
• 3. 遺伝学およびゲノミクスのためのツール：遺伝子発現プロファイリング tools for genetics and genomics gene expression profiling
• 4. 甲状腺結節および非髄様甲状腺癌におけるオンコジーンおよび腫瘍抑制遺伝子 oncogenes and tumor suppressor genes in thyroid nodules and nonmedullary thyroid cancer
• 5. 原発性肝細胞癌の臨床的特徴および診断 clinical features and diagnosis of primary hepatocellular carcinoma

English Journal

• MicroRNAs: Novel mediators of resistance to microtubule-targeting agents.

• Kanakkanthara A, Miller JH.SourceCentre for Biodiscovery and School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand.
• Cancer treatment reviews.Cancer Treat Rev.2013 Apr;39(2):161-70. doi: 10.1016/j.ctrv.2012.07.005. Epub 2012 Aug 16.
• MicroRNAs (miRNAs) are small, non-coding RNAs that act as expression regulators of genes involved in diverse cellular processes, including cell proliferation, differentiation, and apoptosis. Abnormal expression of miRNAs can have profound effects on cellular function, and miRNAs are implicated in tu
• PMID 22902296

• An electrochemical sensor based on label-free functional allosteric molecular beacons for detection target DNA/miRNA.

• Cai Z, Song Y, Wu Y, Zhu Z, James Yang C, Chen X.SourceDepartment of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
• Biosensors & bioelectronics.Biosens Bioelectron.2013 Mar 15;41:783-8. doi: 10.1016/j.bios.2012.10.002. Epub 2012 Oct 8.
• We report two novel electrochemical sensors (E-sensors) for the detection of target DNA and miRNA. The E-sensors were fabricated using label-free functional allosteric molecular beacons (aMBs), which can form streptavidin aptamers to bind to streptavidin peroxidase polymer and so generate catalytic
• PMID 23102830

• microRNAs: fine tuning of erythropoiesis.

• Listowski MA, Heger E, Bogusławska DM, Machnicka B, Kuliczkowski K, Leluk J, Sikorski AF.SourceDepartment of Molecular Biology, University of Zielona, Góra, Poland.
• Cellular & molecular biology letters.Cell Mol Biol Lett.2013 Mar;18(1):34-46. doi: 10.2478/s11658-012-0038-z. Epub 2012 Nov 3.
• Cell proliferation and differentiation is a complex process involving many cellular mechanisms. One of the best-studied phenomena in cell differentiation is erythrocyte development during hematopoiesis in vertebrates. In recent years, a new class of small, endogenous, non-coding RNAs called microRNA
• PMID 23124859

Japanese Journal

• Competitive target gene regulation by promoter methylation and miRNA

• Y-H.Taguchi
• 情報処理学会研究報告. BIO, バイオ情報学 2012-BIO-31(1), 1-6, 2012-10-08
• … miRNA regulation of target genes and promoter methylation were known as primary epigenetic regulation of gene expression. … We found that promoter methylation was miRNA-targeting-specific. … It was also found that miRNA-targeting-specific promoter hypomethylation was related to the miRNA regulation of target genes; …
• NAID 110009459611

• Transcriptome-Wide Characterization of miRNA-Directed and Non-miRNA-Directed Endonucleolytic Cleavage Using Degradome Analysis Under Low Ambient Temperature in Phalaenopsis aphrodite subsp. formosana

• An Feng-Ming,Chan Ming-Tsair
• Plant and Cell Physiology 53(10), 1737-1750, 2012-10
• NAID 40019457295

Related Links

• miRNA（microRNA）とは | コスモ・バイオ株式会社

microRNA（miRNA）は2つの連続したプロセスを経て生合成されます。Primary miRNA転写物（pri-miRNA）はステム-ループのヘアピン構造を1つまたは複数もち、その多くはPol IIを介した転写により産生します。典型的なmiRNAの成熟経路 ...

• miRNA｜研究用語辞典｜研究.net

miRNA 【miRNAとは】 miRNA（micro-RNA）とは、遺伝子発現を抑制する効果を持つ21～25塩基程度の一本鎖RNAです。 ゲノム上にコードされていますが蛋白質へは翻訳されないnon-cording RNAで、分化、細胞増殖、アポトーシスなど ...

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