Protein interactions and complexes in human microRNA biogenesis and function

Encoded in the genome of most eukaryotes, microRNAs (miRNAs) have been proposed to regulate specifically up to 90% of human genes through a process known as miRNA-guided RNA silencing. The aim of this review is to present this process as the integration of a succession of specialized molecular machines exerting well defined functions. The nuclear microprocessor complex initially recognizes and processes its primary miRNA substrate into a miRNA precursor (pre-miRNA). This structure is then exported to the cytoplasm by the Exportin-5 complex where it is presented to the pre-miRNA processing complex. Following pre-miRNA conversion into a miRNA:miRNA* duplex, this complex is assembled into a miRNA-containing ribonucleoprotein (miRNP) complex, after which the miRNA strand is selected. The degree of complementarity of the miRNA for its messenger RNA (mRNA) target guides the recruitment of the miRNP complex. Initially repressing its translation, the miRNP-silenced mRNA is directed to the P-bodies, where the mRNA is either released from its inhibition upon a cellular signal and/or actively degraded. The potency and specificity of miRNA biogenesis and function rely on the distinct protein x protein, protein x RNA and RNA:RNA interactions found in different complexes, each of which fulfill a specific function in a well orchestrated process

Rôle d'ARF3 dans le cytosquelette d'actine chez Saccharomyces cerevisiae

Chez la levure S. cerevisiae, plusieurs protéines participent dans l'organisation du cytosquelette d'actine. L'une d'entre elles, la profiline, est impliquée dans la polymérisation des filaments d'actine. Les cellules pfy1? ont un phénotype anormal, dont la dépolarisation des granules corticaux et l'absence de câbles d'actine visibles. L'équipe du Dr Pallotta a identifié plusieurs protéines impliquées dans un sentier de signalisation menant à cette structure. Deux de ces protéines, Gea1/2p, interagissent avec les protéines Arf. Nous avons donc étudié le rôle d'Arf3p et ainsi déterminé son implication dans la polarisation du cytosquelette d'actine. Sa surexpression dans la souche pfy1-111, un mutant thermosensible, corrige son phénotype. Il existe une interaction génétique entre PFY1 et ARF3. La mutagenèse dirigée de la protéine, sa localisation et une comparaison avec Arf6p humaine a complété l'étude. Nous pouvons conclure que Gea1/2p passent par Arf3p, au moins partiellement, afin de rétablir les phénotypes des cellules déficientes en profiline

Étude de l'interaction entre la ribonucléase Dicer et la cytoskeleton-linking endoplasmic reticulum membrane protein of 63 kDa (CLIMP-63)

La voie endogène des microARN (miARN) est un processus de régulation cellulaire responsable principalement de réguler la traduction des ARN messagers (ARNm). Les miARN sont de petits ARN d'environ 21 à 23 nucleotides qui se lient spécifiquement à des sites de liaison retrouvés dans la région 3' non-codante des ARNm. Bien que le processus régule une panoplie d'ARNm différents, seulement quelques composantes protéiques sont impliquées afin de réaliser la biogenèse des miARN et de médier leur action régulatrice sur l'expression des gènes. La ribonucléase III Dicer est responsable de la génération des miARN, alors que la protéine Argonaute 2 (Ago2) est au centre du complexe effecteur de nature microribonucléoprotéique (miRNP). Afin d'étudier le fonctionnement de la voie des miARN chez l'humain, nous avons élaboré différents protocoles afin de pouvoir monitorer l'activité catalytique des protéines Dicer et Ago2 in vitro. Dicer a précédemment été localisée au reticulum endoplasmique (RE), mais ne possède pas de signal connu de localisation au RE. L'objectif général de mon projet de doctorat était donc d'identifier si des partenaires protéiques peuvent expliquer la présence de Dicer au RE. Dans ce travail, nous avons identifié et caractérisé la «cytoskeleton-linking endoplasmic reticulum membrane protein of 63 kDa» (CLIMP-63) comme nouveau partenaire protéique de Dicer. Impliquant une portion luminale de la CLIMP-63, l'interaction semble se produire à l'intérieur du RE. Dans les cellules humaines, on retrouve un complexe DicerCLIMP-63 de très haut poids moléculaire, une caractéristique possiblement conférée par la formation d'oligomères de la CLIMP-63. Le marquage métabolique des protéines cellulaires a permis d'observer que la CLIMP-63 interagit avec la forme nouvellement synthétisée de Dicer. La CLIMP-63 semble stabiliser la protéine Dicer, car sa surexpression augmente l'expression de la protéine endogène et surexprimée. La diminution de l'expression de la CLIMP-63, quant à elle, influence négativement l'expression de Dicer de même que celle d'un gène rapporteur contenant des sites de liaison à un miARN. L'ensemble de ces résultats suggèrent que la CLIMP-63 est importante pour le bon fonctionnement de la voie endogène des miARN en interagissant et en stabilisant les formes nouvellement synthétisées de la protéine Dicer

Regulation of human Dicer by the resident ER membrane protein CLIMP-63

The ribonuclease Dicer plays a central role in the microRNA pathway by catalyzing the formation of microRNAs, which are known to regulate messenger RNA (mRNA) translation. In order to improve our understanding of the molecular context in which Dicer functions and how it is regulated in human cells, we sought to expand its protein interaction network by employing a yeast two-hybrid screening strategy. This approach led to the identification and characterization of cytoskeleton-linking endoplasmic reticulum (ER) membrane protein of 63 kDa (CLIMP-63) as a novel Dicer-interacting protein. CLIMP-63 interacts with Dicer to form a high molecular weight complex, which is electrostatic in nature, is not mediated by RNA and is catalytically active in pre-microRNA processing. CLIMP-63 is required for stabilizing Dicer protein and for optimal regulation of a reporter gene coupled to the 3′ untranslated region of HMGA2 mRNA in human cells. Interacting with a portion of the luminal domain of CLIMP-63 and within minutes of its synthesis, our results suggest that Dicer transits through the ER, is glycosylated and can be secreted by cultured human cells with CLIMP-63. Our findings define CLIMP-63 as a novel protein interactor and regulator of Dicer function, involved in maintaining Dicer protein levels in human cells

Regulatory RNAs : future perspectives in diagnosis, prognosis, and individualized therapy

With potentially up to 1000 microRNAs (miRNAs) present in the human genome, altogether regulating the expression of thousands of genes, one can anticipate that miRNAs will play a significant role in health and disease. Deregulated protein expression induced by a dysfunctional miRNA-based regulatory system is thus expected to lead to the development of serious, if not lethal, genetic diseases. A relationship among miRNAs, Dicer, and cancer has recently been suggested. Further investigations will help establish specific causal links between dysfunctional miRNAs and diseases. miRNAs of foreign origin, e.g., viruses, may also be used as specific markers of viral infections. In these cases, miRNA expression profiles could represent a powerful diagnostic tool. Regulatory RNAs may also have therapeutic applications, by which disease-causing genes or viral miRNAs could be neutralized, or functional miRNAs be restored. Will bedside miRNA expression profiling eventually assist physicians in providing patients with accurate diagnosis, personalized therapy, and treatment outcome

MicroRNAs in Gene Regulation: When the Smallest Governs It All

Encoded by the genome of most eukaryotes examined so far, microRNAs (miRNAs) are small ~21-nucleotide (nt) noncoding RNAs (ncRNAs) derived from a biosynthetic cascade involving sequential processing steps executed by the ribonucleases (RNases) III Drosha and Dicer. Following their recent identification, miRNAs have rapidly taken the center stage as key regulators of gene expression. In this review, we will summarize our current knowledge of the miRNA biosynthetic pathway and its protein components, as well as the processes it regulates via miRNAs, which are known to exert a variety of biological functions in eukaryotes. Although the relative importance of miRNAs remains to be fully appreciated, deregulated protein expression resulting from either dysfunctional miRNA biogenesis or abnormal miRNA-based gene regulation may represent a key etiologic factor in several, as yet unidentified, diseases. Hence is our need to better understand the complexity of the basic mechanisms underlying miRNA biogenesis and function

Blanc-persiste

Platelets have a crucial role in the maintenance of hemostasis as well as in thrombosis and vessel occlusion, which underlie stroke and acute coronary syndromes. Anucleate platelets contain mRNAs and are capable of protein synthesis, raising the issue of how these mRNAs are regulated. Here we show that human platelets harbor an abundant and diverse array of microRNAs (miRNAs), which are known as key regulators of mRNA translation in other cell types. Further analyses revealed that platelets contain the Dicer and Argonaute 2 (Ago2) complexes, which function in the processing of exogenous miRNA precursors and the control of specific reporter transcripts, respectively. Detection of the receptor P2Y₁₂ mRNA in Ago2 immunoprecipitates suggests that P2Y₁₂ expression may be subjected to miRNA control in human platelets. Our study lends an additional level of complexity to the control of gene expression in these anucleate elements of the cardiovascular system