Elik Chapnik

miRNA malfunction causes spinal motor neuron disease

Defective RNA metabolism is an emerging mechanism involved in ALS pathogenesis and possibly in other neurodegenerative disorders. Here, we show that microRNA (miRNA) activity is essential for long-term survival of postmitotic spinal motor neurons (SMNs) in vivo. Thus, mice that do not process miRNA in SMNs exhibit hallmarks of spinal muscular atrophy (SMA), including sclerosis of the spinal cord ventral horns, aberrant end plate architecture, and myofiber atrophy with signs of denervation. Furthermore, a neurofilament heavy subunit previously implicated in motor neuron degeneration is specifically up-regulated in miRNA-deficient SMNs. We demonstrate that the heavy neurofilament subunit is a target of miR-9, a miRNA that is specifically down-regulated in a genetic model of SMA. These data provide evidence for miRNA function in SMN diseases and emphasize the potential role of miR-9–based regulatory mechanisms in adult neurons and neurodegenerative states.

MicroRNA regulation of the paired-box transcription factor Pax3 confers robustness to developmental timing of myogenesis

Commitment of progenitors in the dermomyotome to myoblast fate is the first step in establishing the body musculature. Pax3 is a crucial transcription factor, important for skeletal muscle development and expressed in myogenic progenitors in the dermomyotome of developing somites and in migratory muscle progenitors that populate the limb buds. Down-regulation of Pax3 is essential to ignite the myogenic program, including up-regulation of myogenic regulators, Myf-5 and MyoD. MicroRNAs (miRNAs) confer robustness to developmental timing by posttranscriptional repression of genetic programs that are related to previous developmental stages or to alternative cell fates. Here we demonstrate that the muscle-specific miRNAs miR-1 and miR-206 directly target Pax3. Antagomir-mediated inhibition of miR-1/miR-206 led to delayed myogenic differentiation in developing somites, as shown by transient loss of myogenin expression. This correlated with increased Pax3 and was phenocopied using Pax3-specific target protectors. Loss of myogenin after antagomir injection was rescued by Pax3 knockdown using a splice morpholino, suggesting that miR-1/miR-206 control somite myogenesis primarily through interactions with Pax3. Our studies reveal an important role for miR-1/miR-206 in providing precision to the timing of somite myogenesis. We propose that posttranscriptional control of Pax3 downstream of miR-1/miR-206 is required to stabilize myoblast commitment and subsequent differentiation. Given that mutually exclusive expression of miRNAs and their targets is a prevailing theme in development, our findings suggest that miRNA may provide a general mechanism for the unequivocal commitment underlying stem cell differentiation.

RNA viruses can hijack vertebrate microRNAs to suppress innate immunity

Currently, there is little evidence for a notable role of the vertebrate microRNA (miRNA) system in the pathogenesis of RNA viruses. This is primarily attributed to the ease with which these viruses mutate to disrupt recognition and growth suppression by host miRNAs. Here we report that the haematopoietic-cell-specific miRNA miR-142-3p potently restricts the replication of the mosquito-borne North American eastern equine encephalitis virus in myeloid-lineage cells by binding to sites in the 3′ non-translated region of its RNA genome. However, by limiting myeloid cell tropism and consequent innate immunity induction, this restriction directly promotes neurologic disease manifestations characteristic of eastern equine encephalitis virus infection in humans. Furthermore, the region containing the miR-142-3p binding sites is essential for efficient virus infection of mosquito vectors. We propose that RNA viruses can adapt to use antiviral properties of vertebrate miRNAs to limit replication in particular cell types and that this restriction can lead to exacerbation of disease severity.

Mononuclear phagocyte miRNome analysis identifies miR-142 as critical regulator of murine dendritic cell homeostasis

The mononuclear phagocyte system comprises cells as diverse as monocytes, macrophages, and dendritic cells (DCs), which collectively play key roles in innate immune responses and the triggering of adaptive immunity. Recent studies have highlighted the role of growth and transcription factors in defining developmental pathways and lineage relations within this cellular compartment. However, contributions of miRNAs to the development of mononuclear phagocytes remain largely unknown. In the present study, we report a comprehensive map of miRNA expression profiles for distinct myeloid populations, including BM-resident progenitors, monocytes, and mature splenic DCs. Each of the analyzed cell populations displayed a distinctive miRNA profile, suggesting a role for miRNAs in defining myeloid cell identities. Focusing on DC development, we found miR-142 to be highly expressed in classic FLT3-L–dependent CD4+ DCs, whereas reduced expression was observed in closely related CD8α+ or CD4- CD8α- DCs. Moreover, mice deficient for miR-142 displayed an impairment of CD4+ DC homeostasis both in vitro and in vivo. Furthermore, loss of miR-142–dependent CD4+ DCs was accompanied by a severe and specific defect in the priming of CD4+ T cells. The results of our study establish a novel role for miRNAs in myeloid cell specification and define miR-142 as a pivotal genetic component in the maintenance of CD4+ DCs.

miR-142 orchestrates a network of actin cytoskeleton regulators during megakaryopoiesis

Genome-encoded microRNAs (miRNAs) provide a posttranscriptional regulatory layer that controls the differentiation and function of various cellular systems, including hematopoietic cells. miR-142 is one of the most prevalently expressed miRNAs within the hematopoietic lineage. To address the in vivo functions of miR-142, we utilized a novel reporter and a loss-of-function mouse allele that we have recently generated. In this study, we show that miR-142 is broadly expressed in the adult hematopoietic system. Our data further reveal that miR-142 is critical for megakaryopoiesis. Genetic ablation of miR-142 caused impaired megakaryocyte maturation, inhibition of polyploidization, abnormal proplatelet formation, and thrombocytopenia. Finally, we characterized a network of miR-142-3p targets which collectively control actin filament homeostasis, thereby ensuring proper execution of actin-dependent proplatelet formation. Our study reveals a pivotal role for miR-142 activity in megakaryocyte maturation and function, and demonstrates a critical contribution of a single miRNA in orchestrating cytoskeletal dynamics and normal hemostasis. DOI: http://dx.doi.org/10.7554/eLife.01964.001

Dgcr8 controls neural crest cells survival in cardiovascular development

DiGeorge syndrome (DGS), characterized genetically by a deletion within chromosome 22q11.2, is associated with a constellation of congenital heart defects. DiGeorge critical region 8 (Dgcr8), a gene that maps to the common deletion region of DGS, encodes a double stranded RNA-binding protein that is essential for miRNA biogenesis. To address the potential contribution of Dgcr8 insufficiency to cardiovascular development, we have inactivated Dgcr8 in cardiac neural crest cells (cNCCs). Dgcr8 mutants displayed a wide spectrum of malformations, including persistent truncus arteriosus (PTA) and ventricular septal defect (VSD). Interestingly, Dgcr8-null cNCCs that properly migrated into the cardiac outflow tract (OFT), proliferate normally and differentiate into vascular smooth muscle cells. However, loss of Dgcr8 causes a significant portion of the cNCCs to undergo apoptosis, causing a decrease in the pool of progenitors required for OFT remodeling. Our data uncover a new role of Dgcr8 in cardiovascular morphogenesis, plausibly as part of transmission mechanism for FGF-dependent survival cue for migrating cNCCs.

Brief Report: miR-290-295 Regulate Embryonic Stem Cell Differentiation Propensities by Repressing Pax6

microRNAs of the miR‐290–295 family are selectively expressed at high levels in mouse embryonic stem cells (mESCs) and have established roles in regulating self‐renewal. However, the potential influence of these microRNAs on cell fate acquisition during differentiation has been overlooked. Here, we show that miR‐290–295 regulate the propensity of mESCs to acquire specific fates. We generated a new miR‐290–295‐null mESC model, which exhibits increased propensity to generate ectoderm, at the expense of endoderm and mesoderm lineages. We further found that in wild‐type cells, miR‐290–295 repress Pax6 and ectoderm differentiation; accordingly, Pax6 knockdown partially rescues the mESCs differentiation impairment that is caused by loss of miR‐290–295. Thus, in addition to regulating self‐renewal, the large reservoir of miR‐290–295 in undifferentiated mESCs fine‐tunes the expression of master transcriptional factors, such as Pax6, thereby regulating the equilibrium of fate acquisition by mESC descendants. Stem Cells 2013;31:2266–2272

Erythrocyte survival is controlled by microRNA-142

Hematopoietic–specific microRNA-142 is a critical regulator of various blood cell lineages, but its role in erythrocytes is unexplored. Herein, we characterize the impact of microRNA-142 on erythrocyte physiology and molecular cell biology, using a mouse loss-of-function allele. We report that microRNA-142 is required for maintaining the typical erythrocyte biconcave shape and structural resilience, for the normal metabolism of reactive oxygen species, and for overall lifespan. microRNA-142 further controls ACTIN filament homeostasis and membrane skeleton organization. The analyses presented reveal previously unappreciated functions of microRNA-142 and contribute to an emerging view of small RNAs as key players in erythropoiesis. Finally, the work herein demonstrates how a housekeeping network of cytoskeletal regulators can be reshaped by a single micro-RNA denominator in a cell type specific manner.

MicroRNA-142 controls thymocyte proliferation

T‐cell development is a spatially and temporally regulated process, orchestrated by well‐defined contributions of transcription factors and cytokines. Here, we identify the noncoding RNA miR‐142 as an additional regulatory layer within murine thymocyte development and proliferation. MiR‐142 deficiency impairs the expression of cell cycle‐promoting genes in mature mouse thymocytes and early progenitors, accompanied with increased levels of cyclin‐dependent kinase inhibitor 1B (Cdkn1b, also known as p27Kip1). By using CRISPR/Cas9 technology to delete the miR‐142‐3p recognition element in the 3’UTR of cdkn1b, we confirm that this gene is a novel target of miR‐142‐3p in vivo. Increased Cdkn1b protein expression alone however was insufficient to cause proliferation defects in thymocytes, indicating the existence of additional critical miR‐142 targets. Collectively, we establish a key role for miR‐142 in the control of early and mature thymocyte proliferation, demonstrating the multifaceted role of a single miRNA on several target genes.

Rac1 functions downstream of miR-142 in regulation of erythropoiesis

Hematopoietic-specific miR-142 is a critical regulator of various blood cell lineages including CD4+ dendritic cells[1][1] and platelet biogenesis in megakaryocytes.[2][2] Furthermore, we recently reported that miR-142 is required in order to maintain the biconcave shape of erythrocytes, their