miR-132 enhances dendritic morphogenesis, spine density, synaptic integration, and survival of newborn olfactory bulb neurons

An emerging role for microRNAs in sexually dimorphic neurobiological systems

Over the past 20 years, our understanding of the basic mechanisms of gene regulation has vastly expanded due to the unexpected roles of small regulatory RNAs, in particular microRNAs (miRNAs). miRNAs add another layer of complexity to the regulation of effector molecules for nearly every physiological process, making them excellent candidate molecules as therapeutic targets, biomarkers, and disease predictors. Hormonal contributions to mature miRNA expression, biosynthetic processing, and downstream functions have only just begun to be investigated. Elucidating the physiological consequences of miRNA sexual dimorphism, and their associated regulatory processes, may be key toward understanding both normal and pathological processes in the brain. This short review provides a basic overview of miRNA biosynthesis, their role in normal brain development, and potential links to neurological diseases. We conclude with a brief discussion of the current knowledge of sex-specific miRNA processes in both the brain and the heart to conceptually integrate the relevance of miRNAs with the overarching theme ("sex differences in health and disease: brain and heart connections") of this special topics issue.

FUS stimulates microRNA biogenesis by facilitating co-transcriptional Drosha recruitment

microRNA abundance has been shown to depend on the amount of the microprocessor components or, in some cases, on specific auxiliary co-factors. In this paper, we show that the FUS/TLS (fused in sarcoma/translocated in liposarcoma) protein, associated with familial forms of Amyotrophic Lateral Sclerosis (ALS), contributes to the biogenesis of a specific subset of microRNAs. Among them, species with roles in neuronal function, differentiation and synaptogenesis were identified. We also show that FUS/TLS is recruited to chromatin at sites of their transcription and binds the corresponding pri-microRNAs. Moreover, FUS/TLS depletion leads to decreased Drosha level at the same chromatin loci. Limited FUS/TLS depletion leads to a reduced microRNA biogenesis and we suggest a possible link between FUS mutations affecting nuclear/cytoplasmic partitioning of the protein and altered neuronal microRNA biogenesis in ALS pathogenesis.

Hypoxia-inducible factor 1a is a Tsc1-regulated survival factor in newborn neurons in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in TSC1 or TSC2 resulting in hyperactivity of the mammalian target of rapamycin and disabling brain lesions. These lesions contain misplaced neurons enriched in hypoxia-inducible factor 1a (HIF1a). However, the relationship between TSC1/2 and HIF1a and the function of HIF1a in TSC neurons remain unexplored. Here, we examine the degree of HIF1a activity and its function in newborn Tsc1(null) neurons in a mouse model of TSC. Using single cell electroporation in the neurogenic subventricular zone (SVZ) of neonatal mice, we deleted Tsc1 and generated olfactory lesions containing misplaced Tsc1(null) neurons as previously reported. These newborn neurons displayed elevated HIF1a-mediated transcriptional activity when compared with Tsc1 heterozygote neurons and a marked resistance to cell death induced by a HIF1a antagonist. Electroporation of Hif1a targeting short hairpin RNA (shRNA) or dominant negative HIF1a constructs resulted in 80-90% loss of Tsc1(null) newborn neurons although sparing SVZ stem cells. Consistent with this later finding, induction of Hif1a shRNA expression during synaptic integration thus bypassing neuron production also resulted in newborn neuron death. Collectively, these results suggest that HIF1a acts as a molecular determinant of newborn neuron survival and that its TSC1-dependent up-regulation gave Tsc1(null) neurons a survival advantage, despite their misplacement in a novel microenvironment.

Adult neural stem cells: an endogenous tool to repair brain injury?

Research on stem cells has developed as one of the most promising areas of neurobiology. In the beginning of the 1990s, neurogenesis in the adult brain was indisputably accepted, eliciting great research efforts. Neural stem cells in the adult mammalian brain are located in the 'neurogenic' areas of the subventricular and subgranular zones. Nevertheless, many reports indicate that they subsist in other regions of the adult brain. Adult neural stem cells have arisen considerable interest as these studies can be useful to develop new methods to replace damaged neurons and treat severe neurological diseases such as neurodegeneration, stroke or spinal cord lesions. In particular, a promising field is aimed at stimulating or trigger a self-repair system in the diseased brain driven by its own stem cell population. Here, we will revise the latest findings on the characterization of active and quiescent adult neural stem cells in the main regions of neurogenesis and the factors necessary to maintain their active and resting states, stimulate migration and homing in diseased areas, hoping to outline the emerging knowledge for the promotion of regeneration in the brain based on endogenous stem cells.

miRNA regulons associated with synaptic function

Differential RNA localization and local protein synthesis regulate synapse function and plasticity in neurons. MicroRNAs are a conserved class of regulatory RNAs that control mRNA stability and translation in tissues. They are abundant in the brain but the extent into which they are involved in synaptic mRNA regulation is poorly known. Herein, a computational analysis of the coding and 3′UTR regions of 242 presynaptic and 304 postsynaptic proteins revealed that 91% of them are predicted to be microRNA targets. Analysis of the longest 3′UTR isoform of synaptic transcripts showed that presynaptic mRNAs have significantly longer 3′UTR than control and postsynaptic mRNAs. In contrast, the shortest 3′UTR isoform of postsynaptic mRNAs is significantly shorter than control and presynaptic mRNAs, indicating they avert microRNA regulation under specific conditions. Examination of microRNA binding site density of synaptic 3′UTRs revealed that they are twice as dense as the rest of protein-coding transcripts and that approximately 50% of synaptic transcripts are predicted to have more than five different microRNA sites. An interaction map exploring the association of microRNAs and their targets revealed that a small set of ten microRNAs is predicted to regulate 77% and 80% of presynaptic and postsynaptic transcripts, respectively. Intriguingly, many of these microRNAs have yet to be identified outside primate mammals, implicating them in cognition differences observed between high-level primates and non-primate mammals. Importantly, the identified miRNAs have been previously associated with psychotic disorders that are characterized by neural circuitry dysfunction, such as schizophrenia. Finally, molecular dissection of their KEGG pathways showed enrichment for neuronal and synaptic processes. Adding on current knowledge, this investigation revealed the extent of miRNA regulation at the synapse and predicted critical microRNAs that would aid future research on the control of neuronal plasticity and etiology of psychiatric diseases.

MicroRNAs shape the neuronal landscape

The nervous system equips us with capability to adapt to many conditions and circumstances. We rely on an armamentarium of intricately formed neural circuits for many of our adaptive strategies. However, this capability also depends on a well-conserved toolkit of different molecular mechanisms that offer not only compensatory responses to a changing world, but also provide plasticity to achieve changes in cellular state that underlie a broad range of processes from early developmental transitions to life-long memory. Among the molecular tools that mediate changes in cellular state, our understanding of posttranscriptional regulation of gene expression is expanding rapidly. Part of the "epigenetic landscape" that shapes the deployment and robust regulation of gene networks during the construction and the remodeling of the brain is the microRNA system controlling both levels and translation of messenger RNA. Here we consider recent advances in the study of microRNA-mediated regulation of synaptic form and function.