The transcription factor NRSF contributes to epileptogenesis by selective repression of a subset of target genes

Epileptogenesis

Epileptogenesis is a chronic process that can be triggered by genetic or acquired factors, and that can continue long after epilepsy diagnosis. In 2015, epileptogenesis is not a treatment indication, and there are no therapies available in clinic to treat individuals at risk of epileptogenesis. However, thanks to active research, a large number of animal models have become available for search of molecular mechanisms of epileptogenesis. The first glimpses of treatment targets and biomarkers that could be developed to become useful in clinic are in sight. However, the heterogeneity of the epilepsy condition, and the dynamics of molecular changes over the course of epileptogenesis remain as challenges to overcome.

Harnessing the master transcriptional repressor REST to reciprocally regulate neurogenesis

Neurogenesis begins in embryonic development and continues at a reduced rate into adulthood in vertebrate species, yet the signaling cascades regulating this process remain poorly understood. Plasma membrane-initiated signaling cascades regulate neurogenesis via downstream pathways including components of the transcriptional machinery. A nuclear factor that temporally regulates neurogenesis by repressing neuronal differentiation is the repressor element 1 (RE1) silencing transcription (REST) factor. We have recently discovered a regulatory site on REST that serves as a molecular switch for neuronal differentiation. Specifically, C-terminal domain small phosphatase 1, CTDSP1, present in non-neuronal cells, maintains REST activity by dephosphorylating this site. Reciprocally, extracellular signal-regulated kinase, ERK, activated by growth factor signaling in neural progenitors, and peptidylprolyl cis/trans isomerase Pin1, decrease REST activity through phosphorylation-dependent degradation. Our findings further resolve the mechanism for temporal regulation of REST and terminal neuronal differentiation. They also provide new potential therapeutic targets to enhance neuronal regeneration after injury.

The Transcription Repressor REST in Adult Neurons: Physiology, Pathology, and Diseases

REST [RE1-silencing transcription factor (also called neuron-restrictive silencer factor)] is known to repress thousands of possible target genes, many of which are neuron specific. To date, REST repression has been investigated mostly in stem cells and differentiating neurons. Current evidence demonstrates its importance in adult neurons as well. Low levels of REST, which are acquired during differentiation, govern the expression of specific neuronal phenotypes. REST-dependent genes encode important targets, including transcription factors, transmitter release proteins, voltage-dependent and receptor channels, and signaling proteins. Additional neuronal properties depend on miRNAs expressed reciprocally to REST and on specific splicing factors. In adult neurons, REST levels are not always low. Increases occur during aging in healthy humans. Moreover, extensive evidence demonstrates that prolonged stimulation with various agents induces REST increases, which are associated with the repression of neuron-specific genes with appropriate, intermediate REST binding affinity. Whether neuronal increases in REST are protective or detrimental remains a subject of debate. Examples of CA1 hippocampal neuron protection upon depolarization, and of neurodegeneration upon glutamate treatment and hypoxia have been reported. REST participation in psychiatric and neurological diseases has been shown, especially in Alzheimer’s disease and Huntington’s disease, as well as epilepsy. Distinct, complex roles of the repressor in these different diseases have emerged. In conclusion, REST is certainly very important in a large number of conditions. We suggest that the conflicting results reported for the role of REST in physiology, pathology, and disease depend on its complex, direct, and indirect actions on many gene targets and on the diverse approaches used during the investigations.

REST represses a subset of the pancreatic endocrine differentiation program

To contribute to devise successful beta-cell differentiation strategies for the cure of Type 1 diabetes we sought to uncover barriers that restrict endocrine fate acquisition by studying the role of the transcriptional repressor REST in the developing pancreas. Rest expression is prevented in neurons and in endocrine cells, which is necessary for their normal function. During development, REST represses a subset of genes in the neuronal differentiation program and Rest is down-regulated as neurons differentiate. Here, we investigate the role of REST in the differentiation of pancreatic endocrine cells, which are molecularly close to neurons. We show that Rest is widely expressed in pancreas progenitors and that it is down-regulated in differentiated endocrine cells. Sustained expression of REST in Pdx1(+) progenitors impairs the differentiation of endocrine-committed Neurog3(+) progenitors, decreases beta and alpha cell mass by E18.5, and triggers diabetes in adulthood. Conditional inactivation of Rest in Pdx1(+) progenitors is not sufficient to trigger endocrine differentiation but up-regulates a subset of differentiation genes. Our results show that the transcriptional repressor REST is active in pancreas progenitors where it gates the activation of part of the beta cell differentiation program.

Rest represses maturation within migrating facial branchiomotor neurons

The vertebrate brain arises from the complex organization of millions of neurons. Neurogenesis encompasses not only cell fate specification from neural stem cells, but also the terminal molecular and morphological maturation of neurons at correct positions within the brain. RE1-silencing transcription factor (Rest) is expressed in non-neural tissues and neuronal progenitors where it inhibits the terminal maturation of neurons by repressing hundreds of neuron-specific genes. Here we show that Rest repression of maturation is intimately linked with the migratory capability of zebrafish facial branchiomotor neurons (FBMNs), which undergo a characteristic tangential migration from hindbrain rhombomere (r) 4 to r6/r7 during development. We establish that FBMN migration is increasingly disrupted as Rest is depleted in zebrafish rest mutant embryos, such that around two-thirds of FBMNs fail to complete migration in mutants depleted of both maternal and zygotic Rest. Although Rest is broadly expressed, we show that de-repression or activation of Rest target genes only within FBMNs is sufficient to disrupt their migration. We demonstrate that this migration defect is due to precocious maturation of FBMNs, based on both morphological and molecular criteria. We further show that the Rest target gene and alternative splicing factor srrm4 is a key downstream regulator of maturation; Srrm4 knockdown partially restores the ability of FBMNs to migrate in rest mutants while preventing their precocious morphological maturation. Rest must localize to the nucleus to repress its targets, and its subcellular localization is highly regulated: we show that targeting Rest specifically to FBMN nuclei rescues FBMN migration in Rest-deficient embryos. We conclude that Rest functions in FBMN nuclei to inhibit maturation until the neurons complete their migration.