Chang-Mei Liu

Fragile X Mental Retardation Protein Regulates Proliferation and Differentiation of Adult Neural Stem/Progenitor Cells

Fragile X syndrome (FXS), the most common form of inherited mental retardation, is caused by the loss of functional fragile X mental retardation protein (FMRP). FMRP is an RNA–binding protein that can regulate the translation of specific mRNAs. Adult neurogenesis, a process considered important for neuroplasticity and memory, is regulated at multiple molecular levels. In this study, we investigated whether Fmrp deficiency affects adult neurogenesis. We show that in a mouse model of fragile X syndrome, adult neurogenesis is indeed altered. The loss of Fmrp increases the proliferation and alters the fate specification of adult neural progenitor/stem cells (aNPCs). We demonstrate that Fmrp regulates the protein expression of several components critical for aNPC function, including CDK4 and GSK3β. Dysregulation of GSK3β led to reduced Wnt signaling pathway activity, which altered the expression of neurogenin1 and the fate specification of aNPCs. These data unveil a novel regulatory role for Fmrp and translational regulation in adult neurogenesis.

MicroRNAs: key participants in gene regulatory networks

microRNAs (miRNAs) are a newly identified and surprisingly large class of endogenous tiny regulatory RNAs. They exhibit various expressional patterns and are highly conserved across species. Recently, several regulatory targets of miRNAs have been predicted. Functional analysis of the potential targets indicated that miRNAs may be involved in a wide range of pivotally biological events. The nature of miRNAs and their intersection with small interfering RNAs endow them with many regulatory advantages over proteins and make them a potent and novel means to regulate gene expression at almost all levels. Here we argue that miRNAs are key participants in gene regulatory network.

Epigenetic Regulation of the Stem Cell Mitogen Fgf-2 by Mbd1 in Adult Neural Stem/Progenitor Cells

Whether and how mechanisms intrinsic to stem cells modulate their proliferation and differentiation are two central questions in stem cell biology. Although exogenous basic fibroblast growth factor 2 (FGF-2/Fgf-2) is commonly used to expand adult neural stem/progenitor cells (NSPCs) in vitro, we do not yet understand the functional significance or the molecular regulation of Fgf-2 expressed endogenously by adult NSPCs. We previously demonstrated that methylated CpG binding protein 1 (MBD1/Mbd1) is a transcriptional repressor of Fgf-2 and is enriched in adult brains. Mbd1 deficiency in mice selectively affected adult neurogenesis and the differentiation of NSPCs. Here we show that an Mbd1 and DNA methylation-mediated epigenetic mechanism regulated the expression of stem cell mitogen Fgf-2 in adult NSPCs. Mbd1 bound to the Fgf-2 promoter and regulates its expression in adult NSPCs. In the absence of functional Mbd1, the Fgf-2 promoter was hypomethylated, and treatment with a DNA methylation inhibitor resulted in increased Fgf-2 expression in adult NSPCs. We further demonstrated that both acute knockdown of Mbd1 or overexpression of Fgf-2 in adult NSPCs inhibited their neuronal differentiation, which could be responsible for the neurogenic deficits observed in Mbd1-deficient mice. These data indicate that intrinsic epigenetic mechanisms play critical roles in the regulation of adult NSPC functions.

Myostatin antisense RNA-mediated muscle growth in normal and cancer cachexia mice.

Myostatin is a negative regulator of myogenesis, and inactivation of myostatin leads to muscle growth. Here we have used modified RNA oligonucleotides targeting the myostatin mRNA and examined the therapeutic potential in normal and cancer cachexia mouse models. We found that the RNA oligonucleotides could suppress the myostatin expression in vivo, leading to the increase in muscle growth both in normal and cachectic mice. We also established that the effect of myostatin inhibition caused by the RNA oligonucleotides may be through the MyoD pathway, as evidenced by a significant upregulation of MyoD expression. Taken together, these results demonstrate the feasibility using antisense strategy for the treatment of muscle wasting conditions.

MicroRNA-138 and SIRT1 form a mutual negative feedback loop to regulate mammalian axon regeneration

Regulated gene expression determines the intrinsic ability of neurons to extend axons, and loss of such ability is the major reason for the failed axon regeneration in the mature mammalian CNS. MicroRNAs and histone modifications are key epigenetic regulators of gene expression, but their roles in mammalian axon regeneration are not well explored. Here we report microRNA-138 (miR-138) as a novel suppressor of axon regeneration and show that SIRT1, the NAD-dependent histone deacetylase, is the functional target of miR-138. Importantly, we provide the first evidence that miR-138 and SIRT1 regulate mammalian axon regeneration in vivo. Moreover, we found that SIRT1 also acts as a transcriptional repressor to suppress the expression of miR-138 in adult sensory neurons in response to peripheral nerve injury. Therefore, miR-138 and SIRT1 form a mutual negative feedback regulatory loop, which provides a novel mechanism for controlling intrinsic axon regeneration ability.

PI3K–GSK3 signalling regulates mammalian axon regeneration by inducing the expression of Smad1

In contrast to neurons in the central nervous system, mature neurons in the mammalian peripheral nervous system can regenerate axons after injury, in part, by enhancing intrinsic growth competence. However, the signalling pathways that enhance the growth potential and induce spontaneous axon regeneration remain poorly understood. Here we reveal that phosphatidylinositol 3-kinase (PI3K) signalling is activated in response to peripheral axotomy and that PI3K pathway is required for sensory axon regeneration. Moreover, we show that glycogen synthase kinase 3 (GSK3), rather than mammalian target of rapamycin, mediates PI3K-dependent augmentation of the growth potential in the peripheral nervous system. Furthermore, we show that PI3K-GSK3 signal is conveyed by the induction of a transcription factor Smad1 and that acute depletion of Smad1 in adult mice prevents axon regeneration in vivo. Together, these results suggest PI3K-GSK3-Smad1 signalling as a central module for promoting sensory axon regeneration in the mammalian nervous system.

MicroRNAs in Adult and Embryonic Neurogenesis

Neurogenesis is defined as a process that includes the proliferation of neural stem/progenitor cells (NPCs) and the differentiation of these cells into new neurons that integrate into the existing neuronal circuitry. MicroRNAs (miRNAs) are a recently discovered class of small non-protein coding RNA molecules implicated in a wide range of diverse gene regulatory mechanisms. More and more data demonstrate that numerous miRNAs are expressed in a spatially and temporally controlled manners in the nervous system, which suggests that miRNAs have important roles in the gene regulatory networks involved in both brain development and adult neural plasticity. This review summarizes the roles of miRNAs-mediated gene regulation in the nervous system with focus on neurogenesis in both embryonic and adult brains.

Effect of RNA oligonucleotide targeting Foxo-1 on muscle growth in normal and cancer cachexia mice

Foxo-1, a member of the Foxo forkhead type transcription factors, is markedly upregulated in skeletal muscle in energy-deprived states such as fasting, cancer and severe diabetes. In this study, we target the Foxo-1 mRNA in a mouse skeletal myoblast cell line C2C12 and in vivo models of normal and cancer cachexia mice by a Foxo-1 specific RNA oligonucleotide. Our results demonstrate that the RNA oligonucleotide can reduce the expression of Foxo-1 in cells and in normal and cachectic mice, leading to an increase in skeletal muscle mass of the mice. In search for the possible downstream target genes of Foxo-1, we show that when Foxo-1 expression is blocked both in cells and in mice, the level of MyoD, a myogenic factor, is increased while a muscle negative regulator GDF-8 or myostatin is suppressed. Taken together, these results show that Foxo-1 pays a critical role in development of muscle atrophy, and suggest that Foxo-1 is a potential molecular target for treatment of muscle wasting conditions.

RNA-binding Protein FXR2 Regulates Adult Hippocampal Neurogenesis by Reducing Noggin Expression

In adult mammalian brains, neurogenesis persists in the subventricular zone of the lateral ventricles (SVZ) and the dentate gyrus (DG) of the hippocampus. Although evidence suggest that adult neurogenesis in these two regions is subjected to differential regulation, the underlying mechanism is unclear. Here, we show that the RNA-binding protein FXR2 specifically regulates DG neurogenesis by reducing the stability of Noggin mRNA. FXR2 deficiency leads to increased Noggin expression and subsequently reduced BMP signaling, which results in increased proliferation and altered fate specification of neural stem/progenitor cells in DG. In contrast, Noggin is not regulated by FXR2 in the SVZ, because Noggin expression is restricted to the ependymal cells of the lateral ventricles, where FXR2 is not expressed. Differential regulation of SVZ and DG stem cells by FXR2 may be a key component of the mechanism that governs the different neurogenic processes in these two adult germinal zones.

The role of small RNAs in human diseases: Potential troublemaker and therapeutic tools

Small RNAs, including short interfering RNAs (siRNAs) and microRNAs (miRNAs), are ubiquitous, versatile repressors of gene expression in plants, animals, and many fungi. They can trigger destruction of homologous mRNA or inhibition of cognate mRNA translation and play an important role in maintaining the stable state of chromosome structure and regulating the expression of protein‐coding genes. Furthermore, the recent research showed that there exists close relationship between small RNAs and human diseases. Several human diseases have surfaced in which miRNAs or their machinery might be implicated, such as some neurological diseases and cancers. The specificity and potency of small RNAs suggest that they might be promising as therapeutic agents. This article will review the role of small RNAs in some human diseases etiology and investigations of taking siRNAs as therapeutic tools for treating viral infection, cancer, and other diseases. We also discuss the potential of miRNAs in gene therapy.