In vivo post-transcriptional regulation of GAP-43 mRNA by overexpression of the RNA-binding protein HuD

The Antidepressant Effect of Resveratrol Is Related to Neuroplasticity Mediated by the ELAVL4-Bdnf mRNA Pathway

Resveratrol, a plant-derived polyphenol, exhibits significant antidepressant effects and notably enhances neuroplasticity in neurological diseases. However, whether the antidepressant function of resveratrol is related to neuroplasticity remains uncertain, and the underlying mechanisms is poorly understood. This study aims to investigate the role and mechanism of resveratrol in neuroplasticity in depression. Here, we adopted the chronic unpredictable mild stress (CUMS) model and resveratrol intervention by oral gavage. Thereafter, behavioral tests confirmed resveratrol's antidepressant effect, and Nissl staining, Golgi staining, and Western blotting (WB) were employed to assess the neuronal plasticity. Moreover, proteomic analysis and WB were used to screen and identify the key proteins. To investigate the downstream target of ELAV-like RNA-binding protein 4 (ELAVL4) (one of candidate genes), the RNA Interactome Database and the National Center for Biotechnology Information databases were utilized to predict the targets of ELAVL4. Finally, Quantitative PCR, WB, and Immunofluorescence were used to verify the prediction. Our results indicate that resveratrol alleviates CUMS-induced depressive-like behaviors accompanied by the restoration of impaired hippocampal neuroplasticity. Then, proteomic analysis shows that 351 differentially expressed proteins (DEPs) decrease after CUMS, while 24 DEPs increase remarkably with the resveratrol treatment. Among which, ELAVL4 is downregulated by CUMS, simultaneously increasing after resveratrol intervention, which acts as a protective protein in this process. Finally, brain-derived neurotrophic factor (Bdnf) mRNA is predicted to be the potential target of ELAVL4 and validated by molecular technologies. In conclusion, our findings demonstrate that resveratrol's antidepressant efficacy is closely associated with ELAVL4, an RNA-binding protein, a mediated neuroplasticity pathway, potentially intersecting with the Bdnf mRNA. Overall, this research sheds light on the role of the ELAVL4-Bdnf mRNA pathway through neuroplasticity in resveratrol's antidepressant action, which provides an mRNA regulation perspective for the development of novel antidepressants and understanding depression pathology.

Role of Post-Transcriptional Regulation in Learning and Memory in Mammals

After many decades, during which most molecular studies on the regulation of gene expression focused on transcriptional events, it was realized that post-transcriptional control was equally important in order to determine where and when specific proteins were to be synthesized. Translational regulation is of the most importance in the brain, where all the steps of mRNA maturation, transport to different regions of the cells and actual expression, in response to specific signals, constitute the molecular basis for neuronal plasticity and, as a consequence, for structural stabilization/modification of synapses; notably, these latter events are fundamental for the highest brain functions, such as learning and memory, and are characterized by long-term potentiation (LTP) of specific synapses. Here, we will discuss the molecular bases of these fundamental events by considering both the role of RNA-binding proteins (RBPs) and the effects of non-coding RNAs involved in controlling splicing, editing, stability and translation of mRNAs. Importantly, it has also been found that dysregulation of mRNA metabolism/localization is involved in many pathological conditions, arising either during brain development or in the adult nervous system.

KHSRP loss increases neuronal growth and synaptic transmission and alters memory consolidation through RNA stabilization

The KH-type splicing regulatory protein (KHSRP) is an RNA-binding protein linked to decay of mRNAs with AU-rich elements. KHSRP was previously shown to destabilize Gap43 mRNA and decrease neurite growth in cultured embryonic neurons. Here, we have tested functions of KHSRP in vivo. We find upregulation of 1460 mRNAs in neocortex of adult Khsrp-/- mice, of which 527 bind to KHSRP with high specificity. These KHSRP targets are involved in pathways for neuronal morphology, axon guidance, neurotransmission and long-term memory. Khsrp-/- mice show increased axon growth and dendritic spine density in vivo. Neuronal cultures from Khsrp-/- mice show increased axon and dendrite growth and elevated KHSRP-target mRNAs, including subcellularly localized mRNAs. Furthermore, neuron-specific knockout of Khsrp confirms these are from neuron-intrinsic roles of KHSRP. Consistent with this, neurons in the hippocampus and infralimbic cortex of Khsrp-/- mice show elevations in frequency of miniature excitatory postsynaptic currents. The Khsrp-/- mice have deficits in trace conditioning and attention set-shifting tasks compared Khsrp+/+ mice, indicating impaired prefrontal- and hippocampal-dependent memory consolidation with loss of KHSRP. Overall, these results indicate that deletion of KHSRP impairs neuronal development resulting in alterations in neuronal morphology and function by changing post-transcriptional control of neuronal gene expression.

Loss of RNA binding protein HuD facilitates the production of the senescence-associated secretory phenotype

HuD, an RNA binding protein, plays a role in the regulation of gene expression in certain types of cells, including neuronal cells and pancreatic β-cells, via RNA metabolism. Its aberrant expression is associated with the pathogenesis of several human diseases. To explore HuD-mediated gene regulation, stable cells expressing short hairpin RNA against HuD were established using mouse neuroblastoma Neuro2a (N2a) cells, which displayed enhanced phenotypic characteristics of cellular senescence. Two approaches, RNA immunoprecipitation (RNA IP)-NanoString profiling and cytokine array, were used to subsequently identify a subset of putative HuD targets that act as senescence-associated secretory phenotype (SASP), including C-C motif ligand 2 (CCL2), CCL20, C-X-C motif chemokine ligand 2 (CXCL2), and interleukin-6 (IL-6). Here, we further demonstrated that HuD regulates the expression of CCL2, a SASP candidate upregulated in cells following HuD knockdown, by binding to the 3′-untranslated region (UTR) of Ccl2 mRNA. Downregulation of HuD increased the level of CCL2 in N2a cells and the brain tissues of HuD knockout (KO) mice. Exposure to γ-irradiation induced cellular senescence in N2a cells and HuD knockdown facilitated stress-induced cellular senescence. Our results reveal that HuD acts as a novel regulator of CCL2 expression, and its aberrant expression may contribute to cellular senescence by regulating SASP production.

RNA-Binding Protein HuD as a Versatile Factor in Neuronal and Non-Neuronal Systems

Simple Summary

Tight regulation of gene expression is critical for various biological processes such as proliferation, development, differentiation, and death; its dysregulation is linked to the pathogenesis of diseases. Gene expression is dynamically regulated by numerous factors at DNA, RNA, and protein levels, and RNA binding proteins (RBPs) and non–coding RNAs play important roles in the regulation of RNA metabolisms. RBPs govern a diverse spectrum of RNA metabolism by recognizing and binding to the secondary structure or the certain sequence of target mRNAs, and their malfunctions caused by aberrant expression or mutation are implicated in disease pathology. HuD, an RBP in the human antigen (Hu) family, has been studied as a pivotal regulator of gene expression in neuronal systems; however, accumulating evidence reveals the significance of HuD in non–neuronal systems including certain types of cancer cells or endocrine cells in the lung, pancreas, and adrenal gland. In addition, the abnormal function of HuD suggests its pathological association with neurological disorders, cancers, and diabetes. Thus, this review discusses HuD–mediated gene regulation in neuronal and non–neuronal systems to address how it works to orchestrate gene expression and how its expression is controlled in the stress response of pathogenesis of diseases.

Abstract

HuD (also known as ELAVL4) is an RNA–binding protein belonging to the human antigen (Hu) family that regulates stability, translation, splicing, and adenylation of target mRNAs. Unlike ubiquitously distributed HuR, HuD is only expressed in certain types of tissues, mainly in neuronal systems. Numerous studies have shown that HuD plays essential roles in neuronal development, differentiation, neurogenesis, dendritic maturation, neural plasticity, and synaptic transmission by regulating the metabolism of target mRNAs. However, growing evidence suggests that HuD also functions as a pivotal regulator of gene expression in non–neuronal systems and its malfunction is implicated in disease pathogenesis. Comprehensive knowledge of HuD expression, abundance, molecular targets, and regulatory mechanisms will broaden our understanding of its role as a versatile regulator of gene expression, thus enabling novel treatments for diseases with aberrant HuD expression. This review focuses on recent advances investigating the emerging role of HuD, its molecular mechanisms of target gene regulation, and its disease relevance in both neuronal and non–neuronal systems.

HuD regulates SOD1 expression during oxidative stress in differentiated neuroblastoma cells and sporadic ALS motor cortex

The neuronal RNA-binding protein (RBP) HuD plays an important role in brain development, synaptic plasticity and neurodegenerative diseases such as Parkinson's (PD) and Alzheimer's (AD). Bioinformatics analysis of the human SOD1 mRNA 3' untranslated region (3'UTR) demonstrated the presence of HuD binding adenine-uridine (AU)-rich instability-conferring elements (AREs). Using differentiated SH-SY5Y cells along with brain tissues from sporadic amyotrophic lateral sclerosis (sALS) patients, we assessed HuD-dependent regulation of SOD1 mRNA. In vitro binding and mRNA decay assays demonstrate that HuD specifically binds to SOD1 ARE motifs promoting mRNA stabilization. In SH-SY5Y cells, overexpression of full-length HuD increased SOD1 mRNA and protein levels while a dominant negative form of the RBP downregulated its expression. HuD regulation of SOD1 mRNA was also found to be oxidative stress (OS)-dependent, as shown by the increased HuD binding and upregulation of this mRNA after H₂O₂ exposure. This treatment also induced a shift in alternative polyadenylation (APA) site usage in SOD1 3'UTR, increasing the levels of a long variant bearing HuD binding sites. The requirement of HuD for SOD1 upregulation during oxidative damage was validated using a specific siRNA that downregulated HuD protein levels to 36% and prevented upregulation of SOD1 and 91 additional genes. In the motor cortex from sALS patients, we found increases in SOD1 and HuD mRNAs and proteins, accompanied by greater HuD binding to this mRNA as confirmed by RNA-immunoprecipitation (RIP) assays. Altogether, our results suggest a role of HuD in the post-transcriptional regulation of SOD1 expression during ALS pathogenesis.

GAP-43 and BASP1 in Axon Regeneration: Implications for the Treatment of Neurodegenerative Diseases

Growth-associated protein-43 (GAP-43) and brain acid-soluble protein 1 (BASP1) regulate actin dynamics and presynaptic vesicle cycling at axon terminals, thereby facilitating axonal growth, regeneration, and plasticity. These functions highly depend on changes in GAP-43 and BASP1 expression levels and post-translational modifications such as phosphorylation. Interestingly, examinations of GAP-43 and BASP1 in neurodegenerative diseases reveal alterations in their expression and phosphorylation profiles. This review provides an overview of the structural properties, regulations, and functions of GAP-43 and BASP1, highlighting their involvement in neural injury response and regeneration. By discussing GAP-43 and BASP1 in the context of neurodegenerative diseases, we also explore the therapeutic potential of modulating their activities to compensate for neuron loss in neurodegenerative diseases.

HuD Binds to and Regulates Circular RNAs Derived From Neuronal Development- and Synaptic Plasticity-Associated Genes

The RNA-binding protein (RBP) HuD is involved in neuronal differentiation, regeneration, synaptic plasticity and learning and memory. RBPs not only bind to mRNAs but also interact with several types of RNAs including circular RNAs (circRNAs), a class of non-coding RNAs generated by pre-mRNA back-splicing. This study explored whether HuD could regulate the expression of neuronal circRNAs. HuD controls target RNA’s fate by binding to Adenylate-Uridylate Rich Elements (AREs). Using bioinformatics analyses, we found HuD-binding ARE-motifs in about 26% of brain-expressed circRNAs. By RNA immunoprecipitation (RIP) from the mouse striatum followed by circRNA arrays, we identified over 600 circRNAs bound to HuD. Among these, 226 derived from genes where HuD also bound to their associated mRNAs including circHomer1a, which we previously characterized as a synaptic HuD target circRNA. Binding of HuD to two additional plasticity–associated circRNAs, circCreb1, and circUfp2, was validated by circRNA-specific qRT-PCR. Interestingly, we found that circUpf2 is also enriched in synaptosomes. Pathway analyses confirmed that the majority of HuD-bound circRNAs originate from genes regulating nervous system development and function. Using striatal tissues from HuD overexpressor (HuD-OE) and knock out (KO) mice for circRNA expression analyses we identified 86 HuD-regulated circRNAs. These derived from genes within the same biological pathways as the HuD RIP. Cross-correlation analyses of HuD-regulated and HuD-bound circRNAs identified 69 regulated in either HuD-OE or HuD-KO and 5 in both sets. These include circBrwd1, circFoxp1, and circMap1a, which derive from genes involved in neuronal development and regeneration, and circMagi1 and circLppr4, originating from genes controlling synapse formation and linked to psychiatric disorders. These circRNAs form competing endogenous RNA (ceRNA) networks including microRNAs and mRNAs. Among the HuD target circRNAs, circBrwd1 and circFoxp1 are regulated in an opposite manner to their respective mRNAs. The expressions of other development- and plasticity-associated HuD target circRNAs such as circSatb2, cirHomer1a and circNtrk3 are also altered after the establishment of cocaine conditioned place preference (CPP). Collectively, these data suggest that HuD interactions with circRNAs regulate their expression and transport, and that the ensuing changes in HuD-regulated ceRNA networks could control neuronal differentiation and synaptic plasticity.

Emerging Roles for 3' UTRs in Neurons

The 3′ untranslated regions (3′ UTRs) of mRNAs serve as hubs for post-transcriptional control as the targets of microRNAs (miRNAs) and RNA-binding proteins (RBPs). Sequences in 3′ UTRs confer alterations in mRNA stability, direct mRNA localization to subcellular regions, and impart translational control. Thousands of mRNAs are localized to subcellular compartments in neurons—including axons, dendrites, and synapses—where they are thought to undergo local translation. Despite an established role for 3′ UTR sequences in imparting mRNA localization in neurons, the specific RNA sequences and structural features at play remain poorly understood. The nervous system selectively expresses longer 3′ UTR isoforms via alternative polyadenylation (APA). The regulation of APA in neurons and the neuronal functions of longer 3′ UTR mRNA isoforms are starting to be uncovered. Surprising roles for 3′ UTRs are emerging beyond the regulation of protein synthesis and include roles as RBP delivery scaffolds and regulators of alternative splicing. Evidence is also emerging that 3′ UTRs can be cleaved, leading to stable, isolated 3′ UTR fragments which are of unknown function. Mutations in 3′ UTRs are implicated in several neurological disorders—more studies are needed to uncover how these mutations impact gene regulation and what is their relationship to disease severity.

Human antigen R-regulated mRNA metabolism promotes the cell motility of migrating mouse neurons

Neocortex development during embryonic stages requires the precise control of mRNA metabolism. Human antigen R (HuR) is a well-studied mRNA-binding protein that regulates mRNA metabolism, and it is highly expressed in the neocortex during developmental stages. Deletion of HuR does not impair neural progenitor cell proliferation or differentiation, but it disturbs the laminar structure of the neocortex. We report that HuR is expressed in postmitotic projection neurons during mouse brain development. Specifically, depletion of HuR in these neurons led to a mislocalization of CDP⁺ neurons in deeper layers of the cortex. Time-lapse microscopy showed that HuR was required for the promotion of cell motility in migrating neurons. PCR array identified profilin 1 (Pfn1) mRNA as a major binding partner of HuR in neurons. HuR positively mediated the stability of Pfn1 mRNA and influenced actin polymerization. Overexpression of Pfn1 successfully rescued the migration defects of HuR-deleted neurons. Our data reveal a post-transcriptional mechanism that maintains actin dynamics during neuronal migration.

Summary: Maintaining actin dynamics is crucial for cell motility. Post-transcriptional regulation plays a pivotal role in supporting actin dynamics during neuronal migration.

RNA-Binding Proteins HuB, HuC, and HuD are Distinctly Regulated in Dorsal Root Ganglia Neurons from STZ-Sensitive Compared to STZ-Resistant Diabetic Mice

The neuron-specific Elav-like Hu RNA-binding proteins were described to play an important role in neuronal differentiation and plasticity by ensuring the post-transcriptional control of RNAs encoding for various proteins. Although Elav-like Hu proteins alterations were reported in diabetes or neuropathy, little is known about the regulation of neuron-specific Elav-like Hu RNA-binding proteins in sensory neurons of dorsal root ganglia (DRG) due to the diabetic condition. The goal of our study was to analyze the gene and protein expression of HuB, HuC, and HuD in DRG sensory neurons in diabetes. The diabetic condition was induced in CD-1 adult male mice with single-intraperitoneal injection of streptozotocin (STZ, 150 mg/kg), and 8-weeks (advanced diabetes) after induction was quantified the Elav-like proteins expression. Based on the glycemia values, we identified two types of responses to STZ, and mice were classified in STZ-resistant (diabetic resistant, glycemia < 260 mg/dL) and STZ-sensitive (diabetic, glycemia > 260 mg/dL). Body weight measurements indicated that 8-weeks after STZ-induction of diabetes, control mice have a higher increase in body weight compared to the diabetic and diabetic resistant mice. Moreover, after 8-weeks, diabetic mice (19.52 ± 3.52 s) have longer paw withdrawal latencies in the hot-plate test than diabetic resistant (11.36 ± 1.92 s) and control (11.03 ± 1.97 s) mice, that correlates with the installation of warm hypoalgesia due to the diabetic condition. Further on, we evidenced the decrease of Elav-like gene expression in DRG neurons of diabetic mice (Elavl2, 0.68 ± 0.05 fold; Elavl3, 0.65 ± 0.01 fold; Elavl4, 0.53 ± 0.07 fold) and diabetic resistant mice (Ealvl2, 0.56 ± 0.07 fold; Elavl3, 0.32 ± 0.09 fold) compared to control mice. Interestingly, Elav-like genes have a more accentuated downregulation in diabetic resistant than in diabetic mice, although hypoalgesia was evidenced only in diabetic mice. The Elav-like gene expression changes do not always correlate with the Hu protein expression changes. To detail, HuB is upregulated and HuD is downregulated in diabetic mice, while HuB, HuC, and HuD are downregulated in diabetic resistant mice compared to control mice. To resume, we demonstrated HuD downregulation and HuB upregulation in DRG sensory neurons induced by diabetes, which might be correlated with altered post-transcriptional control of RNAs involved in the regulation of thermal hypoalgesia condition caused by the advanced diabetic neuropathy.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

The m6A-epitranscriptomic signature in neurobiology: from neurodevelopment to brain plasticity

Research over the past decade has provided strong support for the importance of various epigenetic mechanisms, including DNA and histone modifications in regulating activity-dependent gene expression in the mammalian central nervous system. More recently, the emerging field of epitranscriptomics revealed an equally important role of post-transcriptional RNA modifications in shaping the transcriptomic landscape of the brain. This review will focus on the methylation of the adenosine base at the N6 position, termed N⁶ methyladenosine (m6A), which is the most abundant internal modification that decorates eukaryotic messenger RNAs. Given its prevalence and dynamic regulation in the adult brain, the m6A-epitranscriptome provides an additional layer of regulation on RNA that can be controlled in a context- and stimulus-dependent manner. Conceptually, m6A serves as a molecular switch that regulates various aspects of RNA function, including splicing, stability, localization, or translational control. The versatility of m6A function is typically determined through interaction or disengagement with specific classes of m6A-interacting proteins. Here we review recent advances in the field and provide insights into the roles of m6A in regulating brain function, from development to synaptic plasticity, learning, and memory. We also discuss how aberrant m6A signaling may contribute to neurodevelopmental and neuropsychiatric disorders.

Neuronal RNA-binding protein HuD regulates addiction-related gene expression and behavior

The neuronal RNA-binding protein HuD is involved in synaptic plasticity and learning and memory mechanisms. These effects are thought to be due to HuD-mediated stabilization and translation of target mRNAs associated with plasticity. To investigate the potential role of HuD in drug addiction, we first used bioinformatics prediction algorithms together with microarray analyses to search for specific genes and functional networks upregulated within the forebrain of HuD overexpressing mice (HuD_OE ). When this set was further limited to genes in the knowledgebase of addiction-related genes database (KARG) that contains predicted HuD-binding sites in their 3' untranslated regions (3'UTRs), we found that HuD regulates networks that have been associated with addiction-like behavior. These genes included Bdnf and Camk2a, 2 previously validated HuD targets. Since addiction is hypothesized to be a disorder stemming from altered gene expression causing aberrant plasticity, we sought to test the role of HuD in cocaine conditioned placed preference (CPP), a model of addiction-related behaviors. HuD mRNA and protein were upregulated by CPP within the nucleus accumbens of wild-type C57BL/6J mice. These changes were associated with increased expression of Bdnf and Camk2a mRNA and protein. To test this further, we trained HuD_OE and wild-type mice in CPP and found that HuD_OE mice showed increased cocaine CPP compared with controls. This was also associated with elevated expression of HuD target mRNAs and proteins, CaMKIIα and BDNF. These findings suggest HuD involvement in addiction-related behaviors such as cocaine conditioning and seeking, through increased plasticity-related gene expression.

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Competing Interactions of RNA-Binding Proteins, MicroRNAs, and Their Targets Control Neuronal Development and Function

Post-transcriptional mechanisms play critical roles in the control of gene expression during neuronal development and maturation as they allow for faster responses to environmental cues and provide spatially-restricted compartments for local control of protein expression. These mechanisms depend on the interaction of cis-acting elements present in the mRNA sequence and trans-acting factors, such as RNA-binding proteins (RBPs) and microRNAs (miRNAs) that bind to those cis-elements and regulate mRNA stability, subcellular localization, and translation. Recent studies have uncovered an unexpected complexity in these interactions, where coding and non-coding RNAs, termed competing endogenous RNAs (ceRNAs), compete for binding to miRNAs. This competition can, thereby, control a larger number of miRNA target transcripts. However, competing RNA networks also extend to competition between target mRNAs for binding to limited amounts of RBPs. In this review, we present evidence that competitions between target mRNAs for binding to RBPs also occur in neurons, where they affect transcript stability and transport into axons and dendrites as well as translation. In addition, we illustrate the complexity of these mechanisms by demonstrating that RBPs and miRNAs also compete for target binding and regulation.

Positive feedback between RNA-binding protein HuD and transcription factor SATB1 promotes neurogenesis

The mammalian embryonic lethal abnormal vision (ELAV)-like protein HuD is a neuronal RNA-binding protein implicated in neuronal development, plasticity, and diseases. Although HuD has long been associated with neuronal development, the functions of HuD in neural stem cell differentiation and the underlying mechanisms have gone largely unexplored. Here we show that HuD promotes neuronal differentiation of neural stem/progenitor cells (NSCs) in the adult subventricular zone by stabilizing the mRNA of special adenine-thymine (AT)-rich DNA-binding protein 1 (SATB1), a critical transcriptional regulator in neurodevelopment. We find that SATB1 deficiency impairs the neuronal differentiation of NSCs, whereas SATB1 overexpression rescues the neuronal differentiation phenotypes resulting from HuD deficiency. Interestingly, we also discover that SATB1 is a transcriptional activator of HuD during NSC neuronal differentiation. In addition, we demonstrate that NeuroD1, a neuronal master regulator, is a direct downstream target of SATB1. Therefore, HuD and SATB1 form a positive regulatory loop that enhances NeuroD1 transcription and subsequent neuronal differentiation. Our results here reveal a novel positive feedback network between an RNA-binding protein and a transcription factor that plays critical regulatory roles in neurogenesis.

Limbic Encephalitis: Potential Impact of Adaptive Autoimmune Inflammation on Neuronal Circuits of the Amygdala

Limbic encephalitis is characterized by adaptive autoimmune inflammation of the gray matter structures of the limbic system. It has recently been identified as a major cause of temporal lobe epilepsy accompanied by progressive declarative – mainly episodic – ­memory disturbance as well as a variety of rather poorly defined emotional and behavioral changes. While autoimmune inflammation of the hippocampus is likely to be responsible for declarative memory disturbance, consequences of autoimmune inflammation of the amygdala are largely unknown. The amygdala is central for the generation of adequate homoeostatic behavioral responses to emotionally significant external stimuli following processing in a variety of parallel neuronal circuits. Here, we hypothesize that adaptive cellular and humoral autoimmunity may target and modulate distinct inhibitory or excitatory neuronal networks within the amygdala, and thereby strongly impact processing of emotional stimuli and corresponding behavioral responses. This may explain some of the rather poorly understood neuropsychiatric symptoms in limbic encephalitis.

Molecular determinants of the axonal mRNA transcriptome

Axonal protein synthesis has been shown to play a role in developmental and regenerative growth, as well as in cell body responses to axotomy. Recent studies have begun to identify the protein products that contribute to these autonomous responses of axons. In the peripheral nervous system, intra-axonal protein synthesis has been implicated in the localized in vivo responses to neuropathic stimuli, and there is emerging evidence for protein synthesis in CNS axons in vivo. Despite that hundreds of mRNAs have now been shown to localize into the axonal compartment, knowledge of what RNA binding proteins are responsible for this is quite limited. Here, we review the current state of knowledge of RNA transport mechanisms and highlight recently uncovered mechanisms for dynamically altering the axonal transcriptome. Both changes in the levels or activities of components of the RNA transport apparatus and alterations in transcription of transported mRNAs can effectively shift the axonal mRNA population. Consistent with this, the axonal RNA population shifts with development, with changes in growth state, and in response to extracellular stimulation. Each of these events must impact the transcriptional and transport apparatuses of the neuron, thus directly and indirectly modifying the axonal transcriptome.

KSRP modulation of GAP-43 mRNA stability restricts axonal outgrowth in embryonic hippocampal neurons

The KH-type splicing regulatory protein (KSRP) promotes the decay of AU-rich element (ARE)-containing mRNAs. Although KSRP is expressed in the nervous system, very little is known about its role in neurons. In this study, we examined whether KSRP regulates the stability of the ARE-containing GAP-43 mRNA. We found that KSRP destabilizes this mRNA by binding to its ARE, a process that requires the presence of its fourth KH domain (KH4). Furthermore, KSRP competed with the stabilizing factor HuD for binding to these sequences. We also examined the functional consequences of KSRP overexpression and knockdown on the differentiation of primary hippocampal neurons in culture. Overexpression of full length KSRP or KSRP without its nuclear localization signal hindered axonal outgrowth in these cultures, while overexpression of a mutant protein without the KH4 domain that has less affinity for binding to GAP-43′s ARE had no effect. In contrast, depletion of KSRP led to a rise in GAP-43 mRNA levels and a dramatic increase in axonal length, both in KSRP shRNA transfected cells and neurons cultured from Ksrp^+/− and Ksrp ^−/−embryos. Finally we found that overexpression of GAP-43 rescued the axonal outgrowth deficits seen with KSRP overexpression, but only when cells were transfected with GAP-43 constructs containing 3′ UTR sequences targeting the transport of this mRNA to axons. Together, our results suggest that KSRP is an important regulator of mRNA stability and axonal length that works in direct opposition to HuD to regulate the levels of GAP-43 and other ARE-containing neuronal mRNAs.

Degradation of high affinity HuD targets releases Kv1.1 mRNA from miR-129 repression by mTORC1

Little is known about how a neuron undergoes site-specific changes in intrinsic excitability during neuronal activity. We provide evidence for a novel mechanism for mTORC1 kinase-dependent translational regulation of the voltage-gated potassium channel Kv1.1 messenger RNA (mRNA). We identified a microRNA, miR-129, that repressed Kv1.1 mRNA translation when mTORC1 was active. When mTORC1 was inactive, we found that the RNA-binding protein, HuD, bound to Kv1.1 mRNA and promoted its translation. Unexpectedly, inhibition of mTORC1 activity did not alter levels of miR-129 and HuD to favor binding to Kv1.1 mRNA. However, reduced mTORC1 signaling caused the degradation of high affinity HuD target mRNAs, freeing HuD to bind Kv1.1 mRNA. Hence, mTORC1 activity regulation of mRNA stability and high affinity HuD-target mRNA degradation mediates the bidirectional expression of dendritic Kv1.1 ion channels.

Emerging complexity of the HuD/ELAVl4 gene; implications for neuronal development, function, and dysfunction

Precise control of messenger RNA (mRNA) processing and abundance are increasingly being recognized as critical for proper spatiotemporal gene expression, particularly in neurons. These regulatory events are governed by a large number of trans-acting factors found in neurons, most notably RNA-binding proteins (RBPs) and micro-RNAs (miRs), which bind to specific cis-acting elements or structures within mRNAs. Through this binding mechanism, trans-acting factors, particularly RBPs, control all aspects of mRNA metabolism, ranging from altering the transcription rate to mediating mRNA degradation. In this context the best-characterized neuronal RBP, the Hu/ELAVl family member HuD, is emerging as a key component in multiple regulatory processes--including pre-mRNA processing, mRNA stability, and translation--governing the fate of a substantial amount of neuronal mRNAs. Through its ability to regulate mRNA metabolism of diverse groups of functionally similar genes, HuD plays important roles in neuronal development and function. Furthermore, compelling evidence indicates supplementary roles for HuD in neuronal plasticity, in particular, recovery from axonal injury, learning and memory, and multiple neurological diseases. The purpose of this review is to provide a detailed overview of the current knowledge surrounding the expression and roles of HuD in the nervous system. Additionally, we outline the present understanding of the molecular mechanisms presiding over the localization, abundance, and function of HuD in neurons.

PKC-epsilon activation is required for recognition memory in the rat

Activation of PKCɛ, an abundant and developmentally regulated PKC isoform in the brain, has been implicated in memory throughout life and across species. Yet, direct evidence for a mechanistic role for PKCɛ in memory is still lacking. Hence, we sought to evaluate this in rats, using short-term treatments with two PKCɛ-selective peptides, the inhibitory ɛV1-2 and the activating ψɛRACK, and the novel object recognition task (NORT). Our results show that the PKCɛ-selective activator ψɛRACK, did not have a significant effect on recognition memory. In the short time frames used, however, inhibition of PKCɛ activation with the peptide inhibitor ɛV1-2 significantly impaired recognition memory. Moreover, when we addressed at the molecular level the immediate proximal signalling events of PKCɛ activation in acutely dissected rat hippocampi, we found that ψɛRACK increased in a time-dependent manner phosphorylation of MARCKS and activation of Src, Raf, and finally ERK1/2, whereas ɛV1-2 inhibited all basal activity of this pathway. Taken together, these findings present the first direct evidence that PKCɛ activation is an essential molecular component of recognition memory and point toward the use of systemically administered PKCɛ-regulating peptides as memory study tools and putative therapeutic agents.

HuD promotes BDNF expression in brain neurons via selective stabilization of the BDNF long 3'UTR mRNA

Complex regulation of brain-derived neurotrophic factor (BDNF) governs its intricate functions in brain development and neuronal plasticity. Besides tight transcriptional control from multiple distinct promoters, alternative 3′end processing of the BDNF transcripts generates either a long or a short 3′untranslated region (3′UTR). Previous reports indicate that distinct RNA sequence in the BDNF 3′UTRs differentially regulates BDNF production in the brain to accommodate neuronal activity changes, conceivably through differential interactions with undefined trans-acting factors that regulate stability and translation of these BDNF mRNA isoforms. In this study, we report that the neuronal RNA-binding protein (RBP) HuD interacts with a highly conserved AU-rich element (ARE) specifically located in the BDNF long 3′UTR. Such interaction is necessary and sufficient for selective stabilization of mRNAs that contain the BDNF long 3′UTR in vitro and in vivo. Moreover, in a HuD transgenic mouse model, the BDNF long 3′UTR mRNA is increased in the hippocampal dentate granule cells (DGCs), leading to elevated expression of BDNF protein that is transported and stored in the mossy fiber (MF) terminals. Our results identify HuD as the first trans-acting factor that enhances BDNF expression specifically through the long 3′UTR and a novel mechanism that regulates BDNF protein production in selected neuronal populations by HuD abundance.

Cytoplasmic RNA-binding proteins and the control of complex brain function

The formation and maintenance of neural circuits in the mammal central nervous system (CNS) require the coordinated expression of genes not just at the transcriptional level, but at the translational level as well. Recent evidence shows that regulated messenger RNA (mRNA) translation is necessary for certain forms of synaptic plasticity, the cellular basis of learning and memory. In addition, regulated translation helps guide axonal growth cones to their targets on other neurons or at the neuromuscular junction. Several neurologic syndromes have been correlated with and indeed may be caused by aberrant translation; one important example is the fragile X mental retardation syndrome. Although translation in the CNS is regulated by multiple mechanisms and factors, we focus this review on regulatory mRNA-binding proteins with particular emphasis on fragile X mental retardation protein (FMRP) and cytoplasmic polyadenylation element binding (CPEB) because they have been shown to be at the nexus of translational control and brain function in health and disease.

Increased expression of axogenesis-related genes and mossy fibre length in dentate granule cells from adult HuD overexpressor mice

The neuronal RNA-binding protein HuD plays a critical role in the post-transcriptional regulation of short-lived mRNAs during the initial establishment and remodelling of neural connections. We have generated transgenic mice overexpressing this protein (HuD-Tg) in adult DGCs (dentate granule cells) and shown that their mossy fibres contain high levels of GAP-43 (growth-associated protein 43) and exhibit distinct morphological and electrophysiological properties. To investigate the basis for these changes and identify other molecular targets of HuD, DGCs from HuD-Tg and control mice were collected by LCM (laser capture microscopy) and RNAs analysed using DNA microarrays. Results show that 216 known mRNAs transcripts and 63 ESTs (expressed sequence tags) are significantly up-regulated in DGCs from these transgenic mice. Analyses of the 3'-UTRs (3'-untranslated regions) of these transcripts revealed an increased number of HuD-binding sites and the presence of several known instability-conferring sequences. Among these, the mRNA for TTR (transthyretin) shows the highest level of up-regulation, as confirmed by qRT-PCR (quantitative reverse transcription-PCR) and ISH (in situ hybridization). GO (gene ontology) analyses of up-regulated transcripts revealed a large over-representation of genes associated with neural development and axogenesis. In correlation with these gene expression changes, we found an increased length of the infrapyramidal mossy fibre bundle in HuD-Tg mice. These results support the notion that HuD stabilizes a number of developmentally regulated mRNAs in DGCs, resulting in increased axonal elongation.

RNA-binding protein HuD controls insulin translation

Although expression of the mammalian RNA-binding protein HuD was considered to be restricted to neurons, we report that HuD is present in pancreatic β cells, where its levels are controlled by the insulin receptor pathway. We found that HuD associated with a 22-nucleotide segment of the 5' untranslated region (UTR) of preproinsulin (Ins2) mRNA. Modulating HuD abundance did not alter Ins2 mRNA levels, but HuD overexpression decreased Ins2 mRNA translation and insulin production, and conversely, HuD silencing enhanced Ins2 mRNA translation and insulin production. Following treatment with glucose, HuD rapidly dissociated from Ins2 mRNA and enabled insulin biosynthesis. Importantly, HuD-knockout mice displayed higher insulin levels in pancreatic islets, while HuD-overexpressing mice exhibited lower insulin levels in islets and in plasma. In sum, our results identify HuD as a pivotal regulator of insulin translation in pancreatic β cells.

Identification of continuous epitopes of HuD antibodies related to paraneoplastic diseases/small cell lung cancer

HuD antibodies are associated with small cell lung cancer. To identify relevant epitopes of HuD antibodies, patient sera and a monoclonal antibody were analyzed for their reactivity to linear 20mer peptides spanning the human HuD protein. The HuD monoclonal antibody recognized a single fragment located in the first RNA recognition motif. Thorough analysis identified VRDKITQGSL as the actual epitope. Screening of anti-HuD positive patients and healthy controls identified eight peptides as potential subdominant epitopes. The majority of these peptides were located in the N-terminal end as well as in the linker region between the second and third RNA recognition motifs.

Opposite effects of acute ethanol exposure on GAP-43 and BDNF expression in the hippocampus versus the cerebellum of juvenile rats

The adolescent brain is particularly vulnerable to the effects of alcohol, with intoxications at this developmental age often producing long-lasting effects. The present study addresses the effects of a single acute ethanol exposure on growth-associated protein-43 (GAP-43) and brain-derived neurotrophic factor (BDNF) gene expression in neurons in the cerebellum and hippocampus of adolescent rats. Male postnatal day 23 (P23) Sprague-Dawley rats were exposed to ethanol vapors for 2h and after a recovery period of 2h, the cerebellum and hippocampus were harvested and samples were taken for blood alcohol concentration (BAC) determinations. We found that this exposure resulted in a mean BAC of 174 mg/dL, which resembles levels in human adolescents after binge drinking. Analyses of total RNA and protein by quantitative reverse transcription PCR and western blotting, respectively, revealed that this single ethanol exposure significantly decreased the levels of GAP-43 mRNA and protein in the cerebellum but increased the levels of mRNA and protein in the hippocampus. BDNF mRNA and protein levels were also increased in the hippocampus but not in the cerebellum of these animals. In situ hybridizations revealed that GAP-43 and BDNF mRNA levels were primarily increased by alcohol exposure in hippocampal dentate granule cells and CA3 neurons. Overall, the reported alterations in the expression of the plasticity-associated genes GAP-43 and BDNF in juvenile rats are consistent with the known deleterious effects of binge drinking on motor coordination and cognitive function.

HuD regulates the cpg15 expression via the 3'-UTR and AU-rich element

The candidate plasticity related gene 15 (cpg15) plays important roles in neural development and plasticity. In the present study, we studied the role of the cpg15 3'-untranslated region (UTR) in regulating the expression of the gene. The results showed that the presence of the 3'-UTR significantly decreases, while loss of a putative AU-rich element (ARE) in the 3'-UTR increases the cpg15 expression, indicating that the 3'-UTR and ARE may be essential for regulation of cpg15 expression. In addition, HuD, a neural-specific RNA binding protein, increased the cpg15 expression, which depends on the presence of the 3'-UTR and ARE. RNA-binding protein immunoprecipitation (RIP) assay demonstrated that HuD forms a complex with cpg15 mRNA in the cells of rat hippocampus. Deletion of HuD domains RRM1 plus RRM2 or Hinge region plus RRM3 attenuates the function of HuD in enhancing the cpg15 expression. The results suggest that HuD regulates the cpg15 expression via the 3'-UTR-mediated mechanism, which requires the presence of the ARE.

HuD interacts with survival motor neuron protein and can rescue spinal muscular atrophy-like neuronal defects

Spinal muscular atrophy is an autosomal-recessive neuromuscular disease caused by disruption of the survival of motor neuron (SMN) gene, which promotes cytoplasmic assembly of the splicing core machinery. It remains unclear how a deficiency in SMN results in a disorder leading to selective degeneration of lower motor neurons. We report here that SMN interacts with RNA-binding protein HuD in neurites of motorneuron-derived MN-1 cells. This interaction is mediated through the Tudor domain of SMN and, importantly, naturally occurring Tudor mutations found in patients with severe spinal muscular atrophy (SMA) completely abrogate the interaction, underscoring its relevance to the disease process. We also characterized a regulatory pathway involving coactivator-associated arginine methyltransferase 1 (CARM1) and HuD. Specifically, we show that CARM1 expression is rapidly downregulated, at the protein level, following induction of differentiation through retinoid and neurotrophic signaling. Using purified proteins, we demonstrate that methylation of HuD by CARM1 reduces its interaction with the p21(cip1/waf1) mRNA, showing that CARM1 can directly influence RNA-binding activity. We further demonstrate that this CARM1-dependent regulatory switch mainly controls the activity of HuD in promoting cell-cycle exit, whereas the interaction between HuD and SMN is required for proper recruitment of HuD and its mRNA targets in neuronal RNA granules. Finally, we were able to rescue SMA-like defects in a hypomorphic Smn knockdown MN-1 cell line through overexpression of HuD. Together, these findings extend our understanding of specific role(s) of SMN in motor neurons and provide crucial insights into potential new avenues for SMA therapeutic strategies.

Schizophrenia-like GABAergic gene expression deficits in cerebellar Golgi cells from rats chronically exposed to low-dose phencyclidine

One of the most consistent findings in schizophrenia is the decreased expression of the GABA synthesizing enzymes GAD(67) and GAD(65) in specific interneuron populations. This dysfunction is observed in distributed brain regions including the prefrontal cortex, hippocampus, and cerebellum. In an effort to understand the mechanisms for this GABA deficit, we investigated the effect of the N-methyl-D-aspartate receptor (NMDAR) antagonist phencyclidine (PCP), which elicits schizophrenia-like symptoms in both humans and animal models, in a chronic, low-dose exposure paradigm. Adult rats were given PCP at a dose of 2.58 mg/kg/day i.p. for a month, after which levels of various GABAergic cell mRNAs and other neuromodulators were examined in the cerebellum by qRT-PCR. Administration of PCP decreased the expression of GAD(67), GAD(65), and the presynaptic GABA transporter GAT-1, and increased GABA(A) receptor subunits similar to those seen in patients with schizophrenia. Additionally, we found that the mRNA levels of two Golgi cell selective NMDAR subunits, NR2B and NR2D, were decreased in PCP-treated rats. Furthermore, we localized the deficits in GAD(67) expression solely to these interneurons. Slice electrophysiological studies showed that spontaneous firing of Golgi cells was reduced by acute exposure to low-dose PCP, suggesting that these neurons are particularly vulnerable to NMDA receptor antagonism. In conclusion, our results demonstrate that chronic exposure to low levels of PCP in rats mimics the GABAergic alterations reported in the cerebellum of patients with schizophrenia (Bullock et al., 2008. Am. J. Psychiatry 165, 1594-1603), further supporting the validity of this animal model.

Novel recognition motifs and biological functions of the RNA-binding protein HuD revealed by genome-wide identification of its targets

HuD is a neuronal ELAV-like RNA-binding protein (RBP) involved in nervous system development, regeneration, and learning and memory. This protein stabilizes mRNAs by binding to AU-rich instability elements (AREs) in their 3′ unstranslated regions (3′ UTR). To isolate its in vivo targets, messenger ribonucleoprotein (mRNP) complexes containing HuD were first immunoprecipitated from brain extracts and directly bound mRNAs identified by subsequent GST-HuD pull downs and microarray assays. Using the 3′ UTR sequences of the most enriched targets and the known sequence restrictions of the HuD ARE-binding site, we discovered three novel recognition motifs. Motifs 2 and 3 are U-rich whereas motif 1 is C-rich. In vitro binding assays indicated that HuD binds motif 3 with the highest affinity, followed by motifs 2 and 1, with less affinity. These motifs were found to be over-represented in brain mRNAs that are upregulated in HuD overexpressor mice, supporting the biological function of these sequences. Gene ontology analyses revealed that HuD targets are enriched in signaling pathways involved in neuronal differentiation and that many of these mRNAs encode other RBPs, translation factors and actin-binding proteins. These findings provide further insights into the post-transcriptional mechanisms by which HuD promotes neural development and synaptic plasticity.

A p53-CBP/p300 transcription module is required for GAP-43 expression, axon outgrowth, and regeneration

Transcription regulates axon outgrowth and regeneration. However, to date, no transcription complexes have been shown to control axon outgrowth and regeneration by regulating axon growth genes. Here, we report that the tumor suppressor p53 and its acetyltransferases CBP/p300 form a transcriptional complex that regulates the axonal growth-associated protein 43, a well-characterized pro-axon outgrowth and regeneration protein. Acetylated p53 at K372-3-82 drives axon outgrowth, GAP-43 expression, and binds specific elements on the neuronal GAP-43 promoter in a chromatin environment through CBP/p300 signaling. Importantly, in an axon regeneration model, both CBP and p53 K372-3-82 are induced following axotomy in facial motor neurons, where p53 K372-3-82 occupancy of GAP-43 promoter is enhanced as shown by in vivo chromatin immunoprecipitation. Finally, by comparing wild-type and p53 null mice, we demonstrate that the p53/GAP-43 transcriptional module is specifically switched on during axon regeneration in vivo. These data contribute to the understanding of gene regulation in axon outgrowth and may suggest new molecular targets for axon regeneration.

Diverse molecular functions of Hu proteins

Hu proteins are RNA-binding proteins involved in diverse biological processes. The neuronal members of the Hu family, HuB, HuC, and HuD play important roles in neuronal differentiation and plasticity, while the ubiquitously expressed family member, HuR, has numerous functions mostly related to cellular stress response. The pivotal roles of Hu proteins are dictated by their molecular functions affecting a large number of target genes. Hu proteins affect many post-transcriptional aspects of RNA metabolism, from splicing to translation. In this communication, we will focus on these molecular events and review our current understanding of how Hu proteins mediate them. In particular, emphasis will be put on the nuclear functions of these proteins, which were recently discovered. Three examples including calcitonin/calcitonin gene-related peptide, neurofibromatosis type 1, and Ikaros will be discussed in detail. In addition, an intriguing theme of antagonism between Hu proteins and other AU-rich sequence binding proteins will be discussed.

Characterization of the proneural gene regulatory network during mouse telencephalon development

BACKGROUND

The proneural proteins Mash1 and Ngn2 are key cell autonomous regulators of neurogenesis in the mammalian central nervous system, yet little is known about the molecular pathways regulated by these transcription factors.

RESULTS

Here we identify the downstream effectors of proneural genes in the telencephalon using a genomic approach to analyze the transcriptome of mice that are either lacking or overexpressing proneural genes. Novel targets of Ngn2 and/or Mash1 were identified, such as members of the Notch and Wnt pathways, and proteins involved in adhesion and signal transduction. Next, we searched the non-coding sequence surrounding the predicted proneural downstream effector genes for evolutionarily conserved transcription factor binding sites associated with newly defined consensus binding sites for Ngn2 and Mash1. This allowed us to identify potential novel co-factors and co-regulators for proneural proteins, including Creb, Tcf/Lef, Pou-domain containing transcription factors, Sox9, and Mef2a. Finally, a gene regulatory network was delineated using a novel Bayesian-based algorithm that can incorporate information from diverse datasets.

CONCLUSION

Together, these data shed light on the molecular pathways regulated by proneural genes and demonstrate that the integration of experimentation with bioinformatics can guide both hypothesis testing and hypothesis generation.

RNA–protein interactions and control of mRNA stability in neurons

In addition to transcription, posttranscriptional mechanisms play a vital role in the control of gene expression. There are multiple levels of posttranscriptional regulation, including mRNA processing, splicing, editing, transport, stability, and translation. Among these, mRNA stability is estimated to control about 5-10% of all human genes. The rate of mRNA decay is regulated by the interaction of cis-acting elements in the transcripts and sequence-specific RNA-binding proteins. One of the most studied cis-acting elements is the AU-rich element (ARE) present in the 3' untranslated region (3'UTR) of several unstable mRNAs. These sequences are targets of many ARE-binding proteins; some of which induce degradation whereas others promote stabilization of the mRNA. Recently, these mechanisms were uncovered in neurons, where they have been associated with different physiological phenomena, from early development and nerve regeneration to learning and memory processes. In this Mini-Review, we briefly discuss the general mechanisms of control of mRNA turnover and present evidence supporting the importance of these mechanisms in the expression of an increasing number of neuronal genes.

Alterations in mossy fiber physiology and GAP-43 expression and function in transgenic mice overexpressing HuD

HuD is a neuronal RNA-binding protein associated with the stabilization of mRNAs for GAP-43 and other neuronal proteins that are important for nervous system development and learning and memory mechanisms. To better understand the function of this protein, we generated transgenic mice expressing human HuD (HuD-Tg) in adult forebrain neurons. We have previously shown that expression of HuD in adult dentate granule cells results in an abnormal accumulation of GAP-43 mRNA via posttranscriptional mechanisms. Here we show that this mRNA accumulation leads to the ectopic expression of GAP-43 protein in mossy fibers. Electrophysiological analyses of the mossy fiber to CA3 synapse of HuD-Tg mice revealed increases in paired-pulse facilitation (PPF) at short interpulse intervals and no change in long-term potentiation (LTP). Presynaptic calcium transients at the same synapses exhibited faster time constants of decay, suggesting a decrease in the endogenous Ca(2+) buffer capacity of mossy fiber terminals of HuD-Tg mice. Under resting conditions, GAP-43 binds very tightly to calmodulin sequestering it and then releasing it upon PKC-dependent phosphorylation. Therefore, subsequent studies examined the extent of GAP-43 phosphorylation and its association to calmodulin. We found that despite the increased GAP-43 expression in HuD-Tg mice, the levels of PKC-phosphorylated GAP-43 were decreased in these animals. Furthermore, in agreement with the increased proportion of nonphosphorylated GAP-43, HuD-Tg mice showed increased binding of calmodulin to this protein. These results suggest that a significant amount of calmodulin may be trapped in an inactive state, unable to bind free calcium, and activate downstream signaling pathways. In conclusion, we propose that an unregulated expression of HuD disrupts mossy fiber physiology in adult mice in part by altering the expression and phosphorylation of GAP-43 and the amount of free calmodulin available at the synaptic terminal.

Bcl-2 or Bcl-XL gene therapy reduces apoptosis and increases plasticity protein GAP-43 in PC12 cells

The anti-apoptotic gene replacement could be an option in preventing hypoxia induced neuronal loss-necrosis and/or apoptosis. This intervention is however still controversial. In this paper, we tested the bcl-2 or bcl-XL anti-apoptotic gene transfers using an adenovirus vector in PC12 cells after hypoxia and re-oxygenation. Gene delivery results in a significant increase in both Bcl-2 and Bcl-XL proteins expression. Hypoxia (1h)/re-oxygenation (4-48 h) have a detrimental effect upon cultured cells by inducing increased apoptosis by 30% compared to the controls. After hypoxia the compromised mitochondrial membrane function was detected by decreased tetramethyl-rhodamine-ethylester (TMRE) staining. Anti-apoptotic genes transferred 1h after hypoxia, prevent the cell damage; the number of apoptotic cells has been reduced significantly and the gene transfers prevent mitochondrial membrane damage. Under normoxic conditions or following hypoxia the expression of plasticity protein, growth associated protein 43 (GAP-43) increased significantly by the gene treatment. We can conclude that anti-apoptotic gene transfers are not only cytoprotective as it is already documented before but these genes activate GAP-43 as well. This link on apoptotic signals and cell plasticity is a new finding.

Localization of organic cation/carnitine transporter (OCTN2) in cells forming the blood-brain barrier

Carnitine beta-hydroxy-gamma-(trimethylammonio)butyrate - a compound necessary in the peripheral tissues for a transfer of fatty acids for their oxidation within the cell, accumulates in the brain despite low beta-oxidation in this organ. In order to enter the brain, carnitine has to cross the blood-brain barrier formed by capillary endothelial cells which are in close interaction with astrocytes. Previous studies, demonstrating expression of mRNA coding two carnitine transporters - organic cation/carnitine transporter 2 (OCTN2) and B(0,+) in endothelial cells, did not give any information on carnitine transporters polarity in endothelium. Therefore more detailed experiments were performed on expression and localization of a high affinity carnitine transporter OCTN2 in an in vitro model of the blood-brain barrier by real-time PCR, western blot analysis, and immunocytochemistry. The amount of mRNA was comparable in endothelial cells and kidney, when referred to house-keeping genes, it was, however, significantly lower in astrocytes. Polarity of OCTN2 localization was further studied in an in vitro model of the blood-brain barrier with use of anti-OCTN2 antibodies. Z-axis analysis of the confocal microscope pictures of endothelial cells, with anti-P-glycoprotein antibodies as the marker of apical membrane, showed OCTN2 localization at the basolateral membrane and in the cytoplasmic region in the vicinity of nuclei. Localization of OCTN2 suggest that carnitine can be also transported from the brain, playing an important role in removal of certain acyl esters.

Coordinated expression of HuD and GAP-43 in hippocampal dentate granule cells during developmental and adult plasticity

Previous work from our laboratory demonstrated that the RNA-binding protein HuD binds to and stabilizes the GAP-43 mRNA. In this study, we characterized the expression of HuD and GAP-43 mRNA in the hippocampus during two forms of neuronal plasticity. During post-natal development, maximal expression of both molecules was found at P5 and their levels steadily decreased thereafter. At P5, HuD was also present in the subventricular zone, where it co-localized with doublecortin. In the adult hippocampus, the basal levels of HuD and GAP-43 were lower than during development but were significantly increased in the dentate gyrus after seizures. The function of HuD in GAP-43 gene expression was confirmed using HuD-KO mice, in which the GAP-43 mRNA was significantly less stable than in wild type mice. Altogether, these results demonstrate that HuD plays a role in the post-transcriptional control of GAP-43 mRNA in dentate granule cells during developmental and adult plasticity.

The RNA-binding protein HuD binds acetylcholinesterase mRNA in neurons and regulates its expression after axotomy

After axotomy, expression of acetylcholinesterase (AChE) is greatly reduced in the superior cervical ganglion (SCG); however, the molecular events involved in this response remain unknown. Here, we first examined AChE mRNA levels in the brain of transgenic mice that overexpress human HuD. Both in situ hybridization and reverse transcription-PCR demonstrated that AChE transcript levels were increased by more than twofold in the hippocampus of HuD transgenic mice. Additionally, direct interaction between the HuD transgene product and AChE mRNA was observed. Next, we examined the role of HuD in regulating AChE expression in intact and axotomized rat SCG neurons. After axotomy of the adult rat SCG neurons, AChE transcript levels decreased by 50 and 85% by the first and fourth day, respectively. In vitro mRNA decay assays indicated that the decrease in AChE mRNA levels resulted from changes in the stability of presynthesized transcripts. A combination of approaches performed using the region that directly encompasses an adenylate and uridylate (AU)-rich element within the AChE 3'-untranslated region demonstrated a decrease in RNA-protein complexes in response to axotomy of the SCG and, specifically, a decrease in HuD binding. After axotomy, HuD transcript and protein levels also decreased. Using a herpes simplex virus construct containing the human HuD sequence to infect SCG neurons in vivo, we found that AChE and GAP-43 mRNA levels were maintained in the SCG after axotomy. Together, the results of this study demonstrate that AChE expression in neurons of the rat SCG is regulated via post-transcriptional mechanisms that involve the AU-rich element and HuD.

Associative and spatial learning and memory deficits in transgenic mice overexpressing the RNA-binding protein HuD

HuD is a neuronal specific RNA-binding protein associated with the stabilization of short-lived mRNAs during brain development, nerve regeneration and synaptic plasticity. To investigate the functional significance of this protein in the mature brain, we generated transgenic mice overexpressing HuD in forebrain neurons under the control of the alphaCaMKinII promoter. We have previously shown that one of the targets of HuD, GAP-43 mRNA, was stabilized in neurons in the hippocampus, amygdala and cortex of transgenic mice. Animals from two independent lines expressing different levels of the transgene were subjected to a battery of behavioral tests including contextual fear conditioning and the Morris water maze. Our results show that although HuD is increased after learning and memory, constitutive HuD overexpression impaired the acquisition and retention of both cued and contextual fear and the ability to remember the position of a hidden platform in the Morris water maze. No motor-sensory abnormalities were observed in HuD transgenic mice, suggesting that the poor performance of the mice in these tests reflect a true cognitive impairment. We conclude that posttranscriptional regulation of gene expression by stabilization of specific mRNAs may have to be restricted temporally and spatially for proper acquisition and storage of memories.

The RNA-binding protein HuD: a regulator of neuronal differentiation, maintenance and plasticity

mRNA stability is increasingly recognized as being essential for controlling the expression of a wide variety of transcripts during neuronal development and synaptic plasticity. In this context, the role of AU-rich elements (ARE) contained within the 3' untranslated region (UTR) of transcripts has now emerged as key because of their high incidence in a large number of cellular mRNAs. This important regulatory element is known to significantly modulate the longevity of mRNAs by interacting with available stabilizing or destabilizing RNA-binding proteins (RBP). Thus, in parallel with the emergence of ARE, RBP are also gaining recognition for their pivotal role in regulating expression of a variety of mRNAs. In the nervous system, the member of the Hu family of ARE-binding proteins known as HuD, has recently been implicated in multiple aspects of neuronal function including the commitment and differentiation of neuronal precursors as well as synaptic remodeling in mature neurons. Through its ability to interact with ARE and stabilize multiple transcripts, HuD has now emerged as an important regulator of mRNA expression in neurons. The present review is designed to provide a comprehensive and updated view of HuD as an RBP in the nervous system. Additionally, we highlight the role of HuD in multiple aspects of a neuron's life from early differentiation to changes in mature neurons during learning paradigms and in response to injury and regeneration. Finally, we describe the current state of knowledge concerning the molecular and cellular events regulating the expression and activity of HuD in neurons.

HuD distribution changes in response to heat shock but not neurotrophic stimulation

Cellular stress leads to a change in distribution of RNA-binding proteins. HuR, a member of the ELAV/Hu family of RNA-binding proteins, is nuclear in distribution and following heat shock is found in large cytoplasmic stress granules where translation is inhibited. HuD, another ELAV/Hu RNA-binding protein, stabilizes the GAP-43 mRNA in response to nerve growth factor (NGF) stimulation in PC12 cells. We were interested in determining the nuclear distribution of HuD and if neurotrophic stimulation induced changes in the distribution of HuD. In PC12 cells, we found, as expected, that HuR translocates from the nucleus to the cytoplasm in response to heat shock. In response to heat shock, HuD forms large cytoplasmic stress granules, consistent with a role for HuD in the cessation of translation. In unstimulated cells, HuD is distributed in small granules in the cytoplasm and is consistently present at low levels in the nucleus. Stimulation of PC12 cells with NGF induces neuronal differentiation including outgrowth of neurites and increased levels of GAP-43 protein, whereas HuD remains localized in small cytoplasm granules and is still present in the nucleus. These results suggest that, following neurotrophic stimulation, the lack of changes in HuD distribution are due to continued steady state of HuD nuclear shuttling in PC12 cells, or that HuD is not normally shuttled from the nucleus in response to NGF.