Hzf protein regulates dendritic localization and BDNF-induced translation of type 1 inositol 1,4,5-trisphosphate receptor mRNA

Mechanisms underlying the role of DISC1 in synaptic plasticity

Disrupted in schizophrenia 1 (DISC1) is an important hub protein, forming multimeric complexes by self-association and interacting with a large number of synaptic and cytoskeletal molecules. The synaptic location of DISC1 in the adult brain suggests a role in synaptic plasticity, and indeed, a number of studies have discovered synaptic plasticity impairments in a variety of different DISC1 mutants. This review explores the possibility that DISC1 is an important molecule for organizing proteins involved in synaptic plasticity and examines why mutations in DISC1 impair plasticity. It concentrates on DISC1's role in interacting with synaptic proteins, controlling dendritic structure and cellular trafficking of mRNA, synaptic vesicles and mitochondria. N-terminal directed mutations appear to impair synaptic plasticity through interactions with phosphodiesterase 4B (PDE4B) and hence protein kinase A (PKA)/GluA1 and PKA/cAMP response element-binding protein (CREB) signalling pathways, and affect spine structure through interactions with kalirin 7 (Kal-7) and Rac1. C-terminal directed mutations also impair plasticity possibly through altered interactions with lissencephaly protein 1 (LIS1) and nuclear distribution protein nudE-like 1 (NDEL1), thereby affecting developmental processes such as dendritic structure and spine maturation. Many of the same molecules involved in DISC1's cytoskeletal interactions are also involved in intracellular trafficking, raising the possibility that impairments in intracellular trafficking affect cytoskeletal development and vice versa. While the multiplicity of DISC1 protein interactions makes it difficult to pinpoint a single causal signalling pathway, we suggest that the immediate-term effects of N-terminal influences on GluA1, Rac1 and CREB, coupled with the developmental effects of C-terminal influences on trafficking and the cytoskeleton make up the two main branches of DISC1's effect on synaptic plasticity and dendritic spine stability.

Multilayered Control of Alternative Splicing Regulatory Networks by Transcription Factors

Networks of coordinated alternative splicing (AS) events play critical roles in development and disease. However, a comprehensive knowledge of the factors that regulate these networks is lacking. We describe a high-throughput system for systematically linking trans-acting factors to endogenous RNA regulatory events. Using this system, we identify hundreds of factors associated with diverse regulatory layers that positively or negatively control AS events linked to cell fate. Remarkably, more than one-third of the regulators are transcription factors. Further analyses of the zinc finger protein Zfp871 and BTB/POZ domain transcription factor Nacc1, which regulate neural and stem cell AS programs, respectively, reveal roles in controlling the expression of specific splicing regulators. Surprisingly, these proteins also appear to regulate target AS programs via binding RNA. Our results thus uncover a large "missing cache" of splicing regulators among annotated transcription factors, some of which dually regulate AS through direct and indirect mechanisms.

IP3 receptor mutations and brain diseases in human and rodents

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃ R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃ , which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃ R genes, which have distinct properties of IP₃ -binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃ R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃ Rs, the type 1 receptor (IP₃ R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃ R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃ R1 and its functional implication in human brain diseases, as revealed by IP₃ R mutation studies and analysis of human disease-associated genes. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Specific age-related molecular alterations in the cerebellum of Down syndrome mouse models

Down syndrome, or trisomy 21, has been modeled with various trisomic and transgenic mice to help understand the consequences of an altered gene dosage in brain development and function. Though Down syndrome has been associated with premature aging, little is known about the molecular and cellular alterations that target brain function. To help identify alterations at specific ages, we analyzed the cerebellum of Ts1Cje mice, trisomic for 77 HSA21 orthologs, at three ages-young (4 months), middle-age (12 months), and old (17 months)-compared to age-matched controls. Quantification of neuronal and glial markers (n=11) revealed increases in GFAP, with an age effect, and S100B, with age and genotype effects. The genotype effect on S100B with age was unexpected as Ts1Cje has only two copies of the S100b gene. Interestingly, the different increase in GFAP observed between Ts1Cje (trisomic segment includes Pcp4 gene) and controls was magnified in TgPCP4 mice (1 extra copy of the human PCP4 gene) at the same age. S100B increase was not found in the TgPCP4 confirming a difference of regulation with aging for GFAP and S100B and excluding the calcium signaling regulator, Pcp4, as a potential candidate for increase of S100B in the Ts1Cje. To understand these differences, comparison of GFAP and S100B immunostainings at young and middle-age were performed. Immunohistochemical detection of differences in GFAP and S100B localization with aging implicate S100B+ oligodendrocytes as a new phenotypic target in this specific aging process.

Proposing a new RNA quadruplex structure: j-motif, with possible links to neural development

An RNA-binding protein, hnRNP K, has been studied extensively because of its involvement in neural development through the post-transcriptional regulation of its downstream target genes; however, its binding mode remains unclear. According to structural features of the binding sites, we have presumed the existence of possible unique structures 'j-motifs' that are similar to known i-motifs, the difference being that the initial cluster comprises successive U nucleic acids instead of C. It was suspected that the motifs could be recognized by hnRNP K to regulate the translation levels of target proteins, however, there were virtually no methods to verify their existence except computational methods: regular expression searches and theoretical molecular orbital (MO) calculations. Here, we first show a list of 16 genes having j-motif-like sequences we discovered under refined search conditions. The list was highly related to neural development from both subjective and objective aspects. Additionally, MO calculations revealed the similarity of non-canonical base pairs found in i- and j-motifs qualitatively, leading to a proposal of the possible existence of the j-motifs. When taken into consideration, it was indicated that the j-motifs could be formed and play some role in the neural development.

Regulation of mRNA transport, localization and translation in the nervous system of mammals (Review)

Post-transcriptional control of mRNA trafficking and metabolism plays a critical role in the actualization and fine tuning of the genetic program of cells, both in development and in differentiated tissues. Cis-acting signals, responsible for post-transcriptional regulation, reside in the RNA message itself, usually in untranslated regions, 5′ or 3′ to the coding sequence, and are recognized by trans-acting factors: RNA-binding proteins (RBPs) and/or non-coding RNAs (ncRNAs). ncRNAs bind short mRNA sequences usually present in the 3′-untranslated (3′-UTR) region of their target messages. RBPs recognize specific nucleotide sequences and/or secondary/tertiary structures. Most RBPs assemble on mRNA at the moment of transcription and shepherd it to its destination, somehow determining its final fate. Regulation of mRNA localization and metabolism has a particularly important role in the nervous system where local translation of pre-localized mRNAs has been implicated in developing axon and dendrite pathfinding, and in synapse formation. Moreover, activity-dependent mRNA trafficking and local translation may underlie long-lasting changes in synaptic efficacy, responsible for learning and memory. This review focuses on the role of RBPs in neuronal development and plasticity, as well as possible connections between ncRNAs and RBPs.

Metabolic turnover of synaptic proteins: kinetics, interdependencies and implications for synaptic maintenance

Chemical synapses contain multitudes of proteins, which in common with all proteins, have finite lifetimes and therefore need to be continuously replaced. Given the huge numbers of synaptic connections typical neurons form, the demand to maintain the protein contents of these connections might be expected to place considerable metabolic demands on each neuron. Moreover, synaptic proteostasis might differ according to distance from global protein synthesis sites, the availability of distributed protein synthesis facilities, trafficking rates and synaptic protein dynamics. To date, the turnover kinetics of synaptic proteins have not been studied or analyzed systematically, and thus metabolic demands or the aforementioned relationships remain largely unknown. In the current study we used dynamic Stable Isotope Labeling with Amino acids in Cell culture (SILAC), mass spectrometry (MS), Fluorescent Non-Canonical Amino acid Tagging (FUNCAT), quantitative immunohistochemistry and bioinformatics to systematically measure the metabolic half-lives of hundreds of synaptic proteins, examine how these depend on their pre/postsynaptic affiliation or their association with particular molecular complexes, and assess the metabolic load of synaptic proteostasis. We found that nearly all synaptic proteins identified here exhibited half-lifetimes in the range of 2-5 days. Unexpectedly, metabolic turnover rates were not significantly different for presynaptic and postsynaptic proteins, or for proteins for which mRNAs are consistently found in dendrites. Some functionally or structurally related proteins exhibited very similar turnover rates, indicating that their biogenesis and degradation might be coupled, a possibility further supported by bioinformatics-based analyses. The relatively low turnover rates measured here (∼0.7% of synaptic protein content per hour) are in good agreement with imaging-based studies of synaptic protein trafficking, yet indicate that the metabolic load synaptic protein turnover places on individual neurons is very substantial.

Dendritic protein synthesis in the normal and diseased brain

Synaptic activity is a spatially limited process that requires a precise, yet dynamic, complement of proteins within the synaptic micro-domain. The maintenance and regulation of these synaptic proteins is regulated, in part, by local mRNA translation in dendrites. Protein synthesis within the postsynaptic compartment allows neurons tight spatial and temporal control of synaptic protein expression, which is critical for proper functioning of synapses and neural circuits. In this review, we discuss the identity of proteins synthesized within dendrites, the receptor-mediated mechanisms regulating their synthesis, and the possible roles for these locally synthesized proteins. We also explore how our current understanding of dendritic protein synthesis in the hippocampus can be applied to new brain regions and to understanding the pathological mechanisms underlying varied neurological diseases.

Dendritic mRNA targeting and translation

Selective targeting of specific mRNAs into neuronal dendrites and their locally regulated translation at particular cell contact sites contribute to input-specific synaptic plasticity. Thus, individual synapses become decision-making units, which control gene expression in a spatially restricted and nucleus-independent manner. Dendritic targeting of mRNAs is achieved by active, microtubule-dependent transport. For this purpose, mRNAs are packaged into large ribonucleoprotein (RNP) particles containing an array of trans-acting RNA-binding proteins. These are attached to molecular motors, which move their RNP cargo into dendrites. A variety of proteins may be synthesized in dendrites, including signalling and scaffold proteins of the synapse and neurotransmitter receptors. In some cases, such as the alpha subunit of the calcium/calmodulin-dependent protein kinase II (αCaMKII) and the activity-regulated gene of 3.1 kb (Arg3.1, also referred to as activity-regulated cDNA, Arc), their local synthesis at synapses can modulate long-term changes in synaptic efficiency. Local dendritic translation is regulated by several signalling cascades including Akt/mTOR and Erk/MAP kinase pathways, which are triggered by synaptic activity. More recent findings show that miRNAs also play an important role in protein synthesis at synapses. Disruption of local translation control at synapses, as observed in the fragile X syndrome (FXS) and its mouse models and possibly also in autism spectrum disorders, interferes with cognitive abilities in mice and men.

Cooperative role of the RNA-binding proteins Hzf and HuR in p53 activation

The RNA-binding protein Hzf (hematopoietic zinc finger) plays important roles in mRNA translation in cerebellar Purkinje cells and adipocytes. We along with others have reported that the expression of the Hzf gene is transcriptionally regulated by the p53 tumor suppressor protein. We show here that Hzf regulates p53 expression in cooperation with HuR. Hzf and HuR independently interact with the 3' untranslated region (UTR) of p53 mRNA, which facilitates the cytoplasmic localization of p53 mRNA in the presence of the ARF tumor suppressor protein. In the absence of Hzf and HuR, p53 induction by p19(ARF) is significantly attenuated, and the cells consequently acquire resistance to p19(ARF). Thus, these findings demonstrate that in addition to Mdm2 inhibition, p19(ARF) increases the concentration of p53 through posttranscriptional control of p53 mRNA and suggest critical roles for the RNA-binding proteins Hzf and HuR in p53 induction.

The trafficking of Na(V)1.8

The α-subunit of tetrodotoxin-resistant voltage-gated sodium channel Na(V)1.8 is selectively expressed in sensory neurons. It has been reported that Na(V)1.8 is involved in the transmission of nociceptive information from sensory neurons to the central nervous system in nociceptive [1] and neuropathic [24] pain conditions. Thus Na(V)1.8 has been a promising target to treat chronic pain. Here we discuss the recent advances in the study of trafficking mechanism of Na(V)1.8. These pieces of information are particularly important as such trafficking machinery could be new targets for painkillers.

Neural RNA-binding protein Musashi1 controls midline crossing of precerebellar neurons through posttranscriptional regulation of Robo3/Rig-1 expression

Precisely regulated spatiotemporal gene expression is essential for the establishment of neural circuits. In contrast to the increasing evidence for transcriptional regulation of axon guidance cues and receptors, the role of posttranscriptional regulation in axon guidance, especially in vivo, remains poorly characterized. Here, we demonstrate that the expression of Slit receptor Robo3/Rig-1, which plays crucial roles in axonal midline crossing, is regulated by a neural RNA-binding protein Musashi1 (Msi1). Msi1 binds to Robo3 mRNA through RNA recognition motifs and increases the protein level of Robo3 without affecting its mRNA level. In Msi1-deficient precerebellar neurons, Robo3 protein, but not its mRNA, is dramatically reduced. Moreover, similar to defects in Robo3-deficient mice, axonal midline crossing and neuronal migration of precerebellar neurons are severely impaired in Msi1-deficient mice. Together, these findings indicate that Msi1-mediated posttranscriptional regulation of Robo3 controls midline crossing of precerebellar neurons.

Production of rat monoclonal antibodies against RNA-binding protein Hzf

The hematopoietic zinc finger protein, Hzf, is induced in response to DNA damage or by Arf tumor suppressor in a p53-dependent manner. Recent studies have revealed that Hzf is an RNA-binding protein that regulates localization and translation of specific mRNA. The RNA-binding activity of Hzf is required for the functions of cerebellar purkinje cells and adipocytes, although their molecular mechanisms underlying the mRNA regulation largely remain unknown. To further investigate the molecular function of Hzf, we raised two rat monoclonal antibodies (MAb) against a peptide corresponding to the C-terminal region of the mouse/human Hzf protein. Both MAbs reacted with the native protein expressed in mammalian cells, and were highly efficient in detecting endogenous Hzf by immunoblotting, immunoprecipitation, and immunofluorescence. These MAbs should therefore be useful for further analysis of molecular functions of the Hzf protein and for identification of Hzf-binding proteins.

To localize or not to localize: mRNA fate is in 3'UTR ends

Translation of localized mRNA is a fast and efficient way of reacting to extracellular stimuli with the added benefit of providing spatial resolution to the cellular response. The efficacy of this adaptive response ultimately relies on the ability to express a particular protein at the right time and in the right place. Although mRNA localization is a mechanism shared by most organisms, it is especially relevant in highly polarized cells, such as differentiated neurons. 3'-Untranslated regions (3'UTRs) of mRNAs are critical both for the targeting of transcripts to specific subcellular compartments and for translational control. Here we review recent studies that indicate how, in response to extracellular cues, nuclear and cytoplasmic remodeling of the 3'UTR contributes to mRNA localization and local protein synthesis.

Dendritic LSm1/CBP80-mRNPs mark the early steps of transport commitment and translational control

Messenger RNA (mRNA) transport to neuronal dendrites is crucial for synaptic plasticity, but little is known of assembly or translational regulation of dendritic messenger ribonucleoproteins (mRNPs). Here we characterize a novel mRNP complex that is found in neuronal dendrites throughout the central nervous system and in some axonal processes of the spinal cord. The complex is characterized by the LSm1 protein, which so far has been implicated in mRNA degradation in nonneuronal cells. In brain, it associates with intact mRNAs. Interestingly, the LSm1-mRNPs contain the cap-binding protein CBP80 that associates with (pre)mRNAs in the nucleus, suggesting that the dendritic LSm1 complex has been assembled in the nucleus. In support of this notion, neuronal LSm1 is partially nuclear and inhibition of mRNA synthesis increases its nuclear localization. Importantly, CBP80 is also present in the dendrites and both LSm1 and CBP80 shift significantly into the spines upon stimulation of glutamergic receptors, suggesting that these mRNPs are translationally activated and contribute to the regulated local protein synthesis.

Neural RNA-binding protein Musashi1 inhibits translation initiation by competing with eIF4G for PABP

Musashi1 (Msi1) is an RNA-binding protein that is highly expressed in neural stem cells. We previously reported that Msi1 contributes to the maintenance of the immature state and self-renewal activity of neural stem cells through translational repression of m-Numb. However, its translation repression mechanism has remained unclear. Here, we identify poly(A) binding protein (PABP) as an Msi1-binding protein, and find Msi1 competes with eIF4G for PABP binding. This competition inhibits translation initiation of Msi1's target mRNA. Indeed, deletion of the PABP-interacting domain in Msi1 abolishes its function. We demonstrate that Msi1 inhibits the assembly of the 80S, but not the 48S, ribosome complex. Consistent with these conclusions, Msi1 colocalizes with PABP and is recruited into stress granules, which contain the stalled preinitiation complex. However, Msi1 with mutations in two RNA recognition motifs fails to accumulate into stress granules. These results provide insight into the mechanism by which sequence-specific translational repression occurs in stem cells through the control of translation initiation.

Hzf regulates adipogenesis through translational control of C/EBPalpha

Adipocyte differentiation requires a well-defined programme of gene expression in which the transcription factor C/EBPalpha (CCAAT/enhancer-binding protein) has a central function. Here, we show that Hzf (haematopoietic zinc-finger), a previously identified p53 transcriptional target, regulates C/EBPalpha expression. Hzf is induced during differentiation of preadipocyte cell lines, and its suppression by short hairpin RNA disrupts adipogenesis. In Hzf's absence, expression of C/EBPalpha is severely impaired because of reduced translation of its mRNA. Hzf physically interacts with the 3' untranslated region of C/EBPalpha mRNA to enhance its translation. Taken together, these findings underscore a critical role of Hzf in the adipogenesis regulatory cascade.

Motor coordination defects in mice deficient for the Sam68 RNA-binding protein

The role of RNA-binding proteins in the central nervous system and more specifically their role in motor coordination and learning are poorly understood. We previously reported that ablation of RNA-binding protein Sam68 in mice results in male sterility and delayed mammary gland development and protection against osteoporosis in females. Sam68 however is highly expressed in most regions of the brain especially the cerebellum and thus we investigated the cerebellar-related manifestations in Sam68-null mice. We analyzed the mice for motor function, sensory function, and learning and memory abilities. Herein, we report that Sam68-null mice have motor coordination defects as assessed by beam walking and rotorod performance. Forty-week-old Sam68-null mice (n=12) were compared to their wild-type littermates (n=12). The Sam68-null mice exhibited more hindpaw faults in beam walking tests and fell from the rotating drum at lower speeds and prematurely compared to the wild-type controls. The Sam68-null mice were, however, normal for forelimb strength, tail-hang reflex, balance test, grid walking, the Morris water task, recognition memory, visual discrimination, auditory stimulation and conditional taste aversion. Our findings support a role for Sam68 in the central nervous system in the regulation of motor coordination.

Dendritic mRNA: transport, translation and function

Many cellular functions require the synthesis of a specific protein or functional cohort of proteins at a specific time and place in the cell. Local protein synthesis in neuronal dendrites is essential for understanding how neural activity patterns are transduced into persistent changes in synaptic connectivity during cortical development, memory storage and other long-term adaptive brain responses. Regional and temporal changes in protein levels are commonly coordinated by an asymmetric distribution of mRNAs. This Review attempts to integrate current knowledge of dendritic mRNA transport, storage and translation, placing particular emphasis on the coordination of regulation and function during activity-dependent synaptic plasticity in the adult mammalian brain.

Impaired motor functions in mice lacking the RNA-binding protein Hzf

Local protein synthesis in dendrites plays an important role in some aspects of neuronal development and synaptic plasticity. Neuronal RNA-binding proteins regulate the transport and/or translation of the localized mRNAs. Previously, we reported that hematopoietic zinc finger (Hzf) is one of the neuronal RNA-binding proteins that regulate these processes. The Hzf protein is highly expressed in neuronal cells including hippocampal pyramidal neurons and cerebellar Purkinje cells, and plays essential roles in the dendritic mRNA localization and translation. In the present study we demonstrated that mice lacking Hzf (Hzf(-/-) mice) exhibited severe impairments of motor coordination and cerebellum-dependent motor learning. These findings raise the possibility that the post-transcriptional regulation by Hzf may contribute to some aspects of synaptic plasticity and motor learning in the cerebellum.