Sarm1 deficiency impairs synaptic function and leads to behavioral deficits, which can be ameliorated by an mGluR allosteric modulator

Innate immune responses have been shown to influence brain development and function. Dysregulation of innate immunity is significantly associated with psychiatric disorders such as autism spectrum disorders and schizophrenia, which are well-known neurodevelopmental disorders. Recent studies have revealed that critical players of the innate immune response are expressed in neuronal tissues and regulate neuronal function and activity. For example, Sarm1, a negative regulator that acts downstream of Toll-like receptor (TLR) 3 and 4, is predominantly expressed in neurons. We have previously shown that Sarm1 regulates neuronal morphogenesis and the expression of inflammatory cytokines in the brain, which then affects learning ability, cognitive flexibility, and social interaction. Because impaired neuronal morphogenesis and dysregulation of cytokine expression may disrupt neuronal activity, we investigated whether Sarm1 knockdown affects the synaptic responses of neurons. We here show that reduced Sarm1 expression impairs metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) formation but enhances N-methyl-D-aspartate receptor (NMDAR)-dependent long-term potentiation production in hippocampal CA1 neurons. The expression levels of post-synaptic proteins, including NR2a, NR1, Shank1 and Shank3, are also altered in Sarm1 knockdown mice, suggesting a role for Sarm1 in the maintenance of synaptic homeostasis. The addition of a positive allosteric modulator of mGluR5, CDPPB, ameliorates the LTD defects in slice recording and the behavioral deficits in social interaction and associative memory. These results suggest an important role for mGluR5 signaling in the function of Sarm1. In conclusion, our study demonstrates a role for Sarm1 in the regulation of synaptic plasticity. Through these mechanisms, Sarm1 knockdown results in the impairment of associative memory and social interactions in mice.

Sarm1, a neuronal inflammatory regulator, controls social interaction, associative memory and cognitive flexibility in mice

Impaired neurodevelopment leads to several psychiatric disorders, including autism, schizophrenia and attention deficiency hyperactivity disorder. Our prior study showed that sterile alpha and TIR motif-containing 1 protein (Sarm1) regulates neuronal morphogenesis through at least two pathways. Sarm1 controls neuronal morphogenesis, including dendritic arborization, axonal outgrowth and establishment of neuronal polarity, through the MKK-JNK pathway. Neuronally expressed Sarm1 also regulates the expression of inflammatory cytokines in the brain, which have also been shown to impact brain development and function. Because the reduction of Sarm1 expression negatively influences neuronal development, here we investigated whether Sarm1 controls mouse behaviors. We analyzed two independent Sarm1 transgenic mouse lines using a series of behavioral assays, and found that the reduction of Sarm1 protein levels had a limited effect on locomotion and anxiety. However, Sarm1 knockdown mice exhibited impairments in cued and contextual fear conditioning as well as cognitive flexibility. Moreover, the three-chambered social test, reciprocal social interaction and social transmission of food preference further illustrated deficiencies in Sarm1 knockdown mice in social interaction. These findings suggest that Sarm1, a molecule that regulates innate immunity and neuronal morphogenesis, regulates social behaviors and cognition. We conclude that Sarm1 is involved in immune response, neural development and psychiatric disorders.

Neuronally-expressed Sarm1 regulates expression of inflammatory and antiviral cytokines in brains

Sarm1 is the fifth Toll/IL-1 receptor (TIR) domain-containing adaptor protein identified to regulate TLR downstream signaling. Unlike the other TIR domain-containing adaptor proteins, Sarm1 is predominantly expressed in the brain. Our previous study indicated that Sarm1 regulates dendritic growth, axonal extension and neuronal polarity. Here, we investigated whether Sarm1 is involved in innate immunity in the brain. First, regional and cell-type distribution of Sarm1 in mouse brains was revealed using double immunostaining. Sarm1 was widely distributed in different regions of brains, including the cerebral cortex, hippocampus, amygdala, cerebellum and midbrain. Moreover, Sarm1 is present in both projection and inhibitory neurons, but, interestingly, not in microglial cells--the main immune cells in the brain. These results suggest that Sarm1 is unlikely to regulate microglial activity in a cell-autonomous manner. However, compared with wild type littermates, the RNA expression levels of several inflammatory and antiviral cytokines were altered in the embryonic and adult brains of Sarm1 knockdown transgenic mice. These data imply that Sarm1 influences cytokine expression in neurons. In conclusion, our findings suggest that Sarm1 regulates the innate immune responses of the central nervous system through regulating the inflammatory and anti-virus cytokines produced by neurons.

Calcium/calmodulin-dependent serine protein kinase (CASK) is a new intracellular modulator of P2X3 receptors

ATP-gated P2X3 receptors of sensory ganglion neurons are important transducers of painful stimuli and are modulated by extracellular algogenic substances, via changes in the receptor phosphorylation state. The present study investigated the role of calcium/calmodulin-dependent serine protein kinase (CASK) in interacting and controlling P2X3 receptor expression and function in mouse trigeminal ganglia. Most ganglion neurons in situ or in culture co-expressed P2X3 and CASK. CASK was immunoprecipitated with P2X3 receptors from trigeminal ganglia and from P2X3/CASK-cotransfected human embryonic kidney (HEK) cells. Recombinant P2X3/CASK expression in HEK cells increased serine phosphorylation of P2X3 receptors, typically associated with receptor upregulation. CASK deletion mutants also enhanced P2X3 subunit expression. After silencing CASK, cell surface P2X3 receptor expression was decreased, which is consistent with depressed P2X3 currents. The reduction in P2X3 expression levels was reversed by the proteasomal inhibitor MG-132. Moreover, neuronal CASK/P2X3 interaction was up-regulated by nerve growth factor (NGF) signaling and down-regulated by P2X3 agonist-induced desensitization. These data suggest a novel interaction between CASK and P2X3 receptors with positive outcome for receptor stability and function. As CASK-mediated control of P2X3 receptors was dependent on the receptor activation state, CASK represents an intracellular gateway to regulate purinergic nociceptive signaling.

Neuron-specific regulation on F-actin cytoskeletons: The role of CTTNBP2 in dendritic spinogenesis and maintenance

Dendritic spines are neuron-specific actin-rich subcellular structures and are the location of excitatory synapses. Neurotransmitters released from presynaptic terminals activate the signals modifying the F-actin dynamics and stability and thus control dendritic spine morphology. Many ubiquitously expressed actin-associated proteins, including cortactin, have been shown to regulate dendritic spine morphology and density. Since dendritic spines are neuron-specific structures, neuron-specific proteins are expected to control F-actin cytoskeletons and dendritic spinogenesis. Recently, we demonstrated that cortactin-binding protein 2 (CTTNBP2), a neuron-specific protein, regulates the mobility and distribution of cortactin and controls the density of dendritic spines. This is the first example of a neuron-specific protein that controls the mobility of an F-actin associated protein and influences the dendritic spines. It provides a platform to explore the specific pathway triggering dendritic spinogenesis.

CTTNBP2, but not CTTNBP2NL, regulates dendritic spinogenesis and synaptic distribution of the striatin-PP2A complex

Cortactin-binding protein 2 (CTTNBP2) interacts with cortactin to regulate cortactin mobility and control dendritic spine formation. CTTNBP2 has also been associated with autistic spectrum disorder. The regulation of dendritic spinogenesis could explain the association of CTTNBP2 with autism. Sequence comparison has indicated that CTTNBP2 N-terminal-like protein (CTTNBP2NL) is a CTTNBP2 homologue. To confirm the specific effect of CTTNBP2 on dendritic spinogenesis, here we investigate whether CTTNBP2NL has a similar function to CTTNBP2. Although both CTTNBP2 and CTTNBP2NL interact with cortactin, CTTNBP2NL is associated with stress fibers, whereas CTTNBP2 is distributed to the cortex and intracellular puncta. We also provide evidence that CTTNBP2, but not CTTNBP2NL, is predominantly expressed in the brain. CTTNBP2NL does not show any activity in the regulation of dendritic spinogenesis. In addition to spine morphology, CTTNBP2 is also found to regulate the synaptic distribution of striatin and zinedin (the regulatory B subunits of protein phosphatase 2A [PP2A]), which interact with CTTNBP2NL in HEK293 cells. The association between CTTNBP2 and striatin/zinedin suggests that CTTNBP2 targets the PP2A complex to dendritic spines. Thus we propose that the interactions of CTTNBP2 and cortactin and the PP2A complex regulate spine morphogenesis and synaptic signaling.

From neurodevelopment to neurodegeneration: the interaction of neurofibromin and valosin-containing protein/p97 in regulation of dendritic spine formation

Both Neurofibromatosis type I (NF1) and inclusion body myopathy with Paget's disease of bone and frontotemporal dementia (IBMPFD) are autosomal dominant genetic disorders. These two diseases are fully penetrant but with high heterogeneity in phenotypes, suggesting the involvement of genetic modifiers in modulating patients' phenotypes. Although NF1 is recognized as a developmental disorder and IBMPFD is associated with degeneration of multiple tissues, a recent study discovered the direct protein interaction between neurofibromin, the protein product of the NF1 gene, and VCP/p97, encoded by the causative gene of IBMPFD. Both NF1 and VCP/p97 are critical for dendritic spine formation, which provides the cellular mechanism explaining the cognitive deficits and dementia found in patients. Moreover, disruption of the interaction between neurofibromin and VCP impairs dendritic spinogenesis. Neurofibromin likely influences multiple downstream pathways to control dendritic spinogenesis. One is to activate the protein kinase A pathway to initiate dendritic spine formation; another is to regulate the synaptic distribution of VCP and control the activity of VCP in dendritic spinogenesis. Since neurofibromin and VCP/p97 also regulate cell growth and bone metabolism, the understanding of neurofibromin and VCP/p97 in neurons may be applied to study of cancer and bone. Statin treatment rescues the spine defects caused by VCP deficiency, suggesting the potential role of statin in clinical treatment for these two diseases.

Valosin-containing protein and neurofibromin interact to regulate dendritic spine density

Inclusion body myopathy with Paget disease of bone and frontotemporal dementia (IBMPFD) is an autosomal dominant disorder characterized by progressive myopathy that is often accompanied by bone weakening and/or frontotemporal dementia. Although it is known to be caused by mutations in the gene encoding valosin-containing protein (VCP), the underlying disease mechanism remains elusive. Like IBMPFD, neurofibromatosis type 1 (NF1) is an autosomal dominant disorder. Neurofibromin, the protein encoded by the NF1 gene, has been shown to regulate synaptogenesis. Here, we show that neurofibromin and VCP interact and work together to control the density of dendritic spines. Certain mutations identified in IBMPFD and NF1 patients reduced the interaction between VCP and neurofibromin and impaired spinogenesis. The functions of neurofibromin and VCP in spinogenesis were shown to correlate with the learning disability and dementia phenotypes seen in patients with IBMPFD. Consistent with the previous finding that treatment with a statin rescues behavioral defects in Nf1(+/-) mice and providing further support for our hypothesis that there is crosstalk between neurofibromin and VCP, statin exposure neutralized the effect of VCP knockdown on spinogenesis in cultured hippocampal neurons. The data presented here demonstrate that there is a link between IBMPFD and NF1 and indicate a role for VCP in synapse formation.

Targeted deletion of CASK-interacting nucleosome assembly protein causes higher locomotor and exploratory activities

CASK-interacting nucleosome assembly protein (CINAP) has been shown to interact with the calcium/calmodulin-dependent serine kinase (CASK) and the T-box transcription factor T-brain-1 (Tbr1) thus modulating the expression of N-methyl-D-aspartic acid receptor subunit 2b (NMDAR2b) in cultured hippocampal neurons. To explore the physiological significance of CINAP in vivo, CINAP knockout mice were generated and subjected to biochemical, anatomical, and behavioral analyses. Unexpectedly, CINAP deletion did not impact NMDAR2b expression, and these CINAP knockout mice were consistently comparable to wild-type littermates in terms of immediate memory (assessed with the Y maze) and associative memory (evaluated by conditioned taste aversion and contextual and auditory fear conditioning). Although CINAP deletion did not obviously influence learning and memory behaviors, CINAP knockout mice exhibited higher locomotor and exploratory activities. Compared with wild-type littermates, the horizontal and vertical movements of the CINAP knockout mice were higher in a novel environment; in home cages, rearing, sniffing, and jumping also occurred more frequently in CINAP knockout mice. These observations suggest that although CINAP deletion in mice does not influence learning and memory behaviors, CINAP is required for restriction of locomotor and exploratory activities.

Sarm1, a negative regulator of innate immunity, interacts with syndecan-2 and regulates neuronal morphology

Dendritic arborization is a critical neuronal differentiation process. Here, we demonstrate that syndecan-2 (Sdc2), a synaptic heparan sulfate proteoglycan that triggers dendritic filopodia and spine formation, regulates dendritic arborization in cultured hippocampal neurons. This process is controlled by sterile α and TIR motif-containing 1 protein (Sarm1), a negative regulator of Toll-like receptor 3 (TLR3) in innate immunity signaling. We show that Sarm1 interacts with and receives signal from Sdc2 and controls dendritic arborization through the MKK4-JNK pathway. In Sarm1 knockdown mice, dendritic arbors of neurons were less complex than those of wild-type littermates. In addition to acting downstream of Sdc2, Sarm1 is expressed earlier than Sdc2, which suggests that it has multiple roles in neuronal morphogenesis. Specifically, it is required for proper initiation and elongation of dendrites, axonal outgrowth, and neuronal polarization. These functions likely involve Sarm1-mediated regulation of microtubule stability, as Sarm1 influenced tubulin acetylation. This study thus reveals the molecular mechanism underlying the action of Sarm1 in neuronal morphogenesis.

Bcl11A/CTIP1 mediates the effect of the glutamate receptor on axon branching and dendrite outgrowth

While neuronal activity regulates neurite outgrowth and branching, the details of the underlying mechanisms are still largely unclear. This study investigated the effect of the glutamate receptors on neuritogenesis using cultured hippocampal neurons. At 3-4 days in vitro, NMDA treatment promoted axon branching but not primary axon extension. In contrast, blockade of the NMDA receptor (NMDAR) by AP5 treatment enhanced primary axon extension. NMDAR activation also increased dendrite number and total dendrite length. These results suggest that NMDAR controls axon and dendrite morphogenesis. A previous study demonstrated that knockdown of the zinc finger transcription factor B cell lymphoma 11A-long (Bcl11A-L) reduces deleted in colorectal cancer (DCC) and microtubule-associated protein (MAP) 1b expression, thereby promoting axon branching. Here, glutamate stimulation down-regulated the levels of the Bcl11A-L, DCC, MAP1b, and MAP2c proteins. Over-expression of either Bcl11A-L or DCC countered the effect of NMDA or glutamate on axon branching and dendrite outgrowth, indicating that the Bcl11A-L/DCC pathway is an important downstream effector of glutamate receptors in neurite arborization. Because knockdown of Bcl11A-L did not down-regulate MAP2c, our results suggest that glutamate receptors also use a Bcl11A-L-independent pathway to control dendrite outgrowth. To summarize, this study reveals novel pathways downstream of glutamate receptors that regulate axon and dendrite arborization.

CASK phosphorylation by PKA regulates the protein-protein interactions of CASK and expression of the NMDAR2b gene

Calcium/calmodulin-dependent serine kinase (CASK), a causative gene in X-linked mental retardation, acts as a multi-domain scaffold protein and interacts with more than 20 cellular proteins in different subcellular regions of neurons. It is of interest, therefore, to explore whether post-translational modification regulates CASK's protein-protein interactions. Here, we provide evidence that CASK is phosphorylated by protein kinase A (PKA), identifying residue S562 in the PSD-95-Dlg-ZO-1 domain and residue T724 in the guanylate kinase domain as PKA sites by an in vitro PKA kinase reaction and site-directed mutagenesis. Although the role of S562 phosphorylation is not clear, T724 phosphorylation up-regulates the interaction between CASK and T-box transcription factor T-brain-1 (Tbr-1). NMDAR2b, a downstream target of the CASK-Tbr-1 complex, was then used to explore the significance of CASK phosphorylation by PKA. In cultured cortical neurons, the PKA pathway stimulates both the protein expression and the promoter activity of NMDAR2b. Deletion of the Tbr-1-binding sites greatly reduces the 3'-5'-cyclic AMP responsiveness of the NMDAR2b promoter, and the CASK T724A mutation does not promote the 3'-5'-cyclic AMP responsiveness of NMDAR2b. In conclusion, our data provide evidence that PKA phosphorylates CASK, regulates the nuclear function of CASK, and consequently modulates NMDAR2b expression.

Calcium/calmodulin-dependent serine protein kinase and mental retardation

Calcium/calmodulin-dependent serine protein kinase (CASK) belongs to the membrane-associated guanylate kinase protein family. The members of this protein family function as multiple domain adaptor proteins originally identified at cell junctions and synapses. Insertional mutations or targeted disruption of the CASK gene in mice results in neonatal lethality, indicating an important role for CASK in development. Recently, several reports have also indicated that mutations in the human CASK gene result in X-linked malformations of the brain and mental retardation. At the molecular level, many studies indicate that CASK is critical for synapse formation at both presynaptic and postsynaptic junctions, and in the regulation of gene expression. The known molecular functions of CASK explain, at least partially, mental retardation and brain developmental defects in patients. In this review, recent findings about CASK are summarized and discussed.

Bcl11A/CTIP1 regulates expression of DCC and MAP1b in control of axon branching and dendrite outgrowth

The extension of axon branches is important for target innervation but how axon branching is regulated is currently not well understood. Here, we report that Bcl11A/CTIP1/Evi9, a zinc finger transcription factor, downregulates axon branching. Knockdown of Bcl11A induced axon branching and multi-axon formation, as well as dendrite outgrowth. Due to alternative splicing, a single Bcl11A gene encodes two protein products, Bcl11A-L and -S. Bcl11A-L was found to be the main Bcl11A player in regulation of neurite arborization; Bcl11A-S is an antagonist of Bcl11A-L. Time-lapse study further suggests that Bcl11A-L knockdown enhances axon dynamics and increases the duration of axon outgrowth. Finally, the expression of DCC and MAP1b, two molecules involved in direction and branching of axon outgrowth, is controlled by Bcl11A-L. DCC overexpression rescues the phenotype induced by Bcl11A-L knockdown. In conclusion, this report provides the first evidence that Bcl11A is important for neurite arborization.

SUMOylation of the MAGUK protein CASK regulates dendritic spinogenesis

Membrane-associated guanylate kinase (MAGUK) proteins interact with several synaptogenesis-triggering adhesion molecules. However, direct evidence for the involvement of MAGUK proteins in synapse formation is lacking. In this study, we investigate the function of calcium/calmodulin-dependent serine protein kinase (CASK), a MAGUK protein, in dendritic spine formation by RNA interference. Knockdown of CASK in cultured hippocampal neurons reduces spine density and shrinks dendritic spines. Our analysis of the time course of RNA interference and CASK overexpression experiments further suggests that CASK stabilizes or maintains spine morphology. Experiments using only the CASK PDZ domain or a mutant lacking the protein 4.1–binding site indicate an involvement of CASK in linking transmembrane adhesion molecules and the actin cytoskeleton. We also find that CASK is SUMOylated. Conjugation of small ubiquitin-like modifier 1 (SUMO1) to CASK reduces the interaction between CASK and protein 4.1. Overexpression of a CASK–SUMO1 fusion construct, which mimicks CASK SUMOylation, impairs spine formation. Our study suggests that CASK contributes to spinogenesis and that this is controlled by SUMOylation.

Neurofibromin interacts with CRMP-2 and CRMP-4 in rat brain

Neurofibromin, encoded by the neurofibromatosis type 1 (NF1) gene, regulates the Ras and cAMP pathways and plays a role in proliferation and neuronal morphogenesis. The details of the molecular mechanism of neurofibromin action in these processes are still unclear. In this study, immunoprecipitation and proteomics were used to identify novel proteins from rat brain that interact with neurofibromin. Mass spectrometry analysis showed that two proteins, the collapsin response mediator protein-2 (CRMP-2) and propionyl-CoA carboxylase alpha chain (PCCA), associated with neurofibromin. Immunoprecipitation-immunoblotting analysis confirmed the interactions between neurofibromin and CRMP-2 and CRMP-4, but not CRMP-1, in rat brain. CDK5, a kinase that regulates CRMP-2 in axonal outgrowth, was required for the interaction between neurofibromin and CRMP-2. Since both neurofibromin and CRMP proteins are involved in proliferation and axonal morphogenesis, these results suggest that the interaction with CRMPs contributes to the function of neurofibromin in tumorigenesis and neuronal morphogenesis.

Cdk5 promotes synaptogenesis by regulating the subcellular distribution of the MAGUK family member CASK

Synaptogenesis is a highly regulated process that underlies formation of neural circuitry. Considerable work has demonstrated the capability of some adhesion molecules, such as SynCAM and Neurexins/Neuroligins, to induce synapse formation in vitro. Furthermore, Cdk5 gain of function results in an increased number of synapses in vivo. To gain a better understanding of how Cdk5 might promote synaptogenesis, we investigated potential crosstalk between Cdk5 and the cascade of events mediated by synapse-inducing proteins. One protein recruited to developing terminals by SynCAM and Neurexins/Neuroligins is the MAGUK family member CASK. We found that Cdk5 phosphorylates and regulates CASK distribution to membranes. In the absence of Cdk5-dependent phosphorylation, CASK is not recruited to developing synapses and thus fails to interact with essential presynaptic components. Functional consequences include alterations in calcium influx. Mechanistically, Cdk5 regulates the interaction between CASK and liprin-alpha. These results provide a molecular explanation of how Cdk5 can promote synaptogenesis.

Syndecan-2 induces filopodia and dendritic spine formation via the neurofibromin-PKA-Ena/VASP pathway

Syndecan-2 induced filopodia before spinogenesis; therefore, filopodia formation was used here as a model to study the early downstream signaling of syndecan-2 that leads to spinogenesis. Screening using kinase inhibitors indicated that protein kinase A (PKA) is required for syndecan-2–induced filopodia formation in both human embryonic kidney cells and hippocampal neurons. Because neurofibromin, a syndecan-2–binding partner, activates the cyclic adenosine monophosphate pathway, the role of neurofibromin in syndecan-2–induced filopodia formation was investigated by deletion mutant analysis, RNA interference, and dominant-negative mutant. The results showed that neurofibromin mediates the syndecan-2 signal to PKA. Among actin-associated proteins, Enabled (Ena)/vasodilator-stimulated phosphoprotein (VASP) were predicted as PKA effectors downstream of syndecan-2, as Ena/VASP, which is activated by PKA, induces actin polymerization. Indeed, when the activities of Ena/VASP were blocked, syndecan-2 no longer induced filopodia formation. Finally, in addition to filopodia formation, neurofibromin and Ena/VASP contributed to spinogenesis. This study reveals a novel signaling pathway in which syndecan-2 activates PKA via neurofibromin and PKA consequently phosphorylates Ena/VASP, promoting filopodia and spine formation.

Expression of zinc finger transcription factor Bcl11A/Evi9/CTIP1 in rat brain

Bcl11A/Evi9/CTIP1, a Kruppel-like zinc finger gene, plays an important role in B-cell development. In addition to expression in B lymphocytes, Bcl11A/Evi9/CTIP1 is also highly expressed in the brain, although its function there is still unclear. In the present study, regional and subcellular distributions of Bcl11A/Evi9/CTIP1 in rat brain were investigated by immunostaining and biochemical fractionation. Using antibodies recognizing the first 18 amino acid residues of Bcl11A/Evi9/CTIP1, the distribution of 2 isoforms of Bcl11A/Evi9/CTIP1 gene products, Bcl11A-L/Evi9a and Bcl11A-S/Evi9c, was examined. In rat brain, both Bcl11A-L/Evi9a and Bcl11A-S/Evi9c were expressed, although the amount of Bcl11A-S/Evi9c protein was higher. Bcl11A-S/Evi9c was widely expressed in different regions of the rat brain. In contrast, Bcl11A-L/Evi9a was more restricted, being expressed in the cerebral cortex, hippocampus, and olfactory bulb. At the subcellular level, biochemical fractionation and confocal analysis of adult rat brain revealed that, in addition to being in the nuclei of neurons, fractions of Bcl11A-L/Evi9a and Bcl11A-S/Evi9c could be found in extranuclear locations. Double staining with the synaptic marker synaptophysin indicated a synaptic distribution of Bcl11A/Evi9/CTIP1. Postsynaptic density was also biochemically purified and subjected to immunoblotting using Bcl11A/Evi9/CTIP1 antibodies. The results showed that Bcl11A-L/Evi9a was enriched in the PSD I and PSD II fractions. In contrast, only a trace amount of Bcl11A-S was detected in PSD fractions. Our study also indicated that a fraction of Bcl11A/Evi9/CTIP1 was present in the cytoplasm, even at synapses. To regulate gene expression in the nuclei, nuclear translocation of Bcl11A/Evi9/CTIP1 may be one of the mechanisms controlling neuronal Bcl11A/Evi9/CTIP1 function.

Mice deficient in collapsin response mediator protein-1 exhibit impaired long-term potentiation and impaired spatial learning and memory

Collapsing response mediator protein-1 (CRMP-1) was initially identified in brain and has been implicated in plexin-dependent neuronal function. The high amino acid sequence identity among the five CRMPs has hindered determination of the functions of each individual CRMP. We generated viable and fertile CRMP-1 knock-out (CRMP-1(-/-)) mice with no evidence of gross abnormality in the major organs. CRMP-1(-/-) mice exhibited intense microtubule-associated protein 2 (MAP2) staining in the proximal portion of the dendrites, but reduced and disorganized MAP2 staining in the distal dendrites of hippocampal CA1 pyramidal cells. Immunoreactivity to GAP-43 (growth-associated protein-43) and PSD95 (postsynaptic density-95) (a postsynaptic membrane adherent cytoskeletal protein) was also decreased in the CA1 region of the knock-out mice. These changes were consistent with the mutant mice showing a reduction in long-term potentiation (LTP) in the CA1 region and impaired performance in hippocampal-dependent spatial learning and memory tests. CRMP-1(-/-) mice showed a normal synapsin I labeling pattern in CA1 and normal paired-pulse facilitation. These findings provide the first evidence suggesting that CRMP-1 may be involved in proper neurite outgrowth in the adult hippocampus and that loss of CRMP-1 may affect LTP maintenance and spatial learning and memory.

Neurofibromin signaling and synapses

Neurofibromin, encoded by the Neurofibromatosis type I (NF1) gene, has been shown to regulate the Ras and cAMP signaling pathways. The signaling functions of neurofibromin may account for tumor formation in patients with NF1, as well as influencing neuronal function. Learning defects have been documented in NF1 mutant mice, and in NF1 patients, learning disabilities are common. In this review, the recent studies related to the role of neurofibromin in neuronal morphogenesis will be discussed, which may partly explain why the patients with NF1 have learning defects.