CRMP4 regulates dendritic growth and maturation via the interaction with actin cytoskeleton in cultured hippocampal neurons

Contribution of the dihydropyrimidinase-like proteins family in synaptic physiology and in neurodevelopmental disorders

The dihydropyrimidinase-like (DPYSL) proteins, also designated as the collapsin response mediators (CRMP) proteins, constitute a family of five cytosolic phosphoproteins abundantly expressed in the developing nervous system but down-regulated in the adult mouse brain. The DPYSL proteins were initially identified as effectors of semaphorin 3A (Sema3A) signaling and consequently involved in regulation of growth cone collapse in young developing neurons. To date, it has been established that DPYSL proteins mediate signals for numerous intracellular/extracellular pathways and play major roles in variety of cellular process including cell migration, neurite extension, axonal guidance, dendritic spine development and synaptic plasticity through their phosphorylation status. The roles of DPYSL proteins at early stages of brain development have been described in the past years, particularly for DPYSL2 and DPYSL5 proteins. The recent characterization of pathogenic genetic variants in DPYSL2 and in DPYSL5 human genes associated with intellectual disability and brain malformations, such as agenesis of the corpus callosum and cerebellar dysplasia, highlighted the pivotal role of these actors in the fundamental processes of brain formation and organization. In this review, we sought to establish a detailed update on the knowledge regarding the functions of DPYSL genes and proteins in brain and to highlight their involvement in synaptic processing in later stages of neurodevelopment, as well as their particular contribution in human neurodevelopmental disorders (NDDs), such as autism spectrum disorders (ASD) and intellectual disability (ID).

Phosphorylation of Spastin Promotes the Surface Delivery and Synaptic Function of AMPA Receptors

Synaptic plasticity is essential for cognitive functions such as learning and memory. One of the mechanisms involved in synaptic plasticity is the dynamic delivery of AMPA receptors (AMPARs) in and out of synapses. Mutations of SPAST, which encodes SPASTIN, a microtubule-severing protein, are considered the most common cause of hereditary spastic paraparesis (HSP). In some cases, patients with HSP also manifest cognitive impairment. In addition, mice with Spastin depletion exhibit working and associative memory deficits and reduced AMPAR levels. However, the exact effect and molecular mechanism of Spastin on AMPARs trafficking has remained unclear. Here, we report that Spastin interacts with AMPAR, and phosphorylation of Spastin enhances its interaction with AMPAR subunit GluA2. Further study shows that phosphorylation of Spastin can increase AMPAR GluA2 surface expression and the amplitude and frequency of miniature excitatory synaptic currents (mEPSC) in cultured hippocampal neurons. Moreover, phosphorylation of Spastin at Ser210 is crucial for GluA2 surface expression. Phosphorylation of Spastin K353A, which obliterates microtubule-severing activity, also promotes AMPAR GluA2 subunit trafficking to the surface and increases the amplitude and frequency of mEPSCs in cultured neurons. Taken together, our data demonstrate that Spastin phosphorylation promotes the surface delivery of the AMPAR GluA2 subunit independent of microtubule dynamics.

CRMP4-mediated fornix development involves Semaphorin-3E signaling pathway

Neurodevelopmental axonal pathfinding plays a central role in correct brain wiring and subsequent cognitive abilities. Within the growth cone, various intracellular effectors transduce axonal guidance signals by remodeling the cytoskeleton. Semaphorin-3E (Sema3E) is a guidance cue implicated in development of the fornix, a neuronal tract connecting the hippocampus to the hypothalamus. Microtubule-associated protein 6 (MAP6) has been shown to be involved in the Sema3E growth-promoting signaling pathway. In this study, we identified the collapsin response mediator protein 4 (CRMP4) as a MAP6 partner and a crucial effector in Sema3E growth-promoting activity. CRMP4-KO mice displayed abnormal fornix development reminiscent of that observed in Sema3E-KO mice. CRMP4 was shown to interact with the Sema3E tripartite receptor complex within detergent-resistant membrane (DRM) domains, and DRM domain integrity was required to transduce Sema3E signaling through the Akt/GSK3 pathway. Finally, we showed that the cytoskeleton-binding domain of CRMP4 is required for Sema3E's growth-promoting activity, suggesting that CRMP4 plays a role at the interface between Sema3E receptors, located in DRM domains, and the cytoskeleton network. As the fornix is affected in many psychiatric diseases, such as schizophrenia, our results provide new insights to better understand the neurodevelopmental components of these diseases.

Modelling and Refining Neuronal Circuits with Guidance Cues: Involvement of Semaphorins

The establishment of neuronal circuits requires neurons to develop and maintain appropriate connections with cellular partners in and out the central nervous system. These phenomena include elaboration of dendritic arborization and formation of synaptic contacts, initially made in excess. Subsequently, refinement occurs, and pruning takes places both at axonal and synaptic level, defining a homeostatic balance maintained throughout the lifespan. All these events require genetic regulations which happens cell-autonomously and are strongly influenced by environmental factors. This review aims to discuss the involvement of guidance cues from the Semaphorin family.

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

The lncRNA MALAT1/miR-30/Spastin Axis Regulates Hippocampal Neurite Outgrowth

Spastin, a microtubule-severing enzyme, is important for neurite outgrowth. However, the mechanisms underlying the post-transcriptional regulation of spastin during microtubule-related processes are largely unknown. We demonstrated that the spastin expression level is controlled by a long non-coding RNA (lncRNA) metastasis-associated lung adenocarcinoma transcript 1 (MALAT1)/microRNA-30 (miR-30) axis during neurite outgrowth. The miR-30 expression level decreased in hippocampal neurons with increasing days in culture, and miR-30 overexpression suppressed while miR-30 inhibition promoted neurite outgrowth in hippocampal neurons. Spastin was validated as a target gene of miR-30 using the luciferase reporter assay. The protein expression, microtubule severing activity, and neurite promoting effect of spastin were suppressed by the overexpression of miR-30 mimics and increased by miR-30 inhibitors. MALAT1 expression increased during neurite outgrowth and MALAT1 silencing impaired neurite outgrowth. miR-30 was a sponge target of MALAT1 and MALAT1/miR-30 altered neurite outgrowth in hippocampal neurons. MALAT1 overexpression reversed the inhibitory effect of miR-30 on the activity of a luciferase reporter construct containing spastin, as well as spastin mRNA and protein expression, indicating that spastin was a downstream effector of MALAT1/miR-30. The MALAT1/miR-30 cascade also modulated spastin-induced microtubule severing, and the MALAT1/miR-30/spastin axis regulated neurite outgrowth in hippocampal neurons. This study suggests a new mechanism governing neurite outgrowth in hippocampal neurons involving MALAT1/miR-30-regulated spastin expression.

Ammonia induces calpain-dependent cleavage of CRMP-2 during neurite degeneration in primary cultured neurons

Hyperammonemia in the CNS induces irreversible damages to neurons due to ultimate cell loss. Neurite degeneration, a primary event that leads to neuronal cell death, remains less elucidated especially in hyperammonemia circumstances. Here, we found that the administration of ammonia induced neurite degeneration in cultured cerebellar granule neurons. The resulting altered neuronal morphology, rupture of neurites, and disassembly of the cytoskeleton led to cell death. Calcein and Fluo-4 staining revealed that ammonia induced intracellular calcium dysregulation. Subsequently activated calpain cleaved CRMP-2, a microtubule assembly protein. Pharmacologically inhibition of calpain, but not caspases or GSK-3, suppressed the cleavage of CRMP-2 and reversed neurite degeneration under ammonia treatment. Exposure to ammonia decreased whereas inhibition of calpain restored the amplitude and frequency of miniature excitatory postsynaptic currents. These data suggest a mechanism by which elevated ammonia level may induce neuronal dysfunction via abnormal calcium influx and calpain-dependent CRMP-2 cleavage, leading to abnormal synaptic transmission, cytoskeletal collapse, and neurite degeneration.

Collapsin Response Mediator Proteins: Their Biological Functions and Pathophysiology in Neuronal Development and Regeneration

Collapsin response mediator proteins (CRMPs), which consist of five homologous cytosolic proteins, are one of the major phosphoproteins in the developing nervous system. The prominent feature of the CRMP family proteins is a new class of microtubule-associated proteins that play important roles in the whole process of developing the nervous system, such as axon guidance, synapse maturation, cell migration, and even in adult brain function. The CRMP C-terminal region is subjected to posttranslational modifications such as phosphorylation, which, in turn, regulates the interaction between the CRMPs and various kinds of proteins including receptors, ion channels, cytoskeletal proteins, and motor proteins. The gene-knockout of the CRMP family proteins produces different phenotypes, thereby showing distinct roles of all CRMP family proteins. Also, the phenotypic analysis of a non-phosphorylated form of CRMP2-knockin mouse model, and studies of pharmacological responses to CRMP-related drugs suggest that the phosphorylation/dephosphorylation process plays a pivotal role in pathophysiology in neuronal development, regeneration, and neurodegenerative disorders, thus showing CRMPs as promising target molecules for therapeutic intervention.

Collapsin Response Mediator Protein 4 (CRMP4) Facilitates Wallerian Degeneration and Axon Regeneration following Sciatic Nerve Injury

In contrast to neurons in the CNS, damaged neurons from the peripheral nervous system (PNS) regenerate, but this process can be slow and imperfect. Successful regeneration is orchestrated by cytoskeletal reorganization at the tip of the proximal axon segment and cytoskeletal disassembly of the distal segment. Collapsin response mediator protein 4 (CRMP4) is a cytosolic phospho-protein that regulates the actin and microtubule cytoskeleton. During development, CRMP4 promotes growth cone formation and dendrite development. Paradoxically, in the adult CNS, CRMP4 impedes axon regeneration. Here, we investigated the involvement of CRMP4 in peripheral nerve injury in male and female Crmp4^−/− mice following sciatic nerve injury. We find that sensory axon regeneration and Wallerian degeneration are impaired in Crmp4^−/− mice following sciatic nerve injury. In vitro analysis of dissociated dorsal root ganglion (DRG) neurons from Crmp4^−/− mice revealed that CRMP4 functions in the proximal axon segment to promote the regrowth of severed DRG neurons and in the distal axon segment where it facilitates Wallerian degeneration through calpain-dependent formation of harmful CRMP4 fragments. These findings reveal an interesting dual role for CRMP4 in proximal and distal axon segments of injured sensory neurons that coordinately facilitate PNS axon regeneration.

Ketamine Regulates Phosphorylation of CRMP2 To Mediate Dendritic Spine Plasticity

Ketamine is widely used in infants and young children for anesthesia, and subanesthetic doses of ketamine make neurons form new protrusions and promote synapse formation. However, the precise pathological mechanisms remain to be elucidated. In this study, we demonstrated that ketamine administration significantly increased dendritic spine density and maturity in rat cortical neurons in vivo and in vitro. Western blot analysis showed that CRMP2 protein expression was significantly increased in cerebral cortex of ketamine group, and phosphorylation levels of CRMP at Thr514 and Ser522 were significantly reduced. Furthermore, overexpression of CRMP2 promoted the growth of cortical neuron processes and dendritic spines. Although the dendritic field was more complex after adding ketamine and the density of dendritic spines increased, there was no statistical difference and no obvious superposition effect was observed. Moreover, both Ser522 mutant construction of CRMP2, GFP-CRMP2-522D, and mcherry-CDK5 showed similar inhibitory effects on neurite outgrowth, which could be rescued by ketamine. The frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) were significantly inhibited when GFP-CRMP2-522D and mCherry-CDK5 were transfected into cortical neurons and this trend could also be rescued by ketamine. In general, this study reveals a new mechanism by which ketamine promotes the growth and development of dendritic spines in developing cortical neurons.

Crmp4-KO Mice as an Animal Model for Investigating Certain Phenotypes of Autism Spectrum Disorders

Previous research has demonstrated that the collapsin response mediator protein (CRMP) family is involved in the formation of neural networks. A recent whole-exome sequencing study identified a de novo variant (S541Y) of collapsin response mediator protein 4 (CRMP4) in a male patient with autism spectrum disorder (ASD). In addition, Crmp4-knockout (KO) mice show some phenotypes similar to those observed in human patients with ASD. For example, compared with wild-type mice, Crmp4-KO mice exhibit impaired social interaction, abnormal sensory sensitivities, broader distribution of activated (c-Fos expressing) neurons, altered dendritic formation, and aberrant patterns of neural gene expressions, most of which have sex differences. This review summarizes current knowledge regarding the role of CRMP4 during brain development and discusses the possible contribution of CRMP4 deficiencies or abnormalities to the pathogenesis of ASD. Crmp4-KO mice represent an appropriate animal model for investigating the mechanisms underlying some ASD phenotypes, such as impaired social behavior, abnormal sensory sensitivities, and sex-based differences, and other neurodevelopmental disorders associated with sensory processing disorders.

In Vitro Effects of Chronic Spirolide Treatment on Human Neuronal Stem Cell Differentiation and Cholinergic System Development

Spirolides (SPX) are marine toxins, produced by dinoflagellates that act as potent antagonists of nicotinic acetylcholine receptors. These compounds are not toxic for humans, and since there are no reports of human intoxications caused by this group of toxins they are not yet currently regulated in Europe. Currently 13-desmethyl spirolide C, 13,19-didesmethyl spirolide C, and 20-methyl spirolide G are commercially available as reference materials. Previous work in our laboratory has demonstrated that after 4 days of treatment of primary mice cortical neurons with 13-desmethyl spirolide C, the compound ameliorated the glutamate induced toxicity and increased acetylcholine levels and the expression of the acetylcholine synthesizing enzyme being useful both in vitro and in vivo to decrease the brain pathology associated with Alzheimer's disease. In this work, we aimed to extend the study of the neuronal effects of spirolides in human neuronal cells. To this end, human neuronal progenitor cells CTX0E16 were employed to evaluate the in vitro effect of spirolides on neuronal development. The results presented here indicate that long-term exposure (30 days) of human neuronal stem cells to SPX compounds, at concentrations up to 50 nM, ameliorated the MPP⁺-induced neurotoxicity and increased the expression of neuritic and dendritic markers, the levels of the choline acetyltransferase enzyme and the protein levels of the α7 subunit of nicotinic acetylcholine receptors. These effects are presumably due to the previously described interaction of these compounds with nicotinic receptors containing both α7 and α4 subunits. All together, these data emphasize the idea that SPX could be attractive lead molecules against neurodegenerative disorders.

Proteome and behavioral alterations in phosphorylation-deficient mutant Collapsin Response Mediator Protein2 knock-in mice

CRMP2, alternatively designated as DPYSL2, was the first CRMP family member to be identified as an intracellular molecule mediating the signaling of the axon guidance molecule Semaphorin 3A (Sema3A). In Sema3A signaling, cyclin-dependent kinase 5 (Cdk5) primarily phosphorylates CRMP2 at Ser522. Glycogen synthase kinase-3β (GSK-3β) subsequently phosphorylates the residues of Thr509 and Thr514 of CRMP2. Previous studies showed that CRMP2 is involved in pathogenesis of neurological disorders such as Alzheimer's disease. In Alzheimer's disease, hyper-phosphorylated forms of CRMP2 are accumulated in the paired helical filaments. To get insight into the possible involvement of the phosphorylation of CRMP2 in pathogenesis of neurological disorders, we previously created CRMP2 S522A knock-in (crmp2^ki/ki) mice and demonstrated that the phosphorylation of CRMP2 at Ser522 is involved in normal dendrite patterning in cortical neurons. However, the behavioral impact and in vivo signaling network of the CRMP2 phosphorylation are not fully understood. In this study, we performed behavioral and proteomics analysis of crmp2^ki/ki mice. The crmp2^ki/ki mice appeared healthy and showed no obvious differences in physical characteristics compared to wild-type mice, but they showed impaired emotional behavior, reduced sociality, and low sensitivity to pain stimulation. Through mass-spectrometry-based proteomic analysis, we found that 59 proteins were increased and 77 proteins were decreased in the prefrontal cortex of crmp2^ki/ki mice. Notably, CRMP3, CRMP4, and CRMP5, the other CRMP family proteins, were increased in crmp2^ki/ki mice. KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analyses identified 14 pathways in increased total proteins and 13 pathways in decreased total proteins which are associated with the pathogenesis of Parkinson's, Alzheimer's, and Huntington's diseases. We also detected 20 pathways in increased phosphopeptides and 16 pathways in decreased phosphopeptides including "inflammatory mediator regulation of TRP channels" in crmp2^ki/ki mice. Our study suggests that the phosphorylation of CRMP2 at Ser522 is involved in the signaling pathways that may be related to neuropsychiatric and neurodegenerative diseases and pain.

Phosphorylation and SUMOylation of CRMP2 regulate the formation and maturation of dendritic spines

The posttranslational modifications of CRMP2 play an important role in axon outgrowth, cell polarization and dendritic morphogenesis. However, whether CRMP2 and its posttranslational modifications are involved in dendritic spine development specifically is not completely clear. Here, we show that CRMP2 can promote the formation and maturation of dendritic spines in cultured hippocampal neurons. Overexpression of CRMP2 results in an increase in the density of spines especially the mushroom-shape spines. The amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs) are both enhanced and the intensity of PSD95 is strengthened in the neurons with CRMP2 overexpression. Furthermore, dephosphorylation of CRMP2 at Thr514 and deSUMOylation at Lys374 can further promote the formation and maturation of dendritic spines, whereas, no cross-talk is found between these two posttranslational modifications in the regulation of dendritic spine formation and maturation. Taken together, our data support a model in which phosphorylation and SUMOylation modification of CRMP2 independently promote the formation and maturation of dendritic spines and participate in the process of dendritic spine plasticity.

Human CRMP4 mutation and disrupted Crmp4 expression in mice are associated with ASD characteristics and sexual dimorphism

Autism spectrum disorders (ASD) are more common among boys than girls. The mechanisms responsible for ASD symptoms and their sex differences remain mostly unclear. We previously identified collapsin response mediator protein 4 (CRMP4) as a protein exhibiting sex-different expression during sexual differentiation of the hypothalamic sexually dimorphic nucleus. This study investigated the relationship between the sex-different development of autistic features and CRMP4 deficiency. Whole-exome sequencing detected a de novo variant (S541Y) of CRMP4 in a male ASD patient. The expression of mutated mouse CRMP4 S540Y, which is homologous to human CRMP4 S541Y, in cultured hippocampal neurons derived from Crmp4-knockout (KO) mice had increased dendritic branching, compared to those transfected with wild-type (WT) Crmp4, indicating that this mutation results in altered CRMP4 function in neurons. Crmp4-KO mice showed decreased social interaction and several alterations of sensory responses. Most of these changes were more severe in male Crmp4-KO mice than in females. The mRNA expression levels of some genes related to neurotransmission and cell adhesion were altered in the brain of Crmp4-KO mice, mostly in a gender-dependent manner. These results indicate a functional link between a case-specific, rare variant of one gene, Crmp4, and several characteristics of ASD, including sexual differences.

Endophilin2 Interacts with GluA1 to Mediate AMPA Receptor Endocytosis Induced by Oligomeric Amyloid-β

Amyloid-β (Aβ) plays an important role in Alzheimer's disease (AD), as oligomeric Aβ induces loss of postsynaptic AMPA receptors (AMPARs) leading to cognitive deficits. The loss of postsynaptic AMPARs is mediated through the clathrin-dependent endocytosis pathway, in which endophilin2 is one of the important regulatory proteins. Endophilin2, which is enriched in both the pre- and postsynaptic membrane, has previously been reported to be important for recycling of synaptic vesicles at the presynaptic membrane. However, the role of endophilin2 in oligomeric Aβ-induced postsynaptic AMPAR endocytosis is not well understood. In this study, we show that endophilin2 does not affect constitutive AMPAR endocytosis. Endophilin2 knockdown, but not overexpression, resisted oligomeric Aβ-induced AMPAR dysfunction. Moreover, endophilin2 colocalized and interacted with GluA1, a subunit of AMPAR, to regulate oligomeric Aβ-induced AMPAR endocytosis. Thus, we have determined a role of endophilin2 in oligomeric Aβ-induced postsynaptic AMPAR dysfunction, indicating possible directions for preventing the loss of AMPARs in cognitive impairment and providing evidence for the clinical treatment of AD.