Opening Pandora's jar: a primer on the putative roles of CRMP2 in a panoply of neurodegenerative, sensory and motor neuron, and central disorders

The role of the ubiquitin system in the onset and reversal of neuropathic pain

Neuropathic pain (NP) remains one of the world's most difficult problems, and people suffering from NP have their quality of life affected to a great extent and constantly suffer from pain. Sensitization of injurious receptors, ectopic firing of afferent nerves after nerve injury, and coupling between sympathetic and sensory neurons are involved in the onset or development of NP, but the pathogenesis of NP is still not well understood. We found that the ubiquitin system is involved in the pathogenesis of NP and has a crucial role in it. The ubiquitin system can be involved in the onset or reversal of NP by affecting ion channels, cellular signal transduction, glial cells, and the regulation of non-coding RNAs. This provides new ideas for the treatment of NP. The ubiquitin system may be a new effective target for the treatment of NP. A continued, in-depth understanding of the mechanisms of the ubiquitin system involved in NP could further refine the study of analgesic targets and improve pharmacological studies.

Related cellular signaling and consequent pathophysiological outcomes of ubiquitin specific protease 24

Ubiquitin-specific protease 24 (USP24) is an essential member of the deubiquitinating protease family found in eukaryotes. It engages in interactions with multiple proteins, including p53, MCL-1, E2F4, and FTH1, among others. Through these interactions, USP24 plays a critical role in regulating vital cellular processes such as cell cycle control, DNA damage response, cellular iron autophagy, and apoptosis. Increased levels of USP24 have been observed in various cancer types, including bladder cancer, lung cancer, myeloma, hepatocellular carcinoma, and gastric cancer. However, in certain tumors like kidney cancer, USP24 is significantly downregulated, and the specific mechanism behind this remains unclear. Currently, there are no officially approved USP24 inhibitors available for clinical use. Some existing inhibitors targeting USP24 have shown promising effects in treating malignancies; however, their precise mode of action and information regarding binding sites are not well understood. Moreover, further optimization is required to enhance the selectivity and efficacy of these inhibitors. This review aims to provide a comprehensive overview of recent advancements in understanding the cellular functions of USP24, its association with various diseases, and the development of small-molecule inhibitors that target this protein. In conclusion, USP24 represents a promising therapeutic target for various diseases, and ongoing research will contribute to validating its role and facilitating the development of effective treatments.

CRMP2 Participates in Regulating Mitochondrial Morphology and Motility in Alzheimer's Disease

Mitochondrial bioenergetics and dynamics (alterations in morphology and motility of mitochondria) play critical roles in neuronal reactions to varying energy requirements in health and disease. In Alzheimer's disease (AD), mitochondria undergo excessive fission and become less motile. The mechanisms leading to these alterations are not completely clear. Here, we show that collapsin response mediator protein 2 (CRMP2) is hyperphosphorylated in AD and that is accompanied by a decreased interaction of CRMP2 with Drp1, Miro 2, and Mitofusin 2, which are proteins involved in regulating mitochondrial morphology and motility. CRMP2 was hyperphosphorylated in postmortem brain tissues of AD patients, in brain lysates, and in cultured cortical neurons from the double transgenic APP/PS1 mice, an AD mouse model. CRMP2 hyperphosphorylation and dissociation from its binding partners correlated with increased Drp1 recruitment to mitochondria, augmented mitochondrial fragmentation, and reduced mitochondrial motility. (S)-lacosamide ((S)-LCM), a small molecule that binds to CRMP2, decreased its phosphorylation at Ser 522 and Thr 509/514, and restored CRMP2's interaction with Miro 2, Drp1, and Mitofusin 2. This was paralleled by decreased Drp1 recruitment to mitochondria, diminished mitochondrial fragmentation, and improved motility of the organelles. Additionally, (S)-LCM-protected cultured cortical AD neurons from cell death. Thus, our data suggest that CRMP2, in a phosphorylation-dependent manner, participates in the regulation of mitochondrial morphology and motility, and modulates neuronal survival in AD.

The therapeutic potential of the neuroactive peptides of soluble amyloid precursor protein-alpha in Alzheimer's disease and related neurological disorders

Soluble amyloid precursor protein-alpha (sAPPα) is a multi-functional brain-derived protein that has neuroprotective, neurogenic and neurotropic properties. Moreover, it is known to facilitate synaptic function and promote neural repair. These properties suggest sAPPα may be useful as a therapeutic agent for the treatment of neurological diseases characterized by synaptic failure and neuronal loss, such as occurs in Alzheimer's disease, and for neural repair following traumatic brain injury and stroke. However, sAPPα's relatively large size and the difficulty of ongoing delivery of therapeutics to the brain mean this is not currently practicable. Importantly, however, sAPPα is composed of several neuroactive domains that each possess properties that collectively are remarkably similar to those of sAPPα itself. Here, we review the molecular structure of sAPPα and identify the domains that contribute to its overall functionality. Four peptide motifs present as possible targets for therapeutic development. We review their physiochemical and neuroactive properties, both within sAPPα and as isolated peptides, and discuss their potential for future development as multipurpose therapeutic agents for the treatment of Alzheimer's disease and other disorders of neuronal function. Further, we discuss the role of heparin binding sites, found within sAPPα's structure and overlapping with the neuroactive domains, as sites for interactions with effector proteins and synaptic receptors. The potential role of the neuroactive peptides known as Cationic Arginine-Rich Peptides (CARPs) as neuroprotective motifs is also reviewed. Mechanisms of peptide delivery to the brain are briefly discussed. Finally, we summarise the potential benefits and pitfalls of using the isolated peptides, either individually or in combination, for the treatment of neurological diseases.

Elevated Hippocampal CRMP5 Mediates Chronic Stress-Induced Cognitive Deficits by Disrupting Synaptic Plasticity, Hindering AMPAR Trafficking, and Triggering Cytokine Release

Chronic stress is a critical risk factor for developing depression, which can impair cognitive function. However, the underlying mechanisms involved in chronic stress-induced cognitive deficits remain unclear. Emerging evidence suggests that collapsin response mediator proteins (CRMPs) are implicated in the pathogenesis of psychiatric-related disorders. Thus, the study aims to examine whether CRMPs modulate chronic stress-induced cognitive impairment. We used the chronic unpredictable stress (CUS) paradigm to mimic stressful life situations in C57BL/6 mice. In this study, we found that CUS-treated mice exhibited cognitive decline and increased hippocampal CRMP2 and CRMP5 expression. In contrast to CRMP2, CRMP5 levels strongly correlated with the severity of cognitive impairment. Decreasing hippocampal CRMP5 levels through shRNA injection rescued CUS-induced cognitive impairment, whereas increasing CRMP5 levels in control mice exacerbated memory decline after subthreshold stress treatment. Mechanistically, hippocampal CRMP5 suppression by regulating glucocorticoid receptor phosphorylation alleviates chronic stress-induced synaptic atrophy, disruption of AMPA receptor trafficking, and cytokine storms. Our findings show that hippocampal CRMP5 accumulation through GR activation disrupts synaptic plasticity, impedes AMPAR trafficking, and triggers cytokine release, thus playing a critical role in chronic stress-induced cognitive deficits.

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).

De novo non-synonymous DPYSL2 (CRMP2) variants in two patients with intellectual disabilities and documentation of functional relevance through zebrafish rescue and cellular transfection experiments

Collapsin response mediator protein 2 (Crmp2) is an evolutionarily well-conserved tubulin-binding cytosolic protein that plays critical roles in the formation of neural circuitry in model organisms including zebrafish and rodents. No clinical evidence that CRMP2 variants are responsible for monogenic neurogenic disorders in humans presently exists. Here, we describe two patients with de novo non-synonymous variants (S14R and R565C) of CRMP2 and intellectual disability associated with hypoplasia of the corpus callosum. We further performed various functional assays of CRMP2 variants using zebrafish and zebrafish Crmp2 (abbreviated as z-CRMP2 hereafter) and an antisense morpholino oligonucleotide [AMO]-based experimental system in which crmp2-morphant zebrafish exhibit the ectopic positioning of caudal primary (CaP) motor neurons. Whereas the co-injection of wild-type z-CRMP2 mRNA suppressed the ectopic positioning of CaP motor neurons in Crmp2-morphant zebrafish, the co-injection of R566C or S15R, z-CRMP2, which corresponds to R565C and S14R of human CRMP2, failed to rescue the ectopic positioning. Transfection experiments of zebrafish or rat Crmp2 using plasmid vectors in HeLa cells, with or without a proteasome inhibitor, demonstrated that the expression levels of mutant Crmp2 protein encoded by R565C and S14R CRMP2 variants were decreased, presumably because of increased degradation by proteasomes. When we compared CRMP2-tubulin interactions using co-immunoprecipitation and cellular localization studies, the R565C and S14R mutations weakened the interactions. These results collectively suggest that the CRMP2 variants detected in the present study consistently led to the loss-of-function of CRMP2 protein and support the notion that pathogenic variants in CRMP2 can cause intellectual disabilities in humans.

Neurodegenerative Diseases: From Dysproteostasis, Altered Calcium Signalosome to Selective Neuronal Vulnerability to AAV-Mediated Gene Therapy

Despite intense research into the multifaceted etiology of neurodegenerative diseases (ND), they remain incurable. Here we provide a brief overview of several major ND and explore novel therapeutic approaches. Although the cause (s) of ND are not fully understood, the accumulation of misfolded/aggregated proteins in the brain is a common pathological feature. This aggregation may initiate disruption of Ca⁺⁺ signaling, which is an early pathological event leading to altered dendritic structure, neuronal dysfunction, and cell death. Presently, ND gene therapies remain unidimensional, elusive, and limited to modifying one pathological feature while ignoring others. Considering the complexity of signaling cascades in ND, we discuss emerging therapeutic concepts and suggest that deciphering the molecular mechanisms involved in dendritic pathology may broaden the phenotypic spectrum of ND treatment. An innovative multiplexed gene transfer strategy that employs silencing and/or over-expressing multiple effectors could preserve vulnerable neurons before they are lost. Such therapeutic approaches may extend brain health span and ameliorate burdensome chronic disease states.

Lanthionine Ketimine Ethyl Ester Accelerates Remyelination in a Mouse Model of Multiple Sclerosis

Although over 20 disease modifying therapies are approved to treat Multiple Sclerosis (MS), these do not increase remyelination of demyelinated axons or mitigate axon damage. Previous studies showed that lanthionine ketenamine ethyl ester (LKE) reduces clinical signs in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS and increased maturation of oligodendrocyte (OL) progenitor cells (OPCs) in vitro. In the current study, we used the cuprizone (CPZ) demyelination model of MS to test if LKE could increase remyelination. The corpus callosum (CC) and somatosensory cortex was examined by immunohistochemistry (IHC), electron microscopy and for mRNA expression changes in mice provided 5 weeks of CPZ diet followed by 2 weeks of normal diet in the presence of LKE or vehicle. A significant increase in the number of myelinated axons, and increased myelin thickness was observed in the CC of LKE-treated groups compared to vehicle-treated groups. LKE also increased myelin basic protein and proteolipid protein expression in the CC and cortex, and increased the number of mature OLs in the cortex. In contrast, LKE did not increase the percentage of proliferating OPCs suggesting effects on OPC survival and differentiation but not proliferation. The effects of LKE on OL maturation and remyelination were supported by similar changes in their relative mRNA levels. Interestingly, LKE did not have significant effects on GFAP or Iba1 immunostaining or mRNA levels. These findings suggest that remyelinating actions of LKE can potentially be formulated to induce remyelination in neurological diseases associated with demyelination including MS.

Within- and between-Breed Selection Signatures in the Original and Improved Valachian Sheep

This study explored the genomic diversity and selection signatures in two Slovakian national breeds, the Original Valachian and the Improved Valachian sheep. As they are an important animal genetic resource within the country, but with decreasing population size, our aim is to identify potentially valuable genomic regions. A total of 97 sheep (18 male and 79 female) from the Original Valachian, and 69 sheep (25 male and 44 female) from the Improved Valachian populations were genotyped using the GeneSeek GGP Ovine 50 K chip. The inbreeding levels were assessed with runs of homozygosity (ROH). The selection signatures within breeds were identified based on the top 1% of most homozygous regions within the breed, the so-called ROH islands. The selection signatures between breeds were assessed based on variance in linkage disequilibrium. Overall, we have identified selection signatures with quantitative trait loci (QTL) and genes pointing towards all three production purposes of the Valachian sheep, milk, meat, and wool, including their quality characteristics. Another group with apparent large importance was the various traits related to health and resistance to parasites, which is well in line with the sturdy nature of this breed.

Small molecule targeting NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces pain in chronic constriction injury (CCI) rats

The voltage-gated sodium channel isoform NaV1.7 is a critical player in the transmission of nociceptive information. This channel has been heavily implicated in human genetic pain disorders and is a validated pain target. However, targeting this channel directly has failed, and an indirect approach – disruption of interactions with accessory protein partners – has emerged as a viable alternative strategy. We recently reported that a small-molecule inhibitor of CRMP2 SUMOylation, compound 194, selectively reduces NaV1.7 currents in DRG neurons across species from mouse to human. This compound also reversed mechanical allodynia in a spared nerve injury and chemotherapy-induced model of neuropathic pain. Here, we show that oral administration of 194 reverses mechanical allodynia in a chronic constriction injury (CCI) model of neuropathic pain. Furthermore, we show that orally administered 194 reverses the increased latency to cross an aversive barrier in a mechanical conflict-avoidance task following CCI. These two findings, in the context of our previous report, support the conclusion that 194 is a robust inhibitor of NaV1.7 function with the ultimate effect of profoundly ameliorating mechanical allodynia associated with nerve injury. The fact that this was observed using both traditional, evoked measures of pain behavior as well as the more recently developed operator-independent mechanical conflict-avoidance assay increases confidence in the efficacy of 194-induced anti-nociception.

Maternal deprivation induces cytoskeletal alterations and depressive-like behavior in adult male rats by regulating the AKT/GSK3β/CRMP2 signaling pathway

Early-life adverse events exert persistent effects on brain functions and may increase the risk of psychopathology in adulthood. However, the underlying mechanism remains unclear. The purpose of our study was to study the long-lasting effects of maternal deprivation (MD) on depression-related behaviors and microtubule dynamics, and to illuminate the underlying molecular mechanism. Rat pups were separated from the dams for 360 min (MD) or 15 min (brief maternal separation) each day from postnatal day 4 through 10. Rats with MD experience showed significant depressive-like behaviors in adulthood, while brief maternal separation did not alter the behaviors. Behavioral alterations in the MD group were accompanied by alterations in the AKT/GSK3β/CRMP2 signaling pathway and hyperphosphorylation of CRMP2. CRMP2 interacted and colocalized with the cytoskeleton in the hippocampus, and the overlap of CRMP2 and tubulin staining in the hippocampus of MD rats was decreased. In MD rats, the expression of the α-tubulin isoforms Acet-tubulin and Tyr-tubulin changed, and the ratio of Tyr/Acet-tubulin, which is an important marker of microtubule dynamics, was decreased, indicating decreased microtubule dynamics. Furthermore, regulation of the AKT/GSK3β/CRMP2 signaling pathway by an LY294002 microinjection in the hippocampus resulted in cytoskeletal alterations and depressive-like behaviors in rats. These findings suggest that early-life MD induces depressive-like behaviors and cytoskeletal alterations in adult male rats and that the effects may be partly mediated by the AKT/GSK3β/CRMP2 signaling pathway. An understanding of the mechanism underlying the effect of MD on behaviors is crucial for developing pharmacological and psychological interventions for childhood neglect.

Involvement of CRMP2 in Regulation of Mitochondrial Morphology and Motility in Huntington's Disease

Mitochondrial morphology and motility (mitochondrial dynamics) play a major role in the proper functioning of distant synapses. In Huntington’s disease (HD), mitochondria become fragmented and less motile, but the mechanisms leading to these changes are not clear. Here, we found that collapsin response mediator protein 2 (CRMP2) interacted with Drp1 and Miro 2, proteins involved in regulating mitochondrial dynamics. CRMP2 interaction with these proteins inversely correlated with CRMP2 phosphorylation. CRMP2 was hyperphosphorylated in postmortem brain tissues of HD patients, in human neurons derived from induced pluripotent stem cells from HD patients, and in cultured striatal neurons from HD mouse model YAC128. At the same time, CRMP2 interaction with Drp1 and Miro 2 was diminished in HD neurons. The CRMP2 hyperphosphorylation and dissociation from Drp1 and Miro 2 correlated with increased fission and suppressed motility. (S)-lacosamide ((S)-LCM), a small molecule that binds to CRMP2, decreased its phosphorylation at Thr 509/514 and Ser 522 and rescued CRMP2’s interaction with Drp1 and Miro 2. This was accompanied by reduced mitochondrial fission and enhanced mitochondrial motility. Additionally, (S)-LCM exerted a neuroprotective effect in YAC128 cultured neurons. Thus, our data suggest that CRMP2 may regulate mitochondrial dynamics in a phosphorylation-dependent manner and modulate neuronal survival in HD.

CRMP2 Is Involved in Regulation of Mitochondrial Morphology and Motility in Neurons

Regulation of mitochondrial morphology and motility is critical for neurons, but the exact mechanisms are unclear. Here, we demonstrate that these mechanisms may involve collapsin response mediator protein 2 (CRMP2). CRMP2 is attached to neuronal mitochondria and binds to dynamin-related protein 1 (Drp1), Miro 2, and Kinesin 1 light chain (KLC1). Treating neurons with okadaic acid (OA), an inhibitor of phosphatases PP1 and PP2A, resulted in increased CRMP2 phosphorylation at Thr509/514, Ser522, and Thr555, and augmented Drp1 phosphorylation at Ser616. The CRMP2-binding small molecule (S)-lacosamide ((S)-LCM) prevented an OA-induced increase in CRMP2 phosphorylation at Thr509/514 and Ser522 but not at Thr555, and also failed to alleviate Drp1 phosphorylation. The increased CRMP2 phosphorylation correlated with decreased CRMP2 binding to Drp1, Miro 2, and KLC1. (S)-LCM rescued CRMP2 binding to Drp1 and Miro 2 but not to KLC1. In parallel with CRMP2 hyperphosphorylation, OA increased mitochondrial fission and suppressed mitochondrial traffic. (S)-LCM prevented OA-induced alterations in mitochondrial morphology and motility. Deletion of CRMP2 with a small interfering RNA (siRNA) resulted in increased mitochondrial fission and diminished mitochondrial traffic. Overall, our data suggest that the CRMP2 expression level and phosphorylation state are involved in regulating mitochondrial morphology and motility in neurons.

Nogo-A Induced Polymerization of Microtubule Is Involved in the Inflammatory Heat Hyperalgesia in Rat Dorsal Root Ganglion Neurons

The microtubule, a major constituent of cytoskeletons, was shown to bind and interact with transient receptor potential vanilloid subfamily member 1 (TRPV1), and serves a pivotal role to produce thermal hyperalgesia in inflammatory pain. Nogo-A is a modulator of microtubule assembly and plays a key role in maintaining the function of TRPV1 in inflammatory heat pain. However, whether the microtubule dynamics modulated by Nogo-A in dorsal root ganglion (DRG) neurons participate in the inflammatory pain is not elucidated. Here we reported that the polymerization of microtubules in the DRG neurons, as indicated by the acetylated α-tubulin, tubulin polymerization-promoting protein 3 (TPPP3), and microtubule numbers, was significantly elevated in the complete Freund’s adjuvant (CFA) induced inflammatory pain. Consistent with our previous results, knock-out (KO) of Nogo-A protein significantly attenuated the heat hyperalgesia 72 h after CFA injection and decreased the microtubule polymerization via up-regulation of phosphorylation of collapsin response mediator protein 2 (CRMP2) in DRG. The colocalization of acetylated α-tubulin and TRPV1 in DRG neurons was also reduced dramatically in Nogo-A KO rats under inflammatory pain. Moreover, the down-regulation of TRPV1 in DRG of Nogo-A KO rats after injection of CFA was reversed by intrathecal injection of paclitaxel, a microtubule stabilizer. Furthermore, intrathecal injection of nocodazole (a microtubule disruptor) attenuated significantly the CFA-induced inflammatory heat hyperalgesia and the mechanical pain in a rat model of spared nerve injury (SNI). In these SNI cases, the Nogo-A and acetylated α-tubulin in DRG were also significantly up-regulated. We conclude that the polymerization of microtubules promoted by Nogo-A in DRG contributes to the development of inflammatory heat hyperalgesia mediated by TRPV1.

A screened PirB antagonist peptide antagonizes Aβ42-mediated inhibition of neurite outgrowth in vitro

Alzheimer's disease (AD) is a type of progressive neurodegenerative disease, and amyloid β-protein 42 (Aβ₄₂) serves an important role in the pathological process of development of AD. Paired immunoglobulin-like receptor B (PirB) is a functional receptor for myelin inhibitors of neuron regeneration in the CNS, and it has also been identified to function as a high-affinity receptor for Aβ. Here, we used a phage display to identify a specific PirB antagonist peptide 11(PAP11, PFRLQLS), which could reverse Aβ₄₂-induced neurotoxicity and promote neurite outgrowth in vitro. Immunofluorescence analysis showed that PAP11 colocalized with PirB on the membrane of cortical neurons. Horseradish peroxidase-streptavidin-biotin assay further proved that PAP11 directly binds to PirB and the dissociation constant (K_d) was 0.128μM. PAP11 functionally antagonized the neurite outgrowth inhibitory effect induced by Aβ₄₂ in cortical neurons, and the underlying mechanism was associated with a PirB-ROCK2/CRMP2 signaling pathway. The novel PirB antagonist peptide PAP11 may be a promising candidate therapeutic agent for the treatment of AD and other neurodegenerative diseases. KEY POINTS: • PAP11 was the first PirB antagonist peptide screened by phage display technology. • PAP11 could protect neurons by blocking the binding of Aβ₄₂ and PirB. • PAP11 reverse inhibitory effect of neurite outgrowth through ROCK2/CRMP2 pathway.

Collapsin response mediator protein 5 (CRMP5) modulates susceptibility to chronic social defeat stress in mice

Collapsin response mediator protein 5 (CRMP5), a member of the CRMP family, is expressed in the brain, particularly in the hippocampus, an area of the brain that can modulate stress responses. Social stress has a well-known detrimental effect on health and can lead to depression, but not all individuals are equally sensitive to stress. To date, researchers have not conclusively determined how social stress increases the susceptibility of the brain to depression. Here, we used the chronic social defeat stress (CSDS) model and observed higher hippocampal CRMP5 expression in stress-susceptible (SS) mice than in control and stress-resilient (RES) mice. A negative correlation was observed between the expression levels of CRMP5 and the social interaction (SI) ratio. Reduced hippocampal CRMP5 expression increased the SI ratio in SS mice, whereas CRMP5 overexpression was sufficient to induce social avoidance behaviors in control mice following exposure to subthreshold social stress induced by lentivirus-based overexpression and inducible tetracycline-on strategies to upregulate CRMP5. Interestingly, increased CRMP5 expression in SS and lenti-CRMP5-treated mice also caused serum corticosterone concentrations to increase. These findings improve our understanding of the potential mechanism by which CRMP5 triggers susceptibility to social stress, and they support the further development of therapeutic agents for the treatment of stress disorders in humans.

Evaluation of the effects of the T-type calcium channel enhancer SAK3 in a rat model of TAF1 deficiency

The TATA-box binding protein associated factor 1 (TAF1) is part of the TFIID complex that plays a key role during the initiation of transcription. Variants of TAF1 are associated with neurodevelopmental disorders. Previously, we found that CRISPR/Cas9 based editing of the TAF1 gene disrupts the morphology of the cerebral cortex and blunts the expression as well as the function of the CaV3.1 (T-type) voltage gated calcium channel. Here, we tested the efficacy of SAK3 (ethyl 8'-methyl-2', 4-dioxo-2-(piperidin-1-yl)-2'H-spiro [cyclopentane-1, 3'-imidazo [1, 2-a] pyridine]-2-ene-3-carboxylate), a T-type calcium channel enhancer, in an animal model of TAF1 intellectual disability (ID) syndrome. At post-natal day 3, rat pups were subjected to intracerebroventricular (ICV) injection of either gRNA-control or gRNA-TAF1 CRISPR/Cas9 viruses. At post-natal day 21, the rat pups were given SAK3 (0.25 mg/kg, p.o.) or vehicle for 14 days (i.e. till post-natal day 35) and then subjected to behavioral, morphological, and molecular studies. Oral administration of SAK3 (0.25 mg/kg, p.o.) significantly rescued locomotion abnormalities associated with TAF1 gene editing. SAK3 treatment prevented the loss of cortical neurons and GFAP-positive astrocytes observed after TAF1 gene editing. In addition, SAK3 protected cells from apoptosis. SAK3 also restored the Brain-derived neurotrophic factor/protein kinase B/Glycogen Synthase Kinase 3 Beta (BDNF/AKT/GSK3β) signaling axis in TAF1 edited animals. Finally, SAK3 normalized the levels of three GSK3β substrates - CaV3.1, FOXP2, and CRMP2. We conclude that the T-type calcium channel enhancer SAK3 is beneficial against the deleterious effects of TAF1 gene-editing, in part, by stimulating the BDNF/AKT/GSK3β signaling pathway.

An altered secretome is an early marker of the pathogenesis of CLN6 Batten disease

Neuronal ceroid lipofuscinoses (NCLs) are a group of inherited childhood neurodegenerative disorders. In addition to the accumulation of auto-fluorescent storage material in lysosomes, NCLs are largely characterised by region-specific neuroinflammation that can predict neuron loss. These phenotypes suggest alterations in the extracellular environment-making the secretome an area of significant interest. This study investigated the secretome in the CLN6 (ceroid-lipofuscinosis neuronal protein 6) variant of NCL. To investigate the CLN6 secretome, we co-cultured neurons and glia isolated from Cln6^nclf or Cln6^± mice, and utilised mass spectrometry to compare protein constituents of conditioned media. The significant changes noted in cathepsin enzymes, were investigated further via western blotting and enzyme activity assays. Viral-mediated gene therapy was used to try and rescue the wild-type phenotype and restore the secretome-both in vitro in co-cultures and in vivo in mouse plasma. In Cln6^nclf cells, proteomics revealed a marked increase in catabolic and cytoskeletal-associated proteins-revealing new similarities between the pathogenic signatures of NCLs with other neurodegenerative disorders. These changes were, in part, corrected by gene therapy intervention, suggesting these proteins as candidate in vitro biomarkers. Importantly, these in vitro changes show promise for in vivo translation, with Cathepsin L (CTSL) activity reduced in both co-cultures and Cln6^nclf plasma samples post gene-therapy. This work suggests the secretome plays a role in CLN6 pathogenesis and highlights its potential use as an in vitro model. Proteomic changes present a list of candidate biomarkers for monitoring disease and assessing potential therapeutics in future studies.

Druggability of CRMP2 for Neurodegenerative Diseases

Collapsin response mediator proteins (CRMPs) are ubiquitously expressed phosphoproteins that coordinate cytoskeletal formation and regulate cellular division, migration, polarity, and synaptic connection. CRMP2, the most studied of the five family members, is best known for its affinity for tubulin heterodimers and function in regulating the microtubule network. Accumulating evidence has also demonstrated a key role for CRMP2 in trafficking of voltage- and ligand-gated ion channels. These functions are tightly regulated by post-translational modifications including phosphorylation and SUMOylation (addition of a small ubiquitin like modifier). Over the past decade, it has become increasingly clear that dysregulated post-translational modifications of CRMP2 contribute to the pathomechanisms of diverse diseases, including cancer, neurodegenerative diseases, chronic pain, and bipolar disorder. Here, we review the discovery, functions, and current putative preclinical and clinical therapeutics targeting CRMP2. These potential therapeutics include CRMP2-based peptides that inhibit protein-protein interactions and small-molecule compounds. Capitalizing on the availability of structural information, we identify druggable pockets on CRMP2 and predict binding modes for five known CRMP2-targeting compounds, setting the stage for optimization and de novo drug discovery targeting this multifunctional protein.

Collapsin Response Mediator Protein 2 and Endophilin2 Coordinate Regulation of AMPA Receptor GluA1 Subunit Recycling

The dynamic trafficking of AMPA receptors (AMPARs), which enables the endocytosis, recycling, and exocytosis of AMPARs, is crucial for synaptic plasticity. Endophilin2, which directly interacts with the GluA1 subunit of AMPARs, plays an important role in AMPAR endocytosis. Collapsin response mediator protein 2 (CRMP2) promotes the maturation of the dendritic spine and can transfer AMPARs to the membrane. Although the mechanisms of AMPAR endocytosis and exocytosis are well known, the exact molecular mechanisms underlying AMPAR recycling remain unclear. Here, we report a unique interaction between CRMP2 and endophilin2. Our results showed that overexpression of CRMP2 and endophilin2 increased the amplitude and frequency of miniature excitatory synaptic currents (mEPSCs) and modestly enhanced AMPAR levels in hippocampal neurons. Furthermore, the CRMP2 and endophilin2 overexpression phenotype failed to occur when the interaction between these two proteins was inhibited. Further analysis revealed that this interaction was regulated by CRMP2 phosphorylation. The phosphorylation of CRMP2 inhibited its interaction with endophilin2; this was mainly affected by the phosphorylation of Thr514 and Ser518 by glycogen synthase kinase (GSK) 3β. CRMP2 phosphorylation increased degradation and inhibited the surface expression of AMPAR GluA1 subunits in cultured hippocampal neurons. However, the dephosphorylation of CRMP2 inhibited degradation and promoted the surface expression of AMPAR GluA1 subunits in cultured hippocampal neurons. Taken together, our data demonstrated that the interaction between CRMP2 and endophilin2 was conductive to the recycling of AMPA receptor GluA1 subunits in hippocampal neurons.

DPYSL2 is a novel regulator for neural stem cell differentiation in rats: revealed by Panax notoginseng saponin administration

BACKGROUND

The limited neuronal differentiation of the endogenous or grafted neural stem cells (NSCs) after brain injury hampers the clinic usage of NSCs. Panax notoginseng saponins (PNS) were extensively used for their clinical value, such as in controlling blood pressure, blood glucose, and inhibiting neuronal apoptosis and enhancing neuronal protection, but whether or not it exerts an effect in promoting neuronal differentiation of the endogenous NSCs is completely unclear and the potential underlying mechanism requires further exploration.

METHODS

Firstly, we determined whether PNS could successfully induce NSCs to differentiate to neurons under the serum condition. Mass spectrometry and quantitative polymerase chain reaction (Q-PCR) were then performed to screen the differentially expressed proteins (genes) between the PNS + serum and serum control group, upon which dihydropyrimidinase-like 2 (DPYSL2), a possible candidate, was then selected for the subsequent research. To further investigate the actual role of DPYSL2 in the NSC differentiation, DPYSL2-expressing lentivirus was employed to obtain DPYSL2 overexpression in NSCs. DPYSL2-knockout rats were constructed to study its effects on hippocampal neural stem cells. Immunofluorescent staining was performed to identify the differentiation direction of NSCs after 7 days from DPYSL2 transfection, as well as those from DPYSL2-knockout rats.

RESULTS

Seven differentially expressed protein spots were detected by PD Quest, and DPYSL2 was found as one of the key factors of NSC differentiation in a PNS-treated condition. The results of immunostaining further showed that mainly Tuj1 and GFAP-positive cells increased in the DPYSL2-overexpressed group, while both were depressed in the hippocampal NSCs in the DPYSL2-knockout rat.

CONCLUSIONS

The present study revealed that the differentiation direction of NSCs could be enhanced through PNS administration, and the DPYSL2 is a key regulator in promoting NSC differentiation. These results not only emphasized the effect of PNS but also indicated DPYSL2 could be a novel target to enhance the NSC differentiation in future clinical trials.

Discovery of suppressors of CRMP2 phosphorylation reveals compounds that mimic the behavioral effects of lithium on amphetamine-induced hyperlocomotion

The effective treatment of bipolar disorder (BD) represents a significant unmet medical need. Although lithium remains a mainstay of treatment for BD, limited knowledge regarding how it modulates affective behavior has proven an obstacle to discovering more effective mood stabilizers with fewer adverse side effects. One potential mechanism of action of lithium is through inhibition of the serine/threonine protein kinase GSK3β, however, relevant substrates whose change in phosphorylation may mediate downstream changes in neuroplasticity remain poorly understood. Here, we used human induced pluripotent stem cell (hiPSC)-derived neuronal cells and stable isotope labeling by amino acids in cell culture (SILAC) along with quantitative mass spectrometry to identify global changes in the phosphoproteome upon inhibition of GSK3α/β with the highly selective, ATP-competitive inhibitor CHIR-99021. Comparison of phosphorylation changes to those induced by therapeutically relevant doses of lithium treatment led to the identification of collapsin response mediator protein 2 (CRMP2) as being highly sensitive to both treatments as well as an extended panel of structurally distinct GSK3α/β inhibitors. On this basis, a high-content image-based assay in hiPSC-derived neurons was developed to screen diverse compounds, including FDA-approved drugs, for their ability to mimic lithium's suppression of CRMP2 phosphorylation without directly inhibiting GSK3β kinase activity. Systemic administration of a subset of these CRMP2-phosphorylation suppressors were found to mimic lithium's attenuation of amphetamine-induced hyperlocomotion in mice. Taken together, these studies not only provide insights into the neural substrates regulated by lithium, but also provide novel human neuronal assays for supporting the development of mechanism-based therapeutics for BD and related neuropsychiatric disorders.

TNF-α mediated upregulation of NaV1.7 currents in rat dorsal root ganglion neurons is independent of CRMP2 SUMOylation

Clinical and preclinical studies have shown that patients with Diabetic Neuropathy Pain (DNP) present with increased tumor necrosis factor alpha (TNF-α) serum concentration, whereas studies with diabetic animals have shown that TNF-α induces an increase in Na_V1.7 sodium channel expression. This is expected to result in sensitization of nociceptor neuron terminals, and therefore the development of DNP. For further study of this mechanism, dissociated dorsal root ganglion (DRG) neurons were exposed to TNF-α for 6 h, at a concentration equivalent to that measured in STZ-induced diabetic rats that developed hyperalgesia. Tetrodotoxin sensitive (TTXs), resistant (TTXr) and total sodium current was studied in these DRG neurons. Total sodium current was also studied in DRG neurons expressing the collapsin response mediator protein 2 (CRMP2) SUMO-incompetent mutant protein (CRMP2-K374A), which causes a significant reduction in Na_V1.7 membrane cell expression levels. Our results show that TNF-α exposure increased the density of the total, TTXs and TTXr sodium current in DRG neurons. Furthermore, TNF-α shifted the steady state activation and inactivation curves of the total and TTXs sodium current. DRG neurons expressing the CRMP2-K374A mutant also exhibited total sodium current increases after exposure to TNF-α, indicating that these effects were independent of SUMOylation of CRMP2. In conclusion, TNF-α sensitizes DRG neurons via augmentation of whole cell sodium current. This may underlie the pronociceptive effects of TNF-α and suggests a molecular mechanism responsible for pain hypersensitivity in diabetic neuropathy patients.

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.

Loss of CRMP2 O-GlcNAcylation leads to reduced novel object recognition performance in mice

O-GlcNAcylation is an abundant post-translational modification in the nervous system, linked to both neurodevelopmental and neurodegenerative disease. However, the mechanistic links between these phenotypes and site-specific O-GlcNAcylation remain largely unexplored. Here, we show that Ser517 O-GlcNAcylation of the microtubule-binding protein Collapsin Response Mediator Protein-2 (CRMP2) increases with age. By generating and characterizing a Crmp2S517A knock-in mouse model, we demonstrate that loss of O-GlcNAcylation leads to a small decrease in body weight and mild memory impairment, suggesting that Ser517 O-GlcNAcylation has a small but detectable impact on mouse physiology and cognitive function.

Dysregulation of CRMP2 Post-Translational Modifications Drive Its Pathological Functions

Collapsin response mediator proteins (CRMPs) are a family of ubiquitously expressed, homologous phosphoproteins best known for coordinating cytoskeletal formation and regulating cellular division, migration, polarity, and synaptic connection. CRMP2, the most studied of the five family members, is best known for its affinity for tubulin heterodimers and function in regulating the microtubule network. These functions are tightly regulated by post-translational modifications including phosphorylation, SUMOylation, oxidation, and O-GlcNAcylation. While CRMP2's physiological functions rely mostly on its non-phosphorylated state, dysregulation of CRMP2 phosphorylation and SUMOylation has been reported to be involved in the pathophysiology of multiple diseases including cancer, chronic pain, spinal cord injury, neurofibromatosis type 1, and others. Here, we provide a consolidated update on what is known about CRMP2 signaling and function, first focusing on axonal growth and neuronal polarity, then illustrating the link between dysregulated CRMP2 post-translational modifications and diseases. We additionally discuss the roles of CRMP2 in non-neuronal cells, both in the CNS and regions of the periphery. Finally, we offer thoughts on the therapeutic implications of modulating CRMP2 function in a variety of diseases.

Lacosamide modulates collapsin response mediator protein 2 and inhibits mossy fiber sprouting after kainic acid-induced status epilepticus

Mossy fiber sprouting (MFS) and neuronal loss are important pathological features of chronic epilepsy closely related to the development of spontaneous recurrent seizures. However, the pathological mechanism of MFS remains unclear. Collapsin response mediator protein 2 (CRMP2) is a cytoplasmic protein highly expressed in the nervous system and is involved in axon/dendrite specification and axonal growth. It is possibly associated with the development of MFS. Lacosamide (LCM), a novel antiepileptic drug, was recently found to inhibit the CRMP2-mediated neurite outgrowth. Therefore, we studied the relationships between LCM, CRMP2, and MFS, seeking potential therapeutic targets for epileptogenesis and a better understanding of the mechanism of action of LCM. We used kainic acid to induce status epilepticus in an animal model and examined the resultant changes in protein expression by Western blot and changes in histology by specific staining for cell death and MFS. Our results showed that the expression level of CRMP2 was elevated and the expression level of phosphorylated CRMP2 (p-CRMP2) was reduced following status epilepticus. Administration of LCM not only reversed this effect but also suppressed spontaneous recurrent seizures and reduced MFS and loss of hippocampal neurons. This study reveals that, in addition to its antiseizure efficacy, LCM has a neuroprotective effect and inhibits the development of epilepsy. CRMP2 is possibly involved in the mechanism by which LCM suppresses MFS and is expected to be a new therapeutic target for treating epileptogenesis.

Modulation of CRMP2 via (S)-Lacosamide shows therapeutic promise but is ultimately ineffective in a mouse model of CLN6-Batten disease

CLN6-Batten disease is a rare neurodegenerative disorder with no cure, characterized by accumulation of lipofuscin in the lysosome, glial activation, and neuronal death. Here we test the therapeutic efficacy of modulating collapsin response mediator protein 2 (CRMP2) activity via S-N-benzy-2-acetamido-3-methoxypropionamide ((S)-Lacosamide) in a mouse model of CLN6-Batten disease. Promisingly, mouse neuronal cultures as well as Cln6 patient fibroblasts treated with varying concentrations of (S)-Lacosamide showed positive restoration of lysosomal associated deficits. However, while acute in vivo treatment enhanced glial activation in 3-month-old Cln6 mutant mice, chronic treatment over several months did not improve behavioral or long-term survival outcomes. Therefore, modulation of CRMP2 activity via (S)-Lacosamide alone is unlikely to be a viable therapeutic target for CLN6-Batten disease.

CRMP2-derived peptide ST2-104 (R9-CBD3) protects SH-SY5Y neuroblastoma cells against Aβ25-35-induced neurotoxicity by inhibiting the pCRMP2/NMDAR2B signaling pathway

Collapsin response mediator protein 2 (CRMP2),by regulating voltage-gated calcium channel activity, is a crucial regulator of neuronal excitability. Hyperphosphorylation of CRMP2 has been reported in brains of Alzheimer's disease (AD) patients and other neurodegenerative diseases. CRMP2 acting on N-methyl-d-aspartate receptors (NMDARs) may contribute to AD pathology. A short peptide from CRMP2, designated the Ca²⁺ channel-binding domain 3 (CBD3) peptide, has recently emerged as a Ca²⁺ channel blocker that exerts neuroprotective effects in traumatic brain injury and cerebral ischemia by disrupting pCRMP2/NMDAR interaction to inhibit calcium influx. ST2-104, a nona-arginine (R9)-conjugated CBD3 peptide derived from CRMP2, exerts a beneficial effect on neuropathic pain; however, the effect of ST2-104 on AD and its mechanism of action have not been studied. In this study we investigated the effects of ST2-104 on SH-SY5Y neuroblastoma cells stimulated by Aβ_25-35. To induce neurotoxicity, SH-SY5Y cells were incubated with Aβ_25-35, the shortest toxic fragment of Aβ. CRMP2 expression was manipulated by knockdown or overexpression of CRMP2 before ST2-104 treatment to further explore if the pCRMP2/NMDAR2B signaling pathway is involved in the action of the ST2-104 peptide. The results show that ST2-104 significantly enhanced cell viability, inhibited cell apoptosis, decreased LDH release, suppressed the expression of the pCRMP2 protein, disrupted pCRMP2/NMDAR2B interaction, inhibited Aβ_25-35-induced NMDAR currents, and decreased intracellular Ca²⁺ levels. The effects of ST2-104 was abolished by overexpression of CRMP2 and intensified by knockdown of CRMP2 in SH-SY5Y cells. Taken together, our results support ST2-104 as a possible biologic therapeutic in the face of Aβ_25-35-induced injury via the inhibition of the pCRMP2/NMDAR2B signaling pathway.

Protein tyrosine phosphatase 4A3 (PTP4A3/PRL-3) promotes the aggressiveness of human uveal melanoma through dephosphorylation of CRMP2

Uveal melanoma (UM) is an aggressive tumor in which approximately 50% of patients develop metastasis. Expression of the PTP4A3 gene, encoding a phosphatase, is predictive of poor patient survival. PTP4A3 expression in UM cells increases their migration in vitro and invasiveness in vivo. Here, we show that CRMP2 is mostly dephosphorylated on T514 in PTP4A3 expressing cells. We also demonstrate that inhibition of CRMP2 expression in UM cells expressing PTP4A3 increases their migration in vitro and invasiveness in vivo. This phenotype is accompanied by modifications of the actin microfilament network, with shortened filaments, whereas cells with a inactive mutant of the phosphatase do not show the same behavior. In addition, we showed that the cell cytoplasm becomes stiffer when CRMP2 is downregulated or PTP4A3 is expressed. Our results suggest that PTP4A3 acts upstream of CRMP2 in UM cells to enhance their migration and invasiveness and that a low level of CRMP2 in tumors is predictive of poor patient survival.

Extracellular Application of CRMP2 Increases Cytoplasmic Calcium through NMDA Receptors

Collapsin Response Mediator Protein 2 (CRMP2) is an intracellular protein involved in axon and dendrite growth and specification. In this study, CRMP2 was identified in a conditioned media derived from degenerated sciatic nerves (CM). On cultured rat hippocampal neurons, acute extracellular application of CM or partially purified recombinant CRMP2 produced an increase in cytoplasmic calcium. The increase in cytoplasmic calcium was mostly mediated through NMDA receptors, with a minor contribution of N-type VDCC, and it was maintained as long as CM was present. By using live-labeling of CRMP2, Ca²⁺ channel binding domain 3 (CBD3) peptide derived from CRMP2, and recombinant CRMP2, we demonstrated that that this effect was mediated by an action on the extracellular side of the NMDA receptor. This is the first report of an extracellular action of CRMP2. Prolonged exposure to extracellular CRMP2, may contribute to neuronal calcium dysregulation and neuronal damage.

UBC9 regulates cardiac sodium channel Nav1.5 ubiquitination, degradation and sodium current density

Voltage-gated sodium channel Na_v1.5 is critical for generation and conduction of cardiac action potentials. Mutations and expression level changes of Na_v1.5 are associated with cardiac arrhythmias and sudden death. The ubiquitin (Ub) conjugation machinery utilizes three enzyme activities, E1, E2, and E3, to regulate protein degradation. Previous studies from us and others showed that Nedd4-2 acts as an E3 ubiquitin-protein ligase involved in ubiquitination and degradation of Na_v1.5, however, more key regulators remain to be identified. In this study, we show that UBC9, a SUMO-conjugating enzyme, regulates ubiquitination and degradation of Na_v1.5. Overexpression of UBC9 significantly decreased Na_v1.5 expression and reduced sodium current densities, whereas knockdown of UBC9 expression significantly enhanced Na_v1.5 expression and increased sodium current densities, in both HEK293 cells and primary neonatal cardiomyocytes. Overexpression of UBC9 increased ubiquitination of Na_v1.5, and proteasome inhibitor MG132 blocked the effect of UBC9 overexpression on Na_v1.5 degradation. Co-immunoprecipitation showed that UBC9 interacts with Nedd4-2. UBC9 with mutation C93S, which suppresses SUMO-conjugating activity of UBC9, was as active as wild type UBC9 in regulating Na_v1.5 levels, suggesting that UBC9 regulates Na_v1.5 expression levels in a SUMOylation-independent manner. Our findings thus identify a key structural element of the ubiquitin-conjugation machinery for Na_v1.5 and provide important insights into the regulatory mechanism for ubiquitination and turnover of Na_v1.5.

Mining the Nav1.7 interactome: Opportunities for chronic pain therapeutics

The peripherally expressed voltage-gated sodium Na_V1.7 (gene SCN9A) channel boosts small stimuli to initiate firing of pain-signaling dorsal root ganglia (DRG) neurons and facilitates neurotransmitter release at the first synapse within the spinal cord. Mutations in SCN9A produce distinct human pain syndromes. Widely acknowledged as a "gatekeeper" of pain, Na_V1.7 has been the focus of intense investigation but, to date, no Na_V1.7-selective drugs have reached the clinic. Elegant crystallographic studies have demonstrated the potential of designing highly potent and selective Na_V1.7 compounds but their therapeutic value remains untested. Transcriptional silencing of Na_V1.7 by a naturally expressed antisense transcript has been reported in rodents and humans but whether this represents a viable opportunity for designing Na_V1.7 therapeutics is currently unknown. The demonstration that loss of Na_V1.7 function is associated with upregulation of endogenous opioids and potentiation of mu- and delta-opioid receptor activities, suggests that targeting only Na_V1.7 may be insufficient for analgesia. However, the link between opioid-dependent analgesic mechanisms and function of sodium channels and intracellular sodium-dependent signaling remains controversial. Thus, additional new targets - regulators, modulators - are needed. In this context, we mine the literature for the known interactome of Na_V1.7 with a focus on protein interactors that affect the channel's trafficking or link it to opioid signaling. As a case study, we present antinociceptive evidence of allosteric regulation of Na_V1.7 by the cytosolic collapsin response mediator protein 2 (CRMP2). Throughout discussions of these possible new targets, we offer thoughts on the therapeutic implications of modulating Na_V1.7 function in chronic pain.

Neuronal Conditional Knockout of Collapsin Response Mediator Protein 2 Ameliorates Disease Severity in a Mouse Model of Multiple Sclerosis

We previously showed that treatment with lanthionine ketimine ethyl ester (LKE) reduced disease severity and axonal damage in an experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis and increased neuronal maturation and survival in vitro . A major target of LKE is collapsin response mediator protein 2 (CRMP2), suggesting this protein may mediate LKE actions. We now show that conditional knockout of CRMP2 from neurons using a CamK2a promoter to drive Cre recombinase expression reduces disease severity in the myelin oligodendrocyte glycoprotein (MOG)35–55 EAE model, associated with decreased spinal cord axonal damage, and less glial activation in the cerebellum, but not the spinal cord. Immunohistochemical staining and quantitative polymerase chain reaction show CRMP2 depletion from descending motor neurons in the motor cortex, but not from spinal cord neurons, suggesting that the benefits of CRMP2 depletion on EAE may stem from effects on upper motor neurons. In addition, mice in which CRMP2 S522 phosphorylation was prevented by substitution for an alanine residue also showed reduced EAE severity. These results show that modification of CRMP2 expression and phosphorylation can influence the course of EAE and suggests that treatment with CRMP2 modulators such as LKE act in part by reducing CRMP2 S522 phosphorylation.

Phosphorylated CRMP2 Regulates Spinal Nociceptive Neurotransmission

The collapsin response mediator protein 2 (CRMP2) has emerged as a central node in assembling nociceptive signaling complexes involving voltage-gated ion channels. Concerted actions of post-translational modifications, phosphorylation and SUMOylation, of CRMP2 contribute to regulation of pathological pain states. In the present study, we demonstrate a novel role for CRMP2 in spinal nociceptive transmission. We found that, of six possible post-translational modifications, three phosphorylation sites on CRMP2 were critical for regulating calcium influx in dorsal root ganglion sensory neurons. Of these, only CRMP2 phosphorylated at serine 522 by cyclin-dependent kinase 5 (Cdk5) contributed to spinal neurotransmission in a bidirectional manner. Accordingly, expression of a non-phosphorylatable CRMP2 (S522A) decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), whereas expression of a constitutively phosphorylated CRMP2 (S522D) increased the frequency of sEPSCs. The presynaptic nature of CRMP2's actions was further confirmed by pharmacological antagonism of Cdk5-mediated CRMP2 phosphorylation with S-N-benzy-2-acetamido-3-methoxypropionamide ((S)-lacosamide; (S)-LCM) which (i) decreased sEPSC frequency, (ii) increased paired-pulse ratio, and (iii) reduced the presynaptic distribution of CaV2.2 and NaV1.7, two voltage-gated ion channels implicated in nociceptive signaling. (S)-LCM also inhibited depolarization-evoked release of the pro-nociceptive neurotransmitter calcitonin gene-related peptide (CGRP) in the spinal cord. Increased CRMP2 phosphorylation in rats with spared nerve injury (SNI) was decreased by intrathecal administration of (S)-LCM resulting in a loss of presynaptic localization of CaV2.2 and NaV1.7. Together, these findings indicate that CRMP2 regulates presynaptic excitatory neurotransmission in spinal cord and may play an important role in regulating pathological pain. Novel targeting strategies to inhibit CRMP2 phosphorylation by Cdk5 may have great potential for the treatment of chronic pain.

Protein nitration profile of CD3+ lymphocytes from Alzheimer disease patients: Novel hints on immunosenescence and biomarker detection

Alzheimer's disease (AD) is a progressive form of dementia characterized by increased production of amyloid-β plaques and hyperphosphorylated tau protein, mitochondrial dysfunction, elevated oxidative stress, reduced protein clearance, among other. Several studies showed systemic modifications of immune and inflammatory systems due, in part, to decreased levels of CD3⁺ lymphocytes in peripheral blood in AD. Considering that oxidative stress, both in the brain and in the periphery, can influence the activation and differentiation of T-cells, we investigated the 3-nitrotyrosine (3-NT) proteome of blood T-cells derived from AD patients compared to non-demented (ND) subjects by using a proteomic approach. 3-NT is a formal protein oxidation and index of nitrosative stress. We identified ten proteins showing increasing levels of 3-NT in CD3⁺ T-cells from AD patients compared with ND subjects. These proteins are involved in energy metabolism, cytoskeletal structure, intracellular signaling, protein folding and turnover, and antioxidant response and provide new insights into the molecular mechanism that impact reduced T-cell differentiation in AD. Our results highlight the role of peripheral oxidative stress in T-cells related to immune-senescence during AD pathology focusing on the specific targets of protein nitration that conceivably can be suitable to further therapies. Further, our data demonstrate common targets of protein nitration between the brain and the periphery, supporting their significance as disease biomarkers.

Analysis of lanthionine ketimine ethyl ester in mouse serum, whole blood and tissues using ultrahigh-pressure liquid chromatography/tandem mass spectrometry

RATIONALE

Preclinical studies in the search for treatments for several neurodegenerative diseases have identified lanthionine ketimine (LK) and its monoethyl ester derivative (LKE) as potential candidates. An ultrahigh-pressure liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) assay was developed to evaluate bioavailability by measuring these compounds in mouse serum, whole blood and brain tissue.

METHODS

Following administration of LKE to mice for 3 days in chow at 300 ppm, the animals were sacrificed, and LKE was extracted from serum, whole blood and brain tissues through protein precipitation using cold methanol. To enhance chromatographic separation and electrospray ionization, LK was methylated using diazomethane. Separations were carried out using C₁₈ reversed-phase UHPLC, and quantitative measurements were obtained using on-line triple-quadruple mass spectrometry with positive ion electrospray ionization, collision-induced dissociation and selected reaction monitoring. Tolbutamide was used as internal standard.

RESULTS

LKE showed good recovery ranging from 77-90% in serum and 82-88% in brain tissue. An eight-point standard curve ranging from 0.005 to 4.6 μM was linear (R² 0.998). The average LKE detected in mouse serum was 277.42 nM, while the concentration in whole blood was 38 nM. Neither LK nor LKE was detected in brain tissues.

CONCLUSIONS

A rapid quantitative method to measure LKE in mouse serum, whole blood and brain tissues using UHPLC/MS/MS was developed and validated following FDA guidelines. This method is suitable for bioavailability and pharmacokinetic studies.

Phosphorylation, Dephosphorylation, and Multiprotein Assemblies Regulate Dynamic Behavior of Neuronal Cytoskeleton: A Mini-Review

Cellular localization, assembly and abnormal aggregation of neurofilaments depend on phosphorylation. Pathological processes associated with neurodegeneration exhibit aberrant accumulation of microtubule associated aggregated forms of hyperphosphorylated neuronal protein tau in cell bodies. These processes are critical for the disease progression in patients suffering from Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. In healthy cells, tau is localized in axons. Topographic regulation suggests that whereas the sites of synthesis of kinases and neurofilaments are the cell bodies, and sites of their functional assemblies are axons, phosphorylation/dephosphorylation are the key processes that arrange the molecules at their precise locations. Phosphorylation sites in the dynamic developmental and degenerative processes differ. Not all these processes are well understood. New advancements identify epigenetic factors involved in AD which account for the influence of age-related environment/genome interactions leading to the disease. Progress in proteomics highlights previously found major proteins and adds more to the list of those involved in AD. New key elements of specificity provide determinants of molecular recognition important for the assembly of macromolecular complexes. In this review, we discuss aberrant spatial distribution of neuronal polypeptides observed in neuropathies: aggregation, association with proteins of the neuronal cytoskeleton, and phosphorylation dependent dynamics. Particularly, we emphasize recent advancements in understanding the function and determinants of specific association of molecules involved in Alzheimer's disease with respect to the topographic regulation of phosphorylation in neuronal cytoskeleton and implications for the design of new therapies. Further, we address the role of various filament systems in maintenance of the shape, rigidity and dynamics of the cytoskeleton.

Inhibition of the Ubc9 E2 SUMO-conjugating enzyme-CRMP2 interaction decreases NaV1.7 currents and reverses experimental neuropathic pain

We previously reported that destruction of the small ubiquitin-like modifier (SUMO) modification site in the axonal collapsin response mediator protein 2 (CRMP2) was sufficient to selectively decrease trafficking of the voltage-gated sodium channel NaV1.7 and reverse neuropathic pain. Here, we further interrogate the biophysical nature of the interaction between CRMP2 and the SUMOylation machinery, and test the hypothesis that a rationally designed CRMP2 SUMOylation motif (CSM) peptide can interrupt E2 SUMO-conjugating enzyme Ubc9-dependent modification of CRMP2 leading to a similar suppression of NaV1.7 currents. Microscale thermophoresis and amplified luminescent proximity homogeneous alpha assay revealed a low micromolar binding affinity between CRMP2 and Ubc9. A heptamer peptide harboring CRMP2's SUMO motif, also bound with similar affinity to Ubc9, disrupted the CRMP2-Ubc9 interaction in a concentration-dependent manner. Importantly, incubation of a tat-conjugated cell-penetrating peptide (t-CSM) decreased sodium currents, predominantly NaV1.7, in a model neuronal cell line. Dialysis of t-CSM peptide reduced CRMP2 SUMOylation and blocked surface trafficking of NaV1.7 in rat sensory neurons. Fluorescence dye-based imaging in rat sensory neurons demonstrated inhibition of sodium influx in the presence of t-CSM peptide; by contrast, calcium influx was unaffected. Finally, t-CSM effectively reversed persistent mechanical and thermal hypersensitivity induced by a spinal nerve injury, a model of neuropathic pain. Structural modeling has now identified a pocket-harboring CRMP2's SUMOylation motif that, when targeted through computational screening of ligands/molecules, is expected to identify small molecules that will biochemically and functionally target CRMP2's SUMOylation to reduce NaV1.7 currents and reverse neuropathic pain.

Modeling Complex Neurological Diseases with Stem Cells: A Study of Bipolar Disorder

The pathogenesis of bipolar disorder (BPD) is unknown. Using human-induced pluripotent stem cells (hiPSCs) to unravel pathological mechanisms in polygenic diseases is challenging, with few successful studies to date. However, hiPSCs from BPD patients responsive to lithium have offered unique opportunities to discern lithium's mechanism of action and hence gain insight into BPD pathology. By profiling the proteomics of BPD-hiPSC-derived neurons, we found that lithium alters the phosphorylation state of collapsin response mediator protein-2 (CRMP2). The "set point" for the ratio of pCRMP2:CRMP2 is elevated uniquely in hiPSC-derived neurons from lithium responsive (Li-R) BPD patients, but not other psychiatric and neurological disorders. Utilizing neurons differentiated from human patient stem cells as an in vitro platform, we were able to elucidate the mechanism driving the pathogenesis and pathophysiology of lithium-responsive BPD, heretofore unknown. Importantly, the findings in culture were validated in human postmortem material as well as in animal models of BPD behavior. These data suggest that the "lithium response pathway" in BPD governs CRMP2's phosphorylation, which regulates cytoskeletal organization, particularly in dendritic spines, leading to modulated neural networks that may underlie Li-R BPD pathogenesis. This chapter reviews the methodology of leveraging a functional agent, lithium, to identify unknown pathophysiological pathways with hiPSCs and how to translate this disease modeling approach to other neurological disorders.

Cdk5-mediated CRMP2 phosphorylation is necessary and sufficient for peripheral neuropathic pain

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CRMP2 phosphorylation levels are dysregulated in the SNI model of experimental neuropathy.

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CRMP2 phosphorylation by Cdk5 is increased at the pre-synaptic sites of the dorsal horn of the spinal cord.

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CRMP2 expression is necessary for neuropathic pain.

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Genetic targeting of CRMP2 phosphorylation by Cdk5 reverses neuropathic pain.

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CRMP2 phosphorylation by Cdk5 is sufficient to elicit allodynia.

Neuropathic pain results from nerve injuries that cause ectopic firing and increased nociceptive signal transmission due to activation of key membrane receptors and channels. The dysregulation of trafficking of voltage-gated ion channels is an emerging mechanism in the etiology of neuropathic pain. We identify increased phosphorylation of collapsin response mediator protein 2 (CRMP2), a protein reported to regulate presynaptic voltage-gated calcium and sodium channels. A spared nerve injury (SNI) increased expression of a cyclin dependent kinase 5 (Cdk5)-phosphorylated form of CRMP2 in the dorsal horn of the spinal cord and the dorsal root ganglia (DRG) in the ipsilateral (injured) versus the contralateral (non-injured) sites. Biochemical fractionation of spinal cord from SNI rats revealed the increase in Cdk5-mediated CRMP2 phosphorylation to be enriched to pre-synaptic sites. CRMP2 has emerged as a central node in assembling nociceptive signaling complexes. Knockdown of CRMP2 using a small interfering RNA (siRNA) reversed SNI-induced mechanical allodynia implicating CRMP2 expression as necessary for neuropathic pain. Intrathecal expression of a CRMP2 resistant to phosphorylation by Cdk5 normalized SNI-induced mechanical allodynia, whereas mimicking constitutive phosphorylation of CRMP2 resulted in induction of mechanical allodynia in naïve rats. Collectively, these results demonstrate that Cdk5-mediated CRMP2 phosphorylation is both necessary and sufficient for peripheral neuropathic pain.

Increased Levels of Circulating Glial Fibrillary Acidic Protein and Collapsin Response Mediator Protein-2 Autoantibodies in the Acute Stage of Spinal Cord Injury Predict the Subsequent Development of Neuropathic Pain

Neuropathic pain develops in 40-70% of spinal cord injury (SCI) patients and markedly compromises quality of life. We examined plasma from SCI patients for autoantibodies to glial fibrillary acidic protein (GFAP) and collapsin response mediator protein-2 (CRMP2) and evaluated their relationship to the development of neuropathic pain. In study 1, plasma samples and clinical data from 80 chronic SCI patients (1-41 years post-SCI) were collected and screened for GFAP autoantibodies (GFAPab). Results from study 1 indicated that GFAPab were present in 34 of 80 (42.5%) patients, but circulating levels did not correlate with the occurrence of neuropathic pain. In study 2, longitudinal plasma samples and clinical data were collected from 38 acute SCI patients. The level of GFAPab measured at 16 ± 7 days post-SCI was found to be significantly higher in patients that subsequently developed neuropathic pain (within 6 months post-SCI) than patients who did not (T = 219; p = 0.02). In study 3, we identified CRMP2 as an autoantibody target (CRMP2ab) in 23% of acute SCI patients. The presence of GFAPab and/or CRMP2ab increased the odds of subsequently developing neuropathic pain within 6 months of injury by 9.5 times (p = 0.006). Our results suggest that if a causal link can be established between these autoantibodies and the development of neuropathic pain, strategies aimed at reducing the circulating levels of these autoantibodies may have therapeutic value.

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.

CRMP2 and voltage-gated ion channels: potential roles in neuropathic pain

Neuropathic pain represents a significant and mounting burden on patients and society at large. Management of neuropathic pain, however, is both intricate and challenging, exacerbated by the limited quantity and quality of clinically available treatments. On this stage, dysfunctional voltage-gated ion channels, especially the presynaptic N-type voltage-gated calcium channel (VGCC) (Cav2.2) and the tetrodotoxin-sensitive voltage-gated sodium channel (VGSC) (Nav1.7), underlie the pathophysiology of neuropathic pain and serve as high profile therapeutic targets. Indirect regulation of these channels holds promise for the treatment of neuropathic pain. In this review, we focus on collapsin response mediator protein 2 (CRMP2), a protein with emergent roles in voltage-gated ion channel trafficking and discuss the therapeutic potential of targetting this protein.

Transcriptional regulation of CRMP5 controls neurite outgrowth through Sox5

Transcriptional regulation of proteins involved in neuronal polarity is a key process that underlies the ability of neurons to transfer information in the central nervous system. The Collapsin Response Mediator Protein (CRMP) family is best known for its role in neurite outgrowth regulation conducting to neuronal polarity and axonal guidance, including CRMP5 that drives dendrite differentiation. Although CRMP5 is able to control dendritic development, the regulation of its expression remains poorly understood. Here we identify a Sox5 consensus binding sequence in the putative promoter sequence upstream of the CRMP5 gene. By luciferase assays we show that Sox5 increases CRMP5 promoter activity, but not if the putative Sox5 binding site is mutated. We demonstrate that Sox5 can physically bind to the CRMP5 promoter DNA in gel mobility shift and chromatin immunoprecipitation assays. Using a combination of real-time RT-PCR and quantitative immunocytochemistry, we provide further evidence for a Sox5-dependent upregulation of CRMP5 transcription and protein expression in N1E115 cells: a commonly used cell line model for neuronal differentiation. Furthermore, we report that increasing Sox5 levels in this neuronal cell line inhibits neurite outgrowth. This inhibition requires CRMP5 because CRMP5 knockdown prevents the Sox5-dependent effect. We confirm the physiological relevance of the Sox5-CRMP5 pathway in the regulation of neurite outgrowth using mouse primary hippocampal neurons. These findings identify Sox5 as a critical modulator of neurite outgrowth through the selective activation of CRMP5 expression.