Expression of IP3 receptor isoforms at the nodes of Ranvier in rat sciatic nerve

Missense mutations in inositol 1,4,5-trisphosphate receptor type 3 result in leaky Ca2+ channels and activation of store-operated Ca2+ entry

Mutations in all subtypes of the inositol 1,4,5-trisphosphate receptor Ca2+ release channel are associated with human diseases. In this report, we investigated the functionality of three neuropathy-associated missense mutations in IP3R3 (V615M, T1424M, and R2524C). The mutants only exhibited function when highly over-expressed compared to endogenous hIP3R3. All variants resulted in elevated basal cytosolic Ca2+ levels, decreased endoplasmic reticulum Ca2+ store content, and constitutive store-operated Ca2+ entry in the absence of any stimuli, consistent with a leaky IP3R channel pore. These variants differed in channel function; when stably over-expressed the R2524C mutant was essentially dead, V615M was poorly functional, and T1424M exhibited activity greater than that of the corresponding wild-type following threshold stimulation. These results demonstrate that a common feature of these mutations is decreased IP3R3 function. In addition, these mutations exhibit a novel phenotype manifested as a constitutively open channel, which inappropriately gates SOCE in the absence of stimulation.

Optogenetic stimulation promotes Schwann cell proliferation, differentiation, and myelination in vitro

Schwann cells (SCs) constitute a crucial element of the peripheral nervous system, by structurally supporting the formation of myelin and conveying vital trophic factors to the nervous system. However, the functions of SCs in developmental and regenerative stages remain unclear. Here, we investigated how optogenetic stimulation (OS) of SCs regulates their development. In SC monoculture, OS substantially enhanced SC proliferation and the number of BrdU⁺-S100ß⁺-SCs over time. In addition, OS also markedly promoted the expression of both Krox20 and myelin basic protein (MBP) in SC culture medium containing dBcAMP/NRG1, which induced differentiation. We found that the effects of OS are dependent on the intracellular Ca²⁺ level. OS induces elevated intracellular Ca²⁺ levels through the T-type voltage-gated calcium channel (VGCC) and mobilization of Ca²⁺ from both inositol 1,4,5-trisphosphate (IP₃)-sensitive stores and caffeine/ryanodine-sensitive stores. Furthermore, we confirmed that OS significantly increased expression levels of both Krox20 and MBP in SC-motor neuron (MN) coculture, which was notably prevented by pharmacological intervention with Ca²⁺. Taken together, our results demonstrate that OS of SCs increases the intracellular Ca²⁺ level and can regulate proliferation, differentiation, and myelination, suggesting that OS of SCs may offer a new approach to the treatment of neurodegenerative disorders.

What's the Function of Connexin 32 in the Peripheral Nervous System?

Connexin 32 (Cx32) is a fundamental protein in the peripheral nervous system (PNS) as its mutations cause the X-linked form of Charcot–Marie–Tooth disease (CMT1X), the second most common form of hereditary motor and sensory neuropathy and a demyelinating disease for which there is no effective therapy. Since mutations of the GJB1 gene encoding Cx32 were first reported in 1993, over 450 different mutations associated with CMT1X including missense, frameshift, deletion and non-sense ones have been identified. Despite the availability of a sizable number of studies focusing on normal and mutated Cx32 channel properties, the crucial role played by Cx32 in the PNS has not yet been elucidated, as well as the molecular pathogenesis of CMT1X. Is Cx32 fundamental during a particular phase of Schwann cell (SC) life? Are Cx32 paired (gap junction, GJ) channels in myelinated SCs important for peripheral nerve homeostasis? The attractive hypothesis that short coupling of adjacent myelin layers by Cx32 GJs is required for efficient diffusion of K⁺ and signaling molecules is still debated, while a growing body of evidence is supporting other possible functions of Cx32 in the PNS, mainly related to Cx32 unpaired channels (hemichannels), which could be involved in a purinergic-dependent pathway controlling myelination. Here we review the intriguing puzzle of findings about Cx32 function and dysfunction, discussing possible directions for future investigation.

A dual role for Integrin α6β4 in modulating hereditary neuropathy with liability to pressure palsies

Peripheral myelin protein 22 (PMP22) is a component of compact myelin in the peripheral nervous system. The amount of PMP22 in myelin is tightly regulated, and PMP22 over or under-expression cause Charcot-Marie-Tooth 1A (CMT1A) and Hereditary Neuropathy with Pressure Palsies (HNPP). Despite the importance of PMP22, its function remains largely unknown. It was reported that PMP22 interacts with the β4 subunit of the laminin receptor α6β4 integrin, suggesting that α6β4 integrin and laminins may contribute to the pathogenesis of CMT1A or HNPP. Here we asked if the lack of α6β4 integrin in Schwann cells influences myelin stability in the HNPP mouse model. Our data indicate that PMP22 and β4 integrin may not interact directly in myelinating Schwann cells, however, ablating β4 integrin delays the formation of tomacula, a characteristic feature of HNPP. In contrast, ablation of integrin β4 worsens nerve conduction velocities and non-compact myelin organization in HNPP animals. This study demonstrates that indirect interactions between an extracellular matrix receptor and a myelin protein influence the stability and function of myelinated fibers.

Determining the Roles of Inositol Trisphosphate Receptors in Neurodegeneration: Interdisciplinary Perspectives on a Complex Topic

It is well known that calcium (Ca²⁺) is involved in the triggering of neuronal death. Ca²⁺ cytosolic levels are regulated by Ca²⁺ release from internal stores located in organelles, such as the endoplasmic reticulum. Indeed, Ca²⁺ transit from distinct cell compartments follows complex dynamics that are mediated by specific receptors, notably inositol trisphosphate receptors (IP3Rs). Ca²⁺ release by IP3Rs plays essential roles in several neurological disorders; however, details of these processes are poorly understood. Moreover, recent studies have shown that subcellular location, molecular identity, and density of IP3Rs profoundly affect Ca²⁺ transit in neurons. Therefore, regulation of IP3R gene products in specific cellular vicinities seems to be crucial in a wide range of cellular processes from neuroprotection to neurodegeneration. In this regard, microRNAs seem to govern not only IP3Rs translation levels but also subcellular accumulation. Combining new data from molecular cell biology with mathematical modelling, we were able to summarize the state of the art on this topic. In addition to presenting how Ca²⁺ dynamics mediated by IP3R activation follow a stochastic regimen, we integrated a theoretical approach in an easy-to-apply, cell biology-coherent fashion. Following the presented premises and in contrast to previously tested hypotheses, Ca²⁺ released by IP3Rs may play different roles in specific neurological diseases, including Alzheimer's disease and Parkinson's disease.

Genetic ablation of IP3 receptor 2 increases cytokines and decreases survival of SOD1G93A mice

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disease characterized by the selective death of motor neurons. Disease pathophysiology is complex and not yet fully understood. Higher gene expression of the inositol 1,4,5-trisphosphate receptor 2 gene (ITPR2), encoding the IP₃ receptor 2 (IP₃R2), was detected in sporadic ALS patients. Here, we demonstrate that IP₃R2 gene expression was also increased in spinal cords of ALS mice. Moreover, an increase of IP₃R2 expression was observed in other models of chronic and acute neurodegeneration. Upregulation of IP₃R2 gene expression could be induced by lipopolysaccharide (LPS) in murine astrocytes, murine macrophages and human fibroblasts indicating that it may be a compensatory response to inflammation. Preventing this response by genetic deletion of ITPR2 from SOD1^G93A mice had a dose-dependent effect on disease duration, resulting in a significantly shorter lifespan of these mice. In addition, the absence of IP₃R2 led to increased innate immunity, which may contribute to the decreased survival of the SOD1^G93A mice. Besides systemic inflammation, IP₃R2 knockout mice also had increased IFNγ, IL-6 and IL1α expression. Altogether, our data indicate that IP₃R2 protects against the negative effects of inflammation, suggesting that the increase in IP₃R2 expression in ALS patients is a protective response.

In vitro maturation of the cisternal organelle in the hippocampal neuron's axon initial segment

Regulation of Ca(2+) concentrations is essential to maintain the structure and function of the axon initial segment (AIS). The so-called cisternal organelle of the AIS is a structure involved in this regulation, although little is known as to how this organelle matures and is stabilized. Here we describe how the cisternal organelle develops in cultured hippocampal neurons and the interactions that facilitate its stabilization in the AIS. We also characterize the developmental expression of molecules involved in Ca(2+) regulation in the AIS. Our results indicate that synaptopodin (synpo) positive elements considered to be associated to the cisternal organelle are present in the AIS after six days in vitro. There are largely overlapping microdomains containing the inositol 1,4,5-triphosphate receptor 1 (IP(3)R1) and the Ca(2+) binding protein annexin 6, suggesting that the regulation of Ca(2+) concentrations in the AIS is sensitive to IP(3) and subject to regulation by annexin 6. The expression of synpo, IP(3)R1 and annexin 6 in the AIS is independent of the neuron activity, as it was unaffected by tetrodotoxin blockage of action potentials and it was resistant to detergent extraction, indicating that these proteins interact with scaffolding and/or cytoskeleton proteins. The presence of ankyrin G seems to be required for the acquisition and maintenance of the cisternal organelle, while the integrity of the actin cytoskeleton must be maintained for the expression IP(3)R1 and annexin 6 to persist in the AIS.

Electrical stimulation induces calcium-dependent release of NGF from cultured Schwann cells

Production of nerve growth factor (NGF) from Schwann cells (SCs) progressively declines in the distal stump, if axonal regeneration is staggered across the suture site after peripheral nerve injuries. This may be an important factor limiting the outcome of nerve injury repair. Thus far, extensive efforts are devoted to modulating NGF production in cultured SCs, but little has been achieved. In the present in vitro study, electrical stimulation (ES) was attempted to stimulate cultured SCs to release NGF. Our data showed that ES was capable of enhancing NGF release from cultured SCs. An electrical field (1 Hz, 5 V/cm) caused a 4.1-fold increase in NGF release from cultured SCs. The ES-induced NGF release is calcium dependent. Depletion of extracellular or/and intracellular calcium partially/ completely abolished the ES-induced NGF release. Further pharmacological interventions showed that ES induces calcium influx through T-type voltage-gated calcium channels and mobilizes calcium from 1, 4, 5-trisphosphate-sensitive stores and caffeine/ryanodine-sensitive stores, both of which contributed to the enhanced NGF release induced by ES. In addition, a calcium-triggered exocytosis mechanism was involved in the ES-induced NGF release from cultured SCs. These findings show the feasibility of using ES in stimulating SCs to release NGF, which holds great potential in promoting nerve regeneration by enhancing survival and outgrowth of damaged nerves, and is of great significance in nerve injury repair and neuronal tissue engineering.

Schwannomin-interacting protein-1 isoform IQCJ-SCHIP-1 is a late component of nodes of Ranvier and axon initial segments

Axon initial segments (AISs) and nodes of Ranvier (NRs) are essential regions for saltatory conduction of the action potential along the axon. These two domains are enriched in similar multimolecular complexes, which include voltage-gated sodium channels (Na(v)), NF186 (neurofascin 186), NrCAM (neuron glia-related cell adhesion molecule), and cytoskeleton linkers ankyrin G (AnkG) and betaIV-spectrin. Identification of novel members of these complexes is critical to better understand their formation, function, and maintenance. Here we report that IQCJ-SCHIP-1, a recently identified isoform of schwannomin-interacting protein-1 (SCHIP-1), is a novel component of both AISs and NRs in the central and peripheral nervous systems. We show that IQCJ-SCHIP-1 binds calmodulin in the absence of Ca(2+) and is highly enriched at AISs and NRs. IQCJ-SCHIP-1 accumulation at AISs and NRs is a late event, suggesting that IQCJ-SCHIP-1 is likely to play a role in mature AISs and NRs rather than during their formation. IQCJ-SCHIP-1 was not detected at AISs in the absence of AnkG and interacted in vitro with this protein. IQCJ-SCHIP-1 was also absent from central NRs and AISs of quivering mice, which have a mutation of betaIV-spectrin. We suggest that IQCJ-SCHIP-1 might participate, along with AnkG and betaIV-spectrin, in the stabilization or function of the multimolecular complexes of AISs and NRs, possibly by participating in Ca(2+)-mediated responses.