Gap junction communication in myelinating glia

Oligodendrocytes: Myelination, Plasticity, and Axonal Support

The myelination of axons has evolved to enable fast and efficient transduction of electrical signals in the vertebrate nervous system. Acting as an electric insulator, the myelin sheath is a multilamellar membrane structure around axonal segments generated by the spiral wrapping and subsequent compaction of oligodendroglial plasma membranes. These oligodendrocytes are metabolically active and remain functionally connected to the subjacent axon via cytoplasmic-rich myelinic channels for movement of metabolites and macromolecules to and from the internodal periaxonal space under the myelin sheath. Increasing evidence indicates that oligodendrocyte numbers, specifically in the forebrain, and myelin as a dynamic cellular compartment can both respond to physiological demands, collectively referred to as adaptive myelination. This review summarizes our current understanding of how myelin is generated, how its function is dynamically regulated, and how oligodendrocytes support the long-term integrity of myelinated axons.

Multiple Sclerosis: New Insights into Molecular Pathogenesis and Novel Platforms for Disease Treatment

BACKGROUND

Multiple sclerosis (MS), a chronic inflammatory disorder, affects the central nervous system via myelin degradation. The cause of MS is not fully known, but during recent years, our knowledge has deepened significantly regarding the different aspects of MS, including etiology, molecular pathophysiology, diagnosis and therapeutic options. Myelin basic protein (MBP) is the main myelin protein that accounts for maintaining the stability of the myelin sheath. Recent evidence has revealed that MBP citrullination or deamination, which is catalyzed by Ca2+ dependent peptidyl arginine deiminase (PAD) enzyme leads to the reduction of positive charge, and subsequently proteolytic cleavage of MBP. The overexpression of PAD2 in the brains of MS patients plays an essential role in new epitope formation and progression of the autoimmune disorder. Some drugs have recently entered phase III clinical trials with promising efficacy and will probably obtain approval in the near future. As different therapeutic platforms develop, finding an optimal treatment for each individual patient will be more challenging.

AIMS

This review provides a comprehensive insight into MS with a focus on its pathogenesis and recent advances in diagnostic methods and its present and upcoming treatment modalities.

CONCLUSION

MS therapy alters quickly as research findings and therapeutic options surrounding MS expand. McDonald's guidelines have created different criteria for MS diagnosis. In recent years, ever-growing interest in the development of PAD inhibitors has led to the generation of many reversible and irreversible PAD inhibitors against the disease with satisfactory therapeutic outcomes.

New perspectives in diabetic neuropathy

Diabetes prevalence continues to climb with the aging population. Type 2 diabetes (T2D), which constitutes most cases, is metabolically acquired. Diabetic peripheral neuropathy (DPN), the most common microvascular complication, is length-dependent damage to peripheral nerves. DPN pathogenesis is complex, but, at its core, it can be viewed as a state of impaired metabolism and bioenergetics failure operating against the backdrop of long peripheral nerve axons supported by glia. This unique peripheral nerve anatomy and the injury consequent to T2D underpins the distal-to-proximal symptomatology of DPN. Earlier work focused on the impact of hyperglycemia on nerve damage and bioenergetics failure, but recent evidence additionally implicates contributions from obesity and dyslipidemia. This review will cover peripheral nerve anatomy, bioenergetics, and glia-axon interactions, building the framework for understanding how hyperglycemia and dyslipidemia induce bioenergetics failure in DPN. DPN and painful DPN still lack disease-modifying therapies, and research on novel mechanism-based approaches is also covered.

Channel-mediated ATP release in the nervous system

ATP is well established as a transmitter and modulator in the peripheral and central nervous system. While conventional exocytotic release of ATP at synapses occurs, this transmitter is unusual in also being released into the extracellular space via large-pored plasma membrane channels. This review considers the channels that are known to be permeable to ATP and some of the functions of channel-mediated ATP release. While the possibility of ATP release via channels mediating volume transmission has been known for some time, localised ATP release via channels at specialised synapses made by taste cells to the afferent nerve has recently been documented in taste buds. This raises the prospect that "channel synapses" may occur in other contexts. However, volume transmission and channel synapses are not necessarily mutually exclusive. We suggest that certain glial cells in the brain stem and hypothalamus, which possess long processes and are known to release ATP, may be candidates for both modes of ATP release -channel-mediated volume transmission in the region of their somata and more localised transmission possibly via either conventional or channel synapses from their processes at distal targets. Finally, we consider the different characteristics of vesicular and channel synapses and suggest that channel synapses may be advantageous in requiring less energy than their conventional vesicular counterparts. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Innexin-Mediated Adhesion between Glia Is Required for Axon Ensheathment in the Peripheral Nervous System

Glia are essential to protecting and enabling nervous system function and a key glial function is the formation of the glial sheath around peripheral axons. Each peripheral nerve in the Drosophila larva is ensheathed by three glial layers, which structurally support and insulate the peripheral axons. How peripheral glia communicate with each other and between layers is not well established and we investigated the role of Innexins in mediating glial function in the Drosophila periphery. Of the eight Drosophila Innexins, we found two (Inx1 and Inx2) are important for peripheral glia development. In particular loss of Inx1 and Inx2 resulted in defects in the wrapping glia leading to disruption of the glia wrap. Of interest loss of Inx2 in the subperineurial glia also resulted in defects in the neighboring wrapping glia. Inx plaques were observed between the subperineurial glia and the wrapping glia suggesting that gap junctions link these two glial cell types. We found Inx2 is key to Ca2+ pulses in the peripheral subperineurial glia but not in the wrapping glia, and we found no evidence of gap junction communication between subperineurial and wrapping glia. Rather we have clear evidence that Inx2 plays an adhesive and channel-independent role between the subperineurial and wrapping glia to ensure the integrity of the glial wrap.SIGNIFICANCE STATEMENT Gap junctions are critical for glia communication and formation of myelin in myelinating glia. However, the role of gap junctions in non-myelinating glia is not well studied, yet non-myelinating glia are critical for peripheral nerve function. We found the Innexin gap junction proteins are present between different classes of peripheral glia in Drosophila. Here Innexins form junctions to facilitate adhesion between the different glia but do so in a channel-independent manner. Loss of adhesion leads to disruption of the glial wrap around axons and leads to fragmentation of the wrapping glia membranes. Our work points to an important role for gap junction proteins in mediating insulation by non-myelinating glia.

Functional Roles of Connexins and Gap Junctions in Osteo-Chondral Cellular Components

Gap junctions (GJs) formed by connexins (Cxs) play an important role in the intercellular communication within most body tissues. In this paper, we focus on GJs and Cxs present in skeletal tissues. Cx43 is the most expressed connexin, participating in the formation of both GJs for intercellular communication and hemichannels (HCs) for communication with the external environment. Through GJs in long dendritic-like cytoplasmic processes, osteocytes embedded in deep lacunae are able to form a functional syncytium not only with neighboring osteocytes but also with bone cells located at the bone surface, despite the surrounding mineralized matrix. The functional syncytium allows a coordinated cell activity through the wide propagation of calcium waves, nutrients and anabolic and/or catabolic factors. Acting as mechanosensors, osteocytes are able to transduce mechanical stimuli into biological signals that spread through the syncytium to orchestrate bone remodeling. The fundamental role of Cxs and GJs is confirmed by a plethora of investigations that have highlighted how up- and downregulation of Cxs and GJs critically influence skeletal development and cartilage functions. A better knowledge of GJ and Cx mechanisms in physiological and pathological conditions might help in developing therapeutic approaches aimed at the treatment of human skeletal system disorders.

Astrocytes promote the proliferation of oligodendrocyte precursor cells through connexin 47-mediated LAMB2 secretion in exosomes

BACKGROUND

Oligodendrocyte precursor cells (OPCs) can proliferate and differentiate into oligodendrocytes, the only myelin-forming cells in the central nervous system. Proliferating OPCs promotes remyelination in neurodegenerative diseases. Astrocytes (ASTs) are the most widespread cells in the brain and play a beneficial role in the proliferation of OPCs. Connexin 47 (Cx47) is the main component of AST-OPC gap junctions to regulate OPC proliferation. Nonetheless, the specific mechanism remains unclear.

METHODS AND RESULTS

This study investigates the proliferation mechanism of OPCs connected to ASTs via Cx47. Cx47 siRNA significantly inhibited OPCs from entering the proliferation cycle. Transcriptome sequencing of OPCs and gene ontology enrichment analysis revealed that ASTs enhanced the exosome secretion by OPCs via Cx47. Transmission electron microscopy, Western blot, and nanoparticle tracking analysis indicated that the OPC proliferation was related to extracellular exosomes. Cx47 siRNA decreased the OPC proliferation and exosome secretion in AST-OPC cocultures. Exogenous exosome supplementation alleviated the inhibitory effect of Cx47 siRNA and significantly improved OPC proliferation. Mass spectrometry revealed that LAMB2 was abundant in exosomes. The administration of exogenous LAMB2 induced DNA replication in the S phase in OPCs by activating cyclin D1.

CONCLUSIONS

Collectively, ASTs induce the secretion of exosomes that carry LAMB2 by OPCs via Cx47 to upregulate cyclin D1 thereby accelerating OPC proliferation.

How Does the Immune System Enter the Brain?

Multiple Sclerosis (MS) is considered the most frequent inflammatory demyelinating disease of the central nervous system (CNS). It occurs with a variable prevalence across the world. A rich armamentarium of disease modifying therapies selectively targeting specific actions of the immune system is available for the treatment of MS. Understanding how and where immune cells are primed, how they access the CNS in MS and how immunomodulatory treatments affect neuroinflammation requires a proper knowledge on the mechanisms regulating immune cell trafficking and the special anatomy of the CNS. The brain barriers divide the CNS into different compartments that differ with respect to their accessibility to cells of the innate and adaptive immune system. In steady state, the blood-brain barrier (BBB) limits immune cell trafficking to activated T cells, which can reach the cerebrospinal fluid (CSF) filled compartments to ensure CNS immune surveillance. In MS immune cells breach a second barrier, the glia limitans to reach the CNS parenchyma. Here we will summarize the role of the endothelial, epithelial and glial brain barriers in regulating immune cell entry into the CNS and which immunomodulatory treatments for MS target the brain barriers. Finally, we will explore current knowledge on genetic and environmental factors that may influence immune cell entry into the CNS during neuroinflammation in Africa.

Understanding the Molecular Mechanisms of Succinic Semialdehyde Dehydrogenase Deficiency (SSADHD): Towards the Development of SSADH-Targeted Medicine

Succinic semialdehyde dehydrogenase deficiency (SSADHD) is a rare genetic disorder caused by inefficient metabolic breakdown of the major inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Pathologic brain accumulation of GABA and γ-hydroxybutyrate (GHB), a neuroactive by-product of GABA catabolism, leads to a multitude of molecular abnormalities beginning in early life, culminating in multifaceted clinical presentations including delayed psychomotor development, intellectual disability, hypotonia, and ataxia. Paradoxically, over half of patients with SSADHD also develop epilepsy and face a significant risk of sudden unexpected death in epilepsy (SUDEP). Here, we review some of the relevant molecular mechanisms through which impaired synaptic inhibition, astrocytic malfunctions and myelin defects might contribute to the complex SSADHD phenotype. We also discuss the gaps in knowledge that need to be addressed for the implementation of successful gene and enzyme replacement SSADHD therapies. We conclude with a description of a novel SSADHD mouse model that enables 'on-demand' SSADH restoration, allowing proof-of-concept studies to fine-tune SSADH restoration in preparation for eventual human trials.

Intercellular Communication in the Brain through Tunneling Nanotubes

Simple Summary

Tunneling nanotubes (TNTs) are a means of cell communication which have been recently discovered. They allow the intercellular trafficking of many types of cellular compounds ranging from ions, such as Ca²⁺, to whole organelles such as mitochondria. TNTs are found in many tissues, both in physiological and pathological conditions. They are also found in the brain where they contribute to brain development and function and also to degenerative diseases and glioma.

Abstract

Intercellular communication is essential for tissue homeostasis and function. Understanding how cells interact with each other is paramount, as crosstalk between cells is often dysregulated in diseases and can contribute to their progression. Cells communicate with each other through several modalities, including paracrine secretion and specialized structures ensuring physical contact between them. Among these intercellular specialized structures, tunneling nanotubes (TNTs) are now recognized as a means of cell-to-cell communication through the exchange of cellular cargo, controlled by a variety of biological triggers, as described here. Intercellular communication is fundamental to brain function. It allows the dialogue between the many cells, including neurons, astrocytes, oligodendrocytes, glial cells, microglia, necessary for the proper development and function of the brain. We highlight here the role of TNTs in connecting these cells, for the physiological functioning of the brain and in pathologies such as stroke, neurodegenerative diseases, and gliomas. Understanding these processes could pave the way for future therapies.

Neuroprotective Role of Nutritional Supplementation in Athletes

BACKGROUND

Neurodegenerative disorders belong to different classes of progressive/chronic conditions that affect the peripheral/central nervous system. It has been shown through studies that athletes who play sports involving repeated head trauma and sub-concussive impacts are more likely to experience neurological impairments and neurodegenerative disorders in the long run.

AIMS

The aim of the current narrative review article is to provide a summary of various nutraceuticals that offer promise in the prevention or management of sports-related injuries, especially concussions and mild traumatic brain injuries.

METHODS

This article reviews the various potential nutraceutical agents and their possible mechanisms in providing a beneficial effect in the injury recovery process. A thorough survey of the literature was carried out in the relevant databases to identify studies published in recent years. In the present article, we have also highlighted the major neurological disorders along with the associated nutraceutical(s) therapy in the management of disorders.

RESULTS

The exact pathological mechanism behind neurodegenerative conditions is complex as well as idiopathic. However, mitochondrial dysfunction, oxidative stress as well as intracellular calcium overload are some common reasons responsible for the progression of these neurodegenerative disorders. Owing to the multifaceted effects of nutraceuticals (complementary medicine), these supplements have gained importance as neuroprotective. These diet-based approaches inhibit different pathways in a physiological manner without eliciting adverse effects. Food habits and lifestyle of an individual also affect neurodegeneration.

CONCLUSION

Studies have shown nutraceuticals (such as resveratrol, omega-3-fatty acids) to be efficacious in terms of their neuroprotection against several neurodegenerative disorders and to be used as supplements in the management of traumatic brain injuries. Protection prior to injuries is needed since concussions or sub-concussive impacts may trigger several pathophysiological responses or cascades that can lead to long-term complications associated with CNS. Thus, the use of nutraceuticals as prophylactic treatment for neurological interventions has been proposed.

Connexin and gap junctions: perspectives from biology to nanotechnology based therapeutics

The concept of gap junctions and their role in intercellular communication has been known for around 50 years. Considerable progress has been made in understanding the fundamental biology of connexins in mediating gap junction intercellular communication (GJIC) and their role in various cellular processes including pathological conditions. However, this understanding has not led to development of advanced therapeutics utilizing GJIC. Inadequacies in strategies that target specific connexin protein in the affected tissue, with minimal or no collateral damage, are the primary reason for the lack of development of efficient therapeutic models. Herein, nanotechnology has a role to play, giving plenty of scope to circumvent these problems and develop more efficient connexin based therapeutics. AsODN, antisense oligodeoxynucleotides; BMPs, bone morphogenetic proteins; BMSCs, bone marrow stem cells; BG, bioglass; Cx, Connexin; CxRE, connexin-responsive elements; CoCr NPs, cobalt-chromium nanoparticles; cGAMP, cyclic guanosine monophosphate-adenosine monophosphate; cAMP, cyclic adenosine monophosphate; ERK1/2, extracellular signal-regulated kinase 1/2; EMT, epithelial-mesenchymal transition; EPA, eicosapentaenoic acids; FGFR1, fibroblast growth factor receptor 1; FRAP, fluorescence recovery after photobleaching; 5-FU, 5-fluorouracil; GJ, gap junction; GJIC, gap junctional intercellular communication; HGPRTase, hypoxanthine phosphoribosyltransferase; HSV-TK, herpes virus thymidine kinase; HSA, human serum albumin; HA, hyaluronic acid; HDAC, histone deacetylase; IRI, ischemia reperfusion injury; IL-6, interleukin-6; IL-8, interleukin-8; IONPs, iron-oxide nanoparticles; JNK, c-Jun N-terminal kinase; LAMP, local activation of molecular fluorescent probe; MSCs, mesenchymal stem cells; MMP, matrix metalloproteinase; MI, myocardial infarction; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor kappa B; NO, nitric oxide; PKC, protein kinase C; QDs, quantum dots; ROI, region of interest; RGO, reduced graphene oxide; siRNA, small interfering RNA; TGF-β1, transforming growth factor-β1; TNF-α, tumor necrosis factor-α; UCN, upconversion nanoparticles; VEGF, vascular endothelial growth factor. In this review, we discuss briefly the role of connexins and gap junctions in various physiological and pathological processes, with special emphasis on cancer. We further discuss the application of nanotechnology and tissue engineering in developing treatments for various connexin based disorders.

Connexins in the development and physiology of stem cells

Connexins (Cxs) form gap junction (GJ) channels linking vertebrate cells. During embryogenesis, Cxs are expressed as early as the 4-8 cell stage. As cells differentiate into pluripotent stem cells (PSCs) and during gastrulation, the Cx expression pattern is adapted. Knockdown of Cx43 and Cx45 does not interfere with embryogenic development until the blastula stage, questioning the role of Cxs in PSC physiology and development. Studies in cultivated and induced PSCs (iPSCs) showed that Cx43 is essential for the maintenance of self-renewal and the expression of pluripotency markers. It was found that the role of Cxs in PSCs is more related to regulation of transcription or cell-cell adherence than to formation of GJ channels. Furthermore, a crucial role of Cxs for the self-renewal and differentiation was shown in cultivated adult mesenchymal stem cells. This review aims to highlight aspects that link Cxs to the function and physiology of stem cell development.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Mitochondria in Myelinating Oligodendrocytes: Slow and Out of Breath?

Myelin is a lipid-rich membrane that wraps around axons and facilitates rapid action potential propagation. In the brain, myelin is synthesized and maintained by oligodendrocytes. These cells have a high metabolic demand that requires mitochondrial ATP production during the process of myelination, but they rely less on mitochondrial respiration after myelination is complete. Mitochondria change in morphology and distribution during oligodendrocyte development. Furthermore, the morphology and dynamic properties of mitochondria in mature oligodendrocytes seem different from any other brain cell. Here, we first give a brief introduction to oligodendrocyte biology and function. We then review the current knowledge on oligodendrocyte metabolism and discuss how the available data on mitochondrial morphology and mobility as well as transcriptome and proteome studies can shed light on the metabolic properties of oligodendrocytes.

Effect of Nimodipine and Botulinum Toxin A on Peripheral Nerve Regeneration in Rats: A Pilot Study

BACKGROUND

Peripheral nerve damage is a frequent problem, with an estimated 2.8%-5.0% of trauma admissions involving peripheral nerve injury. End-to-end, tension-free microsurgical repair (neurorrhaphy) is the current gold standard treatment for complete transection (neurotmesis). While neurorrhaphy reapproximates the nerve, it does not address the complex molecular regenerative process. Evidence suggests that botulinum toxin A (BTX) and nimodipine (NDP) may improve functional recovery, but mechanisms of action remain unknown.

METHODS

This research investigates BTX and NDP for their novel capacity to improve neural regeneration in the setting of neurorrhaphy using a Lewis rat tibial nerve neurotmesis model. In a triple-masked, placebo-controlled, randomized study design, we compared functional (rotarod, horizontal ladder walk), electrophysiological (conduction velocity, duration), and stereological (axon count, density) outcomes of rats treated with: NDP+saline injection, BTX+NDP, Saline+placebo, and BTX+placebo. Additional controls included sham surgery +/- BTX.

RESULTS

NDP+saline outperformed other treatment groups in the ladder walk. This group had the fewest deep slips (15.07% versus 30.77% in BTX+NDP, P = 0.122), and the most correct steps (70.53% versus 55.58% in BTX+NDP, P = 0.149) in functional testing. NDP+saline also had the fastest nerve conduction velocity (0.811m/s versus 0.598m/s in BTX+NDP, P = 0.126) among treatment groups. BTX+NDP had the highest axon count (10,012.36 versus 7,738.18 in NDP+Saline, P = 0.009).

CONCLUSION

This study is the first to test NDP with BTX in a multimodal assessment of nerve recovery following neurotmesis and neurorrhaphy. NDP outperformed BTX+NDP functionally. Future work will focus on nimodipine in an effort to improve nerve recovery in trauma patients.

Energy Metabolism in the Inner Retina in Health and Glaucoma

Glaucoma, the leading cause of irreversible blindness, is a heterogeneous group of diseases characterized by progressive loss of retinal ganglion cells (RGCs) and their axons and leads to visual loss and blindness. Risk factors for the onset and progression of glaucoma include systemic and ocular factors such as older age, lower ocular perfusion pressure, and intraocular pressure (IOP). Early signs of RGC damage comprise impairment of axonal transport, downregulation of specific genes and metabolic changes. The brain is often cited to be the highest energy-demanding tissue of the human body. The retina is estimated to have equally high demands. RGCs are particularly active in metabolism and vulnerable to energy insufficiency. Understanding the energy metabolism of the inner retina, especially of the RGCs, is pivotal for understanding glaucoma’s pathophysiology. Here we review the key contributors to the high energy demands in the retina and the distinguishing features of energy metabolism of the inner retina. The major features of glaucoma include progressive cell death of retinal ganglions and optic nerve damage. Therefore, this review focuses on the energetic budget of the retinal ganglion cells, optic nerve and the relevant cells that surround them.

Connexins in neuromyelitis optica: a link between astrocytopathy and demyelination

Neuromyelitis optica, a rare neuroinflammatory demyelinating disease of the CNS, is characterized by the presence of specific pathogenic autoantibodies directed against the astrocytic water channel aquaporin 4 (AQP4) and is now considered as an astrocytopathy associated either with complement-dependent astrocyte death or with astrocyte dysfunction. However, the link between astrocyte dysfunction and demyelination remains unclear. We propose glial intercellular communication, supported by connexin hemichannels and gap junctions, to be involved in demyelination process in neuromyelitis optica. Using mature myelinated cultures, we demonstrate that a treatment of 1 h to 48 h with immunoglobulins purified from patients with neuromyelitis optica (NMO-IgG) is responsible for a complement independent demyelination, compared to healthy donors' immunoglobulins (P < 0.001). In parallel, patients' immunoglobulins induce an alteration of connexin expression characterized by a rapid loss of astrocytic connexins at the membrane followed by an increased size of gap junction plaques (+60%; P < 0.01). This was co-observed with connexin dysfunction with gap junction disruption (-57%; P < 0.001) and increased hemichannel opening (+17%; P < 0.001), associated with glutamate release. Blocking connexin 43 hemichannels with a specific peptide was able to prevent demyelination in co-treatment with patients compared to healthy donors' immunoglobulins. By contrast, the blockade of connexin 43 gap junctions with another peptide was detrimental for myelin (myelin density -48%; P < 0.001). Overall, our results suggest that dysregulation of connexins would play a pathogenetic role in neuromyelitis optica. The further identification of mechanisms leading to connexin dysfunction and soluble factors implicated, would provide interesting therapeutic strategies for demyelinating disorders.

Tau Isoform-Driven CBD Pathology Transmission in Oligodendrocytes in Humanized Tau Mice

The aggregation of abnormally phosphorylated tau protein in neurons and glia is a neuropathological hallmark of several neurodegenerative disorders, collectively known as tauopathies. They are further subclassified based on the preferential pathological aggregation of three carboxyl-terminal repeat domains (3R) and/or 4R tau. Corticobasal degeneration (CBD) is a rare neurodegenerative disorder classified as a 4R tauopathy. In the present study, we extend analysis of CBD-tau cell-type specific pathology transmission with 3R and 4R tau isoform distinguishable changes. We use a humanized tau (hTau) mouse line, which overexpress all six human tau isoforms in a murine tau knockout background and perform intrastriatal inoculation of control and CBD-tau enriched human brain homogenate. We show that CBD-tau causes hyperphosphorylation of tau at Ser202 predominantly in oligodendrocytes. Next, we demonstrate the spread of tau pathology from striatum to the overlaying corpus callosum and further to the contralateral side. Finally, we demonstrate that the almost exclusive oligodendrocyte-based transmission of hyperphosphorylated tau is reflected in the endogenous 4R tau isoform expression and corresponds to subclassification of CBD as a 4R tauopathy. Additionally, we identify functional changes in oligodendrocytes reflected by myelin basic protein abnormalities upon CBD-tau inoculation. These changes are not observed in murine tau knockout mice lacking both human and murine tau. Our study presents not only in vivo tau isoform–driven region- and cell-specific tau pathology, but also underlines that tau pathology seeding and transmission might be oligodendrocyte-based. These results, which need to be extended to more cases, give new insights into why tauopathies might vary greatly in both histopathological and neuroanatomical patterns.

Pathway-Focused Profiling of Oligodendrocytes Over-Expressing miR-125a-3p Reveals Alteration of Wnt and Cell-to-Cell Signaling

MicroRNAs are small post-transcriptional regulators that modulate gene expression by directly interacting with their target transcripts. Since the interaction between miRNAs and target mRNAs does not require a perfect match, one single miRNA can influence the expression of several genes and lead to a very broad array of functional consequences. Recently, we identified miR-125a-3p as a new regulator of oligodendrocyte development, showing that its over-expression is associated to impaired oligodendrocyte maturation. However, whether and how miR-125a-3p over-expression is causally related to oligodendrocyte maturation is still obscure, as well as the pathways responsible for this effect. To shed light on this issue and to identify the underlying molecular mechanisms, we determined the transcriptomic profile of miR-125a-3p over-expressing oligodendrocytes and, by means of two complementary bioinformatic approaches, we have identified pathways and biological processes consistently modulated by miR-125a-3p alteration. This analysis showed that miR-125a-3p is involved in the regulation of cell-cell interactions and Wnt signaling. By means of pathway-focused PCR arrays, we confirmed that miR-125a-3p induces changes in the expression of several genes encoding for adhesion molecules and gap junctions, which play key roles in oligodendrocytes after exposure to pathological demyelinating stimuli. Moreover, the expression changes of different Wnt targets suggest an over-activation of this pathway. Globally, our studies show that miR-125a-3p over-expression can alter signaling pathways and biological processes essential for myelin formation in oligodendrocytes, suggesting that alteration of miR-125a-3p levels may contribute to impairing oligodendrocyte maturation in demyelinating diseases.

Hypomyelinating leukodystrophies - unravelling myelin biology

Hypomyelinating leukodystrophies constitute a subset of genetic white matter disorders characterized by a primary lack of myelin deposition. Most patients with severe hypomyelination present in infancy or early childhood and develop severe neurological deficits, but the clinical presentation can also be mild with onset of symptoms in adolescence or adulthood. MRI can be used to visualize the process of myelination in detail, and MRI pattern recognition can provide a clinical diagnosis in many patients. Next-generation sequencing provides a definitive diagnosis in 80-90% of patients. Genes associated with hypomyelination include those that encode structural myelin proteins but also many that encode proteins involved in RNA translation and some lysosomal proteins. The precise pathomechanisms remain to be elucidated. Improved understanding of the process of myelination, the metabolic axonal support functions of myelin and the proposed contribution of myelin to CNS plasticity provide possible explanations as to why almost all patients with hypomyelination experience slow clinical decline after a long phase of stability. In this Review, we provide an overview of the hypomyelinating leukodystrophies, the advances in our understanding of myelin biology and of the genes involved in these disorders, and the insights these advances have provided into their clinical presentations and evolution.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

Motoneuron expression profiling identifies an association between an axonal splice variant of HDGF-related protein 3 and peripheral myelination

Disorders that disrupt myelin formation during development or in adulthood, such as multiple sclerosis and peripheral neuropathies, lead to severe pathologies, illustrating myelin's crucial role in normal neural functioning. However, although our understanding of glial biology is increasing, the signals that emanate from axons and regulate myelination remain largely unknown. To identify the core components of the myelination process, here we adopted a microarray analysis approach combined with laser-capture microdissection of spinal motoneurons during the myelinogenic phase of development. We identified neuronal genes whose expression was enriched during myelination and further investigated hepatoma-derived growth factor-related protein 3 (HRP3 or HDGFRP3). HRP3 was strongly expressed in the white matter fiber tracts of the peripheral (PNS) and central (CNS) nervous systems during myelination and remyelination in a cuprizone-induced demyelination model. The dynamic localization of HPR3 between axons and nuclei during myelination was consistent with its axonal localization during neuritogenesis. To study this phenomenon, we identified two splice variants encoded by the HRP3 gene: the canonical isoform HRP3-I and a newly recognized isoform, HRP3-II. HRP3-I remained solely in the nucleus, whereas HRP3-II displayed distinct axonal localization both before and during myelination. Interestingly, HRP3-II remained in the nuclei of unmyelinated neurons and glial cells, suggesting the existence of a molecular machinery that transfers it to and retains it in the axons of neurons fated for myelination. Overexpression of HRP3-II, but not of HRP3-I, increased Schwann cell numbers and myelination in PNS neuron-glia co-cultures. However, HRP3-II overexpression in CNS co-cultures did not alter myelination.

Cross-Sectional Study in a Large Cohort of Chinese Patients With GJB1 Gene Mutations

Charcot-Marie-Tooth (CMT) disease is a clinically and genetically heterogeneous group of inherited neuropathies. The GJB1 gene is the pathogenic gene of CMTX1. In this study, we screened a cohort of 465 unrelated Chinese CMT patients from years 2007 to 2019 and 650 controls by direct Sanger sequencing in GJB1 gene or targeted next-generation sequencing (NGS) or whole-exome sequencing (WES). A bidirectional Sanger sequencing would be performed on the 600 bases in the upstream promoter region and 30 bases in the 3′ untranslated region (UTR), if no mutation was found in the coding region of GJB1 of the patient. According to the results, 24 missense mutations, 4 nonsense mutation, 1 entire deletion, 1 intronic mutation, and 4 frameshift mutations in GJB1 were identified. Three of them were novel mutations (c.104 T>C, c.658-659 ins C, and c.811 del G). Moreover, central nervous system involvement was observed in five patients carrying mutations of R15W, V95M, R142W, R164W, and E186K. Our findings expand the mutational spectrum of the GJB1 gene in CMT patients. We also explored the genotype–phenotype correlation according to the collected information in this study. NGS panels for detecting inherited neuropathy should cover the non-coding region of GJB1.

Cuprizone-induced oligodendrocyte loss and demyelination impairs recording performance of chronically implanted neural interfaces

Biological inflammation induced during penetrating cortical injury can disrupt functional neuronal and glial activity within the cortex, resulting in potential recording failure of chronically implanted neural interfaces. Oligodendrocytes provide critical support for neuronal health and function through direct contact with neuronal soma and axons within the cortex. Given their fundamental role to regulate neuronal activity via myelin, coupled with their heightened vulnerability to metabolic brain injury due to high energetic demands, oligodendrocytes are hypothesized as a possible source of biological failure in declining recording performances of intracortical microelectrode devices. To determine the extent of their contribution to neuronal activity and function, a cuprizone-inducible model of oligodendrocyte depletion and demyelination in mice was performed prior to microelectrode implantation. At 5 weeks of cuprizone exposure, mice demonstrated significantly reduced cortical oligodendrocyte density and myelin expression. Mice were then implanted with functional recording microelectrodes in the visual cortex and neuronal activity was evaluated up to 7 weeks alongside continued cuprizone administration. Cuprizone-induced oligodendrocyte loss and demyelination was associated with significantly reduced recording performances at the onset of implantation, which remained relatively stable over time. In contast, recording performances for mice on a normal diet were intially elevated before decreasing over time to the recording level of tcuprizone-treated mice. Further electrophysiological analysis revealed deficits in multi-unit firing rates, frequency-dependent disruptions in neuronal oscillations, and altered laminar communication within the cortex of cuprizone-treated mice. Post-mortem immunohistochemistry revealed robust depletion of oligodendrocytes around implanted microelectrode arrays alongside comparable neuronal densities to control mice, suggesting that oligodendrocyte loss was a possible contributor to chronically impaired device performances. This study highlights potentially significant contributions from the oligodendrocyte lineage population concerning the biological integration and long-term functional performance of neural interfacing technology.

Connexins in oligodendrocytes and astrocytes: Possible factors for demyelination in multiple sclerosis

Increasing evidences support that glial connexins are involved in the demyelination pathology of multiple sclerosis (MS), a chronic inflammatory demyelinating disorder. Here, we review the data from patients with MS and animal models of MS that implicate connexins in demyelination. Connexins expressed in oligodendrocytes and astrocytes show diverse changes at the different phases of MS. Loss of oligodendrocyte or astrocyte connexins contributes to demyelination and exaggerates the pathology of MS. Channel-dependent and -independent connexins are involved in the pathology of demyelination, which is related with myelin integrity, metabolic homeostasis, the brain-blood barrier, the immune cell infiltration, and the inflammatory response. A comprehensive understanding of connexin function in demyelination may provide new therapeutic targets for MS.

Glial tauopathy: Neurons optional?

In some neurodegenerative disorders, tau protein accumulates in astrocytes and/or oligodendrocytes, even though these glial cells produce much less of the protein than do neurons. Testing the hypothesis that the aggregated tau in glia derives from neurons, Narasimhan et al. (https://doi.org/10.1084/jem.20190783) make the unexpected discovery that neuronal tau expression is not required for the formation of glial tau inclusions. The circumstances governing the variable cell-specificity of tauopathy thus remain to be fully defined.

Calcium Signaling in the Oligodendrocyte Lineage: Regulators and Consequences

Cells of the oligodendrocyte lineage express a wide range of Ca²⁺ channels and receptors that regulate oligodendrocyte progenitor cell (OPC) and oligodendrocyte formation and function. Here we define those key channels and receptors that regulate Ca²⁺ signaling and OPC development and myelination. We then discuss how the regulation of intracellular Ca²⁺ in turn affects OPC and oligodendrocyte biology in the healthy nervous system and under pathological conditions. Activation of Ca²⁺ channels and receptors in OPCs and oligodendrocytes by neurotransmitters converges on regulating intracellular Ca²⁺, making Ca²⁺ signaling a central candidate mediator of activity-driven myelination. Indeed, recent evidence indicates that localized changes in Ca²⁺ in oligodendrocytes can regulate the formation and remodeling of myelin sheaths and perhaps additional functions of oligodendrocytes and OPCs. Thus, decoding how OPCs and myelinating oligodendrocytes integrate and process Ca²⁺ signals will be important to fully understand central nervous system formation, health, and function.

RhoA/ROCK Regulates Prion Pathogenesis by Controlling Connexin 43 Activity

Scrapie infection, which converts cellular prion protein (PrP^C) into the pathological and infectious isoform (PrP^Sc), leads to neuronal cell death, glial cell activation and PrP^Sc accumulation. Previous studies reported that PrP^C regulates RhoA/Rho-associated kinase (ROCK) signaling and that connexin 43 (Cx43) expression is upregulated in in vitro and in vivo prion-infected models. However, whether there is a link between RhoA/ROCK and Cx43 in prion disease pathogenesis is uncertain. Here, we investigated the role of RhoA/ROCK signaling and Cx43 in prion diseases using in vitro and in vivo models. Scrapie infection induced RhoA activation, accompanied by increased phosphorylation of LIM kinase 1/2 (LIMK1/2) at Thr508/Thr505 and cofilin at Ser3 and reduced phosphorylation of RhoA at Ser188 in hippocampal neuronal cells and brains of mice. Scrapie infection-induced RhoA activation also resulted in PrP^Sc accumulation followed by a reduction in the interaction between RhoA and p190RhoGAP (a GTPase-activating protein). Interestingly, scrapie infection significantly enhanced the interaction between RhoA and Cx43. Moreover, RhoA and Cx43 colocalization was more visible in both the membrane and cytoplasm of scrapie-infected hippocampal neuronal cells than in controls. Finally, RhoA and ROCK inhibition reduced PrP^Sc accumulation and the RhoA/Cx43 interaction, leading to decreased Cx43 hemichannel activity in scrapie-infected hippocampal neuronal cells. These findings suggest that RhoA/ROCK regulates Cx43 activity, which may have an important role in the pathogenesis of prion disease.

Intercellular Bridge Mediates Ca2+ Signals between Micropatterned Cells via IP3 and Ca2+ Diffusion

Intercellular bridges are plasma continuities formed at the end of the cytokinesis process that facilitate intercellular mass transport between the two daughter cells. However, it remains largely unknown how the intercellular bridge mediates Ca²⁺ communication between postmitotic cells. In this work, we utilize BV-2 microglial cells planted on dumbbell-shaped micropatterned assemblies to resolve spatiotemporal characteristics of Ca²⁺ signal transfer over the intercellular bridges. With the use of such micropatterns, considerably longer and more regular intercellular bridges can be obtained than in conventional cell cultures. The initial Ca²⁺ signal is evoked by mechanical stimulation of one of the daughter cells. A considerable time delay is observed between the arrivals of passive Ca²⁺ diffusion and endogenous Ca²⁺ response in the intercellular-bridge-connected cell, indicating two different pathways of the Ca²⁺ communication. Extracellular Ca²⁺ and the paracrine pathway have practically no effect on the endogenous Ca²⁺ response, demonstrated by application of Ca²⁺-free medium, exogenous ATP, and P2Y₁₃ receptor antagonist. In contrast, the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin and inositol trisphosphate (IP₃) receptor blocker 2-aminoethyl diphenylborate significantly inhibit the endogenous Ca²⁺ increase, which signifies involvement of IP₃-sensitive calcium store release. Notably, passive Ca²⁺ diffusion into the connected cell can clearly be detected when IP₃-sensitive calcium store release is abolished by 2-aminoethyl diphenylborate. Those observations prove that both passive Ca²⁺ diffusion and IP₃-mediated endogenous Ca²⁺ response contribute to the Ca²⁺ increase in intercellular-bridge-connected cells. Moreover, a simulation model agreed well with the experimental observations.

A Neural basis for Octanoic acid regulation of energy balance

Objectives

Nutrient sensing by hypothalamic neurons is critical for the regulation of food intake and energy expenditure. We aimed to identify long- and medium-chain fatty acid species transported into the brain, their effects on energy balance, and the mechanisms by which they regulate activity of hypothalamic neurons.

Methods

Simultaneous blood and cerebrospinal fluid (CSF) sampling was undertaken in rats and metabolic analyses using radiolabeled fatty acid tracers were performed on mice. Electrophysiological recording techniques were used to investigate signaling mechanisms underlying fatty acid-induced changes in activity of pro-opiomelanocortin (POMC) neurons.

Results

Medium-chain fatty acid (MCFA) octanoic acid (C8:0), unlike long-chain fatty acids, was rapidly transported into the hypothalamus of mice and almost exclusively oxidized, causing rapid, transient reductions in food intake and increased energy expenditure. Octanoic acid differentially regulates the excitability of POMC neurons, activating these neurons directly via GPR40 and inducing inhibition via an indirect non-synaptic, purine, and adenosine receptor-dependent mechanism.

Conclusions

MCFA octanoic acid is a central signaling nutrient that targets POMC neurons via distinct direct and indirect signal transduction pathways to instigate changes in energy status. These results could explain the beneficial health effects that accompany MCFA consumption.

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Octanoic acid (C8:0) is rapidly transported from blood to the cerebrospinal fluid.

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Octanoic acid rapidly reduces food intake and increases energy expenditure.

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Octanoic acid targets POMC neurons through direct and indirect signaling pathways.

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Activation of POMC neurons occurs directly through GPR40.

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Inhibition occurs through a nonsynaptic, purine and adenosine receptor-dependent mechanism.

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

Chronic pain, including inflammatory, neuropathic pain, is a serious clinical issue. There are increasing numbers of patients with chronic pain due to the growing number of elderly and it is estimated that about 25% of the global population will develop chronic pain. Chronic pain patients are refractory to medications used to treat acute pain such as opioids and non-steroidal anti-inflammatory drugs. Furthermore, the complexity and diversity of chronic pain mechanisms hinder the development of new analgesics. Thus, a better understanding of the mechanism of chronic pain is needed, which would facilitate the development of novel analgesics based on novel mechanisms. With this goal, connexins (Cxs) could be targeted for the development of new analgesics. Connexins are proteins with 20 subtypes, and function as channels, gap junctions between cells, and hemichannels that sample the extracellular space and release molecules such as neurotransmitters. Furthermore, Cxs could have functions independent of channel activity. Recent studies have shown that Cxs could be crucial in the induction and maintenance of chronic pain, and modulation of the activity or the expression of Cxs ameliorates nociceptive hypersensitivity in multiple chronic pain models. This review will cite novel findings on the role of of Cxs in the nociceptive transduction pathway under the chronic pain state and antinociceptive effects of various molecules modulating activity or expression of Cxs. Also, the potential of Cx modulation as a therapeutic strategy for intractable chronic pain will be discussed.

Evidence of decreased gap junction coupling between astrocytes and oligodendrocytes in the anterior cingulate cortex of depressed suicides

Glial dysfunction is a major pathophysiological feature of mood disorders. While altered astrocyte (AS) and oligodendrocyte-lineage (OL) functions have been associated with depression, the crosstalk between these glial cell types has never been assessed in that context. AS are potent regulators of myelination, in part through gap junction (GJ) channels formed by the heterotypic coupling of AS-specific (Cx30 and Cx43) and OL-specific (Cx32 and Cx47) connexins. This study therefore aimed at addressing the integrity of AS/OL coupling in the anterior cingulate cortex (ACC) of depressed suicides. Using immunofluorescence and confocal imaging, we characterized the distribution of Cx30 and mapped its expression onto OL somas, myelinated axons, and brain vasculature in postmortem brain samples from depressed suicides (N = 48) and matched controls (N = 23). Differential gene expression of key components of the GJ nexus was also screened through RNA-sequencing previously generated by our group, and validated by quantitative real-time PCR. We show that Cx30 expression localized onto OL cells and myelinated fibers is decreased in deep cortical layers of the ACC in male-depressed suicides. This effect was associated with decreased expression of OL-specific connexins, as well as the downregulation of major connexin-interacting proteins essential for the scaffolding, trafficking, and function of GJs. These results provide a first evidence of impaired AS/OL GJ-mediated communication in the ACC of individuals with mood disorders. These changes in glial coupling are likely to have significant impact on brain function, and may contribute to the altered OL function previously reported in this brain region.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Simultaneous alterations of oligodendrocyte-specific CNP, astrocyte-specific AQP4 and neuronal NF-L demarcate ischemic tissue after experimental stroke in mice

Ischemic stroke not only affects neurons, but also glial and vascular elements. The development of novel neuroprotective strategies thus requires an improved pathophysiological understanding of ischemia-affected cell types that comprise the 'neurovascular unit' (NVU). To explore spatiotemporal alterations of oligodendrocytes, astrocytes and neurons after experimental ischemic stroke, we applied a permanent middle cerebral artery occlusion model in mice for 4 and 24 h. Using fluorescence microscopy, the oligodendrocyte marker 2',3'-cyclic nucleotide phosphodiesterase (CNP), the neuronal neurofilament light chain (NF-L) and the astroglial aquaporin-4 (AQP4) were analyzed in regional relation to one another. Immunofluorescence intensities of CNP and NF-L were simultaneously increased in the ischemic neocortex and striatum. AQP4 immunoreactivity was decreased in the ischemic striatum, which represents the initial and potentially strongest affected site of infarction. The more distant ischemic neocortex and infarct border zones exhibited areas with alternately increased or decreased AQP4 immunoreactivity, leading to an increase of fluorescence intensity in total. Further, deformed CNP-immunopositive processes were found around axonal spheroids, indicating a combined affection of oligodendrocytes and neurons due to ischemia. Importantly, altered AQP4 immunosignals were not limited to the ischemic core, but were also detectable in penumbral areas. This applies for CNP and NF-L also, since altered immunosignals of all three markers coincided regionally at both time points. In conclusion, the present study provides evidence for a simultaneous affection of oligodendrocytes, astrocytes and neurons after experimental focal cerebral ischemia. Consequently, CNP, AQP4 and NF-L immunofluorescence alterations can be utilized to identify ischemia-affected tissue. The simultaneity of the described alterations further strengthens the concept of interdependent NVU components and distinguishes NF-L, CNP and AQP4 as highly ischemia-sensitive elements. Consequently, future therapeutic approaches might influence stroke evolution via strategies simultaneously addressing both neuronal and glial functions.

Function and Mechanism of Myelin Regulation in Alcohol Abuse and Alcoholism

Excessive alcohol use has adverse effects on the central nervous system (CNS) and can lead to alcohol use disorders (AUDs). Recent studies have suggested that myelin reductions may directly contribute to CNS dysfunctions associated with AUDs. Myelin consists of compact lipid membranes wrapped around axons to provide electrical insulation and trophic support. Regulation of myelin is considered as a new form of neural plasticity due to its profound impacts on the computation of neural networks. In this review, the authors first discuss experimental evidence showing how alcohol exposure causes demyelination in different brain regions, often accompanied by deficits in cognition and emotion. Next, they discuss postulated molecular and cellular mechanisms underlying alcohol's impact on myelin. It is clear that more extensive investigations are needed in this important but underexplored research field in order to gain a better understanding of the myelin-behavior relationship and to develop new treatment strategies for AUDs.

A Chinese pedigree with a novel mutation in GJB1 gene and a rare variation in DHTKD1 gene for diverse Charcot‑Marie‑Tooth diseases

Charcot‑Marie‑Tooth (CMT) disease is a group of motor and sensory neuropathies with a high degree of pathological and genetic heterogenicity. The present study described 2 patients with CMT in a Chinese Han pedigree. The proband exhibited the classic manifestation of CMT with slowly progressing muscular atrophy and weakness. Electrophysiological examination highlighted axonal and demyelinating features. His mother did not have any symptoms, but did exhibit abnormal electrophysiological results. Next‑generation sequencing technology was employed to screen mutations in the genes associated with inherited motor never diseases. A novel mutation, c.528_530delAGT, in the gap junction protein beta 1 (GJB1) gene for CMTX, and a rare variation, c.2369C>T, in the dehydrogenase E1 and transketolase domain containing 1 (DHTKD1) gene for CMT disease type 2Q (CMT2Q), were identified in the proband and his mother. The results were verified by Sanger sequencing. Although the in silico analysis predicted no change in the 3‑dimensional structure, the clinical and electrophysiological presentation in the pedigree and the high evolutionary conservation of the affected amino acid supported the hypothesis that the c.528_530delAGT mutation in the GJB1 gene may be pathogenic in this pedigree. In silico analysis and high evolutionary conservation suggested the pathogenicity of the c.2369C>T mutation in the DHTKD1 gene; however, the clinical and electrophysiological performances of the proband and his mother did not conform to those of CMT2Q caused by the DHTKD1 gene. The present study provided additional information concerning the range of mutations of the GJB1 gene, which facilitated the understanding of the genotype‑phenotype association of CMT.

Novel gap junction protein beta-1 gene mutation associated with a stroke-like syndrome and central nervous system involvement in patients with X-linked Charcot-Marie-Tooth Type 1: A case report and literature review

Gap junction protein beta-1 (GJB1) gene mutations lead to X-linked Charcot-Marie-Tooth Type 1 (CMTX1). We studied a Chinese family with CMTX1 and identified a novel GJB1 point mutation. Our patient had a transient stroke-like clinical manifestations and magnetic resonance imaging (MRI) changes. An analysis of the genomic DNA of the proband showed a T to C hemizygous mutation in the GJB1 gene at nucleotide position 380, causing a predicted amino acid change from isoleucine to threonine at codon 127, which predicted structural alterations disrupting the function of the GJB1 protein. This novel point mutation expanded the spectrum of GJB1 mutations known to be associated with CMTX1. We performed a PubMed review of CMTX cases with central nervous system involvement in the English-language literature from the past 20 years, and summarized the demographic data, nucleotide and amino acid changes, clinical characteristics, clinical manifestations, and neuroimaging features.

Panglial gap junctions between astrocytes and olfactory ensheathing cells mediate transmission of Ca2+ transients and neurovascular coupling

Astrocytes are arranged in highly organized gap junction-coupled networks, communicating via the propagation of Ca²⁺ waves. Astrocytes are gap junction-coupled not only to neighboring astrocytes, but also to oligodendrocytes, forming so-called panglial syncytia. It is not known, however, whether glial cells in panglial syncytia transmit information using Ca²⁺ signaling. We used confocal Ca²⁺ imaging to study intercellular communication between astrocytes and olfactory ensheathing glial cells (OECs) in in-toto preparations of the mouse olfactory bulb. Our results demonstrate that Ca²⁺ transients in juxtaglomerular astrocytes, evoked by local photolysis of "caged" ATP and "caged" tACPD, led to subsequent Ca²⁺ responses in OECs. This transmission of Ca²⁺ responses from astrocytes to OECs persisted in the presence of neuronal inhibition, but was absent when gap junctional coupling was suppressed with carbenoxolone. When Ca²⁺ transients were directly evoked in OECs by puff application of DHPG, they resulted in delayed Ca²⁺ responses in juxtaglomerular astrocytes, indicating that panglial transmission of Ca²⁺ signals occurred in a bidirectional manner. In addition, panglial transmission of Ca²⁺ signals from astrocytes to OECs resulted in vasoconstriction of OEC-associated blood vessels in the olfactory nerve layer. Our results demonstrate functional transmission of Ca²⁺ signals between different classes of glial cells within gap junction-coupled panglial networks and the resulting regulation of blood vessel diameter in the olfactory bulb.

Connexin and Pannexin-Based Channels in Oligodendrocytes: Implications in Brain Health and Disease

Oligodendrocytes are the myelin forming cells in the central nervous system (CNS). In addition to this main physiological function, these cells play key roles by providing energy substrates to neurons as well as information required to sustain proper synaptic transmission and plasticity at the CNS. The latter requires a fine coordinated intercellular communication with neurons and other glial cell types, including astrocytes. In mammals, tissue synchronization is mainly mediated by connexins and pannexins, two protein families that underpin the communication among neighboring cells through the formation of different plasma membrane channels. At one end, gap junction channels (GJCs; which are exclusively formed by connexins in vertebrates) connect the cytoplasm of contacting cells allowing electrical and metabolic coupling. At the other end, hemichannels and pannexons (which are formed by connexins and pannexins, respectively) communicate the intra- and extracellular compartments, serving as diffusion pathways of ions and small molecules. Here, we briefly review the current knowledge about the expression and function of hemichannels, pannexons and GJCs in oligodendrocytes, as well as the evidence regarding the possible role of these channels in metabolic and synaptic functions at the CNS. In particular, we focus on oligodendrocyte-astrocyte coupling during axon metabolic support and its implications in brain health and disease.

Atypical Auditory Brainstem Response and Protein Expression Aberrations Related to ASD and Hearing Loss in the Adnp Haploinsufficient Mouse Brain

Autism is a wide spread neurodevelopmental disorder with growing morbidity rates, affecting more boys than girls worldwide. Activity-dependent neuroprotective protein (ADNP) was recently recognized as a leading gene accounted for 0.17% of autism spectrum disorder (ASD) cases globally. Respectively, mutations in the human ADNP gene (ADNP syndrome), cause multi-system body dysfunctions with apparent ASD-related traits, commencing as early as childhood. The Adnp haploinsufficient (Adnp^+/-) mouse model was researched before in relations to Alzheimer's disease and autism. Adnp^+/- mice suffer from deficient social memory, vocal and motor impediments, irregular tooth eruption and short stature, all of which corresponds with reported phenotypes in patients with the ADNP syndrome. Recently, a more elaborated description of the ADNP syndrome was published, presenting impediments such as hearing disabilities in > 10% of the studied children. Irregular auditory brainstem response (ABR) has been connected to ASD-related cases and has been suggested as a potential hallmark for autism, allowing diagnosis of ASD risk and early intervention. Herein, we present detriment hearing in the Adnp^+/- mice with atypical ABR and significant protein expression irregularities that coincides with ASD and hearing loss studies in the brain.

Chronic Unpredictable Stress Reduces Immunostaining for Connexins 43 and 30 and Myelin Basic Protein in the Rat Prelimbic and Orbitofrontal Cortices

Background

Astrocytes and oligodendrocytes are pathologically altered in dorsolateral prefrontal and orbitofrontal cortices in major depressive disorder. In rat models of stress (major depressive disorder risk factor) astrocyte gap junction protein connexin 43 (Cx43) is reduced in the prelimbic cortex. Astrocyte connexins are recognized to strongly influence myelin maintenance in the central nervous system. However, it is unknown whether stress-related changes in Cx43 and the other major astrocyte connexin, Cx30, occur in the orbitofrontal cortex, or whether connexin changes are concurrent with disturbances in myelination.

Methods

Frozen sections containing prelimbic cortex and orbitofrontal cortex of rats subjected to 35 days of chronic unpredictable stress and controls (n = 6/group) were immunolabeled for Cx43, Cx30, and myelin basic protein. Density of Cx43 or Cx30 immunoreactive puncta and area fraction of myelin basic protein immunoreactivity were measured in prelimbic cortex and orbitofrontal cortex and results analyzed with t test or Pearson correlations.

Results

Density of Cx43- and Cx30-positive puncta in both prelimbic cortex and orbitofrontal cortex was lower in chronic unpredictable stress-treated than in control rats. In both regions, the area fraction of myelin basic protein immunoreactivity was also lower in chronic unpredictable stress animals. Myelin basic protein area fraction was positively correlated with the density of Cx43-positive puncta in orbitofrontal cortex, and with Cx30 puncta in prelimbic cortex.

Conclusion

Low Cx43 and Cx30 after chronic unpredictable stress in rat prelimbic cortex and orbitofrontal cortex suggests that reduced astrocytic gap junction density may generalize to the entire prefrontal cortex. Concurrent reduction of Cx43-, Cx30-, and myelin basic protein-immunolabeled structures is consistent with a mechanism linking changes in astrocyte gap junction proteins and disturbed myelin morphology in depression.

Conditioned Media From Glial Cells Promote a Neural-Like Connexin Expression in Human Adipose-Derived Mesenchymal Stem Cells

The expression of neuronal and glial connexins (Cxs) has been evaluated in adipose-derived mesenchymal stem cells (ASCs) whose neural differentiation was promoted by a conditioned medium (CM) obtained from cultures of olfactory ensheathing cells (OECs) or Schwann cells (SCs). By immunocytochemistry and flow cytometer analysis it was found that Cx43 was already considerably expressed in naïve ASCs and further increased after 24 h and 7 days from CM exposition. Cx32 and Cx36 were significantly improved in conditioned cultures compared to control ASCs, whereas a decreased expression was noticed in the absence of CM treatments. Cx47 was virtually absent in any conditions. Altogether, high basal levels and induced increases of Cx43 expression suggest a potential attitude of ASCs toward an astrocyte differentiation, whereas the lack of Cx47 would indicate a poor propensity of ASCs to become oligodendrocytes. CM-evoked Cx32 and Cx36 increases showed that a neuronal- or a SC-like differentiation can be promoted by using this strategy. Results further confirm that environmental cues can favor an ASC neural differentiation, either as neuronal or glial elements. Of note, the use of glial products present in CM rather than the addition of chemical agents to achieve such differentiation would resemble "more physiological" conditions of differentiation. As a conclusion, the overexpression of typical neural Cxs would indicate the potential capability of neural-like ASCs to interact with neighboring neural cells and microenvironment.

Astrocytes integrate and drive action potential firing in inhibitory subnetworks

Many brain functions depend on the ability of neural networks to temporally integrate transient inputs to produce sustained discharges. This can occur through cell-autonomous mechanisms in individual neurons and through reverberating activity in recurrently connected neural networks. We report a third mechanism involving temporal integration of neural activity by a network of astrocytes. Previously, we showed that some types of interneurons can generate long-lasting trains of action potentials (barrage firing) following repeated depolarizing stimuli. Here we show that calcium signaling in an astrocytic network correlates with barrage firing; that active depolarization of astrocyte networks by chemical or optogenetic stimulation enhances; and that chelating internal calcium, inhibiting release from internal stores, or inhibiting GABA transporters or metabotropic glutamate receptors inhibits barrage firing. Thus, networks of astrocytes influence the spatiotemporal dynamics of neural networks by directly integrating neural activity and driving barrages of action potentials in some populations of inhibitory interneurons.

Specific types of inhibitory neurons exhibit prolonged, high-frequency barrages of action potentials. Here, the authors show that astrocytes might mediate such barrage firing.

Influence of the extracellular matrix on water mobility in subcortical gray matter

PURPOSE

Water mobility in tissues is related to the microstructure that modulates diffusion and spin relaxation. Previous work has shown that the extracellular matrix (ECM) impacts water diffusion in cartilage. To investigate if similar contributions to image contrast exist for brain, which is characterized by a substantially lower ECM content, diffusion and relaxation were studied in fixed samples from goat and human thalamus before and after enzymatic digestion of ECM compounds. Selected experiments in human corpus callosum were included for comparing subcortical gray matter and white matter.

METHODS

Digestion of matrix components was achieved by treatment with hyaluronidase. Nonlocalized pulsed field gradient measurements were performed with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>b</mml:mi></mml:math> values between 0.6 and 18,000 s/mm² at 3T and temperatures between 0°C and 20°C, in addition to T₁ and T₂ relaxation measurements. The data were fitted to multiexponential models to account for different water compartments. After the measurements, the samples were sliced and stained for ECM-sensitive markers to verify efficient digestion.

RESULTS

Microstructural alterations associated with hyaluronan digestion did not lead to measurable effects on water diffusion or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:math> . However, T₁ of the main relaxographic component, attributed to intra-/extracellular water, decreased by 7%.

CONCLUSION

Investigations with very strong gradients did not reveal a detectable effect on water diffusion or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:math> after hyaluronan removal, indicating that the brain ECM content is too low to produce a detectable effect. The subtle alteration of T₁ upon hyaluronidase treatment might reflect a modulation of intercompartmental water exchange properties.

Gap Junctions in the Nervous System: Probing Functional Connections Using New Imaging Approaches

Gap junctions are channels that physically connect adjacent cells, mediating the rapid exchange of small molecules, and playing an essential role in a wide range of physiological processes in nearly every system in the body, including the nervous system. Thus, altered function of gap junctions has been linked with a plethora of diseases and pathological conditions. Being able to measure and characterize the distribution, function, and regulation of gap junctions in intact tissue is therefore essential for understanding the physiological and pathophysiological roles that gap junctions play. In recent decades, several robust in vitro and in vivo methods have been developed for detecting and characterizing gap junctions. Here, we review the currently available methods with respect to invasiveness, signal-to-noise ratio, temporal resolution and others, highlighting the recently developed chemical tracers and hybrid imaging systems that use novel chemical compounds and/or genetically encoded enzymes, transporters, channels, and fluorescent proteins in order to map gap junctions. Finally, we discuss possible avenues for further improving existing techniques in order to achieve highly sensitive, cell type-specific, non-invasive measures of in vivo gap junction function with high throughput and high spatiotemporal resolution.

Effects of glucocorticoids in depression: Role of astrocytes

Astrocytes or astroglia are heterogeneous cells, similar to neurons, that have different properties in different brain regions. The implications of steroid hormones on glial cells and stress-related pathologies have been studied previously. Glucocorticoids (GCs) that are released in response to stress have been shown to be deleterious to neurons in various brain regions. Further, in the light of the effect of GCs on astrocytes, several reports have shown the crucial role of glia. Still, much remains to be done to understand the stress-astrocytes-glucocorticoid interactions associated with the pathological consequences of various CNS disorders. This review is an attempt to summarize the effects of GCs and stress on astrocytes and its implications in depression.

Altered gene transcription linked to astrocytes and oligodendrocytes in frontal cortex in Creutzfeldt-Jakob disease

Targeted expression of genes coding for proteins specific to astrocytes, oligodendrocytes and myelin was performed in frontal cortex area 8 of Creutzfeldt-Jakob disease methionine/methionine and valine/valine (CJD MM1 and VV2, respectively) compared with controls. GFAP (glial fibrillary acidic protein) mRNA was up-regulated whereas SLC1A2 (solute carrier family 1 member 2, coding for glutamate transporter 1: GLT1), AQ4 (aquaporin 4), MPC1 (mitochondrial pyruvate carrier 1) and UCP5 (mitochondrial uncoupled protein 5) mRNAs were significantly down-regulated in CJD MM1 and CJD VV2, and GJA1 (connexin 43) in CJD VV2. OLIG1 and OLIG2 (oligodendocyte transcription factor 1 and 2, respectively), SOX10 (SRY-Box10) and oligodendroglial precursor cell (OPC) marker NG2 (neuronal/glial antigen) 2 were preserved, but GALC (coding for galactosylceramidase), SLC2A1 (solute carrier family 2 member 1: glucose transporter member 1: GLUT1) and MCT1 (monocarboxylic acid transporter 1) mRNA expression levels were significantly reduced in CJD MM1 and CJD VV2. Expression levels of most genes linked to myelin were not altered in the cerebral cortex in CJD. Immunohistochemistry to selected proteins disclosed individual variations but GFAP, Olig-2, AQ4 and GLUT1 correlated with mRNA levels, whereas GLT1 was subjected to individual variations. However, MPC1, UCP5 and MCT1 decrease was more closely related to the respective reduced neuronal immunostaining. These observations support the idea that molecular deficits linked to energy metabolism and solute transport in astrocytes and oligodendrocytes, in addition to neurons, are relevant in the pathogenesis of cortical lesions in CJD.

Astrocyte pathology in the ventral prefrontal white matter in depression

Astrocyte functions in white matter are less well understood than in gray matter. Our recent study of white matter in ventral prefrontal cortex (vPFC) revealed alterations in expression of myelin-related genes in major depressive disorder (MDD). Since white matter astrocytes maintain myelin, we hypothesized that morphometry of these cells will be altered in MDD in the same prefrontal white matter region in which myelin-related genes are altered. White matter adjacent to vPFC was examined in 25 MDD and 21 control subjects. Density and size of GFAP-immunoreactive (-ir) astrocyte cell bodies was measured. The area fraction of GFAP-ir astrocytes (cell bodies + processes) was also estimated. GFAP mRNA expression was determined using qRT-PCR. The density of GFAP-ir astrocytes was also measured in vPFC white matter of rats subjected to chronic unpredictable stress (CUS) and control animals. Fibrous and smooth GFAP-ir astrocytes were distinguished in human white matter. The density of both types of astrocytes was significantly decreased in MDD. Area fraction of GFAP immunoreactivity was significantly decreased in MDD, but mean soma size remained unchanged. Expression of GFAP mRNA was significantly decreased in MDD. In CUS rats there was a significant decrease in astrocyte density in prefrontal white matter. The decrease in density and area fraction of white matter astrocytes and GFAP mRNA in MDD may be linked to myelin pathology previously noted in these subjects. Astrocyte pathology may contribute to axon disturbances in axon integrity reported by neuroimaging studies in MDD and interfere with signal conduction in the white matter.