Tamoxifen-inducible glia-specific Cre mice for somatic mutagenesis in oligodendrocytes and Schwann cells

The main duct of von Ebner's glands is a source of Sox10 + taste bud progenitors and susceptible to pathogen infections

We have recently demonstrated that Sox10 -expressing ( Sox10 + ) cells give rise to mainly type-III neuronal taste bud cells that are responsible for sour and salt taste. The two tissue compartments containing Sox10 + cells in the surrounding of taste buds include the connective tissue core of taste papillae and von Ebner's glands (vEGs) that are connected to the trench of circumvallate and foliate papillae. In this study, we used inducible Cre mouse models to map the cell lineages of connective tissue (including stromal and Schwann cells) and vEGs and performed single cell RNA-sequencing of the epithelium of Sox10-Cre/tdT mouse circumvallate/vEG complex. In vivo lineage mapping showed that the distribution of traced cells in circumvallate taste buds was closely linked with that in the vEGs, but not in the connective tissue. Sox10 , but not the known stem cells marker Lgr5 , expression was enriched in the cell clusters of main ducts of vEGs that contained abundant proliferating cells, while Sox10-Cre/tdT expression was enriched in type-III taste bud cells and excretory ductal cells. Moreover, multiple genes encoding pathogen receptors are enriched in the vEG main ducts. Our data indicate that the main duct of vEGs is a source of Sox10 + taste bud progenitors and susceptible to pathogen infections.

Small-molecule-induced epigenetic rejuvenation promotes SREBP condensation and overcomes barriers to CNS myelin regeneration

Remyelination failure in diseases like multiple sclerosis (MS) was thought to involve suppressed maturation of oligodendrocyte precursors; however, oligodendrocytes are present in MS lesions yet lack myelin production. We found that oligodendrocytes in the lesions are epigenetically silenced. Developing a transgenic reporter labeling differentiated oligodendrocytes for phenotypic screening, we identified a small-molecule epigenetic-silencing-inhibitor (ESI1) that enhances myelin production and ensheathment. ESI1 promotes remyelination in animal models of demyelination and enables de novo myelinogenesis on regenerated CNS axons. ESI1 treatment lengthened myelin sheaths in human iPSC-derived organoids and augmented (re)myelination in aged mice while reversing age-related cognitive decline. Multi-omics revealed that ESI1 induces an active chromatin landscape that activates myelinogenic pathways and reprograms metabolism. Notably, ESI1 triggered nuclear condensate formation of master lipid-metabolic regulators SREBP1/2, concentrating transcriptional co-activators to drive lipid/cholesterol biosynthesis. Our study highlights the potential of targeting epigenetic silencing to enable CNS myelin regeneration in demyelinating diseases and aging.

Downregulated expression of lactate dehydrogenase in adult oligodendrocytes and its implication for the transfer of glycolysis products to axons

Oligodendrocytes and astrocytes are metabolically coupled to neuronal compartments. Pyruvate and lactate can shuttle between glial cells and axons via monocarboxylate transporters. However, lactate can only be synthesized or used in metabolic reactions with the help of lactate dehydrogenase (LDH), a tetramer of LDHA and LDHB subunits in varying compositions. Here we show that mice with a cell type-specific disruption of both Ldha and Ldhb genes in oligodendrocytes lack a pathological phenotype that would be indicative of oligodendroglial dysfunctions or lack of axonal metabolic support. Indeed, when combining immunohistochemical, electron microscopical, and in situ hybridization analyses in adult mice, we found that the vast majority of mature oligodendrocytes lack detectable expression of LDH. Even in neurodegenerative disease models and in mice under metabolic stress LDH was not increased. In contrast, at early development and in the remyelinating brain, LDHA was readily detectable in immature oligodendrocytes. Interestingly, by immunoelectron microscopy LDHA was particularly enriched at gap junctions formed between adjacent astrocytes and at junctions between astrocytes and oligodendrocytes. Our data suggest that oligodendrocytes metabolize lactate during development and remyelination. In contrast, for metabolic support of axons mature oligodendrocytes may export their own glycolysis products as pyruvate rather than lactate. Lacking LDH, these oligodendrocytes can also "funnel" lactate through their "myelinic" channels between gap junction-coupled astrocytes and axons without metabolizing it. We suggest a working model, in which the unequal cellular distribution of LDH in white matter tracts facilitates a rapid and efficient transport of glycolysis products among glial and axonal compartments.

The level of protein in the maternal murine diet modulates the facial appearance of the offspring via mTORC1 signaling

The development of craniofacial skeletal structures is fascinatingly complex and elucidation of the underlying mechanisms will not only provide novel scientific insights, but also help develop more effective clinical approaches to the treatment and/or prevention of the numerous congenital craniofacial malformations. To this end, we performed a genome-wide analysis of RNA transcription from non-coding regulatory elements by CAGE-sequencing of the facial mesenchyme of human embryos and cross-checked the active enhancers thus identified against genes, identified by GWAS for the normal range human facial appearance. Among the identified active cis-enhancers, several belonged to the components of the PI3/AKT/mTORC1/autophagy pathway. To assess the functional role of this pathway, we manipulated it both genetically and pharmacologically in mice and zebrafish. These experiments revealed that mTORC1 signaling modulates craniofacial shaping at the stage of skeletal mesenchymal condensations, with subsequent fine-tuning during clonal intercalation. This ability of mTORC1 pathway to modulate facial shaping, along with its evolutionary conservation and ability to sense external stimuli, in particular dietary amino acids, indicate that the mTORC1 pathway may play a role in facial phenotypic plasticity. Indeed, the level of protein in the diet of pregnant female mice influenced the activity of mTORC1 in fetal craniofacial structures and altered the size of skeletogenic clones, thus exerting an impact on the local geometry and craniofacial shaping. Overall, our findings indicate that the mTORC1 signaling pathway is involved in the effect of environmental conditions on the shaping of craniofacial structures.

Myelin-Specific microRNA-23a/b Cluster Deletion Inhibits Myelination in the Central Nervous System during Postnatal Growth and Aging

Microribonucleic acids (miRNAs) comprising miR-23a/b clusters, specifically miR-23a and miR-27a, are recognized for their divergent roles in myelination within the central nervous system. However, cluster-specific miRNA functions remain controversial as miRNAs within the same cluster have been suggested to function complementarily. This study aims to clarify the role of miR-23a/b clusters in myelination using mice with a miR-23a/b cluster deletion (KO mice), specifically in myelin expressing proteolipid protein (PLP). Inducible conditional KO mice were generated by crossing miR-23a/b clusterflox/flox mice with PlpCre-ERT2 mice; the offspring were injected with tamoxifen at 10 days or 10 weeks of age to induce a myelin-specific miR-23a/b cluster deletion. Evaluation was performed at 10 weeks or 12 months of age and compared with control mice that were not treated with tamoxifen. KO mice exhibit impaired motor function and hypoplastic myelin sheaths in the brain and spinal cord at 10 weeks and 12 months of age. Simultaneously, significant decreases in myelin basic protein (MBP) and PLP expression occur in KO mice. The percentages of oligodendrocyte precursors and mature oligodendrocytes are consistent between the KO and control mice. However, the proportion of oligodendrocytes expressing MBP is significantly lower in KO mice. Moreover, changes in protein expression occur in KO mice, with increased leucine zipper-like transcriptional regulator 1 expression, decreased R-RAS expression, and decreased phosphorylation of extracellular signal-regulated kinases. These findings highlight the significant influence of miR-23a/b clusters on myelination during postnatal growth and aging.

TEAD1 is crucial for developmental myelination, Remak bundles, and functional regeneration of peripheral nerves

Previously we showed that the hippo pathway transcriptional effectors, YAP and TAZ, are essential for Schwann cells (SCs) to develop, maintain and regenerate myelin . Although TEAD1 has been implicated as a partner transcription factor, the mechanisms by which it mediates YAP/TAZ regulation of SC myelination are unclear. Here, using conditional and inducible knockout mice, we show that TEAD1 is crucial for SCs to develop and regenerate myelin. It promotes myelination by both positively and negatively regulating SC proliferation, enabling Krox20/Egr2 to upregulate myelin proteins, and upregulating the cholesterol biosynthetic enzymes FDPS and IDI1. We also show stage-dependent redundancy of TEAD1 and that non-myelinating SCs have a unique requirement for TEAD1 to enwrap nociceptive axons in Remak bundles. Our findings establish TEAD1 as a major partner of YAP/TAZ in developmental myelination and functional nerve regeneration and as a novel transcription factor regulating Remak bundle integrity.

TEAD1 is crucial for developmental myelination, Remak bundles, and functional regeneration of peripheral nerves

Previously we showed that the hippo pathway transcriptional effectors, YAP and TAZ, are essential for Schwann cells (SCs) to develop, maintain and regenerate myelin (Grove et al., 2017; Grove, Lee, Zhao, & Son, 2020). Although TEAD1 has been implicated as a partner transcription factor, the mechanisms by which it mediates YAP/TAZ regulation of SC myelination are unclear. Here, using conditional and inducible knockout mice, we show that TEAD1 is crucial for SCs to develop and regenerate myelin. It promotes myelination by both positively and negatively regulating SC proliferation, enabling Krox20/Egr2 to upregulate myelin proteins, and upregulating the cholesterol biosynthetic enzymes FDPS and IDI1. We also show stage-dependent redundancy of TEAD1 and that non-myelinating SCs have a unique requirement for TEAD1 to enwrap nociceptive axons in Remak bundles. Our findings establish TEAD1 as a major partner of YAP/TAZ in developmental myelination and functional nerve regeneration and as a novel transcription factor regulating Remak bundle integrity.

Dock1 acts cell-autonomously in Schwann cells to regulate the development, maintenance, and repair of peripheral myelin

Schwann cells, the myelinating glia of the peripheral nervous system (PNS), are critical for myelin development, maintenance, and repair. Rac1 is a known regulator of radial sorting, a key step in developmental myelination, and we previously showed in zebrafish that loss of Dock1, a Rac1-specific guanine nucleotide exchange factor, results in delayed peripheral myelination in development. We demonstrate here that Dock1 is necessary for myelin maintenance and remyelination after injury in adult zebrafish. Furthermore, it performs an evolutionary conserved role in mice, acting cell-autonomously in Schwann cells to regulate peripheral myelin development, maintenance, and repair. Additionally, manipulating Rac1 levels in larval zebrafish reveals that dock1 mutants are sensitized to inhibition of Rac1, suggesting an interaction between the two proteins during PNS development. We propose that the interplay between Dock1 and Rac1 signaling in Schwann cells is required to establish, maintain, and facilitate repair and remyelination within the peripheral nervous system.

A RhoA-mediated biomechanical response in Schwann cells modulates peripheral nerve myelination

Myelin improves axonal conduction velocity and is essential for nerve development and regeneration. In peripheral nerves, Schwann cells depend on bidirectional mechanical and biochemical signaling to form the myelin sheath but the mechanism underlying this process is not understood. Rho GTPases are integrators of "outside-in" signaling that link cytoskeletal dynamics with cellular architecture to regulate morphology and adhesion. Using Schwann cell-specific gene inactivation in the mouse, we discovered that RhoA promotes the initiation of myelination, and is required to both drive and terminate myelin growth at different stages of peripheral myelination, suggesting developmentally-specific modes of action. In Schwann cells, RhoA targets actin filament turnover, via Cofilin 1, actomyosin contractility and cortical actin-membrane attachments. This mechanism couples actin cortex mechanics with the molecular organization of the cell boundary to target specific signaling networks that regulate axon-Schwann cell interaction/adhesion and myelin growth. This work shows that RhoA is a key component of a biomechanical response required to control Schwann cell state transitions for proper myelination of peripheral nerves.

In the mouse cortex, oligodendrocytes regain a plastic capacity, transforming into astrocytes after acute injury

Acute brain injuries evoke various response cascades directing the formation of the glial scar. Here, we report that acute lesions associated with hemorrhagic injuries trigger a re-programming of oligodendrocytes. Single-cell RNA sequencing highlighted a subpopulation of oligodendrocytes activating astroglial genes after acute brain injuries. By using PLP-DsRed1/GFAP-EGFP and PLP-EGFP_mem/GFAP-mRFP1 transgenic mice, we visualized this population of oligodendrocytes that we termed AO cells based on their concomitant activity of astro- and oligodendroglial genes. By fate mapping using PLP- and GFAP-split Cre complementation and repeated chronic in vivo imaging with two-photon laser-scanning microscopy, we observed the conversion of oligodendrocytes into astrocytes via the AO cell stage. Such conversion was promoted by local injection of IL-6 and was diminished by IL-6 receptor-neutralizing antibody as well as by inhibiting microglial activation with minocycline. In summary, our findings highlight the plastic potential of oligodendrocytes in acute brain trauma due to microglia-derived IL-6.

NF-κB is a critical mediator of post-mitotic senescence in oligodendrocytes and subsequent white matter loss

BACKGROUND

Inflammaging represents an accepted concept where the immune system shifts to a low-grade chronic pro-inflammatory state without overt infection upon aging. In the CNS, inflammaging is mainly driven by glia cells and associated with neurodegenerative processes. White matter degeneration (WMD), a well-known process in the aging brain, manifests in myelin loss finally resulting in motor, sensory and cognitive impairments. Oligodendrocytes (OL) are responsible for homeostasis and maintenance of the myelin sheaths, which is a complex and highly energy demanding process sensitizing these cells to metabolic, oxidative and other forms of stress. Yet, the immediate impact of chronic inflammatory stress like inflammaging on OL homeostasis, myelin maintenance and WMD remains open.

METHODS

To functionally analyze the role of IKK/NF-κB signaling in the regulation of myelin homeostasis and maintenance in the adult CNS, we established a conditional mouse model allowing NF-κB activation in mature myelinating oligodendrocytes. IKK2-CA^PLP-CreERT2 mice were characterized by biochemical, immunohistochemical, ultrastructural and behavioral analyses. Transcriptome data from isolated, primary OLs and microglia cells were explored by in silico pathway analysis and validated by complementary molecular approaches.

RESULTS

Chronic NF-κB activation in mature OLs leads to aggravated neuroinflammatory conditions phenocopying brain inflammaging. As a consequence, IKK2-CA^PLP-CreERT2 mice showed specific neurological deficits and impaired motoric learning. Upon aging, persistent NF-κB signaling promotes WMD in these mice as ultrastructural analysis revealed myelination deficits in the corpus callosum accompanied by impaired myelin protein expression. RNA-Seq analysis of primary oligodendrocytes and microglia cells uncovers gene expression signatures associated with activated stress responses and increased post mitotic cellular senescence (PoMiCS) which was confirmed by elevated senescence-associated β-galactosidase activity and SASP gene expression profile. We identified an elevated integrated stress response (ISR) characterized by phosphorylation of eIF2α as a relevant molecular mechanism which is able to affect translation of myelin proteins.

CONCLUSIONS

Our findings demonstrate an essential role of IKK/NF-κB signaling in mature, post-mitotic OLs in regulating stress-induced senescence in these cells. Moreover, our study identifies PoMICS as an important driving force of age-dependent WMD as well as of traumatic brain injury induced myelin defects.

SARM1 detection in myelinating glia: sarm1/Sarm1 is dispensable for PNS and CNS myelination in zebrafish and mice

Since SARM1 mutations have been identified in human neurological disease, SARM1 inhibition has become an attractive therapeutic strategy to preserve axons in a variety of disorders of the peripheral (PNS) and central nervous system (CNS). While SARM1 has been extensively studied in neurons, it remains unknown whether SARM1 is present and functional in myelinating glia? This is an important question to address. Firstly, to identify whether SARM1 dysfunction in other cell types in the nervous system may contribute to neuropathology in SARM1 dependent diseases? Secondly, to ascertain whether therapies altering SARM1 function may have unintended deleterious impacts on PNS or CNS myelination? Surprisingly, we find that oligodendrocytes express sarm1 mRNA in the zebrafish spinal cord and that SARM1 protein is readily detectable in rodent oligodendrocytes in vitro and in vivo. Furthermore, activation of endogenous SARM1 in cultured oligodendrocytes induces rapid cell death. In contrast, in peripheral glia, SARM1 protein is not detectable in Schwann cells and satellite glia in vivo and sarm1/Sarm1 mRNA is detected at very low levels in Schwann cells, in vivo, in zebrafish and mouse. Application of specific SARM1 activators to cultured mouse Schwann cells does not induce cell death and nicotinamide adenine dinucleotide (NAD) levels remain unaltered suggesting Schwann cells likely contain no functionally relevant levels of SARM1. Finally, we address the question of whether SARM1 is required for myelination or myelin maintenance. In the zebrafish and mouse PNS and CNS, we show that SARM1 is not required for initiation of myelination and myelin sheath maintenance is unaffected in the adult mouse nervous system. Thus, strategies to inhibit SARM1 function to treat neurological disease are unlikely to perturb myelination in humans.

Kir4.1 is specifically expressed and active in non-myelinating Schwann cells

Non-myelinating Schwann cells (NMSC) play important roles in peripheral nervous system formation and function. However, the molecular identity of these cells remains poorly defined. We provide evidence that Kir4.1, an inward-rectifying K+ channel encoded by the KCNJ10 gene, is specifically expressed and active in NMSC. Immunostaining revealed that Kir4.1 is present in terminal/perisynaptic SCs (TPSC), synaptic glia at neuromuscular junctions (NMJ), but not in myelinating SCs (MSC) of adult mice. To further examine the expression pattern of Kir4.1, we generated BAC transgenic Kir4.1-CreERT2 mice and crossed them to the tdTomato reporter line. Activation of CreERT2 with tamoxifen after the completion of myelination onset led to robust expression of tdTomato in NMSC, including Remak Schwann cells (RSC) along peripheral nerves and TPSC, but not in MSC. In contrast, activating CreERT2 before and during the onset of myelination led to tdTomato expression in NMSC and MSC. These observations suggest that immature SC express Kir4.1, and its expression is then downregulated selectively in myelin-forming SC. In support, we found that while activating CreERT2 induces tdTomato expression in immature SC, it fails to induce tdTomato in MSC associated with sensory axons in culture. NMSC derived from neonatal sciatic nerve were shown to express Kir4.1 and exhibit barium-sensitive inwardly rectifying macroscopic K+ currents. Thus, this study identified Kir4.1 as a potential modulator of immature SC and NMSC function. Additionally, it established a novel transgenic mouse line to introduce or delete genes in NMSC.

Celsr2 Knockout Alleviates Inhibitory Synaptic Stripping and Benefits Motoneuron Survival and Axon Regeneration After Branchial Plexus Avulsion

Axotomy-induced synaptic stripping modulates survival and axon regeneration of injured motoneurons. Celsr2 is supposed to mediate homophilic interactions of neighboring cells during development, and its role in synaptic stripping remains unknow. In a model of brachial plexus avulsion, Celsr2 knockout improved functional recovery, motoneuron survival, and axon regeneration. Celsr2 was indicated to express in spinal motoneurons, excitatory and inhibitory interneurons, astrocytes, and a subset of oligodendrocytes using Celsr2^LacZ mice. Double immunostaining showed that the coverage of inhibitory and excitatory vesicles on injured motoneurons were remarkably reduced after injury, whereas more inhibitory vesicles were maintained in Celsr2^-/- mutants than control mice. In the ultrastructure, the density of inhibitory F-boutons on injured motoneurons was higher in Celsr2^-/- mutants than controls. Conditional knockout of Celsr2 in astrocytes or oligodendrocytes showed the similar axotomy-induced synaptic withdrawal to the control. RNAseq of injured spinal samples identified 12 MHC I molecules with significant changes between Celsr2^-/- and control mice. After injury, expression of MHC I surrounding injured motoneurons was increased, particularly high in Celsr2^-/- mutants. In conclusion, Celsr2 knockout enhances MHC I signaling, alleviates inhibitory synaptic stripping cell-autonomously, and contributes to motoneuron survival and regeneration, and Celsr2 is a potential target for neural repair.

Schwann cell‐specific RhoA knockout accelerates peripheral nerve regeneration via promoting Schwann cell dedifferentiation

Our previous studies indicated that RhoA knockdown or inhibition could alleviate the proliferation, migration, and differentiation of Schwann cells. However, the role of RhoA in Schwann cells during nerve injury and repair is still unknown. Herein, we developed two lines of Schwann cells conditional RhoA knockout (cKO) mice by breeding RhoA^{flox / flox} mice with Plp^{Cre -ERT2} or Dhh^Cre mice. Our results indicate that RhoA cKO in Schwann cells accelerates axonal regrowth and remyelination after sciatic nerve injury, which enhances the recovery of nerve conduction and hindlimb gait, and alleviates the amyotrophy in gastrocnemius muscle. Mechanistic studies in both in vivo and in vitro models revealed that RhoA cKO could facilitate Schwann cell dedifferentiation via JNK pathway. Schwann cell dedifferentiation subsequently promotes Wallerian degeneration by enhancing phagocytosis and myelinophagy, as well as stimulating the production of neurotrophins (NT-3, NGF, BDNF, and GDNF). These findings shed light on the role of RhoA in Schwann cells during nerve injury and repair, indicating that cell type-specific RhoA targeting could serve as a promising molecular therapeutic strategy for peripheral nerve injury.

Melanoma-associated repair-like Schwann cells suppress anti-tumor T-cells via 12/15-LOX/COX2-associated eicosanoid production

Peripheral glia, specifically the Schwann cells (SCs), have been implicated in the formation of the tumor microenvironment (TME) and in cancer progression. However, in vivo and ex vivo analyses of how cancers reprogram SC functions in different organs of tumor-bearing mice are lacking. We generated Plp1-CreERT/tdTomato mice which harbor fluorescently labeled myelinated and non-myelin forming SCs. We show that this model enables the isolation of the SCs with high purity from the skin and multiple other organs. We used this model to study phenotypic and functional reprogramming of the SCs in the skin adjacent to melanoma tumors. Transcriptomic analyses of the peritumoral skin SCs versus skin SCs from tumor-free mice revealed that the former existed in a repair-like state typically activated during nerve and tissue injury. Peritumoral skin SCs also downregulated pro-inflammatory genes and pathways related to protective anti-tumor responses. In vivo and ex vivo functional assays confirmed immunosuppressive activities of the peritumoral skin SCs. Specifically, melanoma-reprogrammed SCs upregulated 12/15-lipoxygenase (12/15-LOX) and cyclooxygenase (COX)-2, and increased production of anti-inflammatory polyunsaturated fatty acid (PUFA) metabolites prostaglandin E2 (PGE2) and lipoxins A4/B4. Inhibition of 12/15-LOX or COX2 in SCs, or EP4 receptor on lymphocytes reversed SC-dependent suppression of anti-tumor T-cell activation. Therefore, SCs within the skin adjacent to melanoma tumors demonstrate functional switching to repair-like immunosuppressive cells with dysregulated lipid oxidation. Our study suggests the involvement of the melanoma-associated repair-like peritumoral SCs in the modulation of locoregional and systemic anti-tumor immune responses.

Autophagic degradation of CNS myelin maintains axon integrity

(Macro)autophagy is a major lysosome-dependent degradation mechanism which engulfs, removes and recycles unwanted cytoplasmic material, including damaged organelles and toxic protein aggregates. Although a few studies implicate autophagy in CNS demyelinating pathologies, its role, particularly in mature oligodendrocytes and CNS myelin, remains poorly studied. Here, using both pharmacological and genetic inhibition of the autophagic machinery, we provide evidence that autophagy is an essential mechanism for oligodendrocyte maturation in vitro. Our study reveals that two core myelin proteins, namely proteolipid protein (PLP) and myelin basic protein (MBP) are incorporated into autophagosomes in oligodendrocytes, resulting in their degradation. Furthermore, we ablated atg5, a core gene of the autophagic machinery, specifically in myelinating glial cells in vivo by tamoxifen administration (plp-Cre ERT2 ; atg5 f/f ) and showed that myelin maintenance is perturbed, leading to PLP accumulation. Significant morphological defects in myelin membrane such as decompaction accompanied with increased axonal degeneration are observed. As a result, the mice exhibit behavioral deficits. In summary, our data highlight that the maintenance of adult myelin homeostasis in the CNS requires the involvement of a fully functional autophagic machinery.

Olig2 Ablation in Immature Oligodendrocytes Does Not Enhance CNS Myelination and Remyelination

The oligodendrocyte (OL) lineage transcription factor Olig2 is expressed throughout oligodendroglial development and is essential for oligodendroglial progenitor specification and differentiation. It was previously reported that deletion of Olig2 enhanced the maturation and myelination of immature OLs and accelerated the remyelination process. However, by analyzing multiple Olig2 conditional KO mouse lines (male and female), we conclude that Olig2 has the opposite effect and is required for OL maturation and remyelination. We found that deletion of Olig2 in immature OLs driven by an immature OL-expressing Plp1 promoter resulted in defects in OL maturation and myelination, and did not enhance remyelination after demyelination. Similarly, Olig2 deletion during premyelinating stages in immature OLs using Mobp or Mog promoter-driven Cre lines also did not enhance OL maturation in the CNS. Further, we found that Olig2 was not required for myelin maintenance in mature OLs but was critical for remyelination after lysolecithin-induced demyelinating injury. Analysis of genomic occupancy in immature and mature OLs revealed that Olig2 targets the enhancers of key myelination-related genes for OL maturation from immature OLs. Together, by leveraging multiple immature OL-expressing Cre lines, these studies indicate that Olig2 is essential for differentiation and myelination of immature OLs and myelin repair. Our findings raise fundamental questions about the previously proposed role of Olig2 in opposing OL myelination and highlight the importance of using Cre-dependent reporter(s) for lineage tracing in studying cell state progression.SIGNIFICANCE STATEMENT Identification of the regulators that promote oligodendrocyte (OL) myelination and remyelination is important for promoting myelin repair in devastating demyelinating diseases. Olig2 is expressed throughout OL lineage development. Ablation of Olig2 was reported to induce maturation, myelination, and remyelination from immature OLs. However, lineage-mapping analysis of Olig2-ablated cells was not conducted. Here, by leveraging multiple immature OL-expressing Cre lines, we observed no evidence that Olig2 ablation promotes maturation or remyelination of immature OLs. Instead, we find that Olig2 is required for immature OL maturation, myelination, and myelin repair. These data raise fundamental questions about the proposed inhibitory role of Olig2 against OL maturation and remyelination. Our findings highlight the importance of validating genetic manipulation with cell lineage tracing in studying myelination.

Endothelium-Specific Deficiency of Polycystin-1 Promotes Hypertension and Cardiovascular Disorders

Background:

Autosomal dominant polycystic kidney disease is the most frequent hereditary kidney disease and is generally due to mutations in PKD1 and PKD2 , encoding polycystins 1 and 2. In autosomal dominant polycystic kidney disease, hypertension and cardiovascular disorders are highly prevalent, but their mechanisms are partially understood.

Methods:

Since endothelial cells express the polycystin complex, where it plays a central role in the mechanotransduction of blood flow, we generated a murine model with inducible deletion of Pkd1 in endothelial cells ( Cdh5-Cre ERT2 ; Pkd1 fl/fl ) to specifically determine the role of endothelial polycystin-1 in autosomal dominant polycystic kidney disease.

Results:

Endothelial deletion of Pkd1 induced endothelial dysfunction, as demonstrated by impaired flow-mediated dilatation of resistance arteries and impaired relaxation to acetylcholine, increased blood pressure and prevented the normal development of arteriovenous fistula. In experimental chronic kidney disease induced by subtotal nephrectomy, endothelial deletion of Pkd1 further aggravated endothelial dysfunction, vascular remodeling, and heart hypertrophy.

Conclusions:

Altogether, this study provides the first in vivo demonstration that specific deletion of Pkd1 in endothelial cells promotes endothelial dysfunction and hypertension, impairs arteriovenous fistula development, and potentiates the cardiovascular alterations associated with chronic kidney disease.

Ral GTPases are critical regulators of spinal cord myelination and homeostasis

Efficient myelination supports nerve conduction and axonal health throughout life. In the central nervous system, oligodendrocytes (OLs) carry out this demanding anabolic duty in part through biosynthetic pathways controlled by mTOR. We identify Ral GTPases as critical regulators of mouse spinal cord myelination and myelin maintenance. Ablation of Ral GTPases (RalA, RalB) in OL-lineage cells impairs timely onset and radial growth of developmental myelination, accompanied by increased endosomal/lysosomal abundance. Further examinations, including transcriptomic analyses of Ral-deficient OLs, were consistent with mTORC1-related deficits. However, deletion of the mTOR signaling-repressor Pten in Ral-deficient OL-lineage cells is unable to rescue mTORC1 activation or developmental myelination deficiencies. Induced deletion of Ral GTPases in OLs of adult mice results in late-onset myelination defects and tissue degeneration. Together, our data indicate critical roles for Ral GTPases to promote developmental spinal cord myelination, to ensure accurate mTORC1 signaling, and to protect the healthy state of myelin-axon units over time.

Sensory nerves impede the formation of tertiary lymphoid structures and development of protective anti-melanoma immune responses

Peripheral neurons comprise a critical component of the tumor microenvironment (TME). The role of the autonomic innervation in cancer has been firmly established. However, the effect of the afferent (sensory) neurons on tumor progression remains unclear. Utilizing surgical and chemical skin sensory denervation methods, we showed that afferent neurons supported the growth of melanoma tumors in vivo and demonstrated that sensory innervation limited the activation of effective anti-tumor immune responses. Specifically, sensory ablation led to improved leukocyte recruitment into tumors, with decreased presence of lymphoid and myeloid immunosuppressive cells and increased activation of T-effector cells within the TME. Cutaneous sensory nerves hindered maturation of intratumoral high endothelial venules (HEVs) and limited formation of mature tertiary lymphoid-like structures containing organized clusters of CD4+ T cells and B cells. Denervation further increased T-cell clonality and expanded the B-cell repertoire in the TME. Importantly, CD8a depletion prevented denervation-dependent anti-tumor effects. Finally, we observed that gene signatures of inflammation and the content of neuron-associated transcripts inversely correlated in human primary cutaneous melanomas, with the latter representing a negative prognostic marker of patient overall survival. Our results suggest that tumor-associated sensory neurons negatively regulate the development of protective anti-tumor immune responses within the TME, thereby defining a novel target for therapeutic intervention in the melanoma setting.

Progressive axonopathy when oligodendrocytes lack the myelin protein CMTM5

Oligodendrocytes facilitate rapid impulse propagation along the axons they myelinate and support their long-term integrity. However, the functional relevance of many myelin proteins has remained unknown. Here, we find that expression of the tetraspan-transmembrane protein CMTM5 (chemokine-like factor-like MARVEL-transmembrane domain containing protein 5) is highly enriched in oligodendrocytes and central nervous system (CNS) myelin. Genetic disruption of the Cmtm5 gene in oligodendrocytes of mice does not impair the development or ultrastructure of CNS myelin. However, oligodendroglial Cmtm5 deficiency causes an early-onset progressive axonopathy, which we also observe in global and tamoxifen-induced oligodendroglial Cmtm5 mutants. Presence of the Wld^S mutation ameliorates the axonopathy, implying a Wallerian degeneration-like pathomechanism. These results indicate that CMTM5 is involved in the function of oligodendrocytes to maintain axonal integrity rather than myelin biogenesis.

The transcriptional portraits of the neural crest at the individual cell level

Since the discovery of this cell population by His in 1850, the neural crest has been under intense study for its important role during vertebrate development. Much has been learned about the function and regulation of neural crest cell differentiation, and as a result, the neural crest has become a key model system for stem cell biology in general. The experiments performed in embryology, genetics, and cell biology in the last 150 years in the neural crest field has given rise to several big questions that have been debated intensely for many years: "How does positional information impact developmental potential? Are neural crest cells individually multipotent or a mixed population of committed progenitors? What are the key factors that regulate the acquisition of stem cell identity, and how does a stem cell decide to differentiate towards one cell fate versus another?" Recently, a marriage between single cell multi-omics, statistical modeling, and developmental biology has shed a substantial amount of light on these questions, and has paved a clear path for future researchers in the field.

White matter integrity in mice requires continuous myelin synthesis at the inner tongue

Myelin, the electrically insulating sheath on axons, undergoes dynamic changes over time. However, it is composed of proteins with long lifetimes. This raises the question how such a stable structure is renewed. Here, we study the integrity of myelinated tracts after experimentally preventing the formation of new myelin in the CNS of adult mice, using an inducible Mbp null allele. Oligodendrocytes survive recombination, continue to express myelin genes, but they fail to maintain compacted myelin sheaths. Using 3D electron microscopy and mass spectrometry imaging we visualize myelin-like membranes failing to incorporate adaxonally, most prominently at juxta-paranodes. Myelinoid body formation indicates degradation of existing myelin at the abaxonal side and the inner tongue of the sheath. Thinning of compact myelin and shortening of internodes result in the loss of about 50% of myelin and axonal pathology within 20 weeks post recombination. In summary, our data suggest that functional axon-myelin units require the continuous incorporation of new myelin membranes.

Myelin is formed of proteins of long half-lives. The mechanisms of renewal of such a stable structure are unclear. Here, the authors show that myelin integrity requires continuous myelin synthesis at the inner tongue, contributing to the maintenance of a functional axon-myelin unit.

Models and methods to study Schwann cells

Schwann cells (SCs) are fundamental components of the peripheral nervous system (PNS) of all vertebrates and play essential roles in development, maintenance, function, and regeneration of peripheral nerves. There are distinct populations of SCs including: (1) myelinating SCs that ensheath axons by a specialized plasma membrane, called myelin, which enhances the conduction of electric impulses; (2) non‐myelinating SCs, including Remak SCs, which wrap bundles of multiple axons of small caliber, and perysinaptic SCs (PSCs), associated with motor axon terminals at the neuromuscular junction (NMJ). All types of SCs contribute to PNS regeneration through striking morphological and functional changes in response to nerve injury, are affected in peripheral neuropathies and show abnormalities and a diminished plasticity during aging. Therefore, methodological approaches to study and manipulate SCs in physiological and pathophysiological conditions are crucial to expand the present knowledge on SC biology and to devise new therapeutic strategies to counteract neurodegenerative conditions and age‐derived denervation. We present here an updated overview of traditional and emerging methodologies for the study of SCs for scientists approaching this research field.

Deletion of the Sodium-Dependent Glutamate Transporter GLT-1 in Maturing Oligodendrocytes Attenuates Myelination of Callosal Axons During a Postnatal Phase of Central Nervous System Development

The sodium-dependent glutamate transporter GLT-1 (EAAT2, SLC1A2) has been well-described as an important regulator of extracellular glutamate homeostasis in the central nervous system (CNS), a function that is performed mainly through its presence on astrocytes. There is, however, increasing evidence for the expression of GLT-1 in CNS cells other than astrocytes and in functional roles that are mediated by mechanisms downstream of glutamate uptake. In this context, GLT-1 expression has been reported for both neurons and oligodendrocytes (OLGs), and neuronal presynaptic presence of GLT-1 has been implicated in the regulation of glutamate uptake, gene expression, and mitochondrial function. Much less is currently known about the functional roles of GLT-1 expressed by OLGs. The data presented here provide first evidence that GLT-1 expressed by maturing OLGs contributes to the modulation of developmental myelination in the CNS. More specifically, using inducible and conditional knockout mice in which GLT-1 was deleted in maturing OLGs during a peak period of myelination (between 2 and 4 weeks of age) revealed hypomyelinated characteristics in the corpus callosum of preferentially male mice. These characteristics included reduced percentages of smaller diameter myelinated axons and reduced myelin thickness. Interestingly, this myelination phenotype was not found to be associated with major changes in myelin gene expression. Taken together, the data presented here demonstrate that GLT-1 expressed by maturing OLGs is involved in the modulation of the morphological aspects associated with CNS myelination in at least the corpus callosum and during a developmental window that appears of particular vulnerability in males compared to females.

JAB1 deletion in oligodendrocytes causes senescence-induced inflammation and neurodegeneration in mice

Oligodendrocytes are the primary target of demyelinating disorders and progressive neurodegenerative changes may evolve in the CNS. DNA damage and oxidative stress are considered key pathogenic events, but the underlying molecular mechanisms remain unclear. Moreover, animal models do not fully recapitulate human diseases, complicating the path to effective treatments. Here we report that mice with cell autonomous deletion of the nuclear COP9 signalosome component CSN5 (JAB1) in oligodendrocytes develop DNA damage and defective DNA repair in myelinating glial cells. Interestingly, oligodendrocytes lacking JAB1 expression underwent a senescence-like phenotype that fostered chronic inflammation and oxidative stress. These mutants developed progressive CNS demyelination, microglia inflammation and neurodegeneration, with severe motor deficits and premature death. Notably, blocking microglia inflammation did not prevent neurodegeneration, whereas the deletion of p21CIP1 but not p16INK4a pathway ameliorated the disease. We suggest that senescence is key to sustaining neurodegeneration in demyelinating disorders and may be considered a potential therapeutic target.

Río-Hortega's drawings revisited with fluorescent protein defines a cytoplasm-filled channel system of CNS myelin

A century ago this year, Pío del Río-Hortega (1921) coined the term 'oligodendroglia' for the 'interfascicular glia' with very few processes, launching an extensive discovery effort on his new cell type. One hundred years later, we review his original contributions to our understanding of the system of cytoplasmic channels within myelin in the context of what we observe today using light and electron microscopy of genetically encoded fluorescent reporters and immunostaining. We use the term myelinic channel system to describe the cytoplasm-delimited spaces associated with myelin; being the paranodal loops, inner and outer tongues, cytoplasm-filled spaces through compact myelin and further complex motifs associated to the sheath. Using a central nervous system myelinating cell culture model that contains all major neural cell types and produces compact myelin, we find that td-tomato fluorescent protein delineates the myelinic channel system in a manner reminiscent of the drawings of adult white matter by Río-Hortega, despite that he questioned whether some cytoplasmic figures he observed represented artefact. Together, these data lead us to propose a slightly revised model of the 'unrolled' sheath. Further, we show that the myelinic channel system, while relatively stable, can undergo subtle dynamic shape changes over days. Importantly, we capture an under-appreciated complexity of the myelinic channel system in mature myelin sheaths.

Neuronal cholesterol synthesis is essential for repair of chronically demyelinated lesions in mice

Astrocyte-derived cholesterol supports brain cells under physiological conditions. However, in demyelinating lesions, astrocytes downregulate cholesterol synthesis, and the cholesterol that is essential for remyelination has to originate from other cellular sources. Here, we show that repair following acute versus chronic demyelination involves distinct processes. In particular, in chronic myelin disease, when recycling of lipids is often defective, de novo neuronal cholesterol synthesis is critical for regeneration. By gene expression profiling, genetic loss-of-function experiments, and comprehensive phenotyping, we provide evidence that neurons increase cholesterol synthesis in chronic myelin disease models and in patients with multiple sclerosis (MS). In mouse models, neuronal cholesterol facilitates remyelination specifically by triggering oligodendrocyte precursor cell proliferation. Our data contribute to the understanding of disease progression and have implications for therapeutic strategies in patients with MS.

Mek/ERK1/2-MAPK and PI3K/Akt/mTOR signaling plays both independent and cooperative roles in Schwann cell differentiation, myelination and dysmyelination

Multiple signals are involved in the regulation of developmental myelination by Schwann cells and in the maintenance of a normal myelin homeostasis throughout adult life, preserving the integrity of the axons in the PNS. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination in the PNS. In addition, hyperactivation of these signaling pathways in Schwann cells leads to a late onset of various pathological changes in the sciatic nerves. However, it remains poorly understood whether these pathways function independently or sequentially or converge using a common mechanism to facilitate Schwann cell differentiation and myelin growth during development and in causing pathological changes in the adult animals. To address these questions, we analyzed multiple genetically modified mice using simultaneous loss- and constitutive gain-of-function approaches. We found that during development, the Mek/ERK1/2-MAPK pathway plays a primary role in Schwann cell differentiation, distinct from mTOR. However, during active myelination, ERK1/2 is dependent on mTOR signaling to drive the growth of the myelin sheath and regulate its thickness. Finally, our data suggest that peripheral nerve pathology during adulthood caused by hyperactivation of Mek/ERK1/2-MAPK or PI3K is likely to be independent or dependent on mTOR-signaling in different contexts. Thus, this study highlights the complexities in the roles played by two major intracellular signaling pathways in Schwann cells that affect their differentiation, myelination, and later PNS pathology and predicts that potential therapeutic modulation of these pathways in PNS neuropathies could be a complex process.

Murine Esophagus Expresses Glial-Derived Central Nervous System Antigens

Multiple sclerosis (MS) has been considered to specifically affect the central nervous system (CNS) for a long time. As autonomic dysfunction including dysphagia can occur as accompanying phenomena in patients, the enteric nervous system has been attracting increasing attention over the past years. The aim of this study was to identify glial and myelin markers as potential target structures for autoimmune processes in the esophagus. RT-PCR analysis revealed glial fibrillary acidic protein (GFAP), proteolipid protein (PLP), and myelin basic protein (MBP) expression, but an absence of myelin oligodendrocyte glycoprotein (MOG) in the murine esophagus. Selected immunohistochemistry for GFAP, PLP, and MBP including transgenic mice with cell-type specific expression of PLP and GFAP supported these results by detection of (1) GFAP, PLP, and MBP in Schwann cells in skeletal muscle and esophagus; (2) GFAP, PLP, but no MBP in perisynaptic Schwann cells of skeletal and esophageal motor endplates; (3) GFAP and PLP, but no MBP in glial cells surrounding esophageal myenteric neurons; and (4) PLP, but no GFAP and MBP in enteric glial cells forming a network in the esophagus. Our results pave the way for further investigations regarding the involvement of esophageal glial cells in the pathogenesis of dysphagia in MS.

TDP-43 maximizes nerve conduction velocity by repressing a cryptic exon for paranodal junction assembly in Schwann cells

TDP-43 is extensively studied in neurons in physiological and pathological contexts. However, emerging evidence indicates that glial cells are also reliant on TDP-43 function. We demonstrate that deletion of TDP-43 in Schwann cells results in a dramatic delay in peripheral nerve conduction causing significant motor deficits in mice, which is directly attributed to the absence of paranodal axoglial junctions. By contrast, paranodes in the central nervous system are unaltered in oligodendrocytes lacking TDP-43. Mechanistically, TDP-43 binds directly to Neurofascin mRNA, encoding the cell adhesion molecule essential for paranode assembly and maintenance. Loss of TDP-43 triggers the retention of a previously unidentified cryptic exon, which targets Neurofascin mRNA for nonsense-mediated decay. Thus, TDP-43 is required for neurofascin expression, proper assembly and maintenance of paranodes, and rapid saltatory conduction. Our findings provide a framework and mechanism for how Schwann cell-autonomous dysfunction in nerve conduction is directly caused by TDP-43 loss-of-function.

B6-Dct-H2BGFP bitransgenic mice: A standardized mouse model for in vivo characterization of melanocyte development and stem cell differentiation

Melanocyte stem cells (McSCs) are key components of the hair follicle (HF) stem cell system that regenerate differentiated melanocytes during successive HF cycles. To facilitate continued research on melanocyte development and differentiation and McSCs, we backcrossed inducible Dct-H2BGFP mice into the C57BL/6J background (B6-Dct-H2BGFP). We compared the expression pattern of B6-Dct-H2BGFP to that of Dct-H2BGFP mice on a mixed genetic background reported previously. To characterize B6-Dct-H2BGFP mice, we confirmed not only the expression of GFP in all melanocyte lineage cells, but also doxycycline regulation of GFP expression. Furthermore, ex vivo culture of the McSC subsets isolated by fluorescence-activated cell sorting (FACS) showed the propensity of bulge/CD34+ McSCs to differentiate with expression of non-melanocytic, neural crest lineage markers including glia (Gfap and CNPase, 73 ± 1% and 77 ± 2%, respectively), neurons (Tuj1 26 ± 5%), and smooth muscle (α-Sma, 31 ± 9%). In contrast, CD34-/secondary hair germ (SHG) McSCs differentiated into pigmented melanocytes, with higher expression of melanogenic markers Tyr (71 ± 1%), Tyrp1 (68 ± 4%), and Mitf (75 ± 7%). These results establish the utility of B6-Dct-H2BGFP bitransgenic mice for future in vivo studies of melanocytes requiring a defined genetic background.

Dhh-expressing Schwann cell precursors contribute to skin and cochlear melanocytes, but not to vestibular melanocytes

For a long time, melanocytes were believed to be exclusively derived from neural crest cells migrating from the neural tube toward the developing skin. This notion was then challenged by studies suggesting that melanocytes could also be made from neural crest-derived Schwann cell precursors (SCPs) on peripheral nerves. A SCP origin was inferred from lineage tracing studies in mice using a Plp1 promoter-controlled Cre driver transgene (Plp1-CreERT2) and a fluorescent Rosa26 locus-controlled Cre reporter allele (Rosa26^FloxSTOP-YFP ). However, doubts were raised in part because another SCP-directed Cre driver controlled by the Dhh promoter (Dhh-Cre) was apparently unable to label melanocytes when used with a non-fluorescent Rosa26 locus-controlled Cre reporter (Rosa26^{FloxSTOP-LacZ} ). Here, we report that the same Dhh-Cre driver line can efficiently label melanocytes when used in a pure FVB/N background together with the fluorescent instead of the non-fluorescent Rosa26 locus-controlled Cre reporter. Our data further suggest that the vast majority of skin melanocytes are SCP-derived. Interestingly, we also discovered that SCPs contribute inner ear melanocytes in a region-specific manner, extensively contributing to the cochlea but not to the vestibule.

Tropomyosin Receptor Kinase B Expressed in Oligodendrocyte Lineage Cells Functions to Promote Myelin Following a Demyelinating Lesion

The levels of brain-derived neurotrophic factor (BDNF) in the corpus callosum have previously been shown to have a critical impact on oligodendrocyte (OLG) lineage cells during cuprizone-elicited demyelination. In particular, BDNF+/- mice exhibit greater losses in myelin protein levels compared to wild-type mice after cuprizone. To investigate whether OLGs may directly mediate these effects of BDNF during a lesion in vivo, we used the cuprizone model of demyelination with inducible conditional male knockout mice to specifically delete the high-affinity tropomyosin receptor kinase B (TrkB) receptor from proteolipid protein + OLGs during cuprizone-elicited demyelination and subsequent remyelination. The loss of TrkB during cuprizone-elicited demyelination results in an increased sensitivity to demyelination as demonstrated by greater deficits in myelin protein levels, greater decreases in numbers of mature OLGs, increased numbers of demyelinated axons, and decreased myelin thickness. When mice are removed from cuprizone, they exhibit a delayed recovery in myelin proteins and myelin. Our data indicate that following a demyelinating lesion, TrkB in OLGs positively regulates myelin protein expression, myelin itself, and remyelination.

CMTM6 expressed on the adaxonal Schwann cell surface restricts axonal diameters in peripheral nerves

The velocity of nerve conduction is moderately enhanced by larger axonal diameters and potently sped up by myelination of axons. Myelination thus allows rapid impulse propagation with reduced axonal diameters; however, no myelin-dependent mechanism has been reported that restricts radial growth of axons. By label-free proteomics, STED-microscopy and cryo-immuno electron-microscopy we here identify CMTM6 (chemokine-like factor-like MARVEL-transmembrane domain-containing family member-6) as a myelin protein specifically localized to the Schwann cell membrane exposed to the axon. We find that disruption of Cmtm6-expression in Schwann cells causes a substantial increase of axonal diameters but does not impair myelin biogenesis, radial sorting or integrity of axons. Increased axonal diameters correlate with accelerated sensory nerve conduction and sensory responses and perturbed motor performance. These data show that Schwann cells utilize CMTM6 to restrict the radial growth of axons, which optimizes nerve function.

Oligodendrocytes Provide Antioxidant Defense Function for Neurons by Secreting Ferritin Heavy Chain

An evolutionarily conserved function of glia is to provide metabolic and structural support for neurons. To identify molecules generated by glia and with vital functions for neurons, we used Drosophila melanogaster as a screening tool, and subsequently translated the findings to mice. We found that a cargo receptor operating in the secretory pathway of glia was essential to maintain axonal integrity by regulating iron buffering. Ferritin heavy chain was identified as the critical secretory cargo, required for the protection against iron-mediated ferroptotic axonal damage. In mice, ferritin heavy chain is highly expressed by oligodendrocytes and secreted by employing an unconventional secretion pathway involving extracellular vesicles. Disrupting the release of extracellular vesicles or the expression of ferritin heavy chain in oligodendrocytes causes neuronal loss and oxidative damage in mice. Our data point to a role of oligodendrocytes in providing an antioxidant defense system to support neurons against iron-mediated cytotoxicity.

Injured Axons Instruct Schwann Cells to Build Constricting Actin Spheres to Accelerate Axonal Disintegration

After a peripheral nerve lesion, distal ends of injured axons disintegrate into small fragments that are subsequently cleared by Schwann cells and later by macrophages. Axonal debris clearing is an early step of the repair process that facilitates regeneration. We show here that Schwann cells promote distal cut axon disintegration for timely clearing. By combining cell-based and in vivo models of nerve lesion with mouse genetics, we show that this mechanism is induced by distal cut axons, which signal to Schwann cells through PlGF mediating the activation and upregulation of VEGFR1 in Schwann cells. In turn, VEGFR1 activates Pak1, leading to the formation of constricting actomyosin spheres along unfragmented distal cut axons to mediate their disintegration. Interestingly, oligodendrocytes can acquire a similar behavior as Schwann cells by enforced expression of VEGFR1. These results thus identify controllable molecular cues of a neuron-glia crosstalk essential for timely clearing of damaged axons.

Migrating Schwann cells direct axon regeneration within the peripheral nerve bridge

Schwann cells within the peripheral nervous system possess a remarkable regenerative potential. Current research shows that peripheral nerve-associated Schwann cells possess the capacity to promote repair of multiple tissues including peripheral nerve gap bridging, skin wound healing, digit tip repair as well as tooth regeneration. One of the key features of the specialized repair Schwann cells is that they become highly motile. They not only migrate into the area of damaged tissue and become a key component of regenerating tissue but also secrete signaling molecules to attract macrophages, support neuronal survival, promote axonal regrowth, activate local mesenchymal stem cells, and interact with other cell types. Currently, the importance of migratory Schwann cells in tissue regeneration is most evident in the case of a peripheral nerve transection injury. Following nerve transection, Schwann cells from both proximal and distal nerve stumps migrate into the nerve bridge and form Schwann cell cords to guide axon regeneration. The formation of Schwann cell cords in the nerve bridge is key to successful peripheral nerve repair following transection injury. In this review, we first examine nerve bridge formation and the behavior of Schwann cell migration in the nerve bridge, and then discuss how migrating Schwann cells direct regenerating axons into the distal nerve. We also review the current understanding of signals that could activate Schwann cell migration and signals that Schwann cells utilize to direct axon regeneration. Understanding the molecular mechanism of Schwann cell migration could potentially offer new therapeutic strategies for peripheral nerve repair.

EEF1A1 deacetylation enables transcriptional activation of remyelination

Remyelination of the peripheral and central nervous systems (PNS and CNS, respectively) is a prerequisite for functional recovery after lesion. However, this process is not always optimal and becomes inefficient in the course of multiple sclerosis. Here we show that, when acetylated, eukaryotic elongation factor 1A1 (eEF1A1) negatively regulates PNS and CNS remyelination. Acetylated eEF1A1 (Ac-eEF1A1) translocates into the nucleus of myelinating cells where it binds to Sox10, a key transcription factor for PNS and CNS myelination and remyelination, to drag Sox10 out of the nucleus. We show that the lysine acetyltransferase Tip60 acetylates eEF1A1, whereas the histone deacetylase HDAC2 deacetylates eEF1A1. Promoting eEF1A1 deacetylation maintains the activation of Sox10 target genes and increases PNS and CNS remyelination efficiency. Taken together, these data identify a major mechanism of Sox10 regulation, which appears promising for future translational studies on PNS and CNS remyelination.

The molecular mechanisms regulating remyelination are unclear. Here, the authors show that promoting deacetylation of eEF1A1 prevents the translocation of Sox10 outside the nucleus, contributing to maintaining the expression of Sox10 target genes and increasing remyelination efficiency.

Identity and lineage fate of proteolipid protein 1 gene (Plp1)-expressing cells in the embryonic murine spinal cord

BACKGROUND

The proteolipid protein (PLP) is the most abundant protein in the myelin sheath of the central nervous system (CNS). The gene coding PLP, proteolipid protein 1 (Plp1) is highly expressed in oligodendrocytes, the myelin-forming cells in the CNS. Previous studies demonstrate that Plp1 gene is expressed in the embryonic CNS much earlier before the generation of oligodendrocytes. However, the progenitor identity and the fate of Plp1-expressing cells are still elusive.

RESULTS

We employed genetic approaches to permanently label Plp1-expressing cells with the reporter enhanced yellow fluorescence protein (EYFP) and used multicolored immunohistochemistry to characterize their identity and lineage fate. We found that Plp1-expressing cells were initially present without spatial restrictions and later confined to the ventral progenitor domains of the embryonic spinal cord. Our fate-mapping results showed that Plp1-expressing cells during early embryogenesis were multipotent neural progenitor cells that gave rise to not only neurons but also glial progenitor cells whereas they were bipotent glial progenitor cells during later neural development stages and generated oligodendroglial and astroglial lineage cells but not neurons. Intriguingly, postnatal astrocytes generated from embryonic Plp1-expressing glial progenitor cells were present only in the ventral spinal cord.

CONCLUSION

Our study reveals that Plp1-expressing cells during embryonic neural development display dynamic cellular identities and have a broader lineage fate than oligodendroglial lineage.

Axon-dependent expression of YAP/TAZ mediates Schwann cell remyelination but not proliferation after nerve injury

Previously we showed that YAP/TAZ promote not only proliferation but also differentiation of immature Schwann cells (SCs), thereby forming and maintaining the myelin sheath around peripheral axons (Grove et al., 2017). Here we show that YAP/TAZ are required for mature SCs to restore peripheral myelination, but not to proliferate, after nerve injury. We find that YAP/TAZ dramatically disappear from SCs of adult mice concurrent with axon degeneration after nerve injury. They reappear in SCs only if axons regenerate. YAP/TAZ ablation does not impair SC proliferation or transdifferentiation into growth promoting repair SCs. SCs lacking YAP/TAZ, however, fail to upregulate myelin-associated genes and completely fail to remyelinate regenerated axons. We also show that both YAP and TAZ are redundantly required for optimal remyelination. These findings suggest that axons regulate transcriptional activity of YAP/TAZ in adult SCs and that YAP/TAZ are essential for functional regeneration of peripheral nerve.

GNAQQ209L expression initiated in multipotent neural crest cells drives aggressive melanoma of the central nervous system

Primary leptomeningeal melanocytic neoplasms represent a spectrum of rare tumors originating from melanocytes of the leptomeninges, which are the inner two membranes that protect the central nervous system. Like other non-epithelial melanocytic lesions, they bear frequent oncogenic mutations in the heterotrimeric G protein alpha subunits, GNAQ or GNA11. In this study, we used Plp1-creERT to force the expression of oncogenic GNAQ^Q209L in the multipotent neural crest cells of the ventro-medial developmental pathway, beginning prior to melanocyte cell differentiation. We found that this produces leptomeningeal melanocytic neoplasms, including cranial melanocytomas, spinal melanocytomas, and spinal melanomas, in addition to blue nevus-like lesions in the dermis. GNAQ^Q209L drove different phenotypes depending upon when during embryogenesis (E9.5, E10.5, or E11.5) it was induced by tamoxifen and which Cre driver (Plp1-creERT, Tyr-creERT² , or Mitf-cre) was used. Given these differences, we propose that melanocytes go through temporary phases where they become sensitive to the oncogenic effects of GNAQ^Q209L . R26-fs-GNAQ^Q209L ; Plp1-creERT mice will be useful for defining biomarkers for potentially aggressive leptomeningeal melanocytomas and for developing new therapeutics for advanced disease.

Sublamina-specific organization of the blood brain barrier in the mouse olfactory nerve layer

Astrocytes constitute the main glial component of the mammalian blood brain barrier (BBB). However, in the olfactory bulb (OB), the olfactory nerve layer (ONL) is almost devoid of astrocytes, raising the question which glial cells are part of the BBB. We used mice expressing EGFP in astrocytes and tdTomato in olfactory ensheathing cells (OECs), a specialized type of glial cells in the ONL, to unequivocally identify both glial cell types and investigate their contribution to the BBB in the olfactory bulb. OECs were located exclusively in the ONL, while somata of astrocytes were located in deeper layers and extended processes in the inner sublamina of the ONL. These processes surrounded blood vessels and contained aquaporin-4, an astrocytic protein enriched at the BBB. In the outer sublamina of the ONL, in contrast, blood vessels were surrounded by aquaporin-4-negative processes of OECs. Transcardial perfusion of blood vessels with lanthanum and subsequent visualization by electron microscopy showed that blood vessels enwrapped by OECs possessed intact tight junctions. In acute olfactory bulb preparations, injection of fluorescent glucose 6-NBDG into blood vessels resulted in labeling of OECs, indicating glucose transport from the perivascular space into OECs. In addition, Ca²⁺ transients in OECs in the outer sublamina evoked vasoconstriction, whereas Ca²⁺ signaling in OECs of the inner sublamina had no effect on adjacent blood vessels. Our results demonstrate that the BBB in the inner sublamina of the ONL contains astrocytes, while in the outer ONL OECs are part of the BBB.

AMPA Receptor-Mediated Ca2+ Transients in Mouse Olfactory Ensheathing Cells

Ca²⁺ signaling in glial cells is primarily triggered by metabotropic pathways and the subsequent Ca²⁺ release from internal Ca²⁺ stores. However, there is upcoming evidence that various ion channels might also initiate Ca²⁺ rises in glial cells by Ca²⁺ influx. We investigated AMPA receptor-mediated inward currents and Ca²⁺ transients in olfactory ensheathing cells (OECs), a specialized glial cell population in the olfactory bulb (OB), using whole-cell voltage-clamp recordings and confocal Ca²⁺ imaging. By immunohistochemistry we showed immunoreactivity to the AMPA receptor subunits GluA1, GluA2 and GluA4 in OECs, suggesting the presence of AMPA receptors in OECs. Kainate-induced inward currents were mediated exclusively by AMPA receptors, as they were sensitive to the specific AMPA receptor antagonist, GYKI53655. Moreover, kainate-induced inward currents were reduced by the selective Ca²⁺-permeable AMPA receptor inhibitor, NASPM, suggesting the presence of functional Ca²⁺-permeable AMPA receptors in OECs. Additionally, kainate application evoked Ca²⁺ transients in OECs which were abolished in the absence of extracellular Ca²⁺, indicating that Ca²⁺ influx via Ca²⁺-permeable AMPA receptors contribute to kainate-induced Ca²⁺ transients. However, kainate-induced Ca²⁺ transients were partly reduced upon Ca²⁺ store depletion, leading to the conclusion that Ca²⁺ influx via AMPA receptor channels is essential to trigger Ca²⁺ transients in OECs, whereas Ca²⁺ release from internal stores contributes in part to the kainate-evoked Ca²⁺ response. Endogenous glutamate release by OSN axons initiated Ca²⁺ transients in OECs, equally mediated by metabotropic receptors (glutamatergic and purinergic) and AMPA receptors, suggesting a prominent role for AMPA receptor mediated Ca²⁺ signaling in axon-OEC communication.

Comparison of Theiler's Murine Encephalomyelitis Virus Induced Spinal Cord and Peripheral Nerve Lesions Following Intracerebral and Intraspinal Infection

Hallmarks of Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) include spinal cord (SC) inflammation, demyelination and axonal damage occurring approximately 5-8 weeks after classical intracerebral (i.c.) infection. The aim of this study was to elucidate the consequences of intraspinal (i.s.) TMEV infection and a direct comparison of classical i.c. and intraspinal infection. Swiss Jim Lambert (SJL)-mice were i.s. infected with the BeAn strain of TMEV. Clinical investigations including a scoring system and rotarod analysis were performed on a regular basis. Necropsies were performed at 3, 7, 14, 28 and 63 days post infection (dpi) following i.s. and at 4, 7, 14, 28, 56, 98, 147 and 196 dpi following i.c. infection. Serial sections of formalin-fixed, paraffin-embedded SC and peripheral nerves (PN) were investigated using hematoxylin and eosin (HE) and immunohistochemistry. I.s. infected mice developed clinical signs and a deterioration of motor coordination approximately 12 weeks earlier than i.c. infected animals. SC inflammation, demyelination and axonal damage occurred approximately 6 weeks earlier in i.s. infected animals. Interestingly, i.s. infected mice developed PN lesions, characterized by vacuolation, inflammation, demyelination and axonal damage, which was not seen following i.c. infection. The i.s. infection model offers the advantage of a significantly earlier onset of clinical signs, inflammatory and demyelinating SC lesions and additionally enables the investigation of virus-mediated PN lesions.

Disrupted CXCR2 Signaling in Oligodendroglia Lineage Cells Enhances Myelin Repair in a Viral Model of Multiple Sclerosis

CXCR2 is a chemokine receptor expressed on oligodendroglia that has been implicated in the pathogenesis of neuroinflammatory demyelinating diseases as well as enhancement of the migration, proliferation, and myelin production by oligodendroglia. Using an inducible proteolipid protein (Plp) promoter-driven Cre-loxP recombination system, we were able to assess how timed ablation of Cxcr2 in oligodendroglia affected disease following intracranial infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV). Generation of Plp-Cre-ER(T)::Cxcr2flox/flox transgenic mice (termed Cxcr2-CKO mice) allows for Cxcr2 to be silenced in oligodendrocytes in adult mice following treatment with tamoxifen. Ablation of oligodendroglia Cxcr2 did not influence clinical severity in response to intracranial infection with JHMV. Infiltration of activated T cells or myeloid cells into the central nervous system (CNS) was not affected, nor was the ability to control viral infection. In addition, the severity of demyelination was similar between tamoxifen-treated mice and vehicle-treated controls. Notably, deletion of Cxcr2 resulted in increased remyelination, as assessed by g-ratio (the ratio of the inner axonal diameter to the total outer fiber diameter) calculation, compared to that in vehicle-treated control mice. Collectively, our findings argue that CXCR2 signaling in oligodendroglia is dispensable with regard to contributing to neuroinflammation, but its deletion enhances remyelination in a preclinical model of the human demyelinating disease multiple sclerosis (MS).IMPORTANCE Signaling through the chemokine receptor CXCR2 in oligodendroglia is important for developmental myelination in rodents, while chemical inhibition or nonspecific genetic deletion of CXCR2 appears to augment myelin repair in animal models of the human demyelinating disease multiple sclerosis (MS). To better understand the biology of CXCR2 signaling on oligodendroglia, we generated transgenic mice in which Cxcr2 is selectively ablated in oligodendroglia upon treatment with tamoxifen. Using a viral model of neuroinflammation and demyelination, we demonstrate that genetic silencing of CXCR2 on oligodendroglia did not affect clinical disease, neuroinflammation, or demyelination, yet there was increased remyelination. These findings support and extend previous findings suggesting that targeting CXCR2 may offer a therapeutic avenue for enhancing remyelination in patients with demyelinating diseases.

Characterization of Cre recombinase mouse lines enabling cell type-specific targeting of postnatal intervertebral discs

Cre/loxP technology is an important tool for studying cell type-specific gene functions. Cre recombinase mouse lines, including Agc1-CreER^T2 , Col2a1-Cre; Col2a1-CreER^T2 , Shh-Cre, Shh-CreER^T2 , and Osx-Cre, have been proven to be valuable tools to elucidate the biology of long bones, yet the information for their activity in postnatal intervertebral disc (IVD) tissues was very limited. In this study, we used R26-mTmG fluorescent reporter to systematically analyze cell specificity and targeting efficiency of these six mouse lines in IVD tissues at postnatal growing and adult stages. We found that Agc1-CreER^T2 is effective to direct recombination in all components of IVDs, including annulus fibrosus (AF), nucleus pulposus (NP), and cartilaginous endplate (CEP), upon tamoxifen induction at either 2 weeks or 2 months of ages. Moreover, Col2a1-Cre targets most of the cells in IVDs, except for some cells in the outer AF (OAF) and NP. In contrast, the activity of Col2a1-CreER^T2 is mainly limited to the IAF of IVD tissues at either stage of tamoxifen injection. Similarly, Shh-Cre directs recombination specifically in all NP cells, whereas Shh-CreER^T2 is active only in a few NP cells when tamoxifen is administered at either stage. Finally, Osx-Cre targets cells in the CEP, but not in the NP or AF of IVDs tissues at these two stages. Thus, our data demonstrated that all these Cre lines can direct recombination in IVD tissues at postnatal stages with different cell type specificity and/or targeting efficiency, and can, therefore, serve as valuable tools to dissect cell type-specific gene functions in IVD development and homeostasis.

Schwann cell precursors contribute to skeletal formation during embryonic development in mice and zebrafish

Multipotent Schwann cell precursors (SCPs) generate numerous cell types. Here, in both mouse and zebrafish, SCPs contributed to the generation of mesenchymal, chondroprogenitor, and osteoprogenitor cells during embryonic development. These findings reveal a source of cartilage and bone cells and previously unanticipated interactions between the nervous system and skeleton during development.

Immature multipotent embryonic peripheral glial cells, the Schwann cell precursors (SCPs), differentiate into melanocytes, parasympathetic neurons, chromaffin cells, and dental mesenchymal populations. Here, genetic lineage tracing revealed that, during murine embryonic development, some SCPs detach from nerve fibers to become mesenchymal cells, which differentiate further into chondrocytes and mature osteocytes. This occurred only during embryonic development, producing numerous craniofacial and trunk skeletal elements, without contributing to development of the appendicular skeleton. Formation of chondrocytes from SCPs also occurred in zebrafish, indicating evolutionary conservation. Our findings reveal multipotency of SCPs, providing a developmental link between the nervous system and skeleton.

Myelinating Glia-Specific Deletion of Fbxo7 in Mice Triggers Axonal Degeneration in the Central Nervous System Together with Peripheral Neuropathy

Myelination of axons facilitates the rapid propagation of electrical signals and the long-term integrity of axons. The ubiquitin-proteasome system is essential for proper protein homeostasis, which is particularly crucial for interactions of postmitotic cells. In our study, we examined how the E3 ubiquitin ligase FBXO7-SCF (SKP1, Cul1, F-box protein) expressed in myelinating cells affects the axon-myelin unit. Deletion of Fbxo7 in oligodendrocytes and Schwann cells in mice using the Cnp1-Cre driver line led to motor impairment due to hindlimb paresis. It did not result in apoptosis of myelinating cells, nor did it affect the proper myelination of axons or lead to demyelination. It however triggered axonal degeneration in the CNS and resulted in the severe degeneration of axons in the PNS, inducing a full-blown neuropathy. Both the CNS and PNS displayed inflammation, while the PNS was also characterized by fibrosis, massive infiltration of macrophages, and edema. Tamoxifen-induced deletion of Fbxo7, after myelination using the Plp1-CreERT2 line, led to a small number of degenerated axons and hence a very mild peripheral neuropathy. Interestingly, loss of Fbxo7 also resulted in reduced proteasome activity in Schwann cells but not in cerebellar granule neurons, indicating a specific sensitivity of the former cell type. Together, our results demonstrate an essential role for FBXO7 in myelinating cells to support associated axons, which is fundamental to the proper developmental establishment and the long-term integrity of the axon-myelin unit.SIGNIFICANCE STATEMENT The myelination of axons facilitates the fast propagation of electrical signals and the trophic support of the myelin-axon unit. Here, we report that deletion of Fbxo7 in myelinating cells in mice triggered motor impairment but had no effect on myelin biogenesis. Loss of Fbxo7 in myelinating glia, however, led to axonal degeneration in the CNS and peripheral neuropathy of the axonal type. In addition, we found that Schwann cells were particularly sensitive to Fbxo7 deficiency reflected by reduced proteasome activity. Based on these findings, we conclude that Fbxo7 is essential for the support of the axon-myelin unit and long-term axonal health.