HIFα Regulates Developmental Myelination Independent of Autocrine Wnt Signaling

The developing CNS is exposed to physiological hypoxia, under which hypoxia-inducible factor α (HIFα) is stabilized and plays a crucial role in regulating neural development. The cellular and molecular mechanisms of HIFα in developmental myelination remain incompletely understood. A previous concept proposes that HIFα regulates CNS developmental myelination by activating the autocrine Wnt/β-catenin signaling in oligodendrocyte progenitor cells (OPCs). Here, by analyzing a battery of genetic mice of both sexes, we presented in vivo evidence supporting an alternative understanding of oligodendroglial HIFα-regulated developmental myelination. At the cellular level, we found that HIFα was required for developmental myelination by transiently controlling upstream OPC differentiation but not downstream oligodendrocyte maturation and that HIFα dysregulation in OPCs but not oligodendrocytes disturbed normal developmental myelination. We demonstrated that HIFα played a minor, if any, role in regulating canonical Wnt signaling in the oligodendroglial lineage or in the CNS. At the molecular level, blocking autocrine Wnt signaling did not affect HIFα-regulated OPC differentiation and myelination. We further identified HIFα–Sox9 regulatory axis as an underlying molecular mechanism in HIFα-regulated OPC differentiation. Our findings support a concept shift in our mechanistic understanding of HIFα-regulated CNS myelination from the previous Wnt-dependent view to a Wnt-independent one and unveil a previously unappreciated HIFα–Sox9 pathway in regulating OPC differentiation.SIGNIFICANCE STATEMENT Promoting disturbed developmental myelination is a promising option in treating diffuse white matter injury, previously called periventricular leukomalacia, a major form of brain injury affecting premature infants. In the developing CNS, hypoxia-inducible factor α (HIFα) is a key regulator that adapts neural cells to physiological and pathologic hypoxic cues. The role and mechanism of HIFα in oligodendroglial myelination, which is severely disturbed in preterm infants affected with diffuse white matter injury, is incompletely understood. Our findings presented here represent a concept shift in our mechanistic understanding of HIFα-regulated developmental myelination and suggest the potential of intervening with an oligodendroglial HIFα-mediated signaling pathway to mitigate disturbed myelination in premature white matter injury.     

人发角蛋白对脊髓损伤大鼠少突胶质细胞增殖分化效应的影响

目的探讨体内可降解生物材料人发角蛋白(HHK)植入急性冲击性损伤脊髓后,对神经再生过程中少突胶质细胞增殖分化效应的影响。方法采用改制Ⅱ型NYU装置,在建立大鼠急性冲击性脊髓损伤模型基础上,将经过特殊处理后能在体内降解的HHK植入大鼠脊髓损伤部位,对植入后1、4、12、26周损伤脊髓组织进行光学和电镜结构观察。结果第1周时,损伤部位可见少突胶质细胞散在分布于大量浸润的炎症细胞中;第4周时,通过Mallory's磷乌酸苏木素染色,显示HHK周边呈层包绕的增生少突胶质细胞;第12和26周主要为神经再生和髓鞘重建过程,重建中的少突胶质细胞髓鞘内出现较大的空腔,髓鞘层状分离,并形成大小不一的髓鞘小体,重建髓鞘周边可见新生的少突胶质细胞。结论在急性冲击性脊髓损伤后神经再生过程中,HHK对少突胶质细胞的增殖分化及髓鞘再生修复有积极作用,为进一步综合研究HHK对脊髓损伤修复的作用提供了实验依据。     

Localization of glial cell antigens in the brains of young normal mice and the dysmyelinating mutant mice, jimpy and shiverer

Tissue sections from the brains of normal, jimpy, and shiverer mice were immunostained by the peroxidase antiperoxidase method for carbonic anhydrase (CA) and the putative astrocytic "markers" glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP). The cells in normal gray matter that immunostained with anti-CA and anti-GS were similar to one another in size and process elaboration. In the normal gray matter there were relatively few GFAP-positive astrocytes. When present, these cells resembled the CA- and GS-positive cells; however, the GFAP appeared to be concentrated in the astroglial processes, as distinguished from the cell bodies. Glial cell processes, immunostained for CA or GS, surrounded blood vessels and unstained neurons in the normal gray matter. The glial cells in shiverer gray matter were similar to those in the normal gray matter. When stained for GS or GFAP, the glial cells in the jimpy gray matter appeared to be somewhat hypertrophied, and when the glial cells in this mutant were stained for CA, the nuclei appeared to be swollen. It was concluded that some of the CA-positive cells in the gray matter of the normal and of each mutant mouse brain could be astrocytes. The patterns of immunostaining in the white matter emphasized the different complements of glial cells in the mutants. In the normal and shiverer mouse corpus callosum, CA, in particular, was detected only in the oligodendrocytes, their processes, and myelin. However, the data concerning the jimpy mouse suggested that the few CA-positive cells in the corpus callosum of that mutant could be astrocytes.     

Ulip/CRMP proteins are recognized by autoantibodies in paraneoplastic neurological syndromes

Anti-CV2 autoantibodies have recently been discovered in patients with paraneoplastic neurological diseases (PND). These disorders are associated with neuronal degeneration, mediated by autoimmune processes, in patients with systemic cancer. Anti-CV2 autoantibodies recognize a brain protein of 66 kDa developmentally regulated and specifically expressed by a subpopulation of oligodendrocytes in the adult brain. Here, we demonstrate that anti-CV2 sera recognize several post-translationally modified forms of Ulip4/CRMP3, a member of a protein family related to the axonal guidance and homologous to the Unc-33 gene product in Caenorhabditis elegans. The sequence of the human Ulip4/CRMP3 was determined and the gene localized to chromosome 10q25.2-q26, a region mutated in glioblastomas and containing tumour suppressor genes. The identification of the Ulip/CRMP proteins as recognized by anti-CV2 sera should provide new insights into the role of Ulip/CRMPs in oligodendrocytes and into pathophysiology of PND.     

Polydendrocytes in development and myelin repair

Polydendrocytes (NG2 cells) are a distinct type of glia that populate the developing and adult central nervous systems (CNS). In the adult CNS, they retain mitotic activity and represent the largest proliferating cell population. Genetic and epigenetic mechanisms regulate the fate of polydendrocytes, which give rise to both oligodendrocytes and astrocytes. In addition, polydendrocytes actively differentiate into myelin-forming oligodendrocytes in response to demyelination. This review summarizes the current knowledge regarding polydendrocyte development, which provides an important basis for understanding the mechanisms that lead to the remyelination of demyelinated lesions.     

S1P1 deletion in oligodendroglial lineage cells: Effect on differentiation and myelination

Sphingosine 1‐phosphate (S1P) receptors are G protein‐coupled receptors expressed by many cell types, including cells of oligodendrocyte (OLG) lineage. We had previously shown that targeted deletion of S1P₁ in OLG lineage cells did not result in obvious clinical phenotype or altered number of OLGs at 3 months, but there were subtle abnormalities in myelin. In this study, we examined the role of S1P₁ in developmental myelination and cell survival, focusing on age 3 weeks. We found that S1P₁ deficiency led to delayed differentiation of OLG progenitors (OPCs) into OLGs that is independent of p38 phosphorylation. This was accompanied by decreased levels of myelin basic protein (MBP) but not of myelin‐OLG glycoprotein (MOG), and slight decrease in myelin thickness in the corpus callosum of S1P₁ conditional knockout (CKO) mice. S1P₁‐deficient OLGs exhibited slower process extension, which was associated with attenuated phosphorylation of extracellular signal regulated kinases (ERKs) and p21‐activated kinases (PAKs), and with upregulation of tropomodulin1. Basal levels of pAkt were not affected, though expectedly, no response to a selective S1P₁ agonist SEW2871 was observed. S1P₁‐deficient OLGs did not exhibit increased cell death in response to cuprizone, tumor necrosis factor‐α, or deprivation of nutrients and growth factors. We conclude that S1P₁ signaling regulates OLG development, morphological maturation and early myelination. GLIA 2016;64:570–582     

Radial glia give rise to adult neural stem cells in the subventricular zone

Neural stem cells with the characteristics of astrocytes persist in the subventricular zone (SVZ) of the juvenile and adult brain. These cells generate large numbers of new neurons that migrate through the rostral migratory stream to the olfactory bulb. The developmental origin of adult neural stem cells is not known. Here, we describe a lox-Cre-based technique to specifically and permanently label a restricted population of striatal radial glia in newborn mice. Within the first few days after labeling, these radial glial cells gave rise to neurons, oligodendrocytes, and astrocytes, including astrocytes in the SVZ. Remarkably, the rostral migratory stream contained labeled migratory neuroblasts at all ages examined, including 150-day-old mice. Labeling dividing cells with the S-phase marker BrdUrd showed that new neurons continue to be produced in the adult by precursors ultimately derived from radial glia. Furthermore, both radial glia in neonates and radial glia-derived cells in the adult lateral ventricular wall generated self-renewing, multipotent neurospheres. These results demonstrate that radial glial cells not only serve as progenitors for many neurons and glial cells soon after birth but also give rise to adult SVZ stem cells that continue to produce neurons throughout adult life. This study identifies and provides a method to genetically modify the lineage that links neonatal and adult neural stem cells.     

Evidence that the central canal lining of the spinal cord contributes to oligodendrogenesis during postnatal development and adulthood in intact rats

Two waves of oligodendrogenesis in the ventricular zone of the spinal cord (SC‐VZ) during rat development, which take place between embryonic days 14 and 18 (E14–E18) and E20–E21, have been described. In the VZ of the brain, unlike the SC‐VZ, a third wave of oligodendrogenesis occurs during the first weeks of postnatal development. Using immunofluorescence staining of intact rat SC tissue, we noticed the presence of small numbers of Olig2⁺/Sox‐10⁺ cells inside the lining of the central canal (CC) during postnatal development and adulthood. Olig2⁺/Sox‐10⁺ cells appeared inside the lining of the CC shortly after birth, and their number reached a maximum of approximately 0.65 ± 0.14 cell/40‐μm section during the second postnatal week. After the latter development, the number of Olig2⁺/Sox‐10⁺ cells decreased to 0.21 ± 0.07 (P36) and 0.18 ± 0.1 cell/section (P120). At P21, Olig2⁺/Sox‐10⁺ cells inside the CC lining started to express other oligodendroglial markers such as CNPase, RIP, and APC. Olig2⁺/Sox‐10⁺ cells usually did not proliferate inside the CC lining and were only rarely found to be immunoreactive against oligodendrocyte progenitor markers such as NG2 or PDGFRα. Using 5‐bromo‐2‐deoxyuridine administration at P2, P11, P22, or P120–P125, we revealed that these cells arose in the CC lining during postnatal development and adulthood. Our findings confirmed that the CC lining is the source of a small number of cells with an oligodendroglial phenotype during postnatal development and adulthood in the SC of intact rats. J. Comp. Neurol. 522:3194–3207, 2014. © 2014 Wiley Periodicals, Inc.