Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca2+ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination.
cell death; dysmyelination; golli protein; myelination; oligodendrocyte development; BDNF, brain-derived neurotrophic factor; CC, corpus callosum; ClCsp3, cleaved caspase 3; CNP, 2′,3′-cyclic nucleotide phosphodiesterase; DIV, days in vitro; FGFR, fibroblast growth factor receptor; GFAP, glial fibrillary acidic protein; GFP, green fluorescent protein; hemi, hemizygous; JOE, J37-overexpressing; MBP, myelin basic protein; MyAP, myelinated axon profile; OL, oligodendrocyte; OPC, oligodendrocyte precursor cell; P, postnatal day; PDGFRα, platelet-derived growth factor receptor α; PLP, proteolipid protein; TBS-T, Tris-buffered saline with Tween 20; WT, wild-type
OPCs (oligodendrocyte precursor cells) express golli proteins which, through regulation of Ca2+ influx, appear to be important in OPC process extension/retraction and migration. The aim of the present study was to examine further the role of golli in regulating OPC development. The effects of golli ablation and overexpression were examined in primary cultures of OPCs prepared from golli-KO (knockout) and JOE (golli J37-overexpressing) mice. In OPCs lacking golli, or overexpressing golli, differentiation induced by growth factor withdrawal was impaired. Proliferation analysis in the presence of PDGF (platelet-derived growth factor), revealed that golli enhanced the mitogen-stimulated proliferation of OPCs through activation of SOCCs (store-operated Ca2+ channels). PDGF treatment induced a biphasic increase in OPC intracellular Ca2+, and golli specifically increased Ca2+ influx during the second SOCC-dependent phase that followed the initial release of Ca2+ from intracellular stores. This store-operated Ca2+ uptake appeared to be essential for cell division, since specific SOCC antagonists completely blocked the effects of PDGF and golli on OPC proliferation. Additionally, in OPCs overexpressing golli, increased cell death was observed after mitogen withdrawal. This phenomenon could be prevented by exposure to VOCC (voltage-operated Ca2+ channel) blockers, indicating that the effect of golli on cell death involved increased Ca2+ influx through VOCCs. The results showed a clear effect of golli on OPC development and support a role for golli in modulating multiple Ca2+-regulatory events through VOCCs and SOCCs. Our results also suggest that PDGF engagement of its receptor resulting in OPC proliferation proceeds through activation of SOCCs.
apoptosis; calcium influx; cell cycle; golli protein; oligodendrocyte; platelet-derived growth factor (PDGF); 2-APB, 2-aminoethoxydiphenyl borate; bFGF, basic fibroblast growth factor; BrdU, bromodeoxyuridine; [Ca2+]int, intracellular Ca2+ concentration; CNS, central nervous system; DIV, days in vitro; div, days in vitro for mixed cultures; DMEM, Dulbecco’s modified Eagle’s medium; FBS, fetal bovine serum; fura 2/AM, fura 2 acetoxymethyl ester; GC, galactocerebroside; GFP, green fluorescent protein; JOE, golli J37-overexpressing; KO, knockout; MBP, myelin basic protein; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide; NG2, nerve/glial antigen 2; OPC, oligodendrocyte precursor cell; OPC˜GFP, GFP-labelled OPC; PDGF, platelet-derived growth factor; Plp, proteolipid; SOCC, store-operated Ca2+ channel; Tc, cell-cycle time; VOCC, voltage-operated Ca2+ channel