Epigenetic factors up-regulate expression of myelin proteins in the dysmyelinating jimpy mutant mouse

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Sexual dimorphism in the white matter of rodents

Sexual dimorphism of astrocytes and neurons is well documented in many brain and spinal cord structures. Sexual dimorphism of oligodendrocytes (Olgs) and myelin has received less attention. We recently showed that density of Olgs in corpus callosum, fornix, and spinal cord of wild-type male rodents is more densely packed than in females; myelin proteins and myelin gene expression are likewise greater in males than in female rodents. However, glial cell proliferation and cell death were two times greater in female corpus callosum. Endogenous sex hormones, specifically lack of androgens, produce an Olg female phenotype in castrated male mouse. In vitro studies using Olgs culture also showed differences between males and females Olg survival and signaling pathways in response to sexual hormones. Sexual dimorphism of white matter tracts and glia in rodents indicates the necessity for controlling gender in the experimental studies of neurodegenerative disorders. Most importantly, our studies suggest that hormones may contribute to sexual dimorphism observed in certain human diseases including multiple sclerosis.

Proliferation and death of oligodendrocytes and myelin proteins are differentially regulated in male and female rodents

Sexual dimorphism of neurons and astrocytes has been demonstrated in different centers of the brain, but sexual dimorphism of oligodendrocytes and myelin has not been examined. We show, using immunocytochemistry and in situ hybridization, that the density of oligodendrocytes in corpus callosum, fornix, and spinal cord is 20-40% greater in males compared with females. These differences are present in young and aged rodents and are independent of strain and species. Proteolipid protein and carbonic anhydrase-II transcripts, measured by real-time PCR, are approximately two to three times greater in males. Myelin basic protein and 2', 3'-cyclic nucleotide 3'-phosphodiesterase, measured by Western blots, are 20-160% greater in males compared with females. Surprisingly, both generation of new glia and apoptosis of glia, including oligodendrocytes, are approximately two times greater in female corpus callosum. These results indicate that the lifespan of oligodendrocytes is shorter in females than in males. Castration of males produces a female phenotype characterized by fewer oligodendrocytes and increased generation of new glia. These findings indicate that exogenous androgens differentially affect the lifespan of male and female oligodendrocytes, and they can override the endogenous production of neurosteroids. The data imply that turnover of myelin is greater in females than in males. Mu-calpain, a protease upregulated in degeneration of myelin, is dramatically increased at both transcriptional and translational levels in females compared with males. These morphological, molecular, and biochemical data show surprisingly large differences in turnover of oligodendrocytes and myelin between sexes. We discuss the potential significance of these differences to multiple sclerosis, a sexually dimorphic disease, whose progression is altered by exogenous hormones.

Proteolipid protein gene modulates viability and phenotype of neurons

Overexpression or lack of expression of proteolipid protein (PLP) gene by oligodendrocytes causes axonal pathology. It is unclear whether dysfunction of the PLP gene mediates its effects directly on neurons or indirectly by abnormal formation of myelin sheaths. We performed experiments using cocultures and conditioned media (CM) to test the direct effect of PLP gene expression on neurons. Non-glial cell lines were stably transfected with PLP or DM20 (an alternate splice variant of PLP) cDNAs. Immunocytochemistry and enhanced green fluorescent protein expression showed that translated products were synthesized and inserted into the plasma membrane in proper conformation. The number of surviving dorsal root ganglion (DRG) neurons was significantly less than controls when cocultured for 5 d with PLP-expressing cells. The number of degenerating neurons increased in a dose-dependent manner corresponding to increasing numbers of PLP-expressing cells. However, the number of surviving DRG neurons cocultured with DM20-expressing cells was comparable to that of controls, indicating that PLP-specific products contributed to decreased neuron survival. When DRG neurons were cultured with CM from PLP- or DM20-expressing cells, significantly fewer neurons survived with CM of PLP- but not DM20-expressing cells. This suggests that secreted factors from PLP-expressing cells contribute to neuronal death. Increased neuronal death found with PLP-expressing cells cannot be attributed to density-dependent artifacts, because in each experiment the density of different cell lines was similar. This effect of CM may be mediated by a negative pH shift elicited from PLP but not DM20 expression. These results indicate that PLP gene products directly modulate neuron viability.

An immortalized jimpy oligodendrocyte cell line: defects in cell cycle and cAMP pathway

Normal and jimpy oligodendrocytes in secondary cultures were transfected with plasmids containing the SV40 T-antigen gene expressed under the control of the mouse metallothionein-I promoter. Two immortalized stable cell lines, a normal (158N) and jimpy (158JP) cell line, expressed transcripts and proteins of oligodendrocyte markers, including proteolipid protein (PLP), myelin basic protein (MBP), and carbonic anhydrase II (CAII). Galactocerebroside and sulfatide were also detected with immunocytochemistry. Immunoelectron microscopy using gold particles showed that the truncated endogenous jimpy PLP was distributed throughout the cytoplasm and in association with the plasma membrane of cell bodies and processes. The length of the cell cycle in the jimpy oligodendrocytes in the absence of zinc was 31 h, about a 4-h longer cell cycle than the normal line. In the presence of 100 microM zinc, the cell cycle became 3 h shorter for both cell lines, with the jimpy cell cycle duration remaining 4 h longer than the normal line. Interestingly, the jimpy cell line showed a significant deficiency in stimulation via the cAMP pathway. While the level of oligodendrocyte markers (PLP, MBP, and CAII) were significantly increased by dibutyryl cAMP (dbcAMP) treatment in the normal cell line, no changes were observed in the jimpy cell lines. This observation, together with previous results showing jimpy oligodendrocyte's failure to respond to basic fibroblast growth factor (bFGF), suggests a role for PLP in a signal transduction pathway. Jimpy and normal oligodendrocytes transfected with the SV40T antigen gene, driven by the wild-type promoter of mouse metallothionein-I, continue to express properties of oligodendrocytes and therefore provide a powerful model to explore the function of myelin proteins and to dissect the complexity of the jimpy phenotype.

Abnormal Ca(2+) regulation in oligodendrocytes from the dysmyelinating jimpy mouse

Jimpy (jp) is a point mutation in the gene on the X chromosome which codes for the major myelin proteolipid protein. Most oligodendrocytes (OLs) in the jp mouse undergo cell death at the time when they should be actively myelinating. Loss of mature OLs results in severe CNS dysmyelination. Dying jp OLs have the morphology of apoptotic cells but it is not clear how the mutation activates biochemical pathways which lead to programmed death of OLs in jp CNS. There is compelling evidence from a number of systems that high levels of intracellular Ca(2+) ([Ca2+]i) can activate downstream processes which result in both apoptotic and necrotic cell death. To determine whether [Ca2+](i) dysregulation might be involved in the death of jp OLs, we used ratiometric imaging to determine levels of [Ca2+](i) in OLs cultured from jp and normal CNS and in immortalized cell lines derived from jp and normal OLs. Immortalized jp OLs and OLs isolated directly from jp brain both showed a similar elevation in [Ca2+](i) ranging from 60% to 150% over control values. A higher baseline [Ca2+](i) in jp OLs might increase their vulnerability to other insults due to abnormal protein processing or changes in signaling pathways which act as a final trigger for cell death.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

Endogenous opioid system in developing normal and jimpy oligodendrocytes: mu and kappa opioid receptors mediate differential mitogenic and growth responses

The early development of both neurons and neuroglia may be modulated by signaling through opioid mediated pathways. Neurons and astroglia not only express specific types of opiate receptors, but also respond functionally to opioids with altered rates of proliferation and growth. The present study was undertaken to determine if opioids also modulate development of the other major CNS macroglial cell, the oligodendrocyte (OL). Using well-characterized polyclonal antibodies specific for delta-, kappa-, and mu-opiate receptors, OLs grown in vitro were shown to express mu-receptors at a very immature stage prior to expression of kappa-receptors. This developmentally regulated sequence differs from the pattern of expression in neurons and astroglia. delta-receptors are apparently absent from cultured OLs. OLs also have physiologic responses to selective mu- and kappa-receptor agonists and antagonists. Exposure of relatively immature O4+ OLs to the mu-receptor agonist PL017 [H-Tyr-Pro-Phe(N-Me)-D-Pro-NH2] resulted in a significant enhancement in the rate of DNA synthesis. This effect, which was not observed in more mature MBP+ OLs, was entirely blocked by the antagonist naloxone. Although the kappa-receptor pathway appeared to be uninvolved in controlling proliferation, the kappa-receptor antagonist nor-binaltorphimine significantly increased the size of myelin-like membranes produced by the cultured OLs. Interestingly, OLs derived from the jimpy mouse, a mutant characterized by an almost complete lack of CNS myelin and premature death of OLs, were found to be deficient in kappa-opiate receptors. Our findings clearly show that OLs not only express specific opiate receptors, but also respond to changes in their level of stimulation in ways that could profoundly impact nervous system morphology and function. If opiate receptors are expressed by OLs in vivo, their pharmacological manipulation might provide a novel pathway for modulating OL and myelin production both during development and in demyelinated conditions.

In vitro death of jimpy oligodendrocytes: correlation with onset of DM-20/PLP expression and resistance to oligodendrogliotrophic factors

Severe hypomyelination in the jimpy (jp) mouse mutation results from premature death of most oligodendrocytes (OCs). We have applied an immunopanning technique to successfully purify oligodendroblasts (OBs) directly from neonatal jp brainstem in order to determine if their death during differentiation into OCs is preventable in culture by diffusible oligodendrogliotrophic factors. No significant differences in the yield (0.9-1.1 x 10(5) cells/brainstem) or viability (approximately 90%) of OB populations from jp and wild-type (wt) littermates were observed, indicating that cell death occurs at a later stage in the mutant lineage. When cultured in a basally defined, insulin-containing medium, wt and jp OBs died 1-2 days later as their differentiation into GalC+ OCs began. Survival was not enhanced by known trophic factors (ciliary neurotrophic factor, leukemia inhibitory factor, neurotrophin-3) for differentiating rat OCs. In medium conditioned by neonatally derived rat or wt mouse astrocytes, however, wt OBs survived terminal OC differentiation, expressing first GalC, then DM-20/PLP on their surface 1-2 days later, before elaborating myelin-like membrane. By contrast, jp OBs in sister cultures survived differentiation initially as well as their normal counterparts did but rapidly died thereafter, beginning at the time when PLP/DM-20 immunoreactivity became detectable on premature wt GalC+ OCs. Additionally under these conditions, there survived a minor population (<5%) of jp cells, including mature OCs, which expressed stunted membranes and DM-20/PLP immunoreactivity in their cytoplasm, and undifferentiated progenitors. This model supports the concept that OC death in jp is effected by an intrinsic program, one mechanistically related to jp PLP/DM-20 gene expression and refractory to trophic cues in the environment.