Differential sensitivity in the survival of oligodendrocyte cell lines to overexpression of myelin proteolipid protein gene products

The multiple roles of myelin protein genes during the development of the oligodendrocyte

It has become clear that the products of several of the earliest identified myelin protein genes perform functions that extend beyond the myelin sheath. Interestingly, these myelin proteins, which comprise proteolipid protein, 2′,3′-cyclic nucleotide 3′-phosphodiesterase and the classic and golli MBPs (myelin basic proteins), play important roles during different stages of oligodendroglial development. These non-myelin-related functions are varied and include roles in the regulation of process outgrowth, migration, RNA transport, oligodendrocyte survival and ion channel modulation. However, despite the wide variety of cellular functions performed by the different myelin genes, the route by which they achieve these many functions seems to converge upon a common mechanism involving Ca²⁺ regulation, cytoskeletal rearrangements and signal transduction. In the present review, the newly emerging functions of these myelin proteins will be described, and these will then be discussed in the context of their contribution to oligodendroglial development.

RTN3 inducing apoptosis is modulated by an adhesion protein CRELD1

Reticulon3 (RTN3), as a member of the reticulon family, is generally regarded as a novel human apoptosis-inducing protein. But the extensional role of RTN3 remains virtually unknown. Herein, we showed that cysteine rich with EGF like domains 1(CRELD1), a cell adhesion molecule played a critical role in atrioventricular septal defects and it had mutual effect with RTN3 in vitro. Furthermore, we discovered that ectopic CRELD1 could interact with ectopic or endogenous RTN3. CRELD1 bound with RTN3 so as to increase the localization of RTN3 on the plasma membrane and decreased the apoptotic activity of RTN3 moderately. Moreover, the tunicamycin-inducing cell apoptosis was partly suppressed by this kind of interaction mentioned above. These results suggested that CRELD1 could partly change the localization of RTN3 from the endoplasmic reticulum to the plasma membrane and modulate the apoptotic activity of RTN3 through binding with it.

The myelin proteolipid protein gene modulates apoptosis in neural and non-neural tissues

Mutations of the myelin proteolipid protein gene (Plp) are associated with excessive programmed cell death (PCD) of oligodendrocytes. We show for the first time that PLP is a molecule ubiquitously expressed in non-neural tissues during normal development, and that the level of native PLP modulates the level of PCD. We analyze three non-neural tissues, and show that native PLP is expressed in trophoblasts, spermatogonia, and cells of interdigital webbing. The non-neural cells that express high levels of native PLP also undergo PCD. The level of PLP expression modulates the level of PCD because mice that overexpress native PLP have increased PCD and mice deficient in PLP have decreased PCD. We show that overexpression of native PLP causes a dramatic acidification of extracellular fluid that, in turn, causes increased PCD. These studies show that the level of native PLP modulates the amount of PCD during normal development via a pH-dependent mechanism.

Disease-associated mutations cause premature oligomerization of myelin proteolipid protein in the endoplasmic reticulum

Pelizaeus-Merzbacher disease (PMD) is a dysmyelinating disease caused by mutations, deletions, or duplications of the proteolipid protein (PLP) gene. Mutant forms of PLP are retained in the endoplasmic reticulum (ER), and the resulting accumulation of mutant protein is thought to be a direct cause of oligodendrocyte cell death, which is the primary clinical feature of PMD. The molecular mechanisms underlying the toxicity of mutant PLP are however currently unknown. We report here that PMD-linked mutations of PLP are associated with the accelerated assembly of the protein into stable homooligomers that resemble mature, native PLP. Thus although WT PLP forms stable oligomers after an extended maturation period, most likely at the cell surface, mutant forms of PLP rapidly assemble into such oligomers at the ER. Using PLP mutants associated with diseases of varying severity, we show that the formation of stable oligomers correlates with the development of PMD. Based on these findings, we propose that the premature oligomerization of PLP in the ER of oligodendrocytes contributes to the pathology of PMD.

Expression of soma-restricted proteolipid/DM20 proteins in lymphoid cells

A new family of the myelin proteolipid protein (PLP/DM20) gene products, srPLP/DM20, has been identified recently in thymus and brain. In the central nervous system, srPLP/DM20 products are not localized in the myelin membrane, unlike their classic PLP/DM20 counterparts. In the immune system, the classic PLP/DM20 products appear to be expressed predominantly in thymic cortical epithelium. In this study, we examined the cellular expression of sr-PLP/DM20 proteolipids in lymphoid tissues and cells by immunohistochemistry, FACS analysis and RT-PCR. We found that in contrast to the classic PLP/DM20 products, sr-proteins are mainly expressed in developing thymocytes in thymus and in T- and B-lymphocytes in spleen. These results are of importance in our further understanding, not only the different role of these new PLP gene products in central and peripheral tolerance, but also the function of such products in lymphocyte biology.

Do cancer cells die because of Nogo-B?

Nogo-A is a potent neurite outgrowth inhibitory protein in vitro and is suggested to play a role in the lack of regeneration in the central nervous system of adult higher vertebrates. A shorter splice isoform, ASY/Nogo-B, has recently been reported to act as a proapoptotic protein, the loss of which would be typical for cancer cells. Here, we show that the osteosarcoma cell line SaOS-2 and the cell line CHO do express high levels of endogenous Nogo-B and that stable transfectants overexpressing high levels of Nogo-B do not differ significantly from the respective parental wild-type or control cell lines both in respect to cell proliferation and to spontaneous apoptosis or cell death induced by staurosporine and tunicamycin. The deletion of the second transmembrane domain of Nogo-B, which has been claimed to abolish its proapoptotic activity, leads to a shift of the protein from the ER to a cytoplasmic localization, suggesting that ER stress of highly overexpressed Nogo-B may lead to aversive cellular reactions under particular conditions. Our data do not support a function of Nogo-B as a physiological pro-apoptotic protein in certain types of cancer.