Functional impacts of transgenic overexpression of UDP-galactose

Abstract

1 PLP-CGT TRANSGENIC MICE<br /> Galactosylceramide (GalC) and its sulfated derivative (sulfatide) are major lipid components of the myelin membrane constituting about 30% of its lipid content. They fulfill essential functions in oligodendrocyte differentiation, formation and maintenance of myelin. Transgenic mice overexpressing UDPgalactose: ceramide galactosyltransferase (CGT) in oligodendrocytes under the control of the proteolipid protein (PLP) promoter were generated. Elevated CGT activity led to a significant increase in non-hydroxy fatty acid galactosylceramide. In contrast, however, a substantial decrease in the normally predominant alpha-hydroxy fatty acid form of GalC was observed. As a consequence, total GalC levels were only marginally elevated in the transgenic mice. These mice exhibit deficits in motor behavior and develop a progressive hind limbs paralysis. In vitro study of oligodendrocyte development indicates an increase in oligodendrocyte number. This result was confirmed in vivo, where a significant increase of PLP positive cells was observed in the corpus callosum of PLP-CGT mice. Surprisingly, more cells expressing the MBP protein in vitro did not have the morphology of a myelin-forming oligodendrocyte, indicating a possible inhibition of the synthesis of the myelin membrane in vitro. Ultrastructural analysis revealed severe impairment in the formation of compact myelin and a progressive demyelination in the central nervous system, accompanied by axonal degeneration, vacuolation and massive astrogliosis. The composition of myelin proteins was unchanged with the exception of a significant progressive up regulation of the raft-associated MAL protein. MAL up regulation was not due to an increase in mRNA expression, suggesting reduced degradation of MAL. Taken together these results strongly suggest that the HFA- to NFA-GalC ratio is an important factor in the formation and maintenance of compact myelin.<br /> 2 PLP-PST TRANSGENIC MICE<br /> Oligodendrocyte differentiation and myelin formation are two subsequent cellular mechanisms timely separated. During development, oligodendrocyte progenitor cells migrate from their place of origin to their destination, expressing a high level of the polysialic acid (PSA). PSA is a long linear homopolymer of sialic acid. In the vertebrate embryo, PSA appears exclusively attached covalently to a cell surface protein called neural cell adhesion molecule (NCAM). In this context, PSA has been shown to be an important modulator of cell interactions during development, for example, in axonal pathfinding and branching, response of axons to loss of synaptic activity and in the migration of muscle cells. The function of PSA has been proposed to influence not only interactions mediated by NCAM itself, but cell-cell interactions in general. PSA is also known to be involved in the migration, proliferation and differentiation of oligodendrocyte precursors (OPCs). In order to investigate potential physiological consequences of increased level of polysialic acid (PSA) in oligodendrocytes, we generated transgenic mice that overexpress polysialyltransferase (PST/ST8Sia IV) under the control of the proteolipid protein (PLP) promoter. In the central nervous system of PLP-PST mice (PLP-PST), PST was overexpressed by oligodendrocytes. furthermore, increase of PSA concentration in the CNS induces no behavioral deficit of PLP-PST mice. Most importantly, elevation of PSA concentration induces increase of differentiated oligodendrocytes of the spinal cord and brainstem, but a reduction of oligodendrocyte numbers in the corpus callosum. The number of oligodendrocyte progenitor cells was unchanged in all the brain regions investigated. Moreover, biochemical analysis indicates reduction of MBP level during myelination but normal expression of other oligodendrocyte/myelin specific proteins. Structural analysis of the myelin sheath indicates a normal myelination in the CNS and PNS of PLP-PST transgenic mice. Importantly, PLP-PST mice demyelinate at adult age. Demyelination was accompanied by redundant myelinated axons and axonal degeneration leading to vacuolation.