Tuesday, February 21, 2006

Glia and Myelin

Changing places : Nature Reviews Neuroscience



: "Research Highlight
Nature Reviews Neuroscience , | doi:10.1038/nrn1865
Glia: Changing places
Jane Qiu
Although myelin serves similar functions in the PNS and CNS, its main protein constituent differs: the type I integral membrane protein P0 (protein zero) is present in PNS myelin, whereas the tetraspan membrane protein PLP (proteolipid protein) resides in the CNS. It is thought that P0 was initially the primary structural protein of both PNS and CNS myelin � which first appeared 440 million years ago in cartilaginous fishes � but was replaced by PLP in the CNS after the divergence of the bony fishes 400 million years ago. What could be the benefits of the P0 to PLP conversion during evolution?
To address this issue, Yin and colleagues reversed the evolutionary step by generating transgenic mice that expressed P0, rather than PLP, in the CNS. In these P0-CNS animals, the level of P0 expression in the CNS is similar to that of PLP in normal mice, and replacing PLP with this ancestral protein in the CNS does not affect the expression of other myelin proteins. In addition, P0, like PLP, is able to stabilize compact CNS myelin, and its distribution is indistinguishable from that of PLP in wild-type mice. Electron microscopy studies show that P0-CNS myelin is structurally similar to its PNS counterpart, and has greater periodicity (membrane spacing) than that of wild-type CNS myelin.
Next, the researchers studied the effect of replacing PLP with P0 by measuring the animals' motor function. In P0-CNS mice, motor performance was normal at 6 months of age, but declined significantly (by 90%) by 1 year � by which time 50% of the animals had died. These observations are consistent with the precocious accumulation of the amyloid precursor protein �"

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