Abnormal compact myelin in the myelin-deficient rat: absence of proteolipid protein correlates with a defect in the intraperiod line.

Abstract

The cervical spinal cords of 23-day-old myelin-deficient (md) rats, an X chromosome-linked myelin mutant, and their normal littermates were studied by light and electron microscopy, immunocytochemistry, and in situ hybridization. Light microscopy showed that there were scattered myelinated fibers in the md rat, particularly in the lateral and ventral columns. Ultrastructural examination of these fibers showed that the myelin often had many lamellae that were tightly compacted, but in which the intraperiod line was abnormally fused at most places, resulting in a minor alteration of the myelin periodicity. Immunocytochemical staining of adjacent sections following a variety of fixation methods showed that the myelinated fibers were positive for inyelin basic protein but negative for proteolipid protein (PLP). In situ hybridization using cDNA probes to these proteins showed a severe diminution of the mRNAs for both proteins. These findings provide further support for an abnormality in genetic regulation of PLP as has been described in another X chromosome-linked mutant, the jimpy mouse. Despite the lack of PLP, however, a few myelinated fibers are formed in the md rat, but the myelin formed in general lacks a normal intraperiod line, a site at which this protein is thought to be located.Many axons in the central nervous system (CNS) are surrounded by myelin sheaths, multilamellar compact membranes produced by oligodendrocytes. How these membranes are formed and maintained on a molecular basis is unclear, but the study of perturbations of the normal myelinogenesis in myelin mutants has led to progress in unraveling this developmental process. Two molecules, myelin basic protein (MBP) and proteolipid protein (PLP), comprise -60-80o of the total proteins of CNS myelin (1). Several murine mutants display a hypomyelination of the CNS that is accompanied by a lack or dramatic reduction of MBP and/or PLP. In two of such mutants, the shiverer and jimpy mice, considerable progress has been made recently in elucidating the possible molecular basis of their hypomyelination. In the shiverer mouse there is a deletion of a large part of the gene coding MBP (2) and CNS myelin is usually poorly compacted. The CNS and peripheral nervous system (PNS) are negative for MBP when stained immunocytochemically, although the PNS appears structurally normal. The hypomyelination injimpy mice is more severe than in shiverer and appears to be associated with a partial deletion of the coding region of PLP mRNA due to an aberrant splicing of the primary gene transcript (3-6). PLP, an intrinsic membrane protein thought to play a role in compaction of the extracellular leaflets of CNS myelin, is absent or dramatically reduced in jimpy mice (7-9).The myelin-deficient (md) rat is a sex-linked recessive mutant that has a severe CNS myelin deficit similar to jimpy (10,11).Previous morphological studies have failed to find myelin in the CNS of md rats (11,12) and the number of intrafascicular oligodendrocytes appears to be reduced in regions no...

Modeling the Mutational and Phenotypic Landscapes of Pelizaeus-Merzbacher Disease with Human iPSC-Derived Oligodendrocytes

“…[19][20][21][22][23] PLP1's strong cell-type specificity, abundance in myelin, and inter-species conservation all suggest that it fills an indispensable role in oligodendrocyte and myelin biology. However, PLP1's function in oligodendrocytes and myelin has only recently begun to be elucidated [24][25][26][27] and is still very much in question.…”

Transplantation of an oligodendrocyte cell line leading to extensive myelination.

“…Newborn Sprague-Dawley rats were purchased from Taconic Farms and the breeding colony of md rats was maintained as described (23,24). CG4 cells were kindly provided by J.-C. Louis (presently at Amgen Biologicals).…”

“…Recent studies have shown that transplantation ofearly passages of OPs (20) expanded in the presence of growth factors or immortalized with a conditional oncogene (21), resulted in myelination of naked axons within an irradiated and chemically induced spinal cord lesion in adult rats. However, the myelinating potential ofthese grafted cells decreased considerably (20) (24,25).MATERIALS AND METHODS Cells and Animals. Newborn Sprague-Dawley rats were purchased from Taconic Farms and the breeding colony of md rats was maintained as described (23,24).…”

Proteolipid Protein Regulates the Survival and Differentiation of Oligodendrocytes

“…Proteolipid protein (PLP) and DM20, which is an alternatively spliced isoform of the PLP gene (Nave et al, 1987), constitute ϳ50% of the protein in myelin (Greenfield et al, 1971). It is generally accepted that one of the biological functions of PLP is to maintain the structural integrity of the myelin membrane (Duncan et al, 1987;Boison and Stoffel, 1994;Nave, 1994;Boison et al, 1995).…”