Developmental Expression of the Myelin Proteolipid Protein and Basic Protein mRNAs in Normal and Dysmyelinating Mutant Mice

Expression of the myelin proteolipid protein (PLP) was examined in the nuclei and polysomes of 12-27-day-old quaking, jimpy, and shiverer mouse brains and in 2-27-day-old normal brains and compared with expression of the myelin basic proteins (MBPs). Northern blots showed the presence of multiple mouse PLP RNAs, the developmental expression of which coincided with myelination. Two major mouse PLP RNAs, 3.5 and 2.6 kilobases in length, were observed in both cytoplasmic polyribosomes and nuclei, and, in addition, a larger 4.6-kilobase PLP RNA was observed in nuclei. Quantitative measurements with slot blot analyses showed that the levels of PLP and MBP RNAs peaked simultaneously at 18 days in nuclei but that maximal levels of PLP RNA lagged behind MBP RNA by several days in the polysomes. The developmental expression of both major classes of myelin protein mRNAs was affected in all three mutants. In shiverer brains, the levels of PLP mRNA in polysomes and nuclei were only 30-55% of control levels after 15 days. Thus, the deletion of a portion of the MBP gene appeared to have a major effect on the expression of the PLP gene in this mutant. In jimpy mice, where the mutation has been shown to involve the PLP gene, expression of MBP mRNA was also severely reduced, to less than 25% of control values. In quaking brains, the expression of each gene followed its own developmental course, different from each other and different from the normal mouse. The extent to which the expression of PLP and MBP was affected by the quaking mutation depended on the age at which it was examined.     

Axons regulate Schwann cell expression of the major myelin and NGF receptor genes

The elaboration of myelin by Schwann cells is triggered by contact with appropriate peripheral axons. Among the most prominent features of this interaction is the activation and high-level expression of the genes encoding the major myelin proteins P0 and Myelin Basic Protein (MBP). Although the initial induction of these genes is thought to be dependent upon contact with axons, neither the inductive signal of the axon nor the receptor and associated second messenger system of the Schwann cell that transduces this signal has been identified. In this report, we demonstrate that expression of the P0 and MBP genes in rapidly myelinating Schwann cells is sharply reduced upon withdrawal of axons, but that this expression can be substantially restored by agents that raise the intracellular concentration of cyclic AMP. We further show that Schwann cell expression of a third gene, i.e. that encoding the Nerve Growth Factor receptor, is strongly activated by the withdrawal of axons, and that this activation is largely independent of cAMP.     

Morphometric Analysis of Normal, Mutant, and Transgenic CNS: Correlation of Myelin Basic Protein Expression to Myelinogenesis

The neurological mutant mice shiverer (shi) and myelin deficient (shimld) lack a functional gene for the myelin basic proteins (MBP), have virtually no myelin in their CNS, shiver, seize, and die early. Mutant mice homozygous for an MBP transgene have MBP mRNA and MBP in net amounts approximately 25% of normal, have compact myelin, do not shiver or seize, and live normal life spans. We bred mice with various combinations of the normal, transgenic, shi, and shimld genes to produce mice that expressed MBP mRNA at levels of 0, 5, 12.5, 17.5, 50, 62.5, and 100% of normal. The CNS of these mice were analyzed for MBP content, tissue localization of MBP, degree of myelination, axon size, and myelin thickness. MBP protein content correlated with predicted MBP gene expression. Immunocytochemical staining localized MBP to white matter in normal and transgenic shi mice with an intensity of staining comparable to the degree of MBP gene expression. An increase in the percentage of myelinated axons and the thickness of myelin correlated with increased gene expression up to 50% of normal. The percentage of myelinated axons and myelin thickness remained constant at expression levels greater than 50%. The presence of axons loosely wrapped with oligodendrocytic membrane in mice expressing lower amounts of MBP mRNA and protein suggested that the oligodendroglia produced sufficient MBP to elicit axon wrapping but not enough to form compact myelin. Mean axon circumference of myelinated axons was greater than axon circumference of unmyelinated axons at each level of gene expression, further evidence that oligodendroglial cells preferentially myelinate axons of larger caliber.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic AMP-elevating agents induce the expression of MAP kinase phosphatase-1 in PC12 cells

Stimulation of pheochromocytoma PC12 cells by cAMP-elevating agents caused the induction of the immediate early gene 3CH134, which encodes MAP kinase phosphatase-1 (MKP-1). Forskolin was as potent as serum in stimulating MKP-1 gene expression, whereas dibutyryl-cAMP and neuropeptide PACAP were less effective. Induction of the MKP-1 gene was accompanied by neo-synthesis of MKP-1 protein. MAP kinase activation was not involved in the cAMP-induced MKP-1 gene expression. The MAP kinase inactivation, that would result from MKP-1 induction in response to increased intracellular cAMP level, contributes to explain how hormones or neurotransmitters signaling through cAMP influence cell growth and differentiation.     

Developmental expression of the myelin gene MOBP in the rat nervous system

The myelin-associated/oligodendrocyte basic proteins (MOBPs) are recently discovered constituents of myelin and are small, cytoplasmic, and highly basic proteins exclusively expressed postnatally by oligodendrocytes. Due to a clustering of positively charged amino acids observed in the most abundant MOBP isoform similar to myelin basic protein (MBP) and P0, it was speculated that MOBP could function in myelin sheath compaction. The present report strongly supports this view. A direct comparison of MBP and proteolipid protein (PLP) gene expression with that of MOBP by in situ hybridization revealed a very similar regional distribution. It was found that MOBP expression was abundant in the rat CNS at postnatal day 15 (P 15) but is restricted to densely myelinated regions. In contrast to MBP and PLP, expression of MOBP was undetectable in the peripheral nervous system during the entire development. Interestingly, MOBP mRNA was localized in oligodendrocyte processes even at early postnatal stages and throughout development. MOBP showed a very specific timing of expression: in spinal cord and brain, MOBP gene expression occurred significantly later (2–3 days) than that of MBP and PLP, but slightly earlier than myelin oligodendrocyte glycoprotein gene expression. MOBP proteins appeared in spinal cord and brain stem also after MBP protein, suggesting that the MOBPs functionally act after the structural myelin proteins MBP and PLP. Our findings imply a function of MOBP during the late steps of myelin formation, presumably at the initiation of sheath compaction.     

Expressing antisense P0 RNA in Schwann cells perturbs myelination.

Primary Schwann cells were infected in vitro with a recombinant retrovirus expressing a dominant selectable marker, neomycin phosphotransferase (conferring resistance to the drug G418), and antisense P0 RNA under the control of the human beta-actin promoter. A proportion of the G418-resistant cells failed to form myelin when cocultured with dorsal root ganglion neurons under conditions that promote Schwann cell differentiation. These cells expressed high levels of P0 antisense RNA. Among the impaired cells, the majority had segregated and ensheathed individual axon but had not differentiated further. They did not express P0 but did express myelin- associated glycoprotein and galactocerebroside. A minority of partially inhibited Schwann cells were also observed that elaborated thin myelin sheaths containing variable numbers of compacted and noncompacted lamellae. These data indicate that restricting the level of P0 expression inhibits spiralling of the Schwann cell membrane and subsequent compaction.     

Expression of members of the Saccharomyces cerevisiae hsp70 multigene family

The hsp70 multigene family of Saccharomyces cerevisiae is a complex multigene family, composed of members exhibiting complex patterns of regulation. Expression of some members is induced after a heat shock, whereas expression of others is repressed. Some members of the family are expressed during exponential growth. One gene, SSA3, shows an unusual pattern of expression during approach to stationary phase. While most RNAs decrease in abundance, SSA3 RNA levels dramatically increase. The constitutive expression of SSA3 in cells lacking adenylate cyclase activity suggests that cAMP modulates SSA3 expression.