Spatial segregation of mRNA encoding myelin-specific proteins.

Abstract

The cellular and subcellular distributions of mRNAs encoding three myelin-specific proteins-myelin basic protein (MBP), proteolipid protein (PLP), and Po proteinwere studied in tissue sections of developing rat nervous systems by in situ hybridization. The developmental appearance of these mRNAs closely paralleled the appearance of the proteins they encode as determined by immunocytochemistry. mRNA encoding the extrinsic membrane protein, MBP, was concentrated around oligodendrocyte and Schwann cell nuclei during initial stages of myelination; as myelination proceeded, MBP mRNA became distributed diffusely over myelinated fibers. In contrast, mRNAs encoding the intrinsic membrane proteins, PLP and Po, remained concentrated around oligodendrocyte (PLP) and Schwann cell (Po) nuclei at all stages of myelination. These results establish that myelinating cells spatially segregate certain myelin-specific mRNAs. The presence of MBP mRNA within the cytoplasmic domains of myelin internodes indicates that protein sorting during myelination involves transportation of mRNA to specific subcellular sites.Myelin, a multilamellar compact membrane, surrounds many axons in the central and peripheral nervous systems (CNS and PNS). CNS myelin is formed by oligodendrocytes, which individually have the potential to form 30-40 different myelin internodes by extending long slender cytoplasmic processes that ensheath and spirally wrap around axons to form compact myelin. In the PNS, Schwann cells form single myelin internodes. The protein composition of CNS and PNS myelin is well characterized (1). Ultrastructural studies have shown that CNS and PNS myelination occurs in a systematic and predictable manner (2, 3), suggesting that myelin-forming cells utilize very efficient mechanisms for synthesis, transport, and integration ofmyelin components to accomplish the massive expansion of membrane.Formation of myelin represents a terminal phenotypic expression of oligodendrocytes and Schwann cells that must involve the expression of a myelin-specific genetic program. DNAs complementary (cDNA) to the mRNAs that encode several myelin proteins have been cloned recently (4-10). Using these clones, the time courses for expression of myelin-specific mRNAs have been analyzed during development (4-10). The present study focuses on the cellular distribution of three myelin-specific mRNAs. Using in situ hybridization, we determined when these mRNAs could first be detected during early stages of myelination and asked whether spatial segregation of certain mRNAs might occur. (Kodak) melted at 43°C and diluted 1:1 with 0.6 M ammonium acetate. After development of the emulsion, the sections were stained with hematoxylin, dehydrated, and overlaid with coverslips. Sections were photographed with a Zeiss photomicroscope using bright-field and dark-field optics.The cDNA clones used have been well characterized and described elsewhere: proteolipid protein (PLP) (6), MBP (4), and Po (8). The specificity of cDNA hybridization was demonstrated by pretrea...

Visualization of a Single Myelination Process of an Oligodendrocyte in Culture by Video Microscopy.

“…MBPimmunoreactivity was found at high levels in oligodendrocyte processes involved in myelination just before the onset of myelination (unpublished data). These immunocytochemical results are in good agreement with results indicating that MBPmRNA is first detected in the cell body of oligodendrocytes and later in the processes (13,23,26), while proteolipid protein (PLP) transcripts remain confined to the cell body (1, 23). Thus, MBPmay be considered one of the molecules required for onset of myelin sheath formation.…”

nkx2.2apromotes specification and differentiation of a myelinating subset of oligodendrocyte lineage cells in zebrafish

“…Consequently, PLP/DM20 expression marks oligodendrocyte lineage cells as they progress from progenitor to myelinating stages (Baumann and Pham-Dinh, 2001). RNA expressed by the MBP gene, which encodes myelin basic protein, becomes evident after initiation of PLP/DM20 expression and marks myelinating oligodendrocytes (Trapp et al, 1987;Peyron et al, 1997). In zebrafish, although expression of plp/dm20 is never evident at the proliferative ventricular zone of the spinal cord, it slightly precedes expression of mbp (data not shown).…”

Microtubule Organization and Stability in the Oligodendrocyte

“…Mature oligodendrocytes do not contain intermediate filaments, although oligodendrocyte progenitors have been shown to contain vimentin (Raff et al, 1984). A number of authors have noted a relationship between cytoskeletal elements and myelin constituents (Dyer and Benjamins, 1989;Gillespie et al, 1989;Wilson and Brophy, 1989), and microtubules also seem to play a role in the spatial segregation of myelin protein mRNAs in the oligodendrocyte (Colman et al, 1982;Trapp et al, 1987;Amur-Umarjee et al, 1990).…”