A 42 K protein of chick sciatic nerve is immunologically related to PO protein of peripheral nerve myelin

Abstract

The major protein (PO) in PNS myelin is an integral membrane glycoprotein with a molecular weight of about 30 K. The level of PO protein in the developing sciatic nerve of the chicken was monitored by a solid-phase immunoassay and densitometry of Coomassie blue stained polyacrylamide gels. The most rapid rate of accumulation of PO protein occurred after 16 days of embryonic development. In addition to the 30 K PO protein, a number of higher molecular weight proteins could be distinctly detected by immunoblotting. Amongst these high molecular weight proteins, a species with an apparent molecular weight of 42 K was specifically immunostained with epitope-selected polyclonal antibodies against PO protein. This 42 K protein could be first detected after 16 days of embryonic development and increased rapidly following the pattern of myelination in the sciatic nerve. The enzyme endoglycosidase F, which specifically removes N-asparagine linked high mannose and complex carbohydrates from glycoproteins, converted the PO and 42 K proteins to lower molecular weight forms, which could be specifically immunostained by epitope selected polyclonal antibodies to the PO protein. Subcellular fractionation of the 17-day embryonic nerve demonstrated that the 42 K protein was enriched in myelin and microsomal subfractions relative to the total homogenate. These results indicate that the 42 K immuno-crossreactive protein might be chemically and functionally related to the PO protein of the PNS myelin.

The apolipoprotein A-I gene is actively expressed in the rapidly myelinating avian peripheral nerve.

“…Various morphological, physicochemical, and recent biochemical approaches (Lees and Brostoff, 1984;Kirschner et al, 1984;Lemke and Axel, 1985) indicate that the PO protein is an integral membrane protein of the PNS myelin. Recent experiments from our laboratory have demonstrated that the concentration of PO protein triples between day 16 and 18 of embryonic development in the chicken (Nunn et al, 1987). This confirmed previous investigations by Uyemura et al (1979): the ontogeny of PO protein parallels myelin deposition in the peripheral nerve and indicates the period of active myelination in the chick sciatic nerve is between the 14-d embryonic stage and ~5 d posthatch.…”

Molecular cloning of cDNAs that encode the chicken Po protein: Evidence for early expression in avians

“…Whether these different chicken Po mRNAs correspond to different molecular forms of mature Po and, if so, whether they are present in myelin remain to be determined. In this context, it is interesting to note that high-molecular-weight proteins are indeed recognised in nerve by an anti-Po antibody during early devclopnient ol' chicken embryos (Nunn et al, 1987;M . Barbu, personal observations).…”

Secretion and processing of apolipoprotein A-I in the avian sciatic nerve during development

“…The incorporation of [351S-methionine into compact myelin Po increased approximately 25-fold (Fig. 4), while the mass of compact myelin protein per wet weight in- (Uyemura et al, 1979;Nunn et al, 1987). At the 15-day embryonic stage in the chick, the myelin surrounding the axon is very small and loosely-bound, and is largely isolated from the intracellular fraction.…”

“…The ontogeny of the Po protein corresponds to the deposition of myelin in the peripheral nerve (Uyemura et al, 1979;Nunn et al, 1987). In order to correlate apo A-I biosynthesis with the various stages of myelination, endoneurial slices of 15-day embryo, 17-day embryo, and 1 -day-old posthatch chick sciatic nerves, incubated with [351S-methionine for 6 hr at 37"C, were also assayed for nascent Po protein.…”