Effects of Monensin on Posttranslational Processing of Myelin Proteins

Abstract

Rat brain slices were incubated with [3H]palmitic acid and [14C]glycine to label the lipid and protein moieties, respectively, of myelin proteolipid protein (PLP). The effects of monensin on posttranslational processing of proteins were examined by measuring the appearance of [14C]glycine- and [3H]palmitate-labeled proteins in myelin and myelin-like fractions. At 0.01 and 0.10 microM, monensin did not appreciably affect total lipid or protein synthesis; higher concentrations caused increased inhibition. Monensin at 0.10 microM markedly decreased the appearance of [14C]glycine-labeled PLP in myelin, but had little effect on the 14C basic proteins or the incorporation of [3H]palmitic acid into total or myelin PLP. The same relative effect was apparent at higher monensin concentrations. In the myelin-like fraction, monensin at 0.10 microM also depressed entry of [14C]glycine into protein comigrating with PLP, and again had no effect on incorporation of [3H]palmitic acid. In addition, monensin increased the [3H]palmitate label associated with two high-molecular-weight proteins in the myelin-like fraction with no concomitant increase in [14C]glycine label.

Vesicular transport of myelin proteolipid and cerebroside sulfates to the myelin membrane

“…Evidence exists suggesting that cerebroside sulfates and PLP are delivered to the myelin membrane through similar mechanisms. The transport of either of these components is disrupted by monensin (Townsend and Benjamins, 1983;Townsend et al, 1982Townsend et al, , 1984Sat0 et al, 1986) and by colchicine (Bizzozero et al, 1982b;Townsend et al, 1982Townsend et al, , 1984. Furthermore, recent studies from our laboratories strongly suggested the possibility of an associated or coupled transport of both components.…”

“…Burkart et al (1982) suggested that cerebroside sulfates are transported in vesicles that are "pre myelin" membranes associated with lysosomes. There is ample experimental evidence demonstrating that using different substances such as colchicine (Bizzozero et al, 1982a) or monensin (Townsend and Benjamins, 1983), or in the absence of Caz++ or in the presence of C o 2 + + (Pasquini et al, 1987) it is possible to produce a marked decrease in the entry or assembly of either cerebroside sulfates or PLP into myelin. Recently, in our laboratory, we have been able to demonstrate that when the synthesis of cerebroside sulfates is inhibited with L cycloserine, the entry of PLP into myelin is also inhibited, suggesting that both constituents could be cotransported from their site of synthesis in the oligodendroglial cell to the myelin membrane (Pasquini et al, 1989).…”

Transport of proteolipid protein to the plasma membrane does not depend on glycosphingolipid cotransport in oligodendrocyte cultures

“…Proteolipid protein (PLP) is the most abundant myelin protein, constituting about 50% of the total protein content in myelin (Lees and Brostoff, 1984). Studies carried out by several investigators have established that PLP is synthesized at the rough endoplasmic reticulum (RER), processed through the Golgi apparatus (Colman et al, 1982;Nussbaum and Roussel, 1983;Schwob et al, 1985) and transported in vesicles to the myelin membrane (Townsend and Benjamins, 1983). Although there are no lipids unique to myelin, these membranes are remarkably enriched in galactosyl cerebroside (GalCer) and sulfatide (Norton and Cammer, 1984).…”

Proteolipids in developing rat brain

“…Washed total lipid extracts of cerebral hemispheres, medulla oblongata and spinal cord of 10,20,30,40,90,, and 540 day old rats were prepared by the procedure of Folch et al (29). Content of PLP in washed lipid extracts was determined according to Lowry et al (30), modified by (31) and (32).…”

“…The content of proteolipid protein (PLP) in lipid extracts of brain during the development (8,9), its appearance and accumulation in the myelin (10)(11)(12) and other subcellular particles (13), as well as the changes of its amino acid composition (9,14) and the rate of turnover (15,16) were studied. In recent years an intensive study of problems concerning the synthesis of myelin PL, their posttranslational modification, transport and assembly into the myelin membrane has been expanded (17)(18)(19)(20). However, the properties and functions of PL in membranes are, probably, determined to a considerable extent by the lipid component of these complexes as well.…”