Cell-specific fatty acylation of proteins in cultured cells of neuronal and glial origin

Lipopolysaccharide induction of MARCKS-related protein and cytokine secretion are differentially impaired in microglia from LPS-nonresponsive (C3H/HeJ) mice

Many events involved in activation of microglia and leukocytes by lipopolysaccharide (LPS) are mediated by protein kinase C (PKC), and we have recently demonstrated that a major PKC substrate, MARCKS-related protein (MRP), is selectively induced by LPS in murine microglia. In microglia from LPS-nonresponsive (C3H/HeJ) mice, induction of MRP and secretion of CSF-1 required much higher LPS concentrations (> or = 100 ng/ml) than in normal (C3H/OuJ) microglia (< or = 10 ng/ml). By contrast, TNF alpha production was not significantly increased in C3H/HeJ microglia even at 1 microgram LPS/ml. Microglia expressed PKC isoforms alpha, beta, delta, and zeta (but not gamma and epsilon); PKC isoform levels were similar in both normal and C3H/HeJ microglia and no significant change in response to LPS was noted. Our results indicate that LPS alters PKC substrate (rather than kinase) expression, and that the Lpsd mutation in C3H/HeJ mice differentially affects regulation of several gene products implicated in microglial function.

Lipopolysaccharide stimulates differential expression of myristoylated protein kinase C substrates in murine microglia

Microglia rapidly respond to lipoplysaccharide (LPS) by transformation from resting to active states and secretion of several neuro- and immuno-regulators including tumour necrosis factor alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), and interleukin 6 (IL-6). With longer LPS treatment, microglia are converted to reactive or phagocytic states with characteristics similar to macrophages in inflammation and injury processes. We have investigated LPS-mediated changes in two myristoylated substrates of protein kinase C (PKC): MARCKS (myristoylated alaninerich C kinase substrate) and MRP (MARCKS-related protein). Within 6 hours of addition, LPS induced a twofold increase in [3H]myristoylated and immunoreactive MARCKS protein and a sevenfold increase in MRP. The differential effect of LPS on expression of MRP vs. MARCKS was even more dramatic at the level of transcription: S1 nuclease protection assays revealed a 40-fold increase in MRP mRNA levels (maximum at 4-6 hours), whereas a threefold increase was observed for MARCKS. TNF alpha and colony-stimulating factor 1 (CSF-1), two cytokines which are induced by LPS, did not reproduce the observed effect of LPS on MARCKS and MRP gene transcription. CSF-1 also induced differential transcription of MRP, but of lower magnitude (threefold) and more sustained than by LPS. Accordingly, these two substrates for PKC are differentially up-regulated by LPS, apparently independent of TNF alpha or CSF-1.

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.

Dissociation of phosphorylation and translocation of a myristoylated protein kinase C substrate (MARCKS protein) in C6 glioma and N1E-115 neuroblastoma cells

An 80-kDa protein labeled with [3H]myristic acid in C6 glioma and N1E-115 neuroblastoma cells has been identified as the myristoylated alanine-rich C kinase substrate (MARCKS protein) on the basis of its calmodulin-binding, acidic nature, heat stability, and immunochemical properties. When C6 cells preincubated with [3H]myristate were treated with 200 nM 4 beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), labeled MARCKS was rapidly increased in the soluble digitonin fraction (maximal, fivefold at 10 min) with a concomitant decrease in the Triton X-100-soluble membrane fraction. However, phosphorylation of this protein was increased in the presence of beta-TPA to a similar extent in both fractions (maximal, fourfold at 30 min). In contrast, beta-TPA-stimulated phosphorylation of MARCKS in N1E-115 cells was confined to the membrane fraction only and no change in the distribution of the myristoylated protein was noted relative to alpha-TPA controls. These results indicate that although phosphorylation of MARCKS by protein kinase C occurs in both cell lines, it is not directly associated with translocation from membrane to cytosol, which occurs in C6 cells only. The cell-specific translocation of MARCKS appears to correlate with previously demonstrated differential effects of phorbol esters on stimulation of phosphatidylcholine turnover in these two cell lines.