Inhibition of protein kinase C activity in mucolipidosis type-4: a model for a new pathogenetic mechanism in inborn errors of metabolism

Diffuse neuroaxonal involvement in mucolipidosis IV as assessed by proton magnetic resonance spectroscopic imaging

Mucolipidosis IV is an autosomal recessive disorder caused by mutations in MCOLN1, which codes for mucolipin, a transient receptor potential protein. In order to investigate brain metabolic abnormalities in mucolipidosis IV, we studied 14 patients (11 children, 3 adults) by proton magnetic resonance spectroscopic imaging. The ratios of N-acetylaspartate/ creatine-phosphocreatine and N-acetylaspartate/choline-containing compounds in patients with mucolipidosis IV were significantly reduced in all regions of interest except the parietal gray matter and thalamus. The ratios of choline-containing compounds/creatine-phosphocreatine was not significantly reduced in patients compared with controls. The ratio of N-acetylaspartate/creatine-phosphocreatine were significantly lower (P = .005) in the more neurologically impaired patients compared with the least impaired. For every region of interest, except for parietal gray matter, the ratio of N-acetylaspartate/creatine-phosphocreatine was lower in the more motorically impaired patient group. There was no difference for the ratio of N-acetylaspartate/creatine-phosphocreatine between younger and older patients. These findings suggest that mucolipidosis IV is largely a static developmental encephalopathy associated with diffuse neuronal and axonal damage or dysfunction. Mucolipin deficiency impairs motor more than sensory central nervous system pathways.

Modulation of protein kinase C by endogenous sphingosine: inhibition of phorbol dibutyrate binding in Niemann-Pick C fibroblasts

The abnormal and variable increase in levels of free sphingoid bases recently described in fibroblasts from Niemann-Pick C patients allowed us to investigate the modulation of protein kinase C in vivo by endogenous sphingosine. The specific binding of [20-3H]phorbol 12, 13-dibutyrate to the regulatory domain of membrane-bound protein kinase C was significantly decreased in fibroblasts from patients compared with controls. A pronounced difference between the two groups (P<0.0001) was demonstrated in low-density lipoprotein-supplemented medium, i.e. under conditions known to disclose abnormal mobilization of unesterified cholesterol in Niemann-Pick C fibroblasts. Furthermore the degree of impairment of [3H]phorbol 12,13-dibutyrate binding was highly correlated (r=0.95) with the sphingosine levels measured in fibroblasts from those patients. Scatchard analysis of the binding data indicated that Niemann-Pick C and control fibroblasts contained almost the same number of binding sites per cell. A 8-34-fold increase in Kd was measured in Niemann-Pick C fibroblasts with at least a 5-fold increase in sphingosine levels. Removal, by cell fractionation, of membrane-bound protein kinase C from the bulk of sphingosine induced a normalization of Kd values. The overall results suggest that protein kinase C inhibition is directly related to sphingosine accumulation.

Possible role for protein kinase C in the pathogenesis of inborn errors of metabolism

Protein kinase C (PKC) is a ubiquitous enzyme family implicated in the regulation of a large number of short- and long-term intracellular processes. It is hypothesized that modulation of PKC activity may represent, at least in part, a functional link between mutations (genotype) that lead to the pathological accumulation of naturally occurring compounds that affect PKC activity and perturbation of PKC-mediated substrate phosphorylation and cellular function in the corresponding diseases (phenotype). This model provides a unifying putative mechanism by which the phenotypic expression of some inborn errors of metabolism may be explained. Recent studies in a cell-free system of human skin fibroblasts support the hypothesis that alteration of PKC activity may represent the functional link between accumulation of sphingolipids and fatty acyl-CoA esters, and perturbation of cell function in sphingolipidoses and fatty acid oxidation defects, respectively. Further studies will elucidate the effects of these alterations on PKC-mediated short- and long-term cellular functions in these diseases, as well as the possible role of PKC in the pathogenesis of other diseases.