G(M2)-ganglioside metabolism in situ in mucolipidosis IV fibroblasts

Membrane traffic and turnover in TRP-ML1-deficient cells: a revised model for mucolipidosis type IV pathogenesis

The lysosomal storage disorder mucolipidosis type IV (MLIV) is caused by mutations in the transient receptor potential-mucolipin-1 (TRP-ML1) ion channel. The "biogenesis" model for MLIV pathogenesis suggests that TRP-ML1 modulates postendocytic delivery to lysosomes by regulating interactions between late endosomes and lysosomes. This model is based on observed lipid trafficking delays in MLIV patient fibroblasts. Because membrane traffic aberrations may be secondary to lipid buildup in chronically TRP-ML1-deficient cells, we depleted TRP-ML1 in HeLa cells using small interfering RNA and examined the effects on cell morphology and postendocytic traffic. TRP-ML1 knockdown induced gradual accumulation of membranous inclusions and, thus, represents a good model in which to examine the direct effects of acute TRP-ML1 deficiency on membrane traffic. Ratiometric imaging revealed decreased lysosomal pH in TRP-ML1-deficient cells, suggesting a disruption in lysosomal function. Nevertheless, we found no effect of TRP-ML1 knockdown on the kinetics of protein or lipid delivery to lysosomes. In contrast, by comparing degradation kinetics of low density lipoprotein constituents, we confirmed a selective defect in cholesterol but not apolipoprotein B hydrolysis in MLIV fibroblasts. We hypothesize that the effects of TRP-ML1 loss on hydrolytic activity have a cumulative effect on lysosome function, resulting in a lag between TRP-ML1 loss and full manifestation of MLIV.

Rapid detection of the two common mutations in Ashkenazi Jewish patients with mucolipidosis type IV

Among Ashkenazi Jewish individuals with mucolipidosis IV (ML IV), two mutations in the ML IV gene, IVS3-1A --> G and delEX1-EX7, account for more than 95% of disease alleles. The reported method of genotyping for the delEX1-EX7 mutation involves a cumbersome multistep procedure. In the present study, a new simplified one-step procedure is described that detects this mutation in both patients and carriers. An improved procedure is also described for detection of the IVS3-1A --> G mutation. Using these improved procedures, we have characterized the ML IV mutant alleles in 27 patients and 95 of their relatives from 22 families, and in 123 unrelated and unaffected Ashkenazi Jewish controls. Of the 27 ML IV patients, 16 patients (59.3%) were found to be homozygous for the IVS3-1A --> G mutation and 1 patient (3.7%) homozygous for the delEX1-EX7 mutation. Additionally, 9 patients (33.3%) were compound heterozygotes for IVS3-1A --> G/delEX1-EX7. Among the 123 Ashkenazi Jewish controls, two individuals were identified as heteroallelic with one IVS3-1A --> G mutation (carrier frequency: approximately 1 in 61); none showed the delEX1-EX7 mutation. The modifications described here provide a more facile means of genotyping patients and carriers and expand the possibilities for screening at-risk populations.

Delayed lysosomal metabolism of lipids in mucolipidosis type IV fibroblasts after LDL-receptor-mediated endocytosis

We specifically probed the low-density lipoprotein-receptor-dependent endosomal/lysosomal pathway of lipid degradation in control and mucolipidosis type IV fibroblasts using either [choline-methyl-14C]sphingomyelin in complex with apolipoprotein E, or cholesteryl [14C]oleate-labelled low-density lipoprotein as a substrate. Mucolipidosis type IV fibroblasts metabolized [14C]sphingomyelin and cholesteryl [14C]oleate significantly more slowly than controls and fibroblasts from patients with Hurler disease or Niemann-Pick disease type C. So far, no lysosomal enzyme deficiency has been reported for mucolipidosis type IV. Rather, the defect in mucolipidosis type IV cells has recently been suggested to be related to intracellular trafficking. Our results suggest that the defect in mucolipidosis type IV also affects the low-density lipoprotein-receptor-mediated endocytosis pathway.

Mucolipidosis type IV

Mucolipidosis type IV (MLIV) is a neurodegenerative lysosomal storage disorder characterized by psychomotor retardation and ophthalmological abnormalities, including corneal opacities, retinal degeneration, and strabismus. Severely affected as well as milder patients have been described. Over 80% of the MLIV patients are Ashkenazi Jews; the estimated heterozygote frequency in this population is 1/100. The disease is classified as a mucolipidosis due to the simultaneous lysosomal storage of lipids together with water-soluble substances. A broad spectrum of lipids and acid mucopolysaccharides were identified as the storage substances. Kinetic studies demonstrated that this heterogeneous storage stems from an abnormal endocytosis process in cells from MLIV patients of membrane components from late endosomes to the lysosomes and/or delayed efflux to the Golgi apparatus. The MLIV gene was mapped to chromosome 19p13.2--13.3 where a novel gene, MCOLN1, with MLIV-causing mutations, was identified. Two mutations were found among 95% of the Ashkenazi MLIV alleles, including an intronic acceptor splice-site mutation in 72% of the alleles and a partial gene deletion in 23%. Each of these mutations was associated with a defined haplotype in this chromosomal region. Other mutations were mostly identified in single, Ashkenazi and non-Ashkanazi patients, including missense, nonsense nucleotide deletions, and insertions. All mutations but one were identified in patients exhibiting the severe phenotype, an in-frame amino acid deletion was identified in a mild patient. MCOLN1 encodes a 580 aa protein, mucolipin 1, which is a member of a new protein family of unknown function at present, the mucolipins. Mucolipin 1 is a membrane protein with 6 transmembrane domains, a serine lipase, and nuclear localization signal motives. The protein shows homology to a group of calcium channels of the TRP/TRPL family. The involvement of this protein in the endocytosis process of membrane components is currently studied. A population screening operation among the Ashkenazi population for the detection of heterozygotes has been started in Israel as a prevention program.

An established light ear mutant (C57BL/6J-Pdeb(rd1) le) mouse cell line exhibits a block to secretion of lysosomal enzymes

The hypopigment mutant mice, light ear, pallid, and beige, possess defects in melanosomes, lysosomes, and platelet dense granules, suggesting that these organelles share a common biogenesis and processing. Light ear and pallid mutants are animal models for Hermansky Pudlak syndrome, whereas the beige mouse is an animal model for Chediak Higashi syndrome. An established skin cell line from the light ear mouse was tested along with pallid and beige cell lines for mutant effects on secretion of lysosomal hydrolase activities of six different lysosomal glycosidases and the trafficking of N-[5-(5,7-dimethyl BODIPY)-1-pentanoyl]-D-erythrosphingosine (C(5)-DMB-ceramide). There were no consistently significant differences between the pallid and the beige mutant cell lines or between these two mutant lines and the control cell line in the percentage secretion of lysosomal hydrolase activities. The light ear mutant cell line, however, displayed a significantly lower percentage secretion of lysosomal hydrolase activities than all other cell lines tested. The light ear mutant cells processed C(5)-DMB-ceramide completely, as seen in the control cell line, whereas pallid and beige cell lines retained fluorescent material and exhibited a block in the complete processing of C(5)-DMB-ceramide 20 h after labeling. The block to secretion of lyososomal hydrolase activities in the light ear mutant cell line will be useful for further studies on this mutant's lysosomal defect.