Eukaryotic cells are highly compartmentalized into distinct membrane-bound organelles. Molecule transfer between these organelles involves membrane fusion between vesicles, a process that is tightly controlled owing to the unique composition of proteins and lipids in each cell compartment. Central to this fusion process is the function of the SNARE proteins, a family of integral membrane proteins present on both vesicle (v-SNARE) and target (t-SNARE) membranes.11 These molecules are presumed to determine the specificity as well as the rate of the fusion reaction. A second family of proteins called Sec1/Munc18 (SM) proteins also seems to be involved in determining the specificity of SNARE-mediated membrane fusion.12- 14 However, owing to variability in their function in different systems,
there is no consensus as to their exact role, site of action, and relation to SNARE proteins.12 Homozygous loss-of-function mutations in VPS33B encoding a member of the SM family was recently shown to cause ARC syndrome and abnormal trafficking of surface-expressed proteins in the liver.5 Despite the conspicuous presence of cutaneous abnormalities in ARC syndrome, little is currently known about the consequence of abnormal VPS33B function during skin differentiation.
In the present study, a deleterious mutation in VPS33B was shown to result in impaired secretion of lamellar granules, which is likely to contribute to the development of ichthyosis in ARC syndrome. The yeast homologue of the VPS33B protein, Vps33p,
a class C vps protein, is required for vacuolar biogenesis and plays a key role in protein trafficking from Golgi to vacuole.15 Mutations in class C vps proteins have been shown to result in intracellular acid-base imbalance, amino acid pool deficiency, and temperature-sensitive growth failure in yeast.16 Interestingly, lamellar granules are considered as being part or at least being derived from the Golgi system,6 suggesting that cutaneous manifestations in ARC syndrome may be the result of abnormal intracellular organelle trafficking. Surprisingly, loss of function of VPS33B analogues in Drosophila and mouse models leads to relatively minor clinical symptoms. In Drosophila, the hypomorphic carnation (car) allele of the Vps33 homologue causes the carnation eye color phenotype.17 In mice, the Buff (bf) phenotype, characterized by coat-color hypopigmentation, mild platelet storage pool deficiency,
and little if any effect on lysosomal function, is caused by mutations in the Vps33a gene.18 The existence of 2 homologues of the yeast Vps33p protein in multicellular organisms may explain the significant differences between the mouse and fly phenotypes and ARC syndrome.19 Several types of ichthyosis are associated with mutations in genes involved in lamellar granule function. In Harlequin and lamellar ichthyosis, mutations in ABCA12 cause defective lipid transport due to abnormal lipid loading in lamellar granules, and consequent abnormal cornification.20 Likewise, as often seen in ARC syndrome,
severe lamellar ichthyosis developed late after birth in CEDNIK syndrome and is the result of impaired lamellar granule maturation and secretion.9 The present study indicates that ARC syndrome can be considered as part of this revealing group of cornification disorders. Despite the fact that both ARC and CEDNIK syndromes result from abnormal function of proteins involved in vesicle fusion5,21 leading to impaired lamellar granule secretion, lamellar granule maturation was normal in ARC syndrome,
whereas it is markedly abnormal in CEDNIK syndrome.9 Thus, these observations may be interpreted to suggest that in keratinocytes, VPS33B is involved mainly in regulating exocytosis, whereas SNAP29 is additionally involved in vesicle cargo loading in the Golgi system. In conclusion, we have demonstrated that lamellar granule secretion is impaired in ARC syndrome, underscoring the importance of SNARE-mediated vesicle fusion during normal epidermal differentiation.