ABSTRACT
PURPOSE: To characterize the ocular phenotype resulting from mutation of Rab38, a candidate gene for Hermansky-Pudlak syndrome. METHODS: Chocolate mice (cht, Rab38(cht/cht)) and control heterozygous (Rab38(cht/)(+)) and wild-type mice were examined clinically, histologically, ultrastructurally, and electrophysiologically. Mice homozygous for both the Rab38(cht) and the Tyrp1(b) alleles were similarly examined. RESULTS: Rab38(cht/cht) mice showed variable peripheral iris transillumination defects at 2 months of age. Patches of RPE hypopigmentation were noted clinically in 57% of Rab38(cht/cht) eyes and 6% of Rab38(cht/)(+) eyes. Rab38(cht/cht) mice exhibited thinning of the iris and RPE and larger b-wave amplitudes in the scotopic range when compared with the control animals. Compared with wild-type mice, Rab38(cht/cht) melanosomes were smaller and there were fewer in neuroectodermally derived retinal pigment epithelium; in neural crest-derived choroid melanocytes, they were smaller in size only. Mutation of both Rab38 and Tyrp1 produced mice with ocular and coat color pigment dilution greater than that seen with either mutation alone. Comprehensive clinical and pathologic analyses showed no other organ system or blood defects in Rab38(cht/cht) mice. CONCLUSIONS: Rab38(cht/cht) mice show ocular characteristics reminiscent of human oculocutaneous albinism, as well as iris and RPE thinning. The synergistic effects of the Rab38(cht) and Tyrp1(b) alleles suggest that TYRP1 is not the only target of RAB38 trafficking. This mouse line provides a useful model for studying melanosome biology and its role in human ocular diseases.
Subject(s)
Choroid Diseases/genetics , Hermanski-Pudlak Syndrome/genetics , Iris Diseases/genetics , Retinal Diseases/genetics , rab GTP-Binding Proteins/genetics , Alleles , Animals , Blotting, Western , Choroid Diseases/pathology , Disease Models, Animal , Electroretinography , Female , Gene Expression Regulation/physiology , Hair Color , Humans , Iris Diseases/pathology , Male , Melanosomes/ultrastructure , Membrane Glycoproteins/genetics , Mice , Mice, Inbred C57BL , Mice, Mutant Strains , Oxidoreductases/genetics , Phenotype , Pigment Epithelium of Eye/ultrastructure , Polymorphism, Single Nucleotide , Retinal Diseases/pathologyABSTRACT
Small GTPases of the Rab family are key regulators of membrane trafficking. Each Rab shows a characteristic subcellular distribution, and may serve as an important determinant of organelle identity. The molecular mechanisms responsible for targeting Rabs to specific intracellular compartments, however, remain poorly understood. The divergent C-terminal hypervariable region was postulated to contain Rab targeting information. We generated a series of hybrid Rab proteins by exchanging the hypervariable domains of Rab1a, Rab2a, Rab5a, Rab7 and Rab27a, and analysed their subcellular localisations. We found that the various hybrid proteins retained their targeting to the parent organelle and were functionally active. We conclude that the hypervariable region does not contain a general Rab targeting signal. Furthermore, we identified other regions within the RabF and RabSF motifs that are required for specific targeting of Rab27a to secretory granules or melanosomes, and Rab5a to endosomes. We observed only partial overlap between targeting-determining regions in the Rab proteins examined, suggesting that Rab recruitment may be complex and at least partially Rab-specific. Mutations in these targeting-determining regions induced localisation to the ER, an observation that further strengthens our previous finding that ER/Golgi membranes serve as the default location for Rabs that have lost targeting information.
