Mahogunin Ring Finger-1 (Mgrn1) null mutant mice have a pleiotropic phenotype that includes the absence of yellow hair pigment, abnormal head shape, reduced viability and adult-onset spongiform neurodegeneration. Mgrn1 encodes a highly conserved E3 ubiquitin ligase with 4 different isoforms which are differentially expressed and predicted to localize to different subcellular compartments. To test whether loss of specific isoforms causes different aspects of the mutant phenotype, we generated transgenes for each isoform and bred them onto the null mutant background. Mice expressing only isoform I or III appeared completely normal. Isoform II rescued or partially rescued the mutant phenotypes, while isoform IV had little or no effect. Our data show that different Mgrn1 isoforms are not functionally equivalent in vivo and that the presence of only isoform I or III is sufficient for normal development, pigmentation and neuronal integrity.

The dark-like (dal) mutant mouse has a pleiotropic phenotype that includes dark dorsal hairs and reproductive degeneration. Their pigmentation phenotype is similar to Attractin (Atrn) mutants, which also develop vacuoles throughout the brain. In further characterizing the testicular degeneration of dal mutant males, we found that they had reduced serum testosterone and developed vacuoles in their testes. Genetic crosses placed dal upstream of the melanocortin 1 receptor (Mc1r) and downstream of agouti, although dal suppressed the effect of agouti on pigmentation but not body weight. Atrn(mg-3J) and dal showed additive effects on pigmentation, testicular vacuolation, and spongiform neurodegeneration, but transgenic overexpression of Attractin-like-1 (Atrnl1), which compensates for loss of ATRN, did not rescue dal mutant phenotypes. Our results suggest dal and Atrn function in the same pathway and that identification of the dal gene will provide insight into molecular mechanisms of vacuolation in multiple cell types.

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