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The tyrosine kinase Fms, the cell surface receptor for M-CSF and IL-34, is critical for microglial proliferation and differentiation in the brain. Recently, a number of mutations have been identified in Fms as a putative genetic cause of hereditary diffuse leukoencephalopathy with spheroids (HDLS), implying an important role of microglial dysfunction in HDLS pathogenesis. In this study, we initially confirmed that 11 mutations, which reside within the ATP-binding or major tyrosine kinase domain, caused a severe impairment of ligand-induced Fms auto-phosphorylation. Intriguingly, we found that 10 of the 11 mutants also showed a weak cell surface expression, which was associated with a concomitant increase in the low molecular weight hypo-N-glycosylated immature gp130Fms-like species. Indeed, the mutant proteins heavily accumulated to the Golgi-like perinuclear regions. These results indicate that all of the Fms mutations tested severely impair the kinase activity and most of the mutations also impair the trafficking to the cell surface, further suggesting that HDLS is caused by the loss of Fms function.

Previous studies indicated that the acquisition of egg fertilizability during transit through the pars recta portion of the oviduct in Bufo japonicus is accompanied by hydrolytic conversion of the vitelline coat 40- to 52-kDa components to 39-kDa components induced by a 66-kDa serine protease, “oviductin.” In this study, we cloned a 3028-bp cDNA that contained an open reading frame encoding 974 amino acids with a calculated molecular mass of 107.6 kDa, including two protease domains and three repeats of CUB domains. Sequence analysis indicated that the catalytically active 66-kDa protein comprised an N-terminally located oviductin protease and two CUB domains. The oviductin gene was transcribed as a part of 6-kb mRNA that was expressed specifically in the cells lining the bottom of epithelial folds in the oviductal pars recta, and this expression was highly accelerated when the pars recta fragments were cultured in the presence of hCG. Western blot analyses using antibodies against a protease domain revealed that the catalytically inactive 102-kDa proteins in the pars recta granules yield 66-kDa catalytically active and 82- and 59-kDa inactive molecules. We propose that the oviductin translated as 107.6-kDa precursors are processed both N- and C-terminally to give rise to a 66-kDa active form comprising a serine protease and two CUB domains.