The mouse seminal vesicle shape mutation is allelic with Fgfr2

The mouse seminal vesicle shape (svs) mutation is a spontaneous recessive mutation that causes branching morphogenesis defects in the prostate gland and seminal vesicles. Unlike many other mutations that reduce prostatic and/or seminal vesicle branching, the svs mutation dramatically reduces branching without reducing organ growth. Using a positional cloning approach, we identified the svs mutant lesion as a 491 bp insertion in the tenth intron of Fgfr2 that results in changes in the pattern of Fgfr2 alternative splicing. An engineered null allele of Fgfr2 failed to complement the svs mutation proving that a partial loss of FGFR2(IIIb) isoforms causes svs phenotypes. Thus, the svs mutation represents a new type of adult viable Fgfr2 allele that can be used to elucidate receptor function during normal development and in the adult. In the developing seminal vesicles, sustained activation of ERK1/2 was associated with branching morphogenesis and this was absent in svs mutant seminal vesicles. This defect appears to be the immediate downstream effect of partial loss of FGFR2(IIIb) because activation of FGFR2(IIIb) by FGF10 rapidly induced ERK1/2 activation, and inhibition of ERK1/2 activation blocked seminal vesicle branching morphogenesis. Partial loss of FGFR2(IIIb) was also associated with down-regulation of several branching morphogenesis regulators including Shh, Ptch1, Gli1, Gli2, Bmp4, and Bmp7. Together with previous studies, these data suggest that peak levels of FGFR2(IIIb) signaling are required to induce branching and sustain ERK1/2 activation, whereas reduced levels support ductal outgrowth in the prostate gland and seminal vesicles.