Title: the Mechanistic Basis for Sexual Dysfunction in Male Transforming Growth Factor Beta1 Null Mutant Mice Short Title: Sexual Dysfunction in Tgfb1 Null Mice

1 The cytokine transforming growth factor beta 1 (TGFB1) is implicated in male sexual 2 function. Previous behavioral studies show that Tgfb1 null mutant mice mount and 3 display limited intromission behavior with receptive females, but are unable to complete 4 successful copulation. The studies presented here explore the physiological basis for 5 sexual dysfunction in Tgfb1 null mutant males. Scanning electron microscopy revealed 6 that the surface of the penis in Tgfb1 null mutant males was abnormally coated in 7 superficial keratinized epithelial cells. There was a significant reduction in protrusion of 8 penile spines through the superficial tissue in Tgfb1 null mutant mice, and in some mice 9 the spines were almost completely embedded. Histological analysis revealed reduced skin 10 thickness in the penis of Tgfb1 null mutant males. Nerve fibres, endothelial cells, smooth 11 muscle actin, macrophages and neuronal and inducible nitric oxide synthase (NOS) were 12 present in similar abundance and location in Tgfb1 null mutant mice compared to 13 wildtype controls, however an increase in collagen I deposition was detected. Behavioral 14 studies revealed that Tgfb1 null mutant males undergo spontaneous non contact erections, 15 albeit at a reduced rate compared to control mice, and engage in less frequent genital 16 grooming activity. These studies suggest that Tgfb1 null mutation may adversely 17 influence copulatory behavior through effects on both altered structural integrity of the 18 penile skin, and impaired tissue compliance leading to erectile dysfunction. 19