Nerve growth factor (NGF) is produced by keratinocytes and modulates their proliferation and apoptosis. However, it is as yet unknown whether other members of the NGF family of neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), also modulate keratinocyte proliferation in situ. We determined by ELISA and reverse transcriptase-PCR that BDNF, NT-3, and NT-4 are expressed in C57BL/6 mouse skin. By immunofluorescence, the subcutaneous panniculus carnosus muscle and arrector pili muscle showed strong NT-3 immunoreactivity, whereas BDNF-IR was found only in skin nerve bundles. NT-4 immunoreactivity was noted in single epidermal keratinocytes. The high affinity receptor for both BDNF and NT-4, TrkB, was detected in basal and suprabasal epidermal keratinocytes, whereas the high affinity NT-3 receptor, TrkC, was observed in skin nerve bundles. Compared with the corresponding age-matched wild-type mice, BDNF or NT-3-overexpressing transgenic mice showed a significantly increased epidermal thickness and enhanced number of Ki-67-positive (ie, proliferating) epidermal keratinocytes in vivo, whereas the number of these cells was substantially reduced in BDNF knockout mice. In skin organ culture of C57BL/6 mice, BDNF, NT-3, and NT-4 all significantly increased 5-bromo-2'-deoxyuridine incorporation into epidermal keratinocytes. Co-administration of NGF neutralizing antibody failed to abrogate the stimulatory effect of NT-3 on keratinocyte proliferation in skin organ culture. This demonstrates that normal murine epidermal keratinocytes in situ are direct or indirect target cells for these neurotrophins. Therefore, BDNF, NT-3, and NT-4 can also act as "epitheliotrophins" and may thus be intimately involved in the control of epidermal homeostasis.