Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4 act as "epitheliotrophins'' in murine skin

Nerve growth factor (NGF) is produced by keratinocytes and modulates their proliferation and apoptosis. However, it is as yet unknown whether other me mbers of the NGF family of neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), also modulate ke ratinocyte proliferation in situ. We determined by ELISA and reverse transc riptase PCR that BDNF, NT-3, and NT-4 are expressed in C57BL/6 mouse skin. By immunofluorescence, the subcutaneous panniculus carnosus muscle and arre ctor pill muscle showed strong NT-3 immunoreactivity, whereas BDNF-IR was f ound 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, wherea s 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 t hickness and enhanced number of Ki-67-positive (ie, proliferating) epiderma l keratinocytes in vivo, whereas the number of these cells was substantiall y reduced in BDNF knockout mice. In skin organ culture of C57BL/6 mice, BDN F, NT-3, and NT-4 all significantly increased 5-bromo-2'-deoxyuridine incor poration into epidermal keratinocytes. Go-administration of NGF neutralizin g antibody failed to abrogate the stimulatory effect of NT-3 on keratinocyt e proliferation in skin organ culture. This demonstrates that normal murine epidermal keratinocytes in situ are direct or indirect target cells for th ese neurotrophins. Therefore, BDNF, NT-3, and NT-4 can also act as "epithel iotrophins" and may thus be intimately involved in the control of epidermai homeostasis.