In response to cutaneous injury, sensory nerves release
substance P, a proinflammatory
neuropeptide.
Substance P stimulates mitogenesis and migration of keratinocytes, fibroblasts, and endothelial cells.
Neutral endopeptidase (NEP), a cell surface
metallopeptidase, degrades
substance P. Chronic nonhealing
wounds and skin from patients with
diabetes mellitus show increased NEP localization and activity. We hypothesized that increased NEP may retard wound healing and that NEP inhibition would improve closure kinetics in an excisional murine
wound model. NEP
enzyme activity was measured in skin samples from mutant diabetic mice (db/db) and nondiabetic (db/-) littermates by degradation of
glutaryl-ala-ala-phe-4-methoxy-2-naphthylamine. Full-thickness 6-mm dorsal excisional
wounds treated with
normal saline or the NEP inhibitor
thiorphan (10 microM or 25 microM) for 7 days were followed until closure. Histological examination and NEP activity were evaluated in a subset of
wounds. NEP activity in unwounded db/db skin (20.6 pmol MNA/hr/ microg) significantly exceeded activity in db/-skin (7.9 pmol MNA/hr/ microg; p = 0.02). In db/db mice, 25 microM
thiorphan shortened time to closure (18.0 days; p < 0.05) compared to
normal saline (23.5 days). NEP inhibition did not alter closure kinetics in db/-mice. While the inflammatory response appeared enhanced in early
wounds treated with
thiorphan, blinded histological scoring of healed
wounds using a semiquantitative scale showed no difference in
inflammation. Unwounded skin from diabetic mice shows increased NEP activity and NEP inhibition improved
wound closure kinetics without affecting contraction, suggesting that its principal effect was to augment epithelialization.