Transforming growth factor-β and forskolin attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo

Transforming growth factor‐β (TGF‐β) plays a central role in the regulation of Schwann cell (SC) proliferation and differentiation and is essential for the neurotrophic effects of several neurotrophic factors (reviewed by Unsicker and Krieglstein, 2000; Unsicker and Strelau, 2000). However, its role in peripheral nerve regeneration in vivo is not yet understood. Our studies were carried out to characterize (1) the effects of duration of regeneration, and chronic SC denervation on the number of tibial (TIB) motor neurons that regenerated axons over a fixed distance (25 mm into distal common peroneal [CP] nerve stumps), and (2) the effect of in vitro incubation of 6‐month chronically denervated sciatic nerve explants with TGF‐β and forskolin on their capacity to support axonal regeneration in vivo. TIB–CP cross‐suture in Silastic tubing was used, and regeneration into 0–24‐week chronically denervated CP stumps was allowed for either 1.5 or 3 months. Chronically denervated rat sciatic nerve explants (3 × 3 mm2) were incubated in vitro with either DMEM and 15% fetal calf serum (D‐15) plus TGF‐β/forskolin or D‐15 alone for 48 h and placed into a 10‐mm Silastic tube that bridged the proximal and distal nerve stumps of a freshly cut TIB nerve. The number of tibial motor neurons that regenerated axons through the explants and 25 mm into the distal nerve stump after 6 months, and TIB regeneration into the CP nerve stumps, were assessed using retrograde tracers, fluorogold, or fluororuby. We found that all tibial motor neurons regenerate their axons 25 mm into 0–4‐week denervated CP nerve stumps after a regeneration period of 3 months. Reducing regeneration time to 1.5 months and chronic denervation, reduced the number of motor neurons that regenerated axons over 25 mm. Exposure of 6‐month denervated nerve explants to TGF‐β/forskolin increased the number of motor neurons that regenerated through them from 258 ± 13; mean ± SE to 442 ± 22. Hence, acute treatment of atrophic SC with TGF‐β can reactivate the growth‐permissive SC phenotype to support axonal regeneration. GLIA 37:206–218, 2002. © 2002 Wiley‐Liss, Inc.