Botulinum neurotoxin type a modulates schwann cell proliferation: possible implications in peripheral nerve regeneration

Botulinum neurotoxins (BoNTs) are responsible for botulinum, a neuroparalytic syndrome caused by a disfunction of the autonomic nervous system. All serotypes interfere with neural transmission since they selectively cleave the SNARE proteins, involved in neurotransmitter exocytosis. The BoNTs were used as therapeutic drugs for the treatment for neurological and painful disorders. In animal models of neuropathic pain, the peripheral administration of BoNT/A accelerates the regenerative process of the peripheral nerve, suggesting a possible modulation of Schwann cell (SC) proliferation. As previously demonstrated, SCs development is dependent on a balance between NRGs production and ACh levels. In order to evaluate the effects of BoNT/A on SCs, we have analyzed the effects of BoNT/A on SC cultures and ACh release. FACS analysis has demonstrated that BoNT/A causes a significant increase in the percentage of cells in S phase. Moreover BoNT/A induces an increased expression of factors involved in the SC proliferation such as Notch-1, jagged-1, and NRG 1/1 and its receptor erbB2. The immunocytochemistry for cleaved-SNAP25 on the sciatic nerve sections of mice injected with BoNT/A, demonstrates, for the first time, that BoNT/A enters glial cells. Moreover the analysis at scanning electron microscope (SEM) has demonstrated that BoNT/A treatment of cultured SCs causes a strong accumulation of vesicles, suggesting the ability of BoNT/A to block the probably exocytosis of ACh also in Schwann cells. Finally the increased expression of mature form of NGF in SC treated with BoNT/A and the increased fiber length in DRG explants maintained in medium conditioned by SCs treated with BoNT/A, clearly indicated that the neurotoxin can contribute to enhance fiber outgrowth probably via NGF production. These data suggest that BoNT/A may increase the SC proliferation and stimulate the NGF production probably blocking the ACh release. This may contribute to create a permissive environment for peripheral nerve regeneration.