Factors promoting axon growth in the deer antler

Abstract

During their annual regeneration, antlers are innervated by trigeminal sensory axons growing at the highest rate recorded for any adult mammal. Previous analyses established the presence in the antler of nerve growth factor and neurotrophin 3 neurotrophins, which may underlie this rapid nerve growth. We are currently exploring the expression of other molecules that may be involved in such growth (axon growth promoters) combining several gene-expression techniques. Preliminary results indicate the expression of different growth promoters in the antler velvet, five of them not previously described in deer. The expression of these molecules as well as others described in the literature suggests that antler velvet promotes axon growth. However, most promoters expressed in the velvet are also present in unmodified deer skin. Thus, it must be asked why axons grow so fast in the antler? To answer that question, we developed a series of in vitro experiments using sensory neurons from adult and embryo rodents. These studies suggested that soluble proteins secreted by the velvet strongly promote neurite outgrowth. Using specific blocking antibodies, we demonstrated that nerve growth factor is partially responsible for these effects although other yet unidentified proteins seem also to be involved. The studies also showed that neither endocrine serum factors nor antler tissue substrate stimulate neurite outgrowth, although deep velvet layers cause neurite outgrowth orientation.