"Turning up the heat": role of neurotrophic batokines in the postnatal maturation and remodeling of brown adipose tissue in deer mice

Activation of brown adipose tissue (BAT) thermogenesis impacts energy balance and must be tightly regulated. Several neurotrophic factors, expressed in BAT of adult laboratory rodents, have been implicated in remodeling the sympathetic neural network to enhance thermogenesis [e.g., nerve growth factor (NGF), neuregulin-4 (NRG4), and S100b]. Here, we compare, to our knowledge, for the first time, the relative roles of three neurotrophic "batokines" in establishing/remodeling innervation during postnatal development and adult cold stress. We used laboratory-reared , which rely heavily on BAT-based thermogenesis for survival in the wild, beginning between and . BAT sympathetic innervation was enhanced from to , and exogenous NGF, NRG4, and S100b stimulated neurite outgrowth from sympathetic neurons. Endogenous BAT protein stores and/or gene expression of NRG4, S100b, and calsyntenin-3β (which may regulate S100b secretion) remained high and constant during development. However, endogenous NGF was low and mRNA was undetectable. Conditioned media (CM) from cultured BAT slices stimulated neurite outgrowth from sympathetic neurons in vitro, which was inhibited by antibodies against all three growth factors. CM had significant amounts of secreted NRG4 and S100b protein, but not NGF. By contrast, BAT slices from cold-acclimated adults released significant amounts of all three factors relative to thermoneutral controls. These data suggest that although neurotrophic batokines regulate sympathetic innervation in vivo, their relative contributions differ depending on the life stage. They also provide novel insights into the regulation of BAT remodeling and BAT's secretory role, both of which are critical to our understanding of mammalian energy homeostasis. In altricial mice, the developmental shift to endothermy accompanies the establishment of the brown adipose tissue sympathetic neural network. Cultured slices of neonatal BAT secreted high quantities of two predicted neurotrophic batokines: S100b and neuregulin-4, but surprisingly low levels of the classic neurotrophic factor, NGF. Despite low NGF, neonatal BAT-conditioned media was highly neurotrophic. Cold-exposed adults use all three factors to dramatically remodel BAT, suggesting that BAT-neuron communication is life-stage dependent.