Dynamic interactions between olfactory receptor axons and glial cells from the olfactory system of the moth Manduca sexta

Abstract

Across species, glial cells in both peripheral and central nervous systems cooperate extensively with neurons to shape multiple aspects of neural development. In vertebrate and invertebrate olfactory systems, neuron-glia interactions are thought to underlie critical developmental events, including glomerulus formation, and the growth, sorting, and targeting of olfactory receptor neuron (ORN) axons. The olfactory system of the moth Manduca sexta has many similarities to vertebrate olfactory systems, and has been used extensively to explore intercellular interactions involved in the formation of the olfactory pathway. In particular, glial reduction experiments have implicated two types of central olfactory glia, the sorting zone and neuropil-associated glia, in axon sorting and glomerulus stabilization. The developmental roles of a third glial cell type, the antennal nerve glia, remain elusive, yet their peripheral origin and association with ORN axons are similar to mammalian olfactory ensheathing cells. The present body of work uses a defined co-culture system to characterize interactions between ORN axons growing from explants of olfactory receptor epithelium and glial cells from the primary olfactory system of Manduca. We have monitored how particular types of glia, known to influence the behavior of ORN axons in vivo, directly affect the behavior and morphology of individual ORN growth cones in vitro. Time-lapse imaging of neuron-glia cultures revealed that olfactory receptor growth cones elaborate extensively and cease advancement following contact with sorting zone and neuropil-associated glial cells. In contrast, growth cones advance along the surfaces of antennal nerve glial cells without prolonged changes in morphology. Cytoskeletal staining of fixed preparations reinforced live-cell findings, as contact with sorting zone and neuropil-associated glial cells caused statistically significant changes in growth cone morphology. Finally, ORN axons induce antennal nerve glia, but not sorting zone or neuropil glia, to form multicellular arrays through proliferation and process extension. These findings have led to the formation of hypotheses concerning the nature of neuron-glia interactions in vivo.