Figure 9. Mechanisms underlying ectocytosis from ciliated neurons.

Ectocytosis occurs from two different ciliary locations: the cilia tip and the periciliary membrane compartment (PCMC). When ectosomes are shed from the cilia tip, they are environmentally released. Sheath glia (light blue) embeds the ciliary base of ciliated neurons, when ectosomes bud from the base they are concomitantly phagocytosed by their associated sheath glia. Overexpression of ciliary membrane proteins or mutations in genes involved in cilia trafficking, like osm-3, bbs-8, and che-3, fail to balance import and removal of ciliary membrane proteins, leading to local accumulation of cargo in cilia proper and/or to PCMC. It results in increased ectocytosis events to the sheath glia in osm-3 and che-3 and/or to the outside in bbs-8. On the contrary, reduced cilia import of ciliary membrane protein in unc-101 – involved in sorting cilia cargo from Trans Golgi Network – reduced ectocytosis. Besides its crucial role in secretion of extracellular matrix content, we suggest that glia also plays an important function to maintain cilia structure and composition and to recycle ectocytosed material. Cell-specific manipulations of AMsh glia phagocytic activity by expression of DYN-1 dominant negative transgene suggest that a tight regulation of sheath glia phagocytosis contributes to shape the nerve receptive endings. We suggest a model where apical ectocytosis is inherent to most or all ciliated neurons of C. elegans and where neurons and glia cooperate to readily remove basal ectosome from ciliary membranes when cargoes accumulate in PCMC (magenta).