The tubulin code specializes neuronal cilia for extracellular vesicle release

Abstract

Cilia are microtubule‐based organelles that display diversity in morphology, ultrastructure, protein composition, and function. The ciliary microtubules of C. elegans sensory neurons exemplify this diversity and provide a paradigm to understand mechanisms driving ciliary specialization. Only a subset of ciliated neurons in C. elegans are specialized to make and release bioactive extracellular vesicles (EVs) into the environment. The cilia of extracellular vesicle releasing neurons have distinct axonemal features and specialized intraflagellar transport that are important for releasing EVs. In this review, we discuss the role of the tubulin code in the specialization of microtubules in cilia of EV releasing neurons.

Sensory cilia act as a specialized venue for regulated EV biogenesis and signaling

“…EVN ciliary membrane homeostasis of PKD-2 is modulated by a phosphoinositide 5-phosphatase 29 and caveolin 30 . The tubulin code is required for the unique EVN ciliary ultrastructure and EV environmental release 19,23,31 , while kinesin-3 KLP-6 regulates intraflagellar transport and EV biogenesis at both the cilia base and tip 14,22 . The kinesin-3 motor family is characterized by their N-terminal motor domain, a forkhead-associated domain, and a tail possessing lipid-binding domains, the latter endows membrane-associated abilities 32 .…”

“…The KLP-6 mammalian homolog KIF13B - via its lipid-binding C2 domain - maintains a caveolin-enriched membrane domain at the ciliary base in mammalian cells 33 . In addition to C2 domains, the KLP-6 tail contains positively charged amino acids 21 that may enhance protein binding to negatively charged phospholipids in the ciliary membrane or polyglutamylated tubulin in ciliary axoneme 23,31 . We propose a mechanism whereby KLP-6 connects to both the ciliary membrane and axoneme and directs the myristoylated coiled-coil CIL-7 to distal ciliary tip for ciliary EV biogenesis.…”

Transient accumulation and bidirectional movement of KIF13B in primary cilia

“…Cilia are compartmentalized organelles whose assembly, maintenance and function depend on the conserved intraflagellar transport (IFT) machinery that via kinesin-2 and cytoplasmic dynein 2 motors, respectively, brings IFT-A and IFT-B complexes with associated ciliary cargoes into and out of the organelle [2,3]. Ciliary composition and function are additionally regulated by the transition zone at the ciliary base [4], as well as by shedding and environmental release of extracellular vesicles (EVs) from cilia [5,6]. Such EVs act as signal-carrying entities in Caenorhabditis elegans and Chlamydomonas [7][8][9] and may rid cilia of unwanted material in vertebrates [10,11].…”

“…In mammalian cells, release of EVs from cilia requires local polymerization of actin at the EV release site (Nager et al, 2017), whereas in C. elegans the kinesin-3 motor KLP-6 was shown to move anterogradely within male-specific sensory cilia (Morsci and Barr, 2011), where it promotes ciliary targeting and release of polycystin-2-carrying EVs to control male mating behavior (Wang et al, 2014). Several other factors regulating EV biogenesis, shedding and release from cilia in C. elegans have been identified, many of which affect axonemal tubulin post translational modification and hence KLP-6 motility within cilia (Akella and Barr, 2021). It is unknown if vertebrate kinesin-3 motors similarly move within primary cilia and promote ciliary EV release, and to date KLP-6 is the only kinesin apart from IFT kinesin-2 motors with demonstrated intraciliary motility.…”

Editorial overview: Microtubules in nervous system development

“…While we traditionally think about neuronal communication as facilitated through axonal-dendritic connections, we now know that cilia also play an important role. Akella and Barr focus on the cilia of C. elegans sensory neurons and review recent work that uncovered the importance of the tubulin code in building their specialized architecture, regulating intraflagellar transport, and controlling the release of extracellular vesicles (Akella & Barr, 2021).…”