Ectocytosis prevents accumulation of ciliary cargo in C. elegans sensory neurons

Abstract

Cilia are sensory organelles protruding from cell surfaces. Release of Extracellular Vesicles (EVs) from cilia was previously observed in mammals, Chlamydomonas, and in male C. elegans. Using the EV marker TSP-6 (an ortholog of mammalian CD9) and other ciliary receptors, we show that EVs are formed from ciliated sensory neurons in C. elegans hermaphrodites. Release of EVs is observed from two ciliary locations: the cilia tip and/or Periciliary Membrane Compartment (PCMC). Outward budding of EVs from the cilia tip leads to their release into the environment. EVs budding from the PCMC are concomitantly phagocytosed by the associated glial cells. To maintain cilia composition, a tight regulation of cargo import and removal is achieved by the action of Intra-Flagellar Transport (IFT). Unbalanced IFT due to cargo overexpression or mutations in the IFT machinery leads to local accumulation of ciliary proteins. Disposal of excess ciliary proteins via EVs reduces their local accumulation and exports them to the environment and/or to the glia associated to these ciliated neurons. We suggest that EV budding from cilia subcompartments acts as a safeguard mechanism to remove deleterious excess of ciliary material.

Loss of the Extracellular Matrix Protein DIG-1 Causes Glial Fragmentation, Dendrite Breakage, and Dendrite Extension Defects

“…The distribution of these fragments suggests they may be internalized by neighboring cells. Interestingly, the amphid sheath was recently shown to internalize extracellular vesicles released by amphid neurons and, in the absence of the sheath, other neighboring cells perform this function, suggesting that several cells in the head can internalize cellular debris [39]. This phenotype is highly penetrant, with >80% of individual amphid sheath glial cells showing extensive fragmentation (Figure 2D).…”

Unconventional Protein Secretion Dependent on Two Extracellular Vesicles: Exosomes and Ectosomes

“…In addition, recent evidence has demonstrated the ability of EVs to establish surface protein-protein interactions. It appears, therefore, that at least a fraction of EVs tends to show functionally integrated complexes ( Leidal et al, 2020 ; Levy et al, 2021 ; Nikoloff et al, 2021 ; Razzauti and Laurent, 2021 ). At present, populations of single EV type can be isolated by various techniques based on distinct approaches, including monoclonal antibodies ( Levy et al, 2021 ; Lim et al, 2021 ).…”

“…The state of knowledge and techniques are already advanced. Additional developments are expected for the future including their markers and signatures, useful for the identification of subtype-specific EVs and the unconventional secretion of their proteins ( Garcia-Martin et al, 2021 ; Levy et al, 2021 ; Nikoloff et al, 2021 ; Razzauti and Laurent, 2021 ).…”

EV duty vehicles: Features and functions of ciliary extracellular vesicles

“…The best-known example is in C. elegans. In this species, EVs released at the tips of primary cilia of sensory neurons are found in the surrounding extracellular environment, whereas EVs from the PCMC are released into the lumen surrounding the PCMC before being phagocytized by neighboring glial cells (Razzauti and Laurent, 2021). These two EVs subpopulations exert different functions (see below).…”

“…Large EVs (sometimes referred to as ectosomes, microvesicles or microparticles) are extracellular vesicles of 100-500 nm, generated by budding from the plasma membrane and released into the cell environment, while small EVs (also TABLE 1 Microscopic analysis revealed the presence of (A) cilia deformations (e.g., bulb at the tip, membrane bud); (B) vesicles or vesicle-like structures associated with cilia (C) ciliary EVs generated at the ciliary tip (D) alongside the axoneme, or (E) the ciliary base. TSP-6, TSP-7 FBM Razzauti and Laurent, (2021) referred to as exosomes depending on their molecular markers) measure between 50 and 150 nm and are initially formed inside multivesicular bodies (MVB) before being released outside the cell by fusion of the MVB with the plasma membrane (Meldolesi, 2018). It is worth mentioning that this nomenclature is based chiefly on research on EVs deriving from the cellular cytoplasm, from human and mouse models.…”