An endosome-associated actin network involved in directed apical plasma membrane growth

Abstract

The actin cytoskeleton participates in a range of cellular processes. It supports cell shape changes by propagating forces within cells and between cells and their environment. Terminal cells of the Drosophila respiratory system form a subcellular tube by invaginating their apical plasma membrane; cortical actin networks at the basal and apical plasma membranes are critical for proper morphogenesis. Basal actin affects apical membrane morphogenesis, and it is not known how the two separate actin pools communicate. We report here that actin assembles around vesicles of the late endocytic pathway, which are present in the growth cone of the cell, between the tip of the subcellular tube and the leading filopodia of the basal membrane. Actin organized at late endosomes extends towards both membrane compartments. Preventing proper actin nucleation at late endosomes disturbs the directionality of tube growth, uncoupling it from the direction of cell elongation. Severing actin in this area affects tube integrity. These findings demonstrate a role for the late endocytic pathway in organizing actin for proper cell morphogenesis, in addition to its known role in membrane and protein trafficking.

Crosstalk between basal extracellular matrix adhesion and building of apical architecture during morphogenesis

“…Therefore, disrupting MTs by expression of the MT-severing protein Spastin uncouples directed growth of the apical compartment and results in guidance defects ( Gervais and Casanova, 2010 ). Disrupting actin organisation in any subcellular compartment also prevents coordinated apicobasal growth ( Gervais and Casanova, 2010 ; JayaNandanan et al, 2014 ; Levi et al, 2006 ; Okenve-Ramos and Llimargas, 2014 ; Ríos-Barrera and Leptin, 2021 preprint).…”

“…Another actin-based structure involved in subcellular tube formation lies in the space between the tip of the subcellular tube and the cell-growth cone, and it is referred to as the actin core ( Gervais and Casanova, 2010 ; Okenve-Ramos and Llimargas, 2014 ; Oshima et al, 2006 ). We recently showed that late endosomes are responsible for the formation of this structure by promoting actin nucleation through Wash, a Wiscott-Aldrich Syndrome Protein (WASP) member ( MacDonald et al, 2018 ; Mathew et al, 2020 ; Nagel et al, 2017 ; Ríos-Barrera and Leptin, 2021 preprint). Late endosomes and the associated actin core precede subcellular tube growth and branching, and inducing their mislocalisation affects directed apical membrane growth.…”

“…Late endosomes and the associated actin core precede subcellular tube growth and branching, and inducing their mislocalisation affects directed apical membrane growth. Therefore, it is likely that endosome-associated actin ahead of the tube is required to coordinate subcellular tube and cell growth ( Ríos-Barrera and Leptin, 2021 preprint). This model agrees with recent work from Ricolo and Araujo (2020) who have shown that Shortstop (Shot), the only member of the Spectraplakin family in Drosophila , is recruited to the different actin pools in the cell including the actin core, an essential function for proper tube formation.…”

“…Late endosomes have various roles in migration and cytoskeletal organisation throughout metazoans ( MacDonald et al, 2018 ; Palamidessi et al, 2008 ; Ramírez-Santiago et al, 2016 ; Schiefermeier et al, 2014 ). As mentioned for subcellular tube formation in tracheal terminal cells, late endosomes can also mediate interactions between distant plasma membrane domains by regulating the cytoskeleton ( Ríos-Barrera and Leptin, 2021 preprint). Regulation of the cytoskeleton by endosomes has been documented in other processes: peripheral sensory neurons use endosomes as MT organising centres (MTOCs) at branching points, which allows MT growth towards dendritic termini.…”

“…For instance, in filamentous fungi, growth of hyphal tips is regulated by polarised organisation of actin and MTs that generate force and transport molecules to the growing tip. This is coordinated by a collection of secretory vesicles known as Spitzenkörper, which provide membrane material to the growing tip and also allow anchoring of MTs and actin, as late endosomes do during tracheal terminal cell development ( Crampin et al, 2005 ; Ríos-Barrera and Leptin, 2021 preprint; Zheng et al, 2020 ). In addition, similar to late endosomes in subcellular tube formation, Spitzenkörper relocalisation precedes changes in the direction of hyphal growth ( Crampin et al, 2005 ).…”