Drosophila α-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

Abstract

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

The integrin adhesion complex changes its composition and function during morphogenesis of an epithelium

“…However, disruption of actin-fibre polarity could be a cause or consequence of round eggs. Our finding that stress-fibre orientation changes (see also Wahlstrom et al, 2006) shows that stress fibres cannot continuously exert a circumferential contractile force. Therefore, rather than exerting a contractile force, stress fibres might become oriented in response to forces exerted on follicle cells.…”

“…In the case of the imaginal disc cells, strong integrin adhesion is achieved without stress fibres, perhaps because, in elongated cells, the forces driving a return to cuboidal shape will be exerted perpendicular to the basement membrane rather than in the same plane. To date, all mutations that give the round egg phenotype are either transmembrane proteins or cytoplasmic proteins tightly associated with them (Gutzeit, 1991;Bateman et al, 2001;Frydman and Spradling, 2001;Deng et al, 2003;Conder et al, 2007;Mirouse et al, 2009), whereas mutations that disrupt the cytoskeleton alone do not cause round eggs (Wahlstrom et al, 2006). Adding our newly characterized function for aPS2 in the attachment of muscles of the epithelial sheath, this suggests that generating an elongated egg requires the tripartite interaction between the follicular epithelium, the basement membrane and surrounding muscle layer, perhaps directing the expansion of the basement membrane non-uniformly.…”

“…On the basal surface, actin filaments are bundled into stress fibres that terminate at protein aggregates containing integrins (Bateman et al, 2001). Additional components of these focal adhesions have been identified: the receptor tyrosine phosphatase Lar, a-actinin, Enabled and p21-activated kinase (PAK) (Bateman et al, 2001;Deng et al, 2003;Wahlstrom et al, 2006;Conder et al, 2007).…”

Genetic Evidence for Antagonism Between Pak Protein Kinase and Rho1 Small GTPase Signaling in Regulation of the Actin Cytoskeleton During Drosophila Oogenesis

“…If this link between pathways also occurs in the follicular epithelium, it may be that loss of Dpp is suppressing the pak mutant phenotype through disruption of Rho1 signaling. Another possibility is that Dpp regulation of the actin filament cross-linking protein a-actinin in the follicular epithelium is relevant (Wahlstrom et al 2006).…”

Tissue-autonomous EcR functions are required for concurrent organ morphogenesis in the Drosophila embryo

“…By re-investigating the EcRE-LacZ expression pattern at the time of the 20E pulse, we find a broader expression than previously observed, but that the pan-embryonic EcR functions still are not fully reflected by this activity assay. We also characterised and used the hypomorphic usp ActD148 allele (Wahlstrom et al, 2006) to show that reduced Usp functions cause embryonic defects like those produced by reduced EcR functions. Finally, we show that embryonic 20E via EcR instructs the temporal expression of a set of gene regulators, common to the metamorphic 20E response, that are needed for late embryogenesis.…”