The serine/threonine kinase dPak is required for polarized assembly of F-actin bundles and apical–basal polarity in the Drosophila follicular epithelium

Abstract

During epithelial development cells become polarized along their apical-basal axis and some epithelia also exhibit polarity in the plane of the tissue. Mutations in the gene encoding a Drosophila Pak family serine/threonine kinase, dPak, disrupt the follicular epithelium that covers developing egg chambers during oogenesis. The follicular epithelium normally exhibits planar polarized organization of basal F-actin bundles such that they lie perpendicular to the anterior-posterior axis of the egg chamber, and requires contact with the basement membrane for apical-basal polarization. During oogenesis, dPak becomes localized to the basal end of follicle cells and is required for polarized organization of the basal actin cytoskeleton and for epithelial integrity and apical-basal polarity. The receptor protein tyrosine phosphatase Dlar and integrins, all receptors for extracellular matrix proteins, are required for polarization of the basal F-actin bundles, and for correct dPak localization in follicle cells. dpak mutant follicle cells show increased beta(Heavy)-spectrin levels, and we speculate that dPak regulation of beta(Heavy)-spectrin, a known participant in the maintenance of membrane domains, is required for correct apical-basal polarization of the membrane. We propose that dPak mediates communication between the basement membrane and intracellular proteins required for polarization of the basal F-actin and for apical-basal polarity.

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

“…As Pak becomes localized basally when the basal F-actin begins to polarize, it could be negatively regulating Rho1 activation only at this end of the follicle cells. To see whether this was the case we looked for an increase in GTPRho1 at the basal end of follicle cells in pak mutant egg chambers but saw no obvious difference compared to wild type (Conder et al 2007) (Figure 4, B-E, and data not shown).…”

“…pak mutant egg chambers are spherical as they fail to elongate along the A-P axis and they do not develop past stage 10 and therefore never produce mature eggs ( Figure 1C). The inability of pak mutant egg chambers to elongate is likely due to disruptions of the basal F-actin, which is less dense, disorganized, and no longer polarized perpendicularly to the A-P axis (Conder et al 2007). The F-actin architecture in pak mutant egg chambers does not have a distinct orientation as seen in wild-type egg chambers and is highly disrupted.…”

“…We previously showed that Pak, a group I Pak protein, participates in development of the follicular epithelium (FE) surrounding the Drosophila egg chamber through regulation of the actin cytoskeleton and apicobasal polarity (Conder et al 2007;Bahri et al 2010). In middle-stage egg chambers, the F-actin network polarizes to the basal end of the follicle cells where it forms bundles of filaments aligned perpendicularly to the anterior-posterior (A-P) axis of the egg chamber (Gutzeit 1990(Gutzeit , 1991Gutzeit and Haas-Assenbaum 1991).…”

“…In middle-stage egg chambers, the F-actin network polarizes to the basal end of the follicle cells where it forms bundles of filaments aligned perpendicularly to the anterior-posterior (A-P) axis of the egg chamber (Gutzeit 1990(Gutzeit , 1991Gutzeit and Haas-Assenbaum 1991). The role of the F-actin bundles during egg chamber elongation in middle-staged egg chambers has been characterized through analysis of mutants affecting these bundles and has led to a model in which 1 the polarized actin bundles act as a ''molecular corset'' to promote elongation of the egg chambers along the A-P axis through actomyosin contractility along the basal surface of the FE Duffy et al 1998;Bateman et al 2001;Frydman and Spradling 2001;Deng et al 2003;Conder et al 2007;Mirouse et al 2009;Viktorinova et al 2009). The egg chambers continue to elongate as they age through stage 14 and develop into mature eggs ( Figure 1B).…”

“…Trans-heterozygous pak individuals can survive to adulthood but are female sterile and exhibit a number of defects in oogenesis (Hing et al 1999;Conder et al 2007). pak mutant egg chambers are spherical as they fail to elongate along the A-P axis and they do not develop past stage 10 and therefore never produce mature eggs ( Figure 1C).…”