aPKC regulates apical constriction to prevent tissue rupture in the Drosophila follicular epithelium.

Apical-basal polarity is an essential epithelial trait controlled by the evolutionarily conserved PAR-aPKC polarity network. Dysregulation of polarity proteins disrupts tissue organization during development and in disease, but the underlying mechanisms are unclear due to the broad implications of polarity loss. Here, we uncover how Drosophila aPKC maintains epithelial architecture by directly observing tissue disorganization after fast optogenetic inactivation in living adult flies and ovaries cultured ex vivo. We show that fast aPKC perturbation in the proliferative follicular epithelium produces large epithelial gaps that result from increased apical constriction, rather than loss of apical-basal polarity. Accordingly, we can modulate the incidence of epithelial gaps by increasing and decreasing actomyosin-driven contractility. We traced the origin of these large epithelial gaps to tissue rupture next to dividing cells. Live imaging shows that aPKC perturbation induces apical constriction in non-mitotic cells within minutes, producing pulling forces that ultimately detach dividing and neighboring cells. We further demonstrate that epithelial rupture requires a global increase of apical constriction, as it is prevented by the presence of non-constricting cells. Conversely, a global induction of apical tension through light-induced recruitment of RhoGEF2 to the apical side is sufficient to produce tissue rupture. Hence, our work reveals that the roles of aPKC in polarity and actomyosin regulation are separable and provides the first in vivo evidence that excessive tissue stress can break the epithelial barrier during proliferation.

The Pebble/Rho1/Anillin pathway controls polyploidization and axonal wrapping activity in the glial cells of the Drosophila eye.

During development glial cell are crucially important for the establishment of neuronal networks. Proliferation and migration of glial cells can be modulated by neurons, and in turn glial cells can differentiate to assume key roles such as axonal wrapping and targeting. To explore the roles of actin cytoskeletal rearrangements in glial cells, we studied the function of Rho1 in Drosophila developing visual system. We show that the Pebble (RhoGEF)/Rho1/Anillin pathway is required for glia proliferation and to prevent the formation of large polyploid perineurial glial cells, which can still migrate into the eye disc if generated. Surprisingly, this Rho1 pathway is not necessary to establish the total glial membrane area or for the differentiation of the polyploid perineurial cells. The resulting polyploid wrapping glial cells are able to initiate wrapping of axons in the basal eye disc, however the arrangement and density of glia nuclei and membrane processes in the optic stalk are altered and the ensheathing of the photoreceptor axonal fascicles is reduced.

Mitochondrial Dynamics in the Drosophila Ovary Regulates Germ Stem Cell Number, Cell Fate, and Female Fertility.

The fate and proliferative capacity of stem cells have been shown to strongly depend on their metabolic state. Mitochondria are the powerhouses of the cell being responsible for energy production via oxidative phosphorylation (OxPhos) as well as for several other metabolic pathways. Mitochondrial activity strongly depends on their structural organization, with their size and shape being regulated by mitochondrial fusion and fission, a process known as mitochondrial dynamics. However, the significance of mitochondrial dynamics in the regulation of stem cell metabolism and fate remains elusive. Here, we characterize the role of mitochondria morphology in female germ stem cells (GSCs) and in their more differentiated lineage. Mitochondria are particularly important in the female GSC lineage. Not only do they provide these cells with their energy requirements to generate the oocyte but they are also the only mitochondria pool to be inherited by the offspring. We show that the undifferentiated GSCs predominantly have fissed mitochondria, whereas more differentiated germ cells have more fused mitochondria. By reducing the levels of mitochondrial dynamics regulators, we show that both fused and fissed mitochondria are required for the maintenance of a stable GSC pool. Surprisingly, we found that disrupting mitochondrial dynamics in the germline also strongly affects nurse cells morphology, impairing egg chamber development and female fertility. Interestingly, reducing the levels of key enzymes in the Tricarboxylic Acid Cycle (TCA), known to cause OxPhos reduction, also affects GSC number. This defect in GSC self-renewal capacity indicates that at least basal levels of TCA/OxPhos are required in GSCs. Our findings show that mitochondrial dynamics is essential for female GSC maintenance and female fertility, and that mitochondria fusion and fission events are dynamically regulated during GSC differentiation, possibly to modulate their metabolic profile.

Cysteine Oxidative Dynamics Underlies Hypertension and Kidney Dysfunction Induced by Chronic Intermittent Hypoxia.

Previous data showed the lack of efficacy of an adrenoceptor antagonist to revert hypertension induced by chronic intermittent hypoxia (CIH). We hypothesized that, in addition to sympathetic activation, CIH may change the availability and dynamics of cysteine. Temporal variation in total cysteine and its fractions, free reduced, free oxidized and protein-bound (CysSSP), were measured in homogenates of kidney cortex and medulla of Wistar rats. Animals were exposed to CIH for 14, 21 and 60 days and cysteine fractions and fibronectin gene expression were assessed at these time-points. Two different phases in cysteine dynamics were identified. An early phase (14d) characterized by an increase in cysteine oxidation and CysSSP forms. Late events (>21d) were characterized by a global reduction in cysteine, minimum level of CysSSP and maximum overexpression of fibronectin in kidney cortex. In conclusion, cysteine dynamics is influenced by the duration of CIH exposure: first there is a cysteine disulfide stress-like adaptive response followed by a progressive loss of cysteine availability and a decrease in CysSSP fraction. Kidney fibrosis associated to an unbalance in cysteine dynamics might contribute to the inefficacy of available antihypertensive drugs in patients with delayed diagnosis of sleep apnea.