Apical-basal polarity in the gut.

Apical-Basal polarity is a fundamental property of all epithelial cells that underlies both their form and function. The gut is made up of a single layer of intestinal epithelial cells, with distinct apical, lateral and basal domains. Occluding junctions at the apical side of the lateral domains create a barrier between the gut lumen and the body, which is crucial for tissue homeostasis, protection against gastrointestinal pathogens and for the maintenance of the immune response. Apical-basal polarity in most epithelia is established by conserved polarity factors, but recent evidence suggests that the gut epithelium in at least some organisms polarises by novel mechanisms. In this review, we discuss the recent advances in understanding polarity factors by focussing on work in C. elegans, Drosophila, Zebrafish and Mouse.

Dissection, Fixation, and Immunostaining of the Drosophila Midgut.

The Drosophila midgut is mainly composed of highly polarized epithelial cells called enterocytes that establish their apical-basal polarity in a fundamentally different way from other Drosophila epithelia. The roles of polarity factors in the midgut can be studied by generating clones of homozygous mutant cells in the background of wild-type tissue. In this chapter, we will introduce and discuss the procedures for producing positively marked mutant clones in the midgut and describe specific protocols for dissecting, fixing, and immunostaining this tissue.

MARK4 controls ischaemic heart failure through microtubule detyrosination.

Myocardial infarction is a major cause of premature death in adults. Compromised cardiac function after myocardial infarction leads to chronic heart failure with systemic health complications and a high mortality rate1. Effective therapeutic strategies are needed to improve the recovery of cardiac function after myocardial infarction. More specifically, there is a major unmet need for a new class of drugs that can improve cardiomyocyte contractility, because inotropic therapies that are currently available have been associated with high morbidity and mortality in patients with systolic heart failure2,3 or have shown a very modest reduction of risk of heart failure4. Microtubule detyrosination is emerging as an important mechanism for the regulation of cardiomyocyte contractility5. Here we show that deficiency of microtubule-affinity regulating kinase 4 (MARK4) substantially limits the reduction in the left ventricular ejection fraction after acute myocardial infarction in mice, without affecting infarct size or cardiac remodelling. Mechanistically, we provide evidence that MARK4 regulates cardiomyocyte contractility by promoting phosphorylation of microtubule-associated protein 4 (MAP4), which facilitates the access of vasohibin 2 (VASH2)-a tubulin carboxypeptidase-to microtubules for the detyrosination of α-tubulin. Our results show how the detyrosination of microtubules in cardiomyocytes is finely tuned by MARK4 to regulate cardiac inotropy, and identify MARK4 as a promising therapeutic target for improving cardiac function after myocardial infarction.

Drosophila follicle cells are patterned by multiple levels of Notch signaling and antagonism between the Notch and JAK/STAT pathways.

The specification of polar, main-body and stalk follicle cells in the germarium of the Drosophila ovary plays a key role in the formation of the egg chamber and polarisation of its anterior-posterior axis. High levels of Notch pathway activation, resulting from a germline Delta ligand signal, induce polar cells. Here we show that low Notch activation levels, originating from Delta expressed in the polar follicle cells, are required for stalk formation. The metalloprotease Kuzbanian-like, which cleaves and inactivates Delta, reduces the level of Delta signaling between follicle cells, thereby limiting the size of the stalk. We find that Notch activation is required in a continuous fashion to maintain the polar and stalk cell fates. We further demonstrate that mutual antagonism between the Notch and JAK/STAT signaling pathways provides a crucial facet of follicle cell patterning. Notch signaling in polar and main-body follicle cells inhibits JAK/STAT signaling by preventing STAT nuclear translocation, thereby restricting the influence of this pathway to stalk cells. Conversely, signaling by JAK/STAT reduces Notch signaling in the stalk. Thus, variations in the levels of Notch pathway activation, coupled with a continuous balance between the Notch and JAK/STAT pathways, specify the identity of the different follicle cell types and help establish the polarity of the egg chamber.

A translation-independent role of oskar RNA in early Drosophila oogenesis.

The Drosophila maternal effect gene oskar encodes the posterior determinant responsible for the formation of the posterior pole plasm in the egg, and thus of the abdomen and germline of the future fly. Previously identified oskar mutants give rise to offspring that lack both abdominal segments and a germline, thus defining the ;posterior group phenotype'. Common to these classical oskar alleles is that they all produce significant amounts of oskar mRNA. By contrast, two new oskar mutants in which oskar RNA levels are strongly reduced or undetectable are sterile, because of an early arrest of oogenesis. This egg-less phenotype is complemented by oskar nonsense mutant alleles, as well as by oskar transgenes, the protein-coding capacities of which have been annulled. Moreover, we show that expression of the oskar 3' untranslated region (3'UTR) is sufficient to rescue the egg-less defect of the RNA null mutant. Our analysis thus reveals an unexpected role for oskar RNA during early oogenesis, independent of Oskar protein. These findings indicate that oskar RNA acts as a scaffold or regulatory RNA essential for development of the oocyte.