The Drosophila neuroblast polarity cycle at a glance.

Drosophila neural stem cells, or neuroblasts, rapidly proliferate during embryonic and larval development to populate the central nervous system. Neuroblasts divide asymmetrically to create cellular diversity, with each division producing one sibling cell that retains the neuroblast fate and another that differentiates into glia or neurons. This asymmetric outcome is mediated by the transient polarization of numerous factors to the cell cortex during mitosis. The powerful genetics and outstanding imaging tractability of the neuroblast make it an excellent model system for studying the mechanisms of cell polarity. This Cell Science at a Glance article and the accompanying poster explore the phases of the neuroblast polarity cycle and the regulatory circuits that control them. We discuss the key features of the cycle - the targeted recruitment of proteins to specific regions of the plasma membrane and multiple phases of highly dynamic actomyosin-dependent cortical flows that pattern both protein distribution and membrane structure.

Consumption of a polarized membrane reservoir drives asymmetric membrane expansion during the unequal divisions of neural stem cells.

The asymmetric divisions of Drosophila neural stem cells (NSCs) produce unequally sized siblings, with most volume directed into the sibling that retains the NSC fate. Sibling size asymmetry results from the preferential expansion of the NSC sibling surface during division. Here, we show that a polarized membrane reservoir constructed by the NSC in early mitosis provides the source for expansion. The reservoir is formed from membrane domains that contain folds and microvilli that become polarized by apically directed cortical flows of actomyosin early in mitosis. When furrow ingression begins and internal pressure increases, the stores of membrane within the apical reservoir are rapidly consumed. Expansion is substantially diminished in NSCs that lack a reservoir, and membrane expansion equalizes when the reservoir is not polarized. Our results suggest that the cortical flows that remodel the plasma membrane during asymmetric cell division function to satisfy the dynamic surface area requirements of unequally dividing cells.

Actin-dependent membrane polarization reveals the mechanical nature of the neuroblast polarity cycle.

The Par complex directs fate-determinant segregation from the apical membrane of asymmetrically dividing Drosophila neuroblasts. While the physical interactions that recruit the Par complex have been extensively studied, little is known about how the membrane itself behaves during polarization. We examined the membrane dynamics of neuroblasts and surrounding cells using a combination of super-resolution and time-lapse imaging, revealing cellular-scale movements of diverse membrane features during asymmetric division cycles. Membrane domains that are distributed across the neuroblast membrane in interphase become polarized in early mitosis, where they mediate formation of cortical patches of the Par protein atypical protein kinase C (aPKC). Membrane and protein polarity cycles are precisely synchronized and are generated by extensive actin-dependent forces that deform the surrounding tissue. In addition to suggesting a role for the membrane in asymmetric division, our results reveal the mechanical nature of the neuroblast polarity cycle.

A comparison of resveratrol and other polyphenolic compounds on Notch activation and endothelial cell activity.

Resveratrol is a polyphenolic compound produced by plants which makes its way into the human diet through plant-based foods. It has been shown to provide many health benefits, helping to ward of age-related diseases and promoting cardiovascular health. Additionally, resveratrol is a potent activator of the Notch signaling pathway. While resveratrol receives the most attention as a polyphenolic nutraceutical, other compounds with similar structures may be more potent regulators of specific cellular processes. Here, we compare resveratrol, apigenin, chrysin, genistein, luteolin, myricetin, piceatannol, pterostilbene, and quercetin for their ability to regulate Notch signaling. In addition, we compare the ability of these polyphenolic compounds to regulate endothelial cell viability, proliferation, and migration. Out of these compounds we found that resveratrol is the best activator of Notch signaling, however, other similar compounds are also capable of stimulating Notch. We also discovered that several of these polyphenols were able to inhibit endothelial cell proliferation. Finally, we found that many of these polyphenols are potent inhibitors of endothelial migration during wound healing assays. These findings provide the first side-by-side comparison of the regulation of Notch signaling, and endothelial cell proliferation and migration, by nine polyphenolic compounds.

Src kinase phosphorylates Notch1 to inhibit MAML binding.

Notch signaling is a form of intercellular communication which plays pivotal roles at various stages in development and disease. Previous findings have hinted that integrins and extracellular matrix may regulate Notch signaling, although a mechanistic basis for this interaction had not been identified. Here, we reveal that the regulation of Notch by integrins and extracellular matrix is carried out by Src family kinases (SFKs) working downstream of integrins. We identify a physical interaction between the SFK member, c-Src, and the Notch intracellular domain (NICD) that is enhanced by ß3 integrin and the integrin binding ECM protein, MAGP2. Our results demonstrate that c-Src directly phosphorylates the NICD at specific tyrosine residues and that mutation of these phosphorylation sites increases Notch responsive transcriptional activity. Furthermore, we also find that phosphorylation of the NICD by SFKs attenuates Notch mediated transcription by decreasing recruitment of MAML to the Notch co-transcriptional complex. Finally, we also find that SFK activity decreases NICD half-life. Collectively, our results provide important mechanistic data that underlie the emerging role of Notch as a general sensor and responder to extracellular signals.

Beyond the Matrix: The Many Non-ECM Ligands for Integrins.

The traditional view of integrins portrays these highly conserved cell surface receptors as mediators of cellular attachment to the extracellular matrix (ECM), and to a lesser degree, as coordinators of leukocyte adhesion to the endothelium. These canonical activities are indispensable; however, there is also a wide variety of integrin functions mediated by non-ECM ligands that transcend the traditional roles of integrins. Some of these unorthodox roles involve cell-cell interactions and are engaged to support immune functions such as leukocyte transmigration, recognition of opsonization factors, and stimulation of neutrophil extracellular traps. Other cell-cell interactions mediated by integrins include hematopoietic stem cell and tumor cell homing to target tissues. Integrins also serve as cell-surface receptors for various growth factors, hormones, and small molecules. Interestingly, integrins have also been exploited by a wide variety of organisms including viruses and bacteria to support infectious activities such as cellular adhesion and/or cellular internalization. Additionally, the disruption of integrin function through the use of soluble integrin ligands is a common strategy adopted by several parasites in order to inhibit blood clotting during hematophagy, or by venomous snakes to kill prey. In this review, we strive to go beyond the matrix and summarize non-ECM ligands that interact with integrins in order to highlight these non-traditional functions of integrins.

Notch: A multi-functional integrating system of microenvironmental signals.

The Notch signaling cascade is an evolutionarily ancient system that allows cells to interact with their microenvironmental neighbors through direct cell-cell interactions, thereby directing a variety of developmental processes. Recent research is discovering that Notch signaling is also responsive to a broad variety of stimuli beyond cell-cell interactions, including: ECM composition, crosstalk with other signaling systems, shear stress, hypoxia, and hyperglycemia. Given this emerging understanding of Notch responsiveness to microenvironmental conditions, it appears that the classical view of Notch as a mechanism enabling cell-cell interactions, is only a part of a broader function to integrate microenvironmental cues. In this review, we summarize and discuss published data supporting the idea that the full function of Notch signaling is to serve as an integrator of microenvironmental signals thus allowing cells to sense and respond to a multitude of conditions around them.