Emergence and properties of adult mammalian epidermal stem cells.

In this review we discuss how the mammalian interfollicular epidermis forms during development, maintains homeostasis, and is repaired following wounding. Recent studies have provided new insights into the relationship between the stem cell compartment and the differentiating cell layers; the ability of differentiated cells to dedifferentiate into stem cells; and the epigenetic memory of epidermal cells following wounding.

Negatively curved cellular membranes promote BAIAP2 signaling hub assembly.

Plasma membrane deformations are associated with curvature-dependent protein enrichment that contributes to a wide array of cellular functions. While the spatio-temporal protein dynamics at membrane indentations is well characterized, relatively little is known about protein kinetics at outwardly deforming membrane sites. This is in part due to the lack of high throughput approaches to systematically probe the curvature-dependence of protein-membrane interactions. Here, we developed a nanopatterned array for multiplexed analysis of protein dynamics at negatively curved cellular membranes. Taking advantage of this robust and versatile platform, we explored how membrane shape influences the prototypic negative curvature sensing protein BAIAP2 and its effector proteins. We find assembly of multi-protein signaling hubs and increased actin polymerization at outwardly deformed membrane sections, indicative of curvature-dependent BAIAP2 activation. Collectively, this study presents technical and conceptual advancements towards a quantitative understanding of spatio-temporal protein dynamics at negatively curved membranes.

Force-induced changes of α-catenin conformation stabilize vascular junctions independently of vinculin.

Cadherin-mediated cell adhesion requires anchoring via the ß-catenin-α-catenin complex to the actin cytoskeleton, yet, α-catenin only binds F-actin weakly. A covalent fusion of VE-cadherin to α-catenin enhances actin anchorage in endothelial cells and strongly stabilizes endothelial junctions in vivo, blocking inflammatory responses. Here, we have analyzed the underlying mechanism. We found that VE-cadherin-α-catenin constitutively recruits the actin adaptor vinculin. However, removal of the vinculin-binding region of α-catenin did not impair the ability of VE-cadherin-α-catenin to enhance junction integrity. Searching for an alternative explanation for the junction-stabilizing mechanism, we found that an antibody-defined epitope, normally buried in a short α1-helix of the actin-binding domain (ABD) of α-catenin, is openly displayed in junctional VE-cadherin-α-catenin chimera. We found that this epitope became exposed in normal α-catenin upon triggering thrombin-induced tension across the VE-cadherin complex. These results suggest that the VE-cadherin-α-catenin chimera stabilizes endothelial junctions due to conformational changes in the ABD of α-catenin that support constitutive strong binding to actin.

Dendrite tapering actuates a self-organizing signaling circuit for stochastic filopodia initiation in neurons.

How signaling units spontaneously arise from a noisy cellular background is not well understood. Here, we show that stochastic membrane deformations can nucleate exploratory dendritic filopodia, dynamic actin-rich structures used by neurons to sample its surroundings for compatible transcellular contacts. A theoretical analysis demonstrates that corecruitment of positive and negative curvature-sensitive proteins to deformed membranes minimizes the free energy of the system, allowing the formation of long-lived curved membrane sections from stochastic membrane fluctuations. Quantitative experiments show that once recruited, curvature-sensitive proteins form a signaling circuit composed of interlinked positive and negative actin-regulatory feedback loops. As the positive but not the negative feedback loop can sense the dendrite diameter, this self-organizing circuit determines filopodia initiation frequency along tapering dendrites. Together, our findings identify a receptor-independent signaling circuit that employs random membrane deformations to simultaneously elicit and limit formation of exploratory filopodia to distal dendritic sites of developing neurons.

A multiplexed phospholipid membrane platform for curvature sensitive protein screening.

The curvature of lipid membranes plays a key role in many relevant biological processes such as membrane trafficking, vesicular budding and host-virus interactions. In vitro studies on the membrane curvature of simplified biomimetic models in the nanometer range are challenging, due to their complicated nanofabrication processes. In this work, we propose a simple and low-cost platform for curvature sensitive protein screening, prepared through scanning probe lithography (SPL) methods, where lipid bilayer patches of different compositions can be multiplexed onto substrate areas with tailored local curvature. The curvature is imposed by anchoring nanoparticles of the desired size to the substrate prior to lithography. As a proof of principle, we demonstrate that a positive curvature membrane sensitive protein derived from the BAR domain of Nadrin2 binds selectively to lipid patches patterned on substrate areas coated with 100 nm nanoparticles. The platform opens up a path for screening curvature-dependent protein-membrane interaction studies by providing a flexible and easy to prepare substrate with control over lipid composition and membrane curvature.

Receptor-Free Signaling at Curved Cellular Membranes.

Plasma membranes are subject to continuous deformations. Strikingly, some of these transient membrane undulations yield membrane-associated signaling hubs that differ in composition and function, depending on membrane geometry and the availability of co-factors. Here, recent advancements on this ubiquitous type of receptor-independent signaling are reviewed, with a special focus on emerging concepts and technical challenges associated with studying these elusive signaling sites.

ArhGEF37 assists dynamin 2 during clathrin-mediated endocytosis.

Clathrin-mediated endocytosis (CME) engages over 30 proteins to secure efficient cargo and membrane uptake. While the function of most core CME components is well established, auxiliary mechanisms crucial for fine-tuning and adaptation remain largely elusive. In this study, we identify ArhGEF37, a currently uncharacterized protein, as a constituent of CME. Structure prediction together with quantitative cellular and biochemical studies present a unique BAR domain and PI(4,5)P2-dependent protein-membrane interactions. Functional characterization yields accumulation of ArhGEF37 at dynamin 2-rich late endocytic sites and increased endocytosis rates in the presence of ArhGEF37. Together, these results introduce ArhGEF37 as a regulatory protein involved in endocytosis.