Extracellular Caspase-1 induces hair stem cell migration in wounded and inflamed skin conditions.

The wound-healing process is a paradigm of the directed migration of various pools of stem cells from their niche to the site of injury where they replenish damaged cells. Two decades have elapsed since the observation that wounding activates multipotent hair follicle stem cells to infiltrate the epidermis, but the cues that coax these cells out of their niche remain unknown. Here, we report that Caspase-1, a protein classically known as an integral component of the cytosolic inflammasome, is secreted upon wounding and has a non-canonical role in the extracellular milieu. Through its caspase activation recruitment domain (CARD), Caspase-1 is sufficient to initiate the migration of hair follicle stem cells into the epidermis. Uncovering this novel function of Caspase-1 also facilitates a deeper understanding of the mechanistic basis of the epithelial hyperplasia found to accompany numerous inflammatory skin diseases.

A Dynamic Water Channel Affects O2 Stability in [FeFe]-Hydrogenases.

[FeFe]-hydrogenases are capable of reducing protons at a high rate. However, molecular oxygen (O2 ) induces the degradation of their catalytic cofactor, the H-cluster, which consists of a cubane [4Fe4S] subcluster (4FeH ) and a unique diiron moiety (2FeH ). Previous attempts to prevent O2 -induced damage have focused on enhancing the protein's sieving effect for O2 by blocking the hydrophobic gas channels that connect the protein surface and the 2FeH . In this study, we aimed to block an O2 diffusion pathway and shield 4FeH instead. Molecular dynamics (MD) simulations identified a novel water channel (WH ) surrounding the H-cluster. As this hydrophilic path may be accessible for O2 molecules we applied site-directed mutagenesis targeting amino acids along WH in proximity to 4FeH to block O2 diffusion. Protein film electrochemistry experiments demonstrate increased O2 stabilities for variants G302S and S357T, and MD simulations based on high-resolution crystal structures confirmed an enhanced local sieving effect for O2 in the environment of the 4FeH in both cases. The results strongly suggest that, in wild type proteins, O2 diffuses from the 4FeH to the 2FeH . These results reveal new strategies for improving the O2 stability of [FeFe]-hydrogenases by focusing on the O2 diffusion network near the active site.

Stimulation of hair follicle stem cell proliferation through an IL-1 dependent activation of γδT-cells.

The cutaneous wound-healing program is a product of a complex interplay among diverse cell types within the skin. One fundamental process that is mediated by these reciprocal interactions is the mobilization of local stem cell pools to promote tissue regeneration and repair. Using the ablation of epidermal caspase-8 as a model of wound healing in Mus musculus, we analyzed the signaling components responsible for epithelial stem cell proliferation. We found that IL-1α and IL-7 secreted from keratinocytes work in tandem to expand the activated population of resident epidermal γδT-cells. A downstream effect of activated γδT-cells is the preferential proliferation of hair follicle stem cells. By contrast, IL-1α-dependent stimulation of dermal fibroblasts optimally stimulates epidermal stem cell proliferation. These findings provide new mechanistic insights into the regulation and function of epidermal cell-immune cell interactions and into how components that are classically associated with inflammation can differentially influence distinct stem cell niches within a tissue.

Active remodeling of cortical actin regulates spatiotemporal organization of cell surface molecules.

Many lipid-tethered proteins and glycolipids exist as monomers and nanoclusters on the surface of living cells. The spatial distribution and dynamics of formation and breakup of nanoclusters does not reflect thermal and chemical equilibrium and is controlled by active remodeling of the underlying cortical actin. We propose a model for nanoclustering based on active hydrodynamics, wherein cell surface molecules bound to dynamic actin are actively driven to form transient clusters. This consistently explains all of our experimental observations. Using FCS and TIRF microscopy, we provide evidence for the existence of short, dynamic, polymerizing actin filaments at the cortex, a key assumption of the theoretical framework. Our theory predicts that lipid-anchored proteins that interact with dynamic actin must exhibit anomalous concentration fluctuations, and a cell membrane protein capable of binding directly to actin can form nanoclusters. These we confirm experimentally, providing an active mechanism for molecular organization and its spatiotemporal regulation on the plasma membrane.

Dynamic imaging of homo-FRET in live cells by fluorescence anisotropy microscopy.

Multiple lipid and protein components of the plasma membrane of a living cell are organized, both compositionally and functionally, at different spatial and temporal scales. For instance, Rab protein domains in membranes the clathrin-coated pit, or the immunological synapse are exquisite examples of functional compartmentalization in cell membranes. These assemblies consist in part of nanoscale complexes of lipids and proteins and are necessary to facilitate some specific sorting and signaling functions. It is evident that cellular functions require a regulated spatiotemporal organization of components at the nanoscale, often comprising of countable number of molecular species. Here, we describe multiple homo-FRET-based imaging methods that provide information about nanoscale interactions between fluorescently tagged molecules in live cells, at optically resolved spatial resolution.

Nanoclusters of GPI-anchored proteins are formed by cortical actin-driven activity.

Several cell-surface lipid-tethered proteins exhibit a concentration-independent, cholesterol-sensitive organization of nanoscale clusters and monomers. To understand the mechanism of formation of these clusters, we investigate the spatial distribution and steady-state dynamics of fluorescently tagged GPI-anchored protein nanoclusters using high-spatial and temporal resolution FRET microscopy. These studies reveal a nonrandom spatial distribution of nanoclusters, concentrated in optically resolvable domains. Monitoring the dynamics of recovery of fluorescence intensity and anisotropy, we find that nanoclusters are immobile, and the dynamics of interconversion between nanoclusters and monomers, over a range of temperatures, is spatially heterogeneous and non-Arrhenius, with a sharp crossover coinciding with a reduction in the activity of cortical actin. Cholesterol depletion perturbs cortical actin and the spatial scale and interconversion dynamics of nanoclusters. Direct perturbations of cortical actin activity also affect the construction, dynamics, and spatial organization of nanoclusters. These results suggest a unique mechanism of complexation of cell-surface molecules regulated by cortical actin activity.