Effect of allosteric inhibition of non-muscle myosin 2 on its intracellular diffusion

Mechanoregulation of MSC spheroid immunomodulation

Mesenchymal stromal cells (MSCs) are widely used in cell-based therapies and tissue regeneration for their potent secretome, which promotes host cell recruitment and modulates inflammation. Compared to monodisperse cells, MSC spheroids exhibit improved viability and increased secretion of immunomodulatory cytokines. While mechanical stimulation of monodisperse cells can increase cytokine production, the influence of mechanical loading on MSC spheroids is unknown. Here, we evaluated the effect of controlled, uniaxial cyclic compression on the secretion of immunomodulatory cytokines by human MSC spheroids and tested the influence of load-induced gene expression on MSC mechanoresponsiveness. We exposed MSC spheroids, entrapped in alginate hydrogels, to three cyclic compressive regimes with varying stress (L) magnitudes (i.e., 5 and 10 kPa) and hold (H) durations (i.e., 30 and 250 s) L5H30, L10H30, and L10H250. We observed changes in cytokine and chemokine expression dependent on the loading regime, where higher stress regimes tended to result in more exaggerated changes. However, only MSC spheroids exposed to L10H30 induced human THP-1 macrophage polarization toward an M2 phenotype compared to static conditions. Static and L10H30 loading facilitated a strong, interlinked F-actin arrangement, while L5H30 and L10H250 disrupted the structure of actin filaments. This was further examined when the actin cytoskeleton was disrupted via Y-27632. We observed downregulation of YAP-related genes, and the levels of secreted inflammatory cytokines were globally decreased. These findings emphasize the essential role of mechanosignaling in mediating the immunomodulatory potential of MSC spheroids.

Essential role of CD38 in platelet aggregation through the PKC-mediated internalization and activation

Introduction

CD38 is a multifunctional enzyme with a potent Ca2+ mobilizing effect, cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). Here, we aimed to demonstrate the role of CD38 in platelets via protein kinase C (PKC)-mediated internalization and activation.

Methods

Mouse platelets were used in this study. Thrombin, an agonist of platelet function, provoked a prompt and long-lasting increase in intracellular Ca2+ concentration ([Ca2+]i), resulting from an interplay of multifold Ca2+ mobilizing messengers.The signaling pathway was delineated using different inhibitors and techniques such as platelet aggregation assay, intracellular calcium measurements, immunoprecipitation, immunoblotting, and flow cytometry.

Results

We observed a sequential formation of cADPR and NAADP through CD38 activation by PKC of non-muscle myosin heavy chain IIA (MHCIIA), resulting in phospholipase C (PLC) activation in the thrombin-stimulated platelets. These findings reveal that PKC is fundamental in activating CD38 and elicits a physiological response in the murine platelets.

Conclusion

PKC is involved in many signaling pathways. Specifically, PKC is involved in the internalization of CD38 via MHCIIA in CD38+/+ wild-type (WT) and CD38-/- knockout mice (KO). CD38 generates calcium-mobilizing agents that act on specific receptors of the calcium stores. Calcium triggered platelet aggregation while serving as a secondary messenger.

3D cell segregation geometry and dynamics are governed by tissue surface tension regulation

Tissue morphogenesis and patterning during development involve the segregation of cell types. Segregation is driven by differential tissue surface tensions generated by cell types through controlling cell-cell contact formation by regulating adhesion and actomyosin contractility-based cellular cortical tensions. We use vertebrate tissue cell types and zebrafish germ layer progenitors as in vitro models of 3-dimensional heterotypic segregation and developed a quantitative analysis of their dynamics based on 3D time-lapse microscopy. We show that general inhibition of actomyosin contractility by the Rho kinase inhibitor Y27632 delays segregation. Cell type-specific inhibition of non-muscle myosin2 activity by overexpression of myosin assembly inhibitor S100A4 reduces tissue surface tension, manifested in decreased compaction during aggregation and inverted geometry observed during segregation. The same is observed when we express a constitutively active Rho kinase isoform to ubiquitously keep actomyosin contractility high at cell-cell and cell-medium interfaces and thus overriding the interface-specific regulation of cortical tensions. Tissue surface tension regulation can become an effective tool in tissue engineering.

Analysis of senescence-responsive stress fiber proteome reveals reorganization of stress fibers mediated by elongation factor eEF2 in HFF-1 cells

Stress fibers (SFs), which are actomyosin structures, reorganize in response to various cues to maintain cellular homeostasis. Currently, the protein components of SFs are only partially identified, limiting our understanding of their responses. Here we isolate SFs from human fibroblasts HFF-1 to determine with proteomic analysis the whole protein components and how they change with replicative senescence (RS), a state where cells decline in the ability to replicate after repeated divisions. We found that at least 135 proteins are associated with SFs, and 63 of them are up-regulated with RS, by which SFs become larger in size. Among them, we focused on eEF2 (eukaryotic translation elongation factor 2) as it exhibited on RS the most significant increase in abundance. We show that eEF2 is critical to the reorganization and stabilization of SFs in senescent fibroblasts. Our findings provide a novel molecular basis for SFs to be reinforced to resist cellular senescence.

Compliant Substrates Enhance Macrophage Cytokine Release and NLRP3 Inflammasome Formation During Their Pro-Inflammatory Response

Immune cells process a myriad of biochemical signals but their function and behavior are also determined by mechanical cues. Macrophages are no exception to this. Being present in all types of tissues, macrophages are exposed to environments of varying stiffness, which can be further altered under pathological conditions. While it is becoming increasingly clear that macrophages are mechanosensitive, it remains poorly understood how mechanical cues modulate their inflammatory response. Here we report that substrate stiffness influences the expression of pro-inflammatory genes and the formation of the NLRP3 inflammasome, leading to changes in the secreted protein levels of the cytokines IL-1β and IL-6. Using polyacrylamide hydrogels of tunable elastic moduli between 0.2 and 33.1 kPa, we found that bone marrow-derived macrophages adopted a less spread and rounder morphology on compliant compared to stiff substrates. Upon LPS priming, the expression levels of the gene encoding for TNF-α were higher on more compliant hydrogels. When additionally stimulating macrophages with the ionophore nigericin, we observed an enhanced formation of the NLRP3 inflammasome, increased levels of cell death, and higher secreted protein levels of IL-1β and IL-6 on compliant substrates. The upregulation of inflammasome formation on compliant substrates was not primarily attributed to the decreased cell spreading, since spatially confining cells on micropatterns led to a reduction of inflammasome-positive cells compared to well-spread cells. Finally, interfering with actomyosin contractility diminished the differences in inflammasome formation between compliant and stiff substrates. In summary, we show that substrate stiffness modulates the pro-inflammatory response of macrophages, that the NLRP3 inflammasome is one of the components affected by macrophage mechanosensing, and a role for actomyosin contractility in this mechanosensory response. Thus, our results contribute to a better understanding of how microenvironment stiffness affects macrophage behavior, which might be relevant in diseases where tissue stiffness is altered and might potentially provide a basis for new strategies to modulate inflammatory responses.