Advances in Psoriatic Disease Research: Insights From GRAPPA Pilot Research Awardees.

Research progress from the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) pilot award program was presented and discussed at the GRAPPA 2023 annual meeting. Topics included identification of protein biomarkers associated with enthesitis in psoriatic arthritis (PsA), the role of HLA-B27 on gut microbial dysbiosis in PsA, single-cell profiling of synovial fluid vs psoriatic skin lesions in PsA, and the role of mechanotransduction in hyperactivation of transforming growth factor-ß via αVß6 integrin in psoriatic epidermis.

Inflammation modulates intercellular adhesion and mechanotransduction in human epidermis via ROCK2.

Aberrant mechanotransduction and compromised epithelial barrier function are associated with numerous human pathologies including inflammatory skin disorders. However, the cytoskeletal mechanisms regulating inflammatory responses in the epidermis are not well understood. Here we addressed this question by inducing a psoriatic phenotype in human keratinocytes and reconstructed human epidermis using a cytokine stimulation model. We show that the inflammation upregulates the Rho-myosin II pathway and destabilizes adherens junctions (AJs) promoting YAP nuclear entry. The integrity of cell-cell adhesion but not the myosin II contractility per se is the determinative factor for the YAP regulation in epidermal keratinocytes. The inflammation-induced disruption of AJs, increased paracellular permeability, and YAP nuclear translocation are regulated by ROCK2, independently from myosin II activation. Using a specific inhibitor KD025, we show that ROCK2 executes its effects via cytoskeletal and transcription-dependent mechanisms to shape the inflammatory response in the epidermis.

Mechanotransduction in Skin Inflammation.

In the process of mechanotransduction, the cells in the body perceive and interpret mechanical stimuli to maintain tissue homeostasis and respond to the environmental changes. Increasing evidence points towards dysregulated mechanotransduction as a pathologically relevant factor in human diseases, including inflammatory conditions. Skin is the organ that constantly undergoes considerable mechanical stresses, and the ability of mechanical factors to provoke inflammatory processes in the skin has long been known, with the Koebner phenomenon being an example. However, the molecular mechanisms and key factors linking mechanotransduction and cutaneous inflammation remain understudied. In this review, we outline the key players in the tissue's mechanical homeostasis, the available data, and the gaps in our current understanding of their aberrant regulation in chronic cutaneous inflammation. We mainly focus on psoriasis as one of the most studied skin inflammatory diseases; we also discuss mechanotransduction in the context of skin fibrosis as a result of chronic inflammation. Even though the role of mechanotransduction in inflammation of the simple epithelia of internal organs is being actively studied, we conclude that the mechanoregulation in the stratified epidermis of the skin requires more attention in future translational research.

Filamin A mediates isotropic distribution of applied force across the actin network.

Cell sensing of externally applied mechanical strain through integrin-mediated adhesions is critical in development and physiology of muscle, lung, tendon, and arteries, among others. We examined the effects of strain on force transmission through the essential cytoskeletal linker talin. Using a fluorescence-based talin tension sensor (TS), we found that uniaxial stretch of cells on elastic substrates increased tension on talin, which was unexpectedly independent of the orientation of the focal adhesions relative to the direction of strain. High-resolution electron microscopy of the actin cytoskeleton revealed that stress fibers (SFs) are integrated into an isotropic network of cortical actin filaments in which filamin A (FlnA) localizes preferentially to points of intersection between SFs and cortical actin. Knockdown (KD) of FlnA resulted in more isolated, less integrated SFs. After FlnA KD, tension on talin was polarized in the direction of stretch, while FlnA reexpression restored tensional symmetry. These data demonstrate that a FlnA-dependent cortical actin network distributes applied forces over the entire cytoskeleton-matrix interface.

Membrane-cytoskeletal crosstalk mediated by myosin-I regulates adhesion turnover during phagocytosis.

Phagocytosis of invading pathogens or cellular debris requires a dramatic change in cell shape driven by actin polymerization. For antibody-covered targets, phagocytosis is thought to proceed through the sequential engagement of Fc-receptors on the phagocyte with antibodies on the target surface, leading to the extension and closure of the phagocytic cup around the target. We find that two actin-dependent molecular motors, class 1 myosins myosin 1e and myosin 1f, are specifically localized to Fc-receptor adhesions and required for efficient phagocytosis of antibody-opsonized targets. Using primary macrophages lacking both myosin 1e and myosin 1f, we find that without the actin-membrane linkage mediated by these myosins, the organization of individual adhesions is compromised, leading to excessive actin polymerization, slower adhesion turnover, and deficient phagocytic internalization. This work identifies a role for class 1 myosins in coordinated adhesion turnover during phagocytosis and supports a mechanism involving membrane-cytoskeletal crosstalk for phagocytic cup closure.

Mammalian nonmuscle myosin II comes in three flavors.

Nonmuscle myosin II is an actin-based motor that executes numerous mechanical tasks in cells including spatiotemporal organization of the actin cytoskeleton, adhesion, migration, cytokinesis, tissue remodeling, and membrane trafficking. Nonmuscle myosin II is ubiquitously expressed in mammalian cells as a tissue-specific combination of three paralogs. Recent studies reveal novel specific aspects of their kinetics, intracellular regulation and functions. On the other hand, the three paralogs also can copolymerize and cooperate in cells. Here we review the recent advances from the prospective of how distinct features of the three myosin II paralogs adapt them to perform specialized and joint tasks in the cell.

Self-sorting of nonmuscle myosins IIA and IIB polarizes the cytoskeleton and modulates cell motility.

Nonmuscle myosin II (NMII) is uniquely responsible for cell contractility and thus defines multiple aspects of cell behavior. To generate contraction, NMII molecules polymerize into bipolar minifilaments. Different NMII paralogs are often coexpressed in cells and can copolymerize, suggesting that they may cooperate to facilitate cell motility. However, whether such cooperation exists and how it may work remain unknown. We show that copolymerization of NMIIA and NMIIB followed by their differential turnover leads to self-sorting of NMIIA and NMIIB along the front-rear axis, thus producing a polarized actin-NMII cytoskeleton. Stress fibers newly formed near the leading edge are enriched in NMIIA, but over time, they become progressively enriched with NMIIB because of faster NMIIA turnover. In combination with retrograde flow, this process results in posterior accumulation of more stable NMIIB-rich stress fibers, thus strengthening cell polarity. By copolymerizing with NMIIB, NMIIA accelerates the intrinsically slow NMIIB dynamics, thus increasing cell motility and traction and enabling chemotaxis.

Endogenous species of mammalian nonmuscle myosin IIA and IIB include activated monomers and heteropolymers.

BACKGROUND: Class II myosins generate contractile forces in cells by polymerizing into bipolar filaments and pulling on anchored actin filaments. Nonmuscle myosin II (NMII) plays central roles during cell adhesion, migration, cytokinesis, and tissue morphogenesis. NMII is present in virtually all mammalian cell types as tissue-specific combinations of NMIIA, NMIIB, and NMIIC isoforms. It remains poorly understood how the highly dynamic NMII-actin contractile system begins to assemble at new cellular locations during cell migration and how incorporation of different NMII isoforms into this system is coordinated. RESULTS: Using platinum replica electron microscopy in combination with immunogold labeling, we demonstrate that individual activated (phosphorylated on the regulatory light chain and unfolded) NMIIA and NMIIB molecules represent a functional form of NMII in motile cells and that NMIIA and NMIIB copolymerize into nascent bipolar filaments during contractile system assembly. Using subdiffraction stimulated emission depletion microscopy together with a pharmacological block-and-release approach, we report that NMIIA and NMIIB simultaneously incorporate into the cytoskeleton during initiation of contractile system assembly, whereas the characteristic rearward shift of NMIIB relative to NMIIA is established later in the course of NMII turnover. CONCLUSIONS: We show existence of activated NMII monomers in cells, copolymerization of endogenous NMIIA and NMIIB molecules, and contribution of both isoforms, rather than only NMIIA, to early stages of the contractile system assembly. These data change the current paradigms about dynamics and functions of NMII and provide new conceptual insights into the organization and dynamics of the ubiquitous cellular machinery for contraction that acts in multiple cellular contexts.

Regulation of polarity in cells devoid of actin bundle system after treatment with inhibitors of myosin II activity.

Interplay of two cytoskeletal systems--microfilaments and microtubules is essential for directional cell movement. To better understand the role of those cytoskeletal systems in polarization of cells, rat fibroblasts were incubated with drugs inhibiting activity of myosin II: blebbistatin and Y-27632. Both drugs led to disappearance of actin-myosin bundles and mature focal cell-matrix adhesions but did not affect polarization and directional motility. The rate of motility even increased after inhibitor treatment. The characteristic feature of inhibitor-treated fibroblasts was collapse of the cytoplasm accompanied by bundling of microtubules that led to transformation of lamellae into long immobile tails. The only exception was the leading anterior lamella which was not transformed into the tail and supported directional movement of the cell. The tail at the cell rear determined the position of anterior lamella and direction of locomotion. Depolymerization of microtubules by colcemid stopped directional locomotion of inhibitor-treated cells. These data show that integrity of the microtubular system provides the basic mechanism of polarization and orientation which is only modified by interactions with actin-myosin system and cell-substrate adhesions. We suggest that the position of bundled tail microtubules and dispersed microtubules in leading lamella determine polarization in cells lacking stress fibers and focal adhesions. Thus, polarization is based on microtubule-dependent mechanisms both in non-contractile and contractile cells. These mechanisms could switch dependent on circumstances as fibroblasts may acquire non-contractile phenotype, not only after direct inhibition of myosin II but also in certain conditions of microenvironment.