The WAVE2 complex regulates actin cytoskeletal reorganization and CRAC-mediated calcium entry during T cell activation

Hem1 is essential for ruffled border formation in osteoclasts and efficient bone resorption

Bone resorption is highly dependent on the dynamic rearrangement of the osteoclast actin cytoskeleton to allow formation of actin rings and a functional ruffled border. Hem1 is a hematopoietic-specific subunit of the WAVE-complex which regulates actin polymerization and is crucial for lamellipodia formation in hematopoietic cell types. However, its role in osteoclast differentiation and function is still unknown. Here, we show that although the absence of Hem1 promotes osteoclastogenesis, the ability of Hem1-/- osteoclasts to degrade bone was severely impaired. Global as well as osteoclast-specific deletion of Hem1 in vivo revealed increased femoral trabecular bone mass despite elevated numbers of osteoclasts in vivo. We found that the resorption defect derived from the morphological distortion of the actin-rich sealing zone and ruffled border deformation in Hem1-deficient osteoclasts leading to impaired vesicle transport and increased intracellular acidification. Collectively, our data identify Hem1 as a yet unknown key player in bone remodeling by regulating ruffled border formation and consequently the resorptive capacity of osteoclasts.

Coordinated ARP2/3 and glycolytic activities regulate the morphological and functional fitness of human CD8+ T cells

Actin cytoskeleton remodeling sustains the ability of cytotoxic T cells to search for target cells and eliminate them. We here investigated the relationship between energetic status, actin remodeling, and functional fitness in human CD8+ effector T cells. Cell spreading during migration or immunological synapse assembly mirrored cytotoxic activity. Morphological and functional fitness were boosted by interleukin-2 (IL-2), which also stimulated the transcription of glycolytic enzymes, actin isoforms, and actin-related protein (ARP)2/3 complex subunits. This molecular program scaled with F-actin content and cell spreading. Inhibiting glycolysis impaired F-actin remodeling at the lamellipodium, chemokine-driven motility, and adhesion, while mitochondrial oxidative phosphorylation blockade impacted cell elongation during confined migration. The severe morphological and functional defects of ARPC1B-deficient T cells were only partially corrected by IL-2, emphasizing ARP2/3-mediated actin polymerization as a crucial energy state integrator. The study therefore underscores the tight coordination between metabolic and actin remodeling programs to sustain the cytotoxic activity of CD8+ T cells.

WAVE2 Is a Vital Regulator in Myogenic Differentiation of Progenitor Cells through the Mechanosensitive MRTFA-SRF Axis

Skeletal myogenesis is an intricate process involving the differentiation of progenitor cells into myofibers, which is regulated by actin cytoskeletal dynamics and myogenic transcription factors. Although recent studies have demonstrated the pivotal roles of actin-binding proteins (ABPs) as mechanosensors and signal transducers, the biological significance of WAVE2 (Wiskott-Aldrich syndrome protein family member 2), an ABP essential for actin polymerization, in myogenic differentiation of progenitor cells has not been investigated. Our study provides important insights into the regulatory roles played by WAVE2 in the myocardin-related transcription factor A (MRTFA)-serum response factor (SRF) signaling axis and differentiation of myoblasts. We demonstrate that WAVE2 expression is induced during myogenic differentiation and plays a pivotal role in actin cytoskeletal remodeling in C2C12 myoblasts. Knockdown of WAVE2 in C2C12 cells reduced filamentous actin levels, increased globular actin accumulation, and impaired the nuclear translocation of MRTFA. Furthermore, WAVE2 depletion in myoblasts inhibited the expression and transcriptional activity of SRF and suppressed cell proliferation in myoblasts. Consequently, WAVE2 knockdown suppressed myogenic regulatory factors (i.e., MyoD, MyoG, and SMYD1) expressions, thereby hindering the differentiation of myoblasts. Thus, this study suggests that WAVE2 is essential for myogenic differentiation of progenitor cells by modulating the mechanosensitive MRTFA-SRF axis.

PD-1 inhibits T cell actin remodeling at the immunological synapse independently of its signaling motifs

Engagement of the receptor programmed cell death molecule 1 (PD-1) by its ligands PD-L1 and PD-L2 inhibits T cell-mediated immune responses. Blocking such signaling provides the clinical effects of PD-1-targeted immunotherapy. Here, we investigated the mechanisms underlying PD-1-mediated inhibition. Because dynamic actin remodeling is crucial for T cell functions, we characterized the effects of PD-1 engagement on actin remodeling at the immunological synapse, the interface between a T cell and an antigen-presenting cell (APC) or target cell. We used microscopy to analyze the formation of immunological synapses between PD-1+ Jurkat cells or primary human CD8+ cytotoxic T cells and APCs that presented T cell-activating antibodies and were either positive or negative for PD-L1. PD-1 binding to PD-L1 inhibited T cell spreading induced by antibody-mediated activation, which was characterized by the absence of the F-actin-dense distal lamellipodial network at the immunological synapse and the Arp2/3 complex, which mediates branched actin formation. PD-1-induced inhibition of actin remodeling also prevented the characteristic deformation of T cells that contact APCs and the release of cytotoxic granules. We showed that the effects of PD-1 on actin remodeling did not require its tyrosine-based signaling motifs, which are thought to mediate the co-inhibitory effects of PD-1. Our study highlights a previously unappreciated mechanism of PD-1-mediated suppression of T cell activity, which depends on the regulation of actin cytoskeleton dynamics in a signaling motif-independent manner.

Actin cytoskeleton remodeling at the cancer cell side of the immunological synapse: good, bad, or both?

Cytotoxic lymphocytes (CLs), specifically cytotoxic T lymphocytes and natural killer cells, are indispensable guardians of the immune system and orchestrate the recognition and elimination of cancer cells. Upon encountering a cancer cell, CLs establish a specialized cellular junction, known as the immunological synapse that stands as a pivotal determinant for effective cell killing. Extensive research has focused on the presynaptic side of the immunological synapse and elucidated the multiple functions of the CL actin cytoskeleton in synapse formation, organization, regulatory signaling, and lytic activity. In contrast, the postsynaptic (cancer cell) counterpart has remained relatively unexplored. Nevertheless, both indirect and direct evidence has begun to illuminate the significant and profound consequences of cytoskeletal changes within cancer cells on the outcome of the lytic immunological synapse. Here, we explore the understudied role of the cancer cell actin cytoskeleton in modulating the immune response within the immunological synapse. We shed light on the intricate interplay between actin dynamics and the evasion mechanisms employed by cancer cells, thus providing potential routes for future research and envisioning therapeutic interventions targeting the postsynaptic side of the immunological synapse in the realm of cancer immunotherapy. This review article highlights the importance of actin dynamics within the immunological synapse between cytotoxic lymphocytes and cancer cells focusing on the less-explored postsynaptic side of the synapse. It presents emerging evidence that actin dynamics in cancer cells can critically influence the outcome of cytotoxic lymphocyte interactions with cancer cells.

Who wins the combat, CAR or TCR?

Chimeric antigen receptor T (CAR-T) cell therapy has drawn increasing attention over the last few decades given its remarkable effectiveness and breakthroughs in treating B cell hematological malignancies. Even though CAR-T cell therapy has outstanding clinical successes, most treated patients still relapse after infusion. CARs are derived from the T cell receptor (TCR) complex and co-stimulatory molecules associated with T cell activation; however, the similarities and differences between CARs and endogenous TCRs regarding their sensitivity, signaling pathway, killing mechanisms, and performance are still not fully understood. In this review, we discuss the parallel comparisons between CARs and TCRs from various aspects and how these current findings might provide novel insights and contribute to improvement of CAR-T cell therapy efficacy.

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

T cell functions and organ infiltration by leukemic T cells require cortactin

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that is still fatal in many cases. T cell blasts are characterized by hyperactivation and strong proliferative and migratory capacities. The chemokine receptor CXCR4 is involved in mediating malignant T cell properties, and cortactin has been shown to control CXCR4 surface localization in T-ALL cells. We have previously shown that cortactin overexpression is correlated with organ infiltration and relapse in B-ALL. However, the role of cortactin in T cell biology and T-ALL remains elusive. Here, we analyzed the functional relevance of cortactin for T cell activation and migration and the implications for T-ALL development. We found that cortactin is upregulated in response to T cell receptor engagement and recruited to the immune synapse in normal T cells. Loss of cortactin caused reduced IL-2 production and proliferation. Cortactin-depleted T cells showed defects in immune synapse formation and migrated less due to impaired actin polymerization in response to T cell receptor and CXCR4 stimulation. Leukemic T cells expressed much higher levels of cortactin compared to normal T cells that correlated with greater migratory capacity. Xenotransplantation assays in NSG mice revealed that cortactin-depleted human leukemic T cells colonized the bone marrow significantly less and failed to infiltrate the central nervous system, suggesting that cortactin overexpression drives organ infiltration, which is a major complication of T-ALL relapse. Thus, cortactin could serve as a potential therapeutic target for T-ALL and other pathologies involving aberrant T cell responses.

Hypermethylated WASF2: tumor suppressive role in head and neck squamous cell carcinoma

Background

WASF2 regulates actin reorganization during cell migration. WASF2 has been identified as a regulator of the development of gastric cancer, breast cancer, and pancreatic cancer. But its regulatory mechanisms remain unknown. Also, its function was absent in head and neck squamous cell carcinoma (HNSCC). Consequently, we examined the effect of DNA methylation on aberrant WASF2 expression in HNSCC.

Methods

TNMplot, TIMER, GSEA pathway analysis, and the Kaplan-Meier Plotter database were used to analyze the expression, function, and prognostic value of WASF2, as well as the correlation between WASF2 and infiltrating immune cells in HNSCC or pan-cancer analysis. WASF2 promoter methylation levels and the correlation between WASF2 expression and WASF2 promoter methylation in HNSCC were evaluated using the DNMIVD database. The effect of DNA methylation inhibitor on WASF2 expression was demonstrated in the GEO database. Finally, the TISIDB database determined the relationships between WASF2 methylation, immune cell infiltration, and immune inhibitors.

Results

WASF2 was significantly downregulated in HNSCC tissues where WASF2 promoter methylation was elevated. According to the GEO database, treatment with a DNA methylation inhibitor notably restored the mRNA expression of WASF2. WASF2 expression was also a favorable indicator of human papilloma virus (HPV)-positive HNSCC. Its level of promoter methylation had detrimental effects on patient survival. In addition, patients with elevated levels of WASF2 demonstrated active G2/M regulation, TGF-β signaling, Kras signaling, fatty acid metabolism, and p53 pathways. WASF2 was positively associated with tumor-killing immune cells, while WASF2 methylation was positively related to immunosuppressive cells and immune-inhibitors.

Conclusions

Hypermethylated WASF2 acted as a tumor suppressor of HNSCC by regulating tumor formation and immune imbalance.

HEM1 Actin Immunodysregulatory Disorder: Genotypes, Phenotypes, and Future Directions

Cells of the innate and adaptive immune systems depend on proper actin dynamics to control cell behavior for effective immune responses. Dysregulated actin networks are known to play a pathogenic role in an increasing number of inborn errors of immunity. The WAVE regulatory complex (WRC) mediates branched actin polymerization, a process required for key cellular functions including migration, phagocytosis, vesicular transport, and immune synapse formation. Recent reports of pathogenic variants in NCKAP1L, a hematopoietically restricted gene encoding the HEM1 protein component of the WRC, defined a novel disease involving recurrent bacterial and viral infections, autoimmunity, and excessive inflammation (OMIM 141180). This review summarizes the diverse clinical presentations and immunological phenotypes observed in HEM1-deficient patients. In addition, we integrate the pathophysiological mechanisms described in current literature and highlight the outstanding questions for diagnosis and management of the HEM1 actin immunodysregulatory disorder.

Whole-Genome Resequencing of Xiangxi Cattle Identifies Genomic Diversity and Selection Signatures

Understanding the genetic diversity in Xiangxi cattle may facilitate our efforts toward further breeding programs. Here we compared 23 Xiangxi cattle with 78 published genomes of 6 worldwide representative breeds to characterize the genomic variations of Xiangxi cattle. Based on clustering models in population structure analysis, we displayed that Xiangxi cattle had a mutual genome ancestor with Chinese indicine, Indian indicine, and East Asian taurine. Population genetic diversity was analyzed by four methods (nucleotide diversity, inbreeding coefficient, linkage disequilibrium decay and runs of homozygosity), and we found that Xiangxi cattle had higher genomic diversity and weaker artificial selection than commercial breed cattle. Using four testing methods (θπ, CLR, F_ST, and XP-EHH), we explored positive selection regions harboring genes in Xiangxi cattle, which were related to reproduction, growth, meat quality, heat tolerance, and immune response. Our findings revealed the extent of sequence variation in Xiangxi cattle at the genome-wide level. All of our fruitful results can bring about a valuable genomic resource for genetic studies and breed protection in the future.

Targeting the WASF3 complex to suppress metastasis

Wiskott-Aldrich syndrome protein family members (WASF) regulate the dynamics of the actin cytoskeleton, which plays an instrumental role in cancer metastasis and invasion. WASF1/2/3 forms a hetero-pentameric complex with CYFIP1/2, NCKAP1/1 L, Abi1/2/3 and BRK1 called the WASF Regulatory Complex (WRC), which cooperatively regulates actin nucleation by WASF1/2/3. Activation of the WRC enables actin networking and provides the mechanical force required for the formation of lamellipodia and invadopodia. Although the WRC drives cell motility essential for several routine physiological functions, its aberrant deployment is observed in cancer metastasis and invasion. WASF3 expression is correlated with metastatic potential in several cancers and inversely correlates with overall progression-free survival. Therefore, disruption of the WRC may serve as a novel strategy for targeting metastasis. Given the complexity involved in the formation of the WRC which is largely comprised of large protein-protein interfaces, there are currently no inhibitors for WASF3. However, several constrained peptide mimics of the various protein-protein interaction interfaces within the WRC were found to successfully disrupt WASF3-mediated migration and invasion. This review explores the role of the WASF3 WRC in driving metastasis and how it may be selectively targeted for suppression of metastasis.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

Rap1 prevents colitogenic Th17 cell expansion and facilitates Treg cell differentiation and distal TCR signaling

T-cell-specific Rap1 deletion causes spontaneous colitis in mice. In the present study, we revealed that Rap1 deficiency in T cells impaired the preceding induction of intestinal RORγt⁺ Treg cells. In the large intestinal lamina propria (LILP) of T-cell-specific Rap1-knockout mice (Rap1KO mice), Th17 cells were found to increase in a microbiota-dependent manner, and the inhibition of IL-17A production prevented the development of colitis. In the LILP of Rap1KO mice, RORγt⁺ Treg cells were scarcely induced by 4 weeks of age. The expression of CTLA-4 on Rap1-deficient Treg cells was reduced and the expression of CD80 and CD86 on dendritic cells was consequently elevated in Rap1KO mice. When cultured under each polarizing condition, Rap1-deficient naïve CD4⁺ T cells did not show biased differentiation into Th17 cells; their differentiation into Treg cells as well as Th1 and Th2 cells was lesser than that of wild-type cells. Rap1-deficient naïve CD4⁺ T cells were found to exhibit the defective nuclear translocation of NFAT and formation of actin foci in response to TCR engagement. These data suggest that Rap1 amplifies the TCR signaling required for Treg-mediated control of intestinal colitogenic Th17 responses.

Rap1 deficiency in T cells impairs development or maintenance of Rorγt⁺ Tregs in the colonic lamina propria, leading to an increase in pathogenic Th17 cells and development of spontaneous colitis.

Dendritic cell actin dynamics control contact duration and priming efficiency at the immunological synapse

Dendritic cells (DCs) are crucial for the priming of naive T cells and the initiation of adaptive immunity. Priming is initiated at a heterologous cell–cell contact, the immunological synapse (IS). While it is established that F-actin dynamics regulates signaling at the T cell side of the contact, little is known about the cytoskeletal contribution on the DC side. Here, we show that the DC actin cytoskeleton is decisive for the formation of a multifocal synaptic structure, which correlates with T cell priming efficiency. DC actin at the IS appears in transient foci that are dynamized by the WAVE regulatory complex (WRC). The absence of the WRC in DCs leads to stabilized contacts with T cells, caused by an increase in ICAM1-integrin–mediated cell–cell adhesion. This results in lower numbers of activated and proliferating T cells, demonstrating an important role for DC actin in the regulation of immune synapse functionality.

Actin Dynamics at the T Cell Synapse as Revealed by Immune-Related Actinopathies

The actin cytoskeleton is composed of dynamic filament networks that build adaptable local architectures to sustain nearly all cellular activities in response to a myriad of stimuli. Although the function of numerous players that tune actin remodeling is known, the coordinated molecular orchestration of the actin cytoskeleton to guide cellular decisions is still ill defined. T lymphocytes provide a prototypical example of how a complex program of actin cytoskeleton remodeling sustains the spatio-temporal control of key cellular activities, namely antigen scanning and sensing, as well as polarized delivery of effector molecules, via the immunological synapse. We here review the unique knowledge on actin dynamics at the T lymphocyte synapse gained through the study of primary immunodeficiences caused by mutations in genes encoding actin regulatory proteins. Beyond the specific roles of individual actin remodelers, we further develop the view that these operate in a coordinated manner and are an integral part of multiple signaling pathways in T lymphocytes.

Loss of Hem1 disrupts macrophage function and impacts migration, phagocytosis, and integrin-mediated adhesion

Hematopoietic-specific protein 1 (Hem1) is an essential subunit of the WAVE regulatory complex (WRC) in immune cells. WRC is crucial for Arp2/3 complex activation and the protrusion of branched actin filament networks. Moreover, Hem1 loss of function in immune cells causes autoimmune diseases in humans. Here, we show that genetic removal of Hem1 in macrophages diminishes frequency and efficacy of phagocytosis as well as phagocytic cup formation in addition to defects in lamellipodial protrusion and migration. Moreover, Hem1-null macrophages displayed strong defects in cell adhesion despite unaltered podosome formation and concomitant extracellular matrix degradation. Specifically, dynamics of both adhesion and de-adhesion as well as concomitant phosphorylation of paxillin and focal adhesion kinase (FAK) were significantly compromised. Accordingly, disruption of WRC function in non-hematopoietic cells coincided with both defects in adhesion turnover and altered FAK and paxillin phosphorylation. Consistently, platelets exhibited reduced adhesion and diminished integrin αIIbβ3 activation upon WRC removal. Interestingly, adhesion phenotypes, but not lamellipodia formation, were partially rescued by small molecule activation of FAK. A full rescue of the phenotype, including lamellipodia formation, required not only the presence of WRCs but also their binding to and activation by Rac. Collectively, our results uncover that WRC impacts on integrin-dependent processes in a FAK-dependent manner, controlling formation and dismantling of adhesions, relevant for properly grabbing onto extracellular surfaces and particles during cell edge expansion, like in migration or phagocytosis.

Teasing out function from morphology: Similarities between primary cilia and immune synapses

Immune synapses are formed between immune cells to facilitate communication and coordinate the immune response. The reorganization of receptors involved in recognition and signaling creates a transient area of plasma membrane specialized in signaling and polarized secretion. Studies on the formation of the immune synapse between cytotoxic T lymphocytes (CTLs) and their targets uncovered a critical role for centrosome polarization in CTL function and suggested a striking parallel between the synapse and primary cilium. Since these initial observations, a plethora of further morphological, functional, and molecular similarities have been identified between these two fascinating structures. In this review, we describe how advances in imaging and molecular techniques have revealed additional parallels as well as functionally significant differences and discuss how comparative studies continue to shed light on the molecular mechanisms underlying the functions of both the immune synapse and primary cilium.

Biomechanics of T Cell Dysfunctions in Chronic Diseases

Understanding the mechanisms behind T cell dysfunctions during chronic diseases is critical in developing effective immunotherapies. As demonstrated by several animal models and human studies, T cell dysfunctions are induced during chronic diseases, spanning from infections to cancer. Although factors governing the onset and the extent of the functional impairment of T cells can differ during infections and cancer, most dysfunctional phenotypes share common phenotypic traits in their immune receptor and biophysical landscape. Through the latest developments in biophysical techniques applied to explore cell membrane and receptor-ligand dynamics, we are able to dissect and gain further insights into the driving mechanisms behind T cell dysfunctions. These insights may prove useful in developing immunotherapies aimed at reinvigorating our immune system to fight off infections and malignancies more effectively. The recent success with checkpoint inhibitors in treating cancer opens new avenues to develop more effective, targeted immunotherapies. Here, we highlight the studies focused on the transformation of the biophysical landscape during infections and cancer, and how T cell biomechanics shaped the immunopathology associated with chronic diseases.

The Role of the Cytoskeleton in Regulating the Natural Killer Cell Immune Response in Health and Disease: From Signaling Dynamics to Function

Natural killer (NK) cells are innate lymphoid cells, which play key roles in elimination of virally infected and malignant cells. The balance between activating and inhibitory signals derived from NK surface receptors govern the NK cell immune response. The cytoskeleton facilitates most NK cell effector functions, such as motility, infiltration, conjugation with target cells, immunological synapse assembly, and cytotoxicity. Though many studies have characterized signaling pathways that promote actin reorganization in immune cells, it is not completely clear how particular cytoskeletal architectures at the immunological synapse promote effector functions, and how cytoskeletal dynamics impact downstream signaling pathways and activation. Moreover, pioneering studies employing advanced imaging techniques have only begun to uncover the architectural complexity dictating the NK cell activation threshold; it is becoming clear that a distinct organization of the cytoskeleton and signaling receptors at the NK immunological synapse plays a decisive role in activation and tolerance. Here, we review the roles of the actin cytoskeleton in NK cells. We focus on how actin dynamics impact cytolytic granule secretion, NK cell motility, and NK cell infiltration through tissues into inflammatory sites. We will also describe the additional cytoskeletal components, non-muscle Myosin II and microtubules that play pivotal roles in NK cell activity. Furthermore, special emphasis will be placed on the role of the cytoskeleton in assembly of immunological synapses, and how mutations or downregulation of cytoskeletal accessory proteins impact NK cell function in health and disease.

Soluble CD83 Regulates Dendritic Cell-T Cell Immunological Synapse Formation by Disrupting Rab1a-Mediated F-Actin Rearrangement

Dendritic cell-T cell (DC-T) contacts play an important role in T cell activation, clone generation, and development. Regulating the cytoskeletal protein rearrangement of DCs can modulate DC-T contact and affect T cell activation. However, inhibitory factors on cytoskeletal regulation in DCs remain poorly known. We showed that a soluble form of CD83 (sCD83) inhibited T cell activation by decreasing DC-T contact and synapse formation between DC and T cells. This negative effect of sCD83 on DCs was mediated by disruption of F-actin rearrangements, leading to alter expression and localization of major histocompatibility complex class II (MHC-II) and immunological synapse formation between DC and T cells. Furthermore, sCD83 was found to decrease GTP-binding activity of Rab1a, which further decreased colocalization and expression of LRRK2 and F-actin rearrangements in DCs, leading to the loss of MHC-II at DC-T synapses and reduced DC-T synapse formation. Further, sCD83-treated DCs alleviated symptoms of experimental autoimmune uveitis in mice and decreased the number of T cells in the eyes and lymph nodes of these animals. Our findings demonstrate a novel signaling pathway of sCD83 on regulating DC-T contact, which may be harnessed to develop new immunosuppressive therapeutics for autoimmune disease.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

PD-1 suppresses the maintenance of cell couples between cytotoxic T cells and target tumor cells within the tumor

The killing of tumor cells by CD8⁺ T cells is suppressed by the tumor microenvironment, and increased expression of inhibitory receptors, including programmed cell death protein-1 (PD-1), is associated with tumor-mediated suppression of T cells. To find cellular defects triggered by tumor exposure and associated PD-1 signaling, we established an ex vivo imaging approach to investigate the response of antigen-specific, activated effector CD8⁺ tumor-infiltrating lymphocytes (TILs) after interaction with target tumor cells. Although TIL-tumor cell couples readily formed, couple stability deteriorated within minutes. This was associated with impaired F-actin clearing from the center of the cellular interface, reduced Ca²⁺ signaling, increased TIL locomotion, and impaired tumor cell killing. The interaction of CD8⁺ T lymphocytes with tumor cell spheroids in vitro induced a similar phenotype, supporting a critical role of direct T cell-tumor cell contact. Diminished engagement of PD-1 within the tumor, but not acute ex vivo blockade, partially restored cell couple maintenance and killing. PD-1 thus contributes to the suppression of TIL function by inducing a state of impaired subcellular organization.

HEM1 deficiency disrupts mTORC2 and F-actin control in inherited immunodysregulatory disease

Immunodeficiency often coincides with hyperactive immune disorders such as autoimmunity, lymphoproliferation, or atopy, but this coincidence is rarely understood on a molecular level. We describe five patients from four families with immunodeficiency coupled with atopy, lymphoproliferation, and cytokine overproduction harboring mutations in NCKAP1L, which encodes the hematopoietic-specific HEM1 protein. These mutations cause the loss of the HEM1 protein and the WAVE regulatory complex (WRC) or disrupt binding to the WRC regulator, Arf1, thereby impairing actin polymerization, synapse formation, and immune cell migration. Diminished cortical actin networks caused by WRC loss led to uncontrolled cytokine release and immune hyperresponsiveness. HEM1 loss also blocked mechanistic target of rapamycin complex 2 (mTORC2)-dependent AKT phosphorylation, T cell proliferation, and selected effector functions, leading to immunodeficiency. Thus, the evolutionarily conserved HEM1 protein simultaneously regulates filamentous actin (F-actin) and mTORC2 signaling to achieve equipoise in immune responses.

The DISC1-Girdin complex - a missing link in signaling to the T cell cytoskeleton

In this study, using Jurkat cells, we show that DISC1 (disrupted in schizophrenia 1) and Girdin (girders of actin filament) are essential for typical actin accumulation at the immunological synapse. Furthermore, DISC1, Girdin and dynein are bound in a complex. Although this complex initially forms as a central patch at the synapse, it relocates to a peripheral ring corresponding to the peripheral supramolecular activation cluster (pSMAC). In the absence of DISC1, the classic actin ring does not form, cell spreading is blocked, and the dynein complex fails to relocate to the pSMAC. A similar effect is seen when Girdin is deleted. When cells are treated with inhibitors of actin polymerization, the dynein-NDE1 complex is lost from the synapse and the microtubule-organizing center fails to translocate, suggesting that actin and dynein might be linked. Upon stimulation of T cell receptors, DISC1 becomes associated with talin, which likely explains why the dynein complex colocalizes with the pSMAC. These results show that the DISC1-Girdin complex regulates actin accumulation, cell spreading and distribution of the dynein complex at the synapse.This article has an associated First Person interview with the first author of the paper.

Interfacial actin protrusions mechanically enhance killing by cytotoxic T cells

Cytotoxic T lymphocytes (CTLs) kill by forming immunological synapses with target cells and secreting toxic proteases and the pore-forming protein perforin into the intercellular space. Immunological synapses are highly dynamic structures that boost perforin activity by applying mechanical force against the target cell. Here, we used high-resolution imaging and microfabrication to investigate how CTLs exert synaptic forces and coordinate their mechanical output with perforin secretion. Using micropatterned stimulatory substrates that enable synapse growth in three dimensions, we found that perforin release occurs at the base of actin-rich protrusions that extend from central and intermediate locations within the synapse. These protrusions, which depended on the cytoskeletal regulator WASP and the Arp2/3 actin nucleation complex, were required for synaptic force exertion and efficient killing. They also mediated physical deformation of the target cell surface during CTL-target cell interactions. Our results reveal the mechanical basis of cellular cytotoxicity and highlight the functional importance of dynamic, three-dimensional architecture in immune cell-cell interfaces.

Cofilin1-driven actin dynamics controls migration of thymocytes and is essential for positive selection in the thymus

Actin dynamics is essential for T-cell development. We show here that cofilin1 is the key molecule for controlling actin filament turnover in this process. Mice with specific depletion of cofilin1 in thymocytes showed increased steady-state levels of actin filaments, and associated alterations in the pattern of thymocyte migration and adhesion. Our data suggest that cofilin1 is controlling oscillatory F-actin changes, a parameter that influences the migration pattern in a 3-D environment. In a collagen matrix, cofilin1 controls the speed and resting intervals of migrating thymocytes. Cofilin1 was not involved in thymocyte proliferation, cell survival, apoptosis or surface receptor trafficking. However, in cofilin1 mutant mice, impaired adhesion and migration resulted in a specific block of thymocyte differentiation from CD4/CD8 double-positive thymocytes towards CD4 and CD8 single-positive cells. Our data suggest that tuning of the dwelling time of thymocytes in the thymic niches is tightly controlled by cofilin1 and essential for positive selection during T-cell differentiation. We describe a novel role of cofilin1 in the physiological context of migration-dependent cell differentiation.

Roles of the actin cytoskeleton in aging and age-associated diseases

The integrity of the cytoskeleton is essential to diverse cellular processes such as phagocytosis and intracellular trafficking. Disruption of the organization and dynamics of the actin cytoskeleton leads to age-associated symptoms and diseases, ranging from cancer to neurodegeneration. In addition, changes in the integrity of the actin cytoskeleton disrupt the functioning of not only somatic and stem cells but also gametes, resulting in aberrant embryonic development. Strategies to preserve the integrity and dynamics of the cytoskeleton are, therefore, potentially therapeutic to age-related disorders. The objective of this article is to revisit the current understanding of the roles played by the actin cytoskeleton in aging, and to review the opportunities and challenges for the transition of basic research into intervention development. It is hoped that, with the snapshot of evidence regarding changes in actin dynamics with advanced age, insights into future research directions can be attained.

Leucine-rich repeat kinase-2 (LRRK2) modulates microglial phenotype and dopaminergic neurodegeneration

Leucine-rich repeat kinase 2 (LRRK2) is a common gene implicated in Parkinson's disease and many inflammatory processes. Thus, we assessed the role of LRRK2 in the context of endotoxin (lipopolysaccharide, LPS)-induced inflammation of the substantia nigra together with the environmental toxicant, paraquat, that has been implicated in PD. Here we found that LRRK2 ablation prevented the loss of dopaminergic neurons and behavioral deficits (motor) induced by LPS priming followed by paraquat exposure. The LRRK2 ablation also provoked a phenotypic shift in LPS-primed microglia cells. The LRRK2 deficiency reduced their "activated" morphology and upregulation of the inflammatory phagocytic regulator, WAVE2 (critical for actin remodeling), while the chemokine receptor, CX3CR1, was elevated in isolated CD11b+ myeloid cells. Furthermore, LRRK2 knockout attenuated the signs of oxidative stress and morphological changes induced in primary microglia by LPS treatment. However, induced WAVE2 expression together with LPS exposure in microglia overcame the inhibitory effects of LRRK2 knockout, suggesting WAVE2 may be acting downstream of LRRK2. Neither WAVE2 nor did LRRK2 knockout influence LPS-induced cytokine elevations in the microglia. We are the first to show the importance of LRRK2 in neurodegenerative and inflammatory processes in this multi-hit toxin model of PD. These data are consistent with the proposition that LRRK2 and WAVE2 are useful therapeutic targets for PD or other conditions with a prominent neuroinflammatory component.

Multiple actin networks coordinate mechanotransduction at the immunological synapse

Activation of naive T cells by antigen-presenting cells (APCs) is an essential step in mounting an adaptive immune response. It is known that antigen recognition and T cell receptor (TCR) signaling depend on forces applied by the T cell actin cytoskeleton, but until recently, the underlying mechanisms have been poorly defined. Here, we review recent advances in the field, which show that specific actin-dependent structures contribute to the process in distinct ways. In essence, T cell priming involves a tug-of-war between the cytoskeletons of the T cell and the APC, where the actin cytoskeleton serves as a mechanical intermediate that integrates force-dependent signals. We consider each of the relevant actin-rich T cell structures separately and address how they work together at the topologically and temporally complex cell-cell interface. In addition, we address how this mechanobiology can be incorporated into canonical immunological models to improve how these models explain T cell sensitivity and antigenic specificity.

Partial loss of actin nucleator actin-related protein 2/3 activity triggers blebbing in primary T lymphocytes

T lymphocytes utilize amoeboid migration to navigate effectively within complex microenvironments. The precise rearrangement of the actin cytoskeleton required for cellular forward propulsion is mediated by actin regulators, including the actin‐related protein 2/3 (Arp2/3) complex, a macromolecular machine that nucleates branched actin filaments at the leading edge. The consequences of modulating Arp2/3 activity on the biophysical properties of the actomyosin cortex and downstream T cell function are incompletely understood. We report that even a moderate decrease of Arp3 levels in T cells profoundly affects actin cortex integrity. Reduction in total F‐actin content leads to reduced cortical tension and disrupted lamellipodia formation. Instead, in Arp3‐knockdown cells, the motility mode is dominated by blebbing migration characterized by transient, balloon‐like protrusions at the leading edge. Although this migration mode seems to be compatible with interstitial migration in three‐dimensional environments, diminished locomotion kinetics and impaired cytotoxicity interfere with optimal T cell function. These findings define the importance of finely tuned, Arp2/3‐dependent mechanophysical membrane integrity in cytotoxic effector T lymphocyte activities.

The role of actin and myosin in antigen extraction by B lymphocytes

B cells must extract antigens attached to the surface of antigen presenting cells to generate high-affinity antibodies. Antigen extraction requires force, and recent studies have implicated actomyosin-dependent pulling forces generated within the B cell as the major driver of antigen extraction. These actomyosin-dependent pulling forces also serve to test the affinity of the B cell antigen receptor for antigen prior to antigen extraction. Such affinity discrimination is central to the process of antibody affinity maturation. Here we review the evidence that actomyosin-dependent pulling forces generated within the B cell promote affinity discrimination and power antigen extraction. Our take on these critical B cell functions is influenced significantly by the recent identification of formin-generated, myosin-rich, concentric actin arcs in the medial portion of the T cell immune synapse, as B cells appear to contain a similar contractile actomyosin structure.

A Ciliary View of the Immunological Synapse

The primary cilium has gone from being a vestigial organelle to a crucial signaling hub of growing interest given the association between a group of human disorders, collectively known as ciliopathies, and defects in its structure or function. In recent years many ciliogenesis proteins have been observed at extraciliary sites in cells and likely perform cilium-independent functions ranging from regulation of the cytoskeleton to vesicular trafficking. Perhaps the most striking example is the non-ciliated T lymphocyte, in which components of the ciliary machinery are repurposed for the assembly and function of the immunological synapse even in the absence of a primary cilium. Furthermore, the specialization traits described at the immunological synapse are similar to those seen in the primary cilium. Here, we review common regulators and features shared by the immunological synapse and the primary cilium that document the remarkable homology between these structures.

Leucine-rich repeat kinase-2 (LRRK2) modulates paraquat-induced inflammatory sickness and stress phenotype

Background

Leucine-rich repeat kinase 2 (LRRK2) is a common gene implicated in Parkinson’s disease (PD) and is also thought to be fundamentally involved in numerous immune functions. Thus, we assessed the role of LRRK2 in the context of the effects of the environmental toxicant, paraquat, that has been implicated in PD and is known to affect inflammatory processes.

Methods

Male LRRK2 knockout (KO) and transgenic mice bearing the G2019S LRRK2 mutation (aged 6–8 months) or their littermate controls were exposed to paraquat (two times per week for 3 weeks), and sickness measures, motivational scores, and total home-cage activity levels were assessed. Following sacrifice, western blot and ELISA assays were performed to see whether or not LRRK2 expression would alter processes related to plasticity, immune response processes, or the stress response.

Results

Paraquat-induced signs of sickness, inflammation (elevated IL-6), and peripheral toxicity (e.g., organ weight) were completely prevented by LRRK2 knockout. In fact, LRRK2 knockout dramatically reduced not only signs of illness, but also the motivational (nest building) and home-cage activity deficits induced by paraquat. Although LRRK2 deficiency did not affect the striatal BDNF reduction that was provoked by paraquat, it did blunt the corticosterone elevation induced by paraquat, raising the possibility that LRRK2 may modulate aspects of the HPA stress axis. Accordingly, we found that transgenic mice bearing the G2019S LRRK2 mutation had elevated basal corticosterone, along with diminished hippocampal 5-HT1A levels.

Conclusion

We are the first to show the importance of LRRK2 in the peripheral neurotoxic and stressor-like effects of paraquat. These data are consistent with LRRK2 playing a role in the general inflammatory tone and stressor effects induced by environmental toxicant exposure.

Actin Cytoskeleton Straddling the Immunological Synapse between Cytotoxic Lymphocytes and Cancer Cells

The immune system is a fundamental part of the tumor microenvironment. In particular, cytotoxic lymphocytes, such as cytolytic T cells and natural killer cells, control tumor growth and disease progression by interacting and eliminating tumor cells. The actin cytoskeleton of cytotoxic lymphocytes engaged in an immunological synapse has received considerable research attention. It has been recognized as a central mediator of the formation and maturation of the immunological synapse, and its signaling and cytolytic activities. In comparison, fewer studies have explored the organization and function of actin filaments on the target cancer cell side of the immunological synapse. However, there is growing evidence that the actin cytoskeleton of cancer cells also undergoes extensive remodeling upon cytotoxic lymphocyte attack, and that such remodeling can alter physical and functional interactions at the immunological synapse. In this article, we review the current knowledge of actin organization and functions at both sides of the immunological synapse between cytotoxic lymphocytes and cancer cells, with particular focus on synapse formation, signaling and cytolytic activity, and immune evasion.

Origin, Organization, Dynamics, and Function of Actin and Actomyosin Networks at the T Cell Immunological Synapse

The engagement of a T cell with an antigen-presenting cell (APC) or activating surface results in the formation within the T cell of several distinct actin and actomyosin networks. These networks reside largely within a narrow zone immediately under the T cell's plasma membrane at its site of contact with the APC or activating surface, i.e., at the immunological synapse. Here we review the origin, organization, dynamics, and function of these synapse-associated actin and actomyosin networks. Importantly, recent insights into the nature of these actin-based cytoskeletal structures were made possible in several cases by advances in light microscopy.

The role of WASp in T cells and B cells

Wiskott-Aldrich syndrome (WAS) is a form of primary immunodeficiency (PIDs) resulting from mutations of the gene that encodes Wiskott-Aldrich syndrome protein (WASp). WASp is the first identified and most widely studied protein belonging to the actin nucleation-promoting factor family and plays significant role in integrating and transforming signals from critical receptors on the cell surface to actin remodeling. WASp functions in immune defense and homeostasis through the regulation of actin cytoskeleton-dependent cellular processes as well as processes uncoupled with actin polymerization like nuclear transcription programs. In this article, we review the mechanisms of WASp activation through an understanding of its structure. We further discuss the role of WASp in adaptive immunity, paying special attention to some recent findings on the crucial role of WASp in the formation of immunological synapse, the regulation of T follicular helper (Tfh) cells and in the prevention of autoimmunity.

Guided by curvature: shaping cells by coupling curved membrane proteins and cytoskeletal forces

Eukaryote cells have flexible membranes that allow them to have a variety of dynamical shapes. The shapes of the cells serve important biological functions, both for cells within an intact tissue, and during embryogenesis and cellular motility. How cells control their shapes and the structures that they form on their surface has been a subject of intensive biological research, exposing the building blocks that cells use to deform their membranes. These processes have also drawn the interest of theoretical physicists, aiming to develop models based on physics, chemistry and nonlinear dynamics. Such models explore quantitatively different possible mechanisms that the cells can employ to initiate the spontaneous formation of shapes and patterns on their membranes. We review here theoretical work where one such class of mechanisms was investigated: the coupling between curved membrane proteins, and the cytoskeletal forces that they recruit. Theory indicates that this coupling gives rise to a rich variety of membrane shapes and dynamics, while experiments indicate that this mechanism appears to drive many cellular shape changes.This article is part of the theme issue 'Self-organization in cell biology'.

TCR microclusters form spatially segregated domains and sequentially assemble in calcium-dependent kinetic steps

Engagement of the T cell receptor (TCR) by stimulatory ligand results in the rapid formation of microclusters at sites of T cell activation. Whereas microclusters have been studied extensively using confocal microscopy, the spatial and kinetic relationships of their signaling components have not been well characterized due to limits in image resolution and acquisition speed. Here we show, using TIRF-SIM to examine the organization of microclusters at sub-diffraction resolution, the presence of two spatially distinct domains composed of ZAP70-bound TCR and LAT-associated signaling complex. Kinetic analysis of microcluster assembly reveal surprising delays between the stepwise recruitment of ZAP70 and signaling proteins to the TCR, as well as distinct patterns in their disassociation. These delays are regulated by intracellular calcium flux downstream of T cell activation. Our results reveal novel insights into the spatial and kinetic regulation of TCR microcluster formation and T cell activation.

The Actin Cytoskeleton: A Mechanical Intermediate for Signal Integration at the Immunological Synapse

The immunological synapse (IS) is a specialized structure that serves as a platform for cell-cell communication between a T cell and an antigen-presenting cell (APC). Engagement of the T cell receptor (TCR) with cognate peptide-MHC complexes on the APC activates the T cell and instructs its differentiation. Proper T cell activation also requires engagement of additional receptor-ligand pairs, which promote sustained adhesion and deliver costimulatory signals. These events are orchestrated by T cell actin dynamics, which organize IS components and facilitate their signaling. The actin network flows from the edge of the cell inward, driving the centralization of TCR microclusters and providing the force to activate the integrin LFA-1. We recently showed that engagement of LFA-1 slows actin flow, and that this affects TCR signaling. This study highlights the physical nature of the IS, and contributes to a growing appreciation in the field that mechanosensing and mechanotransduction are essential for IS function. Additionally, it is becoming clear that there are multiple types of actin structures at the IS that promote signaling in distinct ways. How the different actin structures contribute to force production and mechanotransduction is just beginning to be explored. In this Perspective, we will feature recent work from our lab and others, that collectively points toward a model in which actin dynamics drive mechanical signaling and receptor crosstalk during T cell activation.

Genetik der Alzheimer-Krankheit

Die Alzheimer-Erkrankung („Alzheimer’s disease“, AD) ist die häufigste Ursache der neurodegenerativen Demenzen. Im Gegensatz zu monogenen und meist frühmanifesten Formen der AD, welche auf hochpenetrante Mutationen in den Genen APP, PSEN1 und PSEN2 zurückzuführen sind, wird die Suszeptibilität für die sporadische, oft spätmanifeste Form der AD durch eine komplexe Wechselwirkung zwischen genetischen und epigenetischen Faktoren wie auch umwelt- und lebensstilbedingten Faktoren bestimmt. Obgleich APOE ε4 der stärkste genetische Risikofaktor für die AD ist, macht der Effekt des APOE ε4 lediglich 27,3 % der geschätzten Heritabilität von 58–79 % aus. Durch den kontinuierlichen technischen Fortschritt von GWAS (genomweite Assoziationsstudien) und automatisierten Sequenziermethoden der nächsten Generation gelingt es Wissenschaftlern in groß angelegten Kollaborationen sukzessive die fehlende Heritabilität aufzudecken. Wichtige Erkenntnisse aus GWAS und Signalweganalysen suggerieren, dass Mikroglia, die residenten Immunzellen des ZNS, eine entscheidende Rolle bei der Pathogenese der AD spielen. Eine beachtliche Anzahl der in genetischen Studien identifizierten Risikogene weisen immunsystembezogene Funktionen auf und werden in höchstem Maße von Mikroglia exprimiert. Durch die Beschreibung von Risikovarianten in CR1, CLU, SPI1, CD33, MS4A, ABCA7, EPHA1, HLA-DRB5/1, INPP5D, TYROBP, TREM2, PLCG2 und ABI3 nimmt die Mikroglia vermittelte Immunantwort bei der Pathogenese der AD eine zentrale Rolle ein. Von besonderer Bedeutung könnte sein, dass die PLCγ2-Variante p.P522R einen protektiven Effekt auf die LOAD („late-onset“ AD; spätmanifeste Form der AD) ausübt und als Enzym ein klassisches Ziel für eine therapeutische Modulation von komplexen Formen der AD darstellt.

Glycerol monolaurate induces filopodia formation by disrupting the association between LAT and SLP-76 microclusters

Glycerol monolaurate (GML) is a monoglyceride with potent antimicrobial properties that suppresses T cell receptor (TCR)-induced signaling and T cell effector function. Actin rearrangement is needed for the interaction of T cells with antigen-presenting cells and for migration to sites of infection. Because of the critical role actin rearrangement plays in T cell effector function, we analyzed the effect of GML on the rearrangement of the actin cytoskeleton after TCR activation. We found that GML-treated human T cells were less adherent than untreated T cells and did not form actin ring structures but instead developed numerous inappropriate actin-mediated filopodia. The formation of these filopodia was not due to disruption of TCR-proximal regulators of actin or microtubule polymerization. Instead, total internal reflection fluorescence microscopy demonstrated mislocalization of actin nucleation protein Arp2 microclusters, but not those containing the adaptor proteins SLP-76 and WASp, or the actin nucleation protein ARPC3, which are necessary for TCR-induced actin rearrangement. Additionally, SLP-76 microclusters colocalized with WASp and WAVE microclusters but not with LAT. Together, our data suggest that GML alters actin cytoskeletal rearrangements and identify diverse functions for GML as a T cell-suppressive agent.

miR-181c-BRK1 axis plays a key role in actin cytoskeleton-dependent T cell function

MicroRNAs are short endogenous noncoding RNAs that play pivotal roles in a diverse range of cellular processes. The miR-181 family is important in T cell development, proliferation, and activation. In this study, we have identified BRK1 as a potential target of miR-181c using a dual selection functional assay and have showed that miR-181c regulates BRK1 by translational inhibition. Given the importance of miR-181 in T cell function and the potential role of BRK1 in the involvement of WAVE2 complex and actin polymerization in T cells, we therefore investigated the influence of miR-181c-BRK1 axis in T cell function. Stimulation of PBMC derived CD3⁺ T cells resulted in reduced miR-181c expression and up-regulation of BRK1 protein expression, suggesting that miR-181c-BRK1 axis is important in T cell activation. We further showed that overexpression of miR-181c or suppression of BRK1 resulted in inhibition of T cell activation and actin polymerization coupled with defective lamellipodia generation and immunological synapse formation. Additionally, we found that BRK1 silencing led to reduced expressions of other proteins in the WAVE2 complex, suggesting that the impairment of T cell actin dynamics was a result of the instability of the WAVE2 complex following BRK1 depletion. Collectively, we demonstrated that miR-181c reduces BRK1 protein expression level and highlighted the important role of miR-181c-BRK1 axis in T cell activation and actin polymerization-mediated T cell functions.

Genome-wide CRISPR screen identifies FAM49B as a key regulator of actin dynamics and T cell activation

Despite decades of research, mechanisms controlling T cell activation remain only partially understood, which hampers T cell-based immune cancer therapies. Here, we performed a genome-wide CRISPR screen to search for genes that regulate T cell activation. Our screen confirmed many of the known regulators in proximal T cell receptor signaling and, importantly, also uncovered a previously uncharacterized regulator, FAM49B (family with sequence similarity 49 member B). FAM49B deficiency led to hyperactivation of Jurkat T cells following T cell receptor stimulation, as indicated by enhancement of CD69 induction, PAK phosphorylation, and actin assembly. FAM49B directly interacted with the active form of the small GTPase Rac, and genetic disruption of the FAM49B-Rac interaction compromised FAM49B function. Thus, FAM49B inhibits T cell activation by repressing Rac activity and modulating cytoskeleton reorganization.

Nanoscale Dynamism of Actin Enables Secretory Function in Cytolytic Cells

Natural killer (NK) cells are innate immune effectors that lyse virally infected and tumorigenic cells through the formation of an immunological synapse. Actin remodeling at the lytic immunological synapse is a critical requirement for multiple facets of cytotoxic function. Activating receptor and integrin signaling leads to the regulated turnover and remodeling of actin, which is required for adhesion, sustained receptor signaling, and ultimately exocytosis. NK cells undergo lytic granule exocytosis in hypodense regions of a pervasive actin network. Although these requirements have been well demonstrated, neither the dynamic regulation of synaptic actin nor its specific function, however, has been determined at a nanoscale level. Here, live-cell super-resolution microscopy demonstrates nanoscale filamentous actin dynamism in NK cell lytic granule secretion. Following cell spreading, the overall content of the branched actin network at an immune synapse is stable over time and contains branched actin fibers and discrete actin foci. Similar actin architecture is generated in cytolytic T cells, although the timescale differs from that of NK cells. Individual filament displacement leads to stochastic clearance formation and disappearance, which are independent of lytic granule positioning. Actin dynamism is dependent upon branched network formation mediated by Arp2/3 and contractility generated by myosin IIA. Importantly, the use of small-molecule inhibitors demonstrates that actin dynamism is ultimately needed for granule secretion. Thus, we describe a requirement for nanoscale actin fiber rearrangement in generating the complex actin architecture that enables lytic granule secretion.

Integrins Modulate T Cell Receptor Signaling by Constraining Actin Flow at the Immunological Synapse

Full T cell activation requires coordination of signals from multiple receptor–ligand pairs that interact in parallel at a specialized cell–cell contact site termed the immunological synapse (IS). Signaling at the IS is intimately associated with actin dynamics; T cell receptor (TCR) engagement induces centripetal flow of the T cell actin network, which in turn enhances the function of ligand-bound integrins by promoting conformational change. Here, we have investigated the effects of integrin engagement on actin flow, and on associated signaling events downstream of the TCR. We show that integrin engagement significantly decelerates centripetal flow of the actin network. In primary CD4⁺ T cells, engagement of either LFA-1 or VLA-4 by their respective ligands ICAM-1 and VCAM-1 slows actin flow. Slowing is greatest when T cells interact with low mobility integrin ligands, supporting a predominately drag-based mechanism. Using integrin ligands presented on patterned surfaces, we demonstrate that the effects of localized integrin engagement are distributed across the actin network, and that focal adhesion proteins, such as talin, vinculin, and paxillin, are recruited to sites of integrin engagement. Further analysis shows that talin and vinculin are interdependent upon one another for recruitment, and that ongoing actin flow is required. Suppression of vinculin or talin partially relieves integrin-dependent slowing of actin flow, indicating that these proteins serve as molecular clutches that couple engaged integrins to the dynamic actin network. Finally, we found that integrin-dependent slowing of actin flow is associated with reduction in tyrosine phosphorylation downstream of the TCR, and that this modulation of TCR signaling depends on expression of talin and vinculin. More generally, we found that integrin-dependent effects on actin retrograde flow were strongly correlated with effects on TCR signaling. Taken together, these studies support a model in which ligand-bound integrins engage the actin cytoskeletal network via talin and vinculin, and tune TCR signaling events by modulating actin dynamics at the IS.

Phosphoproteomics Reveals Regulatory T Cell-Mediated DEF6 Dephosphorylation That Affects Cytokine Expression in Human Conventional T Cells

Regulatory T cells (Tregs) control key events of immune tolerance, primarily by suppression of effector T cells. We previously revealed that Tregs rapidly suppress T cell receptor (TCR)-induced calcium store depletion in conventional CD4⁺CD25⁻ T cells (Tcons) independently of IP₃ levels, consequently inhibiting NFAT signaling and effector cytokine expression. Here, we study Treg suppression mechanisms through unbiased phosphoproteomics of primary human Tcons upon TCR stimulation and Treg-mediated suppression, respectively. Tregs induced a state of overall decreased phosphorylation as opposed to TCR stimulation. We discovered novel phosphosites (T595_S597) in the DEF6 (SLAT) protein that were phosphorylated upon TCR stimulation and conversely dephosphorylated upon coculture with Tregs. Mutation of these DEF6 phosphosites abrogated interaction of DEF6 with the IP₃ receptor and affected NFAT activation and cytokine transcription in primary Tcons. This novel mechanism and phosphoproteomics data resource may aid in modifying sensitivity of Tcons to Treg-mediated suppression in autoimmune disease or cancer.