The Rho GTPase Cdc42 Is Essential for the Activation and Function of Mature B Cells

Comprehensive analysis of cell-extracellular matrix protein Ras suppressor-1 in function and prognosis of gastrointestinal cancers

BACKGROUND

Ras suppressor 1 (RSU1), a highly conserved protein, plays an important role in actin cytoskeleton remodeling and cell-extracellular matrix adhesion. Aberration of RSU1 activity can cause changes in cell adhesion and migration, thereby enhancing tumor proliferation and metastasis. However, the correlation between RSU1 and gastrointestinal cancers (GICs), as well as its prognostic role related to tumor-infiltrating immune cells (TIICs) remains unclear.

AIM

To shows RSU1 plays a potential promoting role in facilitating tumor immune escape in GIC.

METHODS

Differential expression of RSU1 in different tumors and their corresponding normal tissues was evaluated by exploring the Gene Expression Profiling Interactive Analysis (GEPIA) dataset. The correlation between RSU1 expression and prognosis of GIC cancer patients was evaluated by Kaplan-Meier plotter. Then, RSU1-correlated genes were screened and functionally characterized via enrichment analysis. The correlation between RSU1 and TIICs was further characterized using the Tumor Immune Estimation Resource (TIMER). In addition, the correlation between RSU1 and immune cell surface molecules was also analyzed by TIMER.

RESULTS

High RSU1 expression was associated with poor overall survival of gastric cancer patients, exhibiting a hazard ratio (HR) = 1.36, first progression HR = 1.53, and post progression survival HR = 1.6. Specifically, high RSU1 Levels were associated with prognosis of gastric cancer in females, T4 and N3 stages, and Her-2-negative subtypes. Regarding immune-infiltrating cells, RSU1 expression level was positively correlated with infiltration of CD4+ T cells, macrophages, neutrophils, and dendritic cells (DCs) in colorectal adenocarcinoma and stomach adenocarcinoma. RSU1 expression was also predicted to be strongly correlated with immune marker sets in M2 macrophage, DCs and T cell exhaustion in GICs.

CONCLUSION

In gastrointestinal cancers, RSU1 is increased in tumor tissues, and predicts poor survival of patients. Increased RSU1 may be involved in promoting macrophage polarization, DC infiltration, and T cell exhaustion, inducing tumor immune escape and the development of tumors in GICs. We suggest that RSU1 is a promising prognostic biomarker reflecting immune infiltration level of GICs, as well as a potential therapeutic target for precision treatment through improving the immune response.

The clinical phenotype with gastrostomy and abdominal wall infection in a pediatric patient with Takenouchi-Kosaki syndrome due to a heterozygous c.191A > G (p.Tyr64Cys) variant in CDC42: a case report

The CDC42 (cell division cycle homolog 42) gene product, Cdc42 belongs to the Rho GTPase family which plays a pivotal role in the regulation of multiple cellular functions, including cell cycle progression, motility, migration, proliferation, transcription activation, and reactive oxygen species production. The Cdc42 molecule controls various tissue-specific functional pathways underpinning organogenesis as well as developmental integration of the hematopoietic and immune systems. Heterozygous c.191A>G (p.Tyr64Cys) pathogenic variants in CDC42 cause Takenouchi-Kosaki syndrome characterized by a spectrum of phenotypic features comprising psychomotor developmental delay, sensorineural hearing loss, growth retardation, facial dysmorphism, cardiovascular and urinary tract malformations, camptodactyly, accompanied by thrombocytopenia and immunodeficiency of variable degree. Herein, we report a pediatric patient with the Takenouchi-Kosaki syndrome due to a heterozygous p.Tyr64Cys variant in CDC42 manifesting as a congenital malformation complex accompanied by macrothrombocytopenia, poor specific antibody response, B and T cell immunodeficiency, and low serum immunoglobulin A level. We also suggst that feeding disorders, malnutrition, and a gastrointestinal infection could be a part of the phenotypic characteristics of Takenouchi-Kosaki syndrome supporting the hypothesis of immune dysregulation and systemic inflammation occurring in the p.Tyr64Cys variant in CDC42.

Crosstalk between the Rho and Rab family of small GTPases in neurodegenerative disorders

Neurodegeneration is associated with defects in cytoskeletal dynamics and dysfunctions of the vesicular trafficking and sorting systems. In the last few decades, studies have demonstrated that the key regulators of cytoskeletal dynamics are proteins from the Rho family GTPases, meanwhile, the central hub for vesicle sorting and transport between target membranes is the Rab family of GTPases. In this regard, the role of Rho and Rab GTPases in the induction and maintenance of distinct functional and morphological neuronal domains (such as dendrites and axons) has been extensively studied. Several members belonging to these two families of proteins have been associated with many neurodegenerative disorders ranging from dementia to motor neuron degeneration. In this analysis, we attempt to present a brief review of the potential crosstalk between the Rab and Rho family members in neurodegenerative pathologies such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease, and amyotrophic lateral sclerosis (ALS).

S1PR4 deficiency results in reduced germinal center formation but only marginally affects antibody production

Introduction

Splenic B cells exhibit a high expression of the G protein-coupled sphingosine-1-phosphate (S1P) receptor type 4 (S1PR4). Little is known about the functional relevance of S1PR4 expression on those cells.

Methods

In this study, S1PR4-deficient mice were used to study the role of S1PR4-mediated S1P signaling in B cell motility in vitro and for the maintenance of the splenic architecture under steady state conditions as well as in polymicrobial abdominal sepsis in vivo. Finally, the impact of S1PR4 deficiency on antibody production after immunization with T cell dependent antigens was assessed.

Results

Loss of S1PR4 resulted in minor alterations of the splenic architecture concerning the presence of B cell follicles. After sepsis induction, the germinal center response was severely impaired in S1PR4-deficient animals. Splenic B cells showed reduced motility in the absence of S1PR4. However, titres of specific antibodies showed only minor reductions in S1PR4-deficient animals.

Discussion

These observations suggest that S1P signaling mediated by S1PR4 modifies chemokine-induced splenic B cell chemotaxis, thus modulating splenic microarchitecture, GC formation and T-cell dependent antibody production.

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

DOCK8 deficiency causes a skewing to type 2 immunity in the gut with expansion of group 2 innate lymphoid cells

Dedicator of cytokinesis 8 (DOCK8) is a guanine nucleotide exchange factor (GEF) for Cdc42. In humans, homozygous or compound heterozygous deletions in DOCK8 cause a combined immunodeficiency characterized by various allergic diseases including food allergies. Although group 2 innate lymphoid cells (ILC2s) contribute to the development of allergic inflammation by producing interleukin (IL)-5 and IL-13, the role of ILC2s in DOCK8 deficiency has not been fully explored. With the use of cytometry by time-of-flight (CyTOF), we performed high-dimensional phenotyping of intestinal immune cells and found that DOCK8-deficient (Dock8-/-) mice exhibited expansion of ILC2s and other leukocytes associated with type 2 immunity in the small intestine. Moreover, IL-5- and IL-13-producing cells markedly increased in Dock8-/- mice, and the majority of them were lineage-negative cells, most likely ILC2s. Intestinal ILC2s expanded when DOCK8 expression was selectively deleted in hematopoietic cells. Importantly, intestinal ILC2 expansion was also observed in Dock8VAGR mice having mutations in the catalytic center of DOCK8, thereby failing to activate Cdc42. Our findings indicate that DOCK8 is a negative regulator of intestinal ILC2s to inhibit their expansion via Cdc42 activation, and that deletion of DOCK8 causes a skewing to type 2 immunity in the gut.

Pancytopenia, Recurrent Infection, Poor Wound Healing, Heterotopia of the Brain Probably Associated with A Candidate Novel de Novo CDC42 Gene Defect: Expanding the Molecular and Phenotypic Spectrum

CDC42 (cell division cycle protein 42) belongs to the Rho GTPase family that is known to control the signaling axis that regulates several cellular functions, including cell cycle progression, migration, and proliferation. However, the functional characterization of the CDC42 gene in mammalian physiology remains largely unclear. Here, we report the genetic and functional characterization of a non-consanguineous Saudi family with a single affected individual. Clinical examinations revealed poor wound healing, heterotopia of the brain, pancytopenia, and recurrent infections. Whole exome sequencing revealed a de novo missense variant (c.101C > A, p.Pro34Gln) in the CDC42 gene. The functional assays revealed a substantial reduction in the growth and motility of the patient cells as compared to the normal cells control. Homology three-dimensional (3-D) modeling of CDC42 revealed that the Pro34 is important for the proper protein secondary structure. In conclusion, we report a candidate disease-causing variant, which requires further confirmation for the etiology of CDC42 pathogenesis. This represents the first case from the Saudi population. The current study adds to the spectrum of mutations in the CDC42 gene that might help in genetic counseling and contributes to the CDC42-related genetic and functional characterization. However, further studies into the molecular mechanisms that are involved are needed in order to determine the role of the CDC42 gene associated with aberrant cell migration and immune response.

Arhgap25 Deficiency Leads to Decreased Numbers of Peripheral Blood B Cells and Defective Germinal Center Reactions

Rho family GTPases are critical for normal B cell development and function, and their activity is regulated by a large and complex network of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). However, the role of GAPs in B cell development is poorly understood. In this study, we show that the novel Rac-GAP ARHGAP25 is important for B cell development in mice in a CXCR4-dependent manner. We show that Arhgap25 deficiency in mice leads to a significant decrease in peripheral blood B cell numbers as well as defects in mature B cell differentiation. Arhgap25^-/- B cells respond to Ag stimulation in vitro and in vivo but have impaired germinal center formation and decreased IgG1 class switching. Additionally, Arhgap25^-/- B cells show evidence of increased baseline motility and augmented chemotaxis to CXCL12. Taken together, these studies demonstrate an important role for Arhgap25 in peripheral B cell development and Ag response.

Congenital Defects in Actin Dynamics of Germinal Center B Cells

The germinal center (GC) is a transient anatomical structure formed during the adaptive immune response that leads to antibody affinity maturation and serological memory. Recent works using two-photon microscopy reveals that the GC is a highly dynamic structure and GC B cells are highly motile. An efficient selection of high affinity B cells clones within the GC crucially relies on the interplay of proliferation, genome editing, cell-cell interaction, and migration. All these processes require actin cytoskeleton rearrangement to be well-coordinated. Dysregulated actin dynamics may impede on multiple stages during B cell affinity maturation, which could lead to aberrant GC response and result in autoimmunity and B cell malignancy. This review mainly focuses on the recent works that investigate the role of actin regulators during the GC response.

The Lack of WIP Binding to Actin Results in Impaired B Cell Migration and Altered Humoral Immune Responses

Wiskott-Aldrich syndrome protein (WASp) is a main cytoskeletal regulator in B cells. WASp-interacting protein (WIP) binds to and stabilizes WASp but also interacts with actin. Using mice with a mutated actin binding domain of WIP (WIPΔABD), we here investigated the role of WIP binding to actin during B cell activation. We found an altered differentiation of WIPΔABD B cells and diminished antibody affinity maturation after immunization. Mechanistically, WIPΔABD B cells showed impaired B cell receptor (BCR)-induced PI3K signaling and actin reorganization, likely caused by diminished CD81 expression and altered CD19 dynamics on the B cell surface. WIPΔABD B cells displayed reduced in vivo motility, concomitantly with impaired chemotaxis and defective F-actin polarization, HS1 phosphorylation, and polarization of HS1 to F-actin-rich structures after CXCL12 stimulation in vitro. We thus concluded that WIP binding to actin, independent of its binding to WASp, is critical for actin cytoskeleton plasticity in B cells.

Antigen phagocytosis by B cells is required for a potent humoral response

Successful vaccines rely on activating a functional humoral response that results from promoting a proper germinal center (GC) reaction. Key in this process is the activation of follicular B cells that need to acquire antigens and to present them to cognate CD4 T cells. Here, we report that follicular B cells can phagocytose large antigen‐coated particles, a process thought to be exclusive of specialized antigen‐presenting cells such as macrophages and dendritic cells. We show that antigen phagocytosis by B cells is BCR‐driven and mechanistically dependent on the GTPase RhoG. Using Rhog^−/− mice, we show that phagocytosis of antigen by B cells is important for the development of a strong GC response and the generation of high‐affinity class‐switched antibodies. Importantly, we show that the potentiation effect of alum, a common vaccine adjuvant, requires direct phagocytosis of alum–antigen complexes by B cells. These data suggest a new avenue for vaccination approaches by aiming to deliver 1–3 μm size antigen particles to follicular B cells.

Extracellular vesicles from bone marrow-derived mesenchymal stromal cells support ex vivo survival of human antibody secreting cells

Extracellular vesicles (EVs) from bone marrow (BM)-derived mesenchymal stromal cells (BM-MSC) are novel mechanisms of cell-cell communication over short and long distances. BM-MSC have been shown to support human antibody secreting cells (ASC) survival ex vivo, but whether the crosstalk between the MSC-ASC interaction can occur via EVs is not known. Thus, we evaluated the role of EVs in ASC survival and IgG secretion. EVs were isolated from irradiated and non-irradiated primary BM-MSC and were quantified. They were further characterized by electron microscopy (EM) and CD63 and CD81 immuno-gold EM staining. Human ASC were isolated via fluorescence-activated cell sorting (FACS) and cultured ex vivo with the EV fractions, the EV-reduced fractions, or conventional media. IgG Elispots were used to measure the survival and functionality of the ASC. Contents of the EV fractions were evaluated by proteomics. We saw that both irradiated and non-irradiated MSC secretome preparations afforded vesicles of a size consistent with EVs. Both preparations appeared comparable in EM morphology and CD63 and CD81 immuno-gold EM. Both irradiated and non-irradiated EV fractions supported ASC function, at 88% and 90%, respectively, by day 3. In contrast, conventional media maintained only 4% ASC survival by day 3. To identify the specific factors that provided in vitro ASC support, we compared proteomes of the irradiated and non-irradiated EV fractions with conventional media. Pathway analysis of these proteins identified factors involved in the vesicle-mediated delivery of integrin signalling proteins. These findings indicate that BM-MSC EVs provide an effective support system for ASC survival and IgG secretion.

Tuning of in vivo cognate B-T cell interactions by Intersectin 2 is required for effective anti-viral B cell immunity

Wiskott-Aldrich syndrome (WAS) is an immune pathology associated with mutations in WAS protein (WASp) or in WASp interacting protein (WIP). Together with the small GTPase Cdc42 and other effectors, these proteins participate in the remodelling of the actin network downstream of BCR engagement. Here we show that mice lacking the adaptor protein ITSN2, a G-nucleotide exchange factor (GEF) for Cdc42 that also interacts with WASp and WIP, exhibited increased mortality during primary infection, incomplete protection after Flu vaccination, reduced germinal centre formation and impaired antibody responses to vaccination. These defects were found, at least in part, to be intrinsic to the B cell compartment. In vivo, ITSN2 deficient B cells show a reduction in the expression of SLAM, CD84 or ICOSL that correlates with a diminished ability to form long term conjugates with T cells, to proliferate in vivo, and to differentiate into germinal centre cells. In conclusion, our study not only revealed a key role for ITSN2 as an important regulator of adaptive immune-response during vaccination and viral infection but it is also likely to contribute to a better understanding of human immune pathologies.

Methods to Study the Role of Cdc42, Rac1, and Rac2 in B-Cell Cytoskeletal Responses

B-cell migration and adhesion are critical to form a germinal center response, the site for B-cell production of high-affinity antibodies. Here, we describe two assays that can be used to examine B-cell cytoskeletal responses needed during the germinal center response: B-cell spreading and homotypic adhesion. Spreading of B cells is dependent on Cdc42, while Rac1 and Rac2 are necessary for homotypic adhesion. These in vitro assays can be used to examine functional responses of B cells mediated by the cell cytoskeleton, for example when comparing B cells from different gene knockout animals.

The Small Rho GTPases Rac1 and Rac2 Are Important for T-Cell Independent Antigen Responses and for Suppressing Switching to IgG2b in Mice

The Rho GTPases Cdc42, Rac1, and Rac2 coordinate receptor signaling to cell adhesion, migration, and proliferation. Deletion of Rac1 and Rac2 early during B cell development leads to failure in B cell entry into the splenic white pulp. Here, we sought to understand the role of Rac1 and Rac2 in B cell functionality and during the humoral antibody response. To circumvent the migratory deficiency of B cells lacking both Rac1 and Rac2, we took the approach to inducibly delete Rac1 in Rac2^−/− B cells in the spleen (Rac1^BRac2^−/− B cells). Rac1^BRac2^−/− mice had normal differentiation of splenic B cell populations, except for a reduction in marginal zone B cells. Rac1^BRac2^−/− B cells showed normal spreading response on antibody-coated layers, while both Rac2^−/− and Rac1^BRac2^−/− B cells had reduced homotypic adhesion and decreased proliferative response when compared to wild-type B cells. Upon challenge with the T-cell-independent antigen TNP-conjugated lipopolysaccharide, Rac1^BRac2^−/− mice showed reduced antibody response. In contrast, in response to the T-cell-dependent antigen sheep red blood cells, Rac1^BRac2^−/− mice had increased serum titers of IgG1 and IgG2b. During in vitro Ig class switching, Rac1^BRac2^−/− B cells had elevated germline γ2b transcripts leading to increased Ig class switching to IgG2b. Our data suggest that Rac1 and Rac2 serve an important role in regulation of the B cell humoral immune response and in suppressing Ig class switching to IgG2b.

Deletion of Dock10 in B Cells Results in Normal Development but a Mild Deficiency upon In Vivo and In Vitro Stimulations

We sought to identify genes necessary to induce cytoskeletal change in B cells. Using gene expression microarray, we compared B cells stimulated with interleukin-4 (IL-4) and anti-CD40 antibodies that induce B cell spreading, cell motility, tight aggregates, and extensive microvilli with B cells stimulated with lipopolysaccharide that lack these cytoskeletal changes. We identified 84 genes with 10-fold or greater expression in anti-CD40 + IL-4 stimulated B cells, one of these encoded the guanine nucleotide exchange factor (GEF) dedicator of cytokinesis 10 (Dock10). IL-4 selectively induced Dock10 expression in B cells. Using lacZ expression to monitor Dock10 promoter activity, we found that Dock10 was expressed at all stages during B cell development. However, specific deletion of Dock10 in B cells was associated with a mild phenotype with normal B cell development and normal B cell spreading, polarization, motility, chemotaxis, aggregation, and Ig class switching. Dock10-deficient B cells showed lower proliferation in response to anti-CD40 and IL-4 stimulation. Moreover, the IgG response to soluble antigen in vivo was lower when Dock10 was specifically deleted in B cells. Together, we found that most B cell responses were intact in the absence of Dock10. However, specific deletion of Dock10 in B cells was associated with a mild reduction in B cell activation in vitro and in vivo.

Dock10 regulates CD23 expression and sustains B-cell lymphopoiesis in secondary lymphoid tissue

Dock10, a guanine nucleotide exchange factor for the Rho GTPases Rac1 and Cdc42, affects cell morphology, membrane protrusive activity, and cell movement. Dock10 is prominently expressed in lymphoid tissue and upregulated by IL-4 in B cells. To investigate the physiological role of Dock10, WT mice and Dock10 KO mice were used. KO mice showed decreased numbers of B cells in spleen, both follicular B cells and marginal zone B cells, and in peripheral blood, but not in bone marrow. The antiapoptotic effect of IL-4 in vitro, the migratory response to CXCL13 or CCL21 in vitro, and the whole genome expression profile were intact in spleen B cells from KO mice. CD23, the low-affinity receptor for immunoglobulin E, was overexpressed on follicular B cells from KO mice, suggesting that Dock10 negatively regulates membrane CD23 expression. Negative regulation of CD23 expression by Dock10 could play a role in B cell maturation and function.