MALT1 Protease Plays a Dual Role in the Allergic Response by Acting in Both Mast Cells and Endothelial Cells.

The signaling protein MALT1 plays a key role in promoting NF-κB activation in Ag-stimulated lymphocytes. In this capacity, MALT1 has two functions, acting as a scaffolding protein and as a substrate-specific protease. MALT1 is also required for NF-κB-dependent induction of proinflammatory cytokines after FcεR1 stimulation in mast cells, implicating a role in allergy. Because MALT1 remains understudied in this context, we sought to investigate how MALT1 proteolytic activity contributes to the overall allergic response. We compared bone marrow-derived mast cells from MALT1 knockout (MALT1-/-) and MALT1 protease-deficient (MALTPD/PD) mice to wild-type cells. We found that MALT1-/- and MALT1PD/PD mast cells are equally impaired in cytokine production following FcεRI stimulation, indicating that MALT1 scaffolding activity is insufficient to drive the cytokine response and that MALT1 protease activity is essential. In addition to cytokine production, acute mast cell degranulation is a critical component of allergic response. Intriguingly, whereas degranulation is MALT1-independent, MALT1PD/PD mice are protected from vascular edema induced by either passive cutaneous anaphylaxis or direct challenge with histamine, a major granule component. This suggests a role for MALT1 protease activity in endothelial cells targeted by mast cell-derived vasoactive substances. Indeed, we find that in human endothelial cells, MALT1 protease is activated following histamine treatment and is required for histamine-induced permeability. We thus propose a dual role for MALT1 protease in allergic response, mediating 1) IgE-dependent mast cell cytokine production, and 2) histamine-induced endothelial permeability. This dual role indicates that therapeutic inhibitors of MALT1 protease could work synergistically to control IgE-mediated allergic disease.

Critical protein-protein interactions within the CARMA1-BCL10-MALT1 complex: Take-home points for the cell biologist.

The CBM complex, which is composed of the proteins CARMA1, BCL10, and MALT1, serves multiple pivotal roles as a mediator of T-cell receptor and B-cell receptor-dependent NF-κB induction and lymphocyte activation. CARMA1, BCL10, and MALT1 are each proto-oncoproteins and dysregulation of CBM signaling, as a result of somatic gain-of-function mutation or chromosomal translocation, is a hallmark of multiple lymphoid malignancies including Activated B-cell Diffuse Large B-cell Lymphoma. Moreover, loss-of-function as well as gain-of-function germline mutations in CBM complex proteins have been associated with a range of immune dysregulation syndromes. A wealth of detailed structural information has become available over the past decade through meticulous interrogation of the interactions between CBM components. Here, we review key findings regarding the biochemical nature of these protein-protein interactions which have ultimately led the field to a sophisticated understanding of how these proteins assemble into high-order filamentous CBM complexes. To date, approaches to therapeutic inhibition of the CBM complex for the treatment of lymphoid malignancy and/or auto-immunity have focused on blocking MALT1 protease function. We also review key studies relating to the structural impact of MALT1 protease inhibitors on key protein-protein interactions.

Post-transplant lymphoproliferative disorder in pediatric intestinal transplant recipients: A literature review.

Intestinal transplantation is a successful treatment for children with intestinal failure, but has many potential complications. PTLD, a clinically and histologically diverse malignancy, occurs frequently after intestinal transplantation and can be fatal. The management of this disease is particularly challenging. The rejection-prone intestinal allograft requires high levels of immunosuppression, a precondition for PTLD. While EBV infection clearly plays a role in disease pathogenesis, the relatively naïve immune system of children is another likely contributor. As a result, pediatric intestine recipients have a higher risk of developing PTLD than other solid organ recipients. Other risk factors for disease development such as molecular and genomic changes that precipitate malignant transformation are not fully understood, especially among children. Studies on adults have started to describe the molecular pathogenesis of PTLD, but the genomic landscape of the malignancy remains largely undefined in pediatric intestinal transplant patients. In this review, we describe what is known about PTLD in pediatric patients after intestinal transplant and highlight current knowledge gaps to better direct future investigations in the pediatric population.

Immunocompetent murine model of Ewing sarcoma reveals role for TGFß inhibition to enhance immune infiltrates in Ewing tumors during radiation.

Ewing sarcoma (ES) is an aggressive cancer diagnosed in adolescents and young adults. The fusion oncoprotein (EWSR1::FLI1) that drives Ewing sarcoma is known to downregulate TGFBR2 expression (part of the TGFß receptor). Because TGFBR2 is downregulated, it was thought that TGFß likely plays an inconsequential role in Ewing biology. However, the expression of TGFß in the Ewing tumor immune microenvironment (TIME) and functional impact of TGFß in the TIME remains largely unknown given the historical lack of immunocompetent preclinical models. Here, we use single-cell RNAseq analysis of human Ewing tumors to show that immune cells, such as NK cells, are the largest source of TGFß production in human Ewing tumors. We develop a humanized (immunocompetent) mouse model of ES and demonstrate distinct TME signatures and metastatic potential in these models as compared to tumors developed in immunodeficient mice. Using this humanized model, we study the effect of TGFß inhibition on the Ewing TME during radiation therapy, a treatment that both enhances TGFß activation and is used to treat aggressive ES. Utilizing a trivalent ligand TGFß TRAP to inhibit TGFß, we demonstrate that in combination with radiation, TGFß inhibition both increases ES immune cell infiltration and decreases lung metastatic burden in vivo . The culmination of these data demonstrates the value of humanized models to address immunobiologic preclinical questions in Ewing sarcoma and suggests TGFß inhibition as a promising intervention during radiation therapy to promote metastatic tumor control.

CRISPRa-induced upregulation of human LAMA1 compensates for LAMA2-deficiency in Merosin-deficient congenital muscular dystrophy.

Merosin-deficient congenital muscular dystrophy (MDC1A) is an autosomal recessive disorder caused by mutations in the LAMA2 gene, resulting in a defective form of the extracellular matrix protein laminin-α2 (LAMA2). Individuals diagnosed with MDC1A exhibit progressive muscle wasting and declining neuromuscular functions. No treatments for this disorder are currently available. We previously showed that postnatal Lama1 upregulation, achieved through CRISPR activation (CRISPRa), compensates for Lama2 deficiency and prevents neuromuscular pathophysiology in a mouse model of MDC1A. In this study, we assessed the feasibility of upregulating human LAMA1 as a potential therapeutic strategy for individuals with MDC1A, regardless of their mutations. We hypothesized that CRISPRa-mediated upregulation of human LAMA1 would compensate for the lack of LAMA2 and rescue cellular abnormalities in MDC1A fibroblasts. Global transcriptomic and pathway enrichment analyses of fibroblasts collected from individuals carrying pathogenic LAMA2 mutations, compared with healthy controls, indicated higher expression of transcripts encoding proteins that contribute to wound healing, including Transforming Growth Factor-ß (TGF-ß) and Fibroblast Growth Factor (FGF). These findings were supported by wound-healing assays indicating that MDC1A fibroblasts migrated significantly more rapidly than the controls. Subsequently, we treated the MDC1A fibroblasts with SadCas9-2XVP64 and sgRNAs targeting the LAMA1 promoter. We observed robust LAMA1 expression, which was accompanied by significant decreases in cell migration and expression of FGFR2, TGF-ß2, and ACTA2, which are involved in the wound-healing mechanism in MDC1A fibroblasts. Collectively, our data suggest that CRISPRa-mediated LAMA1 upregulation may be a feasible mutation-independent therapeutic approach for MDC1A. This strategy might be adapted to address other neuromuscular diseases and inherited conditions in which strong compensatory mechanisms have been identified.

Successful treatment of refractory metastatic histiocytic sarcoma with alemtuzumab.

BACKGROUND: Histiocytic sarcoma (HS) is an exceedingly rare tumor and carries a dismal prognosis when patients present with advanced-stage disease. Because of the poor response rates to conventional chemotherapy and the rarity of the disease, no standard of care exists for patients with HS. The authors report the single-agent use of the anti-CD52 antibody alemtuzumab in 2 patients who had advanced-stage HS. METHODS: Two patients with chemotherapy-refractory, metastatic HS with tumors that expressed the CD52 antigen received a prolonged course of treatment with the anti-CD52 monoclonal antibody alemtuzumab. RESULTS: Resected tumor samples from both patients demonstrated CD52 expression. Both patients had marked responses to single-agent alemtuzumab. One patient had a complete response with no evidence of disease for >5 years. The second patient had a major response to alemtuzumab and also remained alive with no evidence of disease for >4 years. CONCLUSIONS: Further studies need to be performed to examine CD52 expression and function in HS as well as the role of alemtuzumab in a larger cohort of patients. However, the clinical impact of single-agent alemtuzumab in the 2 patients described in this report was very encouraging. Given the toxicity and frequent inefficacy of conventional chemotherapy, patients with incompletely resected HS should be strongly considered for alemtuzumab therapy.

Thalidomide therapy for aggressive histiocytic lesions in the pediatric population.

Aggressive histiocytic lesions are uncommon in the pediatric population. These neoplasms occur in isolation or after therapy for other types of hematopoietic malignancy such as T-cell acute lymphoblastic leukemia. The etiology of these lesions is poorly understood, and no definitive standard of care has been established for patients with these diagnoses. Here, we report the success of thalidomide treatment for 2 subtypes of histiocytic proliferation--metastatic histiocytic sarcoma and extracutaneous juvenile xanthogranuloma--in pediatric patients. Our findings highlight the importance of considering thalidomide therapy in this unique and difficult to treat patient population.

BRCA1-associated RING domain-1 (BARD1) loss and GBP1 expression enhance sensitivity to DNA damage in Ewing sarcoma.

Ewing sarcoma is a fusion oncoprotein-driven primary bone tumor. A subset of patients (~10%) with Ewing sarcoma are known to harbor germline variants in a growing number of genes involved in DNA damage repair. We recently reported our discovery of a germline mutation in the DNA damage repair protein BARD1 (BRCA1-associated RING domain-1) in a patient with Ewing sarcoma. BARD1 is recruited to the site of DNA double stranded breaks via the poly(ADP-ribose) polymerase (PARP) protein and plays a critical role in DNA damage response pathways including homologous recombination. We thus questioned the impact of BARD1 loss on Ewing cell sensitivity to DNA damage and the Ewing sarcoma transcriptome. We demonstrate that PSaRC318 cells, a novel patient-derived cell line harboring a pathogenic BARD1 variant, are sensitive to PARP inhibition and by testing the effect of BARD1 depletion in additional Ewing sarcoma cell lines, we confirm that BARD1 loss enhances cell sensitivity to PARP inhibition plus radiation. Additionally, RNA-seq analysis revealed that loss of BARD1 results in the upregulation of GBP1 (guanylate-binding protein 1), a protein whose expression is associated with variable response to therapy depending on the adult carcinoma subtype examined. Here, we demonstrate that GBP1 contributes to the enhanced sensitivity of BARD1 deficient Ewing cells to DNA damage. Together, our findings demonstrate the impact of loss-of function mutations in DNA damage repair genes, such as BARD1, on Ewing sarcoma treatment response.

MALT1 Is a Targetable Driver of Epithelial-to-Mesenchymal Transition in Claudin-Low, Triple-Negative Breast Cancer.

MALT1 is the effector protein of the CARMA/Bcl10/MALT1 (CBM) signalosome, a multiprotein complex that drives pro-inflammatory signaling pathways downstream of a diverse set of receptors. Although CBM activity is best known for its role in immune cells, emerging evidence suggests that it plays a key role in the pathogenesis of solid tumors, where it can be activated by selected G protein-coupled receptors (GPCR). Here, we demonstrated that overexpression of GPCRs implicated in breast cancer pathogenesis, specifically the receptors for Angiotensin II and thrombin (AT1R and PAR1), drove a strong epithelial-to-mesenchymal transition (EMT) program in breast cancer cells that is characteristic of claudin-low, triple-negative breast cancer (TNBC). In concert, MALT1 was activated in these cells and contributed to the dramatic EMT phenotypic changes through regulation of master EMT transcription factors including Snail and ZEB1. Importantly, blocking MALT1 signaling, through either siRNA-mediated depletion of MALT1 protein or pharmacologic inhibition of its activity, was effective at partially reversing the molecular and phenotypic indicators of EMT. Treatment of mice with mepazine, a pharmacologic MALT1 inhibitor, reduced growth of PAR1+, MDA-MB-231 xenografts and had an even more dramatic effect in reducing the burden of metastatic disease. These findings highlight MALT1 as an attractive therapeutic target for claudin-low TNBCs harboring overexpression of one or more selected GPCRs. IMPLICATIONS: This study nominates a GPCR/MALT1 signaling axis as a pathway that can be pharmaceutically targeted to abrogate EMT and metastatic progression in TNBC, an aggressive form of breast cancer that currently lacks targeted therapies.

Thrombin-dependent NF-{kappa}B activation and monocyte/endothelial adhesion are mediated by the CARMA3·Bcl10·MALT1 signalosome.

Thrombin is a potent modulator of endothelial function and, through stimulation of NF-κB, induces endothelial expression of intracellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). These cell surface adhesion molecules recruit inflammatory cells to the vessel wall and thereby participate in the development of atherosclerosis, which is increasingly recognized as an inflammatory condition. The principal receptor for thrombin on endothelial cells is protease-activated receptor-1 (PAR-1), a member of the G protein-coupled receptor superfamily. Although it is known that PAR-1 signaling to NF-κB depends on initial PKC activation, the subsequent steps leading to stimulation of the canonical NF-κB machinery have remained unclear. Here, we demonstrate that a complex of proteins containing CARMA3, Bcl10, and MALT1 links PAR-1 activation to stimulation of the IκB kinase complex. IκB kinase in turn phosphorylates IκB, leading to its degradation and the release of active NF-κB. Further, we find that although this CARMA3·Bcl10·MALT1 signalosome shares features with a CARMA1-containing signalosome found in lymphocytes, there are significant differences in how the signalosomes communicate with their cognate receptors. Specifically, whereas the CARMA1-containing lymphocyte complex relies on 3-phosphoinositide-dependent protein kinase 1 for assembly and activation, the CARMA3-containing endothelial signalosome functions completely independent of 3-phosphoinositide-dependent protein kinase 1 and instead relies on ß-arrestin 2 for assembly. Finally, we show that thrombin-dependent adhesion of monocytes to endothelial cells requires an intact endothelial CARMA3·Bcl10·MALT1 signalosome, underscoring the importance of the signalosome in mediating one of the most significant pro-atherogenic effects of thrombin.

The CARMA3-Bcl10-MALT1 signalosome promotes angiotensin II-dependent vascular inflammation and atherogenesis.

The CARMA1, Bcl10, and MALT1 proteins together constitute a signaling complex (CBM signalosome) that mediates antigen-dependent activation of NF-kappaB in lymphocytes, thereby representing a cornerstone of the adaptive immune response. Although CARMA1 is restricted to cells of the immune system, the analogous CARMA3 protein has a much wider expression pattern. Emerging evidence suggests that CARMA3 can substitute for CARMA1 in non-immune cells to assemble a CARMA3-Bcl10-MALT1 signalosome and mediate G protein-coupled receptor activation of NF-kappaB. Here we show that one G protein-coupled receptor, the type 1 receptor for angiotensin II, utilizes this mechanism for activation of NF-kappaB in endothelial and vascular smooth muscle cells, thereby inducing pro-inflammatory signals within the vasculature, a key factor in atherogenesis. Further, we demonstrate that Bcl10-deficient mice are protected from developing angiotensin-dependent atherosclerosis and aortic aneurysms. By uncovering a novel vascular role for the CBM signalosome, these findings illustrate that CBM-dependent signaling has functions outside the realm of adaptive immunity and impacts pathobiology more broadly than previously known.

Protease activity of the API2-MALT1 fusion oncoprotein in MALT lymphoma development and treatment.

Gastric mucosa-associated lymphoid tissue (MALT) lymphoma is a prototypical cancer that occurs in the setting of chronic inflammation and an important model for understanding how deregulated NF-κB transcriptional activity contributes to malignancy. Most gastric MALT lymphomas require ongoing antigenic stimulation for continued tumor growth, and Stage I disease is usually cured by eradicating the causative microorganism, Helicobacter pylori, with antibiotics. However, in a subset of MALT lymphomas, chromosomal translocations are acquired that render the lymphoma antigen-independent. The recurrent translocation t(11;18)(q21;q21) is associated with failure to respond to antibiotic therapy and increased rate of dissemination. This translocation creates the API2-MALT1 fusion oncoprotein, which comprises the amino terminus of inhibitor of apoptosis 2 (API2 or cIAP2) fused to the carboxy terminus of MALT1. A common characteristic of chromosomal translocations in MALT lymphoma, including t(11;18), is that genes involved in the regulation of the NF-κB transcription factor are targeted by the translocations, and these genetic perturbations thereby result in deregulated, constitutive NF-κB stimulation. In the last decade, great insights into the roles of API2 and MALT1 in NF-κB signaling have been made. For example, recent pivotal studies have uncovered the long sought-after proteolytic activity of MALT1 and have demonstrated its critical involvement in the survival of certain lymphomas. Here, we review the current understanding of the role of MALT1 in normal lymphocyte function and lymphomagenesis. We then highlight our recent work that has revealed an intriguing link between the proteolytic activity of the API2-MALT1 fusion and its ability to influence lymphomagenesis by cleaving a key NF-κB regulatory protein, NF-κB-inducing kinase.

An unusual case of membranous nephropathy associated with an ovarian tumor.

Secondary membranous nephropathy (MN) associated with malignancy is not uncommon in adults, but it is rare in children. We report a 6-year-old girl who developed nephrotic-range proteinuria following diagnosis of a Sertoli-Leydig ovarian tumor. A renal biopsy was performed, which led to the diagnosis of MN. The patient maintained normal renal function and gradually showed improvement in proteinuria over several months without the use of corticosteroids or angiotensin-converting enzyme inhibitors. Our case highlights the importance of performing screening urinalyses in children with tumors to recognize the presence of clinically significant, but potentially asymptomatic kidney disease.

Canonical and Non-Canonical Roles of GRK2 in Lymphocytes.

G protein-coupled receptor kinase 2 (GRK2) is emerging as a key integrative signaling node in a variety of biological processes ranging from cell growth and proliferation to migration and chemotaxis. As such, GRK2 is now implicated as playing a role in the molecular pathogenesis of a broad group of diseases including heart failure, cancer, depression, neurodegenerative disease, and others. In addition to its long-known canonical role in the phosphorylation and desensitization of G protein-coupled receptors (GPCRs), recent studies have shown that GRK2 also modulates a diverse array of other molecular processes via newly identified GRK2 kinase substrates and via a growing number of protein-protein interaction binding partners. GRK2 belongs to the 7-member GRK family. It is a multidomain protein containing a specific N-terminal region (referred to as αN), followed by a regulator of G protein signaling homology (RH) domain, an AGC (Protein kinase A, G, C serine/threonine kinase family) kinase domain, and a C-terminal pleckstrin homology (PH) domain. GPCRs mediate the activity of many regulators of the immune system such as chemokines and leukotrienes, and thus GRK proteins may play key roles in modulating the lymphocyte response to these factors. As one of the predominant GRK family members expressed in immune cells, GRK2's canonical and noncanonical actions play an especially significant role in normal immune cell function as well as in the development and progression of disorders of the immune system. This review summarizes our current state of knowledge of the roles of GRK2 in lymphocytes. We highlight the diverse functions of GRK2 and discuss how ongoing investigation of GRK2 in lymphocytes may inform the development of new therapies for diseases associated with lymphocyte dysregulation.

GRK2 suppresses lymphomagenesis by inhibiting the MALT1 proto-oncoprotein.

Antigen receptor-dependent (AgR-dependent) stimulation of the NF-κB transcription factor in lymphocytes is a required event during adaptive immune response, but dysregulated activation of this signaling pathway can lead to lymphoma. AgR stimulation promotes assembly of the CARMA1-BCL10-MALT1 complex, wherein MALT1 acts as (a) a scaffold to recruit components of the canonical NF-κB machinery and (b) a protease to cleave and inactivate specific substrates, including negative regulators of NF-κB. In multiple lymphoma subtypes, malignant B cells hijack AgR signaling pathways to promote their own growth and survival, and inhibiting MALT1 reduces the viability and growth of these tumors. As such, MALT1 has emerged as a potential pharmaceutical target. Here, we identified G protein-coupled receptor kinase 2 (GRK2) as a new MALT1-interacting protein. We demonstrated that GRK2 binds the death domain of MALT1 and inhibits MALT1 scaffolding and proteolytic activities. We found that lower GRK2 levels in activated B cell-type diffuse large B cell lymphoma (ABC-DLBCL) are associated with reduced survival, and that GRK2 knockdown enhances ABC-DLBCL tumor growth in vitro and in vivo. Together, our findings suggest that GRK2 can function as a tumor suppressor by inhibiting MALT1 and provide a roadmap for developing new strategies to inhibit MALT1-dependent lymphomagenesis.

EWS-FLI1 low Ewing sarcoma cells demonstrate decreased susceptibility to T-cell-mediated tumor cell apoptosis.

Metastatic and relapsed Ewing sarcoma typically afflicts the adolescent population and is largely fatal. These bone tumors are most commonly driven by the fusion oncoprotein EWS-FLI1. Ewing tumors demonstrate significant intra-tumoral heterogeneity, and individual tumor cells can express highly variable and dynamic levels of EWS-FLI1. Recent studies revealed that the EWS-FLI1 oncoprotein level (high versus low expression) greatly influences the behavior of Ewing tumor cells. As compared to cells with high EWS-FLI1, Ewing cells in the EWS-FLI1 low state demonstrate an increased propensity for metastasis. In light of these observations, we sought to determine how tumor cell EWS-FLI1 level influences the anti-tumor cell immune response. Since ICAM-1, which can promote tumor cell/T-cell interaction and T-cell activation, is highly expressed on EWS-FLI1 low cells, we hypothesized that EWS-FLI1 low cells would be more susceptible to T-cell mediated tumor cell apoptosis as compared to cells with high EWS-FLI1. Unexpectedly, we found that EWS-FLI1 low cells are more resistant to T-cell mediated apoptosis than EWS-FLI1 high cells. We investigated the potential mechanisms by which EWS-FLI1 level might influence the T-cell anti-tumor response, and discovered that low EWS-FLI1 expression results in upregulation of PD-L1 and PD-L2, both important ligands for the PD-1 immune checkpoint receptor on T-cells. We demonstrated that blocking PD-1 results in a greater increase of T-cell mediated killing of EWS-FLI1 low tumor cells as compared to cells with higher EWS-FLI1 expression. Our studies suggest that Ewing cells in the EWS-FLI1 low expression state may serve as a niche of tumor immune-evasion.

MALT1 is a critical mediator of PAR1-driven NF-κB activation and metastasis in multiple tumor types.

Protease-activated receptor 1 (PAR1), a thrombin-responsive G protein-coupled receptor (GPCR), is implicated in promoting metastasis in multiple tumor types, including both sarcomas and carcinomas, but the molecular mechanisms responsible remain largely unknown. We previously discovered that PAR1 stimulation in endothelial cells leads to activation of NF-κB, mediated by a protein complex comprised of CARMA3, Bcl10, and the MALT1 effector protein (CBM complex). Given the strong association between NF-κB and metastasis, we hypothesized that this CBM complex could play a critical role in the PAR1-driven metastatic progression of specific solid tumors. In support of our hypothesis, we demonstrate that PAR1 stimulation results in NF-κB activation in both osteosarcoma and breast cancer, which is suppressed by siRNA-mediated MALT1 knockdown, suggesting that an intact CBM complex is required for the response in both tumor cell types. We identify several metastasis-associated genes that are upregulated in a MALT1-dependent manner after PAR1 stimulation in cancer cells, including those encoding the matrix remodeling protein, MMP9, and the cytokines, IL-1ß and IL-8. Further, exogenous expression of PAR1 in MCF7 breast cancer cells confers highly invasive and metastatic behavior which can be blocked by CRISPR/Cas9-mediated MALT1 knockout. Importantly, we find that PAR1 stimulation induces MALT1 protease activity in both osteosarcoma and breast cancer cells, an activity that is mechanistically linked to NF-κB activation and potentially other responses associated with aggressive phenotype. Several small molecule MALT1 protease inhibitors have recently been described that could therefore represent promising new therapeutics for the prevention and/or treatment of PAR1-driven tumor metastasis.