Genetic immunodeficiency and autoimmune disease reveal distinct roles of Hem1 in the WAVE2 and mTORC2 complexes

Abstract

One sentence summary: Hem1 loss of function mutations cause a congenital immunodysregulatory disease and reveal its role regulating WAVE2 and mTORC2 signaling. AbstractImmunodeficiency often coincides with immune hyperresponsiveness such as autoimmunity, lymphoproliferation, or atopy, but the molecular basis of this paradox is typically unknown. We describe four families with immunodeficiency coupled with atopy, lymphoproliferation, cytokine overproduction, hemophagocytic lymphohistocytosis, and autoimmunity. We discovered loss-offunction variants in the gene NCKAP1L, encoding the hematopoietic-specific Hem1 protein. Three mutations cause Hem1 protein and WAVE regulatory complex (WRC) loss, thereby disrupting actin polymerization, synapse formation, and immune cell migration. Another mutant, M371V encodes a stable Hem1 protein but abrogates binding of the Arf1 GTPase and identifies Arf1 as a critical Hem1 regulator. All mutations reduce the cortical actin barrier to cytokine release explaining immune hyperresponsiveness. Finally, Hem1 loss blocked mTORC2-dependent AKT phosphorylation, T cell proliferation, and effector cytokine production during T cell activation. Thus, our data show that Hem1 independently governs two key regulatory complexes, the WRC and mTORC2, and how Hem1 loss causes a combined immunodeficiency and immune hyperresponsiveness disease.Inborn errors of immunity can paradoxically cause life-threatening infections coupled with lymphoproliferation or autoimmunity by genetically altering global cellular regulatory systems. (1, 2) The actin cytoskeleton is a global regulator of cell migration, phagocytosis, immune synapse formation, cell division, vesicle release, and cytotoxicity. Other global cell regulators are the mTOR complexes 1 (mTORC1) and 2 (mTORC2) that control cell metabolism and signaling. How these two systems are molecularly coordinated during immune responses is unknown. The WAVE regulatory complex (WRC), an obligate heteropentamer containing isoforms of the Cyfip1/2, Hem1/2, Abi1/2, HSPC300 and WAVE1/2/3 proteins, dynamically regulates F-actin polymerization for cell migration and immune synapse function and generates a static cortical actin network (CAcN) that controls cell deformation and restricts cytoplasmic vesicle release.(3-6) Diverse signals, including the small GTPases Rac1 and Arf1, acidic phospholipids, kinases, and cell surface receptors can cooperatively recruit and activate the WRC.(7-12) Rac1 has two binding sites on Cyfip1/2, but none on Hem1. The Arf1 docking site in the WRC is unknown. Genetic diseases have been described for some actin regulatory proteins, including WASP, ARPC1B, and Rac2, but not yet for WRC components.Rapamycin-insensitive mTORC2, a protein complex comprised of mTOR, Rictor, mSIN1, mLST8, Protor1/2, and Deptor, phosphorylates and activates AGC kinases (AKT, SGK1, and PKC) to promote cell survival/proliferation, T cell differentiation, and is required for regulated cell migration through regulation of the actin cytoskeleton.(13-17) Thus, the up...

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

“…This has severe consequences for the host, as inefficiently cleared pathogens, apoptotic and necrotic cell bodies are a major source of inflammation and rise of autoantigens, respectively. Indeed, numerous studies have suggested that inefficiently cleared cells can promote the development of autoimmune diseases, such as observed in recently described human patients harboring Hem1 mutations (102).…”

When Actin is Not Actin’ Like It Should: A New Category of Distinct Primary Immunodeficiency Disorders

“…Recently, 5 patients have been identified with loss-of-function mutations in NCKAP1L [74]. These bi-allelic missense mutations lead to either a loss of HEM1 protein or abrogated binding of HEM1 to the GTPase ADP-ribosylation factor 1, thereby leading to destabilization of the WRC or disruption of ADP-ribosylation factor 1-WRC binding and activation, respectively.…”

“…This defective F-actin accumulation was also observed in the NK-cell-target synapse. Reduction of the cortical actin barrier in HEM1-deficient T-cells resulted in exaggerated degranulation and IL-10, perforin, and granzyme A and B release, which could explain the patients’ autoinflammatory phenotype [74]. Increased degranulation has also been observed in ARPC1B-deficient [31] and MKL1-deficient [28] neutrophils.…”

“…These bi-allelic missense mutations lead to either a loss of HEM1 protein or abrogated binding of HEM1 to the GTPase ADP-ribosylation factor 1, thereby leading to destabilization of the WRC or disruption of ADP-ribosylation factor 1-WRC binding and activation, respectively. These patients suffered from a severe immunodeficiency with recurrent bacterial and viral skin infections, otitis media, upper respiratory tract infections, as well as immune system hyperreactivity which included atopic and allergic disease [74].…”

“…Neutrophils of these patients showed impaired migration in response to fMLF, C5a, and IL-8. More specifically, HEM1-deficient neutrophils could not form a broad leading edge or maintain directional persistence, but instead became dramatically elongated with competing leading edges moving parallel to or against the chemotactic gradient [74]. Impaired migration is also observed in HEM1-deficient mouse neutrophils, as well as impaired actin polymerization [75].…”