The Channel-Kinase TRPM7 as Novel Regulator of Immune System Homeostasis

Abstract

The enzyme-coupled transient receptor potential channel subfamily M member 7, TRPM7, has been associated with immunity and immune cell signalling. Here, we review the role of this remarkable signalling protein in lymphocyte proliferation, differentiation, activation and survival. We also discuss its role in mast cell, neutrophil and macrophage function and highlight the potential of TRPM7 to regulate immune system homeostasis. Further, we shed light on how the cellular signalling cascades involving TRPM7 channel and/or kinase activity culminate in pathologies as diverse as allergic hypersensitivity, arterial thrombosis and graft versus host disease (G_VHD), stressing the need for TRPM7 specific pharmacological modulators.

Keywords: SMAD; TH17; TRPM7; calcium signalling; graft versus host disease; hypersensitivity; inflammation; kinase; lymphocytes; regulatory T cells; thrombosis.

Figure 1

TRPM7 topology. Each TRPM7 protein consists of six transmembrane segments (1 to 6) with the channel pore located between segment 5 and 6. Within the N-terminus are melastatin homology domains (MHD), characteristic for TRPM family members. The cytoplasmic C-terminus contains a transient receptor potential domain (TRP), a coiled-coil domain (CC) and a kinase substrate domain (SD) upstream the atypical α-type serine/threonine protein kinase domain (KD). Mutation at the catalytic side of the kinase (K1646R) abolishes kinase activity without affecting current activity. Deletion of the KD at different amino acids (aa) results in either enhanced or reduced current activity. The black star indicates the location of the point mutation, crosses mark the kinase deletion. TRPM7 is negatively regulated by depletion of phosphatidylinositol 4,5-bisphosphate (PIP₂), physiologic free cytosolic magnesium concentrations [Mg²⁺]_c, Mg-ATP, polyamines, low pH and chloride (Cl⁻) and bromide (Br⁻) concentrations.

Figure 2

Role of TRPM7 kinase in calcium signalling and proliferation of T cells. Upon T cell receptor (TCR) binding, phospholipase C (PLC) is activated and hydrolyses phosphatidylinositol 4,5-biphosphate (PIP₂) to inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). DAG in conjunction with Ca²⁺ activates protein kinase C (PKC), thus inducing cell proliferation. IP₃ induces Ca²⁺ release from the endoplasmic reticulum (ER) via IP₃ receptor (IP₃R), followed by the translocation of the stromal interaction molecule (STIM) to the plasma membrane. STIM triggers Ca²⁺ influx from the extracellular space via ORAI/CRACM channels. Ca²⁺ is rapidly removed from the cytosol by the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), refilling the ER Ca²⁺ stores. The prolonged increase in cytosolic Ca²⁺ concentrations leads to the activation of calcineurin, resulting in the dephosphorylation and nuclear translocation of nuclear factor of activated T cells (NFAT) and subsequent cytokine expression, e.g., interleukin 2 (IL-2), triggering clonal expansion of T cells.

Figure 3

TRPM7 kinase in T cell signalling and transcriptional regulation. Upon binding of tumour growth factor β1 (TGF-β1), the TGF-β receptor complex (TGFβRI/II) initiates the phosphorylation of the c-terminal SXS-motif of SMAD2. Results gained from TRPM7 kinase deficient murine T cells suggest an additional mechanism by which the TRPM7 kinase phosphorylates SMAD2 directly, once it is anchored to the plasma membrane. Phosphorylated SMAD2 interacts with SMAD4 and promotes the transcription of Itgae, Il-17 and Rorc genes. The interleukin 6 (IL-6) dependent STAT3 as well as Erk1/2 phosphorylation pathway is unaltered in TRPM7 kinase deficient murine T cells.

Figure 4

TRPM7 kinase is essential for T cell differentiation into the pro-inflammatory T_H17 cell type and the development of graft versus host disease. (A) Naïve CD4⁺ T cells differentiate into pro-inflammatory T_H17 cells in the presence of TGF-β and IL-6. For the differentiation into anti-inflammatory regulatory T cells (T_regs), the cytokines TGF-β and IL-2 are required. (B) Genetic inactivation of TRPM7 kinase activity (Trpm7^R/R) results in reduced T_H17 cell differentiation, indicated via diminished Rorc and Il-17 mRNA expression as well as IL-17 serum levels, while T_reg cell differentiation, evident via FoxP3 expression and IL-10 serum levels, is unaltered. In addition, integrin αE (CD103) expression is reduced in TRPM7 kinase deficient T cells. (C) Transplantation of bone marrow cells (BMC, C57BL/6) in conjunction with splenocytes (SPL, C57BL/6) triggers the development of graft versus host disease (G_VHD) in lethally irradiated mice with different genetic background (BALB/c), manifesting in massive tissue destruction of the intestine but also lung and skin tissues. TRPM7 kinase deficient BMC and SPL transplantation does not induce inflammation in the intestine and ameliorates or even prevents disease progression, suggesting TRPM7 kinase inhibition as valid tool for the treatment of G_VHD. n.a. (not altered).