Interaction of calmodulin with Bcl10 modulates NF-kappaB activation

Metal-ion-chelating phenylalanine nanostructures reverse immune dysfunction and sensitize breast tumour to immune checkpoint blockade

An immunosuppressive tumour microenvironment strongly influences response rates in patients receiving immune checkpoint blockade-based cancer immunotherapies, such as programmed death-1 (PD-1) and programmed death-ligand 1 (PD-L1). Here we demonstrate that metal-ion-chelating L-phenylalanine nanostructures synergize with short-term starvation (STS) to remodel the immunosuppressive microenvironment of breast and colorectal tumours. These nanostructures modulate the electrophysiological behaviour of dendritic cells and activate them through the NLRP3 inflammasome and calcium-mediated nuclear factor-κB pathway. STS promotes the cellular uptake of nanostructures through amino acid transporters and plays a key role in dendritic cell maturation and tumour-specific cytotoxic T lymphocyte responses. This study demonstrates the potential role of metal-ion-chelating L-phenylalanine nanostructures in activating immune responses and the effect of STS treatment in improving nanomaterial-mediated cancer immunotherapy.

Dynamic Na+/H+ exchanger 1 (NHE1) - calmodulin complexes of varying stoichiometry and structure regulate Ca2+-dependent NHE1 activation

Calmodulin (CaM) engages in Ca²⁺-dependent interactions with numerous proteins, including a still incompletely understood physical and functional interaction with the human Na⁺/H⁺-exchanger NHE1. Using nuclear magnetic resonance (NMR) spectroscopy, isothermal titration calorimetry, and fibroblasts stably expressing wildtype and mutant NHE1, we discovered multiple accessible states of this functionally important complex existing in different NHE1:CaM stoichiometries and structures. We determined the NMR solution structure of a ternary complex in which CaM links two NHE1 cytosolic tails. In vitro, stoichiometries and affinities could be tuned by variations in NHE1:CaM ratio and calcium ([Ca²⁺]) and by phosphorylation of S648 in the first CaM-binding α-helix. In cells, Ca²⁺-CaM-induced NHE1 activity was reduced by mimicking S648 phosphorylation and by mutation of the first CaM-binding α-helix, whereas it was unaffected by inhibition of Akt, one of several kinases phosphorylating S648. Our results demonstrate a diversity of NHE1:CaM interaction modes and suggest that CaM may contribute to NHE1 dimerization and thereby augment NHE1 regulation. We propose that a similar structural diversity is of relevance to many other CaM complexes.

ETS1 and PAX5 transcription factors recruit AID to Igh DNA

B lymphocytes optimize antibody responses by class switch recombination (CSR), which changes the expressed constant region exon of the immunoglobulin heavy chain (IgH), and by somatic hypermutation (SH) that introduces point mutations in the variable regions of the antibody genes. Activation-induced cytidine deaminase (AID) is the key mutagenic enzyme that initiates both these antibody diversification processes by deaminating cytosine to uracil. Here we asked the question if transcription factors can mediate the specific targeting of the antibody diversification by recruiting AID. We have recently reported that AID is together with the transcription factors E2A, PAX5 and IRF4 in a complex on key sequences of the Igh locus. Here we report that also ETS1 is together with AID in this complex on key sequences of the Igh locus in splenic B cells of mice. Furthermore, we show that both ETS1 and PAX5 can directly recruit AID to DNA sequences from the Igh locus with the specific binding site for the transcription factor. Taken together, our findings support the notion of a targeting mechanism for the selective diversification of antibody genes with limited genome wide mutagenesis by recruitment of AID by PAX5 and ETS1 in a transcription factor complex.

Calmodulin as a protein linker and a regulator of adaptor/scaffold proteins

Calmodulin (CaM) is a universal regulator for a huge number of proteins in all eukaryotic cells. Best known is its function as a calcium-dependent modulator of the activity of enzymes, such as protein kinases and phosphatases, as well as other signaling proteins including membrane receptors, channels and structural proteins. However, less well known is the fact that CaM can also function as a Ca²⁺-dependent adaptor protein, either by bridging between different domains of the same protein or by linking two identical or different target proteins together. These activities are possible due to the fact that CaM contains two independently-folded Ca²⁺ binding lobes that are able to interact differentially and to some degree separately with targets proteins. In addition, CaM can interact with and regulates several proteins that function exclusively as adaptors. This review provides an overview over our present knowledge concerning the structural and functional aspects of the role of CaM as an adaptor protein and as a regulator of known adaptor/scaffold proteins.

Involvement of calmodulin and calmodulin kinase II in tumor necrosis factor alpha-induced survival of bone marrow derived macrophages

We previously showed that survival signaling in TNFα-treated, human THP1-derived macrophages (TDMs) has an obligatory requirement for constitutive Ca(2+) influx through a mechanism involving calmodulin/calmodulin kinase II (CAM/CAMKII). We also demonstrated that such requirement also applies to the protective actions of TNFα in murine bone marrow-derived macrophages (BMDMs) and that TRPC3 channels mediate constitutive Ca(2+) influx. Using a pharmacological approach we here examined if in BMDMs, similarly to TDMs, TNFα-induced survival signaling also involves CAM/CAMKII. In BMDMs, TNFα induced rapid activation of the survival pathways NFκB, AKT and p38MAPK. All these routes were activated in a PI3K-dependent fashion. Activation of AKT and NFκB, but not that of p38MAPK, was abrogated by the CAM inhibitor W7, while KN-62, a CAMKII inhibitor, prevented activation of AKT and p38MAPK but not that of NFκB. Inhibition of CAM or CAMKII completely prevented the protective actions of TNFα. Our observations indicate that in BMDMs CAM and CAMKII have differential contributions to the components of TNFα-dependent survival signaling and underscore a complex interplay among canonical survival routes. These findings set a signaling framework to understand how constitutive Ca(2+) influx couples to macrophage survival in BMDMs.

Molecular mechanisms of treg-mediated T cell suppression

CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4(+)CD25(-) conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca(2+)), NFAT, and NF-κB signaling in Tcons by Tregs.

CaMKII targets Bcl10 in T-cell receptor induced activation of NF-κB

Recognition of antigen by T- or B-cell receptors leads to formation of an immunological synapse and initiation of signalling events that collaborate to determine the nature of the adaptive immune response. Activation of NF-κB transcription factors has a key role in regulation of numerous genes with important functions in immune responses and inflammation and is of great importance for lymphocyte activation and differentiation. The activation of NF-κB depends on changes in intracellular Ca(2+) levels, and both calmodulin (CaM) and a CaM-dependent kinase, CaMKII, help regulate NF-κB activation after T-cell receptor (TCR) stimulation, but the mechanisms are not well characterized. Here we have analyzed the functional role of CaMKII in the signalling pathway from the TCR to activation of IKK, the kinase that phosphorylates the NF-κB inhibitor IκB. We show that CaMKII is recruited to the immunological synapse where it interacts with and phosphorylates the signalling adaptor protein Bcl10. Furthermore, phosphorylation of the CARD domain of Bcl10 by CaMKII regulates the interactions within the important Carma1, Bcl10, Malt1 signalling complex and the essential signal induced ubiquitinations of Bcl10 and IKKγ. We propose a novel mechanism whereby Ca(2+) signals can be integrated at the immunological synapse through CaMKII-dependent phosphorylation of Bcl10.

A cytosolic STIM2 preprotein created by signal peptide inefficiency activates ORAI1 in a store-independent manner

Calcium (Ca(2+)) influx through the plasma membrane store-operated Ca(2+) channel ORAI1 is controlled by Ca(2+) sensors of the stromal interaction molecule (STIM) family. STIM1 responds to endoplasmic reticulum (ER) Ca(2+) store depletion by redistributing and activating ORAI1 from regions of the ER juxtaposed to the plasma membrane. Unlike STIM1, STIM2 can regulate ORAI1 in a store-dependent and store-independent manner, but the mechanism by which this is achieved is unknown. Here we find that STIM2 is translated from a highly conserved methionine residue and is directed to the ER by an incredibly long 101-amino acid signal peptide. We find that although the majority of the total STIM2 population resides on the ER membrane, a second population escapes ER targeting to accumulate as a full-length preprotein in the cytosol, signal peptide intact. Unlike STIM2, preSTIM2 localizes to the inner leaflet of the plasma membrane where it interacts with ORAI1 to regulate basal Ca(2+) concentration and Ca(2+)-dependent gene transcription in a store-independent manner. Furthermore, a third protein comprising a fragment of the STIM2 signal peptide is released from the ER membrane into the cytosol where it regulates gene transcription in a Ca(2+)-independent manner. This study establishes a new model for STIM2-mediated regulation of ORAI1 in which two distinct proteins, STIM2 and preSTIM2, control store-dependent and store-independent modes of ORAI1 activation.

The Ca2+-dependent phosphatase calcineurin controls the formation of the Carma1-Bcl10-Malt1 complex during T cell receptor-induced NF-kappaB activation

T cell receptor (TCR) ligation induces increased diacylglycerol and Ca(2+) levels in T cells, and both secondary messengers are crucial for TCR-induced nuclear factor of activated T cells (NF-AT) and NF-κB signaling pathways. One prominent calcium-dependent enzyme involved in the regulation of NF-AT and NF-κB signaling pathways is the protein phosphatase calcineurin. However, in contrast to NF-AT, which is directly dephosphorylated by calcineurin, the molecular basis of the calcium-calcineurin dependence of the TCR-induced NF-κB activity remains largely unknown. Here, we demonstrate that calcineurin regulates TCR-induced NF-κB activity by controlling the formation of a protein complex composed of Carma1, Bcl10, and Malt1 (CBM complex). For instance, increased calcium levels induced by ionomycin or thapsigargin augmented the phorbol 12-myristate 13-acetate-induced formation of the CBM complex and activation of NF-κB, whereas removal of calcium by the calcium chelator EGTA-acetoxymethyl ester (AM) attenuated both processes. Furthermore, inhibition of the calcium-dependent phosphatase calcineurin with the immunosuppressive agent cyclosporin A (CsA) or FK506 as well as siRNA-mediated knockdown of calcineurin A strongly affected the PMA + ionomycin- or anti-CD3 + CD28-induced CBM complex assembly. Mechanistically, the positive effect of calcineurin on the CBM complex formation seems to be linked to a dephosphorylation of Bcl10. For instance, Bcl10 was found to be hyperphosphorylated in Jurkat T cells upon treatment with CsA or EGTA-AM, and calcineurin dephosphorylated Bcl10 in vivo and in vitro. Furthermore, we show here that calcineurin A interacts with the CBM complex. In summary, the evidence provided here argues for a previously unanticipated role of calcineurin in CBM complex formation as a molecular basis of the inhibitory function of CsA or FK506 on TCR-induced NF-κB activity.