Monster Protein Controls Calcium Entry and Fights Infection
2008; Cell Press; Volume: 28; Issue: 1 Linguagem: Inglês
10.1016/j.immuni.2007.12.009
ISSN1097-4180
AutoresAmy N. Radermacher, Gerald R. Crabtree,
Tópico(s)Ion Channels and Receptors
ResumoSustained calcium signaling in T cells is critical for development and activation. In this issue of Immunity, Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar demonstrate that the huge scaffold protein, AHNAK1, interacts with L-type calcium channels, regulates Ca2+ influx, and defends against Leishmania major infection. Sustained calcium signaling in T cells is critical for development and activation. In this issue of Immunity, Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar demonstrate that the huge scaffold protein, AHNAK1, interacts with L-type calcium channels, regulates Ca2+ influx, and defends against Leishmania major infection. Calcium ions (Ca2+) are essential second messengers. Their widespread roles in biology are paralleled within the immune system. Engagement of several different tyrosine kinase and nontyrosine kinase receptors stimulate Ca2+ influx in many immune cells including macrophages, T cells, B cells, natural killer cells, mast cells, and dendritic cells. In T cells, antigen engagement of the T cell receptor (TCR) initiates an influx of Ca2+. This activates calcineurin to dephosphorylate NFATc transcription factors. Subsequent nuclear localization and formation of NFATc complexes turn on and off genes required for T cell development, activation, cytokine production, and anergy (Wu et al., 2007Wu H. Peisley A. Graef I.A. Crabtree G.R. Trends Cell Biol. 2007; 17: 251-260Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). The primary channel machinery used to maintain increased intracellular Ca2+ concentrations in T cells is the calcium release-activated channel (CRAC). TCR triggering increases intracellular inositol 1,4,5 phosphate (InsP3), opening InsP3 receptors in the endoplasmic reticulum (ER). This transient Ca2+ release opens CRAC channels in the plasma membrane and results in a sustained Ca2+ rise. Recent studies identified STIM1 as the mammalian ER Ca2+ sensor and Orai1 (also known as CRACM1) as an element of the mammalian CRAC channel. In resting T cells, both Orai1 and STIM1 are distributed primarily throughout the plasma membrane and ER, respectively. Upon store depletion, Orai1 and STIM1 redistribute to sites where the ER and plasma membrane are in close proximity and STIM1 activates Orai1 by a yet unknown mechanism (Feske et al., 2006Feske S. Gwack Y. Prakriya M. Srikanth S. Puppel S.H. Tanasa B. Hogan P.G. Lewis R.S. Daly M. Rao A. Nature. 2006; 441: 179-185Crossref PubMed Scopus (1703) Google Scholar, Liou et al., 2005Liou J. Kim M.L. Heo W.D. Jones J.T. Myers J.W. Ferrell Jr., J.E. Meyer T. Curr. Biol. 2005; 15: 1235-1241Abstract Full Text Full Text PDF PubMed Scopus (1626) Google Scholar, Vig et al., 2006Vig M. Peinelt C. Beck A. Koomoa D.L. Rabah D. Koblan-Huberson M. Kraft S. Turner H. Fleig A. Penner R. Kinet J.P. Science. 2006; 312: 1220-1223Crossref PubMed Scopus (1076) Google Scholar). In addition to the CRAC channel, T cells express among others the L-type voltage-gated Ca2+ (Cav1) channels. These channels are composed of transmembrane α1, δ, and γ subunits and the extracellular α2 and intracellular β subunits. Within Cav1, one of four possible α1 subunits (Cav1.1 to Cav1.4) can associate with one of four possible β subunits (β1 to β4). The β subunit is required for proper membrane insertion of the α1 subunit. After insertion, β becomes an allosteric modulator of Cav1. Typically connected with activation in excitable cells such as neurons and cardiomyocytes, these voltage-gated channels are surprisingly expressed in T cells (Badou et al., 2006Badou A. Jha M.K. Matza D. Mehal W.Z. Freichel M. Flockerzi V. Flavell R.A. Proc. Natl. Acad. Sci. USA. 2006; 103: 15529-15534Crossref PubMed Scopus (90) Google Scholar). Analysis of β3 null and β4 mutant mice (Badou et al., 2006Badou A. Jha M.K. Matza D. Mehal W.Z. Freichel M. Flockerzi V. Flavell R.A. Proc. Natl. Acad. Sci. USA. 2006; 103: 15529-15534Crossref PubMed Scopus (90) Google Scholar) show that both the initial and sustained rise in Ca2+ are attenuated after TCR triggering, implicating Cav1 in the regulation of intracellular Ca2+ concentrations after T cell activation. Why are these voltage-gated channels expressed in nonexcitable T cells? The authors find that KCl-induced depolarization does not open Cav1. Therefore, during T cell activation, Cav1 appear to open by a mechanism different than that in excitable cells. In this issue of Immunity, Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar propose that the giant scaffold protein AHNAK1 controls plasma membrane localization of Cav1 channels. AHNAK1 was originally identified as a tumor-related protein involved in proliferation. It is located primarily in the nucleus, but is also found in the cytoplasm and at the plasma membrane (Haase, 2007Haase H. Cardiovasc. Res. 2007; 73: 19-25Crossref PubMed Scopus (52) Google Scholar). Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar examined the role of AHNAK1 in T cells by analyzing Ahnak1−/− mice. In wild-type mice, AHNAK1 is expressed strongly in CD4+ T cells and weakly in thymocytes. Thymocyte development, splenic lymphoid composition, and memory T cell distribution appeared to be normal in the Ahnak1−/− mice. After activation, null CD4+ T cells show substantially reduced proliferation and IL-2 production. The proliferation defect is in part caused by the failure to make IL-2 because addition of IL-2 to T cells partially overcame this defect. To investigate the defective IL-2 production by Ahnak1−/− mice, the authors examined NFAT localization in null T cells after stimulation and found a marked reduction in NFATc1 and c2 nuclear localization because of attenuated Ca2+ signaling. Both the initial rise and plateau of intracellular Ca2+ were reduced. This suggested that the Ca2+ signaling defect caused the proliferation defect. Indeed, Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar were able to rescue proliferation in mice with ionomycin, which artificially drains intracellular stores and stimulates Ca2+ influx by a mechanism that resembles the physiologic mechanism of Ca2+ entry. Previously, AHNAK1 had been reported to interact with the Cav1 β subunit in other cells types (Hohaus et al., 2002Hohaus A. Person V. Behlke J. Schaper J. Morano I. Haase H. FASEB J. 2002; 16: 1205-1216Crossref PubMed Scopus (103) Google Scholar). Along these lines, Matza et al., 2007Matza D. Badou A. Kobayashi K.S. Goldsmith-Pestana K. Masuda Y. Komuro A. McMahon-Pratt D. Marchesi V.T. Flavell R.A. Immunity. 2008; 28 (this issue): 64-74Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar found what appears to be a robust interaction between endogenous AHNAK1 and endogenous Cav1 β2 (but not β3) in T cells. Direct interaction with β1 and β4 was not tested because of a lack of available antibodies. AHNAK1 may interact with β4 because this same group has previously shown that β3 null;β4 mutant mice displayed a similar attenuated Ca2+ phenotype (Badou et al., 2006Badou A. Jha M.K. Matza D. Mehal W.Z. Freichel M. Flockerzi V. Flavell R.A. Proc. Natl. Acad. Sci. USA. 2006; 103: 15529-15534Crossref PubMed Scopus (90) Google Scholar). The potential effect of this interaction was investigated by analyzing expression and cellular distribution of Cav1 in Ahnak1−/− mice. Protein expression of Cav1.1 α1 and Cav1.2 α1 increased after stimulation in wild-type, but not null, T cells. Surprisingly, mRNA expression was upregulated similarly in null and wild-type T cells. Therefore, AHNAK1 must influence post-translational processing of α1 subunits. Consistent with this, activated null T cells had 50% less Cav1.1 α1 protein at their plasma membrane than wild-type T cells, but similar amounts of intracellular protein. Hence, the authors propose that AHNAK1 is required for intact membrane expression of Cav1 complexes (Figure 1). Although supported by the observation that naive Ahnak1−/− T cells have 80% less Cav1.1 α1 protein at their plasma membrane, a role for AHNAK1 in other post-translational processing events is also possible. The expression of Cav1 α1 subunits was analyzed only after stimulation at 0 and 48 hr. A time course would yield much more information; especially because the attenuated Ca2+ signaling profile occurs on a shorter time scale, minutes to hours after activation. To understand the role of AHNAK1 in immune responses in vivo, the authors examined the response of the Ahnak1−/− mice to the Leishmania major parasite. Mice that mount a T helper 1 (Th1) response to L. major usually heal, whereas mice that mount a Th2 response usually die (Locksley et al., 1987Locksley R.M. Heinzel F.P. Sadick M.D. Holaday B.J. Gardner Jr., K.D. Ann. Inst. Pasteur (Paris). 1987; 138: 744-749Crossref Scopus (165) Google Scholar). The authors found that intradermal foot infection was not cleared well by null mice. As expected, IFN-γ production was severely reduced in the Ahnak1−/− mice. Unexpectedly, the null mice produced substantially higher amounts of IL-4. This is surprising because in vitro T cell activation in null mice was defective. It is generally thought that calcineurin-NFAT signaling is upstream of the decision to make both Th1 and Th2 cells because the activation of genes such as T-bet and production of both cell types is blocked by cyclosporine A (Wu et al., 2007Wu H. Peisley A. Graef I.A. Crabtree G.R. Trends Cell Biol. 2007; 17: 251-260Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). Thus, Ahnak1−/− mice would be expected to respond poorly to all activation signals. Instead, it appears that AHNAK1 may have a role in T helper differentiation. It would be interesting to analyze proliferation of CD4+ T cells after infection. Additional analysis of other in vivo activation and infection models would yield insight into the physiological role of AHNAK1. More broadly, it is surprising that development in Ahnak1−/− mice is normal, because calcineurin-NFAT signaling is essential for the activation of Erk kinase and subsequent positive selection in the thymus as well as the development of many other mammalian organs and cell types (Gallo et al., 2007Gallo E.M. Winslow M.M. Cante-Barrett K. Radermacher A.N. Ho L. McGinnis L. Iritani B. Neilson J.R. Crabtree G.R. Nature. 2007; 450: 731-735Crossref PubMed Scopus (45) Google Scholar, Wu et al., 2007Wu H. Peisley A. Graef I.A. Crabtree G.R. Trends Cell Biol. 2007; 17: 251-260Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). Perhaps this is because the product of a second AHNAK-like gene is expressed in these cell types. If AHNAK is really a component of a broadly acting developmental pathway upstream of calcineurin-NFAT, one would expect double knockout mice to have multiple development defects in organogenesis as well as a complete block in T cell development similar to the calcineurin and NFATc null mice. This study also raises important mechanistic questions: what component of total Ca2+ influx is dependent on this pathway? How does AHNAK1 regulate Cav1 channels? Because AHNAK1 is found in the nucleus and cytoplasm in addition to near the membrane, it could be responsible for the proper processing of Cav1 rather than directly involved in Ca2+ conductance. Does this Ca2+ influx pathway synergize with better characterized mechanisms? The production of mutant mice and the characterization of their defects should provide the essential reagents to answer these questions. A Scaffold Protein, AHNAK1, Is Required for Calcium Signaling during T Cell ActivationMatza et al.ImmunityJanuary 18, 2008In BriefEngagement of the T cell antigen receptor (TCR) during antigen presentation initiates a coordinated action of a large number of signaling proteins and ion channels. AHNAK1 is a scaffold protein, highly expressed by CD4+ T cells, and is a critical component for calcium signaling. We showed that AHNAK1-deficient mice were highly susceptible to Leishmania major infection. AHNAK1-deficient CD4+ T cells responded poorly to TCR stimulation in vitro with low proliferation and low Interleukin-2 production. Full-Text PDF Open Archive
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