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Binding, Immunophilins leads to Inhibition, Calcineurin Activity
Key Event Relationship Overview
AOPs Referencing Relationship
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
The phosphatase activity of calcineurin (CN) is well known to be inhibited by CN inhibitors such as FK506 and cyclosporine A through complex formation with immunophilins.
Immunophilins are a general class of proteins that exhibit peptidyl-propyl isomerase (PPIase) activity, such as FKBP (FK506-binding protein) or cyclophilin (Barik. 2006). FKBP and cyclophilin bind with calcineurin (CN)-inhibitors FK506 and cyclosporin A to form complexes, which inhibit CN activity (Barik. 2006).
While FKBP12, FKBP12.6, FKBP13, and FKBP52 are all part of the FK506-binding FKBP family, FKBP12 has a significant involvement in the mechanism of action for FK506-induced immunosuppression (Siekierka et al. 1989, Kang et al. 2008).
FKBP12 is a 12-kDa protein localized in cytoplasm and has been isolated from Jurkat T-cells as a receptor that binds with the CN inhibitor FK506 (Bram et al. 1993). FKBP12 has an FK506-binding domain (FKBD) that comprises 108 amino acids, and is expressed in T‑cells, B‑cells, Langerhans cells, and mast cells (Siekierka et al. 1990, Panhans-Gross et al. 2001, Hultsch et al. 1991).
Cyclophilin and FKBP both exhibit PPIase activity, but no structural similarities have been found between them. Additionally, while immunophilin complexes formed with either substance do inhibit CN phosphatase activity, the PPlase activity and the inhibition of activity that they indicate are unrelated to CN regulation.
CN is a heterodimer that comprises a catalytic subunit (CnA) and a Ca-binding regulatory subunit (CnB). CnA handles phosphatase activity as well as calmodulin binding, and CnB regulates intracellular calcium and CnA (Klee et al. 1988, Zhang et al. 1996). CnA is a 59kDa protein with a serine-threonine phosphatase domain. A FK506-FKBP complex binds directly to CnA in the cell, causing steric hindrance of substrate binding to CN, which in turn inhibits phosphatase activity of CN (Schreiber and Crabtree 1992, Liu et al. 1993, Bierer et al. 1993, Bram et al. 1993, Rao et al. 1997, Liu et al. 1991). Cyclophilin-Cyclosprine A (CsA) complexes also function in the same manner, binding directly to CnA in the cell, which in turn inhibits CN phosphatase activity.
The nuclear factor of activated T cells (NFAT) is a substrate of calcineurin (Rao et al. 1997). When CN activates through stimulus from outside of the cell, it binds directly to the N‑terminal of NFAT in cytoplasm, after which dephosphorylation of SP motifs exposes nuclear localization signal (NLS) and covers nuclear export signal (NES), thereby promoting nuclear localization of NFAT (Matsuda and Koyasu 2000, Zhu and McKeon 1999). When T-cell activation takes place, T-cell receptor (TCR)-mediated stimulus increases the intracellular concentration of calcium and activates CnB, which subsequently induces CnA phosphatase activation, leading to dephosphorylation of NFAT followed by nuclear localization.
When CN activity is inhibited by the binding of immunophilin complexes, dephosphorylation does not occur in NFAT, thereby interfering with nuclear localization.
Evidence Supporting this KER
The molecular structures and functions of calcineurin and NFAT are evident based on sufficient scientific findings. The well-known mechanisms for inhibition of calcineurin phosphatase activity by calcineurin inhibitors such as FK506 and cyclosporine A is initiated by their complex formations with their respective immunophilin species. Immunophilins are general classes of proteins that exhibit PPlase activity, but modification of these functions are unrelated to inhibition of CN activity and thought to arise in the molecular structure of the complexes (Schreiber and Crabtree 1992, Liu et al. 1993, Bierer et al. 1993, Bram et al. 1993, Rao et al. 1997, Liu et al. 1991).
It is also well known that inhibition of CN phosphatase activity interfere the dephosphorylation of NFAT leading to suppression of its nuclear localization.
Uncertainties and Inconsistencies
CN and NFAT are expressed in T cells and other immune cells including B cells, DC and NKT cells, and cytokine productions from these immune cells and expression of IL-2 receptors (IL-2R) in DCs are lowered due to the inhibition of CN phosphatase activity by CN inhibitor treatment. Among them, reduced production of IL-2 and IL-4 from T cells plays a major role in suppression of TDAR as a result of lowed proliferation, differentiation and class switching of B cells, and there have been no reports showing that CN inhibitor-induced reduction of cytokines other than IL-2 and IL-4 as well as reduced expression of IL-2R resulted in TDAR suppression.
FKBP12, a specific immmunophilin that bind with FK506, is also an accessory molecule that bind to IP3 and Ryanodine receptors, both of which are Ca channel located on the membrane of endoplasmic reticulum and participating in the regulation of intracellular Ca concentration. When binding with FK506, FKBP12 leaves from these receptors to increase the influx of Ca2+ from the endoplasmic reticulum to cytoplasm, which is expected to increase CN activity; however, FK506 treatment suppresses NFAT nuclear localization. In addition, FKBP12-knock out mice show no changes in immune functions including T cell functions. These facts suggest that inhibition of CN-NFAT system induced by FK506 treatment result from direct inhibition of CN phosphatase activity by FK506-FKBP12 complex and not by affecting Ryanodine and IP3 receptors associated with FKBP12.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
FKBP is found in a wide variety of organisms, from prokaryotes to multicellular organisms (Siekierka et al. 1989). Multiple subfamilies of FKBP have been reported, with at least eight types having been found in mammals. FKBP12 is reported to be expressed in B-cells, Langerhans cells, and mast cells as well as in T-cells of humans, mice, and other mammalian species.
Cyclophilins have been found in mammals, plants, insects, fungi, and bacteria. They are structurally conserved throughout evolution and all have PPIase activity (Wang P et al. 2005).
CN is broadly distributed throughout the body, including T- and B-cells, and the structure of CnA and CnB is highly conserved from yeasts to humans. Also highly conserved are the amino acid sequences of the catalytic and regulatory domains of CnA isoforms from different organisms (Kincaid. 1993).
NFAT expresses in B cells, mast cells, neutrophil granulocytes, dendritic cells, macrophages, and natural killer cells as well as T cells from humans, rodents, and other mammalian species (Rao et al. 1997).
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