Calcineurin (PP2B) is a Ca²⁺/calmodulin-dependent serine/threonine phosphatase — the canonical T-cell activation switch and the molecular target of cyclosporine A + tacrolimus. T-cell activation flow: TCR engagement → LCK / ZAP70 / LAT → PLCγ1 → IP3 → ER Ca²⁺ release → STIM1 / STIM2 oligomerization → ORAI1 channel activation at plasma membrane → sustained Ca²⁺ influx (CRAC current, store-operated). Sustained cytoplasmic Ca²⁺ → calmodulin binds + activates calcineurin. Active calcineurin dephosphorylates the regulatory domain of NFAT (NFATc1-c4 isoforms) at multiple Ser sites → unmasks nuclear-localization signal → NFAT nuclear translocation → cooperates with AP-1 (Fos-Jun, PMA-inducible) at composite NFAT-AP-1 sites → drives IL-2, IL-4, IFN-γ, CD40L, FasL, GM-CSF transcription → T-cell proliferation + effector differentiation. Immunosuppressants — direct calcineurin inhibitors (CNIs): cyclosporine binds cyclophilin A → cyclo-CypA complex inhibits calcineurin; tacrolimus binds FKBP12 → tacro-FKBP12 complex inhibits calcineurin (same active site, different immunophilin). Side-effect profile (similar across CNIs): nephrotoxicity (vasoconstriction + tubular toxicity), neurotoxicity, HTN, hyperglycemia (post-transplant diabetes), hyperlipidemia, gingival hyperplasia (cyclosporine > tacrolimus), hirsutism (cyclosporine), alopecia + tremor (tacrolimus). Pimecrolimus is the topical FKBP-calcineurin inhibitor for atopic dermatitis. mTOR inhibitors (rapamycin, everolimus) act downstream of FKBP12 but inhibit mTORC1, NOT calcineurin — different mechanism. JAK inhibitors block downstream cytokine signaling (parallel target). Cross-links: cyp phase1 overview (CYP3A4 metabolism — many DDIs), transporter phase3 overview (P-gp substrates), mtor signaling (parallel arm), insulin glucose homeostasis (post-transplant DM).
Organ Systems
immune-hematologic
cardiovascular
integumentary
Pathway Steps
TCR / CD3 engagement → LCK / ZAP70 / LAT → PLCγ1 — via CD4/CD8 co-receptor for LCK; LAT scaffold. T-cell activation begins when the TCR/CD3 complex recognizes peptide-MHC. The Src-family kinase LCK phosphorylates CD3 ITAMs, recruiting ZAP-70, which builds the LAT signalosome and activates PLCγ1 — the proximal cascade converting antigen recognition into the second messengers that drive the calcium and Ras/MAPK arms.
PLCγ1 → IP3 → ER Ca²⁺ release — via depletes ER Ca²⁺ stores. PLCγ1 cleaves PIP2 into IP3 and DAG. IP3 opens IP3 receptors on the ER to release stored Ca²⁺ — the initial, transient calcium signal. DAG simultaneously launches the parallel PKC/Ras-AP-1 arm that is also required for full activation.
STIM1 / STIM2 (ER sensor) → ORAI1 channel activation (plasma membrane) — via CRAC current — store-operated Ca²⁺ entry; sustained influx. ER Ca²⁺ depletion is sensed by STIM1/STIM2, which oligomerize and gate ORAI1 channels in the plasma membrane — store-operated Ca²⁺ entry (CRAC). This sustained influx, not the brief ER release, maintains the high cytosolic calcium needed downstream; ORAI1/STIM mutations cause immunodeficiency.
↑[Ca²⁺]cyt + calmodulin → calcineurin activation — via Ca²⁺/CaM displaces autoinhibitory domain of calcineurin A subunit. Sustained high cytosolic Ca²⁺ binds calmodulin, which activates the phosphatase calcineurin (PP2B). Calcineurin is unusual as a Ca²⁺/calmodulin-regulated Ser/Thr phosphatase — the node that converts the calcium signal into a dephosphorylation event, and the direct target of the major transplant immunosuppressants.
calcineurin active → NFAT dephosphorylation (multi-site Ser) — via unmasks NLS → nuclear translocation; rapid (minutes). Active calcineurin dephosphorylates NFAT at multiple serines, unmasking a nuclear localization signal so NFAT enters the nucleus. Kinases (GSK-3, CK1) rapidly re-phosphorylate it when calcium falls, so NFAT activity tracks sustained calcium in real time — a built-in coincidence detector.
nuclear NFAT + AP-1 (PMA-induced Fos-Jun) → IL-2 / IL-4 / IFN-γ / CD40L transcription — via cooperative binding at composite NFAT-AP-1 elements → T-cell effector programme. In the nucleus NFAT must partner with AP-1 (Fos/Jun, supplied by the DAG-PKC-Ras arm) to transactivate IL-2, IL-4, IFN-γ, and CD40L. This NFAT/AP-1 requirement enforces that the calcium and Ras arms fire together — productive activation versus the anergy that NFAT alone induces.
cyclosporine + cyclophilin A → calcineurin inhibition (drug action) — via cyclo-CypA complex blocks calcineurin substrate access. Cyclosporine first binds the immunophilin cyclophilin A; the cyclosporine-cyclophilin complex then binds and inhibits calcineurin. Blocking calcineurin prevents NFAT dephosphorylation and IL-2 transcription — the basis of its use in transplantation, and of its dose-limiting nephrotoxicity.
tacrolimus + FKBP12 → calcineurin inhibition (drug action) — via tacro-FKBP12 binds same calcineurin site as cyclo-CypA — different immunophilin. Tacrolimus (FK506) reaches the same target by a parallel route: it binds the immunophilin FKBP12, and the tacrolimus-FKBP12 complex inhibits calcineurin. Two structurally different drugs thus converge on one step — both calcineurin inhibitors — explaining their shared efficacy and overlapping nephro/neurotoxicity.
tacrolimus (inhibitor) — calcineurin (via FKBP12). more potent than CsA; transplant default; P-gp + CYP3A4 substrate
pimecrolimus (inhibitor) — calcineurin (topical, FKBP-binding). atopic dermatitis; weaker than tacrolimus; FDA boxed warning for malignancy
rapamycin (inhibitor) — mTORC1 (parallel arm via same FKBP12 — NOT calcineurin). transplant maintenance; combines with CNI for synergistic + sparing dose