Cardiac defects and altered ryanodine receptor function in mice lacking FKBP12

Termination of Ca2+ release by a local inactivation of ryanodine receptors in cardiac myocytes

In heart, a robust regulatory mechanism is required to counteract the regenerative Ca2+-induced Ca2+ release from the sarcoplasmic reticulum. Several mechanisms, including inactivation, adaptation, and stochastic closing of ryanodine receptors (RyRs) have been proposed, but no conclusive evidence has yet been provided. We probed the termination process of Ca2+ release by using a technique of imaging local Ca2+ release, or "Ca2+ spikes", at subcellular sites; and we tracked the kinetics of Ca2+ release triggered by L-type Ca2+ channels. At 0 mV, Ca2+ release occurred and terminated within 40 ms after the onset of clamp pulses (0 mV). Increasing the open-duration and promoting the reopenings of Ca2+ channels with the Ca2+ channel agonist, FPL64176, did not prolong or trigger secondary Ca2+ spikes, even though two-thirds of the sarcoplasmic reticulum Ca2+ remained available for release. Latency of Ca2+ spikes coincided with the first openings but not with the reopenings of L-type Ca2+ channels. After an initial maximal release, even a multi-fold increase in unitary Ca2+ current induced by a hyperpolarization to -120 mV failed to trigger additional release, indicating absolute refractoriness of RyRs. When the release was submaximal (e.g., at +30 mV), tail currents did activate additional Ca2+ spikes; confocal images revealed that they originated from RyRs unfired during depolarization. These results indicate that Ca2+ release is terminated primarily by a highly localized, use-dependent inactivation of RyRs but not by the stochastic closing or adaptation of RyRs in intact ventricular myocytes.

Independent inhibition of calcineurin and K+ currents by the immunosuppressant FK-506 in rat ventricle

FK-506 increases the cytosolic Ca2+ concentration transient in rat ventricular myocytes by prolonging the action potential through inhibition of the K+ currents Ito and IK [J. Physiol. (Lond.) 501: 509-516, 1997]. Physiological and biochemical techniques were used in parallel to examine the electrophysiological mechanisms and the role of calcineurin inhibition in these effects. FK-506 prolonged the recovery of Ito from inactivation. Thus Ito inhibition was frequency dependent, with no decrease at 0.2 Hz (recorded at +50 mV from -70 mV) but a 40% decrease at 2.0 Hz. In contrast, inhibition of IK ( approximately 60%) was time and voltage independent. At 25 microM, FK-506 (by 65%) and cyclosporin A (by 57%) inhibited calcineurin activity in myocyte extracts. However, only FK-506 increased the cytosolic Ca2+ concentration transient in field-stimulated myocytes. Furthermore, FK-506 was still active on K+ currents when cells were dialyzed with 10 mM EGTA. These results demonstrate that calcineurin inhibition is not responsible for the functional effects of FK-506 in heart and suggest that IK and Ito are modulated by FK-506-binding proteins or directly by FK-506.

Cardiac-specific overexpression of mouse cardiac calsequestrin is associated with depressed cardiovascular function and hypertrophy in transgenic mice

Calsequestrin is a high capacity Ca2+-binding protein in the sarcoplasmic reticulum (SR) lumen. To elucidate the functional role of calsequestrin in vivo, transgenic mice were generated that overexpressed mouse cardiac calsequestrin in the heart. Overexpression (20-fold) of calsequestrin was associated with cardiac hypertrophy and induction of a fetal gene expression program. Isolated transgenic cardiomyocytes exhibited diminished shortening fraction (46%), shortening rate (60%), and relengthening rate (60%). The Ca2+ transient amplitude was also depressed (45%), although the SR Ca2+ storage capacity was augmented, as suggested by caffeine application studies. These alterations were associated with a decrease in L-type Ca2+ current density and prolongation of this channel's inactivation kinetics without changes in Na+-Ca2+ exchanger current density. Furthermore, there were increases in protein levels of SR Ca2+-ATPase, phospholamban, and calreticulin and decreases in FKBP12, without alterations in ryanodine receptor, junctin, and triadin levels in transgenic hearts. Left ventricular function analysis in Langendorff perfused hearts and closed-chest anesthetized mice also indicated depressed rates of contraction and relaxation of transgenic hearts. These findings suggest that calsequestrin overexpression is associated with increases in SR Ca2+ capacity, but decreases in Ca2+-induced SR Ca2+ release, leading to depressed contractility in the mammalian heart.

FKBP12 modulates gating of the ryanodine receptor/calcium release channel

Excitation-contraction (EC) coupling in muscle requires the activation of intracellular calcium release channels (CRC). Four type 1 ryanodine receptor (RyR1) molecules form each tetrameric CRC. Each RyR1 contains a binding site for the FK506 binding protein (FKBP12), a cis-trans peptidyl-prolyl isomerase that is required for coordinated gating of the four RyR1 subunits comprising the channel. When FKBP12 is bound to RyR1, it stabilizes the four subunits that form each CRC. We propose that binding of one FKBP12 to each RyR1 lowers the energy of twisted-amide peptidyl-prolyl bonds and stabilizes RyR1 in a conformation that permits coordinated gating of the four RyR1 subunits.

Redesigning an FKBP-ligand interface to generate chemical dimerizers with novel specificity

FKBP ligand homodimers can be used to activate signaling events inside cells and animals that have been engineered to express fusions between appropriate signaling domains and FKBP. However, use of these dimerizers in vivo is potentially limited by ligand binding to endogenous FKBP. We have designed ligands that bind specifically to a mutated FKBP over the wild-type protein by remodeling an FKBP-ligand interface to introduce a specificity binding pocket. A compound bearing an ethyl substituent in place of a carbonyl group exhibited sub-nanomolar affinity and 1,000-fold selectivity for a mutant FKBP with a compensating truncation of a phenylalanine residue. Structural and functional analysis of the new pocket showed that recognition is surprisingly relaxed, with the modified ligand only partially filling the engineered cavity. We incorporated the specificity pocket into a fusion protein containing FKBP and the intracellular domain of the Fas receptor. Cells expressing this modified chimeric protein potently underwent apoptosis in response to AP1903, a homodimer of the modified ligand, both in culture and when implanted into mice. Remodeled dimerizers such as AP1903 are ideal reagents for controlling the activities of cells that have been modified by gene therapy procedures, without interference from endogenous FKBP.

An Arabidopsis immunophilin, AtFKBP12, binds to AtFIP37 (FKBP interacting protein) in an interaction that is disrupted by FK506

AtFKBP12 is an Arabidopsis cDNA that encodes a protein similar to the mammalian immunophilin, FKBP12. AtFKBP12 was used as 'bait' in a yeast 2-hybrid system to screen for cDNAs in Arabidopsis encoding proteins that bind to FKBP12. Two partial cDNAs were recovered encoding the C-terminus of a protein we have called Arabidopsis thaliana FKBP12 interacting protein 37 (AtFIP37). AtFIP37 is similar to a mammalian protein, FAP48, that also binds to FKBP12. The interaction between AtFKBP12 and AtFIP37 in the 2-hybrid system, as assessed by histidine auxotrophy and beta-galactosidase activity, was disrupted by FK506, but not by cyclosporin A, a drug that binds to cyclophilin A. AtFIP37 was also shown to bind in vitro to AtFKBP12 in GST-fusion protein binding assays. The binding was abolished by prior incubation of AtFKBP12 with FK506. These findings indicate that an Arabidopsis FKBP12 ortholog encodes a protein that binds FK506 and that the interaction between AtFKBP12 and AtFIP37 may involve the FK506 binding site of AtFKBP12. The interaction provides interesting new opportunities for controlling protein:protein interactions in vivo in plants.

Coupled gating between individual skeletal muscle Ca2+ release channels (ryanodine receptors)

Excitation-contraction coupling in skeletal muscle requires the release of intracellular calcium ions (Ca2+) through ryanodine receptor (RyR1) channels in the sarcoplasmic reticulum. Half of the RyR1 channels are activated by voltage-dependent Ca2+ channels in the plasma membrane. In planar lipid bilayers, RyR1 channels exhibited simultaneous openings and closings, termed "coupled gating." Addition of the channel accessory protein FKBP12 induced coupled gating, and removal of FKBP12 uncoupled channels. Coupled gating provides a mechanism by which RyR1 channels that are not associated with voltage-dependent Ca2+ channels can be regulated.

TGF-beta-signaling with small molecule FKBP12 antagonists that bind myristoylated FKBP12-TGF-beta type I receptor fusion proteins

BACKGROUND

Growth arrest in many cell types is triggered by transforming growth factor beta (TGF-beta), which signals through two TGF-beta receptors (type I, TGF-beta RI, and type II, TGF-beta). In the signaling pathway, TGF-beta binds to the extracellular domain of TGF-betaRII, which can then transphosphorylate TGF-betaRI in its glycine/serine (GS)-rich box. Activated TGF-betaRI phosphorylates two downstream effectors, Smad2 and Smad3, leading to their translocation into the nucleus. Cell growth is arrested and plasminogen activator inhibitor 1 (PAI-1) is upregulated. We investigated the role of the immunophilin FKBP12, which can bind to the GS box of TGF-betaRI, in TGF-beta signaling.

RESULTS

Overexpression of myristoylated TGF-betaRI and TGF-betaRII cytoplasmic tails caused constitutive nuclear translocation of a green-fluorescent-protein-Smad2 construct in COS-1 cells, and constitutive activation of a PAI-1 reporter plasmid in mink lung cells. Fusing FKBP12 to TGF-betaRI resulted in repression of autosignaling that could be alleviated by FK506M or rapamycin (two small molecules that can bind to FKBP12). Mutation of the FKBP12-binding site in the FKBP1-TGF-betaRI fusion protein restored constitutive signaling. An acidic mutation in the FKBP12-TGF-betaRI protein allowed FKBP12 antagonists to activate signaling in the absence of TGF-betaRII. Further mutations in the TGF-betaRI FKBP12-binding site resulted in TGF-beta signaling that was independent of both TGF-betaRII and FKBP12 antagonists.

CONCLUSIONS

Fusing FKBP12 to TGF-betaRI results in a novel receptor that is activated by small molecule FKBP12 antagonists. These results suggest that FKBP12 binding to TGF-betaRI is inhibitory and that FKBP12 plays a role in inhibiting TGF-beta superfamily signals.