Calcineurin and the heartbeat, an evolving story

Rescue of Heart Failure by Mitochondrial Recovery

Heart failure (HF) is a multifactorial disease brought about by numerous, and oftentimes complex, etiological mechanisms. Although well studied, HF continues to affect millions of people worldwide and current treatments can only prevent further progression of HF. Mitochondria undoubtedly play an important role in the progression of HF, and numerous studies have highlighted mitochondrial components that contribute to HF. This review presents an overview of the role of mitochondrial biogenesis, mitochondrial oxidative stress, and mitochondrial permeability transition pore in HF, discusses ongoing studies that attempt to address the disease through mitochondrial targeting, and provides an insight on how these studies can affect future research on HF treatment.

Mitochondrial calcium homeostasis as potential target for mitochondrial medicine

Mitochondria are crucial in different intracellular pathways of signal transduction. Mitochondria are capable of decoding a variety of extracellular stimuli into markedly different intracellular actions, ranging from energy production to cell death. The fine modulation of mitochondrial calcium (Ca²⁺) homeostasis plays a fundamental role in many of the processes involving this organelle. When mitochondrial Ca²⁺ homeostasis is compromised, different pathological conditions can occur, depending on the cell type involved. Recent data have shed light on the molecular identity of the main proteins involved in the handling of mitochondrial Ca²⁺ traffic, opening fascinating and ambitious new avenues for mitochondria-based pharmacological strategies.

Post-conditioning with cyclosporine A fails to reduce the infarct size in an in vivo porcine model

BACKGROUND

Cyclosporine A has generated intense interest in the field of cardioprotection due to its ability to protect the mitochondria at reperfusion by blocking the opening of the mitochondrial permeability transition pore. The aim of our study was to examine the cardioprotective effect of Sandimmun, a clinically available formulation of cyclosporine A, in an in vivo large mammal model.

METHODS

Forty-eight pigs were randomly allocated to one of three groups: (i) Control group (Con, n=19), (ii) Cyclosporine group, (Cyclo, n=19) Sandimmun 10 mg/kg i.v. bolus 5 min before reperfusion and (iii) Pre-conditioning group (Precon, n=10) two cycles of 10 min ischemia interspersed with 30-min reperfusion. The study was further sub-divided into a metabolic protocol, evaluating myocardial metabolism by measuring changes in the interstitial lactate concentration, and a coronary flow protocol. All animals were subjected to 40 min of left anterior descending coronary artery occlusion, followed by 180 min of reperfusion before histochemical staining and assessment of infarct size by planimetry.

RESULTS

Infarct sizes were measured as: Con 51.4 +/- 16.5%, Cyclo 47.3 +/- 15.7% and Precon 2.4 +/- 3.6%, with no significant difference between the Con and Cyclo groups but a highly significant difference between the Precon and Cyclo and Con groups (P<0.0001 for both comparisons). In the Cyclo group, the interstitial lactate concentration was significantly increased compared with the Con group at 6-min reperfusion, although significantly lower at 14 min presumably due to accelerated washout.

CONCLUSION

In this large animal model, a 10 mg/kg bolus administration of Sandimmun 5 min before reperfusion did not reduce the infarct size.

A pore way to die: the role of mitochondria in reperfusion injury and cardioprotection

In addition to their normal physiological role in ATP production and metabolism, mitochondria exhibit a dark side mediated by the opening of a non-specific pore in the inner mitochondrial membrane. This mitochondrial permeability transition pore (MPTP) causes the mitochondria to breakdown rather than synthesize ATP and, if unrestrained, leads to necrotic cell death. The MPTP is opened in response to Ca2+ overload, especially when accompanied by oxidative stress, elevated phosphate concentration and adenine nucleotide depletion. These conditions are experienced by the heart and brain subjected to reperfusion after a period of ischaemia as may occur during treatment of a myocardial infarction or stroke and during heart surgery. In the present article, I review the properties, regulation and molecular composition of the MPTP. The evidence for the roles of CyP-D (cyclophilin D), the adenine nucleotide translocase and the phosphate carrier are summarized and other potential interactions with outer mitochondrial membrane proteins are discussed. I then review the evidence that MPTP opening mediates cardiac reperfusion injury and that MPTP inhibition is cardioprotective. Inhibition may involve direct pharmacological targeting of the MPTP, such as with cyclosporin A that binds to CyP-D, or indirect inhibition of MPTP opening such as with preconditioning protocols. These invoke complex signalling pathways to reduce oxidative stress and Ca2+ load. MPTP inhibition also protects against congestive heart failure in hypertensive animal models. Thus the MPTP is a very promising pharmacological target for clinical practice, especially once more specific drugs are developed.

The role of the mitochondrial permeability transition pore in heart disease

Like Dr. Jeckyll and Mr. Hyde, mitochondria possess two distinct persona. Under normal physiological conditions they synthesise ATP to meet the energy needs of the beating heart. Here calcium acts as a signal to balance the rate of ATP production with ATP demand. However, when the heart is overloaded with calcium, especially when this is accompanied by oxidative stress, mitochondria embrace their darker side, and induce necrotic cell death of the myocytes. This happens acutely in reperfusion injury and chronically in congestive heart failure. Here calcium overload, adenine nucleotide depletion and oxidative stress combine forces to induce the opening of a non-specific pore in the mitochondrial membrane, known as the mitochondrial permeability transition pore (mPTP). The molecular nature of the mPTP remains controversial but current evidence implicates a matrix protein, cyclophilin-D (CyP-D) and two inner membrane proteins, the adenine nucleotide translocase (ANT) and the phosphate carrier (PiC). Inhibition of mPTP opening can be achieved with inhibitors of each component, but targeting CyP-D with cyclosporin A (CsA) and its non-immunosuppressive analogues is the best described. In animal models, inhibition of mPTP opening by either CsA or genetic ablation of CyP-D provides strong protection from both reperfusion injury and congestive heart failure. This confirms the mPTP as a promising drug target in human cardiovascular disease. Indeed, the first clinical trials have shown CsA treatment improves recovery after treatment of a coronary thrombosis with angioplasty.

Cyclophilin D gene ablation protects mice from ischemic renal injury

Increased oxidative stress and intracellular calcium levels and mitochondrial overloading of calcium during ischemic renal injury (IRI) favor mitochondrial membrane permeability transition pore (MPTP) opening and subsequent necrotic cell death. Cyclophilin D (CypD) is an essential component of MPTP, and recent findings implicate its role in necrotic, but not apoptotic, cell death. To evaluate the role of CypD following IRI, we tested the hypothesis that CypD gene ablation protects mice from IRI. Renal function as assessed by plasma levels of both creatinine and blood urea nitrogen was significantly reduced in CypD knockout (CypD(-/-)) mice compared with wild-type mice during the 5-day post-ischemia period. Erythrocyte trapping, tubular cell necrosis, tubular dilatation, and neutrophil infiltration were significantly decreased in CypD(-/-) mice. To define the mechanisms by which CypD deficiency protect the kidneys, an in vitro model of IRI was employed. Inhibition of CypD using cyclosporin A in oxidant-injured cultured proximal tubular cells (PTC) prevented mitochondrial membrane depolarization, reduced LDH release, ATP depletion and necrotic cell death. Similarly, oxidant-injured CypD(-/-) PTC primary cultures were protected from cytotoxicity and necrosis. To conclude, CypD gene ablation offers both functional and morphological protection in mice following IRI by decreasing necrotic cell death possibly via inhibition of MPTP and ATP depletion.

Recent progress in elucidating the molecular mechanism of the mitochondrial permeability transition pore

The mitochondrial permeability transition pore (MPTP) plays a key role in cell death, especially necrosis, and mediates the injury tissues such as the heart and brain experience following ischaemia and reperfusion. However, the molecular identity of the MPTP remains uncertain. Knockout studies have confirmed a role for cyclophilin-D (CyP-D) in pore opening, probably mediated by its peptidyl-prolyl cis-trans isomerase activity that facilitates a conformational change in an inner membrane protein. However, similar knockout studies have cast doubt on the central role of the adenine nucleotide translocase (ANT), previously regarded as a leading contender for the membrane component that forms the transmembrane channel of the MPTP. Here we review the evidence for and against a role for the ANT in MPTP opening and conclude that it usually plays a regulatory role rather than provide the transmembrane pore component. We suggest that the protein fulfilling the latter role is the mitochondrial phosphate carrier (PiC) and summarise recent evidence in support of this proposal. Our data are consistent with a model for the MPTP in which a calcium-triggered conformational change of the PiC, facilitated by CyP-D, induces pore opening. We propose that this is enhanced by an association of the PiC with the "c" conformation of the ANT. Agents that modulate pore opening may act on either or both the PiC and the ANT.

Structures of calcineurin and its complexes with immunophilins-immunosuppressants

Calcineurin (CN) is a Ca(2+)/calmodulin-dependent serine/threonine protein phosphatase and is involved in many physiological processes such as T-cell activation and cardiac hypertrophy. The crystal structures of CN and its complexes with FKBP12-FK506 and cyclophilin-cyclosporin showed that the two structurally unrelated immunophilins-immunosuppressants bind to a common composite surface made up of the residues from both catalytic subunit and regulatory subunit of CN. The recognition of the immunophilins and immunosuppressive drugs is achieved by common but few distinct CN residues. However, the binding pattern of FKBP12-FK506 such as hydrogen bonding is significantly different from that of CyPA-CsA. This common but distinct recognition may indicate capacity of the composition surface for binding of other inhibitory proteins. The recognition site and the active site are adjacent and form an "L" shaped cleft. This implies that the immunophilin recognition site may also serve as a recognition site to define the narrow substrate specificity of calcineurin.