Mitochondrial Calcium in heart failure with preserved ejection fraction—friend or foe?

In this issue of Acta Physiologica, Miranda-Silva et al1 explore mitochondrial and cytosolic calcium (Ca) dynamics during excitation-contraction coupling in a metabolic syndrome–based model of heart failure with preserved ejection fraction (HFpEF). Heart failure with preserved ejection fraction is a disease with significant morbidity and mortality. To date, potential therapeutics have not been shown to confer a survival benefit in patients. A hallmark feature of the disease is the presence of metabolic syndrome with changes in mitochondrial respiration, increased oxidative stress, inflammation and impaired energy consumption.2 Only recently for instance, HFpEF caused by metabolic disease has been associated with altered states of nitric oxide (NO) production and increased inducible NO synthase (iNOS) levels.3 iNOS exposed to reactive oxygen species (ROS) can become uncoupled due to a depletion of tetrahydrobiopterin (TH4) leading to increased O2- production and a switch towards pro-oxidative states. The group could link iNOS to decreased XBP1 levels potentially compromising the degradation of proteins. Increased iNOS and an altered oxidative cellular state directly affect a multitude of proteins involved in Ca cycling, including proteins relevant for mitochondrial Ca homeostasis. Mitochondrial Ca homeostasis is an important centrepiece of metabolic and mechanical regulation in cardiomyocytes. The uptake of Ca into mitochondria alters ATP production and even controls apoptosis. In addition, mitochondria might directly shape the Ca transient on a beat to beat basis and control local cytosolic [Ca] during the cardiac cycle. Ca uptake is maintained due to the activity of the mitochondrial Ca uniporter (MCU), while the mitochondrial sodium/Ca exchanger (mNCX) affects Ca efflux. Interestingly, mitochondrial Ca uptake has shown to profoundly affect Ca signalling during excitation contraction coupling and even partially affect the Ca transient.4 This might also be related to the ATP dependence of sarcoplasmic reticulum (SR) Ca uptake proteins like the SR Ca ATPase (SERCA), and the close spatial relation between the SR and mitochondria in a Ca microdomain.5 Using sophisticated state-of-the art methods (including a mitochondrial-targeted FRET-based Ca sensor, circumventing potential cytosolic Ca signal ‘contamination’), Miranda-Silva et al show that mitochondrial Ca transients in HFpEF rats are of higher amplitude and exhibit changes in rise kinetics as compared to lean, hypertensive animals. Decreased mitochondrial Ca levels leading to decreased ATP production and altered reactive oxygen generation are considered hallmark feature of heart failure with reduced ejection fraction (HFrEF). These changes have been shown to be related to increased levels of cytosolic Na, increased mitochondrial NCX-mediated Ca efflux and an impaired MCU activity. The paper by Miranda-Silva et al for the first time extends the concept of Ca-mediated heart failure–related ‘mitochondriopathy’ towards a model of metabolic syndrome mediated HFpEF. As opposed to findings in HFrEF, the authors show that mitochondrial Ca uptake is augmented which was paralleled by the observation of an increased cytosolic [Ca] and delayed diastolic Ca removal. In addition, in vitro relaxation was delayed as it is expected in a model of HFpEF. This not only indicates profound differences between HFrEF and HFpEF when it comes to mitochondrial remodelling, but also shows potential beneficial, mitochondria-related compensatory mechanisms that are active in HFpEF but not HFrEF. Cytosolic Ca removal mainly relies on the ATP-dependent function of SERCA and decreased SERCA activity has been associated with both HFrEF and HFpEF. Mitochondrial Ca uptake is an important modifier of ATP production via the enhanced oxidation of NAD(P) and the inhibition of Ca efflux could be linked to improved NADH/NAD balanced in the setting of HFrEF.6 In the light of results from other groups, where mitochondrial [Ca] was a function of cytosolic [Ca], the presented data allows to speculate on a potential compensatory mechanism: As proposed by the authors enhanced ATP production might counterbalance a hampered SERCA function. Nevertheless, the authors show that in vitro relaxation was slowed, indicating that the proposed compensation was insufficient to significantly affect cytosolic Ca removal or to overcome changes in SR Ca leak. Even though changes in SR Ca leak are not investigated in the study and the effects of ATP on ryanodine receptors are rather speculative, altered mitochondrial ATP production might increase ryanodine receptor open probability and, by that, counteract positive effects on SERCA function.7 In addition, increased mitochondrial Ca might lead to mitochondria permeability transition (MPT) pore opening with negative effects on cell survival. Indicative of this negative effect of increased mitochondrial Ca and subsequent MPT, the authors show pronounced Ca dependent mitochondrial swelling in HFpEF (Figure 1). Interestingly, the authors also show that mitochondrial Ca uptake was related to MCU activity rather than MCU expression. Considering the high Kd of the MCU and a mitochondrial Ca accumulation independent of the free cytosolic [Ca], the experiments argue for cytosolic Ca microdomains in close vicinity to the mitochondrial Ca uptake machinery as well as potential unknown modifiers of MCU activity. While metabolic dysfunction has been shown to be of great importance for the pathophysiology of HFpEF, the specific role of mitochondrial Ca uptake has not yet been elucidated. The intricacies of mitochondrial remodelling become apparent when data from other groups in models mimicking conditions associated with the development of HFpEF are considered. For example, Luptak et al could show that mitochondrial ATP production is markedly reduced in metabolic heart disease.8 In human HFpEF, a decreased creatine phosphate/ATP ratio was observed signifying energy deficiency as well.9 Several drugs and substances targeting mitochondrial bioenergetics are currently evaluated in clinical trials for patients with HFpEF.10 In the light of these partly mitochondrial-targeted therapies, the understanding of impaired energy metabolism as well as its relation to altered cytosolic Ca appears pivotal. Stimulation of mitochondrial biogenesis and an enhancement of energy supply were deemed possible by activation of nitric oxide–related pathways. However, increased iNOS3 and nitrosative stress had rather malicious effects (in the same metabolic syndrome HFpEF model investigated by Miranda-Silva et al), which allowed to understand why prominent clinical HFpEF trials targeting nitric oxide might have failed to reach their endpoint (eg SOCRATES Preserved). The results presented by Miranda-Silva et al provide novel additional insights into the pathophysiology of HFpEF by elucidating mitochondrial Ca homeostasis in a well-characterized HFpEF model. The authors provide evidence that cytosolic Ca release and diastolic Ca levels are increased and that Ca removal kinetics are altered—changes that appear to be closely linked to mitochondrial Ca homeostasis. While these results underscore the tight coupling between mitochondrial and cytosolic Ca homeostasis, the observed altered oxidative capacity as well as decreased electron transport function also further corroborate an important contribution of mitochondria towards (de)-compensatory mechanisms in HFpEF pathophysiology. No conflict of interest related to this editorial.