Identification and optimization of a novel inhibitor of mitochondrial calpain 10

Modeling the Effects of Calcium Overload on Mitochondrial Ultrastructural Remodeling

Mitochondrial cristae are dynamic invaginations of the inner membrane and play a key role in its metabolic capacity to produce ATP. Structural alterations caused by either genetic abnormalities or detrimental environmental factors impede mitochondrial metabolic fluxes and lead to a decrease in their ability to meet metabolic energy requirements. While some of the key proteins associated with mitochondrial cristae are known, very little is known about how the inner membrane dynamics are involved in energy metabolism. In this study, we present a computational strategy to understand how cristae are formed using a phase-based separation approach of both the inner membrane space and matrix space, which are explicitly modeled using the Cahn–Hilliard equation. We show that cristae are formed as a consequence of minimizing an energy function associated with phase interactions which are subject to geometric boundary constraints. We then extended the model to explore how the presence of calcium phosphate granules, entities that form in calcium overload conditions, exert a devastating inner membrane remodeling response that reduces the capacity for mitochondria to produce ATP. This modeling approach can be extended to include arbitrary geometrical constraints, the spatial heterogeneity of enzymes, and electrostatic effects to mechanize the impact of ultrastructural changes on energy metabolism.

Calcium phosphate precipitation inhibits mitochondrial energy metabolism

Early studies have shown that moderate levels of calcium overload can cause lower oxidative phosphorylation rates. However, the mechanistic interpretations of these findings were inadequate. And while the effect of excessive calcium overload on mitochondrial function is well appreciated, there has been little to no reports on the consequences of low to moderate calcium overload. To resolve this inadequacy, mitochondrial function from guinea pig hearts was quantified using several well-established methods including high-resolution respirometry and spectrofluorimetry and analyzed using mathematical modeling. We measured key mitochondrial variables such as respiration, mitochondrial membrane potential, buffer calcium, and substrate effects for a range of mitochondrial calcium loads from near zero to levels approaching mitochondrial permeability transition. In addition, we developed a computer model closely mimicking the experimental conditions and used this model to design experiments capable of eliminating many hypotheses generated from the data analysis. We subsequently performed those experiments and determined why mitochondrial ADP-stimulated respiration is significantly lowered during calcium overload. We found that when calcium phosphate levels, not matrix free calcium, reached sufficient levels, complex I activity is inhibited, and the rate of ATP synthesis is reduced. Our findings suggest that calcium phosphate granules form physical barriers that isolate complex I from NADH, disrupt complex I activity, or destabilize cristae and inhibit NADH-dependent respiration.

Mitochondrial calcium handling has been studied for nearly half a century. As we understand it today, low concentrations (1–10 nmol/mg mitochondria) of calcium exert beneficial effects on energy transduction. And high concentrations (>500 nmol/mg mitochondria) lead to respiratory uncoupling and membrane damage. But relatively little is known about the effect of moderate calcium concentrations (10–500 nmol/mg mitochondria) on mitochondrial function. At these concentrations, mitochondrial membrane integrity remains intact and energized, while ATP synthesis becomes significantly impaired. Prior studies have postulated several possible mechanisms, but the precise consequence of calcium overload on mitochondrial ATP production remained obscure. In this study, we combine experimental and computational approaches to show that calcium phosphate precipitation, as opposed to matrix free calcium, inhibits respiratory function at complex I just enough to limit proton pumping during oxidative phosphorylation and decrease ATP synthesis rates.

Molecular dynamic simulations of the catalytic subunit of calpains 1, 2, 5, and 10: Structural analysis with an aim toward drug design

Calpains are cysteine proteases involved in the development of several human chronic illnesses such as neurodegenerative diseases, cardiovascular ailments, diabetes, and obesity which constitutes them into possible therapeutic targets. Here, using molecular dynamic simulations and docking, we studied the binding of known inhibitors to representative members of classical and nonclassical calpains. Our aim is to gain better understanding on the inhibition mechanism of calpains and to develop better and more specific inhibitors. Our atomistic models confirmed the importance of calcium ions for the structure of calpains and, as a consequence, their functionality. With these models and their subsequent use in molecular docking, essential structural requirements were identified for the binding of ligands to the calpain catalytic site that provide useful information for the design of new selective calpain inhibitors.

Calpain-10 Activity Underlies Angiotensin II-Induced Aldosterone Production in an Adrenal Glomerulosa Cell Model

Aldosterone is a steroid hormone important in the regulation of blood pressure. Aberrant production of aldosterone results in the development and progression of diseases including hypertension and congestive heart failure; therefore, a complete understanding of aldosterone production is important for developing more effective treatments. Angiotensin II (AngII) regulates steroidogenesis, in part through its ability to increase intracellular calcium levels. Calcium can activate calpains, proteases classified as typical or atypical based on the presence or absence of penta-EF-hands, which are involved in various cellular responses. We hypothesized that calpain, in particular calpain-10, is activated by AngII in adrenal glomerulosa cells and underlies aldosterone production. Our studies showed that pan-calpain inhibitors reduced AngII-induced aldosterone production in 2 adrenal glomerulosa cell models, primary bovine zona glomerulosa and human adrenocortical carcinoma (HAC15) cells, as well as CYP11B2 expression in the HAC15 cells. Although AngII induced calpain activation in these cells, typical calpain inhibitors had no effect on AngII-elicited aldosterone production, suggesting a lack of involvement of classical calpains in this process. However, an inhibitor of the atypical calpain, calpain-10, decreased AngII-induced aldosterone production. Consistent with this result, small interfering RNA (siRNA)-mediated knockdown of calpain-10 inhibited aldosterone production and CYP11B2 expression, whereas adenovirus-mediated overexpression of calpain-10 resulted in increased AngII-induced aldosterone production. Our results indicate that AngII-induced activation of calpain-10 in glomerulosa cells underlies aldosterone production and identify calpain-10 or its downstream pathways as potential targets for the development of drug therapies for the treatment of hypertension.

Calpain 10 homology modeling with CYGAK and increased lipophilicity leads to greater potency and efficacy in cells

Calpain 10 is a ubiquitously expressed mitochondrial and cytosolic Ca(2+)-regulated cysteine protease in which overexpression or knockdown leads to mitochondrial dysfunction and cell death. We previously identified a potent and specific calpain 10 peptide inhibitor (CYGAK), but it was not efficacious in cells. Therefore, we created a homology model using the calpain 10 amino acid sequence and calpain 1 3-D structure and docked CYGAK in the active site. Using this model we modified the inhibitor to improve potency 2-fold (CYGAbuK). To increase cellular efficacy, we created CYGAK-S-phenyl-oleic acid heterodimers. Using renal mitochondrial matrix CYGAK, CYGAK-OC, and CYGAK-ON had IC(50)'s of 70, 90, and 875 nM, respectively. Using isolated whole renal mitochondria CYGAK, CYGAK-OC, and CYGAK-ON had IC(50)'s of 95, 196, and >10,000 nM, respectively. Using renal proximal tubular cells (RPTC) in primary culture, 30 min exposures to CYGAK-OC and CYGAbuK-OC decreased cellular calpain activity approximately 20% at 1 μM, and concentrations up to 100 μM had no additional effect. RPTC treated with 10 μM CYGAK-OC for 24 h induced accumulation of ATP synthase β and NDUFB8, two calpain 10 substrates. In summary, we used molecular modeling to improve the potency of CYGAK, while creating CYGAK-oleic acid heterodimers to improve efficacy in cells. Since calpain 10 has been implicated in type 2 diabetes and renal aging, the use of this inhibitor may contribute to elucidating the role of calpain 10 in these and other diseases.

Calpains, mitochondria, and apoptosis

Mitochondrial activity is critical for efficient function of the cardiovascular system. In response to cardiovascular injury, mitochondrial dysfunction occurs and can lead to apoptosis and necrosis. Calpains are a 15-member family of Ca(2+)-activated cysteine proteases localized to the cytosol and mitochondria, and several have been shown to regulate apoptosis and necrosis. For example, in endothelial cells, Ca(2+) overload causes mitochondrial calpain 1 cleavage of the Na(+)/Ca(2+) exchanger leading to mitochondrial Ca(2+) accumulation. Also, activated calpain 1 cleaves Bid, inducing cytochrome c release and apoptosis. In renal cells, calpains 1 and 2 promote apoptosis and necrosis by cleaving cytoskeletal proteins, which increases plasma membrane permeability and cleavage of caspases. Calpain 10 cleaves electron transport chain proteins, causing decreased mitochondrial respiration and excessive activation, or inhibition of calpain 10 activity induces mitochondrial dysfunction and apoptosis. In cardiomyocytes, calpain 1 activates caspase 3 and poly-ADP ribose polymerase during tumour necrosis factor-α-induced apoptosis, and calpain 1 cleaves apoptosis-inducing factor after Ca(2+) overload. Many of these observations have been elucidated with calpain inhibitors, but most calpain inhibitors are not specific for calpains or a specific calpain family member, creating more questions. The following review will discuss how calpains affect mitochondrial function and apoptosis within the cardiovascular system.

Mitochondrial calpain 10 is degraded by Lon protease after oxidant injury

Calpain 10 is ubiquitously expressed and is one of four mitochondrial matrix proteases. We determined that over-expression or knock-down of mitochondrial calpain 10 results in cell death, demonstrating that mitochondrial calpain 10 is required for viability. Thus, we studied calpain 10 degradation in isolated mitochondrial matrix, mitochondria and in renal proximal tubular cells (RPTC) under control and toxic conditions. Using isolated renal cortical mitochondria and mitochondrial matrix, calpain 10 underwent rapid degradation at 37°C that was blocked with Lon inhibitors but not by calpain or proteasome inhibitors. While exogenous Ca(2+) addition, Ca(2+) chelation or exogenous ATP addition had no effect on calpain 10 degradation, the oxidants tert-butyl hydroperoxide (TBHP) or H(2)O(2) increased the rate of degradation. Using RPTC, mitochondrial and cytosolic calpain 10 increased in the presence of MG132 (Lon/proteasome inhibitor) but only cytosolic calpain 10 increased in the presence of epoxomicin (proteasome inhibitor). Furthermore, TBHP and H(2)O(2) oxidized mitochondrial calpain 10, decreased mitochondrial, but not cytosolic calpain 10, and pretreatment with MG132 blocked TBHP-induced degradation of calpain 10. In summary, mitochondrial calpain 10 is selectively degraded by Lon protease under basal conditions and is enhanced under and oxidizing conditions, while cytosolic calpain 10 is degraded by the proteasome.

Activation of mitochondrial μ-calpain increases AIF cleavage in cardiac mitochondria during ischemia-reperfusion

Ubiquitous calpains (calpain I and II) are generally recognized as cytosolic proteins. Recently, mitochondrial localized calpain I (μ-calpain) has been identified. Activation of mito-μ-calpain cleaves apoptosis inducing factor (AIF), a flavoprotein located within the mitochondrial intermembrane space, in liver mitochondria, but not in brain mitochondria. We first tested if activation of mito-μ-calpain cleaves AIF in isolated heart mitochondria. A decrease in AIF content within mitochondria increases cardiac injury during ischemia-reperfusion by augmenting oxidative stress. We hypothesize that the activation of mito-μ-calpain by calcium overload during ischemia-reperfusion results in decreased AIF content within mitochondria by cleaving AIF. The μ-calpain was present within mouse heart mitochondria, mostly in the intermembrane space. Exogenous calcium treatment induced a calpain-dependent decrease of mitochondrial AIF content in isolated mouse heart mitochondria. This process was blocked by a calpain inhibitor (MDL-28170). The Mitochondrial μ-calpain activity was increased by 160 ± 15% during ischemia-reperfusion compared to time control. In contrast, the mitochondrial AIF content was decreased by 52 ± 7% during reperfusion vs. time control in the buffer perfused mouse heart. Inhibition of mito-μ-calpain using MDL-28170 decreased cardiac injury by preserving AIF content within mitochondria during ischemia-reperfusion. Thus, activation of mito-μ-calpain is required to release AIF from cardiac mitochondria. Inhibition of calpains using MDL-28170 decreases cardiac injury by inhibiting both cytosolic calpains and mito-μ-calpain during ischemia-reperfusion.