The L‐type Ca²⁺ channel facilitates abnormal metabolic activity in the cTnI‐G203S mouse model of hypertrophic cardiomyopathy

Abstract

Key pointsr Genetic mutations in cardiac troponin I (cTnI) are associated with development of hypertrophic cardiomyopathy characterized by myocyte remodelling, disorganization of cytoskeletal proteins and altered energy metabolism.r The L-type Ca 2+ channel is the main route for calcium influx and is crucial to cardiac excitation and contraction. The channel also regulates mitochondrial function in the heart by a functional communication between the channel and mitochondria via the cytoskeletal network.r We find that L-type Ca 2+ channel kinetics are altered in cTnI-G203S cardiac myocytes and that activation of the channel causes a significantly greater increase in mitochondrial membrane potential and metabolic activity in cTnI-G203S cardiac myocytes. r We propose that L-type Ca 2+ channel antagonists, such as diltiazem, might be effective in reducing the cardiomyopathy by normalizing mitochondrial metabolic activity.Abstract Genetic mutations in cardiac troponin I (cTnI) account for 5% of families with hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy is associated with disorganization of cytoskeletal proteins and altered energy metabolism. The L-type Ca 2+ channel (I Ca-L ) plays an important role in regulating mitochondrial function. This involves a functional communication between the channel and mitochondria via the cytoskeletal network. We investigate the role of I Ca-L in regulating mitochondrial function in 25-to 30-week-old cardiomyopathic mice expressing the human disease-causing mutation Gly203Ser in cTnI (cTnI-G203S). The inactivation rate of I Ca-L is significantly faster in cTnI-G203S myocytes [cTnI-G203S: τ 1 = 40.68 ± 3.22, n = 10 vs. wild-type (wt): τ 1 = 59.05 ± 6.40, n = 6, P < 0.05]. Activation of I Ca-L caused a greater increase in mitochondrial membrane potential ( m , 29.19 ± 1.85%, n = 15 vs. wt: 18.84 ± 2.01%, n = 10, P < 0.05) and metabolic activity (24.40 ± 6.46%, n = 8 vs. wt: 9.98 ± 1.57%, n = 9, P < 0.05). The responses occurred because of impaired communication between I Ca-L and F-actin, involving lack of dynamic movement of actin-myosin and block of the mitochondrial voltage-dependent anion channel. Similar responses were observed in precardiomyopathic mice. I Ca-L antagonists nisoldipine and diltiazem decreased m to basal levels. We conclude that the Gly203Ser mutation

The L-type Ca2+ channel: A mediator of hypertrophic cardiomyopathy

“…5 Cardiac myocytes isolated from mice with disease causing cardiac troponin I mutation Gly203Ser (cTnI-G203S), exhibited altered I Ca-L kinetics, and a significantly greater increase in C m and mitochondrial oxygen consumption (measured as an increase in flavoprotein oxidation and formation of formazan from tetrazolium salt) in response to activation of the channel. 6 Consistent with our original hypothesis, the alterations in C m were not due to alterations in intracellular calcium, because the responses occurred under calcium-free conditions. Further to this, diastolic intracellular calcium levels were similar in cTnI-G203S versus wt mice under basal conditions, and in response to activation of I Ca-L .…”

“…Further to this, diastolic intracellular calcium levels were similar in cTnI-G203S versus wt mice under basal conditions, and in response to activation of I Ca-L . 6 We demonstrated that the response involved F-actin and b-tubulin and that the hyperpolarisation of the C m occurred due to increased block of mitochondrial VDAC by b-tubulin. 6,7 This finding is supported by studies demonstrating that b-tubulin associates with and regulates the open/closed state of mitochondrial VDAC.…”

“…6 We demonstrated that the response involved F-actin and b-tubulin and that the hyperpolarisation of the C m occurred due to increased block of mitochondrial VDAC by b-tubulin. 6,7 This finding is supported by studies demonstrating that b-tubulin associates with and regulates the open/closed state of mitochondrial VDAC. 8 Overall, our findings provide important mechanistic insight into the development of Gly203Ser hypertrophic cardiomyopathy.…”

“…In our recent study, we demonstrated that exposure of cTnI-G203S myocytes to diltiazem significantly normalizes C m in response to activation of I Ca-L . 6 These data suggest that I Ca-L antagonists may be effective in reducing the cardiomyopathy by altering mitochondrial function. It is important to note that similar alterations in I Ca-L kinetics and mitochondrial metabolic activity were recorded in cardiac myocytes isolated from cTnI-G203S mice that not yet developed the cardiomyopathy.…”

“…It is important to note that similar alterations in I Ca-L kinetics and mitochondrial metabolic activity were recorded in cardiac myocytes isolated from cTnI-G203S mice that not yet developed the cardiomyopathy. 6 That is, myocytes isolated from pre-cardiomyopathic hearts also exhibited altered I Ca-L kinetics, and a significantly greater increase in C m and metabolic activity in response to activation of I Ca-L . This finding is of major significance because it indicates that alterations in I Ca-L and mitochondrial function in mice exhibiting the Gly203Ser mutation precede development of the disease.…”