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Cardiac BIN1 folds T-tubule membrane, controlling ion flux and limiting arrhythmia.

Hong T, Yang H, Zhang SS, Cho HC, Kalashnikova M, Sun B, Zhang H, Bhargava A, Grabe M, Olgin J, Gorelik J, Marbán E, Jan LY, Shaw RM - Nat. Med. (2014)

Bottom Line: Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts.We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs.When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

View Article: PubMed Central - PubMed

Affiliation: 1] Cedars-Sinai Heart Institute, Cedars-Sinai Medical Center, Los Angeles, California, USA. [2].

ABSTRACT
Cardiomyocyte T tubules are important for regulating ion flux. Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts. Here we find that cardiac T tubules normally contain dense protective inner membrane folds that are formed by a cardiac isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased, which does not change overall cardiomyocyte morphology but leads to free diffusion of local extracellular calcium and potassium ions, prolonging action-potential duration and increasing susceptibility to ventricular arrhythmias. We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs. BIN1+13+17 recruits actin to fold the T-tubule membrane, creating a 'fuzzy space' that protectively restricts ion flux. When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

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Bin1 deletion increases extracellular Ca2+ diffusion. (a) Representative patch clamp recording of the LTCC mediated ICa from a WT cardiomyocyte in response to quick change from 2 mM extracellular calcium solution to calcium free 5 mM EGTA solution. (b) Kinetics of ICa current changes using the protocol described in (a) were fitted with one plateau followed by one phase exponential decay. X0 is the initial delay before ICa decays. (c) Comparison of X0 for WT and Bin1 HT. Data are presented as mean ± SEM, P = 0.0001 by student’s t-test (cardiomyocytes are from 3 mice for each genotype). (d) A diagram describing the salient features of a mathematical model for calcium diffusion. (e) Kinetics of ICa current decay computed using the model in (d). The normalized calcium concentration in the slow diffusion zone serves as a surrogate for the calcium current since it is directly related to the inward Ca2+ driving force. The model of WT T-tubules containing a slow diffusion zone matches the experimental data (black curve – model, black circles – data). Removal of the diffusion barrier at the left side of the T-tubule in (a) results in a shorter initial delay as observed in the Bin1 HT experiments (red curve – model, red squares – data).
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Figure 2: Bin1 deletion increases extracellular Ca2+ diffusion. (a) Representative patch clamp recording of the LTCC mediated ICa from a WT cardiomyocyte in response to quick change from 2 mM extracellular calcium solution to calcium free 5 mM EGTA solution. (b) Kinetics of ICa current changes using the protocol described in (a) were fitted with one plateau followed by one phase exponential decay. X0 is the initial delay before ICa decays. (c) Comparison of X0 for WT and Bin1 HT. Data are presented as mean ± SEM, P = 0.0001 by student’s t-test (cardiomyocytes are from 3 mice for each genotype). (d) A diagram describing the salient features of a mathematical model for calcium diffusion. (e) Kinetics of ICa current decay computed using the model in (d). The normalized calcium concentration in the slow diffusion zone serves as a surrogate for the calcium current since it is directly related to the inward Ca2+ driving force. The model of WT T-tubules containing a slow diffusion zone matches the experimental data (black curve – model, black circles – data). Removal of the diffusion barrier at the left side of the T-tubule in (a) results in a shorter initial delay as observed in the Bin1 HT experiments (red curve – model, red squares – data).

Mentions: What is the function of T-tubule membrane folds? One possibility, suggested by the calcium density, is that the folds create an extracellular microenvironment distinct from the bulk extracellular space. We measured the dynamics of calcium ion diffusion into T-tubules by recording the ICa current through LTCCs while rapidly changing extracellular calcium. The baseline measurements revealed that Bin1 HT cardiomyocytes have similar total protein expression level of Cav1.2 (Supplementary Fig. 4a) and, of the membrane-inserted channels, preserved preferential localization of LTCCs to T-tubules. The Bin1 HT cardiomyocytes also have less total membrane LTCCs and less overall LTCC current (Supplementary Fig. 4b), consistent with our previous report that BIN1 helps LTCC forward trafficking18. Next, the activity of the surface LTCCs was studied by recording ICa decay kinetics following a rapid switch of extracellular perfusate from physiological calcium concentration to zero calcium (buffered with 5 mM EGTA; Fig. 2a). After an initial delay, ICa decayed exponentially until all extracellular calcium inside the T-tubules was removed or chelated (Fig. 2b). In Bin1 HT cardiomyocytes, the time length of the initial delay (X0) is shortened by 64 ms compared to that of WT cardiomyocytes (239 ± 5 ms vs. 303 ± 10 ms, Fig. 2c, P = 0.0001). To test whether shortened X0 could be due to increased ion diffusion within the T-tubules, a simple mathematical model of calcium diffusion was implemented containing a slow diffusion zone (T-tubule folds, Fig. 1eh), a rapid diffusion zone (T-tubule central lumen, tomography in Fig. 1e) and EGTA chelation (Fig. 2d). The theoretical decay curve generated from the model superimposes with the experimental data in WT cardiomyocytes. Removal of the diffusion barrier results in a shortened X0, as seen in the Bin1 HT cardiomyocytes (Fig. 2e). The autonomous mathematical algorithm that determined best-fit T-tubule diffusion and anatomical parameters yielded values that are remarkably consistent with those previously measured (Supplementary Table 1), providing computational support for a model of BIN1-induced restricted extracellular calcium diffusion.


Cardiac BIN1 folds T-tubule membrane, controlling ion flux and limiting arrhythmia.

Hong T, Yang H, Zhang SS, Cho HC, Kalashnikova M, Sun B, Zhang H, Bhargava A, Grabe M, Olgin J, Gorelik J, Marbán E, Jan LY, Shaw RM - Nat. Med. (2014)

Bin1 deletion increases extracellular Ca2+ diffusion. (a) Representative patch clamp recording of the LTCC mediated ICa from a WT cardiomyocyte in response to quick change from 2 mM extracellular calcium solution to calcium free 5 mM EGTA solution. (b) Kinetics of ICa current changes using the protocol described in (a) were fitted with one plateau followed by one phase exponential decay. X0 is the initial delay before ICa decays. (c) Comparison of X0 for WT and Bin1 HT. Data are presented as mean ± SEM, P = 0.0001 by student’s t-test (cardiomyocytes are from 3 mice for each genotype). (d) A diagram describing the salient features of a mathematical model for calcium diffusion. (e) Kinetics of ICa current decay computed using the model in (d). The normalized calcium concentration in the slow diffusion zone serves as a surrogate for the calcium current since it is directly related to the inward Ca2+ driving force. The model of WT T-tubules containing a slow diffusion zone matches the experimental data (black curve – model, black circles – data). Removal of the diffusion barrier at the left side of the T-tubule in (a) results in a shorter initial delay as observed in the Bin1 HT experiments (red curve – model, red squares – data).
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Figure 2: Bin1 deletion increases extracellular Ca2+ diffusion. (a) Representative patch clamp recording of the LTCC mediated ICa from a WT cardiomyocyte in response to quick change from 2 mM extracellular calcium solution to calcium free 5 mM EGTA solution. (b) Kinetics of ICa current changes using the protocol described in (a) were fitted with one plateau followed by one phase exponential decay. X0 is the initial delay before ICa decays. (c) Comparison of X0 for WT and Bin1 HT. Data are presented as mean ± SEM, P = 0.0001 by student’s t-test (cardiomyocytes are from 3 mice for each genotype). (d) A diagram describing the salient features of a mathematical model for calcium diffusion. (e) Kinetics of ICa current decay computed using the model in (d). The normalized calcium concentration in the slow diffusion zone serves as a surrogate for the calcium current since it is directly related to the inward Ca2+ driving force. The model of WT T-tubules containing a slow diffusion zone matches the experimental data (black curve – model, black circles – data). Removal of the diffusion barrier at the left side of the T-tubule in (a) results in a shorter initial delay as observed in the Bin1 HT experiments (red curve – model, red squares – data).
Mentions: What is the function of T-tubule membrane folds? One possibility, suggested by the calcium density, is that the folds create an extracellular microenvironment distinct from the bulk extracellular space. We measured the dynamics of calcium ion diffusion into T-tubules by recording the ICa current through LTCCs while rapidly changing extracellular calcium. The baseline measurements revealed that Bin1 HT cardiomyocytes have similar total protein expression level of Cav1.2 (Supplementary Fig. 4a) and, of the membrane-inserted channels, preserved preferential localization of LTCCs to T-tubules. The Bin1 HT cardiomyocytes also have less total membrane LTCCs and less overall LTCC current (Supplementary Fig. 4b), consistent with our previous report that BIN1 helps LTCC forward trafficking18. Next, the activity of the surface LTCCs was studied by recording ICa decay kinetics following a rapid switch of extracellular perfusate from physiological calcium concentration to zero calcium (buffered with 5 mM EGTA; Fig. 2a). After an initial delay, ICa decayed exponentially until all extracellular calcium inside the T-tubules was removed or chelated (Fig. 2b). In Bin1 HT cardiomyocytes, the time length of the initial delay (X0) is shortened by 64 ms compared to that of WT cardiomyocytes (239 ± 5 ms vs. 303 ± 10 ms, Fig. 2c, P = 0.0001). To test whether shortened X0 could be due to increased ion diffusion within the T-tubules, a simple mathematical model of calcium diffusion was implemented containing a slow diffusion zone (T-tubule folds, Fig. 1eh), a rapid diffusion zone (T-tubule central lumen, tomography in Fig. 1e) and EGTA chelation (Fig. 2d). The theoretical decay curve generated from the model superimposes with the experimental data in WT cardiomyocytes. Removal of the diffusion barrier results in a shortened X0, as seen in the Bin1 HT cardiomyocytes (Fig. 2e). The autonomous mathematical algorithm that determined best-fit T-tubule diffusion and anatomical parameters yielded values that are remarkably consistent with those previously measured (Supplementary Table 1), providing computational support for a model of BIN1-induced restricted extracellular calcium diffusion.

Bottom Line: Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts.We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs.When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

View Article: PubMed Central - PubMed

Affiliation: 1] Cedars-Sinai Heart Institute, Cedars-Sinai Medical Center, Los Angeles, California, USA. [2].

ABSTRACT
Cardiomyocyte T tubules are important for regulating ion flux. Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts. Here we find that cardiac T tubules normally contain dense protective inner membrane folds that are formed by a cardiac isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased, which does not change overall cardiomyocyte morphology but leads to free diffusion of local extracellular calcium and potassium ions, prolonging action-potential duration and increasing susceptibility to ventricular arrhythmias. We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs. BIN1+13+17 recruits actin to fold the T-tubule membrane, creating a 'fuzzy space' that protectively restricts ion flux. When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

Show MeSH
Related in: MedlinePlus