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Ex vivo stretch reveals altered mechanical properties of isolated dystrophin-deficient hearts.

Barnabei MS, Metzger JM - PLoS ONE (2012)

Bottom Line: Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes.During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts.In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan- and laminin-α(2) mutant mice, but not in dysferlin- mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Biology and Physiology, University of Minnesota Medical School, Minneapolis, Minnesota, United States of America.

ABSTRACT
Duchenne muscular dystrophy (DMD) is a progressive and fatal disease of muscle wasting caused by loss of the cytoskeletal protein dystrophin. In the heart, DMD results in progressive cardiomyopathy and dilation of the left ventricle through mechanisms that are not fully understood. Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes. To expand upon these findings, here we have subjected the left ventricles of dystrophin-deficient mdx hearts to mechanical stretch. Unexpectedly, isolated mdx hearts showed increased left ventricular (LV) compliance compared to controls during stretch as LV volume was increased above normal end diastolic volume. During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts. This suggests that the mechanical properties of the intact heart cannot be modeled as a simple extrapolation of findings in single cardiac myocytes. To explain these findings, a model is proposed in which disruption of the dystrophin-glycoprotein complex perturbs cell-extracellular matrix contacts and promotes the apparent slippage of myocytes past each other during LV distension. In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan- and laminin-α(2) mutant mice, but not in dysferlin- mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes. Collectively, these findings suggest a novel and cell-death independent mechanism for the progressive pathological LV dilation that occurs in DMD.

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Whole-organ passive compliance of isolated hearts from other models of muscular dystrophy.A) Left, altered compliance of β-sarcoglycan- and dy2J hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. B) Estimation of the true LV volume-EDP relationship, plotted as in Figure 1F. C) Left, normal compliance of dysferlin- hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. D) Estimation of the true LV volume-EDP relationship. E) Volume required to reach 8 mmHg EDP within the LV (Vinit). n = 5–7. * - p<0.05 for dy2J versus C57BL/6, ∧ - p<0.05 for β-sarcoglycan- versus C57BL/6. Statistical comparisons made by two-way ANOVA with main effects for strain, volume, and an interaction of the two. Values expressed as mean ± SEM.
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pone-0032880-g004: Whole-organ passive compliance of isolated hearts from other models of muscular dystrophy.A) Left, altered compliance of β-sarcoglycan- and dy2J hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. B) Estimation of the true LV volume-EDP relationship, plotted as in Figure 1F. C) Left, normal compliance of dysferlin- hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. D) Estimation of the true LV volume-EDP relationship. E) Volume required to reach 8 mmHg EDP within the LV (Vinit). n = 5–7. * - p<0.05 for dy2J versus C57BL/6, ∧ - p<0.05 for β-sarcoglycan- versus C57BL/6. Statistical comparisons made by two-way ANOVA with main effects for strain, volume, and an interaction of the two. Values expressed as mean ± SEM.

Mentions: Based on the findings above, we hypothesized that increased whole-organ compliance observed in mdx hearts is due to disruption of DGC-mediated connectivity between the extracellular matrix and the intracellular architecture of the cell. To test this hypothesis, we determined the whole-organ compliance of other mouse models of muscular dystrophy in which the DGC or its binding partners are disrupted: β-sarcoglycan- mice and dy2j mice. β-sarcoglycan is a DGC protein normally expressed in the heart, the lack of which causes Limb-Girdle Muscular Dystrophy Type 2E [44], [45]. Dy2j mice carry a mutation in the LAMA2 gene which causes abnormal splicing of the laminin-α2 transcript and expression of a truncated protein [46]. The α2 heavy chain subunit of the extracellular matrix protein laminin is bound by α-dystroglycan, facilitating the interaction of the DGC with the extracellular matrix. [47]. Mutations in LAMA2 cause merosin-deficient muscular dystrophy [48]. Similar to mdx mice, the hearts of both β-sarcoglycan and dy2j hearts showed reduced LV volumes at normal LVEDP and increased whole-organ compliance compared to WT (C57BL/6) hearts (Figure 4a, b, e). Here, similar to mdx mice, we observed increased compliance despite smaller Vinit for β-sarcoglycan and dy2j hearts (Figure 4). To determine if the observed effects on compliance in mdx, dy2j, and β-sarcoglycan- hearts are due to specific effects on the DGC or other effects of membrane dysfunction in muscular dystrophy, we also performed the stretch protocol on dysferlin- hearts. Dysferlin is a membrane repair protein which is not associated with the DGC, the lack of which causes Limb Girdle Muscular Dystrophy Type 2B [49]. These mice have no defect in expression of DGC proteins [50]. Results showed no differences in LV volumes or whole-organ compliance of dysferlin- and WT control (129S1/SvImJ) hearts (Figure 4c, d, e). Collectively, the findings of altered whole-organ compliance in β-sarcoglycan and dy2J hearts, but not in dysferlin- hearts, support the hypothesis that specific disruption of the cytoskeleton-DGC-extracellular matrix alters the mechanical properties of the myocardium and increases whole-organ compliance.


Ex vivo stretch reveals altered mechanical properties of isolated dystrophin-deficient hearts.

Barnabei MS, Metzger JM - PLoS ONE (2012)

Whole-organ passive compliance of isolated hearts from other models of muscular dystrophy.A) Left, altered compliance of β-sarcoglycan- and dy2J hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. B) Estimation of the true LV volume-EDP relationship, plotted as in Figure 1F. C) Left, normal compliance of dysferlin- hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. D) Estimation of the true LV volume-EDP relationship. E) Volume required to reach 8 mmHg EDP within the LV (Vinit). n = 5–7. * - p<0.05 for dy2J versus C57BL/6, ∧ - p<0.05 for β-sarcoglycan- versus C57BL/6. Statistical comparisons made by two-way ANOVA with main effects for strain, volume, and an interaction of the two. Values expressed as mean ± SEM.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3298453&req=5

pone-0032880-g004: Whole-organ passive compliance of isolated hearts from other models of muscular dystrophy.A) Left, altered compliance of β-sarcoglycan- and dy2J hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. B) Estimation of the true LV volume-EDP relationship, plotted as in Figure 1F. C) Left, normal compliance of dysferlin- hearts as shown by plotting LVEDP against volume added above Vinit during ex vivo stretch. D) Estimation of the true LV volume-EDP relationship. E) Volume required to reach 8 mmHg EDP within the LV (Vinit). n = 5–7. * - p<0.05 for dy2J versus C57BL/6, ∧ - p<0.05 for β-sarcoglycan- versus C57BL/6. Statistical comparisons made by two-way ANOVA with main effects for strain, volume, and an interaction of the two. Values expressed as mean ± SEM.
Mentions: Based on the findings above, we hypothesized that increased whole-organ compliance observed in mdx hearts is due to disruption of DGC-mediated connectivity between the extracellular matrix and the intracellular architecture of the cell. To test this hypothesis, we determined the whole-organ compliance of other mouse models of muscular dystrophy in which the DGC or its binding partners are disrupted: β-sarcoglycan- mice and dy2j mice. β-sarcoglycan is a DGC protein normally expressed in the heart, the lack of which causes Limb-Girdle Muscular Dystrophy Type 2E [44], [45]. Dy2j mice carry a mutation in the LAMA2 gene which causes abnormal splicing of the laminin-α2 transcript and expression of a truncated protein [46]. The α2 heavy chain subunit of the extracellular matrix protein laminin is bound by α-dystroglycan, facilitating the interaction of the DGC with the extracellular matrix. [47]. Mutations in LAMA2 cause merosin-deficient muscular dystrophy [48]. Similar to mdx mice, the hearts of both β-sarcoglycan and dy2j hearts showed reduced LV volumes at normal LVEDP and increased whole-organ compliance compared to WT (C57BL/6) hearts (Figure 4a, b, e). Here, similar to mdx mice, we observed increased compliance despite smaller Vinit for β-sarcoglycan and dy2j hearts (Figure 4). To determine if the observed effects on compliance in mdx, dy2j, and β-sarcoglycan- hearts are due to specific effects on the DGC or other effects of membrane dysfunction in muscular dystrophy, we also performed the stretch protocol on dysferlin- hearts. Dysferlin is a membrane repair protein which is not associated with the DGC, the lack of which causes Limb Girdle Muscular Dystrophy Type 2B [49]. These mice have no defect in expression of DGC proteins [50]. Results showed no differences in LV volumes or whole-organ compliance of dysferlin- and WT control (129S1/SvImJ) hearts (Figure 4c, d, e). Collectively, the findings of altered whole-organ compliance in β-sarcoglycan and dy2J hearts, but not in dysferlin- hearts, support the hypothesis that specific disruption of the cytoskeleton-DGC-extracellular matrix alters the mechanical properties of the myocardium and increases whole-organ compliance.

Bottom Line: Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes.During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts.In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan- and laminin-α(2) mutant mice, but not in dysferlin- mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Biology and Physiology, University of Minnesota Medical School, Minneapolis, Minnesota, United States of America.

ABSTRACT
Duchenne muscular dystrophy (DMD) is a progressive and fatal disease of muscle wasting caused by loss of the cytoskeletal protein dystrophin. In the heart, DMD results in progressive cardiomyopathy and dilation of the left ventricle through mechanisms that are not fully understood. Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes. To expand upon these findings, here we have subjected the left ventricles of dystrophin-deficient mdx hearts to mechanical stretch. Unexpectedly, isolated mdx hearts showed increased left ventricular (LV) compliance compared to controls during stretch as LV volume was increased above normal end diastolic volume. During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts. This suggests that the mechanical properties of the intact heart cannot be modeled as a simple extrapolation of findings in single cardiac myocytes. To explain these findings, a model is proposed in which disruption of the dystrophin-glycoprotein complex perturbs cell-extracellular matrix contacts and promotes the apparent slippage of myocytes past each other during LV distension. In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan- and laminin-α(2) mutant mice, but not in dysferlin- mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes. Collectively, these findings suggest a novel and cell-death independent mechanism for the progressive pathological LV dilation that occurs in DMD.

Show MeSH
Related in: MedlinePlus