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Cytoskeletal mechanics in pressure-overload cardiac hypertrophy
H Tagawa1, N Wang, T Narishige
1Department of Medicine, Gazes Cardiac Research Institute, Medical University of South Carolina, Charleston, USA.
Insights
Increased microtubule density in cardiac hypertrophy causes cellular dysfunction by increasing viscous load, not myofilament issues. This study quantifies the mechanical changes in heart cells using magnetic twisting cytometry.
Area of Science:
- Cardiovascular Biology
- Cellular Mechanics
- Biophysics
Background:
- Pressure-overload cardiac hypertrophy leads to cellular contractile dysfunction.
- The underlying cause of this dysfunction has been debated, with focus on myofilaments versus other cytoskeletal components.
Purpose of the Study:
- To investigate the physical mechanisms by which increased microtubule density in cardiac hypertrophy mechanically overloads the cardiocyte contractile apparatus.
- To quantify changes in cytoskeletal stiffness and apparent viscosity in pressure-overloaded cardiac cells.
Main Methods:
- Magnetic twisting cytometry was used to measure cytoskeletal stiffness and apparent viscosity in isolated cardiac cells (cardiocytes).
- Measurements were performed on cardiocytes from pressure-overloaded right ventricles (RV) and control left ventricles (LV) of cats with induced pulmonary artery banding.
- The effects of microtubule depolymerization (colchicine) and hyperpolymerization (taxol) on cytoskeletal properties were assessed.
Main Results:
- Cytoskeletal stiffness nearly doubled in hypertrophied RV cardiocytes compared to normal LV cardiocytes.
- Cytoskeletal apparent viscosity increased nearly fourfold in hypertrophied RV cardiocytes.
- Colchicine normalized these properties in hypertrophied cells, while taxol induced abnormal values in normal cells, confirming the role of microtubules.
Conclusions:
- Increased microtubule density, not myofilament abnormality, is the primary cause of contractile dysfunction in pressure-overload cardiac hypertrophy.
- This increased microtubule density imposes a significant viscous load on the cardiocyte contractile apparatus.
- Targeting microtubule dynamics may offer therapeutic strategies for cardiac hypertrophy.
Abstract:
We have shown that the cellular contractile dysfunction characteristic of pressure-overload cardiac hypertrophy results not from an abnormality intrinsic to the myofilament portion of the cardiocyte cytoskeleton but rather from an increased density of the microtubule component of the extramyofilament portion of the cardiocyte cytoskeleton. To determine how, in physical terms, this increased microtubule density mechanically overloads the contractile apparatus at the cellular level, we measured cytoskeletal stiffness and apparent viscosity in isolated cardiocytes via magnetic twisting cytometry, a technique by which magnetically induced force is applied directly to the cytoskeleton through integrin-coupled ferromagnetic beads coated with Arg-Gly-Asp (RGD) peptide. Measurements were made in two groups of cardiocytes from cats with right ventricular (RV) hypertrophy induced by pulmonary artery banding: (1) those from the pressure-overloaded RV and (2) those from the normally loaded same-animal control left ventricle (LV). Cytoskeletal stiffness increased almost twofold, from 8.53 +/- 0.77 dyne/cm2 in the normally loaded LV cardiocytes to 16.46 +/- 1.32 dyne/cm2 in the hypertrophied RV cardiocytes. Cytoskeletal apparent viscosity increased almost fourfold, from 20.97 +/- 1.92 poise in the normally loaded LV cardiocytes to 87.85 +/- 6.95 poise in the hypertrophied RV cardiocytes. In addition to these baseline data showing differing stiffness and, especially, apparent viscosity in the two groups of cardiocytes, microtubule depolymerization by colchicine was found to return both the stiffness and the apparent viscosity of the pressure overload-hypertrophied RV cells fully to normal. Conversely, microtubule hyperpolymerization by taxol increased the stiffness and apparent viscosity values of normally loaded LV cardiocytes to the abnormal values given above for pressure-hypertrophied RV cardiocytes. Thus, increased microtubule density constitutes primarily a viscous load on the cardiocyte contractile apparatus in pressure-overload cardiac hypertrophy.