Microtubules linked to cardiomyocyte function in heart failure

Originally Published By 2 Minute Medicine®. Reused on MPR with permission.

1. Post-translational detyrosination of microtubules results in resistance to cardiomyocyte sarcomere shortening and stretch.

2. Pharmacologic and genetic strategies to inhibit the detyrosination of microtubules improves contractility in cultured human cardiomyocytes.

Evidence Rating Level: 2 (Good)

Study Rundown: While heart failure represents a leading cause of death worldwide, there are currently few targeted therapeutics for this condition; most patients with heart failure ultimately depend on ventricular assist devices or heart transplant, options that are made difficult both by limitations such as access and complications such as infection and organ rejection. In this study, the authors present a novel targeted therapeutic approach for treating human heart failure by modulating microtubule structure.

The authors build off of previous work in murine myocytes, in which they showed that detyrosination (dTyr) of microtubules results in a switch in interactions with the sarcomere, the contractile apparatus of the cardiomyocyte. In particular, detyrosination was found to increase the viscoelastic resistance to sarcomere contraction and relaxation in murine myocytes.

To determine how the cardiac cytoskeleton might be altered in human heart failure, the authors used left ventricular myocardium samples from 105 non-failing and failing human hearts and compared their proteomic expression profiles. These analyses revealed a consistent upregulation and stabilization of microtubules and intermediate filaments in heart failure samples. In addition to an increase in microtubule density, failing hearts also displayed a significant increase in the ratio of dTyr to total microtubules. Treatment of failing myocytes, isolated from failing cardiac tissues, with colchicine (microtubule depolymerizer) or parthenolide (inhibitor of the tubulin detyrosinating enzyme) robustly reduced viscoelasticity in failing myocytes. Furthermore, adenoviral-mediated overexpression of tubulin tyrosine ligase (catalyzes the re-addition of tyrosine residues to α-tubulin tails) in cultured human cardiomyocytes was found to improve contractility of these cells. These results suggest that altering microtubule dynamics profoundly influences cardiomyocyte function. Pharmacologic or genetic approaches to prevent the detyrosination of microtubules may provide a widely applicable strategy to treat heart failure that results from a variety of etiologies.

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Relevant Reading: Cardiac cytoskeleton and heart failure

In-Depth [pre-clinical study]: This study used left ventricular myocardial samples from 105 failing and non-failing human hearts. Non-failing hearts were divided into two groups: normal or compensated hypertrophy. Failing hearts were divided into three groups: ischemic cardiomyopathy (ICM), dilated cardiomyopathy (DCM), and hypertrophic cardiomyopathy (HCM). The hypertrophic cardiomyopathy group was further subdivided into preserved (HCMpEF) or reduced ejection fraction groups (HCMrEF). Western blot was performed on 102 hearts, and 34 of these hearts were also used for mass spectrometry, while 22 hearts were used for primary isolation of cardiomyocytes for functional characterization.

Principal component analyses of proteomics of 34 human left ventricular tissues revealed that HCMpEF, HCMrEF, and DCM groups cluster together, distinct from the ICM and normal/compensated hypertrophy groups. Enrichment analyses revealed cytoskeletal genes amongst the top enriched gene ontology groups. Genes of the intermediate filament cytoskeleton including DES (desmin), SYNM (synemin), VIM (vimentin), NES (nestin) and LMNA (Lamin-A/C) were upregulated in failing hearts, as were genes of the  microtubule cytoskeleton including TUBB, TUBB2C, TUBB4B, TUBB6, TUBA1B, TUBA1A. Immunofluorescent imaging of surface and interior microtubules confirmed an increased microtubule network density and increase in the ratio of dTyr-microtubules/total microtubules. Microtubule-dependent viscoelasticity of primary isolated human myocytes was measured via nanoindentation techniques and plotted as stiffness (elastic modulus) versus velocity of indentation. Suppression of dytyrosinated microtubules with parthenolide was found to improve contractility in failing human cardiomyocytes, quantified by average sarcomere shortening compared to DMSO-treated controls. Genetic modification of tubulin tyrosination via adenoviral-mediated overexpression of tubulin tyrosine ligase reduced stiffness (reduced elastic modulus) and improved contractility (greater % sarcomere shortenining) in cultured human cardiomyocytes compared to myocytes infected with a null encoding adenovirus.

Image: PD

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