Microtubule-induced restriction of nuclear deformability in hypertrophic cardiomyopathy
Authors: Duursma I , Nollet E E , Jansen V , Bedi K , Margulies K B , Van Der Wel N N , Van Der Velden J , Kirby T J , Kuster D W D 
Published: May 2024

Abstract

Funding Acknowledgements
Type of funding sources: Foundation. Main funding source(s): Leducq foundation

Background
Hypertrophic cardiomyopathy (HCM) is characterized by abnormal thickening of the left ventricular (LV) wall in conjunction with diastolic dysfunction with a prevalence of ~1:200. HCM pathophysiology is incompletely understood. We hypothesized that misregulated mechanotransduction plays an important role in the disease process. Extensive cytoskeletal remodeling has been observed in HCM patients, with a robust increase in tubulin and desmin levels, with a concomitant increase in stabilizing post-translational modifications, which we previously showed contributes to diastolic dysfunction. Microtubules are connected to the nucleus by the LINC complex. Little is known about the interaction of the non-sarcomeric cytoskeleton with the nucleus and the role this may have in the development of HCM.

Purpose
Relate cytoskeleton changes to nuclear and chromatin morphology in HCM patient myocardium and assess nuclear deformability in contracting cardiomyocytes with and without microtubule destabilizing drugs.

Methods
Nuclear morphology and chromatin organization was assessed using electron microscopy in cardiac septal tissue from 19 patients with obstructive HCM and compared to samples from 4 non-failing donors (NF). Further nuclear changes were studied using wild type (WT) and homozygous Mybpc3 c.2373InsG mice (Mybpc3c.2373InsG, KI), which have a HCM phenotype. Nuclear morphology was assessed in slices of 6 WT and 6 Mybpc3c.2373InsG paraffin-embedded mouse hearts. To study nuclear deformability during contractions, adult cardiomyocytes were isolated, stained with live cell dyes and imaged live while electrically paced. Lastly, DNA damaged was assessed in 6 WT and 6 Mybpc3c.2373InsG paraffin-embedded mouse hearts.

Results
Cardiomyocyte nuclei of HCM patients had a significantly higher number of invaginations (0.32±0.09 invaginations/µm in HCM versus 0.18±0.03 invaginations/µm in NF hearts), were more irregular of shape and had altered chromatin organization compared to those from NF hearts. Nuclei of Mybpc3c.2373InsG mice were considerably larger (322±75 µm³ versus 221±24 µm³). While sarcomere shortening is similar (7.0±1.4% in Mybpc3c.2373InsG versus 6.5±1.7% in WT), nuclei from Mybpc3c.2373InsG mice are less deformable than WT nuclei (figure). Addition of microtubule destabilizer nocodazole or detyrosination inhibitor EPO-Y restored nuclear deformability to WT levels, indicating a role of detyrosinated microtubules in nuclear deformability. In addition, nuclei from Mybpc3c.2373InsG have more extensive DNA damage (19.7±3.4 foci in Mybpc3c.2373InsG versus 9.1±3.2 foci in WT).

Conclusion
Extensive nuclear changes are observed in HCM patients and a HCM mouse model. Cytoskeletal remodeling results in less deformable nuclei that might contribute to chromatin alterations and increased DNA damage. Nuclear deformability can be restored by microtubule modifying drugs, specifically those that decrease microtubule detyrosination.