Microtubule Bundles in the Adult Cardiomyocyte

Abstract

The role of Microtubules (MTs) in mechanosignalling and mechanical properties has seen a recent surge of interest among the cardiac and skeletal muscle community. We recently reported that MTs buckle and bear load during the cardiac cycle. This has a significant effect on contractile mechanics, suggesting that myocyte function is tunable by any regulatory pathway that alters the mechanical behavior of MTs. Other work ongoing in our lab indicates that a number of Microtubule Associated Proteins (MAPs) are robustly upregulated in human heart disease. Multi-MT assemblies or bundles associated with these and other MAPs have been observed and studied extensively in other contexts, including unicellular motile structures and neuronal axons, but have not been examined in cardiac myocytes. The number of MTs involved, their geometry and the relative abundance of such structures compared to isolated MTs have significant implications for the mechanical properties of a myocyte. Using targeted overexpression of several MAPs in adult rat cardiomyocytes, we examine the structure, magnitude and prevalence of MT bundles by electron microscopy. We find that single MTs are the most prevalent MT elements in the unmanipulated myocyte, accompanied by comparatively rare and small groupings of 2-3 MTs with wide spacing. MTs are preferentially located at the interface between myofibril and mitochondria. Manipulation of MT binding partners can modestly increase the size and likelihood of MT bundles, and also dramatically reduces the inter-tubule spacing to be in-line with canonical bundle spacing. When we further characterized the functional consequences of MT bundling on cardiac contractility, we were surprised to find that the promotion of bundling, at least with certain MAPs, does not appear to impair contraction by reinforcing compliant microtubules, but rather uncouples the MTs from the process of contraction-associated buckling.