Cardiac microtubules mediate transverse (t)-tubule growth and homeostasis
Transverse (t)-tubules enable rapid, synchronous Ca release required for efficient cardiac contraction by bringing L-type Ca channels into close apposition with ryanodine receptors. In heart failure with reduced ejection fraction (HFrEF), t-tubule disorganisation and loss occur alongside cardiac microtubule remodelling, contributing to impaired Ca handling and contractile dysfunction. Despite their canonical function in contraction, how t-tubules develop is unknown. Microtubules support delivery of L-type Ca channels to t-tubules via Amphiphysin-II/BIN1, yet whether microtubules directly regulate t-tubule formation and maintenance is unclear. Here, we investigated a role for microtubules in t-tubule development and homeostasis. Neonatal rat ventricular myocytes (NRVMs), which lack endogenous t-tubules, were used as a reductionist model in which BIN1 overexpression induces nascent membrane tubules. Microtubule depolymerisation with nocodazole before BIN1 overexpression impaired BIN1-driven tubule formation, reducing tubule density and length. Dynein inhibition with EHNA produced similar effects, indicating a requirement for microtubule-based motor activity during tubule elongation. Knockdown of the microtubule +TIP tracking protein CLIP-170 also reduced BIN1-driven tubule density, implicating BIN1-CLIP-170-dependent microtubule capture in tubule initiation. Microtubules were also required to maintain existing tubules. In NRVMs with established BIN1-driven tubules, microtubule depolymerisation, microtubule stabilisation or dynein inhibition each reduced tubule density and length. Consistent with this, acute microtubule depolymerisation or stabilisation disrupted native t-tubule networks in isolated adult sheep left atrial myocytes. Together, these findings identify cardiac microtubules as active regulators of t-tubule architecture. We propose that BIN1-dependent tubule formation requires CLIP-170-mediated microtubule plus-end capture and dynein-dependent elongation, while ongoing microtubule dynamics are necessary to preserve mature t-tubule structure.