Myosin Binding Protein H-Like Regulates Myofilament Content In Atrial And A Subset Of Ventricular Conduction System Cardiomyocytes

A premature stop variant in the MYBPHL gene associates with human dilated cardiomyopathy and cardiac conduction system disease. MYBPHL is highly expressed in the atria and encodes myosin binding protein H-like (MyBP-HL), a previously unstudied myofilament component with homology to MyBP-C. Deletion of the Mybphl gene in mice also produces dilated cardiomyopathy and arrhythmias, confirming MyBP-HL's importance in the heart. Using novel tools, we identified MyBP-HL-positive ventricular cardiomyocytes in wild-type mouse hearts, finding that Mybphl+/− ventricles only have 10% as many MyBP-HL-positive cells. MyBP-HL co-localized with the VCS marker contactin-2 (Cntn2) near the atrioventricular (AV) node and in a subset of Cntn2-positive Purkinje fibers. Surface telemetry revealed third-degree AV block following propranolol administration, and intracardiac pacing revealed prolonged action potential propagation through the His system. In the absence of MyBP-HL, atrial cardiomyocyte morphology is unchanged despite an increase in total atrial size. Single-cell unloaded shortening measurements showed no difference in contractile function between wild-type, Mybphl-/+, and Mybphl−/− atrial cardiomyocytes under baseline conditions. Mass-spectrometry (MS) of proteins from atrial wild-type, Mybphl-/+, and Mybphl−/− tissue revealed an inverse relationship between MyBP-HL and cMyBP-C levels. The MS data was combined with super-resolution imaging to model the myofilament occupancy of both these proteins. Atrial cardiomyocytes showed MyBP-HL co-localization with cMyBP-C in the C-zone of the A-band, but in a position closer to the M-line. In the absence of MyBP-HL, cMyBP-C is able to occupy additional binding sites closer to the M-line than normal. This inverse relationship was confirmed using a cMyBP-C null mouse model that showed increased MyBP-HL when cMyBP-C was reduced. These data outline a key role of MyBP-HL in regulating myofilament protein content in atrial and a specific subset of ventricular cardiomyocytes.