Abstract Body: Background: Left ventricular noncompaction (LVNC) is a type of cardiomyopathy, which patients manifest clinical features of heart failure, arrhythmias, thrombotic events, and sudden death. We previously found that the ablation of Numb(Nb), and its homologue Numbl(Nl) at early stage, causes LVNC defects resulting in embryonic lethality. In this study, we found that mice with Nb and Nl deletion at later stage survive to adulthood and develop toLVNC cardiomyopathy.
Method: We applied aMHC-Cre to delete and generate Nb and Nl double knockout (ADKO); examine the structure and functions of the ADKO hearts via echocardiography (ECHO); determine the heart arrhythmia via telemetry; determine the Nb subcellular localization via a Nb overexpression line (R26-Flag:mCherry:Numb); and determine the mechanistic function of NFPs in autophagy via autophagic reporter, electron microscopy, and molecular and biochemical assays.
Results: The cTnT-Cre mediated Nb and Nl deletion causes LVNC and embryonic lethality, while the ADKO can survive to adulthood and died between eight- and twelve-month-old. The ADKO hearts display LVNC by two months of age, a reduced systolic function by four months old, and left ventricular remodeling and dilatation starting at six months old. ADKO hearts manifest major clinical features of LVNC, including arrhythmias and thrombosis. ADKO hearts exhibit significant accumulation of damaged mitochondria, autophagosomes and autophagolysosomes compared to controls. Biochemical analyses reveal elevated levels of p62 and a higher ratio of LC3II/LC3I in ADKO hearts. NFP null cells and ADKO hearts exhibit abnormal autophagic flux, characterized by arrested at autophagolysosome, increased expression of Lamp1 and Lamp2, but reduced levels of cathepsin D. Mass spectrometry analyses reveal Numb's interaction with proteins involved in endocytosis and autophagy. Consistently, Numb localizes to autophagosomes and autophagolysosomes.
Conclusion: NFPs regulate left ventricular compaction at both embryonic and postnatal stages, suggesting that LVNC can be both congenital and acquired. As LVNC mice progress, they develop dilated cardiomyopathy (DCM) at later stage, suggesting that LVNC can occur independently at early stage, and is associated with DCM at later stage. Deletion of NFPs in cardiomyocytes disrupts endocytosis and autophagic flux. Our studies suggest that NFPs mediated endocytosis and autophagy are essential for left ventricular compaction.