The role of microtubule-associated protein 1S (MAP1S) in regulating autophagy in the heart

Abstract

Autophagy is an important process to maintain cellular homeostasis in many cell types including cardiomyocytes. One type of selective autophagy which degrades defective mitochondria is called mitophagy. In the heart, defective autophagy and/or mitophagy in response to pathological stimuli may lead to the development of adverse cardiac remodelling and eventually heart failure. The microtubule-associated protein 1S (MAP1S) has previously been identified as an interacting partner of the major autophagy regulator LC3; however, its role in the heart is still unknown. In this study I hypothesised that MAP1S may play an essential role in regulating autophagy in the heart. I used mice with genetic knockout of the Map1s gene (MAP1S-/-) and neonatal rat cardiomyocytes (NRCM) with siRNA-mediated gene silencing to study the role of MAP1S in the heart and in cardiomyocytes. In response to autophagic stimulation using rapamycin and chloroquine treatment (Rap/Chl), MAP1S-deficient cardiomyocytes displayed reduction in autophagic flux with an indication of autophagososme-lysosome fusion impairment. This finding was supported by data from electron microscopy analysis of Rap/Chl- induced MAP1S-/- mice, which showed evidence of higher numbers of lysosomal structures as well as indications of altered autophagosome-lysosome fusion in MAP1S-/- mice. Furthermore, in vitro analyses using GFP-LC3 + MitoTracker co-staining and an mt-mKeima reporter system suggested that MAP1S-deficient cardiomyocytes were characterized by impairment of mitochondrial binding with autophagosomes. In addition, analysis of mitochondrial function using a Seahorse analyser showed that MAP1S depletion resulted in the reduction of mitochondrial function. Equally important, MAP1S-knockdown cardiomyocytes exhibited increased apoptosis. To study the role of MAP1S in pathological conditions in vivo, I subjected MAP1S-/- mice to myocardial infarction. Following MI, there was significantly higher mortality in MAP1S-/- mice than in WT controls, despite a comparable degree of infarction between groups as assessed by cTnI level and the fibrotic infarct area. Echocardiography analysis also suggested a reduction in ejection fraction in MAP1S-/- mice compared to WT after MI. Importantly, TUNEL assay indicated higher apoptosis in MAP1S-/- mice which might contribute to the low survival rate. This phenotype might be attributable to altered autophagy or mitophagy in the knockout animals. Taken together, my findings indicate that MAP1S plays an essential role in regulating autophagy and mitophagy in the heart. Ablation of MAP1S reduces survival and leads to the severe impairment of cardiac function after MI.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xin Wang (Supervisor) & Delvac Oceandy (Supervisor)