Date of Graduation

8-2015

Document Type

Dissertation (PhD)

Program Affiliation

Cell and Regulatory Biology

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

Shane Cunha, Ph.D.

Committee Member

Russell Broaddus, M.D./Ph.D.

Committee Member

Mong-Hong Lee, Ph.D.

Committee Member

Ruth Heidelberger, M.D./Ph.D.

Committee Member

Dat Tran, M.D.

Abstract

The mammalian target of rapamycin complex 1 (mTORC1) activity is paramount in the regulation of electrical activities in the brain and the heart. In the brain, the tumor suppressor gene TSC2 encodes the protein product tuberin that interacts with hamartin to form a heterodimer Tuberous Sclerosis Complex (TSC) that regulates mTORC1. When TSC2 is disrupted, mTORC1 activity becomes dysregulated resulting in abnormal electrical activities in the brain manifesting in the form of epileptic seizures. In the heart, mTORC1 activity is triggered by a sustained increase in hemodynamic pressure causing the heart to electrically remodel. A likely candidate serving as the mediator between mTORC1-dependent electrical remodeling in the brain and heart is the adaptor protein ankyrin. In the heart, ankyrin-B targets and maintains the membrane expression of ion channels and transporters that are critical for maintaining calcium ion homeostasis, which underlies normal excitation-contraction coupling. Sustained mTORC1 activity in the heart decreases the expression of ankyrin-B and alters the electrical conductance between the atria and ventricles. These effects are reversed with the administration of the mTORC1 inhibitor rapamycin. In addition, we identified and characterized two functionally and spatially distinct full-length ankyrin-B isoforms – AnkB-188 and AnkB-212. AnkB-188 selectively interacts with the sodium-calcium exchanger (NCX1) increasing its membrane expression, overall current, and targeting to the sarcoplasmic reticulum/transverse-tubule of neonatal cardiomyocytes. Whereas AnkB-212 does not increase NCX1 membrane expression or current, but uniquely localizes to the sarcomeric M-line. Knockdown of either isoform results in abnormal contraction rhythms in vitro, but only the M-line population appears to be regulated by mTORC1. Collectively, the data support the hypothesis that mTORC1 regulates electrical remodeling of the stressed heart by decreasing the expression of the ankyrin-B population at the M-line.

Keywords

ankyrin, mTOR, tuberous sclerosis complex, cardiovascular disease, arrhythmia, rapamycin, heart failure