Date of Award

Summer 8-4-2025

Embargo Period

8-4-2031

Document Type

Dissertation - MUSC Only

Degree Name

Doctor of Philosophy (PhD)

Department

Regenerative Medicine and Cell Biology

College

College of Graduate Studies

First Advisor

Henry Sucov

Second Advisor

Ge tao

Third Advisor

Kristine Deleon-Pennell

Fourth Advisor

Wenjian Gan

Fifth Advisor

Robin Muise-Helmericks

Abstract

The mammalian adult heart has limited ability to regenerate, partly because most cardiomyocytes (CMs) are polyploid. The frequency of potentially regenerative diploid CMs in the adult heart is determined by multiple polymorphic genes. By identifying these genes, it might be possible to clarify mechanisms that influence whether CMs entering the cell cycle can complete it or instead fail during karyokinesis or cytokinesis.

BALB/cJ and BALB/cByJ mice are highly related sister strains that diverge substantially in mononuclear CM frequency and CM ploidy. Our evidence suggests that the difference in CM ploidy is an autosomal trait, whereas the variation in mononuclear CM frequency is X-linked. Additionally, we identified a large deletion in the Cyth1 gene that arose uniquely in BALB/cByJ mice, creating a null allele. The deletion also results in ectopic transcription of the downstream gene Dnah17, although this transcript is unlikely to encode a protein. By evaluating the natural null allele from BALB/cByJ and an engineered knockout allele in the C57BL/6J background, we determined that absence of Cyth1 alone does not influence CM ploidy. The ready availability of BALB/cByJ mice may be helpful to other investigations of Cyth1 in other biological processes.

A genome-wide association study of the BXD mouse strains implicated a locus on Chr 3 that influences the percentage of mononuclear CMs. My analysis confirmed that Spg20, located within the locus, is more highly expressed in C57BL/6J compared to DBA/2J mice. A SNP in the 3’ untranslated sequence (rs30203612) in DBA/2J mice may underlie the differential expression. The Spg20 protein (Spartin) is known in other cell types to colocalize with the ESCRT III protein Ist1 at the midbody during the cytokinesis, where it regulates the microtubule-severing protein (Spastin). Immunostaining showed that Spartin is recruited to the midbody in primary neonatal CMs during cytokinesis. This work points to a potential molecular mechanism linking Spartin with the ESCRT complex in cytokinesis, influencing CM cell cycle completion.

In addition to genetic factors, we also found that the corncob bedding used in mouse cages affects CM phenotype. In summary, these research findings contribute to a better understanding of karyokinesis and cytokinesis in CMs, paving the way for heart regeneration.

Rights

Copyright is held by the author. All rights reserved.

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Available for download on Monday, August 04, 2031

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