Cardiac side population cells: moving toward the center stage in cardiac regeneration

Abstract

Over the past decade, extensive work in animal models and humans has identified the presence of adult cardiac progenitor cells, capable of cardiomyogenic differentiation and likely contributors to cardiomyocyte turnover during normal development and disease. Among cardiac progenitor cells, there is a distinct subpopulation, termed "side population" (SP) progenitor cells, identified by their unique ability to efflux DNA binding dyes through an ATP-binding cassette transporter. This review highlights the literature on the isolation, characterization, and functional relevance of cardiac SP cells. We review the initial discovery of cardiac SP cells in adult myocardium as well as their capacity for functional cardiomyogenic differentiation and role in cardiac regeneration after myocardial injury. Finally, we discuss recent advances in understanding the molecular regulators of cardiac SP cell proliferation and differentiation, as well as likely future areas of investigation required to realize the goal of effective cardiac regeneration.

Figure 1. A representative FACS plot of CSP cells

Blue plots and green plots indicate CD31 negative and CD31 positive cells, respectively. CSP cells are identified as both Hoechst blue low and Hoechst red low population in the FACS plot (enclosed population).

Figure 2. Dye efflux capacity of CSP cells is calcium independent

(A). A representative FACS plot of CSP isolation with regular Hoechst 33342 incubation medium (Ca²⁺ concentration of 1.8mM). (B). Verapamil inhibits SP phenotype completely. (C). Ca²⁺ is chelated from the incubation media with EGTA. CSP population is not affected by Ca²⁺ deprivation, suggesting that verapamil inhibits ABC transporters through a Ca²⁺ independent manner.

Figure 3. The expression of ABC transporters on CSP cells

CSP cells are positive for both Mdr1 and Abcg2 (CSP cells were isolated from 8 weeks old C57BL6 mouse). Scale bar indicates 5μm.