Group B streptococcal beta-hemolysin/cytolysin directly impairs cardiomyocyte viability and function.
BACKGROUND
Group B Streptococcus (GBS) is a leading cause of neonatal sepsis where myocardial dysfunction is an important contributor to poor outcome. Here we study the effects of the GBS pore-forming beta-hemolysin/cytolysin (Bh/c) exotoxin on cardiomyocyte viability, contractility, and calcium transients.


RESULTS
HL-1 cardiomyocytes exposed to intact wild-type (WT) or isogenic Deltabeta h/c mutant GBS, or to cell-free extracts from either strain, were assessed for viability by trypan blue exclusion and for apoptosis by TUNEL staining. Functionality of exposed cardiomyocytes was analyzed by visual quantitation of the rate and extent of contractility. Mitochondrial membrane polarization was measured in TMRE-loaded cells exposed to GBS beta h/c. Effects of GBS beta h/c on calcium transients were studied in fura-2AM-loaded primary rat ventricular cardiomyocytes. Exposure of HL-1 cardiomyocytes to either WT GBS or beta h/c extracts significantly reduced both rate and extent of contractility and later induced necrotic and apoptotic cell death. No effects on cardiomyocyte viability or function were observed after treatment with Deltabeta h/c mutant bacteria or extracts. The beta h/c toxin was associated with complete and rapid loss of detectable calcium transients in primary neonatal rat ventricular cardiomyocytes and induced a loss of mitochondrial membrane polarization. These effects on viability and function were abrogated by the beta h/c inhibitor, dipalmitoyl phosphatidylcholine (DPPC).


CONCLUSIONS
Our data show a rapid loss of cardiomyocyte viability and function induced by GBS beta h/c, and these deleterious effects are inhibited by DPPC, a normal constituent of human pulmonary surfactant.. These findings have clinical implications for the cardiac dysfunction observed in neonatal GBS infections.
Department of Pediatrics, University of California San Diego, La Jolla, California, United States of America.
Mary E
ME
Hensler
Shigeki
S
Miyamoto
Victor
V
Nizet
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D006460:Hemolysin Proteins / Q000378:metabolism*; D006801:Humans; D020287:In Situ Nick-End Labeling; D007231:Infant, Newborn; D008564:Membrane Potentials; D008929:Mitochondria, Heart / Q000502:physiology; D009206:Myocardium / Q000166:cytology*; D054353:Perforin / Q000502:physiology*; D051381:Rats; D013290:Streptococcal Infections / Q000503:physiopathology; D013292:Streptococcus agalactiae / Q000502:physiology*
D006460:Hemolysin Proteins; D054353:Perforin; D002118:Calcium