Cardiac Critical Care Research Program
Innate immune system such as Toll-like receptor 4 (TLR4) represents the first line of defense against infection. In addition to its pivotal role in host immunity1,2, it has been recently demonstrated that TLR4 is an important functional contributor to tissue inflammation and cell survival in response to non-infectious injury3-6. We and other investigators have demonstrated that TLR4 signaling plays an important role in ischemic myocardial injury in the heart and regulates apoptotic pathways in isolated cardiomyocytes7-10. The goals of our investigation are to define the role of cardiac TLR4 in protecting cardiomyocytes in models of ischemia-reperfusion injury (IRI) and to identify the downstream mechanisms that mediate these effects. We anticipate that insights gained from the proposed studies will serve as a foundation for the future development of novel therapeutic approaches for the management of ischemic myocardial injury.
The laboratory investigation is based on the following three hypotheses:
1) that TLR4 activation via its signaling protein IRAK-1 represents an important survival mechanism in the heart;
2) that nitric oxide synthase 2 (NOS2) mediates the TLR4-induced survival benefits; and
3) that augmentation of cardiac TLR4 signaling will reduce myocardial damage and produce a meaningful functional rescue in IRI.
To test these hypotheses, we will modulate tissue expression of TLR4 signaling molecules by germline manipulation, generation of chimeric mice via bone-marrow ablation and reconstitution, and by adenovirus-mediated somatic gene transfer. Specifically, we propose:
Aim 1: To explore the mechanisms by which TLR4 protects cardiomyocytes in in vitro models of apoptosis. We will examine how TLR4 modulates the two established pathways of apoptotic signaling.
Aim 2: To determine how NOS2 and NO contribute to TLR4-mediated anti-apoptotic effects in isolated cardiomyocytes. We will explore TLR4 regulation of NOS2 signaling and the mechanisms by which NOS2 and NO improve cardiomyocyte survival. Both cGMP-dependent and cGMP-independent (protein S-nitrosylation) signaling will be examined.
Aim 3: To ascertain the role of cardiac TLR4 in protecting myocardium against IRI. In the chimeric TLR4-/- mice and IRI models, we will define the specific role of cardiac (vs. bone-marrow-derived) TLR4 in protecting myocardium against ischemic injury. The role of NOS2 in mediating TLR4?s cardio-protective effect will be explored in intact heart.
Aim 4: To evaluate the anatomic and functional consequences of manipulating IRAK-1 in mouse models of IRI. We will test the impact of IRAK-1 deletion or cardiac IRAK-1 expression (via gene transfer) on IRI.
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- Takeda K, Kaisho T, Akira S. Toll-like receptors. Annu Rev Immunol. 2003;21:335-76.
- Jiang D, Liang J, Fan J, Yu S, Chen S, Luo Y, Prestwich GD, Mascarenhas MM, Garg HG, Quinn DA, Homer RJ, Goldstein DR, Bucala R, Lee PJ, Medzhitov R, Noble PW. Regulation of lung injury and repair by Toll-like receptors and hyaluronan. Nat Med. 2005;11:1173-9.
- Zhang X, Shan P, Qureshi S, Homer R, Medzhitov R, Noble PW, Lee PJ. Cutting edge: TLR4 deficiency confers susceptibility to lethal oxidant lung injury. J Immunol. 2005;175:4834-8.
- Qureshi ST, Zhang X, Aberg E, Bousette N, Giaid A, Shan P, Medzhitov RM, Lee PJ. Inducible activation of TLR4 confers resistance to hyperoxia-induced pulmonary apoptosis. J Immunol. 2006;176:4950-8.
- Glezer I, Lapointe A, Rivest S. Innate immunity triggers oligodendrocyte progenitor reactivity and confines damages to brain injuries. Faseb J. 2006;20:750-2.
- Oyama J, Blais C, Jr., Liu X, Pu M, Kobzik L, Kelly RA, Bourcier T. Reduced myocardial ischemia-reperfusion injury in toll-like receptor 4-deficient mice. Circulation. 2004;109:784-9.
- Chong AJ, Shimamoto A, Hampton CR, Takayama H, Spring DJ, Rothnie CL, Yada M, Pohlman TH, Verrier ED. Toll-like receptor 4 mediates ischemia/reperfusion injury of the heart. J Thorac Cardiovasc Surg. 2004;128:170-9.
- Chao W, Shen Y, Zhu X, Zhao H, Novikov M, Schmidt U, Rosenzweig A. Lipopolysaccharide improves cardiomyocyte survival and function after serum deprivation. J Biol Chem. 2005;280:21997-2005.
- Zhu X, Zhao H, Graveline AR, Buys ES, Schmidt U, Bloch KD, Rosenzweig A, Chao W. MyD88 and NOS2 are essential for toll-like receptor 4-mediated survival effect in cardiomyocytes. Am J Physiol Heart Circ Physiol. 2006;291:H1900-9.