Effect of toll-like receptor 4 in left ventricular cavity dimensions

Elizabeth Belcher, Salome J. Stanford, Timothy W. Evans and Jane A. Mitchell. Cardiothoracic Pharmacology and Critical Care, NHLI, Imperial College, Dovehouse Street, London, SW3 6LY, UK.

Infection is now a recognised risk factor for cardiovascular death. Toll-like receptors (TLRs) participate in the recognition of bacterial structures (Medzhitov et al., 1997). TLRs are critically involved in early, innate immune responses to infection in macrophages (Akira et al., 2001) and blood vessels (Jimenez, et al., 2005). TLR4 mediates responses to most Gram-negative bacterial LPSs. A role for TLR4 in the myocardial dysfunction associated with heart disease is beginning to emerge. Specifically, TLR4 has been implicated in atherothrombosis (Fuster et al., 2005), pressure overload cardiac hypertrophy (Ha et al., 2005), ishcaemia reperfusion injury (Chong et al., 2004) and dilated cardiomyopathy (Satoh et al., 2004); conditions all associated with changes in ventricular muscle mass and ventricular cavity dimension. The aim of this study was to determine the effects of TLR4 deletion on ventricular structure.

TLR4-deficient mice (a kind gift, Takeuchi et al., 2000) were used throughout. C57Bl/6 mice were used as controls. Adult male animals (25-30 g) were anaesthetised with intraperitoneal pentobarbitone and killed by cervical dislocation. Hearts from C57/Bl6 mice and TLR4 deficient mice were fixed in 4% buffered formalin, paraffin-embedded and sectioned (4 μm). Sections were placed on a hot block (56°C), dewaxed in histoclear, rehydrated in graded ethanol and stained with haematoxylin and eosin. Hearts were visualised by Zeiss AxioCam and analysed by Openlab software (v3.04, Improvision). Analysis of data was by unpaired t-test with Welch’s correction.

There was no difference in heart weight between the two groups (Control 0.116 ± 0.019 g n=7; TLR4-/- 0.150 ± 0.016 g n=8 p=value). Total heart area was increased in TLR4-/- hearts compared to control (Control 16.27 ± 1.169 mm2; TRL4-/- 20.18 ± 1.131 mm2; p<0.05). Left ventricular cavity size was increased in TLR4 -/- hearts compared to controls (Control 1.770 ± 0.255 mm2; TLR4-/- 2.644 ± 0.181 mm2, p=0.0175). Right ventricular cavity size was not significantly different between the two groups (Control 1.833 ± 0.324 mm2; TLR4-/- 2.950 ± 0.494 mm2). Right ventricular muscle area was not significantly increased in TLR4-/- hearts (Control 1.992 ± 0.229 mm2; TLR4-/- 2.571 ± 0.140 mm2; p=0.056). There was no difference in left ventricular muscle area between the two groups (Control 10.43 ± 0.738 mm2; TLR4-/- 11.74 ± 0.777 mm2).

TLR4 may play a role in regulation of myocardial remodelling in the absence of pathogen activation. These data support that the notion that activation of TLR4 by endogenous ligands may act as a physiological sensor in the cardiovascular system. These findings have importance implications for our understanding of how TLR4 influences cardiac function.

Akira et al., (2001) Nat Immunol 2:675;
Chong et al., (2004) J Cardiovasc Surg 128(2);170-9;
Fuster et al., (2005) J Am Coll Cardiol 46(6);937-54;
Ha et al., (2005); Cardiovasc Res 18 [Epub ahead of print];
Jimenez et al., (2005) Proc Natl Acad Sci 102(12):4637-42;
Medzhitov et al., (1997) Nature 388:394;
Satoh et al., (2204) Lab Invest 84(2): 173-81;
Takeuchi et al., (2000) J Immunol 165: 5392-6.

Supported by the MRC.