Characterisation of a novel GTP cyclohydrolase 1 feedback regulatory protein (GFRP) over expressing mouse in a model of septic shock

Abstract

Sepsis, a leading cause of death in intensive care patients worldwide, occurs when the body elicits an overwhelming inflammatory response to a microbial infection. Initial hypotension can escalate to serious circulatory collapse (septic shock) with ensuing multiple organ failure. Nitric oxide (NO) is produced in excess by inducible nitric oxide synthase (iNOS), contributing to the hypotension observed. Tetrahydrobiopterin (BH4) is an essential cofactor for NOS and is produced from GTP by GTP cyclohydrolase 1 (GCH1). GCH1 can be regulated by BH4, through allosteric feedback inhibition, when GCH1 is bound to GCH1 regulatory feedback protein (GFRP). Our study investigates whether allosteric regulation of GCH1 by GFRP in the cardiovascular system is an important mechanism regulating BH4 and NO in vivo, using GFRP over expressing mice, in an experimental model of septic shock. Refined techniques of radiotelemetry and echocardiography were used to investigate cardiovascular changes following cecal ligation and puncture (CLP), a clinically relevant model of sepsis. Temporal changes in BH4 were also measured.The characteristic hypotension, cardiac output reduction and vascular tissue BH4 increases seen following CLP, consistent with a profound inflammatory response and excessive NO production, were less pronounced in GFRP over expressing mice than in wildtype littermates. These findings suggest that altering the ratio between GFRP and GCH1 may impact on the bioavailability of BH4 and indicate that pharmacological modulation of the GCH1-GFRP complex may provide a means to limit the pathophysiological NO production that occurs in septic shock patients.