Characterisation of substance P and transient receptor potential melastatin channel messenger RNA and protein expression in acute and chronic neurological disorders.

Abstract

Traumatic brain injury (TBI) is the leading cause of death and disability in people under 40 years of age, with motor vehicle incidents accounting for the majority of severe TBI cases. Despite the public health burden of TBI, there are no effective treatment options available, with survivors often left with debilitating long-term deficits. Following TBI, a cascade of pathophysiological processes is initiated in the central nervous system, including oedema, inflammation, magnesium decline and oxidative stress. These factors play a role in the high morbidity and mortality following TBI, however, their underlying molecular mechanisms remain poorly understood. Parkinson’s Disease (PD) is a common neurodegenerative disease and affects approximately 1 % of the population over 65 years of age. PD is characterised by the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, leading to a reduction of dopamine levels in the striatum. The pathogenesis of PD is poorly understood, but is likely to involve oxidative stress and inflammatory processes. Current treatments that replace dopamine lose efficacy after several years. Treatments for TBI and PD are thus urgently required; this requires a greater understanding of the pathophysiology of these disorders at a molecular level. Recent studies from our laboratory have demonstrated a link between the neuropeptide, substance P (SP), and the development of cerebral oedema and neurologic deficits following TBI, which are attenuated with the administration of an NK-1 (neurokinin-1, SP receptor) antagonist. In addition, studies using a rat model of PD have similarly established a putative role for SP in this disease process. Transient receptor potential melastatin (TRPM) channels are a diverse family of ion channels, many of which are highly expressed in the brain. It is likely that TRPM7 and TRPM6 regulate cellular magnesium homeostasis. TRPM7 and TRPM2 are critical mediators of ischaemic neuronal death, and mutations in the TRPM7 and TRPM2 genes confer a genetic susceptibility to parkinsonism. The function of TRPM3 is not well understood, but evidence suggests it may be involved in brain function. The aims of the present thesis were to: quantify the mRNA level and protein expression of SP, TRPM2, TRPM3, TRPM6 and TRPM7 channels following TBI in human clinical cases and over a time course of experimental TBI in rats; and to characterise the mRNA level of SP, TRPM2, TRPM3 and TRPM7 channels in both clinical PD cases and two rodent models of PD (early and late disease stage), and the protein expression of TRPM channels in early experimental PD. We demonstrate an upregulation of SP expression in clinical and experimental TBI, supporting our previous studies implicating SP release with TBI pathophysiology. Changes in TRPM channel expression at both the transcript and protein level were also observed following both TBI and in PD, suggesting that TRPM channels may contribute to the oxidative stress, inflammation and neuronal death associated with these disorders. This thesis provides novel information regarding the molecular mechanisms underlying TBI and PD, which is relevant to the development of effective treatment strategies.