Postgraduate Studies Projects

Project 1 - The presence of metabolic abnormalities in young adults and their effects on organ damage – implication of the sympathetic nervous system

Supervisors: Elisabeth Lambert, Gavin Lambert, Tye Dawood

Email: elisabeth.lambert@bakeridi.edu.au; tye.dawood@bakeridi.edu.au

Phone: 8532 1345 or 8532 1279

The metabolic syndrome, characterized by abdominal obesity, insulin resistance, hypertension, dyslipidaemia, hyperglycaemia and a systemic pro-inflammatory state is a common disorder in Australia. Persons with the metabolic syndrome are at an increased risk of incident diabetes and cardiovascular and kidney disease, thereby making it a major public health issue. Our current project is addressing the issue of the presence of obesity in young adults and its association with metabolic abnormalities on early organ damage. Previous work from our laboratory has established the importance of sympathetic nervous system (SNS) activation in metabolic and cardiovascular function. Dysfunction in the sympathetic control of the cardiovascular and metabolic function might be responsible for the related cardiovascular risk. Our recent finding of an increased SNS activity in young obese subjects compared with their lean counterparts suggests that such activation in the SNS at a young age may lead to early organ damage, therefore compromising the cardiovascular system later in life. Furthermore, our preliminary data implicates stress in the generation of some symptoms associated with the metabolic syndrome.

There is some evidence that at least 3 key organs may be affected by obesity:

• The heart: Studies have shown that obesity and metabolic syndrome are associated with compromised structure and function of the heart in subjects aged over 45 years. In obese, otherwise healthy younger women, without any associated metabolic disorder, early abnormalities in left ventricular structure and function were detectable.
• The kidneys: Recent findings have suggested an association between the metabolic syndrome and chronic kidney disease. There is now evidence that chronic kidney disease is characterized by an activated SNS, which may adversely affect prognosis. In young healthy men, high metabolic risk was recently shown to be associated with glomerular hyperfiltration well before overt manifestations of cardiovascular disease were evident.
• The endothelium: Vascular endothelial dysfunction, which represents the earliest abnormality in the development of vascular disease, has been demonstrated in overweight patients with insulin resistance and with visceral obesity.

This study is looking at whether:

• SNS activation in young obese subjects is associated with demonstrable early defects in cardiac, renal and endothelial function and
• the damage will be exacerbated by the presence of metabolic abnormalities. The project would be suitable for somebody with an interest in clinical research, biochemistry and also psychology/psychiatry.

Project 2 - Understanding Fontan physiology: Improving outcomes in those with non correctable congenital heart conditions

Supervisor: Gavin Lambert / Elisabeth Lambert

Email: gavin.lambert@bakeridi.edu.au,                                   elisabeth.lambert@bakeridi.edu.au                                              

Since its conception in 1971, the Fontan procedure has been used as the final stage of palliation for all children born with congenital heart disease who could not be offered a two-ventricle repair. This procedure comprises the redirecting of the systemic veins to drain directly into the pulmonary arteries, without passing through the heart. This is a unique group of patients in which a functionally single ventricle is used to drive the systemic circulation following a series of palliative procedures. It has recently been shown that morbidity related to Fontan circulation increases with increasing duration of post-surgical follow-up. While the mechanisms underpinning Fontan failure remain to be identified there exists evidence implicating the possible involvement of the sympathetic nervous system.

We will examine sympathetic nervous system function in Fontan patients using direct nerve recording techniques. We will determine the relationship between elevated sympathetic activity in Fontan patients and (1) the presence and degree of diastolic dysfunction, (2) endothelial function, and (3) arterial compliance. We will test the following hypotheses:

• That sympathetic activity is elevated in Fontan patients.
• That there exists a relation between the extent of diastolic dysfunction and the magnitude of sympathetic activation in Fontan patients.
• That endothelin-1 levels are elevated in Fontan patients and are associated with endothelial dysfunction and the degree of sympathetic activation.
• That arterial compliance in Fontan patients is impaired and is related to the degree of sympathetic activation.

Clinical significance

Heart transplantation is the ultimate andonly definitive treatment for Fontan failure. The mechanisms underpinning Fontan failure remain poorly understood, thereby making therapy difficult to optimise. Knowledge of the mechanisms at play in Fontan patients may pave the way for the development of novel therapeutic approaches (e.g. endothelin antagonism or sympathetic nervous system inhibition) in order to improve morbidity and mortality in this vulnerable patient group.

Project 3 - High sympathetic activity in major depressive disorder- a cause for concern?

Supervisor:              Gavin Lambert

Email:                        gavin.lambert@bakeridi.edu.au;                                                                      

Phone:                      8532 1346

There is strong evidence that patients with major depressive disorder (MDD) are at increased risk of developing coronary heart disease. While the mechanism of increased cardiac risk attributable to MDD at this stage is not known, recent work by us, and others, implicates brain serotonin, serotonin transporter genotype (5-HTT) and the sympathetic nervous system as possible candidates involved in generating increased risk. Using high internal jugular vein blood sampling we demonstrated that brain serotonin turnover was elevated in patients with MDD and was influenced by the 5-HTT genotype, with carriage of the short allele being associated with an over 2-fold increase in brain serotonin turnover (Barton, Arch Gen Psych 2008). In parallel, using direct cardiac catheterization techniques coupled with state of the art noradrenaline isotope dilution methodology we also showed that whole body and cardiac sympathetic nervous activity in patients with MDD follows a bimodal distribution, with values in some patients being extraordinarily high and others being marginally lower than those found in healthy subjects (Barton, J Hyperten 2007). Further analysis, presented as pilot data in our application, indicates a link between carriage of the short allele of the 5-HTT gene and elevated sympathetic nervous activity in MDD.

While at this stage MDD patients with high sympathetic activity do not present with any evidence of current clinical cardiac disease, we, and others, have provided a growing body of evidence linking elevated sympathetic activity and exaggerated sympathetic responses to stress to early stages of end organ dysfunction and markers of disease development. Of particular note is the association of chronic sympathetic nervous activation to: (a) the development of left ventricular hypertrophy (LVH) and diastolic dysfunction, (b) abnormal blood pressure regulation, and (c) the development of insulin resistance. In this project we will determine the physiological consequences of sympathetic activation in patients with MDD.

Clinical significance

There is a clear need to identify the underlying mechanisms responsible for MDD and their linkage to the heart and vascular system. Further knowledge of the mechanisms responsible for generating cardiac risk may pave the way for novel and perhaps relatively simple therapeutic strategies (eg -blockers, central sympatholytic agents, renin angiotensin system blockers, statins, diet, exercise) to be administered in those with MDD in order to modify cardiac risk.

Project 4 - Stress and the metabolic syndrome: Making it a target for therapy

Supervisor: Gavin Lambert / Tye dawaood

Email: gavin.lambert@bakeridi.edu.au, tye.dawood@bakeridi.edu.au    

Phone: 8532 1346 or 8532 1279

A growing body of evidence links "stress" and the development of the metabolic syndrome. Recent research strongly suggests that activation of both the sympathetic nervous system (SNS) and the hypothalamic-pituitary-adrenal (HPA) axis, the two most prominent biological "stress pathways", are involved in generating the metabolic abnormalities that characterise the metabolic syndrome.

Hypotheses

1. Stress pathways are deregulated in the metabolic syndrome.
2. HPA axis and SNS activation are modified in the metabolic syndrome at rest and during stress.
3. Weight loss will have an effect on stress responses in the metabolic    syndrome.

Aims

1. To examine stress pathways in the metabolic syndrome.
2. To investigate the role and mechanisms of action of the HPA axis and SNS activity in the metabolic syndrome at rest and during stress.
3. To determine the effect of weight loss on stress responses in the metabolic syndrome.

Research Plan

We propose to investigate the SNS and HPA axis in subjects with the metabolic syndrome, before and following a 3 month weight loss period. Our pilot data indicate that those subjects with a higher anxiety profile exhibit elevated sympathetic activity, higher plasma cortisol levels and a worse metabolic profile compared with those with lower stress levels. Sympathetic activity will be measured using clinical microneurography and from blood samples, obtained from an arterial catheter, for determination of the rate of spillover of noradrenalmine into plasma. Blood will be collected for the measurement of HPA activity (ie plasma cortisol and ACTH), and insulin and glucose determinations. Measurements will be taken prior to and after mental stress. Moreover, a dexamethasone suppression test will be performed to assess the effectiveness of the negative feedback control mechanisms controlling the HPA axis. In parallel, to determine the central nervous system mechanisms involved, we will use our well-established sheep model of metabolic syndrome to investigate SNS and HPA activation. Our sheep model of metabolic syndrome has high levels of insulin and glucose which are of a similar magnitude to those observed in patients with the metabolic syndrome. The sheep will be subjected to a psychological stressor, and measurements taken for ACTH, cortisol and catecholamines. Furthermore, we will conduct hypophysial portal sampling from metabolic syndrome sheep that are exposed to stress and measure the levels of CRH and AVP in hypophysial portal blood. This technique is unique to our laboratory and allows investigation of mechanisms which are not possible in human or rodent models.

Outcomes and Significance

Weight loss is an important component of treating the metabolic syndrome. We hypothesise that subjects who suffer from the metabolic syndrome will have improved responses to stress after a period of weight loss. The mechanisms, by which stress responses are activated in the metabolic syndrome and the attenuation of responses in some individuals, and not others, are not well understood. We aim to provide a better understanding of the role of stress in the metabolic syndrome and the attenuation of the stress response seen in lean individuals by focusing on the HPA axis and activity of the SNS, which are the major systems activated by stress. Such knowledge is essential for the development of novel therapies that may counteract the activation of stress pathways leading to the characteristic pattern of abnormalities seen in the metabolic syndrome.

Project 5 - The Neural Biology of Takotsubo (Stress) Cardiomyopathy

Supervisor: Murray Esler

Email: murray.esler@bakeridi.edu.au

It has always seemed plausible that short-term mental stress can act as a trigger for the development of myocardial infarction and sudden death in patients with existing heart disease, but the case for this was inconclusive. In recent years systematic evidence has been gathered at times of disasters, including war, missile attacks on civilians, and earthquakes, which strongly supports this proposition of a mental stress-heart attack link. A cardiac endpoint of severe mental stress totally different from the above is now increasingly recognised, following the initial description by Sato and colleagues in 1990. This is a unique form of cardiomyopathy, the octopus pot (Takotsubo), or "Broken Heart" Syndrome. The origin of the Japanese name derives from the fancied resemblance of the afflicted heart, with its bulbous, stunned apex but a normally contacting base to a narrow-mouthed octopus trap. The biological mechanisms of Takotsubo cardiomyopathy are uncertain, and the optimal treatment remains unknown, perhaps to be expected in a disorder where the precise pathogenesis is uncertain. This project will attempt to delineate the mediating neural biology of Takotsubo cardiomyopathy, studied in the acute phase, to test for genetic vulnerabilities predisposing towards this disorder, to test for biological vulnerabilities to Takotsubo cardiomyopathy relating to age and gender, and to evaluate potential psychological and social predisposition to the condition.

Neural Mechanisms of Takotsubo Cardiomyopathy

We will evaluate whether Takotsubo cardiomyopathy is mediated by extreme activation of the cardiac sympathetic outflow, by applying the technique of isotope dilution to measure cardiac noradrenaline spillover in the disorder and in comparator groups. We will test whether specific neural characteristics, namely impairment of neuronal reuptake of noradrenaline, co-release of adrenaline from sympathetic nerves, and release of NPY act as components of pathogenesis.

Genetic Predisposition

We will assess whether Takotsubo sufferers are genetically predisposed to extreme activation of the sympathetic nervous system with mental stress, through (i) A predisposing polymorphism of the serotonin transporter (5HTT) gene, the presence of the promoter region short form, which leads to an intensified CNS stress response, and enhanced sympathetic outflow from the brain, and/or (ii) Genetic augmentation of sympathetic neuronal responses through influences on the synthesis, storage, vesicular recycling/release and reuptake of noradrenaline, which we will investigate with the quantitative analysis of the relevant sympathetic nerve proteins, specifically tyrosine hydroxylase, the vesicular monoamine transporter (VMAT2), dynamin I and the noradrenaline transporter. These proteins will be accessed by biopsy of a small subcutaneous forearm vein. Subcutaneous veins have a dense sympathetic innervation.

Do Sympathetic Nerve Changes Associated with Healthy Aging Predispose to Takotsubo Cardiomyopathy?

We will test whether age and female gender predispose to the development of Takotsubo cardiomyopathy via identifiable neural mechanisms in sympathetic nerves, specifically through the very pronounced increase in sympathetic nervous system activity which occurs with ageing in females, and by impairment of neuronal noradrenaline reuptake and through co-release of adrenaline in sympathetic nerves in older men and women.

Psychological and Social Vulnerability

Psychological and social vulnerability to Takotsubo Cardiomyopathy presumably exists. We will evaluate whether this takes the form of pre-existing depressive illness, anxiety disorder (including panic disorder, and increased anxiety sensitivity, specifically of the "cardiac reactor" form) and social isolation. There is preliminary evidence that underlying depressive illness, anxiety disorder and social isolation may be contributors. Methodology will be used which allows discrimination of the current psychological state, which will be influenced by the recent stress (and often bereavement), from the antecedent psychological picture.

Project 6 - A longitudinal study of the effect of substantial weight loss on hypertension and target organ damage in obese subjects

Supervisors: Associate Professor John Dixon, Dr Gavin Lambert

Email: john.dixon@bakeridi.edu.au, gavin.lambert@bakeridi.edu.au

Phone:  8532 1115 or 8532 1346

Obesity and hypertension are two of the most common medical problems encountered in today's society. The fact that the two conditions are associated is undeniable, but the precise relationship and especially the underlying pathophysiological mechanisms, are still poorly understood. Previous work by our group has examined the effects of weight loss in obese patients in order to learn more about various other obesity-related comorbidities such as Type II Diabetes, asthma, depression and sleep apnea. We now hope to extend our previous studies to better describe the mechanisms linking obesity and hypertension. An ideal human model to explore these important relationships has been patients undergoing weight loss surgery using the laparoscopic adjustable gastric band (LAGB), because of the substantial sustained weight loss achieved through LAGB surgery.

The current project is a longitudinal study of 220 obese patients who have elected to undergo LAGB surgery with the surgical team at LapSurgery Australia, collaborators of our group. We aim to characterise obese subjects who do and do not develop hypertension and those who do and do not respond to substantial weight loss with a reduction in blood pressure. A range of clinical and psychometric assessments will be performed at baseline and again following 10% weight loss, at 1 and 2 years following surgery. The parameters to be measured include:

• Anthropometry and body composition by DEXA scan
• 24-hour blood pressure
• sympathetic nerve activity, measured by microneurography, a specialised technique of our group
• endothelial function and cardiac structure and function, measured by ECG and echocardiography
• circulating levels of lipids, glucose, adipokines and inflammatory markers, through plasma assays
• tissue expression of adipokines and markers of metabolic and inflammatory activity, using biopsies obtained during surgery
• renal function by measurement of RAS activation and urinalysis
• markers of target organ damage, such as retinopathy
• sleep architecture and disturbance, by overnight polysomnography
• diet, quality of life and psychological health (depression and anxiety), through questionnaires and structured interviews.

This study contains sufficient scope for a PhD project, with the student able to become involved in the recruitment and co-ordination of study participants, conducting many of the above clinical tests and performing laboratory analyses on samples obtained. The project would suit someone with interests in clinical research and or/ molecular biology, as well as the massive health problems of obesity and hypertension.

Project 7 - Neurogenic mechanisms of cardiovascular risk in metabolic syndrome obesity: Benefits of dietary weight loss with our without exercise training

Supervisor: Dr Nora E. Straznicky

Email: nora.straznicky@bakeridi.edu.au

Phone: 8532 1371

The metabolic syndrome (MetS) is an increasingly prevalent multi-dimensional risk factor for cardiovascular disease and type 2 diabetes. Its aetiology is complex and incompletely understood, but thought to involve the interplay between metabolic susceptibility, lifestyle factors and the acquisition of excess visceral adiposity. Scientific studies performed over the last two decades strongly support the relevance of the sympathetic nervous system (SNS) in both the pathogenesis and target organ complications of MetS obesity.

Weight loss and exercise are recommended as first-line treatments for the MetS and individually, both these life-style modalities cause sympathoinhibition and improvement in MetS components. We have recently completed a randomized controlled trial of dietary weight loss alone or together with aerobic exercise in untreated, middle-aged MetS subjects. We now wish to measure a range of pro-inflammatory and anti-inflammatory markers in blood to examine (1) the effects of lifestyle interventions and (2) the inter-relationships between changes in inflammatory markers and SNS activity, anthropometric, metabolic and cardiovascular parameters.