Effects of anti-inflammatory bioactives on diabetes-induced changes in cognition-related gene expression in the hippocampus

Abstract

Research Doctorate - Doctor of Philosophy (PhD)Diabetes is a metabolic disorder that leads to other health complications over a period of time and is the cause for considerable morbidity and mortality world-wide. The complications of diabetes in the peripheral tissues are well characterized including coronary heart disease, retinal degeneration, renal disorders, and also micro and macro vascular complications. Over the last few decades much emphasis has been placed on the complications of diabetes that occur in the central nervous system (CNS). One such neuropathology due to diabetes in CNS is cognitive impairment. The hippocampus, the limbic structure, is involved in higher brain functions and appears to be particularly vulnerable to diabetes. This thesis presents studies conducted on hippocampal gene expression profiles under diabetic and non-diabetic conditions which would give us a clear understanding of the molecular mechanisms that underpin cognitive impairment due to diabetes. To understand gene expression profile in the hippocampus region of the brain, microarray technology was used and the genes of interest were further confirmed by quantitative real-time polymerase chain reaction (RT-qPCR). Investigation of two dietary interventions, fish oil rich in docosahexaenoic acid (DHA) and resveratrol (RES) were also carried out to examine the mechanism by which these naturally occurring compounds may act to provide neuronal protection thereby aiding in the prevention and delay of cognitive impairment due to diabetes. The first research chapter (Chapter 3), presents a study on the comparison of gene expression profile in the hippocampus of a streptozotocin (STZ) induced diabetic mice and a vehicle treated control animals. Here it is demonstrated that diabetes causes significant alterations in the genes that plays a crucial role in synaptic function and plasticity and also for neurogenesis, both of which are required for normal cognitive functions. Six weeks after diabetes was established in these mice, a number of genes had altered expression including genes involved in epigenetic regulation, and this included histone deacetylase (Hdac) 4, 9 and 11. Interestingly Hdac 4 and 9 are abundantly present in the hippocampus and are required for hippocampal dependent learning, memory and synaptic plasticity. Glycogen synthase kinase beta (Gsk3β) which has been shown to have a crucial role in metabolic and neurodevelopmental functions and considered to be an important regulator of synaptic functions, also exhibited significant decrease in the STZ induced diabetic mice as opposed to the buffer treated control group. The marked decrease of these genes that are crucial for higher functions in the hippocampus underscores the impact of uncontrolled diabetes on the hippocampus. Furthermore genes that have been linked to neurological disorder and cognitive dysfunction such as apolipoprotien E (ApoE) showed increased expression in diabetic mice as opposed to the non- diabetic control group. These findings implicate the abnormal transcription of genes which could disrupt normal cognitive functions in the hippocampus and also underscores epigenetic mechanisms involved in disease conditions like diabetes. The second research chapter (Chapter 4) explores whether a dietary intervention had any effect on the hippocampal gene expression profile in non- diabetic mice. A vast body of evidence indicates that DHA and RES have a beneficial effect on cognitive functions. In this chapter, we also sought to examine the effects of DHA and RES individually or in combination on hippocampal gene expression. Here it is demonstrated that both DHA and RES modulate inflammatory signals; neurotrophin signalling and neurogenesis related mechanisms. In the combined intervention group, additional genes such as Sod1, Sod2, Hmox1 and ApoE in pathways such as antioxidants and apo-lipoprotein pathways were also implicated. This study provides insight into the effects of dietary modification that modulate gene expressions favourably for normal cognitive functions and prevent or delay neurodegeneration in disease conditions such as diabetes or those seen in aging. The third research chapter (chapter 5), presents a study aimed at exploring the effects of a RES dietary supplementation on hippocampal gene expression profile in STZ induced diabetic mice as opposed to a non-supplemented diabetic group. Here it is demonstrated that dietary supplementation with RES significantly increases the expression of histone deacetylase 4(Hdac4), superoxide dismutase 2(Sod2), heme oxygease 1(Hmox1), and decreases the expression of pro-inflammatory interleukin22 (Il22), interleukin 15 (Il15), suppressor of cytokine signalling 2(Socs2), suppressor of cytokine signalling 5 (Socs5) exhibiting that RES has diverse biological properties, in addition to its already established properties as a potent antioxidant and anti-inflammatory compound. The fourth research chapter (chapter 6), presents a study aimed at exploring the effects of dietary supplementation with DHA on hippocampal gene expression profile in STZ induced diabetic mice. Here we demonstrated that dietary intervention of DHA had anti-inflammatory effects as opposed to the non- supplemented diabetic group. Dietary supplementation of DHA also exhibited an increased expression of genes that are involved in neuronal protection and survival. DHA supplementation showed an increase in insulin like growth factor II (IgfII) and also sirtuin deacetylase 1(Sirt1) expression. A decrease in pro-inflammatory tumor necrosis factor- alpha (Tnf-α), interleukin 6 (Il6) and apolipoprotien E (ApoE) was seen in DHA supplemented diabetic mice as opposed to non- supplemented diabetic mice group. Together these studies have investigated the molecular mechanism that underpins cognitive dysfunction that are apparent in diabetes and have also explored the effect of nutritional intervention with RES and DHA in the hippocampus of STZ induced diabetic and non-diabetic mice. Of the many therapeutic approaches nutritional intervention appears promising and evidence from the present study reinforces the potential of nutritional intervention to modulate the complex aberrant metabolic insults that initiate and underpin cognitive impairments due to diabetes