THE MOLECULAR INTERACTION BETWEEN TYPE II DIABETES AND ALZHEIMER’S DISEASE THROUGH CROSS-SEEDING OF PROTEIN MISFOLDING

With the population of the world aging, the prominence of diseases such as Type II Diabetes (T2D) and Alzheimer’s disease (AD) are on the rise. In addition, patients with T2D have an increased risk of developing AD compared to age-matched individuals, and the number of AD patients with T2D is higher than among aged-matched non-AD patients. AD is a chronic and progressive dementia characterized by amyloid-beta (Aβ) plaques, neurofibrillary tangles (NFTs), neuronal loss, brain inflammation, and cognitive impairment. T2D involves the dysfunctional use of pancreatic insulin by the body resulting in insulin resistance, hyperglycemia, hyperinsulinemia, pancreatic beta cell (β-cell) death, and other complications. T2D and AD are considered protein misfolding disorders (PMDs). PMDs are characterized by the presence of misfolded protein aggregates, such as in T2D pancreas (islet amyloid polypeptide - IAPP) and in AD brain (amyloid– Aβ) of affected individuals. The misfolding and accumulation of these proteins follows a seeding-nucleation model where misfolded soluble oligomers act as nuclei to propagate misfolding by recruiting other native proteins. Cross-seeding occurs when oligomers composed by one protein seed the aggregation of a different protein. Our hypothesis is that the pathological interactions between T2D and AD may in part occur through cross-seeding of protein misfolding. To test this hypothesis, we examined how each respective aggregate (Aβ or IAPP) affects the disparate disease pathology through in vitro and in vivo studies. Assaying Aβ aggregates influence on T2D pathology, IAPP+/+/APPSwe+/- double transgenic (DTg) mice exhibited exacerbated T2D-like pathology as seen in elevated hyperglycemia compared to controls; in addition, IAPP levels in the pancreas are highest compared to controls. Moreover, IAPP+/+/APPSwe+/- animals demonstrate abundant plaque formation and greater plaque density in cortical and hippocampal areas in comparison to controls. Indeed, IAPP+/+/APPSwe+/- exhibit a colocalization of both misfolded proteins in cerebral plaques suggesting IAPP may directly interact with Aβ and aggravate AD pathology. In conclusion, these studies suggest that cross-seeding between IAPP and Aβ may occur, and that these protein aggregates exacerbate and accelerate disease pathology, respectively. Further mechanistic studies are necessary to determine how these two proteins interact and aggravate both pancreatic and brain pathologies.

CONFORMATIONAL CHANGES IN THE EXTRACELLULAR DOMAIN OF GLUTAMATE RECEPTORS

The family of membrane protein called glutamate receptors play an important role in the central nervous system in mediating signaling between neurons. Glutamate receptors are involved in the elaborate game that nerve cells play with each other in order to control movement, memory, and learning. Neurons achieve this communication by rapidly converting electrical signals into chemical signals and then converting them back into electrical signals. To propagate an electrical impulse, neurons in the brain launch bursts of neurotransmitter molecules like glutamate at the junction between neurons, called the synapse. Glutamate receptors are found lodged in the membranes of the post-synaptic neuron. They receive the burst of neurotransmitters and respond by fielding the neurotransmitters and opening ion channels. Glutamate receptors have been implicated in a number of neuropathologies like ischemia, stroke and amyotrophic lateral sclerosis. Specifically, the NMDA subtype of glutamate receptors has been linked to the onset of Alzheimer’s disease and the subsequent degeneration of neuronal cells. While crystal structures of AMPA and kainate subtypes of glutamate receptors have provided valuable information regarding the assembly and mechanism of activation; little is known about the NMDA receptors. Even the basic question of receptor assembly still remains unanswered. Therefore, to gain a clear understanding of how the receptors are assembled and how agonist binding gets translated to channel opening, I have used a technique called Luminescence Resonance Energy Transfer (LRET). LRET offers the unique advantage of tracking large scale conformational changes associated with receptor activation and desensitization. In this dissertation, LRET, in combination with biochemical and electrophysiological studies, were performed on the NMDA receptors to draw a correlation between structure and function. NMDA receptor subtypes GluN1 and GluN2A were modified such that fluorophores could be introduced at specific sites to determine their pattern of assembly. The results indicated that the GluN1 subunits assembled across each other in a diagonal manner to form a functional receptor. Once the subunit arrangement was established, this was used as a model to further examine the mechanism of activation in this subtype of glutamate receptor. Using LRET, the correlation between cleft closure and activation was tested for both the GluN1 and GluN2A subunit of the NMDA receptor in response to agonists of varying efficacies. These investigations revealed that cleft closure plays a major role in the mechanism of activation in the NMDA receptor, similar to the AMPA and kainate subtypes. Therefore, suggesting that the mechanism of activation is conserved across the different subtypes of glutamate receptors.

Hypertension and cognitive decline among patients of Alzheimer's disease

Objective. The main aim of our study is to assess the effect of hypertension on the decline in cognitive impairment among Alzheimer’s patients. Methods. We analyzed the data of AD patients enrolled in Baylor ADMDC in a prospective study design. We divided AD patients into two groups based on the definition of hypertension. We described a decline in cognitive impairment as a change of 5 points in mini-mental state examination score (MMSE) from the baseline visit. Results. Independent of covariates, AD patients with hypertension did not exhibit a significant decline in cognitive impairment after adjustment of covariates, age, race and education (Hazard Ratio (HR) = 1.07, p value 0.58, 95% confidence interval 0.84-1.39) than AD patients without hypertension. In addition, AD patients with hypertension did not experience decline in cognitive impairment sooner than AD patients without hypertension. (P value 0.83). Conclusions . Hypertension is not associated with cognitive impairment over time among patients with Alzheimer’s disease enrolled in Baylor ADMDC after other potential confounders were taken into account. These findings should not be interpreted as a basis for discouraging appropriate medical treatment of hypertension in AD patients. Greater efforts should be made to improve the recognition of hypertension as a modifiable risk factor for decline in cognitive impairment in AD population. ^

Association between cardiorespiratory fitness and Alzheimer's mortality: A prospective cohort study

Context. Alzheimer’s disease is a major source of morbidity and mortality in aging societies. Preventive measures, such as increasing cardiorespiratory fitness, to reduce the risk of Alzheimer’s disease mortality have not been sufficiently examined.^ Objective. To examine the association between levels of cardiorespiratory fitness and Alzheimer’s disease mortality.^ Design, Setting, and Patients. A prospective cohort study of 53,911 men and 18,876 women (mean age, 51.4 [SD, 10.0] years; range 20-88) enrolled in the Cooper Center Longitudinal Study who completed a baseline health examination during 1970-2006. The primary exposure, cardiorespiratory fitness, was assessed via a maximal exercise test. Fitness was categorized according to age- and sex-specific tertiles based on the participants’ distribution of maximal treadmill exercise test duration, in metabolic equivalent tasks (METs). The main outcome measure was Alzheimer’s disease mortality, defined as the underlying or contributing cause of death using the National Death Index and death certificates through December 31, 2006.^ Results. There were 175 Alzheimer’s disease deaths during a mean follow up of 37 years and 1,309,170 person-years of exposure. Women in the high fitness category had a 70% reduction in risk of Alzheimer’s mortality compared to women in the low fitness category (HR=0.3; 95% CI, 0.1-0.8; P=.02), while adjusting for potential confounders. Similarly, women in the moderate fitness category had a 70% reduction in risk for AD mortality compared to women in the low fit category (HR=0.3; 95% CI, 0.1-0.7; P=.005). Among men, the relationship between fitness level and AD mortality risk was examined but none were of statistical significance. The adjusted comparison of men in the high fitness category to low fit men yielded an HR of 0.9 (95% CI, 0.6-1.5; P=.79), while moderately fit men compared to low fit men yielded an HR of 1.3 (95% CI, 0.9-1.9; P=.21).^ Conclusions. Higher levels of cardiorespiratory fitness were associated with decreased risk of AD mortality, in women. No statistically significant association was found among men. Physical fitness may be an important protective factor against Alzheimer’s disease death in women, further supporting its clinical and public health values.^