Doctor of Medicine and Doctor of Philosophy (MDPhD)
Mental Health and Addictions Research
Schizophrenia (SCZ) is a severe and complex disorder that presents in young adults and evolves to chronicity causing pervasive deterioration in personal, social, and professional functioning. SCZ is better conceptualized as a syndrome as the concrete underlying etiopathological mechanisms have not been identified. Nonetheless, there are reports of subtle abnormal post-mortem findings, moderate brain deficits, and severe neurocognitive impairments. Although the etiology of SCZ is unknown, evidence suggests a strong genetic contribution to the disorder. The apolipoprotein E gene (APOE) codifies for the apoE protein, which is involved in a wide range of functions from cholesterol metabolism to synaptic plasticity. One of the three main allele polymorphisms of APOE, APOE-ε4, is associated with increased risk of Alzheimer’s disease, decreased hippocampal volume, and neuropathological findings. However, data in youth indicates that it may also be associated with cognitive performance or brain development. Samples of post-mortem brain tissue from SCZ patients and healthy controls from Brodmann area 9 were used to quantify the amount of apoE, cholesterol, methylation of the promoter region of reelin, reelin mRNA, apoE receptor 2 (apoER2), and very-low density lipoprotein receptor (VLDLR). In addition, a sample of first-episode psychosis patients and healthy controls was recruited. Subjects underwent neurocognitive testing as well as brain imaging at baseline and 9 to 12 months after. Our data demonstrated higher levels of methylation of the reelin promoter region and decreased expression of apoER2 in SCZ samples, but there were no differences in apoE, cholesterol, reelin or VLDLR. In SCZ data suggested dysregulation of apoE and cholesterol in both grey and white matter. In FEP, there was severe impairment in verbal memory, but APOE-ε4 was associated with improved verbal memory over time in SCZ. APOE-ε4 status, but not memory capacity was associated with smaller hippocampal volume.APOE-ε4 is associated with different phenotypes of opposing effects in SCZ and may be associated with the syndromic expression of the disorder.
Synaptic dysfunction likely contributes to abnormal brain function in schizophrenia. Patient symptoms indicate that the striatum is involved in this disease. The three neuronal soluble-NSF-attachment receptor (SNARE) proteins (SNAP-25, syntaxin-1 and VAMP) interact at the presynaptic neuronal membrane to facilitate neurotransmission, and are thus key players in synaptic function. SNARE abnormalities have already been reported in cortical and hippocampal brain regions in schizophrenia. Their involvement in striatal dysfunction has not been investigated. Normal synaptic function requires the SNAREs to physically interact with each other, but little is known about how altered SNARE protein levels in schizophrenia relate to SNARE protein interactions. Multiple isoforms of each SNARE exist in the brain, may affect SNARE protein interactions and synaptic transmission differently, and may diverge functionally. SNARE isoform expression in schizophrenia is unknown. Thus, abnormalities in SNARE protein expression or function may underlie or contribute to brain dysfunction and disease. In this thesis, SNARE protein levels were measured in human post mortem brain samples of schizophrenia subjects for the first time in the striatum. The functional consequences of SNARE alterations were investigated by developing a novel ELISA assay to measure SNARE protein interactions. The possible confounding effects of medications were addressed in several ways, including the use of striatal tissue from animals exposed to antipsychotic medications. Alterations in SNAP-25 and syntaxin-1 protein levels were further dissected by measuring protein isoforms. Syntaxin-1 isoforms were assayed by quantitative immunoblotting. A mass-spectrometry based assay was developed and used to measure SNAP-25 protein isoform levels. The results of these investigations suggest that SNARE protein alterations in schizophrenia are restricted to distinct functional regions of the striatum, perturb SNARE protein interactions, involve specific protein isoforms, and may occur independent of patient treatment with antipsychotic medications. Furthermore, these studies contribute a new, high-throughput method for measuring SNARE protein interactions, and for the first time, a means of detecting and quantifying SNAP-25 isoforms in brain tissue.