Cyclic nucleotide signalling in Alzheimer's disease postmortem brain

Abstract: The aim of this study was to determine the nature and significance of altered guanylyl cyclase (GC) and adenylyl cyclase (AC) signalling in Alzheimer's disease (AD) postmortem brain. Paper I describes a study of particulate and soluble GC activities in postmortem temporal cortex from a series of AD and matched control subjects. Particulate GC activity was not significantly different between groups. In contrast, the Vmax values for basal and sodium nitroprusside-stimulated soluble GC activities were approximately 50% lower in the Alzheimer's disease cases, compared to controls. These results provide the first evidence for a loss of nitric oxide responsive GC activity in AD brain. Paper II was concerned with determining whether disrupted AC in AD brain is accompanied by altered cAMP-dependent protein kinase (PKA) activity. It was shown that GTP[gamma]S-stimulated AC activity was significantly lower in AD superior temporal cortex, but not cerebellum, compared to values from a series of matched control cases. Neither basal or forskolin-stimulated AC activities were significantly different between the AD and control brain regions. No significant differences were seen in either particulate or soluble fraction PKA activities between the AD and control brain regions. From this it was concluded that disrupted AC signalling in AD brain occurs specifically at the level of Gs-protein - enzyme interactions and is not accompanied by an altered PKA activity. The second half of this thesis was concerned with determining whether alterations in the levels of PKA and cGMP-dependent protein kinase (PKG) showed a relationship to the AD pathologies of neurofibrillary changes and amyloid deposition. Prior to this, an autoradiography characterisation study (paper III) was performed in rat brain to determine the optimal conditions for distinguishing [3H]cGMP binding to cytosolic PKG from that to cGMP-stimulated phosphodiesterase (cGS-PDE). In paper IV, autoradiographic [3H]cAMP binding to cytosolic and particulate PKA was measured in sections of entorhinal cortex / hippocampal formation from 23 cases that had been staged for Alzheimer's disease-related neurofibrillary changes and amyloid deposits according to Braak and Braak. [3H]cAMP binding to cytosolic PKA showed statistically significant reductions in the entorhinal cortex with respect to neurofibrillary changes. In contrast, other regions including the subiculum, CA1-CA4 subfields of the hippocampus and dentate gyrus showed no significant alterations of cytosolic PKA with respect to neurofibrillary changes. [3H]cAMP binding to cytosolic PKA also showed significant declines in the entorhinal cortex and subiculum. with respect to staging for amyloid deposits. [3H]cAMP binding to particulate PKA did not show significant relationships to staging for either neurofibrillary changes or amyloid deposition in either the entorhinal cortex or any of the hippocampal subregions. These findings suggest that whereas [3H]cAMP binding to cytosolic PKA in the entorhinal cortex is reduced with progression of neurofibrillary and amyloid pathology, other hippocampal regions show a preservation of cytosolic and particulate PKA even in late stage pathologies. Paper V describes [3H)cGMP binding to cytosolic PKG and cGS-PDE in the same series of staged postmortem brains used in paper IV. [3H]cGMP binding to cGMP-dependent protein kinase (PKG) showed significant correlations with the progression of neurofibrillary changes in the entorhinal cortex and hippocampal sub-regions. A significant deficit of [3H]cGMP binding to cGS-PDE was found in the entorhinal cortex, subiculum, CA3-CA4 subfields and dentate gyrus, but not in the CAl-CA2 subfields. [3H]cGMP binding to PKG showed no significant changes in any regions with staging for amyloid deposits. In contrast, a significant decrease of [3H]cGMP binding to cGS-PDE was seen with arnyloid deposits in the entorhinal cortex, subiculum, CA4 subfield and dentate gyrus, but not the CA1-CA3 regions. This thesis indicates that altered cyclic nucleotide signalling occurs in both a cellular and regional specific manner in AD brain. Alterations in specific components of the GC and AC signalling cascades also show relationships to the progression of AD pathologies, possibly via a contribution to altered arnyloid precursor protein metabolism and tau phosphorylation.