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Differential Localization of Hic-5 and Paxillin in the Brain of Alzheimer's Disease Subjects

Alzheimer's disease (AD) is a neurodegenerative disorder that results from a loss of synaptic transmission and ultimately results in cell death. However, the mechanisms that induce neuronal cell death remain elusive. Amyloid plaques composed of amyloid fibrils (Ab) and neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau (pTau) are the main pathogenic hallmarks of AD. Ab and NFT generation is influenced by reactive oxygen species and altered signaling pathways. Focal adhesion proteins assemble into intracellular complexes involved in integrin-mediated communication between the extracellular matrix (ECM) and the actin cytoskeleton, regulating many cell physiological processes. Interestingly, recent studies report that integrins bind to Ab fibrils, mediating Ab signal transmission from extracellular sites of Ab deposits into the cell and ultimately to the nucleus. Hydrogen peroxide-inducible-clone 5 (Hic-5) and paxillin are members of the group III LIM domain protein family that localize to both the nucleus and focal adhesions. Hic-5 and paxillin are expressed in numerous regions of the rat brain including cerebellum, striatum, prefrontal cortex, hippocampus, hypothalamus, thalamus, and spinal cord. While little is known about the specific roles of paxillin and Hic-5 in regulating focal adhesion signaling and gene expression within brain, non-genomic roles for both paxillin and Hic-5 in brain have been described. For example, in cultured neurons, paxillin is rapidly phosphorylated in the presence of fibrillar b-amyloid and colocalized with pTau, leading to altered focal adhesion turnover and loss of synaptic integrity. A functional role for Hic-5 in the brain was revealed by its ability to (1) decrease surface levels of dopamine transporter (DAT) in rat midbrain neuronal cultures and to (2) negatively affect dopamine uptake. A direct interaction between Hic-5 and DAT may be responsible for this effect. While these reports suggest biologic functions of Hic-5 and paxillin in brain, a detailed analysis of paxillin and Hic-5 expression and distribution in normal or AD brain has not been performed. Given the in vitro association between paxillin and b-amyloid-induced toxicity and Hic-5 response to oxidative stress, a blinded retrospective cross-sectional study of the human hippocampus for Hic-5 and paxillin was performed. The expression and subcellular distribution of Hic-5 and paxillin in AD and control hippocampus were determined by immunohistochemistry (IHC) from early and late-stage AD and age-matched control subjects. IHC was also used to examine the subcellular distribution of specific phosphorylated isoforms of paxillin. Laser scanning confocal microscopy (LSCM) was used to visualize or demonstrate colocalization of Hic-5, paxillin and phosphorylated isoforms of paxillin. Observations demonstrate changes in the subcellular distribution of Hic-5, paxillin and specific phosphorylated isoforms of paxillin within particular regions of the hippocampus in AD brain. Hic-5 and phosphorylated isoforms of paxillin colocalize with NFTs, while paxillin is predominantly found in reactive astrocytes (stellate-shaped) in the hippocampus of AD brains. Thus, important scaffolding proteins that link various intracellular signaling pathways to the ECM are modified and exhibit altered subcellular distribution in hippocampus during AD.