PAK1 Protein Expression in the Auditory Cortex of Schizophrenia Subjects

Deficits in auditory processing are among the best documented endophenotypes in schizophrenia, possibly due to loss of excitatory synaptic connections. Dendritic spines, the principal post-synaptic target of excitatory projections, are reduced in schizophrenia. p21-activated kinase 1 (PAK1) regulates both the actin cytoskeleton and dendritic spine density, and is a downstream effector of both kalirin and CDC42, both of which have altered expression in schizophrenia. This study sought to determine if there is decreased auditory cortex PAK1 protein expression in schizophrenia through the use of quantitative western blots of 25 schizophrenia subjects and matched controls. There was no significant change in PAK1 level detected in the schizophrenia subjects in our cohort. PAK1 protein levels within subject pairs correlated positively with prior measures of total kalirin protein in the same pairs. PAK1 level also correlated with levels of a marker of dendritic spines, spinophilin. These latter two findings suggest that the lack of change in PAK1 level in schizophrenia is not due to limited sensitivity of our assay to detect meaningful differences in PAK1 protein expression. Future studies are needed to evaluate whether alterations in PAK1 phosphorylation states, or alterations in protein expression of other members of the PAK family, are present in schizophrenia

Effect of subject variables on PAK1 expression in schizophrenia.

<p>Each point represents the mean within pair difference in log PAK1 expression. Horizontal bars are group means. No differences were significant.</p

Effects of postmortem interval on PAK1 expression.

<p><b>A</b>) Example western blot of PAK1 and Tubulin in mice in which the interval from sacrifice to brain harvesting was experimentally varied between 0 and 48 hours. MW, Molecular Weight Markers. <b>B</b>) Optical densities for PAK1 and Tubulin in mice in which the interval from sacrifice to brain harvesting was experimentally varied between 0 and 48 hours. Two sets of mice (each set containing one mouse for each PMI) were tested. Sets were run concurrently on separate gels, and the run was repeated. Mean (SEM) values for the four observations at each time point are shown. Time points sharing the same superscript letter differ significantly.</p

PAK1 Signaling Pathway.

<p>PAK1 resides in inactive homo- or heterodimers. Binding of the Rho-GTPase, RAC1 (or CDC42) causes dissociation of the dimers and activation of PAK1. PAK1 activates LIMK1, which inhibits cofilin-mediated f-actin depolymerization. PAK1 may be subject to activation by non-GTPase mechanisms, and can activate other effector pathways, some of which (e.g. MLCK and MLC) may also impact spine dynamics. Blue indicates promotion of dendritic spine persistence, red indicates promotion of spine elimination.</p

PAK1 Expression in schizophrenia.

<p><b>A</b>) Detection of PAK1 in human gray matter extracts. Bands corresponding to the predicted molecular weights of PAK1 and Tubulin are readily detected in human gray matter extracts from subjects with schizophrenia (S) and matched control subjects (C). MW, Molecular Weight Markers. <b>B</b>) Comparison of PAK1 expression (normalized to tubulin expression) for the 25 pairs of subjects. Each point represents a pair of subjects. The diagonal reference line represents a control: schizophrenia ratio of one. Points above the line indicate increased expression in schizophrenia subjects, points below the line indicate decreased expression in schizophrenia subjects. <b>C</b>) Mean (SD) expression PAK1:tubulin in schizophrenia and control subjects. <b>D</b>) Correlation of within pair differences in log PAK1 expression with the corresponding measure of log total kalirin protein expression (r = 0.55, p = 0.004). <b>E</b>) Correlation of within pair differences in PAK1 expression with the corresponding measure of spinophilin protein expression (r = 0.61, p = 0.004).</p