Toward Neuroproteomics in Biological Psychiatry: A Systems Approach Unravels Okadaic Acid-Induced Alterations in the Neuronal Phosphoproteome.

Neuroproteomics is an evolving field of postgenomic medicine, highlighting the convergence of psychiatry/neurology and proteomics, yet compared with neurogenetics, it has received little attention. This study in rat primary neuronal cultures provides an example of a neuroproteomic approach relevant to the study of psychiatric disease pathophysiology, focusing on Alzheimer's disease. In this context, okadaic acid (OA) is routinely used in experimental designs to investigate phosphorylation-mediated events. It is a potent protein phosphatase (PP) inhibitor, particularly of PP1 and PP2A. Typically, a single protein and its phosphorylation level are monitored upon OA exposure. Although useful, this can be misleading as protein phosphorylation-mediated events involve complex signaling cascades and an array of kinases, phosphatases, and substrates. Bearing in mind the involvement of multiple pathways and cascade cross talk, this study employed a systems approach to analyze OA-induced molecular responses through PP inhibition. We showed that upon OA exposure, the recovery rate of 245 phosphoproteins significantly increased, while that of 75 significantly decreased. The prominent biological processes affected included anatomical structural development, transport, cell differentiation, and signal transduction. The associated phosphointeraction networks identified nodes representing OA-responsive phosphoproteins. Many of these are key players of signaling cascades relevant to a range of pathologies. In summary, the data presented results from a neuroproteomic preclinical study offering an array of phosphoproteins as potential targets for future diagnostic and therapeutic strategies in biological psychiatry. We note, however, the nonspecificity of targeting PPs themselves and emphasize the need for future neuroproteomic approaches toward systems psychiatry.

Altered protein phosphorylation as a resource for potential AD biomarkers.

The amyloidogenic peptide, Aß, provokes a series of events affecting distinct cellular pathways regulated by protein phosphorylation. Aß inhibits protein phosphatases in a dose-dependent manner, thus it is expected that the phosphorylation state of specific proteins would be altered in response to Aß. In fact several Alzheimer's disease related proteins, such as APP and TAU, exhibit pathology associated hyperphosphorylated states. A systems biology approach was adopted and the phosphoproteome, of primary cortical neuronal cells exposed to Aß, was evaluated. Phosphorylated proteins were recovered and those whose recovery increased or decreased, upon Aß exposure across experimental sets, were identified. Significant differences were evident for 141 proteins and investigation of their interactors revealed key protein clusters responsive to Aß treatment. Of these, 73 phosphorylated proteins increased and 68 decreased upon Aß addition. These phosphorylated proteins represent an important resource of potential AD phospho biomarkers that should be further pursued.

Aß Influences Cytoskeletal Signaling Cascades with Consequences to Alzheimer's Disease.

Abnormal signal transduction events can impact upon the cytoskeleton, affecting the actin and microtubule networks with direct relevance to Alzheimer's disease (AD). Cytoskeletal anomalies, in turn, promote atypical neuronal responses, with consequences for cellular organization and function. Neuronal cytoskeletal modifications in AD include neurofibrillary tangles, which result from aggregates of hyperphosphorylated tau protein. The latter is a microtubule (MT)-binding protein, whose abnormal phosphorylation leads to MT instability and consequently provokes irregularities in the neuronal trafficking pathways. Early stages of AD are also characterized by synaptic dysfunction and loss of dendritic spines, which correlate with cognitive deficit and impaired brain function. Actin dynamics has a prominent role in maintaining spine plasticity and integrity, thus providing the basis for memory and learning processes. Hence, factors that disrupt both actin and MT network dynamics will compromise neuronal function and survival. The peptide Aß is the major component of senile plaques and has been described as a pivotal mediator of neuronal dystrophy and synaptic loss in AD. Here, we review Aß-mediated effects on both MT and actin networks and focus on the relevance of the elicited cytoskeletal signaling events targeted in AD pathology.