Proteomic Analyses of PKA and AKAP Signaling in Cocaine Addiction

Abstract

The development and application of proteomics techniques allows for ab initio identification of changes in protein expression and modifications which drive cellular processes. In the case of behavioral neuroscience, these techniques may be applied toward identification of candidate proteins and cellular pathways within specific nuclei that are affected by experience or training, and testing of subsequent hypotheses in behavioral models. Accordingly, proteomic techniques have been applied to identify protein changes following chronic cocaine abuse. These studies have examined changes in the nucleus accumbens (NAc) from human overdose victims, as well as non-human primates and rodents (for review, see Hemby (2010)). From these, a number of important themes have emerged; for example, changes in cellular metabolism, cytoskeletal dynamics, and signal transduction were highly represented. Of particular note, elements of the cAMP, adenylate cyclase, and PKA signaling pathway have been identified by traditional biochemical methods and genomic analyses, as well as proteomic methods (Hemby, 2010). For example, ChIP-chip microarray analysis of genes influenced by cocaine exposure has indicated increased histone acetylation at the promoters of the PKA catalytic and RII regulatory subunits, as well as A kinase anchoring protein 9 (AKAP9) (also known as Yotiao) and AKAP8 (AKAP95) (Renthal et al, 2009). Moreover, increased binding of transcription factor ΔFosB was identified at the promoters for PKA RIIα, as well as AKAP8 following cocaine vs saline exposure. AKAPs constitute a family of more than 50 proteins across vertebrates and invertebrates, which mediate scaffolding and localization of PKA and other signaling molecules within specific cellular subcompartments (Sanderson and Dell'Acqua, 2011). An isobaric tag for relative and absolute quantitation proteomic analysis of a rat postsynaptic density-enriched subfraction following cocaine self-administration and extinction identified upregulation of AKAP5 (also known as AKAP79/150) (Reissner et al, 2011). AKAP150 serves to complex PKA and other signaling molecules with ionotropic glutamate receptors (Sanderson and Dell'Acqua, 2011). Disruption of AKAP scaffolding by microinjection of a cell permeable inhibitor peptide in the NAc (vs control peptide) led to decreased reinstatement of cocaine seeking, indicating a functional role for AKAPs in the neuropathology of drug abuse (Reissner et al, 2011). Downstream of PKA, Boudreau et al (2009) observed increased AMPA surface expression, as well as PKA (and pERK2) activation over time during withdrawal in cocaine sensitized rats. They went on to use a mass spectrometry approach to identify PKA substrates whose phosphorylation increased over time of withdrawal, employing proteomics to analyze effector proteins downstream of PKA. The repeated identification of the PKA signaling pathway in the cellular adaptations induced by cocaine underscores the importance of this pathway in the addiction process. However, the ubiquitous nature of AKAP and PKA signaling in cellular pathways and subcompartments (eg, synapse vs nucleus, organelles, etc) complicates application of this pathway toward candidate pharmacotherapeutic targets for psychostimulant abuse. In the specific case of AKAP150, however, proteomic and functional studies indicate that synaptic upregulation promotes reinstatement behavior; thus, development of a structurally specific inhibitor targeting this member of the AKAP family may aid inhibition of craving and drug seeking, as a pharmacological adjuvant to cognitive therapy and counseling.