DISC1 & GSK3β modulate PDE4 activity: functional integration of psychiatric associated signalling pathways

Following the discovery of the DISC1 gene in 2000, subsequent research has led to DISC1 becoming one of the most promising candidate genes for psychiatric disorders. Acting as a scaffold protein, DISC1 has a large number of interacting proteins and is involved in a series of intracellular signalling pathways. Amongst these binding proteins are two enzymes, PDE4 and GSK3β, that were originally implicated in psychiatric disease by virtue of their inhibition by psychoactive drugs. PDE4 enzymes are inhibited by rolipram, which possesses anti-depressant and anti-psychotic activity, while GSK3β is one of the major targets of lithium, a potent mood stabiliser. Both these enzymes are intricately involved in the PI3K/AKT, cAMP, and MAPK signalling pathways, all of which have a number of downstream outcomes with potential relevance to psychiatric disorders. The Millar and Porteous laboratory had established that DISC1 modulates PDE4 activity, but this predated awareness of GSK3 as another DISC1 interactor whose binding site overlapped with that of PDE4. Since cAMP is a key regulator of signalling pathways in the brain, I hypothesised that not only DISC1, but also GSK3β may be involved in the regulation of PDE4 activity to control local cAMP levels and gradients. To investigate this hypothesis, I characterised SHSY5Y cells as a model for measuring PDE4 activity, and performed a series of genetic and pharmacological manipulations on this system. Inhibition of GSK3β resulted in a decrease of basal PDE4 activity that was amplified by DISC1 overexpression. Wild type cells that were treated with forskolin exhibited a significant increase in PDE4 activity, which was suppressed by GSK3β inhibition and both overexpression and knockdown of DISC1. Further experiments confirmed that none of these changes were a result of differences in PDE4 mRNA or protein expression. Thus I have provided evidence that suggests tonic activation of PDE4 by GSK3β and evidence for modulation of PDE4 activity by DISC1. I provide evidence for the localisation of PDE4B & PDE4D with key psychiatric associated receptors in structures resembling developing dendritic spines; furthermore, agonism of NMDA receptors results in a significant increase in PDE4 activity in primary neurons. These results are a simple demonstration of an emerging principle in psychiatric research: that none of the signalling pathways implicated in psychiatric disease are acting in isolation. There are likely to be multiple points of integration between these pathways, with the demonstrated DISC1-GSK3β-PDE4 interaction forming one of these points. My results add an important new element to the understanding of how the DISC1 complex may regulate intracellular signalling in response to extracellular cues