PDE9A-mediated regulation of cGMP: Developing a biomarker for a novel therapy for Alzheimer's disease

Cyclic nucleotide signaling is integral to the regulation of synaptic plasticity. Given the synaptic dysfunction in Alzheimer's disease (AD), the pharmacological modulation of these signaling cascades is an attractive avenue towards novel treatments. Phosphodiesterases (PDEs) regulate the spatial and temporal aspects of cyclic nucleotide signaling. Here we report on the localization of PDE9A in the brain and the role of this enzyme in regulating cGMP signaling. The regional distribution of PDE9A mRNA and protein was evaluated using RT-PCR, in-situ hybridization, Western blot analysis and immunohistochemistry. We measured regional brain and CSF cGMP levels following targeted deletion of the PDE9 gene and inhibition of PDE9 activity with a selective inhibitor. PDE9A mRNA is expressed widely throughout the central nervous system (CNS), albeit at low levels. Western blot analysis of human and rat tissue showed PDE9A protein in striatum, prefrontal cortex, hippocampus, and cerebellum. Immunochemistry further revealed PDE9A protein in human hippocampal, cortical, and cerebellar Purkinje neurons. Consistent with this distribution, genetic deletion of PDE9A in mice resulted in significant elevations of cGMP throughout the brain - specifically, in striatum (~300%), hippocampus (~200%), cortex (~80%), and cerebellum (~280%) relative to wild type. Notably, concentrations of cGMP in cerebrospinal fluid (CSF) increased by ~250% relative to wild type. Pharmacological inhibition of PDE9A in both rat and mouse also caused dose-dependent elevation in cGMP in these same tissue regions as well as in CSF. Given the involvement of cGMP signaling in several aspects of synaptic plasticity thought to underlie hippocampal memory formation, our findings support further investigation of PDE9A inhibitors as a potential treatment for the cognitive deficits of AD. Our results also indicate that cGMP in CSF may serve as a useful biomarker for PDE9A inhibition in clinical trials.