Regulation of cAMP-phosphodiesterases by phosphatidic acid binding

Understanding PDE4's function in Alzheimer's disease; a target for novel therapeutic approaches

Phosphodiesterases (PDEs) have long been considered as targets for the treatment of Alzheimer's disease (AD) and a substantial body of evidence suggests that one sub-family from the super-family of PDEs, namely PDE4D, has particular significance in this context. This review discusses the role of PDE4 in the orchestration of cAMP response element binding signaling in AD and outlines the benefits of targeting PDE4D specifically. We examine the limited available literature that suggests PDE4 expression does not change in AD brains together with reports that show PDE4 inhibition as an effective treatment in this age-related neurodegenerative disease. Actually, aging induces changes in PDE4 expression/activity in an isoform and brain-region specific manner that proposes a similar complexity in AD brains. Therefore, a more detailed account of AD-related alterations in cellular/tissue location and the activation status of PDE4 is required before novel therapies can be developed to target cAMP signaling in this disease.

The upstream conserved regions (UCRs) mediate homo- and hetero-oligomerization of type 4 cyclic nucleotide phosphodiesterases (PDE4s)

PDE4s (type 4 cyclic nucleotide phosphodiesterases) are divided into long and short forms by the presence or absence of conserved N-terminal domains termed UCRs (upstream conserved regions). We have shown previously that PDE4D2, a short variant, is a monomer, whereas PDE4D3, a long variant, is a dimer. In the present study, we have determined the apparent molecular masses of various long and short PDE4 variants by size-exclusion chromatography and sucrose density-gradient centrifugation. Our results indicate that dimerization is a conserved property of all long PDE4 forms, whereas short forms are monomers. Dimerization is mediated by the UCR domains. Given their high sequence conservation, the UCR domains mediate not only homo-oligomerization, but also hetero-oligomerization of distinct PDE4 long forms as detected by co-immunoprecipitation assays and FRET microscopy. Endogenous PDE4 hetero-oligomers are, however, low in abundance compared with homo-dimers, revealing the presence of mechanisms that predispose PDE4s towards homo-oligomerization. Oligomerization is a prerequisite for the regulatory properties of the PDE4 long forms, such as their PKA (protein kinase A)-dependent activation, but is not necessary for PDE4 protein-protein interactions. As a result, individual PDE4 protomers may independently mediate protein-protein interactions, providing a mechanism whereby PDE4s contribute to the assembly of macromolecular signalling complexes.

PDE4 as a target for cognition enhancement

INTRODUCTION

The second messengers cAMP and cGMP mediate fundamental aspects of brain function relevant to memory, learning, and cognitive functions. Consequently, cyclic nucleotide phosphodiesterases (PDEs), the enzymes that inactivate the cyclic nucleotides, are promising targets for the development of cognition-enhancing drugs.

AREAS COVERED

PDE4 is the largest of the 11 mammalian PDE families. This review covers the properties and functions of the PDE4 family, highlighting procognitive and memory-enhancing effects associated with their inactivation.

EXPERT OPINION

PAN-selective PDE4 inhibitors exert a number of memory- and cognition-enhancing effects and have neuroprotective and neuroregenerative properties in preclinical models. The major hurdle for their clinical application is to target inhibitors to specific PDE4 isoforms relevant to particular cognitive disorders to realize the therapeutic potential while avoiding side effects, in particular emesis and nausea. The PDE4 family comprises four genes, PDE4A-D, each expressed as multiple variants. Progress to date stems from characterization of rodent models with selective ablation of individual PDE4 subtypes, revealing that individual subtypes exert unique and non-redundant functions in the brain. Thus, targeting specific PDE4 subtypes, as well as splicing variants or conformational states, represents a promising strategy to separate the therapeutic benefits from the side effects of PAN-PDE4 inhibitors.

Analysis of a novel diacylglycerol kinase from Dictyostelium discoideum: DGKA

Diacylglycerol kinases (DGKs) catalyze the ATP-dependent phosphorylation of diacylglycerols to generate phosphatidic acid and have been investigated in prokaryotic and eukaryotic organisms. Recently, a protein that is significantly similar to human DGK-theta, DGKA, was identified in Dictyostelium discoideum. It has been shown to possess DGK activity when assayed using a medium-chain diacylglycerol, 1,2-dioctanoyl-sn-glycerol (DiC8). A complete understanding of DGK catalytic and regulatory mechanisms, as well as physiological roles, requires an understanding of its biochemical and kinetic properties. This report presents an analysis of these properties for DGKA. The enzyme catalyzes the phosphorylation of DiC8, and another medium-chain DAG, DiC6 (1,2-dihexanoyl-sn-glycerol), in a Michaelis-Menten manner. Interestingly, the kinetics of DGKA using physiologically relevant long-chain DAGs was dependent on substrate surface concentration and the detergent that was used. DGKA displayed Michaelis-Menten kinetics with respect to bulk substrate concentration (1,2-dioleoyl-sn-glycerol) in octyl glucoside mixed micelles when the surface substrate concentration was at or below 3.5 mol %. At higher surface concentrations, however, there was a sigmoidal relationship between the initial velocity and bulk substrate concentration. In contrast, DGKA displayed sigmoidal kinetics with respect to bulk substrate concentrations at all surface concentrations in Triton X-100 mixed micelles. Finally, we show the catalytic activity of DGKA was significantly enhanced by phosphatidylserine (PS) and phosphatidic acid (PA).