Increased Energy Expenditure and Protection From Diet-Induced Obesity in Mice Lacking the cGMP-Specific Phosphodiesterase PDE9

Cyclic nucleotide phosphodiesterases as drug targets

Cyclic nucleotides are synthesized by adenylyl and/or guanylyl cyclase, and downstream of this synthesis, the cyclic nucleotide phosphodiesterase families (PDEs) specifically hydrolyze cyclic nucleotides. PDEs control cyclic adenosine-3',5'monophosphate (cAMP) and cyclic guanosine-3',5'-monophosphate (cGMP) intracellular levels by mediating their quick return to the basal steady state levels. This often takes place in subcellular nanodomains. Thus, PDEs govern short-term protein phosphorylation, long-term protein expression, and even epigenetic mechanisms by modulating cyclic nucleotide levels. Consequently, their involvement in both health and disease is extensively investigated. PDE inhibition has emerged as a promising clinical intervention method, with ongoing developments aiming to enhance its efficacy and applicability. In this comprehensive review, we extensively look into the intricate landscape of PDEs biochemistry, exploring their diverse roles in various tissues. Furthermore, we outline the underlying mechanisms of PDEs in different pathophysiological conditions. Additionally, we review the application of PDE inhibition in related diseases, shedding light on current advancements and future prospects for clinical intervention. SIGNIFICANCE STATEMENT: Regulating PDEs is a critical checkpoint for numerous (patho)physiological conditions. However, despite the development of several PDE inhibitors aimed at controlling overactivated PDEs, their applicability in clinical settings poses challenges. In this context, our focus is on pharmacodynamics and the structure activity of PDEs, aiming to illustrate how selectivity and efficacy can be optimized. Additionally, this review points to current preclinical and clinical evidence that depicts various optimization efforts and indications.

Phosphorylation-Dependent Regulation of Guanylyl Cyclase (GC)-A and Other Membrane GC Receptors

Receptor guanylyl cyclases (GCs) are single membrane spanning, multidomain enzymes, that synthesize cGMP in response to natriuretic peptides or other ligands. They are evolutionarily conserved from sea urchins to humans and regulate diverse physiologies. Most family members are phosphorylated on 4 to 7 conserved serines or threonines at the beginning of their kinase homology domains. This review describes studies that demonstrate that phosphorylation and dephosphorylation are required for activation and inactivation of these enzymes, respectively. Phosphorylation sites in GC-A, GC-B, GC-E, and sea urchin receptors are discussed, as are mutant receptors that mimic the dephosphorylated inactive or phosphorylated active forms of GC-A and GC-B, respectively. A salt bridge model is described that explains why phosphorylation is required for enzyme activation. Potential kinases, phosphatases, and ATP regulation of GC receptors are also discussed. Critically, knock-in mice with glutamate substitutions for receptor phosphorylation sites are described. The inability of opposing signaling pathways to inhibit cGMP synthesis in mice where GC-A or GC-B cannot be dephosphorylated demonstrates the necessity of receptor dephosphorylation in vivo. Cardiac hypertrophy, oocyte meiosis, long-bone growth/achondroplasia, and bone density are regulated by GC phosphorylation, but additional processes are likely to be identified in the future.

The therapeutic potential of phosphodiesterase 9 (PDE9) inhibitors: a patent review (2018-present)

INTRODUCTION

Phosphodiesterase 9 (PDE9) has been demonstrated as a potential target for neurological disorders and cardiovascular diseases, such as Alzheimer's disease and heart failure. For the last few years, a series of PDE9 inhibitors with structural diversities have been developed and patented by researchers and pharmaceutical companies, providing insights into first-in-class therapies of PDE9 drug candidates.

AREA COVERED

This review provides an overview of PDE9 inhibitors in patents from 2018 to the present.

EXPERT OPINION

Only a few of the current PDE9 inhibitors are highly selective over other PDEs, which limits their application in pharmacological and clinical research. The design and development of highly selective PDE9 inhibitors remain the top priority in future research. The advantages of targeting PDE9 rather than other PDEs in treating neurodegenerative diseases need to be explained thoroughly. Besides, application of PDE9 inhibitor-based combination therapies sheds light on treating diabetes and refractory heart diseases. Finally, PDE9 inhibitors should be further explored in clinical indications beyond neurological disorders and cardiovascular diseases.

PDE9A polymorphism and association analysis with growth performance and gastrointestinal weight of Hu sheep

Phosphodiesterase 9A (PDE9A) plays a crucial role in activating the cGMP-dependent signaling pathway and may have important effects on the growth and development of the gastrointestinal tract in Hu sheep. In this study, we analyzed the single nucleotide polymorphisms of PDE9A in 988 Hu sheep and their correlation with growth performance, feed efficiency, and gastrointestinal development. Additionally, we examined the expression level of different PDE9A genotypes in the gastrointestinal tract of Hu sheep by using fluorescence quantitative PCR. The results revealed a moderate level of polymorphism (0.25 < PIC < 0.50) at the g.286248617 T > C mutation site located in the first intron of PDE9A in Hu sheep, with three genotypes: CC, CT, and TT. The weights of the omasum, colon, and cecum were significantly greater in the CC genotype than in the TT genotype (P < 0.05), and the expression level of PDE9A in the tissues of the rumen, ileum, cecum, and colon was notably lower in the CC genotype individuals (P < 0.05). These findings suggest that the polymorphism of PDE9A affects the weight of the stomach, colon, and cecum in Hu sheep through expression regulation. Overall, the results of this study suggest that the g.286248617 T > C mutation site in the first intron of PDE9A can serve as a potential molecular marker for breeding practices related to the gastrointestinal weight of Hu sheep.

An overview of phosphodiesterase 9 inhibitors: Insights from skeletal structure, pharmacophores, and therapeutic potential

Cyclic nucleotide phosphodiesterase 9 (PDE9), a specifically hydrolytic enzyme with the highest affinity for cyclic guanosine monophosphate (cGMP) among the phosphodiesterases family, plays a critical role in many biological processes. Consequently, the development of PDE9 inhibitors has received increasing attention in recent years, with several compounds undergoing clinical trials for the treatment of central nervous system (CNS) diseases such as Alzheimer's disease, schizophrenia, and psychotic disorders, as well as heart failure and sickle cell disease. This review analyzes the recent primary literatures and patents published from 2004 to 2023, focusing on the structure, pharmacophores, selectivity, and therapeutic potential of PDE9 inhibitors. It hoped to provide a comprehensive overview of the field's current state to inform the development of novel PDE9 inhibitors.

Dysregulated Cyclic Nucleotide Metabolism in Alcohol-Associated Steatohepatitis: Implications for Novel Targeted Therapies

BACKGROUND

Cyclic nucleotides are second messengers, which play significant roles in numerous biological processes. Previous work has shown that cAMP and cGMP signaling regulates various pathways in liver cells, including Kupffer cells, hepatocytes, hepatic stellate cells, and cellular components of hepatic sinusoids. Importantly, it has been shown that cAMP levels and enzymes involved in cAMP homeostasis are affected by alcohol. Although the role of cyclic nucleotide signaling is strongly implicated in several pathological pathways in liver diseases, studies describing the changes in genes regulating cyclic nucleotide metabolism in ALD are lacking.

METHODS

Male C57B/6 mice were used in an intragastric model of alcohol-associated steatohepatitis (ASH). Liver injury, inflammation, and fibrogenesis were evaluated by measuring plasma levels of injury markers, liver tissue cytokines, and gene expression analyses. Liver transcriptome analysis was performed to examine the effects of alcohol on regulators of cyclic AMP and GMP levels and signaling. cAMP and cGMP levels were measured in mouse livers as well as in livers from healthy human donors and patients with alcohol-associated hepatitis (AH).

RESULTS

Our results show significant changes in several phosphodiesterases (PDEs) with specificity to degrade cAMP (Pde4a, Pde4d, and Pde8a) and cGMP (Pde5a, Pde6d, and Pde9a), as well as dual-specificity PDEs (Pde1a and Pde10a) in ASH mouse livers. Adenylyl cyclases (ACs) 7 and 9, which are responsible for cAMP generation, were also affected by alcohol. Importantly, adenosine receptor 1, which has been implicated in the pathogenesis of liver diseases, was significantly increased by alcohol. Adrenoceptors 1 and 3 (Adrb), which couple with stimulatory G protein to regulate cAMP and cGMP signaling, were significantly decreased. Additionally, beta arrestin 2, which interacts with cAMP-specific PDE4D to desensitize G-protein-coupled receptor to generate cAMP, was significantly increased by alcohol. Notably, we observed that cAMP levels are much higher than cGMP levels in the livers of humans and mice; however, alcohol affected them differently. Specifically, cGMP levels were higher in patients with AH and ASH mice livers compared with controls. As expected, these changes in liver cyclic nucleotide signaling were associated with increased inflammation, steatosis, apoptosis, and fibrogenesis.

CONCLUSIONS

These data strongly implicate dysregulated cAMP and cGMP signaling in the pathogenesis of ASH. Future studies to identify changes in these regulators in a cell-specific manner could lead to the development of novel targeted therapies for ASH.

The 10th International Conference on cGMP 2022: recent trends in cGMP research and development-meeting report

Increasing cGMP is a unique therapeutic principle, and drugs inhibiting cGMP-degrading enzymes or stimulating cGMP production are approved for the treatment of various diseases such as erectile dysfunction, coronary artery disease, pulmonary hypertension, chronic heart failure, irritable bowel syndrome, or achondroplasia. In addition, cGMP-increasing therapies are preclinically profiled or in clinical development for quite a broad set of additional indications, e.g., neurodegenerative diseases or different forms of dementias, bone formation disorders, underlining the pivotal role of cGMP signaling pathways. The fundamental understanding of the signaling mediated by nitric oxide-sensitive (soluble) guanylyl cyclase and membrane-associated receptor (particulate) guanylyl cyclase at the molecular and cellular levels, as well as in vivo, especially in disease models, is a key prerequisite to fully exploit treatment opportunities and potential risks that could be associated with an excessive increase in cGMP. Furthermore, human genetic data and the clinical effects of cGMP-increasing drugs allow back-translation into basic research to further learn about signaling and treatment opportunities. The biannual international cGMP conference, launched nearly 20 years ago, brings all these aspects together as an established and important forum for all topics from basic science to clinical research and pivotal clinical trials. This review summarizes the contributions to the "10th cGMP Conference on cGMP Generators, Effectors and Therapeutic Implications," which was held in Augsburg in 2022 but will also provide an overview of recent key achievements and activities in the field of cGMP research.

Pharmacologic blockade of the natriuretic peptide clearance receptor promotes weight loss and enhances insulin sensitivity in type 2 diabetes

While natriuretic peptides (NPs) are primarily known for their renal and cardiovascular actions, NPs stimulate lipolysis in adipocytes and induce a thermogenic program in white adipose tissue (WAT) that resembles brown fat. The biologic effects of NPs are negatively regulated by the NP clearance receptor (NPRC), which binds and degrades NPs. Knockout (KO) of NPRC protects against diet induced obesity and improves insulin sensitivity in obese mice. To determine if pharmacologic blockade of NPRC enhanced the beneficial metabolic actions of NPs in type 2 diabetes, we blocked NP clearance in a mouse model of type 2 diabetes using the specific NPRC ligand ANP(4-23). We found that treatment with ANP(4-23) caused a significant decrease in body weight by increasing energy expenditure and reducing fat mass without a change in lean body mass. The decrease in fat mass was associated with a significant improvement in insulin sensitivity and reduced serum insulin levels. These beneficial effects were accompanied by a decrease in infiltrating macrophages in adipose tissue, and reduced expression of inflammatory markers in both serum and WAT. These data suggest that inhibiting NP clearance may be an effective pharmacologic approach to promote weight loss and enhance insulin sensitivity in type 2 diabetes. Optimizing the therapeutic approach may lead to useful therapies for obesity and type 2 diabetes.

Protein kinase D1 (Prkd1) deletion in brown adipose tissue leads to altered myogenic gene expression after cold exposure, while thermogenesis remains intact

Brown adipose tissue (BAT) has in recent times been rediscovered in adult humans, and together with work from preclinical models, has shown to have the potential of providing a variety of positive metabolic benefits. These include lower plasma glucose, improved insulin sensitivity, and reduced susceptibility to obesity and its comorbidities. As such, its continued study could offer insights to therapeutically modulate this tissue to improve metabolic health. It has been reported that adipose-specific deletion of the gene for protein kinase D1 (Prkd1) in mice enhances mitochondrial respiration and improves whole-body glucose homeostasis. We sought to determine whether these effects were mediated specifically through brown adipocytes using a Prkd1 brown adipose tissue (BAT) Ucp1-Cre-specific knockout mouse model, Prkd1BKO . We unexpectedly observed that upon both cold exposure and β3 -AR agonist administration, Prkd1 loss in BAT did not alter canonical thermogenic gene expression or adipocyte morphology. We took an unbiased approach to assess whether other signaling pathways were affected. RNA from cold-exposed mice was subjected to RNA-Seq analysis. These studies revealed that myogenic gene expression is altered in Prkd1BKO BAT after both acute and extended cold exposure. Given that brown adipocytes and skeletal myocytes share a common precursor cell lineage expressing myogenic factor 5 (Myf5), these data suggest that loss of Prkd1 in BAT may alter the biology of mature brown adipocytes and preadipocytes in this depot. The data presented herein clarify the role of Prkd1 in BAT thermogenesis and present new avenues for the further study of Prkd1 function in BAT.

Inhibition of phosphodiesterase type 9 reduces obesity and cardiometabolic syndrome in mice

Central obesity with cardiometabolic syndrome (CMS) is a major global contributor to human disease, and effective therapies are needed. Here, we show that cyclic GMP-selective phosphodiesterase 9A inhibition (PDE9-I) in both male and ovariectomized female mice suppresses preestablished severe diet-induced obesity/CMS with or without superimposed mild cardiac pressure load. PDE9-I reduces total body, inguinal, hepatic, and myocardial fat; stimulates mitochondrial activity in brown and white fat; and improves CMS, without significantly altering activity or food intake. PDE9 localized at mitochondria, and its inhibition in vitro stimulated lipolysis in a PPARα-dependent manner and increased mitochondrial respiration in both adipocytes and myocytes. PPARα upregulation was required to achieve the lipolytic, antiobesity, and metabolic effects of PDE9-I. All these PDE9-I-induced changes were not observed in obese/CMS nonovariectomized females, indicating a strong sexual dimorphism. We found that PPARα chromatin binding was reoriented away from fat metabolism-regulating genes when stimulated in the presence of coactivated estrogen receptor-α, and this may underlie the dimorphism. These findings have translational relevance given that PDE9-I is already being studied in humans for indications including heart failure, and efficacy against obesity/CMS would enhance its therapeutic utility.