The Role of PDE8 in T Cell Recruitment and Function in Inflammation

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

Phosphodiesterase 8 (PDE8): Distribution and Cellular Expression and Association with Alzheimer's Disease

Phosphodiesterase 8 (PDE8), as a member of PDE superfamily, specifically promotes the hydrolysis and degradation of intracellular cyclic adenosine monophosphate (cAMP), which may be associated with pathogenesis of Alzheimer's disease (AD). However, little is currently known about potential role in the central nervous system (CNS). Here we investigated the distribution and expression of PDE8 in brain of mouse, which we believe can provide evidence for studying the role of PDE8 in CNS and the relationship between PDE8 and AD. Here, C57BL/6J mice were used to observe the distribution patterns of two subtypes of PDE8, PDE8A and PDE8B, in different sexes in vivo by western blot (WB). Meanwhile, C57BL/6J mice were also used to demonstrate the distribution pattern of PDE8 in selected brain regions and localization in neural cells by WB and multiplex immunofluorescence staining. Furthermore, the triple transgenic (3×Tg-AD) mice and wild type (WT) mice of different ages were used to investigate the changes of PDE8 expression in the hippocampus and cerebral cortex during the progression of AD. PDE8 was found to be widely expressed in multiple tissues and organs including heart, kidney, stomach, brain, and liver, spleen, intestines, and uterus, with differences in expression levels between the two subtypes of PDE8A and PDE8B, as well as two sexes. Meanwhile, PDE8 was widely distributed in the brain, especially in areas closely related to cognitive function such as cerebellum, striatum, amygdala, cerebral cortex, and hippocampus, without differences between sexes. Furthermore, PDE8A was found to be expressed in neuronal cells, microglia and astrocytes, while PDE8B is only expressed in neuronal cells and microglia. PDE8A expression in the hippocampus of both female and male 3×Tg-AD mice was gradually increased with ages and PDE8B expression was upregulated only in cerebral cortex of female 3×Tg-AD mice with ages. However, the expression of PDE8A and PDE8B was apparently increased in both cerebral cortex and hippocampus in both female and male 10-month-old 3×Tg-AD mice compared WT mice. These results suggest that PDE8 may be associated with the progression of AD and is a potential target for its prevention and treatment in the future.

A mini-review: phosphodiesterases in charge to balance intracellular cAMP during T-cell activation

T-cell activation is a pivotal process of the adaptive immune response with 3',5'-cyclic adenosine monophosphate (cAMP) as a key regulator of T-cell activation and function. It governs crucial control over T-cell differentiation and production of pro-inflammatory cytokines, such as IFN-γ. Intriguingly, levels of intracellular cAMP differ between regulatory (Treg) and conventional T-cells (Tcon). During cell-cell contact, cAMP is transferred via gap junctions between these T-cell subsets to mediate the immunosuppressive function of Treg. Moreover, the activation of T-cells via CD3 and CD28 co-stimulation leads to a transient upregulation of cAMP. Elevated intracellular cAMP levels are balanced precisely by phosphodiesterases (PDEs), a family of enzymes that hydrolyze cyclic nucleotides. Various PDEs play distinct roles in regulating cAMP and cyclic guanosine monophosphate (cGMP) in T-cells. Research on PDEs has gained growing interest due to their therapeutic potential to manipulate T-cell responses. So far, PDE4 is the best-described PDE in T-cells and the first PDE that is currently targeted in clinical practice to treat autoimmune diseases. But also, other PDE families harbor additional therapeutic potential. PDE2A is a dual-substrate phosphodiesterase which is selectively upregulated in Tcon upon activation. In this Mini-Review, we will highlight the impact of cAMP regulation on T-cell activation and function and summarize recent findings on different PDEs regulating intracellular cAMP levels in T-cells.

Dynamics of Changes in the cAMP/cGMP Concentration Ratio in the Thymus and Spleen of Laboratory Mice during Vaccination against Plague and Tularemia against the Background of Immunomodulation

Vaccine strains Yersinia pestis EV NIIEG at a dose of 103 CFU and Francisella tularensis 15 NIIEG at a dose of 102 CFU induced changes in the concentration of cyclic nucleotides in the thymus and spleen of white mice. Antigen-induced changes in the cAMP/cGMP ratio in immunocompetent organs had a phase or oscillatory character, which seems to be related to the regulation of postvaccination immunoreactivity in the body. Synthetic organoselenium compound 974zh stimulated an increase in the amplitude of cAMP/cGMP oscillations, indicating its stimulating effect on the immunogenic properties of vaccine strains at doses an order of magnitude below the standard doses.

Effects of cortisol, epinephrine, and bisphenol contaminants on the transcriptional landscape of marine mammal blubber

Top ocean predators such as marine mammals are threatened by intensifying anthropogenic activity, and understanding the combined effects of multiple stressors on their physiology is critical for conservation efforts. We investigated potential interactions between stress hormones and bisphenol contaminants in a model marine mammal, the northern elephant seal (NES). We exposed precision-cut adipose tissue slices (PCATS) from blubber of weaned NES pups to cortisol (CORT), epinephrine (EPI), bisphenol A (BPA), bisphenol S (BPS), or their combinations (CORT-EPI, BPA-EPI, and BPS-EPI) ex vivo and identified hundreds of genes that were differentially regulated in response to these treatments. CORT altered expression of genes associated with lipolysis and adipogenesis, whereas EPI and CORT-EPI-regulated genes were associated with responses to hormones, lipid and protein turnover, immune function, and transcriptional and epigenetic regulation of gene expression, suggesting that EPI has wide-ranging and prolonged impacts on the transcriptional landscape and function of blubber. Bisphenol treatments alone had a weak impact on gene expression compared with stress hormones. However, the combination of EPI with bisphenols altered expression of genes associated with inflammation, cell stress, DNA damage, regulation of nuclear hormone receptor activity, cell cycle, mitochondrial function, primary ciliogenesis, and lipid metabolism in blubber. Our results suggest that CORT, EPI, bisphenols, and their combinations impact cellular, immune, and metabolic homeostasis in marine mammal blubber, which may affect the ability of marine mammals to sustain prolonged fasting during reproduction and migration, renew tissues, and mount appropriate responses to immune challenges and additional stressors.

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.

Upregulation of Phosphodiesterase 2A Augments T Cell Activation by Changing cGMP/cAMP Cross-Talk

3′,5′-cyclic adenosine monophosphate (cAMP) is well-known for its diverse immunomodulatory properties, primarily inhibitory effects during T cell activation, proliferation, and production of pro-inflammatory cytokines. A decrease in cAMP levels, due to the hydrolyzing activity of phosphodiesterases (PDE), is favoring inflammatory responses. This can be prevented by selective PDE inhibitors, which makes PDEs important therapeutic targets for autoimmune disorders. In this study, we investigated the specific roles of PDE2A and PDE3B in the regulation of intracellular cAMP levels in different mouse T cell subsets. Unexpectedly, T cell receptor (TCR) activation led to a selective upregulation of PDE2A at the protein level in conventional T cells (Tcon), whereas no changes were detected in regulatory T cells (Treg). In contrast, protein expression of PDE3B was significantly higher in both non-activated and activated Tcon subsets as compared to Treg, with no changes upon TCR engagement. Live-cell imaging of T cells expressing a highly sensitive Förster resonance energy transfer (FRET)-based biosensor, Epac1-camps, has enabled cAMP measurements in real time and revealed stronger responses to the PDE2A inhibitors in activated vs non-activated Tcon. Importantly, stimulation of intracellular cGMP levels with natriuretic peptides led to an increase of cAMP in non-activated and a decrease of cAMP in activated Tcon, suggesting that TCR activation changes the PDE3B-dependent positive to PDE2A-dependent negative cGMP/cAMP cross-talk. Functionally, this switch induced higher expression of early activation markers CD25 and CD69. This constitutes a potentially interesting feed-forward mechanism during autoimmune and inflammatory responses that may be exploited therapeutically.