Both direct and indirect evidence demonstrate a central role for the cAMP-dependent protein kinase (PKA) signaling pathway in the regulation of energy balance and metabolism across multiple systems. However, the ubiquitous pattern of PKA expression across cell types poses a challenge in pinpointing its tissue-specific regulatory functions and further characterizing its many downstream effects in certain organs or cells. Mouse models of PKA deficiency and over-expression and studies in living cells have helped clarify PKA function in adipose tissue (AT), liver, adrenal, pancreas, and specific brain nuclei, as they pertain to energy balance and metabolic dysregulation. Limited studies in humans suggest differential regulation of PKA in AT of obese compared to lean individuals and an overall dysregulation of PKA signaling in obesity. Despite its complexity, under normal physiologic conditions, the PKA system is tightly regulated by changes in cAMP concentrations upstream via adenylate cyclase and downstream by phosphodiesterase-mediated cAMP degradation to AMP and by changes in PKA holoenzyme stability. Adjustments in the PKA system appear to be important to the development and maintenance of the obese state and its associated metabolic perturbations. In this review we discuss the important role of PKA in obesity and its involvement in resistance to obesity, through studies in humans and in mouse models, with a focus on the regulation of PKA in energy expenditure, intake behavior, and lipid and glucose metabolism.

The habenula (Hb) is a bilateral, evolutionarily conserved epithalamic structure connecting forebrain and midbrain structures that has gained attention for its roles in depression, 1 addiction, 2-5 rewards processing, 6 and motivation. 7,8 Of its two major subdivisions, the medial (MHb) and lateral Hb (LHb), MHb circuitry and function are poorly understood relative to LHb. 9 Prkar2a codes for cAMP-dependent protein kinase (PKA) regulatory subunit IIα (RIIα), a component of the PKA holoenzyme at the center of one of the major cell-signaling pathways conserved across systems and species. Type 2 regulatory subunits (RIIα, RIIβ) determine the subcellular localization of PKA, and unlike other PKA subunits, Prkar2a has minimal brain expression except in the MHb. 10 We previously showed that RIIα knockout (RIIαKO) mice resist diet-induced obesity (DIO). 11 In the present study, we report that RIIαKO mice have decreased consumption of palatable, "rewarding" foods and increased motivation for voluntary exercise. Prkar2a deficiency led to decreased habenular PKA enzymatic activity and impaired dendritic localization of PKA catalytic subunits in MHb neurons. Re-expression of Prkar2a in the Hb rescued this phenotype confirming differential roles for Prkar2a in regulating the drives for palatable foods and voluntary exercise. Our findings show that in the MHb decreased PKA signaling and dendritic PKA activity decrease motivation for palatable foods, while enhancing the motivation for exercise, a desirable combination of behaviors.

Background High childhood obesity rates coincide with increased incidence of non-alcoholic fatty liver disease (NAFLD) and other comorbidities. Understanding the genetics of susceptibility to obesity and its comorbidities could guide intervention. The cyclic-AMP dependent protein kinase (PKA) signaling pathway regulates energy balance, glucose homeostasis and lipid metabolism. We hypothesized that PKA-related gene variants may be associated with obesity or associated metabolic conditions. Methods We included 457 youths from the Yale Obesity Clinic into the Pathogenesis of Youth-Onset Diabetes cohort (NCT01967849); a variety of clinical tests were performed to characterize NAFLD. Exon sequencing of 54 PKA pathway genes was performed. Variants were confirmed by Sanger sequencing. Clinical data were analyzed, correcting for NAFLD status and BMI z-score with adjustments for multiple comparisons. FRET and PKA enzymatic assays were performed in HEK293 cells transfected with the PRKAR1B p.R115K construct. In silico structural analysis for this variant was done. Results We identified the variant PRKAR1B p.R115K in 4 unrelated, African American patients. Analyses compared this variant group to other African American patients in the cohort. PRKAR1B p.R115K was associated with favorable circulating lipoprotein levels. Analysis of FRET and PKA enzymatic assay showed stronger interaction between the R1β mutant and PKA catalytic subunit Cα and decreased basal PKA activity compared to wildtype (p<.0001). Structural analysis revealed that p.R115K may hinder conformational changes resulting from cAMP binding at cAMP-binding domain A. Conclusions Data suggest PRKAR1B p.R115K affects cAMP-signaling and may favorably modulate lipoprotein profile in African American youth, protecting them from some adverse metabolic outcomes.

Context: Nonalcoholic fatty liver disease (NAFLD) is estimated to affect nearly 10% of Americans age 2-19 and about 38% of those affected are obesei. NAFLD is characterized by triglyceride accumulation in hepatocytes and can progress to nonalcoholic steatohepatitis, end stage liver disease and hepatocellular carcinoma. The underlying causes of NAFLD in youth are unclear although obesity, insulin resistance, type 2 diabetes mellitus and metabolic syndrome are risk factors. Genome-wide association studies and candidate gene studies have found several single nucleotide polymorphisms that affect susceptibility to and progression of NAFLD, but clinical translation for some of these genetics is lackingii. Study design: Because mouse models of dysregulated PKA signaling demonstrate the centrality of this pathway in hepatic lipid metabolism and glucose homeostasis, we hypothesized that defects in hepatic PKA signaling genes could affect susceptibility to or severity of NAFLD in children. We asked whether identified variants might be associated with differences in clinical markers in a cohort of obese pediatric patients (non-NAFLD, n=295; NAFLD, n=165) followed at Yale Medical School, where clinical data and genomic DNA were collected. Exon sequencing of 54 PKA-related candidate genes included those coding for PKA subunits, PDEs and other proteins integral to the hepatic PKA system. Variants were ranked by allele frequency and potential pathogenicity. Ongoing analyses aim to identify associations between single variants and potential additive effects with clinical parameters (anthropometric, liver function, glucose metabolism, plasma lipids). Results: Gene variants were identified in ABCA1, ADCY4, ADCY5, AKAP7, CREB3L1, CREB3L4, CREM, CYP27A1, DHCR7, ERN1, GYS2, IL6, IL10RB, MC2R, PDE1B, PDE2A, PDE3B, PDE4A, PDE7B, PDE10A, PDE11A, PPARGC1B, PRKAR2A, and PRKAR1B. Reported variants met criteria of high to moderate impact based on 9 in silico scores that predict pathogenicity. Allele frequency ranged from 2.5 to over 50 times higher in our cohort than the general population. One or more variant was identified in 34.9% of non-NAFLD and 19.4% of NAFLD patients (p=.0004). Conclusion: We report PKA-related gene variants among a cohort of pediatric obese patients that might serve as useful predictors of risk of NAFLD or obesity. Further analyses will help determine whether any of these variants may play a functional role in NAFLD. Endnotes i Schwimmer JB, Deutsch R, Kahen T, Lavine JE, Stanley C, Behling C. Pediatrics. 2006;118(4):1388. ii Vespasiani-Gentilucci U, Gallo P, Dell’Unto C et al. World J Gastroenterol. 2018;24(43):4835-4845.