cAMP signaling affects age-associated deterioration of pacemaker beating interval dynamics

Sinoatrial node (SAN) beating interval variability (BIV) and the average beating interval (BI) are regulated by a coupled-clock system, driven by Ca2+-calmodulin activated adenylyl cyclase, cAMP, and downstream PKA signaling. Reduced responsiveness of the BI and BIV to submaximal, [X]50, β-adrenergic receptor (β-AR) stimulation, and phosphodiesterase inhibition (PDEI) have been documented in aged SAN tissue, whereas the maximal responses, [X]max, do not differ by age. To determine whether age-associated dysfunction in cAMP signaling leads to altered responsiveness of BI and BIV, we measured cAMP levels and BI in adult (2-4 months n = 27) and aged (22-26 months n = 25) C57/BL6 mouse SAN tissue in control and in response to β-AR or PDEI at X50 and [X]max. Both cAMP and average BI in adult SAN were reduced at X50, whereas cAMP and BI at Xmax did not differ by age. cAMP levels and average BI were correlated both within and between adult and aged SAN. BIV parameters in long- and short-range terms were correlated with cAMP levels for adult SAN. However, due to reduced cAMP within aged tissues at [X]50, these correlations were diminished in advanced age. Thus, cAMP level generated by the coupled clock mechanisms is tightly linked to average BI. Reduced cAMP level at X50 in aged SAN explains the reduced responsiveness of the BI and BIV to β-AR stimulation and PDEI.

Proteomes of primary skin fibroblasts from healthy individuals reveal altered cell responses across the life span

Changes in the proteome of different human tissues with advancing age are poorly characterized. Here, we studied the proteins present in primary skin fibroblasts collected from 82 healthy individuals across a wide age spectrum (22-89 years old) who participated in the GESTALT (Genetic and Epigenetic Signatures of Translational Aging Laboratory Testing) study of the National Institute on Aging, NIH. Proteins were extracted from lysed fibroblasts and subjected to liquid chromatography-mass spectrometry analysis, and the expression levels of 9341 proteins were analyzed using linear regression models. We identified key pathways associated with skin fibroblast aging, including autophagy, scavenging of reactive oxygen species (ROS), ribosome biogenesis, DNA replication, and DNA repair. Changes in these prominent pathways were corroborated using molecular and cell culture approaches. Our study establishes a framework of the global proteome governing skin fibroblast aging and points to possible biomarkers and therapeutic targets.

Overexpression of a Neuronal Type Adenylyl Cyclase (Type 8) in Sinoatrial Node Markedly Impacts Heart Rate and Rhythm

Heart rate (HR) and HR variability (HRV), predictors of over-all organism health, are widely believed to be driven by autonomic input to the sinoatrial node (SAN), with sympathetic input increasing HR and reducing HRV. However, variability in spontaneous beating intervals in isolated SAN tissue and single SAN cells, devoid of autonomic neural input, suggests that clocks intrinsic to SAN cells may also contribute to HR and HRV in vivo. We assessed contributions of both intrinsic and autonomic neuronal input mechanisms of SAN cell function on HR and HRV via in vivo, telemetric EKG recordings. This was done in both wild type (WT) mice, and those in which adenylyl cyclase type 8 (ADCY8), a main driver of intrinsic cAMP-PKA-Ca²⁺ mediated pacemaker function, was overexpressed exclusively in the heart (TG^AC8). We hypothesized that TG^AC8 mice would: (1) manifest a more coherent pattern of HRV in vivo, i.e., a reduced HRV driven by mechanisms intrinsic to SAN cells, and less so to modulation by autonomic input and (2) utilize unique adaptations to limit sympathetic input to a heart with high levels of intrinsic cAMP-Ca²⁺ signaling. Increased adenylyl cyclase (AC) activity in TG^AC8 SAN tissue was accompanied by a marked increase in HR and a concurrent marked reduction in HRV, both in the absence or presence of dual autonomic blockade. The marked increase in intrinsic HR and coherence of HRV in TG^AC8 mice occurred in the context of: (1) reduced HR and HRV responses to β-adrenergic receptor (β-AR) stimulation; (2) increased transcription of genes and expression of proteins [β-Arrestin, G Protein-Coupled Receptor Kinase 5 (GRK5) and Clathrin Adaptor Protein (Dab2)] that desensitize β-AR signaling within SAN tissue, (3) reduced transcripts or protein levels of enzymes [dopamine beta-hydorxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT)] required for catecholamine production in intrinsic cardiac adrenergic cells, and (4) substantially reduced plasma catecholamine levels. Thus, mechanisms driven by cAMP-PKA-Ca²⁺ signaling intrinsic to SAN cells underlie the marked coherence of TG^AC8 mice HRV. Adaptations to limit additional activation of AC signaling, via decreased neuronal sympathetic input, are utilized to ensure the hearts survival and prevent Ca²⁺ overload.

Sarcoplasmic reticulum Ca2+ cycling protein phosphorylation in a physiologic Ca2+ milieu unleashes a high-power, rhythmic Ca2+ clock in ventricular myocytes: relevance to arrhythmias and bio-pacemaker design

Basal phosphorylation of sarcoplasmic reticulum (SR) Ca(2+) proteins is high in sinoatrial nodal cells (SANC), which generate partially synchronized, spontaneous, rhythmic, diastolic local Ca(2+) releases (LCRs), but low in ventricular myocytes (VM), which exhibit rare diastolic, stochastic SR-generated Ca(2+) sparks. We tested the hypothesis that in a physiologic Ca(2+) milieu, and independent of increased Ca(2+) influx, an increase in basal phosphorylation of SR Ca(2+) cycling proteins will convert stochastic Ca(2+) sparks into periodic, high-power Ca(2+) signals of the type that drives SANC normal automaticity. We measured phosphorylation of SR-associated proteins, phospholamban (PLB) and ryanodine receptors (RyR), and spontaneous local Ca(2+) release characteristics (LCR) in permeabilized single, rabbit VM in physiologic [Ca(2+)], prior to and during inhibition of protein phosphatase (PP) and phosphodiesterase (PDE), or addition of exogenous cAMP, or in the presence of an antibody (2D12), that specifically inhibits binding of the PLB to SERCA-2. In the absence of the aforementioned perturbations, VM could only generate stochastic local Ca(2+) releases of low power and low amplitude, as assessed by confocal Ca(2+) imaging and spectral analysis. When the kinetics of Ca(2+) pumping into the SR were increased by an increase in PLB phosphorylation (via PDE and PP inhibition or addition of cAMP) or by 2D12, self-organized, "clock-like" local Ca(2+) releases, partially synchronized in space and time (Ca(2+) wavelets), emerged, and the ensemble of these rhythmic local Ca(2+) wavelets generated a periodic high-amplitude Ca(2+) signal. Thus, a Ca(2+) clock is not specific to pacemaker cells, but can also be unleashed in VM when SR Ca(2+) cycling increases and spontaneous local Ca(2+) release becomes partially synchronized. This unleashed Ca(2+) clock that emerges in a physiological Ca(2+) milieu in VM has two faces, however: it can provoke ventricular arrhythmias; or if harnessed, can be an important feature of novel bio-pacemaker designs.

Oxysterols in heart failure

Oxysterols are biologically active molecules that result from the oxidation of cholesterol. Several oxysterols are found in macrophages and macrophage-derived 'foam cells' in atherosclerotic tissue. Lipophilic oxysterols penetrate cell membranes and, therefore, their concentrations can reach harmful levels in endothelial and smooth muscle cells located in close proximity to the atherosclerotic plaques or inflammatory zones. New findings suggest that the effects of oxysterols on cardiomyocytes can lead to cell hypertrophy and death. This may make oxysterols one of the major factors precipitating morbidity in atherosclerosis-induced cardiac diseases and inflammation-induced heart complications. The pathological actions of oxysterols on muscle cells were shown to depend on dysfunctional Ca(2+) signaling; however, the mechanisms of the effects remain to be elucidated. Understanding the effects of oxysterols could lead to therapies that modulate malfunction of cardiomyocytes. This review discusses the experimental findings and the relevance of oxysterols to heart failure, and suggests strategies for important future investigations.

Testosterone regulates 25-hydroxycholesterol production in testicular macrophages

Recently, we found that testicular macrophages produce 25-hydroxycholesterol (25-HC) and express 25-hydroxylase, the enzyme that converts cholesterol to 25-HC. In addition, 25-HC may be an important paracrine factor mediating the known interactions between macrophages and neighboring Leydig cells, because it is efficiently converted to testosterone by Leydig cells. The purpose of the present study was to determine if testosterone can regulate the production of 25-HC in rat testicular macrophages, representing a potential negative-feedback loop from Leydig cells. We found that expression of 25-hydroxylase mRNA and production of 25-HC by cultured testicular macrophages were significantly inhibited by testosterone at 10 micro g/ml. This dose of testosterone did not have an effect on cell viability and did not change the rate of mRNA degradation in the presence of actinomycin D. These studies indicate that production of 25-HC is negatively regulated by testosterone, which may be representative of a paracrine negative-feedback loop.

25-hydroxycholesterol is produced by testicular macrophages during the early postnatal period and influences differentiation of Leydig cells in vitro

Leydig cells develop inappropriately in animals lacking testicular macrophages. We have recently found that macrophages from adult animals produce 25-hydroxycholesterol, an oxysterol involved in the differentiation of hepatocytes and keratinocytes. Therefore, we hypothesized that testicular macrophages also produce 25-hydroxycholesterol during the early postnatal period and that this oxysterol plays a role in the differentiation of Leydig cells. We assessed the production of 25-hydroxycholesterol and 25-hydroxylase mRNA by cultured testicular macrophages from rats at 10, 20, and 40 days of age. We also tested the long-term effects of 25-hydroxycholesterol on basal and LH-stimulated testosterone production, and 3beta-hydroxysteroid dehydrogenase activity as end points of Leydig cell differentiation in vitro. We found that testicular macrophages from animals at all ages produced both 25-hydroxycholesterol and 25-hydroxylase mRNA, with macrophages from 10-day-old animals having the highest steady-state levels of message. We also found that chronic exposure of Leydig cells to 25-hydroxycholesterol increased basal production of testosterone but decreased LH-stimulated steroidogenesis at all ages. Finally, 25-hydroxycholesterol increased 3beta-hydroxysteroid dehydrogenase activity in both progenitor and immature Leydig cells. These findings support the hypothesis that testicular macrophages play an important role in the differentiation of Leydig cells through the secretion of 25-hydroxycholesterol.

Pregnenolone synthesis in immature rat Sertoli cells

It has been reported that testicular Sertoli cells can be induced to synthesize the steroidogenic acute regulatory (StAR) protein. StAR mediates the rate-limiting step of steroidogenesis, which is the transfer of cholesterol to the inner mitochondrial membrane. Since Sertoli cells are thought to be unable to utilize cholesterol for the synthesis of steroids the role of StAR in these cells was questioned. In the present studies we have corroborated the induction of StAR protein in immature cultured Sertoli cells in response to either trophic hormone or cAMP analog stimulation. Further, we have shown that long term stimulation of Sertoli cells with cAMP analog results in the induction of P450scc enzyme and increased pregnenolone production. In this manner, the Sertoli cell may resemble its ovarian homolog, the granulosa cell, more closely than previously thought with regards to its steroidogenic capacity. Thus, StAR may play the same role in Sertoli cells as it does in other steroidogenic tissues.