The alveolar epithelium serves as a barrier between the body and the external environment. To maintain efficient gas exchange, the alveolar epithelium has evolved to withstand and rapidly respond to an assortment of inhaled, injury-inducing stimuli. However, alveolar damage can lead to loss of alveolar fluid barrier function and exuberant, non-resolving inflammation that manifests clinically as acute respiratory distress syndrome (ARDS). This review discusses recent discoveries related to mechanisms of alveolar homeostasis, injury, repair, and regeneration, with a contemporary emphasis on virus-induced lung injury. In addition, we address new insights into how the alveolar epithelium coordinates injury-induced lung inflammation and review maladaptive lung responses to alveolar damage that drive ARDS and pathologic lung remodeling.

To test the hypothesis that the activation of the growth hormone-releasing hormone (GHRH) receptor signalling pathway within the myocardium both prevents and reverses diastolic dysfunction and pathophysiologic features consistent with heart failure with preserved ejection fraction (HFpEF). Impaired myocardial relaxation, fibrosis, and ventricular stiffness, among other multi-organ morbidities, characterize the phenotype underlying the HFpEF syndrome. Despite the rapidly increasing prevalence of HFpEF, few effective therapies have emerged. Synthetic agonists of the GHRH receptors reduce myocardial fibrosis, cardiomyocyte hypertrophy, and improve performance in animal models of ischaemic cardiomyopathy, independently of the growth hormone axis. CD1 mice received 4- or 8-week continuous infusion of angiotensin-II (Ang-II) to generate a phenotype with several features consistent with HFpEF. Mice were administered either vehicle or a potent synthetic agonist of GHRH, MR-356 for 4-weeks beginning concurrently or 4-weeks following the initiation of Ang-II infusion. Ang-II-treated animals exhibited diastolic dysfunction, ventricular hypertrophy, interstitial fibrosis, and normal ejection fraction. Cardiomyocytes isolated from these animals exhibited incomplete relaxation, depressed contractile responses, altered myofibrillar protein phosphorylation, and disturbed calcium handling mechanisms (ex vivo). MR-356 both prevented and reversed the development of the pathological phenotype in vivo and ex vivo. Activation of the GHRH receptors increased cAMP and cGMP in cardiomyocytes isolated from control animals but only cAMP in cardiac fibroblasts, suggesting that GHRH-A exert differential effects on cardiomyocytes and fibroblasts. These findings indicate that the GHRH receptor signalling pathway(s) represents a new molecular target to counteract dysfunctional cardiomyocyte relaxation by targeting myofilament phosphorylation and fibrosis. Accordingly, activation of GHRH receptors with potent, synthetic GHRH agonists may provide a novel therapeutic approach to management of the myocardial alterations associated with the HFpEF syndrome. Proposed mechanism of action for GHRH-As to modulate the Excitation-Contraction Coupling (ECC). It is well known that activation of GHRHRs (GPCR) induces the adenylyl cyclase (AC)/cyclic AMP/protein kinase A (PKA) pathway. PKA can both phosphorylate sarcomeric and calcium handling proteins and activate soluble guanylyl cyclase (sGC). This pathway also promotes reparative mechanisms at transcriptional level. On the other hand, has been well demonstrated that GHRH-As also promote activation of AKT (PKB) among other kinases. Endothelial Nitric Oxide Synthase eNOS) is also activated by GHRH. The generated nitric oxide (NO) in turn activates sGC to produce the second messenger cGMP, an activator of the protein kinase G (PKG). This kinase is also able to phosphorylate the above-mentioned proteins. This pathway has been proposed to be activated downstream GHRHRs through activation of PI3K by the Gβγ proteins or β-arrestins (among other possible mechanisms).

Efferocytosis is a process whereby apoptotic cells are cleared to maintain tissue homeostasis. In the lungs, efferocytosis has been implicated in several acute and chronic inflammatory diseases. A long-standing method to study efferocytosis in vivo is to instill apoptotic cells into the lungs to evaluate macrophage uptake. However, this approach provides nonphysiologic levels of cells to the airspaces, where there is preferential access to the alveolar macrophages. To circumvent this limitation, we developed a new method to study efferocytosis of damaged alveolar type 2 (AT2) epithelial cells in vivo. A reporter mouse that expresses TdTomato in AT2 epithelial cells was injured with influenza (strain PR8) to induce apoptosis of AT2 cells. We were able to identify macrophages that acquire red fluorescence after influenza injury, indicating efferocytosis of AT2 cells. Furthermore, evaluation of macrophage populations led to the surprising finding that lung interstitial macrophages were the primary efferocyte in vivo. In summary, we present a novel finding that the interstitial macrophage, not the alveolar macrophage, primarily mediates clearance of AT2 cells in the lungs after influenza infection. Our method of studying efferocytosis provides a more physiologic approach in evaluating the spatiotemporal dynamics of apoptotic cell clearance in vivo and opens new avenues to study the mechanisms by which efferocytosis regulates inflammation.

Objective: To test the hypothesis that the activation of the growth hormone-releasing hormone (GHRH) receptor signaling pathway within the myocardium both prevents and reverses heart failure with preserved ejection fraction (HFpEF). Background: HFpEF is characterized by impaired myocardial relaxation, fibrosis and ventricular stiffness. Despite the rapidly increasing prevalence of HFpEF, no effective therapies have emerged. Synthetic agonists of the GHRH receptors reduce myocardial fibrosis, hypertrophy and improve performance, independently of the growth-hormone axis. Methods: We generated a HFpEF-like phenotype with continuous infusion of angiotensin-II (Ang-II) in CD1 mice. Mice were injected with either vehicle or a potent synthetic agonist of the growth hormone-releasing hormone, MR-356. Results: Ang-II treated animals had diastolic dysfunction, ventricular hypertrophy, and normal ejection fraction and isolated cardiomyocytes (ex vivo) exhibited incomplete relaxation, depressed contractile responses and altered myofibrillar protein phosphorylation. Calcium handling mechanisms were disturbed in cardiomyocytes from mice with HFpEF. MR-356 both prevented and reversed the development of the pathological phenotype in vivo and ex vivo. Conclusion: These findings indicate that the GHRH receptor signaling pathway represents a new molecular target to counteract HFpEF-associated cardiomyocyte dysfunction by targeting myofilament phosphorylation. Accordingly, activation of the GHRH receptor with potent synthetic GHRH agonists may provide a novel therapeutic approach to management of the HFpEF syndrome. Condensed Heart failure with preserved ejection fraction (HFpEF) is characterized by a remodeled myocardium conferring ventricular stiffness and diastolic dysfunction. There are no effective therapies. Agonists of growth hormone-releasing hormone (GHRH) receptors have beneficial effects on the heart. We hypothesize that activation of GHRH receptors suppresses this HFpEF phenotype. Treatment with a synthetic agonist of GHRH, prevented the development of the pathological phenotype in a murine model of HFpEF-induced by chronic angiotensin-II infusion. These findings indicate that activation of GHRH receptors represents a novel molecular strategy to counteract HFpEF-associated cardiomyocyte dysfunction and provide a potential approach to management of HFpEF syndrome. Highlights: A synthetic growth hormone-releasing hormone agonist (GHRH-A) prevents and reverses the pathological remodeling in a mouse model of HFpEF induced by infusion of low dose Ang II. GHRH-A improves intracellular calcium handling by reducing the sarcoplasmic reticulum calcium leakage and enhancing phospholamban phosphorylation. GHRH-A treatment prevents and reverses diastolic dysfunction by enhancing the rate of sarcomere re-lengthening. Activation of the GHRH receptor with the GHRH-A, MR-356, leads to targeting myofibrillar proteins and desensitizing myofilaments in response to calcium.