Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

Molecular Signaling Mechanisms and Function of Natriuretic Peptide Receptor-A in the Pathophysiology of Cardiovascular Homeostasis

The discovery of atrial, brain, and C-type natriuretic peptides (ANP, BNP, and CNP) and their cognate receptors has greatly increased our knowledge of the control of hypertension and cardiovascular homeostasis. ANP and BNP are potent endogenous hypotensive hormones that elicit natriuretic, diuretic, vasorelaxant, antihypertrophic, antiproliferative, and antiinflammatory effects, largely directed toward the reduction of blood pressure (BP) and cardiovascular diseases (CVDs). The principal receptor involved in the regulatory actions of ANP and BNP is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which produces the intracellular second messenger cGMP. Cellular, biochemical, molecular, genetic, and clinical studies have facilitated understanding of the functional roles of natriuretic peptides (NPs), as well as the functions of their receptors, and signaling mechanisms in CVDs. Transgenic and gene-targeting (gene-knockout and gene-duplication) strategies have produced genetically altered novel mouse models and have advanced our knowledge of the importance of NPs and their receptors at physiological and pathophysiological levels in both normal and disease states. The current review describes the past and recent research on the cellular, molecular, genetic mechanisms and functional roles of the ANP-BNP/NPRA system in the physiology and pathophysiology of cardiovascular homeostasis as well as clinical and diagnostic markers of cardiac disorders and heart failure. However, the therapeutic potentials of NPs and their receptors for the diagnosis and treatment of cardiovascular diseases, including hypertension, heart failure, and stroke have just begun to be expanded. More in-depth investigations are needed in this field to extend the therapeutic use of NPs and their receptors to treat and prevent CVDs.

Genetic Ablation and Guanylyl Cyclase/Natriuretic Peptide Receptor-A: Impact on the Pathophysiology of Cardiovascular Dysfunction

Mice bearing targeted gene mutations that affect the functions of natriuretic peptides (NPs) and natriuretic peptide receptors (NPRs) have contributed important information on the pathogenesis of hypertension, kidney disease, and cardiovascular dysfunction. Studies of mice having both complete gene disruption and tissue-specific gene ablation have contributed to our understanding of hypertension and cardiovascular disorders. These phenomena are consistent with an oligogenic inheritance in which interactions among a few alleles may account for genetic susceptibility to hypertension, renal insufficiency, and congestive heart failure. In addition to gene knockouts conferring increased risks of hypertension, kidney disorders, and cardiovascular dysfunction, studies of gene duplications have identified mutations that protect against high blood pressure and cardiovascular events, thus generating the notion that certain alleles can confer resistance to hypertension and heart disease. This review focuses on the intriguing phenotypes of Npr1 gene disruption and gene duplication in mice, with emphasis on hypertension and cardiovascular events using mouse models carrying Npr1 gene knockout and/or gene duplication. It also describes how Npr1 gene targeting in mice has contributed to our knowledge of the roles of NPs and NPRs in dose-dependently regulating hypertension and cardiovascular events.

Emerging concepts of receptor endocytosis and concurrent intracellular signaling: Mechanisms of guanylyl cyclase/natriuretic peptide receptor-A activation and trafficking

Endocytosis is a prominent clathrin-mediated mechanism for concentrated uptake and internalization of ligand-receptor complexes, also known as cargo. Internalization of cargo is the fundamental mechanism for receptor-dependent regulation of cell membrane function, intracellular signal transduction, and neurotransmission, as well as other biological and physiological activities. However, the intrinsic mechanisms of receptor endocytosis and contemporaneous intracellular signaling are not well understood. We review emerging concepts of receptor endocytosis with concurrent intracellular signaling, using a typical example of guanylyl cyclase/natriuretic peptide receptor-A (NPRA) internalization, subcellular trafficking, and simultaneous generation of second-messenger cGMP and signaling in intact cells. We highlight the role of short-signal motifs located in the carboxyl-terminal regions of membrane receptors during their internalization and subsequent receptor trafficking in organelles that are not traditionally studied in this context, including nuclei and mitochondria. This review sheds light on the importance of future investigations of receptor endocytosis and trafficking in live cells and intact animals in vivo in physiological context.

Clathrin-dependent internalization, signaling, and metabolic processing of guanylyl cyclase/natriuretic peptide receptor-A

Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP), have pivotal roles in renal hemodynamics, neuroendocrine signaling, blood pressure regulation, and cardiovascular homeostasis. Binding of ANP and BNP to the guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) induces rapid internalization and trafficking of the receptor via endolysosomal compartments, with concurrent generation of cGMP. However, the mechanisms of the endocytotic processes of NPRA are not well understood. The present study, using ¹²⁵I-ANP binding assay and confocal microscopy, examined the function of dynamin in the internalization of NPRA in stably transfected human embryonic kidney-293 (HEK-293) cells. Treatment of recombinant HEK-293 cells with ANP time-dependently accelerated the internalization of receptor from the cell surface to the cell interior. However, the internalization of ligand-receptor complexes of NPRA was drastically decreased by the specific inhibitors of clathrin- and dynamin-dependent receptor internalization, almost 85% by monodansylcadaverine, 80% by chlorpromazine, and 90% by mutant dynamin, which are specific blockers of endocytic vesicle formation. Visualizing the internalization of NPRA and enhanced GFP-tagged NPRA in HEK-293 cells by confocal microscopy demonstrated the formation of endocytic vesicles after 5 min of ANP treatment; this effect was blocked by the inhibitors of clathrin and by mutant dynamin construct. Our results suggest that NPRA undergoes internalization via clathrin-mediated endocytosis as part of its normal itinerary, including trafficking, signaling, and metabolic degradation.

Endocytosis and Trafficking of Natriuretic Peptide Receptor-A: Potential Role of Short Sequence Motifs

The targeted endocytosis and redistribution of transmembrane receptors among membrane-bound subcellular organelles are vital for their correct signaling and physiological functions. Membrane receptors committed for internalization and trafficking pathways are sorted into coated vesicles. Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP) bind to guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and elicit the generation of intracellular second messenger cyclic guanosine 3',5'-monophosphate (cGMP), which lowers blood pressure and incidence of heart failure. After ligand binding, the receptor is rapidly internalized, sequestrated, and redistributed into intracellular locations. Thus, NPRA is considered a dynamic cellular macromolecule that traverses different subcellular locations through its lifetime. The utilization of pharmacologic and molecular perturbants has helped in delineating the pathways of endocytosis, trafficking, down-regulation, and degradation of membrane receptors in intact cells. This review describes the investigation of the mechanisms of internalization, trafficking, and redistribution of NPRA compared with other cell surface receptors from the plasma membrane into the cell interior. The roles of different short-signal peptide sequence motifs in the internalization and trafficking of other membrane receptors have been briefly reviewed and their potential significance in the internalization and trafficking of NPRA is discussed.

Regulation of innate immune responses by transmembrane interactions: lessons from the TLR family

The mammalian innate immune response is responsible for the early stages of defense against invading pathogens. One of the major receptor families facilitating innate immune activation is the Toll-like receptor (TLR) family. These receptors are type 1 membrane proteins spanning the membrane with a single transmembrane domain (TMD). All TLRs form homo- and hetero-dimers within membranes and new data suggest that the single transmembrane domain of some of these receptors is involved in their dimerization and function. Newly identified TLR dimers are continuously reported but only little is known about the importance of the TMDs for their dimer assembly and signaling regulation. Uncontrolled or untimely activation of TLRs is related to a large number of pathologies ranging from cystic fibrosis to sepsis and cancer. In this review we will focus on the contribution of the TMDs of innate immune receptors - specifically TLR2-to their regulation and function. In addition, we will address the current issues remaining to be solved regarding the mechanistic insights of this regulation. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Cyclic nucleotide phosphodiesterase 1A: a key regulator of cardiac fibroblast activation and extracellular matrix remodeling in the heart

Cardiac fibroblasts become activated and differentiate to smooth muscle-like myofibroblasts in response to hypertension and myocardial infarction (MI), resulting in extracellular matrix (ECM) remodeling, scar formation and impaired cardiac function. cAMP and cGMP-dependent signaling have been implicated in cardiac fibroblast activation and ECM synthesis. Dysregulation of cyclic nucleotide phosphodiesterase (PDE) activity/expression is also associated with various diseases and several PDE inhibitors are currently available or in development for treating these pathological conditions. The objective of this study is to define and characterize the specific PDE isoform that is altered during cardiac fibroblast activation and functionally important for regulating myofibroblast activation and ECM synthesis. We have found that Ca(2+)/calmodulin-stimulated PDE1A isoform is specifically induced in activated cardiac myofibroblasts stimulated by Ang II and TGF-β in vitro as well as in vivo within fibrotic regions of mouse, rat, and human diseased hearts. Inhibition of PDE1A function via PDE1-selective inhibitor or PDE1A shRNA significantly reduced Ang II or TGF-β-induced myofibroblast activation, ECM synthesis, and pro-fibrotic gene expression in rat cardiac fibroblasts. Moreover, the PDE1 inhibitor attenuated isoproterenol-induced interstitial fibrosis in mice. Mechanistic studies revealed that PDE1A modulates unique pools of cAMP and cGMP, predominantly in perinuclear and nuclear regions of cardiac fibroblasts. Further, both cAMP-Epac-Rap1 and cGMP-PKG signaling was involved in PDE1A-mediated regulation of collagen synthesis. These results suggest that induction of PDE1A plays a critical role in cardiac fibroblast activation and cardiac fibrosis, and targeting PDE1A may lead to regression of the adverse cardiac remodeling associated with various cardiac diseases.

Guanylyl cyclase / atrial natriuretic peptide receptor-A: role in the pathophysiology of cardiovascular regulation

Atrial natriuretic factor (ANF), also known as atrial natriuretic peptide (ANP), is an endogenous and potent hypotensive hormone that elicits natriuretic, diuretic, vasorelaxant, and anti-proliferative effects, which are important in the control of blood pressure and cardiovascular events. One principal locus involved in the regulatory action of ANP and brain natriuretic peptide (BNP) is guanylyl cyclase / natriuretic peptide receptor-A (GC-A/NPRA). Studies on ANP, BNP, and their receptor, GC-A/NPRA, have greatly increased our knowledge of the control of hypertension and cardiovascular disorders. Cellular, biochemical, and molecular studies have helped to delineate the receptor function and signaling mechanisms of NPRA. Gene-targeted and transgenic mouse models have advanced our understanding of the importance of ANP, BNP, and GC-A/NPRA in disease states at the molecular level. Importantly, ANP and BNP are used as critical markers of cardiac events; however, their therapeutic potentials for the diagnosis and treatment of hypertension, heart failure, and stroke have just begun to be realized. We are now just at the initial stage of molecular therapeutics and pharmacogenomic advancement of the natriuretic peptides. More investigations should be undertaken and ongoing ones be extended in this important field.

The functional genomics of guanylyl cyclase/natriuretic peptide receptor-A: perspectives and paradigms

The cardiac hormones atrial natriuretic peptide and B-type natriuretic peptide (brain natriuretic peptide) activate guanylyl cyclase (GC)-A/natriuretic peptide receptor-A (NPRA) and produce the second messenger cGMP. GC-A/NPRA is a member of the growing family of GC receptors. The recent biochemical, molecular and genomic studies on GC-A/NPRA have provided important insights into the regulation and functional activity of this receptor protein, with a particular emphasis on cardiac and renal protective roles in hypertension and cardiovascular disease states. The progress in this field of research has significantly strengthened and advanced our knowledge about the critical roles of Npr1 (coding for GC-A/NPRA) in the control of fluid volume, blood pressure, cardiac remodeling, and other physiological functions and pathological states. Overall, this review attempts to provide insights and to delineate the current concepts in the field of functional genomics and signaling of GC-A/NPRA in hypertension and cardiovascular disease states at the molecular level.

Protein homeostasis and aging: The importance of exquisite quality control

All cells count on precise mechanisms that regulate protein homeostasis to maintain a stable and functional proteome. A progressive deterioration in the ability of cells to preserve the stability of their proteome occurs with age and contributes to the functional loss characteristic of old organisms. Molecular chaperones and the proteolytic systems are responsible for this cellular quality control by assuring continuous renewal of intracellular proteins. When protein damage occurs, such as during cellular stress, the coordinated action of these cellular surveillance systems allows detection and repair of the damaged structures or, in many instances, leads to the complete elimination of the altered proteins from inside cells. Dysfunction of the quality control mechanisms and intracellular accumulation of abnormal proteins in the form of protein inclusions and aggregates occur in almost all tissues of an aged organism. Preservation or enhancement of the activity of these surveillance systems until late in life improves their resistance to stress and is sufficient to slow down aging. In this work, we review recent advances on our understanding of the contribution of chaperones and proteolytic systems to the maintenance of cellular homeostasis, the cellular response to stress and ultimately to longevity.

Small peptide recognition sequence for intracellular sorting

Increasing evidence indicate that complex arrays of short signals and recognition peptide sequence ensure accurate trafficking and distribution of transmembrane receptors and/or proteins and their ligands into intracellular compartments. Internalization and subsequent trafficking of cell-surface receptors into the cell interior is mediated by specific short-sequence peptide signals within the cytoplasmic domains of these receptor proteins. The short signals usually consist of small linear amino acid sequences, which are recognized by adaptor coat proteins along the endocytic and sorting pathways. In recent years, much has been learned about the function and mechanisms of endocytic pathways responsible for the trafficking and molecular sorting of membrane receptors and their ligands into intracellular compartments, however, the significance and scope of the short-sequence motifs in these cellular events is not well understood. Here a particular emphasis has been given to the functions of short-sequence signal motifs responsible for the itinerary and destination of membrane receptors and proteins moving into subcellular compartments.