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

Abstract

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.

Keywords: atrial natriuretic peptide; guanylyl cyclase; internalization; natriuretic peptide receptors; receptor trafficking; short sequence motifs.

Figure 1

Diagrammatic representation of ligand-dependent activation and post-binding events of NPRA, NPRB, and NPRC: ANP binding activates NPRA, which leads to enhanced production of second messenger cGMP. The bound ligand-receptor complexes of NPRA are internalized into the intracellular compartments and a large proportion of ligand-bound receptors are degraded in the lysosomal compartments, while a small population of internalized receptors recycles back to the plasma membrane. An increased accumulation of intracellular cGMP activates cGMP-dependent protein kinase (PKG), which plays a critical role in ANP-dependent biological responsiveness. The second messenger, cGMP can also activate phosphodiesterases (PDEs) as well as cGMP-gated ion channels to activate ANP-dependent cellular and physiological functions. CNP activates NPRB, which is also internalized, degraded in lysosomes, and recycled back to the cell surface. Ligand- binding to NPRC is suggested to lower cyclic adenosine 3',5'-monophosphate (cAMP) levels and to increase inositoltrisphosphate (IP₃) in target cells. The bound ligand-receptor complexes of NPRC are internalized, degraded in the lysosomal compartments, and recycle back to the membrane [35] JMD, juxtamembrane domain; KHD, kinase like homology domain; DD, dimerization domain; GCD, guanylyl cyclase catalytic domain.

Figure 2

Schematic representation of ligand-mediated receptor endocytosis and trafficking pathways: Ligand-binding to cell surface receptors leads to a selective recruitment of ligand-receptor complexes (cargo) into clathrin-coated pits. The coated pit represents a small area of the plasma membrane, which invaginates and pinches off into vesicles in the cytoplasmic compartments. The coated vesicles trigger the recruitment of APs for example AP-1, AP-2, AP-3, AP-4, and AP-5 as well as other interacting protein molecules such as disabled-2 (Dab-2) and Golgi-localizing gamma adaptor homology domain binding protein (GGA). The clathrin-dependent routes require dynamin to achieve the fission of membrane invaginations and subsequent internalization of vesicle into the cell interior. The ligand-receptor complexes within the vesicles entering via clathrin pathway are usually directed to early endosomes. Within the endosomes, ligand-receptor complexes are sorted and directed to various subcellular compartments, where the internalized receptors are either routed to degradative compartments such as late endosomes, multi vascular bodies, and/or lysosomes, or recycled back to the plasma membrane via recycling endosomes. Alternatively, the internalized cargo may be sequestered in endosomes for a longer period of time and continue to spark signaling events. Some early and late endosomes also contain membrane structures in the lumen, which are referred to as multi-vesicular bodies (MVBs). The endosomes and lysosomes can also transmit and receive cargo from trans-Golgi network (TGN) involving vesicular intermediate carriers. The key proteins involved in the trafficking of molecules at different locations may include: AP-1, AP-2, AP-3, AP-4, AP-5, Dab-2, and GGA.