The Importance of Natriuretic Peptides
Natriuretic peptides (NP) are produced by the heart and the blood vessels. The body increases its production of natriuretic peptides when humans experience heart disease, liver disease, or kidney failure as these diseases, and others, expand overall fluid volumes within the body. Natriuresis occurs when an abnormally large amount of salt is excreted into the urine, and the term is derived from the Latin word natrium, which means «sodium». It also comes from the Greek word ouresis, which means «making water».
There are four types of NP protein hormones in humans that are responsible for causing natriuresis. They are A-type (atrial natriuretic peptide [ANP]); B-type natriuretic peptide (BNP); C-type natriuretic peptide (CNP); and D-type (dendroaspis natriuretic peptide [DNP]), for which all are small-protein molecules. These peptides naturally decrease the amount of sodium in the blood, similar to how diuretic drugs work. This process helps to lower high blood pressure and other medical conditions, thus lowering blood volume as sodium takes water with it into the urine. In fact, natriuretic peptides are sometimes referred to as the body’s natural diuretics.
This process will be explored more in depth later on in the article, but A-type natriuretic peptides work to reduce blood volume, which then results in reduced heart output and blood pressure. B-type natriuretic peptides are also called brain natriuretic peptides. They are produced, stored and released by the ventricles of the heart when the ventricles expand and stretch due to the collection of extra fluids. Acting as a diuretic, B-type natriuretic peptides result due to fluid overload and increasing blood volume, directly occurring from congestive heart failure. This peptide eases strain on the damaged heart as it relaxes blood vessels and eliminates excess and damaging fluids into the urine.
C-type natriuretic peptides are formed by cells that line blood vessels. They naturally relax blood vessels, thus helping to decrease blood pressure. They differ from A-type and B-type natriuretic peptides in that they do not perform natriuretic-type activities. Here it is easy to understand just how important natriuretic peptides are as natural responders to disease processes. Now we'll look further into NPs and how they were discovered, as well as physiologic functions and therapeutic applications.
History of Natriuretic Peptides
The natriuretic peptides are a family of ring-shaped vasoactive hormones showing considerable sequence homology. In 1956, Kisch discovered that atrial cells were comprised of elaborate Golgi networks similar to secretory cells. In 1964, Jamieson and Palade found that atrial myocytes contained spherical, electron opague granuals. Around the same time, Henry and colleagues uncovered the correlation of balloon atrial distension with increased urine output in dogs.
In 1968, DeBold and colleagues initiated atrial granule characterization and, in doing so, they revealed that granule content changed in response to electrolyte and water alterations. It has been more than three decades since deBold and colleagues released their 1981 seminal report demonstrating the existence of atrial natriuretic peptide (ANP) where they elaborately linked Kisch and Henry's findings. Their observations concluded that ANP links the kidneys and heart in cardiorenal homeostasis. DeBold discovered that by infusing extracts of atrial tissue into rats, copious amounts of natriuresis resulted. This is how the first natriuretic peptide was discovered. This led to the isolation and cloning of ANP, which was heralded as the first robust natriuretic. It also was found to have efficacious properties in diuretic and vasorelaxant acitivites.
Of the four natriuretic peptides, the B-type natriuretic peptide (BNP) has realized the development of commercial immunoassays, as has the N terminus of the prohormone, N-terminal pro BNP (NT-proBNP). Vast clinical studies are being conducted in relation to BNP/NT-proBNP, which will be touched upon later in this article.
BNP was originally discovered in the porcine (pig) brain, where it was believed to be a neurotransmitter, thus the name - brain natriuretic peptide was given, though now it is simply known as B-natriuretic peptide. In fact, there is a larger concentration of BNP in the heart ventricles. Later studies conducted by Mukoyama and colleagues in 1990 and 1991 found that BNP is even more highly concentrated in cardiac ventricles of patients with heart failure. Over the years, a variety of names have been assigned to natriuretic peptides including atrial natriuretic factor, atriopeptin, cardionatrin, and cardiodilatin.
C-type natriuretic peptide (CNP) was discovered in 1991 by Sudoh and colleagues from porcine brain extracts where it was found to relax smooth muscle. ANP, BNP and CNP have the following commonalities:
• Are products of separate genes
• Are similar in amino acid structure
• Contain 17-residue disulfide rings
Types of Natriuretic Peptides
BNP serves as a ventricular dilatation marker because the amount of BNP in the ventricles is in direct correlation to the amount of ventricular stretch – that which is caused by excess fluids in the heart. Because the messenger RNA for proBNP is unpredictable, there is active regulation of BNP levels according to ventricular wall tension. The initial transcription product contains a leader sequence, which is cleaved to yield proBNP, followed by a cleaving either on secretion, or afterwards, to NT-proBNP and BNP.
There have been found to exist in two receptors:
1. Natriuretic peptide receptor-A (NPR-A)
2. B (NPR-B)
NPR-A and NPR-B each contain:
• An extracellular binding site
• A transmembrane domain
• An intracellular domain with a protein, kinase-like region
• A guanyl cyclase site
• An intact ring structure for receptor binding
The receptor binding produces a guanosine monophosphate (GMP) as a second messenger, with BNP having a half-life of only 20 minutes, and NtpBNP a half-life of 60 minutes. At any rate, BNP binds to NPR-C, the third natriuretic peptide, which ultimately results in further binding, endocytosis, and lysosomal degradation. Additionally, the ring structure is cleaved by a membrane-bound neutral endopeptidase located in the kidneys. This, then, inactivates the molecule found in the kidneys and vascular tree, which renders the molecule inactive.
Returning to the atrial natriuretic peptide, it flags in an endocrine and paracrine mode to decrease blood pressure and cardiac hypertrophy. On the other hand, BNP serves to reduce ventricular fibrosis. C-type natriuretic peptide (CNP) works to stimulate long-bone growth. ANP and BNP activate the transmembrane guanylyl cyclase, natriuretic peptide receptor-A (NPR-A). CNP activates a related cyclase, natriuretic peptide receptor-B (NPR-B). Both receptors propel the synthesis of cGMP, which facilitates most effects and functions of natriuretic peptides that are known to date. The third natriuretic peptide receptor, natriuretic peptide receptor-C (NPR-C), moves natriuretic peptides from the circulation through receptors via internalization and degradation.
All natriuretic peptides are synthesized as preprohormones. The human gene encoding ANP is called NPPA (GeneID 4878) and is located on chromosome 1 at location 1p36.21. NPPA is approximately 2 Kb in length and consists of 3 exons and 2 introns. The resulting mRNA gives rise to a 151 amino acid polypeptide, known as prepro-ANP. The first 25 amino acids constitute a signal sequence that is cleaved to yield a 126 amino acid peptide called proANP, which is the major form of ANP stored in the atrial granules (Oikawa et al. 1984). Upon release from these granules, pro-ANP is rapidly cleaved by corin, a transmembrane cardiac serine protease. Corin has been found to be expressed on the atrial cardiomyocytes surface.
Now we'll delve a bit more into the three known natriuretic peptide binding proteins. All members are comprised of a significant (~450 amino acid) extracellular ligand binding domain and 20 residues of a sole membrane-span. Natriuretic peptide receptors A and B also contain a significant intracellular domain that consists of the following domains:
• Kinase homology
• Dimerization
• Carboxyl-terminal guanylyl cyclase
This means that NPR-A and NPR-B signal by facilitating the synthesis of the intracellular signaling molecule cGMP. Conversely, NPR-C is only comprised of a 37-residue intracellular domain. As well, it does not exhibit guanylyl cyclase activity. Its main function is to manage local natriuretic peptide accumulations through receptor-mediated internalization and degradation, as mentioned above.
Contrasting & Comparing NPR-A, -B, and -C
All three of the natriuretic peptides have unique properties, but there are some similarities as well. Beginning with NPR-A and/or its mRNA, it is expressed in kidney, lung, adipose, adrenal, brain, heart, testis, and vascular smooth muscle tissue. Natriuretic peptide receptor-A (NPR-A) is the principal receptor of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). Its domain contains, on an extracellular level, 3 intramolecular disulfide bonds as well as 5 N-linked glycosylation sites.
Natriuretic peptide receptor-B (NPR-B) is the primary receptor of C-type natriuretic peptide (CNP). It is similar to NPR-A in that it expresses comparable topology, glycosylation, and intramolecular disulfide bonding. NPR-B and/or its mRNA are found in bone, brain, fibroblasts, heart, kidney, liver, lung, uterine, and vascular smooth muscle tissue.
NPR-C is the most richly expressed natriuretic peptide receptor of the three. In 1986, Leitman and colleagues found it to be contained in almost all ANP binding sites within endothelial cells. NPR-C and/or its mRNA are found in adrenal, brain, heart, kidney, mesentery, and vascular smooth-muscle tissue. Natriuretic peptide receptor-C (NPR-C)'s domain contains large amounts of extracellular ligand-binding, for which Chang (et al. 1989), Fuller (et al. 1988), and Porter (et al. 1990) found to be 30-percent-to-35-percent identical to NPR-A and NPR-B within the span of a single membrane region. Interestingly, these investigations revealed only 37 intracellular amino acids. Later, in 2008, Rose and Giles discovered that it displays no enzymatic activity, though they found it to signal in a G protein-dependent manner.
Natriuretic Peptides: Physiologic Functions and Therapeutic Applications
Beginning with NPR-C functions, also referred to as the clearance receptor, and as mentioned prior, are to clear circulating natriuretic peptides through the process of receptor-mediated internalization and degradation. Osteocrin binds NPR-C, but not with either NPR-A or NPR-B, as discovered by Moffatt and colleagues in 2007. Osteocrin is believed to contend with CNP for binding to NPR-C in bone, and therefore, increase local CNP levels during critical periods for bone growth and development.
During heart failure, high levels of ANP and BNP are expressed and are thought to counteract this condition due to their natriuretic, diuretic, and vasodilating activities while inhibiting aldosterone and renin secretion. According to new evidence, ANP and BNP also express their circulating hormones as local paracrine and/or autocrine contributors. And in a clinical setting, ANP and BNP could someday be utilized as therapeutic agents for heart failure, though further investigation is needed for this to occur.
Natriuretic peptides have also been found to regulate blood pressure, and to inhibit cardiac hypertrophy (caused by prolonged hypertension) and remodeling. Here, cardiac hypertrophy is regulated by ANP and NPR-A, where remodeling is regulated by the ANP/BNP/NPR-A and the CNP/NPR-B pathways. In addition to regulating blood pressure, natriuretic peptides inhibit cardiac hypertrophy and remodeling. Hypertrophy is regulated by ANP and NPR-A, whereas remodeling is regulated by both the ANP/BNP/NPR-A and the CNP/NPR-B pathways. It has been discovered that levels of NPR-A deficient mice is much higher than that of other genetic models that result in similar hypertension levels. This suggests that NPR-A causes a growth inhibitory message in the heart.
Natriuretic Peptides in Subarachnoid Hemorrhage; Carbon Monoxide Poisoning; Hyperthyroidism; and Therapeutic Agents
Patients with subarachnoid hemorrhage (SAH) show an increased urine output and urinary excretion of sodium as well as higher BNP levels than the controls in a study that involved 50 patients with SAH. It is believed that the heart releases both ANP and BNP after acute SAH when compared to patients with mild-to-moderate SAH. This also appeared evident in patients with elevated cranial pressure as compared to those without who had a more favorable outcome.
In another study involving 15 patients who suffered carbon monoxide poisoning, there was a positive correlation between the levels of COHb and NT-proBNP. This may assist in earlier diagnoses of those patients with cardiotoxicity and CO poisoning.
In yet another study that enrolled 67 patients with clinical hyperthyroidism (along with normal patients), thyroid functions were found to be affected by serum NT-proBNP levels in that they show a direct stimulatory effect on thyroid hormones.
Finally, it is highly conceivable that NPs or their agents and antagonists could be used for the treatment of cardiac failure due to the valuable evidence and effects of NPs on the cardiovascular system. However, one notable difference is that NT-proBNP is more stable, and thus, more predictable.
BNP has become commercially available for immunoassay systems for research applications as well as clinical practices. The future awaits more uses and applications of these formidable natriuretic peptides, along with existing findings so as to be even more applicable to the treatment of disease and for life-saving diagnostics.
References
1. Ellis R. Levin, M.D., David G. Gardner, M.D., and Willis K. Samson, Ph.D. Natriuretic Peptides. N Engl J Med 1998; 339:321-328. [Article].
2. Cho Y, Somer BG, Amatya A. Natriuretic peptides and their therapeutic potential. Heart Dis. 1999 Nov-Dec;1(5):305-28.. [PubMed].