Discovery of peptides as key regulators of metabolic and cardiovascular crosstalk

SUMMARY

Peptides are fundamental regulators of metabolism, with several already developed as drugs, including glucagon-like peptide-1-based peptide therapeutics for diabetes and obesity. Despite their established importance, our understanding of their biosynthesis, modifications, receptor interactions, and signaling pathways remains incomplete. Advances in peptidomics and proteomics, particularly mass spectrometry, have facilitated peptide discovery and characterization, revealing novel roles for known peptides and uncovering previously unrecognized post-translational modifications. With the increasing prevalence of metabolic diseases driven by obesity, understanding the regulatory functions of peptide hormones has significant therapeutic potential. This review discusses the latest insights into peptide biology, highlighting key examples of peptides controlling tissue crosstalk, as well as how multi-omics technologies, computational approaches, and AI-driven methods are likely to expand our knowledge of peptide-mediated metabolic regulation.

INTRODUCTION

Peptides are potent regulators of numerous biological functions, and many peptides have already been developed into therapeutic drugs. Peptides can act as signaling molecules that coordinate cellular processes such as metabolism,¹ immune response,² and neuroendocrine regulation.³ They can be secreted by a variety of tissues, including the pancreas,⁴ gastrointestinal tract,⁵ hypothalamus,⁶ and adipose tissue,⁷ making them essential for interorgan communication.

Historically, the discovery of peptide hormones has largely been dependent on classical biochemical methods, such as chromatography-based fractionation guided by bioactivity and Edman degradation to determine the exact sequence.⁸ These early studies were dependent on the isolation and structural characterization of peptides from biological extracts.⁹ However, as our techniques advanced, molecular biology approaches, including mRNA expression analysis¹⁰ and recombinant protein technology,¹¹ provided deeper insights into peptide biosynthesis, post-translational modifications (PTMs), and receptor interactions. More recently, mass spectrometry (MS)-based peptidomics and computational modeling have expanded our ability to identify novel peptides.^12,13 In this review, we will discuss the approaches to the discovery of the peptides known to date, as well as highlight key examples of how interorgan metabolic and cardiovascular crosstalk is mediated by peptide hormones.

A HISTORICAL PERSPECTIVE ON THE CHALLENGES IN PEPTIDE DISCOVERY

Most peptide hormones were discovered between the early and late 20th century using biochemical methods, with the hypothalamus, gastrointestinal tract, and pancreas being the main organs of synthesis. The identification of gastrin in 1905, a 17-mer peptide made in the gastrointestinal tract, laid the foundation for understanding how peptides stimulate acid secretion.¹⁴ Another early example is cholecystokinin (CCK), which was identified using classical biochemical and physiological methods in 1928 by Ivy and Oldberg. They generated crude extracts from the small intestine that demonstrated the capacity to stimulate the gallbladder to contract and release bile, suggesting the existence of a “secretin.” Edman degradation and immunohistochemical methods later revealed that CCK is a 33-mer peptide hormone not only produced in the small intestine but also present in the brain, where it regulates satiety and food intake in rats and rhesus monkeys.¹⁵

In the early days, biochemical purification methods, often referred to as “bucket” biochemistry, were used to identify new peptides. The lack of advanced analytical techniques meant that purification was slow and prone to contamination, often requiring repeated fractionation using basic chromatography and centrifugation. For example, groundbreaking work to identify corticotropin-releasing hormone (CRH), led by Wylie Vale and his colleagues at the Salk Institute, undertook the overwhelming task of extracting 490,000 fragments of ovine (sheep) hypothalami.¹⁶ This extensive effort allowed for the identification of CRH through fractionation. However, these experiments were difficult due to the instability of peptides,^17,18 difficulties in their purification,¹⁹ and lack of sensitive detection techniques.¹⁶ They required large tissue samples and were prone to contamination, leading to slow and inefficient identification processes. Modern approaches, including MS-based peptidomics^12,16 and bioinformatics-driven predictions,^20,21 have improved peptide identification but still face obstacles, which are described in detail in the following sections. Table 1 lists the known peptide hormones to date and their functions, receptors, and use as therapeutics.

Table 1.

Known peptides in metabolism

Site of synthesis	Hormone	Receptor	Discovery method	Signaling pathway	Target tissues	Drugs	Size (aa)	Reference to first paper	Primary function
Hypothalamus	arginine vasopressin	V1a, V1b, V2 receptors	peptide isolation and sequencing	cAMP (V2), IP3/DAG (V1a, V1b)	liver, kidney, blood vessels, pituitary	desmopressin (diabetes insipidus, nocturnal enuresis, and bleeding disorders)	9	du Vigneaud et al., 195422	regulates water retention and blood pressure
	corticotropin-releasing hormone (CRH)	CRH receptor 1, CRH receptor 2	peptide isolation and sequencing	cAMP	pituitary	corticorelin (diagnostic testing for adrenal function)	41	Vale et al.19	regulates stress response by stimulating adrenocorticotropic hormone (ACTH) release from the anterior pituitary gland
	growth hormone-releasing hormone (GHRH)	GHRH receptor	peptide isolation and sequencing	cAMP	pituitary	sermorelin (diagnose and treat GH deficiency)	44	Guillemin et al., 198223	stimulates GH secretion from the anterior pituitary gland
	gonadotropin-releasing hormone (GnRH)	GnRH receptor	peptide isolation and sequencing	IP3/DAG	pituitary	leuprolide, nafarelin (hormone therapy in prostate cancer, endometriosis, and infertility)	10	Schally et al., 197124	stimulates the synthesis and release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland
	oxytocin	oxytocin receptor	peptide isolation and sequencing	IP3/DAG	uterus, mammary glands	carbetocin, pitocin (induce labor and prevent postpartum hemorrhage)	9	du Vigneaud et al., 195325	stimulates uterine contractions and milk ejection
	prolactin-releasing hormone (PRH)	unknown, hypothesized PRH receptor	hypothetical concept, inferred from prolactin secretion	possibly cAMP-PKA	pituitary	none known	31	McCann et al., 197226	stimulates the synthesis and release of prolactin from the anterior pituitary gland and modulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary
	somatostatin	somatostatin receptor (SSTR1 -5)	radioimmunoassay and sequencing	Gi-protein inhibition of cAMP	pituitary, eye, gastrointestinal (GI), pancreas, adrenal	octreotide, lanreotide (acromegaly and neuroendocrine tumors)	14	Brazeau et al.6	inhibits the secretion of GH, TSH, insulin, glucagon, and gastrin
	neuropeptide Y (NPY)	Y1, Y2, Y4, Y5 receptors	peptide isolation from brain extracts	Gi/o protein, inhibition of cAMP	eye, heart, adipose, pancreas, liver	none known	36	Tatemoto et al.8	stimulates food intake and appetite and modulates stress and anxiety responses, circadian rhythm, and cardiovascular function
	orexin (hypocretin)	orexin receptor 1 (OX1R), OX2R	genetic screening for hypothalamic peptides	cAMP, IP3/DAG	brown adipose tissue (BAT), brain	suvorexant (orexin receptor antagonists) for insomnia	33	Sakurai et al.27	stimulates food intake and appetite and regulates wakefulness and arousal
	nesfatin-1	unknown	genetic analysis of nucleobindin-2	calcium signaling, inhibition of food intake	pancreas, adipose tissue, heart, GI	none known	82	Oh-I et al.11	regulates feeding, glucose homeostasis, reproduction, and cardiovascular function
	thyrotropin-releasing hormone (TRH)	TRH receptor	peptide isolation and sequencing	IP3/DAG	pituitary	protirelin (diagnostic testing of thyroid function)	3	Burgus et al., 196928	stimulates TSH and prolactin release from the anterior pituitary gland
Anterior pituitary	adrenocorticotropin hormone	melanocortin receptor 2 (MC2R)	protein purification and sequencing	cAMP-PKA pathway	adrenal cortex	cosyntropin (adrenal insufficiency diagnosis)	39	Collip et al., 193329	stimulates cortisol release from the adrenal cortex
	parathyroid hormone (PTH)	PTH receptor 1, PTH receptor 2	peptide isolation from parathyroid glands	cAMP, IP3/DAG	bone, kidney, GI	teriparatide (osteoporosis)	84	Collip, 192530	increases blood calcium levels
Heart	atrial natriuretic peptide	natriuretic peptide receptors A, B	peptide isolation from atrial tissue	cGMP	heart, kidney, adrenal, adipose, muscle	nesiritide (heart failure)	28	de Bold et al., 198131	reduces blood volume and pressure by promoting sodium excretion
GI tract	cholecystokinin (CCK)	CCK1, CCK2 receptors	peptide isolation from intestinal mucosa	IP3/DAG	gallbladder, pancreas, stomach	sincalide (gallbladder function tests)	33	Ivy and Oldberg14	stimulates bile and pancreatic enzyme secretion
	gastrin	CCK2 receptor	peptide isolation from gastric mucosa	IP3/DAG	Gl, pancreas	pentagastrin (gastric secretion tests)	17	Edkins, 190532	stimulates acid secretion
	ghrelin	GH secretagogue receptor (GHSR)	peptide isolation from stomach	Gq-mediated signaling	brain, heart, GI, pancreas	macimorelin (adult GH deficiency diagnosis)	28	Kojima et al., 199933	stimulates feeding
	glucagon-like peptide 1 (GLP-1)	GLP-1 receptor	peptide isolation from intestinal L-cells	cAMP	brain, heart, bone, pancreas, stomach, kidney, muscle, liver	exenatide, liraglutide (type 2 diabetes, obesity)	30	Bell et al., 198334	enhances insulin secretion, inhibits glucagon release, slows gastric emptying, and promotes satiety
	GLP-2	GLP-2 receptor	peptide isolation from intestinal L-cells	cAMP	brain, heart, bone, pancreas, stomach, kidney, muscle, liver	teduglutide (short bowel syndrome)	33	Drucker et al.35	promotes intestinal growth, enhances nutrient absorption, and improves gut barrier function
	glucose-dependent insulinotropic polypeptide (GIP)	GIP receptor	peptide isolation from intestinal K-cells	cAMP	brain, bone, pancreas, muscle, adipose	tirzepatide (dual GLP-1/GIP agonist for diabetes and obesity)	42	Dupre et al., 197336	stimulates insulin secretion in response to glucose
	obestatin	GPR39 (uncertain)	derived from the same gene as ghrelin	unknown	brain, heart, pancreas, GI, adipose	none known	23	Zhang et al.37	potentially suppresses appetite and reduces GI motility
	oxyntomodulin	GLP-1 receptor, glucagon receptor	peptide isolation from intestine	cAMP	brain, liver, GI, pancreas, kidney, adipose	none known	37	Bataille et al., 198338	reduces appetite and increases energy expenditure
	motilin	motilin receptor	peptide isolation from duodenal mucosa	cAMP, IP3/DAG	gallbladder, pancreas, stomach, brain	erythromycin (motilin receptor agonist used as a prokinetic agent)	22	Brown et al.5	regulates GI motility by smooth muscle contraction in the stomach and small intestine
	peptide YY (PYY)	Y1, Y2 receptors	peptide isolation from intestinal L-cells	Gi/o protein, inhibition of cAMP	brain, GI, pancreas	none known	36	Tatemoto et al., 198039	reduces appetite and inhibits gastric motility
	secretin	secretin receptor	peptide isolation from intestinal S-cells	cAMP	pancreas, liver, stomach	synthetic secretin (secretin stimulation test)	27	Bayliss et al., 190240	stimulates bicarbonate secretion
Liver and multiple other tissues	insulin-like growth factor-1 (IGF-1)	IGF-1 receptor	peptide isolation from blood	tyrosine kinase, PI3K/Akt	muscles, connective tissue, cartilage, bone	mecasermin (GH insensitivity syndrome)	70	Salmon and Daughaday, 195741	promotes growth and development and enhances glucose uptake
	adropin	unknown	bioinformatics and proteomics	energy metabolism regulation	liver, adipose, heart, muscle	none known	76	Kumar et al., 200842	regulates energy homeostasis and lipid metabolism
Pancreas	glucagon	glucagon receptor	peptide isolation from pancreas	cAMP	brain, liver, GI, pancreas, kidney, adipose	recombinant glucagon (hypoglycemia)	29	Kimball et al., 192343	raises blood glucose levels by promoting glycogen breakdown
	amylin	calcitonin receptor, RAMPs	peptide isolation from pancreas	cAMP	pancreas, GI, kidney, brain, bone	pramlintide (diabetes treatment, obesity)	37	Cooper et al., 198744	regulates blood glucose levels, slows gastric emptying, promotes satiety, and inhibits glucagon secretion
	pancreatic polypeptide	Y4 receptor	peptide isolation from pancreas	Gi/o protein, inhibition of cAMP	GI, gallbladder, pancreas	none known	36	Lin et al., 197245	regulates pancreatic secretions and GI motility
	enterostatin	unknown	isolated from small intestine	involved in appetite suppression	GI	none known	23	Brown et al., 197146	regulates fat intake by reducing appetite for dietary fats
	vasoactive intestinal peptide (VIP)	VPAC1, VPAC2 receptors	identified in GI tract	involves smooth muscle relaxation and vasodilation	GI, smooth muscle	none known	28	Said et al., 197047	relaxes smooth muscle and increases water and electrolyte secretion
Adipose tissue	apelin	APJ receptor	isolated from heart tissue	involved in cardiovascular regulation	brain, lung, liver, heart, kidney	none known	77	Tatemoto et al.48	regulates cardiovascular functions, blood pressure, fluid homeostasis, angiogenesis, and heart development
Thymus	thymosin	unknown	peptide purification from thymus extracts	PI3K/Akt	T cells	thymosin β4 (clinical trials)	5	Goldstein et al., 196649	stimulates T cell development

Peptides, their organ of expression, size, time of discovery, receptor, drug targets, and functions are shown. aa, amino acid. V1a, V1b, V2 receptors, subtypes of the vasopressin receptor; Y1, Y2, Y4, Y5 receptors, different subtypes of NPY receptors; GPR39, G-protein-coupled receptor 39; VPAC1, VPAC2 receptors, vasoactive intestinal peptide receptors; APJ, the apelin receptor; cAMP, cyclic adenosine monophosphate; IP3, inositol trisphosphate; DAG, diacylglycerol; PKA, protein kinase A; cGMP, cyclic guanosine monophosphate; PI3K, phosphoinositide 3-kinase; AKT, protein kinase B.

PEPTIDE DISCOVERY USING MOLECULAR BIOLOGY: FROM IDENTIFICATION TO FUNCTIONAL INSIGHTS

Peptide discovery through molecular biology has historically focused on transcriptomics and gene expression analyses. For instance, the cocaine- and amphetamine-regulated transcript (CART) peptide was first identified through differential gene expression analysis in the brain following psychostimulant exposure.¹⁰ While this discovery was significant, the real breakthrough came when subsequent studies demonstrated its physiological role in energy homeostasis, feeding behavior, and stress responses through central actions and the hypothalamus-adipose axis.⁵⁰ Hypothalamic CART drives sympathetic outflow to brown adipose tissue, increasing UCP-1 expression and thermogenesis.⁵¹ CART is also produced in pancreatic β cells, where it increases glucose-stimulated insulin release and suppresses glucagon secretion.⁵² In addition, leptin-regulated CART released from sympathetic neurons has been shown to inhibit osteoclast-mediated bone resorption.⁵³

Similarly, nesfatin-1 (gene name nucleobindin-2 [NUCB2]), an 82-mer peptide first identified through transcriptomic profiling,¹¹ was found to regulate appetite and glucose metabolism by modulating the hypothalamic control of energy balance.^11,54 Immunohistochemical staining mapped nesfatin-1 to specific hypothalamic nuclei involved in appetite control, such as the paraventricular nucleus (PVN). Functional assays, including peripheral and central injections of nesfatin-1 in rodent models, demonstrated significant reductions in food intake and body weight. Beyond its central actions, nesfatin serves as a bona fide interorgan messenger. Intestinal nesfatin-1 activates a gut-brain-liver circuit via MC4R-cAMP-GLP-1 signaling to suppress hepatic glucose production and increase hepatic insulin sensitivity.⁵⁵ However, nesfatin-1 also elevates blood pressure, which has reduced the enthusiasm for this peptide as an anti-obesity target.⁵⁶

These findings underscore how molecular biology approaches have not only facilitated the discovery of new peptides but have also been pivotal in linking them to specific metabolic and neuroendocrine pathways. Single-cell transcriptomics and spatial transcriptomics have further refined our understanding of tissue-specific peptide expression, revealing how different cell populations contribute to systemic metabolic regulation.⁵⁷ For example, single-cell RNA sequencing (scRNA-seq) studies have uncovered previously uncharacterized peptide-expressing cell types in the pancreas, hypothalamus, and adipose tissue.⁵⁸

PEPTIDE DISCOVERY USING MS: WHAT HAVE WE LEARNED?

Proteomics and peptidomics have huge potential to provide new peptide targets. Peptidomics, in theory, allows for large-scale profiling of endogenous peptides in different physiological and pathological states, enhancing our understanding of their roles in metabolic regulation³⁵ and disease processes.⁵⁹ Targeted methods for peptide identification and quantification include tandem MS (MS/MS), which provides mass-to-charge ratios of peptide fragments, allowing for the exact quantification of peptides in complex biological samples, and has replaced Edman degradation.^60,61 For example, the sequences of orexin-A and orexin-B were identified using MS. First, high-resolution high-performance liquid chromatography (HPLC) fractions from rat brain extracts were used to screen for G-protein-coupled receptor (GPCR) agonist activity using a panel of over 50 stable cell lines, each expressing a unique orphan GPCR. Tissue fractions were then applied to these cell lines, and an intracellular calcium increase was observed in cells expressing the orphan receptor HFGAN72, later renamed as the orexin receptor (OX1R).²⁷

However, using untargeted MS-based peptidomics alone to identify bioactive peptides has not been very successful, likely because of challenges due to peptide degradation and identification following peptide extraction from biological materials. Proteomics generally relies on enzymatic digestion using trypsin to generate peptide fragments for protein identification, whereas peptidomics avoids enzymes to maintain peptides in their native, post-translationally modified forms.⁶¹ While the absence of digestion simplifies sample preparation in peptidomics, it complicates data analysis, especially for larger peptides. Annotating prohormones and their peptides is challenging due to the PTMs involved and the often short, conserved bioactive sequences within variable prohormones. Therefore, homology alone is insufficient for accurate peptide predictions across species.

One key challenge is distinguishing biologically active peptides from degradation fragments in complex biological samples. The majority of peptides detected in peptidomic studies originate from protein degradation rather than functional hormones, complicating analysis.^17,18,62,63 MS also has a limited dynamic range for detecting low-abundant peptides, and peptidomics is particularly vulnerable to minor protein degradation, which overshadows signals from bioactive peptides. In fact, >95%–99% of peptides detected in complex samples (e.g., serum or brain extracts) represent degradation fragments from abundant proteins, with only a small fraction reflecting true bioactive peptides.^63–66 For example, PeptideAtlas¹⁸ is a database of detected tryptic and non-tryptic peptides collected from multiple LC-MS experiments. PeptideAtlas has facilitated the validation of specific neuropeptides by providing peptide fragmentation data, including bradykinin variants identified from human plasma.¹⁶ However, no entirely new peptides have been discovered exclusively through PeptideAtlas; instead, it supports the reanalysis and validation of some known peptides.⁶⁷ While the peptide coverage from proglucagon is >85%, there are no uniquely mapping non-tryptic peptides from proglucagon detected, such as 98–127 corresponding to GLP-1 (7–36). Instead, 98–117 is detected, which is a peptide fragment ending in lysine and is likely a tryptic fragment.⁶⁸ Therefore, using PeptideAtlas alone to distinguish between degradation fragments versus true biological cleavage fragments is unlikely to be successful. Additionally, PTMs significantly influence peptide function but remain difficult to predict and detect.^69,70

Future efforts should focus on integrating functional assays with computational and omics-based discovery.¹³ Advances in chemical stabilization strategies, such as C-terminal amidation^69,71 and N-terminal pyroglutamate formation,⁷² could enhance peptide stability and bioavailability.^69–71 One of the major contributions of proteomics is in revealing previously unknown PTMs, tissue-specific peptide variants, and dynamic interactions between peptides and other signaling molecules across tissues. Refining MS methodologies to improve sensitivity and specificity will be crucial for identifying novel bioactive peptides with therapeutic potential.^12,16 As α-amidation^73,74 has proven to be one of the hallmarks of bioactive peptides,⁷⁵ assays specific to C-terminal α-amides have been developed and successfully used to identify bioactive peptides such as neurokinin A,⁷⁶ substance P,⁷⁷ and many others. A recent class of modifications known as “capped peptides” involves both C-terminal α-amidation and N-terminal pyroglutamate (Pyro-Glu) formation, which stabilize the peptide and enhance its biological activity.⁷⁸ Examples include CAP-TAC1 and CAP-GDF15, where the addition of these modifications protects the peptide termini from enzymatic degradation and can improve receptor binding affinity. These modifications were initially predicted computationally and later confirmed by MS.⁷⁹ In summary, while omics may not have discovered entirely new peptides, they have transformed our understanding of known peptides, uncovering their roles in tissue-specific interactions, their regulation through PTMs, and their contributions to complex metabolic regulation.

PEPTIDE DISCOVERY USING BIOINFORMATICS: PROMISE OR HYPE?

Computational in silico methods also have the potential to speed up the discovery of new peptides and will likely become invaluable for predicting peptide sequences and structures,⁷² but these methods are still in their infancy.⁸⁰ Several databases and tools are available for peptide hormone discovery, providing curated information on known peptides and their sequences, structures, and biological activities.^67,81–83 Algorithms based on machine learning, homology modeling, and molecular dynamics simulations can predict potential peptide hormones.⁸⁴ These in silico approaches can, in principle, rapidly screen large datasets, identifying candidates for further experimental validation.⁸⁵ Recent computational models, such as DeepNeuropePred²⁰ and MultiPep,²¹ offer promising tools for predicting peptide cleavage sites. However, their accuracy is limited by incomplete datasets and a lack of experimental validation.^20,70

DeepNeuropePred⁸⁶ is a learning method for detecting cleavage sites in neuropeptide precursors by predicting the neuropeptide cleavage sites from precursors independent of species. Here, 717 precursors as the independent training dataset were used to predict the cleavage sites of a test set of 31 precursors. Five of the predicted peptides were validated against a published MS dataset of a stick insect, Carausius morosus, but no other peptides have been identified using this method. While the performance of this tool against a training dataset was excellent, this paper did not describe any new peptide candidates.

MultiPep^20,21 is another machine learning method to predict peptide fragments by identifying cleavage sites. They first identified 75 neuropeptide precursors in the rhesus monkey genome and used a supervised machine learning algorithm for classification to predict cleavage sites. This algorithm separates data points into distinct categories—in this case, identifying where cleavage sites are likely to occur in precursor proteins. These predictions were then compared to assignments based on homology to human sequences, achieving a cleavage classification accuracy of over 97% for both human and rhesus datasets. Similar methods include methods for the prediction of peptides using cleavage site annotation.^59,87,88 While potentially useful, the above-mentioned methods have not been used to identify novel peptides.

More recently, Secher et al.⁸⁹ developed an algorithm to streamline peptide complexity, facilitating the identification of biologically relevant peptides. When applied to the rat hypothalamus, this pipeline enabled the successful identification of thousands of neuropeptides, as well as their associated PTMs. In their approach, peptidomics was conducted under various conditions, and peptides were then selected based on their annotations as prohormones. Unlike PeptideAtlas, the samples were not trypsinized and were computationally filtered for known prohormones. However, a limitation of this method is that new, unannotated prohormones are excluded from the analysis. As a proof of concept, this method enabled large-scale neuropeptide identification in the rat hypothalamus, revealing a wide array of PTMs. The study presents 54 potentially novel peptides derived from 21 precursor proteins. Unfortunately, this promising study did not test the bioactivity of these peptides.

In summary, while some tools and analytical frameworks for peptidomics primarily support the validation and mapping of known peptides, others have enabled the prediction of novel peptide fragments and cleavage variants. The major limitation is that peptide prediction needs to be combined not only with MS for detection but also with appropriate screening methods to identify the biologically relevant peptides.

PEPTIDES AS KEY REGULATORS OF METABOLIC AND CARDIOVASCULAR CROSSTALK

Classical purification techniques, along with transcriptomics and proteomics, have led to the discovery of previously unknown peptide functions and novel therapeutic targets.¹³ For example, the development of GLP-1 receptor agonists, directly inspired by the role of GLP-1 in glucose homeostasis, has revolutionized the treatment of type 2 diabetes and obesity.^1,90 Many peptides function as hormones, traveling through the bloodstream to affect distant organs, while others act locally, serving as short-range signals between neighboring cells (Table 1). This section focuses on apelin, obestatin, and amylin as three examples of metabolic and cardiovascular crosstalk. These peptides have been identified as mediators of interorgan crosstalk, particularly in the regulation of cardiovascular function, metabolic homeostasis, and energy balance. These peptides have also drawn increasing attention for their potential therapeutic applications in conditions such as heart failure, obesity, and diabetes. Understanding their signaling mechanisms provides valuable insights as examples of how these peptides can control systemic physiology.

Apelin: A vascular and metabolic mediator

Apelin, a 77-amino-acid peptide, coordinates various physiological processes, including glucose metabolism, cardiovascular function, and fluid homeostasis (Figure 1). Apelin binds to the apelin receptor (APJ),⁹¹ a GPCR structurally similar to the angiotensin II receptor but with distinct physiological functions. The crystal structure of APJ complexed with G proteins has revealed insights into the molecular interactions that facilitate apelin binding and downstream signaling. Upon apelin binding, the APJ receptor undergoes a conformational change that allows coupling of the Gαi and Gq/i subtypes.⁹¹ APJ activation leads to elevated intracellular calcium levels and activates many signaling pathways, including PI3K/AKT, MAPK/ERK, and AMPK, depending on the cell type, which play roles in cellular proliferation, survival, and migration. Apelin acts directly on vascular endothelial cells. This interaction stimulates several downstream effects, including the activation of endothelial nitric oxide synthase (eNOS) in endothelial cells, leading to nitric oxide (NO) production. NO acts as a vasodilator, contributing to the regulation of blood pressure and vascular tone, promoting vasodilation, and enhancing blood flow. The NO-mediated response is particularly beneficial in managing hypertension and heart failure.⁹² Furthermore, apelin is also expressed in the kidney, where it interacts with vasopressin in renal tubules to enhance fluid absorption and sodium balance. In chronic heart failure, reduced cardiac output leads to decreased blood flow to the kidneys, which triggers the kidneys to retain more sodium and water in an attempt to increase blood volume and pressure. This compensatory mechanism often worsens fluid overload, contributing to edema and further stressing the heart. Apelin can help counteract the effects of fluid overload in chronic heart failure by promoting vasodilation and inhibiting sodium and water reabsorption in the kidneys.⁹³ In other peripheral metabolic tissues, apelin controls glucose homeostasis by increasing glucose uptake and insulin sensitivity, mainly in skeletal muscle and adipose tissue.⁹⁴ The mechanism proposed is direct activation of the AMPK pathway, leading to increased glucose transport and utilization in muscle cells, complementing insulin’s action.⁷ Apelin can also improve lipid metabolism by promoting lipolysis, which could be useful in metabolic syndrome and diabetes management.⁹⁵ However, apelin-induced lipolysis could potentially lead to increased circulating free fatty acids, which, in prolonged activation, might contribute to lipotoxicity or insulin resistance in tissues sensitive to lipid overload, such as the liver or pancreas.

Figure 1. The physiological and pharmacological effects of Apelin and its receptor and signaling mechanisms.

(A) Apelin-13 exerts diverse effects across multiple organs.

(B) Schematic representation of Apelin receptor (APJ) signaling through Apelin-13 and Apelin-36. The right image shows a molecular model of the Apelin receptor bound to Apelin-13, with associated G protein subunits, highlighting the receptor’s structure and interaction with signaling components.

Ongoing clinical trials of apelin receptor agonists include studies targeting heart failure, muscle atrophy, obesity, and chronic kidney disease (CKD). Another pressing question is the pharmacokinetics and safety of apelin analogs or agonists in clinical applications, as there are currently no robust trials assessing the long-term effects or potential desensitization of the APJ receptor.⁴⁸ A key area of debate is the specificity of apelin’s receptor interactions and signaling pathways. Although APJ is structurally similar to AT1R, it mediates distinct physiological effects that are not yet fully understood. This is particularly true for tissue-specific signaling. APJ activation promotes vasodilation, metabolic regulation, and fluid homeostasis, in contrast to the vasoconstrictive and pro-inflammatory effects of angiotensin II signaling. Some studies suggest that APJ may form heterodimers with AT1R, influencing angiotensin-mediated responses,^96,97 but the physiological relevance of this interaction remains unclear. Additionally, APJ exhibits ligand-dependent signaling bias, where different ligands or cellular contexts preferentially activate specific pathways, complicating efforts to develop selective therapeutics.⁴⁸ For instance, while apelin binding typically activates Gαi and Gq/i pathways, synthetic agonists may trigger alternative downstream signaling responses with distinct physiological outcomes.⁹⁸ Furthermore, APJ has been proposed to function as a decoy receptor for angiotensin II, potentially regulating angiotensin-induced cardiovascular effects and protecting against hypertension and heart failure.⁹⁹ However, the mechanisms underlying this interaction remain speculative, with some studies reporting a modulatory role for APJ in angiotensin II signaling, while others suggest independent and distinct signaling pathways.¹⁰⁰ Therefore, there is a need to understand the molecular mechanisms governing APJ signaling and its therapeutic potential, particularly in the context of tissue-specific responses, biased agonism, and receptor crosstalk.

Obestatin: Bridging gastrointestinal function and systemic metabolism

Obestatin, a peptide hormone derived from the same precursor as ghrelin, primarily acts like a hormone, although its exact physiological role and mechanisms are still being clarified. Obestatin was initially identified for its antagonistic effects of ghrelin, primarily in the regulation of food intake.^37,101 However, recent studies suggest that the role of obestatin extends far beyond simple antagonism, contributing significantly to the coordination of various organ systems, particularly in inflammatory and metabolic pathways (Figure 2). The role of obestatin in cardiovascular functions includes modulation of blood pressure and inflammation. Studies suggest that obestatin has vasodilatory effects, likely mediated through increased NO production, contributing to improved blood flow in both peripheral and central circulations.¹⁰² Obestatin exerts protective effects within the gastrointestinal tract, particularly in reducing inflammation and promoting mucosal healing in conditions like colitis.¹⁰³ It achieves this through the downregulation of pro-inflammatory cytokines, suggesting a role in maintaining gut barrier integrity.¹⁰⁴ In adipose tissue, it inhibits lipogenesis and stimulates lipolysis through activation of GPR39 receptors, enhancing insulin sensitivity.¹⁰⁵ In the liver, obestatin has protective roles in non-alcoholic fatty liver disease, reducing lipid accumulation and improving insulin signaling.¹⁰⁶ These actions highlight the potential of obestatin in regulating both local tissue metabolism and systemic energy homeostasis, which is especially relevant to conditions like obesity and type 2 diabetes. Obestatin supports pancreatic β cell health, increasing insulin secretion and improving glucose tolerance in hyperglycemic conditions.¹⁰⁷ Studies have shown that obestatin’s interaction with incretin receptors, including GLP-1R, potentiates insulin release while also providing protective effects against β cell apoptosis.¹⁰⁸This makes obestatin an interesting candidate for modulating glucose homeostasis and insulin resistance. Several controversies and unanswered questions remain about the physiological role of obestatin and its therapeutic potential. A major debate exists around obestatin’s receptor interactions, particularly its association with GPR39, which remains contested, as evidence on binding specificity and its signaling efficacy has been inconsistent. Another area of ambiguity is the extent to which obestatin functions independently versus synergistically or antagonistically with ghrelin, particularly in energy homeostasis and appetite regulation. Although obestatin was initially proposed as a ghrelin antagonist, subsequent studies suggest more complex, potentially context-dependent interactions that are not yet fully understood. Therapeutically, it is unclear whether long-term administration of obestatin would sustain its beneficial effects or lead to desensitization or downregulation of its signaling pathways. Thus, the exact mechanisms, receptor interactions, and therapeutic viability of obestatin require further studies to validate if it has any potential clinical applications.

Figure 2. Processing and physiological effects of proghrelin-derived peptide obestatin.

(Top) Proghrelin undergoes posttranslational processing in X/A-like cells to produce ghrelin and obestatin. (Bottom) Obestatin has various effects on multiple organs, including regulation of gastrointestinal (GI) motility and feeding behavior.

Amylin: Coordinating insulin secretion with appetite control

Amylin, a pancreatic peptide hormone, interacts with multiple organs, including the brain, kidneys, and pancreas. Amylin, co-secreted with insulin from pancreatic β cells, plays a vital role in glucose regulation and appetite control.¹⁰⁹ Amylin also works by slowing gastric emptying and inhibiting glucagon release, thereby stabilizing postprandial glucose levels.⁴ Pramlintide, a synthetic analog of amylin, has been approved for the treatment of both type 1 and type 2 diabetes mellitus since 2005. Amylin does not have a unique receptor but instead binds to calcitonin receptors. It acts by mediating signaling through these GPCRs modified by receptor activity-modifying proteins (RAMPs). Different RAMP subtypes give rise to amylin’s diverse physiological roles by modulating receptor binding and signaling.¹¹⁰ However, the exact binding specificity and tissue distribution of amylin receptors are not entirely clear, especially since different RAMP subtypes (e.g., RAMP1, RAMP2, and RAMP3) appear to generate distinct receptor responses and signaling outcomes. Amylin also affects cardiovascular health and energy expenditure by having an effect on sympathetic nervous activity. Amylin has been shown to promote thermogenesis in brown adipose tissue in mice, which increases total energy expenditure and protects against obesity in a RAMP1-dependent manner.^111,112 In the cardiovascular system, amylin indirectly affects blood pressure and heart rate through sympathetic activation, thus contributing to an integrated response in stressful or energy-demanding situations.¹¹³ Amylin also works as a satiety hormone, mainly in the area postrema (AP) of the brainstem, to control meal size and promote satiety.¹¹⁴ However, pramlintide has been shown to promote only modest weight loss, primarily by reducing food intake. This has led to an increased interest in combining it with other weight-loss medications, such as GLP-1 receptor agonists. Next-generation amylin-GLP-1RA analogs, such as cagrilintide,¹¹⁵ have been designed with improved pharmacokinetics to allow for once-weekly dosing and have shown promise in achieving more robust weight loss. These candidates are being evaluated for their enhanced receptor binding, prolonged half-life, and improved tolerability profiles.

In summary, small peptides like apelin, obestatin, and amylin play crucial roles in interorgan crosstalk, coordinating physiological processes essential for maintaining metabolic and cardiovascular health. These peptides exemplify the diverse ways in which interorgan peptide signaling supports systemic energy balance and opens pathways for novel therapeutic interventions.

OUTSTANDING CHALLENGES

Peptides have large translational potential, and because of this, the field is now rapidly evolving, with discoveries and technological advancements continuously emerging. There is still a significant gap in understanding how many peptides are made and modified, their specific receptors, and the downstream signaling pathways they modulate. Despite their established existence in biomarker studies, our understanding of their diverse functions and mechanisms remains incomplete. Given the difficulty in finding the most relevant biological context for these new peptides, most of the peptides we study today for their mechanisms and functions were discovered decades ago. The growing prevalence of metabolic diseases, including obesity, diabetes, and cardiovascular disorders, necessitates a better understanding of the underlying mechanisms of metabolic regulation, including peptides that regulate obesity and cardiovascular disease. Since peptide hormones are key regulators in these processes, elucidating their roles could lead to new pharmacology.