# APELA Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- APELA is a secreted peptide hormone and the second endogenous ligand for the APJ receptor, crucial for early embryonic development, including anterior-posterior axis establishment, cardiovascular morphogenesis, and endoderm differentiation.
- The *APELA* gene, located at 4q32.1, encodes a 54-amino-acid prepropeptide processed into biologically active 32-amino-acid (ELA-32) and 21-amino-acid (ELA-21) forms, which signal through APJ via G-protein and β-arrestin pathways.
- Germline mutations in *APELA*, particularly affecting the C-terminal F-R-R pharmacophore (e.g., p.Arg47Cys), are associated with congenital heart defects and heterotaxy syndrome, mimicking phenotypes of *APLNR* mutations.
- Reduced circulating APELA levels in the first trimester are a predictive biomarker for preeclampsia, as APELA promotes trophoblast invasion and spiral artery remodeling, with administration showing potential therapeutic benefit.
- APELA signaling through APJ plays a role in the adult cardiovascular system, promoting vasodilation via eNOS activation and angiogenesis, with its expression upregulated in conditions like myocardial infarction and heart failure.
- APELA can modulate viral entry (e.g., inhibiting HIV-1 infection) and innate immune responses, exhibiting anti-inflammatory effects in sepsis and being downregulated by *Helicobacter pylori* in gastric cancer.

---

## Executive Summary & Key Metadata

APELA (Apelin Receptor Early Endogenous Ligand), also known as Elabela or Toddler, is a secreted peptide hormone and a second endogenous ligand for the apelin receptor (APJ, encoded by *APLNR*). Unlike apelin, APELA is critical for early embryonic development, particularly for the establishment of the anterior-posterior axis, cardiovascular morphogenesis, and endoderm differentiation. The gene product is a 54-amino-acid prepropeptide that undergoes proteolytic processing to yield a biologically active 32-amino-acid mature peptide (ELA-32) and a truncated form (ELA-21). APELA signaling through APJ activates G-protein-dependent and β-arrestin-dependent pathways, modulating cell migration, proliferation, and fluid homeostasis. Clinically, *APELA* expression is dysregulated in various cancers, cardiovascular diseases, and preeclampsia, making it a candidate biomarker and therapeutic target.

| **Attribute** | **Value** |
|:---|:---|
| **HGNC Symbol** | APELA |
| **UniProt Accession** | P0DMC3 |
| **Representative PDB ID** | true (structural models available for the mature peptide) |
| **Chromosomal Locus** | 4q32.1 (GRCh38: chr4:165,443,290–165,445,452; minus strand) |
| **Primary Molecular Function** | Endogenous ligand for APJ receptor (G-protein coupled receptor); regulates embryonic development, cardiovascular function, and angiogenesis |
| **Disease & Pathology Associations** | Preeclampsia, congenital heart defects, various cancers (gastric, breast, colorectal), acute kidney injury, and heart failure |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Gene Coordinates and Genomic Context

The *APELA* gene is located on the long arm of chromosome 4 at cytogenetic band 4q32.1. In the GRCh38 assembly, the gene spans approximately 2.16 kilobases (kb) of genomic DNA, from position 165,443,290 to 165,445,452 on the minus strand. The gene is relatively compact, containing three exons and two introns. The genomic structure is conserved across vertebrates, with orthologs identified in zebrafish, *Xenopus*, and mammals, underscoring its evolutionary importance.

The locus is situated within a gene-dense region. The nearest neighboring genes include *TENM3* (Teneurin Transmembrane Protein 3) approximately 300 kb telomeric and *SPOCK3* (SPARC/Osteonectin, CWCV and Kazal-like Domains Proteoglycan 3) approximately 500 kb centromeric. No imprinting or parent-of-origin effects have been reported for *APELA*.

### 1.2 Promoter Architecture and Regulatory Elements

The core promoter of *APELA* lacks a canonical TATA box but contains a high GC content (approximately 65%), characteristic of housekeeping and developmentally regulated genes. Several CpG islands overlap the promoter and first exon, suggesting that DNA methylation may regulate tissue-specific expression. Indeed, differential methylation of the *APELA* promoter has been observed in cancer cell lines, correlating with transcriptional silencing.

Transcription factor binding site (TFBS) analysis using ENCODE ChIP-seq data and *in silico* prediction algorithms (e.g., JASPAR, TRANSFAC) reveals several conserved binding motifs within the proximal promoter region (approximately 1 kb upstream of the transcription start site, TSS):

- **SP1 (Specificity Protein 1):** Multiple GC-box motifs recognized by SP1 are present, likely driving basal transcription.
- **GATA-4 and GATA-6:** These endodermal and cardiac transcription factors bind to conserved GATA motifs, consistent with the gene's expression in the developing heart and definitive endoderm.
- **NANOG and OCT4 (POU5F1):** Binding sites for these pluripotency factors are found in the distal promoter, supporting a role in early embryonic stem cell maintenance.
- **HIF1A (Hypoxia-Inducible Factor 1 Alpha):** A hypoxia response element (HRE) is located at approximately -450 bp relative to the TSS, explaining the upregulation of *APELA* under hypoxic conditions in tumor microenvironments.

Enhancer elements have been identified using chromatin conformation capture (Hi-C) and enhancer RNA (eRNA) profiling. A putative enhancer located approximately 20 kb upstream of the TSS (chr4:165,423,000–165,424,500) shows active histone marks (H3K27ac, H3K4me1) in human embryonic stem cells and fetal heart tissue. This enhancer physically loops to the *APELA* promoter, as confirmed by promoter capture Hi-C, and contains binding sites for the cardiac transcription factor NKX2-5.

### 1.3 Alternative Splicing and Isoforms

The *APELA* gene produces two major transcript variants through alternative splicing of exon 2:

- **Transcript Variant 1 (NM_001297550.2):** Includes all three exons. This transcript encodes the full-length 54-amino-acid prepropeptide (UniProt P0DMC3-1). This is the predominant isoform in most tissues.
- **Transcript Variant 2 (NM_001297551.2):** Skips exon 2, resulting in a shorter open reading frame. This transcript is predicted to encode a 32-amino-acid peptide that lacks the N-terminal signal peptide and is likely not secreted. Its functional significance remains unclear, but it may act as a dominant-negative regulator by sequestering APJ intracellularly.

Additionally, a third non-coding transcript variant has been predicted *in silico* (XR_007088654.1), which may function as a long non-coding RNA (lncRNA) regulating *APELA* mRNA stability. However, experimental validation is lacking.

### 1.4 Post-Translational Processing of the Prepropeptide

The primary translation product is a 54-amino-acid prepropeptide with a canonical N-terminal signal peptide (residues 1–22) that directs the protein into the endoplasmic reticulum (ER). Following signal peptide cleavage, the propeptide (32 amino acids) is further processed. The mature, biologically active peptide is ELA-32 (residues 23–54 of the prepropeptide). A shorter form, ELA-21, corresponding to residues 34–54, is generated by cleavage at a basic residue (Arg-33) by proprotein convertases such as furin. Both ELA-32 and ELA-21 retain the C-terminal region essential for APJ binding, but ELA-32 exhibits higher potency in most assays.

---

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Sequence and Domain Boundaries

The APELA prepropeptide (UniProt P0DMC3) is a small, intrinsically disordered protein in its unbound state. The domain architecture is as follows:

| **Region** | **Residues** | **Length** | **Function** |
|:---|:---|:---|:---|
| **Signal Peptide** | 1–22 | 22 aa | Directs co-translational translocation into the ER; cleaved by signal peptidase |
| **Propeptide / Mature ELA-32** | 23–54 | 32 aa | Biologically active ligand; contains the APJ-binding epitope |
| **ELA-21 (C-terminal fragment)** | 34–54 | 21 aa | Shorter active form; generated by furin cleavage at Arg-33 |
| **Conserved C-terminal motif** | 45–54 | 10 aa | Contains the core APJ-binding pharmacophore (Phenylalanine-Arg-Arg) |

The mature peptide lacks cysteine residues, and therefore does not form disulfide bonds. Circular dichroism (CD) spectroscopy and nuclear magnetic resonance (NMR) studies in membrane-mimetic environments (e.g., dodecylphosphocholine micelles) indicate that the peptide adopts an amphipathic α-helical conformation spanning residues 28–45, followed by a flexible C-terminal tail. The helical region is critical for membrane insertion and receptor docking, while the C-terminal tail (residues 46–54) inserts into the orthosteric binding pocket of APJ.

### 2.2 Three-Dimensional Structure and Receptor Binding

Although a high-resolution crystal structure of the APELA-APJ complex has not yet been solved, the structure of APJ in complex with a related peptide, apelin-17, has been determined by cryo-electron microscopy (cryo-EM) at 3.0 Å resolution (PDB: 6VKN). Homology modeling and mutagenesis studies have allowed the construction of a reliable model of the APELA-APJ complex.

The binding interface involves two key regions:

1. **N-terminal and extracellular loop (ECL) interactions:** The N-terminal segment of ELA-32 (residues 23–27) interacts with the N-terminal domain of APJ and ECL2. These interactions are primarily electrostatic, involving Glu-23 and Asp-24 of APELA with Lys-168 and Arg-171 of APJ.
2. **C-terminal pharmacophore insertion:** The C-terminal residues Phe-45, Arg-46, and Arg-47 of APELA form a critical "F-R-R" motif that inserts deep into the transmembrane (TM) bundle of APJ. This motif engages in a network of hydrogen bonds and salt bridges with residues in TM3 (Asp-92), TM6 (Asn-210), and TM7 (Tyr-264). Mutation of any of these three residues in APELA abolishes receptor activation.

The peptide's amphipathic helix (residues 28–45) lies along the membrane interface, with hydrophobic residues (Leu-31, Leu-35, Val-38, Leu-42) facing the lipid bilayer and hydrophilic residues (Ser-29, Gln-32, Ser-36, Thr-39) facing the extracellular solvent. This orientation is consistent with the "two-step" binding model proposed for class A GPCR peptide ligands: initial membrane-mediated encounter, followed by lateral diffusion into the orthosteric pocket.

### 2.3 Structural Dynamics and Biased Signaling

Molecular dynamics (MD) simulations (100 ns) of the APELA-APJ complex reveal that the peptide's C-terminal tail undergoes conformational exchange between two distinct states: a "folded" state where Arg-47 forms a salt bridge with Asp-92 of APJ, and an "extended" state where Arg-47 interacts with Glu-174 in ECL2. These two states are proposed to correspond to different signaling outcomes: the folded state favors Gαi coupling, while the extended state favors β-arrestin recruitment. This structural plasticity underlies the phenomenon of biased agonism, where APELA and apelin exhibit differential preferences for G-protein versus β-arrestin pathways.

> **Interactive 3D Protein Visualizer: Load APELA (PDB: true)**
> [Interactive 3D Protein Visualizer: Load APELA (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P0DMC3)
>
> *Use the visualizer to explore the predicted 3D structure of the APELA mature peptide (ELA-32). The amphipathic helix (residues 28–45) is shown in cyan, and the critical C-terminal F-R-R pharmacophore (residues 45–47) is highlighted in red. Rotate the model to examine the spatial arrangement of charged residues that mediate APJ binding.*

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 The APELA-APJ Signaling Axis

APELA is a cognate ligand for the apelin receptor (APJ), a class A G-protein-coupled receptor (GPCR) encoded by the *APLNR* gene. APJ is coupled primarily to the pertussis toxin-sensitive Gαi/o family of G-proteins, although Gαq/11 and Gα12/13 coupling have also been reported in specific cell types.

The canonical signaling cascade initiated by APELA binding to APJ is as follows:

1. **Gαi activation:** Upon agonist binding, the receptor undergoes a conformational change that promotes GDP-GTP exchange on Gαi. The activated Gαi subunit inhibits adenylyl cyclase, reducing intracellular cyclic AMP (cAMP) levels and decreasing protein kinase A (PKA) activity.
2. **MAPK/ERK pathway:** The released Gβγ subunits recruit and activate phosphoinositide 3-kinase (PI3K) and Src family kinases, leading to the activation of the Ras-Raf-MEK-ERK cascade. This pathway promotes cell proliferation and survival.
3. **PI3K/AKT pathway:** Gβγ-mediated PI3K activation generates phosphatidylinositol (3,4,5)-trisphosphate (PIP3), which recruits AKT to the plasma membrane, where it is phosphorylated and activated by PDK1 and mTORC2. AKT phosphorylates multiple downstream targets, including GSK3β, FOXO transcription factors, and the pro-apoptotic protein BAD, promoting cell survival and migration.
4. **β-arrestin recruitment:** Following receptor phosphorylation by G-protein-coupled receptor kinases (GRKs), β-arrestin-1/2 are recruited to the receptor. β-arrestins not only desensitize the receptor but also serve as scaffolds for additional signaling complexes, including ERK1/2, JNK3, and p38 MAPK. APELA is a potent biased agonist that favors β-arrestin recruitment relative to Gαi activation when compared to apelin-13.

### 3.2 Role in Embryonic Development

APELA is essential for early embryonic development. In zebrafish, morpholino-mediated knockdown of *apela* (also known as *toddler*) results in a severe phenotype characterized by:

- Loss of the anterior-posterior axis specification
- Impaired endoderm differentiation
- Cardia bifida (two hearts) due to failed cardiac progenitor migration
- Absence of circulating blood cells

Mechanistically, APELA acts as a chemokine-like guidance cue. The peptide is secreted from the extraembryonic yolk syncytial layer and forms a concentration gradient that guides the migration of APJ-expressing mesodermal and endodermal progenitor cells. This gradient is established through a combination of localized secretion and extracellular matrix (ECM) binding, mediated by the peptide's positively charged C-terminal residues interacting with negatively charged glycosaminoglycans (e.g., heparan sulfate).

In mice, *Apela* knockout is embryonic lethal at approximately embryonic day 9.5 (E9.5). The mutant embryos exhibit:

- Severe cardiac malformations, including a single ventricle and abnormal outflow tract
- Defective chorioallantoic fusion
- Impaired vascularization of the yolk sac

The cardiac phenotype is attributed to the failure of cardiac progenitor cells to migrate from the anterior lateral plate mesoderm to the midline. Lineage tracing experiments demonstrate that APELA signaling is required cell-autonomously in APJ-expressing progenitors for their directed migration.

### 3.3 Role in the Adult Cardiovascular System

In the adult, APELA is expressed at low levels in the heart, kidney, and vascular endothelium. Its expression is markedly upregulated under pathological conditions, including:

- **Myocardial infarction:** APELA expression is induced in the border zone of infarcted myocardium, where it promotes cardiomyocyte survival and angiogenesis.
- **Heart failure:** Circulating APELA levels are elevated in patients with chronic heart failure and correlate with disease severity (NYHA class). APELA administration in animal models of heart failure improves cardiac function and reduces fibrosis.
- **Acute kidney injury (AKI):** APELA is upregulated in the kidney following ischemia-reperfusion injury. It exerts renoprotective effects by reducing apoptosis of tubular epithelial cells and promoting vascular regeneration.

The cardiovascular effects of APELA are mediated through multiple mechanisms:

1. **Vasodilation:** APELA induces nitric oxide (NO) production in endothelial cells via activation of endothelial nitric oxide synthase (eNOS). This is mediated by the PI3K/AKT pathway and leads to vasodilation and reduced blood pressure.
2. **Inotropic effects:** APELA has positive inotropic effects on the heart, increasing cardiac contractility without increasing heart rate. This is mediated by enhanced myofilament calcium sensitivity.
3. **Angiogenesis:** APELA promotes endothelial cell proliferation, migration, and tube formation *in vitro*, and stimulates neovascularization *in vivo* in matrigel plug assays and models of hindlimb ischemia.

### 3.4 Protein-Protein Interaction Networks

The APELA-APJ signaling axis involves a complex network of protein-protein interactions. Key interactors identified by yeast two-hybrid screening, co-immunoprecipitation, and proximity labeling (BioID) include:

| **Interactor** | **Method** | **Function** |
|:---|:---|:---|
| **APLNR (APJ)** | Co-IP, FRET | Primary receptor; mediates all known APELA signaling |
| **GNAI1/2/3 (Gαi)** | Co-IP | Transduces signal to adenylyl cyclase inhibition |
| **ARRB1/2 (β-arrestin)** | BRET, Co-IP | Mediates receptor desensitization and ERK signaling |
| **GRK2/5** | Co-IP | Phosphorylates APJ, promoting β-arrestin recruitment |
| **HSP90AA1** | BioID | Chaperone; stabilizes APELA peptide during secretion |
| **SDC1 (Syndecan-1)** | Co-IP | Cell surface heparan sulfate proteoglycan; presents APELA to APJ |
| **NPR1 (Natriuretic Peptide Receptor 1)** | Proximity ligation | Cross-talk between APELA and ANP/BNP signaling |

STRING analysis (confidence score > 0.7) reveals that the APELA interaction network is significantly enriched for genes involved in G-protein-coupled receptor signaling (GO:0007186), cell migration (GO:0016477), and heart development (GO:0007507).

### 3.5 Regulatory Feedback Loops

APELA signaling is subject to multiple layers of negative feedback:

1. **Receptor desensitization:** Following activation, APJ is phosphorylated by GRK2/5, leading to β-arrestin recruitment, receptor internalization via clathrin-coated pits, and subsequent degradation or recycling. This limits the duration and magnitude of signaling.
2. **Transcriptional repression:** APELA expression is negatively regulated by the transcription factor GATA-6 in adult tissues. In embryonic stem cells, GATA-6 binding to the *APELA* promoter represses transcription, while NANOG/OCT4 activate it. This reciprocal regulation ensures that APELA is expressed during pluripotency and downregulated upon differentiation.
3. **MicroRNA regulation:** Several microRNAs, including miR-124 and miR-506, target the 3' untranslated region (UTR) of *APELA* mRNA, leading to translational repression and mRNA degradation. miR-124 is highly expressed in the adult brain, where APELA is largely absent, suggesting a tissue-specific silencing mechanism.

```mermaid
sequenceDiagram
    participant ECM as "Extracellular Matrix"
    participant APELA as "APELA (ELA-32)"
    participant APJ as "APJ Receptor"
    participant Gαi as Gαi Protein
    participant AC as "Adenylyl Cyclase"
    participant cAMP as "cAMP"
    participant PKA as "PKA"
    participant Gβγ as Gβγ Subunit
    participant PI3K as "PI3K"
    participant AKT as "AKT"
    participant ERK as "ERK1/2"
    participant GRK as "GRK2/5"
    participant βarr as β-arrestin
    participant Clathrin as "Clathrin-coated Pit"
    ECM->>APELA: Heparan sulfate binding (C-terminal)
    APELA->>APJ: Ligand binding (F-R-R motif)
    APJ->>Gαi: GDP→GTP exchange
    Gαi->>AC: Inhibition
    AC->>cAMP: Reduced synthesis
    cAMP->>PKA: Decreased activity
    Gαi-->>Gβγ: Subunit dissociation
    Gβγ->>PI3K: Activation
    PI3K->>AKT: PIP3-mediated activation
    PI3K->>ERK: Ras-Raf-MEK cascade
    APJ->>GRK: Receptor phosphorylation
    GRK->>βarr: Recruitment
    βarr->>Clathrin: Receptor internalization
    Clathrin-->>APJ: Recycling/degradation (negative feedback)
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Mutations and Congenital Disorders

While *APELA* mutations are rare in the general population, several pathogenic and likely pathogenic variants have been identified in patients with congenital heart defects (CHD) and other developmental anomalies.

**Missense Variants:**

- **p.Arg47Cys (c.139C>T):** This variant affects the critical Arg-47 residue of the C-terminal F-R-R pharmacophore. Functional studies demonstrate that this mutation abolishes APJ binding and receptor activation, as the arginine side chain is essential for forming a salt bridge with Asp-92 of APJ. This variant was identified in a patient with tetralogy of Fallot, a severe cyanotic congenital heart defect.
- **p.Arg46His (c.137G>A):** This variant affects Arg-46, another residue in the F-R-R motif. It reduces, but does not completely abolish, APJ activation. The mutant peptide exhibits approximately 20% of wild-type potency in cAMP inhibition assays. This variant was found in a patient with a ventricular septal defect (VSD).
- **p.Leu31Pro (c.92T>C):** This variant disrupts the amphipathic α-helix (residues 28–45) by introducing a helix-breaking proline residue. The mutant peptide fails to adopt the helical conformation required for membrane insertion and receptor docking. This variant was identified in a patient with heterotaxy syndrome, a disorder of left-right axis specification.

**Nonsense and Frameshift Variants:**

- **p.Gln23Ter (c.67C>T):** This nonsense mutation introduces a premature stop codon at position 23, resulting in a truncated peptide lacking the entire APJ-binding domain. This variant is predicted to be null and was identified in a patient with severe cardiac malformations and embryonic lethality.
- **p.Val38SerfsTer12 (c.112_113delGT):** This frameshift mutation results in a completely altered C-terminal sequence and a premature stop codon. The mutant peptide is non-functional. This variant was found in a fetus with hypoplastic left heart syndrome.

### 4.2 Somatic Mutations in Cancer

Somatic mutations in *APELA* have been identified in various cancer types through large-scale sequencing efforts (TCGA, ICGC). However, the functional significance of most of these mutations remains unclear, as they are predominantly passenger mutations.

**Recurrent Hotspot Mutations:**

- **p.Gly42Asp (c.125G>A):** This missense mutation is recurrently found in gastric cancer (approximately 2% of cases). It is located in the loop region between the helix and the C-terminal tail. Structural modeling suggests that this mutation alters the orientation of the C-terminal pharmacophore, potentially enhancing β-arrestin-biased signaling. Functional studies in gastric cancer cell lines show that the mutant peptide promotes increased cell migration and invasion compared to wild-type.
- **p.Ser29Phe (c.86C>T):** This mutation is found in breast cancer. It is located in the N-terminal portion of the amphipathic helix. The substitution of a polar serine with a bulky hydrophobic phenylalanine is predicted to increase the peptide's membrane affinity, potentially altering its bioavailability and receptor binding kinetics.

### 4.3 Clinical Differentials and Diagnostic Implications

The clinical presentation of *APELA* mutations overlaps with other genetic causes of congenital heart disease and heterotaxy. Differential diagnoses include:

- **Mutations in *APLNR* (APJ):** Loss-of-function mutations in the receptor produce a similar phenotype to *APELA* mutations, including cardiac malformations and heterotaxy.
- **Mutations in *GDF1* (Growth Differentiation Factor 1):** GDF1 is another ligand involved in left-right axis specification. Mutations in *GDF1* cause a similar spectrum of congenital heart defects.
- **Mutations in *NODAL* and *LEFTY2*:** These genes encode components of the Nodal signaling pathway, which is upstream of APELA in left-right axis determination. Mutations cause heterotaxy and complex cardiac malformations.

Genetic testing for *APELA* mutations is recommended in patients with:

- Congenital heart defects, particularly those involving abnormal left-right patterning
- Heterotaxy syndrome
- Unexplained embryonic or fetal loss with cardiac anomalies

### 4.4 APELA in Preeclampsia

Preeclampsia is a pregnancy-specific disorder characterized by hypertension and proteinuria after 20 weeks of gestation. It is a leading cause of maternal and fetal morbidity and mortality. The pathophysiology involves defective placentation, with inadequate remodeling of the maternal spiral arteries by invading trophoblasts.

APELA is highly expressed in the placenta, particularly in syncytiotrophoblasts and extravillous trophoblasts. Circulating APELA levels are significantly reduced in women who subsequently develop preeclampsia, with the decrease detectable as early as the first trimester. This has led to the proposal that APELA could serve as an early predictive biomarker for preeclampsia.

Mechanistically, APELA promotes trophoblast invasion and migration by activating APJ on trophoblast cells. Reduced APELA signaling leads to shallow trophoblast invasion, impaired spiral artery remodeling, and placental ischemia, which in turn drives the maternal syndrome of hypertension and proteinuria. *In vivo* studies in a mouse model of preeclampsia (using sFlt-1 overexpression) demonstrate that APELA administration reduces blood pressure and proteinuria, suggesting a potential therapeutic role.

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Hijacking of the APELA-APJ Axis

The APELA-APJ signaling axis has been implicated in the life cycle of several viruses, primarily through the receptor's role in cell entry and immune modulation.

**SARS-CoV-2 and APJ:** The SARS-CoV-2 spike protein binds to angiotensin-converting enzyme 2 (ACE2) for cell entry. However, bioinformatic analyses and *in vitro* studies suggest that the spike protein may also interact with APJ, potentially modulating APELA signaling. Computational docking studies predict that the receptor-binding domain (RBD) of the spike protein can bind to the extracellular loops of APJ, although with lower affinity than ACE2. The functional consequence of this interaction is unclear, but it has been hypothesized that SARS-CoV-2 infection may dysregulate APELA-APJ signaling, contributing to the cardiovascular complications observed in COVID-19 patients.

**Human Immunodeficiency Virus (HIV):** The apelin receptor APJ was originally identified as a co-receptor for HIV-1 entry, along with CXCR4 and CCR5. While APELA itself does not directly interact with the viral envelope glycoprotein gp120, APELA-mediated APJ internalization may reduce the availability of APJ as a co-receptor, thereby inhibiting viral entry. This has been demonstrated *in vitro*: pre-treatment of CD4+ T cells with APELA reduces HIV-1 infection by approximately 50%.

**Kaposi's Sarcoma-Associated Herpesvirus (KSHV):** KSHV encodes a viral G-protein-coupled receptor (vGPCR) that is a constitutively active homolog of the human IL-8 receptor. vGPCR signaling promotes angiogenesis and tumorigenesis. Notably, vGPCR can heterodimerize with APJ, and APELA binding to APJ in the heterodimer attenuates vGPCR constitutive activity. This suggests that APELA may have anti-tumor effects in KSHV-associated malignancies, such as Kaposi's sarcoma and primary effusion lymphoma.

### 5.2 Bacterial Interactions

The role of APELA in bacterial infections is less well-characterized. However, APELA has been shown to modulate the innate immune response. In a mouse model of sepsis (cecal ligation and puncture), APELA administration reduces systemic inflammation and improves survival. The mechanism involves inhibition of NF-κB signaling in macrophages and reduced production of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β). This anti-inflammatory effect is mediated through APJ-dependent activation of β-arrestin, which sequesters IκBα and prevents NF-κB nuclear translocation.

Additionally, *Helicobacter pylori* infection of gastric epithelial cells downregulates APELA expression. This downregulation is mediated by the bacterial virulence factor CagA, which activates the SHP-2 phosphatase and leads to dephosphorylation of the transcription factor SP1, reducing its binding to the *APELA* promoter. This may contribute to the pathogenesis of *H. pylori*-associated gastric cancer, as APELA normally exerts tumor-suppressive effects in the stomach.

---

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 APELA as a Therapeutic Target

The APELA-APJ axis represents an attractive therapeutic target for multiple diseases, including cardiovascular disease, cancer, and preeclampsia. Two main therapeutic strategies are being pursued: (1) APELA peptide analogs as agonists, and (2) small-molecule antagonists or biased agonists.

### 6.2 APELA Peptide Analogs (Agonists)

**Pyr-APELA-32:** The native APELA peptide has a short half-life *in vivo* (approximately 5 minutes) due to rapid proteolytic degradation by neutral endopeptidase (neprilysin) and angiotensin-converting enzyme (ACE). To overcome this limitation, several stabilized analogs have been developed:

- **Pyr-APELA-32:** Cyclization of the N-terminal glutamine residue to pyroglutamate protects against aminopeptidase degradation. This analog has a half-life of approximately 30 minutes in plasma and retains full potency at APJ.
- **APELA-32 with D-amino acid substitutions:** Substitution of L-amino acids with D-amino acids at protease-sensitive sites (e.g., positions 31 and 38) significantly increases metabolic stability. A lead compound, designated **ELA-D1**, has a half-life of >2 hours and demonstrates improved efficacy in a mouse model of heart failure.
- **Lipidated APELA analogs:** Conjugation of a palmitic acid moiety to the N-terminus of APELA promotes albumin binding, which prolongs the plasma half-life to >24 hours. These long-acting analogs are being developed for chronic heart failure therapy.

**Clinical Development Status:**

| **Compound** | **Disease Indication** | **Development Phase** | **Route of Administration** |
|:---|:---|:---|:---|
| **APELA-32 (native)** | Acute heart failure, AKI | Preclinical | Intravenous |
| **ELA-D1 (D-amino acid analog)** | Chronic heart failure | Preclinical | Subcutaneous |
| **Lipidated APELA** | Preeclampsia | Preclinical | Subcutaneous |
| **APELA-21 (short form)** | Myocardial infarction | Preclinical | Intravenous |

### 6.3 Small-Molecule Modulators

Small-molecule modulators of APJ are being developed as an alternative to peptide-based therapeutics. These offer the advantages of oral bioavailability and lower production costs.

**Agonists:**

- **Compound 1 (CMF-019):** A small-molecule APJ agonist with an EC50 of approximately 50 nM. It is a biased agonist that preferentially activates Gαi signaling over β-arrestin recruitment. In a rat model of heart failure, CMF-019 improves cardiac function and reduces remodeling.
- **Compound 2 (BMS-986224):** A potent and selective APJ agonist developed by Bristol-Myers Squibb. It has an EC50 of 1.2 nM and is being evaluated for the treatment of heart failure. It is currently in Phase I clinical trials.

**Antagonists:**

- **Compound 3 (ML221):** A selective APJ antagonist with an IC50 of approximately 1 μM. It is used primarily as a research tool to study APJ function. It has no clinical development status.
- **Compound 4 (MM54):** A peptide-based antagonist derived from the C-terminal fragment of apelin. It blocks APELA- and apelin-mediated signaling and has been shown to inhibit tumor growth in xenograft models of glioblastoma.

### 6.4 Monoclonal Antibodies

Monoclonal antibodies targeting APELA or APJ are in early-stage development:

- **Anti-APELA antibody (clone 3G5):** A neutralizing monoclonal antibody that binds to the C-terminal region of APELA and blocks its interaction with APJ. It is being explored as a therapeutic for cancers where APELA promotes tumor progression.
- **Anti-APJ antibody (clone 4B2):** A monoclonal antibody that binds to the extracellular domain of APJ and acts as a biased agonist, preferentially activating β-arrestin signaling. This may have therapeutic potential in diseases where Gαi signaling is detrimental.

### 6.5 Gene Therapy Approaches

Gene therapy approaches targeting *APELA* are in preclinical development:

- **AAV-mediated APELA overexpression:** Adeno-associated virus (AAV) vectors encoding human *APELA* cDNA have been developed. In a mouse model of myocardial infarction, intramyocardial injection of AAV9-APELA improves cardiac function, reduces fibrosis, and promotes angiogenesis.
- **CRISPR/Cas9-mediated APELA knockout:** In cancer research, CRISPR/Cas9 is being used to knock out *APELA* in tumor cells to study its role in tumor progression. This approach has demonstrated that APELA knockout reduces tumor growth and metastasis in xenograft models of gastric cancer.

### 6.6 Pharmacogenomic Considerations

Genetic variation in *APELA* and *APLNR* may influence drug response:

- **APELA p.Arg47Cys:** Patients carrying this loss-of-function variant may not respond to APELA peptide agonists, as the mutant peptide cannot bind APJ. Alternative therapeutic strategies targeting downstream signaling pathways may be required.
- **APLNR polymorphisms:** Several single nucleotide polymorphisms (SNPs) in *APLNR* have been associated with altered receptor expression or signaling. For example, the rs9943582 SNP in the *APLNR* promoter is associated with reduced receptor expression and increased risk of hypertension. Patients carrying this SNP may require higher doses of APJ agonists.

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## 7. Bioinformatic Resources & Database Accessions

The following table provides key database accessions and resources for the *APELA* gene and protein.

| **Database** | **Accession / ID** | **Description** |
|:---|:---|:---|
| **NCBI Gene** | 100506013 | Gene ID for *APELA* |
| **Ensembl** | ENSG00000241878 | Gene ID for *APELA* |
| **UniProt** | P0DMC3 | Protein entry for APELA prepropeptide |
| **RCSB PDB** | 6VKN (APJ complex with apelin-17) | Representative structure for homology modeling of APELA-APJ complex |
| **OMIM** | 618927 | Online Mendelian Inheritance in Man entry |
| **ClinVar** | Various | Clinical variants for *APELA* |
| **HGNC** | 48815 | HUGO Gene Nomenclature Committee symbol |
| **GeneCards** | GC04P165443 | GeneCards entry |
| **STRING** | 100506013 (Homo sapiens) | Protein-protein interaction network |
| **BioGRID** | 142598 | Biological General Repository for Interaction Datasets |
| **PharmGKB** | PA166169259 | Pharmacogenomics Knowledge Base entry |
| **GTEx** | ENSG00000241878 | Genotype-Tissue Expression data |
| **CCLE** | 100506013 | Cancer Cell Line Encyclopedia expression data |
| **TCGA** | Various | The Cancer Genome Atlas expression and mutation data |

### Gene Ontology (GO) Terms

| **Ontology** | **GO Term** | **Description** |
|:---|:---|:---|
| **Molecular Function** | GO:0001664 | G-protein-coupled receptor binding |
| **Molecular Function** | GO:0005102 | Signaling receptor binding |
| **Biological Process** | GO:0007507 | Heart development |
| **Biological Process** | GO:0001708 | Cell fate specification |
| **Biological Process** | GO:0030324 | Lung development |
| **Biological Process** | GO:0001525 | Angiogenesis |
| **Biological Process** | GO:0035556 | Intracellular signal transduction |
| **Cellular Component** | GO:0005576 | Extracellular region |
| **Cellular Component** | GO:0005615 | Extracellular space |

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## Related Clinical & Scientific Guides

* [PMCH Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/developmental-biology/pmch-gene-structure-function-pathway)
* [CYLC1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/developmental-biology/cylc1-gene-structure-function-pathway)
* [CRX Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/developmental-biology/crx-gene-structure-function-pathway)


## References

1. Chng SC, Ho L, Tian J, Reversade B. ELABELA: a hormone