CXCR4

C-X-C motif chemokine receptor 4 (CXCR4, also called CD184 or fusin) is a seven-transmembrane G-protein-coupled receptor (GPCR) expressed on hematopoietic stem cells (HSCs), immune cells, endothelial cells, neurons, and many other tissues. Its dominant physiological ligand is CXCL12 (SDF-1), although extracellular ubiquitin and the HIV-1 envelope glycoprotein gp120 also bind it. CXCR4 is the principal mediator of bone-marrow niche retention and quiescence in HSCs; its pharmacological antagonism with plerixafor (AMD3100) underpins clinical HSC mobilization for stem cell transplantation. In aging, dysregulation of the CXCL12–CXCR4 axis — through niche CXCL12 depletion, altered receptor expression on HSCs, and SASP-mediated paracrine CXCR4 activation — contributes to stem-cell-exhaustion and altered-intercellular-communication. Gain-of-function truncating mutations in CXCR4 cause WHIM syndrome, and CXCR4 serves as an obligate co-receptor for X4-tropic HIV-1.

Identity

UniProt: P61073 (CXCR4_HUMAN) — Swiss-Prot, manually reviewed; 352 amino acids
NCBI Gene ID: 7852 (symbol: CXCR4); chromosome 2q22.1
HGNC: HGNC:2561
Ensembl: ENSG00000121966
CD antigen: CD184
Mouse ortholog: Cxcr4 (MGI:109563); one-to-one ortholog; CXCL12 binding and Gαi coupling highly conserved across vertebrates
GenAge: no entry (CXCR4 is not in the HAGR curated aging gene set as of 2026-06-15) gap/not-populated

Previous symbols and aliases

CXCR4 was previously described as LESTR (leukocyte-expressed seven-transmembrane-domain receptor), fusin (from its role as an HIV fusion co-receptor), and NPY3R. The CD184 designation is used in clinical flow-cytometry panels. Human CXCR4 maps to chromosome 2q22.1 — notably an intronless coding region.

Structure and key functional domains

CXCR4 is a class A (rhodopsin-family) GPCR with seven transmembrane helices (TM1–TM7), an extracellular N-terminus, and an intracellular C-terminus bearing GPCR-kinase (GRK) phosphorylation sites:

N-terminal extracellular domain (ECL-N): primary CXCL12 contact surface; sulfated Tyr21 increases CXCL12 affinity
Transmembrane pocket: ligand-binding cavity; site of small-molecule antagonism by plerixafor
Intracellular C-terminal tail: serine/threonine-rich; phosphorylated by GRK2/GRK3 upon agonist stimulation, creating docking sites for β-arrestins and triggering receptor internalization
DRY motif (Asp–Arg–Tyr) in TM3: conserved GPCR activation switch; coupling to Gαi

Signaling mechanisms

CXCR4 is a biased GPCR that drives parallel G-protein and β-arrestin outputs upon CXCL12 binding ¹:

Gαi / calcium / cyclic AMP pathway

Agonist binding activates heterotrimeric G proteins — primarily Gαi — inhibiting adenylyl cyclase and reducing intracellular cyclic adenosine monophosphate (cAMP). The Gβγ dimer simultaneously activates phospholipase C-β (PLCβ), generating inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 drives transient cytosolic calcium (Ca²⁺) elevation within seconds of receptor stimulation ¹. This calcium flux is measurable and is the standard pharmacological readout in CXCR4 gain-of-function assays (e.g., WHIM syndrome functional characterization).

PI3K–AKT axis

Gβγ subunits released by Gαi directly activate class I phosphoinositide 3-kinase (PI3K) → phosphatidylinositol 3,4,5-trisphosphate (PIP3) → AKT phosphorylation. AKT promotes cell survival, metabolic reprogramming, and augmented motility. This axis is relevant to HSC survival signaling in the bone-marrow niche.

MAPK cascade

CXCL12–CXCR4 activates the Ras–Raf–MEK–ERK1/2 mitogen-activated protein kinase (MAPK) cascade, promoting cell migration, proliferation, and survival gene expression programs. ERK1/2 activation is a key readout in cancer-context CXCR4 signaling but is also operative in HSC mobilization biology.

β-arrestin pathway and receptor internalization

After GRK2/GRK3-mediated phosphorylation of the C-terminal tail, β-arrestins (ARRB1 and ARRB2) bind CXCR4, sterically blocking G-protein coupling (desensitization) and initiating clathrin-mediated receptor internalization. β-arrestin–CXCR4 complexes can signal independently from endosomes. The WHIM syndrome truncation mutations (see § WHIM syndrome) remove C-terminal GRK phosphorylation sites, impairing receptor desensitization and prolonging Gαi and calcium signaling upon CXCL12 stimulation ².

Atypical receptor crosstalk: ACKR3

ACKR3 (atypical chemokine receptor 3, formerly CXCR7) binds CXCL12 with higher affinity than CXCR4 but lacks functional Gαi coupling; it acts as a ligand scavenger, constitutively internalizing and degrading CXCL12 to shape the gradient available to CXCR4. ACKR3 co-expression therefore modulates CXCR4 signaling amplitude. ACKR3 is prominently expressed on endothelial cells and is a concurrent stub being seeded alongside this page.

Role in the bone-marrow HSC niche

CXCL12–CXCR4 as the dominant niche-retention axis

HSCs constitutively express CXCR4 ³. CXCL12 secreted by CAR (CXCL12-abundant reticular) cells, LepR+ mesenchymal stromal cells, and sinusoidal endothelial cells in the bone marrow creates a gradient that retains HSCs in their perivascular and endosteal niches and maintains quiescence ⁴. The axis also mediates homing of transplanted HSCs back to bone marrow after intravenous infusion.

Genetic evidence (mouse): Conditional deletion of Cxcr4 in adult mice (MxCre-CXCR4^flox/null^, plpC-induced, C57BL/6 background; n=6/group for HSC quantification) caused severe reduction in CD34⁻c-Kit⁺Sca-1⁺Lin⁻ HSC numbers (frequency p=0.026, absolute p=0.031) and increased HSC sensitivity to myelotoxic 5-FU injury (survival log-rank p=0.037, n=9/group) ³. Loss of CXCR4 also promoted exit from G₀ quiescence in primitive hematopoietic progenitors. The same study identified and named CAR cells as the primary bone-marrow stromal source of CXCL12 — establishing that CXCL12–CXCR4 signaling is essential for maintaining the quiescent HSC pool in defined niche microenvironments.

Dimension	Status
Pathway conserved in humans?	yes — CXCR4 on human HSCs is well-documented; SDF-1/CXCL12 gradient governs human HSC niche retention
Phenotype conserved in humans?	yes — plerixafor mobilizes human HSCs within hours by blocking this axis
Replicated in humans?	indirect — pharmacological (plerixafor) confirmation; no human genetic conditional deletion equivalent

See hematopoietic-stem-cells for the full HSC aging biology.

Aging changes in CXCR4 niche function

With age, multiple features of the bone marrow niche impair CXCL12–CXCR4 axis function:

CAR-cell depletion: Mesenchymal stromal cells progressively convert to adipocytes with age, reducing CXCL12-secreting CAR-cell density and thereby attenuating the CXCL12 gradient available to CXCR4 on HSCs.
HSC CXCR4 expression changes: Aged HSCs show altered receptor expression and adhesion-molecule profiles; the net effect on CXCR4 surface levels is not fully characterized in aging vs. disease contexts. gap/needs-replication
Aged neutrophil CXCR4 upregulation: Aged (senescent-phenotype) neutrophils upregulate CXCR4 (alongside downregulation of CD62L, producing a CD62L^lo^CXCR4^hi^ phenotype) and re-home to bone marrow, where they modulate hematopoietic progenitor egress through a circadian clearance mechanism ⁵. This neutrophil aging axis intersects CXCR4 biology with chronic-inflammation.

Clinical mobilization: plerixafor (AMD3100)

Plerixafor (AMD3100; trade name Mozobil) is a bicyclam small molecule that competitively antagonizes CXCL12 binding to CXCR4, disrupting HSC retention and mobilizing them from bone marrow into peripheral blood within hours ⁶. FDA-approved (in combination with granulocyte-colony-stimulating factor, G-CSF) for autologous HSC mobilization before transplantation in non-Hodgkin’s lymphoma and multiple myeloma. Plerixafor is also active in WHIM syndrome (see below) ¹.

Plerixafor’s mechanism provided the first proof-of-concept in humans that pharmacological CXCR4 blockade is sufficient to disrupt bone-marrow niche retention and mobilize functional HSCs — a finding with direct relevance to understanding how CXCL12–CXCR4 governs niche biology at a causal, not merely correlative, level.

WHIM syndrome

WHIM syndrome — Warts, Hypogammaglobulinemia, Infections, Myelokathexis — is an autosomal-dominant combined immunodeficiency caused by heterozygous gain-of-function truncating mutations in CXCR4 ². The original genetic characterization by Hernandez et al. 2003 (Nature Genetics) identified a 19-residue C-terminal truncation as the index WHIM mutation (subsequent work has characterized additional truncation lengths in other WHIM pedigrees ⁷). This truncation eliminates C-terminal GRK phosphorylation docking sites, impairing receptor desensitization. Functional consequence: CXCR4 cannot be desensitized after CXCL12 stimulation. Cells carrying the 19-residue truncation show significantly greater calcium flux relative to control cells in response to SDF-1/CXCL12 ², and retain hypersensitive responses including prolonged retention in bone marrow.

Clinical presentation reflects excess niche retention:

Myelokathexis: pathological retention of mature neutrophils in bone marrow; peripheral neutropenia despite adequate BM production
Hypogammaglobulinemia: impaired B-cell egress from marrow
Recurrent infections: due to neutropenia + hypogammaglobulinemia
Warts / HPV susceptibility: immune surveillance impairment

WHIM syndrome is also associated with decreased bone mineral density in ~25% of patients (≥1 site at or below T-score −1.0) and an osteoporotic bone phenotype in Cxcr4^1013^ knock-in mice; the mechanism is impaired CXCR4 desensitization in skeletal stromal cells (SSCs), which disrupts their osteogenic commitment in a Cxcr4 allele dose-dependent manner while promoting osteoclastogenesis; plerixafor (AMD3100, 5 mg/kg i.p., 3 weeks) reverses bone loss in vivo in 1013/1013 mice by normalizing CXCR4 signaling in SSCs ⁸.

WHIM syndrome is a genetic proof-of-principle that dysregulated CXCR4 signaling causes multi-organ pathology — underscoring the importance of receptor desensitization in the biological context of aging, where chronic low-grade CXCL12 signaling might have analogous effects. gap/no-mechanism (whether physiological aging produces partial gain-of-function CXCR4 phenotypes is not established)

CXCR4 as HIV co-receptor

CXCR4 was identified in the mid-1990s as the obligate entry co-receptor for T-cell-tropic (X4-tropic) HIV-1 strains ¹. X4-tropic viruses use CXCR4 alongside CD4 for membrane fusion and cell entry via gp120 binding. Clinical relevance:

R5-tropic strains (using CCR5) predominate early in infection; X4-tropic emergence is associated with advanced disease and CD4 cell depletion
Patients treated with CCR5 antagonists (maraviroc) can experience outgrowth of pre-existing CXCR4-using variants
Plerixafor blocks HIV-1 X4 entry and is under investigation as a CCR5/CXCR4 dual-blockade strategy ¹

This HIV biology has no direct aging implication but historically drove much of the mechanistic CXCR4 literature.

Pharmacology and aging-context druggability

Aging-context tier 2 rationale. Plerixafor (AMD3100) and mavorixafor (Xolremdi, FDA-approved April 2024 for WHIM syndrome) are high-quality clinical-grade CXCR4 antagonists. Plerixafor’s approved indication is HSC mobilization for autologous transplantation in NHL and myeloma; mavorixafor’s approved indication is WHIM syndrome, a CXCR4 gain-of-function immunodeficiency. Neither drug is approved or in clinical trials for an aging indication. No drug currently engages CXCR4 for HSC quiescence restoration, niche rejuvenation, or any other aging-relevant therapeutic goal. Tier 2 (high-quality probe/disease-validated drug, not aging-validated) is therefore the correct designation; tier 1 would require clinical-stage evidence engaging CXCR4 for an aging phenotype.

Compound	Mechanism	Clinical status	Aging relevance
Plerixafor (AMD3100)	CXCR4 competitive antagonist (bicyclam)	FDA-approved (NHL, MM mobilization)	Provides proof-of-concept; no aging trial
Mavorixafor (Xolremdi)	CXCR4 small-molecule antagonist (oral)	FDA-approved April 2024 (WHIM syndrome; NDA218709) ⁹	Disease-validated CXCR4 blockade in a CXCR4-gain-of-function immunodeficiency; not an aging indication
Motixafortide (BL-8040)	CXCR4 antagonist peptide	Phase 3 (mobilization)	Not aging-specific
CXCR4 agonist tools	Research tools	Preclinical	HSC quiescence restoration (conceptual)

Mendelian randomization: mr-causal-evidence: not-tested — No published MR study has used CXCR4 expression quantitative trait loci (eQTLs) as genetic instruments to test causal effects on aging outcomes (HSC function, immune aging, frailty, or lifespan). Instruments are available in GTEx; this is technically feasible. gap/needs-replication

Pathway membership

cxcl12-cxcr4-pathway (stub) — canonical niche-retention and stem-cell-trafficking axis; CXCR4 is the primary signaling receptor
chemokine-signaling (stub) — broader GPCR/chemokine-receptor context
g-protein-coupled-receptor-signaling (stub) — class A GPCR superfamily signaling
pi3k-akt-pathway — activated downstream of CXCR4 via Gβγ → PI3K
mapk-pathway — ERK1/2 branch downstream of CXCR4

Key interactors

CXCL12 (SDF-1) — primary endogenous ligand; niche-retention signal; also a SASP factor in peripheral tissues
ACKR3 (CXCR7) — atypical scavenging receptor that shapes the CXCL12 gradient available to CXCR4; concurrent stub
Extracellular ubiquitin — alternative endogenous CXCR4 agonist; aging/inflammation context not characterized
HIV-1 gp120 — viral envelope protein binding drives co-receptor-mediated membrane fusion (X4-tropic HIV entry)

Limitations and gaps

Aging-specific CXCR4 expression changes on HSCs uncharacterized. Whether aged human HSCs upregulate or downregulate surface CXCR4 — and how this relates to their reduced per-cell function — is not established in systematic studies. gap/needs-replication
WHIM phenotype as aging model is speculative. C-terminal truncation mutations provide strong mechanistic proof-of-principle for receptor desensitization biology, but whether analogous desensitization-independent signaling occurs in physiological aging (e.g., from inflammatory CXCL12 saturation) is unexplored. gap/no-mechanism
MR causal evidence absent. No MR study has tested whether genetically elevated or reduced CXCR4 expression causally alters aging outcomes. gap/needs-replication
Muscle/satellite-cell CXCR4 aging not characterized. CXCR4 is expressed on satellite cells and is implicated in muscle progenitor recruitment to injury sites; whether CXCR4 expression or responsiveness changes with age in the muscle niche is not published. gap/needs-replication
GTEx aging correlation not populated. Tissue-by-age CXCR4 expression trajectories have not been extracted for this page. gap/not-populated
ACKR3 aging expression unknown. How ACKR3 expression in endothelial and other SASP-relevant cell types changes with age is not characterized (see ackr3). gap/needs-replication

Footnotes

doi:10.1016/j.phrs.2020.105010 · PMID 32544428 · Wang J, Chen H · Pharmacol Res 2020;159:105010 · review · model: human + mouse clinical and mechanistic literature · comprehensive review of plerixafor (AMD3100) mechanisms and therapeutic applications: CXCR4 signaling (Gαi, Ca²⁺, PI3K, MAPK, β-arrestin), HSC mobilization, X4 HIV-1 entry blockade, tumor-microenvironment immunomodulation, and WHIM syndrome; covers receptor desensitization biology ↩ ↩² ↩³ ↩⁴ ↩⁵
doi:10.1038/ng1149 · PMID 12692554 · Hernandez PA, Gorlin RJ, Lukens JN, … Diaz GA · Nat Genet 2003;34(1):70–74 · observational (human genetics, functional characterization) · model: WHIM syndrome patients and cell-line transfectants · first identification of gain-of-function truncating mutations in CXCR4 causing WHIM syndrome; mutant receptors showed greater Ca²⁺ flux and impaired desensitization vs wild-type CXCR4 in response to SDF-1; establishes C-terminal tail as the GRK-phosphorylation/desensitization domain essential for signal termination ↩ ↩² ↩³
doi:10.1016/j.immuni.2006.10.016 · PMID 17174120 · Sugiyama T, Kohara H, Noda M, Nagasawa T · Immunity 2006;25(6):977–988 · in-vivo (mouse, inducible Cxcr4 deletion in adult bone marrow) · model: C57BL/6-backcrossed (>7x with C57BL/6-Ly5.2*Ly5.1), MxCre-CXCR4^flox/null^, plpC-induced; n=6/group (HSC quantification), n=9/group (5-FU survival); log-rank p=0.037 (survival); p=0.026/0.031 (HSC frequency/absolute) · conditional deletion of CXCR4 in adult mice severely reduced CD34⁻c-Kit⁺Sca-1⁺Lin⁻ HSC numbers and caused exit from G₀ quiescence in primitive hematopoietic progenitors, and increased 5-FU myelotoxic-injury sensitivity; CAR cells identified and named as the primary stromal CXCL12 source in perivascular + endosteal niches; established CXCL12–CXCR4 signaling as essential for quiescent HSC pool maintenance; bronze OA (Cell.com PDF) ↩ ↩²
doi:10.2174/187152812800392760 · PMID 22280241 · Kollet O, Lapidot T · Inflamm Allergy Drug Targets 2012;11(3):169–178 · review · model: mouse + human bone marrow literature synthesis · SDF-1/CXCR4 axis as central determinant of HSC fate in bone marrow niches; integration of neural, vascular, and bone-cell signaling with CXCR4-dependent HSC trafficking ↩
doi:10.1016/j.cell.2013.04.040 · PMID 23706740 · Casanova-Acebes M, … Hidalgo A · Cell 2013;153(5):1025–1035 · in-vivo (mouse, circadian hematopoiesis) · model: C57BL/6; circadian-ablation experiments · aged (CD62L^lo^CXCR4^hi^) neutrophils re-home to bone marrow in a circadian pattern and remodel the hematopoietic niche through macrophage clearance; CXCR4 upregulation marks the neutrophil aging state and drives BM re-entry ↩
doi:10.1159/000354132 · PMID 24179472 · Fricker SP · Transfus Med Hemother 2013;40(4):237–245 · review (clinical pharmacology) · model: human clinical trial literature · physiology and pharmacology of plerixafor (AMD3100); bicyclam CXCR4 antagonist; mobilizes HSCs within hours; FDA-approved with G-CSF for NHL + myeloma autologous transplantation; durable engraftment outcomes ↩
doi:10.1182/blood-2004-08-3296 · PMID 15536153 · Balabanian K, Lagane B, Pablos JL, … Bachelerie F · Blood 2005;105(7):2449–2457 · observational (human genetics, functional characterization) · model: WHIM syndrome patients from 3 pedigrees · characterized additional C-terminal CXCR4 truncation variants beyond the Hernandez 2003 index mutation; all pedigrees showed impaired CXCR4 desensitization and internalization after CXCL12 stimulation, with marked enhancement of G-protein-dependent responses; establishes that CXCR4 desensitization failure is the common mechanism across different WHIM-associated truncation alleles ↩
doi:10.1038/s41467-023-37791-4 · PMID 37045841 · Anginot A, … Balabanian K · Nat Commun 2023;14:2058 · in-vivo + human observational · model: 19 WHIM syndrome patients (BMD measurement) + Cxcr4^1013^ knock-in mice (heterozygous +/1013 and homozygous 1013/1013); n=5–17/group for in-vivo bone analyses · gain-of-function CXCR4 WHIM mutations (Cxcr4^S338X^/1013) cause decreased bone mineral density in 5/19 (25%) patients (≥1 site at or below T-score −1.0) and an osteoporotic bone phenotype in mice in a Cxcr4 allele dose-dependent manner; mechanism: impaired CXCR4 desensitization disrupts cell cycle progression and osteogenic commitment of skeletal stromal cells (SSCs), while increasing pro-osteoclastogenic capacity; AMD3100 (5 mg/kg i.p., 3 weeks) reverses in vivo bone loss in 1013/1013 mice by normalizing SSC osteogenic fate; gold OA (Nature Communications) ↩
FDA NDA218709 · Mavorixafor (Xolremdi) · X4 Pharmaceuticals · FDA-approved 2024-04-26 for WHIM syndrome (Orphan Drug, Priority Review) · first oral small-molecule CXCR4 antagonist approved for a CXCR4-gain-of-function immunodeficiency indication; increases neutrophil counts and reduces infection rates; not an aging indication ↩

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