FGF23-alpha-Klotho endocrine axis in phosphate and vitamin D homeostasis

The bone-kidney endocrine axis through which the hormone FGF23 lowers serum phosphate and active vitamin D. In osteocytes and osteoblasts FGF23 is synthesised as a secreted proprotein whose activity is set post-translationally at a proprotein-convertase site (R176XXR179): the Golgi polypeptide GalNAc-transferase GALNT3 O-glycosylates Thr178, which shields the site from furin-type cleavage and permits secretion of intact, active hormone, whereas FAM20C phosphorylation of Ser180 opposes that glycosylation and favours cleavage into inactive N- and C-terminal fragments. Circulating intact FGF23 acts on kidney (and parathyroid) cells that express the single-pass transmembrane co-receptor alpha-Klotho, which tethers the FGF23 C-terminal tail to the FGFR1c ectodomain and converts the canonical receptor tyrosine kinase into a high-affinity FGF23 receptor. Signalling through the Klotho-FGFR1c complex reduces proximal-tubule sodium-phosphate cotransport (NaPi-2a/SLC34A1 and NaPi-2c/SLC34A3), causing phosphaturia, and lowers 1,25-dihydroxyvitamin D by suppressing the 25-hydroxyvitamin D 1-alpha-hydroxylase CYP27B1 and inducing the 24-hydroxylase CYP24A1. Loss of function at the hormone, glycosylation or co-receptor step causes hyperphosphatemic familial tumoral calcinosis, and cleavage-resistant FGF23 or excess osteocytic FGF23 production causes hypophosphatemic rickets.

MODULE:fgf23_klotho_phosphate_homeostasisDRAFTSignaling Pathwaymodules/fgf23_klotho_phosphate_homeostasis.yaml
phosphate ion homeostasisGO:0055062
GO:0055062
phosphate ion homeostasis
The axis is the principal hormonal system lowering serum phosphate; FGF23, alpha-Klotho and GALNT3 reviews all carry this process or its regulation.
PMID:17086194
Klotho converts canonical FGF receptor into a specific receptor for FGF23.
Klotho binds FGF23 and, with FGFR1(IIIc), reconstitutes the FGF23 receptor (basis for the co-receptor and receptor annotons).
PMID:29342138
α-Klotho is a non-enzymatic molecular scaffold for FGF23 hormone signalling.
Crystal structure of the FGF23-FGFR1c-alpha-Klotho complex: Klotho tethers FGFR1c by its D3 domain and FGF23 by its C-terminal tail.
PMID:16638743
Polypeptide GalNAc-transferase T3 and familial tumoral calcinosis. Secretion of fibroblast growth factor 23 requires O-glycosylation.
Secretion of intact FGF23 requires GALNT3-dependent O-glycosylation (basis for the glycosylation step and its positive effect on the hormone).
PMID:15040831
FGF-23 is a potent regulator of vitamin D metabolism and phosphate homeostasis.
FGF23 reduces renal 1alpha-hydroxylase mRNA, increases 24-hydroxylase mRNA and reduces NaPi-2a, establishing the renal effector arm.
file:human/FGF23/FGF23-ai-review.yaml
FGF23 gene review (human)
Hormone activity (GO:0005179) with negative regulation of sodium-dependent phosphate transport and of vitamin D biosynthesis are the core functions of UniProtKB:Q9GZV9.
file:human/KL/KL-ai-review.yaml
KL gene review (human)
Coreceptor activity (GO:0015026) at the plasma membrane is the core function of alpha-Klotho (UniProtKB:Q9UEF7).
file:human/GALNT3/GALNT3-ai-review.yaml
GALNT3 gene review (human)
Polypeptide N-acetylgalactosaminyltransferase activity (GO:0004653) at the Golgi membrane, with FGF23 Thr178 as its non-redundant physiological substrate, is the core function of UniProtKB:Q14435.
file:human/FGFR1/FGFR1-ai-review.yaml
FGFR1 gene review (human)
Fibroblast growth factor receptor activity (GO:0005007) at the plasma membrane is a core function of UniProtKB:P11362 (review in draft).
file:human/CYP27B1/CYP27B1-ai-review.yaml
CYP27B1 gene review (human)
Calcidiol 1-monooxygenase activity (GO:0004498) at the mitochondrial inner membrane is the core function of UniProtKB:O15528.
5Nodes
4Parts
0Variant Sets
0Variants
5Annotons
3Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:fgf23_klotho_phosphate_homeostasis deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Function consistency and core coverage

5 gene-function check(s) reflected in core functions · 0 conflict(s) · 0 evidence or coverage gap(s) (5 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
galnt3_activity GALNT3
UniProtKB:Q14435
Gene 		polypeptide N-acetylgalactosaminyltransferase activity
GO:0004653 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
fgf23_activity FGF23
UniProtKB:Q9GZV9
Gene 		hormone activity
GO:0005179 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
klotho_activity KL
UniProtKB:Q9UEF7
Gene 		coreceptor activity
GO:0015026 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
fgfr1_activity FGFR1
UniProtKB:P11362
Gene 		fibroblast growth factor receptor activity
GO:0005007 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
cyp27b1_activity CYP27B1
UniProtKB:O15528
Gene 		calcidiol 1-monooxygenase activity
GO:0004498 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).

Feedback loops (advisory, prose-driven: a regulator counts as "feedback" when its own description says so)

✓ every negative regulator described as feedback has an upstream edge closing the loop.

  • fgf23_receptor_step ⊣ calcitriol_synthesis_step [closed]
    fgf23_receptor_step is regulated by fgf23_hormone_step: feedback loop closed

Gene-review completeness (5/5 grounded genes reviewed)

4 complete review(s) · 5 with deep research · 0 missing review · 0 reviewed but lacking deep research

Gene Review Complete Deep research
CYP27B1 O15528 ✓ ✓ ✓
FGF23 Q9GZV9 ✓ ✓ ✓
FGFR1 P11362 ✓ ✓ ✓
GALNT3 Q14435 ✓ 34/35 ✓
KL Q9UEF7 ✓ ✓ ✓

Details

FGF23-alpha-Klotho phosphate homeostasis axisSignaling Pathwayfgf23_klotho_phosphate_homeostasis
phosphate ion homeostasisGO:0055062

Scope is the bone-kidney endocrine arm; parathyroid suppression of PTH secretion by FGF23 is noted in the FGF23 review but not modelled as a part. Parts are restricted to genes with ai-gene-review reviews (FGF23, GALNT3, KL, FGFR1 in draft, CYP27B1). Steps not yet modelled because the human genes have no review: FAM20C phosphorylation of FGF23 Ser180 (opposes GALNT3 glycosylation), furin-type proprotein-convertase cleavage at R176XXR179 (inactivation), the proximal-tubule sodium-phosphate cotransporters SLC34A1 (NaPi-2a) and SLC34A3 (NaPi-2c) whose abundance FGF23 reduces, and CYP24A1 (24-hydroxylase) which FGF23 induces. GO has no term for regulation of vitamin D catabolism (see the FGF23 review's suggested questions). Osteocyte regulators of FGF23 expression (DMP1, PHEX, ENPP1) are upstream regulatory context; DMP1 is reviewed but its mechanism of restraining FGF23 is undefined, so it is not modelled as a part. Generic FGF-FGFR signalling, including the endocrine FGF ligand annoton, is in the fgfr_signaling module; this module is the hormone-specific physiological axis. Family-level PANTHER ids are deliberately omitted; participants are concrete human gene products.

Connections

GALNT3 O-glycosylation of Thr178 protects FGF23 from furin-type cleavage, allowing secretion of intact, active hormone.
fgf23_hormone_step -> fgf23_receptor_step Positively Regulates
Intact FGF23 binds and activates the alpha-Klotho-FGFR1c receptor complex.
Klotho-FGFR1c signalling represses renal CYP27B1 expression, lowering calcitriol production.
Part 1: protective O-glycosylation of the FGF23 proprotein-convertase site in bone cells
GALNT3 O-glycosylation of FGF23 Thr178Reactionfgf23_glycosylation_step

Annotons

GALNT3: polypeptide N-acetylgalactosaminyltransferase
galnt3_activity
Participant: Gene Product: GALNT3
Gene Product:
GALNT3UniProtKB:Q14435

Function

polypeptide N-acetylgalactosaminyltransferase activityGO:0004653
Substrates: UDP-N-acetyl-alpha-D-galactosamine FGF23 Thr178 (within the R176HTR179 convertase site)
Products: O-GalNAc-Thr178 FGF23 UDP

Processes

protein O-linked glycosylation via N-acetylgalactosamineGO:0016266

Locations

Golgi membraneGO:0000139

Golgi-resident GalNAc-transferase that glycosylates FGF23 at Thr178, protecting the R176XXR179 site from furin-type cleavage so that intact, bioactive FGF23 is secreted. Loss of function causes hyperphosphatemic familial tumoral calcinosis with low intact FGF23.

Part 2: circulating phosphaturic hormone
Intact FGF23 acting as an endocrine hormoneMolecular Functionfgf23_hormone_step

Annotons

FGF23: phosphaturic hormone
fgf23_activity
Participant: Gene Product: FGF23
Gene Product:
FGF23UniProtKB:Q9GZV9

Function

hormone activityGO:0005179

Processes

negative regulation of sodium-dependent phosphate transportGO:2000119 negative regulation of vitamin D biosynthetic processGO:0010957

Locations

extracellular regionGO:0005576

Bone-derived secreted hormone that binds the Klotho-FGFR1c complex on target cells. Its activity is set by the balance between GALNT3 O-glycosylation (protective) and FAM20C phosphorylation plus furin-type cleavage (inactivating) at the R176XXR179 site.

Part 3: Klotho-dependent FGF23 receptor complex on renal tubular (and parathyroid) cells
alpha-Klotho-FGFR1c FGF23 receptor complexSignaling Pathwayfgf23_receptor_step

Annotons

alpha-Klotho: obligate FGF23 co-receptor
klotho_activity
Participant: Gene Product: KL
Gene Product:
KLUniProtKB:Q9UEF7

Function

coreceptor activityGO:0015026

Processes

fibroblast growth factor receptor signaling pathwayGO:0008543

Locations

plasma membraneGO:0005886

Transmembrane co-receptor whose expression defines the FGF23 target tissues (kidney tubule, parathyroid); confers high-affinity FGF23 binding on FGFR1c. Loss of function causes hyperphosphatemic tumoral calcinosis with high FGF23 (end-organ resistance).

FGFR1 (IIIc isoform): signalling receptor tyrosine kinase
fgfr1_activity
Participant: Gene Product: FGFR1
Gene Product:
FGFR1UniProtKB:P11362

Function

fibroblast growth factor receptor activityGO:0005007

Processes

fibroblast growth factor receptor signaling pathwayGO:0008543

Locations

plasma membraneGO:0005886

Principal FGF23 signalling receptor (FGFR3c and FGFR4 contribute less); Klotho-bound FGFR1c activates FRS2-ERK1/2 signalling that reduces NaPi-2a/NaPi-2c and represses CYP27B1. General FGFR signalling is modelled in the fgfr_signaling module.

Part 4: renal vitamin D activation step repressed by FGF23 signalling
CYP27B1 1-alpha-hydroxylation of calcidiol to calcitriolReactioncalcitriol_synthesis_step

Annotons

CYP27B1: 25-hydroxyvitamin D 1-alpha-hydroxylase
cyp27b1_activity
Participant: Gene Product: CYP27B1
Gene Product:
CYP27B1UniProtKB:O15528

Function

calcidiol 1-monooxygenase activityGO:0004498
Substrates: calcidiol (25-hydroxyvitamin D3)
Products: calcitriol (1-alpha,25-dihydroxyvitamin D3)

Processes

vitamin D3 biosynthetic processGO:1901755

Locations

mitochondrial inner membraneGO:0005743

Proximal-tubule mitochondrial P450 that produces calcitriol; its transcription is suppressed by FGF23-Klotho-FGFR signalling, closing the feedback loop in which calcitriol induces FGF23 in bone.