GPNMB protein (Glycoprotein Nonmetastatic Melanoma Protein B, also known as Osteoactivin or DC-HIL) is a type I transmembrane glycoprotein encoded by the GPNMB gene. GPNMB is highly expressed in microglia, macrophages, osteoclasts, and melanocytes, where it functions as an anti-inflammatory receptor, phagocytosis mediator, and neuroprotective factor. In the context of neurodegeneration, GPNMB is strongly upregulated in disease-associated microglia (DAM) and lipid-laden microglia in Alzheimer's disease, Parkinson's disease, and ALS. GPNMB is a top GWAS hit for PD risk, and its soluble ectodomain (sGPNMB) serves as both a cerebrospinal fluid biomarker and a neuroprotective signaling molecule.
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| Protein Name | Glycoprotein Nonmetastatic Melanoma Protein B |
| Gene | GPNMB |
| UniProt ID | Q14956 |
| PDB IDs | 7SJJ |
| Molecular Weight | 63.5 kDa (predicted), 90-120 kDa (glycosylated) |
| Subcellular Localization | Cell surface, endosomes, lysosomes, melanosome-like organelles |
| Protein Family | PMEL/NMB family (type I transmembrane glycoproteins) |
| Associated Diseases | PD, AD, ALS, Gaucher disease |
¶ Domain Architecture
GPNMB is a 572-amino acid type I transmembrane glycoprotein:
- Signal peptide (residues 1-22)
- Extracellular domain (residues 23-480): Contains the PKD (polycystic kidney disease) domain (residues 23-247) with RGD integrin-binding motif, and a heavily glycosylated mucin-like region (12 N-glycosylation sites)
- Transmembrane domain (residues 481-501): Single-pass alpha helix
- Cytoplasmic tail (residues 502-572): Contains a di-leucine endosomal sorting motif and a half ITIM (immunoreceptor tyrosine-based inhibitory motif)
- Soluble ectodomain (sGPNMB): Cleaved by ADAM10 near the transmembrane domain; the shed ectodomain signals through CD44 and integrins
- Heavy N-glycosylation: 12 N-linked glycans on the ectodomain; glycosylation patterns differ between cell types
- Proteolytic shedding: ADAM10 constitutive cleavage releases sGPNMB; regulated shedding is enhanced by inflammatory stimuli
- Lysosomal targeting: Di-leucine motif in the cytoplasmic tail directs GPNMB to late endosomes/lysosomes
GPNMB is a key anti-inflammatory receptor on microglia and macrophages:
- Binds CD44 on neighboring cells to suppress pro-inflammatory cytokine production
- Signals through the cytoplasmic ITIM-like motif to recruit SHP-1/SHP-2 phosphatases
- Downregulates NF-κB and MAPK signaling, reducing TNF-α, IL-6, and IL-1β production
- sGPNMB acts as a paracrine neuroprotective factor by binding neuronal CD44 and integrins
¶ Phagocytosis and Lipid Handling
- GPNMB promotes microglial phagocytosis of apoptotic neurons, myelin debris, and amyloid-beta
- Essential for lipid handling in lipid-laden microglia — GPNMB+ microglia efficiently process cholesterol-rich myelin debris
- GPNMB expression is strongly induced by lysosomal stress and lipid overload
- Localizes to lysosomes and melanosome-like organelles
- Participates in lysosomal biogenesis; transcriptionally regulated by TFEB/TFE3
- Supports lysosomal membrane integrity under stress conditions
GPNMB is one of the strongest GWAS risk genes for PD:
- rs199347 (intronic variant) increases PD risk (OR ~1.1) by altering GPNMB expression
- GPNMB is massively upregulated in substantia nigra microglia in PD patients
- sGPNMB levels are elevated in PD CSF and correlate with disease severity
- α-Synuclein aggregates induce GPNMB in activated microglia
- sGPNMB promotes neuronal survival through ERK/AKT signaling
- GBA1 mutations (Gaucher/PD overlap) drive lysosomal stress and GPNMB induction
- GPNMB is a hallmark marker of disease-associated microglia (DAM) — the microglial phenotype found around amyloid plaques
- GPNMB+ microglia show enhanced phagocytic capacity but also contribute to chronic inflammation when overwhelmed
- sGPNMB levels are elevated in AD CSF and plasma
- GPNMB expression correlates with TREM2 and APOE expression in plaque-associated microglia
- GPNMB is upregulated in spinal cord microglia of ALS patients and SOD1-G93A mice
- sGPNMB may serve as a CSF biomarker for microglial activation in ALS
- GPNMB promotes phagocytic clearance of motor neuron debris
- GBA1-deficient macrophages massively upregulate GPNMB due to glucosylceramide accumulation
- GPNMB is one of the most elevated proteins in Gaucher disease serum
- The GBA1-GPNMB axis connects lysosomal storage disorders to PD risk
- sGPNMB as a therapeutic: Recombinant sGPNMB ectodomain administration is neuroprotective in PD models
- Anti-GPNMB antibodies: Glembatumumab vedotin (CDX-011) targets GPNMB-expressing cells in cancer; repurposing for neuroinflammation is under investigation
- GPNMB as a biomarker: CSF and plasma sGPNMB levels track microglial activation and may serve as pharmacodynamic biomarkers for anti-inflammatory therapies