A breakthrough discovery reveals how the SLC45A4 gene fine-tunes polyamine transport in sensory neurons, reshaping our understanding of pain and opening new paths for targeted therapies.
Study: SLC45A4 is a pain gene encoding a neuronal polyamine transporter. Image Credit: Gorodenkoff / Shutterstock
In a recent study published in the journal Nature, an international team of researchers showed that solute carrier family 45 member 4 (SLC45A4) is a pain gene that encodes a neuronal polyamine transporter.
Chronic pain affects one in five adults and has an adverse effect on the quality of life. Unfortunately, available treatments are often inadequate, with poor tolerability and efficacy. Polyamines, such as spermidine, spermine, and putrescine, are regulatory metabolites reported to contribute to chronic pain. They play crucial roles in nucleic acid synthesis and stability, cell signaling, and growth.
Polyamines are implicated in neurological disorders, such as stroke and epilepsy. They can regulate neuronal excitability through ion channel interactions and have been linked to pain. Polyamines exhibit altered levels in pain states in humans and modulate pain behavior in animal models. Nevertheless, the systems influencing polyamine transport in the nervous system remain unclear.
The study and findings
In the present study, researchers showed that SLC45A4 encodes a neuronal membrane polyamine transporter genetically linked to human chronic pain. First, they performed a genome-wide association study (GWAS) of the enhanced pain phenotyping questionnaire data from the United Kingdom Biobank (UKB).
The team identified 29 single-nucleotide variants (SNVs) at a genome-wide significance level associated with pain intensity. These included two independent loci with lead SNVs: rs3905668 near the MSL complex subunit 2 (MSL2) gene and rs10625280 (previously misstated as rs1062580) that maps to the SLC45A4 gene. This association was replicated in the Million Veteran Program (MVP) and FinnGen cohorts. rs10625280 was located within an SLC45A4 intron, with a subset of SNVs exhibiting linkage disequilibrium. One of these, a missense variant, rs3739238, showed significant associations with pain intensity and broader health traits like osteoarthritis and immune dysfunction in phenome-wide analyses.
SLC45A4 has been proposed as a proton-coupled sucrose transporter based on homology to the Arabidopsis thaliana sucrose transporter, but recent studies report conflicting functions. As such, the researchers performed a correlation analysis between expression and metabolomics datasets to identify possible substrates. SLC45A4 expression in over 1,000 cell lines was positively correlated with γ-aminobutyric acid (GABA) levels. Cryo-EM structural analysis revealed a unique autoinhibitory “plug domain” essential for polyamine recognition and transport regulation.
Next, the team analyzed substrates involved in GABA synthesis via the arginine-ornithine-putrescine pathway. They found a marked reduction in thermal stability of SLC45A4 in the presence of biogenic amines, with spermidine and spermine inducing the most significant response. Cell-based assays confirmed that SLC45A4 is a broad-specificity polyamine transporter with differential affinity for substrates (highest for putrescine and cadaverine). Next, the team characterized the expression of SLC45A4 in neural tissues.
Published mouse RNA-sequencing datasets indicate predominant Slc45a4 mRNA expression in dorsal root ganglia (DRG) sensory neurons. Quantitative reverse-transcription polymerase chain reaction confirmed enriched expression of Slc45a4 in the DRG across the sensory neuraxis. Published human data also indicate SLC45A4 expression in sensory neurons. Further, Slc45a4-/- knockout (KO) mice were generated to evaluate the links between pain perception and SLC45A4 function. These mice exhibited a transient “salt and pepper” coat color defect linked to disrupted melanoblast differentiation and spermidine’s established role in melanin production, supporting spermidine’s role in melanin production.
All polyamines were abundant in the spinal cord, DRG, and brain of wild-type mice. However, Slc45a4 loss increased DRG putrescine and reduced spinal spermidine levels. While all polyamines were detected in the plasma of wild-type and KO mice, KO mice had elevated Spd levels. Next, the team performed behavioral testing at 10 weeks of age when coat color had normalized. There were no deficits in open-field behavior tests of exploration and locomotion.
However, in the rotarod task, KO mice had a higher maximum final speed and latency to fall from the rotarod than wild-type mice, indicating increased motor endurance that may be associated with reduced ventral horn GABAergic inhibition. Moreover, KO mice had a higher latency to respond to being placed on a hot plate than wild-type mice, suggesting hyposensitivity. However, no differences were observed in the reflex withdrawal latency to dry ice. Since polyamines are involved in GABA synthesis, the team investigated whether GABA levels were altered in KO mice.
While brain GABA levels were normal in KO mice, spinal GABA levels were significantly reduced. Further analyses showed that GABA levels were significantly lower only in the ventral horn of KO mice. A patch-clamp analysis of sensory neurons was performed to explore peripheral pain modulation mechanisms, focusing on nociceptors that bind to isolectin B4 (IB4) (predominantly non-peptidergic) and those that do not bind to IB4 (predominantly peptidergic).
While IB4+ nociceptors were normal in KO mice, the suprathreshold excitability of IB4⁻ nociceptors and C-polymodal nociceptors in response to dynamic and static current injections was selectively reduced in KO mice. Moreover, C-mechano-heat-nociceptors, also called C-polymodal nociceptors, in KO mice showed selective and marked hypoexcitability in response to suprathreshold mechanical and heat stimuli. Mechanical pain pathways remained intact, indicating modality-specific effects.
Conclusions
The findings demonstrate that SLC45A4 encodes a neuronal membrane polyamine transporter with genetic associations to human pain. It is expressed in sensory neurons, and its ablation alters polyamine homeostasis, pain perception, and thermal coding in mice. Mechanical pain responses following SLC45A4 KO remained intact, likely due to preserved functions of other mechanoreceptor populations. Overall, SLC45A4 may be a potential molecular target for modulating thermal and chemical pain perception while preserving mechanical sensitivity.