Expression of GCH1 and SPR increases seven days following SNI.
Like trusty sidekicks in old Westerns, cofactors are supposed to quietly ride shotgun in enzymatic reactions. However, Tegeder et al. report that the enzymatic cofactor tetrahydrobiopterin and the enzymes involved in its production exacerbate pain in a recent article in Nature Medicine.
Tetrahydrobiopterin is a cofactor in nitric oxide, dopamine and serotonin synthesis and phenylalanine metabolism. Tetrahydrobiopterin is produced from guanosine triphosphate (GTP) via several serial enzymatic reactions. The authors previously identified two of the enzymes involved in tetrahydrobiopterin synthesis, GTP cyclohydrolase (Gch1) and sepiapterin reductase (Spr), in a high-throughput search for genes induced by sciatic nerve injury in the rat dorsal root ganglion (DRG).
Neuropathic pain is caused by nerve injury and can be difficult to treat. People with neuropathic pain commonly have allodynia, which is pain in response to stimuli that are normally not painful, and hyperalgesia, which is excessive pain in response to painful stimuli. Spared nerve injury (SNI), which induces neuropathy in the rat, increased expression of Gch1 and Spr mRNA in rat DRGs. SNI also increased neopterin and biopterin, which are both products in the tetrahydrobiopterin synthesis pathway, in rat DRGs.
Tetrahydrobiopterin synthesis inhibitors reduced neuropathic and inflammatory pain. The GCH1 inhibitor DAHP and the SPR inhibitor NAS blocked SNI-induced increases in biopterin production and mechanical and cold allodynia. In rats, the injection of inflammatory agents into the paw causes pain. DAHP and NAS blocked inflammation-induced increases in heat hyperalgesia and biopterin production.
Tetrahydrobiopterin treatment increased inflammation-induced heat hyperalgesia and SNI-induced mechanical and cold allodynia. Does tetrahydrobiopterin exacerbate pain by driving excessive enzymatic activity? Nitric oxide increased in SNI relative to uninjured rats, and DAHP treatment blocked SNI-induced increases in nitric oxide. The authors blocked SNI-induced cold and mechanical allodynia with the nitric oxide synthase inhibitor L-NAME, suggesting that tetrahydrobiopterin exacerbates pain by increasing nitric oxide production.
The authors found 15 single nucleotide polymorphisms (SNPs) in GCH1 in people. Five SNPs associated with persistent leg pain one year after surgery to repair a herniated disk. One haplotype, or set of SNPs, associated with reduced pain and had an allelic frequency of 15%. People homozygous for the haplotype had less sensitivity to thermal and mechanical pain than people heterozygous or homozygous for the more common allele.
The pain-associated SNPs localized to the GCH1 promoter, suggesting that regulation of GCH1 expression might be important in pain. Expression of GCH1 mRNA was similar in immortalized leukocytes from people with zero, one and two copies of the 'pain-protective' allele. The adenylyl cyclase activator forskolin increased the expression of GCH1 mRNA in leukocytes from people without the pain-protective haplotype, but did not alter the expression of GCH1 mRNA in leukocytes from people with at least one copy. Forskolin stimulated biopterin release in leukocytes from people without the pain-protective allele, but it reduced biopterin release in leukocytes from people homozygous for the pain-protective allele.
These data indicate that GCH1 regulation is important in pain and may suggest that DAHP could be used to treat people suffering from chronic neuropathic pain.