EXPRESSION OF GLUTAMATE RECEPTORS AND TRANSPORTERS IN HUMAN SUBCHONDRAL BONE IN OSTEOARTHRITIS

Abstract

Purpose of the study: Since the neurotransmitter glutamate mediates nociceptive and pathological processes in arthritis, we have investigated how glutamate receptor and transporter expression varies with anatomical site or disease severity in subchondral bone of patients with osteoarthritis.

Methods and results: Subchondral bone was sampled from tibial cuts derived from total knee arthroplasty (n=2, TKR, Kellgren Lawrence grade 3) and from tibial drill hole sites from high tibial osteotomy (n=2, HTO, KL grades 2 and 3) for osteoarthritis. RNA was extracted, reverse transcribed and RT-PCR performed for the housekeeping gene GAPDH, the glutamate transporter EAAT-1, and glutamate receptors (NR2A and KA1). Quantitative RT-PCR assessed differences in the expression of EAAT-1, a dominant negative splice variant called EAAT-1ex9skip and osteocalcin after nor-malisation to GAPDH.

Good quality RNA was obtained from bone cores removed from drill holes during HTO surgery, with GAPDH, EAAT-1, NR2A and KA1 expression detected. Osteocalcin expression was high indicating RNA was derived from osteoblasts and osteocytes, but did not vary with anatomical site or disease status. End-stage RT-PCR indicated differential expression of EAAT-1 between medial and lateral bone samples in total knee arthroplasty, however these differences were not significant by quantitative RT-PCR. In one patient, EAAT-1 expression was significantly reduced in the anterior zone versus the middle or posterior zones (ANOVA, p< 0.001). EAAT-1ex9skip represented a significant proportion of the total EAAT-1 mRNA expression in bone from TKR patients, but appeared less abundant in HTO samples.

Conclusion: We have shown for the first time that glutamate transporters and receptors are highly expressed in subchondral bone of patients with osteoarthritis and that EAAT-1 expression may vary with anatomical location and pathology. Activation of these receptors and transporters by the increased synovial fluid concentrations of glutamate that occur in arthritis may contribute to pathological changes and nociception.