Expression and regulation of the methionine cycle genes in the mouse cochlea

Abstract

The impairment in cochlear homocysteine (Hcy) metabolism has been associated with hearing loss. Epidemiological studies revealed a correlation between hearing loss and increased plasma Hcy. Hcy represents a key point in the remethylation pathway of the methionine cycle. Betaine homocysteine S-methyltransferases (BHMT) are responsible for the remethylation of Hcy. Cochlear BHMT and Hcy metabolism have been reported to have organ-specific traits. Hearing was evaluated by auditory brainstem response (ABR) recording of 2-3 month-old mice of different strains. Cochlear samples were used for analysis of selected methionine cycle enzymes by RT-qPCR and Western blotting. The methionine cycle was studied in control and VS noise-exposed C57BL/6J mice (105 dB SPL, 2-20 kHz, 30 min). Two days post-noise, exposed mice showed increased ABR thresholds and reduced cochlear expression of Bhmt and Bhmt2. No changes were observed in Mat2a, Mat2b and Ahcy expression or in BHMT, BHMT2, MATα2, MATß and AHCY levels. As reported, a BHMT band of 68 kDa was found in C57BL/6J cochleae. In contrast, the canonical BHMT band of 45 kDa was detected in Ola:MF1 mice, suggesting a strain-specific regulation. To further explore the role of BHMT, we studied the hearing phenotype of Bhmt+/+, Bhmt+/- and Bhmt-/- C57BL/6x129sv mice. Noise exposure caused a significant and irreversible increase in ABR thresholds in Bhmt-/- with respect to Bhmt+/+ mice. Bhmt-/- but not Bhmt+/+ mouse cochleae showed significantly increased levels of Bhmt2, no differences were detected in Mat2a, Mat2b and Ahcy. These data suggest that BHMT plays a central role in cochlear methionine metabolism and that Bhmt2 could carry out a compensatory role in cochlear protection against noise injury in the absence of Bhmt.