Figure 6. Combined spinal cord and peripheral nerve injury inhibits methyl cycling, SAM bioavailability, and MS activity; and MS inhibition abolishes folate-mediated axonal regeneration.
Injury to the rat spinal cord and left sciatic nerve causes a diminution in the SAM/SAH ratio (n = 6 [normal]; n = 6 [injured]. Student’s t test; mean ± SEM; *P < 0.05) (A), suggesting a direct effect of injury on DNA methylation. To confirm this observation, spinal axonal regeneration was measured in the SCRM model on the “conditioned” side after treatment with N2O. N2O is a specific inhibitor of the MS enzyme; thus it interferes with the entry of active folate into the methionine methylation cycle. N2O suppressed axon growth to below control levels. n = 10 (untreated); n = 7 (FA); n = 8 (N2O); n = 8 (FA). One-way ANOVA with Bonferroni’s correction; mean ± SEM; *P < 0.05 (B). To determine whether N2O suppresses MS, MS activity and SAH levels were measured in the spinal cord after N2O exposure. N2O suppressed MS activity (C) with no change in the enzyme protein levels by Western analysis (not shown) and caused a corresponding rise in SAH (D). Note the drop in MS activity after injury and after N2O treatment. Recovery from the N2O effect occurs by 3 days (C). N, no injury; S, spinal cord injury; N0 and S0, No N2O treatment; N1, N2O given the day before removing the spinal tissue; N2, N2O given 3 days before removing the spinal tissue; S1, N2O treatment every other day for 2 weeks; S2, N2O given 2 weeks before removing spinal tissue. n = 3 in each group; each specimen was run in triplicate. One-way ANOVA with Bonferroni’s correction; mean ± SEM, *P < 0.05.