Heme deficiency is associated with senescence and causes suppression of N-methyl-D-aspartate receptor subunits expression in primary cortical neurons

Heme is a crucial component of many pharmacological and toxicological processes, and studies have suggested that heme deficiency may play a role in cellular ageing. A model of ageing neurons was established using prolonged cultures of BALB/c mouse primary cortical neurons. Aged neurons displayed a senescent phenotype and a marked up-regulation of cathepsin-L expression. Down-regulation of the candidate neuron-specific genes for N-methyl-D-aspartate (NMDA) receptor subunits (NMDA zeta 1 and -epsilon 2) and neurofilament light peptide (NF-L) were found to be characteristic of the aging process as reported in vivo (Brain Res 907: 71-83, 2001; Brain Res Mol Brain Res 99: 40-45, 2002). In contrast, the genes for the controlling enzymes of heme synthesis and degradation (5-aminolevulinate synthase 1 and heme oxygenase 1, respectively) were up-regulated, implying depletion of a regulatory heme pool. Inhibition of heme synthesis (by 70-80%) at different enzymic steps by succinyl acetone and N-methylprotoporphyrin IX resulted in the earlier lowered expression of NMDA zeta 1 and -epsilon 2 and NF-L. Exogenous hemin added to heme- depleted cells rescued the expression of these neuron-specific genes. Culture of cortical neurons from BALB/c Fech(m1Pas) mutant mice demonstrating depressed heme synthesis showed premature senescence and reduced expression of NMDA zeta 1 and -epsilon 2 receptor subunits and NF-L compared with wild-type cells. Our findings suggest that reduced availability of heme in neurons associated with senescence may have significant effects on synaptic function.