MTHFR and hyperhomocysteinemia: Slc38a2 expression has been found to be induced in rat brain subjected to prolonged systemic hypertonicity [32] and upregulated in hypothalamic cell line N25/2 subjected to complete amino acid starvation [33], while Mthfd2, the regulatory enzyme of mitochondrial folate cycle, was found to be upregulated in mice with methylenetetrahydrofolate reductase deficiency and hyperhomocysteinemia [34] and linked to DNA polymerase gamma expression, a common cause of mitochondrial neurodegeneration [35].