SLC15A1 and amyotrophic lateral sclerosis: Are the mitochondrial alterations evident in ALS associated with changes in the levels of the peptide transporters, PEPT1/2, in the mitochondrial membrane, thereby impacting the levels of exogenous melatonin uptake into mitochondria via PEPT1/2 and the sulfation metabolites of melatonin via the organic anion transporter (OAT)3, as shown in cancer cells [204]?