Lastly, we observed an association with age. In particular, with increasing age, the fractions Ex3 and Ex4 significantly increased and the fractions of some nonneuronal types decreased (Fig. 2F and fig. S19). These changes may be associated with differential expression of specific genes, e.g., the gene for somatostatin (SST), known to be associated with aging and neurotransmission (Fig. 2F) (30). Also, SST exhibits increasing promoter methylation with age, perhaps explaining its decreasing expression. Other genes known to be associated with brain aging, such as those for EGR1 (early growth response) and CP (ceruloplasmin), exhibit different trends (Fig. 2F and figs. S20 and S21) (21, 31).
