With a graying population and increasing longevity, it is essential to identify life transition in later years and discern heterogeneity among older people. Subclassifying the elderly population to inspect the subdivisions for pathophysiological differences is particularly important for the investigation of age-related illnesses. For this purpose, using 24- and 28-month-old mice to represent the “young-old” and “old-old”, respectively, we compared their skeletal muscle transcriptomes and found each in a distinct stage: early/gradual (E-aging) and late/accelerated aging phase (L-aging). Principal component analysis showed that the old-old transcriptomes were largely disengaged from the forward transcriptomic trajectory generated in the younger-aged group, indicating a substantial change in gene expression profiles during L-aging. By calculating the transcriptomic distance, it was found that the 28-month group was closer to the two-month group than to the 24-month group. The divergence rate per month for the transcriptomes was the highest in L-aging, twice as fast as the rate in E-aging. Indeed, many of the L-aging genes were significantly altered in transcription, although the changes did not seem random but rather coordinated in a variety of functional gene sets. Of 2,707 genes transcriptionally altered during E-aging, two-thirds were also significantly changed during L-aging, to either downturning or upturning way. The downturn genes were related to mitochondrial function and translational gene sets, while the upturn genes were linked to inflammation-associated gene sets. Our results provide a transcriptomic muscle signature that distinguishes old-old mice from young-old mice. This can help to methodically examine muscle disorders in the elderly.

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SETDB1 is a histone H3-lysine 9-specific methyltransferase that fulfills epigenetic functions inside the nucleus; however, when overexpressed, SETDB1 majorily localizes in the cytoplasm. SETDB1 has a single nuclear-localization-signal (NLS) motif and two successive nuclear-export-signal (NES1 and NES2) motifs in the N-terminus, suggesting that SETDB1 localization is the consequence of a balance between the two antithetic motifs. Here, we performed a series of motif deletions to characterize their effects on the cellular movement of SETDB1. Given the cytoplasmic localization of GFP-SETDB1 in the whole form, without the NES motifs, GFP-SETDB1 was not nuclear, and 3xNLS addition plus NES removal held the majority of GFP-SETDB1 within the nucleus. The results indicated that the cytoplasmic localization of GFP-SETDB1 is the combined result of weak NLS and robust NESs. In ATF7IP-overexpressing cells, GFP-SETDB1 entered the nucleus only in the presence of the NES1 motif; neither the NES2 nor NLS motif was necessary. Since subcellular fractionation results showed that ATF7IP was nuclear-only, an intermediary protein may interact specifically with the NES1 motif after stimulation by ATF7IP. When GFP-SETDB1 had either NES1 or NES2, it was precipitated (in immunoprecipitation) and colocalized (in immunofluorescence) with ATF7IP, indicating that GFP-SETDB1 interacts with ATF7IP through the NES motifs in the nucleus. The regulated nuclear entry of SETDB1 is assumed to set a tight restriction on its abundance within the nucleus, thereby ensuring balanced nuclear SETDB1 levels.