Novel Alterations in Chromatin Structure during Aging Reveal Cell-Specific Differential Expression of SUMO Proteins
Xiaoyin Shan, Ph.D., Cleresa Roberts, BS, Catherine Calvert, MD, Yemin Lam, Ph.D., Ivona Percec, M.D., Ph.D..
University Of Pennsylvania, Philadelphia, PA, USA.
PURPOSE: Human adipose derived stem cells (ASCs) are considered to be an ideal adult stem cell for regenerative medicine applications due to their wide availability, ease of harvest, multilineage differentiation potential, trophic factor secretion, and low immunogenicity and oncogenic capacity. Unfortunately, aging negatively affects the therapeutic efficacy of ASCs. Currently, the precise molecular mechanisms governing adult stem cell aging remains poorly understood. Therefore, elucidating the mechanisms of ASC aging is critical for improving treatment outcomes for human stem-cell based therapies and aging prevention. METHODS: Subcutaneous abdominal tissue was obtained from healthy patients of various ages (25-74) undergoing abdominoplasty. ASCs were isolated via standard collagenase protocols. Dermal fibroblasts were isolated from human skin specimens obtained from healthy patients of various ages (25-64). Chromatin accessibility profiles were examined using ATAC-seq technology. Genomic nucleosome occupancy was mapped using NucleoATAC software. Pathway enrichment of genes display specific promoter nucleosome positioning was analyzed using Reactome Pathway Database. Small ubiquitin-like modifier (SUMO) protein expression levels at basal and after heat shock were assessed using the dot-blot assay. RESULTS: We initially demonstrated that ASC genomes overall contain more peaks from Tn5 fragmentation than those of fibroblasts. Furthermore, we observed that the position of nucleosomes flanking transcription start sites (TSS) is globally well maintained in aging ASCs but not in aging fibroblasts. However, subtle age-dependent changes in nucleosome positioning were detected in the promoter region of genes involved in protein sumoylation pathways in both ASCs and fibroblasts. In ASCs, the nucleosome positions in the promoter region of these genes, especially the one at the upstream of TSS (-1 nucleosome), are more consistently narrowly distributed in the older age group. In contrast, the -1 nucleosome positioning in older fibroblasts is variable. Global SUMO protein expressions under stress and non-stress conditions indicates that SUMO expression in old ASCs is more sensitive to heat shock than in young ASCs, whereas in fibroblasts, this age-dependent sensitivity is minor. CONCLUSIONS: Our data suggest a significant role for nucleosome positioning in sumoylation pathway regulation in response to stress during aging of adult stem cells. The distinctive difference in the chromatin structures described here between human ASCs and fibroblasts will contribute to the elucidation of mechanisms regulating gene expression during chronological aging in both stem cells and differentiated cells. The novel findings of age-dependent changes in chromatin structure and regulation of stress response in human ASCs offer significant implications for therapeutic application in regenerative medicine.
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