Rejuvenating the Aged Intestinal System
Aging disrupts the homeostasis of all tissues in the body, including the intestinal epithelium, known for its robust regenerative capacity, which is crucial for ensuring nutrient absorption and barrier protection. The key players in this regenerative mechanism are intestinal stem cells (ISCs), located at the base of the intestinal crypts, pockets of epithelial cells at the base of the villus. These cells sustain the tissue by undergoing daily divisions and committing to secretory or absorptive lineages. Notably, the proliferative potential of ISCs becomes jeopardized as part of the aging process leading to well-known dysfunctions: nutrient malabsorption, compromised recovery in response to intestinal injuries and acute inflammation, impaired communication between epithelial cells and the intestinal microbiota, as well as severe side effects during chemotherapy treatments. At the cellular level, several molecular processes, some of which are partially understood, lead to an increase of the pool of differentiated cells at the expense of the stem cells compartment in aged epithelium, owing to a reduced proliferation. Furthermore, as ISCs respond to regulatory signals from the surrounding microenvironment, the niche, and, particularly, molecules secreted by neighboring immune cells or by gut microbiota, therefore aging must alter the homeostasis of these compartments as well. Indeed, previous studies have confirmed that aged gut microbiota triggers a chronic systemic inflammatory state, as a part of a process scientists term inflammaging; besides, the disruption of the homeostatic crosstalk between immune cells from lamina propria and the neighboring intestinal epithelium results in the release of stimuli skewing ISCs fate towards differentiation. Here, to unravel how the communication between immune cells and ISCs is impaired during aging, the authors of this study dissected the transcriptional and cellular aspects of their interaction by comparing young and elderly mice.
Total RNA sequencing of intestinal crypts from young and elderly mice showed a significant transcriptional variance upon aging, with a high number of differentially expressed genes. To note, transcriptional changes in inflammation-related pathways, in particular in the Antigen Presenting Pathway (APP), were enriched in both genders as a common signature of aging; this data was supported by the overall increase of cells expressing major histocompatibility complex class II (MHCII) on their surface, a hallmark of ongoing inflammatory process. Strikingly, many of these aging-related transcriptional changes occur at stem cell level, as shown by RNA sequencing on ISCs isolated from young and old mice, which recapitulated what observed in the whole crypts, including APP upregulation, and, additionally, revealed new upregulated pathways, including those related to p53, IGF-1, calcium signaling. Focusing on the balance between stem and differentiated cells within the crypts, single cell level analysis revealed that both male and female old mice experience an exhaustion of stem cells pool concomitantly to an increase of secretory cells. The observed phenotypes, which include cells expressing MHCII and an elevated presence of secretory cells compared to stem cells, suggested that the aged intestine was actually responding to a pro-inflammatory environment. Experiments performed in intestinal organoid cultures displayed that aged intestinal crypts counteract to extrinsic factors released from a perturbated surrounding; indeed, the lamina propria, physically situated close to the epithelium and housing immune cells, experiences a cell-type composition rearrangement during aging, accumulating pro-inflammatory cytotoxic CD4 T and ILC2 cells, which in turn release increased pro-inflammatory signals, particularly, the Interferon gamma (IFN). Consequently, the authors investigated the role of IFN; treating organoids with IFN was able to mimic the in vivo intestinal crypts aging, suggesting that IFN triggers ISC transition to a more proliferating state and to a transcriptionally-primed (towards secretory lineage) state expressing MHCII. Ultimately, it all leads to a depletion of the stem pool. Additionally, aged crypts, by turning into a presenting antigen tissue, trigger an immune response in terms of increase of CD4+ T cells an ILC2 in the lamina propria, as assessed by co-culturing immune cells and IFN pre-treated MHCII+ - to mimic the aged phenotype - young intestinal organoids. Finally, the scientists demonstrated that ISCs functional impairment is due to the activation of a downstream target of IFN, the gene Stat1, which is upregulated during aging, as well as upon IFN administration, and directly binds the promoters of the secretory markers and APP genes.
To delve into the clinical relevance of this discovery, aged mice carrying a profound intestinal damage due to the treatment with the chemotherapeutic drug (5FU) were cured with a molecule capable of blocking IFN. The pre-treatment restored the regenerative capacity of the intestine in these animals to the level of the young ones and rescued the previously observed aging-associated alterations. Remarkably these mice did not lose weight and suffer after the chemotherapeutic-driven intestinal injury. If translated to human these results can advance the current clinical practice, increase the patient well-being and improve the suitability of anti-cancer treatments to a wider pool of people.