Interleukin-6 is an activator of pituitary stem cells upon local damage, a competence quenched in the aging gland

Abstract

Stem cells in the adult pituitary are quiescent yet show acute activation upon tissue injury. The molecular mechanisms underlying this reaction are completely unknown. We applied single-cell transcriptomics to start unraveling the acute pituitary stem cell activation process as occurring upon targeted endocrine cell–ablation damage. This stem cell reaction was contrasted with the aging (middle-aged) pituitary, known to have lost damage-repair capacity. Stem cells in the aging pituitary show regressed proliferative activation upon injury and diminished in vitro organoid formation. Single-cell RNA sequencing uncovered interleukin-6 (IL-6) as being up-regulated upon damage, however only in young but not aging pituitary. Administering IL-6 to young mice promptly triggered pituitary stem cell proliferation, while blocking IL-6 or associated signaling pathways inhibited such reaction to damage. By contrast, IL-6 did not generate a pituitary stem cell activation response in aging mice, coinciding with elevated basal IL-6 levels and raised inflammatory state in the aging gland (inflammaging). Intriguingly, in vitro stem cell activation by IL-6 was discerned in organoid culture not only from young but also from aging pituitary, indicating that the aging gland’s stem cells retain intrinsic activatability in vivo, likely impeded by the prevailing inflammatory tissue milieu. Importantly, IL-6 supplementation strongly enhanced the growth capability of pituitary stem cell organoids, thereby expanding their potential as an experimental model. Our study identifies IL-6 as a pituitary stem cell activator upon local damage, a competence quenched at aging, concomitant with raised IL-6/inflammatory levels in the older gland. These insights may open the way to interfering with pituitary aging.

Decoding the activated stem cell phenotype of the vividly maturing neonatal pituitary

“…To obtain a granular view on the cell type and activity landscape in the neonatal maturing pituitary, we performed single-cell RNA-sequencing (scRNA-seq) analysis of the major endocrine anterior pituitary (AP) from neonatal mouse (postnatal day 7, PD7), and contrasted the data with adult AP (Vennekens et al, 2021). After quality control and exclusion of doublets and dead and low-quality cells (Figure 1 - figure supplement 1A; (Vennekens et al, 2021)), the neonatal AP cell transcriptomic data were integrated with our recent scRNA-seq dataset of adult AP (Vennekens et al, 2021) followed by filtering-out of ambient RNA (Figure 1A). Unsupervised clustering and annotation using established lineage markers revealed all main endocrine cell types, as well as distinct clusters of pericytes and of stem, mesenchymal, immune and endothelial cells (Figure 1A and B – figure supplement 1B).…”

“…To further explore the stem cells’ nature of neonatal pituitary, we applied our recently established pituitary organoid model (B. Cox et al, 2019; Vennekens et al, 2021). We have shown that organoids developing from adult mouse AP originate from the SOX2 + stem cells and reflect their phenotype and activation state (e.g.…”

“…higher organoid number following damage; (B. Cox et al, 2019; Vennekens et al, 2021)), thus serving as a valuable pituitary stem-cell biology research model and activation readout tool (B. Cox et al, 2019; Vennekens et al, 2021).…”

“…One factor that may be responsible for activating the stem cells in the neonatal pituitary is IL-6 which we recently uncovered as pituitary stem cell-activating factor in adult mouse (Vennekens et al, 2021). Intriguingly, gene expression of Il6 is low in neonatal AP when compared to adult gland (Figure 2 - figure supplement 1F).…”

“…We have recently shown that the MC cluster is part of the formerly designated folliculostellate cell population in the (adult) AP (Vennekens et al, 2021), a heterogeneous cell group encompassing, among others, stem cells as well as paracrine-supportive cells (Allaerts et al, 2005; J. Chen et al, 2009; Fauquier et al, 2008; Vennekens et al, 2021).…”