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

Abstract

The pituitary represents the endocrine master regulator. In mouse, the gland undergoes vivid maturation immediately after birth. Here, we in detail portrayed the stem cell compartment of neonatal pituitary. Single-cell RNA-sequencing pictured an active gland, revealing proliferative stem as well as hormonal (progenitor) cell populations. The stem cell pool displayed a hybrid epithelial/mesenchymal phenotype, characteristic of development-involved tissue stem cells. Organoid culturing recapitulated the stem cells' phenotype, interestingly also reproducing their paracrine activity. The pituitary stem cell-activating interleukin-6 (IL-6) advanced organoid growth, although the neonatal stem cell compartment was not visibly affected in IL-6-/- mice, possibly due to cytokine family redundancy. Further transcriptomic analysis exposed a pronounced WNT pathway in the neonatal gland, shown to be involved in stem cell activation and to overlap with the (fetal) human pituitary transcriptome. Following local damage, the neonatal gland efficiently regenerates, despite absence of additional stem cell proliferation, or upregulated IL-6 or WNT expression, all in line with the already high stem cell activation status, thereby divulging striking differences with adult pituitary. Together, our study decodes the stem cell compartment of neonatal pituitary, exposing an activated state in the vividly maturing gland. Understanding stem cell activation is key to potential pituitary regenerative prospects.

The human pituitary master gland stripped to single-cell resolution

“…Sc transcriptomebased pituitary stem cell heterogeneity was already reported earlier in mouse, revealing two major subclusters diverging in SOX2 and SOX9 expression levels and displaying specific markers (e.g. TROP2) 4,5 . It would be informative to map the human subclusters on the reported mouse subpopulations.…”

“…Pseudotime trajectory analysis between uncommitted and committing cells would further support the authors' functional interpretations. A surprising finding is that the "(likely) committing" progenitor cells increase from pediatric to older age, which feels counterintuitive and is different from mouse pituitary in which cell remodeling activity contrarily declines from neonatal to adult age 4,5,8,9 . Caution is warranted when extracting quantitative data from very low abundance cells (and limited sample numbers), and one should keep in mind that hormone expression increases toward adulthood, entailing higher risks of ambient transcripts.…”

“…Moreover, instead of analyzing only one fragment per pituitary 1 , whole-gland pulverization (as done for one pituitary 1 ) will strengthen the outcome generalizability. The study zoomed in on the projected stem cell population, thereby epitomizing the booming interest in these mysterious cells [3][4][5][6] . Authors designated the stem cell population mainly by SOX9 expression, whereas in rodents, pituitary stem cells are primarily assigned through the overarching SOX2 [3][4][5] while SOX9 expression may indicate a further developed (progenitor) stage 7 , although not clearly settled yet.…”

“…The study zoomed in on the projected stem cell population, thereby epitomizing the booming interest in these mysterious cells [3][4][5][6] . Authors designated the stem cell population mainly by SOX9 expression, whereas in rodents, pituitary stem cells are primarily assigned through the overarching SOX2 [3][4][5] while SOX9 expression may indicate a further developed (progenitor) stage 7 , although not clearly settled yet. Then, authors subclustered the projected stem cell compartment into eight subpopulations (Fig.…”

“…Their annotation was not included which would rationally back translational efforts from in silico to real-life implications (e.g. using mouse or organoid models) 4,5 . Remarkably, the projected stem cell population also contained "committing" or "likely committing" progenitor cells, marked by GATA3 and POU1F1/POMC expression, respectively, which suggests that clustering based on SOX9 expression may have expanded the genuine (i.e.…”