Decoding the activated stem cell phenotype of the neonatally maturing pituitary

The pituitary represents the endocrine master regulator. In mouse, the gland undergoes active 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 advanced organoid growth, although the neonatal stem cell compartment was not visibly affected in Il6^−/− mice, likely 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 exposing striking differences with adult pituitary. Together, our study decodes the stem cell compartment of neonatal pituitary, exposing an activated state in the maturing gland. Understanding stem cell activation is key to potential pituitary regenerative prospects.

The pituitary gland is a pea-sized structure found just below the brain that produces hormones controlling everything from growth and stress to reproduction and immunity. To perform its role, the pituitary gland needs specialised hormone-producing cells, but it also contains stem cells. These stem cells can divide to produce more cells like themselves, or differentiate into cells of different types, including hormone-producing cells.

In mice, the stem cells of the pituitary gland appear to be activated in the first few weeks after birth, and later become ‘quiescent’ (or lazy) in the adult pituitary gland. However, it remains unclear how the activated state found in the maturing gland is established and regulated.

To answer this question, Laporte et al. used single-cell RNA sequencing, a technique that allows researchers to profile which genes are active in individual cells, which can provide vital information about the state and activity of a tissue. The researchers compared the cells of the maturing pituitary gland of newborn mice to the cells in the established gland of adult mice. This analysis revealed that the maturing pituitary gland is a dynamic tissue, with populations of cells that are actively dividing (including the stem cells), which the mature pituitary gland lacks. Additionally, Laporte et al. established the molecular basis for the activated state of the stem cells in the maturing pituitary gland, which relies on the activation of a cell signalling pathway called WNT.

To confirm these findings, Laporte et al. used an organoid system that allowed them to recapitulate the stem cell compartment of the maturing pituitary gland in a dish. When Laporte et al. blocked WNT signalling in these organoids, the organoids failed to form or divide. Furthermore, blocking the pathway directly in newborn mice reduced the number of dividing stem cells in the pituitary gland. Both findings support the notion that WNT signalling is required to establish the activated state of the maturing pituitary gland in newborn mice.

Laporte et al. also wanted to know whether the newborn pituitary gland responded to injury differently than the adult gland. It had already been established that the adult pituitary stem cells become activated upon injury, and that the gland has some regenerative capacity. However, when Laporte et al. injured the newborn pituitary gland, the gland was able to fully regenerate, despite the stem cells not becoming more activated. This is likely because these cells are already activated (or ‘primed’), and do not require further activation to divide and repair the gland with the help of other proliferating cells.

With these results, Laporte et al. shed light on the activated state of the stem cells in the pituitary gland of newborn mice. This provides insight into the role of these stem cells, as well as unveiling possible routes towards regenerating pituitary tissue. This could eventually prove useful in medicine, in cases when the pituitary gland is damaged or removed.

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

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.

Novel Pituitary Organoid Model as Powerful Tool to Unravel Pituitary Stem Cell Biology Across Ages and Disease

The pituitary gland harbors a population of stem cells. However, role and regulation of these cells remain poorly understood. We recently established organoids from mouse pituitary as a novel research tool to explore pituitary stem cell biology (Cox et al., J. Endocrinol. 2019; 240:287-308). In general, organoids represent 3D in vitro cell configurations that develop and self-organize from (single) tissue stem cells under well-defined culture conditions that typically mirror the stem cell niche and/or embryogenic processes. Organoids reliably recapitulate key aspects of the original organ, including of its stem cell compartment. Moreover, organoids are long-term expandable while retaining these properties. We demonstrated that pituitary organoids originate from the resident (SOX2⁺) stem cells, largely phenocopy these cells and retain the stemness phenotype during expansive culture. Interestingly, the organoids show confident in vivo translatability and, when developed from transgenically damaged gland, recapitulate the activation status of the stem cells as observed in situ following injury. Now, we found that the organoids also mirror the stem cells’ phenotype and biology in physiological conditions in which the stem cell compartment is either activated or compromised. Organoids from the neonatal maturing pituitary reproduce phenotypical and functional aspects of its activated stem cells, whereas organoids from aging gland mimic the declined functional state of the stem cells in old pituitary. Interestingly, this functional decay was found to be reverted during organoid culture, indicating that the old pituitary stem cells retain intrinsic functionality but are in vivo restrained by an obstructive microenvironment, not present in the organoid culture. Indeed, using single-cell transcriptomics and in vivo analysis, we found that the aging pituitary suffers from a prevailing inflammatory state (inflammaging) which appears to raise the threshold for stem cell activation. Interestingly, comparison of young and old pituitary led us to the discovery of pituitary stem cell activators. Finally, we found that activated parameters of organoid formation are also observed when tumorigenesis takes place in the gland, again mimicking the in situ stem cell activation that is occurring in this perturbed, pathological condition. Taken together, we identified, and applied, our new pituitary organoid model as advanced and powerful tool to gain profound insight into pituitary stem cell behavior across life and disease, which is expected to eventually translate into restorative and rejuvenative tactics when pituitary function is compromised by damage or age. In this context, our single-cell transcriptome database has strong potential to unveil appealing targets.

Pituitary Remodeling Throughout Life: Are Resident Stem Cells Involved?

The pituitary gland has the primordial ability to dynamically adapt its cell composition to changing hormonal needs of the organism throughout life. During the first weeks after birth, an impressive growth and maturation phase is occurring in the gland during which the distinct hormonal cell populations expand. During pubertal growth and development, growth hormone (GH) levels need to peak which requires an adaptive enterprise in the GH-producing somatotrope population. At aging, pituitary function wanes which is associated with organismal decay including the somatopause in which GH levels drop. In addition to these key time points of life, the pituitary's endocrine cell landscape plastically adapts during specific (patho-)physiological conditions such as lactation (need for PRL) and stress (engagement of ACTH). Particular resilience is witnessed after physical injury in the (murine) gland, culminating in regeneration of destroyed cell populations. In many other tissues, adaptive and regenerative processes involve the local stem cells. Over the last 15 years, evidence has accumulated that the pituitary gland houses a resident stem cell compartment. Recent studies propose their involvement in at least some of the cell remodeling processes that occur in the postnatal pituitary but support is still fragmentary and not unequivocal. Many questions remain unsolved such as whether the stem cells are key players in the vivid neonatal growth phase and whether the decline in pituitary function at old age is associated with decreased stem cell fitness. Furthermore, the underlying molecular mechanisms of pituitary plasticity, in particular the stem cell-linked ones, are still largely unknown. Pituitary research heavily relies on transgenic in vivo mouse models. While having proven their value, answers to pituitary stem cell-focused questions may more diligently come from a novel powerful in vitro research model, termed organoids, which grow from pituitary stem cells and recapitulate stem cell phenotype and activation status. In this review, we describe pituitary plasticity conditions and summarize what is known on the involvement and phenotype of pituitary stem cells during these pituitary remodeling events.

Traumatic brain injury and resultant pituitary dysfunction: insights from experimental animal models

PURPOSE

Traumatic brain injury (TBI) is a major worldwide cause of disability, often burdening young people with serious lifelong health problems. A frequent clinical complication is post-traumatic hypopituitarism (PTHP) manifesting in several hypothalamus-pituitary axes. The head trauma-induced mechanisms underlying PTHP remain largely unknown. Several hypotheses have been proposed including direct damage to the pituitary gland and hypothalamus, vascular events and autoimmunity. This review aims to provide a summary of the currently limited number of studies exploring hypothalamus-pituitary dysfunction in experimental animal TBI models.

RESULTS

Although the impact of different forms of TBI on a number of hypothalamus-pituitary axes has been investigated, consequences for pituitary tissue and function have only scarcely been described. Moreover, mechanisms underlying the endocrine dysfunctions remain under explored.

CONCLUSIONS

Studies on TBI-induced pituitary dysfunction are still scarce. More research is needed to acquire mechanistic insights into the pathophysiology of PTHP which may eventually open up the horizon toward better treatments, including pituitary-regenerative approaches.

Organoids from pituitary as a novel research model toward pituitary stem cell exploration

The pituitary is the master endocrine gland, harboring stem cells of which the phenotype and role remain poorly characterized. Here, we established organoids from mouse pituitary with the aim to generate a novel research model to study pituitary stem cell biology. The organoids originated from the pituitary cells expressing the stem cell marker SOX2 were long-term expandable, displayed a stemness phenotype during expansive culture and showed specific hormonal differentiation ability, although limited, after subrenal transplantation. Application of the protocol to transgenically injured pituitary harboring an activated stem cell population, resulted in more numerous organoids. Intriguingly, these organoids presented with a cystic morphology, whereas the organoids from undamaged gland were predominantly dense and appeared more limited in expandability. Transcriptomic analysis revealed distinct epithelial phenotypes and showed that cystic organoids more resembled the pituitary phenotype, at least to an immature state, and displayed in vitro differentiation, although yet moderate. Organoid characterization further exposed facets of regulatory pathways of the putative stem cells of the pituitary and advanced new injury-activated markers. Taken together, we established a novel organoid research model revealing new insights into the identity and regulation of the putative pituitary stem cells. This organoid model may eventually lead to an interesting tool to decipher pituitary stem cell biology in both healthy and diseased gland.

Pituitary stem cell regulation: who is pulling the strings?

The pituitary gland plays a pivotal role in the endocrine system, steering fundamental processes of growth, metabolism, reproduction and coping with stress. The adult pituitary contains resident stem cells, which are highly quiescent in homeostatic conditions. However, the cells show marked signs of activation during processes of increased cell remodeling in the gland, including maturation at neonatal age, adaptation to physiological demands, regeneration upon injury and growth of local tumors. Although functions of pituitary stem cells are slowly but gradually uncovered, their regulation largely remains virgin territory. Since postnatal stem cells in general reiterate embryonic developmental pathways, attention is first being given to regulatory networks involved in pituitary embryogenesis. Here, we give an overview of the current knowledge on the NOTCH, WNT, epithelial-mesenchymal transition, SHH and Hippo pathways in the pituitary stem/progenitor cell compartment during various (activation) conditions from embryonic over neonatal to adult age. Most information comes from expression analyses of molecular components belonging to these networks, whereas functional extrapolation is still very limited. From this overview, it emerges that the 'big five' embryonic pathways are indeed reiterated in the stem cells of the 'lazy' homeostatic postnatal pituitary, further magnified en route to activation in more energetic, physiological and pathological remodeling conditions. Increasing the knowledge on the molecular players that pull the regulatory strings of the pituitary stem cells will not only provide further fundamental insight in postnatal pituitary homeostasis and activation, but also clues toward the development of regenerative ideas for improving treatment of pituitary deficiency and tumors.