Pituitary Regeneration: It'll Knock Your SOX Off!

Tissue regeneration provides promise for improved treatments for many human pathologies affecting neural and cardiac tissues, pancreas, bone, and cochlear hair cells (1–5). Recent studies in the pituitary gland have demonstrated that although cell turnover is normally low, pituitary stem cells do exist and can regenerate pituitary endocrine cell types in response to physiological stressors (6, 7). Increasing our understanding of and ability to manipulate pituitary regeneration is important for improving therapeutics for patients with hypopituitarism.

Pituitary stem cells reside primarily in the marginal-zone, a layer of cells bordering the pituitary cleft, and express several stem cell markers, including sex-determining region Y box 2 (SOX2) (6, 8, 9). Additional stem cell niches may exist in the pituitary gland as a subset of SOX2⁺ cells are scattered throughout the anterior pituitary (6, 7, 10, 11). Deletion of Sox2 causes severe pituitary hypoplasia and reduced differentiation of somatotropes and thyrotropes due to insufficient renewal of periluminal stem cells (12). Additional evidence for the presence of pituitary stem cells comes from studies showing that SOX2⁺ pituitary cells from embryos and adults have the ability to differentiate into multiple pituitary endocrine cell types in culture and in vivo, suggesting that these cells are multipotent (8, 13).

In this issue of Endocrinology, an elegant study by Willems et al (33) provides insight into pituitary regeneration. Previously, this group used a transgenic mouse model (GHCre/iDTR) to target somatotropes for ablation by diphtheria toxin (DT) (14). Treatment with DT for 3 days obliterated nearly all somatotropes. In young adult mice, somatotropes were able to regenerate, and somatotrope populations were partially restored. The process of regeneration involved expansion of the SOX2⁺ marginal-zone niche and the appearance of cells doubly positive for cytoplasmic SOX2 and GH, which likely represent cells transitioning from progenitors to terminally differentiated somatotropes (14).

In the current study, Willems et al (33) expand their investigation by addressing the role of recovery period and age in regenerative capacity. Using the same model (GHCre/iDTR), they demonstrate that increasing the recovery period to 19 months does not significantly increase restoration of somatotrope numbers. Interestingly, after ablation of somatotropes in older mice, regeneration no longer occurrs, suggesting that regenerative capacity of the pituitary gland is lost with age. The loss of regenerative capacity in older animals correlates with a significant reduction in stem cells, especially those containing nuclear SOX2. Thus, nuclear SOX2 may be an indicator of healthy stem cells that are able to contribute to regenerative capacity.

To determine how pituitary stem cells respond to extended ablation of somatotropes, Willems et al (33) treated young GHCre/iDTR mice with DT for 10 days. Interestingly, they find that somatotrope regeneration does not occur under these conditions, suggesting that continual ablation of somatotropes exhausts regenerative capacity of stem cells in young adult pituitary glands. Stem cell exhaustion has been observed in other systems as well. Deletion of the forkhead transcription factor family members, Foxo1, Foxo3, and Foxo4, in neural stem cells of mice results in aberrant proliferation and increased brain size. The excessive proliferation of neural stem cells eventually leads to exhaustion of their regenerative capacity ultimately causing a reduction in neural stem cells and neural degeneration (15). These data suggest that regulation of proliferation is important for maintaining stem cell health and regenerative capacity. Similar results were observed with deletion of Foxo family members in hematopoietic stem cells, which causes these cells to aberrantly enter into the cell cycle, ultimately exhausting their capacity for self-renewal and regeneration (16).

Willems et al (33) next investigated pituitary stem cell transcriptomes to get a global perspective of the mechanism of pituitary regeneration. They compared stem cells from regenerating and nonregenerating pituitary glands, identifying several differentially expressed genes including those involved in regulating cell proliferation, differentiation and communication. Genes from signaling pathways important in embryogenesis, including the sonic hedgehog and wingless-type mouse mammary tumor virus integration site, more commonly known as WNT, pathways, are significantly up-regulated in stem cells from regenerating pituitary glands. Although differences in expression for members of the NOTCH signaling family were not statistically significant, possibly due to limited sample size, they demonstrated a trend of differential expression.

NOTCH signaling is active in the pituitary gland and is important for maintaining cells in an undifferentiated state until the appropriate time for differentiation arrives (17–19). In this way, NOTCH signaling regulates the balance between the POU1F1 (POU domain, class 1, transcription factor 1 also known as PIT1) linage, which consists of thyrotropes, somatotropes, and lactotropes, and the corticotrope lineage. Persistent expression of activated NOTCH maintains progenitor cells in an undifferentiated state for too long, past the time when conditions are appropriate for corticotrope differentiation, resulting in decreased numbers of corticotrope cells (19). Loss of NOTCH signaling results in a failure to maintain pituitary stem cells and reduction of the POU1F1 lineage (18). Interestingly, Davis et al (20) find that anterior pituitary cell types are born concurrently, suggesting that hormone-producing pituitary cells are not specified by the timing of their final S-phase. It may be that pituitary specification correlates with the particular phase of the cell cycle during determination events or time spent in G₁ phase before differentiation, as has been shown for neurogenesis (21).

Finely orchestrated regulation of the cell cycle, including how much time cells spend in particular phases of the cell cycle may be important for pituitary cell specification, as has been shown for neural tissues (22). In the pituitary gland, the cell cycle inhibitors, p57Kip2 and p27Kip1 control cell cycle exit and reentry but are not required for differentiation, suggesting a requirement for prodifferentiation factors as well as cell cycle regulators in this process (23, 24). Transcription factors that target genes coding for cell regulators may be important for initiating cell specification and differentiation. One intriguing candidate for regulating specification and differentiation of somatotropes is the forkhead transcription factor, FOXO1. FOXO1 inhibits cell cycle progression and regulates differentiation in several tissues by regulating expression of genes coding for cell cycle inhibitors such as p27Kip1 and cyclin D1 (25–28). FOXO1 is present in embryonic pituitary cells at the time of somatotrope differentiation, and thus may be important for this process (29).

Ultimately, our goal is to improve human health. By understanding how the balance between pituitary cell proliferation and differentiation is regulated we can improve therapeutics for patients with hypopituitarism and pituitary adenomas. A detailed analysis of the timing of cell cycle phases during pituitary cell differentiation may provide important information regarding the regulation of cell specification. Identification of molecules that can be manipulated to affect pituitary cell cycle and differentiation could provide improved therapies for patients with pituitary pathologies. The NOTCH pathway and FOXO1 are both druggable and therefore represent possible targets for therapeutics (30–32). Future studies will be needed to provide the level of understanding necessary for specifically manipulating the molecules that regulate pituitary regeneration.

Acknowledgments

Disclosure Summary: The author has nothing to disclose.

For article see page 705

Abbreviations

 
• DT

  diphtheria toxin

 
• FOXO1

  forkhead transcription factor

 
• SOX2

  sex-determining region Y box 2.