Endocrine Abstracts

Pituitary stem cells have been characterized in the postnatal pituitary. We now know they are organized in a niche and co-express specific markers such as Sox2, Sox9 or Gfra2. Although many studies by our group and others have been dedicated to its characterization in situ it is under discussion their role in the maintenance and turnover of the pituitary in physiological conditions or physiological pituitary challenges. It’s not known if the stem cells are required and which molecular mechanisms are implicated in recruitment/differentiation. We established a model of hypothyroidism in rodents similar to human conditions in which levels of thyroxine are maintained just below the lower normal cut-off. We studied pituitary extracts in a precise time-course for stem cell and differentiation markers of thyrotropes. We have found that Shh is increased immediately after the establishing of the hypothyroidism. Following this, we purified the Gfra2+ stem cell population from vehicle and short-term hypothyroid animals and grown them as spheres in absence of serum. Spheres grow during the days of culture duplicating from day 1 to day 5 when they reach a plateau. Gfra2+ cells obtained from hypothyroid animals produce a significantly higher level of spheres per well both at day 1 and at day 5. When cultured in presence of cyclopamine, a Shh inhibitor, the number of spheres is significantly reduced in the hypothyroid Gfra2+ but not in the control wells. We used immunofluorescence techniques to see what happen in the intact pituitary niche in vivo. A genetic mouse model of tracing where Gfra2/Sox2 positive cells are induced to express GFP long-term after the tamoxifen injection was followed in a time-course under the same conditions of above vehicle/hypothyroidism. There was a significant increase of the Sox2 positive cell in long-term hypothyroid mice compared with vehicle treated. Tracing the GFP+ population through a time-course, we detected a significant increase in the double GFP/TSH+ cells in the adenopituitary of hypothyroid mice compared to vehicle treated. This data confirm that Sox2 positive cells recruited from the pituitary niche are able to differentiate into TSH producing cells. In summary, our results indicate that the Gfra2/Sox2 population, the pituitary stem cells, are activated when a mild hypothyroidism is induced. Results in vitro and in vivo confirm that initially (short-term hypothyroidism) the stem cells are driven to proliferate and expand while later (long-term hypothyroidism) differentiate into thyrotropes.