Caractérisation morphologique, biochimique et physiologique des protéines de jonction lacunaire, les connexines 46 et 50, dans les cellules folliculo-stellaires TtT/GF de l’hypophyse antérieure

The folliculo-stellate (FS) cells of the anterior pituitary are star-shaped and extend long cytoplasmic processes forming pseudo-follicles encircling hormone-secreting cells. Dispersed throughout the anterior pituitary gland, FS cells are joined to form a continuous three dimensional network through communicating gap junctions. One of the primary roles of FS cells is the maintenance of the anterior pituitary microenvironment, accomplished through the expression and secretion of various growth factors and cytokines. These chemical messengers, including bFGF, VEGF, IL-6 and IL-1 mediate a range of cellular processes such as hormone gene expression. Our aim is to study whether intercellular communication among FS cells contributes to the modulatory activity of the FS cells within the anterior pituitary gland. To pursue this, we use the TtT/GF cell line that shares many morphological, physiological and biochemical characteristics with FS cells.   Gap junctions are formed by the joining of two connexons/hemichanels from adjacent cells that link their cytoplasms allowing for the passive diffusion of small molecules. Connexons/hemichannels are themselves formed by the oligomerization of six connexin (Cx) proteins from the family α, β or γ, which then migrate into the lipid bilayer of the cell membrane.  FS cells express Cx43 (α-connexin), which is regulated in response to growth factors and cytokines. Changes in the anterior pituitary microenvironment due to signaling molecules results in modifications to Cx43, particularly in the phosphorylation status of the protein. Such alterations yield alterations in the physiological behaviour of Cx43 gap junctions such as permeability and turnover.  Tissues generally express more than one connexin type and to date, Cx43 has been the sole connexin to be identified in FS cells. The ocular lens expresses the α-connexins: Cx43, Cx46 and Cx50, which are modulated by growth factors that are also present in the anterior pituitary. Based on these facts, we hypothesize that Cx46 and Cx50 are also expressed by the FS cells and contribute to the FS modulatory role in the anterior pituitary gland.   In the present study, we have identified and characterized Cx46 and Cx50 in the TtT/GF cell line. We identified Cx46 and Cx50 transcripts through northern blots and identified the corresponding protein products using antibodies and western blot analyses. Through confocal microscopy, we determined that Cx46 co-localized with the organelle markers: trans-Golgi, early endosomes and lysosomes. Cx50 co-localized with markers for the ER, cis-Golgi and early endosomes. An isolation procedure using a non-ionic detergent we showed that neither Cx46 nor Cx50 were associated to lipid rafts or caveolae. However, confocal microscopy showed a cytoplasmic co-localization between Cx50 and flotillin-1.  We pursued a finding that localized Cx46 to the nucleus and using a nuclear isolation technique, demonstrated that several isoforms of Cx46 are exclusively located in the nuclear compartment. Furthermore, with confocal microscopy we found a co-localization of Cx46 with a nucleolus/coiled body marker.  We demonstrated an effect of bFGF on the temporal expression patterns of Cx46 and Cx50 and showed that Cx46 levels decreased over longer exposures to the growth factor while Cx50 levels transiently increased. Lastly, drastic changes were noted in an in situ study of Cx46 and Cx50 in the male and female mink anterior pituitary during the annual reproductive cycle.  Our study indicates that addition to Cx43, FS cells also express Cx46 and Cx50. We also demonstrated that Cx46 and Cx50 localize to different sub-cellular structures, suggesting different roles in the FS cells. While they may not play a major role in intercellular communication in quiescent FS cells, our results suggest that Cx46 and Cx50 may serve other functions: Cx46 isoforms may contribute to ribosome biogenesis and Cx50 may have communication-related responsibilities in stimulated cells. Importantly, our identification and characterization studies provide a foundation on which future studies can be built.

Keyphrases: Cx, Cellules folliculo-stellaires, TtT/GF, bFGF, Connexin, Gap junction, Folliculo-stellate cells, Anterior pituitary, Mink