Modulation of dye‐coupling and proliferation in cultured rat thymic epithelium by factors involved in thymulin secretion

Abstract

Cultures of rat thymic epithelium were used to measure the effect of thymulin secretagogues on dye-coupling and proliferation. Dye-coupling was assessed after the injection of lucifer yellow dextran which cannot permeate the connexin pore of gap junctions and the smaller, permeant cascade blue. In addition to gap junctional communication, larger intercellular bridges were demonstrated by the transfer of lucifer yellow dextran between cells. The extent of intercellular communication was found to be influenced by both cell density and the number of passages. In control cultures, intercellular communication was reduced in cell groups of low ( 20 cells\group) or high cell densities ( 100 cells\group) compared with groups of 20-60 cells. The highest coupling indices were found in subcultures 20-30. Taking these factors into account, significant decreases in coupling index were observed after pretreatment of test cultures with factors known to influence the secretion of thymulin (5 U\ml interleukin 1 (α and β), 1 µ progesterone, 1 µ oestrogen, 1 µ testosterone, 1 ng\ml adrenocorticotropic hormone, 100 n rat growth hormone) but 7n5 ng\ml thymulin had no effect on dye-coupling. The nonspecific gap junction uncoupler, octanol, abolished dyecoupling. Cellular proliferation, as measured by the uptake of tritiated thymidine, showed that the same factors that reduced coupling also increased proliferation. None of these factors affected the number of multinucleate cells present, except interleukin-1β which caused a significant reduction in the average number of nuclei per cell. Thus rat thymic epithelium in vitro provides a model for the study of the direct action of factors on cells of the thymic microenvironment.

Neuropeptides Exert Direct Effects on Rat Thymic Epithelial Cells in Culture

“…Cells of all morphologies showed dye coupling. Transfer of dextran-conjugated dye occurred between some cells of syncitia with three or more constituent cells, as previously reported (Head et al, 1997). The CI was highly significantly reduced (P < 0.0001) both by y-aminobutyric acid (GABA) (CI 0.12 + 0.08, n 52) and NPY (CI 0.13 + 0.09, n 45).…”

“…Earlier studies with procion yellow suggested the presence of a functional syncytium (Kendall, 1985(Kendall, , 1986 and intercellular communication by dye coupling was characterized for cultured mouse and human thymic epithelium (Alves et al, 1994(Alves et al, , 1995. Using cultured rat thymic epithelium, we have previously shown that the extent of cell coupling is modulated by factors known to affect the secretory activity of the cells (Head et al, 1996a(Head et al, , 1996bHead et al, 1997). In this study, we extend the work to examine the effect of selected neuropeptides using the same cell-model system.…”

“…High levels of connexin 43 are found in rapidly proliferating lymphoid and hematopoietic centers (Rosendaal et al, 1994;Krenacs and Rosendaal, 1995). In our previous study of factors influencing secretion (Head et al, 1997), decreasing dye coupling inversely increased proliferation. In this study, this relationship is only true for CGRP and to a lesser extent, histamine.…”

“…This suggests that neuropeptide-induced uncoupling does not involve calcium. Those neuropeptides that reduce thymulin levels had minimal or no effect on dye coupling (similar to the effects of thymulin itself ;Head et al, 1997).…”

Use of Cultured Thymic Tissues for the Regeneration of the Thymus

“…Many authors have reported that the expression of these hormones is regulated by extrathymic biomolecules including pituitary hormones [78], steroids [79] and thyroid hormones [80]. Intrathymic biomolecules also associated with modulation of thymic hormones include growth hormone and insulin-like growth factor-I [81], IL-1 [82] and neuropeptides [83]. It has also been reported that thymic hormones can exert an effect on gonadotrophin expression in pituitary cells in culture, illustrating that a feedback link exists between the two [84].…”