Space-Flight- and Microgravity-Dependent Alteration of Mast Cell Population and Protease Expression in Digestive Organs of Mongolian Gerbils

Abstract

Mast cell (MC)-specific proteases are of particular interest for space biology and medicine due to their biological activity in regulating targets of a specific tissue microenvironment. MC tryptase and chymase obtain the ability to remodel connective tissue through direct and indirect mechanisms. Yet, MC-specific protease expression under space flight conditions has not been adequately investigated. Using immunohistochemical stainings, we analyzed in this study the protease profile of the jejunal, gastric, and hepatic MC populations in three groups of Mongolian gerbils-vivarium control, synchronous experiment, and 12-day orbital flight on the Foton-M3 spacecraft-and in two groups-vivarium control and anti-orthostatic suspension-included in the experiment simulating effects of weightlessness in the ground-based conditions. After a space flight, there was a decreased number of MCs in the studied organs combined with an increased proportion of chymase-positive MCs and MCs with a simultaneous content of tryptase and chymase; the secretion of specific proteases into the extracellular matrix increased. These changes in the expression of proteases were observed both in the mucosal and connective tissue MC subpopulations of the stomach and jejunum. Notably, the relative content of tryptase-positive MCs in the studied organs of the digestive system decreased. Space flight conditions simulated in the synchronous experiment caused no similar significant changes in the protease profile of MC populations. The space flight conditions resulted in an increased chymase expression combined with a decreased total number of protease-positive MCs, apparently due to participating in the processes of extracellular matrix remodeling and regulating the state of the cardiovascular system.

Keywords: Mongolian gerbils; chymase; mast cells; space flight; the digestive system; tryptase.

Figure 1

Gastric mast cells of Mongolian gerbils. Experimental groups: (A–E) vivarium control, (F,G) synchronous experiment, (H,I) space flight, (J–L) antiorthostatic suspension. Notes: (A) Localization of tryptase-positive MCs at different levels in the lamina propria. (B,C) Chymase-positive (B) and tryptase-positive (C) MCs in the submucosa. (D) Attachment of MCs to the gastric proper glands of the connective tissue subpopulation; immunohistochemical staining of mast cell tryptase. (E) Secretion of chymase to submucosal fibroblast (presumably). (F) Localization of tryptase-positive mast cells in the gastric mucosa. (G) A submucosal mast cell containing both tryptase and chymase, distributed predominantly along the periphery of the secretory granules. (H) The appearance of chymase-positive MCs in the area of the proper glands of the gastric mucosa. (I) Two distinct MC phenotypes in the submucosa, with visualized chymase secretion into the extracellular matrix. (J) A mucosal mast cell filled with large tryptase-positive granules, with evident secretion into the intercellular substance. (K,L) Chymase-positive MCs in the perivascular region of the submucosal muscular artery (K) and mucosal intrinsic plasty (L). Scale: (A) 50 µm, the rest—10 µm.

Figure 2

Jejunal mast cells of Mongolian gerbils. Experimental groups: (A–E) vivarium control, (F–H) synchronous experiment, (I–O) space flight, (P,Q) antiorthostatic suspension. Notes: (A) Distribution of tryptase-positive MCs in all structures of the jejunum; chymase-positive MCs are absent in the mucosa. (B) Grouping of tryptase-positive MCs. (C) Distribution of tryptase along the periphery of contacting mast cell granules. (D) A mast cell is filled with tryptase-positive granules. Part of them are transported to the extracellular matrix (arrowed). (E) Localization of chymase in large submucosal MC granules; part of them are secreted (arrowed). (F) Mucosal mast cells in contact with immunocompetent cells (arrowed) and smooth myocytes of the muscularis lamina (double-arrowed). (G) Chymase-positive cells in the muscle layer. (H) Chymase-positive MCs at the base of the mucosal villus with signs of degranulation. (I) Severe degranulation of tryptase-positive MC granules at the mucosal–submucosal interface. (J) Migration of tryptase-positive MCs in the stroma of the jejunal villus. (K) Different mechanisms of tryptase degranulation into the extracellular matrix (arrowed). (L) Chymase-positive MCs in the muscle layer. (M) Increase in the number of chymase-positive MCs in the mucosa. (N,O) MCs with simultaneous tryptase and chymase expression. (P) Tryptase-positive mucosal MCs with signs of degranulation. (Q) Active secretion of chymase-positive granules into a specific mucosal tissue microenvironment. Scale: (A) 100 µm, the rest—10 µm.

Figure 3

Hepatic mast cells of Mongolian gerbils. Experimental groups: (A–D) vivarium control, (E–H) space flight, (I–K) antiorthostatic suspension. Techniques: (A,E,I) staining with toluidine blue, (B,F,J) determination of chloroacetate esterase activity, (C,D,G,H,K) immunohistochemical staining of specific MC proteases. Notes: (A–D) Mast cells in various morphofunctional states in the stroma of portal triads. The contact formation of MCs with each other is typical. (E) Degranulation of MCs in the liver lobule. (F). MCs in the stroma of the portal tract; close contact with pericyte (arrowed). (G) A chymase-positive MC in the liver lobule. (H) Chymase- and tryptase-positive MCs in the region of the portal tract. (I) High content of mast cells in the periportal region; active secretion of biosynthetic products into the extracellular matrix (arrowed). (J) MCs of the portal tract in a state of active secretion (arrowed). (K) Mast cells with diverse protease expression in the portal tract region. Scale: 10 µm.