Cholinergic epithelial cell with chemosensory traits in murine thymic medulla

Abstract

Specialized epithelial cells with a tuft of apical microvilli (“brush cells”) sense luminal content and initiate protective reflexes in response to potentially harmful substances. They utilize the canonical taste transduction cascade to detect “bitter” substances such as bacterial quorum-sensing molecules. In the respiratory tract, most of these cells are cholinergic and are approached by cholinoceptive sensory nerve fibers. Utilizing two different reporter mouse strains for the expression of choline acetyltransferase (ChAT), we observed intense labeling of a subset of thymic medullary cells. ChAT expression was confirmed by in situ hybridization. These cells showed expression of villin, a brush cell marker protein, and ultrastructurally exhibited lateral microvilli. They did not express neuroendocrine (chromogranin A, PGP9.5) or thymocyte (CD3) markers but rather thymic epithelial (CK8, CK18) markers and were immunoreactive for components of the taste transduction cascade such as Gα-gustducin, transient receptor potential melastatin-like subtype 5 channel (TRPM5), and phospholipase Cβ2. Reverse transcription and polymerase chain reaction confirmed the expression of Gα-gustducin, TRPM5, and phospholipase Cβ2. Thymic “cholinergic chemosensory cells” were often in direct contact with medullary epithelial cells expressing the nicotinic acetylcholine receptor subunit α3. These cells have recently been identified as terminally differentiated epithelial cells (Hassall’s corpuscle-like structures in mice). Contacts with nerve fibers (identified by PGP9.5 and CGRP antibodies), however, were not observed. Our data identify, in the thymus, a previously unrecognized presumptive chemosensitive cell that probably utilizes acetylcholine for paracrine signaling. This cell might participate in intrathymic infection-sensing mechanisms.

Identification of cholinergic chemosensory cells in mouse tracheal and laryngeal glandular ducts

“…Slides were evaluated with an epifluorescence microscope (Zeiss, Jena, Germany) using appropriate filter sets. Controls for the specificity of antibodies used were tested previously [1] for antigustducin and anti-PLCβ2 antibody by preincubation of these antibodies with the corresponding peptides (100 g/ml, Santa Cruz) for 1 h at room temperature before use, and [2] for TRPM5 with the preimmune serum instead of the primary antibody . Controls for the specificity of the secondary reagents were performed by omitting the primary antibody.…”

“…urethra, thymus, thyroid gland, heart, auditory tube, airways and lung [1][2][3][4][5][6][7]. In mucosal epithelia, these chemosensory cells express one or more members of the taste receptor family 1 (Tas1R) (sweet and umami) and Tas2R (bitter) that are coupled to various G-proteins (e.g.…”

Cholinergic chemosensory cells of the thymic medulla express the bitter receptor Tas2r131

“…Positive selection ensures survival of those T cells which react with self-MHC on the surface of epithelial cells, whereas negative selection eliminates those T cells that react with self-antigens presented by medullary epithelial and dendritic cells. Autoimmune regulator (AIRE) plays a key role in negative selection by inducing expression of tissue-restricted antigens in medullary thymic epithelial cells (mTECs) [1][2][3]. AIRE deficiency results in autoimmunity [1].…”

Cholinergic urethral brush cells are widespread throughout placental mammals

“…In mice, structurally similar cholinergic epithelial cells expressing components of the taste transduction cascade have been identified in the gastrointestinal tract including the gallbladder [19,20], the auditory tube [21], and the thymic medulla [22]. Hence, it is tempting to assume the presence of a general surveillance system constituting of cholinergic epithelial cells monitoring the chemical composition of their immediate surroundings and releasing acetylcholine upon stimulation to initiate appropriate defense mechanisms in the presence of hazardous compounds.…”

“…In the rat, brush cells are found in the lung alveoli (type III alveolar cells), whereas they are absent at this location in mice (reviewed in [12]). Their cholinergic phenotype has been primarily shown for mice [7,9,11,19,22,23], where additional populations of non-cholinergic brush cells also exist in trachea and urethra [10,13], and beyond that evidence for acetylcholine synthesis has been provided for rat bronchi as well as rat and human urethra by means of choline acetyltransferase (ChAT) antibodies [13,24].…”