Espin cytoskeletal proteins in the sensory cells of rodent taste buds

Abstract

Espins are multifunctional actin-bundling proteins that are highly enriched in the microvilli of certain chemosensory and mechanosensory cells, where they are believed to regulate the integrity and/or dimensions of the parallel-actin-bundle cytoskeletal scaffold. We have determined that, in rats and mice, affinity purified espin antibody intensely labels the lingual and palatal taste buds of the oral cavity and taste buds in the pharyngo-laryngeal region. Intense immunolabeling was observed in the apical, microvillar region of taste buds, while the level of cytoplasmic labeling in taste bud cells was considerably lower. Taste bud cells contain tightly packed collections of sensory cells (light, or type II plus type III) and supporting cells (dark, or type I), which can be distinguished by microscopic features and cell type-specific markers. On the basis of results obtained using an antigen-retrieval method in conjunction with double immunofluorescence for espin and sensory taste cell-specific markers, we propose that espins are expressed predominantly in the sensory cells of rat circumvallate taste buds. In confocal images, we counted 21.5±0.3 espin-positive cells/taste bud, in agreement with a previous report showing 20.7±1.3 light cells/ taste bud when counted at the ultrastructural level. The espin antibody labeled spindle-shaped cells with round nuclei and showed 100% colocalization with cell-specific markers recognizing all type II [inositol 1,4,5-trisphosphate receptor type III (IP 3 R 3 ),α-gustducin, protein-specific gene product 9.5 (PGP9.5)] and a subpopulation of type III (IP 3 R 3 , PGP9.5) taste cells. On average, 72%, 50%, and 32% of the espin-positive taste cells were labeled with antibodies to IP 3 R 3 , α-gustducin, and PGP9.5, respectively. Upon sectional analysis, the taste buds of rat circumvallate papillae commonly revealed a multi-tiered, espin-positive apical cytoskeletal apparatus. One espin-positive zone, a collection of ~3 μm-long microvilli occupying the taste pore, was separated by an espindepleted zone from a second espin-positive zone situated lower within the taste pit. This latter zone included espin-positive rod-like structures that occasionally extended basally to a depth of 10-12 μm into the cytoplasm of taste cells. We propose that the espin-positive zone in the taste pit coincides with actin bundles in association with the microvilli of type II taste cells, whereas the espin-positive microvilli in the taste pore are the single microvilli of type III taste cells.

Espins and the actin cytoskeleton of hair cell stereocilia and sensory cell microvilli

“…In addition to hair cell stereocilia, espins are detected at high levels in the microvillar PABs of a number of other sensory cell types, including taste receptor cells, solitary chemoreceptor cells, vomeronasal sensory neurons and Merkel cells [4,5] (fig. 2D-G).…”

“…The actin-bundling proteins villin and fimbrin/plastin have been detected in the microvilli of supporting (type I) and sensory (type II) cells in taste buds [46]. In contrast, espins appear to be expressed at high levels only in the sensory cells, specifically the type II and type III cells [4,5] (fig. 2D).…”

“…2D). A consequence is that, when labeled with espin antibody, taste buds often reveal a multi-tiered apical cytoskeletal apparatus, in which the espin-containing microvillar/rootlet PABs of the type II cells appear separated from the espin-containing microvillar PAB of type III cells by an espin-deficient zone, which presumably contains the microvilli of type I cells [5] (fig. 2D).…”

“…Nevertheless, relatively little is known regarding the role of this activity in situ. It is noteworthy, however, that espins are present in hair cells and taste bud cells relatively early in their differentiation, at a time when stereociliary or microvillar PABs are forming [5,6]. In addition, it remains possible that espin WH2 domaindependent PAB assembly occurs locally in cells in situ on a smaller scale, without resulting in the accumulation of such a large PAB.…”

“…1A). Although present in multiple actin-rich structures, espins are especially prevalent in the PABs of hair cell stereocilia and the microvilli of other sensory cells, where they appear to be distributed uniformly along the PAB at the core [4,5,7].…”