Nucleoside triphosphate diphosphohydrolase‐2 is the ecto‐ATPase of type I cells in taste buds

Abstract

The presence of one or more calcium-dependent ecto-ATPases (enzymes that hydrolyze extracellular 5′-triphosphates) in mammalian taste buds was first shown histochemically. Recent studies have established that dominant ecto-ATPases consist of enzymes now called nucleoside triphosphate diphosphohydrolases (NTPDases). Massively parallel signature sequencing (MPSS) from murine taste epithelium provided molecular evidence suggesting that NTPDase2 is the most likely member present in mouse taste papillae. Immunocytochemical and enzyme histochemical staining verified the presence of NTPDase2 associated with plasma membranes in a large number of cells within all mouse taste buds. To determine which of the three taste cell types expresses this enzyme, double label assays were performed using antisera directed against the glial glutamate/aspartate transporter, (GLAST), the transduction pathway proteins phospholipaseC β2 (PLCβ2) or the G protein subunit α-gustducin, and serotonin (5HT) as markers of type I, II or III taste cells, respectively. Analysis of the double labeled sections indicates that NTPDase2 immunoreactivity is found on cell processes that often envelop other taste cells, reminiscent of type I cells. In agreement with this observation, NTPDase2 was located to the same membrane as GLAST, indicating that this enzyme is present in type I cells. The presence of ecto-ATPase in taste buds likely reflects the importance of ATP as an intercellular signaling molecule in this system. KeywordsGustatory; ecto-ATPase; NTPDase2; taste cells; ATP signaling; mouse; taste bud Taste buds, the sensory endorgans mediating taste, reside in the epithelium of the tongue, palate and larynx. In rodents, lingual taste buds occur in three types of papillae: fungiform papillae scattered over the anterior portion of the tongue, circumvallate papillae on the posterior dorsal surface, and foliate papillae along the posterior lateral surface. Taste buds also occur at the oral entrance to the nasoincisor ducts, soft palate, epiglottis and larynx.Taste buds comprise several proliferative basal cells and 50-80 elongate epithelial cells which are heterogeneous in their morphologic and cytochemical characteristics. The differentiated taste cells in rodents are classified into three morphologic types: type I, type II, and type III as determined by cell structure, shape of the nucleus and apical processes (Murray and Murray, reviewed in Finger and Simon, 2000). These cell types usually correlate with particular histochemical features. For example, immunoreactivity for the G-protein subunit, a-gustducin, is localized to a subset of type II taste cells (Boughter et al., 1997;Yang et al., 2000b The presence of calcium-dependent ecto-ATPases (enzymes that hydrolyze extracellular 5′-triphosphates such as ATP) in mammalian taste buds has long been known as demonstrated histochemically (Iwayama and Nada, 1967;Zalewski, 1968;Iwayama, 1969;Akisaka and Oda, 1977;Barry, 1992). Since these early reports, different families of these ecto-enzymes hav...

Cellular function and molecular structure of ecto-nucleotidases

“…It is co-expressed with NTPDase3 and eN in salivary cells and stratified epithelia of the gastrointestinal tract [40]. It is also found on taste buds [51] and was identified in a variety of tumor cells [37]. In the rodent brain, it is highly expressed by adult neural stem cells, where it has been implicated in the generation of new nerve cells, by non-myelinating Schwann cells of the peripheral nervous system, the satellite glia of dorsal root ganglia, and enteric glia [52][53][54][55][56][57].…”

Taste perception, associated hormonal modulation, and nutrient intake

“…[17][18][19] These cells express a plasma membranebound nucleotidase, which degrades extracellular adenosine triphosphate (ATP) and restricts neurotransmitter spread. 20,21 Regulation of the extracellular ionic environment within the taste bud seems to be a key function. 22 Because of their role in terminating synaptic transmission, these cells are considered to play a supporting, glial-like function within the taste bud.…”

Signal transduction and information processing in mammalian taste buds

“…This wrapping alone might limit ATP diffusion within the taste bud. However, in addition, type I cells express a potent ecto-ATPase [144] that breaks down ATP. This ecto-ATPase would further limit ATP spread during taste stimulation.…”