Identification and localization of aquaporin water channels in human salivary glands

Abstract

Aquaporin (AQP) water channels are expressed in a variety of fluid-transporting epithelia and are likely to play a significant role in salivary secretion. Our aim was to identify and localize the aquaporins expressed in human salivary glands. Total RNA was extracted from human parotid, submandibular, sublingual, and labial glands and from human brain. Expression of aquaporin mRNA was assessed by RT-PCR using specific primers for human AQP1, AQP3, AQP4, and AQP5. All four aquaporins were detected by RT-PCR in all of the glands, and the sequences were confirmed after further amplification with nested primers. Cleaned PCR products were then used as (32)P-labeled cDNA probes in a semiquantitative Northern blot analysis using glyceraldehyde-3-phosphate dehydrogenase as reference. Only AQP1, AQP3, and AQP5 mRNAs were present at significant levels. AQP localization was determined by immunohistochemistry on paraffin sections using affinity-purified primary antibodies and peroxidase-linked secondary antibodies. Each salivary gland type showed a broadly similar staining pattern: AQP1 was localized to the capillary endothelium and myoepithelial cells; AQP3 was present in the basolateral membranes of both mucous and serous acinar cells; AQP4 was not detected; and AQP5 was expressed in the luminal and canalicular membranes of both types of acinar cell. We conclude that AQP3 and AQP5 together may provide a pathway for transcellular osmotic water flow in the formation of the primary saliva.

Selective Down-Regulation of Aquaporin-1 in Salivary Glands in Primary Sjögren's Syndrome

“…Indirect immunofluorescence detection showed that the distribution of AQP-1 labeling was confined to the basal region of acini and intercalated ducts as shown previously, probably in myoepithelial cells (Gresz et al, 2001). AQP-1 labeling patterns were similar in both control and pSS glands (Fig.…”

“…By contrast, AQP-3 labeling was present in the basolateral membrane of the acinar cells, as described previously, but not in intercalated or striated ducts (Gresz et al, 2001). This pattern was similar in glands from healthy control subjects and patients with pSS, and the intensity of the labeling seemed similar between control subjects and patients with pSS.…”

“…The presence of M3R (Beroukas et al, 2002) and AQP-1 (Gresz et al, 1999(Gresz et al, , 2001; present study) on salivary gland myoepithelial cells also raises the interesting possibility that some of the effects of acetylcholine on secretion may be mediated indirectly through an action on the myoepithelial cells rather than through a direct effect of acetylcholine on the acinar cells. Because myoepithelial cells embrace acini, they could constitute a barrier to water flow into the acinar cells; the presence of AQP-1 in myoepithelial cells might ensure that water flow into the basal aspect of the acinar epithelial cells is not restricted (Gresz et al, 2001).…”

“…AQP isoforms are differentially expressed in different subcellular compartments in salivary glands: AQP-3 has been reported to be localized to the basolateral surface of acinar epithelial cells, where it presumably allows water inflow into these cells, and AQP-1 is expressed in myoepithelial cells (Gresz et al, 2001). We therefore studied the distribution of AQP-1 and AQP-3 isoforms in control and pSS labial salivary gland (LSG) biopsies using high-resolution confocal microscopy and quantitative image analysis.…”

Up-regulation of M3-Muscarinic Receptors in Labial Salivary Gland Acini in Primary Sjögren's Syndrome

“…M3R receptors have also been localized to myoepithelial cells in rat lacrimal gland (Lemullois et al, Beroukas et al 1996), and stimulation of these receptors causes contraction of the myoepithelial cells (Satoh et al, 1997). The presence of M3R (present study) and aquaporin-1 (Gresz et al, 2001) on salivary gland myoepithelial cells also raises the interesting possibility that some of the effects of acetylcholine on secretion may be mediated indirectly through an action on the myoepithelial cells, rather than a direct effect of acetylcholine on the acinar cells. We have also shown that M3R is likely to be present in the basal membrane of acinar cells, where it would mediate the direct effects of acetylcholine on secretion.…”