Tear secretion by lacrimal glands in transgenic mice lacking water channels AQP1, AQP3, AQP4 and AQP5

The expression of three aquaporin (AQP)-type water channels has been reported in the lacrimal gland: AQP5 in the apical membranes of acinar and duct cells, AQP4 in the basolateral membranes of acinar cells, and AQP1 in microvascular endothelia. Recent experiments indicate that water movement through AQP5 in the salivary gland is important in saliva secretion. To investigate the role of aquaporins in lacrimal gland function, basal and pilocarpine-stimulated tear secretion was compared in wildtype mice and knockout mice lacking AQP1, AQP4 and AQP5, as well as AQP3, which was found here to be expressed in the basolateral membrane of acinar cells. Tear fluid was collected in anesthetized mice using microcapillary tubes before and at 4 min intervals after pilocarpine administration. Tear fluid volumes were (in microliter per 4 min, S.E.): 0.69 +/- 0.06 (wildtype mice), 0.70 +/- 0.07 (AQP1 -/-), 0.81 +/- 0.13 (AQP3 -/-), 0.62 +/- 0.14 (AQP4 -/-), and 0.78 +/- 0.09 (AQP5 -/-) (differences not significant). Chloride concentrations (average 155 +/- 13 mM) measured by a fluorescence assay were also not different in tear fluid collected from wildtype vs aquaporin null mice. These findings provide direct evidence against an essential role for aquaporins in lacrimal gland fluid secretion. The requirement for aquaporins in salivary but not lacrimal gland secretion, may involve the substantially slower fluid secretion rate across lacrimal gland acinar cells.     

Mildly abnormal retinal function in transgenic mice without Müller cell aquaporin-4 water channels

PURPOSE: Immunocytochemistry showed strong aquaporin (AQP)-4 water channel expression in Müller cells in mouse retina and fibrous astrocytes in optic nerve. This study was designed to test the hypothesis that AQP4 is required for vision by comparing electroretinograms and retinal morphology in wild-type mice and transgenic knockout mice with no AQP4. METHODS: Electroretinograms were recorded over a 10(5)-fold range of flash intensities in dark-adapted mice and analyzed for a- and b-wave amplitude and latency, a-wave normalized slope, and oscillatory potential amplitude and latency. AQP4 protein was localized in mouse retina by immunocytochemistry, and retinal morphology was studied by light and electron microscopy. RESULTS: Significantly reduced electroretinogram b-wave potentials were recorded in 10-month-old null mice with smaller changes in 1-month-old mice. Immunocytochemistry showed strong AQP4 protein expression in retina of wild-type mice. Morphologic analysis of retina by light and electron microscopy showed no differences in retinal ultrastructure. CONCLUSIONS: Retinal function is mildly impaired in AQP4-null mice, suggesting a role for AQP4 in Müller cell fluid balance. These results support the paradigm that AQP4 expression in supportive cells in the nervous system facilitates neural signal transduction in nearby electrically excitable cells.     

Physiological importance of aquaporins: lessons from knockout mice

The phenotype analysis of transgenic mice deficient in specific aquaporin water channels has provided new insights into the role of aquaporins in organ physiology. AQP1-deficient mice are polyuric and are unable to concentrate their urine in response to water deprivation or vasopressin administration. AQP1 deletion reduces osmotic water permeability in the proximal tubule, thin descending limb of Henle and vasa recta, resulting in defective proximal tubule fluid absorption and medullary countercurrent exchange. Mice lacking AQP3, a basolateral membrane water channel expressed mainly in the cortical collecting duct, are remarkably polyuric but are able to generate a partly concentrated urine after water deprivation. In contrast, mice lacking AQP4, a water channel expressed mainly in the inner medullary collecting duct, manifest only a mild defect in maximum urinary concentrating ability. These data, together with phenotype analyses of the brain, lung, salivary gland, and gastrointestinal organs, support the paradigm that aquaporins can facilitate near-isosmolar transepithelial fluid absorption/secretion as well as rapid vectorial water movement driven by osmotic gradients. The phenotype data obtained from aquaporin knockout mice suggest the utility of aquaporin blockers as novel diuretic agents.     

Features of transient hypogammaglobulinaemia in infants screened for immunological abnormalities

The incidence of transient hypogammaglobulinaemia of infancy (THI) detected in a major paediatric centre over a 10 year period was examined. A total of 2468 subjects less than 2 years of age had an IgG measurement taken between July 1979 and March 1990. Subjects with known immunodeficiencies were excluded. Fifteen patients were classified as having THI with an initial IgG level less than the fifth centile followed by a second measurement within the normal range. A further 24 patients were identified as having possible THI with a single low IgG concentration. There were 60,174 live births each year in Victoria in the years 1979-88. This gives an incidence of proved THI of 23 per 10(6) births, and including proved and probable THI an incidence of 61 per 10(6) live births. Of those patients with proved THI 12/15 had symptoms of either atopic disease or food allergy/intolerance and three had gastrointestinal symptoms without any evidence of atopic disease. At presentation 12/15 (80%) were IgA deficient and 9/15 had IgM concentrations less than the 20th centile for age. It is suggested that in view of the preponderance of atopic and food intolerant patients that subclinical protein loss from the bowel due to allergic inflammation may be a contributing factor to the development of THI in some patients.     

Dynamic computed tomography of the head and neck: differential diagnostic value

A retrospective review of the dynamic CT studies performed in our institution on head and neck lesions, excluding the brain, was carried out. Five basic types of density vs. time curves were obtained. Dynamic CT scanning is valuable in the differential diagnosis, management, and followup of such cases; its usefulness as an imaging modality in diagnosis and followup of hemangiomas is stressed.     

Drug discovery and epithelial physiology

PURPOSE OF REVIEW: Small-molecule inhibitors and activators of gene products or cell functions can be valuable research tools for analyzing gene function ('chemical genetics'), and as leads for the development of new therapies ('drug discovery'). The recent National Institutes of Health roadmap highlights small-molecule discovery and applications in cellular and in-vivo systems as an important new research direction. The purpose of this review is to explain the small-molecule discovery process for investigators doing research in an academic setting, with emphasis on advances and directions in epithelial transport physiology. RECENT FINDINGS: The small-molecule discovery process involves the identification and validation of gene or phenotype targets, the screening of collections of small compounds for activity against the target, and the evaluation and optimization of compounds of interest. Many potential targets in renal epithelial physiology are suitable for small-molecule identification. Although small-molecule discovery in epithelial biology is in its infancy, recent advances have been reported in modulating the function of epithelial chloride channels, including the cystic fibrosis transmembrane conductance regulator and ClC-type chloride channels. SUMMARY: Small-molecule discovery by the screening of chemical libraries is feasible in the academic setting, and holds great potential for the elucidation of gene function and complex regulatory pathways, and the identification of lead drug candidates for rare diseases and diseases of limited commercial interest. The rapid chemical turn-off of gene function addresses the concerns of compensatory/developmental changes in cell and animal models of gene deletion.