Mice lacking the basolateral Na-K-2Cl cotransporter have impaired epithelial chloride secretion and are profoundly deaf.

In chloride-secretory epithelia, the basolateral Na-K-2Cl cotransporter (NKCC1) is thought to play a major role in transepithelial Cl(-) and fluid transport. Similarly, in marginal cells of the inner ear, NKCC1 has been proposed as a component of the entry pathway for K(+) that is secreted into the endolymph, thus playing a critical role in hearing. To test these hypotheses, we generated and analyzed an NKCC1-deficient mouse. Homozygous mutant (Nkcc1(-/-)) mice exhibited growth retardation, a 28% incidence of death around the time of weaning, and mild difficulties in maintaining their balance. Mean arterial blood pressure was significantly reduced in both heterozygous and homozygous mutants, indicating an important function for NKCC1 in the maintenance of blood pressure. cAMP-induced short circuit currents, which are dependent on the CFTR Cl(-) channel, were reduced in jejunum, cecum, and trachea of Nkcc1(-/-) mice, indicating that NKCC1 contributes to cAMP-induced Cl(-) secretion. In contrast, secretion of gastric acid in adult Nkcc1(-/-) stomachs and enterotoxin-stimulated fluid secretion in the intestine of suckling Nkcc1(-/-) mice were normal. Finally, homozygous mutants were deaf, and histological analysis of the inner ear revealed a collapse of the membranous labyrinth, consistent with a critical role for NKCC1 in transepithelial K(+) movements involved in generation of the K(+)-rich endolymph and the endocochlear potential.

Balance and hearing deficits in mice with a null mutation in the gene encoding plasma membrane Ca2+-ATPase isoform 2.

Plasma membrane Ca2+-ATPase isoform 2 (PMCA2) exhibits a highly restricted tissue distribution, suggesting that it serves more specialized physiological functions than some of the other isoforms. A unique role in hearing is indicated by the high levels of PMCA2 expression in cochlear outer hair cells and spiral ganglion cells. To analyze the physiological role of PMCA2 we used gene targeting to produce PMCA2-deficient mice. Breeding of heterozygous mice yielded live homozygous mutant offspring. PMCA2-null mice grow more slowly than heterozygous and wild-type mice and exhibit an unsteady gait and difficulties in maintaining balance. Histological analysis of the cerebellum and inner ear of mutant and wild-type mice revealed that null mutants had slightly increased numbers of Purkinje neurons (in which PMCA2 is highly expressed), a decreased thickness of the molecular layer, an absence of otoconia in the vestibular system, and a range of abnormalities of the organ of Corti. Analysis of auditory evoked brainstem responses revealed that homozygous mutants were deaf and that heterozygous mice had a significant hearing loss. These data demonstrate that PMCA2 is required for both balance and hearing and suggest that it may be a major source of the calcium used in the formation and maintenance of otoconia.

TGFbeta2 knockout mice have multiple developmental defects that are non-overlapping with other TGFbeta knockout phenotypes.

The growth and differentiation factor transforming growth factor-beta2 (TGFbeta2) is thought to play important roles in multiple developmental processes. Targeted disruption of the TGFbeta2 gene was undertaken to determine its essential role in vivo. TGFbeta2-null mice exhibit perinatal mortality and a wide range of developmental defects for a single gene disruption. These include cardiac, lung, craniofacial, limb, spinal column, eye, inner ear and urogenital defects. The developmental processes most commonly involved in the affected tissues include epithelial-mesenchymal interactions, cell growth, extracellular matrix production and tissue remodeling. In addition, many affected tissues have neural crest-derived components and simulate neural crest deficiencies. There is no phenotypic overlap with TGFbeta1- and TGFbeta3-null mice indicating numerous non-compensated functions between the TGFbeta isoforms.

Optimal visualization of immunogold-silver staining of hepatic PEPCK with epipolarized light microscopy.

We used immunogold-silver staining to localize phosphoenolpyruvate carboxykinase in 10 microns cryosections of 4% paraformaldehyde perfusion-fixed normal male rat liver. The resolution and sensitivity of detection were improved by epipolarized light microscopy of 0.5 microns semi-thin plastic sections prepared from these pre-embedding immunogold-silver-enhanced 10-microns thick cryosections. Epipolarized light combined with transmitted light simultaneously demonstrated antigenic sites (visualized with epipolarized light illumination) and tissue morphology (revealed by transmitted light). To optimize the conditions for high resolution, an oil immersion objective lens (x 100) with adjustable iris diaphragm was used with different intensity settings for both light sources. Our observations indicate that if the intensity of the transmitted light is too high, the visibility of the gold-cored silver grains by epipolarized illumination is decreased; if the intensity of epipolarized light is too strong, haloes appear around the gold-cored silver particles. By adjusting the aperture in the objective lens and the neutral density filter in the transmitted light pathway to balance the intensities of transmitted and epipolarized light, an optimal image is obtained that shows the maximal number of antigenic sites and excellent morphology.

Glycogen metabolism in cultured chick hepatocytes: a morphological study.

Ultrastructural and autoradiographic observations of cultured chick hepatocytes under the following conditions are described: Induction of glycogen synthesis with glucose alone and glucose plus insulin, and glucagon-induced glycogen breakdown. Profiles of hepatocytes cultured in medium containing 10 mM glucose showed typical cellular organelles and occasionally a few glycogen granules. After incubation of hepatocytes with 3H-glucose, silver grains were found over these sparse glycogen granules, indicating a low level of glycogen synthesis by a few cells. After addition of 75 mM glucose for 1 hr about 3% of the profiles of cells showed glycogen, and by 24 hr half of the hepatocytes had glycogen. Addition of insulin plus glucose induced glycogen synthesis in 82% of the cells after 6 hr, and by 24 hr almost every cellular profile showed glycogen particles. Morphologically, glycogen accumulation was similar whether the cells were stimulated by high glucose or by glucose plus insulin: glycogen granules appeared in restricted regions of the cytoplasm, which were rich in smooth endoplasmic reticulum (SER), and peroxisomes were found close to the newly deposited glycogen particles. At maximum glycogen accumulation the association of SER and peroxisomes with glycogen was less obvious. Glycogenolysis induced by incubation of glycogen-rich hepatocytes with glucagon resulted in proliferation of SER in the glycogen regions of the cells. These observations are compatible with the concept of regions in the hepatocyte cytoplasm specialized for glycogen metabolism. Possible roles for SER and peroxisomes found near glycogen particles and other organelles in hepatic glycogen metabolism are discussed.

SERGE, the subcellular site of initial hepatic glycogen deposition in the rat: a radioautographic and cytochemical study.

Hormonal control of hepatic glycogen and blood glucose levels is one of the major homeostatic mechanisms in mammals: glycogen is synthesized when portal glucose concentration is sufficiently elevated and degraded when glucose levels are low. We have studied initial events of hepatic glycogen synthesis by injecting the synthetic glucocorticoid dexamethasone (DEX) into adrenalectomized rats fasted overnight. Hepatic glycogen levels are very low in adrenalectomized rats, and DEX causes rapid deposition of the complex carbohydrate. Investigation of the process of glycogen deposition was performed by light and electron microscopic (EM) radioautography using [3H]galactose as a glycogen precursor. Rats injected with DEX for 2-3 h and [3H]galactose one hour before being killed displayed an increasing number of intensely labeled hepatocytes. EM radioautography revealed silver grains over small (+/- 1 micron) ovoid or round areas of the cytosome that were rich in smooth endoplasmic reticulum (SER) and contained a high concentration of small dense particles. These distinct areas or foci of SER and presumptive glycogen (SERGE) were most numerous during initial periods of glycogen synthesis. After longer exposure to DEX (4-5 h) more typical deposits of cytoplasmic glycogen were evident in the SERGE regions. Several criteria indicated that the SERGE foci contained glycogen or presumptive glycogen: resemblance of the largest dense particles to beta-glycogen particles in EM; association of 3H-carbohydrate with the foci; removal of particles and label with alpha-amylase; and positive reaction with periodic acid-chromic acid-silver methenamine. The concentration of SER in the small foci and the association of newly formed glycogen particles with elements of SER suggest a role for this organelle in the initial synthesis of glycogen.

Effects of glucagon on hepatic microsomal glucose-6-phosphatase in vivo.

The smooth endoplasmic reticulum (SER) has been implicated in glycogen deposition and depletion. It has been suggested that SER proliferation plays a role in elevated glucose release during rapid glycogenolysis (Striffler et al., Am. J. Anat. 160: 363, 1981). In these studies, the role of SER in glucagon-stimulated hepatic glucose release was examined by assessing changes in microsomal glucose-6-phosphatase (G-6-Pase) and membrane cholesterol to phospholipid ratios. In control fed rats given 6 i.p. injections of glucagon 350 micrograms/injection) at hourly intervals, percentage hepatic glycogen decreased nearly 30 fold, with liver homogenate G-6-Pase (U/mg protein) increasing 50% (p less than .02 relative to vehicle-injected controls) from .055 +/- .003 at 0h (n = 12) to .081 +/- .004 at 6h (n = 11). The increase in homogenate G-6-Pase was reflected in the isolated SER fraction by a 48% rise (p less than .01 relative to controls) in activity from a 0h value of .210 +/- .010 (n = 10) to a peak activity of .310 +/- .027 U/mg microsomal protein at 5 h (n = 12). G-6-Pase also increased in the rough endoplasmic reticulum (RER) reaching .242 +/- .023 U/mg protein at 4h (n = 14), but then declining to .209 +/- .019 U/mg protein at 6 h (n = 11). The changes in G-6-Pase were accompanied by alterations in the ratio of microsomal cholesterol to phospholipid, which decreased by 50% (p less than .002) in both RER and SER fractions signifying changes in membrane lipid environment. Ultrastructural analysis revealed a marked reduction in hepatic glycogen and conspicuous presence of elements of the SER in regions of the cytoplasm where glycogen was or presumably had been located.

Effects of glucagon on hepatic glycogen and smooth endoplasmic reticulum.

The action of glucagon on hepatic glycogen and smooth endoplasmic reticulum (SER) was studied in samples of liver taken sequentially from anesthetized rats. The physiological state of each animal was assessed by continuously monitoring aortic blood pressure and blood lactatepyruvate ratios. High hepatic glycogen levels were established by using 10--12 hr fasted control-fed rats infused continuously with glucose. In rats receiving glucose only, hepatic glycogen levels remained above 5.0% during the 4-hr period of glucose administration. Centrilobular hepatocytes displayed an abundance of glycogen which often appeared dispersed with elements of SER between the glycogen particles. Periportal cells had dense clumps of glycogen with few vesicles of SER restricted to the periphery of the glycogen masses. The addition of glucagon to the glucose infusate caused a marked stimulation of glycogenolysis. In these rats, the hepatic glycogen level (X +/- SE) was 6.74 +/- .15% 1 hr after glucose and declined after initiation of glucagon infusion as follows: 5.86 +/- .29% (15 min), 4.89 +/- .26% (1 hr), 2.16 +/- .40% (2 hr), and 1.66 +/- .29% (3 hr). The fine structure of hepatocytes showed a dramatic response to the administration of glucagon. The glycogen regions of the cells were noticeably decreased in size and number of glycogen granules 3 hr after initiation of glucagon infusion, and SER was abundant in both periportal and centrilobular hepatocytes. The interpretation offered is that glucagon induces the formation of new SER membranes which participate in glycogen breakdown and/or glucose release from hepatocytes.

Ultrastructural observations on FUdR-induced cell death and subsequent elimination of cell debris.

Twelve-day mouse embryos were treated with fluorodeoxyuridine (FUdR) and sacrificed at various time intervals after treatment. The neuroepithelial cells were then examined to determine by electron microscopy the primary site of action of the drug. About two hours after treatment mitotic activity ceased and a number of cells were found with a normal interphase nucleus, but with a cytoplasm in which the ribosomes had lost their normal polysomal configuration and were dispersed as single ribosomes. At about the same time cells were seen with an accumulation of chromatin at the nuclear membrane and the segregation of chromatin masses within the somewhat denser karyoplasm. Concurrent with the nuclear changes was the appearance of a condensed cytosome containing monoribosomes. The condensation of the nucleus and cytoplasm was followed by fragmentation of the cell into membrane bound bodies. Since condensed cells always contained monodispersed ribosomes, it seems likely that the dispersal of the ribosomes is the first morphological sign of the action of FUdR. Since about half of the neuroepithelial population underwent cell degeneration, the second goal of this experiment was to study the fate of the dying cells. Some fragments from condensed cells were found within apparently normal neuroepithelial cells, indicating phagocytosis. In addition macrophages were seen containing phagosomes from fragmented cells. Most of the fragments, however, remained free and were not immediately phagocytosed. These unphagocytosed fragments lysed and became ghosts, thereby giving the neureopithlium a vacuolated appearance. Hence, cellular debris was eliminated partially by neuroepithelial cells, partially by macrophages and to a great extent by lysis.