Avian salt gland: a model for the study of membrane biogenesis

Phenotypic Plasticity in Animals Exposed to Osmotic Stress - Is it Always Adaptive?

Hyperplasia and hypertrophy are elements of phenotypic plasticity adjusting organ size and function. Because they are costly, we assume that they are beneficial. In this review, the authors discuss examples of tissue and organ systems that respond with plastic changes to osmotic stress to raise awareness that we do not always have sufficient experimental evidence to conclude that such processes provide fitness advantages. Changes in hydranth architecture in the hydroid Cordylophora caspia or variations in size in the anal papillae of insect larvae upon changes in medium salinity may be adaptive or not. The restructuring of salt glands in ducklings upon salt-loading is an example of phenotypic plasticity which indeed seems beneficial. As the genomes of model species are recently sequenced and the animals are easy to rear, these species are suitable study objects to investigate the biological significance of phenotypic plasticity and to study potential epigenetic and other mechanisms underlying phenotypic changes.

Coping with excess salt: adaptive functions of extrarenal osmoregulatory organs in vertebrates

In all organisms, changing environmental conditions require appropriate regulatory measures to physiologically adjust to the altered situation. Uptake of excess salt in non-mammalian vertebrates having limited or no access to freshwater is balanced by extrarenal salt excretion through specialized structures called 'salt glands'. Nasal salt glands of marine birds are usually fully developed in very early stages of their lives since individuals of these species are exposed to salt soon after hatching. In individuals of other bird species, salt uptake may occur infrequently. In these animals, glands are usually quiescent and glandular cells are kept in a fairly undifferentiated state. This is the situation in 'naive' ducklings, Anas platyrhynchos, which have never been exposed to excess salt. When these animals become initially osmotically stressed, the nasal glands start to secrete a moderately hypertonic sodium chloride solution but secretory performance is meager. Within 48 h after the initial stimulus, however, the number of cells per gland is elevated by a factor of 2-3, the secretory cells differentiate and acquire full secretory capacity. During this differentiation process, extensive surface specializations are formed. The number of mitochondria is increased and metabolic enzymes and transporters are upregulated. These adaptive growth and differentiation processes result in a much higher efficiency of salt excretion in acclimated ducklings compared with naive animals. Receptors and signal transduction pathways in salt gland cells controling the adaptive processes seem to be the same as those controling salt secretion, namely muscarinic acetylcholine receptors and receptors for vasoactive intestinal peptide. This review focusses on signal transduction pathways activated by muscarinic receptors which seem to fine-tune salt secretion in salt-adapted ducklings and may control adaptive growth and differentiation processes in the nasal gland of naive animals.

Vertebrate salt glands: short- and long-term regulation of function

Excess salt loads in most non-mammalian vertebrates are dealt with by a variety of extra-renal salt-secreting structures collectively described as salt glands. The best studied of these are the supra-orbital nasal salt glands of birds. Two distinct types of response to osmoregulatory disturbances are shown by this structure: a progressive adaptive response on initial exposure to a salt load that results in the induction and enhancement of the secretory performance or capabilities of the gland; and the rapid activation of existing osmoregulatory mechanisms in the adapted gland in response to immediate osmoregulatory imbalance. Not only is the time-frame of these two types of response very different, but the responses usually involve fundamentally different processes: e.g., the growth and differentiation of osmoregulatory structures and their components in the former case, compared with the rapid activation of ion channels, pumps etc. in the latter. Despite marked differences in the nature and time-frame of these responses, they both are apparently triggered by neuronally released acetylcholine, which acts at muscarinic receptors on the secretory cells to induce an inositol phosphate-dependent increase in cytosolic-free calcium concentrations ([Ca2+]i). Therefore, the question arises as to how the cells produce the appropriate distinct response using a single common signal (i.e., an increase in [Ca2+]i). Examination of the features of this signaling pathway in the two conditions described, reveals that they each are uniquely tuned to generate a response with the characteristics appropriate for the cells' requirements. This tuning of the signal involves often rather subtle changes in the overall signaling pathway that are part of the adaptive differentiation process.

In vivo and in vitro induction of c-fos in avian exocrine salt gland cells

Osmotic stress in ducklings (Anas platyrhynchos) results in salt secretion and adaptive cell proliferation and differentiation in the nasal glands. We investigated whether osmotic stress in vivo or muscarinic ACh receptor activation in vitro changed the expression levels of the cellular protooncogene products Fos and Jun, which may play a role in the initiation of the adaptive processes. Using Fos- and Jun-specific polyclonal antisera in Western blot experiments, we demonstrated that Jun is constitutively expressed in nasal gland tissue, whereas Fos is not detectable in tissue from unstressed (naive) animals. Under conditions of osmotic stress imposed by replacing the drinking water of the animals with a 1% NaCl solution, Jun protein remains constant in nasal gland tissue, whereas Fos protein is transiently upregulated. Treatment of cultured nasal gland tissue with muscarinic agonists results in a transcriptionally regulated expression of Fos in an atropine-sensitive manner. Immunohistochemical experiments show that Fos accumulation occurs in the nuclei of the secretory cells. These results indicate that the activation of the c-fos gene induced by muscarinic ACh receptor-mediated signaling pathways may play an important role in the initiation of adaptive growth and differentiation processes in nasal glands of osmotically stressed ducklings.

Identification and cDNA cloning of 35-kDa phosphatidic acid phosphatase (type 2) bound to plasma membranes. Polymerase chain reaction amplification of mouse H2O2-inducible hic53 clone yielded the cDNA encoding phosphatidic acid phosphatase

We previously described the purification of an 83-kDa phosphatidic acid phosphatase (PAP) from the porcine thymus membranes (Kanoh, H., Imai, S.-i., Yamada, K. and Sakane, F.(1992) J. Biol. Chem. 267, 25309-25314). However, we found that a minor 35-kDa protein could account for the PAP activity when the purified enzyme preparation was further analyzed. We thus determined the N-terminal sequence of the 35-kDa candidate protein and prepared antipeptide antibody against the determined sequence, MFDKTRLPYVALDVL. The antibody almost completely precipitated the purified enzyme activity. Furthermore, the antibody precipitated from the radioiodinated enzyme preparation a single 35-kDa protein, which was converted to a 29-kDa form when treated with N-glycanase. We also found that the immunoprecipitable PAP activity was exclusively associated with the plasma membranes of porcine thymocytes. These results indicated that the 35-kDa glycosylated protein represents the plasma membrane-bound (type 2) PAP. We surprisingly noted that the N-terminal sequence of the porcine PAP was almost completely conserved in the internal sequence encoded by a mouse partial cDNA clone, hic53, reported as a H2O2-inducible gene (Egawa, K., Yoshiwara, M., Shibanuma, M., and Nose, K.(1995) FEBS Lett. 372, 74-77). We thus amplified from the mouse kidney RNA the hic53 clone by polymerase chain reaction, and obtained a cDNA encoding a novel protein of 283 amino acid residues with a calculated Mr of 31,894. Methionine reported as an internal residue was found to serve as an initiator, and the C-terminal 64 residues were lacking in hic53. The protein contains several putative membrane-spanning domains and two N-glycosylation sites. When transfected into 293 cells, the cDNA gave more than 10-fold increase of the membrane-bound PAP activity, which could be precipitated by the antipeptide antibody. In [35S]methionine-labeled cells, the translational product was confirmed to be a 35-kDa protein, which became 30 kDa in cells treated with tunicamycin, an inhibitor of N-glycosylation. We thus succeeded first in identifying the porcine type 2 PAP and subsequently in determining the primary structure of a mouse homolog of the PAP.

Adaptive differentiation of avian exocrine cells alters their pHi response to mAChR activation

Muscarinic receptor-mediated changes in intracellular pH (pHi) were measured in isolated 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-loaded cells, suspended in bicarbonate-containing media, from the exocrine nasal gland of freshwater-fed ducklings (Anas platyrhynchos). The pHi recovery from an acid load was sensitive to amiloride, required sodium ions in the external medium, and was independent of added bicarbonate. These findings are consistent with the hypothesis that the pHi recovery was mediated by a Na+/H+ exchanger. Muscarinic activation of cells resulted in a sustained cytosolic alkalinization that was sensitive to atropine and that was blocked by amiloride. Activation of protein kinase C (PKC) or inhibition of protein phosphatases mimicked the effect of receptor activation on pHi, whereas inhibitors of PKC blocked the response, indicating that phosphorylation of a major pHi control mechanism results in a shift of pHi to more alkaline values. In contrast, fully differentiated salt gland cells isolated from nasal glands of salt-stressed ducklings responded to muscarinic receptor activation with a transient cytosolic acidification. These findings raise the question whether the cytosolic alkalinization in muscarinic acetylcholine receptor-activated naive cells may serve as a signal or a permissive factor for the initiation of adaptive growth and/or differentiation processes observed in the salt glands of salt-stressed birds.

Fiber type- and position-dependent expression of a myosin light chain-CAT transgene detected with a novel histochemical stain for CAT

We recently generated and characterized transgenic mice in which regulatory sequences from a myosin light chain gene (MLC1f/3f) are linked to the chloramphenicol acetyltransferase (CAT) gene. Transgene expression in these mice is specific to skeletal muscle and graded along the rostrocaudal axis: adult muscles derived from successively more caudal somites express successively higher levels of CAT. To investigate the cellular basis of these patterns of expression, we developed and used a histochemical stain that allows detection of CAT in individual cells. Our main results are as follows: (a) Within muscles, CAT is detected only in muscle fibers and not in associated connective tissue, blood vessels, or nerves. Thus, the tissue specificity of transgene expression observed by biochemical assay reflects a cell-type specificity demonstrable histochemically. (b) Within individual muscles, CAT levels vary with fiber type. Like the endogenous MLC1f/3f gene, the transgene is expressed at higher levels in fast-twitch (type II) than in slow-twitch (type I) muscle fibers. In addition, CAT levels vary among type II fiber subtypes, in the order IIB greater than IIX greater than IIA. (c) Among muscles that are similar in fiber type composition, the average level of CAT per fiber varies with rostrocaudal position. This position-dependent variation in CAT level is apparent even when fibers of a single type are compared. From these results, we conclude that fiber type and position affect CAT expression independently. We therefore infer the existence of separate fiber type-specific and positionally graded transcriptional regulators that act together to determine levels of transgene expression.

Biochemical and immunochemical determination of (Na+ + K+)-ATPase content in subcellular fractions prepared from the avian salt gland: evidence for enzyme heterogeneity

Subcellular membrane fractions were prepared from the salt glands of osmotically-stressed ducklings. Two fractions were characterized biochemically with respect to (Na+ + K+)-ATPase, alkaline phosphodiesterase I, succinate dehydrogenase, esterase, and galactosyltransferase activities and immunochemically with respect to (Na+ + K+)-ATPase. The ratios of the estimates of the (Na+ + K+)-ATPase contents obtained biochemically and immunochemically from the two fractions differed by more than 2 X. The results are consistent with the presence of at least two molecular species of (Na+ + K+)-ATPase, unevenly distributed between the two fractions.

Partial immunochemical identity between (Na+ + K+)-ATPase and a membrane component extractable with chloroform: methanol

An IgG fraction prepared from an antiserum against a holoenzyme preparation of (Na+ + K+)-ATPase precipitated a single antigen when samples of holoenzyme were subjected to crossed immunoelectrophoresis but precipitated an additional, immunochemically-related antigen when a plasma membrane-enriched fraction was subjected to crossed immunoelectrophoresis under the same conditions. The immunochemically-related antigen could be extracted from the plasma membrane fraction with CHCl3:CH3OH.