Acid-base regulation in fishes: cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na(+)/H(+) exchange (NHE) and vacuolar-type H(+)-ATPase transport H(+) from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H(+)-ATPase drives H(+) excretion and is indirectly coupled to Na(+) uptake (via Na(+) channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H(+)-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl(-)/HCO(3)(-) anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance.     

Immune regulation by helminth parasites: cellular and molecular mechanisms

Immunology was founded by studying the body's response to infectious microorganisms, and yet microbial prokaryotes only tell half the story of the immune system. Eukaryotic pathogens--protozoa, helminths, fungi and ectoparasites--have all been powerful selective forces for immune evolution. Often, as with lethal protozoal parasites, the focus has been on acute infections and the inflammatory responses they evoke. Long-lived parasites such as the helminths, however, are more remarkable for their ability to downregulate host immunity, protecting themselves from elimination and minimizing severe pathology in the host.     

Formation of the middle ear: recent progress on the developmental and molecular mechanisms

The middle ear allows animals to hear while moving in an aerial medium. It is composed of a cavity harbouring a chain of three ossicles that transmit vibrations produced by airborne sound in the tympanic membrane into the inner ear, where they are converted into neural impulses. The middle ear develops in the branchial arches, and this requires sequential interactions between the epithelia and the underlying mesenchyme. Gene-inactivation experiments have identified genes required for the formation of different middle ear components. Some encode for signalling molecules, including Endothelin1 and Fgf8, probable mediators of epithelial-mesenchymal interactions. Other genes, including Eya1, Prx1, Hoxa1, Hoxa2, Dlx1, Dlx2, Dlx5, and Gsc, are most likely involved in patterning and morphogenetic processes in the neural crest-derived mesenchyme. Mechanisms controlling formation of a functional tympanic membrane are also discussed. Basically, the tympanic ring, which serves as support for the tympanic membrane, directs invagination of the first pharyngeal cleft ectoderm to form the external acoustic meatus (EAM), which provides the outer layer of the membrane. Gsc and Prx1 are essential for tympanic ring development. While invaginating, the EAM controls skeletogenesis in the underlying mesenchyme to form the manubrium of the malleus, the link between the membrane and the middle ear ossicles.     

Ion channels: recent progress and prospects

Determination of the crystal structure of the KcsA potassium channel and its subsequent refinement at 2 A resolution have stimulated much interest in modelling of ion channels. Here we review the recent developments in ion channels research, focusing especially on the question of structure-function relationships, and discuss how permeation models based on Brownian and molecular dynamics simulations can be used fruitfully in this endeavour.     

Estrogens and the skeleton: cellular and molecular mechanisms

Postmenopausal women lose bone mineral density and this loss can be prevented by estrogen administration. Although the skeletal effects of estrogens have been regarded previously as indirect, estrogen receptors have been discovered in cultured human osteoblasts and related cell lines. The UMR106 cell line derived from a rat osteogenic osteosarcoma is such an osteoblast model. We have shown direct effects of estradiol (E) on these cells in vitro, inhibiting growth and stimulating alkaline phosphatase activity (AP) corrected for cell number. This response was maximal at E conc. of 10(-10) M in serum and Phenol Red free medium, was metabolite specific and cell cycle-dependent. These cells contain high affinity binding sites with a Kd of 0.5 nM. Estrogen receptors were detected by the monoclonal antibody H-222 on Western blot after initial immunoprecipitation to concentrate the proteins. E treatment increased several enzymes including creatine kinase and LDH isoenzymes along with increments in intracellular transferrin. Transforming growth factor-beta is secreted by these cells. Secretion of this peptide was stimulated by E. TGF-beta mediated the transient growth inhibition associated with E treatment. Insulin like growth factors (IGF) are also secreted by these cells with IGF-II concentrations in the culture medium being eight times higher than IGF-I levels. E treatment increased the concentrations of both IGFs in the culture medium after a 3 day incubation. Exposure of E treated cells manifested a mitogenic response and reduced AP, indicating that E induced receptors for IGFs. These findings establish direct effects of E on osteoblastic cells in vitro and demonstrate responses to E at many levels. These osteoblast responses in vitro suggest an important role for sex steroids in the development and function of the osteoblast lineage.     

Ion regulation in invertebrates: molecular and integrative aspects

The subject of ion regulation in invertebrates is discussed, using a variety of invertebrate model species and approaches that range from the whole-organism level to tissue, subcellular, and molecular levels to illustrate the future direction of the field. These organisms inhabit a variety of aquatic, freshwater, and terrestrial environments, showing specific adaptations to each environment. This overview discusses mechanisms of metal detoxification and the presence of Cl-ATPase in marine organisms to avoid excess intracellular Cl(-); Ca(2+) regulation and endocrine aspects of adaptations to transitional (semiterrestrial) environments; adaptations to Ca(2+)-poor freshwater, particularly the reabsorption of Ca(2+) through specific transporters found in the urine; and finally, ionoregulatory mechanisms for life on land, such as Ca(2+) conservation during molting in isopods and the presence of K(+) channels in insect Malpighian tubules. Convergent mechanisms for dealing with similar problems in dissimilar habitats are discussed, taking into consideration that invertebrates will continue to serve as model systems for the evolution of ionoregulation in different habitats.     

Mechanisms of ion and acid-base regulation at the gills of freshwater fish.

This review examines the branchial mechanisms utilized by freshwater fish to regulate internal acid-base status and presents a model to explain the underlying basis of the compensatory processes. Rainbow trout, Oncorhynchus mykiss, and brown bullhead, Ictalurus nebulosus, were examined under a variety of experimental treatments which induced respiratory and metabolic acid-base disturbances. Acid-base regulation was achieved by appropriate adjustments of Na+ and Cl- net fluxes across the gills which, in turn, were accomplished by variable contributions of three different branchial mechanisms: 1) differential changes in Na+ and Cl- diffusive effluxes, 2) changes in internal substrate (H+, HCO3-) availability, and 3) morphological adjustments to the gill epithelium. Differential diffusive efflux of Na+ over Cl- was involved only during periods of metabolic alkalosis. The importance of internal substrate availability was demonstrated using a two-substrate model. According to the model, ionic flux rates (J(in)Cl-, J(in)Na+) are determined not only by the concentration of the external ion (Na+, Cl-) but also by the concentration of the internal counterion (H+, HCO3-). This system provides for an "automatic negative feedback" to aid in the compensation of metabolic acid-base disturbances. Morphological alteration of the gill epithelia and the associated regulation of chloride cell (CC) fractional area is an essential third mechanism which is especially important during respiratory acid-base disturbances. Specifically, fish vary the availability of the CC associated Cl-/HCO3- exchange mechanism by physical covering/uncovering of CCs by adjacent pavement cells.     

Retinoid-mediated regulation of mood: possible cellular mechanisms

Vitamin A and its derivatives, the retinoids, have long been studied for their ability to alter central nervous system (CNS) development. Increasingly, it is recognized that sufficient levels of retinoids may also be required for adult CNS function. However, excess dietary vitamin A, due to the consumption of supplements or foods rich in vitamin A, has been reported to induce psychosis. In addition, 13-cis-retinoic acid (13-cis-RA, isotretinoin), the active ingredient in the acne treatment Accutane, has been reported to cause adverse psychiatric events, including depression and suicidal ideation. Nevertheless, epidemiological studies have reported no consistent link between Accutane use and clinical depression in humans. Using an animal model, we have recently shown that 13-cis-RA induces an increase in depression-related behavior. Impairments in spatial learning and memory have also been demonstrated following 13-cis-RA treatment in mice. This review focuses on the behavioral and possible cellular effects of retinoid deficiency or excess in the adult brain in relation to altered mood. Specifically, we discuss the effect of retinoids on depression-related behaviors and whether norepinephrinergic, dopaminergic, or serotonergic neurotransmitter systems may be impaired. In addition, we consider the evidence that adult neurogenesis, a process implicated in the pathophysiology of depression, is reduced by retinoid signaling. We suggest that 13-cis-RA treatment may induce depression-related behaviors by decreasing adult neurogenesis and/or altering the expression of components of serotonergic neurotransmitter system, thereby leading to impaired serotonin signaling.     

Neuronal plasticity and cellular immunity: shared molecular mechanisms

It is becoming evident that neurons express an unusual number of molecules that were originally thought to be specific to immune functions. One such molecule, class I major histocompatibility complex, is required in the activity-dependent refinement and plasticity of connections in the developing and adult central nervous system, demonstrating that molecules can perform critical roles in both systems. Recent studies reveal striking parallels between cellular signaling mechanisms in the immune and nervous systems that may provide unexpected insights into the development, function, and diseases of both systems.     

Natural agents: cellular and molecular mechanisms of photoprotection

The skin is the largest organ of the body that produces a flexible and self-repairing barrier and protects the body from most common potentially harmful physical, environmental, and biological insults. Solar ultraviolet (UV) radiation is one of the major environmental insults to the skin and causes multi-tiered cellular and molecular events eventually leading to skin cancer. The past decade has seen a surge in the incidence of skin cancer due to changes in life style patterns that have led to a significant increase in the amount of UV radiation that people receive. Reducing excessive exposure to UV radiation is desirable; nevertheless this approach is not easy to implement. Therefore, there is an urgent need to develop novel strategies to reduce the adverse biological effects of UV radiation on the skin. A wide variety of natural agents have been reported to possess substantial skin photoprotective effects. Numerous preclinical and clinical studies have elucidated that natural agents act by several cellular and molecular mechanisms to delay or prevent skin cancer. In this review article, we have summarized and discussed some of the selected natural agents for skin photoprotection.     

Growth hormone secretion: molecular and cellular mechanisms and in vivo approaches

Growth hormone (GH) release is under the direct control of hypothalamic releasing hormones, some being also produced peripherally. The role of these hypothalamic factors has been understood by in vitro studies together with such in vivo approaches as stalk sectioning. Secretion of GH is stimulated by GH-releasing hormone (GHRH) and ghrelin (acting via the GH secretagogue [GHS] receptor [GHSR]), and inhibited by somatostatin (SRIF). Other peptides/proteins influence GH secretion, at least in some species. The cellular mechanism by which the releasing hormones affect GH secretion from the somatotrope requires specific signal transduction systems (cAMP and/or calcium influx and/or mobilization of intracellular calcium) and/ or tyrosine kinase(s) and/or nitric oxide (NO)/cGMP. At the subcellular level, GH release (at least in response to GHS) is accomplished by the following. The GH-containing secretory granules are moved close to the cell surface. There is then transient fusion of the secretory granules with the fusion pores in the multiple secretory pits in the somatotrope cell surface.     

The cellular mechanisms and regulation of metastasis formation

Data accumulating in biochemistry, molecular and cell biology, and experimental oncology indicate that metastasis, which is the formation of secondary tumor growth foci during cancer progression, is a highly determinate and regulated process. The process includes the emergence of a metastatic cell population possessing special properties to allow cell dissemination and seeding in distant organs and the formation of a specific microenvironment in target organs. Among other changes, metastatic cells display plasticity, being capable of switching their motility to the most efficient mode depending on the properties of surrounding tissues; start expressing specific surface receptors to ensure their migration to target organs; and acquire certain features of stem cells, being capable of surviving and reproducing in an alien microenvironment. A specific niche, known as the premetastatic niche, develops in the target organ in a strong coordination with the above changes to stimulate the initiation and growth of a prospective metastasis. The review considers the recent findings related to the mechanisms that regulate the emergence of the metastatic cell population, the formation of premetastatic niches, and the coordination of the two processes.     

Seizures in the developing brain: cellular and molecular mechanisms of neuronal damage, neurogenesis and cellular reorganization

Epilepsy is a common neurological disorder that occurs more frequently in children than in adults. The extent that prolonged seizure activity, i.e. status epilepticus (SE), and repeated, brief seizures affect neuronal structure and function in both the immature and mature brain has been the subject of increasing clinical and experimental research. Earlier studies suggest that seizure-induced effects in the immature brain compared with the adult brain are different. This is manifested as differences in neuronal vulnerability, cellular and synaptic reorganization and regenerative processes. The focus of this review is first to give a short overview of currently used experimental models of epilepsy in immature rats, and then discuss more thoroughly seizure-induced acute and sub-acute cellular and molecular alterations, highlight the contribution of inflammatory-like reactions and intracellular cytoskeleton to the insult, and reveal changes in the structure and function of inhibitory GABA(A) and excitatory glutamate receptors. The role of seizure-activated reparative, plastic processes, synaptic remodelling, neurogenesis as well as the long-term consequences of seizures are briefly outlined. The main emphasis is put on studies carried out in experimental animals, and the focus of interest is the hippocampus, the brain area of great vulnerability in epilepsy. In vitro studies are discussed only to limited extent. Collectively, recent studies suggest that the deleterious effects of seizures may not solely be a consequence of neuronal damage and loss per se, but could be due to the fact that seizures interfere with the highly regulated developmental processes in the immature brain.     

Significance and mechanisms of cellular regulation of the immune response

The conditions known to favor the induction of delayed-type hypersensitivity (DTH), IgM and IgG antibody production, can be accounted for on the postulate that their precursors require the formation of different numbers of inductive complexes between their receptors, antigen, and the antigen-specific factor derived from helper T cells. The postulate that DTH precursors require the least, IgM B cells an intermediate number, and IgG precursors the most, accounts for the following facts: i) antigens with few foreign sites, for which there are relatively few helper T cell clones, induce only DTH; ii) medium doses of antigens that bear many foreign sites induce a humoral response; whereas iii) low doses that do not result in efficient collaboration induce DTH; and iv) high doses that partially block collaboration also lead to the induction of DTH. Furthermore, the conditions under which unresponsiveness can be induced at the humoral level in immunological competent animals are just those that give rise to the induction of DTH; the induction of a humoral response is also known to result in unresponsiveness at the DTH level. Therefore it seems very likely that these unresponsive states reflect the cellular regulation responsible for the exclusiveness between the induction of DTH and humoral immunity observed in the whole animal. Theoretically, this exclusiveness is due to the action of regulatory T cells. The biological significance of the way in which the induction of different classes is regulated is discussed. Experimental evidence is described that tests the following predictions: i) the class of response induced is due to the action of suppressor and repressor T cells, and ii) it is the number of inductive complexes formed that determines the class of response induced; DTH precursors require the least number and IgG precursors the most.