Distribution of F-actin in bipolar and horizontal cells of bass retinas

Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes may occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system. We compared membrane properties of neurons isolated from the retina of goldfish (Carassius auratus), an organism well-adapted to extreme environmental change, during the summer and winter months. Goldfish were maintained in a facility under constant environmental conditions throughout the year. Analysis of whole-retina phospholipid composition using mass spectrometry-based lipidomics revealed a two-fold increase in phosphatidylethanolamine species during the winter, suggesting an increase in cell membrane fluidity. Atomic force microscopy was used to produce localized, nanoscale-force deformation of neuronal membranes. Measurement of Young's modulus indicated increased membrane-cortical stiffness (or decreased elasticity) in neurons isolated during the winter. Voltage-clamp electrophysiology was used to assess physiological changes in neurons between seasons. Winter neurons displayed a hyperpolarized reversal potential (Vrev) and a significantly lower input resistance (Rin) compared to summer neurons. This was indicative of a decrease in membrane excitability during the winter. Subsequent measurement of intracellular Ca2+ activity using Fura-2 microspectrofluorometry confirmed a reduction in action potential activity, including duration and action potential profile, in neurons isolated during the winter. These studies demonstrate chemical and biophysical changes that occur in retinal neurons of goldfish throughout the year without exposure to seasonal cues, and suggest a novel mechanism of seasonal regulation of retinal activity.

Fixation strategies for retinal immunohistochemistry

Immunohistochemical and ex vivo anatomical studies have provided many glimpses of the variety, distribution, and signaling components of vertebrate retinal neurons. The beauty of numerous images published to date, and the qualitative and quantitative information they provide, indicate that these approaches are fundamentally useful. However, obtaining these images entailed tissue handling and exposure to chemical solutions that differ from normal extracellular fluid in composition, temperature, and osmolarity. Because the differences are large enough to alter intercellular and intracellular signaling in neurons, and because retinae are susceptible to crush, shear, and fray, it is natural to wonder if immunohistochemical and anatomical methods disturb or damage the cells they are designed to examine. Tissue fixation is typically incorporated to guard against this damage and is therefore critically important to the quality and significance of the harvested data. Here, we describe mechanisms of fixation; advantages and disadvantages of using formaldehyde and glutaraldehyde as fixatives during immunohistochemistry; and modifications of widely used protocols that have recently been found to improve cell shape preservation and immunostaining patterns, especially in proximal retinal neurons.

Intracellular supply of phospholipids for biliary secretion: evidence for a nonvesicular transport component

Phospholipids (PL) for biliary secretion could be supplied from the endoplasmic reticulum (ER) to the plasma membrane by cytosolic transfer proteins or transport vesicles. Therefore, we studied whether biliary secretions of PL and apolipoprotein A-I (apo A-I), as markers for the ER-to-Golgi vesicular transport pathway, are tightly coupled in isolated perfused rat livers with enhanced secretion (+60%) of PL after withdrawal of the cholesterol synthesis inhibitor pravastatin (0.1% of chow, fed for 7 days). Blocking agents dissociated the secretion of apo A-I and PL. Brefeldin A as well as cycloheximide inhibited biliary secretion of apo A-I (-52%; -68%), however, not of PL. Both bilirubin ditaurate and taurodehydrocholic acid reduced biliary secretion of PL (-27%; -79%), but not of apo A-I. The data support the concept that PL destined for biliary secretion bypass the vesicular transport pathway of apo A-I through the Golgi compartment, most likely via cytosolic transfer proteins.

Delineation of a novel hepatic route for the selective transfer of unesterified sterols from high-density lipoproteins to bile: studies using the perfused rat liver

Cholesterol is principally excreted from the body in bile as unesterified cholesterol (UC). Using the unesterified plant sterol, sitostanol (SIT), as a nonexchangeable analog for UC, we have found that high-density lipoproteins (HDL), but not low-density lipoproteins, provide a vehicle for the direct delivery of cholesterol to bile. To determine the mechanism for preferential cholesterol transport from HDL to bile, isolated rat livers were perfused with a reconstituted HDL, made with radiolabeled unesterified SIT, UC, and cholesteryl esters (CE). Total biliary sterol secretion was independent of the concentration of HDL added to perfusions, but with increasing HDL-SIT perfused, the proportion of SIT to cholesterol in bile was linearly increased. Biliary SIT secretion was rapid (detected within 2 to 4 minutes after reconstituted HDL was added to perfusions) and was dependent on the immediate presence of SIT in the perfusate, but independent of the amount of SIT that had accumulated in the liver. The ratio of SIT to UC was seven- to ninefold greater in bile than in the liver, consistent with preferential mobilization of membrane sterols delivered from HDL. Although radiolabeled UC as well as SIT was taken up from HDL by the liver and secreted in bile, net UC uptake could not be quantitated because of both UC exchange and a sizable enrichment of HDL with UC mass that approximated the SIT removed during the passage of HDL through the liver. These results are consistent with sterol transport to bile from HDL by a direct plasma membrane pathway and by a mechanism that appears to involve substitution of unesterified (exogenous) sterol from HDL for plasma membrane UC during transport. By this process, HDL can promote reverse cholesterol transport from peripheral tissues to bile, even without an increase in biliary cholesterol secretion.

Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling

We have studied the role of actin fiber assembly on calcium signaling in astrocytes. We found that (1) after astrocytes have been placed in culture, it takes several hours for organization of the definitive actin cytoskeleton. Actin organization and the number of cells engaged in calcium signaling increased in parallel. (2) Disruption of the actin cytoskeleton attenuated the calcium wave propagation; cytochalasin D treatment reduced the number of astrocytes engaged in calcium signaling. (3) Propagation of calcium waves depends on cytoskeletal function; inhibition of myosin light chain kinase suppressed wave activity. (4) Astrocytic calcium signaling is mediated by release of ATP and purinergic receptor stimulation, because agents that interfere with this cascade attenuated or reduced calcium signaling. Because purinergic receptors are fully functional shortly after plating and not affected by cytochalasin D, these observations indicate that cytoskeleton organization is a prerequisite for interastrocytic calcium signaling mediated by release of ATP.

Interocular effect of actin depolymerization on spinule formation in teleost retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles whereas darkness induces a reduction in the number of spinules. Horizontal cells show nematosomes whose size decreases as the number of spinules increases. We have investigated the involvement of actin filaments in spinule formation, by using cytochalasin D through intraocular injection into an eye. The ultrastructural analysis reveals that cytochalasin D impairs spinule formation and nematosome-size reduction in both treated and contralateral untreated retinas.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Short-term feeding of a diet enriched in phospholipids increases bile formation and the bile acid transport maximum in rats

Earlier studies suggested that the secretory rate maximum (SRm) of bile acid and the cholestasis which occurs after the SRm is reached may be determined by the hepatic or extrahepatic biliary phospholipid pool. We therefore investigated whether bile formation and the bile acid SRm could be influenced by feeding a diet enriched in phospholipids. Male rats were fed phospholipid (PLD) or triacylglycerol (TgD)-enriched diet for 3 days, and bile formation as well as biliary lipid output were measured on the 4th day. In other similarly fed groups, cholic acid was infused in stepwise increasing doses to determine the effect of PLD on the SRm of cholic acid. The plasma lipid levels were significantly lower in PLD and TgD diets compared to basal diet. But, while the levels of total cholesterol (CH), HDL-CH, and phospholipid (PH) were not significantly altered by PLD compared to TgD, the triacylglycerol levels were markedly increased by PLD. In the liver of PLD fed rats, triacylglycerol and CH ester contents decreased by 39 and 62%, respectively, while free CH and PH contents were not significantly changed. The PLD significantly augmented spontaneous bile flow, bile acid, PH and CH secretion rates compared to TgD diet (65, 124, 164 and 654%, respectively). The enhanced biliary secretory function was associated with an increase in pericanalicular vacuoles and diverticuli in centrilobular hepatocytes. Compared to TgD fed rats, PLD rats showed a 2-fold decrease in the ratio of cholic acid/chenodeoxycholic acid in bile and a significant decrease in the % contribution of taurine conjugated BA. The PH fatty acids in bile were similar in both groups except that in PLD group the % contribution of C18:2 was higher than in TgD group. No differences were found in plasma membrane CH/PH content or total fatty acid composition. During bile acid infusion, the SRm and the total cholic acid secreted were significantly higher in the PLD than in the TgD rats. Moreover, the cholestatic response observed after high bile acid dose was markedly reduced by PLD. The results show that short-term feeding of PLD induces changes in CH and bile acid metabolism which result in enhanced biliary output of CH and PH. The enhanced pool of biliary lipid may protect plasma membranes from the deleterious effects of high bile acid concentrations.

Spinule-type neurite outgrowth from horizontal cells during light adaptation in the carp retina: an actin-dependent process

Dendrites of horizontal cells in the carp retina which invaginate the cone pedicles form numerous spinules during light adaptation. We have analyzed the contribution of cytoskeletal elements to this process. Isolated horizontal cells and frozen sections were screened with phalloidin for the existence of F-actin. F-actin was present in all types of horizontal cells and particularly enriched in the distal parts of the dendrites. Electron microscopical analysis demonstrated that interruption of the F-actin polymerization with cytochalasin B inhibited the formation of spinules during light adaptation. The persistence of spinules was also affected. Cytochalasin B also prevented the light-independent, phorbol ester-induced formation of spinules. Cytochalasin B only affected the morphology of the lateral, spinule-forming dendrites of cone horizontal cells within the cone pedicles, leaving the central, non spinule-forming dendrites of cone horizontal cells and the processes of rod horizontal cells within rod spherules unaffected. Whereas cytochalasin B prevented the protrusion of spinules, the spinule-associated membrane densities were only slightly affected. The two main characteristics of spinules, protrusion and membrane densities are therefore independently regulated processes.

Cytochalasin inhibits light-dependent synaptic plasticity of horizontal cells in teleost retina

Previous studies have shown that the horizontal cell-->cone photoreceptor negative feedback synapse in teleost fish retinae is 'plastic', being suppressed in the dark and potentiated by light adaptation. The possible involvement of filamentous actin in ultrastructural and electrophysiological aspects of this plasticity has been investigated using cytochalasins, which inhibit actin turnover, in the cyprinid fish (roach) retinae. Cytochalasin B or D (40 microM) inhibited both the light-dependent formation and maintenance of spinules, and enhancement of the feedback interaction involved in generation of biphasic spectral responses in horizontal cells. The results suggest that actin turnover is essential for both ultrastructural and electrophysiological plasticity of horizontal cell feedback and that spinules could mediate this dynamic interaction.

Glial and neuronal markers in bass retinal horizontal and Müller cells

Retinal horizontal cells (HCs) are second-order neurons that integrate information from photoreceptors over large retinal areas, mediating the lateral spread of visual signals in the distal retina. The 'glial' vs. 'neuronal' nature of the HC has been widely debated. For example, carbonic anhydrase (CA), glutamine synthetase (GS), and glial fibrillary acidic protein (GFAP) are considered 'glial' markers, yet both CA and GFAP have been previously reported in HCs of the teleost retina in species-specific patterns. In contrast, the neurofilament triplet (NFT) proteins are considered 'neuronal' markers; these proteins have been immunolocalized to a mammalian HC, but are absent from teleost HCs. We have studied these cytochemical characteristics in HCs from the white bass, by immunolabeling both cryosections of intact retina and freshly isolated, identified cells attached to coverslips. We found that both HCs (neurons) and Müller cells (MCs; glia) immunolabeled with antisera to CA. Both type 1 (external) HCs and MCs immunolabeled with an antibody to vimentin. Only MCs immunolabeled with antisera to GS and GFAP. Neither HC perikarya (and their major dendrites) nor MCs immunolabeled with an antibody to the 160-kDa subunit of NFT protein. Thus, bass HCs and MCs share the presence of CA and vimentin epitopes and absence of the NFT 160-kDa epitope. Moreover, retinal cell isolation, by itself, does not affect cell-type specific immunolabeling patterns in identified cells, except for what may be lost with the finer processes of the various cells. Isolated cell studies can aid in interpreting immunolabeling patterns observed in the intact retina, especially in retinal layers where several cell types may be present.