Modulation of connexon densities in gap junctions of horizontal cell perikarya and axon terminals in fish retina: effects of light/dark cycles,interruption of the optic nerve and application of dopamine

Summary In the fish retina, connexon densities of gap junctions in the outer horizontal cells are modulated in response to different light or dark adaptation times and wavelengths. We have examined whether the connexon density is a suitable parameter of gap junction coupling under in situ conditions. Short-term light adaptation evoked low connexon densities, regardless of whether white or red light was used. Short-term dark adaptation evoked high connexon densities; this was more pronounced in the axon terminal than in perikaryal gap junctions. Under a 12 h red light/12 h dark cycle, a significant difference in connexon densities between the light and the dark period could be established in the gap junctions of the perikarya and axon terminals. Under a white light/dark cycle, only the gap junctions of axon terminals showed a significant difference. Crushing of the optic nerve resulted in an increase in connexon densities; this was more pronounced in axon terminals than in perikarya. Dopamine injected into the right eye of white-light-adapted animals had no effect. However, dopamine prevented the effect of optic-nerve crushing on connexon density. The reaction of axon-terminal gap junctions to different conditions thus resembles that of perikaryal gap junctions, but is more intense. Axon terminals are therefore thought to play an important role in the adaptation process.     

The gap theory in forest dynamics

Since the late 1970s, ecologists interested in forest dynamics have focused their attention to the responses of individuals, populations and communities to “gaps” which are openings created in the forest canopy. This review intended to introduce some collective knowledge on major subjects of the gap theory in forest dynamics, in relation to gap-disturbance regimes, tree regeneration behaviour and community structure.     

Current perspectives on plasmodesmata: structure and function

Recent studies on plasmodesmata have shown that these important intercellular passages for communication and transport are much more sophisticated in both structure and regulatory abilities than previously imagined. A complex, but not well understood, substructure has been revealed by a variety of increasingly reliable ultrastructural techniques. Proteinaceous particles are seen within the cytoplasmic sleeve surrounding the desmotubule. Dye-coupling studies have provided experimental evidence for the physical pathway of solute movement, supporting conclusions about substructural dimensions within plasmodesmata drawn from the ultrastructural studies. Calcium has been identified as a major factor in the regulation of intercellular communication via plasmodesmata. Evidence from studies on virus movement through plasmodesmata suggests a direct interaction between virallycoded movement proteins and plasmodesmata in the systemic spread of many viruses. There is increasing evidence, albeit indirect, that in some plant species phloem loading may involve transport of photoassimilate entirely within the symplast from mesophyll cells to the sieve element-companion cell complexes of minor veins.     

The design of single-stranded nucleic acid knots

A general strategy is described for the synthesis of single-stranded nucleic acid knots. Control of nucleic acid sequence is used to direct the formation of secondary structures that produce the target topology. The key feature of the strategy is the equation of a half-turn of double helical DNA or RNA with a node in a knot. By forming nodes from complementary DNA sequences, it appears possible to direct the assembly of any simple knot. Stabilization of individual nodes may be achieved by constructing them from long regions containing both B-DNA and Z-DNA. Control over the braiding of DNA that acts as a link between node-forming domains can be realized by condensing the nodes into well-defined DNA structures, such as extended domains of linear duplex, branched junctions, antijunctions or mesojunctions. Further topological control may be derived from the pairing of linker regions to complementary single-stranded molecules, thereby preventing them from braiding in an undesirable fashion.     

Interactions between intrinsic regulation and neural modulation of acetylcholinesterase in fast and slow skeletal muscles

1. Initiation of subsynaptic sarcolemmal specialization and expression of different molecular forms of AChE were studied in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle of the rat under different experimental conditions in order to understand better the interplay of neural influences with intrinsic regulatory mechanisms of muscle cells. 2. Former junctional sarcolemma still accumulated AChE and continued to differentiate morphologically for at least 3 weeks after early postnatal denervation of EDL and SOL muscles. In noninnervated regenerating muscles, postsynaptic-like sarcolemmal specializations with AChE appeared (a) in the former junctional region, possibly induced by a substance in the former junctional basal lamina, and (b) in circumscribed areas along the whole length of myotubes. Therefore, the muscle cells seem to be able to produce a postsynaptic organization guiding substance, located in the basal lamina. The nerve may enhance the production or accumulation of this substance at the site of the future motor end plate. 3. Significant differences in the patterns of AChE molecular forms in EDL and SOL muscles arise between day 4 and day 10 after birth. The developmental process of downregulation of the asymmetric AChE forms, eliminating them extrajunctionally in the EDL, is less efficient in the SOL. The presence of these AChE forms in the extrajunctional regions of the SOL correlates with the ability to accumulate AChE in myotendinous junctions. The typical distribution of the asymmetric AChE forms in the EDL and SOL is maintained for at least 3 weeks after muscle denervation. 4. Different patterns of AChE molecular forms were observed in noninnervated EDL and SOL muscles regenerating in situ. In innervated regenerates, patterns of AChE molecular forms typical for mature muscles were instituted during the first week after reinnervation. 5. These results are consistent with the hypothesis that intrinsic differences between slow and fast muscle fibers, concerning the response of their AChE regulating mechanism to neural influences, may contribute to different AChE expression in fast and slow muscles, in addition to the influence of different stimulation patterns.     

Initiation of acellular extrinsic fiber cementum on human teeth

Summary The development of acellular extrinsic fiber cementum (AEFC) has never before been studied in human teeth. We have therefore examined the initiation of AEFC in the form of a collagenous fiber fringe and its attachment to the underlying dentinal matrix, in precisely selected, erupting human premolars with roots developed to 50%–60% of their final length. Freshly extracted teeth were prefixed in Karnovsky's fixative, decalcified in EDTA and subdivided into about 10 blocks each, cut from the mesial and distal root surfaces, vertical to and along the root axis. The blocks were postfixed in osmium tetroxide, embedded in Epon and cut for light- and electron-microscopic investigation. Starting at the advancing edge of the root, within a region extending about 1 mm coronal to this edge, fibroblast-like cells were seen closely covering the external root surface. Along the first 100 m from the root edge, these cells extended cytoplasmic processes and contacted the dentinal collagen fibrils. Between these cells and the dentinal matrix, new collagen fibrils and very short collagen fibers gradually developed. Within the second 100 m from the root edge, this resulted in the formation of a cell-fiber fringe network. Newly formed fibers of the fringe were directly attached to the non-mineralized matrix containing dentinal collagen fibrils and could be distinguished from the latter by differences in fibril orientation. During the process of dentin mineralization, the transitional zone between the fiber-fringe base and the dentinal matrix, i.e., the future dentino-cemental junction, also mineralized. It is suggested that this fiber fringe is the base of AEFC, which later increases in thickness by fiber extension and subsequent mineralization.Abbreviations AEFC acellular extrinsic fiber cementum - AIFC acellular intrinsic fiber cementum - CIFC cellular intrinsic fiber cementum - CMSC cellular mixed stratified cementum - ARE advancing root edge - CP cytoplasmic process - D dentin - DCJ dentinocemental junction - E enamel - EBL external basal lamina - EC epithelial cell - EDTA ethylene diaminetetraacetic acid - ERM epithelial rests of Malassez - FF fiber fringe - HRS Hertwig's epithelial root sheath - IBL internal basal lamina - MD mineralized dentin - NMD non-mineralized dentin - OB odontoblast - PD predentin - PL periodontal ligament     

The formation of myeloid bodies in retinular cells of the pupal compound eyes of silkworm moths (Bombyx mori) exposed to a constant bright light

Summary To study the three-dimensional structure of tight junction fibrils, the epithelia of the jejunum and epididymis of adult mice were examined by the freezefracture technique in unfixed and in aldehyde-fixed specimens. The fibrils have a stronger affinity for the protoplasmic (P) face of the lipid bilayer in fixed material, and for the external (E) face in unfixed and rapidly frozen material. Therefore we can observe the fibrils both from the outside and inside of the cell. Fibrils appearing on the P-face are smoothly contoured ridges and rows of hemispherical particles, while those appearing on the E-face are exclusively rows of hemispherical particles. Based on these observations, we wish to propose a new fibril model for the tight junction. There are two distinctive types of junctional elements. One type is composed of a smooth and continuous strand in the external view of the cell, but is studded with hemispherical bulgings in its internal view. This type will be referred to as the continuous type. The other type is bead-like, and will be referred to as the particle type. The relative proportion of these two types of elements appearing within a tight junction network differs among tissues.     

Lowering of pH does not directly affect the junctional resistance of crayfish lateral axons

Summary The effect of pH was tested on the junction between crayfish lateral axons. By means of a glass capillary inserted into one of the axons, one side of the nunction was perfused with solutions of known pH while the junctional resistance,R _j, was monitored. Integrity of the gap junction was checked electron microscopically.R _j remained unchanged when the pH of the perfusate was lowered from 7.1 to 6.0. However, when the pH of the unperfused side of the junction was lowered by substituting acetate for chloride in the external solution,R _j rose, attesting to the integrity of the junction and its capacity to uncouple in the perfused state. We suggest that H⁺ does not affect the junctional channels directly, but acts through an intermediary which is inactivated or removed by the perfusion.     

Quantitative gap junction alterations in mammalian heart cells quickly frozen or chemically fixed after electrical uncoupling

Summary The gap junction morphology was quantified in freeze-fracture replicas prepared from rat auricles that had been either quickly frozen at 6 K or chemically fixed by glutaraldehyde, in a state of normal cell-to-cell conduction or in a state of electrical uncoupling. The general appearance of the gap junctions was similar after both preparative procedures. A quantitative analysis of three gap junctional dimensions provided the following measurements in the quickly frozen conducting auricles (mean±sd): (a) P-face particles' diameter 8.27±0.74 nm (n =5709), (b) P-face particles' center-to-center distance 10.78±2.12 nm (n=4800), and (c) E-face pits' distance 9.99±2.19 nm (n=1600). Corresponding values obtained from chemically fixed tissues were decreased by about 3% for the particle's diameter and about 5% for the particles' and pits' distances. Electrical uncoupling by the action of either 1 mM 2–4-dinitrophenol (DNP), or 3.5 mMn-Heptan-1-ol (heptanol), induced a decrease of the particle's diameter, which amounted to –0.69±0.01 nm (mean ±se) in the quickly frozen preparations and –0.71±0.01 nm in the chemically fixed ones. The particles' distance was decreased by –0.96±0.04 nm in the quickly frozen samples and by –0.90 ±0.03 nm in the chemically fixed ones and the E-face pits' distance was similarly reduced. All differences were statistically significant (P<0.001 for all dimensions). Electrical recoupling after the heptanol effect promoted a return of these gap junctional dimensions towards normal values, which was about 50% complete within 20 min. It is concluded that very similar morphological alterations of the gap junctional structure are induced in the mammalian heart by different treatments promoting electrical uncoupling and that these conformational changes appear independently of the preparative procedure. The suggestion that the observed decrease of the particles' diameter is genuinely related to the closing mechanism of the unit cell-to-cell channel set in thei centers is thus confirmed.     

Downregulation of cell-to-cell communication by the viralsrc gene is blocked by TMB-8 and recovery of communication is blocked by vanadate

Summary The viralsrc gene downregulates junctional communication, closing cell-to-cell membrane channels presumably by way of the phosphoinositide signal route. We show that TMB-8 [8-N, N-(diethylamino) octyl-3,4,5-trimethoxybenzoate] counteracts this downregulation in cells transformed by temperature-sensitive mutant Rous sarcoma virus: TMB-8 (36–72 m) raises junctional permeability when applied during activity ofsrc protein kinase, i.e., at steady permissive temperature; and TMB-8 inhibits the fall of junctional permeability, when the activity ofsrc protein kinase gets turned on. TMB-8 also (reversibly) inhibits the growth of the cells at permissive temperature and reverses the morphological changes associated with transformation. The morphological reversal lags several hours behind the junctional-permeability reversal. Communication recovers within a few minutes when the activity of thesrc protein kinase is turned off (in absence of TMB-8). Sodium orthovanadate (20 m) prevents this recovery, but it has no major effect on junctional permeability on its own. We discuss possible modes of action of these agents on critical stages of the signal route, related to intracellular Ca²⁺ and protein kinase C.