Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomical relations between GABAergic synapses and dendrodendritic gap junctions

Immunocytochemical and electron microscopic methods were used to examine the GABAergic innervation of the inferior olivary nucleus in adult rats. This neuronal system was visualized with an antibody against glutamic acid decarboxylase (GAD, EC 4.1.1.15), the GABA-synthesizing enzyme. A GAD-positive reaction product was encountered only in short segments of preterminal axons and in axon terminals. Their relative number per unit area of neuropil was very similar in all olivary subnuclei. Despite this homogeneity in density, obvious intraregional differences existed. Some regions were strongly immunoreactive (the "c" subgroup, the beta nucleus, and the mediolateral outgrowth of the medial accessory olive), whereas others were weakly labeled (the dorsomedial cell column and the central zones of the medial accessory and principal olives). The strongly immunoreactive areas contained the largest and most intensively labeled axon terminals. Areas of weak labeling were filled with small, weakly immunoreactive nerve terminals. Thus, variations in size and in intensity of labeling create a specific pattern of GABA innervation, revealed by an almost continuous gradient between the above-mentioned extremes. The GAD-positive axon terminals established conventional synapses with dendrites (94% of the samples) or with cell bodies (6%). The vast majority of these synapses were type II (84%) and only a small proportion formed type I synaptic contacts (16%), regardless of the nature of the postsynaptic element. Immunoreactive terminals were also involved in the complex synaptic arrangements--the glomeruli, which characterize the olivary neuropil. Within these formations, olivary neurons were electrotonically coupled through dendrodendritic gap junctions. There was a constant association between GAD-positive axon terminals and small dendritic appendages linked by gap junctions. This association was revealed not only by the systematic presence of immunolabeled terminals directly apposed to the dendritic appendages but, more importantly, by the frequent presence of type II synapses straddling both elements. These synapses were in close proximity to the low-resistance pathways represented by the gap junctions. The strategic location of these GABA synapses is discussed in relation to recent findings indicating the possibility of a synaptic modulation of the electrical coupling: the release of GABA, by increasing nonjunctional membrane conductance, could shunt the coupling between olivary neurons. The functional decoupling of selected gap junctions would be responsible for the spatial organization of the olivary electrotonic coupling.

Regulation of a novel pathway for cell death by lysosomal aspartic and cysteine proteinases

PC12 cells undergo apoptosis when cultured under conditions of serum deprivation. In this situation, the activity of caspase-3-like proteinases was elevated, and the survival rate could be maintained by treatment with acetyl-DEVD-cho, a specific inhibitor of caspase-3. In a culture of PC12 cells treated with acetyl-DEVD-cho, where caspase-3-like proteinases are not activated, CA074, a specific inhibitor of cathepsin B induced active death of the cells. Cathepsin B antisense oligonucleotides showed a similar effect to CA074 on the induction of active cell death. By double staining of terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling and activated caspase-3, the dying cells treated with CA074 were positive for terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling staining but negative for activated caspase-3. Ultrastructurally, the cells were relatively large and had nuclei with chromatin condensation. The initiation of cell death by CA074 or the cathepsin B antisense were inhibited by the addition of pepstatin A, a lysosomal aspartic proteinase inhibitor, or by cathepsin D antisense. To examine whether this cell death pathway was present in cell types other than PC12 cells, we analysed dorsal root ganglion neurons obtained from rat embryos on the 15th gestational day, a time when they require nerve growth factor for survival and differentiation in culture. When cultured in the absence of nerve growth factor, the neurons survived in the presence of acetyl-DEVD-cho or acetyl-YVAD-cho. Under these conditions, CA074 reduced the survival rate of the neurons, which was subsequently restored by the further addition of pepstain A. These results suggest that a novel pathway for initiating cell death exists which is regulated by lysosomal cathepsins, and in which cathepsin D acts as a death factor. We speculate that this death-inducing activity is normally suppressed by cathepsin B.

Morphological and biochemical signs of age-related neurodegenerative changes in klotho mutant mice

Klotho mutant mice, defective in the klotho gene, develop multiple age-related disorders with very short lifespans. Introduction of the exogenous klotho gene into these mutant mice leads to an improvement in their phenotypes, while overexpression of this gene in wild-type mice significantly extends their lifespan. These observations suggest that the klotho gene/protein has an anti-aging function. Since there have been only a few reports with some disagreement about results on the CNS of the mutant mice, we tried to clarify whether the CNS neurons generate aging-like features, even in premature stages, using biochemical and morphological approaches. Results obtained from the mutant mice, when compared with wild-type mice, were as follows. Neurofilaments (NFs) were increased significantly in axons, with the subunit proteins showing a significant enhancement in phosphorylation or expression of NF-H or NF-L, respectively. Microtubules in Purkinje cell dendrites were closer to each other, and in the CNS tissue tubulin was unaltered, but microtubule-associated protein (MAP) 2 was significantly reduced in expression. Neuronal cellular organelles were morphologically disordered. Lysosomes, cathepsin D and light chain 3 of MAP1A/B (LC3) were augmented with the appearance of putative autophagy-related structures. Antiapoptotic Bcl-xL and proapoptotic Bax were reduced and enhanced, respectively, and mitogen-activated protein kinase was reduced. Synapse-related proteins and structures were decreased. Neuronal degeneration was evident in hippocampal pyramidal cells, and possibly in Purkinje cells. Astrocytic glial filaments and glial fibrillary acidic protein were increased in density and expression, respectively. Together, the CNS neuronal alterations in klotho mutant mice were quite similar to those found in aged animals, including even premature death, so this mouse should be a more appropriate animal model for CNS aging than those previously reported.

Cytoplasmic architecture of the axon terminal: filamentous strands specifically associated with synaptic vesicles

Cytoplasmic architecture of axon terminals in rat central nervous tissue was examined by quick-freeze deep-etch method to determine how synaptic vesicles and their associated cytoplasmic environment are organized in the terminal and to know how these structures participate in the mechanism for neurotransmitter release. The axoplasm is divisible into two domains: one occupied by mitochondria in the middle of the terminal, called the mitochondrial domain, the other situated in the periphery and exclusively filled with spherical synaptic vesicles, 50-60 nm in diameter, the synaptic vesicle domain. The most characteristic feature of the mitochondrial domain was the appearance of many microtubules connected with mitochondria by filamentous strands. Large vesicles, 80-100 nm in diameter, were preferentially associated with the mitochondrial domain, and linked with microtubules wherever they appeared. The cytoplasmic matrix of the synaptic vesicle domain showed a more fibrillar texture than that of the mitochondrial domain because of the distribution of filamentous strands associated with synaptic vesicles. These strands were significantly thicker and longer (mean 11.7 nm thick and 42.7 nm long) than those linking membrane-bound organelles to microtubules (mean 8.3 nm thick and 23.0 nm long), and connected vesicles to one another or to the plasma membrane, making a complicated network around the vesicles. Further, both strands were significantly different in dimension from actin filaments (mean 9.9 nm thick and 73.5 nm long) showing 5-nm axial periodicity. These strands, especially synaptic vesicle-associated ones including their network, were readily broken down in the most part by detergent treatment or chemical fixation, indicating that they are very delicate in nature. Granular materials, which are spherical and vary in size (6-20 nm in diameter), are also more conspicuous in the synaptic vesicle domain than in the mitochondrial domain. More fibrillar and granular cytoplasmic structure of the synaptic vesicle domain may be crucial for synaptic vesicles to perform an essential role in releasing the transmitter.

Electron microscopic observation and single-stranded DNA binding activity of the Mcm4,6,7 complex

Mcm2-7 proteins that play an essential role in eukaryotic DNA replication contain DNA-dependent ATPase motifs in a central domain that, from yeast to mammals, is highly conserved. Our group has reported that a DNA helicase activity is associated with a 600 kDa human Mcm4, 6 and 7 complex. The structure of the Mcm4,6,7 complex was visualized by electron microscopy after negative staining with uranyl acetate. The complex contained toroidal forms with a central channel and also contained structures with a slit. Gel-shift analysis indicated that the level of affinity of the Mcm4,6,7 complex for single-stranded DNA was comparable to that of SV40 T antigen, although the Mcm4,6,7 complex required longer single-stranded DNA for the binding than did SV40 T antigen. The nucleoprotein complexes of Mcm4,6,7 and single-stranded DNA were visualized as beads in a queue or beads on string-like structures. The formation of these nucleoprotein complexes was erased by Mcm2 that is a potential inhibitor of the Mcm4,6,7 helicase. We also found that the DNA helicase activity of Mcm4,6,7 complex was inhibited by the binding of Mcm3,5 complex. These results support the notion that the Mcm4,6,7 complex functions as a DNA helicase and the formation of 600 kDa complex is essential for the activity.

Identification of cellular compartments involved in processing of cathepsin E in primary cultures of rat microglia

Cathepsin E is a major nonlysosomal, intracellular aspartic proteinase that localizes in various cellular compartments such as the plasma membrane, endosome-like organelles, and the endoplasmic reticulum (ER). To learn the segregation mechanisms of cathepsin E into its appropriate cellular destinations, the present studies were initiated to define the biosynthesis, processing, and intracellular localization as well as the site of proteolytic maturation of the enzyme in primary cultures of rat brain microglia. Immunohistochemical and immunoblot analyses revealed that cathepsin E was the most abundant in microglia among various brain cell types, where the enzyme existed predominantly as the mature enzyme. Immunoelectron microscopy studies showed the presence of the enzyme predominantly in the endosome-like vacuoles and partly in the vesicles located in the trans-Golgi area and the lumen of ER. In the primary cultured microglial cells labeled with [35S]methionine, >95% of labeled cathepsin E were represented by a 46-kDa polypeptide (reduced form) after a 30-min pulse. Most of it was proteolytically processed via a 44-kDa intermediate to a 42-kDa mature form within 4 h of chase. This processing was completely inhibited by bafilomycin A1, a specific inhibitor of vacuolar-type H+-ATPase. Brefeldin A, a blocker for the traffic of secretory proteins from the ER to the Golgi complex, also inhibited the processing of procathepsin E and enhanced its degradation. Procathepsin E, after pulse-labeling, showed complete susceptibility to endoglycosidase H, whereas the mature enzyme almost acquired resistance to endoglycosidases H as well as F. The present studies provide the first evidence that cathepsin E in microglia is first synthesized as the inactive precursor bearing high-mannose oligosaccharides and processed to the active mature enzyme with complex-type oligosaccharides via the intermediate form and that the final proteolytic maturation step occurs in endosome-like acidic compartments.

Phosphorylation of neurofilament H subunit as related to arrangement of neurofilaments

To find out what causes differences in phosphorylation states in neurofilaments (NF), we selected two types of dendrite, one provided with very few NFs (Purkinje cell) and the other with relatively many (anterior horn cell). We examined these with four monoclonal antibodies selected by the Western blot analysis, two (NE14 and SMI31) recognizing only phosphorylated, SMI32 recognizing only nonphosphorylated, and N52 recognizing phosphorylation-independent epitopes of NF-H. The immunoperoxidase labeling of dendrites, and also of perikarya, in both neurons was detectable with all four antibodies. After the tissue was treated with Triton X-100, the labeling was still detectable with SMI32 or N52, but undetectable with NE14 and SMI31. The brain homogenate Triton-extracted supernatant after centrifugation at 100,000g for 1 hr showed the staining of NE14, SMI31, and N52 but not that of SMI32. In Purkinje cell dendrite and perikaryon, NFs always appeared singly. In the immunogold labeling, they were labeled only with SMI32 or N52. Labeling by NE14 or SMI31 was distributed throughout the cytoplasm and hardly associated with NFs. In the anterior horn cell dendrite and perikaryon, NFs appeared both singly and in bundles. They were predominantly labeled with SMI32 or N52 when they were single, and with NE14, SMI31, or N52 when they were bundled. Even in one NF, portions that appeared single were labeled mostly with SMI32 or N52, while the remainder, to which other NFs approached closely, were labeled mostly with NE14, SMI31, or N52. Thus, when NFs appear singly, NF-H in their projections or cross-bridges with other organelles is not phosphorylated, while when NFs are bundled, NF-H is phosphorylated in crossbridges between NF core filaments. These data may explain why the NF-H is heavily phosphorylated in axons, where NFs are abundant, and not in dendrites and perikarya, where NFs are sparse.

Selective localization of Bcl-2 to the inner mitochondrial and smooth endoplasmic reticulum membranes in mammalian cells

Bcl-2, an anti-apoptotic protein, is believed to be localized in the outer mitochondrial membrane, endoplasmic reticulum, and nuclear envelope. However, Bcl-2 has also been suggested as playing a role in the maintenance of mitochondrial membrane potential, indicating its possible association with the inner mitochondrial membrane. We therefore further examined the exact localization of Bcl-2 in mitochondria purified from wild-type and bcl-2-transfected PC12 cells and pre- and postnatal rat brains. Double immunostaining demonstrated that Bcl-2 was co-localized with subunit beta of F1F0ATPase in the inner mitochondrial membrane. Biochemical analysis of isolated mitochondria using digitonin and trypsin suggests an association of Bcl-2 with the inner mitochondrial membrane. More interestingly, the majority of Bcl-2 disappeared from the inner membrane of mitochondria when cultured under serum deprivation. These results suggest that Bcl-2 acts as an anti-apoptotic regulator by localizing mainly to the inner mitochondrial and smooth ER membranes.

An ultrastructural and immunohistochemical study of PC12 cells during apoptosis induced by serum deprivation with special reference to autophagy and lysosomal cathepsins

In addition to the caspase family of proteinases, cathepsin D, a lysosomal aspartic proteinase, has been suggested to act as a proapoptotic mediator in mammalian cells. To further understand the roles of cathepsins B and D in apoptosis of the cells, we examined the precise alteration processes of ultrastructures and immunoreactivity for these enzymes in PC12 cells cultured under serum deprivation. Laser scanning microscopy showed immunoreactivity for cathepsins B and D to be finely distributed in the cytoplasm of PC12 cells at the onset of culture under serum deprivation. At 3 h after the onset of culture, the immunoreactivity for cathepsin B slightly decreased in the cells, while immunodeposits for cathepsin D in the cells became more intense and larger in size than those at 0 h. Positive staining for TUNEL in nuclei of the cells appeared at 6 h, though fewer in number. Corresponding to the increase in the number of TUNEL-positive cells at 12 h and 24 h, the immunoreactivity for cathepsin B was drastically diminished in the cells, whereas that for cathepsin D was significantly augmented, especially in TUNEL-positive cells. Electron microscopically, autophagic vacuoles/autolysosomes appeared in the cytoplasm of the cells 3 h after the onset of culture. A distinct nuclear change showing relatively condensed chromatin first appeared in the peripheral part of the nuclei at 6 h. The number of PC12 cells having nuclei with chromatin condensation increased especially at 24 h, while these cells showed shrinkage of both their cytoplasm and nuclei. Dense bodies and autophagic vacuoles with limiting membranes were seen in these cells. These results showing the occurrence of autophagy and imbalance of protein amounts between cathepsins B and D during apoptosis may argue for our hypothesis that these enzymes are, in part, involved in the cell death cascade for PC12 cells following serum deprivation.

Regulation of neurofilament interactions in vitro by natural and synthetic polypeptides sharing Lys-Ser-Pro sequences with the heavy neurofilament subunit NF-H: neurofilament crossbridging by antiparallel sidearm overlapping

Neurofilaments are organised into parallel bundles in axons through crossbridges formed by lateral projections of neurofilament subunits. Pure neurofilaments form gels in vitro, consisting of interconnected parallel arrays of filaments regulated by the phosphorylation level of neurofilament subunits. Neurofilament-associated polypeptides sharing phosphorylated epitopes with the repetitive lysine-serine-proline (Lys-Ser-Pro) motifs of the neurofilament heavy subunit sidearm are characterised: they regulate in vitro the neurofilament gelation kinetics in a concentration- and phosphorylation-dependent manner. Studies with synthetic peptides show that interactions between neurofilaments involve both acid and base amino acid residues of neurofilament sidearms and demonstrate the opposite effects of peptides containing either one (inhibition) or two (activation) Lys-Ser-Pro motifs. Electron microscopy reveals an organised network of native neurofilament sidearms, regulated by the phosphorylation level of neurofilament subunits, suggesting a structural transition between intra- and inter-neurofilament sidearm interactions. These results favour the hypothesis of a mechanism of neurofilament crossbridging through the variable antiparallel overlapping of the phosphorylable Lys-Ser-Pro domains of neurofilament sidearms from adjacent filaments, following an equilibrium regulated by neurofilament-associated proteins, bivalent cations and the phosphorylation level of Lys-Ser-Pro motifs from both neurofilament sidearms and neurofilament-associated proteins.

Neurofilaments in health and disease

With dendritic neurofilaments (NFs) and NF reassembly experiments, the phosphorylation of NF-H was found related to development of crossbridges, resulting in alignment of core filaments. When treated with aluminum chloride, rabbits died acutely with tetanic spasm in which NFs were accumulated in neuronal perikarya and proximal axons. Compared with axonal NFs, the NFs accumulated in the perikarya were composed of less-developed cross-bridges and more irregularly aligned core filaments, and their NF-H, although it became phosphorylated, was less phosphorylated. Transgenic mice expressing NF-H-beta-galactosidase protein also showed NF accumulation in the perikarya, which was similar in organization and NF-H phosphorylation to that in aluminum-treated rabbits, but NFs were almost absent from the axonal compartment in these mice that did not show any overt phenotype. Jimpy mutant mice, with dysmyelinated axons and a short lifespan, showed a significant increase in NF density in the axonal compartment. NF-H and its mRNA were drastically enhanced in expression in these mice, whereas enhancement in expression of NF-L and its mRNA was slight. Most increased NF-H, and probably NF-M also, in the axons was of the nonphosphrylated form. NFs that increased in the axons were also constructed of irregularly organized core filaments linked with fewer crossbridges. Another dysmyelinating mutant type of mice, shiverer mice, also showed similar morphological, immunocytochemical, and behavioral characteristics. Taken together, axonal NF accumulation rather than that in the perikarya must be toxic for neurons to provoke axonal degeneration, possibly resulting in reduction of lifespan. In other transgenic mice, however, the elimination of NFs from the axonal compartment seems to make the neuron vulnerable. Nevertheless, because overexpression of NF-H displayed severe neurological disorder while elimination of this protein appeared to be more resistant to some neurotoxic agent, NF-H appears to function as an exacerbation factor when it exists in the neurologically disordered condition. However, as NF-H is provided with a unique carboxy-terminal tail domain that is highly phosphorylated in the axon and because disruption of its gene affected the survival of axons, which did not develop normal axonal caliber, NF-H should play an important role in healthy neurons.

Effects of desmin gene knockout on mice heart mitochondria

In heart tissue from mice lacking the intermediate filament (IF) desmin, mitochondria show an abnormal shape and distribution (Thornell et al., 1997). In the present study we have isolated heart mitochondria from desmin null (D-/-) and control (D+/+) mice, and analyzed their composition by SDS-PAGE, immunoblotting, and enzyme measurements. We found both in vitro and in situ that the conventional kinesin, the microtubule-associated plus-end directed motor, was frequently associated with D+/+ heart mitochondria, but not with D-/- heart mitochondria, suggesting that the positioning of mitochondria in heart is a dynamic event involving the IF desmin, the molecular motor kinesin, and, most likely, the microtubules (MT) network. Furthermore, an increased capacity in energy production was found, as indicated by a threefold higher creatine kinase activity in heart mitochondria from D-/- compared to D+/+ mice. We also observed a significantly lower amount of cytochrome c in heart mitochondria from D-/- mice, and a relocalization of Bcl-2, which may indicate an apoptotic condition in the cell leading to the earlier reported pathological events, such as cardiomyocytes degeneration and calcinosis of the heart (Thornell et al., 1997).

Fine structure of the ependyma and intercellular junctions in the area postrema of the rat

Ependymal cells and their junctional complexes in the area postrema of the rat were studied in detail by tracer experiments using horseradish peroxidase (HRP) and colloidal lanthanum and by freeze-etch techniques, in addition to routine electron microscopy. The ependyma of the area postrema is characterized as flattened cells possessing very few cilia, a moderate amount of microvilli, a well-developed Golgi apparatus and rough endoplasmic reticulum. Numerous vesicles or tubular formations with internal dense content were found to accumulate in the basal processes of ependymal cells; the basal process makes contact with the perivascular basal lamina. It is suggested that the dense material in the tubulovesicular formations is synthesized within the ependymal cell and discharged into the perivascular space. The apical junctions between adjacent ependymal cells display very close apposition, with a gap of 2--3 nm, but no fusion of adjacent plasma membranes; they thus represent a transitional form between the zonulae adhaerentes present in the ordinary mural ependyma and the zonulae occludentes in the choroidal epithelium. A direct intercommunication between the ventricular cerebrospinal fluid (CSF) and the blood vascular system indicates that a region exists lacking a blood-ventricular CSF barrier.

Expression of an inwardly rectifying K(+) channel, Kir4.1, in satellite cells of rat cochlear ganglia

Satellite cells are glial cells wrapped around somata of sensory and autonomic ganglion neurons. Neither their functional roles nor electrical properties have been fully clarified so far. Using immunohistochemistry, we found that inwardly rectifying K(+) channel subunit Kir4.1 (also called Kir1.2 or K(AB)-2) was expressed prominently in the satellite cells of cochlear ganglia. The Kir4.1 immunoreactivity was localized specifically at the myelin sheaths of satellite cells wrapping the somata of the ganglion neurons. Developmental expression of Kir4.1 in satellite cells paralleled development of the action potential in the auditory nerve. These results suggest that this channel in satellite cells may be responsible for the regulation of K(+) extruded from the ganglion neurons during excitation.

Electrical myotonia of rabbit skeletal muscles by HMG-CoA reductase inhibitors

HMG-CoA reductase (HCR) inhibitors are effective cholesterol-lowering agents in the treatment of hypercholesterolemia. Using intracellular microelectrodes, we studied the pathomechanism of myotonia experimentally induced in rabbits by HCR inhibitors, simvastatin, and pravastatin. The external intercostal muscle of rabbits showed some electrophysiologic characteristics of myotonia including repetitive firing after administration of simvastatin (50 mg/kg per day, for 4 weeks). The relative chloride conductance, though reduced in both, was more affected in simvastatin-administered muscles. In normal muscles perfused with a solution containing the inhibitors, both simvastatin and pravastatin produced membrane hyperexcitability with repetitive firing similar to that seen in simvastatin-administered rabbits. The minimum concentrations required to cause repetitive firing was 0.3 mg/L for simvastatin and 30 mg/L for pravastatin. These results indicate that HCR inhibitors induce some characteristics of myotonia by blocking the chloride channel in the muscle membrane.

Dephosphorylation of the largest neurofilament subunit protein influences the structure of crossbridges in reassembled neurofilaments

Phosphorylation-dependent change in electrophoretic mobility is the most unique characteristic of NF-H, the largest molecular mass subunit of the neurofilament. We dephosphorylated NF-H using Escherichia coli alkaline phosphatase, then reassembled it into neurofilaments with NF-M and NF-L, and into NF-H filaments with NF-H alone. We compared these dephosphorylated filaments with control: projections by low-angle rotary-shadow, crossbridges by quick-freeze deep-etch, and core filament packing density by thin-section electron microscopy. Projections in the dephosphorylated filaments were basically similar in structure to those in control, although there was a tendency for them to be wider and less dense, especially in NF-H filaments. Dephosphorylated filaments were still able to form crossbridges between core filaments, but their crossbridges were significantly wider, less dense, more branched and more irregular than crossbridges in control, and core filaments were more densely packed. These structural differences may be brought about by the removal of phosphate groups from NF-H tail and consequent reduction of electrostatic repulsion between adjacent crossbridges extending from the same core filament. The results indicate that phosphorylation of NF-H is necessary for forming well developed crossbridges, straight and at constant intervals, like those of in vivo axonal neurofilaments.

Abnormal expression of neurofilament proteins in dysmyelinating axons located in the central nervous system of jimpy mutant mice

Myelination in the peripheral nervous system is considered to increase the phosphorylation level of neurofilament proteins in the axon, resulting in an increase in axonal calibre. To understand the relationship between myelination and neurofilament proteins in axons, we examined jimpy mutant mice with a point mutation in the proteolipid protein gene and dysmyelination in the central nervous system. The jimpy mice exhibited a characteristic similarity in neurofilament nature to the myelin-deficient mice in the peripheral nervous system reported previously. The following novel results were obtained in the jimpy mice: dysmyelinated axons, in which the amount of non-phosphorylated neurofilament-H was drastically increased without a significant reduction of the phosphorylated form, compared with the control myelinated axons, did not suffer any decrease in their diameters. Expression levels of all neurofilament subunit proteins and their mRNAs were enhanced in the central nervous system tissue. Because the above biochemical data were obtained from the cytoskeletal fraction, at least some of the increased neurofilament-H and -M proteins appeared to be coassembled into neurofilaments but remained non-phosphorylated. Axonal neurofilaments of the jimpy were, probably due to this abnormal stoichiometry and phosphorylation state in neurofilaments, more compact and random in alignment with less prominent cross-bridges than those of the control, providing possible evidence for disturbing the axonal transport of other organelles. These results suggest that myelination regulates both the expression and phosphorylation of neurofilament proteins, and is essential for the cytoplasmic organization of myelinated axons.

Expression and polarized distribution of an inwardly rectifying K+ channel, Kir4.1, in rat retinal pigment epithelium

1. In the eye, different substances and ions including potassium (K+) are transported between neural retina and choroid via the subretinal space. Inwardly rectifying K+ channels (Kir) on the apical membrane of retinal pigment epithelial (RPE) cells are thought to play an essential role in K+ transport in the subretinal space. 2. Single-channel recordings from the apical membrane of RPE cells exhibited functional expression of a Kir channel with properties identical to those of Kir4.1, while recordings from the basolateral membrane showed no detectable Kir channel currents. 3. The expression of Kir4.1 mRNA in RPE cells was confirmed by RT-PCR analysis and in situ hybridization. Furthermore, using immunohistochemistry, we found that Kir4.1 was prominently expressed in RPE cells and localized specifically on the processes on their apical membrane. 4. Developmental studies revealed that expression of Kir4.1 started to appear 10 days or later after birth in RPE cells, in parallel with the maturation of retinal neuronal activity as represented by the a- and b-waves of the electroretinogram. 5. These data suggest that Kir4.1 is one of the Kir channels involved in RPE-mediated control of K+ ions in the subretinal space.

Myelin-associated oligodendrocytic basic protein is essential for normal arrangement of the radial component in central nervous system myelin

We previously reported that myelin-associated oligodendrocytic basic protein (MOBP) was abundantly expressed in the central nervous system (CNS) myelin, and shared several characteristics with myelin basic protein (MBP). In particular, a cluster of positively charged amino acids was considered to facilitate compaction of the cytoplasmic face of the myelin sheath, as in the case of MBP. However, the contribution of MOBP in forming and maintaining the myelin sheath still remains unclear. Recent investigations showed that one isoform of MOBP was expressed in the embryo prior to myelination, and MOBP isoforms were colocalized with the microtubular network and nucleus in vitro. To explore the role of MOBP in vivo, we generated MOBP-deficient mice and analysed the CNS myelin. Surprisingly, the compact myelin was formed, however, the myelin from MOBP-deficient mice exposed to hexachlorophene, a known dysmyelinating agent, showed widening of the major dense lines. These results suggest that MOBP is not essential for myelin formation, but reinforces the apposition of the cytoplasmic faces of the myelin sheath. A striking phenotype of MOBP-deficient mice was the presence of the straight 'condensed' radial component. This component has been described as a tight junction-like complex running radially and zig-zag through the CNS myelin sheath between inner and outer mesaxons. These results suggest that MOBP is essential for normal arrangement of the radial component.

Postnatal development of the inferior olivary complex in the rat: IV. Synaptogenesis of GABAergic afferents, analyzed by glutamic acid decarboxylase immunocytochemistry

The postnatal maturation of the GABAergic innervation of the rat inferior olive was studied with an antiserum to glutamic acid decarboxylase (GAD), the GABA-synthesizing enzyme. GAD-positive axons were present at a very low density in the periolivary and interlamellar regions of newborn rats, as well as in certain precise areas of the lamellae, at the mediodorsal limit. The immature distribution indicates that the GABAergic projections reach the inferior olive shortly before birth and that the greater part of synaptogenesis and the establishment of the adult organization occurs postnatally. Light and electron microscopic analyses disclosed that the maturation of this system of olivary afferents passes through three well-defined stages: (1) During the first, or immature stage (from PO to P5), GAD immunoreactivity is not confined to axon terminals, as in adult rats. The labeled fibers penetrate progressively into the periphery of the lamellae and reach their centers in an irregular manner by the end of the immature stage. This staggered invasion of the lamellae accentuates intraregional olivary differences and begins to take the adult configuration. As fiber penetration advances, the density of labeled axons establishing synaptic contacts increases, while the number of completely immunostained fibers decreases. This distribution prevails until the end of the immature stage and suggests that the GABAergic afferent projections remain in a "waiting compartment" from their prenatal arrival until the moment they invade the olivary parenchyma. (2). The second stage is designated as an intermediate stage of maturation and lasts from P7 to P10. During this period, GAD axoplasmic compartmentation occurs, and henceforth only axon terminals exhibit GAD immunoreactivity. Concomitantly, intraregional differences in the pattern of innervation become more marked, because of the continuing irregular distribution of the growing labeled axons. This intermediate maturational stage is also characterized by a rapid increase in labeled axon terminals bearing synaptic complexes and by the formation of complex synaptic arrangements, the protoglomeruli. From the beginning of protoglomeruli formation, GAD-positive axon terminals are one of their constituents, and they are systematically localized at the periphery of the incipient dendritic protrusions. (3) The final stage of maturation takes place from P10 to P15. During this stage, the adultlike pattern of GABAergic innervation of the inferior olive is attained. Toward P15, intraregional differences in GAD immunoreactivity are similar to those of the adult rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of serotonergic afferents on quantal release at central inhibitory synapses

Although most examples of modulation of synaptic transmission have been obtained from excitatory rather than from inhibitory connections, serotonin (5HT) is now shown to cause a presynaptic facilitation of release of the inhibitory neurotransmitter glycine. Brief local injections of this amine, or application of a 5HT uptake blocker, produce a long-lasting enhancement of both spontaneous and evoked inhibitory currents in the teleost Mauthner cell. Quantal analysis showed that the probability of release is increased. Focal recording indicated that 5HT acts directly on the inhibitory terminals, possibly reducing potassium conductances. Double staining with specific antibodies demonstrated a morphological substrate for this effect. Nerve endings that contain 5HT contact inhibitory terminals directly apposed to postsynaptic glycine receptors.

Plasma membrane organization of astrocytes in elasmobranchs with special reference to the brain barrier system

The structural machinery contributing to the blood-brain barrier in elasmobranchs has been examined mainly using freeze-fracture techniques. Capillary endothelial cells, which show local aggregations of pinocytotic vesicles and infrequent fenestrations, are connected by poorly developed tight junctions. Astrocytic processes investing the capillary are linked by well-developed tight junctions between lateral membranes immediately beneath the perivascular space. The tight junctions consist of continuous strands of multiple layers coursing circumferentially around the astrocytic processes parallel to one another as well as to the perivascular space. The presence of intramembrane particles (IMPs) within E-face grooves may result in discontinuities in IMP rows on the P-face. Thus, in compensation for the capillary endothelium, perivascular astrocytes constitute the morphological site of the blood-brain barrier in elasmobranchs. Continuous strands of tight junctions are also detected between astrocytic processes forming the glia limitans at the brain surface. These may act as a barrier between meningeal connective tissue and brain parenchyma. Astrocytic membranes have numerous IMPs of 8-9 nm in diameter on their P-faces. These IMPs are uniformly distributed so that astrocytic membranes are easily distinguished from neuronal membranes even in the neuropil. Ependymal cells also have numerous IMPs in all their membrane domains. Orthogonal arrays are not detected in either astrocytic or ependymal plasma membranes.

Cytochemical characteristics of astrocytic plasma membranes specialized with numerous orthogonal arrays

Astrocytic membranes contacting the basal lamina are found to be less affected by filipin than subjacent lateral membranes. An abrupt change in density of lesions induced by filipin creates a border between subpial and lateral membranes at the glia limitans. This means that orthogonal array-crowded membranes may contain relatively less cholesterol than other astrocytic membrane domains. Another possible explanation for filipin resistance is also considered in relation to aggregated intramembrane particles of orthogonal arrays and/or membrane-associated filamentous elements including the basal lamina. The polygonal particle junction between astrocytic processes located just below the subpial membrane is strongly resistant to the action of filipin. Both membrane-associated enzymes, i.e. alkaline phosphatase (AlkPase) and Na+,K+-ATPase are commonly detected only in perivascular astrocytic membranes, and not in subpial membranes, suggesting a regional differentiation in function of astrocytic membranes. There are variations in the reactive deposits particularly of those for Na+,K+-ATPase. It is apparent that the distribution polarity of orthogonal arrays is not connected with that of either AlkPase or Na+,K+-ATPase. Judging from the relative resistance to filipin, however, astrocytes throughout the C.N.S., having domains specialized with orthogonal arrays, may possess a unique stabilizing mechanism for their own membranes contacting the basal lamina.

Roles of cyclic GMP and inositol trisphosphate in phototransduction of the molluscan extraocular photoreceptor

The internal messengers mediating the photocurrent of the molluscan extraocular photoreceptor, A-P-1, were examined. In the dark, pressure-injection of cGMP into the A-P-1, voltage-clamped at resting levels, produced a rapid outward current, associated with an increase in conductance. However, the cGMP-induced current and increase in conductance were suppressed by subsequent photostimulation, suggesting hydrolysis of cGMP by light. The steady-state I/V relation for the cGMP-induced current was non-linear. The I/V relation for the instantaneous cGMP-induced current, measured 50 ms after the beginning of a voltage step, was linear, and reversed at the membrane potential, -67 mV, which corresponded to the K+ equilibrium potential of A-P-1 in 10 mM K+ normal saline. These findings indicate that the internal cGMP induces a voltage- and time-dependent K+ current. Since the photocurrent results from the suppression of a voltage- and time-dependent K+ current similar to above, the photocurrent is considered to be equivalent to the suppression of the cGMP-induced current. Short pressure-injection of GDP-beta-S into A-P-1 reduced the subsequent photocurrent. The photocurrent was also suppressed after an external application of Pertussis toxin. On the other hand, the photocurrent was amplified by prior pressure-injection of inositol 1,4,5-trisphosphate (IP3). However, a short pressure-injection of neomycin into A-P-1 depressed the subsequent photocurrent. These results suggested that the cGMP-induced (dark) current is mediated by cGMP, and that hydrolysis of cGMP by light leads to the photocurrent, then being modified by another messenger, IP3, to be amplified.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential localization of annexin V to the axon terminal

To examine the participation of annexin V, a member of Ca(2+)-dependent phospholipid-binding proteins, in the process of synaptic vesicle exocytosis, rat central nervous tissue was analysed using biochemical and morphological techniques. By both fluorescence and confocal laser scanning microscopy, immunoreactivity for annexin V was predominantly localized around neuronal somata and dendrites, and the reactivity was mostly co-labeled with that for synaptophysin. The annexin V immunoreactivity was also detectable, but less intensely, in neuronal perikarya, glial cells and endothelial cells. Both immunoblot and immunoelectron microscopic analyses with intact tissues, synaptosomes and purified synaptic vesicles showed that annexin V was expressed in neurons, preferentially concentrated in axon terminals and associated with synaptic vesicles. Purified synaptic vesicles were relatively homogeneously distributed in the medium where Ca2+ was removed and thus the amount of annexin V was reduced drastically. The vesicles tended to be clustered in the fraction where endogenous annexin V is maintained, and the clusters were more conspicuous when purified human annexin V was added. Synaptic vesicles forming the clusters were not directly fused with each other but separated by a 10-15 nm gap that corresponded well with the size of single annexin V molecules. In axon terminals, globular structures 12-13 nm in diameter, similar in dimension to annexin V molecules, were distinctly found to be attached to the cytoplasmic surface of both vesicle membranes when the two vesicles were close to each other. These results suggest that annexin V belongs to the group of synaptic vesicle-associated proteins. Although its localization and significance in non-neuronal cells were not analysed here, at least in the axon terminal annexin V may participate in the cluster formation of synaptic vesicles by linking with the cytoplasmic surface of the vesicles in a Ca(2+)-dependent manner.