Ultrastructure of acetylcholine receptor aggregates parallels mechanisms of aggregation.

BACKGROUND: Acetylcholine receptors become aggregated at the developing neuromuscular synapse shortly after contact by a motorneuron in one of the earliest manifestations of synaptic development. While a major physiological signal for receptor aggregation (agrin) is known, the mechanism(s) by which muscle cells respond to this and other stimuli have yet to be worked out in detail. The question of mechanism is addressed in the present study via a quantitative examination of ultrastructural receptor arrangement within aggregates. RESULTS: In receptor rich cell membranes resulting from stimulation by agrin or laminin, or in control membrane showing spontaneous receptor aggregation, receptors were found to be closer to neighboring receptors than would be expected at random. This indicates that aggregation proceeds heterogeneously: nanoaggregates, too small for detection in the light microscope, underlie developing microaggregates of receptors in all three cases. In contrast, the structural arrangement of receptors within nanoaggregates was found to depend on the aggregation stimulus. In laminin induced nanoaggregates receptors were found to be arranged in an unstructured manner, in contrast to the hexagonal array of about 10 nm spacing found for agrin induced nanoaggregates. Spontaneous aggregates displayed an intermediate amount of order, and this was found to be due to two distinct population of nanoaggregates. CONCLUSIONS: The observations support earlier studies indicating that mechanisms by which agrin and laminin-1 induced receptor aggregates form are distinct and, for the first time, relate mechanisms underlying spontaneous aggregate formation to aggregate structure.

Developmental changes in the ultrastructure of the Sauromatum guttatum (Araceae) mitochondria.

Electron microscopic studies show that the electron density of the mitochondria of the thermogenic appendix of Sauromatum guttatum inflorescences changes during development. Two to 3 days before heat-production, the mitochondria accumulate osmiophilic, electron-dense material between the inner and outer membranes. During heat-production and the release of volatiles compounds the osmiophilic material disappears from the inter membrane space. Deposits with the same electron density are also found in the endoplasmic reticulum. It is concluded that the mitochondria may also have the capacity to accumulate that material in the inter membrane space.

Further studies of the glandular tissue of the Sauromatum guttatum (Araceae) appendix.

Electron microscopic studies showed that the trans-Golgi network (trans indicates the polarity of cisternae within the Golgi apparatus; it is opposite to the cis-face that is adjacent to the rough endoplasmic reticulum) was involved in the processing of the osmiophilic material present in the appendix of the inflorescence of Sauromatum guttatum. This material accumulated in the rough endoplasmic reticulum and in special pockets of the plasma membrane prior to heat production. Associations between the endoplasmic reticulum and trans-Golgi network were observed. The Golgi apparatus was composed of 5-6 dictyosomes on one side and one or two somewhat detached cisternae on the other side. Various nonosmiophilic Golgi-derived vesicles were observed: small ones covered with spike-like material, large ones with a smooth surface, and irregularly shaped ones. These electron-translucent vesicles seemed to accumulate in specific localities at the plasma membrane surface in the vicinity of the osmiophilic material; they were not found when the aroma was released. During heat production, the Golgi structures shrank and the activity of the trans-Golgi network seemed to be reduced. At the same time, coated pits were seen at the plasma membrane surface. In some cells, hypertrophic Golgi apparatuses were seen with only 2-3 dictyosomes that contained granulated material in their lumens. Finally, the osmiophilic material was also found in the plasmodesmata.

Structural organization of developing acetylcholine receptor aggregates.

We report the first quantitative ultrastructural analysis of newly formed acetylcholine receptor aggregates. Aggregates were induced in Xenopus muscle cell cultures with agrin, labeled with gold particles, and detected using high resolution scanning electron microscopy. Aggregates are readily discernible at the ultrastructural level within 2 h of stimulation by agrin. The size and density profiles of the developing aggregates show that receptors reach maximal density very quickly in small "nano-aggregates" and that the aggregation process is not limited by the diffusion rate of the receptor. Quantitative analysis of label locations indicates that the receptor distribution within aggregates is nonrandom. Instead, the newly aggregated receptors appear to be bound to a localized scaffold conforming to a hexagonal (close-packed) geometry with a spacing of approximately 9.9 nm.

High levels of circulating beta-amyloid peptide do not cause cerebral beta-amyloidosis in transgenic mice.

We have established transgenic mice that constitutively overproduce the signal sequence and the 99-amino-acid carboxyl-terminal region of the human beta-amyloid precursor protein. The transgenic mice strongly expressed the transgene in multiple tissues under the control of a cytomegalovirus enhancer/chick beta-actin promoter. There were exceptionally high levels of beta-amyloid peptides in the plasma (approximately 17 times or more compared with the human plasma level). Although some transgenic mice from one founder line developed amyloidosis in the intestine, no neuropathology was found in transgenic mice up to age 29 months. Given the absence of cerebral beta-amyloidosis despite extremely high levels of circulating beta-amyloid peptides in the transgenic mice, the results suggest that local cerebral metabolism of beta-amyloid precursor protein may play a predominant role in cerebral beta-amyloidosis in transgenic mice. Such transgenic mice may be useful for the investigation of the etiology of the disease and for the establishment of therapeutic strategies.

Axon arbors and synaptic connections of hippocampal mossy cells in the rat in vivo.

The axon collateralization patterns and synaptic connections of intracellularly labeled and electrophysiologically identified mossy cells were studied in rat hippocampus. Light microscopic analysis of 11 biocytin-filled cells showed that mossy cell axon arbors extended through an average of 57% of the total septotemporal length of the hippocampus (summated two-dimensional length, not adjusted for tissue shrinkage). Axon collaterals were densest in distant lamellae rather than in lamellae near the soma. Most of the axon was concentrated in the inner one-third of the molecular layer, with the hilus containing an average of only 26% of total axon length and the granule cell layer containing an average of only 7%. Ultrastructural analysis was carried out on three additional intracellularly stained mossy cells, in which axon collaterals and synaptic targets were examined in serial sections of chosen axon segments. In the central and subgranular regions of the hilus, mossy cell axons established a low density of synaptic contacts onto dendritic shafts, neuronal somata, and occasional dendritic spines. Most hilar synapses were made relatively close to the mossy cell somata. At greater distances from the labeled mossy cell (1-2 mm along the septotemporal axis), the axon collaterals ramified predominantly within the inner molecular layer and made a high density of asymmetric synaptic contacts almost exclusively onto dendritic spines. Quantitative measurements indicated that more than 90% of mossy cell synaptic contacts in the ipsilateral hippocampus are onto spines of proximal dendrites of presumed granule cells. These results are consistent with a primary mossy cell role in an excitatory associational network with granule cells of the dentate gyrus.

Pathway of terpene excretion by the appendix of Sauromatum guttatum.

Electron microscopy of the cells of the thermogenic appendix of Sauromatum guttatum has revealed a fusion event between pocket-like structures of the rough endoplasmic reticulum (rER) and the plasma membrane. As a result of the fusion event, many regions of the plasma membrane have paired unit membranes (four leaflets instead of two). The fusion allows the transfer of osmiophilic material from the rER pockets to the plasma membrane, where the osmiophilic material is confined to bilayer, pocket-like structures. A clear correlation is found between the presence of the osmiophilic compound and sesquiterpenes. Prior to heat production, the rER- and plasma-membrane pockets are electron dense, and sesquiterpenes are detectable only in tissue extracts. On the day of heat production, electron-translucent pockets are subsequently found and the stored sesquiterpenes are released to the atmosphere. Three sesquiterpenes have been identified by gas chromatography-mass spectrometry as alpha-copaene and beta- and alpha-caryophyllene.

Beta amyloid is neurotoxic in hippocampal slice cultures.

We examined the neurotoxicity of the 40 amino acid fragment of beta amyloid peptide (A beta 1-40) in cultured hippocampal slices. When injected into area CA3, A beta 1-40 produced widespread neuronal damage. Injection of the reverse sequence peptide, A beta 40-1, or vehicle alone produced little damage. The distribution A beta 1-40 was highly correlated with the area of neuronal damage. Thioflavine S and electron microscopic analysis confirmed that injected A beta 1-40 formed 7-9 nm AD type amyloid fibrils in the cultures. A beta 1-40 also altered the number of GFAP immunoreactive astrocytes and ED-1 immunoreactive microglia/macrophages within and around the A beta 1-40 deposit. The observed neurotoxicity of A beta 1-40 in hippocampal slice cultures provides evidence that this peptide may be responsible for the neurodegeneration observed in AD.

Physiologic and morphologic characteristics of granule cell circuitry in human epileptic hippocampus.

Morphological and electrophysiological techniques were used to examine granule cells and their mossy fiber axons in nine surgically resected hippocampal specimens from temporal lobe epilepsy (TLE) patients. Timm histochemistry showed mossy fiber sprouting into the inner molecular layer (IML) of the dentate in a subset of tissue samples. In slices from five tissue samples, stimulus-induced bursting activity could be induced with a low concentration (2.5 microM) of bicuculline; bursts were sensitive to the N-methyl-D-aspartate (NMDA) blocker, APV. There was a general correlation between such sprouting and experimentally induced hyperexcitability. Fourteen granule cells from five tissue samples were intracellularly stained [with lucifer yellow (LY) or neurobiotin]. Axons from a subset of these neurons showed axon collaterals reaching into the IML, but this axon projection pattern for single cells was not directly correlated with degree of mossy fiber sprouting shown grossly by Timm staining. Electron microscopic examination of intracellularly stained elements showed mossy fiber axon terminals making asymmetric synaptic contacts (including autapses on the granule cell dendrite) with dendritic shafts and spines in both apical and basal domains. These data are consistent with the hypothesis that mossy fiber sprouting provides a structural basis for recurrent excitation of granule cells, but does not provide direct support of the hypothesis that mossy fiber sprouting causes hyperexcitability. The data suggest that granule cell bursting activity is at least in part a function of compromised synaptic inhibition, since levels of gamma-aminobutyric acid (GABA) blockade that are generally subthreshold for burst induction were epileptogenic in some tissue samples from human epileptic hippocampus.

Localization of Kv1.1 and Kv1.2, two K channel proteins, to synaptic terminals, somata, and dendrites in the mouse brain.

Multiple voltage-gated potassium (K) channel gene products are likely to be involved in regulating neuronal excitability of any single neuron in the mammalian brain. Here we show that two closely related voltage-gated K channel proteins, mKv1.1 and mKv1.2, are present in multiple subcellular locations including cell somata, juxta-paranodal regions of myelinated axons, synaptic terminals, unmyelinated axons, specialized junctions among axons, and proximal dendrites. Staining patterns of the two channel polypeptides overlap in some areas of the brain, yet each has a unique pattern of expression. For example, in the hippocampus, both mKv1.1 and mKv1.2 proteins are present in axons, often near or at synaptic terminals in the middle molecular layer of the dentate gyrus, while only mKv1.1 is detected in axons and synaptic terminals in the hilar/CA3 region. In the cerebellum, both channel proteins are localized to axon terminals and specialized junctions among axons in the plexus region of basket cells. Strong differential staining is observed in the olfactory bulb, where mKv1.2 is localized to cell somata and axons, as well as to proximal dendrites of the mitral cells. This overlapping yet differential pattern of expression and specific subcellular localization may contribute to the unique profile of excitability displayed by a particular neuron.

Overexpression of a C-terminal portion of the beta-amyloid precursor protein in mouse brains by transplantation of transformed neuronal cells.

The role of beta-amyloid protein and its precursor protein is a central question in the pathogenesis of Alzheimer's disease. We have established several transformants from a mouse embryonic carcinoma cell line, which overproduce a C-terminal region of the beta-amyloid precursor protein from the integrated DNA constructs. These stable transformants degenerated to varying extents when undergoing neural differentiation mediated by retinoic acid. To test the neurotoxicity and the amyloidogenicity of the transgene product and its proteolytic derivatives in vivo, two stable transformants were neuronally differentiated and transplanted into the hippocampal regions of syngeneic mice. Similarly, either a nontransformant or a transformant bearing a cDNA construct for yeast major apurinic endonuclease was transplanted to the contralateral regions of the same mice. Three weeks after transplantation, grafts were identified around needle tracts or in hippocampal regions. The regions where transformants overproducing the C-terminal region were grafted were highly reactive to antibodies raised against beta-amyloid protein and its precursor protein, in contrast to the contralateral regions. At 2 and 5 months after neurotransplantation, remarkable distortion and shrinkage characterized the hippocampus on the sides injected with the transformants overproducing the C-terminal region. This shrinkage was associated particularly with a loss of the hippocampal granule cells. beta-Amyloid protein immunoreactive granular deposits in the neuropil were also found in the same sides. Hippocampal blood vessel walls were also stained with the antibodies. These walls were surrounded by astrocytic processes, suggesting involvement of astroglial cells in vascular deposits of beta-amyloid protein. The results are consistent with the hypothesis that the C-terminal region or its derivatives are neurotoxic and amyloidogenic.

Dynorphin opioids present in dentate granule cells may function as retrograde inhibitory neurotransmitters.

The granule cell population response to perforant path stimulation decreased significantly within seconds following release of endogenous dynorphin from dentate granule cells. The depression was blocked by the opioid receptor antagonists naloxone and norbinaltorphimine, suggesting that the effect was mediated by dynorphin activation of kappa 1 type opioid receptors. Pharmacological application of dynorphin B in the molecular layer was effective at reducing excitatory synaptic transmission from the perforant path, but application in the hilus had no significant effect. These results suggest that endogenous dynorphin peptides may be released from a local source within the dentate molecular layer. By light microscopy, dynorphin-like immunoreactivity (dynorphin-LI) was primarily found in granule cell axons in the hilus and stratum lucidum with only a few scattered fibers evident in the molecular layer. At the extreme ventral pole of the hippocampus, a diffuse band of varicose processes was also seen in the molecular layer, but this band was not present in more dorsal sections similar to those used for the electrophysiological studies. Electron microscopic analysis of the molecular layer midway along the septotemporal axis revealed that dynorphin-LI was present in dense-core vesicles in both spiny dendrites and unmyelinated axons with the majority (74%) of the dynorphin-LI-containing dense-core vesicles found in dendrites. Neuronal processes containing dynorphin-LI were observed throughout the molecular layer. The results suggest that dynorphin release from granule cell processes in the molecular layer regulates excitatory inputs entering the hippocampus from cerebral cortex, thus potentially counteracting such excitation-induced phenomena as epileptogenesis or long-term potentiation.

Somatostatin-containing neurons in rat organotypic hippocampal slice cultures: light and electron microscopic immunocytochemistry.

Light and electron microscopic immunocytochemical techniques were used to study the interneuron population staining for somatostatin (SRIF) in cultured slices of rat hippocampus. The SRIF immunoreactive somata were most dense in stratum oriens of areas CA1 and CA3, and in the dentate hilus. Somatostatin immunoreactive cells in areas CA1 and CA3 were characteristically fusiform in shape, with dendrites that extended both parallel to and into the alveus. The axonal plexus in areas CA1 and CA3 was most dense in stratum lacunosum-moleculare and in stratum pyramidale. Electron microscopic analysis of this area revealed that the largest number of symmetric synaptic contacts from SRIF immunoreactive axons were onto pyramidal cell somata and onto dendrites in stratum lacunosum-molecular. In the dentate gyrus, SRIF somata and dendrites were localized in the hilus. Hilar SRIF immunoreactive neurons were fusiform in shape and similar in size to those seen in CA1 and CA3. Axon collaterals coursed throughout the hilus, projected between the granule cells and into the outer molecular layer. The highest number of SRIF synaptic contacts in the dentate gyrus were seen on granule cell dendrites in the outer molecular layer. Synaptic contacts were also observed on hilar neurons and granular cell somata. SRIF synaptic profiles were seen on somata and dendrites of interneurons in all regions. The morphology and synaptic connectivity of SRIF neurons in hippocampal slice cultures appeared generally similar to intact hippocampus.

Somatostatin-immunoreactivity in the hippocampus of mouse, rat, guinea pig, and rabbit.

The hippocampi of species commonly used for in vitro physiologic studies were examined to determine if there were species-specific and regional differences in somatostatin immunoreactivity. The distributions of somatostatin-immunoreactive somata and fiber plexuses were determined, and the concentration of somatostatin along the septotemporal axis of the hippocampus was measured using a radioimmunoassay. There are many similarities in the patterns of somatostatin immunoreactivity in the hippocampi of mice, rats, guinea pigs, and rabbits. All species had a relatively even distribution of somatostatin-positive perikarya across three fields of the hippocampus (dentate gyrus, CA3, and CA1-2), a similar distribution of somatostatin-immunoreactive perikarya across the strata of the CA1-2 field and the dentate gyrus; and more somatostatin-positive cells in temporal than in septal hippocampus. However, there are species-specific differences in the distribution of somatostatin-immunoreactive perikarya across the strata of CA3. In addition, unlike the other species examined, mice appeared not to have a somatostatin-immunoreactive fiber plexus in the molecular layer of the dentate gyrus. The functional significance of these differences remains to be determined.

Development of dopamine-beta-hydroxylase-positive fiber innervation of the rat hippocampus.

Development of the noradrenergic fiber innervation of the rat hippocampus by the locus coeruleus was examined immunohistochemically in fixed tissue from animals aged 4 days through 55 days postnatal. The presence of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) immunoreactive cells and fibers was evaluated in sections of hippocampus and locus coeruleus. Large, multipolar TH- and DBH-positive cells with long beaded fibers were visible within locus coeruleus at all ages; no immunopositive cell bodies were found in hippocampus. In hippocampal sections from mature animals (PN55), the highest density of DBH-stained fibers was found in stratum lucidum of CA3 and in the hilus and inner molecular layer of the dentate gyrus. Whereas similar patterns of fiber positivity were found at PN21 and PN10 (although with somewhat reduced density of immunopositive fibers), the pattern was quite different at PN4. Although fiber staining was relatively sparse at PN4, relative density of DBH fibers was highest in stratum radiatum of CA1 and subiculum. This change in staining pattern suggests that noradrenergic function in hippocampus may change as the rat matures. Double immunofluorescence techniques showed an overlap of DBH and TH positive fibers in all hippocampal regions at all ages. DBH immunostaining appeared to be somewhat more sensitive than the TH staining. These data made it impossible to confirm the presence of significant numbers of nonnoradrenergic, catecholamine-containing fibers in hippocampus.

Development of dopamine-beta-hydroxylase-positive fiber innervation in co-cultured hippocampus-locus coeruleus organotypic slices.

Development of the noradrenergic innervation of the rat hippocampus by the nucleus locus coeruleus was examined immunohistochemically in the roller tube organotypic cultured slice preparation. Slices of rat hippocampus and locus coeruleus were co-cultured on glass coverslips for 2-6 weeks and evaluated for the presence of dopamine-beta-hydroxylase (DBH) and tyrosine hydroxylase (TH) immunoreactive cells and fibers. Large, multipolar DBH- and TH-positive cells were visible within the locus coeruleus; an occasional cell appeared near or just within co-cultured hippocampal tissue and in connecting fiber tracts. DBH-positive cells tended to concentrate near the edges of locus coeruleus tissue. Locus coeruleus slices cultured alone showed little indication of fiber outgrowth in any direction. In co-cultures, however, beaded DBH- and TH-positive fibers were directed toward the hippocampus. The majority of these fibers entered the hippocampus in the hilar/CA3 region and formed extensive collateral branches. Light microscopy suggests that DBH-positive fiber growth was densest at or near the pyramidal cell layer in CA3b and CA3c and in the infragranular region of the dentate hilus. This pattern of noradrenergic innervation of hippocampus by co-cultured locus coeruleus in vitro appears very similar to the pattern established in vivo (see Moudy et al., companion article, this issue).

Transgenic animal models for Alzheimer's disease.

The neuropathology of Alzheimer's disease is characterized by the deposition of abnormal protein aggregates. The main constituent of the deposition is beta-amyloid protein. A seminal role of this protein is supported by the discovery of point mutations in the gene of its precursor protein in certain forms of familial Alzheimer's disease. In vitro (cultured neuronal cells), overexpression of the precursor protein or a part of the precursor leads to degeneration of neurons, suggesting neurotoxicity of its derivatives. At this time, all of the reported transgenic mice bearing DNA construct for the precursor or a part of the precursor, however, have not developed convincing pathological changes similar to what is observed in patients with Alzheimer's disease. This interesting discrepancy between in vitro and in vivo suggests suppressors in vivo which ameliorate beta-amyloid precursor protein derivative-mediated neurotoxicity.

Heteromultimeric K+ channels in terminal and juxtaparanodal regions of neurons.

Voltage-gated potassium (K+) channels display a wide variety of conductances and gating properties in vivo. This diversity can be attributed not only to the presence of many K(+)-channel gene products, but also to the possibility that different K(+)-channel subunits co-assemble to form heteromultimeric channels in vivo. When expressed in Xenopus oocytes or transfected cells, K(+)-channel polypeptides assemble to form tetramers. Certain combinations of Shaker-like subunits have been shown to co-assemble, forming heteromultimeric channels with distinct properties. It is not known, however, whether K(+)-channel polypeptides form heteromultimeric channels in vivo. Here we describe the co-localization of two Shaker-like voltage-gated K(+)-channel proteins, mKv1.1 and mKv1.2, in the juxtaparanodal regions of nodes of Ranvier in myelinated axons, and in terminal fields of basket cells in mouse cerebellum. We also show that mKv1.1 and mKv1.2 can be coimmunoprecipitated with specific antibodies that recognize only one of them. These data indicate that the two polypeptides occur in subcellular regions where rapid membrane repolarization may be important and that they form heteromultimeric channels in vivo.

Electron microscopy of intracellularly labeled neurons in the hippocampal slice preparation.

We have assessed the properties of three intracellular markers, horseradish peroxidase, biocytin/Neurobiotin, and Lucifer Yellow, and have compared their usefulness as neuronal markers for light and electron microscopic visualization. Neurons in the acute slice preparation of rat hippocampus were filled with one of these markers, and the marker was converted to an optical and electron-dense reaction product. Dimethylsulfoxide (DMSO) greatly facilitated penetration of recognition reagents while preserving membrane integrity. The markers were compared with respect to injection parameters, mobility and recognition, stability and visibility, and ultrastructural clarity. Horseradish peroxidase (HRP)-labeled neurons, recognized histochemically with diaminobenzedine (DAB), were easily visualized by the density of the DAB reaction product; however, the electron density was often so great as to obscure ultrastructural details. Biocytin (BC)-/Neurobiotin (NB)-labeled neurons were recognized by avidin-HRP, followed by histochemical localization of HRP with DAB. The optically dense reaction product gave complete visualization of the soma and processes at the light microscopic level. The electron density was homogeneously distributed throughout the cell, so that ultrastructural features were easily identified. Lucifer Yellow (LY), a fluorescent marker, was converted to an optical and electron-dense reaction product via immunocytochemical staining with a rabbit anti-LY antibody, followed by goat anti-rabbit IgG-HRP and DAB histochemical localization. Similar to BC/NB, the reaction product was evenly dispersed, providing good light microscopic and ultrastructural clarity. Under our experimental conditions, BC/NB and LY were superior markers that could be used routinely to label neurons, and give excellent visualization not only at the light but also at the electron microscopic level.