Smooth endoplasmic reticulum and axonal transport

The moonlighting protein c-Fos activates lipid synthesis in neurons, an activity that is critical for cellular differentiation and cortical development

Differentiation of neuronal cells is crucial for the development and function of the nervous system. This process involves high rates of membrane expansion, during which the synthesis of membrane lipids must be tightly regulated. In this work, using a variety of molecular and biochemical assays and approaches, including immunofluorescence microscopy and FRET analyses, we demonstrate that the proto-oncogene c-Fos (c-Fos) activates cytoplasmic lipid synthesis in the central nervous system and thereby supports neuronal differentiation. Specifically, in hippocampal primary cultures, blocking c-Fos expression or its activity impairs neuronal differentiation. When examining its subcellular localization, we found that c-Fos co-localizes with endoplasmic reticulum markers and strongly interacts with lipid-synthesizing enzymes, whose activities were markedly increased in vitro in the presence of recombinant c-Fos. Of note, the expression of c-Fos dominant-negative variants capable of blocking its lipid synthesis-activating activity impaired neuronal differentiation. Moreover, using an in utero electroporation model, we observed that neurons with blocked c-Fos expression or lacking its AP-1-independent activity fail to initiate cortical development. These results highlight the importance of c-Fos-mediated activation of lipid synthesis for proper nervous system development.

Lipid Metabolism in Neurons: A Brief Story of a Novel c-Fos-Dependent Mechanism for the Regulation of Their Synthesis

The mechanisms that coordinately regulate lipid synthesis in the nervous system together with the high rates of membrane biogenesis needed to support cell growth are largely unknown as are their subcellular site of synthesis. c-Fos, a well-known AP-1 transcription factor, has emerged as a unique protein with the capacity to associate to specific enzymes of the pathway of synthesis of phospholipids at the endoplasmic reticulum and activate their synthesis to accompany genomic decisions of growth. Herein, we discuss this effect of c-Fos in the context of neuronal differentiation and also with respect to pathologies of the nervous system such as the development and growth of tumors. We also provide insights into the sub-cellular sites where this regulation occurs at the endoplasmic reticulum membranes and the molecular mechanism by which c-Fos exerts this activity.

Functional symmetry of endomembranes

In higher eukaryotic cells pleiomorphic compartments composed of vacuoles, tubules and vesicles move from the endoplasmic reticulum (ER) and the plasma membrane to the cell center, operating in early biosynthetic trafficking and endocytosis, respectively. Besides transporting cargo to the Golgi apparatus and lysosomes, a major task of these compartments is to promote extensive membrane recycling. The endocytic membrane system is traditionally divided into early (sorting) endosomes, late endosomes and the endocytic recycling compartment (ERC). Recent studies on the intermediate compartment (IC) between the ER and the Golgi apparatus suggest that it also consists of peripheral ("early") and centralized ("late") structures, as well as a third component, designated here as the biosynthetic recycling compartment (BRC). We propose that the ERC and the BRC exist as long-lived "mirror compartments" at the cell center that also share the ability to expand and become mobilized during cell activation. These considerations emphasize the functional symmetry of endomembrane compartments, which provides a basis for the membrane rearrangements taking place during cell division, polarization, and differentiation.

Nociceptor structural specialization in canine and rodent testicular "free" nerve endings

The receptive fields (RFs) of polymodal nociceptor units of canine testis consist of small fascicles of branching axons ending as clusters within the thin vascular layer overlying the seminiferous tubules. This propitious arrangement enabled serial reconstruction of electron micrographs of a flat, punctate zone, identified during recording of impulse discharge of a single physiologically characterized nociceptor "unit." The RFs showed multiple axons, with some branching and sequential vesicle-containing swellings, similar to what have been called terminals and assumed to constitute impulse generating sites in other tissues. Consistent with this interpretation is the local presence of: (1) clusters of small mitochondria, (2) glycogen granules, and (3) sectors of axolemma denuded of Schwann cell processes and thus "free" endings directly contacting the epithelial basal lamina. The most distal sectors showed additional morphologic properties: (1) zones of vesicles embedded in an extensive granular matrix, (2) preterminal arrays of "axonal reticulum" derived from the smooth endoplasmic reticulum displacing axonal cytoskeletal elements, (3) a variety of sizes and electron lucency in clear, spherical vesicles and a few granular or dense-core vesicles, and (4) specialization in the last Schwann cell of the fascicle. These latter features may reflect differences in the specialized receptor mechanisms of nociceptors that are difficult to detect without extensive serial electron microscopic analysis. Alternatively, these features may constitute a regional specialization of the testis. The term free nerve ending is perhaps an insufficient and inaccurate descriptor of the morphology of nociceptors. These findings are considered in the context of their possible relation to the sensitized vanilloid receptor mechanism unique to nociceptors.

Movement of endoplasmic reticulum in the living axon is distinct from other membranous vesicles in its rate, form, and sensitivity to microtubule inhibitors

The endoplasmic reticulum (ER) is the major membranous component present throughout the axon. Although other membranous structures such as synaptic vesicles are known to move via fast axonal transport, the dynamics of ER in the axon still remains unknown. To study the dynamics of ER in the axon, we have directly visualized the movement of two ER-specific membrane proteins, the sarcoplasmic/endoplasmic reticulum calcium-ATPase and the inositol 1,4,5-trisphosphate receptor, both of which were tagged with green fluorescence protein (GFP) and expressed in cultured chick dorsal root ganglion neurons. In contrast to GFP-tagged synaptophysin that moved as vesicles at 1 microm/sec predominantly in the anterograde direction in the typical style of fast axonal transport, the two ER proteins did not move in a discrete vesicular form. Their movement determined by the fluorescence recovery after photobleaching technique was bi-directional, 10-fold slower (approximately 0.1 microm/sec), and temperature-sensitive. The rate of movement of ER was also sensitive to low doses of vinblastine and nocodazole that did not affect the rate of synaptophysin-GFP, further suggesting that it is also distinct from the well-documented movement of membranous vesicles in its relation with microtubules.

Localization and targeting of SCG10 to the trans-Golgi apparatus and growth cone vesicles

SCG10 is a membrane-associated, microtubule-destabilizing protein of neuronal growth cones. Using immunoelectron microscopy, we show that in the developing cortex of mice, SCG10 is specifically localized to the trans face Golgi complex and apparently associated with vesicular structures in putative growth cones. Consistent with this, subcellular fractionation of rat forebrain extracts demonstrates that the protein is enriched in the fractions containing the Golgi apparatus and growth cone particles. In isolated growth cone particles, SCG10 was found to be particularly concentrated in the growth cone vesicle fraction. To evaluate the molecular determinants of the specific targeting of SCG10 to growth cones, we have transfected PC12 cells and primary neurons in culture with mutant and fusion cDNA constructs. Deletion of the amino-terminal domain or mutations within this domain that prevented palmitoylation at cysteines 22 and 24 abolished Golgi localization as well as growth cone targeting, suggesting that palmitoylation of the amino-terminal domain is a necessary signal for Golgi sorting and possibly transport of SCG10 to growth cones. Fusion proteins consisting of the amino-terminal domain of SCG10 and the cytosolic proteins stathmin or glutathione-S-transferase colocalized with a Golgi marker, alpha-mannosidase II, and accumulated in growth cones of both axons and dendrites. These results reveal a novel axonal/dendritic growth cone targeting sequence that involves palmitoylation.

The synthesis and transport of lipids for axonal growth and nerve regeneration

Neurons are unique polarized cells in which the growing axon is often located up to a meter or more from the cell body. Consequently, the intracellular movement of membrane lipids and proteins between cell bodies and axons poses a special challenge. The mechanisms of lipid transport within neurons are, for the most part, unknown although lipid transport via vesicles and via cholesterol- and sphingolipid-rich 'rafts' are considered likely mechanisms. Very active anterograde and retrograde transport of lipid-containing vesicles occurs between the cell body and distal axons. However, it is becoming clear that the axon need not obtain all of its membrane constituents from the cell body. For example, the synthesis of phosphatidylcholine, the major membrane phospholipid, occurs in axons, and its synthesis at this location is required for axonal elongation. In contrast, cholesterol synthesis appears to occur only in cell bodies, and cholesterol is efficiently delivered from cell bodies to axons by anterograde transport. Cholesterol that is required for axonal growth can also be exogenously supplied from lipoproteins to axons of cultured neurons. Several studies have suggested a role for apolipoprotein E in lipid delivery for growth and regeneration of axons after a nerve injury. Alternatively, or in addition, apolipoprotein E has been proposed to be a ligand for receptors that mediate signal transduction cascades. Lipids are also transported from axons to myelin, although the importance of this process for myelination is not clear.

Targeting of endoplasmic reticulum-associated proteins to axons and dendrites in rotavirus-infected neurons

To analyze sorting and compartmentalization of molecules in neuronal endomembranes, the distribution of endogenous proteins associated with the endoplasmic reticulum (ER), intermediate compartment, the Golgi apparatus in cultures of dorsal root ganglion (DRG), and hippocampal neurons was compared with that of newly synthesized ER-associated rotavirus proteins. The endogenous ER-retained immunoglobulin heavy chain binding protein, protein disulfide isomerase, and a peptide containing the KDEL amino acid sequence appeared in the soma and dendrites up to their first branching, but not in axons. However, two other endogenous ER-associated proteins, calreticulin and calnexin, occurred in axons as well as in the somatodendritic domains. The ER-associated rotavirus proteins, VP7 and NSP4, were widely distributed in cell bodies and dendrites. The former appeared also in axons and its localization partially overlapped with that of calreticulin and calnexin. One intermediate compartment protein, ER-Golgi-intermediate compartment-protein-53 (ERGIC-53), extended beyond the first division of the dendrites and did not, as the small guanosine 5'-triphosphate (GTP)-binding protein rab2, appear in axons. The location of rab2 to small vesicles was distinct from that of rotavirus VP7. Cis/medial Golgi cistern proteins were restricted to the cell bodies and proximal dendrites. This study emphasizes the marked heterogeneity in the targeting to axons and dendrites of proteins associated with ER and intermediate compartments. Therefore, the composition of axonal ER-retained molecules differs from that in the soma and this variation may reflect differences in functions between the ER compartments. Viral proteins are useful reporters for such heterogeneities and rotavirus VP7 may be a tool to reveal sorting signals for targeting of vesicular proteins to axons via a nonclassical Golgi-independent mechanism. Such signals may also determine viral targeting to different regions of the brain.

Regional distribution of phospholipids and polyphosphatidyl inositides in the rabbit's spinal cord

The plasticity of the membrane phospholipids in general and stimulated phosphoinositides turnover in particular are the subjects in a variety of neural paradigms studying the molecular mechanisms of neuronal changes under normal and pathological conditions. The regional modifiability of phospholipids (SM, PC, PS, PI, PA + DG, PE), polyphosphatidylinositides (PI, PIP, PIP2) and diacylglycerol-dependent incorporation of CDP-choline into phosphatidylcholine in the gray matter, white matter, dorsal horns, intermediate zone and ventral horns of the rabbit's spinal cord was studied. We have found 1. a significant increase in the concentration of SM, PC, PS, DG + PA and PE in the white matter in comparison to the gray one, 2. the highest concentration of the outer membrane leaflet-bound phospholipids in the dorsal horns and the inner membrane phospholipids in the intermediate zone in comparison to the gray matter, 3. a substantial amount of labeled polyphosphatidylinositides (poly-PI(s)) in the spinal cord white matter with descending order PIP > PI > PIP2, 4. similar incorporation of myo-2-[3H]inositol into all poly-PI(s) in ventral horns and intermediate zone, but a different, lower incorporation into PI and PIP and higher into PIP2 in the dorsal horns, 5. higher diacylglycerol-dependent incorporation of CDP-choline into PC in the regionally undivided gray matter than in the white matter taken as a whole, 6. the high proportion of diacylglycerol-dependent incorporation of CDP-choline into PC in both the ventral and dorsal horns, whereas that in the intermediate zone remained low.

Sorting signals and targeting of infectious agents through axons: an annotation to the 100 years' birth of the name "axon"

A brief review is given on mechanisms by which axons may be initiated during development and by which the polarity of neurons is maintained by selective sorting and delivery of molecules to axons and dendrites. The use of viruses as tools to study targeting of newly synthesized proteins to axons is described. Emphasis is then given to the hazards that are presented to the individual by the retrograde transport of infectious agents in axons to the brain. Borna disease virus, prions, and Listeria monocytogenes are examined briefly as examples of these mechanisms. These agents have attracted interest previously in veterinary medicine for the most part, but they may present potential and substantial threats to human health. Such infectious agents also represent a new type of virus, a new principle for disease transmission, and a new mechanism for intracellular transport, respectively.

Colocalization of gamma-aminobutyric acid with vasopressin, vasoactive intestinal peptide, and somatostatin in the rat suprachiasmatic nucleus

The seemingly contradictory observations in previous publications that gamma-aminobutyric acid (GABA) is detected in all cell bodies of the suprachiasmatic nucleus (SCN) and that terminals originating from the SCN are only 20-30% GABA positive prompted us to investigate whether this might be explained by a preference of colocalization in terminals of certain peptidergic neurons in the SCN or by a day/night rhythm in GABA synthesis. At three different circadian times, animals were perfusion fixed, and their SCNs were stained for vasopressin (VP), somatostatin (SOM), or vasoactive intestinal polypeptide (VIP). Subsequently, the number of GABA peptide-positive terminals was determined using GABA postembedding staining in ultrathin sections. It appeared that the highest percentage of colocalization with GABA was detected in VIP terminals (38%) and the lowest in VP terminals (15%). No differences in colocalization percentages could be observed in any parameter at any circadian time. In the dorsomedial hypothalamus, one of the target areas of the VP and VIP fibers from the SCN, a colocalization of GABA within VP and VIP terminals was found similar to that in the SCN. In the region of the somatostatin-containing neurons in the SCN, a number of axoaxonal contacts could be observed that sometimes exhibited synaptic specializations. In nearly all cases, the axoaxonic terminals contained GABA and/or SOM. The conclusion is that the high level of intrinsic GABAergic connections in the SCN represents a putatively powerful mechanism to synchronize or shut down the activity of the SCN. We discuss the possibility that, depending on the firing frequency of the neurons, the colocalization of GABA with all peptides under investigation allows for the selection of which transmitter is released, the peptidergic one or the amino acid.

Ultrastructural localization of SNAP-25 within the rat spinal cord and peripheral nervous system

Synaptosomal associated protein of 25 kDa (SNAP-25) has been implicated in the membrane fusion machinery of neurotransmitter release and axonal growth. Using immunocytochemistry, we have analyzed the distribution and ultrastructural localization of SNAP-25 in selected areas of the central and peripheral nervous systems of adult rats. We show that the protein is specifically expressed in the trans face of the Golgi apparatus and in the axonal compartment. In axons and nerve endings, SNAP-25 is localized to discrete areas of the membranes of most organelles such as the axoplasmic reticulum, the axolemma, the outer membrane of mitochondria and synaptic vesicles. This wide distribution of SNAP-25 suggests that the protein is involved in the fusion of membranes in the whole axonal compartment of neurons.

Continuous network of endoplasmic reticulum in cerebellar Purkinje neurons

Purkinje neurons in rat cerebellar slices injected with an oil drop saturated with 1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC16(3) or DiI] to label the endoplasmic reticulum were observed by confocal microscopy. DiI spread throughout the cell body and dendrites and into the axon. DiI spreading is due to diffusion in a continuous bilayer and is not due to membrane trafficking because it also spreads in fixed neurons. DiI stained such features of the endoplasmic reticulum as densities at branch points, reticular networks in the cell body and dendrites, nuclear envelope, spines, and aggregates formed during anoxia nuclear envelope, spines, and aggregates formed during anoxia in low extracellular Ca2+. In cultured rat hippocampal neurons, where optical conditions provide more detail, DiI labeled a clearly delineated network of endoplasmic reticulum in the cell body. We conclude that there is a continuous compartment of endoplasmic reticulum extending from the cell body throughout the dendrites. This compartment may coordinate and integrate neuronal functions.

Evidence that the major membrane lipids, except cholesterol, are made in axons of cultured rat sympathetic neurons

Membrane lipids and proteins required for axonal growth and regeneration are generally believed to be synthesized in the cell bodies of neurons and transported into the axons. However, we have demonstrated recently that, in cultured rat sympathetic neurons, axons themselves have the capacity to synthesize phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine. In these experiments, we employed a compartment model of neuron culture in which pure axons grow in a fluid environment separate from that containing the cell bodies. In the present study, we again used compartmented cultures to confirm and extend the previous results. We have shown that three enzymes of phosphatidylcholine biosynthesis via the CDP-choline pathway are present in axons. We have also shown that the rate-limiting step in the biosynthesis of phosphatidylcholine by this route in neurons, and locally in axons, is catalyzed by the enzyme CTP:phosphocholine cytidylytransferase. The biosynthesis of other membrane lipids, such as phosphatidylserine, phosphatidylethanolamine derived by decarboxylation of phosphatidylserine, phosphatidylinositol, and fatty acids, also occurs in axons. However, the methylation pathway for the conversion of phosphatidylethanolamine into phosphatidylcholine appears to be a quantitatively insignificant route for phosphatidylcholine synthesis in neurons. Moreover, our data provided no evidence for the biosynthesis of another important membrane lipid, cholesterol, in axons.

The endoplasmic reticulum-associated VP7 of rotavirus is targeted to axons and dendrites in polarized neurons

Rotavirus, which matures and is retained in the endoplasmic reticulum, was used to examine how polarized dorsal root ganglion and spinal cord neurons distributed cytoplasmic and endoplasmic reticulum-associated proteins. A remarkable observation was that NS28, a trans-endoplasmic reticulum-membrane protein which functions as a receptor for budding particles, remained in the cell body during the whole course of infection (48 h) while the VP7 glycoprotein, which is endoplasmic reticulum associated and usually retained in the endoplasmic reticulum, was targeted to axons already 4 h post infection. VP7 was furthermore transported in an endo-beta-N-acetylglucosaminidase H sensitive form through the secretory pathway. The segregated appearances of NS28 and the endo-beta-N-acetylglucosaminidase H sensitive VP7 indicate that VP7 enters a transport compartment separate from NS28. Brefeldin A treatment rapidly disintegrated the Golgi apparatus of the neurons and rapidly blocked axonal transport of Sendai virus glycoproteins, while axonal transport of rotavirus VP7 was not blocked, suggesting that VP7 uses an intracellular pathway in neurons which does not involve the Golgi apparatus.

The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport

Immunofluorescence and immunogold labeling, together with sucrose gradient separation and Western blot analysis of microsomal subfractions, were employed in parallel to probe the endoplasmic reticulum in the cell body and dendrites of rat cerebellar Purkinje neurons. Two markers, previously investigated in non-nerve cells, the membrane protein p91 (calnexin) and the lumenal protein BiP, were found to be highly expressed and widely distributed to the various endoplasmic reticulum sections of Purkinje neurons, from the cell body to dendrites and dendritic spines. An antibody (denominated anti-rough-surfaced endoplasmic reticulum), which recognized two membrane proteins, p14 and p40, revealed a similar immunogold labeling pattern. However, centrifugation results consistent with a widespread distribution were obtained for p14 only, while p40 was concentrated in the rough microsome-enriched subfractions. The areas enriched in the inositol 1,4,5-triphosphate receptor and thus presumably specialized in Ca2+ transport (stacks of multiple smooth-surfaced cisternae; the dendritic spine apparatus) also exhibited labeling for BiP and p91, and were positive for the anti-rough-surfaced endoplasmic reticulum antibody (presumably via the p14 antigen). Additional antibodies, that yielded inadequate immunocytochemical signals, were employed only by Western blotting of the microsomal subfractions, while the ryanodine receptor was studied by specific binding. The latter receptor and the Ca2+ ATPase, known in other species to be concentrated in Purkinje neurons, exhibited bimodal distributions with a peak in the light and another in the heavy subfractions. A similar distribution was also observed with another lumenal protein, protein disulfide isomerase. Taken as a whole, the results that we have obtained suggest the existence in the endoplasmic reticulum of Purkinje neurons of two levels of organization; the first identified by widespread, probably general markers (BiP, p91, possibly p14 and others), the second by specialization markers, such as the inositol 1,4,5-triphosphate receptor and, possibly, p40, which appear restricted to areas where specific functions appear to be localized.

Tubulomembranous lesions in p-bromophenylacetylurea neuropathy reflect local stasis of fast axonal transport: evidence from electron microscopic autoradiography

The source of the membranous materials that accumulate in distal axons of rats intoxicated with p-bromophenylacetylurea (BPAU) was studied by electron microscopy. 35S-Methionine was injected into the ventral horn of the spinal cord at 2, 14, and 35 days after injection of BPAU. Three days later, samples of the deep peroneal nerves were obtained, and autoradiographs of thin cross sections were prepared. Organellar accumulations were absent from vehicle-treated control nerves and rare in the clinically latent period after administration of BPAU. In later stages of neuropathy, approximately 20% of the myelinated axons in any specific section were swollen and packed with tubules, membranes, and mitochondria. Numerous silver grains were located over the accumulated organelles, and the coincidence was statistically significant. The results indicate a sporadic local stasis of fast-transported proteins and provide a plausible explanation for axonal damage in BPAU neuropathy.

Isolation and characterization of rapid transport vesicle subtypes from rabbit optic nerve

Subcellular fractionation of rabbit optic nerve resolves three populations of membranes that are rapidly labelled in the axon. The lightest membranes are greater than 200 nm and are relatively immobile. The intermediate density membranes consist of 84 nm vesicles which disappear from the nerve with kinetics identical to those of the rapid component. A third population of membranes, displaying a distinct protein profile, is present in the most dense region of the gradient. Immunological characterization of these membranes suggests the following. (1) The lightest peak contains rapidly transported glucose transporter and most of the total glucose transporters present in the nerve; this peak is therefore enriched in axolemma. (2) The intermediate peak contains rapidly transported glucose transporters and synaptophysin, an integral synaptic vesicle protein, and about half of the total synaptophysin; this peak therefore contains transport vesicles bound for both the axolemma and the nerve terminal, and these subpopulations can be separated by immunoadsorption with specific antibodies against the aforementioned proteins. (3) The heaviest peak contains rapidly transported synaptophysin and tachykinin neuromodulators and about half of the total synaptophysin, and 80% of the total tachykinins present in the nerve; this peak appears to represent a class of synaptic vesicle precursor bound for the nerve terminal exclusively. (4) Synaptophysin is present in the membranes of vesicles carrying tachykinins. (5) Both the intermediate and the heaviest peaks are enriched in kinesin heavy chain, suggesting that both vesicle classes may be transported by the same mechanism.

Intraneuronal distribution of a synaptic vesicle membrane protein: antibody binding sites at axonal membrane compartments and trans-Golgi network and accumulation at nodes of Ranvier

The distribution of a cholinergic synaptic vesicle-specific transmembrane glycoprotein (Buckley and Kelly, 1985, J. Cell Biol. 100, 1284-1294) was investigated in the entire electromotor neuron of Torpedo marmorata using a monoclonal antibody and immunocytochemistry at the light- and electron-microscopical level (immunoperoxidase, colloidal gold). In the nerve, terminal binding of immunogold particles is restricted to synaptic vesicles. In the axon a number of additional membrane compartments like multivesicular bodies, vesiculotubular structures, lamellar bodies and electron-dense granules share the surface located synaptic vesicle-specific transmembrane glycoprotein-epitope. Membranous structures likely to represent the axoplasmic reticulum inside axons and nerve terminals are not labelled. Antibody-binding membrane compartments are accumulated at nodes of Ranvier. In the perikaryon the tubules of the trans-Golgi network as well as multivesicular bodies, lamellar bodies, electron-lucent vesicles, granules with electron-dense core and peroxisomes are labelled. Immunotransfer blots of isolated synaptic vesicles and tissue extracts of electric organ display a 100,000 mol. wt band of broad electrophoretic mobility typical of the synaptic vesicle-specific transmembrane glycoprotein. Extracts of electromotor nerve and electric lobe contain in addition a strong band at 85,000 mol. wt and a few lower molecular weight bands. We suggest that the synaptic vesicle originates directly from the trans-Golgi network. The endoplasmic reticulum is not involved in vesicle formation or retrieval. On retrograde transport the vesicle membrane compartment is likely to fuse with other intra-axonal (endosomal?) organelles.

Reduced axonal transport of the G4 molecular form of acetylcholinesterase in the rat sciatic nerve during aging

Aging in the sciatic nerve of the rat is characterized by various alterations, mainly cytoskeletal impairment, the presence of residual bodies and glycogen deposits, and axonal dystrophies. These alterations could form a mechanical blockade in the axoplasm and disturb the axoplasmic transports. However, morphometric studies on the fiber distribution indicate that the increase of the axoplasmic compartment during aging could obviate this mechanical blockade. Analysis of the axoplasmic transport, using acetylcholinesterase (AChE) molecular forms as markers, demonstrates a reduction in the total AChE flow rate, which is entirely accounted for by a significant bidirectional 40-60% decrease in the rapid axonal transport of the G4 molecular form. However, the slow axoplasmic flow of G1 + G2 forms, as well as the rapid transport of the A12 form of AChE, remain unchanged. Our results support the hypothesis that the alterations observed in aged nerves might be related either to the impairment in the rapid transport of specific factor(s) or to modified exchanges between rapidly transported and stationary material along the nerves, rather than to a general defect in the axonal transport mechanisms themselves.

Immunoelectron microscopic localization of synaptophysin in a Golgi subcompartment of developing hypothalamic neurons

Synaptophysin, previously identified as an integral membrane glycoprotein (mol. wt 38,000) characteristic of presynaptic vesicles of mature neurons, provides a molecular marker to study the origin, formation and traffic of synaptic vesicles. Using the monoclonal antibody SY38 against this polypeptide we have localized synaptophysin by immunofluorescence and electron microscope immunoperoxidase methods in cultured mouse hypothalamic neurons taken from 16-day-old fetuses which achieve synaptogenesis after 10-12 days in vitro. We have compared the localization of synaptophysin in perikarya and nerve endings as a function of age (2-19 days in vitro) and of treatment of mature neurons with nocodazole. Using immunofluorescence microscopy, synaptophysin was already detected in neuronal soma at 2 days in vitro, where the initiation of neurite development is observed. At the electron microscope level, virtually all mature synaptic boutons and varicosities showed an extensive synaptophysin labeling of synaptic vesicles at 12-13 days in culture whereas neurites showed only very few labeled vesicles. In neuronal soma taken before synapse formation (6 days in vitro), synaptophysin was selectively localized in membranes of the innermost cisternae of the Golgi zone and in vesicles of variable size and shape in the core of the Golgi zone. In contrast, after synapse formation, synaptophysin labeling was barely detected in the Golgi zone of neurons but a very strong labeling of synaptic vesicles in synaptic boutons was observed. Treatment of mature neurons (12 days in vitro) with nocodazole (10(-5) M) resulted in a conspicuous synaptophysin staining of the innermost trans-Golgi cisternae and numerous vesicles in the cytoplasm. Furthermore, an accumulation of labeled synaptic vesicles on the presynaptic membrane of nerve terminals was found. The data suggest that synaptophysin is released from the Golgi apparatus in a vesicular form, after glycosylation, and is then transported to nerve endings by a mechanism which requires integrity of microtubules.

The Golgi apparatus-complex of neurons and astrocytes studied with an anti-organelle antibody

An antiserum reacting with a 135-kDa antigen of rat liver Golgi apparatus-complex was used to stain, by light microscopic and ultrastructural immunocytochemistry, sections of rat cerebellum and by immunoblot homogenates of whole brain, isolated neurons and a fraction of enriched neuronal Golgi apparatus. In sections of rat cerebellum fixed with periodate-lysine-paraformaldehyde and immunostained with the direct peroxidase or peroxidase-antiperoxidase methods, the Golgi apparatus-complex in perikarya of neurons and glia was stained. Occasionally, nuclear envelopes and cisternae of the rough endoplasmic reticulum of neurons and glia were stained. Immunostain was not observed in peripheral dendrites, axons and presynaptic terminals. In striking contrast, peripheral smooth cisternae of astrocytic perikarya and processes were stained. Immunoblots of whole-brain membrane fractions, homogenates of isolated neurons and an enriched neuronal fraction of the Golgi apparatus-complex showed a principal single band of 64-kDa apparent mol. wt. We have concluded that the putative 64-kDa antigen(s) is distributed in cisternae of the Golgi apparatus-complex and occasionally in the nuclear envelope and rough reticulum, within the perikarya of neurons and glia. A second important distribution of the 64-kDa antigen(s), involving peripheral cisternae in perikarya and processes of astrocytes, is consistent with the hypothesis that the Golgi apparatus-complex of these cells extends to the periphery of these cells. The functional implications of the peripheral localization of the 64-kDa antigen(s) in astrocytes are discussed.

Absolute specificity for retrograde fast axonal transport displayed by lipid droplets originating in the axon of an identified Aplysia neuron in vitro

Lipid droplets were found to form all along the axon of the giant cerebral neuron (GCN) of the sea hare Aplysia californica when the cell was placed in culture. The emission of yellow fluorescence by the droplets after exposure of the neuron to Nile red and their uniformly dark appearance in electron micrographs of axons fixed with glutaraldehyde and osmium tetroxide identified them as lipid droplets. In contrast to lipid droplets in fat cells and certain other cell types, these droplets were bounded by a membrane, indicating that the lipid droplet is a type of organelle that is membranated in some situations but not others. As observed by video-enhanced contrast-differential interference contrast microscopy, the droplets grew manyfold in place in the axon to diameters of 1-3 micron within 2-3 days. Often they formed coherent tandem arrays of 3-15 droplets. Droplets were usually essentially stationary but occasionally moved tens of microns by fast axonal transport, the largest spherical organelles to have been observed to undergo transport. They usually moved as singlets, sometimes as tandem arrays. The direction of transport was always retrograde (towards the cell body). Thus, an organelle need neither originate nor be modified in the axon terminal to be specified for retrograde transport. Whether or not an organelle is formed in the cell body might determine directionality. Alternatively, size might be a determining factor, with large organelles specified for retrograde transport.

Increased synthesis of membrane macromolecules is an early response of retinal neurons to trimethyltin intoxication

We studied the synthesis and axonal transport of proteins and glycoproteins in the visual system of adult Long-Evans rats that had received 4 weekly doses of trimethyltin hydroxide (TMT, 4 mg/kg b. wt.) by gastric intubation. One week following the last dose, an in vitro assay was used to study the rate of incorporation of radioactive precursors into various macromolecules of isolated retinas. Retinas from TMT-treated rats showed increased apparent rates of synthesis, relative to retinas from control rats, for proteins [( 35S]methionine precursor) and glycoproteins [( 3H]fucose precursor). Gel electrophoretic analysis of newly synthesized proteins indicated that the increased synthesis was a generalized effect, i.e. it was not restricted to a select subset of proteins. The axonal transport of these macromolecules by retinal ganglion cells to axons (optic tract) and nerve endings (superior colliculus) was examined in vivo following intraocular precursor injection. The amount of material transported, relative to that synthesized in the retina, was not appreciably altered in TMT-treated rats, indicating that TMT did not selectively impair axonal transport. The biochemical changes were accompanied by minimal ultrastructural alterations and little neuronal necrosis in the retina. We suggest that TMT induces increased synthesis of membrane macromolecules in retinal neurons; this may reflect an early reactive (compensatory) response rather than a regressive (degenerative) response of retinal neurons to TMT. Our data do not support the hypothesis that TMT induces a functional impairment of neuronal endoplasmic reticulum or Golgi apparatus.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 3. The peripheral axon lesion with special reference to the nerve roots

Progressive axonopathy is an autosomal recessive inherited neuropathy of Boxer dogs with lesions in the CNS and PNS. This paper describes the axonal changes in the lumbar and cervical nerve roots and tibial nerve. By 2 months of age the proximal paranodal areas of many larger diameter fibres show small axonal swellings, sometimes with attenuation or loss of the associated myelin sheath. Axoplasmic changes within swollen and non-swollen fibres include disorganization of the peripheral neurofilaments and small accumulations of vesicles and vesiculo-tubular profiles, particularly in the sub-axolemmal area. Occasional fibres, more often in the cervical roots, are massively distended with disorganized neurofilaments. The frequency of the membranous accumulations decreases with progression of the disease. Many axons show a markedly irregular or corrugated outline and are surrounded by an attenuated sheath. The peripheral axonal cytoskeleton is disorganized and misaligned, whereas the central structures maintain a more normal arrangement. Regenerating axonal clusters are common in the cervical ventral roots but occur infrequently in the lumbar roots. Similar axonal changes occur in the peripheral nerves but at a much lower frequency. Any membranous accumulations or cytoskeletal disorganization are more probable in the proximal tibial nerves, while the frequency of axonal degeneration and regeneration increases distally. The morphological appearances indicate gross disturbances in axon-sheath cell relationships and suggest that abnormalities in the transport of various axoplasmic organelles may be involved in the pathogenesis of the axonal lesion.

Retrograde axonal transport of phospholipid in rat sciatic nerve

Retrograde axonal transport of phospholipid was studied in rat sciatic motoneuron axons by placing collection crushes on the nerve at intervals after injection of [methyl-3H]choline into the lumbosacral spinal cord, and allowing labelled material undergoing anterograde or retrograde movement to accumulate adjacent to the collection crushes. Control experiments showed that the accumulations of label were not a result of local uptake of circulating precursor. The majority of the 3H label was associated with phosphatidylcholine. Accumulation of label at the distal collection crush, representing retrograde transport, was observed subsequent to the anterograde transport of phospholipid. In comparison with a previous study on retrograde transport of protein, the following points were noted: (1) onset of retrograde transport occurred at approximately the same time after precursor injection (10-20 h) for both protein and phospholipid; (2) retrograde transport of lipids was more prolonged: maximum retrograde transport occurred later for phospholipid (approximately 30 h) than for protein (15-20 h), and declined to half-maximum between 49 and 99 h, compared to a corresponding value of 24-28 h for protein; (3) the proportion of total anterograde-transported activity subsequently undergoing retrograde transport was less in the case of phospholipid, at least over the time interval studied (up to 99 h after precursor injection). The similar times of onset of retrograde transport of phospholipid and protein support the concept of retrograde transport as a recycling mechanism returning to the cell body membrane fragments that were earlier transported into the axon. Coordinated retrograde transport of labelled protein and phospholipid components of the recycled membranes would be predicted.(ABSTRACT TRUNCATED AT 250 WORDS)

Target size of neurotoxic esterase and acetylcholinesterase as determined by radiation inactivation

The target size of neurotoxic esterase (NTE), the putative target site for the initiation of organophosphorus-compound-induced delayed neurotoxicity, and acetylcholinesterase (AChE) from hen brain were examined by determining the rate at which the activities of the esterases were destroyed by ionizing irradiation. Samples of hen brain were prepared by slowly drying a microsomal preparation under vacuum. The dried samples were then irradiated with electrons from a 1 MeV Van de Graaff generator. The doses ranged from 0 to 28 Mrad. The radiation doses were calibrated by the rate of inactivation of T1-bacteriophage plaque induction. Following the irradiation procedure, the samples were resuspended in buffer and enzymic activity was measured. The target size of NTE from hen brain was determined to be about 105 kDa, whereas hen brain AChE was found to have a target size of about 53 kDa. The target size of NTE was found to be similar in experiments with rat brain and cat brain. In addition, commercial preparations of electric-eel electric-organ AChE and horse serum butyrylcholinesterase were found to have target sizes that were identical with each other, and also were very similar to that of AChE from hen brain.

Retrograde axonal transport of receptor-bound opiate in the vagus and delayed accumulation in the nodose ganglion

Opiate receptors measured in vivo with [3H]lofentanil in the rat vagus nerve were found to accumulate on both sides of a ligature. The time-course of accumulation was completely different in the proximal and the distal segments; the labelling was maximal 4 h after injection of [3H]lofentanil above the ligature but 16-24 h below the ligature. In unligated rats, a peak of radioactivity appeared in the nodose ganglion 16 h after injection; vagotomy, vinblastine or chronic treatment with capsaicin prevented the appearance of this delayed accumulation in the ganglion. These foregoing experiments suggest that opiate may act in the cell body of sensory neurones after being internalized at the nerve terminals and then transported retrogradely through fast axoplasmic mechanisms.

The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. II. Axons and terminals

The morphology and cytochemistry of the endoplasmic reticulum (ER) in axons and terminals of a number of different types of neurons in brains from mice were investigated ultrastructurally. The neurohypophysis received particular attention because the morphology and enzyme cytochemical activities of many of the preterminal swellings of hypothalamo-neurohypophysial axons are altered by chronic salt-stress. Membrane contrast and enzyme cytochemical staining techniques were employed to characterize the axonal reticulum and to determine if organelles representing the lysosomal system in the axon and the tubular profiles participating in the anterograde axonal transport of native horseradish peroxidase (HRP) are associated with the ER. Potential enzyme cytochemical markers for the axonal ER included glucose-6-phosphatase (G6Pase), thiamine pyrophosphatase, nucleoside diphosphatase, and acid hydroxylase activities. The anterograde transport of HRP was analyzed in undamaged hypothalamo-neurohypophysial neurons and in facial and hypoglossal motoneurons of mice receiving the protein in the lateral cerebral ventricle. The ER pervaded the axon and appeared as parallel, 20-40-nm-wide tubules interconnected by oblique anastomoses. Membrane thickness of the axonal reticulum measured 60-100 A, which is similar to that of the perikaryal ER. Enzyme cytochemical activities associated with the ER or lysosomes were not conspicuous in axons and terminals under normal conditions but became prominent in some axons and preterminal swellings manifesting an autophagic appearance within neurohypophyses from salt-stressed mice. Only G6Pase activity was a marker for the ER in these axons and preterminals. Many ER profiles in non-incubated sections and in G6Pase cytochemical preparations of salt-stressed neurohypophyses were wrapped around or interspersed among secretory granules, multilamellar bodies, and vacuoles that may represent forms of lysosomes involved in autophagy and crinophagy. Acid hydrolase activities were localized within the vacuoles as well as within 80-130-nm-wide, blunt-ended tubules in pituitary stalk axons; similar reactive tubules were confluent with large secondary lysosomes in neurosecretory cell bodies and may be derived from these lysosomes. Morphologically identical tubules transporting HRP in the anterograde direction were observed only in the salt-stressed hypothalamo-neurohypophysial neuron. The HRP-positive tubules very likely are affiliated with the lysosomal system.

Quantitative ultrastructural stereology of synapses in nucleus dorsalis after a peripheral nerve injury at birth

Utilizing recent techniques in quantitative stereology, this investigation studied the synaptology of nucleus dorsalis (Clarke's column) in 12-week-old rats whose sciatic nerves were crushed in the 1st postnatal day. Four morphometric variables were analyzed at the levels of L1 and L3 spinal cord segments: total surface area of synaptic contact zones per unit volume (SV), total length of synaptic contact zones per unit area (LA), average length of synaptic membrane (L), and numerical density of synapses per unit volume (NV). The original raw data were corrected for Holmes's effect. The results indicated that peripheral nerve crush at birth induced a transganglionic change in central sensory terminals with a loss of numerous synapses. A significant loss (P less than 0.001) of about 32% in the SV and LA and a significant loss (P less than 0.001) of about 36% in the NV were observed on the experimental side. There was no preferential loss of synapses in either segment. The mean synaptic membrane length showed no significant difference between the control and experimental sides. The control values of the four morphometric variables calculated for L3 were lower than those calculated for L1. The loss of synapses after a peripheral nerve lesion was probably due to the loss of sensory neurons and their central processes, but there were other possibilities.

Depression of fast axonal transport produced by tullidora

The fast axoplasmic transport of labeled proteins was studied in cats showing hindlimb paralysis 4-7 weeks after a single oral dose of tullidora (Karwinskia humboldtiana) toxins. The isotope (3H-leucine) was injected into the spinal ganglion and the contralateral spinal cord of the seventh lumbar segment in order to study transport in sensory and motor fibers. The axoplasmic transport in motor fibers of the sciatic nerve was clearly altered in tullidora-treated cats. The majority of these animals showed a gradual decline of radioactivity from the cord to the periphery instead of the clear-cut wave front always seen in normal cats. An apparent wave was seen in three treated cats but the wave peak was behind the normal position and the slope of the wave front was reduced. While the rate of transport indicated by the farthest extent of the foot of the slope was not in all cases significantly changed, the results all indicated a hindered transport by the reduced slope front in the distal segments of the motor axons. In contrast, the axoplasmic transport appeared normal in the sensory fibers of all but one tullidora-treated cat. Light and electron microscopy of medial gastrocnemius and sural (cutaneous) nerves revealed axonal constrictions and axolemal irregularities associated with organelle retention after tullidora treatment. Also, some mitochondria appeared swollen. These changes were more frequent and intense in the motor nerve fibers than in the cutaneous nerve fibers.

Is protease activity involved in fast axonal transport?

N-alpha-p-Tosyl-L-Lysine Chloromethyl Ketone (TLCK), a protease inhibitor, was found to irreversibly inhibit rapid axonal transport of protein in vitro in the frog sciatic nerve. TLCK exerted its action at the axonal level and seemed to depress the rate rather than the amount of transported protein. The efficiency of TLCK as a protease inhibitor was demonstrated by polyacrylamide gel electrophoresis, which showed that degradation of high molecular weight proteins (presumably neurofilament subunits) into a 25000 dalton protein could be induced by exposing the frog nerves to triton-X and prevented by the presence of TLCK. Findings that TLCK, at a transport inhibiting concentration (0.1 mM), had little or no effects on either protein synthesis or ATP levels, suggest that TLCK did not affect transport due to general cytotoxic properties. The effects of TLCK is discussed in relation to possible roles of protease activity in axonal transport.

Localization of axonally transported label in chick retinal ganglion cell axons after intravitreal injections of wheat germ agglutinin conjugated to horseradish peroxidase

We have studied the subcellular localization of peroxidase-labeled organelles after anterograde axonal transport by chick retinal ganglion cells that had been exposed 23-25 h earlier to wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). After intravitreal injection of WGA-HRP, we found in the optic tectum that 82% of labeled organelles were located within axons and axon terminals. The organelles included: tubules and cisternae of the smooth endoplasmic reticulum, hypolemmal cisternae, vesicles, dense bodies and multivesticular bodies. We also measured the distances between the centers of the labeled organelles and the plasma membrane of these profiles. The density of organelles (number of organelles/micron 2) was plotted as a function of distance from the plasma membrane. Irrespective of the dose of lectin-peroxidase injected, labeled organelles were most densely concentrated in a 30 nm wide annular zone centered 75 nm in from the plasma membrane. In axon terminals the labeled organelles were most concentrated 75-90 nm in from the plasma membrane. Assuming that the peroxidase label indicates the presence of WGA-HRP, we conclude that after anterograde axonal transport the lectin accumulates in lysosomal organelles and elements of the smooth endoplasmic reticulum. Therefore, in contrast to the more restricted localization of [125I]WGA as inferred from electron microscopic autoradiography after uptake and transport by the same cell type, WGA-HRP-labeled organelles are found more diffusely within the axoplasm, particularly in axon terminals. Furthermore, peroxidase-labeled organelles in dendritic, glial or neuronal cell bodies in the tectum were seen less frequently than expected based on evidence of frequent transfer to second cells after intravitreal injections of [125I]WGA. Thus, we infer that at these concentrations WGA labeled with HRP may not be transferred intercellularly as efficiently as even lower concentrations of iodinated WGA are apparently transferred.

Neurotrophic and neuronotrophic effects in the regenerating newt limb bud after electrical stimulation of brachiospinal nerves

An earlier work demonstrated that electrical stimulation of newt brachiospinal nerves produces a 20% increase in protein synthesis in the regenerating limb bud at 6 h post-stimulation. The present study shows that if stimulation of nerve cell bodies is prevented by placing procaine between the cell bodies and the stimulating electrode, there is no increase in limb bud protein synthesis compared to the non-stimulated, contralateral control limb bud. Similarly, if colchicine is applied to the brachiospinal nerves at the site of and prior to stimulation, there is no increase in limb bud protein synthesis after stimulation. Colchicine applied to brachiospinal nerves in the absence of stimulation results in a reduction of limb bud protein synthesis that is of the same magnitude as the increase seen with stimulation. The results suggest that the neurotrophic increase in limb bud protein synthesis after stimulation is under the control of the cell body and that this control is mediated by changes in fast axonal transport. A neuronotrophic increase in axonal density in the stimulated side limb bud is seen at the same time as the increase in protein synthesis after stimulation.

Delayed neuronal recovery and neuronal death in rat hippocampus following severe cerebral ischemia: possible relationship to abnormalities in neuronal processes

Mechanisms involved in the postischemic delay in neuronal recovery or death in rat hippocampus were evaluated by light and electron microscopy at 3, 15, 30, and 120 min and 24, 36, 48, and 72 h following severe cerebral ischemia that was produced by permanent occlusion of the vertebral arteries and 30-min occlusion of the common carotid arteries. During the early postischemic period, neurons in the Ca1 and Ca3 regions both showed transient mitochondrial swelling followed by the disaggregation of polyribosomes, decrease in rough endoplasmic reticulum (RER), loss of Golgi apparatus (GA) cisterns, and decrease in GA vesicles . Recovery of these organelles in Ca3 neurons was first noted between 24 and 36 h and was accompanied by a marked proliferation of smooth endoplasmic reticulum (SER). Many Ca1 neurons initially recovered between 24 and 36 h, but subsequent cell death at 48-72 h was often preceded by peripheral chromatolysis, constriction and shrinkage of the proximal dendrites, and cytoplasmic dilatation that was continuous with focal expansion of RER cisterns. Because SER accumulates in resistant Ca3 neurons and proximal neuronal processes are damaged in vulnerable Ca1 neurons, we hypothesize that delayed cell recovery or death in vulnerable and resistant postischemic hippocampal neurons is related to abnormalities in neuronal processes.

Microsomal opiate receptors: characterization of smooth microsomal and synaptic membrane opiate receptors

In continuing studies on smooth microsomal and synaptic membranes from rat forebrain, we compared the binding properties of opiate receptors in these two discrete subcellular populations. Receptors in both preparations were saturable and stereospecific. Scatchard and Hill plots of [3H]naloxone binding to microsomes and synaptic membranes were similar to plots for crude membranes. Both synaptic membranes and smooth microsomes contained similar enrichments of low- and high-affinity [3H]naloxone binding sites. No change in the affinity of the receptors was observed. When [3H]D-ala2-D-leu5-enkephalin was used as ligand, microsomes possessed 60% fewer high-affinity sites than did synaptic membranes, and a large number of low-affinity sites. In competition binding experiments microsomal opiate receptors lacked the sensitivity to (guanyl-5'-yl)imidodiphosphate [Gpp(NH)p] shown by synaptic and crude membrane preparations. In this respect microsomal opiate receptors resembled membranes that were experimentally guanosine triphosphate (GTP)-uncoupled with N-ethylmaleimide (NEM). Agonist binding to microsomal and synaptic membrane opiate receptors was decreased by 100 mM NaCl. Like NEM-treated crude membranes, microsomal receptors were capable of differentiating agonist and antagonists in the presence of 100 mM NaCl. MnCl2 (50-100 microM) reversed the effects of 100 mM NaCl and 50 microM GTP on binding of the mu-specific agonist [3H]dihydromorphine in both membrane populations. Since microsomal receptors are unable to distinguish agonists from antagonists in the presence of Gpp(NH)p, they are a convenient source of guanine nucleotide-uncoupled opiate receptors.

Acrylamide-induced remodelling of perikarya in rat superior cervical ganglia

To extend investigations on how acrylamide affects neuronal perikarya, we studied post-ganglionic cell bodies in the superior cervical ganglia of control and intoxicated rats (50 mg/kg/day for 9-10 days) with light and electron microscopy and with stereology. By light microscopy, perikarya of intoxicated rats showed peripheral Nissl bodies and nuclear eccentricity. Ultrastructurally, many experimental neurons showed augmented Nissl bodies, often extending from nucleus to plasma membrane; cisternae of granula endoplasmic bodies often appeared in orderly stacked configurations. Intoxicated neurons had increased numbers of normal-appearing mitochondria; some mitochondrial profiles appeared in clumps and others were hypertrophied. Compared to control neurons where Golgi complex most often aligned in a circumnuclear position, experimentals showed reductions in amount and loss of orientation. In some perikarya the pattern of eccentricity of nucleus, peripheral mantle of Nissl, and increased mitochondria gave a distinctive chromatolysis-like appearance. Ultrastructural stereologic analysis showed significantly increased volume fractions of Nissl (P less than 0.001) and mitochondria (P less than 0.002), a trend towards decreased Golgi, and no change in lysosomes, confirming the ultrastructural findings. These data indicate that cell body remodelling is a widespread phenomenon, specific for different neuronal types, and that it probably acts importantly in the pathogenesis of disease.

Uptake of calcium by the endoplasmic reticulum of the frog photoreceptor

We studied retinal photoreceptors of Rana pipiens by using techniques designed to investigate calcium localization. Particularly useful were methods in which intracellular sites of calcium uptake were detected by incubation of saponin-treated isolated retinas in calcium-containing media, with oxalate present as a trapping agent. With these procedures, cell compartments accumulate deposits, which can be shown to contain calcium by x-ray microanalysis. Calcium accumulation was prominent in the rough endoplasmic reticulum in the myoid region. In addition, deposits were observed in agranular reticulum and in certain Golgi-associated compartments of the myoid region, in mitochondria, in axonal reticulum, and in agranular reticulum of presynaptic terminals. Calcium was also detected in the endoplasmic reticulum of retinas fixed directly upon isolation, by a freeze-substitution method. The factors influencing accumulation of calcium in the endoplasmic reticulum were evaluated by a semiquantitative approach based on determining the relative frequency of calcium oxalate crystals under varying conditions. Calcium accumulation was markedly enhanced by ATP. Studies with a nonhydrolyzable ATP analogue (adenylyl- imidodiphosphate ) and with inhibitors of the sarcoplasmic reticulum Ca2+-Mg2+ ATPase (mersalyl and tetracaine) indicated that this ATP-dependent calcium uptake reflects an energy-dependent process roughly comparable to that in the sarcoplasmic reticulum.

Characterization of translation products of the polyadenylated RNA of free and membrane-bound polyribosomes of rat forebrain

Poly(A)+ RNA (polyadenylated RNA) isolated from membrane-bound and free polyribosomes was translated in reticulocyte lysates, and the products were analysed by two-dimensional gel electrophoresis. Several translation products were specific to membrane-bound polyribosomal mRNA, including polypeptides of 47kDa, 35kDa and 21 kDa, whereas others (e.g. of 37 kDa, 17 kDa and 14 kDa) were specific to free polyribosomal mRNA. Although many products were common to both mRNA species, cross-contamination could be ruled out on the basis of the presence of these and other specific products. The common products included a 68 kDa microtubule-associated protein, tubulin, actin, the brain form of creatine kinase, neuron-specific enolase and protein 14-3-3 and calmodulin, all of which were identified on the basis of two-dimensional gel and peptide analyses. The 35 kDa protein product of membrane-specific mRNA was co-translationally processed in vitro by microsomal membranes, resulting in its cleavage to 33 kDa (and partial glycosylation). The 33 kDa processed protein (but not the 35 kDa precursor) was integrated into both dog pancreas and rat brain microsomal membranes. The occurrence of the enzymes and calmodulin as products of membrane-bound polyribosomal mRNA is discussed in the light of their presence on rat brain synaptic plasma membranes [Lim, Hall, Leung, Mahadevan & Whatley (1983) J. Neurochem. 41, 1177-1182] and their existence in a specific component of axonal flow. It is suggested that some of these translation products of the rough endoplasmic reticulum may represent proteins destined for the plasma membrane. However, the identity and location of the 35 kDa membrane-specific product (or its processed form) still remain unestablished.

The tridimensional structure of Nissl bodies: a stereoscopic study in ventral horn cells of rat spinal cord

The tridimensional structure of rough endoplasmic reticulum was examined with both high and low voltage electron microscopes in large ventral horn cells of rat spinal cord, by combining stereoscopic techniques with the use of thick sections selectively impregnated with heavy metal salts. In all neurons examined Nissl bodies appeared as well defined clusters of densely stained and profusely anastomosed plate-, ribbon-, and thread-like cisternae. Plate-like cisternae were variable in size, often showed a shallow curvature, and usually ran in short parallel arrays, separated from one another by fairly constant intervals. All gave rise at their edges to several ribbon-like extensions which occasionally decreased in width distally, turning into thin, thread-like cisternae. Characteristically, these ribbon-like structures would emerge at an angle from their plate of origin and smoothly curve away from the plane of the plate to merge with ribbons or threads arising from adjacent or more distant plates. Most plate-like cisternae were found at the periphery of Nissl bodies and tended to be oriented parallel to their surface. In contrast, the center of Nissl bodies was almost exclusively occupied by a complex network of ribbon- and thread-like cisternae. It is suggested that the basic plate/ribbon association here described in spinal motoneurons might be a constant feature of Nissl body architecture in various neuronal types, while the size, orientation, and relative proportion of plate-like cisternae may vary according to the metabolic state and/or functional specialization of the cells.

Distribution of proteins migrating with fast axonal transport. Their relationship to smooth endoplasmic reticulum

The cross-sectional distribution of 3H-labeled proteins moving with the front and plateau of fast axonal transport was analyzed by quantitative electron microscopic autoradiography in sciatic nerves of rats following injection of a 1:1 mixture of [3H]proline and [3H]lysine into lumbar ventral horns. While in the front the transported proteins were uniformly distributed within the axons excluding the axolemma, which was not labeled, in the plateau 90% of 3H-labeled proteins were uniformly distributed and 10% were exclusively located at a 80-nm annulus with its outer edge at the axolemma. Ultrastructural morphometry showed that the density of smooth endoplasmic reticulum (SER) in a 160 nm wide subaxolemmal annulus was approximately twice that in the remaining axon, where it was uniform. These findings: (a) are consistent with the concept that membranous materials are released from the fast transport, move laterally and are inserted into the axolemma; (b) indicate that a portion of SER, largely located at the periphery of the axon, is not involved in fast transport.

Two anatomical pathways for the renewal of surface glycoproteins in chloride cells of fish gills

The cytoplasm of chloride cells found in the epithelium lining the gills of guppies (Lebistes reticulatus) contains, in addition to the Golgi apparatus and cisternae of endoplasmic reticulum, two distinct membranous components, the vesiculotubular and the tubular systems. While the latter is connected to the laterobasal plasma membrane, the former, made up of small vesicles and short membranous tubules, is seen mainly between the Golgi apparatus and the apical cavity which invaginates the apex of the cell. The role of these two systems in the transport of glycoproteins from the Golgi apparatus to the cell surface was investigated in fishes maintained in fresh and salt water, injected with 3H-fucose, and sacrificed at various intervals thereafter (10 and 30 min; 2.5, 8, 15.5, 24, and 48 hours). The distribution of the label was analyzed by quantitative radioautography in sections examined with the light and electron microscopes. The light microscopic data suggested that the label incorporated in the supranuclear region, where the Golgi apparatus is located, migrated toward the apical and the laterobasal regions of the chloride cells. The relative concentration of the tracer over the various components of the cytoplasm of these cells was calculated from data collected on electron microscope radioautographs at various intervals after 3H-fucose injection. The curves obtained supported the view that glycoproteins synthesized in the Golgi apparatus were transported to the apical surface via the vesiculotubular system, and to the laterobasal membrane via the tubular system.

Ultrastructural networks in growth cones and neurites of cultured central nervous system neurons

We have examined growth cones and neurites of cultured central nervous system neurons by high-voltage electron microscopy. Embryonic chicken retina cells were cultured on polylysine-treated and Formvar-coated gold grids for 2-6 days, fixed, and critical point dried. Growth cones and neurites were examined as unembedded whole mounts. Three-dimensional images from stereo-pair electron micrographs of these regions showed a high degree of ultrastructural articulation, with distinct, non-tapering filaments (5-9 nm in diameter) joining both cytoskeletal and membranous components. In the central regions of growth cones, interconnected structures included microtubules, large membranous sacs (up to 400 nm), and irregular vesicles (25-75 nm). A denser filamentous network was prevalent at the edges of growth cones. This network, which frequently adjoined the surface membrane, linked vesicles of uniform size (35-40 nm). Such vesicles often were seen densely packed in growth cone protrusions that were about the size of small synaptic boutons. Prevalent structural interconnections within growth cones conceivably could play a logistic role in specific membrane assembly, intracellular transport, endocytosis, and secretion. Because such processes are not unique to growth cones, the extensive linkages we have observed may have implications for cytoplasmic structure in general.