ER and Golgi trafficking in axons,dendrites, and glial processes

Both neurons and glia in mammalian brains are highly ramified. Neurons form complex neural networks using axons and dendrites. Axons are long with few branches and form pre-synaptic boutons that connect to target neurons and effector tissues. Dendrites are shorter, highly branched, and form post-synaptic boutons. Astrocyte processes contact synapses and blood vessels in order to regulate neuronal activity and blood flow, respectively. Oligodendrocyte processes extend toward axons to make myelin sheaths. Microglia processes dynamically survey their environments. Here, we describe the local secretory system (ER and Golgi) in neuronal and glial processes. We focus on Golgi outpost functions in acentrosomal microtubule nucleation, cargo trafficking, and protein glycosylation. Thus, satellite ER and Golgi are critical for local structure and function in neurons and glia.     

Cell adhesion molecules: signalling functions at the synapse

Many cell adhesion molecules are localized at synaptic sites in neuronal axons and dendrites. These molecules bridge pre- and postsynaptic specializations but do far more than simply provide a mechanical link between cells. In this review, we will discuss the roles these proteins have during development and at mature synapses. Synaptic adhesion proteins participate in the formation, maturation, function and plasticity of synaptic connections. Together with conventional synaptic transmission mechanisms, these molecules are an important element in the trans-cellular communication mediated by synapses.     

Neural Organization of the Median Ocellus of the Dragonfly : II. Synaptic structure

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.     

The postnatal development of the hypoglossal nucleus in the rat]

Using light and electron microscopy the neurons, glial cells and capillaries in hypoglossal nucleus of the rats have been examined up to 20 days after birth. The neuronal nuclei are usually situated ecentrically. The mitochondria and extensively developed Golgi-zones occupy the perinuclear region. The microtubules and lysosomes become more numerous with aging. At the earliest periods rough endoplasmic reticulum (ER) occupies the neuronal periphery, whereas after 14th day it is extended to the perinuclear region also. The ER forms elongated and concentric lamellated bodies and subsurface cisternae. At this time nucleolus like bodies are also numerous in the cytoplasm. After 4th and 6th days the extensive growth of dendrites, containing many cell organelles, and axons rich in microtubules are observed. Only at the birthday do neurons contain glycogen deposit. After 1st day the glycogen leaves the pericaryon, but it persists a long time in the neuronal processes. The symmetrical and asymmetrical contacts are characteristic for the examined period. The axo-somatic and axo-dendritic synapses are more abundant, but "double synapses" are also established. More synaptic boutons possess besides synaptic vesicles dense-core vesicles at the earlier periods. The quantity of asymmetric synapses increases with differentiation. Extensive cell degeneration has been established between 8 and 18th days. At 4 and 6 days the glial cells penetrate from subependymal layer and they have satellite neuronal position. This is more pronounced between 14 and 18 days when the oligodendrocytes are more numerous and active. At the same time fibrous astrocyte like cells are appeared. Microglial cells were not observed. Capillary differentiation, expressed by changes of the endothelial cells, pericytes and connective tissue cells, continues after birth also.     

Axonal transport of mitochondria to synapses depends on milton,a novel Drosophila protein

A protein required to localize mitochondria to Drosophila nerve terminals has been identified genetically. Photoreceptors mutant for milton show aberrant synaptic transmission despite normal phototransduction. Without Milton, synaptic terminals and axons lack mitochondria, although mitochondria are numerous in neuronal cell bodies. In contrast, synaptic vesicles continue to be transported to and concentrated at synapses. Milton protein is associated with mitochondria and is present primarily in axons and synapses. A likely explanation of the apparent trafficking defect is offered by the coimmunoprecipitation of Milton and kinesin heavy chain. Transfected into HEK293T cells, Milton induces a redistribution of mitochondria within the cell. We propose that Milton is a mitochondria-associated protein required for kinesin-mediated transport of mitochondria to nerve terminals.     

Evidence for local corticotropin releasing factor (CRF)-immunoreactive neuronal circuits in the paraventricular nucleus of the rat hypothalamus

Summary The interrelationships of corticotropin-releasing factor (CRF) immunoreactive neuronal cell bodies and processes have been examined in the paraventricular nucleus (PVN) of adrenalectomized-dexamethesone treated rats. Antisera generated against ovine CRF (oCRF) were used in the peroxidase-anti-peroxidase-complex (PAP)-immunocytochemical method at both the light and electron microscopic levels. In this experimental model, a great number of CRF-immunoreactive neurons were detected in the parvocellular subdivisions of the PVN and a few scattered labelled parvocellular neurons were also observed within the magnocellular subunits. Characteristic features of immunolabeled perikarya included hypertrophied rough endoplasmic reticulum with dilated endoplasmic cisternae, well developed Golgi complexes and increased numbers of neurosecretory granules. These features are interpreted to indicate accelerated hormone synthesis as a result of adrenalectomy. Afferent fibers communicated with dendrites and somata of CRF-immunoreactive neurons via both symmetrical and asymmetrical synapses. Some neurons exhibited somatic appendages and these structures were also observed to receive synaptic terminals. Within both the PVN and its adjacent neuropil, CRF-immunoreactive axons demonstrated varicosites which contained accumulations of densecore vesicles. CRF-containing axons were observed to branch into axon collaterals. These axons or axon collaterals established axo-somatic synapses on CRF-producing neurons in the parvocellular regions of the PVN, while in the magnocellular area of the nucleus they were found in juxtaposition with unlabeled magnocellular neuronal cell bodies or in synaptic contact with their dendrites. The presence of CRF-immunoreactive material in presynaptic structures suggests that the neurohormone may participate in mechanisms of synaptic transfer.These ultrastructural data indicate that the function of the paraventricular CRF-synthesizing neurons is adrenal steroid hormone dependent. They also provide morphological evidence for the existence of a neuronal ultrashort feedback mechanism within the PVN for the regulation of CRF production and possibly that of other peptide hormones contained within this complex.Supported by NIH grant NS 19266 to WKP     

Evidence for local corticotropin releasing factor (CRF)-immunoreactive neuronal circuits in the paraventricular nucleus of the rat hypothalamus. An electron microscopic immunohistochemical analysis

The interrelationships of corticotropin-releasing factor (CRF) immunoreactive neuronal cell bodies and processes have been examined in the paraventricular nucleus (PVN) of adrenalectomized-dexamethasone treated rats. Antisera generated against ovine CRF (oCRF) were used in the peroxidase-anti-peroxidase-complex (PAP)-immunocytochemical method at both the light and electron microscopic levels. In this experimental model, a great number of CRF-immunoreactive neurons were detected in the parvocellular subdivisions of the PVN and a few scattered labelled parvocellular neurons were also observed within the magnocellular subunits. Characteristic features of immunolabeled perikarya included hypertrophied rough endoplasmic reticulum with dilated endoplasmic cisternae, well developed Golgi complexes and increased numbers of neurosecretory granules. These features are interpreted to indicate accelerated hormone synthesis as a result of adrenalectomy. Afferent fibers communicated with dendrites and somata of CRF-immunoreactive neurons via both symmetrical and asymmetrical synapses. Some neurons exhibited somatic appendages and these structures were also observed to receive synaptic terminals. Within both the PVN and its adjacent neuropil, CRF-immunoreactive axons demonstrated varicosites which contained accumulations of densecore vesicles. CRF-containing axons were observed to branch into axon collaterals. These axons or axon collaterals established axo-somatic synapses on CRF-producing neurons in the parvocellular regions of the PVN, while in the magnocellular area of the nucleus they were found in juxtaposition with unlabeled magnocellular neuronal cell bodies or in synaptic contact with their dendrites. The presence of CRF-immunoreactive material in presynaptic structures suggests that the neurohormone may participate in mechanisms of synaptic transfer. These ultrastructural data indicate that the function of the paraventricular CRF-synthesizing neurons is adrenal steroid hormone dependent. They also provide morphological evidence for the existence of a neuronal ultrashort feed-back mechanism within the PVN for the regulation of CRF production and possibly that of other peptide hormones contained within this complex.     

Ionotropic glutamate receptor trafficking--there and back again

Glutamate is a major excitatory neurotransmitter in brain. It engages mainly ionotropic glutamate receptors of AMPA and NMDA type. Thus, regulation of the number and properties of the receptors is crucial for correct neuronal communication, but also contributes to various forms of synaptic plasticity, namely neuronal development, learning and memory. Glutamate receptors are not static components of synapses. On the contrary, they are continuously delivered and removed from postsynaptic membranes and this process is regulated by synaptic activity, Receptor trafficking to synapses is a multi-step process, involving exit from endoplasmic reticulum, transport along dendrites, incorporation to postsynaptic membrane and finally removing them from synapses. The transport is regulated by numerous proteins, especially those bearing PDZ domains, or by receptors themselves.     

Molecular motors in neurons: transport mechanisms and roles in brain function, development, and disease

The kinesin, dynein, and myosin superfamily molecular motors have fundamental roles in neuronal function, plasticity, morphogenesis, and survival by transporting cargos such as synaptic vesicle precursors, neurotransmitter and neurotrophic factor receptors, and mRNAs within axons, dendrites, and synapses. Recent studies have begun to clarify the mechanisms of cargo selection and directional transport in subcellular compartments. Furthermore, molecular genetics has revealed unexpected roles for molecular motors in brain wiring, neuronal survival, neuronal plasticity, higher brain function, and control of central nervous system and peripheral nervous system development. Finally, it is also evident that molecular motors are critically involved in neuronal disease pathogenesis. Thus, molecular motor research is becoming an exciting frontier of neuroscience.     

Electron microscopic localization of neuron-specific enolase in rat and mouse brain

The cellular distribution and intracellular localization of neuron-specific enolase (NSE) has been studied by electron microscopic immunocytochemistry in the brain of the rat and of the mouse. Although the intensity of staining was less in the mouse, the same structures were positive in both species. In the cerebrum, the neuronal perikarya and dendrites were intensely stained, but staining was almost entirely absent in the presynaptic terminals. The deep neurons of the brain stem were also positive. In the cerebellum, perikarya, axons, and parallel fibers of the granule cell neurons were stained as were the synaptic vesicles and presynaptic membranes of the synapses between the parallel fibers and the Purkinje cell dendrites. Golgi cell dendrites, basket cells and their axons, and mossy fibers were also positive. In contrast, the Purkinje cells including their dendrites, and the climbing fibers that formed synapses with the Purkinje cell dendrites were not stained. The majority of the myelinated axons in both the cerebrum and the cerebellum did not stain, but the fibrillary astrocytic processes between myelinated axons in the white matter did. Oligodendroglia, protoplasmic astrocytes, Bergmann glia, astrocytes investing capillaries, and vascular endothelial cells were negative for reaction product. In the positively staining cells and their processes, the positivity was dispersed throughout the cytoplasm and corresponded most closely to the distribution of ribosomes, the granular endoplasmic reticulum, and microtubules. Nuclei, mitochondria, the cisternae of the Golgi complex, myelin lamellae, and most membranes were not stained.     

Development of cell type-specific connectivity patterns of converging excitatory axons in the retina

To integrate information from different presynaptic cell types, dendrites receive distinct patterns of synapses from converging axons. How different afferents in?vivo establish specific connectivity patterns with the same dendrite is poorly understood. Here, we examine the synaptic development of three glutamatergic bipolar cell types converging onto?a common postsynaptic retinal ganglion cell. We find that after axons and dendrites target appropriate synaptic layers, patterns of connections among these neurons?diverge through selective changes in the conversion of axo-dendritic appositions to synapses. This process is differentially regulated by neurotransmission, which is required for the shift from single to multisynaptic appositions of one bipolar cell type but not for maintenance and elimination, respectively, of connections from the other two types. Thus, synaptic specificity among converging excitatory inputs in the?retina emerges via differential synaptic maturation of axo-dendritic appositions and is shaped by neurotransmission in a cell type-dependent manner.     

Independently Outgrowing Neurons and Geometry-Based Synapse Formation Produce Networks with Realistic Synaptic Connectivity

Neuronal signal integration and information processing in cortical networks critically depend on the organization of synaptic connectivity. During development, neurons can form synaptic connections when their axonal and dendritic arborizations come within close proximity of each other. Although many signaling cues are thought to be involved in guiding neuronal extensions, the extent to which accidental appositions between axons and dendrites can already account for synaptic connectivity remains unclear. To investigate this, we generated a local network of cortical L2/3 neurons that grew out independently of each other and that were not guided by any extracellular cues. Synapses were formed when axonal and dendritic branches came by chance within a threshold distance of each other. Despite the absence of guidance cues, we found that the emerging synaptic connectivity showed a good agreement with available experimental data on spatial locations of synapses on dendrites and axons, number of synapses by which neurons are connected, connection probability between neurons, distance between connected neurons, and pattern of synaptic connectivity. The connectivity pattern had a small-world topology but was not scale free. Together, our results suggest that baseline synaptic connectivity in local cortical circuits may largely result from accidentally overlapping axonal and dendritic branches of independently outgrowing neurons.     

Electron microscopic analysis of tyrosine hydroxylase, dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase immunoreactive innervation of the hypothalamic paraventricular nucleus in the rat

The catecholaminergic innervation of the hypothalamic paraventricular nucleus (PVN) of the rat was studied by preembedding immunocytochemical methods utilizing specific antibodies which were generated against catecholamine synthesizing enzymes. Phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive terminals contained 80-120 nm dense core granules and 30-50 nm clear synaptic vesicles. The labeled boutons terminated on cell bodies and dendrites of both parvo- and magnocellular neurons of PVN via asymmetric synapses. The parvocellular subnuclei received a more intense adrenergic innervation than did the magnocellular regions of the nucleus. Dopamine-beta-hydroxylase (DBH)-immunopositive axons were most numerous in the periventricular zone and the medial parvocellular subnucleus of PVN. Labeled terminal boutons contained 70-100 nm dense granules and clusters of spherical, electron lucent vesicles. Dendrites, perikarya and spinous structures of paraventricular neurons were observed to be the postsynaptic targets of DBH axon terminals. These asymmetric synapses frequently exhibited subsynaptic dense bodies. Paraventricular neurons did not demonstrate either PNMT or DBH immunoreactivity. The fibers present within the nucleus which contained these enzymes are considered to represent extrinsic afferent connections to neurons of the PVN. Tyrosine hydroxylase (TH)-immunoreactivity was found both in neurons and neuronal processes within the PVN. In TH-cells, the immunolabel was associated with rough endoplasmic reticulum, free ribosomes and 70-120 nm dense granules. Occasionally, nematosome-like bodies and cilia were observed in the TH-perikarya. Unlabeled axons established en passant and bouton terminaux type synapses with these TH-immunopositive cells. TH-immunoreactive axons terminated on cell bodies as well as somatic and dendritic spines of paraventricular parvocellular neurons. TH-containing axons were observed to deeply invaginate into both dendrites and perikarya of magnocellular neurons. These observations provide ultrastructural evidence for the participation of central catecholaminergic neuronal systems in the regulation of the different neuronal and neuroendocrine functions which have been related to hypothalamic paraventricular neurons.     

Translational control of synaptic plasticity

Synapses, points of contact between axons and dendrites, are conduits for the flow of information in the circuitry of the central nervous system. The strength of synaptic transmission reflects the interconnectedness of the axons and dendrites at synapses; synaptic strength in turn is modified by the frequency with which the synapses are stimulated. This modulation of synaptic strength, or synaptic plasticity, probably forms the cellular basis for learning and memory. RNA metabolism, particularly translational control at or near the synapse, is one process that controls long-lasting synaptic plasticity and, by extension, memory formation and consolidation. In the present paper, I review some salient features of translational control of synaptic plasticity.     

AMPA receptor trafficking: a road map for synaptic plasticity

Most excitatory transmission in the brain is mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPA receptors). Therefore, the presence of these receptors at synapses has to be carefully regulated in order to ensure correct neuronal communication. Interestingly, AMPA receptors are not static components of synapses. On the contrary, they are continuously being delivered and removed in and out of synapses in response to neuronal activity. This dynamic behavior of AMPA receptors is an important mechanism to modify synaptic strength during brain development and also during experience-dependent plasticity. AMPA receptor trafficking involves an intricate network of protein-protein interactions that start with the biosynthesis of the receptors, continues with their transport along dendrites, and ends with their local insertion and removal from synapses. The molecular and cellular mechanisms that regulate each of these processes, and their importance for synaptic plasticity, are now starting to be unraveled.     

Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.     

Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey

Summary Synaptic junctions are found in all parts of the nucleus, being almost as densely distributed between cell laminae as within these laminae.In addition to the six classical cell laminae, two thin intercalated laminae have been found which lie on each side of lamina 1. These laminae contain small neurons embedded in a zone of small neural processes and many axo-axonal synapses occur there.Three types of axon form synapses in all cell laminae and have been called RLP, RSD and F axons. RLP axons have large terminals which contain loosely packed round synaptic vesicles, RSD axons have small terminals which contain closely packed round vesicles and F axons have terminals intermediate in size containing many flattened vesicles.RLP axons are identified as retinogeniculate fibers. Their terminals are confined to the cell laminae, where they form filamentous contacts upon large dendrites and asymmetrical regular synaptic contacts (with a thin postsynaptic opacity) upon large dendrites and F axons. RSD axons terminate within the cellular laminae and also between them. They form asymmetrical regular synaptic contacts on small dendrites and on F axons. F axons, which also occur throughout the nucleus, form symmetrical regular contacts upon all portions of the geniculate neurons and with other F axons. At axo-axonal junctions the F axon is always postsynaptic.Supported by Grant R 01 NB 06662 from the USPHS and by funds of the Neurological Sciences Group of the Medical Research Council of Canada. Most of the observations were made while R. W. Guillery was a visiting professor in the Department of Physiology at the University of Montreal. We thank the Department of Physiology for their support and Mr. K. Watkins, Mrs. E. Langer and Mrs. B. Yelk for their skillful technical assistance.     

Ultrastructure of synapses of the parietal and visceral ganglia of Helix pomatia

Types of synaptic contacts and peculiarities of their distribution in the neuropil of the parietal and visceral ganglia of the edible snail (Helix pomatia) CNS have been studied electron microscopically. Ultrastructure of dendrites and axons has been identified. Dendrites with spinous++ processes, polymorphism of synaptic contacts have been revealed. Besides axo-axonal synapses, axo-dendritic synapses are demonstrated on the trunks and on the spinous processes of the dendrites, as well as dendro-dendritic and serial synapses. Unevenness in distribution of synaptic contacts is shown in the neuropil. The areas of the greatest concentration of the synapses are the "synaptic fields". Peculiarities in distribution of the synaptic contacts are demonstrated in the parietal and visceral ganglia.     

Emerging aspects of membrane traffic in neuronal dendrite growth

Polarized growth of the neuron would logically require some form of membrane traffic to the tip of the growth cone, regulated in conjunction with other trafficking processes that are common to both neuronal and non-neuronal cells. Unlike axons, dendrites are endowed with membranous organelles of the exocytic pathway extending from the cell soma, including both rough and smooth endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Dendrites also have satellite Golgi-like cisternal stacks known as Golgi outposts that have no membranous connections with the somatic Golgi. Golgi outposts presumably serve both general and specific local trafficking needs, and could mediate membrane traffic required for polarized dendritic growth during neuronal differentiation. Recent findings suggest that dendritic growth, but apparently not axonal growth, relies very much on classical exocytic traffic, and is affected by defects in components of both the early and late secretory pathways. Within dendrites, localized processes of recycling endosome-based exocytosis regulate the growth of dendritic spines and postsynaptic compartments. Emerging membrane traffic processes and components that contribute specifically to dendritic growth are discussed.     

Participation of puncta adherentia junctions in the formation of synaptic connections between a dentate fascia graft and the host brain

The fetal dentate fascia of Wistar rats on the 20th day of gestation was heterotopically grafted into the somatosensory neocortex of adult rats. Granule cells of a graft projected their axons (mossy fibers) to the host brain and established synaptic contacts with inappropriate targets. The organization of ectopic mossy fiber synapses was studied by electron microscopy. It was shown that ectopic synapses reproduce the structural determinants of hippocampal giant synapses and induce a subcellular reorganization of postsynaptic neocortex dendrites. Using morphometric analysis, a significant increase was found in the number of discrete puncta adherentia junctions and their total length in ectopic synapses as compared with the control group. The data obtained indicate that puncta adherentia contacts participate in the structural and chemical adaptation of neuronal targets to alien axons growing from transplants.