Recording Large Extracellular Spikes in Microchannels along Many Axonal Sites from Individual Neurons

The numerous connections between neuronal cell bodies, made by their dendrites and axons, are vital for information processing in the brain. While dendrites and synapses have been extensively studied, axons have remained elusive to a large extent. We present a novel platform to study axonal physiology and information processing based on combining an 11,011-electrode high-density complementary metal-oxide semiconductor microelectrode array with a poly(dimethylsiloxane) channel device, which isolates axons from somas and, importantly, significantly amplifies recorded axonal signals. The combination of the microelectrode array with recording and stimulation capability with the microfluidic isolation channels permitted us to study axonal signal behavior at great detail. The device, featuring two culture chambers with over 30 channels spanning in between, enabled long-term recording of single spikes from isolated axons with signal amplitudes of 100 μV up to 2 mV. Propagating signals along axons could be recorded with 10 to 50 electrodes per channel. We (i) describe the performance and capabilities of our device for axonal electrophysiology, and (ii) present novel data on axonal signals facilitated by the device. Spontaneous action potentials with characteristic shapes propagated from somas along axons between the two compartments, and these unique shapes could be used to identify individual axons within channels that contained many axonal branches. Stimulation through the electrode array facilitated the identification of somas and their respective axons, enabling interfacing with different compartments of a single cell. Complex spike shapes observed in channels were traced back to single cells, and we show that more complicated spike shapes originate from a linear superposition of multiple axonal signals rather than signal distortion by the channels.     

A microfluidic culture platform for CNS axonal injury, regeneration and transport

Investigation of axonal biology in the central nervous system (CNS) is hindered by a lack of an appropriate in vitro method to probe axons independently from cell bodies. Here we describe a microfluidic culture platform that polarizes the growth of CNS axons into a fluidically isolated environment without the use of targeting neurotrophins. In addition to its compatibility with live cell imaging, the platform can be used to (i) isolate CNS axons without somata or dendrites, facilitating biochemical analyses of pure axonal fractions and (ii) localize physical and chemical treatments to axons or somata. We report the first evidence that presynaptic (Syp) but not postsynaptic (Camk2a) mRNA is localized to developing rat cortical and hippocampal axons. The platform also serves as a straightforward, reproducible method to model CNS axonal injury and regeneration. The results presented here demonstrate several experimental paradigms using the microfluidic platform, which can greatly facilitate future studies in axonal biology.     

Monastrol, a selective inhibitor of the mitotic kinesin Eg5, induces a distinctive growth profile of dendrites and axons in primary cortical neuron cultures

Various factors including some motor proteins regulate microtubule (MT) transport and influence the formation of neuronal processes. Eg5, a slow and non-processive (+)-end directed motor molecule, is expressed in developing and differentiated neurons. However, how Eg5 works in neurons is still elusive. Thus, we treated primary rat cortical neuron cultures with monastrol, a specific inhibitor of Eg5, to investigate its role in neurons. Immature neurons treated with monastrol extended longer processes than control within a few hours. After 3 days, immature neurons treated with monastrol had longer dendrites but slightly shorter axons than control. This difference in growth between dendrites and axons became more prominent as the cells differentiated until 5 days. Interestingly, MT distributions in the cell bodies of monastrol-treated neurons appeared somewhat circular surrounding the nucleus, while MTs in the cell bodies of control neurons were primarily distributed in the MT organizing center (MTOC) just beside the nucleus. In mature neurons, monastrol treatment induced the axonal clusters of tubulins, grossly not affecting dendrites. Taken together, we conclude that Eg5 acts distinctively on dendrites and axons in neurons and suggest a putative model of how Eg5 works distinctively on dendrites and axons.     

Production and Isolation of Axons from Sensory Neurons for Biochemical Analysis Using Porous Filters

Neuronal axons use specific mechanisms to mediate extension, maintain integrity, and induce degeneration. An appropriate balance of these events is required to shape functional neuronal circuits. The protocol described here explains how to use cell culture inserts bearing a porous membrane (filter) to obtain large amounts of pure axonal preparations suitable for examination by conventional biochemical or immunocytochemical techniques. The functionality of these filter inserts will be demonstrated with models of developmental pruning and Wallerian degeneration, using explants of embryonic dorsal root ganglion. Axonal integrity and function is compromised in a wide variety of neurodegenerative pathologies. Indeed, it is now clear that axonal dysfunction appears much earlier in the course of the disease than neuronal soma loss in several neurodegenerative diseases, indicating that axonal-specific processes are primarily targeted in these disorders. By obtaining pure axonal samples for analysis by molecular and biochemical techniques, this technique has the potential to shed new light into mechanisms regulating the physiology and pathophysiology of axons. This in turn will have an impact in our understanding of the processes that drive degenerative diseases of the nervous system.     

Clustering and activity tuning of Kv1 channels in myelinated hippocampal axons

Precise localization of axonal ion channels is crucial for proper electrical and chemical functions of axons. In myelinated axons, Kv1 (Shaker) voltage-gated potassium (Kv) channels are clustered in the juxtaparanodal regions flanking the node of Ranvier. The clustering can be disrupted by deletion of various proteins in mice, including contactin-associated protein-like 2 (Caspr2) and transient axonal glycoprotein-1 (TAG-1), a glycosylphosphatidylinositol-anchored cell adhesion molecule. However, the mechanism and function of Kv1 juxtaparanodal clustering remain unclear. Here, using a new myelin coculture of hippocampal neurons and oligodendrocytes, we report that tyrosine phosphorylation plays a critical role in TAG-1-mediated clustering of axonal Kv1.2 channels. In the coculture, myelin specifically ensheathed axons but not dendrites of hippocampal neurons and clustered endogenous axonal Kv1.2 into internodes. The trans-homophilic interaction of TAG-1 was sufficient to position Kv1.2 clusters on axonal membranes in a neuron/HEK293 coculture. Mutating a tyrosine residue (Tyr⁴⁵⁸) in the Kv1.2 C terminus or blocking tyrosine phosphorylation disrupted myelin- and TAG-1-mediated clustering of axonal Kv1.2. Furthermore, Kv1.2 voltage dependence and activation threshold were reduced by TAG-1 coexpression. This effect was eliminated by the Tyr⁴⁵⁸ mutation or by cholesterol depletion. Taken together, our studies suggest that myelin regulates both trafficking and activity of Kv1 channels along hippocampal axons through TAG-1.     

Ultrastructure of heart synapses]

The nerve endings in the intramural ganglia of the rat's heart are connected by synaptic and non-synaptic junctions with the dendrites and neuron bodies. Peri-membrane indurations of synaptic complexes may look relatively symmetrical or asymmetrical depending on the orientation of the section in relation to the elements of the Akert's presynaptic lattice. In non-synaptic junctions the indurations are symmetrical, but the presynaptic one may be more complicated in its structure. The synaptic complexes are disposed in the field of synaptic dilatations of axons and in the sites of interlacing thin "preterminal" parts of axons with dendrites. They connect preganglionic fibres with dendrites, neuron bodies and with the filamentous and fungiform thorns.     

Endocytotic elimination and domain-selective tethering constitute a potential mechanism of protein segregation at the axonal initial segment

The axonal initial segment is a unique subdomain of the neuron that maintains cellular polarization and contributes to electrogenesis. To obtain new insights into the mechanisms that determine protein segregation in this subdomain, we analyzed the trafficking of a reporter protein containing the cytoplasmic II-III linker sequence involved in sodium channel targeting and clustering. Here, we show that this reporter protein is preferentially inserted in the somatodendritic domain and is trapped at the axonal initial segment by tethering to the cytoskeleton, before its insertion in the axonal tips. The nontethered population in dendrites, soma, and the distal part of axons is subsequently eliminated by endocytosis. We provide evidence for the involvement of two independent determinants in the II-III linker of sodium channels. These findings indicate that endocytotic elimination and domain-selective tethering constitute a potential mechanism of protein segregation at the axonal initial segment of hippocampal neurons.     

Biochemical and immunological analyses of cytoskeletal domains of neurons

We have used cultured sympathetic neurons to identify microtubule proteins (tubulin and microtubule-associated proteins [MAPs]) and neurofilament (NF) proteins in pure preparations of axons and also to examine the distribution of these proteins between axons and cell bodies + dendrites. Pieces of sympathetic ganglia containing thousands of neurons were plated onto culture dishes and allowed to extend neurites. Dendrites remained confined to the ganglionic explant or cell body mass (CBM), while axons extended away from the CBM for several millimeters. Axons were separated from cell bodies and dendrites by dissecting the CBM away from cultures, and the resulting axonal and CBM preparations were analyzed using biochemical, immunoblotting, and immunoprecipitation methods. Cultures were used after 17 d in vitro, when 40-60% of total protein was in the axons. The 68,000-mol-wt NF subunit is present in both axons and CBM in roughly equal amounts. The 145,000- and 200,000-mol-wt NF subunits each consist of several variants which differ in phosphorylation state; poorly and nonphosphorylated species are present only in the CBM, whereas more heavily phosphorylated forms are present in axons and, to a lesser extent, the CBM. One 145,000-mol-wt NF variant was axon specific. Tubulin is roughly equally distributed between CBM and axon-like neurites of explant cultures. MAP-1a, MAP-1b, MAP-3, and the 60,000-mol-wt MAP are also present in the CBM and axon-like neurites and show distribution patterns similar to that of tubulin. In contrast, MAP-2 was detected only in the CBM, while tau and the 210,000-mol-wt MAP were greatly enriched in axons compared to the CBM. In immunostaining analyses, MAP-2 localized to cell bodies and dendrite-like neurites, but not to axon-like neurites, whereas antibodies to tubulin and MAP-1b localized to all regions of the neurons. The regional differences in composition of the neuronal cytoskeleton presumably generate corresponding differences in its structure, which may, in turn, contribute to the morphological differences between axons and dendrites.     

The establishment of polarity by hippocampal neurons: the relationship between the stage of a cell's development in situ and its subsequent development in culture

Neurons removed from the embryonic hippocampus and placed into culture develop structurally and functionally distinct axonal and dendritic processes. The central issue addressed in this study concerns the extent to which the sequence of events which results in the differentiation of neurites by hippocampal neurons in culture is influenced by the cell's state of development in situ. [3H]thymidine was administered to pregnant rats either on Embryonic Day 15 (E15) or on E18.5 to label hippocampal neurons at known stages of their development. All fetuses were sacrificed on E19. Some of the fetal brains were sectioned and examined by autoradiography to determine the location of labeled cells in the hippocampus. The remaining brains were used to prepare hippocampal cell cultures. Neurons labeled at E18.5 remained confined to the ventricular zone at E19. Those labeled at E15 had completed their migration to the cortical plate. Other data suggest that the former cells had not yet initiated process outgrowth, while the latter cells had begun to elaborate both axons and dendrites. When introduced into culture, both populations of cells developed axons and dendrites and both compartmentalized MAP2 to the dendritic domain. Moreover, despite marked differences in their developmental state at the time of introduction into culture, both underwent the same sequence of developmental events leading to axonal and dendritic development. In a few cases cells that incorporated [3H]thymidine in situ at E18.5 apparently underwent mitosis in culture. These neurons also developed axons and dendrites appropriately. These results indicate that hippocampal neurons become polarized in culture, even if they have never developed axons or dendrites in situ, and do so as efficiently as cells that have become polarized before being placed into culture. Moreover, they indicate that the same sequence of events leading to the establishment of polarity occurs for hippocampal neurons with different developmental histories prior to culturing.     

Chronic excitotoxin-induced axon degeneration in a compartmented neuronal culture model

Glutamate excitotoxicity is a major pathogenic process implicated in many neurodegenerative conditions, including AD (Alzheimer''s disease) and following traumatic brain injury. Occurring predominantly from over-stimulation of ionotropic glutamate receptors located along dendrites, excitotoxic axonal degeneration may also occur in white matter tracts. Recent identification of axonal glutamate receptor subunits within axonal nanocomplexes raises the possibility of direct excitotoxic effects on axons. Individual neuronal responses to excitotoxicity are highly dependent on the complement of glutamate receptors expressed by the cell, and the localization of the functional receptors. To enable isolation of distal axons and targeted excitotoxicity, murine cortical neuron cultures were prepared in compartmented microfluidic devices, such that distal axons were isolated from neuronal cell bodies. Within the compartmented culture system, cortical neurons developed to relative maturity at 11 DIV (days in vitro) as demonstrated by the formation of dendritic spines and clustering of the presynaptic protein synaptophysin. The isolated distal axons retained growth cone structures in the absence of synaptic targets, and expressed glutamate receptor subunits. Glutamate treatment (100 μM) to the cell body chamber resulted in widespread degeneration within this chamber and degeneration of distal axons in the other chamber. Glutamate application to the distal axon chamber triggered a lesser degree of axonal degeneration without degenerative changes in the untreated somal chamber. These data indicate that in addition to current mechanisms of indirect axonal excitotoxicity, the distal axon may be a primary target for excitotoxicity in neurodegenerative conditions.     

Differentiated neurons retain the capacity to generate axons from dendrites

Cutting the axon of a morphologically polarized neuron (stage 3) close to the cell body causes another neurite to grow as an axon [1-3]. Stage 3 neurons still lack molecular segregation of axonal and dendritic proteins, however. Axonal and dendritic compartments acquire their distinct composition at stage 4 (4-5days in culture), when proteins such as the microtubule-associated protein 2 (MAP-2) and the glutamate receptor subunit GluR1 localize to the dendrites and disappear from the axon [4,5]. We investigated whether cultured hippocampal neurons retained axon/dendrite plasticity after axons and dendrites have created their distinct cytoskeletal architecture and acquired their specific membrane composition. We found that axotomy of stage 4 neurons transformed a dendrite into an axon. Using axonal and dendritic markers, we tested whether cytoskeletal changes could cause similar transformations, and found that actin depolymerization induced multiple axons in unpolarized neurons. Moreover, depletion of actin filaments from both morphologically and molecularly polarized cells also resulted in the growth of multiple axons from pre-existing dendrites. These results imply that dendrites retain the potential to become axons even after molecular segregation has occurred and that the dendritic fate depends on the integrity of the actin cytoskeleton.     

Reversal of axonal transport at a nerve crush.

Abstract— —We have compared retrograde axonal transport of ³H-labeled protein in normal rat motor and sensory axons, and axons which were injured by a distal ligation of the sciatic nerve. After injection of L-[³H]leucine into the vicinity of the neuron cell bodies, labeled protein was transported into the axons. A premature return of protein towards the cell bodies occurred in the injured axons, which we interpret as a reversal of axonal transport occurring at the site of injury. We estimate that reversal of transport occurred within 1.9–2.4 h of the arrival of labeled protein at the injury, and that the minimum velocity of the subsequent retrograde transport was 112–133 mm day^?1. The ability of the injured axons to reverse transport developed about 0.8 h after making the injury. A large fraction of the orthograde transported protein was returned towards the cell body: it is estimated that by 28 h after labeled protein in sensory axons reached the injury, 46% of the³H-labeled protein originally transported to the injury site had been returned. In intact sensory nerves at this time only 15% of the transported protein had returned. It is suggested that axonal injury produces a sudden increase in the return of newly synthesized protein to the cell body, and that this might serve as a signal for chromatolysis.     

The neuroanatomy of an amphibian embryo spinal cord

Horseradish peroxidase has been used to stain spinal cord neurons in late embryos of the clawed toad (Xenopus laevis). It has shown clearly the soma, dendrites and axonal projections of spinal sensory, motor and interneurons. On the basis of light microscopy we describe nine differentiated spinal cord neuron classes. These include the Rohon-Beard cells and extramedullary cells which are both primary sensory neurons, one class of motoneurons that innervate the segmental myotomes, two classes of interneurons with decussating axons, three classes of interneurons with ipsilateral axons and a previously undescribed class of ciliated ependymal cells with axons projecting ipsilaterally to the brain. We believe that all differentiated neuron classes are described and that this anatomical account is the most complete for any vertebrate spinal cord.     

Interactions between entorhinal axons and target hippocampal neurons: a role for glutamate in the development of hippocampal circuitry

A coculture system consisting of input axons from entorhinal cortex explants and target hippocampal pyramidal neurons was used to demonstrate that glutamate, released spontaneously from afferent axons, can influence both dendritic geometry of target neurons and formation of presumptive synaptic sites. Dendritic outgrowth was reduced in hippocampal neurons growing on entorhinal axons when compared with neurons growing off the axons. Presumptive presynaptic sites were observed in association with hippocampal neuron dendrites and somas. HPLC analysis showed that glutamate was released from the explants in an activity- and Ca2(+)-dependent manner. The general glutamate receptor antagonist D-glutamylglycine significantly increased dendritic outgrowth in pyramidal neurons associated with entorhinal axons and reduced presumptive presynaptic sites. Tetrodotoxin and reduction of extracellular Ca2+ also promoted dendritic outgrowth and reduced the formation of presumptive synaptic sites. The results suggest that the neurotransmitter glutamate may play important roles in the development of hippocampal circuitry.     

A novel adenoviral vector-mediated mouse model of Charcot-Marie-Tooth type 2D (CMT2D)

Charcot-Marie-Tooth disease type 2D is a hereditary axonal and glycyl-tRNA synthetase (GARS)-associated neuropathy that is caused by a mutation in GARS. Here, we report a novel GARS-associated mouse neuropathy model using an adenoviral vector system that contains a neuronal-specific promoter. In this model, we found that wild-type GARS is distributed to peripheral axons, dorsal root ganglion (DRG) cell bodies, central axon terminals, and motor neuron cell bodies. In contrast, GARS containing a G240R mutation was localized in DRG and motor neuron cell bodies, but not axonal regions, in vivo. Thus, our data suggest that the disease-causing G240R mutation may result in a distribution defect of GARS in peripheral nerves in vivo. Furthermore, a distributional defect may be associated with axonal degradation in GARS-associated neuropathies.     

In vitro regulation of neuronal morphogenesis and polarity by astrocyte-derived factors

Mesencephalic neurons were cultured from 2 to 5 days in mesencephalic (CM Gmes) or striatal (CM Gstr) astrocyte conditioned media or in the soluble (S100) and insoluble (P100) fractions prepared from these media by ultracentrifugation. CM Gmes as well as all soluble fractions induced dendritic and axonal elongation, whereas CM Gstr and the insoluble fractions promoted axonal growth only. The study of the shape of the neuronal cell bodies and the measurement of their adhesion to the substratum revealed that axons elongated under low adhesion conditions, but that dendrite growth was highly dependent upon adhesion and spreading of the neuronal soma. This different dependency of axonal and dendritic elongation upon spreading is explained by a model in which we consider the respective viscosities of axons and dendrites. From these observations and speculations we propose that axons and dendrites have different modes of elongation and that the primary effect of the astrocyte-derived factors capable of regulating neuronal polarity is to modify the adhesion of the neurons to their culture substratum.     

Gamma-tubulin distribution in the neuron: implications for the origins of neuritic microtubules

Axons and dendrites contain dense microtubule (MT) assays that are not attached to a traditional MT nucleating structure such as the centrosome. Nevertheless, the MTs within these neurites are highly organized with respect to their polarity, and consist of a regular 13-protofilament lattice, the two known characteristics of MTs nucleated at the centrosome. These observations suggest either that axonal and dendritic MTs arise at the centrosome, or that they are nucleated locally, following a redistribution of MT nucleating material from the centrosome during neuronal development. To begin distinguishing between these possibilities, we have determined the distribution of gamma-tubulin within cultured sympathetic neurons. gamma-tubulin, a newly discovered protein which is specifically localized to the pericentriolar region of nonneuronal cells (Zheng, Y., M. K. Jung, and B. R. Oakley. 1991. Cell. 65:817-823; Stearns, T., L. Evans, and M. Kirschner. 1991. Cell. 65:825-836), has been shown to play a critical role in MT nucleation in vivo (Joshi, H. C., M. J. Palacios, L. McNamara, and D. W. Cleveland. 1992. Nature (Lond.). 356:80-83). Because the gamma-tubulin content of individual cells is extremely low, we relied principally on the high degree of resolution and sensitivity afforded by immunoelectron microscopy. Our studies reveal that, like the situation in nonneuronal cells, gamma-tubulin is restricted to the pericentriolar region of the neuron. Furthermore, serial reconstruction analyses indicate that the minus ends of MTs in both axons and dendrites are free of gamma-tubulin immunoreactivity. The absence of gamma-tubulin from the axon was confirmed by immunoblot analyses of pure axonal fractions obtained from explant cultures. The observation that gamma-tubulin is restricted to the pericentriolar region of the neuron provides compelling support for the notion that MTs destined for axons and dendrites are nucleated at the centrosome, and subsequently released for translocation into these neurites.     

Immunolocalization of a candidate pheromone receptor in the antenna of the male moth, Heliothis virescens

Pheromone recognition in insects is thought to involve distinct receptor proteins in the dendritic membrane of antennal sensory neurons. We have generated antibodies directed against a peptide derived from the sequence of the candidate pheromone receptor HR13 from Heliothis virescens. The antibodies specifically labelled the cell bodies of a distinct neuron population housed in male-specific pheromone-sensitive sensilla. Combining antibody staining with in situ hybridization the reactive cells were found to express the HR13 gene. In addition, dendrites projecting into sensilla hairs as well as the axonal processes of immunoreactive cells were labelled. Labelling of axons has allowed visualization of their fasciculation within antennal segments and permits tracking of axons as they merge into the antennal nerve. The HR13 protein was first detected 1 day before eclosion. Thus, the distribution of HR13 protein in the antennal neurons of the male moth strongly suggests a role of the HR13 receptor in recognition of pheromones.     

Evidence that the Major Membrane Lipids, Except Cholesterol, Are Made in Axons of Cultured Rat Sympathetic Neurons

Abstract: 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 cytidylyltransferase. 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.