Independent control of the local environment of somas and neurites

The sympathetic nervous system in development and disease

The sympathetic nervous system prepares the body for 'fight or flight' responses and maintains homeostasis during daily activities such as exercise, eating a meal or regulation of body temperature. Sympathetic regulation of bodily functions requires the establishment and refinement of anatomically and functionally precise connections between postganglionic sympathetic neurons and peripheral organs distributed widely throughout the body. Mechanistic studies of key events in the formation of postganglionic sympathetic neurons during embryonic and early postnatal life, including axon growth, target innervation, neuron survival, and dendrite growth and synapse formation, have advanced the understanding of how neuronal development is shaped by interactions with peripheral tissues and organs. Recent progress has also been made in identifying how the cellular and molecular diversity of sympathetic neurons is established to meet the functional demands of peripheral organs. In this Review, we summarize current knowledge of signalling pathways underlying the development of the sympathetic nervous system. These findings have implications for unravelling the contribution of sympathetic dysfunction stemming, in part, from developmental perturbations to the pathophysiology of peripheral neuropathies and cardiovascular and metabolic disorders.

Neuromuscular Activity Induces Paracrine Signaling and Triggers Axonal Regrowth after Injury in Microfluidic Lab-On-Chip Devices

Peripheral nerve injuries, including motor neuron axonal injury, often lead to functional impairments. Current therapies are mostly limited to surgical intervention after lesion, yet these interventions have limited success in restoring functionality. Current activity-based therapies after axonal injuries are based on trial-error approaches in which the details of the underlying cellular and molecular processes are largely unknown. Here we show the effects of the modulation of both neuronal and muscular activity with optogenetic approaches to assess the regenerative capacity of cultured motor neuron (MN) after lesion in a compartmentalized microfluidic-assisted axotomy device. With increased neuronal activity, we observed an increase in the ratio of regrowing axons after injury in our peripheral-injury model. Moreover, increasing muscular activity induces the liberation of leukemia inhibitory factor and glial cell line-derived neurotrophic factor in a paracrine fashion that in turn triggers axonal regrowth of lesioned MN in our 3D hydrogel cultures. The relevance of our findings as well as the novel approaches used in this study could be useful not only after axotomy events but also in diseases affecting MN survival.

Design of Cultured Neuron Networks in vitro with Predefined Connectivity Using Asymmetric Microfluidic Channels

The architecture of neuron connectivity in brain networks is one of the basic mechanisms by which to organize and sustain a particular function of the brain circuitry. There are areas of the brain composed of well-organized layers of neurons connected by unidirectional synaptic connections (e.g., cortex, hippocampus). Re-engineering of the neural circuits with such a heterogeneous network structure in culture may uncover basic mechanisms of emergent information functions of these circuits. In this study, we present such a model designed with two subpopulations of primary hippocampal neurons (E18) with directed connectivity grown in a microfluidic device with asymmetric channels. We analysed and compared neurite growth in the microchannels with various shapes that promoted growth dominantly in one direction. We found an optimal geometric shape features of the microchannels in which the axons coupled two chambers with the neurons. The axons grew in the promoted direction and formed predefined connections during the first 6 days in vitro (DIV). The microfluidic devices were coupled with microelectrode arrays (MEAs) to confirm unidirectional spiking pattern propagation through the microchannels between two compartments. We found that, during culture development, the defined morphological and functional connectivity formed and was maintained for up to 25 DIV.

Studying axonal degeneration and regeneration using in vitro and in vivo models: the translational potential

ABSTRACT: 

Since the initial studies by Cajal, multiple models of peripheral nerve degeneration and regeneration have been developed to address the ever-increasing complexity of the mechanisms involved in regeneration. In vitro models offer the principal benefit of a system that can be readily manipulated to address specific mechanistic questions in a deconstructed system. However, in vitro models can be overly simplified and intricacies of the interactions between neurons and glia can be lost. In vivo animal models seek to remedy some of these shortcomings, but most in vivo animal systems fail to precisely model human nerve regeneration. Rodent models of chronic nerve regeneration have been developed to better recapitulate human nerve regeneration, but are not widely used. An important development in the field has been the establishment of experimental nerve regeneration in humans, involving the reinnervation of the epidermis after cutaneous axotomy or topical capsaicin application. Use of such human models will likely accelerate the development and evaluation of new drugs that enhance peripheral nerve regeneration.

Engineering a functional neuro-muscular junction model in a chip

A large open-reservoir, fluidically-isolated, compartmentalized microfluidic co-culture platform was utilized as an in vitro model for establishing neuro-muscular junction formation.

Environmental toxins trigger PD-like progression via increased alpha-synuclein release from enteric neurons in mice

Pathological studies on Parkinson's disease (PD) patients suggest that PD pathology progresses from the enteric nervous system (ENS) and the olfactory bulb into the central nervous system. We have previously shown that environmental toxins acting locally on the ENS mimic this PD-like pathology progression pattern in mice. Here, we show for the first time that the resection of the autonomic nerves stops this progression. Moreover, our results show that an environmental toxin (i.e. rotenone) promotes the release of alpha-synuclein by enteric neurons and that released enteric alpha-synuclein is up-taken by presynaptic sympathetic neurites and retrogradely transported to the soma, where it accumulates. These results strongly suggest that pesticides can initiate the progression of PD pathology and that this progression is based on the transneuronal and retrograde axonal transport of alpha-synuclein. If confirmed in patients, this study would have crucial implications in the strategies used to prevent and treat PD.

Compartmented chambers for studying neurotrophic factor action

Neurotrophic factors released by target tissues maintain the survival and differentiation of innervating neurons. The manner by which these target-derived neurotrophic proteins communicate with innervating neurons has been actively pursued for over three decades. The present chapter describes a technique for preparing and maintaining compartmented chambers for culturing neurons derived from either superior cervical ganglia (sympathetic neurons) or dorsal root ganglia (sensory neurons). This system recapitulates the selective stimulation of neuron terminals that occurs in vivo following release of target-derived neurotrophins.

Biochip∕laser cell deposition system to assess polarized axonal growth from single neurons and neuron∕glia pairs in microchannels with novel asymmetrical geometries

Axon path-finding plays an important role in normal and pathogenic brain development as well as in neurological regenerative medicine. In both scenarios, axonal growth is influenced by the microenvironment including the soluble molecules and contact-mediated signaling from guiding cells and cellular matrix. Microfluidic devices are a powerful tool for creating a microenvironment at the single cell level. In this paper, an asymmetrical-channel-based biochip, which can be later incorporated into microfluidic devices for neuronal network study, was developed to investigate geometric as well as supporting cell control of polarized axonal growth in forming a defined neuronal circuitry. A laser cell deposition system was used to place single cells, including neuron-glia pairs, into specific microwells of the device, enabling axonal growth without the influence of cytophilic∕phobic surface patterns. Phase microscopy showed that a novel "snag" channel structure influenced axonal growth in the intended direction 4:1 over the opposite direction. In heterotypic experiments, glial cell influence over the axonal growth path was observed with time-lapse microscopy. Thus, it is shown that single cell and heterotypic neuronal path-finding models can be developed in laser patterned biochips.

Wnt5a mediates nerve growth factor-dependent axonal branching and growth in developing sympathetic neurons

Nerve growth factor (NGF) is a potent survival and axon growth factor for neuronal populations in the peripheral nervous system. Although the mechanisms by which target-derived NGF influences survival of innervating neurons have been extensively investigated, its regulation of axonal growth and target innervation are just being elucidated. Here, we identify Wnt5a, a member of the Wnt family of secreted growth factors, as a key downstream effector of NGF in mediating axonal branching and growth in developing sympathetic neurons. Wnt5a is robustly expressed in sympathetic neurons when their axons are innervating NGF-expressing targets. NGF:TrkA signaling enhances neuronal expression of Wnt5a. Wnt5a rapidly induces axon branching while it has a long-term effect on promoting axon extension. Loss of Wnt5a function revealed that it is necessary for NGF-dependent axonal branching and growth, but not survival, in vitro. Furthermore, Wnt5a(-/-) mice display reduced innervation of NGF-expressing target tissues, and a subsequent increase in neuronal apoptosis, in vivo. Wnt5a functions in developing sympathetic neurons by locally activating protein kinase C in axons. Together, our findings define a novel regulatory pathway in which Wnt5a, expressed in sympathetic neurons in response to target-derived NGF, regulates innervation of peripheral targets.

Preparation and maintenance of dorsal root ganglia neurons in compartmented cultures

Neurons extend axonal processes that are far removed from the cell body to innervate target tissues, where target-derived growth factors are required for neuronal survival and function. Neurotrophins are specifically required to maintain the survival and differentiation of innervating sensory neurons but the question of how these target-derived neurotrophins communicate to the cell body of innervating neurons has been an area of active research for over 30 years. The most commonly accepted model of how neurotrophin signals reach the cell body proposes that signaling endosomes carry this signal retrogradely along the axon. In order to study retrograde transport, a culture system was originally devised by Robert Campenot, in which cell bodies are isolated from their axons. The technique of preparing these compartmented chambers for culturing sensory neurons recapitulates the selective stimulation of neuron terminals that occurs in vivo following release of target-derived neurotrophins. Retrograde signaling events that require long-range microtubule dependent retrograde transport have important implications for the treatment of neurodegenerative disorders.

A model for neuronal competition during development

We report that developmental competition between sympathetic neurons for survival is critically dependent on a sensitization process initiated by target innervation and mediated by a series of feedback loops. Target-derived nerve growth factor (NGF) promoted expression of its own receptor TrkA in mouse and rat neurons and prolonged TrkA-mediated signals. NGF also controlled expression of brain-derived neurotrophic factor and neurotrophin-4, which, through the receptor p75, can kill neighboring neurons with low retrograde NGF-TrkA signaling whereas neurons with high NGF-TrkA signaling are protected. Perturbation of any of these feedback loops disrupts the dynamics of competition. We suggest that three target-initiated events are essential for rapid and robust competition between neurons: sensitization, paracrine apoptotic signaling, and protection from such effects.

In vitro, lidocaine-induced axonal injury is prevented by peripheral inhibition of the p38 mitogen-activated protein kinase, but not by inhibiting caspase activity

BACKGROUND

All local anesthetics (LAs) are, to some extent, neurotoxic. Toxicity studies have been performed in dissociated neuron cultures, immersing both axon and soma in LA. This approach, however, does not accurately reflect the in vivo situation for peripheral nerve blockade, where LA is applied to the axon alone.

METHODS

We investigated lidocaine neurotoxicity in compartmental sensory neuron cultures, which are composed of one central compartment containing neuronal cell bodies and a peripheral compartment containing their axons, allowing for selective incubation. We applied lidocaine +/- neuroprotective drugs to neuronal somata or axons, and assessed neuron survival and axonal outgrowth.

RESULTS

Lidocaine applied to the peripheral compartment led to a decreased number of axons (to 59% +/- 9%), without affecting survival of cell bodies. During axonal incubation with lidocaine, the p38 mitogen-activated protein kinase inhibitor SB203580 (10 microM) attenuated axonal injury when applied to the axon (insignificant reduction of maximal axonal distance to 93% +/- 9%), but not when applied to the cell body (deterioration of maximal axonal length to 48% +/- 6%). Axonal co-incubation of lidocaine with the caspase inhibitor z-vad-fmk (20 microM) was not protective.

CONCLUSIONS

Whereas inhibition of either p38 mitogen-activated protein kinase or caspase activity promote neuronal survival after LA treatment of dissociated neuronal cultures, axonal degeneration induced by lidocain (40 mM) is prevented by p38 MAP kinase but not by caspase inhibition. We conclude that processes leading to LA-induced neurotoxicity in dissociated neuronal culture may be different from those observed after purely axonal application.

NGF augments the autophosphorylation of Ret via inhibition of ubiquitin-dependent degradation

Nerve growth factor (NGF) is required for the trophic maintenance of postnatal sympathetic neurons. A significant portion of the growth-promoting activity of NGF is from NGF-dependent phosphorylation of the heterologous receptor tyrosine kinase, Ret. We found that NGF applied selectively to distal axons of sympathetic neurons maintained in compartmentalized cultures activated Ret located in these distal axons. Inhibition of either proteasomal or lysosomal degradation pathways mimicked the effect of NGF on Ret activation. Likewise, NGF inhibited the degradation of Ret induced by glial cell line-derived neurotrophic factor-dependent activation, a process that requires ubiquitination and proteasomal degradation. NGF induced the accumulation of autophosphorylated Ret predominantly in the plasma membrane, in contrast to GDNF, which promoted the internalization of activated Ret. An accretion of monoubiquitinated, but not polyubiquitinated, Ret occurred in NGF-treated neurons, in contrast to glial cell line-derived neurotrophic factor that promoted the robust polyubiquitination of Ret. Thus, NGF stimulates Ret activity in mature sympathetic neurons by inhibiting the ongoing ubiquitin-mediated degradation of Ret before its internalization and polyubiquitination.

A multielectrode microcompartment culture platform for studying signal transduction in the nervous system

This paper describes the design, fabrication, and characterization of a microfabricated compartmented culture system (micro-CCS) useful for electrophysiological signaling studies in cultured neurons. The focus of the paper is the process of interfacing the micro-CCS with cultured neurons and to demonstrate the applicability of the system for biochemical-mediated electrophysiological studies. Moreover, we show that we can record action potentials from cultured neurons through the extracellular compartmented application of elevated levels of K(+) ions. Finally, we show that we can isolate the electrophysiological effects of the sodium channel blocker tetrodotoxin in one of the compartments of a two compartment culture while recording electrophysiological data from both compartments.

Glaucoma: thinking in new ways-a rôle for autonomous axonal self-destruction and other compartmentalised processes?

Glaucoma is a common neurodegenerative disease that affects retinal ganglion cells (RGCs). Substantial effort is being expended to determine how RGCs die in glaucoma. As in other neurodegenerative diseases, the majority of effort focuses on characterising apoptotic self-destruct pathways. However, apoptosis is not the only self-destruct mechanism that may be activated in neurons. It is now known that neurons have distinct classes of self-destruct programme that are spatially compartmentalised. In addition to the well-described intracellular suicide machinery in the neuronal soma, responsible for apoptosis, there is another, molecularly distinct, self-destruct programme localised in the axon. Evidence also supports the existence of compartmentalised degeneration programmes in synapses and dendrites. RGCs are no exception to this. Recent data, from in vitro studies and from an inherited mouse model of glaucoma, suggest that molecularly distinct degenerative pathways underlie the destruction of RGC somata and RGC axons. In various neurodegenerative diseases, axons, dendrites and synapses often degenerate well before the cells die, and there is increasing evidence that this is important for the production of clinical symptoms and signs. We hypothesise that such compartmentalised and autonomous programmes are of critical importance in the pathophysiology of glaucoma, and we suggest that studies of these processes are essential for a complete understanding of this complex disease.

Application of Rho antagonist to neuronal cell bodies promotes neurite growth in compartmented cultures and regeneration of retinal ganglion cell axons in the optic nerve of adult rats

Inactivation of Rho promotes neurite growth on inhibitory substrates and axon regeneration in vivo. Here, we compared axon growth when neuronal cell bodies or injured axons were treated with a cell-permeable Rho antagonist (C3-07) in vitro and in vivo. In neurons plated in compartmented cultures, application of C3-07 to either cell bodies or distal axons promoted axonal growth on myelin-associated glycoprotein substrates. In vivo, an injection of C3-07 into the eye promoted regeneration of retinal ganglion cell (RGC) axons in the optic nerve after microcrush lesion. Delayed application of C3-07 promoted RGC growth across the lesion scar. Application of C3-07 completely prevented RGC cell death for 1 week after axotomy. To investigate the mechanism by which Rho inactivation promotes RGC growth, we studied slow axonal transport. Reduction in slow transport of cytoskeletal proteins was observed after axotomy, but inactivation of Rho did not increase slow axonal transport rates. Together, our results indicate that application of a Rho antagonist at the cell body is neuroprotective and overcomes growth inhibition but does not fully prime RGCs for active growth.

Functions and mechanisms of retrograde neurotrophin signalling

Neuronal connections are established and refined through a series of developmental programs that involve axon and dendrite specification, process growth, target innervation, cell death and synaptogenesis. Many of these developmental events are regulated by target-derived neurotrophins and their receptors, which signal retrogradely over long distances from distal-most axons to neuronal cell bodies. Recent work has established many of the cellular and molecular events that underlie retrograde signalling and the importance of these events for both development and maintenance of proper neural connectivity.

Cholesterol homeostasis in neurons and glial cells

Cholesterol is highly enriched in the brain compared to other tissues. Essentially all cholesterol in the brain is synthesized endogenously since plasma lipoproteins are unable to cross the blood-brain barrier. Cholesterol is transported within the central nervous system in the form of apolipoprotein E-containing lipoprotein particles that are secreted mainly by glial cells. Cholesterol is excreted from the brain in the form of 24-hydroxycholesterol. Apolipoprotein E and cholesterol have been implicated in the formation of amyloid plaques in Alzheimer's disease. In addition, the progressive neurodegenerative disorder Niemann-Pick C disease is characterized by defects in intracellular trafficking of cholesterol.

Glial lipoproteins stimulate axon growth of central nervous system neurons in compartmented cultures

The role of lipoproteins secreted by cortical glial cells in axon growth of central nervous system (CNS) neurons was investigated. We first established compartmented cultures of CNS neurons (retinal ganglion cells). Addition of glial cell-conditioned medium (GCM) to distal axons increased the rate of axon extension by approximately 50%. Inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase in glial cells diminished the secretion of cholesterol and apolipoprotein E, and prevented the growth stimulatory effect of GCM. When glia-derived lipoproteins containing apolipoprotein E were provided to distal axons, axon extension was stimulated to the same extent as by GCM. In contrast, addition of lipoproteins to cell bodies failed to enhance growth. The growth stimulatory effect of glial lipoproteins was abrogated in the presence of receptor-associated protein, RAP, indicating involvement of receptor(s) of the low density lipoprotein receptor family in stimulation of axonal extension. These observations suggest that glial cells stimulate axon growth of CNS neurons by providing lipoproteins containing cholesterol and apolipoprotein E to distal axons.

Trafficking of cholesterol from cell bodies to distal axons in Niemann Pick C1-deficient neurons

Niemann Pick type C (NPC) disease is a progressive neurodegenerative disorder. In cells lacking functional NPC1 protein, endocytosed cholesterol accumulates in late endosomes/lysosomes. We utilized primary neuronal cultures in which cell bodies and distal axons reside in separate compartments to investigate the requirement of NPC1 protein for transport of cholesterol from cell bodies to distal axons. We have recently observed that in NPC1-deficient neurons compared with wild-type neurons, cholesterol accumulates in cell bodies but is reduced in distal axons (Karten, B., Vance, D. E., Campenot, R. B., and Vance, J. E. (2002) J. Neurochem. 83, 1154-1163). We now show that NPC1 protein is expressed in both cell bodies and distal axons. In NPC1-deficient neurons, cholesterol delivered to cell bodies from low density lipoproteins (LDLs), high density lipoproteins, or cyclodextrin complexes was transported into axons in normal amounts, whereas transport of endogenously synthesized cholesterol was impaired. Inhibition of cholesterol synthesis with pravastatin in wild-type and NPC1-deficient neurons reduced axonal growth. However, LDLs restored a normal rate of growth to wild-type but not NPC1-deficient neurons treated with pravastatin. Thus, although LDL cholesterol is transported into axons of NPC1-deficient neurons, this source of cholesterol does not sustain normal axonal growth. Over the lifespan of NPC1-deficient neurons, these defects in cholesterol transport might be responsible for the observed altered distribution of cholesterol between cell bodies and axons and, consequently, might contribute to the neurological dysfunction in NPC disease.

Cholesterol accumulates in cell bodies, but is decreased in distal axons, of Niemann-Pick C1-deficient neurons

Niemann-Pick type-C (NPC) disease is characterized by a progressive loss of neurons and an accumulation of unesterified cholesterol within the endocytic pathway. Unlike other tissues, however, NPC1-deficient brains do not accumulate cholesterol but whether or not NPC1-deficient neurons accumulate cholesterol is not clear. Therefore, as most studies on cholesterol homeostasis in NPC1-deficient cells have been performed in fibroblasts we have investigated cholesterol homeostasis in cultured murine sympathetic neurons lacking functional NPC1. These neurons did not display obvious abnormalities in growth or morphology and appeared to respond normally to nerve growth factor. Filipin staining revealed numerous cholesterol-filled endosomes/lysosomes in NPC1-deficient neurons and the mass of cholesterol in cell bodies was greater than in wild-type neurons. Surprisingly, however, the cholesterol content of NPC1-deficient and wild-type neurons as a whole was the same. This apparent paradox was resolved when the cholesterol content of NPC1-deficient distal axons was found to be less than of wild-type axons. Cholesterol sequestration in cell bodies did not depend on exogenously supplied cholesterol since the cholesterol accumulated before birth and did not disperse when neurons were cultured without exogenous cholesterol. The altered cholesterol distribution between cell bodies and axons suggests that transport of cholesterol, particularly that synthesized endogenously, from cell bodies to distal axons is impaired in NPC1-deficient neurons.

Ceramide inhibits axonal growth and nerve growth factor uptake without compromising the viability of sympathetic neurons

Ceramide inhibits axonal growth of cultured rat sympathetic neurons when the ceramide content of distal axons, but not cell bodies, is increased (Posse de Chaves, E. I., Bussiere, M. Vance, D. E., Campenot, R. B., and Vance, J.E. (1997) J. Biol. Chem. 272, 3028-3035). We now report that inhibition of growth does not result from cell death since although ceramide is a known apoptotic agent, C(6)-ceramide given to the neurons for 24 h did not cause cell death but instead protected the neurons from death induced by deprivation of nerve growth factor (NGF). We also find that a pool of ceramide generated from sphingomyelin in distal axons, but not cell bodies, inhibits axonal growth. Analysis of endogenous sphingomyelinase activities demonstrated that distal axons are rich in neutral sphingomyelinase activity but contain almost no acidic sphingomyelinase, which is concentrated in cell bodies/proximal axons. Together, these observations are consistent with the idea that generation of ceramide from sphingomyelin by a neutral sphingomyelinase in axons inhibits axonal growth. Furthermore, we demonstrate that treatment of distal axons with ceramide inhibits the uptake of NGF and low density lipoproteins by distal axons by approximately 70 and 40%, respectively, suggesting that the inhibition of axonal growth by ceramide might be due, at least in part, to impaired endocytosis of NGF. However, inhibition of endocytosis of NGF by ceramide could not be ascribed to decreased phosphorylation of TrkA.

Calsyntenin-1, a proteolytically processed postsynaptic membrane protein with a cytoplasmic calcium-binding domain

In a screen for proteins released from synapse-forming spinal cord neurons, we found the proteolytically cleaved N-terminal fragment of a transmembrane protein localized in the postsynaptic membrane of both excitatory and inhibitory synapses. We termed this protein calsyntenin-1, because it binds synaptic Ca2+ with its cytoplasmic domain. By binding Ca2+, calsyntenin-1 may modulate Ca2+-mediated postsynaptic signals. Proteolytic cleavage of calsyntenin-1 in its extracellular moiety generates a transmembrane stump that is internalized and accumulated in the spine apparatus of spine synapses. Therefore, the synaptic Ca2+ modulation by calsyntenin-1 may be subject to regulation by extracellular proteolysis in the synaptic cleft. Thus, calsyntenin-1 may link extracellular proteolysis in the synaptic cleft and postsynaptic Ca2+ signaling.

Uptake of lipoproteins for axonal growth of sympathetic neurons

Lipoproteins originating from axon and myelin breakdown in injured peripheral nerves are believed to supply cholesterol to regenerating axons. We have used compartmented cultures of rat sympathetic neurons to investigate the utilization of lipids from lipoproteins for axon elongation. Lipids and proteins from human low density lipoproteins (LDL) and high density lipoproteins (HDL) were taken up by distal axons and transported to cell bodies, whereas cell bodies/proximal axons internalized these components from only LDL, not HDL. Consistent with these observations, the impairment of axonal growth, induced by inhibition of cholesterol synthesis, was reversed when LDL or HDL were added to distal axons or when LDL, but not HDL, were added to cell bodies. LDL receptors (LDLRs) and LR7/8B (apoER2) were present in cell bodies/proximal axons and distal axons, with LDLRs being more abundant in the former. Inhibition of cholesterol biosynthesis increased LDLR expression in cell bodies/proximal axons but not distal axons. LR11 (SorLA) was restricted to cell bodies/proximal axons and was undetectable in distal axons. Neither the LDL receptor-related protein nor the HDL receptor, SR-B1, was detected in sympathetic neurons. These studies demonstrate for the first time that lipids are taken up from lipoproteins by sympathetic neurons for use in axonal regeneration.

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.

Elevation of ceramide within distal neurites inhibits neurite growth in cultured rat sympathetic neurons

Sphingolipids are abundant constituents of neuronal membranes and have been implicated in intracellular signaling. We show that two analogs of glycosphingolipid biosynthetic intermediates, fumonisin B1 (which inhibits dihydroceramide synthesis) and DL-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) (which inhibits glucosylceramide synthesis) decrease glycosphingolipid synthesis in rat sympathetic neurons. Although both fumonisin and PPMP inhibit glycosphingolipid synthesis, these inhibitors have differential effects on ceramide metabolism in axons. threo-PPMP, but not erythro-PPMP or fumonisin, induces an accumulation of [3H]palmitate-labeled ceramide and impairs axonal growth. Moreover, exogenously added, cell-permeable C6-ceramide, but not C6-dihydroceramide, mimicks the effect of PPMP. Our studies suggest that the lipid second messenger ceramide acts in distal axons, but not cell bodies, as a negative regulator of neurite growth.

Alkylphosphocholines inhibit choline uptake and phosphatidylcholine biosynthesis in rat sympathetic neurons and impair axonal extension

At least 50% of the major axonal membrane lipid, phosphatidylcholine, of rat sympathetic neurons is synthesized in situ in axons [Posse de Chaves, Vance, Campenot and Vance (1995) J. Cell Biol. 128, 913-918]. In the same study we reported that, in a choline-deficient model for neuron growth, phosphatidylcholine synthesis in cell bodies is neither necessary nor sufficient for growth of distal axons. Rather, the local synthesis of phosphatidylcholine in distal axons is required for normal axon growth. We have now used three alkylphosphocholines (hexadecylphosphocholine, dodecylphosphocholine and octadecylphosphocholine) as inhibitors of PtdCho biosynthesis in a compartmented model for culture of rat sympathetic neurons. The experiments reveal that alkylphosphocholines decrease the uptake of choline into these neurons and inhibit PtdCho synthesis, but not via an effect on the activity of the enzyme CTP: phosphocholine cytidylyltransferase. We also show that when the distal axons, but not the cell bodies, are exposed to alkylphosphocholines, axonal elongation is inhibited, which is consistent with the hypothesis that phosphatidylcholine synthesis in axons, but not in cell bodies, is required for axonal elongation. The inhibitory effect of alkylphosphocholines on axon growth is most likely not mediated via a decrease in the activity of protein kinase C, since when this enzyme activity is down-regulated by treatment of the cells with phorbol ester, the alkylphosphocholines retain their ability to inhibit axonal growth.

Neurotropic factors, retrograde axonal transport and cell signalling

In vitro studies have recently identified receptors and signal transduction systems for many neurotrophic factors. In vivo, however, target-derived factors act over distances that are too great to be accounted for by simple diffusion of factors or classical second messengers. The active translocation of neurotrophic factors from the axon to the cell body by receptor-mediated retrograde transport provides a means by which factors presented at distal sites may influence somal signal transduction. We hypothesize that retrograde transport of receptors and other receptor-associated proteins leads to signalling at the cell body.

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.

Cholinergic differentiation of rat sympathetic neurons in culture: Effects of factors applied to distal neurites

Cholinergic properties are induced in sympathetic neurons by several factors applied to entire neurons in culture. Evidence from work with the rat sweat gland model indicates that factors located in target tissues can induce cholinergic differentiation in vivo. We now report that when leukemia inhibitory factor (LIF), heart cell-conditioned medium (HCCM), or dermal fibroblast-conditioned medium (DFCM) is applied to only distal neurites in compartmented cultures of rat sympathetic neurons, the neurons exhibit an increase in specific choline acetyltransferase activity and a concomitant decrease in levels of tyrosine hydroxylase. LIF, HCCM, and DFCM also induce neurite fasciculation, thus suggesting an additional role of cholinergic switching factors in regulating axon-axon and/or axon-substrate adhesion. These results demonstrate that rat sympathetic neurons have the cellular machinery to respond to cholinergic differentiation cues located in peripheral targets, analogous to the response to nerve growth factor.

Biosynthesis of membrane lipids in rat axons

Compartmented cultures of sympathetic neurons from newborn rats were employed to test the hypothesis that the lipids required for maintenance and growth of axonal membranes must be synthesized in the cell body and transported to the axons. In compartmented cultures the distal axons grow into a compartment separate from that containing the cell bodies and proximal axons, in an environment free from other contaminating cells such as glial cells and fibroblasts. There is virtually no bulk flow of culture medium or small molecules between the cell body and axonal compartments. When [methyl-3H]choline was added to the cell body-containing compartment the biosynthesis of [3H]-labeled phosphatidylcholine and sphingomyelin occurred in that compartment, with a gradual transfer of lipids (less than 5% after 16 h) into the axonal compartment. Surprisingly, addition of [methyl-3H]choline to the compartment containing only the distal axons resulted in the rapid incorporation of label into phosphatidylcholine and sphingomyelin in that compartment. Little retrograde transport of labeled phosphatidylcholine and sphingomyelin (less than 15%) into the cell body compartment occurred. Moreover, there was minimal transport of the aqueous precursors of these phospholipids (e.g., choline, phosphocholine and CDP-choline) between cell compartments. Similarly, when [3H]ethanolamine was used as a phospholipid precursor, the biosynthesis of phosphatidylethanolamine occurred in the pure axons, and approximately 10% of the phosphatidylethanolamine was converted into phosphatidylcholine. Experiments with [35S]methionine demonstrated that proteins were made in the cell bodies, but not in the axons. We conclude that axons of rat sympathetic neurons have the capacity to synthesize membrane phospholipids. Thus, a significant fraction of the phospholipids supplied to the membrane during axonal growth may be synthesized locally within the growing axon.

Growth of sympathetic nerve fibers in culture does not require extracellular calcium

A compartmented culture system in which distal neurites from newborn rat sympathetic neurons entered a fluid environment separate from that bathing the cell bodies and proximal neurites was used to investigate effects of extracellular Ca2+ deprivation on nerve fiber growth. Neurites readily grew into, elongated for many days within, and regenerated after neuritotomy within distal compartments substantially deprived of Ca2+ (0 added Ca2+, 0.5-5 mM EGTA), provided Ca2+ was supplied to the cell bodies. The Ca2(+)-deprived neurites generally extended at rates 20%-35% slower than controls. Growth of neurites did, however, cease within 2 days when the cell bodies were deprived of Ca2+, and the neurites and cell bodies eventually degenerated. These results show that neither extracellular Ca2+ nor the influx of Ca2+ at or near the growth cone is required for sustained neurite growth. They also rule out the possibility that the promotion of neurite growth by nerve growth factor is mediated, by the influx of extracellular Ca2+.

Size distribution of rhodamine-labelled microspheres retrogradely transported in cultured neurons

Rhodamine-labelled latex microspheres (RLM), 10-110 nm in diameter, were used as a retrograde tracer in cultured sympathetic neurons. Retrogradely transported RLM were isolated from the cells and measured. These were all smaller than 50 nm in diameter and represented not more than 2% of the original RLM population. The permeability of RLM as a function of size was also estimated and showed a steep decrease between 20 and 30 nm from 100% to less than 20%. Therefore, RLM with a mean diameter of 20 nm would be optimal for retrograde tracing.

Identification of proteins secreted from axons of embryonic dorsal-root-ganglia neurons

Secretion of proteins from the growth cone has been implicated in axon growth and synapse formation and might be involved in the transmission of a variety of axon-derived regulatory signals during neurogenesis. In order to identify axonally secreted proteins, dorsal-root-ganglia neurons from chicken embryos were cultured in a compartmentalized cell culture system that allows separate access to neuronal cell somas and axons. The proteins synthesized by the neurons were metabolically labeled by addition of [35S]methionine to the compartment containing the cell somas; the proteins released from the axons were harvested from the culture medium of the axonal compartment. Two-dimensional gel electrophoresis revealed two axonally secreted proteins with apparent molecular mass of 132-140 kDa and 54-60 kDa; they were termed axonin-1 and axonin-2, respectively. Both axonins were found to be secreted from a variety of neuronal cell cultures, but not from any of the nonneuronal cultures investigated, and hence might be neuron-specific. Virtual absence of these proteins from the axonal protein pattern suggests constitutive secretion. The information acquired on coordinates and spot morphology of these proteins in two-dimensional gel electrophoresis provides a useful assay for their purification.

Electron microscopic visualization of fluorescent microspheres used as a neuronal tracer

A method is described to visualize rhodamine-labelled microspheres (RLM) at the electron microscopic level using potassium permanganate for negative contrast. Retrogradely labelled nerve cells in the thalamus of the rat and in tissue culture are examined. In addition, it is demonstrated that single beads as small as 50 nm in diameter can be identified with the fluorescent microscope.

Coordinate regulation of the expression of axonal proteins by the axonal microenvironment

The axonal functions that act in the formation of the neuronal network have been shown to occur in close interdependence with the tissue that surrounds the growing axons. However, little is known about the molecular building blocks underlying axonal functions, although more than 400 axonal proteins have been identified. In view of the existence of such a large number of axonal proteins, we have initiated a project to determine the molecules involved in the implementation of particular axonal functions by a selective approach. On the assumption that plasticity in the expression of axonal functions in response to specific features of the local axonal environment may be based on changes in the expression of particular axonal proteins, the axonal proteins of dorsal root ganglion (DRG) neurons were screened for those whose expression responds to environmental influences. DRG neurons were grown in a compartmental cell system that offers separate access to neuronal somas and to their axons and the axons were locally exposed to different populations of cells from the peripheral or central nervous system. The axonal proteins were metabolically labeled and subjected to two-dimensional gel electrophoresis. Computerized quantitation of the individual axonal proteins revealed that the cocultured cells modulate the synthesis of a few axonal proteins of DRG neurons differentially. The data on the abundance of the newly expressed proteins under varying local environmental conditions were condensed as expression profiles. Comparison of expression profiles and cluster analysis of quantitative gel analysis data revealed that the environmentally modulated proteins subdivide into clusters with common distinct expression profiles under the influence of nonneuronal cells from the peripheral nervous system, nonneuronal cells of the central nervous system, and spinal cord cells, which are composed of neurons and nonneuronal cells. By means of this new, characteristic attribute assigned to environmentally modulated axonal proteins, working hypotheses were made as to their functional role.

Binding, uptake and retrograde axonal transport of herpes virus suis in sympathetic neurons

Newborn rat dissociated sympathetic neurons were grown in a chamber culture system, where a Teflon ring sealed with silicon grease separated the axonal plexus from the corresponding nerve cell bodies. The binding of 35S-labeled herpes virus suis (HVS) to the neurites was partially inhibited by an excess of unlabeled HVS as well as by concanavalin A, indicating the presence of specific binding sites for the virus. Specific binding was a prerequisite for the subsequent uptake and retrograde transport of HVS to the nerve cell bodies. Predominantly free nucleocapsids were detected by electron microscopy in the axons at the time of retrograde transport, both in culture and in vivo, suggesting the possibility that nucleocapsids without lipid membrane and not contained in cellular membrane compartments can be transported by retrograde axonal transport.

Morphological properties and membrane channels of the growth cones induced in PC12 cells by nerve growth factor

Large growth cones were produced in vitro by nerve growth factor (NGF) treatment of multinucleate cells produced by chemical fusion of cells of the neuron-like clone PC12. These endings were studied both at the light microscopic and ultrastructural levels. The activity of ionic channels at growth cones was recorded with intracellular microelectrodes, patch recording of single channels, and whole cone recording from mechanically isolated growth cones. Morphologically, these large growth cones were characterized by the presence of microspikes and filopodia, by the presence of actin demonstrated immunohistochemically, and by the presence of catecholamine fluorescence. At the ultrastructural level they contained a broad spectrum of organelles with a distribution characteristic of neuronal growth cones, including dense core vesicles, abundant smooth membrane cisternae, microtubules, and a filamentous network. The presence of channels capable of generating action potentials was revealed by intracellular microelectrode recording from the growth cone in the presence of locally applied tetraethylammonium (TEA). TEA appeared to block outward current channels that could effectively shunt inward current activated by depolarization. Action potentials elicited by depolarizing current in the presence of TEA could be blocked reversibly by Cd2+, a specific blocker of Ca channels. These action potentials were often followed by a long after-hyperpolarization lasting hundreds of milliseconds. This after-hyperpolarization was similar to that recorded in the cell body of PC12 cells where it appears to be mediated by Ca-activated K current. Single channel recording from outside-out excised patches of membrane from the growth cones perfused with KF revealed the presence of voltage sensitive Na channels, Ca-activated K channels, and K channels resembling delayed rectifier K channels. Macroscopic currents recorded from mechanically isolated growth cones in the "whole cone" configuration showed rapid inward currents at potentials greater than or equal to -40 mV, followed by delayed outward currents at more positive potentials, a finding providing additional evidence for the presence of Na and K channels in growth cones.

A few axonal proteins distinguish ventral spinal cord neurons from dorsal root ganglion neurons

A series of proteins putatively involved in the generation of axonal diversity was identified. Neurons from ventral spinal cord and dorsal root ganglia were grown in a compartmented cell-culture system which offers separate access to cell somas and axons. The proteins synthesized in the neuronal cell somas and subsequently transported into the axons were selectively analyzed by 2-dimensional gel electrophoresis. The patterns of axonal proteins were substantially less complex than those derived from the proteins of neuronal cell bodies. The structural and functional similarity of axons from different neurons was reflected in a high degree of similarity of the gel pattern of the axonal proteins from sensory ganglia and spinal cord neurons. Each axonal type, however, had several proteins that were markedly less abundant or absent in the other. These neuron-population enriched proteins may be involved in the implementation of neuronal diversity. One of the proteins enriched in dorsal root ganglia axons had previously been found to be expressed with decreased abundance when dorsal root ganglia axons were co-cultured with ventral spinal cord cells under conditions in which synapse formation occurs (P. Sonderegger, M. C. Fishman, M. Bokoum, H. C. Bauer, and P.G. Nelson, 1983, Science [Wash. DC], 221:1294-1297). This protein may be a candidate for a role in growth cone functions, specific for neuronal subsets, such as pathfinding and selective axon fasciculation or the initiation of specific synapses. The methodology presented is thus capable of demonstrating patterns of protein synthesis that distinguish different neuronal subsets. The accessibility of these proteins for structural and functional studies may contribute to the elucidation of neuron-specific functions at the molecular level.

Axonal proteins of presynaptic neurons during synaptogenesis

Changes occur in the synthesis and axonal transport of neuronal proteins in dorsal-root ganglia axons as a result of contact with cells from the spinal cord during synapse formation. Dorsal-root ganglia cells were cultured in a compartmental cel culture system that allows separate access to neuronal cell bodies and their axons. When cells from the ventral spinal cord were cultured with the dorsal-root ganglia axons, synapses were established within a few days. Metabolic labeling and two-dimensional electrophoresis revealed that four of more than 300 axonal proteins had changed in their expression by the time synapses were established. The highly selective nature of these changes suggests that the proteins involved may be important in the processes of axon growth and synapse formation and their regulation by the regional environment.

Mechanism of uptake and retrograde axonal transport of noradrenaline in sympathetic neurons in culture: reserpine-resistant large dense-core vesicles as transport vehicles

The uptake and retrograde transport of noradrenaline (NA) within the axons of sympathetic neurons was investigated in an in vitro system. Dissociated neurons from the sympathetic ganglia of newborn rats were cultured for 3-6 wk in the absence of non-neuronal cells in a culture dish divided into three chambers. These allowed separate access to the axonal networks and to their cell bodies of origin. [3H]NA (0.5 X 10(-6) M), added to the axon chambers, was taken up by the desmethylimipramine- and cocaine-sensitive neuronal amine uptake mechanisms, and a substantial part was rapidly transported retrogradely along the axons to the nerve cell bodies. This transport was blocked by vinblastine or colchicine. In contrast with the storage of [3H]NA in the axonal varicosities, which was totally prevented by reserpine (a drug that selectively inactivates the uptake of NA into adrenergic storage vesicles), the retrograde transport of [3H]NA was only slightly diminished by reserpine pretreatment. Electron microscopic localization of the NA analogue 5-hydroxydopamine (5-OHDA) indicated that mainly large dense-core vesicles (700-1,200-A diam) are the transport compartment involved. Whereas the majority of small and large vesicles lost their amine dense-core and were resistant to this drug. It, therefore, seems that these vesicles maintained the amine uptake and storage mechanisms characteristic for adrenergic vesicles, but have lost the sensitivity of their amine carrier for reserpine. The retrograde transport of NA and 5-OHDA probably reflects the return of used synaptic vesicle membrane to the cell body in a form that is distinct from the membranous cisternae and prelysosomal structures involved in the retrograde axonal transport of extracellular tracers.