Retrograde support of neuronal survival without retrograde transport of nerve growth factor

Up-regulation of phosphorylated CREB but not c-Jun in bladder afferent neurons in dorsal root ganglia after cystitis

We examined the changes of two transcription factors, CREB and c-Jun, in dorsal root ganglia (DRG) after acute (8 or 48 hours) or chronic (10 days) cyclophosphamide (CYP)-induced cystitis. Results showed an increase in the number of p-CREB-immunoreactive (-IR) cells in the L1 and L2 DRG (5-7-fold; P < or = 0.05) as well as L6 and S1 DRG (2-4-fold; P < or = 0.05) after acute and chronic cystitis. The number of p-CREB-IR cells in the L4-L5 DRG was not altered with cystitis. The number of c-Jun-IR cells increased in the L1-L2 DRG (L1: 10-fold; L2: 8-fold; P < or = 0.05) only with chronic cystitis, although it increased in the L6-S1 DRG with CYP-induced cystitis of acute (2-3-fold; P < or = 0.05) and chronic (6-10-fold; P < or = 0.05) duration. After CYP treatment, the percentage of bladder afferent cells expressing p-CREB immunoreactivity (3-7-fold; P < or = 0.05) increased in L1, L2, L6, and S1 DRG. The increase occurred 8 hours post-CYP injection and was maintained with chronic cystitis. There were few c-Jun-IR cells in the bladder afferent population. These results demonstrate that CYP induces p-CREB and c-Jun expression in DRG in a time-dependent manner. However, c-Jun expression is not associated with bladder afferent neurons. Resiniferatoxin reduced CYP-induced up-regulation of p-CREB in DRG, suggesting that cystitis can reveal an altered CREB phosphorylation that may be mediated by capsaicin-sensitive bladder afferents. Colocalization of p-CREB and Trk receptor(s) showed that a subpopulation of p-CREB-IR cells expressed p-Trk with cystitis. These results suggest that up-regulation of p-CREB may be mediated by a neurotrophin/Trk signaling pathway.

Cystitis-induced upregulation of tyrosine kinase (TrkA, TrkB) receptor expression and phosphorylation in rat micturition pathways

This study examined tyrosine kinase receptor (Trk) expression and phosphorylation in lumbosacral dorsal root ganglia (DRG) after acute (8 or 48 hours) or chronic (10 days) cyclophosphamide (CYP)-induced cystitis. Increases in the number of TrkA-immunoreactive (IR) cell profiles were detected in the L1 and L6 DRG (four-fold; P < or = 0.01) and the S1 DRG (1.5-fold; P < or = 0.05) but not in the L2, L4, and L5 DRG with CYP-induced cystitis of acute and chronic duration compared with control rats. The number of TrkB-IR cell profiles increased in the L1 and L2 DRG (L1: 2.6-fold; L2: 1.4-fold; P < or = 0.05) and in the L6 and S1 DRG (L6: 2.2-fold; S1: 1.3-fold; P < or = 0.05) only after acute CYP treatment (8 hours). After CYP treatment, the percentage of bladder afferent cell profiles expressing TrkA-IR (approximately 50%; P < or = 0.05) increased in L1 and L6 DRG. The percentage of bladder afferent cell profiles expressing TrkB-IR (approximately 45%; P < or = 0.05) in L1, L2, L6, and S1 DRG also increased compared with control cell profiles. The increase in TrkA-IR in bladder afferent cells occurred 8 hours after CYP treatment and was maintained in L1 DRG with chronic (10 days) CYP-induced cystitis. However, the increase in bladder afferent cells expressing TrkB-IR only occurred at the most acute time point examined (8 hours). TrkA-IR and TrkB-IR cell profiles also demonstrated phosphorylated Trk-IR with acute and/or chronic CYP-induced cystitis. These results demonstrated that CYP-induced cystitis increases the expression and phosphorylation of Trk receptors in lumbosacral DRG. Expression of neurotrophic factors in the inflamed urinary bladder may contribute to this increased expression, and neurotrophic factor and Trk interactions may play unique roles in decreased urinary tract plasticity with CYP-induced cystitis.

Internalization of inactive EGF receptor into endosomes and the subsequent activation of endosome-associated EGF receptors. Epidermal growth factor

Despite intensive efforts to understand cell signaling from endosomes, there is no direct evidence demonstrating that endosomal signaling is sufficient to activate signal transduction pathways or that endosomal signaling can produce biological responses. The lack of breakthrough is due in part to the inability to generate endosomal signals in isolation from plasma membrane signals. In this Protocol, we describe a system in which epidermal growth factor (EGF) receptor (EGFR) is specifically activated when it is endocytosed into endosomes. We treated cells with EGF in the presence of AG1478, a specific EGFR tyrosine kinase inhibitor, and monensin, which blocks recycling of EGFR. This treatment led to the internalization of nonactivated EGF-EGFR complex into endosomes. The endosome-associated EGFR was then activated by removing AG1478 and monensin. During this procedure, we did not observe any detectable surface EGFR phosphorylation. We also achieved specific activation of endosome-associated EGFR without using monensin. Specific activation of endosome-associated EGFR provides a unique tool to study endosomal signaling of EGFR. This method may also be applied to other receptor tyrosine kinases to study whether they, too, can signal from endosomes.

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.

Cyclic AMP induces transactivation of the receptors for epidermal growth factor and nerve growth factor, thereby modulating activation of MAP kinase, Akt, and neurite outgrowth in PC12 cells

In PC12 cells, a well studied model for neuronal differentiation, an elevation in the intracellular cAMP level increases cell survival, stimulates neurite outgrowth, and causes activation of extracellular signal-regulated protein kinase 1 and 2 (ERK1/2). Here we show that an increase in the intracellular cAMP concentration induces tyrosine phosphorylation of two receptor tyrosine kinases, i.e. the epidermal growth factor (EGF) receptor and the high affinity receptor for nerve growth factor (NGF), also termed Trk(A). cAMP-induced tyrosine phosphorylation of the EGF receptor is rapid and correlates with ERK1/2 activation. It occurs also in Panc-1, but not in human mesangial cells. cAMP-induced tyrosine phosphorylation of the NGF receptor is slower and correlates with Akt activation. Inhibition of EGF receptor tyrosine phosphorylation, but not of the NGF receptor, reduces cAMP-induced neurite outgrowth. Expression of dominant-negative Akt does not abolish cAMP-induced survival in serum-free media, but increases cAMP-induced ERK1/2 activation and neurite outgrowth. Together, our results demonstrate that cAMP induces dual signaling in PC12 cells: transactivation of the EGF receptor triggering the ERK1/2 pathway and neurite outgrowth; and transactivation of the NGF receptor promoting Akt activation and thereby modulating ERK1/2 activation and neurite outgrowth.

Localized synaptic potentiation by BDNF requires local protein synthesis in the developing axon

Brain-derived neurotrophic factor (BDNF) is known to promote neuronal survival, guide axonal pathfinding, and participate in activity-dependent synaptic plasticity. In Xenopus nerve-muscle cultures, localized contact of a single BDNF-coated bead with the presynaptic axon resulted in potentiation of transmitter secretion at the developing synapses, but only when the bead was placed within 60 microm from the synapse. The localized potentiation induced by BDNF is accompanied by a persistent local elevation of [Ca(2+)](i) in the axon and requires constitutive presynaptic protein translation, even for axons severed from the cell body. Thus, presynaptic local TrkB signaling and protein synthesis allow a localized source of BDNF to potentiate transmitter secretion from nearby synapses, a property suited for spatially restricted synaptic modification by neurotrophins.

Endosomal signaling of epidermal growth factor receptor stimulates signal transduction pathways leading to cell survival

In spite of intensified efforts to understand cell signaling from endosomes, there is no direct evidence demonstrating that endosomal signaling is sufficient to activate signal transduction pathways and no evidence to demonstrate that endosomal signaling is able to produce a biological outcome. The lack of breakthrough is due in part to the lack of means to generate endosomal signals without plasma membrane signaling. In this paper, we report the establishment of a system to specifically activate epidermal growth factor (EGF) receptor (EGFR) when it endocytoses into endosomes. We treated cells with EGF in the presence of AG-1478, a specific EGFR tyrosine kinase inhibitor, and monensin, which blocks the recycling of EGFR. This treatment led to the internalization of nonactivated EGF-EGFR complexes into endosomes. The endosome-associated EGFR was then activated by removing AG-1478 and monensin. During this procedure we did not observe any surface EGFR phosphorylation. We also achieved specific activation of endosome-associated EGFR without using monensin. By using this system, we provided original evidence demonstrating that (i) the endosome can serve as a nucleation site for the formation of signaling complexes, (ii) endosomal EGFR signaling is sufficient to activate the major signaling pathways leading to cell proliferation and survival, and (iii) endosomal EGFR signaling is sufficient to suppress apoptosis induced by serum withdrawal.

Neurotrophic factors and axonal growth

Neuronal morphological differentiation is regulated by numerous polypeptide growth factors (neurotrophic factors). Recently, significant progress has been achieved in clarifying the roles of neurotrophins as well as glial cell line-derived neurotrophic factor family members in peripheral axon elongation during development. Additionally, advances have been made in defining the signal transduction mechanisms employed by these factors in mediating axon morphological responses. Several studies addressed the role of neurotrophic factors in regenerative axon growth and suggest that signaling mechanisms in addition to those triggered by receptor tyrosine kinases may be required for successful peripheral nervous system regeneration. Finally, recent investigations demonstrate that neurotrophic factors can enhance axon growth after spinal cord injuries.

Local and target-derived brain-derived neurotrophic factor exert opposing effects on the dendritic arborization of retinal ganglion cells in vivo

The dendritic and axonal arbors of developing retinal ganglion cells (RGCs) are exposed to two sources of BDNF: RGC dendrites are exposed to BDNF locally within the retina, and RGC axons are exposed to BDNF at the target, the optic tectum. Our previous studies demonstrated that increasing tectal BDNF levels promotes RGC axon terminal arborization, whereas increasing retinal BDNF levels inhibits RGC dendritic arborization. These results suggested that differential neurotrophic action at the axon versus dendrite might be responsible for the opposing effects of BDNF on RGC axonal versus dendritic arborization. To explore this possibility, we examined the effects of altering BDNF levels at the optic tectum on the elaboration of RGC dendritic arbors in the retina. Increasing tectal BDNF levels resulted in a significant increase in dendritic branching, whereas neutralizing endogenous tectal BDNF with function-blocking antibodies significantly decreased dendritic arbor complexity. Thus, RGC dendritic arbors react in opposing manners to retinal- versus tectal-derived BDNF. Alterations in retinal BDNF levels, however, did not affect axon terminal arborization. Thus, RGC dendritic arborization is controlled in a complementary manner by both local and target-derived sources of BDNF, whereas axon arborization is modulated solely by neurotrophic interactions at the target. Together, our results indicate that developing RGCs modulate dendritic arborization by integrating signals from discrete sources of BDNF in the eye and brain. Differential integration of spatially discrete neurotrophin signals within a single neuron may therefore finely tune afferent and efferent neuronal connectivity.

Evidence for reduced accumulation of exogenous neurotrophin by aged sympathetic neurons

The present study investigated the potential for neurotrophin uptake by cerebrovascular axons and subsequent accumulation in the aged superior cervical ganglion (SCG) following a two week intracerebroventricular infusion of nerve growth factor (NGF). In the SCG from aged rats, NGF protein levels declined significantly compared with the SCG from young adult rats. Following NGF infusion, perivascular axons from both young adult and aged rats showed intense NGF immunostaining. In addition, significant increases in NGF protein were shown using enzyme-linked immunosorbent assay (ELISA) and in counts of NGF immunopositive cell bodies in the SCG when compared with age-matched controls. NGF accumulation in ganglia from aged rats, however, was significantly less when compared with ganglia from young adult rats. The results of the present study suggest that NGF protein is significantly reduced in aged ganglia with the neurons retaining some capacity to take up and transport exogenous neurotrophin. Even so, the potential for NGF accumulation is dramatically reduced in aged rats when compared with that of young adult rats. While previous results have shown robust NGF-induced neurotransmitter responses by sympathetic neurons from the aged animal, the present finding of reduced accumulation of NGF in aged sympathetic neurons suggests an age-related difference in the utilization or transport of NGF.

The role of the cytoskeleton in the life cycle of viruses and intracellular bacteria: tracks, motors, and polymerization machines

Recent advances in microbiology implicate the cytoskeleton in the life cycle of some pathogens, such as intracellular bacteria, Rickettsia and viruses. The cellular cytoskeleton provides the basis for intracellular movements such as those that transport the pathogen to and from the cell surface to the nuclear region, or those that produce cortical protrusions that project the pathogen outwards from the cell surface towards an adjacent cell. Transport in both directions within the neuron is required for pathogens such as the herpesviruses to travel to and from the nucleus and perinuclear region where replication takes place. This trafficking is likely to depend on cellular motors moving on a combination of microtubule and actin filament tracks. Recently, Bearer et al. reconstituted retrograde transport of herpes simplex virus (HSV) in the giant axon of the squid. These studies identified the tegument proteins as the viral proteins most likely to recruit retrograde motors for the transport of HSV to the neuronal nucleus. Similar microtubule-based intracellular movements are part of the biological behavior of vaccinia, a poxvirus, and of adenovirus. Pathogen-induced surface projections and motility within the cortical cytoplasm also play a role in the life cycle of intracellular pathogens. Such motility is driven by pathogen-mediated actin polymerization. Virulence depends on this actin-based motility, since virulence is reduced in Listeria ActA mutants that lack the ability to recruit Arp2/3 and polymerize actin and in vaccinia virus mutants that cannot stimulate actin polymerization. Inhibition of intracellular movements provides a potential strategy to limit pathogenicity. The host cell motors and tracks, as well as the pathogen factors that interact with them, are potential targets for novel antimicrobial therapy.

Retrograde signaling by the neurotrophins follows a well-worn trk

The mechanism that allows a neuron to send cues received at its terminal to its cell body and nucleus has proved elusive. However, a recent study by Howe and colleagues indicates that neurotrophin signaling via the trkA receptor requires formation of a signaling endosome containing NGF and trkA. Thus, endocytosis of the neurotrophin-receptor complex is a crucial step in the generation of intracellular signaling platforms required for activation and compartmentalization of signaling events.

All signaling is local?

The mechanism of signal transmission following ligand stimulation of receptor tyrosine kinases in living cells is poorly understood. Recent studies have visualized the spatio-temporal pattern of EGF signaling, indicating that receptor density is an important factor in the mechanism of lateral propagation of local EGF signaling.

Signal transduction and endocytosis: close encounters of many kinds

Binding of hormones, growth factors and other cell modulators to cell-surface receptors triggers a complex array of signal-transduction events. The activation of many receptors also accelerates their endocytosis. Endocytic transport is important in regulating signal transduction and in mediating the formation of specialized signalling complexes. Conversely, signal-transduction events modulate specific components of the endocytic machinery. Recent studies of protein tyrosine kinases and G-protein-coupled receptors have shed new light on the mechanisms and functional consequences of this bidirectional interplay between signalling and membrane-transport networks.

Up-regulation of tyrosine kinase (Trka, Trkb) receptor expression and phosphorylation in lumbosacral dorsal root ganglia after chronic spinal cord (T8-T10) injury

Previous studies have demonstrated changes in urinary bladder neurotrophic factors after bladder dysfunction. We have hypothesized that retrograde transport of neurotrophin(s) from the bladder to lumbosacral dorsal root ganglia (DRG) may play a role in bladder reflex reorganization after spinal cord injury (SCI). In this study, we determined whether the expression of tyrosine kinase receptors (TrkA, TrkB) is altered in lumbosacral DRG after SCI through immunofluorescence techniques. Complete transection of the spinal cord (T8-T10) was performed in female Wistar rats (120-150 g), and animals were studied 5-6 weeks after SCI. One week before killing, Fast Blue (FB) was injected into the bladder to label bladder afferent cells in the L1, L2, L6, and S1 DRG. After SCI, a significant increase in the number of TrkA-immunoreactive (IR) positive cells was detected in the L6-S1 DRG (L6: 1.9-fold, P < or = 0.01; S1: 1.7-fold, P < or = 0.05) and in the L1 DRG (3.0-fold; P < or = 0.01) but not in the L4-L5 DRG compared with spinal-intact (control) rats. After SCI, a significant increase in the number of TrkB-IR cells was also detected in the L6-S1 DRG (L6: 2.2-fold, P < or = 0.01; S1: 1.5-fold, P < or = 0.05) and in the L1-L2 DRG (L1: 1.5-fold, P < or = 0.01; L2: 1.3-fold, P < or = 0.05) but not in the L4-L5 DRG compared with control rats. After SCI, the percentage of FB-labeled cells expressing TrkA immunoreactivity (approximately 68%) or TrkB immunoreactivity (approximately 65%) in L1 and L6 DRG significantly (P < or = 0.01) increased compared with control (20-30%) DRG. After SCI, the percentage of TrkA-IR cells expressing phosphorylated (p)-Trk immunoreactivity significantly increased (1.5- to 2.3-fold increase) in the L1, L6, and S1 DRG. The percentage of TrkB-IR cells expressing p-Trk immunoreactivity after SCI also increased (1.3-fold increase) in the L1 and L6 DRG. These results demonstrate that (1) TrkA and TrkB immunoreactivity is increased in bladder afferent cells after SCI and (2) TrkA and TrkB receptors are phosphorylated in DRG after SCI. Neuroplasticity of lower urinary tract reflexes after SCI may be mediated by both nerve growth factor and brain-derived neurotrophic factor.

High-resolution imaging demonstrates dynein-based vesicular transport of activated Trk receptors

Target-derived neurotrophins signal from nerve endings to the cell body to influence cellular and nuclear responses. The retrograde signal is conveyed by neurotrophin receptors (Trks) themselves. To accomplish this, activated Trks may physically relocalize from nerve endings to the cell bodies. However, alternative signaling mechanisms may also be used. To identify the vehicle wherein the activated Trks are located and transported, and to identify associated motor proteins that would facilitate transport, we use activation-state specific antibodies in concert with immunoelectron microscopy and deconvolution microscopy. We show that the'activated Trks within rat sciatic nerve axons are preferentially localized to coated and uncoated vesicles. These vesicles are moving in a retrograde direction and so accumulate distal to a ligation site. The P-Trk containing vesicles, in turn, colocalize with dynein components, and not with kinesins. Collectively, these results indicate activated Trk within axons travel in vesicles and dynein is the motor that drives these vesicles towards the cell bodies.

Location, location, location: a spatial view of neurotrophin signal transduction

Neurotrophins were originally identified as target-derived factors that regulate the survival and differentiation of innervating neurons. However, neurotrophins can also be released by presynaptic cells to stimulate postsynaptic neurons. Recent studies indicate that differences exist between the signaling pathways activated by neurotrophin stimulation of nerve terminals (retrograde signaling) and neurotrophin stimulation of cell bodies. Retrograde signaling relies on the formation of signaling endosomes, vesicles containing activated Trk receptors and their ligands. Signaling endosomes travel from the nerve terminals to remote cell bodies, where they selectively activate a novel MAP kinase, Erk5, as well as PI3 kinase, and thereby stimulate neuronal survival. The differences in the signaling pathways activated by neurotrophins, which depends on the location of stimulation, provide a mechanism by which neurons can interpret the 'where' as well as the 'what' of growth factor stimulation.