Retrograde signaling by the neurotrophins follows a well-worn trk

NGF-dependent neurons and neurobiology of emotions and feelings: Lessons from congenital insensitivity to pain with anhidrosis

NGF is a well-studied neurotrophic factor, and TrkA is a receptor tyrosine kinase for NGF. The NGF-TrkA system supports the survival and maintenance of NGF-dependent neurons during development. Congenital insensitivity to pain with anhidrosis (CIPA) is an autosomal recessive genetic disorder due to loss-of-function mutations in the NTRK1 gene encoding TrkA. Individuals with CIPA lack NGF-dependent neurons, including NGF-dependent primary afferents and sympathetic postganglionic neurons, in otherwise intact systems. Thus, the pathophysiology of CIPA can provide intriguing findings to elucidate the unique functions that NGF-dependent neurons serve in humans, which might be difficult to evaluate in animal studies. Preceding studies have shown that the NGF-TrkA system plays critical roles in pain, itching and inflammation. This review focuses on the clinical and neurobiological aspects of CIPA and explains that NGF-dependent neurons in the peripheral nervous system play pivotal roles in interoception and homeostasis of our body, as well as in the stress response. Furthermore, these NGF-dependent neurons are likely requisite for neurobiological processes of 'emotions and feelings' in our species.

Myenteric expression of nerve growth factor and the p75 neurotrophin receptor regulate axonal remodeling as a consequence of colonic inflammation in mice

Nerve growth factor (NGF) levels increase in response to inflammation of the mammalian colon. The precise cellular sources of colonic NGF synthesis, however, remain elusive. Using lines of transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the NGF promoter, we found a subpopulation of adendritic EGFP(+) neurons in the myenteric plexus. These colonic EGFP(+) neurons display positive immunostaining for calretinin but not nitric oxide synthase 1 (NOS1) two biomarkers of mouse myenteric neurons. A loss of NGF expression in null mutant postnatal mice does not affect the survival of these EGFP(+) neurons. Induction of colonic inflammation confirms local increases in NGF mRNA/protein levels, which coincide with heightened detection of EGFP by myenteric neurons. Though NOS1(+) myenteric neurons display positive immunostaining for trkA (the receptor required for NGF binding/signaling), transgenic overexpression of NGF by smooth muscle cells in the colon does not alter the survival, somal size, or axonal density of trkA-expressing NOS1(+) myenteric neurons. Mice lacking functional p75NTR (the second receptor required for NGF binding) exhibit significantly less axonal damage among NOS1(+) myenteric neurons, in response to chemically induced colonic inflammation. Likewise, trkA-expressing sympathetic axons that innervate the myenteric ganglia display less damage in the absence of p75NTR. These data are the first to implicate calretinin(+) myenteric neurons as a source of NGF in the murine colon, and that in response to colonic inflammation, increases in NGF can exaggerate damage of intrinsic NOS1(+) axons and extrinsic sympathetic axons that co-express trkA and p75NTR.

NGF in Early Embryogenesis, Differentiation, and Pathology in the Nervous and Immune Systems

The physiology of NGF is extremely complex, and although the study of this neurotrophin began more than 60 years ago, it is far from being concluded. NGF, its precursor molecule pro-NGF, and their different receptor systems (i.e., TrkA, p75NTR, and sortilin) have key roles in the development and adult physiology of both the nervous and immune systems. Although the NGF receptor system and the pathways activated are similar for all types of cells sensitive to NGF, the effects exerted during embryonic differentiation and in committed mature cells are strikingly different and sometimes opposite. Bearing in mind the pleiotropic effects of NGF, alterations in its expression and synthesis, as well as variations in the types of receptor available and in their respective levels of expression, may have profound effects and play multiple roles in the development and progression of several diseases. In recent years, the use of NGF or of inhibitors of its receptors has been prospected as a therapeutic tool in a variety of neurological diseases and injuries. In this review, we outline the different roles played by the NGF system in various moments of nervous and immune system differentiation and physiology, from embryonic development to aging. The data collected over the past decades indicate that NGF activities are highly integrated among systems and are necessary for the maintenance of homeostasis. Further, more integrated and multidisciplinary studies should take into consideration these multiple and interactive aspects of NGF physiology in order to design new therapeutic strategies based on the manipulation of NGF and its intracellular pathways.

Pea3 transcription factor family members Etv4 and Etv5 mediate retrograde signaling and axonal growth of DRG sensory neurons in response to NGF

Nerve growth factor (NGF) is a target-derived neurotrophic growth factor that controls many aspects of sensory and sympathetic neuronal development. The identification of transcription factors and downstream target genes that mediate NGF-dependent neuronal differentiation and target field innervation is currently a major challenge. Here, we show that the Pea3 transcription factor family members Etv4 and Etv5 are expressed by developing TrkA-positive dorsal root ganglion (DRG) neurons during the period of target innervation. Real-time PCR assays indicated that Etv4 and Etv5 mRNAs are significantly induced by NGF in different neuronal cells, suggesting that they could be involved in the biological responses induced by this neurotrophin. Interestingly, distal axon application of NGF in compartmentalized cultures of rat DRG sensory neurons was sufficient to induce a significant increase in Etv4 and Etv5 mRNA expression. Pharmacological assays also revealed that activation of MEK/ERK (MAPK) pathway is required for Etv4 and Etv5 gene induction in response to NGF. Downregulation of Etv4 and Etv5 using small interference RNA knockdown experiments inhibited NGF-induced neurite outgrowth of rat sensory neurons, while overexpression of full-length Etv4 or Etv5 potentiated neuronal differentiation in response to this neurotrophin. Together, these data establish Etv4 and Etv5 as essential molecules of the transcriptional program linking neurotrophin signaling to sensory neuronal differentiation, and suggest that they can be involved in NGF-mediated target innervation.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Up-regulation of brain-derived neurotrophic factor is regulated by extracellular signal-regulated protein kinase 5 and by nerve growth factor retrograde signaling in colonic afferent neurons in colitis

Brain-derived neurotrophic factor (BDNF) plays an essential role in sensory neuronal activation in response to visceral inflammation. Here we report that BDNF up-regulation in the primary afferent neurons in the dorsal root ganglia (DRG) in a rat model of colitis is mediated by the activation of endogenous extracellular signal-regulated protein kinase (ERK) 5 and by nerve growth factor (NGF) retrograde signaling. At 7 days of colitis, the expression level of BDNF is increased in conventional neuronal tracing dye Fast Blue labeled primary afferent neurons that project to the distal colon. In these neurons, the phosphorylation (activation) level of ERK5 is also increased. In contrast, the level of phospho-ERK1/2 is not changed in the DRG during colitis. Prevention of the ERK5 activation in vivo with an intrathecal application of the MEK inhibitor PD98059 significantly attenuates the colitis-induced increases in BDNF expression in the DRG. Further studies show that BDNF up-regulation in the DRG is triggered by NGF retrograde signaling which also involves activation of the MEK/ERK pathways. Application of exogenous NGF exclusively to the compartment containing DRG nerve terminals in an ex vivo ganglia-nerve preparation markedly increases the BDNF expression level in the DRG neuronal cell body that is placed in a different compartment; this BDNF elevation is attenuated by U0126, PD98059 and a specific ERK5 inhibitor BIX02188. These results demonstrate the mechanisms and pathways by which BDNF expression is elevated in primary sensory neurons following visceral inflammation that is mediated by increased activity of ERK5 and is likely to be triggered by the elevated NGF level in the inflamed viscera.

Comparison of nerve growth factor receptor binding models using heterodimeric muteins

Nerve growth factor (NGF) is a homodimer that binds to two distinct receptor types, TrkA and p75, to support survival and differentiation of neurons. The high-affinity binding on the cell surface is believed to involve a heteroreceptor complex, but its exact nature is unclear. We developed a heterodimer (heteromutein) of two NGF muteins that can bind p75 and TrkA on opposite sides of the heterodimer, but not two TrkA receptors. Previously described muteins are Δ9/13 that is TrkA negative and 7-84-103 that is signal selective through TrkA. The heteromutein (Htm1) was used to study the heteroreceptor complex formation and function, in the putative absence of NGF-induced TrkA dimerization. Cellular binding assays indicated that Htm1 does not bind TrkA as efficiently as wild-type (wt) NGF but has better affinity than either homodimeric mutein. Htm1, 7-84-103, and Δ9/13 were each able to compete for cold-temperature, cold-chase stable binding on PC12 cells, indicating that binding to p75 was required for a portion of this high-affinity binding. Survival, neurite outgrowth, and MAPK signaling in PC12 cells also showed a reduced response for Htm1, compared with wtNGF, but was better than the parent muteins in the order wtNGF > Htm1 > 7-84-103 >> Δ9/13. Htm1 and 7-84-103 demonstrated similar levels of survival on cells expressing only TrkA. In the longstanding debate on the NGF receptor binding mechanism, our data support the ligand passing of NGF from p75 to TrkA involving a transient heteroreceptor complex of p75-NGF-TrkA.

Activation of extracellular signal-regulated protein kinase 5 is essential for cystitis- and nerve growth factor-induced calcitonin gene-related peptide expression in sensory neurons

Background

Cystitis causes considerable neuronal plasticity in the primary afferent pathways. The molecular mechanism and signal transduction underlying cross talk between the inflamed urinary bladder and sensory sensitization has not been investigated.

Results

In a rat cystitis model induced by cyclophosphamide (CYP) for 48 h, the mRNA and protein levels of the excitatory neurotransmitter calcitonin gene-related peptide (CGRP) are increased in the L6 dorsal root ganglia (DRG) in response to bladder inflammation. Cystitis-induced CGRP expression in L6 DRG is triggered by endogenous nerve growth factor (NGF) because neutralization of NGF with a specific NGF antibody reverses CGRP up-regulation during cystitis. CGRP expression in the L6 DRG neurons is also enhanced by retrograde NGF signaling when NGF is applied to the nerve terminals of the ganglion-nerve two-compartmented preparation. Characterization of the signaling pathways in cystitis- or NGF-induced CGRP expression reveals that the activation (phosphorylation) of extracellular signal-regulated protein kinase (ERK)5 but not Akt is involved. In L6 DRG during cystitis, CGRP is co-localized with phospho-ERK5 but not phospho-Akt. NGF-evoked CGRP up-regulation is also blocked by inhibition of the MEK/ERK pathway with specific MEK inhibitors U0126 and PD98059, but not by inhibition of the PI3K/Akt pathway with inhibitor LY294002. Further examination shows that cystitis-induced cAMP-responsive element binding protein (CREB) activity is expressed in CGRP bladder afferent neurons and is co-localized with phospho-ERK5 but not phospho-Akt. Blockade of NGF action in vivo reduces the number of DRG neurons co-expressing CGRP and phospho-CREB, and reverses cystitis-induced increases in micturition frequency.

Conclusions

A specific pathway involving NGF-ERK5-CREB axis plays an essential role in cystitis-induced sensory activation.

Interleukin-17A increases neurite outgrowth from adult postganglionic sympathetic neurons

Inflammation can profoundly alter the structure and function of the nervous system. Interleukin (IL)-17 has been implicated in the pathogenesis of several inflammatory diseases associated with nervous system plasticity. However, the effects of IL-17 on the nervous system remain unexplored. Cell and explant culture techniques, immunohistochemistry, electrophysiology, and Ca2+ imaging were used to examine the impact of IL-17 on adult mouse sympathetic neurons. Receptors for IL-17 were present on postganglionic neurons from superior mesenteric ganglia (SMG). Supernatant from activated splenic T lymphocytes, which was abundant in IL-17, dramatically enhanced axonal length of SMG neurons. Importantly, IL-17-neutralizing antiserum abrogated the neurotrophic effect of splenocyte supernatant, and incubation of SMG neurons in IL-17 (1 ng/ml) significantly potentiated neurite outgrowth. The neurotrophic effect of IL-17 was accompanied by inhibition of voltage-dependent Ca2+ influx and was recapitulated by incubation of neurons in a blocker of N-type Ca2+ channels (ω-conotoxin GVIA; 30 nM). IL-17-induced neurite outgrowth in vitro appeared to be independent of glia, as treatment with a glial toxin (AraC; 5 μM) did not affect the outgrowth response to IL-17. Moreover, application of the cytokine to distal axons devoid of glial processes enhanced neurite extension. An inhibitor of the NF-κB pathway (SC-514; 20 μM) blocked the effects of IL-17. These data represent the first evidence that IL-17 can act on sympathetic somata and distal neurites to enhance neurite outgrowth, and identify a novel potential role for IL-17 in the neuroanatomical plasticity that accompanies inflammation.

Recruitment of actin modifiers to TrkA endosomes governs retrograde NGF signaling and survival

The neurotrophins NGF and NT3 collaborate to support development of sympathetic neurons. Although both promote axonal extension via the TrkA receptor, only NGF activates retrograde transport of TrkA endosomes to support neuronal survival. Here, we report that actin depolymerization is essential for initiation of NGF/TrkA endosome trafficking and that a Rac1-cofilin signaling module associated with TrkA early endosomes supports their maturation to retrograde transport-competent endosomes. These actin-regulatory endosomal components are absent from NT3/TrkA endosomes, explaining the failure of NT3 to support retrograde TrkA transport and survival. The inability of NT3 to activate Rac1-GTP-cofilin signaling is likely due to the labile nature of NT3/TrkA complexes within the acidic environment of TrkA early endosomes. Thus, TrkA endosomes associate with actin-modulatory proteins to promote F-actin disassembly, enabling their maturation into transport-competent signaling endosomes. Differential control of this process explains how NGF but not NT3 supports retrograde survival of sympathetic neurons.

Treadmill exercise suppresses muscle cell apoptosis by increasing nerve growth factor levels and stimulating p-phosphatidylinositol 3-kinase activation in the soleus of diabetic rats

We investigated the effects of treadmill exercise performed regularly for 6 weeks on the levels of nerve growth factor (NGF), tyrosine kinase A and p75 receptors, phosphatidylinositol 3-kinase (PI3-K), mitogen-activated protein kinase/extracellular signal-regulated kinase (Erk) 1,2, cyclic AMP response element-binding protein (CREB), and caspase-3 in the soleus of rats with streptozotocin (STZ)-induced diabetes. Thirty-two male Sprague-Dawley rats were divided into the following four groups: (1) normal control group (NCG; n = 8), (2) normal exercise group (NEG; n = 8), (3) diabetes control group (DCG; n = 8), and (4) diabetes exercise group (DEG; n = 8). Diabetes was induced by intraperitoneal injection of STZ (55 mg/kg dissolved in 0.05 M citrate buffer, pH 4.5). Rats were subjected to treadmill exercise 5 days a week for 6 weeks. The protein level of NGF significantly increased in the NEG and DEG (p < 0.001), whereas the levels of tyrosine kinase A and p75 receptors significantly increased in the NEG (p < 0.001). The levels of t-PI3-K, p-PI3-K, and p-CREB, and the p-CREB/t-CREB ratio significantly increased in the NEG (p < 0.001, respectively). The p-PI3-K/t-PI3-K ratio significantly increased in the DEG (p < 0.001). The p-Erk1/t-Erk1 ratio significantly increased in the NEG (p < 0.001), whereas the p-Erk2/t-Erk2 ratio significantly decreased in the DCG and DEG (p < 0.001). The caspase-3 level significantly increased in the DCG compared with that in the DEG (p < 0.001). These results suggest that treadmill exercise increases NGF levels and accelerates p-PI3-K activation in order to suppress apoptotic cell death in the soleus muscle of diabetic rats.

A perspective on neuronal cell death signaling and neurodegeneration

Although neuronal cell death through apoptotic pathways represents a common feature of dysferopathies, the canonical apoptotic changes familiar from nonneuronal cells are late events. Loss of neuronal function occurs at a much early time, when synaptic-based neuronal connectivity fails. In this context, apoptotic pathways may normally serve a cleanup role, rather than a pathogenic one. Reframing the consideration of cell death in the nervous system to include the early stages of axonal degeneration provides a better understanding of the roles played by various apoptotic signaling pathways in neurodegenerative diseases. Focusing on disease-specific mechanisms that initiate the sequence that eventually leads to neuronal loss should facilitate development of therapies that preserve neuronal function and neuronal numbers.

Adrenergic stimulation mediates visceral hypersensitivity to colorectal distension following heterotypic chronic stress

BACKGROUND & AIMS

Chronic stress exacerbates or causes relapse of symptoms such as abdominal pain and cramping in patients with irritable bowel syndrome. We investigated whether chronic stress increases plasma norepinephrine and sensitizes colon-specific dorsal root ganglion (DRG) neurons by increasing expression of nerve growth factor (NGF) in the colon wall.

METHODS

Heterotypic chronic stress (HeCS) was applied to male Wistar rats and neurologic and molecular responses were analyzed. Tissues were analyzed for NGF expression.

RESULTS

HeCS significantly increased visceromoter response to colorectal distension; expression of NGF increased in colonic muscularis externa and mucosa/submucosa. Rheobase decreased, resting membrane potential was depolarized, and electrogenesis of action potentials increased in colon-specific thoracolumbar DRG neurons. Luminal administration of resiniferatoxin in distal colon, systemic administration of anti-NGF antibody, or inhibition of the NGF receptor trkA by k252a or antisense oligonucleotides in thoracolumbar DRG blocked the chronic stress-induced visceral hypersensitivity to colorectal distension. Blockade of alpha1/alpha2- and beta1/beta2-adrenergic receptors prevented the stress-induced visceral hypersensitivity and increased expression of NGF in the colon wall. HeCS did not induce any inflammatory response in the colon wall.

CONCLUSIONS

The peripheral stress mediator norepinephrine induces visceral hypersensitivity to colorectal distension in response to HeCS by increasing the expression of NGF in the colon wall, which sensitizes primary afferents in the absence of an inflammatory response.

Schizophrenia: the "BLOC" may be in the endosomes

Genome-wide association studies have identified multiple genetic polymorphisms associated with schizophrenia. These polymorphisms conform to a polygenic disease model in which multiple alleles cumulatively increase the risk of developing disease. Two genes linked to schizophrenia, DTNBP1 and MUTED, encode proteins that belong to the endosome-localized Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1). BLOC-1 plays a key role in endosomal trafficking and as such has been found to regulate cell-surface abundance of the D2 dopamine receptor, the biogenesis and fusion of synaptic vesicles, and neurite outgrowth. These functions are pertinent to both neurodevelopment and synaptic transmission, processes tightly regulated by selective cell-surface delivery of membrane proteins to and from endosomes. We propose that cellular processes, such as endosomal trafficking, act as convergence points in which multiple small effects from polygenic genetic polymorphisms accumulate to promote the development of schizophrenia.

Treadmill exercise improves cognitive function and facilitates nerve growth factor signaling by activating mitogen-activated protein kinase/extracellular signal-regulated kinase1/2 in the streptozotocin-induced diabetic rat hippocampus

This study aimed to investigate the effects of regular treadmill exercise on nerve growth factor (NGF) expression, the improvement of cognitive function in the hippocampus of diabetic rats, and to understand the molecular mechanisms through which the relevant signaling factors act. We investigated the effects of regular treadmill exercise for 6 weeks on NGF, tyrosine kinase receptor A (TrkA), p75 receptor, phosphatidylinositol 3-kinase (PI3-K), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (Erk1/2), cyclic AMP response element-binding protein (CREB), and caspase-3 protein levels; we also assessed cell survival and cognitive function. Forty male Sprague-Dawley rats were divided into four groups: (1) normal control group (NCG: n=10); (2) normal exercise group (NEG: n=10); (3) diabetes control group (DCG: n=10), and (4) diabetes exercise group (DEG: n=10). Diabetes was induced by injecting streptozotocin (STZ; 55 mg/kg dissolved in 0.05 M citrate buffer, pH 4.5, i.p.) into rats. Rats were subjected to treadmill exercise for 5 days a week over 6 weeks, and the speed of the treadmill was gradually increased. In a passive avoidance test, the retention latency in the DCG was significantly shorter than that in the DEG (P<0.05). Increased 5-bromo-2'-deoxyuridine-5'-mono-phosphate (BrdU)-labeled cells (P<0.001) and significant increases in NGF and TrkA protein levels were observed in the hippocampal dentate gyrus in the NEG and DEG (P<0.001 and P<0.01, respectively). The p75 receptor protein level significantly increased in the NEG but decreased in the DCG (P<0.001). The p-PI3-K and t-CREB protein levels significantly increased in the NEG (P<0.001 and P<0.05, respectively), whereas t-Erk1/2 significantly decreased in the DCG (P<0.01, P<0.01, respectively). p-Erk1/2 and p-CREB protein levels significantly increased in the NEG and DEG (P<0.001, P<0.001, and P<0.01, respectively). Caspase-3 protein levels significantly increased in the DCG (P<0.001). These results show that treadmill exercise improves cognitive function, increases the number of BrdU-labeled cells, and increases NGF levels, by the activation of the MAPK/Erk1/2 signaling pathway in the hippocampus of diabetic rats.

High-resolution fractionation of signaling endosomes containing different receptors

Receptor endocytosis is regulated by ligand binding, and receptors may signal after endocytosis in signaling endosomes. We hypothesized that signaling endosomes containing different types of receptors may be distinct from one another and have different physical characteristics. To test this hypothesis, we developed a high-resolution organelle fractionation method based on mass and density, optimized to resolve endosomes from other organelles. Three different types of receptors undergoing ligand-induced endocytosis were localized predominately in endosomes that were resolved from one another using this method. Endosomes containing activated receptor tyrosine kinases (RTKs), TrkA and EGFR, were similar to one another. Endosomes containing p75(NTR) (in the tumor necrosis receptor superfamily) and PAC1 (a G-protein-coupled receptor) were distinct from each other and from RTK endosomes. Receptor-specific endosomes may direct the intracellular location and duration of signal transduction pathways to dictate response to signals and determine cell fate.

Alix and ALG-2 are involved in tumor necrosis factor receptor 1-induced cell death

Alix/AIP1 regulates cell death in a way involving interactions with the calcium-binding protein ALG-2 and with proteins of ESCRT (endosomal sorting complex required for transport). Using mass spectrometry we identified caspase-8 among proteins co-immunoprecipitating with Alix in dying neurons. We next demonstrated that Alix and ALG-2 interact with pro-caspase-8 and that Alix forms a complex with the TNFalpha receptor-1 (TNF-R1), depending on its capacity to bind ESCRT proteins. Thus, Alix and ALG-2 may allow the recruitment of pro-caspase-8 onto endosomes containing TNF-R1, a step thought to be necessary for activation of the apical caspase. In line with this, expression of Alix deleted of its ALG-2-binding site (AlixDeltaALG-2) significantly reduced TNF-R1-induced cell death, without affecting endocytosis of the receptor. In a more physiological setting, we found that programmed cell death of motoneurons, which can be inhibited by AlixDeltaALG-2, is regulated by TNF-R1. Taken together, these results highlight Alix and ALG-2 as new actors of the TNF-R1 pathway.

The coming of age of axonal neurotrophin signaling endosomes

Neurons of both the central and the peripheral nervous system are critically dependent on neurotrophic signals for their survival and differentiation. The trophic signal is originated at the axonal terminals that innervate the target(s). It has been well established that the signal must be retrogradely transported back to the cell body to exert its trophic effect. Among the many forms of transmitted signals, the signaling endosome serves as a primary means to ensure that the retrograde signal is delivered to the cell body with sufficient fidelity and specificity. Recent evidence suggests that disruption of axonal transport of neurotrophin signals may contribute to neurodegenerative diseases such as Alzheimer's disease and Down syndrome. However, the identity of the endocytic vesicular carrier(s), and the mechanisms involved in retrogradely transporting the signaling complexes remain a matter of debate. In this review, we summarize current insights that are mainly based on classical hypothesis-driven research, and we emphasize the urgent needs to carry out proteomics to resolve the controversies in the field.

Overexpression of functional TrkA receptors after internalisation in human airway smooth muscle cells

Trafficking of the TrkA receptor after stimulation by NGF is of emerging importance in structural cells in the context of airway inflammatory diseases. We have recently reported the expression of functional TrkA receptors in human airway smooth muscle cells (HASMC). We have here studied the TrkA trafficking mechanisms in these cells. TrkA disappearance from the cell membrane was induced within 5 min of NGF (3pM) stimulation. Co-immunoprecipitation of clathrin-TrkA was revealed, and TrkA internalisation inhibited either by clathrin inhibitors or by siRNA inducing downregulation of endogenous clathrin. TrkA internalised receptors were totally degraded in lysosomes, with no recycling phenomenon. Newly synthesized TrkA receptors were thereafter re-expressed at the cell membrane within 10 h. TrkA re-synthesis was inhibited by blockade of clathrin-dependent internalisation, but not of TrkA receptors lysosomal degradation. Finally, we observed that NGF multiple stimulations progressively increased TrkA expression in HASMC, which was associated with an increase in NGF/TrkA-dependent proliferation. In conclusion, we show here the occurrence of clathrin-dependent TrkA internalisation and lysosomal degradation in the airway smooth muscle, followed by upregulated re-synthesis of functional TrkA receptors and increased proliferative effect in the human airway smooth muscle. This may have pathophysiological consequences in airway inflammatory diseases.

Age-dependent loss of NGF signaling in the rat basal forebrain is due to disrupted MAPK activation

The loss of nerve growth factor (NGF) and its high affinity receptor TrkA has been implicated in the loss of cholinergic tone and function in Alzheimer's disease (AD) and normal aging. We employed an animal model of aging, the aged rat, which also exhibits memory loss and NGF alterations. Basal forebrain TrkA levels increased after injection of NGF in the hippocampus within 1h in young rats, but this response was diminished in aged animals as determined by Western blot analysis. Further, NGF activated MAPK pathways without changing total ERK levels and the activation of these pathways was also diminished in aged animals. The exogenous NGF injection did not appear to activate the PI-3K pathway or alter total levels of Akt significantly. These data shed light on mechanisms of NGF signaling in the CNS, and alterations in this signaling cascade associated with age and memory loss. These findings might lead to development of novel treatment therapies for the memory loss associated with AD and other age-associated neurodegenerative diseases.

ERK-mediated NGF signaling in the rat septo-hippocampal pathway diminishes with age

RATIONALE

Degeneration of basal forebrain cholinergic neurons (BFCNs) plays an important role in aging and Alzheimer's disease (AD) pathology. This degeneration may be a result of disrupted nerve growth factor (NGF) signaling. Aged rats have memory deficits, BFCN degeneration, and disrupted NGF signaling.

OBJECTIVE

In this study we identify a rapid NGF signaling pathway in BFCNs and the second messenger system associated with that signaling. We also identify age-dependent alterations in this signaling pathway.

MATERIALS AND METHODS

After cognitive assessment using the Morris water maze, rats were given an intra-hippocampal NGF injection. Basal forebrain immunohistochemical analysis, confocal microscopy, and inhibitor studies were performed.

RESULTS

An increase in immunoreactivity for the NGF receptor TrkA was found in cell bodies of BFCNs 15 min and 1 h post-NGF injection. Immunohistochemistry studies with phospho-ERK and phospho-AKT antibodies showed that this rapid signaling occurred through MAP kinase, but not PI-3 kinase pathways. MAPK inhibitor studies attenuated the NGF-induced effects. Both TrkA and phospho-ERK (extracellular signal-regulated kinase) immunoreactivities were diminished in aged rats and phospho-ERK immunoreactivity-correlated with aged rat performance in the Morris water maze.

CONCLUSIONS

Rapid NGF signaling likely occurs in the rat CNS through the MAPK signaling pathway. This rapid signaling pathway is diminished in aged rats compared to young ones and may contribute to memory deficits observed in aged rats. As cholinergic degeneration coupled with altered levels of NGF and TrkA receptors are also seen in human aging and AD, ERK-related dysfunction may be relevant in human conditions as well.

Nerve growth factor in treatment and pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) is still unknown. In addition, this terrible neurodegenerative disease will increase exponentially over the next two decades due to longer lifespan and an aging "baby-boomer" generation. All treatments currently approved for AD have moderate efficacy in slowing the rate of cognitive decline in patients, and no efficacy in halting progression of the disease. Hence, there is an urgent need for new drug targets and delivery methods to slow or reverse the progression of AD. One molecule that has received much attention in its potential therapeutic role in AD is nerve growth factor (NGF). This review will demonstrate data from humans and animals which promote NGF as a potential therapeutic target by (1) outlining the hypothesis behind using NGF for the treatment of AD, (2) reviewing both the normal and AD altered signaling pathways and effects of NGF in the central nervous system (CNS), and (3) examining the results of NGF treatment obtained from animal models of AD and AD patients.

Dependence of neurotrophic factor activation of Trk tyrosine kinase receptors on cellular sialidase

A direct link between receptor glycosylation and activation following natural ligand interaction has not been observed. Here, we discover a membrane sialidase-controlling mechanism that depends on ligand binding to its receptor to induce enzyme activity which targets and desialylates the receptor and, consequently, causes the induction of receptor dimerization and activation. We also identify a specific sialyl alpha-2,3-linked beta-galactosyl sugar residue of TrkA tyrosine kinase receptor, which is rapidly targeted and hydrolyzed by the sialidase. Trk-expressing cells and primary cortical neurons following stimulation with specific neurotrophic growth factors express a vigorous membrane sialidase activity. Neuraminidase inhibitors, Tamiflu, BCX1812, and BCX1827, block sialidase activity induced by nerve growth factor (NGF) in TrkA-PC12 cells and by brain-derived neurotrophic factor (BDNF) in primary cortical neurons. In contrast, the neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, specific for plasma membrane ganglioside Neu3 and Neu2 sialidases has no inhibitory effect on NGF-induced pTrkA. The GM1 ganglioside specific cholera toxin subunit B applied to TrkA-PC12 cells has no inhibitory effect on NGF-induced sialidase activity. Neurite outgrowths induced by NGF-treated TrkA-PC12 and BDNF-treated PC12(nnr5) stably transfected with TrkB receptors (TrkB-nnr5) cells are significantly inhibited by Tamiflu. Our results establish a novel mode of regulation of receptor activation by its natural ligand and define a new function for cellular sialidases.

Differential responses of intrinsic and extrinsic innervation of smooth muscle cells in rat colitis

Intestinal smooth muscle cells receive neural input from axons that originate within the intestine, as well as from axons of extrinsic origin. In the inflamed intestine, altered motility may arise from damage to the axon/smooth muscle cell relationship, but the extent of change is unknown. Western blotting, histology and immunocytochemistry were used in the TNBS model of colitis in the rat to evaluate intrinsic and extrinsic axon numbers, which were then correlated with circular smooth muscle cell (CSMC) number during the time course from the acute onset of colitis to apparent recovery, at Day 35 post TNBS. Total axon profiles in the circular smooth muscle layer were reduced by nearly 50% on Day 4 of colitis, to 428 +/- 82 axons/section from 757 +/- 125 in control (n = 8-14 animals). The intrinsic innervation density (axon number per CSMC) dropped sharply by Day 2 to less than 30% of control. Although CSMC number nearly tripled during colitis, innervation density was restored to control levels by Day 6 due to a coordinated three-fold increase in axon number. The subpopulation of extrinsic axons expressing tyrosine hydroxylase showed a unique pattern during colitis, with no initial decrease in axon number, followed by axonal proliferation between Days 6 and 16 post-TNBS. We conclude that loss of intrinsic axons is an early event in colitis, and although reversed by axonal proliferation, transient denervation may promote CSMC hyperplasia as seen in earlier work in vitro. Axonal proliferation of both intrinsic and extrinsic axons is identified as a major homeostatic mechanism, with distinct patterns of damage and repair suggesting a structural basis for the altered motility seen in the inflamed colon.

Connective tissue growth factor CCN2 interacts with and activates the tyrosine kinase receptor TrkA

Connective tissue growth factor (CTGF) is implicated as a factor promoting tissue fibrosis in several disorders, including diabetic nephropathy. However, the molecular mechanism(s) by which it functions is not known. CTGF rapidly activates several intracellular signaling molecules in human mesangial cells (HMC), including extracellular signal-related kinase 1/2, Jun NH(2)-terminal kinase, protein kinase B, CaMK II, protein kinase Calpha, and protein kinase Cdelta, suggesting that it functions via a signaling receptor. Treating HMC with CTGF stimulated tyrosine phosphorylation of proteins 75 to 80 and 140 to 180 kD within 10 min, and Western blot analysis of anti-phosphotyrosine immunoprecipitates identified the neurotrophin receptor TrkA (molecular weight approximately 140 kD). Cross-linking rCTGF to cell surface proteins with 3,3'-dithiobis(sulfosuccinimidylpropionate) revealed that complexes formed with TrkA and with the general neurotrophin co-receptor p75(NTR). rCTGF stimulated phosphorylation of TrkA (tyr 490, 674/675). K252a, a known selective inhibitor of Trk, blocked this phosphorylation, CTGF-induced activation of signaling proteins, and CTGF-dependent induction of the transcription factor TGF-beta-inducible early gene in HMC. It is concluded that TrkA serves as a tyrosine kinase receptor for CTGF.

p75NTR-Immunoreactivity in the subventricular zone of adult male rats: Expression by cycling cells

While the study of in vitro regulation of neural stem cell lineage from both embryonic and adult neurospheres is greatly advanced, much less is known about factors acting in situ for neural stem cell lineage in adult brain. We reported that neurotrophin low affinity receptor p75(NTR) is present in the subventricular zone (SVZ) in adult male rats. We then characterized co-distribution of markers associated with precursor cells (nestin and PSA-NCAM) with growth factor receptors (p75(NTR), trkA, EGFr) and proliferation-associated antigens (Ki67 and BrDU-uptake) in adult male rat by immunocytochemistry and confocal laser scan microscopy. Distribution of p75(NTR)-immunoreactivity (IR) was investigated using different mono- and polyclonal antisera. p75(NTR-) is not co-distributed with glial fibrillary acid protein. It was found to be co-distributed with a small number of nestin-IR cells, whereas no coexistence with PSA-NCAM-IR was observed. Conversely, p75(NTR)-IR was present in numerous dividing cells (Ki-67-positive) and co-distributed with EGFr. In order to verify the possible association between p75(NTR) and cell death, we investigated co-distribution of p75(NTR)-IR with nuclear condensation images as visualized by Hoechst 33258 staining. While few images indicating nuclear condensation were observed in the SVZ, no coexistence with p75(NTR) was found. TrkA- and trkB-IR was not found in the SVZ. We also investigated p75(NTR) immunostaining on post-natal day 1 and day 16, because of the dramatic reduction of proliferating cells in SVZ over this time-interval. p75(NTR)-IR was not increased in the early post-natal phase. Thus, p75(NTR) seems to be associated with cell cycle regulation in SVZ in adult rat brain.

Bcl-xS induces an NGF-inhibitable cytochrome c release

Bcl-x(S), a pro-apoptotic member of the Bcl-2 protein family, is localized in the mitochondrial outer membrane and induces caspase-dependent and nerve growth factor (NGF)-inhibitable apoptosis in PC12 cells. The mechanism of action of Bcl-x(S) and how NGF inhibits this death are not fully understood. It is still unknown whether Bcl-x(S) induces mitochondrial cytochrome c release, and which apoptotic step NGF inhibits. We show that Bcl-x(S) induces cytochrome c release and caspase-3 activation in several cell types, and that in PC12 cells, these events are inhibited by NGF treatment. The survival effect of NGF was inhibited by inhibitors of protein kinase C (PKC), phosphatidylinositol-3-kinase (PI 3-kinase), and the mitogen-activated protein kinase kinase (MEK) inhibitors GF109203X, LY294002, and U0126. These findings show that cytochrome c release and caspase-3 activation participate in Bcl-x(S)-induced apoptosis, and that NGF inhibits Bcl-x(S)-induced apoptosis at the mitochondrial level via the PKC, PI 3-kinase, and MEK signaling pathways.

Alsin Is a Rab5 and Rac1 Guanine Nucleotide Exchange Factor

ALS2 is the gene mutated in a recessive juvenile form of amyotrophic lateral sclerosis (ALS2). ALS2 encodes a large protein termed alsin, which contains a number of predicted cell signaling and protein trafficking sequence motifs. To gain insight into the overall function of alsin and to begin to evaluate its role in motor neuron maintenance, we examined the subcellular localization of alsin and the biochemical activities associated with its individual subdomains. We found that the Vps9p domain of alsin has Rab5 guanine nucleotide exchange activity. In addition, alsin interacted specifically with and acted as a guanine nucleotide exchange factor for Rac1. Immunofluorescence and fractionation experiments in both fibroblasts and neurons revealed that alsin is a cytosolic protein, with a significant portion associated with small, punctate membrane structures. Many of these membrane structures also contained Rab5 or Rac1. Upon overexpression of full-length alsin, the overexpressed material was largely cytosolic, indicating that the association with membrane structures could be saturated. We also found that alsin was present in membrane ruffles and lamellipodia. These data suggest that alsin is involved in membrane transport events, potentially linking endocytic processes and actin cytoskeleton remodeling.

Orchestrating development and function: retrograde BMP signaling in the Drosophila nervous system

Recent work has shown that bone morphogenetic protein (BMP) growth factors regulate development of the larval neuromuscular junction (NMJ) of Drosophila. Intriguingly, the same BMP growth factors also influence the expression of circulating hormones that modulate the physiological properties of NMJs. Together, the results suggest that retrograde growth factor signaling by BMPs integrates neuromuscular development and function at both local and global levels in the animal.

Spinal shock revisited: a four-phase model

Spinal shock has been of interest to clinicians for over two centuries. Advances in our understanding of both the neurophysiology of the spinal cord and neuroplasticity following spinal cord injury have provided us with additional insight into the phenomena of spinal shock. In this review, we provide a historical background followed by a description of a novel four-phase model for understanding and describing spinal shock. Clinical implications of the model are discussed as well.

Regulation of growth cone actin filaments by guidance cues

The motile behaviors of growth cones at the ends of elongating axons determine pathways of axonal connections in developing nervous systems. Growth cones express receptors for molecular guidance cues in the local environment, and receptor-guidance cue binding initiates cytoplasmic signaling that regulates the cytoskeleton to control growth cone advance, turning, and branching behaviors. The dynamic actin filaments of growth cones are frequently targets of this regulatory signaling. Rho GTPases are key mediators of signaling by guidance cues, although much remains to be learned about how growth cone responses are orchestrated by Rho GTPase signaling to change the dynamics of polymerization, transport, and disassembly of actin filaments. Binding of neurotrophins to Trk and p75 receptors on growth cones triggers changes in actin filament dynamics to regulate several aspects of growth cone behaviors. Activation of Trk receptors mediates local accumulation of actin filaments, while neurotrophin binding to p75 triggers local decrease in RhoA signaling that promotes lengthening of filopodia. Semaphorin IIIA and ephrin-A2 are guidance cues that trigger avoidance or repulsion of certain growth cones, and in vitro responses to these proteins include growth cone collapse. Dynamic changes in the activities of Rho GTPases appear to mediate responses to these cues, although it remains unclear what the changes are in actin filament distribution and dynamic reorganization that result in growth cone collapse. Growth cones in vivo simultaneously encounter positive and negative guidance cues, and thus, growth cone behaviors during axonal pathfinding reflect the complex integration of multiple signaling activities.

The Role of BDNF in Epilepsy and Other Diseases of the Mature Nervous System

The neurotrophin brain-derived neurotrophic factor (BDNF) is ubiquitous in the central nervous system (CNS) throughout life. In addition to trophic effects on target neurons, BDNF appears to be part of a general mechanism for activity-dependent modification of synapses in the developing and adult nervous system. Thus, diseases of abnormal trophic support (such as neurodegenerative diseases) and diseases of abnormal excitability (such as epilepsy and central pain sensitization) can be related in some cases to abnormal BDNF signaling. For example, various studies have shown that BDNF is upregulated in areas implicated in epileptogenesis, and interference with BDNF signal transduction inhibits the development of the epileptic state. Further study of the cellular and molecular mechanisms by which BDNF influences cell survival and excitability will likely provide novel concepts and targets for the treatment of diverse CNS diseases.

Stem cells and nervous tissue repair: from in vitro to in vivo

Recent development in stem cell biology has indicated a new possible approach for the treatment of neurological diseases. However, in spite of tremendous hope generated, we are still on the way to understand if the use of stem cells to repair mature brain and spinal cord is a reliable possibility. In particular, we know very little on the in situ regulation of adult neural stem, and this also negatively impact on cell transplant possibilities. In this chapter we will discuss issues concerning the role and function of stem cells in neurological diseases, with regard to the impact of features of degenerating neurons and glial cells on in situ stem cells. Stem cell location and biology in the adult brain, brain host reaction to transplantation, neural stem cell reaction to experimental injuries and possibilities for exogenous regulation are the main topics discussed.

Inhibition of rat sympathetic neuron apoptosis by ceramide. Role of p75NTR in ceramide generation

C6-ceramide protects sympathetic neurons from apoptosis caused by nerve growth factor (NGF) deprivation. Here, we report for the first time that ceramide generated "de novo" is also anti-apoptotic. Moreover, C6-ceramide is converted to long-chain ceramides in a process inhibited by fumonisin B1. The anti-apoptotic effect of C6-ceramide is due to the short analogue as to the long-chain ceramides. C6-ceramide shares mechanisms of action with NGF. C6-ceramide induces TrkA phosphorylation and selective activation of the phosphatidyl inositol 3-kinase (PI3-kinase)/Akt pathway but not the MAPK/ERK pathway. Importantly, the PI3-kinase inhibitor LY294002 abolishes the pro-survival effect of C6-ceramide. We identified a novel way to activate retrograde-mediated neuronal survival in the absence of NGF. Using compartmented cultures we show that addition of C6-ceramide exclusively to distal axons is sufficient to abort nuclear apoptosis. Our system offers a very unique alternative to understand the molecular bases of retrograde signaling in the absence of retrograde transport of neurotrophins. In search for a natural ligand that leads to ceramide generation we examined the activation of the sphingomyelin (SM) cycle downstream the p75 neurotrophin receptor (p75NTR). We found that in sympathetic neurons, selective activation of p75NTR by brain-derived neurotrophin factor or NGF plus K252a induces elevation of ceramide that correlates with SM hydrolysis. However, p75NTR activation does not generate sufficient ceramide to block apoptosis probably due to the rapid decrease in p75NTR expression that occurs upon NGF withdrawal.

EphB-ephrinB bi-directional endocytosis terminates adhesion allowing contact mediated repulsion

Eph receptors and their membrane-associated ephrin ligands mediate cell-cell repulsion to guide migrating cells and axons. Repulsion requires that the ligand-receptor complex be removed from the cell surface, for example by proteolytic processing of the ephrin ectodomain. Here we show that cell contact-induced EphB-ephrinB complexes are rapidly endocytosed during the retraction of cells and neuronal growth cones. Endocytosis occurs in a bi-directional manner that comprises of full-length receptor and ligand complexes. Endocytosis is sufficient to promote cell detachment and seems necessary for axon withdrawal during growth cone collapse. Here, we show a mechanism for the termination of adhesion and the promotion of cell repulsion after intercellular (trans) interaction between two transmembrane proteins.