TrkA glycosylation regulates receptor localization and activity

TrkAIII promotes microtubule nucleation and assembly at the centrosome in SH-SY5Y neuroblastoma cells, contributing to an undifferentiated anaplastic phenotype

The alternative TrkAIII splice variant is expressed by advanced stage human neuroblastomas (NBs) and exhibits oncogenic activity in NB models. In the present study, employing stable transfected cell lines and assays of indirect immunofluorescence, immunoprecipitation, Western blotting, microtubule regrowth, tubulin kinase, and tubulin polymerisation, we report that TrkAIII binds α -tubulin and promotes MT nucleation and assembly at the centrosome. This effect depends upon spontaneous TrkAIII activity, TrkAIII localisation to the centrosome and pericentrosomal area, and the capacity of TrkAIII to bind, phosphorylate, and polymerise tubulin. We propose that this novel role for TrkAIII contributes to MT involvement in the promotion and maintenance of an undifferentiated anaplastic NB cell morphology by restricting and augmenting MT nucleation and assembly at the centrosomal MTOC.

Neuroprotective effects of reactive oxygen species mediated by BDNF-independent activation of TrkB

Reactive oxygen species (ROS) have diverse biological consequences in the mammalian CNS, but the molecular targets mediating these pleiotropic effects are incompletely understood. Like ROS, the neurotrophin receptor, TrkB receptor tyrosine kinase, has diverse effects in the developing and mature mammalian brain. Our discovery that zinc can transactivate TrkB, together with the finding that ROS can trigger zinc release from cytosolic zinc binding proteins, led us to hypothesize that ROS can transactivate TrkB in CNS neurons by a zinc-dependent mechanism. We found that both exogenous H(2)O(2) and endogenous ROS activate TrkB signaling by a Src family kinase-dependent but brain-derived neurotrophic factor-independent mechanism in cultured rat cortical neurons. Exogenous H(2)O(2) enhances cytosolic zinc content in a metallothionein-3 (MT-3)-requiring manner. Both exogenous H(2)O(2) and endogenous ROS mediated transactivation of TrkB requires intracellular zinc and MT-3. The ROS-triggered transactivation of TrkB exerts neuroprotective effects, because inhibition of TrkB kinase activity or uncoupling Shc signaling from TrkB exacerbates neuronal cell death induced by H(2)O(2). Thus, we propose a molecular signaling event whereby ROS induce release of zinc from cytosolic MT-3, the increased cytosolic zinc transactivates TrkB, and the enhanced Shc signaling downstream from TrkB promotes prosurvival effects. We suggest that such neuroprotective effects mediated by ROS are operative in diverse acute and chronic neurological disorders.

N-glycans of SREC-I (scavenger receptor expressed by endothelial cells): essential role for ligand binding, trafficking and stability

Scavenger receptor expressed by endothelial cells (SREC-I) mediates the endocytosis of chemically modified lipoproteins such as acetylated low-density lipoprotein (Ac-LDL) and oxidized LDL and is implicated in atherogenesis. We produced recombinant SREC-I in Chinese hamster ovary-K1 cells and identified three potential glycosylation sites, Asn(289), Asn(382) and Asn(393), which were all glycosylated. To determine the function of N-glycans in SREC-I, we characterized SREC-I mutant proteins by intracellular distribution and the cellular incorporation rate of Ac-LDL. N382Q/N393Q and N289Q/N382Q/N393Q were sequestered in the endoplasmic reticulum, resulting in a severe reduction in the cellular incorporation of Ac-LDL. N382Q showed a normal cell surface residency and an enhanced affinity for Ac-LDL, resulting in an elevated Ac-LDL cellular incorporation. These results indicate that the N-glycan of Asn(393) regulates the intracellular sorting of SREC-I and that the N-glycan of Asn(382) controls ligand-binding affinity. Furthermore, we detected an enhanced trypsin sensitivity of the N289Q. Glycan structure analyses revealed that the core-fucosylated bi-antennary is the common major structure at all glycosylation sites. In addition, tri- and tetra-antennary were detected as minor constituents at Asn(289). A bisecting GlcNAc was also detected at Asn(382) and Asn(393). Structural analyses and homology modeling of SREC-I suggest that the N-glycan bearing a β1-6GlcNAc branch at Asn(289) protects from proteinase attack and thus confers a higher stability on SREC-I. These data indicate that Asn(289)-, Asn(382)- and Asn(393)-linked N-glycans of SREC-I have distinct functions in regulating proteolytic resistance, ligand-binding affinity and subcellular localization, all of which might be involved in the development of atherogenesis.

Signaling endosomes and growth cone motility in axon regeneration

During development and regeneration, growth cones guide neurites to their targets by altering their motility in response to extracellular guidance cues. One class of cues critical to nervous system development is the neurotrophins. Neurotrophin binding to their cognate receptors stimulates their endocytosis into signaling endosomes. Current data indicate that the spatiotemporal localization of signaling endosomes can direct diverse processes regulating cell motility, including membrane trafficking, cytoskeletal remodeling, adhesion dynamics, and local translation. Recent experiments manipulating signaling endosome localization in neuronal growth cones support these views and place the neurotrophin signaling endosome in a central role regulating growth cone motility during axon growth and regeneration.

Neurotrophin receptor TrkC is an entry receptor for Trypanosoma cruzi in neural, glial, and epithelial cells

Trypanosoma cruzi, the agent of Chagas' disease, infects a variety of mammalian cells in a process that includes multiple cycles of intracellular division and differentiation starting with host receptor recognition by a parasite ligand(s). Earlier work in our laboratory showed that the neurotrophin-3 (NT-3) receptor TrkC is activated by T. cruzi surface trans-sialidase, also known as parasite-derived neurotrophic factor (PDNF). However, it has remained unclear whether TrkC is used by T. cruzi to enter host cells. Here, we show that a neuronal cell line (PC12-NNR5) relatively resistant to T. cruzi became highly susceptible to infection when overexpressing human TrkC but not human TrkB. Furthermore, trkC transfection conferred an ∼3.0-fold intracellular growth advantage. Sialylation-deficient Chinese hamster ovarian (CHO) epithelial cell lines Lec1 and Lec2 also became much more permissive to T. cruzi after transfection with the trkC gene. Additionally, NT-3 specifically blocked T. cruzi infection of the TrkC-NNR5 transfectants and of naturally permissive TrkC-bearing Schwann cells and astrocytes, as did recombinant PDNF. Two specific inhibitors of Trk autophosphorylation (K252a and AG879) and inhibitors of Trk-induced MAPK/Erk (U0126) and Akt kinase (LY294002) signaling, but not an inhibitor of insulin-like growth factor 1 receptor, abrogated TrkC-mediated cell invasion. Antibody to TrkC blocked T. cruzi infection of the TrkC-NNR5 transfectants and of cells that naturally express TrkC. The TrkC antibody also significantly and specifically reduced cutaneous infection in a mouse model of acute Chagas' disease. TrkC is ubiquitously expressed in the peripheral and central nervous systems, and in nonneural cells infected by T. cruzi, including cardiac and gastrointestinal muscle cells. Thus, TrkC is implicated as a functional PDNF receptor in cell entry, independently of sialic acid recognition, mediating broad T. cruzi infection both in vitro and in vivo.

Antidepressant drugs transactivate TrkB neurotrophin receptors in the adult rodent brain independently of BDNF and monoamine transporter blockade

Background

Antidepressant drugs (ADs) have been shown to activate BDNF (brain-derived neurotrophic factor) receptor TrkB in the rodent brain but the mechanism underlying this phenomenon remains unclear. ADs act as monoamine reuptake inhibitors and after prolonged treatments regulate brain bdnf mRNA levels indicating that monoamine-BDNF signaling regulate AD-induced TrkB activation in vivo. However, recent findings demonstrate that Trk receptors can be transactivated independently of their neurotrophin ligands.

Methodology

In this study we examined the role of BDNF, TrkB kinase activity and monoamine reuptake in the AD-induced TrkB activation in vivo and in vitro by employing several transgenic mouse models, cultured neurons and TrkB-expressing cell lines.

Principal Findings

Using a chemical-genetic TrkB^F616A mutant and TrkB overexpressing mice, we demonstrate that ADs specifically activate both the maturely and immaturely glycosylated forms of TrkB receptors in the brain in a TrkB kinase dependent manner. However, the tricyclic AD imipramine readily induced the phosphorylation of TrkB receptors in conditional bdnf^−/− knock-out mice (132.4±8.5% of control; P = 0.01), indicating that BDNF is not required for the TrkB activation. Moreover, using serotonin transporter (SERT) deficient mice and chemical lesions of monoaminergic neurons we show that neither a functional SERT nor monoamines are required for the TrkB phosphorylation response induced by the serotonin selective reuptake inhibitors fluoxetine or citalopram, or norepinephrine selective reuptake inhibitor reboxetine. However, neither ADs nor monoamine transmitters activated TrkB in cultured neurons or cell lines expressing TrkB receptors, arguing that ADs do not directly bind to TrkB.

Conclusions

The present findings suggest that ADs transactivate brain TrkB receptors independently of BDNF and monoamine reuptake blockade and emphasize the need of an intact tissue context for the ability of ADs to induce TrkB activity in brain.

Functional characterization of human cancer-derived TRKB mutations

Cancer originates from cells that have acquired mutations in genes critical for controlling cell proliferation, survival and differentiation. Often, tumors continue to depend on these so-called driver mutations, providing the rationale for targeted anticancer therapies. To date, large-scale sequencing analyses have revealed hundreds of mutations in human tumors. However, without their functional validation it remains unclear which mutations correspond to driver, or rather bystander, mutations and, therefore, whether the mutated gene represents a target for therapeutic intervention. In human colorectal tumors, the neurotrophic receptor TRKB has been found mutated on two different sites in its kinase domain (TRKB^T695I and TRKB^D751N). Another site, in the extracellular part of TRKB, is mutated in a human lung adenocarcinoma cell line (TRKB^L138F). Lastly, our own analysis has identified one additional TRKB point mutation proximal to the kinase domain (TRKB^P507L) in a human melanoma cell line. The functional consequences of all these point mutations, however, have so far remained elusive. Previously, we have shown that TRKB is a potent suppressor of anoikis and that TRKB-expressing cells form highly invasive and metastatic tumors in nude mice. To assess the functional consequences of these four TRKB mutations, we determined their potential to suppress anoikis and to form tumors in nude mice. Unexpectedly, both colon cancer-derived mutants, TRKB^T695I and TRKB^D751N, displayed reduced activity compared to that of wild-type TRKB. Consistently, upon stimulation with the TRKB ligand BDNF, these mutants were impaired in activating TRKB and its downstream effectors AKT and ERK. The two mutants derived from human tumor cell lines (TRKB^L138F and TRKB^P507L) were functionally indistinguishable from wild-type TRKB in both in-vitro and in-vivo assays. In conclusion, we fail to detect any gain-of-function of four cancer-derived TRKB point mutations.

Spinal 5-HT7 receptor activation induces long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of serotonin-dependent respiratory plasticity known as phrenic long term facilitation (pLTF). Episodic spinal serotonin-2 (5-HT2) receptor activation on or near phrenic motor neurons is necessary for pLTF. A hallmark of pLTF is the requirement for serotonin-dependent synthesis of brain-derived neurotrophic factor (BDNF), and activation of its high affinity receptor, TrkB. Activation of spinal Gs protein-coupled adenosine 2A receptors (GsPCRs) elicits a unique form of long-lasting phrenic motor facilitation (PMF), but via unique mechanisms (BDNF independent TrkB trans-activation).We hypothesized that other GsPCRs elicit PMF, specifically serotonin-7 (5-HT7) receptors, which are expressed in phrenic motor neurons. Cervical spinal (C4) injections of a selective 5-HT7 receptor agonist, AS-19 (10 μM, 5 μl; 3 × 5 min), in anaesthetized, vagotomized and ventilated male Sprague-Dawley rats elicited long-lasting PMF (>120 min), an effect prevented by pretreatment with a 5-HT7 receptor antagonist (SB 269970; 5mM, 7 μl).GsPCR activation 'trans-activates'TrkB by increasing synthesis of an immature TrkB isoform. Spinal injection of a TrkB inhibitor (k252a) and siRNAs that prevent TrkB (but not BDNF) mRNA translation both blocked 5-HT7 agonist-induced PMF, confirming a requirement for TrkB synthesis and activity. k252a affected late PMF (≥ 90 min) only. Spinal inhibition of the PI3K/AKT pathway blocked 5-HT7 agonist-induced PMF, whereas MEK/ERK inhibition delayed, but did not block, PMF. An understanding of signalling mechanisms giving rise to PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders associated with respiratory insufficiency, such as spinal injury and motor neuron disease.

Mutual effects of caveolin and nerve growth factor signaling in pig oligodendrocytes

Signaling of growth factors may depend on the recruitment of their receptors to specialized microdomains. Previous reports on PC12 cells indicated an interaction of raft-organized caveolin and TrkA signaling. Because porcine oligodendrocytes (OLs) respond to nerve growth factor (NGF), we were interested to know whether caveolin also plays a role in oligodendroglial NGF/TrkA signaling. OLs expressed caveolin at the plasma membrane but also intracellularly. This was partially organized in the classically Omega-shaped invaginations, which may represent caveolae. We could show that caveolin and TrkA colocalize by using a discontinuous sucrose gradient (Song et al. [1996] J. Biol. Chem. 271:9690-9697), MACS technology, and immunoprecipitation. However, differential extraction of caveolin and TrkA with Triton X-100 at 4 degrees C indicated that caveolin and TrkA are probably not exclusively present in detergent-resistant, caveolin-containing rafts (CCRs). NGF treatment of OLs up-regulated the expression of caveolin-1 (cav-1) and stimulated tyrosine-14 phosphorylation of cav-1. Furthermore, OLs were transfected with cav-1-specific small interfering RNA (siRNA). A knockdown of cav-1 resulted in a reduced activation of downstream components of the NGF signaling cascade, such as p21Ras and mitogen-activated protein kinase (MAPK) after NGF exposure of OLs. Subsequently, increased oligodendroglial process formation via NGF was impaired. The present study indicates that CCRs/caveolin could play a modulating role during oligodendroglial differentiation and regeneration.

Dependence of pathogen molecule-induced toll-like receptor activation and cell function on Neu1 sialidase

The signaling pathways of mammalian Toll-like receptors (TLR) are well characterized, but the initial molecular mechanisms activated following ligand interactions with the receptors remain poorly defined. Here, we show a membrane controlling mechanism that is initiated by ligand binding to TLR-2, -3 and-4 to induce Neu1 sialidase activity within minutes in live primary bone marrow (BM) macrophage cells and macrophage and dendritic cell lines. Central to this process is that Neu1 and not Neu2,-3 and-4 forms a complex with TLR-2,-3 and-4 on the cell surface of naïve macrophage cells. Neuraminidase inhibitors BCX1827, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA), zanamivir and oseltamivir carboxylate have a limited significant inhibition of the LPS-induced sialidase activity in live BMC-2 macrophage cells but Tamiflu (oseltamivir phosphate) completely blocks this activity. Tamiflu inhibits LPS-induced sialidase activity in live BMC-2 cells with an IC(50) of 1.2 microM compared to an IC(50) of 1015 microM for its hydrolytic metabolite oseltamivir carboxylate. Tamiflu blockage of LPS-induced Neu1 sialidase activity is not affected in BMC-2 cells pretreated with anticarboxylesterase agent clopidogrel. Endotoxin LPS binding to TLR4 induces Neu1 with subsequent activation of NFkappaB and the production of nitric oxide and pro-inflammatory IL-6 and TNFalpha cytokines in primary and macrophage cell lines. Hypomorphic cathepsin A mice with a secondary Neu1 deficiency respond poorly to LPS-induced pro-inflammatory cytokines compared to the wild-type or hypomorphic cathepsin A with normal Neu1 mice. Our findings establish an unprecedented mechanism for pathogen molecule-induced TLR activation and cell function, which is critically dependent on Neu1 sialidase activity associated with TLR ligand treated live primary macrophage cells and macrophage and dendritic cell lines.

An agonistic mAb directed to the TrkC receptor juxtamembrane region defines a trophic hot spot and interactions with p75 coreceptors

The D5 domain of TrkC receptors is a docking site for Neurotrophin-3 (NT-3), but other domains may be relevant for function or harmonizing signals with p75(NTR) coreceptors. We report a monoclonal antibody (mAb) 2B7 targeting the juxtamembrane domain of TrkC. mAb 2B7 binds to murine and human TrkC receptors and is a functional agonist that affords activation of TrkC, AKT, and MAPK. These signals result in cell survival but not in cellular differentiation. Monomeric 2B7 Fabs also affords cell survival. Binding of 2B7 mAb and 2B7 Fabs to TrkC are blocked by NT-3 in a dose-dependent manner but not by pro-NT-3. Expression of p75(NTR) coreceptors on the cell surface block the binding and function of mAb 2B7, whereas NT-3 binding and function are enhanced. mAb 2B7 defines a previously unknown neurotrophin receptor functional hot spot; that exclusively generates survival signals; that can be activated by non-dimeric ligands; and potentially unmasks a site for p75-TrkC interactions.

The neuroblastoma tumour-suppressor TrkAI and its oncogenic alternative TrkAIII splice variant exhibit geldanamycin-sensitive interactions with Hsp90 in human neuroblastoma cells

Hsp90 chaperones stabilize many tyrosine kinases including several oncogenes, which are inhibited or induced to degrade by the Hsp90 inhibitor geldanamycin (GA). As a consequence, GA has been developed for future chemotherapeutic use in several tumour types including neuroblastoma (NB). Alternative splicing of the neurotrophin receptor tyrosine kinase TrkA may have a pivotal function in regulating NB behaviour, with reports suggesting that tumour-suppressing signals from TrkA may be converted to oncogenic signals by stress-regulated alternative TrkAIII splicing. Within this context, it is important to know whether Hsp90 interacts with TrkA variants in NB cells and how GA influences this. Here, we report that both TrkAI and TrkAIII are Hsp90 clients in human NB cells. TrkAI exhibits GA-sensitive interaction with Hsp90 required for receptor endoplasmic reticulum export, maturation, cell surface stabilization and ligand-mediated activation, whereas TrkAIII exhibits GA-sensitive interactions with Hsp90 required for spontaneous activity and to a lesser extent stability. We show that GA inhibits proliferation and induces apoptosis of TrkAI expressing NB cells, whereas TrkAIII reduces the sensitivity of NB cells to GA-induced elimination. Our data suggest that GA-sensitive interactions with Hsp90 are critical for both TrkAI tumour suppressor and TrkAIII oncogenic function in NB and that TrkAIII expression exerts a negative impact on GA-induced NB cell eradication, which can be counteracted by a novel TrkAIII-specific peptide nucleic acid inhibitor.

The alternative TrkAIII splice variant targets the centrosome and promotes genetic instability

The hypoxia-regulated alternative TrkAIII splice variant expressed by human neuroblastomas exhibits oncogenic potential, driven by in-frame exon 6 and 7 alternative splicing, leading to omission of the receptor extracellular immunoglobulin C(1) domain and several N-glycosylation sites. Here, we show that the TrkAIII oncogene promotes genetic instability by interacting with and exhibiting catalytic activity at the centrosome. This function depends upon intracellular TrkAIII accumulation and spontaneous interphase-restricted activation, in cytoplasmic tyrosine kinase (tk) domain orientation, predominantly within structures that closely associate with the fully assembled endoplasmic reticulum intermediate compartment and Golgi network. This facilitates TrkAIII tk-mediated binding of gamma-tubulin, which is regulated by endogenous protein tyrosine phosphatases and geldanamycin-sensitive interaction with Hsp90, paving the way for TrkAIII recruitment to the centrosome. At the centrosome, TrkAIII differentially phosphorylates several centrosome-associated components, increases centrosome interaction with polo kinase 4, and decreases centrosome interaction with separase, the net results of which are centrosome amplification and increased genetic instability. The data characterize TrkAIII as a novel internal membrane-associated centrosome kinase, unveiling an important alternative mechanism to "classical" cell surface oncogenic receptor tk signaling through which stress-regulated alternative TrkAIII splicing influences the oncogenic process.

Receptor tyrosine kinases and respiratory motor plasticity

Protein kinases are a family of enzymes that transfer a phosphate group from adenosine tri-phosphate to an amino acid residue on a protein. The receptor tyrosine kinases (RTKs) are expressed on the outer cell membrane, bind extracellular protein ligands, and phosphorylate tyrosine residues on other proteins-essentially permitting communication between cells. Such activity regulates multiple aspects of cellular physiology including cell growth and differentiation, adhesion, motility, cell death, and morphological and synaptic plasticity. This review will focus on the role of RTKs in respiratory motor plasticity, with particular emphasis on long-term changes in respiratory motoneuron function. Reflecting the predominant literature, specific attention will be devoted to the role of tropomyosin-related kinase type B (TrkB) activation on phrenic motoneuron activity. However, many RTKs share similar patterns of expression and mechanisms of ligand-induced activation and downstream signaling. Thus, a perspective based on TrkB-induced phrenic motor plasticity may provide insight into the potential roles of other RTKs in the neural control of breathing. Finally, understanding how different RTKs affect respiratory motor output in the long-term may provide future avenues for pharmacological development with the goal of increasing respiratory motor output in disorders such as obstructive sleep apnea and after spinal cord injury. This is best illustrated in recent studies where we have used small, highly diffusible molecules to transactivate TrkB receptors near phrenic motoneurons to improve breathing after cervical spinal cord injury.

NGF-induced Tyro3 and Axl function as survival factors for differentiating PC12 cells

Tyro3 and Axl, two members of the TAM family of receptor tyrosine kinases, play important regulatory roles in a variety of tissues, including the central nervous, reproductive, immune, and vascular systems. We have found that expression of Tyro3 and Axl on PC12 cells is upregulated by nerve growth factor (NGF). PI3K inhibitor LY294002, which is known to inhibit NGF-induced PC12 differentiation, blocked up-regulation of Tyro3 and Axl. NGF regulates Tyro3 and Axl expression by activating their transcription. Both Tyro3 and Axl were associated with the NGF receptor, and protected PC12 cells from stress or toxin-induced cell death. Gas6, a common ligand for both Tyro3 and Axl, was able to replace NGF to support PC12 growth in serum-free medium, and to prevent cell death following serum deprivation. In summary, both Tyro3 and Axl receptors are upregulated by NGF on the differentiating PC12, where they collaborate with TrkA to support neuronal differentiation and survival.

Effects of optic nerve injury, glaucoma, and neuroprotection on the survival, structure, and function of ganglion cells in the mammalian retina

Glaucoma is an optic neuropathy that originates with pressure-induced damage to the optic nerve. This results in the retrograde degeneration of ganglion cells in the retina, and a progressive loss of vision. Over the past several years, a number of studies have described the structural and functional changes that characterize ganglion cell degeneration in the glaucomatous eye, and following optic nerve injury. In addition, a variety of different strategies for providing neuroprotection to the injured retina have been proposed. Many of these are based on the use of brain-derived neurotrophic factor (BDNF), a particularly potent neuroprotectant in the mammalian eye and the basis of our research in this area. Of particular importance is the fact that BDNF not only promotes ganglion cell survival following damage to the optic nerve, but also helps to preserve the structural integrity of the surviving neurons, which in turn results in enhanced visual function. The studies presented here describe these attributes, and serve as the foundation for ongoing work that suggests a need to think beyond the eye in the development of future treatment strategies.

Spinal adenosine A2a receptor activation elicits long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of spinal, brain-derived neurotrophic factor (BDNF)-dependent respiratory plasticity known as phrenic long-term facilitation. Ligands that activate G(s)-protein-coupled receptors, such as the adenosine 2a receptor, mimic the effects of neurotrophins in vitro by transactivating their high-affinity receptor tyrosine kinases, the Trk receptors. Thus, we hypothesized that A2a receptor agonists would elicit phrenic long-term facilitation by mimicking the effects of BDNF on TrkB receptors. Here we demonstrate that spinal A2a receptor agonists transactivate TrkB receptors in the rat cervical spinal cord near phrenic motoneurons, thus inducing long-lasting (hours) phrenic motor facilitation. A2a receptor activation increased phosphorylation and new synthesis of an immature TrkB protein, induced TrkB signaling through Akt, and strengthened synaptic pathways to phrenic motoneurons. RNA interference targeting TrkB mRNA demonstrated that new TrkB protein synthesis is necessary for A2a-induced phrenic motor facilitation. A2a receptor activation also increased breathing in unanesthetized rats, and improved breathing in rats with cervical spinal injuries. Thus, small, highly permeable drugs (such as adenosine receptor agonists) that transactivate TrkB receptors may provide an effective therapeutic strategy in the treatment of patients with ventilatory control disorders, such as obstructive sleep apnea, or respiratory insufficiency after spinal injury or during neurodegenerative diseases.

N-acetylglucosaminyltransferase V modifies TrKA protein, regulates the receptor function

1. N-Acetylglucosaminyltransferases V (GnT-V/Mgat5) play a pivotal role in the processing of N-linked glycoproteins in the Golgi apparatus. The aim of the present study is to investigate whether the N-acetylglucosaminyltransferase V is able to modify TrKA, the high-affinity tyrosine kinase-type receptor for NGF, and thereby to regulate the receptor function. 2. Plasmids of the pcDNA3/GnT-V and pcDNA3 were transfected into PC12 cells. Expression of GnT-V protein was detected by Western blot. TrKA protein was examined by immunoprecipitation. Endocytosis of TrKA was investigated by the method of receptor internalization. 3. We report here that over-expression GnT-V directly modifies TrKA protein, accompanied by marked enhancement of axon outgrowth in rat pheochromocytoma cells (PC12) elicited by a low dose of NGF that alone is insufficient to induce neuronal differentiation. Further study indicated that modification of TrKA glycoprotein could directly enhance NGF-activated autophosphorylation of immunoprecipitated TrKA in vitro. To further elucidate the mechanism, we study the different time point of endocytosis of TrKA receptor. The results show that TrKA of GnT-V gene-transfected PC12 Cells delayed their removal by constitutive endocytosis as compared to the mock cells, suggesting high expression of GnT-V may affect their receptor TrKA endocytosis. 4. These results strongly suggest that N-acetylglucosaminyltransferase V functioning as a specific endogenous role of NGF receptor function, which appear to be due, at least in part, to the promotion of differentiation. This work is an important step toward intriguing innovative therapeutic strategies targeting glycosyltransferase.

Adenosine receptor A2A-R contributes to motoneuron survival by transactivating the tyrosine kinase receptor TrkB

Neurotrophins are potent survival factors for developing and injured neurons. However, they are not being used to treat neurodegenerative diseases because of difficulties in administration and numerous side effects that have been encountered in previous clinical trials. Their biological activities use Trk (tropomyosin-related kinase) transmembrane tyrosine kinases. Therefore, one alternative approach is to use transactivation pathways such as adenosine 2A receptor agonists, which can activate Trk receptor signaling independent of neurotrophin binding. However, the relevance in vivo and applicability of these transactivation events during neurodegenerative and injury conditions have never been extensively studied. Here we demonstrate that motoneuron survival after facial nerve lesioning is significantly enhanced by transactivation of Trk receptor tyrosine kinases by adenosine agonists. Moreover, survival of motoneurons directly required the activation of the BDNF receptor TrkB and an increase in Akt (AKT8 virus oncogene cellular homolog) activity. The ability of small molecules to activate a trophic response by using Trk signaling provides a unique mechanism to promote survival signals in motoneurons and suggests new strategies for using transactivation in neurodegenerative diseases.

Primary human osteoblasts and bone cancer cells as models to study glycodynamics in bone

Bone cells produce many glycoproteins potentially involved in the maintenance of healthy bone tissues. Two cytokines produced in inflamed joints, tumor necrosis factor (TNF)alpha and transforming growth factor (TGF)beta, have previously been shown to alter cellular glycosylation which may potentially affect the expression and function of glycoproteins. In order to evaluate models to study the glycodynamics of bone cells, we examined primary human osteoblastic cells from osteoarthritis patients, and compared these to human osteosarcoma cells MG63 and SJSA-1. We showed here for the first time that all of the human osteoblastic cells actively synthesize complex N- and O-glycan chains of bone cell glycoproteins, with quantitative differences between cell types. TNFalpha-induced apoptosis or TGFbeta-induced cell differentiation and proliferation had significant effects on both cell surface carbohydrates and glycosyltransferase activities of osteoblasts and osteosarcoma cells. The results indicate that cultured human bone-derived osteoblastic cells are good models to examine the glycodynamics of osteoblasts under conditions of cell growth and cell death. The changes induced by cytokines can result in altered cell surface functions which may be of importance in osteoarthritis, osteoporosis and other bone diseases.

Remarkable leukemogenic potency and quality of a constitutively active neurotrophin receptor, ΔTrkA

Neurotrophins and their receptors play a key role in neurogenesis and survival. However, we and others have recently obtained evidence for a potential involvement of this receptor system in leukemia. To investigate mechanisms underlying the leukemogenic potential of activated neurotrophin receptor signaling, we analyzed in vivo leukemogenesis mediated by deltaTrkA, a mutant of TRKA (tropomyosin-related kinase A) isolated from a patient with acute myeloid leukemia (AML). Retroviral expression of deltaTrkA in myeloid 32D cells induced AML in syngeneic C3H/Hej mice (n=11/11, latency approximately 4 weeks). C57Bl/6J mice transplanted with deltaTrkA-transduced primary lineage negative (Lin-) bone marrow cells died of a transient polyclonal AML (n=7/15, latency of <12 days). Serial transplantation of AML cells did not re-induce this disease but rather acute lymphoblastic leukemia (ALL, latency >78 days). All primary recipients surviving the early AML developed clonal ALL or myeloid leukemia (latency >72 days) that required additional genetic lesions. PI3K and mTOR-raptor were identified as the crucial mediators of leukemic transformation, whereas STAT and MAP kinase signaling pathways were not activated. Thus, our findings reveal potent and unique transforming properties of altered neurotrophin receptor signaling in leukemogenesis, and encourage further analyses of neurotrophin receptors and downstream signaling events in hematological malignancies.

Critical role for TrkB kinase function in anoikis suppression, tumorigenesis, and metastasis

Anoikis, or cell death induced by cell detachment, provides protection against the metastatic spread of tumor cells. We have previously shown that the neurotrophic receptor tyrosine kinase TrkB suppresses anoikis in rat intestinal epithelial cells and renders them highly tumorigenic and metastatic. Because TrkB is overexpressed in several aggressive human cancers, first attempts are being made to target TrkB in cancer therapy. However, the mechanisms underlying TrkB-mediated anoikis suppression, tumorigenesis, and metastasis still remain largely elusive. Although, to date, most attempts to neutralize TrkB in tumors aim to inactivate its kinase activity, it is unclear whether TrkB kinase activity is required for its oncogenic functions. Indeed, it has been suggested that also other properties of the receptor contribute to functions that are relevant to tumor cell survival. Specifically, several adhesion motifs reside within the extracellular domains of TrkB. In line with this, TrkB-expressing epithelial cells form large cellular aggregates in suspension cultures, possibly facilitating tumor cell survival. Therefore, we set out to study the relative contributions of TrkB's kinase activity and its adhesion domains to anoikis suppression and oncogenicity. On the basis of a structure-function analysis, we report that TrkB kinase activity is required and, unexpectedly, also sufficient for anoikis suppression, tumor formation, and experimental metastasis. Thus, TrkB can act tumorigenically independent of its adhesion motifs. These results suggest that targeting the enzymatic activity of TrkB might be beneficial in cancer therapy.

Alternative TrkAIII splicing: a potential regulated tumor-promoting switch and therapeutic target in neuroblastoma

An association between elevated tyrosine kinase receptor (Trk)-A expression and better prognosis; the absence of mutation-activated TrkA oncogenes; the induction of apoptosis, growth arrest, morphological differentiation and inhibition of xenograft growth; and angiogenesis by TrkA gene transduction, provide the basis for the current concept of an exclusively tumor-suppressor role for TrkA in the aggressive pediatric tumor, neuroblastoma. This concept, however, has recently been challenged by the discovery of a novel hypoxia-regulated alternative TrkAIII splice variant, initial data for which suggest predominant expression in advanced-stage neuroblastoma. TrkAIII exhibits neuroblastoma xenograft tumor-promoting activity associated with the induction of a more angiogenic and stress-resistant neuroblastoma phenotype and antagonises nerve growth factor/TrkAI antioncogenic signaling. In this short review, the authors integrate this novel information into a modified concept that places alternative TrkA splicing as a potential pivotal regulator of neuroblastoma behavior and identifies the TrkAIII alternative splice variant as a potential biomarker of patient prognosis and novel therapeutic target.

Ultrastructural localization of the neurotrophin receptor (TrkA) in cultured rat pheochromocytoma PC12 Cells: three-dimensional image analysis by high voltage electron microscopy

Nerve growth factor (NGF) is a well-known neurotrophic factor and the NGF signaling through the receptor, TrkA, plays important roles in regulating neuronal differentiation and survival. A recent study has demonstrated that the TrkAs expressed in undifferentiated PC12 cells were associated with caveolae, which were invaginated small pits on the plasma membrane. Caveolae are frequently seen in many cell types such as endothelial cells, fibroblasts and hepatocytes, but few in neurons. In the present study, we performed immunocytochemistry of TrkA in differentiated PC12 cells and analyzed the ultrastructural localization of TrkA by conventional electron microscopy and high-voltage electron microscopic (HVEM) tomography. The TrkA immunoreactivities were mainly associated with the cytoplasmic vesicles (10-30 nm in diameter) and a part of the plasma membrane. The HVEM tomography showed that the TrkA immunoreactivities were often assembled into ring-like structures (400-800 nm in diameter) near the plasma membrane, unlike typical flask-shaped invaginations of caveolae (50-100 nm in diameter). These results suggest that TrkA are not localized in the caveolae, at least in differentiated PC12 cells, but other invaginations are involved in a novel process of internalization of ligand-bound TrkA.

Localization of TrkC to Schwann cells and effects of neurotrophin-3 signaling at neuromuscular synapses

Neurotrophins and their receptors, the Trks, are differentially expressed among the cell types that make up neuromuscular and other synapses, but the function and directionality of neurotrophin signaling at synapses are poorly understood. Here we demonstrate, via immunostaining, Western blotting, and RT-PCR analyses, that TrkC, the receptor for neurotrophin-3 (NT3), is expressed by mouse perisynaptic and myelinating Schwann cells from birth through adulthood and is unaltered after denervation. Analyses of transgenic mice in which the NT3 coding sequence is replaced by lacZ showed that NT3 is expressed in motor neurons and Schwann cells during perinatal development, but not in adult mice. In muscle, NT3 is expressed by intrafusal muscle fibers within spindles, as has been previously reported. Surprisingly, NT3 is also expressed in extrafusal muscle fibers during perinatal life and in adults. Genetic approaches were used to explore the roles of NT3 and TrkC signaling at neuromuscular synapses. Overexpression of NT3 in muscle fibers during development resulted in an increased number of perisynaptic Schwann cells at neuromuscular synapses, without altering synaptic size, suggesting that muscle-derived NT3 might act as a mitogen or trophic factor for Schwann cells. Conditional deletion of NT3 from motor neurons did not alter the number of Schwann cells or other aspects of neuromuscular synaptic structure, suggesting that motor-neuron-derived NT3 is not required for normal development of perisynaptic Schwann cells or synapses. Together, these results demonstrate that NT3 expression is developmentally regulated in skeletal muscle and may modulate the number of Schwann cells at neuromuscular synapses.

Trypanosome trans-sialidase mediates neuroprotection against oxidative stress, serum/glucose deprivation, and hypoxia-induced neurite retraction in Trk-expressing PC12 cells

Trypanosome trans-sialidase (TS) is a sialic acid-transferring enzyme and a novel ligand of tyrosine kinase (TrkA) receptors but not of neurotrophin receptor p75NTR. Here, we show that TS targets TrkB receptors on TrkB-expressing pheochromocytoma PC12 cells and colocalizes with TrkB receptor internalization and phosphorylation (pTrkB). Wild-type TS but not the catalytically inactive mutant TSDeltaAsp98-Glu induces pTrkB and mediates cell survival responses against death caused by oxidative stress in TrkA- and TrkB-expressing cells like those seen with nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). These same effects are not observed in Trk deficient PC12(nnr5) cells, but are re-established in PC12(nnr5) cells stably transfected with TrkA or TrkB, are partially blocked by inhibitors of tyrosine kinase (K-252a), mitogen-activated protein/mitogen-activated kinase (PD98059) and completely blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K). Both TrkA- and TrkB-expressing cells pretreated with TS or their natural ligands are protected against cell death caused by serum/glucose deprivation or from hypoxia-induced neurite retraction. The cell survival effects of NGF and BDNF against oxidative stress are significantly inhibited by the neuraminidase inhibitor, Tamiflu. Together, these observations suggest that trypanosome TS mimics neurotrophic factors in cell survival responses against oxidative stress, hypoxia-induced neurite retraction and serum/glucose deprivation.

Structural and mechanistic insights into nerve growth factor interactions with the TrkA and p75 receptors

Nerve growth factor engages two structurally distinct transmembrane receptors, TrkA and p75, which have been proposed to create a "high-affinity" NGF binding site through formation of a ternary TrkA/NGF/p75 complex. To define a structural basis for the high-affinity site, we have determined the three-dimensional structure of a complete extracellular domain of TrkA complexed with NGF. The complex reveals a crab-shaped homodimeric TrkA structure, but a mechanism for p75 coordination is not obvious. We investigated the heterodimerization of membrane-bound TrkA and p75, on intact mammalian cells, using a beta-gal protein-protein interaction system. We find that NGF dimerizes TrkA and that p75 exists on the cell surface as a preformed oligomer that is not dissociated by NGF. We find no evidence for a direct TrkA/p75 interaction. We propose that TrkA and p75 likely communicate through convergence of downstream signaling pathways and/or shared adaptor molecules, rather than through direct extracellular interactions.

TrkAIII expression in the thymus

The alternative TrkAIII splice variant is expressed by murine and human thymus. Alternative TrkAIII splicing predominates in postembryonic day E13 (E17 and E18), postnatal murine (3 week and 3 month) and human thymuses, with TrkAIII mRNA expressed by selected thymocyte subsets and thymic epithelial cells (TECs) and a 100 kDa immunoprecipitable TrkAIII-like protein detected in purified thymocyte and whole thymus extracts. FACS and immunohistochemical analysis indicate a non-cell surface localisation for the TrkAIII-like protein in cortical CD4+/CD8+ double positive and, to a lesser extent, single positive thymocyte subsets at the cortex/medulla boundary and in Hassle's corpuscles, reticular epithelial and dendritic cells of the thymic medulla. TrkA(I/II) expression, on the other hand, predominates in sub-capsular regions of the thymus. TrkAIII-like immunoreactivity at the cortex/medulla boundary associates with regions of thymocyte proliferation and not apoptosis. A potential role for thymic hypoxia in thymocyte alternative TrkAIII splicing is supported by reversal to TrkAI splicing by normoxic but not hypoxic culture and induction of Jurkat T cell alternative TrkAIII splicing by the hypoxia mimic CoCl2. In contrast, TEC expression of TrkAIII predominates in both normoxic and hypoxic culture conditions. The data support a potential role for TrkAIII in thymic development and function, of particular relevance to intermediate stage CD4+/CD8+ thymocyte subsets and TECs, which potentially reflects a reversible thymocyte and more permanent TEC adaptation to thymic environment. Since intracellular TrkAIII neither binds nor responds to NGF and can impede regular NGF/TrkA signalling (Tacconelli et al., Cancer Cell, 2004), its expression would be expected to provide an alternative and/or impediment to regular NGF/TrkA signalling within the developing and developed thymus of potential functional importance.

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.

The endocrine-disrupting compound, nonylphenol, inhibits neurotrophin-dependent neurite outgrowth

Endocrine-disrupting compounds (EDCs) may interfere with neuronal development due to high levels of accumulation in biological tissue and potentially aberrant steroid signaling. Treatment of dissociated embryonic Xenopus spinal cord neurons with the EDC, nonylphenol (NP), did not alter cell survival or neurite outgrowth but inhibited neurotrophin-induced neurite outgrowth, effects that were recapitulated by treatment with comparable concentrations of 17 beta-estradiol (E2) and beta-estradiol 6-(O-carboxy-methyl)oxime: BSA (E2-BSA), but not a synthetic androgen. Effects of NP were not inhibited by the nuclear estrogen receptor antagonist, ICI 182,780, but were inhibited by the G protein antagonist, pertussis toxin. Nerve growth factor (NGF)-induced neurite outgrowth in Xenopus neurons was shown to require MAPK signaling. NP did not affect TrkA expression, MAPK signaling, or phosphatidylinositol 3' kinase-Akt-glycogen synthase kinase 3 beta (PI3K-Akt-GSK3 beta) signaling in Xenopus. The ability of NP to inhibit NGF-induced neurite outgrowth without altering survival was recapitulated in the rat pheochromocytoma (PC12) cell line. As with Xenopus neurons, the inhibitory actions of NP in PC12 cells were not antagonized by ICI 182,780 and did not involve alterations in signaling along either the MAPK or PI3K-Akt-GSK3 beta pathways. NP did significantly inhibit the ability of NGF to increase protein kinase A activity in this cell line. These data have important implications with respect to potentially deleterious effects of NP exposure during early neural development and highlight the fact that bioaccumulation of EDCs, such as NP, may elicit very disparate effects along divergent signaling pathways than those that arise from the actions of physiological levels of endogenous estrogens.

Intracellular retention, degradation, and signaling of glycosylation-deficient FGFR2 and craniosynostosis syndrome-associated FGFR2C278F

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are known to play a critical role in a variety of fundamental processes, including wound healing, angiogenesis, and development of multiple organ systems. Mutations in the FGFR gene family have been linked to a series of syndromes (the craniosynostosis syndromes) whose primary phenotype involves aberrant development of the craniofacial skeleton. Craniosynostosis syndrome-linked FGFR mutations have been shown to be gain of function in terms of receptor activation and have been presumed to result in increased levels of FGF/FGFR signaling. Unfortunately, studies attempting to link expression of mutant FGFRs with changes in cellular phenotype have yielded conflicting results. In an effort to better understand the biochemical consequences of these mutations on receptor function, here we have investigated the effect of the FGFR2C278F mutation of Crouzon craniosynostosis syndrome on receptor trafficking, ubiquitination, degradation, and signaling. We find that FGFR2C278F exhibits diminished glycosylation, increased degradation, and limited cellular sublocalization in the osteoblastic cell line, MC3T3E1(C4). Additionally, we show that trafficking and autoactivation of wild type FGFR2 is glycosylation-dependent. Both FGFR2C278F and unglycosylated wild type FGFR2 signal through phospholipase Cgamma in a ligand-independent manner as well as exhibit dramatically increased binding to the adaptor protein, Frs2. These findings suggest that autoactive FGFR2 can signal from intracellular compartments. Based upon our results, we propose that the functional signaling of craniosynostosis mutant, autoactive receptors is limited in some cell types by protective cellular responses, such as increased trafficking to lysosomes and proteasomes for degradation.

Long-term adeno-associated viral vector-mediated expression of truncated TrkB in the adult rat facial nucleus results in motor neuron degeneration

Adult facial motor neurons continue to express full-length TrkB tyrosine kinase receptor (TrkB.FL), the high-affinity receptor for the neurotrophins BDNF and neurotrophic factor-4/5 (NT-4/5), suggesting that they remain dependent on target-derived and locally produced neurotrophins in adulthood. Studies on the role of TrkB signaling in the adult CNS have been hampered by the early lethality of bdnf, nt-4/5, and trkB knock-out mice. We disrupted TrkB.FL signaling in adult facial motor neurons using adeno-associated viral vector-mediated overexpression of a naturally occurring dominant-negative TrkB receptor, TrkB.T1. Expression of TrkB.T1 resulted in neuronal atrophy and downregulation of NeuN (neuronal-specific nuclear protein) and ChAT expression in facial motor neurons. A subset of transduced neurons displayed signs of motor neuron degeneration that included dendritic beading and rounding of the soma at 2 months of TrkB.T1 expression. Cell counts revealed a significant reduction in motor neuron number in the facial nucleus at 4 months after onset of expression of TrkB.T1, suggesting that a proportion of TrkB.T1-expressing motor neurons became undetectable as a result of severe atrophy or was lost because of cell death. In contrast, overexpression of TrkB.FL did not result in a decrease in facial motor neuron number. Our results indicate that a subset of facial motor neurons remains dependent on TrkB ligands for the maintenance of structural and molecular characteristics in adulthood.

Role of N-glycans in growth factor signaling

Secreted proteins and membrane proteins are frequently post-translationally modified by oligosaccharides. Therefore, many glycoproteins are involved in signal transduction. One example is growth factor receptors, which are membrane proteins that often contain oligosaccharides. The oligosaccharides in those growth factor receptors play crucial roles in receptor functions. An analysis of glycosyltransferase-transfectants revealed that the branching structures of oligosaccharide also serve as important determinants. For example, N-glycans of epidermal growth factor receptor (EGFR) are involved in receptor sorting, ligand binding and dimerization. The addition of a bisecting GlcNAc to N-glycans increases the endocytosis of EGFR. N-glycans of Trk, a high affinity nerve growth factor receptor, also affect its function. Thus, oligosaccharides play an important role in growth factor signaling.

Critical role for Kalirin in nerve growth factor signaling through TrkA

Kalirin is a multidomain guanine nucleotide exchange factor (GEF) that activates Rho proteins, inducing cytoskeletal rearrangement in neurons. Although much is known about the effects of Kalirin on Rho GTPases and neuronal morphology, little is known about the association of Kalirin with the receptor/signaling systems that affect neuronal morphology. Our experiments demonstrate that Kalirin binds to and colocalizes with the TrkA neurotrophin receptor in neurons. In PC12 cells, inhibition of Kalirin expression using antisense RNA decreased nerve growth factor (NGF)-induced TrkA autophosphorylation and process extension. Kalirin overexpression potentiated neurotrophin-stimulated TrkA autophosphorylation and neurite outgrowth in PC12 cells at a low concentration of NGF. Furthermore, elevated Kalirin expression resulted in catalytic activation of TrkA, as demonstrated by in vitro kinase assays and increased NGF-stimulated cellular activation of Rac, Mek, and CREB. Domain mapping demonstrated that the N-terminal Kalirin pleckstrin homology domain mediates the interaction with TrkA. The effects of Kalirin on TrkA provide a molecular basis for the requirement of Kalirin in process extension from PC12 cells and for previously observed effects on axonal extension and dendritic maintenance. The interaction of TrkA with the pleckstrin homology domain of Kalirin may be one example of a general mechanism whereby receptor/Rho GEF pairings play an important role in receptor tyrosine kinase activation and signal transduction.

Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor

A decrease in the number of nicotinic-acetylcholine receptors (nAChRs) in the brain is thought to contribute to the cognitive dysfunction associated with diseases as diverse as Alzheimer's disease and schizophrenia. Interestingly, nicotine and similar compounds have been shown to enhance memory function and increase the expression of nAChRs and therefore, could have a therapeutic role in the aforementioned diseases. Nicotine has also been shown to exert positive effects on certain neurotrophins such as nerve growth factor (NGF), and therefore could play a role beyond mere symptomatic therapy. However, to date, comprehensive studies of nicotine's effects on the expression of specific acetylcholine (ACh) receptor subtypes, key cholinergic proteins (that are regulated by NGF) such as choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) are lacking. Studies to further investigate the effects of nicotine on NGF especially its high- and low-affinity receptors are also needed. In the present study, male Wistar rats exposed a relatively low dosage of nicotine (0.35 mg/kg every 12 h) for 14 days demonstrated improved memory performance (assessed in two separate water maze testing methods) when compared with controls. Autoradiographic experiments indicated that nicotine increased [3H]-epibatidine, [125I]-alpha-bungarotoxin and [3H]-AFDX384, but not [3H]-pirenzepine binding sites in several learning- and memory-related brain areas. The expression of ChAT, VAChT, as well as tropomyosin-receptor kinase A (TrkA) NGF receptors and phospho-TrK receptors was increased by nicotine in the hippocampus. No changes were observed in the levels of the NGF peptide or low affinity p75 neurotrophin receptors (p75NTR), however. These results suggest that repeated exposure to nicotine results in positive effects on central cholinergic markers and memory function, which may be mediated via effects on high-affinity NGF receptors.

Human Tumorous Imaginal Disc 1 (TID1) Associates with Trk Receptor Tyrosine Kinases and Regulates Neurite Outgrowth in nnr5-TrkA Cells

The human tumorous imaginal disc 1 (TID1) proteins including TID1(L) and TID1(S), members of the DnaJ domain protein family, are involved in multiple intracellular signaling pathways such as apoptosis induction, cell proliferation, and survival. Here we report that TID1 associates with the Trk receptor tyrosine kinases and regulates nerve growth factor (NGF)-induced neurite outgrowth in PC12-derived nnr5 cells. Binding assays and transfection studies showed that the carboxyl-terminal end of TID1 (residues 224-429) bound to Trk at the activation loop (Tyr(P)(683)-Tyr(684)(P)(684) in rat TrkA) and that TID1 was tyrosine phosphorylated by Trk both in yeast and in transfected cells. Moreover endogenous TID1 was also tyrosine phosphorylated by and co-immunoprecipitated with Trk in neurotrophin-stimulated primary rat hippocampal neurons. Overexpression studies showed that both TID1(L) and TID1(S) significantly facilitated NGF-induced neurite outgrowth in TrkA-expressing nnr5 cells possibly through a mechanism involving increased activation of mitogen-activated protein kinase. Consistently knockdown of endogenous TID1, mediated with specific short hairpin RNA, significantly reduced NGF-induced neurite growth in nnr5-TrkA cells. These data provide the first evidence that TID1 is a novel intracellular adaptor that interacts with the Trk receptor tyrosine kinases in an activity-dependent manner to facilitate Trk-dependent intracellular signaling.

Transactivation of Trk neurotrophin receptors by G-protein-coupled receptor ligands occurs on intracellular membranes

Neurotrophins, such as NGF and BDNF, activate Trk receptor tyrosine kinases through receptor dimerization at the cell surface followed by autophosphorylation and intracellular signaling. It has been shown that activation of Trk receptor tyrosine kinases can also occur via a G-protein-coupled receptor (GPCR) mechanism, without involvement of neurotrophins. Two GPCR ligands, adenosine and pituitary adenylate cyclase-activating polypeptide (PACAP), can activate Trk receptor activity to increase the survival of neural cells through stimulation of Akt activity. To investigate the mechanism of Trk receptor transactivation, we have examined the localization of Trk receptors in PC12 cells and primary neurons after treatment with adenosine agonists and PACAP. In contrast to neurotrophin treatment, Trk receptors were sensitive to transcriptional and translational inhibitors, and they were found predominantly in intracellular locations particularly associated with Golgi membranes. Biotinylation and immunostaining experiments confirm that most of the transactivated Trk receptors are found in intracellular membranes. These results indicate that there are alternative modes of activating Trk receptor tyrosine kinases in the absence of neurotrophin binding at the cell surface and that receptor signaling may occur and persist inside of neuronal cells.

Ltrk is differentially expressed in developing and adult neurons of theLymnaea central nervous system

The Trk receptor family plays diverse roles in both development and plasticity of the vertebrate nervous system. Ltrk is a related receptor that is expressed in the CNS of the mollusk Lymnaea, although little is known of its cellular distribution. This study provides three independent lines of evidence (based on RT-PCR, in situ hybridization, and immunohistochemistry) that Ltrk is universally expressed by neurons and dorsal body cells of both the juvenile and the adult Lymnaea CNS. The highest level of expression by neuronal somata occurs in the late juvenile stage, whereas axon collaterals express high levels throughout the animal's life span. Our data support multifunctional roles for Ltrk that parallel those of its mammalian counterparts.

Neurotrophin and Trk expression by cells of the human lamina cribrosa following oxygen-glucose deprivation

Background

Ischemia within the optic nerve head (ONH) may contribute to retinal ganglion cell (RGC) loss in primary open angle glaucoma (POAG). Ischemia has been reported to increase neurotrophin and high affinity Trk receptor expression by CNS neurons and glial cells. We have previously demonstrated neurotrophin and Trk expression within the lamina cribrosa (LC) region of the ONH. To determine if ischemia alters neurotrophin and Trk protein expression in cells from the human LC, cultured LC cells and ONH astrocytes were exposed to 48 hours of oxygen-glucose deprivation (OGD). Also cells were exposed to 48 hours of OGD followed by 24 hours of recovery in normal growth conditions. Cell number, neurotrophin and Trk receptor protein expression, neurotrophin secretion, and Trk receptor activation were examined.

Results

Cell number was estimated using an assay for cell metabolism following 24, 48 and 72 hours of OGD. A statistically significant decrease in LC and ONH astrocyte cell number did not occur until 72 hours of OGD, therefore cellular protein and conditioned media were collected at 48 hours OGD. Protein expression of NGF, BDNF and NT-3 by LC cells and ONH astrocytes increased following OGD, as did NGF secretion. Recovery from OGD increased BDNF protein expression in LC cells. In ONH astrocytes, recovery from OGD increased NGF protein expression, and decreased BDNF secretion. Trk A expression and activation in LC cells was increased following OGD while expression and activation of all other Trk receptors was decreased. A similar increase in Trk A expression and activation was observed in ONH astrocytes following recovery from OGD.

Conclusions

In vitro conditions that mimic ischemia increase the expression and secretion of neurotrophins by cells from the ONH. Increased Trk A expression and activation in LC cells following OGD and in ONH astrocytes following recovery from OGD suggest autocrine/paracrine neurotrophin signaling could be a response to ONH ischemia in POAG. Also, the increase in NGF, BDNF and NT-3 protein expression and NGF secretion following OGD also suggest LC cells and ONH astrocytes may be a paracrine source of neurotrophins for RGCs.

TrkA alternative splicing: a regulated tumor-promoting switch in human neuroblastoma

We identify a novel alternative TrkA splice variant, TrkAIII, with deletion of exons 6, 7, and 9 and functional extracellular IG-C1 and N-glycosylation domains, that exhibits expression restricted to undifferentiated early neural progenitors, human neuroblastomas (NBs), and a subset of other neural crest-derived tumors. This NGF-unresponsive isoform is oncogenic in NIH3T3 cells and promotes tumorigenic NB cell behavior in vitro and in vivo (cell survival, xenograft growth, angiogenesis) resulting from spontaneous tyrosine kinase activity and IP3K/Akt/NF-kappaB but not Ras/MAPK signaling. TrkAIII antagonizes NGF/TrkAI signaling, which is responsible for NB growth arrest and differentiation through Ras/MAPK, and its expression is promoted by hypoxia at the expense of NGF-responsive receptors, providing a mechanism for converting NGF/TrkA/Ras/MAPK antioncogenic signals to TrkAIII/IP3K/Akt/NF-kappaB tumor-promoting signals during tumor progression.

Glycosylation of the receptor guanylate cyclase C: role in ligand binding and catalytic activity

GC-C (guanylate cyclase C) is the receptor for heat-stable enterotoxins, guanylin and uroguanylin peptides. Ligand binding to the extracellular domain of GC-C activates the guanylate cyclase domain leading to accumulation of cGMP. GC-C is expressed as differentially glycosylated forms in HEK-293 cells (human embryonic kidney-293 cells). In the present study, we show that the 145 kDa form of GC-C contains sialic acid and galactose residues and is present on the PM (plasma membrane) of cells, whereas the 130 kDa form is a high mannose form that is resident in the endoplasmic reticulum and serves as the precursor for the PM-associated form. Ligand-binding affinities of the differentially glycosylated forms are similar, indicating that glycosylation of GC-C does not play a role in direct ligand interaction. However, ligand-stimulated guanylate cyclase activity was observed only for the fully mature form of the receptor present on the PM, suggesting that glycosylation had a role to play in imparting a conformation to the receptor that allows ligand stimulation. Treatment of cells at 20 degrees C led to intracellular accumulation of a mature glycosylated form of GC-C that now showed ligand-stimulated guanylate cyclase activity, indicating that localization of GC-C was not critical for its catalytic activity. To determine if complex glycosylation was required for ligand-stimulated activation of GC-C, the receptor was expressed in HEK-293 cells that were deficient in N -acetylglucosaminyltransferase 1. This minimally glycosylated form of the receptor was expressed on the cell surface and could bind a ligand with an affinity comparable with the 145 kDa form of the receptor. However, this form of the receptor was poorly activated by the ligand. Therefore our studies indicate a novel role for glycosidic modification of GC-C during its biosynthesis, in imparting subtle conformational changes in the receptor that allow for ligand-mediated activation and perhaps regulation of basal activity.

Ethanol exposure alters neurotrophin receptor expression in the rat central nervous system: Effects of neonatal exposure

The detrimental effects of ethanol exposure during nervous system development have been well established. The cellular mechanisms of ethanol neurotoxicity, however, have not been clearly defined. Recent studies suggest that neurotrophin signaling pathways may be involved in ethanol-mediated neuronal death. The present investigation, therefore, was designed to examine ethanol-induced alterations in neurotrophin receptor protein levels in the developing central nervous system (CNS) following chronic ethanol treatment administered during the early neonatal period. For this study, rats were exposed to ethanol via vapor inhalation from postnatal day 4 (P4) to P10. Brains were then dissected on P10 or P21, and Western blots used to quantify expression of neurotrophin receptors TrkA, TrkB, TrkC, and p75. This early postnatal ethanol treatment produced significant alterations in receptor levels in hippocampus, septum, cerebral cortex, and cerebellum. The alterations seen were variable, with decreases generally found in hippocampus and cerebellum, increases noted in septum, and changes in both directions occurring in cortex. These alterations were generally more prevalent in males than in females. While most of the receptor changes observed were transient, sustained or delayed alterations were occasionally seen in hippocampus, cortex, and cerebellum. These results suggest that developmental ethanol exposure modulates expression of these neurotrophin receptors throughout the CNS, alterations which could have wide-ranging effects on functional CNS development. The possible linkage between such changes and abnormalities encountered in the fetal alcohol syndrome are considered.

Ethanol exposure alters neurotrophin receptor expression in the rat central nervous system: Effects of prenatal exposure

Developmental ethanol exposure produces significant central nervous system (CNS) abnormalities. The cellular mechanisms of ethanol neurotoxicity, however, remain elusive. Recent data implicate altered neurotrophin signaling pathways in ethanol-mediated neuronal death. The present study investigated ethanol-induced alterations in neurotrophin receptor proteins in the rat CNS following chronic ethanol treatment during gestation, via liquid diet to pregnant dams. Brains were dissected on P1 and P10, and Western blots for the neurotrophin receptors TrkA, TrkB, TrkC, and p75 were quantified. Such ethanol treatment produced significant changes in neurotrophin receptor levels in the hippocampus, septum, cerebral cortex, and cerebellum. Receptor levels in hippocampus, septum, and cerebellum, tended to be decreased, while levels in cortex were consistently increased. Males were generally more affected than females. While most of these alterations were transient, sustained or delayed changes were present in P10 septum, cortex, and cerebellum. These results indicate that developmental ethanol exposure produces major changes in the normal physiological levels of the neurotrophin receptors throughout the CNS. These changes in the receptor complement during critical prenatal stages could relate to the anomalous development of the CNS seen in the fetal alcohol syndrome. This relationship is discussed, together with the potential biological effects of such dramatic changes in neurotrophin receptor expression.

Trafficking of TrkA-green fluorescent protein chimerae during nerve growth factor-induced differentiation

A chimera of the nerve growth factor (NGF) receptor, TrkA, and green fluorescent protein (GFP) was engineered by expressing GFP in phase with the carboxyl terminus of TrkA. TrkA-GFP becomes phosphorylated on tyrosine residues in response to NGF and is capable of initiating signaling cascades leading to prolonged MAPK activation and differentiation in PC12 nnr5 cells. TrkA constructs, progressively truncated in the carboxyl-terminal domain, were prepared as GFP chimerae in order to identify which part of the receptor intracellular domain is involved in its trafficking. Immunofluorescence observations show that TrkA-GFP is found mainly in cell surface membrane ruffles and in endosomes. Biochemical analysis indicated that the cytoplasmic domain of TrkA is not necessary for correct maturation and cell surface translocation of the receptor. An antibody against the extracellular domain of TrkA (RTA) was used as ligand to stimulate internalization and phosphorylation of TrkA. Co-localization studies with anti-phosphorylated TrkA antibodies support a role for such complexes in the propagation of signaling from the cell surface, resulting in the activation of TrkA in areas of the endosome devoid of receptor-ligand complexes. Confocal time-lapse analysis reveals that the TrkA-GFP chimera shows highly dynamic trafficking between the cell surface and internal locations. TrkA-positive vesicles were estimated to move 0.46 +/- 0.09 microm/s anterograde and 0.48 +/- 0.07 microm/s retrograde. This approach and the fidelity of the biochemical properties of the TrkA-GFP demonstrate that real-time visualization of trafficking of tyrosine kinase receptors in the presence or absence of the ligand is feasible.

MuSK glycosylation restrains MuSK activation and acetylcholine receptor clustering

MuSK, a muscle-specific receptor tyrosine kinase that is activated by agrin, has a critical role in neuromuscular synapse formation. In cultured myotubes, agrin stimulates the rapid phosphorylation of MuSK, leading to MuSK activation and tyrosine phosphorylation and clustering of acetylcholine receptors. Agrin, however, fails to stimulate tyrosine phosphorylation of MuSK that is force-expressed in myoblasts and fibroblasts, indicating that myotubes contain an additional activity that is required for agrin to stimulate MuSK. Certain glycosyltransferases are expressed selectively at synaptic sites in skeletal muscle, raising the possibility that carbohydrate modifications of MuSK, catalyzed by glycosyltransferases expressed selectively in myotubes, may be essential for agrin to bind and activate MuSK. We identifed two N-linked glycosylation sites in MuSK, and we expressed MuSK mutants lacking one or both N-linked sites into MuSK mutant myotubes to determine whether N-linked carbohydrate modifications of MuSK have a role in MuSK activation. We found that N-linked glycosylation restrains ligand-independent tyrosine phosphorylation of MuSK and downstream signaling but is not necessary for agrin to stimulate MuSK.

Molecular kinetics of nerve growth factor receptor trafficking and activation

A growing body of evidence indicates a close relationship between tyrosine kinase receptor trafficking and signaling. Biochemical and molecular analyses of the expression, fate, and kinetics of membrane trafficking of the nerve growth factor (NGF) receptor TrkA were performed in PC12 cells. Pulse-chase experiments indicate that TrkA is synthesized as a 110-kDa N-glycosylated precursor that leads to the mature 140-kDa form of the receptor with a half-life of conversion of approximately 24 +/- 0.5 min. Neuraminidase digestion shows that modification of the carbohydrate moiety of the receptor by sialylation occurs during maturation. The 140-kDa form is rapidly translocated to the cell surface as assessed by cell surface biotinylation performed on intact PC12 cells. Mature receptor half-life is approximately 138 +/- 4 min and is shortened to 86 +/- 8 min by NGF treatment. Flow cytometric analysis indicates that NGF induces clearing of this receptor from the cell surface within minutes of treatment. The addition of NGF decreases the half-life of cell surface gp140(TrkA) from 100 to 35 min and leads to enhanced lysosomal degradation of the receptor. The process of NGF-induced TrkA internalization is clearly affected by interfering with ligand binding to p75(NTR). An analysis of receptor activation kinetics also shows that receptor signaling primarily takes place from an intracellular location. Together, these data show that the primary effect of NGF treatment is a p75(NTR)-modulated decrease in TrkA transit time at the cell surface.

Role of low-affinity p75 receptor in nerve growth factor-inducible growth arrest of PC12 cells

Mutant PC12 cell clones (PC84 cells) were obtained by transfection with nerve growth factor (NGF) cDNA. These cells secreted active NGF, extended short processes, and proliferated faster than the parental PC12 cells. These features are of great interest because the parental PC12 cells cease proliferation and extend long processes when transfected with NGF cDNA. PC84 cells expressed a high level of acetylcholinesterase activity and neurofilament M, which indicates that PC84 cells were differentiated. The inhibition of TrkA by K252a diminished the short processes of PC84 cells but had no effect on their fast proliferation. The expression level of TrkA in PC84 cells was comparable to that in PC12 cells; whereas that of another NGF receptor, p75, was significantly lower. These data suggest that the decrease of p75 contributed to the continuous growth of PC84 cells, which was confirmed by suppressing p75 activity of PC12 cells with the antisense oligonucleotide of p75 or with anti-p75 neutralizing antibody. The treated cells did not cease proliferation in the presence of NGF and extended short processes. Our results suggest that NGF signaling via TrkA affects the differentiation characteristics of PC12 cells but that an additional signaling via p75 is necessary for the growth arrest of the cells.

Constitutive TrkA activity in receptor-overexpressing PC12 clones

We have studied ligand-independent signaling by the nerve growth factor receptor TrkA in PC12 clones, under conditions of receptor overexpression. Our results indicate that TrkA-overexpressing PC12 clones display constitutive receptor activation, involving both the mature, 140-kDa form and the immature, intracellular 110-kDa form of the receptor. Phosphorylation of Tyr 674/675, located in the activation loop domain and reflecting TrkA kinase activity, appears particularly prominent in the immature form of the receptor. Constitutive receptor activation is able to chronically stimulate the PI-3 kinase/Akt as well as the mitogen-activated protein kinase pathways, leading to ligand-independent neurite extension. Under conditions of overexpression, a significant fraction of the receptor is retained intracellularly by thiol-mediated mechanisms. Exposure of the cells to reducing agents promotes translocation of the intracellular pool of the receptor to the plasma membrane and suppresses ligand-independent neurite outgrowth. Our results suggest that the levels of expression of TrkA, both intracellularly and at the cell surface, may act to modulate its kinase activity and generate ligand-independent downstream signaling.