Expression of the AMF/neuroleukin receptor in developing and adult brain cerebellum

GRP78-Mediated Signaling Contributes to Axonal Growth Resulting in Motor Function Recovery in Spinal Cord-Injured Mice

Promoting axonal growth is essential for repairing damaged neuronal connections and motor function in spinal cord injury (SCI). Neuroleukin (NLK) exerts axonal growth activity in vitro and in vivo, but the mechanism remains unclear. This study reveals that the 78-kDa glucose-regulated protein (GRP78) is a NLK neuronal receptor that contributes to recovery from SCI. Binding and immunoprecipitation assays indicated that NLK binds to GRP78. Pretreatment to cultured neurons with a GRP78-neutralizing antibody suppressed NLK-induced axonal growth. Blocking cell surface GRP78 inhibited neuronal NLK-induced Akt activation. Treatment with an Akt inhibitor suppressed NLK-induced axonal growth. Continuous administration of NLK into the lateral ventricle of SCI mice increased axonal density in the injured region and restored motor function, which was not observed when NLK was simultaneously administered with a GRP78-neutralizing antibody. These results indicate that GRP78 regulates the NLK-induced axonal growth activity; NLK-GRP78 signaling promotes motor function recovery in SCI, presenting as a potential therapeutic target.

Extracellular Neuroleukin Enhances Neuroleukin Secretion From Astrocytes and Promotes Axonal Growth in vitro and in vivo

Under pathological conditions in the central nervous system (CNS), including spinal cord injury, astrocytes show detrimental effects against neurons. It is also known that astrocytes sometimes exert beneficial effects, such as neuroprotection and secretion of axonal growth factors. If beneficial effects of astrocytes after injury could be induced, dysfunction of the injured CNS may improve. However, a way of promoting beneficial functions in astrocytes has not been elucidated. In the current study, we focused on neuroleukin (NLK), which is known to have axonal growth activities in neurons. Although NLK is secreted from astrocytes, the function of NLK in astrocytes is poorly understood. We aimed to clarify the mechanism of NLK secretion in astrocytes and the functional significance of secreted NLK from astrocytes. Stimulation of cultured astrocytes with recombinant NLK significantly elevated the secretion of NLK from astrocytes. Furthermore, astrocyte conditioned medium treated with NLK increased axonal density in cultured cortical neurons. Recombinant NLK itself directly increased axonal density in cultured neurons. These results indicated that NLK secreted from astrocytes acted as an axonal growth factor and that secretion was stimulated by extracellular NLK. To elucidate a direct binding molecule of NLK on astrocytes, drug affinity responsive target stability (DARTS) analysis was performed. A 78 kDa glucose regulated protein (GRP78) was identified as a receptor for NLK, which was related to the secretion of NLK from astrocytes. When NLK was injected into the lesion site of spinal cord injured mice, axonal density in the injured region was significantly increased and hindlimb motor function improved. These results suggested that NLK-GRP78 signalling was important for the beneficial effects of astrocytes. This study strengthens the potential of astrocytes for use as therapeutic targets in CNS traumatic injury.

Oxidation Resistance 1 Modulates Glycolytic Pathways in the Cerebellum via an Interaction with Glucose-6-Phosphate Isomerase

Glucose metabolism is essential for the brain: it not only provides the required energy for cellular function and communication but also participates in balancing the levels of oxidative stress in neurons. Defects in glucose metabolism have been described in neurodegenerative disease; however, it remains unclear how this fundamental process contributes to neuronal cell death in these disorders. Here, we investigated the molecular mechanisms driving the selective neurodegeneration in an ataxic mouse model lacking oxidation resistance 1 (Oxr1) and discovered an unexpected function for this protein as a regulator of the glycolytic enzyme, glucose-6-phosphate isomerase (GPI/Gpi1). Initially, we present a dysregulation of metabolites of glucose metabolism at the pre-symptomatic stage in the Oxr1 knockout cerebellum. We then demonstrate that Oxr1 and Gpi1 physically and functionally interact and that the level of Gpi1 oligomerisation is disrupted when Oxr1 is deleted in vivo. Furthermore, we show that Oxr1 modulates the additional and less well-understood roles of Gpi1 as a cytokine and neuroprotective factor. Overall, our data identify a new molecular function for Oxr1, establishing this protein as important player in neuronal survival, regulating both oxidative stress and glucose metabolism in the brain.

Gp78 E3 Ubiquitin Ligase: Essential Functions and Contributions in Proteostasis

As per the requirement of metabolism and fitness, normal cellular functions are controlled by several proteins, and their interactive molecular and signaling events at multiple levels. Protein quality control (PQC) mechanisms ensure the correct folding and proper utilization of these proteins to avoid their misfolding and aggregation. To maintain the optimum environment of complex proteome PQC system employs various E3 ubiquitin ligases for the selective degradation of aberrant proteins. Glycoprotein 78 (Gp78) is an E3 ubiquitin ligase that prevents multifactorial deleterious accumulation of different misfolded proteins via endoplasmic reticulum-associated degradation (ERAD). However, the precise role of Gp78 under stress conditions to avoid bulk misfolded aggregation is unclear, which can act as a crucial resource to establish the dynamic nature of the proteome. Present article systematically explains the detailed molecular characterization of Gp78 and also addresses its various cellular physiological functions, which could be crucial to achieving protein homeostasis. Here, we comprehensively represent the current findings of Gp78, which shows its PQC roles in different physiological functions and diseases; and thereby propose novel opportunities to better understand the unsolved questions for therapeutic interventions linked with different protein misfolding disorders.

Selective multifaceted E3 ubiquitin ligases barricade extreme defense: Potential therapeutic targets for neurodegeneration and ageing

Efficient and regular performance of Ubiquitin Proteasome System and Autophagy continuously eliminate deleterious accumulation of nonnative protiens. In cellular quality control system, E3 ubiquitin ligases are significant employees for defense mechanism against abnormal toxic proteins. Few findings indicate that lack of functions of E3 ubiquitin ligases can be a causative factor of neurodevelopmental disorders, neurodegeneration, cancer and ageing. However, the detailed molecular pathomechanism implying E3 ubiquitin ligases in cellular functions in multifactorial disease conditions are not well understood. This article systematically represents the unique characteristics, molecular nature, and recent developments in the knowledge of neurobiological functions of few crucial E3 ubiquitin ligases. Here, we review recent literature on the roles of E6-AP, HRD1 and ITCH E3 ubiquitin ligases in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic interventions.

Secretome profiling of differentiated neural mes-c-myc A1 cell line endowed with stem cell properties

Neural stem cell proliferation and differentiation play a crucial role in the formation and wiring of neuronal connections forming neuronal circuits. During neural tissues development, a large diversity of neuronal phenotypes is produced from neural precursor cells. In recent years, the cellular and molecular mechanisms by which specific types of neurons are generated have been explored with the aim to elucidate the complex events leading to the generation of different phenotypes via distinctive developmental programs that control self-renewal, differentiation, and plasticity. The extracellular environment is thought to provide instructive influences that actively induce the production of specific neuronal phenotypes. In this work, the secretome profiling of differentiated neural mes-c-myc A1 (A1) cell line endowed with stem cell properties was analyzed by applying a shotgun LC-MS/MS approach. The results provide a list of secreted molecules with potential relevance for the functional and biological features characterizing the A1 neuronal phenotype. Proteins involved in biological processes closely related to nervous system development including neurites growth, differentiation of neurons and axonogenesis were identified. Among them, proteins belonging to extracellular matrix and cell-adhesion complexes as well as soluble factors with well established neurotrophic properties were detected. The presented work provides the basis to clarify the complex extracellular protein networks implicated in neuronal differentiation and in the acquisition of the neuronal phenotype. This article is part of a Special Issue entitled: An Updated Secretome.

Autocrine motility factor receptor is involved in the process of learning and memory in the central nervous system

The autocrine motility factor receptor (AMFR) is a multifunctional protein involved in cellular adhesion, proliferation, motility and apoptosis. Our study showed that increased AMFR protein expression in the hippocampus of KM mice correlated with enhanced capacity for learning and memory following the shuttle-box test and was significantly elevated in the highest score group. Also, AMF and AMFR mRNA expression positively correlates with the mRNA expression of the synapse marker synaptophysin (Syp). Aging studies in the senescence-accelerated mouse strain (SAM) prone/8 (SAMP8), an animal model of Alzheimer's disease (AD), revealed significantly decreased mRNA and protein expression of AMF and AMFR in the hippocampus. This is especially true for AMFR and AMF protein expression compared with age-matched SAM resistant/1 (SAMR1) mouse strain as the control. Additionally, the low mRNA expression of AMFR could be up-regulated by the four nootropic traditional Chinese medicinal prescriptions (TCMPs): Ba-Wei-Di-Huang decoction (BW), Huang-Lian-Jie-Du decoction (HL), Dang-Gui-Shao-Yao-San (DSS) and Tiao-Xin-Fang decoction (TXF). AMFR protein expression could be up-regulated by two TCMPs, Liu-Wei-Di-Huang decoction (LW) and BW. This indicated that AMFR is involved in the process of learning and memory in the central nervous system. These results may provide useful clues for understanding the etiology of AD.

Raft-dependent endocytosis of autocrine motility factor/phosphoglucose isomerase: a potential drug delivery route for tumor cells

Background

Autocrine motility factor/phosphoglucose isomerase (AMF/PGI) is the extracellular ligand for the gp78/AMFR receptor overexpressed in a variety of human cancers. We showed previously that raft-dependent internalization of AMF/PGI is elevated in metastatic MDA-435 cells, but not metastatic, caveolin-1-expressing MDA-231 cells, relative to non-metastatic MCF7 and dysplastic MCF10A cells suggesting that it might represent a tumor cell-specific endocytic pathway.

Methodology/Principal Findings

Similarly, using flow cytometry, we demonstrate that raft-dependent endocytosis of AMF/PGI is increased in metastatic HT29 cancer cells expressing low levels of caveolin-1 relative to metastatic, caveolin-1-expressing, HCT116 colon cells and non-metastatic Caco-2 cells. Therefore, we exploited the raft-dependent internalization of AMF/PGI as a potential tumor-cell specific targeting mechanism. We synthesized an AMF/PGI-paclitaxel conjugate and found it to be as efficient as free paclitaxel in inducing cytotoxicity and apoptosis in tumor cells that readily internalize AMF/PGI compared to tumor cells that poorly internalize AMF/PGI. Murine K1735-M1 and B16-F1 melanoma cells internalize FITC-conjugated AMF/PGI and are acutely sensitive to AMF/PGI-paclitaxel mediated cytotoxicity in vitro. Moreover, following in vivo intratumoral injection, FITC-conjugated AMF/PGI is internalized in K1735-M1 tumors. Intratumoral injection of AMF/PGI-paclitaxel induced significantly higher tumor regression compared to free paclitaxel, even in B16-F1 cells, known to be resistant to taxol treatment. Treatment with AMF/PGI-paclitaxel significantly prolonged the median survival time of tumor bearing mice. Free AMF/PGI exhibited a pro-survival role, reducing the cytotoxic effect of both AMF/PGI-paclitaxel and free paclitaxel suggesting that AMF/PGI-paclitaxel targets a pathway associated with resistance to chemotherapeutic agents. AMF/PGI-FITC uptake by normal murine spleen and thymus cells was negligible both in vitro and following intravenous injection in vivo where AMF/PGI-FITC was selectively internalized by subcutaneous B16-F1 tumor cells.

Conclusions/Significance

The raft-dependent endocytosis of AMF/PGI may therefore represent a tumor cell specific endocytic pathway of potential value for drug delivery to tumor cells.

Gene expression and functional studies of the optic nerve head astrocyte transcriptome from normal African Americans and Caucasian Americans donors

PURPOSE

To determine whether optic nerve head (ONH) astrocytes, a key cellular component of glaucomatous neuropathy, exhibit differential gene expression in primary cultures of astrocytes from normal African American (AA) donors compared to astrocytes from normal Caucasian American (CA) donors.

METHODS

We used oligonucleotide Affymetrix microarray (HG U133A & HG U133A 2.0 chips) to compare gene expression levels in cultured ONH astrocytes from twelve CA and twelve AA normal age matched donor eyes. Chips were normalized with Robust Microarray Analysis (RMA) in R using Bioconductor. Significant differential gene expression levels were detected using mixed effects modeling and Statistical Analysis of Microarray (SAM). Functional analysis and Gene Ontology were used to classify differentially expressed genes. Differential gene expression was validated by quantitative real time RT-PCR. Protein levels were detected by Western blots and ELISA. Cell adhesion and migration assays tested physiological responses. Glutathione (GSH) assay detected levels of intracellular GSH.

RESULTS

Multiple analyses selected 87 genes differentially expressed between normal AA and CA (P<0.01). The most relevant genes expressed in AA were categorized by function, including: signal transduction, response to stress, ECM genes, migration and cell adhesion.

CONCLUSIONS

These data show that normal astrocytes from AA and CA normal donors display distinct expression profiles that impact astrocyte functions in the ONH. Our data suggests that differences in gene expression in ONH astrocytes may be specific to the development and/or progression of glaucoma in AA.

Expression analysis of neuroleukin, calmodulin, cortactin, and Rho7/Rnd2 in the intact and injured mouse brain

Subtracted cDNA libraries from the mouse developing inferior colliculus were previously constructed between postnatal day (P) 6 and 10. In the P10-P6 subtracted library, neuroleukin, calmodulin I, cortactin, and Rho7 were identified. The goal of the present study was to analyze their distribution, at the mRNA and protein levels, in both the adult and the developing mouse brain. The four molecules showed a wide expression throughout the brain, with a neuronal-enriched localization in structures such as the cortex, the hippocampus, the cerebellum, and the inferior colliculus. The level of expression of their corresponding mRNAs increased during brain postnatal development. The expression of these molecules was also investigated 2 weeks after a mechanical lesion in the adult cerebral cortex. Neuroleukin and cortactin were found to be expressed by reactive astrocytes, while there were no changes in the expression of calmodulin and Rho7. The expression of neuroleukin, calmodulin, cortactin, and Rho7 is discussed in the context of their putative role in the maturation of the brain and in the axonal regeneration process.

Acid-induced conformational changes in phosphoglucose isomerase result in its increased cell surface association and deposition on fibronectin fibrils

Phosphoglucose isomerase (PGI) is a glycolytic enzyme that exhibits extracellular cytokine activity as autocrine motility factor, neuroleukin, and maturation factor and that has been recently implicated as an autoantigen in rheumatoid arthritis. In contrast to its receptor-mediated endocytosis at neutral pH, addition of 25 microg/ml of either Alexa 568- or FITC-conjugated PGI to NIH-3T3 cells at progressively acid pH results in its quantitatively increased association with cell surface fibrillar structures that is particularly evident at pH 5. A similar pH-dependent cell surface association of PGI is observed for first passage human chondrocytes obtained from osteoarthritic joints. At acid pH, PGI colocalizes with fibronectin fibrils, and this association occurs directly upon addition of PGI to the cells. In contrast to the receptor-mediated endocytosis of PGI, fibril association of 25 microg/ml PGI at pH 5 is not competed with an excess (2 mg/ml) of unlabeled PGI. PGI binding at acid pH is therefore neither saturable nor mediated by its receptor. PGI is enzymatically active as a dimer and we show here by non-denaturing gel electrophoresis as well as by glutaraldehyde cross-linking that it exists at neutral pH in a tetrameric form. Increasingly acid pH results in the appearance of PGI monomers that correlates directly with its enhanced cell surface association. However, glutaraldehyde cross-linked PGI is endocytosed at neutral pH and still exhibits enhanced cell surface binding at pH 5. Circular dichroism analysis revealed pH-dependent changes in the near but not the far UV spectra indicating that the tertiary structure of the protein is specifically altered at pH 5. Conformational changes of PGI and exposure of the monomer-monomer interface under acidic conditions, such as those encountered in the synovial fluid of arthritic joints, could therefore result in its deposition on the surface of joints and the induction of an autoimmune response.

Species specificity of the cytokine function of phosphoglucose isomerase

Phosphoglucose isomerase (PGI) is a cytosolic glycolytic enzyme that also functions as an extracellular cytokine (neuroleukin/autocrine motility factor (AMF)/maturation factor). Contrary to mammalian PGI, bacterial PGI was not internalized by the PGI/AMF receptor (gp78/AMF-R) and neither bacterial nor yeast PGI competed with mammalian PGI for receptor binding and internalization. Furthermore, while the bacterial, yeast and mammalian preparations all exhibited isomerase activity, only mammalian PGI stimulated the motility of NIH-3T3 fibroblasts. The conserved active site of PGI is therefore not sufficient for receptor binding and cytokine activity of PGI. However, synthetic peptides corresponding to distinct peripheral mammalian PGI sequences did not inhibit internalization of mammalian PGI. Our data therefore argue that the cytokine activity of PGI is specific to mammalian PGI but cannot exclude the possibility that the receptor binding motif of PGI is complex and includes elements within and without the active site.

Dynamic visualization of a joint-specific autoimmune response through positron emission tomography

In the K/BxN mouse model of rheumatoid arthritis, the transfer of autoantibodies specific for glucose-6-phosphate isomerase (GPI) into naïve mice rapidly induces joint-specific inflammation similar to that seen in human rheumatoid arthritis. The ubiquitous expression of GPI and the systemic circulation of anti-GPI immunoglobulin G (IgG) seem incongruous with the tissue specificity of this disease. By using PET (positron emission tomography), we show here that purified anti-GPI IgG localizes specifically to distal joints in the front and rear limbs within minutes of intravenous injection, reaches saturation by 20 min and remains localized for at least 24 h. In contrast, control IgG does not localize to joints or cause inflammation. The rapid kinetics of anti-GPI IgG joint localization supports a model in which autoantibodies bind directly to pre-existing extracellular GPI in normal healthy mouse joints.

Effect of rabies virus infection on gene expression in mouse brain

A variety of molecular genetic approaches were used to study the effect of rabies virus (RV) infection on host gene expression in mouse brain. The down-regulation of gene expression was found to be a major effect of RV infection by using subtraction hybridization. However, a combination of techniques identified approximately 39 genes activated by infection. These included genes involved in regulation of cell metabolism, protein synthesis, synaptic activity, and cell growth and differentiation. Northern blot analysis to monitor temporal activation of several of these genes following infection revealed essentially two patterns of activation: (i) an early response with up-regulation beginning within 3 days after infection and correlating with transcription of RV nuclear protein; and (ii) a late response with enhanced expression occurring at days 6-7 after infection and associated with peak RV replication. The gene activation patterns and the known functions of their products suggest that a number of host genes may be involved in the replication and spread of RV in the brain.