STAT3 as a downstream mediator of Trk signaling and functions

P60TRP interferes with the GPCR/secretase pathway to mediate neuronal survival and synaptogenesis

In the present study, we show that overexpression of the G-protein-coupled receptor (GPCR)-associated sorting protein p60TRP (transcription regulator protein) in neural stem cells (NSCs) and in a transgenic mouse model modulates the phosphorylation and proteolytic processing of amyloid precursor protein (App), N-cadherin (Cdh2), presenilin (Psen) and τ protein (Mapt). Our results suggest that p60TRP is an inhibitor of Bace1 (β-site App cleaving enzyme) and Psen. We performed several apoptosis assays [Annexin-V, TdT-mediated dUTP Nick-End Labeling (TUNEL), caspase-3/7] using NSCs and PC12 cells (overexpressing p60TRP and knockdown of p60TRP) to substantiate the neuroprotective role of p60TRP. Functional analyses, both in vitro and in vivo, revealed that p60TRP promotes neurosynaptogenesis. Characterization of the cognitive function of p60TRP transgenic mice using the radial arm water maze test demonstrated that p60TRP improved memory and learning abilities. The improved cognitive functions could be attributed to increased synaptic connections and plasticity, which was confirmed by the modulation of the γ-aminobutyric acid receptor system and the elevated expression of microtubule-associated protein 2, synaptophysin and Slc17a7 (vesicle glutamate transporter, Vglut1), as well as by the inhibition of Cdh2 cleavage. In conclusion, interference with the p60TRP/ GPCR/secretase signalling pathway might be a new therapeutic target for the treatment of Alzheimer's disease (AD).

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.

Axonal transcription factors signal retrogradely in lesioned peripheral nerve

Retrograde axonal injury signalling stimulates cell body responses in lesioned peripheral neurons. The involvement of importins in retrograde transport suggests that transcription factors (TFs) might be directly involved in axonal injury signalling. Here, we show that multiple TFs are found in axons and associate with dynein in axoplasm from injured nerve. Biochemical and functional validation for one TF family establishes that axonal STAT3 is locally translated and activated upon injury, and is transported retrogradely with dynein and importin α5 to modulate survival of peripheral sensory neurons after injury. Hence, retrograde transport of TFs from axonal lesion sites provides a direct link between axon and nucleus.

The novel protein MANI modulates neurogenesis and neurite-cone growth

Neuronal regeneration and axonal re-growth in the injured mammalian central nervous system remains an unsolved field. To date, three myelin-associated proteins [Nogo or reticulon 4 (RTN4), myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMG)] are known to inhibit axonal regeneration via activation of the neuronal glycosylphosphatidylinositol-anchored Nogo receptor [NgR, together with p75 neurotrophin receptor (p75NTR) and Lingo-1]. In the present study we describe the novel protein MANI (myelin-associated neurite-outgrowth inhibitor) that localizes to neural membranes. Functional characterization of MANI overexpressing neural stem cells (NSCs) revealed that the protein promotes differentiation into catecholaminergic neurons. Yeast two-hybrid screening and co-immunoprecipitation experiments confirmed the cell division cycle protein 27 (Cdc27) as an interacting partner of Mani. The analyses of Mani-overexpressing PC12 cells demonstrated that Mani retards neuronal axonal growth as a positive effector of Cdc27 expression and activity. We show that knockdown of Cdc27, a component of the anaphase-promoting complex (APC), leads to enhanced neurite outgrowth. Our finding describes the novel MANI-Cdc27-APC pathway as an important cascade that prevents neurons from extending axons, thus providing implications for the potential treatment of neurodegenerative diseases.

Novel anti-cancer compounds for developing combinatorial therapies to target anoikis-resistant tumors

Anoikis, a cell death pathway induced by loss of normal cell-matrix attachment or upon adhesion to a non-native matrix, ensures the balance between proliferative potential of normal cells and maintenance of tissue integrity. Thereby, anoikis serves as a potential molecular barrier against oncogenic transformation of normal cells. Cancer cells acquire anoikis resistance for survival and distant metastatic progression. During the acquisition of anoikis resistance, tumors modulate multiple cell signaling parameters through changes in the expression of up-stream receptors and by dynamically calibrating the dependency on down-stream signaling cascades. Many compounds that target the tumor-acquired switches in integrins, tumor antigens, growth factors, metabolic pathways, oxidative and osmotic-stress signaling are in various phases of pre-clinical and clinical development. Combinatorial approaches maximize the therapeutic efficacy and minimize the activation of alternate signaling pathways, which will otherwise contribute to drug resistance. In this regard, an integrated analysis of the mechanisms of action of potential drugs and lead compounds that can target significant nodes of anoikis signaling networks will provide a rational frame-work for further development and clinical use of respective agents, by formulating more effective combinatorial therapies, in patients with distinct drug-sensitivity profiles.

The adaptor protein SH2B3 (Lnk) negatively regulates neurite outgrowth of PC12 cells and cortical neurons

SH2B adaptor protein family members (SH2B1-3) regulate various physiological responses through affecting signaling, gene expression, and cell adhesion. SH2B1 and SH2B2 were reported to enhance nerve growth factor (NGF)-induced neuronal differentiation in PC12 cells, a well-established neuronal model system. In contrast, SH2B3 was reported to inhibit cell proliferation during the development of immune system. No study so far addresses the role of SH2B3 in the nervous system. In this study, we provide evidence suggesting that SH2B3 is expressed in the cortex of embryonic rat brain. Overexpression of SH2B3 not only inhibits NGF-induced differentiation of PC12 cells but also reduces neurite outgrowth of primary cortical neurons. SH2B3 does so by repressing NGF-induced activation of PLCγ, MEK-ERK1/2 and PI3K-AKT pathways and the expression of Egr-1. SH2B3 is capable of binding to phosphorylated NGF receptor, TrkA, as well as SH2B1β. Our data further demonstrate that overexpression of SH2B3 reduces the interaction between SH2B1β and TrkA. Consistent with this finding, overexpressing the SH2 domain of SH2B3 is sufficient to inhibit NGF-induced neurite outgrowth. Together, our data demonstrate that SH2B3, unlike the other two family members, inhibits neuronal differentiation of PC12 cells and primary cortical neurons. Its inhibitory mechanism is likely through the competition of TrkA binding with the positive-acting SH2B1 and SH2B2.

Mitochondrial localized STAT3 is involved in NGF induced neurite outgrowth

Background

Signal transducer and activator of transcription 3 (STAT3) plays critical roles in neural development and is increasingly recognized as a major mediator of injury response in the nervous system. Cytokines and growth factors are known to phosphorylate STAT3 at tyrosine⁷⁰⁵ with or without the concomitant phosphorylation at serine⁷²⁷, resulting in the nuclear localization of STAT3 and subsequent transcriptional activation of genes. Recent evidence suggests that STAT3 may control cell function via alternative mechanisms independent of its transcriptional activity. Currently, the involvement of STAT3 mono-phosphorylated at residue serine⁷²⁷ (P-Ser-STAT3) in neurite outgrowth and the underlying mechanism is largely unknown.

Principal Findings

In this study, we investigated the role of nerve growth factor (NGF) induced P-Ser-STAT3 in mediating neurite outgrowth. NGF induced the phosphorylation of residue serine⁷²⁷ but not tyrosine⁷⁰⁵ of STAT3 in PC12 and primary cortical neuronal cells. In PC12 cells, serine but not tyrosine dominant negative mutant of STAT3 was found to impair NGF induced neurite outgrowth. Unexpectedly, NGF induced P-Ser-STAT3 was localized to the mitochondria but not in the nucleus. Mitochondrial STAT3 was further found to be intimately involved in NGF induced neurite outgrowth and the production of reactive oxygen species (ROS).

Conclusion

Taken together, the findings herein demonstrated a hitherto unrecognized novel transcription independent mechanism whereby the mitochondria localized P-Ser-STAT3 is involved in NGF induced neurite outgrowth.

The STAT3 beacon: IL-6 recurrently activates STAT 3 from endosomal structures

Endocytic trafficking plays an important role in signal transduction. Signal transducer and activator of transcription 3 (STAT3) and mitogen-activate protein kinase (MAPK) have both been localized to endosomal structures and are dependent upon endocytosis for downstream function. While the dependence of MAPK signaling upon endosomes has been well characterized, the involvement of endosomes in regulating STAT3 signaling has not been defined. Consequently, this study evaluated the role of endosomes in the initiation, modulation, amplification and persistence of interleukin-6(IL-6)-induced STAT3 signal transduction and transcription, and utilized IL-6-induced MAPK signaling as a comparator. Using pharmacologic treatment and temperature control of endocytic trafficking, pulse-chase treatments and in vitro kinase assays, STAT3 was found to interact with endosomes in a markedly different fashion than MAPK. STAT3 was activated by direct interaction with internal structures upstream of the late endosome following IL-6 exposure and persistent STAT3 signaling depended upon recurrent activation from endocytic structures. Further, STAT3 subcellular localization was not dependent upon endocytic trafficking. Instead, STAT3 transiently interacted with endosomes and relocated to the nucleus by an endosome-independent mechanism. Finally, endocytic trafficking played a central role in regulating STAT3 serine 727 phosphorylation through crosstalk with the MAPK signaling system. Together, these data reveal endosomes as central to the genesis, course and outcome of STAT3 signal transduction and transcription.

Identification of novel target genes of nerve growth factor (NGF) in human mastocytoma cell line (HMC-1 (V560G c-Kit)) by transcriptome analysis

Background

Nerve growth factor (NGF) is a potent growth factor that plays a key role in neuronal cell differentiation and may also play a role in hematopoietic differentiation. It has been shown that NGF induced synergistic action for the colony formation of CD34 positive hematopoietic progenitor cells treated with macrophage-colony stimulating factor (M-CSF or CSF-1), or stem cell factor (SCF). However, the exact role of NGF in hematopoietic system is unclear. It is also not clear whether NGF mediated signals in hematopoietic cells are identical to those in neuronal cells.

Results

To study the signal transduction pathways induced by NGF treatment in hematopoietic cells, we utilized the mastocytoma cell line HMC-1(V560G c-Kit) which expresses the NGF receptor, tropomyosin-receptor-kinase (Trk)A, as well as the constitutively activated SCF receptor, V560G c-Kit, which can be inhibited completely by treatment with the potent tyrosine kinase inhibitor imatinib mesylate (imatinib). NGF rescues HMC-1(V560G c-Kit) cells from imatinib mediated cell death and promotes proliferation. To examine the NGF mediated proliferation and survival in these cells, we compared the NGF mediated upregulated genes (30 and 120 min after stimulation) to the downregulated genes by imatinib treatment (downregulation of c-Kit activity for 4 h) by transcriptome analysis. The following conclusions can be drawn from the microarray data: Firstly, gene expression profiling reveals 50% overlap of genes induced by NGF-TrkA with genes expressed downstream of V560G c-Kit. Secondly, NGF treatment does not enhance expression of genes involved in immune related functions that were down regulated by imatinib treatment. Thirdly, more than 55% of common upregulated genes are involved in cell proliferation and survival. Fourthly, we found Kruppel-like factor (KLF) 2 and Smad family member 7 (SMAD7) as the NGF mediated novel downstream genes in hematopoietic cells. Finally, the downregulation of KLF2 gene enhanced imatinib induced apoptosis.

Conclusion

NGF does not induce genes which are involved in immune related functions, but induces proliferation and survival signals in HMC-1(V560G c-Kit) cells. Furthermore, the current data provide novel candidate genes, KLF2 and SMAD7 which are induced by NGF/TrkA activation in hematopoietic cells. Since the depletion of KLF2 causes enhanced apoptosis of HMC-1(V560G c-Kit), KLF2 may play a role in the NGF mediated survival signal.

Calcitonin gene-related peptide as a regulator of neuronal CaMKII-CREB, microglial p38-NFκB and astroglial ERK-Stat1/3 cascades mediating the development of tolerance to morphine-induced analgesia

Tolerance to morphine-induced analgesia is an intractable phenomenon, often hindering its prolonged applications in the clinics. The enhanced pronociceptive actions of spinal pain-related molecules such as calcitonin gene-related peptide (CGRP) may underlie this phenomenon and could be a promising target for intervention. We demonstrate here how CGRP regulates the development of morphine analgesic tolerance at the spinal level. A 7-day treatment with morphine led to tolerance to its analgesic effects and enhanced expression of CGRP and its receptor subunits calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1). Activation of several cell-type-specific kinase transcription factor cascades is required to mediate this tolerance, including calcium/calmodulin-dependent protein kinase II (CaMKII) and cAMP response element-binding protein (CREB) in neurons, p38 and nuclear factor kappa B (NFκB) in microglia and extracellular signal-regulated protein kinase (ERK) and signal transducer and activator of transcription 1 and 3 (Stat1/3) in astrocytes, because inhibitors of CaMKII, p38 and ERK pathways correspondingly reduced the increases in phosphorylated CREB, acetylated-NFκB and phosphorylated Stat1/3 levels and attenuated the development of tolerance. Interestingly, these cascades were linked to the regulation of glutamatergic N-methyl-d-aspartate (NMDA) receptor expression. Chronic morphine-induced behavioural responses and biochemical events were all subjugated to modulation by disrupting CGRP receptor signaling. Together, these data suggest that CGRP contributes to the development of tolerance to morphine-induced analgesia by regulating the activation of the neuronal CaMKII-CREB, microglial p38-NFκB and astroglial ERK-Stat1/3 cascades. Targeting CGRP-associated signaling molecules may prolong or restore morphine's analgesic properties upon a chronic exposure.

A trifluoromethyl analog of verbenachalcone promotes neurite outgrowth and cell proliferation of NeuroScreen-1 cells

Past research has shown that natural products of plant and marine origins and their congeners enhance the actions of neuritogenic factors of the central nervous system (CNS) such as nerve growth factor (NGF). However, the role of fluorine substitutions in their structure-activity relationship (SAR) has not been explored. We have synthesized a trifluoromethyl analog of verbenachalcone (VC), a pharmacologically active natural compound previously shown to potentiate NGF activity. This analog, designated C278, enhances neurite outgrowth and proliferation of NeuroScreen-1™ (NS-1) cells, a subclone of PC12 pheochromocytoma cells. C278 increases the percentage of neurite bearing cells in the presence of suboptimal doses of NGF in comparison with controls treated with NGF alone. In addition, C278 stimulates cell growth in reduced serum and serum-free cell culture conditions based on our observation of increases in cell number and metabolic assessment with MTT reduction and resazurin assays. The addition of C278 partially restored inhibition of NGF-induced neurite outgrowth by the mitogen-activated protein kinase kinase (MEK) inhibitors PD98059 and U0126. Short-term sequential exposure of cells to U0126, C278, and NGF enhanced phosphorylation of extracellular signal-regulated kinase (ERK) in comparison with cells treated with only the MEK inhibitor and NGF. C278 also attenuated cell growth arrest caused by exposure to PD98059, U0126 and the phosphatidylinositol-3 kinase (PI3K) inhibitor, LY294002 but did not alter phosphorylation of Akt, a classic downstream target of PI3K during cell survival. These data suggest that C278 promotes NGF-dependent neurite outgrowth in NS-1 cells through a MEK signaling pathway by a mechanism that alters short-term activation of ERK. In contrast, C278 promotes PI3K-mediated survival independently of Akt phosphorylation.

SOCS3-mediated blockade of JAK/STAT3 signaling pathway reveals its major contribution to spinal cord neuroinflammation and mechanical allodynia after peripheral nerve injury

Neuropathic pain after peripheral nerve injury, associated with local neuroinflammation in the spinal cord, is a severe incapacitating condition with which clinical treatment remains challenging. Inflammatory molecules signal through various intracellular transduction pathways, activation of which may amplify and cause spreading of the inflammatory response. We showed recently that spinal nerve lesion leads to rapid activation of Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) signal transduction pathway in dorsal spinal cord microglia in relation with enhanced levels of spinal interleukin-6 (IL-6) protein. Here, we selectively inactivated JAK/STAT3 signaling in rat dorsal spinal cord glia through local, lentiviral-mediated production of the suppressor of cytokine signaling SOCS3, a physiologic inhibitory protein of JAK/STAT3, and analyzed its consequences in a preclinical model of neuropathic pain. The targeted blockade of JAK/STAT3 activity prevented the abnormal expression of IL-6, CC chemokine ligand CCL2, and activating transcription factor ATF3 induced in the spinal cord by chronic constriction injury of the sciatic nerve (CCI) and substantially attenuated mechanical hypersensitivity (allodynia) in rats. In naive rats, intrathecal administration of a proalgesic cytokine IL-6 rapidly activated microglial JAK/STAT3 and induced downstream changes closely resembling CCI-evoked alterations. We identified downstream mechanisms through which JAK/STAT3 pathway activation leads to the spreading of neuroinflammation. Our findings reveal that JAK/STAT3 signaling plays a major role in spinal cord plasticity and mechanical allodynia associated with peripheral nerve injury.

Lentiviral shRNA silencing of BDNF inhibits in vivo multiple myeloma growth and angiogenesis via down-regulated stroma-derived VEGF expression in the bone marrow milieu

Bone marrow (BM) neovascularization and vascular endothelial growth factor (VEGF) expression in multiple myeloma (MM) correlate with disease progression. Brain derived neurotrophic factor (BDNF) is highly expressed by malignant plasma cells isolated from the majority of MM patients. Recently, BDNF was identified as a potential proangiogenic factor for the promotion of endothelial cell survival, induction of neoangiogenesis in ischemic tissues, and increase of VEGF expression in neuroblastoma. Since tropomyosin receptor kinase B (TrkB), the receptor of BDNF, is expressed by stromal cells within the BM milieu, here we sought to evaluate the involvement of BDNF/TrkB in myeloma-marrow stroma interaction and its effects on BM angiogenesis. TrkB was abundantly expressed by bone marrow stromal cells (BMSCs) isolated from healthy donors. Stimulation of BMSCs with BDNF induced a time- and dose- dependent increase in VEGF secretion, which was completely abolished by K252alpha, an inhibitor of TrkB. BDNF triggered activation of signal transducer and activator of transcription 3 (STAT3) and activator protein-1 (AP-1), whereas STAT3 was involved in mediating VEGF expression. We further delineated the biological significance of BDNF in MM by using lentiviral short-interfering RNA (shRNA). When myeloma cells were cocultured with BMSCs in a noncontact Transwell system, VEGF levels in supernatants were significantly decreased when BDNF expression was knocked down. Furthermore, silencing of BDNF expression significantly inhibited xenograft tumor growth and angiogenesis, and prolonged survival in mouse model. Our studies demonstrate that BDNF, as a potential stimulator of angiogenesis, contributes to MM tumorgenesis; it mediates stromal-MM cell interactions via selective activation of specific receptor TrkB and downstream signal transducer STAT3, regulating VEGF secretion.

Role of STAT3 in in vitro transformation triggered by TRK oncogenes

TRK oncoproteins are chimeric versions of the NTRK1/NGF receptor and display constitutive tyrosine kinase activity leading to transformation of NIH3T3 cells and neuronal differentiation of PC12 cells. Signal Transducer and Activator of Transcription (STAT) 3 is activated in response to cytokines and growth factors and it has been recently identified as a novel signal transducer for TrkA, mediating the functions of NGF in nervous system. In this paper we have investigated STAT3 involvement in signalling induced by TRK oncogenes. We showed that TRK oncogenes trigger STAT3 phosphorylation both on Y705 and S727 residues and STAT3 transcriptional activity. MAPK pathway was involved in the induction of STAT3 phosphorylation. Interestingly, we have shown reduced STAT3 protein level in NIH3T3 transformed foci expressing TRK oncogenes. Overall, we have unveiled a dual role for STAT3 in TRK oncogenes-induced NIH3T3 transformation: i) decreased STAT3 protein levels, driven by TRK oncoproteins activity, are associated to morphological transformation; ii) residual STAT3 transcriptional activity is required for cell growth.

Nemo-like kinase is involved in NGF-induced neurite outgrowth via phosphorylating MAP1B and paxillin

Nerve growth factor (NGF) promotes neurite outgrowth through regulating cytoskeletal organization and cell adhesion. These activities are modulated by protein phosphorylation. Nemo-like kinase (NLK) is an evolutionarily conserved MAP kinase-like kinase that phosphorylates several transcription factors. Although NLK is known to be expressed at relatively high levels in the nervous system, its function is not well understood. We found that NGF promotes the translocation of NLK to PC12 cells' leading edges, and triggers NLK kinase activity in them. Activated NLK directly phosphorylates microtubule-associated protein-1B (MAP1B) and the focal adhesion adaptor protein, paxillin. Knockdown of NLK attenuates the phosphorylation of both paxillin and MAP1B and inhibits both the NGF-induced re-distribution of F-actin and neurite outgrowth. We also discovered that NLK is a LiCl-sensitive kinase. LiCl is known to block NGF-induced neurite outgrowth and the phosphorylation of MAP1B and paxillin in PC12 cells. Therefore, the effects of LiCl are mediated in part by blocking NLK activity. These results suggest that NLK controls the dynamics of the cytoskeleton downstream of NGF signaling.

Extracellular Signal-regulated Kinase Activation Is Required for Serine 727 Phosphorylation of STAT3 in Schwann Cells in vitro and in vivo

In the peripheral nerves, injury-induced cytokines and growth factors perform critical functions in the activation of both the MEK/ERK and JAK/STAT3 pathways. In this study, we determined that nerve injury-induced ERK activation was temporally correlated with STAT3 phosphorylation at the serine 727 residue. In cultured Schwann cells, we noted that ERK activation is required for the serine phosphorylation of STAT3 by neuropoietic cytokine interleukin-6 (IL-6). Serine phosphorylated STAT3 by IL-6 was transported into Schwann cell nuclei, thereby indicating that ERK may regulate the transcriptional activity of STAT3 via the induction of serine phosphorylation of STAT3. Neuregulin-1 (NRG) also induced the serine phosphorylation of STAT3 in an ERK-dependent fashion. In contrast with the IL-6 response, serine phosphorylated STAT3 induced by NRG was not detected in the nucleus, thus indicating the non-nuclear function of serine phosphorylated STAT3 in response to NRG. Finally, we determined that the inhibition of ERK prevented injury-induced serine phosphorylation of STAT3 in an ex-vivo explants culture of the sciatic nerves. Collectively, the results of this study show that ERK may be an upstream kinase for the serine phosphorylation of STAT3 induced by multiple stimuli in Schwann cells after peripheral nerve injury.

Role of signal transducer and activator of transcription 3 in neuronal survival and regeneration

Signal Transducers and Activators of Transcription (STATs) comprise a family of transcription factors that mediate a wide variety of biological functions in the central and peripheral nervous systems. Injury to neural tissue induces STAT activation, and STATs are increasingly recognized for their role in neuronal survival. In this review, we discuss the role of STAT3 during neural development and following ischemic and traumatic injury in brain, spinal cord and peripheral nerves. We focus on STAT3 because of the expanding body of literature that investigates protective and regenerative effects of growth factors, hormones and cytokines that use STAT3 to mediate their effect, in part through transcriptional upregulation of neuroprotective and neurotrophic genes. Defining the endogenous molecular mechanisms that lead to neuroprotection by STAT3 after injury might identify novel therapeutic targets against acute neural tissue damage as well as chronic neurodegenerative disorders.

SH2B1beta enhances fibroblast growth factor 1 (FGF1)-induced neurite outgrowth through MEK-ERK1/2-STAT3-Egr1 pathway

Genetic studies have established the crucial roles of FGF signaling, FGF-induced gene expression and morphogenesis during embryogenesis. In this study, we showed that overexpressing a signaling adaptor protein, SH2B1beta, enhanced FGF1-induced neurite outgrowth in PC12 cells. SH2B1beta has previously been shown to promote nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF)-induced neurite outgrowth, in part, through prolonging NGF and GDNF-induced signaling. To delineate how SH2B1beta promotes FGF1-induced neurite outgrowth, we examined its role in FGF1-dependent signaling. Our data suggest that SH2B1beta enhances and prolongs FGF1-induced MEK-ERK1/2 and PI3K-AKT pathways. We also provided the first evidence that FGF1 induces the phosphorylation of signal transducer and activator of transcription 3 (STAT3) at serine 727 [pSTAT3(S727)] in PC12 cells. SH2B1beta enhances this phosphorylation and the expression of the immediate early gene, Egr1. Through inhibitor assays, we have further shown that MEK-ERK1/2 is required for FGF1-induced neurite outgrowth, pSTAT3(S727) and Egr1 expression. Moreover, inhibiting Rho kinase, ROCK, enhances FGF1-induced neurite outgrowth through pSTAT3(S727)-independent manner. Taken together, our results demonstrate, for the first time, that SH2B1beta enhances FGF1-induced neurite outgrowth in PC12 cells mainly through MEK-ERK1/2-STAT3-Egr1 pathway.

A novel mechanism of methylglyoxal cytotoxicity in neuroglial cells

Methylglyoxal (MGO) is an endogenous dicarbonyl compound that is highly produced in hyperglycemic conditions. It forms advanced glycation endproducts that are believed to contribute, as etiological factors, to the pathophysiology of diabetic complications. In addition, MGO suppresses cell viability through the induction of apoptosis in vitro. In this study, we have, for the first time, demonstrated the effect of MGO on the gp130 cytokine-induced signal transducer and activator of transcription 3 (STAT3) responses in RT4 schwannoma, PC12 pheochromocytoma and U87MG glioma cells. At dose that very mildly affects cell viability, MGO rapidly induces endocytotic degradation of gp130, which involves the di-leucine internalization motif in the cytoplasmic domain of gp130, without affecting other growth factor receptors. Concomitant inhibition of basal and interleukin-6-induced STAT3 activation was observed following pre-treatment with MGO. The inhibitory effect of MGO on the gp130/STAT3 signaling was prevented by the pre-treatment with an advanced glycation endproduct scavenger aminoguanidine. Finally, these deleterious effects of MGO on STAT3 signaling led to down-regulation of a STAT3 target gene, Bcl-2, and sensitized cellular toxicity induced by H(2)O(2) and etoposide. Our data indicate that MGO affects cell viability via desensitization of gp130/STAT3 signaling, which is the key signaling pathway for cell survival, and thereby promotes cytotoxicity.

Profiling of behavioral changes and hippocampal gene expression in mice chronically treated with the SSRI paroxetine

INTRODUCTION

Monoamine-based antidepressants inhibit neurotransmitter reuptake within short time. However, it commonly takes several weeks until clinical symptoms start to resolve--indicating the involvement of effects distant from reuptake inhibition.

OBJECTIVE

To unravel other mechanisms involved in drug action, a "reverse" pharmacological approach was applied to determine antidepressant-induced alterations of hippocampal gene expression.

MATERIALS AND METHODS

The behavioral response to long-term paroxetine administration of male DBA/2Ola mice was assessed by the forced swim test (FST), the modified hole board (mHB), and the dark/light box. Hippocampi of test-naive mice were dissected, and changes in gene expression by paroxetine treatment were investigated by means of microarray technology.

RESULTS AND DISCUSSION

Robust effects of paroxetine on passive stress-coping behavior in the FST were observed. Furthermore, anxiolytic properties of long-term antidepressant treatment could be identified in DBA mice in both, the mHB and dark/light box. Analysis of microarray results revealed a list of 60 genes differentially regulated by chronic paroxetine treatment. Preproenkephalin 1 and inhibin beta-A showed the highest level of transcriptional change. Furthermore, a number of candidates involved in neuroplasticity/neurogenesis emerged (e.g., Bdnf, Gfap, Vim, Sox11, Egr1, Stat3). Seven selected candidates were confirmed by in situ hybridization. Additional immunofluorescence colocalization studies of GFAP and vimentin showed more positive cells to be detected in long-term paroxetine-treated DBA mice.

CONCLUSION

Candidate genes identified in the current study using a mouse strain validated for its responsiveness to long-term paroxetine treatment add, in our opinion, to unraveling the mechanism of action of paroxetine as a representative for SSRIs.

Tyrphostin ErbB2 Inhibitors AG825 and AG879 Have Non-specific Suppressive Effects on gp130/ STAT3 Signaling

Although the interaction between gp130 and the ErbB family has frequently been shown in cancer cells, the mechanism of this interaction remains unclear and controversial. In the present study, we found that specific tyrphostin inhibitors of ErbB2 (AG825 and AG879), but not ErbB1 inhibitor (AG1478), suppressed IL-6-induced tyrosine phosphorylation of STAT3 in schwannoma cells. However, biochemical evidence for transactivation of ErbB2 by IL-6 was not observed. Additionally, the inhibition of ErbB2 expression, with either a specific RNAi or transfection of an ErbB2 mutant lacking the intracellular domain did not inhibit the IL-6-induced tyrosine phosphorylation of STAT3. Thus, it seems that tyrphostins, which are known as specific inhibitors of the ErbB2 kinase, may have non-specific suppressive effects on the IL-6/STAT3 pathway.

Interleukin-6 inhibits receptor activator of nuclear factor kappaB ligand-induced osteoclastogenesis by diverting cells into the macrophage lineage: key role of Serine727 phosphorylation of signal transducer and activator of transcription 3

Osteoclasts are bone-resorptive cells that differentiate from hematopoietic precursors upon receptor activator of nuclear factor kappaB ligand (RANKL) activation. Previous studies demonstrated that IL-6 indirectly stimulates osteoclastogenesis through the production of RANKL by osteoblasts. However, few data described the direct effect of IL-6 on osteoclasts. To investigate this effect, we used several models: murine RAW264.7 cells, mouse bone marrow, and human blood monocytes. In the three models used, the addition of IL-6 inhibited RANKL-induced osteoclastogenesis. Furthermore, IL-6 decreased the expression of osteoclast markers and up-modulated macrophage markers. To elucidate this inhibition, signal transducer and activator of transcription (STAT) 3, the main signaling molecule activated by IL-6, was analyzed. Addition of two STAT3 inhibitors completely abolished RANKL-induced osteoclastogenesis, revealing a key role of STAT3. We demonstrated that a basal level of phosphorylated-STAT3 on Serine(727) associated with an absence of phosphorylation on Tyrosine(705) is essential for osteoclastogenesis. Furthermore, a decrease of Serine(727) phosphorylation led to an inhibition of osteoclast differentiation, whereas an increase of Tyrosine(705) phosphorylation upon IL-6 stimulation led to the formation of macrophages instead of osteoclasts. In conclusion, we showed for the first time that IL-6 inhibits RANKL-induced osteoclastogenesis by diverting cells into the macrophage lineage, and demonstrated the functional role of activated-STAT3 and its form of phosphorylation in the control of osteoclastogenesis.

Cyclin-dependent kinase 5 supports neuronal survival through phosphorylation of Bcl-2

Accumulating evidence indicates that deregulation of cyclin-dependent kinase 5 (Cdk5) activity is associated with apoptosis in various neurodegenerative disease models. Interestingly, recent studies suggest that Cdk5 may also favor neuronal survival. Nonetheless, whether Cdk5 is directly required for neuronal survival during development remains enigmatic. In the current study, we established the pivotal role of Cdk5 in neuronal survival during development by demonstrating that reduction or absence of Cdk5 activity markedly exacerbated neuronal death in cultures and in vivo. Interestingly, the antiapoptotic protein Bcl-2 (B-cell lymphoma protein 2) was identified as a novel substrate of Cdk5. We found that Cdk5-mediated phosphorylation of Bcl-2 at Ser70 was required for the neuroprotective effect of Bcl-2. Intriguingly, inhibition of this phosphorylation conferred proapoptotic property to Bcl-2. Furthermore, overexpression of a Bcl-2 mutant lacking the Cdk5 phosphorylation site abolished the protective effect of Cdk5 re-expression in Cdk5(-/-) neurons, suggesting that Ser70 phosphorylation of Bcl-2 contributed to Cdk5-mediated neuronal survival. Our observations revealed that Cdk5-mediated Bcl-2 phosphorylation is pivotal for the antiapoptotic effect of Bcl-2 and contributes to the maintenance of neuronal survival by Cdk5. Our study has also identified Cdk5 as a regulator of Bcl-2 function in neuronal apoptosis.

Erythropoietin promotes axonal growth in a model of neuronal polarization

Erythropoietin (EPO) enhances neurogenesis, neuroprotection and regeneration. Here, we examined the effects of EPO on axonal and dendritic growth in a model of neuronal polarization. EPO did not effect survival or the polarized morphology of hippocampal neurons but its effect on neurite outgrowth depended upon the stage of polarization. When added to neurons in the process of establishing polarity (0-2 days in vitro (DIV)), it enhanced axonal and dendritic growth, while EPO added to early polarized cultures at 3-4 DIV promoted the growth of axons but not dendrites. EPO stimulated the phosphorylation of Akt at serine-473 and co-incubation of the Akt/PI-3 kinase pathway inhibitor LY294002 with EPO abolished its effects on Akt phosphorylation and axonal growth. However, while Leukemia Inhibitory Factor (LIF) similarly stimulated phosphorylation of Akt, it had no effect on axonal or dendritic growth, indicating that AKT phosphorylation is necessary but not sufficient for neurite outgrowth in hippocampal neurons.

Regulation of neurite outgrowth by G(i/o) signaling pathways

Neurogenesis is a long and winding journey. A neural progenitor cell migrates long distances, differentiates by forming a single axon and multiple dendrites, undergoes maturation, and ultimately survives. The initial formation of neurites during neuronal differentiation, commonly referred to as "neurite outgrowth," can be induced by a large repertoire of signals that stimulate an array of receptors and downstream signaling pathways. The G(i/o) family of heterotrimeric G-proteins are abundantly expressed in the brain and enriched at neuronal growth cones. Recent evidence has uncovered several G(i/o)-coupled receptors that induce neurite outgrowth and has begun to elucidate the underlying molecular mechanisms. Emerging data suggests that signals from several G(i/o)-coupled receptors converge at the transcription factor STAT3 to regulate neurite outgrowth and at Rac1 and Cdc42 to regulate cytoskeletal reorganization. Physiologically, signaling through G(i/o)-coupled cannabinoid receptors is critical for pro percentral nervous system development. As the mechanisms by which G(i/o)-coupled receptors regulate neurite outgrowth are clarified, it is becoming evident that modulating signals from G(i/o) and their receptors has great potential for the treatment of neurodegenerative diseases.

Roles of unphosphorylated STATs in signaling

The seven members of the signal transducer and activator of transcription (STAT) family of transcription factors are activated in response to many different cytokines and growth factors by phosphorylation of specific tyrosine residues. The STAT1 and STAT3 genes are specific targets of activated STATs 1 and 3, respectively, resulting in large increases in the levels of these unphosphorylated STATs (U-STATs) in response to the interferons (STAT1) or ligands that active gp130, such as IL-6 (STAT3). U-STATs drive gene expression by novel mechanisms distinct from those used by phosphorylated STAT (P-STAT) dimers. In this review, we discuss the roles of U-STATs in transcription and regulation of gene expression.

BDNF and the diseased nervous system: a delicate balance between adaptive and pathological processes of gene regulation

It is clear that brain-derived neurotrophic factor (BDNF) plays a crucial role in organizing the response of the genome to dynamic changes in the extracellular environment that enable brain plasticity. BDNF has emerged as one of the most important signaling molecules for the developing nervous system as well as the impaired nervous system, and multiple diseases, such as Alzheimer's, Parkinson's, Huntington's, epilepsy, Rett's syndrome, and psychiatric depression, are linked by their association with potential dysregulation of BDNF-driven signal transduction programs. These programs are responsible for controlling the amount of activated transcription factors, such as cAMP response element binding protein, that coordinate the expression of multiple brain proteins, like ion channels and early growth response factors, whose job is to maintain the balance of excitation and inhibition in the nervous system. In this review, we will explore the evidence for BDNF's role in gene regulation side by side with its potential role in the etiology of neurological diseases. It is hoped that by bringing the datasets together in these diverse fields we can help develop the foundation for future studies aimed at understanding basic principles of gene regulation in the nervous system and how they can be harnessed to develop new therapeutic opportunities.

Epigenetic regulation of kappa opioid receptor gene in neuronal differentiation

The gene of mouse kappa opioid receptor (KOR) utilizes two promoters, P1 and P2. P1 is active in various brain areas and constitutively in P19 mouse embryonal carcinoma cells. P2 is active in limited brain stem areas of adult animals and only in late differentiated cells of P19 induced for neuronal differentiation in the presence of nerve growth factor (NGF). NGF response of P2 was found to be mediated by a specific binding site for transcription factor activation protein 2 (AP2) located in P2. Electrophoretic gel shift assay showed specific binding of this AP2 site by AP2beta, but not AP2alpha. Knockdown of endogenous AP2beta with siRNA abolished the stimulating effect of NGF on the expression of transcripts driven by P2. Binding of endogenous AP2beta on the endogenous KOR P2 chromatin region was also confirmed by chromatin immunoprecipitation. The effect of NGF was inhibited by LY2942002 (phosphatidylinositol 3-kinase, PI3K inhibitor), suggesting that PI3K was involved in signaling pathway mediating the effect of NGF stimulation on KOR P2. The chromatin of P2 in P19 was found to be specifically modified following NGF stimulation, which included demethylation at Lys9 and dimethylation at Lys4 of histone H3 and was consistent with the increased recruitment of RNA polymerase II to this promoter. This study presents the first evidence for epigenetic changes occurred on a specific KOR promoter triggered by NGF in cells undergoing neuronal differentiation. This epigenetic change is mediated by recruited AP2beta to this promoter and involves the PI3K system.

Cdk5 is involved in BDNF-stimulated dendritic growth in hippocampal neurons

Neurotrophins are key regulators of neuronal survival and differentiation during development. Activation of their cognate receptors, Trk receptors, a family of receptor tyrosine kinases (RTKs), is pivotal for mediating the downstream functions of neurotrophins. Recent studies reveal that cyclin-dependent kinase 5 (Cdk5), a serine/threonine kinase, may modulate RTK signaling through phosphorylation of the receptor. Given the abundant expression of both Cdk5 and Trk receptors in the nervous system, and their mutual involvement in the regulation of neuronal architecture and synaptic functions, it is of interest to investigate if Cdk5 may also modulate Trk signaling. In the current study, we report the identification of TrkB as a Cdk5 substrate. Cdk5 phosphorylates TrkB at Ser478 at the intracellular juxtamembrane region of TrkB. Interestingly, attenuation of Cdk5 activity or overexpression of a TrkB mutant lacking the Cdk5 phosphorylation site essentially abolishes brain-derived neurotrophic factor (BDNF)–triggered dendritic growth in primary hippocampal neurons. In addition, we found that Cdk5 is involved in BDNF-induced activation of Rho GTPase Cdc42, which is essential for BDNF-triggered dendritic growth. Our observations therefore reveal an unanticipated role of Cdk5 in TrkB-mediated regulation of dendritic growth through modulation of BDNF-induced Cdc42 activation.

Accurate transmission of information in the nervous system requires the precise formation of contact points between neurons. Regulation of these contact sites involves fine tuning the number and branching of dendritic processes on neurons. Throughout development, several secreted factors act to regulate dendrite number and branching. One important family of these factors is neurotrophins, which are indispensable for the survival and development of neurons. For example, stimulation of hippocampal neurons with one neurotrophin, brain-derived neurotrophic factor (BDNF), increases the number of dendrites directly extending from the cell body. Here, we report that BDNF-stimulated dendritic growth requires phosphorylation of the BDNF receptor, TrkB, by a kinase known as cyclin-dependent kinase 5 (Cdk5). Inhibiting phosphorylation of TrkB by Cdk5 essentially abolishes the induction of dendrites by BDNF. Our observations reveal that Cdk5 serves as a regulator of neurotrophin function. Since Cdk5 and neurotrophins both play essential roles in neuronal development, our findings suggest that the interplay between Cdk5 and TrkB may also be implicated in the regulation of other biological processes during development.

Dendritic growth stimulated by brain-derived neurotrophic factor (BDNF) requires phosphorylation of the BDNF receptor, TrkB, by a kinase known as cyclin-dependent kinase 5 (Cdk5). This study identifies a novel interplay between Cdk5 and TrkB.

The roles of cyclin-dependent kinase 5 in dendrite and synapse development

Since the isolation of cyclin-dependent kinase 5 (Cdk5), this proline-directed serine/threonine kinase has been demonstrated as an important regulator of neuronal migration, neuronal survival and synaptic functions. Recently, a number of players implicated in dendrite and synapse development have been identified as Cdk5 substrates. Neurite extension, synapse and spine maturation are all modulated by a myriad of extracellular guidance cues or trophic factors. Cdk5 was recently demonstrated to regulate signaling downstream of some of these extracellular factors, in addition to modulating Rho GTPase activity, which regulates cytoskeletal dynamics. In this communication, we summarize our existing knowledge on the pathways and mechanisms through which Cdk5 affects dendrite, synapse and spine development.

JAK/STAT pathway mediates retinal ganglion cell survival after acute ocular hypertension but not under normal conditions

Intraocular pressure (IOP) elevation is an important cause of glaucoma. Animal models of ocular hypertension have been widely used to mimic glaucoma to investigate the mechanisms underlying retinal ganglion cell (RGC) death and search for possible cure. The aim of the present study was to examine the role of JAK/STAT pathway in RGC viability in normal condition or after acute IOP elevation. Retinal explants obtained from intact or IOP-elevated eyes were firstly used to examine the effect of the JAK/STAT pathway inhibitors, AG490 and Jak Inhibitor I, on RGC viability in vitro. The role of this signal pathway was further investigated and confirmed in vivo. AG490 and Jak Inhibitor I were applied into the left eye on days 3, 9, and 15 post 2-h IOP elevation at 110mmHg. Fluorescence dye Fluorogold was used to retrogradely label surviving RGCs. Because macrophage recruitment was seen in the IOP-elevated eyes after inhibition of this pathway, clodronate liposomes were used to remove phagocytic cells in the eye and examine the role of JAK/STAT pathway in RGC survival independent of macrophages. Activities and location of JAK/STAT pathway in the retina were examined using Western blotting and immunohistochemistry. We found that inhibition of JAK/STAT pathway did not affect RGC survival in the retinal explants derived from intact eye but caused RGC death in the retinal explants that were derived from IOP-elevated eye. Importantly, the detrimental effect of JAK/STAT pathway inhibition on RGC survival was also observed in vivo following acute IOP elevation, but not in intact eye. In addition, both in vitro and in vivo experiments confirmed a detrimental action of phagocytic cells following acute IOP elevation and the pathway inhibition. Compatible with what were observed in vivo, Western blotting and immunohistochemistry showed that JAK/STAT activities were not present in intact retina, but acute IOP elevation activated JAK/STAT pathway in the retina, in the regions of inner nuclear layer and ganglion cell layer, including RGCs. The IOP elevation-induced JAK/STAT activities were effectively abolished by intravitreal application of AG490. This study thus shows that (1) acute IOP elevation activates JAK/STAT pathway in RGCs, and (2) JAK/STAT pathway mediates RGC survival following IOP elevation but not under normal condition.

Unphosphorylated STAT3 accumulates in response to IL-6 and activates transcription by binding to NFkappaB

gp130-linked cytokines such as interleukin-6 (IL-6) stimulate the formation of tyrosine-phosphorylated signal transducer and activator of transcription 3 (P-STAT3), which activates many genes, including the STAT3 gene itself. The resulting increase in the concentration of unphosphorylated STAT3 (U-STAT3) drives a second wave of expression of genes such as RANTES, IL6, IL8, MET, and MRAS that do not respond directly to P-STAT3. Thus, U-STAT3 sustains cytokine-dependent signaling at late times through a mechanism completely distinct from that used by P-STAT3. Many U-STAT3-responsive genes have kappaB elements that are activated by a novel transcription factor complex formed when U-STAT3 binds to unphosphorylated NFkappaB (U-NFkappaB), in competition with IkappaB. The U-STAT3/U-NFkappaB complex accumulates in the nucleus with help from the nuclear localization signal of STAT3, activating a subset of kappaB-dependent genes. Additional genes respond to U-STAT3 through an NFkappaB-independent mechanism. The role of signal-dependent increases in U-STAT3 expression in regulating gene expression is likely to be important in physiological responses to gp130-linked cytokines and growth factors that activate STAT3, and in cancers that have constitutively active P-STAT3.