Dok-4 regulates GDNF-dependent neurite outgrowth through downstream activation of Rap1 and mitogen-activated protein kinase

Analgesic targets identified in mouse sensory neuron somata and terminal pain translatomes

The relationship between transcription and protein expression is complex. We identified polysome-associated RNA transcripts in the somata and central terminals of mouse sensory neurons in control, painful (plus nerve growth factor), and pain-free conditions (Nav1.7-null mice). The majority (98%) of translated transcripts are shared between male and female mice in both the somata and terminals. Some transcripts are highly enriched in the somata or terminals. Changes in the translatome in painful and pain-free conditions include novel and known regulators of pain pathways. Antisense knockdown of selected somatic and terminal polysome-associated transcripts that correlate with pain states diminished pain behavior. Terminal-enriched transcripts included those encoding synaptic proteins (e.g., synaptotagmin), non-coding RNAs, transcription factors (e.g., Znf431), proteins associated with transsynaptic trafficking (HoxC9), GABA-generating enzymes (Gad1 and Gad2), and neuropeptides (Penk). Thus, central terminal translation may well be a significant regulatory locus for peripheral input from sensory neurons.

Small-molecule inhibition of the METTL3/METTL14 complex suppresses neuroblastoma tumor growth and promotes differentiation

The N6-methyladenosine (m6A) RNA modification is an important regulator of gene expression. m6A is deposited by a methyltransferase complex that includes methyltransferase-like 3 (METTL3) and methyltransferase-like 14 (METTL14). High levels of METTL3/METTL14 drive the growth of many types of adult cancer, and METTL3/METTL14 inhibitors are emerging as new anticancer agents. However, little is known about the m6A epitranscriptome or the role of the METTL3/METTL14 complex in neuroblastoma, a common pediatric cancer. Here, we show that METTL3 knockdown or pharmacologic inhibition with the small molecule STM2457 leads to reduced neuroblastoma cell proliferation and increased differentiation. These changes in neuroblastoma phenotype are associated with decreased m6A deposition on transcripts involved in nervous system development and neuronal differentiation, with increased stability of target mRNAs. In preclinical studies, STM2457 treatment suppresses the growth of neuroblastoma tumors in vivo. Together, these results support the potential of METTL3/METTL14 complex inhibition as a therapeutic strategy against neuroblastoma.

Optogenetic dissection of RET signaling reveals robust activation of ERK and enhanced filopodia-like protrusions of regenerating axons

RET (REarranged during Transfection) is a receptor tyrosine kinase that transduces various external stimuli into biological functions, such as survival and differentiation, in neurons. In the current study, we developed an optogenetic tool for modulating RET signaling, termed optoRET, combining the cytosolic region of human RET with a blue-light-inducible homo-oligomerizing protein. By varying the duration of photoactivation, we were able to dynamically modulate RET signaling. Activation of optoRET recruited Grb2 (growth factor receptor-bound protein 2) and stimulated AKT and ERK (extracellular signal-regulated kinase) in cultured neurons, evoking robust and efficient ERK activation. By locally activating the distal part of the neuron, we were able to retrogradely transduce the AKT and ERK signal to the soma and trigger formation of filopodia-like F-actin structures at stimulated regions through Cdc42 (cell division control 42) activation. Importantly, we successfully modulated RET signaling in dopaminergic neurons of the substantia nigra in the mouse brain. Collectively, optoRET has the potential to be developed as a future therapeutic intervention, modulating RET downstream signaling with light.

Intracellular RET signaling pathways activated by GDNF

Activation of REarranged during Transfection (RET) proto-oncogene is responsible for various human cancers such as papillary and medullary thyroid carcinomas and non-small cell lung carcinomas. RET activation in these tumors is caused by point mutations or gene rearrangements, resulting in constitutive activation of RET tyrosine kinase. Physiologically, RET is activated by glial cell line-derived neurotrophic factor (GDNF) ligands that bind to coreceptor GDNF family receptor alphas (GFRαs), leading to RET dimerization. GDNF-GFRα1-RET signaling plays crucial roles in the development of the enteric nervous system, kidney and lower urinary tract as well as in spermatogenesis. Intracellular tyrosine phosphorylation in RET and recruitment of adaptor proteins to phosphotyrosines are essential for various biological functions. Significance of intracellular RET signaling pathways activated by GDNF is discussed and summarized in this review.

Hippo kinases MST1 and MST2 control the differentiation of the epididymal initial segment via the MEK-ERK pathway

Although the roles of the Hippo pathway in organogenesis and tumorigenesis have been well studied in multiple organs, its role in sperm maturation and male fertility has not been investigated. The initial segment (IS) of the epididymis plays a critical role in sperm maturation. IS differentiation is governed by ERK1/2, but the mechanisms of ERK1/2 activation in IS are not fully understood. Here we show that double knockout (dKO) of mammalian sterile 20-like kinases 1 and 2 (Mst1 and Mst2), homologs of Hippo in Drosophila, in the epididymal epithelium led to male infertility in mice. Sperm in the cauda epididymides of mutant mice were immotile with flagellar angulation and severely disorganized structures. Loss of Mst1/2 activated YAP and increased proliferation and cell death in all the segments of epididymis. The mutant mice showed substantially suppressed MEK/ERK signaling in the IS and failed IS differentiation. Deletion of Yap restored the reduced MEK/ERK signaling, and partially rescued the defective IS differentiation and fertility in Mst1/2 dKO mice. Our results demonstrate that YAP inhibits the MEK/ERK pathway in IS epithelial cells, and MST1/2 control IS differentiation and fertility at least partially by repressing YAP. Taken together, the Hippo pathway is essential for sperm maturation and male fertility.

Synaptic and Gene Regulatory Mechanisms in Schizophrenia, Autism, and 22q11.2 Copy Number Variant-Mediated Risk for Neuropsychiatric Disorders

BACKGROUND

22q11.2 copy number variants are among the most highly penetrant genetic risk variants for developmental neuropsychiatric disorders such as schizophrenia (SCZ) and autism spectrum disorder (ASD). However, the specific mechanisms through which they confer risk remain unclear.

METHODS

Using a functional genomics approach, we integrated transcriptomic data from the developing human brain, genome-wide association findings for SCZ and ASD, protein interaction data, and gene expression signatures from SCZ and ASD postmortem cortex to 1) organize genes into the developmental cellular and molecular systems within which they operate, 2) identify neurodevelopmental processes associated with polygenic risk for SCZ and ASD across the allelic frequency spectrum, and 3) elucidate pathways and individual genes through which 22q11.2 copy number variants may confer risk for each disorder.

RESULTS

Polygenic risk for SCZ and ASD converged on partially overlapping neurodevelopmental modules involved in synaptic function and transcriptional regulation, with ASD risk variants additionally enriched for modules involved in neuronal differentiation during fetal development. The 22q11.2 locus formed a large protein network during development that disproportionately affected SCZ-associated and ASD-associated neurodevelopmental modules, including loading highly onto synaptic and gene regulatory pathways. SEPT5, PI4KA, and SNAP29 genes are candidate drivers of 22q11.2 synaptic pathology relevant to SCZ and ASD, and DGCR8 and HIRA are candidate drivers of disease-relevant alterations in gene regulation.

CONCLUSIONS

This approach offers a powerful framework to identify neurodevelopmental processes affected by diverse risk variants for SCZ and ASD and elucidate mechanisms through which highly penetrant, multigene copy number variants contribute to disease risk.

Evolutionary and expression analyses reveal a pattern of ancient duplications and functional specializations in the diversification of the Downstream of Kinase (DOK) genes

Downstream of Kinase (DOK) proteins represent a multigenic family of adaptors that includes negative regulators of immune cell signaling. Using phylogenetics and intron/exon structure data, we show here that the seven human DOK genes (DOK1 to DOK7) form three highly divergent groups that emerged before the protostome-deuterostome split: DOK1/2/3, DOK4/5/6, and DOK7. For two of these three groups (DOK1/2/3 and DOK4/5/6), further gene duplications occurred in vertebrates and so while chordates only have three DOK genes, vertebrates have seven DOK genes over the three groups. From our expression analysis in humans, we show that each group of DOK genes has a distinct pattern of expression. The DOK1/2/3 group is immune specific, yet each of the three genes in the group has a distinct pattern of expression in immune cells. This immune specificity could thus be ancestral, with the DOK1/2/3 gene also being immune-related in protostomes. The DOK4/5/6 and DOK7 groups represent genes that are much less expressed in immune system than the DOK1/2/3 group. Interestingly, we identify a novel tyrosine based motif that is specific to the vertebrate DOK4/5/6 sequences. The evolution of the DOK genes is thus marked by a pattern of ancient duplications and functional specializations.

Essential Role of Linx/Islr2 in the Development of the Forebrain Anterior Commissure

Linx is a member of the leucine-rich repeat and immunoglobulin family of membrane proteins which has critical roles in the development of the peripheral nervous system and forebrain connectivity. A previous study showed that Linx is expressed in projection neurons in the cortex and in cells that comprise the passage to the prethalamus that form the internal capsule, indicating the involvement of Linx in axon guidance and cell-cell communication. In this study, we found that Linx-deficient mice develop severe hydrocephalus and die perinatally by unknown mechanisms. Importantly, mice heterozygous for the linx gene exhibited defects in the development of the anterior commissure in addition to hydrocephalus, indicating haploinsufficiency of the linx gene in forebrain development. In N1E-115 neuroblastoma cells and primary cultured hippocampal neurons, Linx depletion led to impaired neurite extension and an increase in cell body size. Consistent with this, but of unknown significance, we found that Linx interacts with and upregulates the activity of Rho-kinase, a modulator of many cellular processes including cytoskeletal organization. These data suggest a role for Linx in the regulation of complex forebrain connectivity, and future identification of its extracellular ligand(s) will help clarify this function.

The GDNF Family: A Role in Cancer?

The glial cell line-derived neurotrophic factor (GDNF) family of ligands (GFLs) comprising of GDNF, neurturin, artemin, and persephin plays an important role in the development and maintenance of the central and peripheral nervous system, renal morphogenesis, and spermatogenesis. Here we review our current understanding of GFL biology, and supported by recent progress in the area, we examine their emerging role in endocrine-related and other non-hormone-dependent solid neoplasms. The ability of GFLs to elicit actions that resemble those perturbed in an oncogenic phenotype, alongside mounting evidence of GFL involvement in tumor progression, presents novel opportunities for therapeutic intervention.

Activation of PERK Elicits Memory Impairment through Inactivation of CREB and Downregulation of PSD95 After Traumatic Brain Injury

The PKR-like ER kinase (PERK), a transmembrane protein, resides in the endoplasmic reticulum (ER). Its activation serves as a key sensor of ER stress, which has been implicated in traumatic brain injury (TBI). The loss of memory is one of the most common symptoms after TBI, but the precise role of PERK activation in memory impairment after TBI has not been well elucidated. Here, we have shown that blocking the activation of PERK using GSK2656157 prevents the loss of dendritic spines and rescues memory deficits after TBI. To elucidate the molecular mechanism, we found that activated PERK phosphorylates CAMP response element binding protein (CREB) and PSD95 directly at the S129 and T19 residues, respectively. Phosphorylation of CREB protein prevents its interaction with a coactivator, CREB-binding protein, and subsequently reduces the BDNF level after TBI. Conversely, phosphorylation of PSD95 leads to its downregulation in pericontusional cortex after TBI in male mice. Treatment with either GSK2656157 or overexpression of a kinase-dead mutant of PERK (PERK-K618A) rescues BDNF and PSD95 levels in the pericontusional cortex by reducing phosphorylation of CREB and PSD95 proteins after TBI. Similarly, administration of either GSK2656157 or overexpression of PERK-K618A in primary neurons rescues the loss of dendritic outgrowth and number of synapses after treatment with a PERK activator, tunicamycin. Therefore, our study suggests that inhibition of PERK phosphorylation could be a potential therapeutic target to restore memory deficits after TBI.SIGNIFICANCE STATEMENT Traumatic brain injury (TBI) is the leading cause of death and disability around the world and affects 1.7 million Americans each year. Here, we have shown that TBI-activated PKR-like ER kinase (PERK) is responsible for memory deficiency, which is the most common problem in TBI patients. A majority of PERK's biological activities have been attributed to its function as an eIF2α kinase. However, our study suggests that activated PERK mediates its function via increasing phosphorylation of CAMP response element binding protein (CREB) and PSD95 after TBI. Blocking PERK phosphorylation rescues spine loss and memory deficits independently of phosphorylation of eIF2α. Therefore, our study suggests that CREB and PSD95 are novel substrates of PERK, so inhibition of PERK phosphorylation using GSK2656157 would be beneficial against memory impairment after TBI.

Binding and inhibition of the ternary complex factor Elk-4/Sap1 by the adapter protein Dok-4

The adapter protein Dok-4 (downstream of kinase-4) has been reported as both an activator and inhibitor of Erk and Elk-1, but lack of knowledge about the identity of its partner molecules has precluded any mechanistic insight into these seemingly conflicting properties. We report that Dok-4 interacts with the transactivation domain of Elk-4 through an atypical phosphotyrosine-binding domain-mediated interaction. Dok-4 possesses a nuclear export signal and can relocalize Elk-4 from nucleus to cytosol, whereas Elk-4 possesses two nuclear localization signals that restrict interaction with Dok-4. The Elk-4 protein, unlike Elk-1, is highly unstable in the presence of Dok-4, through both an interaction-dependent mechanism and a pleckstrin homology domain-dependent but interaction-independent mechanism. This is reversed by proteasome inhibition, depletion of endogenous Dok-4 or lysine-to-arginine mutation of putative Elk-4 ubiquitination sites. Finally, Elk-4 transactivation is potently inhibited by Dok-4 overexpression but enhanced by Dok-4 knockdown in MDCK renal tubular cells, which correlates with increased basal and EGF-induced expression of Egr-1, Fos and cylcinD1 mRNA, and cell proliferation despite reduced Erk activation. Thus, Dok-4 can target Elk-4 activity through multiple mechanisms, including binding of the transactivation domain, nuclear exclusion and protein destabilization, without a requirement for inhibition of Erk.

mRNA expression of DOK1-6 in human breast cancer

AIM: To examine the expression of downstream of tyrosine kinase (DOK)1-6 genes in normal and breast cancer tissue and correlated this with several clinico-pathological and prognostic factors.

METHODS: DOK1-6 mRNA extraction and reverse transcription were performed on fresh frozen breast cancer tissue samples (n = 112) and normal background breast tissue (n = 31). Tissues were collected between 1991 and 1996 at two centres and all patients underwent mastectomy and ipsilateral axillary node dissection. All tissues were randomly numbered and the details were only made known after all analyses were completed. Transcript levels of expression were determined using real-time polymerase chain reaction and analyzed against TNM stage, tumour grade and clinical outcome over a 10-year follow-up period.

RESULTS: DOK-2 and DOK-6 expression decreased with increasing TNM stage. DOK-6 expression decreased with increasing Nottingham Prognostic Index (NPI) [NPI-1 vs NPI-3 (mean copy number 15.4 vs 0.22, 95%CI: 2.7-27.6, P = 0.018) and NPI-2 vs NPI-3 (mean copy number 7.6 vs 0.22, 95%CI: 0.1-14.6, P = 0.048)]. After a median follow up period of 10 years, higher levels of DOK-2 expression were found among patients who remained disease-free compared to those who developed local or distant recurrence (mean copy number 3.94 vs 0.0000096, 95%CI: 1.0-6.85, P = 0.0091), and distant recurrence (mean copy number 3.94 vs 0.0025, 95%CI: 1.0-6.84, P = 0.0092). Patients who remained disease-free had higher levels of DOK-6 expression compared to those who died from breast cancer.

CONCLUSION: Decreasing expression levels of DOK-2 and DOK-6 with increased breast tumour progression supports the notion that DOK-2 and DOK-6 behave as tumour suppressors in human breast cancer.

BDNF and NT4 play interchangeable roles in gustatory development

A limited number of growth factors are capable of regulating numerous developmental processes, but how they accomplish this is unclear. The gustatory system is ideal for examining this issue because the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) have different developmental roles although both of them activate the same receptors, TrkB and p75. Here we first investigated whether the different roles of BDNF and NT4 are due to their differences in temporal and spatial expression patterns. Then, we asked whether or not these two neurotrophins exert their unique roles on the gustatory system by regulating different sets of downstream genes. By using Bdnf(Nt4/Nt4) mice, in which the coding region for BDNF is replaced with NT4, we examined whether the different functions of BDNF and NT4 are interchangeable during taste development. Our results demonstrated that NT4 could mediate most of the unique roles of BDNF during taste development. Specifically, caspase-3-mediated cell death, which was increased in the geniculate ganglion in Bdnf(-/-) mice, was rescued in Bdnf(Nt4/Nt4) mice. In BDNF knockout mice, tongue innervation was disrupted, and gustatory axons failed to reach their targets. However, disrupted innervation was rescued and target innervation is normal when NT4 replaced BDNF. Genome wide expression analyses revealed that BDNF and NT4 mutant mice exhibited different gene expression profiles in the gustatory (geniculate) ganglion. Compared to wild type, the expression of differentiation-, apoptosis- and axon guidance-related genes was changed in BDNF mutant mice, which is consistent with their different roles during taste development. However, replacement of BDNF by NT4 rescued these gene expression changes. These findings indicate that the functions of BDNF and NT4 in taste development are interchangeable. Spatial and temporal differences in BDNF and NT4 expression can regulate differential gene expression in vivo and determine their specific roles during development.

Doxycycline-regulated GDNF expression promotes axonal regeneration and functional recovery in transected peripheral nerve

Increased production of neurotrophic factors (NTFs) is one of the key responses seen following peripheral nerve injury, making them an attractive choice for pro-regenerative gene therapies. However, the downside of over-expression of certain NTFs, including glial cell line-derived neurotrophic factor (GDNF), was earlier found to be the trapping and misdirection of regenerating axons, the so-called 'candy-store' effect. We report a proof-of-principle study on the application of conditional GDNF expression system in injured peripheral nerve. We engineered Schwann cells (SCs) using dendrimers or lentiviral transduction with the vector providing doxycycline-regulated GDNF expression. Injection of GDNF-modified cells into the injured peripheral nerve followed by time-restricted administration of doxycycline demonstrated that GDNF expression in SCs can also be controlled locally in the peripheral nerves of the experimental animals. Cell-based GDNF therapy was shown to increase the extent of axonal regeneration, while controlled deactivation of GDNF effectively prevented trapping of regenerating axons in GDNF-enriched areas, and was associated with improved functional recovery.

MOR is not enough: identification of novel mu-opioid receptor interacting proteins using traditional and modified membrane yeast two-hybrid screens

The mu-opioid receptor (MOR) is the G-protein coupled receptor primarily responsible for mediating the analgesic and rewarding properties of opioid agonist drugs such as morphine, fentanyl, and heroin. We have utilized a combination of traditional and modified membrane yeast two-hybrid screening methods to identify a cohort of novel MOR interacting proteins (MORIPs). The interaction between the MOR and a subset of MORIPs was validated in pulldown, co-immunoprecipitation, and co-localization studies using HEK293 cells stably expressing the MOR as well as rodent brain. Additionally, a subset of MORIPs was found capable of interaction with the delta and kappa opioid receptors, suggesting that they may represent general opioid receptor interacting proteins (ORIPS). Expression of several MORIPs was altered in specific mouse brain regions after chronic treatment with morphine, suggesting that these proteins may play a role in response to opioid agonist drugs. Based on the known function of these newly identified MORIPs, the interactions forming the MOR signalplex are hypothesized to be important for MOR signaling and intracellular trafficking. Understanding the molecular complexity of MOR/MORIP interactions provides a conceptual framework for defining the cellular mechanisms of MOR signaling in brain and may be critical for determining the physiological basis of opioid tolerance and addiction.

Zic2 is required for enteric nervous system development and neurite outgrowth: a mouse model of enteric hyperplasia and dysplasia

From a forward genetic screen in mice we isolated a novel Zic2 allele with abnormal aggregation of meconium in the gastrointestinal tract. Zic2(m1Nisw) mutant embryos show an increase in the number of enteric neurons in vivo and disorganization of the neurite network. Explant culture of Zic2(m1Nisw) gastrointestinal tract show extensive neurite outgrowth, suggesting that Zic2 is a negative regulator of nerve fiber growth. These studies demonstrate a previously unknown function of Zic2 and provide a novel animal model of enteric nervous system dysplasia and hyperplasia.

Glial cell line-derived neurotrophic factor (GDNF) induces neuritogenesis in the cochlear spiral ganglion via neural cell adhesion molecule (NCAM)

Glial cell line-derived neurotrophic factor (GDNF) increases survival and neurite extension of spiral ganglion neurons (SGNs), the primary neurons of the auditory system, via yet unknown signaling mechanisms. In other cell types, signaling is achieved by the GPI-linked GDNF family receptor α1 (GFRα1) via recruitment of transmembrane receptors: Ret (re-arranged during transformation) and/or NCAM (neural cell adhesion molecule). Here we show that GDNF enhances neuritogenesis in organotypic cultures of spiral ganglia from 5-day-old rats and mice. Addition of GFRα1-Fc increases this effect. GDNF/GFRα1-Fc stimulation activates intracellular PI3K/Akt and MEK/Erk signaling cascades as detected by Western blot analysis of cultures prepared from rats at postnatal days 5 (P5, before the onset of hearing) and 20 (P20, after the onset of hearing). Both cascades mediate GDNF stimulation of neuritogenesis, since application of the Akt inhibitor Wortmannin or the Erk inhibitor U0126 abolished GDNF/GFRα1-Fc stimulated neuritogenesis in P5 rats. Since cultures of P5 NCAM-deficient mice failed to respond by neuritogenesis to GDNF/GFRα1-Fc, we conclude that NCAM serves as a receptor for GDNF signaling responsible for neuritogenesis in early postnatal spiral ganglion.

Stimulation of angiogenesis, neurogenesis and regeneration by side population cells from dental pulp

Mesenchymal stem cells (MSCs) have been used for cell therapy in various experimental disease models. However, the regenerative potential of MSCs from different tissue sources and the influence of the tissue niche have not been investigated. In this study, we compared the regenerative potential of dental pulp, bone marrow and adipose tissue-derived CD31(-) side population (SP) cells isolated from an individual porcine source. Pulp CD31(-) SP cells expressed the highest levels of angiogenic/neurotrophic factors and had the highest migration activity. Conditioned medium from pulp CD31(-) SP cells produced potent anti-apoptotic activity and neurite outgrowth, compared to those from bone marrow and adipose CD31(-) SP cells. Transplantation of pulp CD31(-) SP cells in a mouse hindlimb ischemia model produced higher blood flow and capillary density than transplantation of bone marrow and adipose CD31(-) SP cells. Motor function recovery and infarct size reduction were greater with pulp CD31(-) SP cells. Pulp CD31(-) SP cells induced maximal angiogenesis, neurogenesis and pulp regeneration in ectopic transplantation models compared to other tissue sources. These results demonstrate that pulp stem cells have higher angiogenic, neurogenic and regenerative potential and may therefore be superior to bone marrow and adipose stem cells for cell therapy.

Promoting engraftment of transplanted neural stem cells/progenitors using biofunctionalised electrospun scaffolds

With the brain's limited capacity for repair, new and innovative approaches are required to promote regeneration. While neural transplantation for a number of neural disease/injuries have been demonstrated, major limitations in the field include poor cell survival and integration. This, in part, is due to the non-conducive environment of the adult brain, failing to provide adequate chemical and physical support for new neurons. Here we examine the capacity of fibrous poly ε-caprolactone (PCL) scaffolds, biofunctionalised with immobilised glial cell-derived neurotrophic factor (GDNF), to influence primary cortical neural stem cells/progenitors in vitro and enhance integration of these cells following transplantation into the brain parenchyma. Immobilisation of GDNF was confirmed prior to in vitro culturing and at 28 days after implantation into the brain, demonstrating long-term delivery of the protein. In vitro, we demonstrate that PCL with immobilised GDNF (iGDNF) significantly enhances cell viability and neural stem cell/progenitor proliferation compared to conventional 2-dimensional cultureware. Upon implantation, PCL scaffolds including iGDNF enhanced the survival, proliferation, migration, and neurite growth of transplanted cortical cells, whilst suppressing inflammatory reactive astroglia.

RET PLCγ phosphotyrosine binding domain regulates Ca2+ signaling and neocortical neuronal migration

The receptor tyrosine kinase RET plays an essential role during embryogenesis in regulating cell proliferation, differentiation, and migration. Upon glial cell line-derived neurotrophic factor (GDNF) stimulation, RET can trigger multiple intracellular signaling pathways that in concert activate various downstream effectors. Here we report that the RET receptor induces calcium (Ca²⁺) signaling and regulates neocortical neuronal progenitor migration through the Phospholipase-C gamma (PLCγ) binding domain Tyr1015. This signaling cascade releases Ca²⁺ from the endoplasmic reticulum through the inositol 1,4,5-trisphosphate receptor and stimulates phosphorylation of ERK1/2 and CaMKII. A point mutation at Tyr1015 on RET or small interfering RNA gene silencing of PLCγ block the GDNF-induced signaling cascade. Delivery of the RET mutation to neuronal progenitors in the embryonic ventricular zone using in utero electroporation reveal that Tyr1015 is necessary for GDNF-stimulated migration of neurons to the cortical plate. These findings demonstrate a novel RET mediated signaling pathway that elevates cytosolic Ca²⁺ and modulates neuronal migration in the developing neocortex through the PLCγ binding domain Tyr1015.

Intracellular signal transduction and modification of the tumor microenvironment induced by RET/PTCs in papillary thyroid carcinoma

RET gene rearrangements (RET/PTCs) represent together with BRAF point mutations the two major groups of mutations involved in papillary thyroid carcinoma (PTC) initiation and progression. In this review, we will examine the mechanisms involved in RET/PTC-induced thyroid cell transformation. In detail, we will summarize the data on the molecular mechanisms involved in RET/PTC formation and in its function as a dominant oncogene, on the activated signal transduction pathways and on the induced gene expression modifications. Moreover, we will report on the effects of RET/PTCs on the tumor microenvironment. Finally, a short review of the literature on RET/PTC prognostic significance will be presented.

Rap1GAP interacts with RET and suppresses GDNF-induced neurite outgrowth

Glial cell line-derived neurotrophic factor (GDNF) was originally recognized for its ability to promote survival of midbrain dopaminergic neurons, but it has since been demonstrated to be crucial for the survival and differentiation of many neuronal subpopulations, including motor neurons, sympathetic neurons, sensory neurons and enteric neurons. To identify possible effectors or regulators of GDNF signaling, we performed a yeast two-hybrid screen using the intracellular domain of RET, the common signaling receptor of the GDNF family, as bait. Using this approach, we identified Rap1GAP, a GTPase-activating protein (GAP) for Rap1, as a novel RET-binding protein. Endogenous Rap1GAP co-immunoprecipitated with RET in neural tissues, and RET and Rap1GAP were co-expressed in dopaminergic neurons of the mesencephalon. In addition, overexpression of Rap1GAP attenuated GDNF-induced neurite outgrowth, whereas suppressing the expression of endogenous Rap1GAP by RNAi enhanced neurite outgrowth. Furthermore, using co-immunoprecipitation analyses, we found that the interaction between RET and Rap1GAP was enhanced following GDNF treatment. Mutagenesis analysis revealed that Tyr981 in the intracellular domain of RET was crucial for the interaction with Rap1GAP. Moreover, we found that Rap1GAP negatively regulated GNDF-induced ERK activation and neurite outgrowth. Taken together, our results suggest the involvement of a novel interaction of RET with Rap1GAP in the regulation of GDNF-mediated neurite outgrowth.

The roles of Dok family adapters in immunoreceptor signaling

The mammalian Dok protein family has seven members (Dok-1-Dok-7). The Dok proteins share structural similarities characterized by the NH2-terminal pleckstrin homology and phosphotyrosine-binding domains followed by SH2 target motifs in the COOH-terminal moiety, indicating an adapter function. Indeed, Dok-1 was originally identified as a 62 kDa protein that binds with p120 rasGAP, a potent inhibitor of Ras, upon tyrosine phosphorylation by a variety of protein tyrosine kinases. Among the Dok family, only Dok-1, Dok-2, and Dok-3 are preferentially expressed in hematopoietic/immune cells. Dok-1 and its closest relative Dok-2 act as negative regulators of the Ras-Erk pathway downstream of many immunoreceptor-mediated signaling systems, and it is believed that recruitment of p120 rasGAP by Dok-1 and Dok-2 is critical to their negative regulation. By contrast, Dok-3 does not bind with p120 rasGAP. However, accumulating evidence has demonstrated that Dok-3 is a negative regulator of the activation of JNK and mobilization of Ca2+ in B-cell receptor-mediated signaling, where the interaction of Dok-3 with SHIP-1 and Grb2 appears to be important. Here, we review the physiological roles and underlying mechanisms of Dok family proteins.

Downstream of tyrosine kinase/docking protein 6, as a novel substrate of tropomyosin-related kinase C receptor, is involved in neurotrophin 3-mediated neurite outgrowth in mouse cortex neurons

BACKGROUND

The downstream of tyrosine kinase/docking protein (Dok) adaptor protein family has seven members, Dok1 to Dok7, that act as substrates of multiple receptor tyrosine kinase and non-receptor tyrosine kinase. The tropomyosin-related kinase (Trk) receptor family, which has three members (TrkA, TrkB and TrkC), are receptor tyrosine kinases that play pivotal roles in many stages of nervous system development, such as differentiation, migration, axon and dendrite projection and neuron patterning. Upon related neurotrophin growth factor stimulation, dimerisation and autophosphorylation of Trk receptors can occur, recruiting adaptor proteins to mediate signal transduction.

RESULTS

In this report, by using yeast two-hybrid assays, glutathione S-transferase (GST) precipitation assays and coimmunoprecipitation (Co-IP) experiments, we demonstrate that Dok6 selectively binds to the NPQY motif of TrkC through its phosphotyrosine-binding (PTB) domain in a kinase activity-dependent manner. We further confirmed their interaction by coimmunoprecipitation and colocalisation in E18.5 mouse cortex neurons, which provided more in vivo evidence. Next, we demonstrated that Dok6 is involved in neurite outgrowth in mouse cortex neurons via the RNAi method. Knockdown of Dok6 decreased neurite outgrowth in cortical neurons upon neurotrophin 3 (NT-3) stimulation.

CONCLUSIONS

We conclude that Dok6 interacts with the NPQY motif of the TrkC receptor through its PTB domain in a kinase activity-dependent manner, and works as a novel substrate of the TrkC receptor involved in NT-3-mediated neurite outgrowth in mouse cortex neurons.

Analysis of DOK-6 function in downstream signaling of RET in human neuroblastoma cells

Point mutations and structural alterations of the RET tyrosine kinase gene cause multiple endocrine neoplasia type 2 (MEN 2) and papillary thyroid carcinoma, respectively. RET activation by glial cell line-derived neurotrophic factor (GDNF) is essential for the development of the enteric nervous system and the kidney. The signal through RET tyrosine kinase requires several adaptor proteins including the DOK (downstream of kinase) family of proteins. Of the seven members of the DOK protein family, DOK-1, -4, -5, and -6 have been reported to play roles in the GDNF-RET signaling pathway. Although DOK-6 has been shown to bind to RET and promote GDNF-induced neurite outgrowth in mouse Neuro2A cells, DOK-6 function in human cells remains unclear. In the present study, we investigated the role of DOK-6 in GDNF-RET signaling in human cells including neuroblastoma cells. DOK-6 was constitutively localized to the plasma membrane via its pleckstrin homology (PH) domain, and was phosphorylated following RET activation via a MEN2A mutation or GDNF stimulation. However, DOK-6 could not significantly affect downstream signaling and neurite outgrowth in human neuroblastoma cells. The binding affinity of the DOK-6 phosphotyrosine-binding (PTB) domain to RET was much lower than that of the DOK-1, DOK-4, and SHC PTB domains to RET. These findings indicate that DOK-6 is involved in RET signaling with less influence when compared with DOK-1, DOK-4, and SHC.

Muscle-conditioned media and cAMP promote survival and neurite outgrowth of adult spinal cord motor neurons

Embryonic spinal cord motor neurons (MNs) can be maintained in vitro for weeks with a cocktail of trophic factors and muscle-derived factors under serum-containing conditions. Here we investigated the beneficial effects of muscle-derived factors in the form of muscle-conditioned medium (MCM) on the survival and neurite outgrowth of adult rat spinal cord MNs under serum-free conditions. Ventral horn dissociated cell cultures from the cervical enlargement were maintained in the presence of one or more of the following factors: brain-derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), a cell permeant cyclic adenosine-3',5'-monophosphate (cAMP) analog and MCM. The cell cultures were immunostained with several antibodies recognizing a general neuronal marker the microtubule-associated protein 2 (MAP2) and either one or more motor neuronal markers: the non-phosphorylated neurofilament heavy isoform (SMI32), the transcription factors HB9 and Islet-1 and the choline acetyl transferase. We found that treatment with MCM together with the cAMP analog was sufficient to promote selective survival and neurite outgrowth of adult spinal cord MNs. These conditions can be used to maintain adult spinal cord MNs in dissociated cultures for several weeks and may have therapeutic potential following spinal cord injury or motor neuropathies. More studies are necessary to evaluate how MCM and the cAMP analog act in synergy to promote the survival and neurite outgrowth of adult MNs.

Dok-4 is a novel negative regulator of T cell activation

Dok-4 (downstream of tyrosine kinase-4) is a recently identified member of the Dok family of adaptor proteins, which are characterized by an amino-terminal pleckstrin homology domain, a phosphotyrosine-binding domain, and a carboxyl-terminal region containing several tyrosines and poly-proline-rich motifs. Two members of the Dok family, Dok-1 and Dok-2, have already been described as negative regulators in T cells. However, the function of Dok-4, which is also expressed in T cells, remains unknown. In this study, we report that Dok-4 is phosphorylated after TCR engagement and shuttled within the cytoplasm of T cells before being recruited to the polarized microtubule organizing center after the formation of the immunological synapse. Loss-of-function experiments using RNA interference constructs show that Dok-4 is a negative regulator of ERK phosphorylation, IL-2 promoter activity, and T cell proliferation. Exogenous expression of wild-type Dok-4 induces a significant activation of Rap1, which is involved in the regulation of ERK. The pleckstrin homology domain of Dok-4 is required both for its cytoplasmic shuttling and relocalization as well as for its inhibitory properties on T cell activation. Thus, Dok-4 represents a novel negative regulator of T cells.

Transcriptional regulation of IRS5/DOK4 expression in non-small-cell lung cancer cells

The insulin-receptor substrate family plays important roles in cellular growth, signaling, and survival. Two new members of this family have recently been isolated: IRS5/Dok4 and IRS6/Dok5. This study examines the expression of IRS5/DOK4 in a panel of lung cancer cell lines and tumor specimens. The results demonstrate that expression of IRS5/DOK4 is frequently altered with both elevated and decreased expression in non-small-cell lung cancer (NSCLC) tumor specimens. The altered expression of IRS5/DOK4 observed in tumor samples is not due to aberrant methylation. In vitro cell culture studies demonstrate that treatment of NSCLC cell lines with the histone deacetylase inhibitor trichostatin A (TSA) upregulates IRS5/DOK4. This finding indicates that expression is regulated epigenetically at the level of chromatin remodeling. Chromatin immunoprecipitation experiments confirm that the IRS5/DOK4 promoter has enhanced histone hyperacetylation following treatments with TSA. Finally, hypoxia was demonstrated to downregulate IRS5/DOK4 expression. This expression was restored by TSA. The clinical relevance of altered IRS5/DOK4 expression in NSCLC requires further evaluation.

The guanine nucleotide exchange factor, C3G regulates differentiation and survival of human neuroblastoma cells

Neuronal differentiation involving neurite growth is dependent on environmental cues which are relayed by signalling pathways to actin cytoskeletal remodelling. C3G, the exchange factor for Rap1, functions in pathways leading to actin reorganization and filopodia formation, processes required during neurite growth. In the present study, we have analyzed the function of C3G, in regulating neuronal cell survival and plasticity. Human neuroblastoma cells, IMR-32 induced to differentiate by serum starvation or by treatment with nerve growth factor (NGF) or forskolin showed enhanced C3G protein levels. Transient over-expression of C3G stimulated neurite growth and also increased responsiveness to NGF and serum deprivation induced differentiation. C3G-induced neurite growth was dependent on both its catalytic and N-terminal regulatory domains, and on the functions of Cdc42 and Rap1. Knockdown of C3G using small hairpin RNA inhibited forskolin and NGF-induced morphological differentiation of IMR-32 cells. Forskolin-induced differentiation was dependent on catalytic activity of C3G. Forskolin and NGF treatment resulted in phosphorylation of C3G at Tyr504 predominantly in the Golgi. C3G expression induced the cell cycle inhibitor p21 and C3G knockdown enhanced cell death in response to serum starvation. These findings demonstrate a novel function for C3G in regulating survival and differentiation of human neuroblastoma cells.

Roles of induced expression of MAPK phosphatase-2 in tumor development in RET-MEN2A transgenic mice

Germline mutations in the RET tyrosine kinase gene are responsible for the development of multiple endocrine neoplasia 2A and 2B (MEN2A and MEN2B). However, knowledge of the fundamental principles that determine the mutant RET-mediated signaling remains elusive. Here, we report increased expression of mitogen-activated protein kinase phosphatase-2 (MKP-2) in carcinomas developed in transgenic mice carrying RET with the MEN2A mutation (RET-MEN2A). The expression of MKP-2 was not only induced by RET-MEN2A or RET-MEN2B mutant proteins but also by the activation of endogenous RET by its ligand, glial cell line-derived neurotrophic factor (GDNF). MKP-2 expression was also evident in the MKK-f cell line, which was established from a mammary tumor developed in a RET-MEN2A transgenic mouse. Inhibition of MKP-2 attenuated the in vitro and in vivo proliferation of MKK-f cells, which was mediated by the suppression of cyclin B1 expression. Furthermore, we found that MKP-2 is highly expressed in medullary thyroid carcinomas derived from MEN2A patients. These findings suggest that the increased expression of MKP-2 may play a crucial role in oncogenic signaling downstream of mutant RET, leading to deregulation of cell cycle.

Exogenous application of persephin influences phosphatidylinositol-3 kinase and MAPK/ERK signalling and enhances proliferation during early development in retinospheres

Persephin (PSPN), a member of the glial cell line-derived neurotrophic factor family, and its implication in the retina is not well understood but might be an interesting therapeutic target for degenerative diseases. Although, PSPN is lost in the chicken during evolution, its target, the GDNF family receptor alpha 4 (GFRalpha4), is still expressed in a temporal and spatial pattern in the developing retina. We used this "knockout-precondition" to study the bioactivity and the effect of exogenous PSPN application and subsequent GFRalpha activation during retinal development in vitro without impairments of endogenous PSPN. Retinospheres, derived from dissociated chicken retina of embryonic day 6, were treated with PSPN and intracellular signalling was monitored. Additionally, PSPN was added during cultivation of the retinospheres and immunhistochemical stainings and Western blotting were performed to evaluate changes in proliferation, apoptosis and differentiation. Exogenous applied PSPN enhanced phosphatidylinositol-3-kinase (PI-3K) signalling and decreased signalling of mitogen-activated protein kinases (MAPK). Most importantly early retinal proliferation was enhanced and glutamine synthetase expression was decreased whereas differentiation of major retinal cell types was not changed. In contrast to GDNF, PSPN is exclusively influencing early progenitors whereas differentiation is not effected and seems to be regulated through PSPN-independent mechanisms. Since the binding site of PSPN and therefore the target of potential therapeuticals, is well conserved among species and is with high probability not able to bind other members of the GDNF-family, these results might be assigned to other species including mammals and humans.

Dysregulated RET signaling in thyroid cancer

Numerous biologic processes and such diseases as cancer depend on activation of tyrosine kinase receptors. The RET tyrosine kinase receptor was discovered two decades ago as a transforming gene and was subsequently implicated in the formation of papillary and medullary thyroid carcinoma. This article examines the data about the mechanism of activation of downstream signal transduction pathways by RET oncoproteins. Collectively, these findings have advanced the understanding of the processes underlying thyroid carcinoma formation.

Insulin, PKC signaling pathways and synaptic remodeling during memory storage and neuronal repair

Protein kinase C (PKC) is involved in synaptic remodeling, induction of protein synthesis, and many other processes important in learning and memory. Activation of neuronal protein kinase C correlates with, and may be essential for, all phases of learning, including acquisition, consolidation, and reconsolidation. Protein kinase C activation is closely tied to hydrolysis of membrane lipids. Phospholipases C and A2 produce 1,2-diacylglycerol and arachidonic acid, which are direct activators of protein kinase C. Phospholipase C also produces inositol triphosphate, which releases calcium from internal stores. Protein kinase C interacts with many of the same pathways as insulin; therefore, it should not be surprising that insulin signaling and protein kinase C activation can both have powerful effects on memory storage and synaptic remodeling. However, investigating the possible roles of insulin in memory storage can be challenging, due to the powerful peripheral effects of insulin on glucose and the low concentration of insulin in the brain. Although peripheral for insulin, synthesized in the beta-cells of the pancreas, is primarily involved in regulating glucose, small amounts of insulin are also present in the brain. The functions of this brain insulin are inadequately understood. Protein kinase C may also contribute to insulin resistance by phosphorylating the insulin receptor substrates required for insulin signaling. Insulin is also responsible insulin-long term depression, a type of synaptic plasticity that is also dependent on protein kinase C. However, insulin can also activate PKC signaling pathways via PLC gamma, Erk 1/2 MAP kinase, and src stimulation. Taken together, the available evidence suggests that the major impact of protein kinase C and its interaction with insulin in the mature, fully differentiated nervous system appears to be to induce synaptogenesis, enhance memory, reduce Alzheimer's pathophysiology, and stimulate neurorepair.

Integrin signaling is integral to regeneration

The inability of the adult injured mammalian spinal cord to successfully regenerate is not well understood. Studies suggest that both extrinsic and intrinsic factors contribute to regeneration failure. In this review, we focus on intrinsic factors that impact regeneration, in particular integrin receptors and their downstream signaling pathways. We discuss studies that address the impact of integrins and integrin signaling pathways on growth cone guidance and motility and how lessons learned from these studies apply to spinal cord regeneration in vivo.

A SAGE-based screen for genes expressed in sub-populations of neurons in the mouse dorsal root ganglion

Background

The different sensory modalities temperature, pain, touch and muscle proprioception are carried by somatosensory neurons of the dorsal root ganglia. Study of this system is hampered by the lack of molecular markers for many of these neuronal sub-types. In order to detect genes expressed in sub-populations of somatosensory neurons, gene profiling was carried out on wild-type and TrkA mutant neonatal dorsal root ganglia (DRG) using SAGE (serial analysis of gene expression) methodology. Thermo-nociceptors constitute up to 80 % of the neurons in the DRG. In TrkA mutant DRGs, the nociceptor sub-class of sensory neurons is lost due to absence of nerve growth factor survival signaling through its receptor TrkA. Thus, comparison of wild-type and TrkA mutants allows the identification of transcripts preferentially expressed in the nociceptor or mechano-proprioceptor subclasses, respectively.

Results

Our comparison revealed 240 genes differentially expressed between the two tissues (P < 0.01). Some of these genes, CGRP, Scn10a are known markers of sensory neuron sub-types. Several potential markers of sub-populations, Dok4, Crip2 and Grik1/GluR5 were further analyzed by quantitative RT-PCR and double labeling with TrkA,-B,-C, c-ret, parvalbumin and isolectin B4, known markers of DRG neuron sub-types. Expression of Grik1/GluR5 was restricted to the isolectin B4+ nociceptive population, while Dok4 and Crip2 had broader expression profiles. Crip2 expression was however excluded from the proprioceptor sub-population.

Conclusion

We have identified and characterized the detailed expression patterns of three genes in the developing DRG, placing them in the context of the known major neuronal sub-types defined by molecular markers. Further analysis of differentially expressed genes in this tissue promises to extend our knowledge of the molecular diversity of different cell types and forms the basis for understanding their particular functional specificities.

Sprouty2 regulates growth and differentiation of human neuroblastoma cells through RET tyrosine kinase

The Sprouty (SPRY) family of proteins includes important regulators of downstream signaling initiated by receptor tyrosine kinases. In the present study, we investigated the role of SPRY proteins in intracellular signaling via the RET receptor tyrosine kinase activated by glial cell line-derived neurotrophic factor (GDNF). Expression of SPRY1, SPRY2, SPRY3 and SPRY4 in HEK293T cells transfected with RET and GDNF receptor family alpha1 (GFRalpha1) genes significantly reduced sustained ERK activation as well as ELK-1 activation. Because expression of SPRY2 was efficiently induced by GDNF in TGW human neuroblastoma cells expressing RET and GFRalpha1, we further investigated the role of SPRY2 in the growth and differentiation of TGW cells. Expression of wild-type SPRY2 (WT-SPRY2) decreased the growth of TGW cells. In contrast, expression of a dominant negative form of SPRY2 (MT-SPRY2, with a mutated tyrosine residue) enhanced cell proliferation. In addition, expression of WT-SPRY2 reduced GDNF-dependent neurite outgrowth of TGW cells, whereas expression of MT-SPRY2 enhanced it. Taken together, our results suggest that SPRY2 regulates GDNF-dependent proliferation and differentiation of TGW neuroblastoma cells mediated by RET tyrosine kinase.

Identification of Dok-4b, a Dok-4 splice variant with enhanced inhibitory properties

Dok adapter proteins have been primarily implicated in negative regulation of tyrosine kinase signaling, but Dok-4 has been reported to exert both inhibitory and stimulatory effects. We have identified a splice variant of Dok-4, Dok-4b, which contains a 39 aa insert within the its C-terminal region. The approximately 45kDa Dok-4b protein was detected in several human epithelial cell lines. Based on genomic sequences, Dok-4b was also predicted to exist in primates and possibly bovines, but not in rodents or other species. Compared to Dok-4, Dok-4b inhibited the tyrosine kinase-induced activation of both Erk and Elk-1 more strongly. Truncation of the C-terminal region of Dok-4 (Dok-4 DeltaCT) also enhanced the inhibitory activity of Dok-4, whereas expression of the isolated C-terminal domain enhanced Elk-1 activation, suggesting that the N-terminus and C-terminus of Dok-4 possess opposing inhibitory and stimulatory properties, respectively, the balance of which is altered by alternative splicing of Dok-4 to Dok-b.

RET and neuroendocrine tumors

The RET proto-oncogene encodes a receptor tyrosine kinase that is a main component of the signaling pathway activated by the glial cell line-derived neurotrophic factor family ligands. Gene targeting studies revealed that signaling through RET plays a crucial role in neuronal and renal organogenesis. It is well-known that germline mutations in RET lead to the human inherited diseases, multiple endocrine neoplasia type 2 (MEN 2) and Hirschsprung's disease, and that somatic rearrangements of RET cause papillary thyroid carcinoma. Due to marked advances in understanding of the molecular mechanisms of the development of MEN 2, a consensus on MEN 2 management associated with RET status is being reached and currently put into general use as a guideline. In this review, we summarize progress in the study of RET from bench to bedside, focusing on pathophysiology of neuroendocrine tumors.