Overriding Adaptive Resistance to Sorafenib Through Combination Therapy With Src Homology 2 Domain-Containing Phosphatase 2 Blockade in Hepatocellular Carcinoma.

BACKGROUND AND AIMS: The survival benefit of sorafenib for patients with hepatocellular carcinoma (HCC) is unsatisfactory due to the development of adaptive resistance. Increasing evidence has demonstrated that drug resistance can be acquired by cancer cells by activating a number of signaling pathways through receptor tyrosine kinases (RTKs); nevertheless, the detailed mechanism for the activation of these alternative pathways is not fully understood. APPROACH AND RESULTS: Given the physiological role of Src homology 2 domain-containing phosphatase 2 (SHP2) as a downstream effector of many RTKs for activation of various signaling cascades, we first found that SHP2 was markedly up-regulated in our established sorafenib-resistant cell lines as well as patient-derived xenografts. Upon sorafenib treatment, adaptive resistance was acquired in HCC cells through activation of RTKs including AXL, epidermal growth factor receptor, EPH receptor A2, and insulin-like growth factor 1 receptor, leading to RAS/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK), and AKT reactivation. We found that the SHP2 inhibitor SHP099 abrogated sorafenib resistance in HCC cell lines and organoid culture in vitro by blocking this negative feedback mechanism. Interestingly, this sensitization effect was also mediated by induction of cellular senescence. SHP099 in combination with sorafenib was highly efficacious in the treatment of xenografts and genetically engineered models of HCC. CONCLUSIONS: SHP2 blockade by SHP099 in combination with sorafenib attenuated the adaptive resistance to sorafenib by impeding RTK-induced reactivation of the MEK/ERK and AKT signaling pathways. SHP099 in combination with sorafenib may be a safe therapeutic strategy against HCC.

Discoidin domain receptor functions in physiological and pathological conditions.

The discoidin domain receptors, DDR1 and DDR2, are nonintegrin collagen receptors that are members of the receptor tyrosine kinase family. Both DDRs bind a number of different collagen types and play important roles in embryo development. Dysregulated DDR function is associated with progression of various human diseases, including fibrosis, arthritis, and cancer. By interacting with key components of the extracellular matrix and displaying distinct activation kinetics, the DDRs form a unique subfamily of receptor tyrosine kinases. DDR-facilitated cellular functions include cell migration, cell survival, proliferation, and differentiation, as well as remodeling of extracellular matrices. This review summarizes the current knowledge of DDR-ligand interactions, DDR-initiated signal pathways and the molecular mechanisms that regulate receptor function. Also discussed are the roles of DDRs in development and disease progression.

AXL is a key regulator of inherent and chemotherapy-induced invasion and predicts a poor clinical outcome in early-stage colon cancer.

PURPOSE: Despite the use of 5-fluorouracil (5-FU)-based adjuvant treatments, a large proportion of patients with high-risk stage II/III colorectal cancer will relapse. Thus, novel therapeutic strategies are needed for early-stage colorectal cancer. Residual micrometastatic disease from the primary tumor is a major cause of patient relapse. EXPERIMENTAL DESIGN: To model colorectal cancer tumor cell invasion/metastasis, we have generated invasive (KRASMT/KRASWT/+chr3/p53-null) colorectal cancer cell subpopulations. Receptor tyrosine kinase (RTK) screens were used to identify novel proteins that underpin the migratory/invasive phenotype. Migration/invasion was assessed using the XCELLigence system. Tumors from patients with early-stage colorectal cancer (N = 336) were examined for AXL expression. RESULTS: Invasive colorectal cancer cell subpopulations showed a transition from an epithelial-to-mesenchymal like phenotype with significant increases in migration, invasion, colony-forming ability, and an attenuation of EGF receptor (EGFR)/HER2 autocrine signaling. RTK arrays showed significant increases in AXL levels in all invasive sublines. Importantly, 5-FU treatment resulted in significantly increased migration and invasion, and targeting AXL using pharmacologic inhibition or RNA interference (RNAi) approaches suppressed basal and 5-FU-induced migration and invasion. Significantly, high AXL mRNA and protein expression were found to be associated with poor overall survival in early-stage colorectal cancer tissues. CONCLUSIONS: We have identified AXL as a poor prognostic marker and important mediator of cell migration/invasiveness in colorectal cancer. These findings provide support for the further investigation of AXL as a novel prognostic biomarker and therapeutic target in colorectal cancer, in particular in the adjuvant disease in which EGFR/VEGF-targeted therapies have failed.

Deletion of the receptor tyrosine kinase Tyro3 inhibits synovial hyperplasia and bone damage in arthritis.

OBJECTIVE: To test whether the tyrosine kinase Tyro3 affects arthritis. Tyro3, the ligand of growth arrest-specific protein 6 (GAS6) is a receptor tyrosine kinase involved in cell survival. Tyro3 and GAS6 are expressed in the arthritic synovium, and in vitro studies have shown their role in osteoclast differentiation. METHODS: Bone was assessed by micro CT and histomorphometry in Tyro3-deficient (Tyro3(-/-)) and wild-type mice. Arthritis was induced in both genotypes, and Gas6 level was measured by ELISA. Synovitis, synovial hyperplasia, bone erosion, osteoclast activation and osteoclast gene expression were assessed by histomorphometry and reverse transcriptase-PCR, respectively. In vitro osteoclast differentiation assays were performed in Tyro3(-/-) and wild-type mice. Furthermore, effects of Tyro3 and GAS6 on human synovial fibroblast proliferation and osteoclastogenesis were assessed in human cells. RESULTS: Tyro3(-/-) mice had significantly higher bone mass than wild-type littermates. Induction of arthritis increased GAS6 serum levels. Arthritic Tyro3(-/-) mice showed less synovial hyperplasia, osteoclast numbers and bone damage compared with controls. In vivo expression of osteoclast-associated receptor and receptor activator of nuclear factor-κB and in vitro osteoclastogenesis were impaired in Tyro3(-/-) mice. GAS6 also induced synovial fibroblast proliferation and osteoclast differentiation in human cells in Tyro3-dependent manner. CONCLUSIONS: These findings indicate that Tyro3 is a critical signal for synovial hyperplasia, osteoclast differentiation and bone erosion during arthritis. GAS6 and Tyro3 therefore constitute therapeutic targets to inhibit synovial hyperplasia and associated bone erosion.

Mer receptor tyrosine kinase promotes invasion and survival in glioblastoma multiforme.

The infiltration of glioma cells into adjacent tissue is one of the major obstacles in the therapeutic management of malignant brain tumours, in most cases precluding complete surgical resection. Consequently, malignant glioma patients almost invariably experience tumour recurrences. Within the brain, glioma cells migrate rapidly either amoeboidly or mesenchymally to invade surrounding structures, in dependence on the extracellular environment. In addition, radiotherapy, frequently applied as adjuvant therapeutic modality, may enhance tumour cell mobility. Here, we show that the receptor tyrosine kinase Mer (MerTK) is overexpressed in glioblastoma multiforme (GBM) and that this is accompanied with increased invasive potential. MerTK expression is maintained in primary GBM-derived tumour spheres under stem cell culture conditions but diminishes significantly in serum-containing cultures with concomitant downregulation of Nestin and Sox2. Depletion of MerTK disrupts the rounded morphology of glioma cells and decreases their invasive capacity. Furthermore, the expression and phosphorylation of myosin light chain 2 are strongly associated with MerTK activity, indicating that the effect of MerTK on glioma cell invasion is mediated by actomyosin contractility. Finally, DNA damage robustly triggers the upregulation and phosphorylation of MerTK, which protects cells from apoptosis. This effect is strongly impaired upon MerTK depletion or overexpression of an inactive MerTK mutant. Collectively, our data suggests that MerTK is a novel therapeutic target in the treatment of the malignant gliomas.

The yin, the yang, and the angiopoietin-1.

Twenty years after the discovery of the vascular endothelial Tie receptor tyrosine kinases and 15 years after the discovery of the Tie2 ligand, angiopoietin-1 (Angpt1, also known as Ang1), a study published in the current issue of the JCI reveals an unexpected loss-of-function phenotype of mice conditionally deleted of the Angpt1 gene. The results suggest that Angpt1 is needed as a vascular stabilizing factor that organizes and limits the angiogenesis response and protects from pathological consequences, such as tissue fibrosis.

[Tyrosine kinases in etiopathogenesis and therapy of malignant diseases--C-kit activating mutations].

INTRODUCTION: In the last 15 years, the introduction of molecular biology methods and techniques for identifying mutations and measuring gene expression levels of mutated genes since recently, have enabled precise molecular diagnostics, classification and assessment of prognosis and therapeutic response of malignant disease to specific therapies. The increased knowledge of the cancer genome and the introduction of multiple new technologies in cancer research have significantly improved the drug discovery process, leading to key success in targeted cancer therapeutics, including tyrosine kinase inhibitors. Tyrosine kinase inhibitors are the molecular targeted neoadjuvant and adjuvant therapy of various malignancies. Many more results which are expected from ongoing trials are necessary to specify the appropriate dosages, stages at which to start the treatment, and which therapeutic combinations to apply.

RON receptor tyrosine kinase in human gliomas: expression, function, and identification of a novel soluble splice variant.

Malignant gliomas are incurable because of their diffuse infiltration of the surrounding brain. The recepteur d'origine nantais (RON) receptor tyrosine kinase is highly expressed in several epithelial cancer types and mediates tumorigenic, pro-invasive as well as metastatic effects. Analyzing RON expression in human gliomas, we found that different splice variants with known oncogenic activity are expressed in glioblastomas (GBM). In addition, the RON ligand macrophage-stimulating protein (MSP) is secreted by cultured GBM cells. MSP showed no mitogenic effect on GBM cells but displayed significant chemotactic activity for several GBM cell lines. We identified a novel splice variant, RONDelta90, which is generated by a transcript missing exon 6. As a result of a frameshift, translation is terminated in exon 7, resulting in a truncated soluble protein. RONDelta90 transcripts are expressed in normal human brain as well as in low grade astrocytomas but only in approximately 50% of highly malignant astrocytomas. In addition, RONDelta90 is detectable in supernatants of GBM cell lines. We cloned the RONDelta90 cDNA, and purified the recombinant protein from transfected cells. RONDelta90 inhibited MSP-induced phosphorylation of cellular RON and also attenuated basal activation levels. In addition, RONDelta90 inhibited MSP-induced glioma cell migration as well as random motility. To conclude, RONDelta90 is a novel soluble receptor variant with antagonistic activity that may act as a physiological modulator of RON signaling. The expression of several oncogenic RON splice variants in malignant gliomas suggests that these could represent candidate targets for treatment with agents inhibiting RON activity.

Dynamic gene and protein expression patterns of the autism-associated met receptor tyrosine kinase in the developing mouse forebrain.

The establishment of appropriate neural circuitry depends on the coordination of multiple developmental events across space and time. These events include proliferation, migration, differentiation, and survival-all of which can be mediated by hepatocyte growth factor (HGF) signaling through the Met receptor tyrosine kinase. We previously found a functional promoter variant of the MET gene to be associated with autism spectrum disorder, suggesting that forebrain circuits governing social and emotional function may be especially vulnerable to developmental disruptions in HGF/Met signaling. However, little is known about the spatiotemporal distribution of Met expression in the forebrain during the development of such circuits. To advance our understanding of the neurodevelopmental influences of Met activation, we employed complementary Western blotting, in situ hybridization, and immunohistochemistry to comprehensively map Met transcript and protein expression throughout perinatal and postnatal development of the mouse forebrain. Our studies reveal complex and dynamic spatiotemporal patterns of expression during this period. Spatially, Met transcript is localized primarily to specific populations of projection neurons within the neocortex and in structures of the limbic system, including the amygdala, hippocampus, and septum. Met protein appears to be principally located in axon tracts. Temporally, peak expression of transcript and protein occurs during the second postnatal week. This period is characterized by extensive neurite outgrowth and synaptogenesis, supporting a role for the receptor in these processes. Collectively, these data suggest that Met signaling may be necessary for the appropriate wiring of forebrain circuits, with particular relevance to the social and emotional dimensions of behavior.

Acanthosis nigricans: a practical approach to evaluation and management.

Acanthosis nigricans is a dermatosis characterized by thickened, hyperpigmented plaques, typically of the intertriginous surfaces and neck. Common in some populations, its prevalence depends on race. Clinicians should recognize acanthosis nigricans; it heralds disorders ranging from endocrinologic disturbances to malignancy. In this review, we discuss the pathogenesis of acanthosis nigricans and its clinical implications and management.

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters.

BACKGROUND: A crucial event in the development of the vertebrate neuromuscular junction (NMJ) is the postsynaptic enrichment of muscle acetylcholine (ACh) receptors (AChRs). This process involves two distinct steps: the local clustering of AChRs at synapses, which depends on the activation of the muscle-specific receptor tyrosine kinase MuSK by neural agrin, and the global dispersal of aneural or "pre-patterned" AChR aggregates, which is triggered by ACh or by synaptogenic stimuli. We and others have previously shown that tyrosine phosphatases, such as the SH2 domain-containing phosphatase Shp2, regulate AChR cluster formation in muscle cells, and that tyrosine phosphatases also mediate the dispersal of pre-patterned AChR clusters by synaptogenic stimuli, although the specific phosphatases involved in this latter step remain unknown. RESULTS: Using an assay system that allows AChR cluster assembly and disassembly to be studied separately and quantitatively, we describe a previously unrecognized role of the tyrosine phosphatase Shp2 in AChR cluster disassembly. Shp2 was robustly expressed in embryonic Xenopus muscle in vivo and in cultured myotomal muscle cells, and treatment of the muscle cultures with an inhibitor of Shp2 (NSC-87877) blocked the dispersal of pre-patterned AChR clusters by synaptogenic stimuli. In contrast, over-expression in muscle cells of either wild-type or constitutively active Shp2 accelerated cluster dispersal. Significantly, forced expression in muscle of the Shp2-activator SIRPalpha1 (signal regulatory protein alpha1) also enhanced the disassembly of AChR clusters, whereas the expression of a truncated SIRPalpha1 mutant that suppresses Shp2 signaling inhibited cluster disassembly. CONCLUSION: Our results suggest that Shp2 activation by synaptogenic stimuli, through signaling intermediates such as SIRPalpha1, promotes the dispersal of pre-patterned AChR clusters to facilitate the selective accumulation of AChRs at developing NMJs.

Curcumin blocks RON tyrosine kinase-mediated invasion of breast carcinoma cells.

We have recently shown that macrophage-stimulating protein (MSP) promotes the invasion of recepteur d'origine nantais (RON), a tyrosine kinase receptor-positive MDA-MB-231, MDA-MB-468 breast cancer cells, and also identified the regulatory elements required for RON gene expression. In this report, we have analyzed the efficacy of a chemopreventive agent, curcumin, in blocking RON tyrosine kinase-mediated invasion of breast cancer cells. Reverse transcription-PCR and Western analysis indicated the down-regulation of the RON message and protein, respectively, in MDA-MB-231 and MDA-MB-468 cells. Significantly, curcumin-mediated inhibition of RON expression resulted in the blockade of RON ligand, MSP-induced invasion of breast cancer cells. We have identified two putative nuclear factor-kappaB p65 subunit binding sites on the RON promoter. Using chromatin immunoprecipitation analysis and site-directed mutagenesis of the RON promoter, we have confirmed the binding of p65 to the RON promoter. Our data show that curcumin reduces RON expression by affecting p65 protein expression and transcriptional activity. Treatment of MDA-MB-231 cells with pyrrolidine dithiocarbamate, an inhibitor of p65, or small interfering RNA knockdown of p65, blocked RON gene expression and MSP-mediated invasion of MDA-MB-231 cells. This is the first report showing the regulation of human RON gene expression by nuclear factor-kappaB and suggests a potential therapeutic role for curcumin in blocking RON tyrosine kinase-mediated invasion of carcinoma cells.

Activation of tyrosine kinase receptor signaling pathway by rasagiline facilitates neurorescue and restoration of nigrostriatal dopamine neurons in post-MPTP-induced parkinsonism.

The anti-Parkinson monoamine oxidase (MAO)-B inhibitor rasagiline (Azilect) was shown to possess neuroprotective activities, involving the induction of brain-derived- and glial cell line-derived neurotrophic factors (BDNF, GDNF). Employing conventional neurochemical techniques, transcriptomics and proteomic screening tools combined with a biology-based clustering method, we show that rasagiline, given chronically post-MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), exerts neurorescue/neurotrophic activity in mice midbrain dopamine neurons. Rasagiline induced the activation of cell signaling mediators associated with neurotrophic factors responsive-tyrosine kinase receptor (Trk) pathway including ShcC, SOS, AF6, Rin1 and Ras and the increase in the Trk-downstream effector phosphatidylinositol 3 kinase (PI3K) protein. Confirmatory Western and immunohistochemical analyses indicated activation of the substrate of PI3K, Akt and phosphorylative inactivation of glycogen synthase kinase-3beta and Raf1. Thus, the activation of Ras-PI3K-Akt survival pathway may contribute to rasagiline-mediated neurorescue effect. It is interesting to determine whether a similar effect is seen in parkinsonian patients after long-term treatment with rasagiline.

Role of tyrosine kinase receptors in angiotensin II AT2 receptor signaling: involvement in neurite outgrowth and in p42/p44mapk activation in NG108-15 cells.

NG108-15 cells, which have a rounding-up morphology when cultured in serum-supplemented medium, extend neurites when stimulated for 3 d with angiotensin II (Ang II). The aim of the present study was to investigate whether growth factor receptors are necessary for mediating the effects of Ang II. A 3-d treatment with AG879, an inhibitor of nerve growth factor receptor TrkA, strongly affected neurite outgrowth and phosphorylation of p42/p44(mapk) induced by Ang II. PD168393, an inhibitor of epidermal growth factor (EGF) receptor slightly decreased Ang II-induced neurite outgrowth, whereas AG213, an inhibitor of both platelet-derived growth factor receptor and EGF receptor, stimulated neurite outgrowth and p42/p44(mapk) phosphorylation on its own, without affecting further stimulation with Ang II. Moreover, Ang II induced the phosphorylation of TrkA (maximum at 5 min of incubation in the presence of serum or at 20 min in cells depleted in serum for 2 h) and a rapid increase in Rap1 activity, both effects abolished in cells preincubated with 10 microm AG879. In summary, the present results demonstrate that AT(2) receptor-induced sustained activation of p42/p44(mapk) and corresponding neurite outgrowth are mediated by phosphorylation of the nerve growth factor TrkA receptor. However, the results also point out that the presence of other growth factors, such as EGF or PDFG, may interfere with the effect of Ang II. Altogether, the current findings clearly indicate that the effects of the AT(2) receptor on neurite outgrowth dynamics are modulated by the presence of growth factors in the culture medium.

Ryk-mediated Wnt repulsion regulates posterior-directed growth of corticospinal tract.

Guidance cues along the longitudinal axis of the CNS are poorly understood. Wnt proteins attract ascending somatosensory axons to project from the spinal cord to the brain. Here we show that Wnt proteins repel corticospinal tract (CST) axons in the opposite direction. Several Wnt genes were found to be expressed in the mouse spinal cord gray matter, cupping the dorsal funiculus, in an anterior-to-posterior decreasing gradient along the cervical and thoracic cord. Wnts repelled CST axons in collagen gel assays through a conserved high-affinity receptor, Ryk, which is expressed in CST axons. Neonatal spinal cord secretes diffusible repellent(s) in an anterior-posterior graded fashion, with anterior cord being stronger, and the repulsive activity was blocked by antibodies to Ryk (anti-Ryk). Intrathecal injection of anti-Ryk blocked the posterior growth of CST axons. Therefore, Wnt proteins may have a general role in anterior-posterior guidance of multiple classes of axons.

Neurotrophin-3 is a target-derived neurotrophic factor for penile erection-inducing neurons.

The aim of this study was to determine whether the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin (NT)-3 could act as endogenous target-derived trophic factors for erection-inducing, i.e. penis-projecting major pelvic ganglion (MPG) neurons, and/or penile sensory neurons in adult rat. This was accomplished by studying the expression of NT mRNAs in the penis and their cognate receptors in the MPG and dorsal root ganglia (DRGs), and the retrograde axonal transport of radioiodinated NTs injected into the corpora cavernosa. Northern hybridization showed that NGF, BDNF, and NT-3 mRNAs are expressed in the shaft of the penis. In situ hybridization combined with usage of the retrograde tracer Fluoro-Gold showed that TrkC and p75 receptors are expressed in penis-projecting neurons of the MPG whereas the mRNAs for TrkA and TrkB receptors were undetectable. However, all the NT receptor mRNAs were expressed in penile sensory neurons of sacral level 1 (S1) DRG. (125)I-NT-3 injected into the shaft of the penis was retrogradely transported into the MPG and S1 DRG, whereas radioiodinated NGF and BDNF were transported specifically into the S1 DRG, thus confirming the existence of functional NT receptors in these penile neurons. In conclusion, these data suggest that NT-3 may act as a target-derived neurotrophic factor for both erection-inducing and penile sensory neurons, whereas NGF and BDNF may be more important for the sensory innervation of the penis.

Direct cell cycle regulation by the fibroblast growth factor receptor (FGFR) kinase through phosphorylation-dependent release of Cks1 from FGFR substrate 2.

Fibroblast growth factors (FGFs) are upstream activators of the mitogen-activated protein kinase pathway and mitogens in a wide variety of cells. However, whether the mitogen-activated protein kinase pathway solely accounts for the induction of cell cycle or antiapoptotic activity of the FGF receptor (FGFR) tyrosine kinase is not clear. Here we report that cell cycle inducer Cks1, which triggers ubiquitination and degradation of p27(Kip1), associates with the unphosphorylated form of FGFR substrate 2 (FRS2), an adaptor protein that is phosphorylated by FGFR kinases and recruits downstream signaling molecules. FGF-dependent activation of FGFR tyrosine kinases induces FRS2 phosphorylation, causes release of Cks1 from FRS2, and promotes degradation of p27(Kip1) in 3T3 cells. Since degradation of p27(Kip1) is a key regulatory step in activation of the cyclin E/A-Cdk complex during the G(1)/S transition of the cell cycle, the results suggest a novel mitogenic pathway whereby FGF and other growth factors that activate FRS2 directly activate cyclin-dependent kinases.

[GnRH deficiency: new insights from genetics]. / Déficience en GnRH: le nouvel eclairage apporté par la génétique.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Hypogonadotropic hypogonadism (HH) results from impaired secretion of GnRH. The patients display with delayed puberty, micropenis and cryptorchidism in the male reflecting gonadotropin insufficiency, and amenorrhea in the female. Kallmann's syndrome (KS) is defined by the association of HH and anosmia or hyposmia (absent smelling sense). Segregation analysis in familial cases has demonstrated diverse inheritance patterns, suggesting the existence of several genes regulating GnRH secretion. The X-linked form of the disease was associated with a genetic defect in the KALI gene located on the Xp22.3 region. KAL1 gene encodes an extracellular matrix glycoprotein anosmin-1, which facilitates neuronal growth and migration. Abnormalities in the migratory processes of the GnRH neurons with the olfactory neurons explain the association of HH with anosmia. Recently, mutations in the FGF recepteur 1 (FGFR1) gene were found in KS with autosomal dominant mode of inheritance. The role of FGFR1 in the function of reproduction requires further investigation. Besides HH with anosmia, there are isolated HH (IHH). No human GnRH mutations have been reported although hypogonadal mice due to a GnRH gene deletion exist. In patients with idiopathic HH and without anosmia an increasing number of GnRH receptor (GnRHR) mutations have been described which represent about 50% of familial cases. The clinical features are highly variable and there is a good relationship between genotype and phenotype. A complete loss of function is associated with the most severe phenotype with resistance to pulsatile GnRH treatment, absence of puberty and cryptorchidism in the male. In contrast, milder loss of function mutations causes incomplete failure of pubertal development. The preponderant role of GnRH in the secretion of LH by the gonadotrophs explains the difference of the phenotype between male and female with partial GnRH resistance. Affected females can have spontaneous telarche and normal breast development while affected males exhibit no pubertal development but normal testis volume, a feature described as "fertile-eunuch". High-dose pulsatile GnRH has been used to induce ovulation. Another gene, called GPR54, responsible for idiopathic HH has been recently described by segregation analysis in two different consanguineous families. The GPR54 gene is an orphan receptor, and its putative ligand is the product of the KISS-1 gene, called metastine. Their roles in the function of reproduction are still unknown.

Receptor tyrosine kinase signaling in gliomagenesis: pathobiology and therapeutic approaches.

Gliomas are a large collection of primary central nervous system tumors that arise from glia, astrocytes and oligodendrocytes or their precursors. They are graded on a scale of I to IV based on their degree of malignancy as judged by variable histological features. Genetic and biochemical evidences have proven that gliomagenesis involves a stepwise accumulation of genetic lesions affecting either signal transduction pathways activated by receptor tyrosine kinases (RTKs) or cell cycle growth arrest pathways. Many of these observed molecular alterations are now being used to compliment clinical diagnosis. Genetic alterations affecting RTK signaling results in the activation of several downstream pathways, such as the PI3-kinase/Akt and Ras/Raf/MEK/MAPK pathways, which provides a number of novel targets for glioma therapy. This article aims to present a broad understanding of the receptor tyrosine kinase signaling networks involved in gliomagenesis. Molecular classification of primary glial tumors and elucidation of cooperative interactions between different genetic lesions will eventually allow us to target distinct glioma subsets and will provide a more rational approach to adjuvant therapies for this refractory disease.

Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections.

The development of connections between thalamic afferents and their cortical target cells occurs in a highly precise manner. Thalamic axons enter the cortex through deep cortical layers, then stop their growth in layer 4 and elaborate terminal arbors specifically within this layer. The mechanisms that underlie target layer recognition for thalamocortical projections are not known. We compared the growth pattern of thalamic explants cultured on membrane substrates purified from cortical layer 4, the main recipient layer for thalamic axons, and cortical layer 5, a non-target layer. Thalamic axons exhibited a reduced growth rate and an increased branching density on their appropriate target membranes compared with non-target substrate. When confronted with alternating stripes of both membrane substrates, thalamic axons grew preferentially on their target membrane stripes. Enzymatic treatment of cortical membranes revealed that growth, branching and guidance of thalamic axons are independently regulated by attractive and repulsive cues differentially expressed in distinct cortical layers. These results indicate that multiple membrane-associated molecules collectively contribute to the laminar targeting of thalamic afferents. Furthermore, we found that interfering with the function of Eph tyrosine kinase receptors and their ligands, ephrins, abolished the preferential branching of thalamic axons on their target membranes, and that recombinant ephrin-A5 ligand elicited a branch-promoting activity on thalamic axons. We conclude that interactions between Eph receptors and ephrins mediate branch formation of thalamic axons and thereby may play a role in the establishment of layer-specific thalamocortical connections.