Ephexin1 Is Required for Eph-Mediated Limb Trajectory of Spinal Motor Axons

Neuronal guanine nucleotide exchange factor promotes the axonal growth and cancer cell proliferation via Ephrin-A3/EphA2 axis in lung adenocarcinoma

BACKGROUND

Neural infiltration has been found in various cancers and the infiltrating nerves influence tumor growth and dissemination. In non-small cell lung cancer, pan-neuronal marker PGP9.5 was detected by immunohistochemical staining and its high expression correlated with poor prognosis. However, the existence of nerve fibers and the mechanism driving neural infiltration remains unclear.

METHOD

We first used immunohistochemical staining to assess the density of nerve fibers in patients with lung adenocarcinoma of different tumor sizes. Following that, we performed differential expression analysis and univariate Cox prognostic analysis, using public datasets and cell experiments to identify the gene that triggers neural infiltration and is associated with cancer progression and unfavorable prognosis. Finally, molecular biology experiments and a subcutaneous tumor model were used to deeply analyze the mechanism that the gene regulates neural infiltration and tumor progression.

RESULTS

In lung adenocarcinoma patients, the density of PGP9.5 positive nerve fibers within tumors larger than 2 cm in diameter is significantly higher than that in tumors smaller than 2 cm. Bioinformatics analysis suggested NGEF, KIF4A, and PABPC1 could be the genes that trigger neural infiltration and are associated with cancer progression and unfavorable prognosis. Subsequent co-culture experiments with neurons showed that the increased expression of NGEF in lung cancer cells significantly enhanced axonal growth in neurons. Meanwhile, GSE30219 datasets indicated that patients exhibiting high levels of NGEF expression are associated with larger tumor sizes, higher lymph node involvement, and reduced overall survival rates. At the level of molecular mechanisms, the knockdown of Ephrin-A3 in ND7/23 neurons or the use of ALW-II-41-27 resulted in a significant decrease in neurite outgrowth when co-cultured with LA795 cells. In animal model, NGEF overexpression significantly promoted tumor growth and increased the density of nerve fibers, and these effects were inhibited by ALW-II-41-27.

CONCLUSIONS

NGEF facilitates the infiltration of nerve and the growth of cancer cells in lung adenocarcinoma through the Ephrin-A3/EphA2 pathway, suggesting that NGEF is a promising target for disrupting interactions between nerves and tumors. Biomaterials that focus on NGEF are anticipated to be a potential treatment option for lung cancer.

Rho GTPase Signaling: A Molecular Switchboard for Regulating the Actin Cytoskeleton in Axon Guidance

Axon pathfinding is a highly dynamic process regulated by the interactions between cell-surface guidance receptors and guidance cues present in the extracellular environment. During development, precise axon pathfinding is crucial for the formation of functional neural circuits. The spatiotemporal expression of axon guidance receptors helps the navigating axon make correct decisions in a complex environment comprising both attractive and repulsive guidance cues. Axon guidance receptors initiate distinct signaling cascades that eventually influence the cytoskeleton at the growing tip of an axon, called the growth cone. The actin cytoskeleton is the primary target of these guidance signals and plays a key role in growth cone motility, exploration, and behavior. Of the many regulatory molecules that modulate the actin cytoskeleton in response to distinct guidance signals, Rho GTPases play central roles. Rho GTPases are molecular switchboards; their ON (GTP-bound) and OFF (GDP-bound) switches are controlled by their interactions with proteins that regulate the exchange of GDP for GTP or with the proteins that promote GTP hydrolysis. Various upstream signals, including axon guidance signals, regulate the activity of these Rho GTPase switch regulators. As cycling molecular switches, Rho GTPases interact with and control the activities of downstream effectors, which directly influence actin reorganization in a context-dependent manner. A deeper exploration of the spatiotemporal dynamics of Rho GTPase signaling and the molecular basis of their involvement in regulating growth cone actin cytoskeleton can unlock promising therapeutic strategies for neurodevelopmental disorders linked to dysregulated Rho GTPase signaling. This review not only provides a comprehensive overview of the field but also highlights recent discoveries that have considerably advanced our understanding of the complex regulatory roles of Rho GTPases in modulating actin cytoskeleton arrangement at the growth cone during axon guidance.

Interactions between EGFR and EphA2 promote tumorigenesis through the action of Ephexin1

The cell signaling factors EGFR, EphA2, and Ephexin1 are associated with lung and colorectal cancer and play an important role in tumorigenesis. Although the respective functional roles of EGFR and EphA2 are well known, interactions between these proteins and a functional role for the complex is not understood. Here, we showed that Ephexin1, EphA2, and EGFR are each expressed at higher levels in lung and colorectal cancer patient tissues, and binding of EGFR to EphA2 was associated with both increased tumor grade and metastatic cases in both cancer types. Treatment with Epidermal Growth Factor (EGF) induced binding of the RR domain of EGFR to the kinase domain of EphA2, and this binding was promoted by Ephexin1. Additionally, the AKT-mediated phosphorylation of EphA2 (at Ser897) promoted interactions with EGFR, pointing to the importance of this pathway. Two mutations in EGFR, L858R and T790M, that are frequently observed in lung cancer patients, promoted binding to EphA2, and this binding was dependent on Ephexin1. Our results indicate that the formation of a complex between EGFR, EphA2, and Ephexin1 plays an important role in lung and colorectal cancers, and that inhibition of this complex may be an effective target for cancer therapy.

Akt-mediated Ephexin1-Ras interaction promotes oncogenic Ras signaling and colorectal and lung cancer cell proliferation

ABSTRCT

Ephexin1 was reported to be highly upregulated by oncogenic Ras, but the functional consequences of this remain poorly understood. Here, we show that Ephexin1 is highly expressed in colorectal cancer (CRC) and lung cancer (LC) patient tissues. Knockdown of Ephexin1 markedly inhibited the cell growth of CRC and LC cells with oncogenic Ras mutations. Ephexin1 contributes to the positive regulation of Ras-mediated downstream target genes and promotes Ras-induced skin tumorigenesis. Mechanically, Akt phosphorylates Ephexin1 at Ser16 and Ser18 (pSer16/18) and pSer16/18 Ephexin1 then interacts with oncogenic K-Ras to promote downstream MAPK signaling, facilitating tumorigenesis. Furthermore, pSer16/18 Ephexin1 is associated with both an increased tumor grade and metastatic cases of CRC and LC, and those that highly express pSer16/18 exhibit poor overall survival rates. These data indicate that Ephexin1 plays a critical role in the Ras-mediated CRC and LC and pSer16/18 Ephexin1 might be an effective therapeutic target for CRC and LC.

TDP-43 is essential for Eph receptor-class-specific spinal motor axon trajectory into the limb

During the neural circuit formation, neuronal growth cones must be guided precisely to their neuronal or muscle targets, which can be achieved by the activation of membrane-bound guidance receptors at the periphery. However, the mechanisms that regulate the temporal availability of these receptors remain largely unknown. TAR DNA binding protein-43 (TDP-43) has been proposed to bind with the mRNAs of guidance receptors, thus prompting us to investigate its role in axon guidance of the spinal lateral motor column (LMC) neurons into the limb mesenchyme. We first identified the TDP-43 expression in the LMC neurons at the stage of axons growth into the limb using in situ mRNA hybridization. The loss and gain of TDP-43 function in chick LMC neurons redirected their axon trajectory with opposite effects. In mice, a spinal motor neuron-specific TDP-43 deletion led to the misrouting of LMC axons. Further, ectopic TDP-43 expression increased EphB protein levels in LMC neurons, suggesting that TDP-43 mediates LMC pathfinding by regulating EphB expression. Finally, TDP-43 levels influenced the growth preference of LMC neurites against ephrin-B, but not Netrin-1 and Semaphorin ligands. Our results demonstrate that TDP-43 is essential for the ephrinB:EphB signaling-mediated axon trajectory selection of LMC subtypes into the limb.

Vav2 is required for Netrin-1 receptor-class-specific spinal motor axon guidance

BACKGROUND

Proper guidance of neuronal axons to their targets is required to assemble neural circuits during the development of the nervous system. However, the mechanism by which the guidance of axonal growth cones is regulated by specific intermediaries activated by receptor signaling pathways to mediate cytoskeleton dynamics is unclear. Vav protein members have been proposed to mediate this process, prompting us to investigate their role in the limb selection of the axon trajectory of spinal lateral motor column (LMC) neurons.

RESULTS

We found Vav2 and Vav3 expression in LMC neurons when motor axons grew into the limb. Vav2, but not Vav3, loss-of-function perturbed LMC pathfinding, while Vav2 gain-of-function exhibited the opposite effects, demonstrating that Vav2 plays an important role in motor axon growth. Vav2 knockdown also attenuated the redirectional phenotype of LMC axons induced by Dcc, but not EphA4, in vivo and lateral LMC neurite growth preference to Netrin-1 in vitro. This study showed that Vav2 knockdown and ectopic nonphosphorylable Vav2 mutant expression abolished the Src-induced stronger growth preference of lateral LMC neurites to Netrin-1, suggesting that Vav2 is downstream of Src in this context.

CONCLUSIONS

Vav2 is essential for Netrin-1-regulated LMC motor axon pathfinding through Src interaction.

Paxillin Is Required for Proper Spinal Motor Axon Growth into the Limb

To assemble the functional circuits of the nervous system, the neuronal axonal growth cones must be precisely guided to their proper targets, which can be achieved through cell-surface guidance receptor activation by ligand binding in the periphery. We investigated the function of paxillin, a focal adhesion protein, as an essential growth cone guidance intermediary in the context of spinal lateral motor column (LMC) motor axon trajectory selection in the limb mesenchyme. Using in situ mRNA detection, we first show paxillin expression in LMC neurons of chick and mouse embryos at the time of spinal motor axon extension into the limb. Paxillin loss-of-function and gain-of-function using in ovo electroporation in chick LMC neurons, of either sex, perturbed LMC axon trajectory selection, demonstrating an essential role of paxillin in motor axon guidance. In addition, a neuron-specific paxillin deletion in mice led to LMC axon trajectory selection errors. We also show that knocking down paxillin attenuates the growth preference of LMC neurites against ephrins in vitro, and erythropoietin-producing human hepatocellular (Eph)-mediated retargeting of LMC axons in vivo, suggesting paxillin involvement in Eph-mediated LMC motor axon guidance. Finally, both paxillin knockdown and ectopic expression of a nonphosphorylable paxillin mutant attenuated the retargeting of LMC axons caused by Src overexpression, implicating paxillin as a Src target in Eph signal relay in this context. In summary, our findings demonstrate that paxillin is required for motor axon guidance and suggest its essential role in the ephrin-Eph signaling pathway resulting in motor axon trajectory selection.SIGNIFICANCE STATEMENT During the development of neural circuits, precise connections need to be established among neurons or between neurons and their muscle targets. A protein family found in neurons, Eph, is essential at different stages of neural circuit formation, including nerve outgrowth and pathfinding, and is proposed to mediate the onset and progression of several neurodegenerative diseases, such as Alzheimer's disease. To investigate how Ephs relay their signals to mediate nerve growth, we investigated the function of a molecule called paxillin and found it important for the development of spinal nerve growth toward their muscle targets, suggesting its role as an effector of Eph signals. Our work could thus provide new information on how neuromuscular connectivity is properly established during embryonic development.

Emerging Roles of Ephexins in Physiology and Disease

Dbl (B-cell lymphoma)-related guanine nucleotide exchange factors (GEFs), the largest family of GEFs, are directly responsible for the activation of Rho family GTPases and essential for a number of cellular events such as proliferation, differentiation and movement. The members of the Ephexin (Eph-interacting exchange protein) family, a subgroup of Dbl GEFs, initially were named for their interaction with Eph receptors and sequence homology with Ephexin1. Although the first Ephexin was identified about two decades ago, their functions in physiological and pathological contexts and regulatory mechanisms remained elusive until recently. Ephexins are now considered as GEFs that can activate Rho GTPases such as RhoA, Rac, Cdc42, and RhoG. Moreover, Ephexins have been shown to have pivotal roles in neural development, tumorigenesis, and efferocytosis. In this review, we discuss the known and proposed functions of Ephexins in physiological and pathological contexts, as well as their regulatory mechanisms.