Mirko H. H. Schmidt

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Acetylation-dependent regulation of endothelial Notch signalling by the SIRT1 deacetylase

Notch signalling is a key intercellular communication mechanism that is essential for cell specification and tissue patterning, and which coordinates critical steps of blood vessel growth. Although subtle alterations in Notch activity suffice to elicit profound differences in endothelial behaviour and blood vessel formation, little is known about the regulation and adaptation of endothelial Notch responses. Here we report that the NAD+-dependent deacetylase SIRT1 acts as an intrinsic negative modulator of Notch signalling in endothelial cells. We show that acetylation of the Notch1 intracellular domain (NICD) on conserved lysines controls the amplitude and duration of Notch responses by altering NICD protein turnover. SIRT1 associates with NICD and functions as a NICD deacetylase, which opposes the acetylation-induced NICD stabilization. Consequently, endothelial cells lacking SIRT1 activity are sensitized to Notch signalling, resulting in impaired growth, sprout elongation and enhanced Notch target gene expression in response to DLL4 stimulation, thereby promoting a non-sprouting, stalk-cell-like phenotype. In vivo, inactivation of Sirt1 in zebrafish and mice causes reduced vascular branching and density as a consequence of enhanced Notch signalling. Our findings identify reversible acetylation of the NICD as a molecular mechanism to adapt the dynamics of Notch signalling, and indicate that SIRT1 acts as rheostat to fine-tune endothelial Notch responses.

miR-126 and miR-126*: New Players in Cancer

Cancer progression is characterized by autarky in growth signals, insensitivity to growth-restrictive signals, evasion of apoptosis, a limitless potential to replicate, sustained angiogenesis, and tissue invasion, including metastasis. The regulation of these cellular processes relies on a fine-tuned control of molecular signal cascades. In recent years, short noncoding RNAs termed microRNAs (miRNAs) have been described as a novel class of molecular regulators. These affect various signaling cascades during the progression of neoplastic diseases by the regulation of gene expression on the post-transcriptional level. The novel endothelial cell–derived secreted protein epidermal growth factor–like domain 7 (EGFL7) has been suggested to control vascular tubulogenesis. Further, the two biologically active miRNAs miR-126 and its complement miR-126*, which are encoded by intron 7 of the egfl7 gene, have been described to mediate vascular functions. Knock-out studies in zebrafish and mice suggested a major role of miR-126 in angiogenesis and vascular integrity, which was mediated by the repression of inhibitors of VEGF-induced proliferation in endothelial cells. Recent studies revealed the distribution and function of miR-126 and miR-126* in various types of cancer, and assigned a role to both miRNAs as suppressors of tumor formation. Indeed, miR-126 and miR-126* have been reported to impair cancer progression through signaling pathways that control tumor cell proliferation, migration, invasion, and survival. Conversely, miR-126 and miR-126* may have a supportive role in the progression of cancer as well, which might be mediated by the promotion of blood vessel growth and inflammation. In this work, we will summarize the current knowledge on functions of miR-126/miR-126* that are relevant for cancer formation, and we will discuss their potential clinical use as predictive markers of survival and application as novel therapeutic targets for the treatment of neoplastic diseases.

Epidermal growth factor receptor signaling intensity determines intracellular protein interactions, ubiquitination, and internalization.

Ligand activation of the epidermal growth factor receptor (EGFR) causes the binding of Cbls, which leads to EGFR polyubiquitination and internalization through endophilin complexes that contain the adaptor protein SH3-domain encoding, expressed in tumorigenic astrocytes/Cbl-interacting protein of 85 kDa/regulator of ubiquitous kinase (SETA/CIN85/Ruk). In cells grown at high density, high levels of SETA interfered in the recruitment of Casitas B-lineage (Cbl) proteins to the EGFR and reduced its polyubiquitination, suggesting that SETA has a regulatory function in the formation of the EGFR–Cbl–endophilin complex and in EGFR down-regulation. In a situation where there is EGFR signaling but no internalization or down-regulation, as is the case with the EGFR with exons 2–7 deleted (ΔEGFR) oncogene, these proteins were absent altogether. By using mAb 806, which recognizes an EGFR-activation state and preferentially immunoprecipitates ΔEGFR, we show that ΔEGFR did not interact with Cbls, SETA, or endophilin A1, providing a mechanistic explanation for its lack of internalization. As would be expected by the absence of Cbl proteins in the ΔEGFR complex, the mutant receptor was also not polyubiquitinated. The intracellular C terminus and tyrosine autophosphorylation pattern of ΔEGFR are similar to wild-type EGFR, but it signals at a lower intensity as determined by levels of EGFR phosphotyrosine. To test the implication that the lack of interaction with the Cbl–SETA–endophilin complex is because of differences in signal intensity, EGFR-expressing cells were treated with tyrphostin AG1478 EGFR inhibitor. Attenuation of wild-type EGFR signal to levels similar to that found in ΔEGFR resulted in the dissociation of SETA and Cbl proteins and a concomitant attenuation of receptor internalization.

Regulation of epidermal growth factor receptor trafficking by lysine deacetylase hdac6

HDAC6 sets a brake that slows down the delivery of activated epidermal growth factor receptors to the degradative compartment. Setting a Speed Limit on EGFR Traffic Receptor tyrosine kinases interact with ligands at the cell surface to trigger intracellular signaling cascades. In some cases, these receptors are internalized, a process that can either enable them to initiate signaling cascades from intracellular membranes or target them for lysosomal degradation. Lissanu Deribe et al. connect acetylation of the microtubule cytoskeleton to regulation of delivery of epidermal growth factor receptors (EGFRs) to lysosomes. HDAC6, a cytoplasmic lysine deacetylase, was identified as binding to the inactive EGFR, stimulating deacetylation of α-tubulin, and decreasing the rate of delivery of EGFR from the early endosome to late endosomes or lysosomes. Phosphorylation of HDAC6, which decreased its activity, by activated EGFR created a negative feedback loop, leading to increased degradation of activated EGFRs. Binding of epidermal growth factor (EGF) to its receptor leads to receptor dimerization, assembly of protein complexes, and activation of signaling networks that control key cellular responses. Despite their fundamental role in cell biology, little is known about protein complexes associated with the EGF receptor (EGFR) before growth factor stimulation. We used a modified membrane yeast two-hybrid system together with bioinformatics to identify 87 candidate proteins interacting with the ligand-unoccupied EGFR. Among them was histone deacetylase 6 (HDAC6), a cytoplasmic lysine deacetylase, which we found negatively regulated EGFR endocytosis and degradation by controlling the acetylation status of α-tubulin and, subsequently, receptor trafficking along microtubules. A negative feedback loop consisting of EGFR-mediated phosphorylation of HDAC6 Tyr570 resulted in reduced deacetylase activity and increased acetylation of α-tubulin. This study illustrates the complexity of the EGFR-associated interactome and identifies protein acetylation as a previously unknown regulator of receptor endocytosis and degradation.

Alix/AIP1 Antagonizes Epidermal Growth Factor Receptor Downregulation by the Cbl-SETA/CIN85 Complex

ABSTRACT The assembly of the Cbl-SETA/CIN85-endophilin complex at the C terminus of the epidermal growth factor receptor (EGFR) following ligand activation mediates its internalization and ubiquitination. We found that the SETA/CIN85-interacting protein Alix/AIP1, which also binds endophilins, modulates this complex. Alix was found to associate indirectly with EGFR, regardless of its activation state, and with ΔEGFR, which signals at low intensity and does not bind Cbls or SETA/CIN85. In agreement with this, Alix interaction did not occur via SETA/CIN85. However, SETA/CIN85 and Alix were capable of mutually promoting their interaction with the EGFR. Increasing the level of Alix weakened the interaction between SETA/CIN85 and Cbl and reduced the tyrosine phosphorylation of c-Cbl and the level of ubiquitination of EGFR, SETA/CIN85, and Cbls. This antagonism of the Cbl-SETA/CIN85 complex by Alix was reflected in its diminution of EGFR internalization. In agreement with this, small interfering RNA-mediated knockdown of Alix promoted EGFR internalization and downregulation. It has been suggested that SETA/CIN85 promotes receptor internalization by recruiting endophilins. However, Alix was also capable of increasing the level of endophilin associated with EGFR, implying that this is not sufficient to promote receptor internalization. We propose that Alix inhibits EGFR internalization by attenuating the interaction between Cbl and SETA/CIN85 and by inhibiting Cbl-mediated ubiquitination of the EGFR.

Epidermal growth factor-like domain 7 (EGFL7) modulates Notch signalling and affects neural stem cell renewal

Epidermal growth factor-like domain 7 (EGFL7) is a secreted factor implicated in cellular responses such as cell migration and blood vessel formation; however the molecular mechanisms underlying the effects of EGFL7 are largely unknown. Here we have identified transmembrane receptors of the Notch family as EGFL7-binding molecules. Secreted EGFL7 binds to a region in Notch involved in ligand-mediated receptor activation, thus acting as an antagonist of Notch signalling. Expression of EGFL7 in neural stem cells (NSCs) in vitro decreased Notch-specific signalling and consequently, reduced proliferation and self-renewal of NSCs. Such altered Notch signalling caused a shift in the differentiation pattern of cultured NSCs towards an excess of neurons and oligodendrocytes. We identified neurons as a source of EGFL7 in the brain, suggesting that brain-derived EGFL7 acts as an endogenous antagonist of Notch signalling that regulates proliferation and differentiation of subventricular zone-derived adult NSCs.

SETA/CIN85/Ruk and its binding partner AIP1 associate with diverse cytoskeletal elements, including FAKs, and modulate cell adhesion

The adaptor protein SETA/CIN85/Ruk is involved in regulating diverse signal transduction pathways, including the internalization of tyrosine kinase receptors via the Cbl ubiquitin ligases, and attenuating PI3K activity by interaction with its regulatory subunit. Here we present evidence for a new aspect of SETA function, based on the initial observation that it co-localizes with actin in microfilaments and at focal adhesions, and with microtubules. Although there was no evidence for direct molecular interactions between SETA and cytoskeletal proteins, the SETA-interacting protein AIP1, which is a rat ortholog of the Xenopus src substrate Xp95, strongly interacted with structural proteins of the cytoskeleton, including actin and tubulins. Both SETA and AIP1 interacted with focal adhesion kinase (FAK) and proline rich tyrosine kinase 2 (PYK-2), and c-Cbl interacted with PYK-2. AIP1, which interacted more strongly than either SETA or c-Cbl, required an intact consensus tyrosine kinase phosphorylation sequence at Y319 to bind to focal adhesion kinases, which suggests that phosphorylation is an important mediator of this complex. SETA, which interacted as a dimer with focal adhesion kinases, promoted the interaction between PYK-2 and AIP1. Direct analysis of the impact of these proteins on cell adhesion, by use of an electrical cell-substrate impedance sensor (ECIS), showed that SETA promoted cell adhesion while AIP1 and c-Cbl reduced it. Furthermore, the ability of AIP1 and AIP1 mutants to decrease cell adhesion in ECIS analysis correlated with their presence in PYK-2 complexes, providing a direct link between AIP1-mediated molecular interactions and cellular behavior. Transfection of AIP1 also reduced the level of phosphorylation of endogenous PYK-2 and FAK, suggesting that this protein may directly regulate focal adhesion kinases, and thereby cell adhesion. These data are the first to implicate the adaptor protein SETA and its binding partner AIP1 as being involved with the cytoskeleton and in the regulation of cell adhesion, and suggest that they may be part of the focal adhesion kinase regulatory complex.

Angiopoietin-2 Impairs Revascularization After Limb Ischemia

Angiopoietins play important roles in the formation of neovessels and complex vascular networks. Angiopoietin (Ang)-1 and Ang-2 belong to a family of growth factors that display opposing effects on the activation of Tie2 (tyrosine kinase with immunoglobulin and epidermal growth factor homology domain 2). Endothelial Ang-2 expression is associated with vessel destabilization and regulates a balance between vascular regression and growth. To elucidate, in particular, the role of Ang-2 after arterial artery occlusion in the mouse limb, we applied a transgenic animal model with targeted Ang-2 expression in endothelial cells. We show here that restoration of blood flow in Ang-2:Tie1 transgenic mice is dramatically impaired when Ang-2 expression is induced in the vasculature. The defective restoration of perfusion in Ang-2 transgenic mice is evidenced by reduced collateral artery growth, which typically occurs to compensate for flow deficits after occlusion of the large conductance artery. Furthermore, reduced movement capacities and higher incidents of necrosis are consequently observed in the transgenic limbs as compared with controls. Mechanistically, the observed effects are attributed to defective smooth muscle cell recruitment in Ang-2 transgenic mice. Moreover, distinct Ang-2 levels in the genetically modified animals clearly correlated with the magnitude of reduced perfusion. In conclusion, our studies define Ang-2 as an important molecule for the progression of collateral artery growth and angiogenesis during ischemia and suggest precise Ang-2 dosage activities to accomplish blood vessel growth.

EGFL7 meets miRNA-126: an angiogenesis alliance.

Blood vessels form de novo through the tightly regulated programs of vasculogenesis and angiogenesis. Both processes are distinct but one of the steps they share is the formation of a central lumen, when groups of cells organized as vascular cords undergo complex changes to achieve a tube-like morphology. Recently, a protein termed epidermal growth factor-like domain 7 (EGFL7) was described as a novel endothelial cell-derived factor involved in the regulation of the spatial arrangement of cells during vascular tube assembly. With its impact on tubulogenesis and vessel shape EGFL7 joined the large family of molecules governing blood vessel formation. Only recently, the molecular mechanisms underlying EGFL7s effects have been started to be elucidated and shaping of the extracellular matrix (ECM) as well as Notch signaling might very well play a role in mediating its biological effects. Further, findings in knock-out animal models suggest miR-126, a miRNA located within the egfl7 gene, has a major role in vessel development by promoting VEGF signaling, angiogenesis and vascular integrity. This review summarizes our current knowledge on EGFL7 and miR-126 and we will discuss the implications of both bioactive molecules for the formation of blood vessels.