Integrin alpha5beta1 function is regulated by XGIPC/kermit2 mediated endocytosis during Xenopus laevis gastrulation

SH3BP4 promotes neuropilin-1 and α5-integrin endocytosis and is inhibited by Akt

Cells probe their surrounding matrix for attachment sites via integrins that are internalized by endocytosis. We find that SH3BP4 regulates integrin surface expression in a signaling-dependent manner via clathrin-coated pits (CCPs). Dephosphorylated SH3BP4 at S246 is efficiently recruited to CCPs, while upon Akt phosphorylation, SH3BP4 is sequestered by 14-3-3 adaptors and excluded from CCPs. In the absence of Akt activity, SH3BP4 binds GIPC1 and targets neuropilin-1 and α5/β1-integrin for endocytosis, leading to inhibition of cell spreading. Similarly, chemorepellent semaphorin-3a binds neuropilin-1 to activate PTEN, which antagonizes Akt and thus recruits SH3BP4 to CCPs to internalize both receptors and induce cell contraction. In PTEN mutant non-small cell lung cancer cells with high Akt activity, expression of non-phosphorylatable active SH3BP4-S246A restores semaphorin-3a induced cell contraction. Thus, SH3BP4 links Akt signaling to endocytosis of NRP1 and α5/β1-integrins to modulate cell-matrix interactions in response to intrinsic and extrinsic cues.

Possible Role of Activin in the Adiponectin Paradox-Induced Progress of Alzheimer's Disease

Accumulating evidence suggests that the adiponectin (APN) paradox might be involved in promoting aging-associated chronic diseases such as Alzheimer's disease (AD). In human brain, APN regulation of the evolvability of amyloidogenic proteins (APs), including amyloid-β (Aβ) and tau, in developmental/reproductive stages, might be paradoxically manifest as APN stimulation of AD through antagonistic pleiotropy in aging. The unique mechanisms underlying APN activity remain unclear, a better understanding of which might provide clues for AD therapy. In this paper, we discuss the possible relevance of activin, a member of transforming growth factor β (TGFβ) superfamily of peptides, to antagonistic pleiotropy effects of APN. Notably, activin, a multiple regulator of cell proliferation and differentiation, as well as an endocrine modulator in reproduction and an organizer in early development, might promote aging-associated disorders, such as inflammation and cancer. Indeed, serum activin, but not serum TGFβ increases during aging. Also, activin/TGFβ signal through type II and type I receptors, both of which are transmembrane serine/threonine kinases, and the serine/threonine phosphorylation of APs, including Aβ42 serine 8 and αS serine 129, may confer pathological significance in neurodegenerative diseases. Moreover, activin expression is induced by APN in monocytes and hepatocytes, suggesting that activin might be situated downstream of the APN paradox. Finally, a meta-analysis of genome-wide association studies demonstrated that two SNPs relevant to the activin/TGFβ receptor signaling pathways conferred risk for major aging-associated disease. Collectively, activin might be involved in the APN paradox of AD and could be a significant therapeutic target.

Integrin trafficking in cells and tissues

Cell adhesion to the extracellular matrix is fundamental to metazoan multicellularity and is accomplished primarily through the integrin family of cell-surface receptors. Integrins are internalized and enter the endocytic-exocytic pathway before being recycled back to the plasma membrane. The trafficking of this extensive protein family is regulated in multiple context-dependent ways to modulate integrin function in the cell. Here, we discuss recent advances in understanding the mechanisms and cellular roles of integrin endocytic trafficking.

Roles for Xenopus aquaporin-3b (aqp3.L) during gastrulation: Fibrillar fibronectin and tissue boundary establishment in the dorsal margin

Aquaporins and aquaglyceroporins are a large family of membrane channel proteins that allow rapid movement of water and small, uncharged solutes into and out of cells along concentration gradients. Recently, aquaporins have been gaining recognition for more complex biological roles than the regulation of cellular osmotic homeostasis. We have identified a specific expression pattern for Xenopus aqp3b (also called aqp3.L) during gastrulation, where it is localized to the sensorial (deep) layer of the blastocoel roof and dorsal margin. Interference with aqp3b expression resulted in loss of fibrillar fibronectin matrix in Brachet's cleft at the dorsal marginal zone, but not on the free surface of the blastocoel. Detailed observation showed that the absence of fibronectin matrix correlated with compromised border integrities between involuted mesendoderm and noninvoluted ectoderm in the marginal zone. Knockdown of aqp3b also led to delayed closure of the blastopore, suggesting defects in gastrulation movements. Radial intercalation was not affected in aqp3b morphants, while the data presented are consistent with impeded convergent extension movements of the dorsal mesoderm in response to loss of aqp3b. Our emerging model suggests that aqp3b is part of a mechanism that promotes proper interaction between cells and the extracellular matrix, thereby playing a critical role in gastrulation.

The GIPC1-Akt1 Pathway Is Required for the Specification of the Eye Field in Mouse Embryonic Stem Cells

During early patterning of the neural plate, a single region of the embryonic forebrain, the eye field, becomes competent for eye development. The hallmark of eye field specification is the expression of the eye field transcription factors (EFTFs). Experiments in fish, amphibians, birds, and mammals have demonstrated largely conserved roles for the EFTFs. Although some of the key signaling events that direct the synchronized expression of these factors to the eye field have been elucidated in fish and frogs, it has been more difficult to study these mechanisms in mammalian embryos. In this study, we have used two different methods for directed differentiation of mouse embryonic stem cells (mESCs) to generate eye field cells and retina in vitro to test for a role of the PDZ domain-containing protein GIPC1 in the specification of the mammalian eye primordia. We find that the overexpression of a dominant-negative form of GIPC1 (dnGIPC1), as well as the downregulation of endogenous GIPC1, is sufficient to inhibit the development of eye field cells from mESCs. GIPC1 interacts directly with IGFR and participates in Akt1 activation, and pharmacological inhibition of Akt1 phosphorylation mimics the dnGIPC1 phenotype. Our data, together with previous studies in Xenopus, support the hypothesis that the GIPC1-PI3K-Akt1 pathway plays a key role in eye field specification in vertebrates.

GAIP interacting protein C-terminus regulates autophagy and exosome biogenesis of pancreatic cancer through metabolic pathways

GAIP interacting protein C terminus (GIPC) is known to play an important role in a variety of physiological and disease states. In the present study, we have identified a novel role for GIPC as a master regulator of autophagy and the exocytotic pathways in cancer. We show that depletion of GIPC-induced autophagy in pancreatic cancer cells, as evident from the upregulation of the autophagy marker LC3II. We further report that GIPC regulates cellular trafficking pathways by modulating the secretion, biogenesis, and molecular composition of exosomes. We also identified the involvement of GIPC on metabolic stress pathways regulating autophagy and microvesicular shedding, and observed that GIPC status determines the loading of cellular cargo in the exosome. Furthermore, we have shown the overexpression of the drug resistance gene ABCG2 in exosomes from GIPC-depleted pancreatic cancer cells. We also demonstrated that depletion of GIPC from cancer cells sensitized them to gemcitabine treatment, an avenue that can be explored as a potential therapeutic strategy to overcome drug resistance in cancer.

Functional proteomics, human genetics and cancer biology of GIPC family members

GIPC1, GIPC2 and GIPC3 consist of GIPC homology 1 (GH1) domain, PDZ domain and GH2 domain. The regions around the GH1 and GH2 domains of GIPC1 are involved in dimerization and interaction with myosin VI (MYO6), respectively. The PDZ domain of GIPC1 is involved in interactions with transmembrane proteins [IGF1R, NTRK1, ADRB1, DRD2, TGFβR3 (transforming growth factorβ receptor type III), SDC4, SEMA4C, LRP1, NRP1, GLUT1, integrin α5 and VANGL2], cytosolic signaling regulators (APPL1 and RGS19) and viral proteins (HBc and HPV-18 E6). GIPC1 is an adaptor protein with dimerizing ability that loads PDZ ligands as cargoes for MYO6-dependent endosomal trafficking. GIPC1 is required for cell-surface expression of IGF1R and TGFβR3. GIPC1 is also required for integrin recycling during cell migration, angiogenesis and cytokinesis. On early endosomes, GIPC1 assembles receptor tyrosine kinases (RTKs) and APPL1 for activation of PI3K-AKT signaling, and G protein-coupled receptors (GPCRs) and RGS19 for attenuation of inhibitory Gα signaling. GIPC1 upregulation in breast, ovarian and pancreatic cancers promotes tumor proliferation and invasion, whereas GIPC1 downregulation in cervical cancer with human papillomavirus type 18 infection leads to resistance to cytostatic transforming growth factorβ signaling. GIPC2 is downregulated in acute lymphocytic leukemia owing to epigenetic silencing, while Gipc2 is upregulated in estrogen-induced mammary tumors. Somatic mutations of GIPC2 occur in malignant melanoma, and colorectal and ovarian cancers. Germ-line mutations of the GIPC3 or MYO6 gene cause nonsyndromic hearing loss. As GIPC proteins are involved in trafficking, signaling and recycling of RTKs, GPCRs, integrins and other transmembrane proteins, dysregulation of GIPCs results in human pathologies, such as cancer and hereditary deafness.

Neuropilin-1 stimulates tumor growth by increasing fibronectin fibril assembly in the tumor microenvironment

The tumor microenvironment, including stromal myofibroblasts and associated matrix proteins, regulates cancer cell invasion and proliferation. Here, we report that neuropilin-1 (NRP-1) orchestrates communications between myofibroblasts and soluble fibronectin that promote α5β1 integrin-dependent fibronectin fibril assembly, matrix stiffness, and tumor growth. Tumor growth and fibronectin fibril assembly were reduced by genetic depletion or antibody neutralization of NRP-1 from stromal myofibroblasts in vivo. Mechanistically, the increase in fibronectin fibril assembly required glycosylation of serine 612 of the extracellular domain of NRP-1, an intact intracellular NRP-1 SEA domain, and intracellular associations between NRP-1, the scaffold protein GIPC, and the nonreceptor tyrosine kinase c-Abl that augmented α5β1 fibronectin fibril assembly activity. Analysis of human cancer specimens established an association between tumoral NRP-1 levels and clinical outcome. Our findings indicate that NRP-1 activates the tumor microenvironment, thereby promoting tumor growth. These results not only identify new molecular mechanisms of fibronectin fibril assembly but also have important implications for therapeutic targeting of the myofibroblast in the tumor microenvironment.