Intercellular transfer regulation of the paracrine activity of GPI-anchored Cripto-1 as a Nodal co-receptor

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins II: Intercellular Transfer of Matter (Inheritance?) That Matters

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of the plasma membrane (PM) bilayer by covalent linkage to a typical glycolipid and expressed in all eukaryotic organisms so far studied. Lipolytic release from PMs into extracellular compartments and intercellular transfer are regarded as the main (patho)physiological roles exerted by GPI-APs. The intercellular transfer of GPI-APs relies on the complete GPI anchor and is mediated by extracellular vesicles such as microvesicles and exosomes and lipid-free homo- or heteromeric aggregates, and lipoprotein-like particles such as prostasomes and surfactant-like particles, or lipid-containing micelle-like complexes. In mammalian organisms, non-vesicular transfer is controlled by the distance between donor and acceptor cells/tissues; intrinsic conditions such as age, metabolic state, and stress; extrinsic factors such as GPI-binding proteins; hormones such as insulin; and drugs such as anti-diabetic sulfonylureas. It proceeds either "directly" upon close neighborhood or contact of donor and acceptor cells or "indirectly" as a consequence of the induced lipolytic release of GPI-APs from PMs. Those displace from the serum GPI-binding proteins GPI-APs, which have retained the complete anchor, and become assembled in aggregates or micelle-like complexes. Importantly, intercellular transfer of GPI-APs has been shown to induce specific phenotypes such as stimulation of lipid and glycogen synthesis, in cultured human adipocytes, blood cells, and induced pluripotent stem cells. As a consequence, intercellular transfer of GPI-APs should be regarded as non-genetic inheritance of (acquired) features between somatic cells which is based on the biogenesis and transmission of matter such as GPI-APs and "membrane landscapes", rather than the replication and transmission of information such as DNA. Its operation in mammalian organisms remains to be clarified.

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

A Small Change With a Twist Ending: A Single Residue in EGF-CFC Drives Bilaterian Asymmetry

Asymmetries are essential for proper organization and function of organ systems. Genetic studies in bilaterians have shown signaling through the Nodal/Smad2 pathway plays a key, conserved role in the establishment of body asymmetries. Although the main molecular players in the network for the establishment of left-right asymmetry (LRA) have been deeply described in deuterostomes, little is known about the regulation of Nodal signaling in spiralians. Here, we identified orthologs of the egf-cfc gene, a master regulator of the Nodal pathway in vertebrates, in several invertebrate species, which includes the first evidence of its presence in non-deuterostomes. Our functional experiments indicate that despite being present, egf-cfc does not play a role in the establishment of LRA in gastropods. However, experiments in zebrafish suggest that a single amino acid mutation in the egf-cfc gene in at least the common ancestor of chordates was the necessary step to induce a gain of function in LRA regulation. This study shows that the egf-cfc gene likely appeared in the ancestors of deuterostomes and "protostomes", before being adopted as a mechanism to regulate the Nodal pathway and the establishment of LRA in some lineages of deuterostomes.

Biological Role of the Intercellular Transfer of Glycosylphosphatidylinositol-Anchored Proteins: Stimulation of Lipid and Glycogen Synthesis

Glycosylphosphatidylinositol-anchored proteins (GPI-APs), which are anchored at the outer leaflet of plasma membranes (PM) only by a carboxy-terminal GPI glycolipid, are known to fulfill multiple enzymic and receptor functions at the cell surface. Previous studies revealed that full-length GPI-APs with the complete GPI anchor attached can be released from and inserted into PMs in vitro. Moreover, full-length GPI-APs were recovered from serum, dependent on the age and metabolic state of rats and humans. Here, the possibility of intercellular control of metabolism by the intercellular transfer of GPI-APs was studied. Mutant K562 erythroleukemia (EL) cells, mannosamine-treated human adipocytes and methyl-ß-cyclodextrin-treated rat adipocytes as acceptor cells for GPI-APs, based on their impaired PM expression of GPI-APs, were incubated with full-length GPI-APs, prepared from rat adipocytes and embedded in micelle-like complexes, or with EL cells and human adipocytes with normal expression of GPI-APs as donor cells in transwell co-cultures. Increases in the amounts of full-length GPI-APs at the PM of acceptor cells as a measure of their transfer was assayed by chip-based sensing. Both experimental setups supported both the transfer and upregulation of glycogen (EL cells) and lipid (adipocytes) synthesis. These were all diminished by serum, serum GPI-specific phospholipase D, albumin, active bacterial PI-specific phospholipase C or depletion of total GPI-APs from the culture medium. Serum inhibition of both transfer and glycogen/lipid synthesis was counteracted by synthetic phosphoinositolglycans (PIGs), which closely resemble the structure of the GPI glycan core and caused dissociation of GPI-APs from serum proteins. Finally, large, heavily lipid-loaded donor and small, slightly lipid-loaded acceptor adipocytes were most effective in stimulating transfer and lipid synthesis. In conclusion, full-length GPI-APs can be transferred between adipocytes or between blood cells as well as between these cell types. Transfer and the resulting stimulation of lipid and glycogen synthesis, respectively, are downregulated by serum proteins and upregulated by PIGs. These findings argue for the (patho)physiological relevance of the intercellular transfer of GPI-APs in general and its role in the paracrine vs. endocrine (dys)regulation of metabolism, in particular. Moreover, they raise the possibility of the use of full-length GPI-APs as therapeutics for metabolic diseases.

Whence CRIPTO: The Reemergence of an Oncofetal Factor in 'Wounds' That Fail to Heal

There exists a set of factors termed oncofetal proteins that play key roles in ontogeny before they decline or disappear as the organism's tissues achieve homeostasis, only to then re-emerge in cancer. Although the unique therapeutic potential presented by such factors has been recognized for more than a century, their clinical utility has yet to be fully realized1. This review highlights the small signaling protein CRIPTO encoded by the tumor derived growth factor 1 (TDGF1/Tdgf1) gene, an oft cited oncofetal protein whose presence in the cancer literature as a tumor promoter, diagnostic marker and viable therapeutic target continues to grow. We touch lightly on features well established and well-reviewed since its discovery more than 30 years ago, including CRIPTO's early developmental roles and modulation of SMAD2/3 activation by a selected set of transforming growth factor β (TGF-β) family ligands. We predominantly focus instead on more recent and less well understood additions to the CRIPTO signaling repertoire, on its potential upstream regulators and on new conceptual ground for understanding its mode of action in the multicellular and often stressful contexts of neoplastic transformation and progression. We ask whence it re-emerges in cancer and where it 'hides' between the time of its fetal activity and its oncogenic reemergence. In this regard, we examine CRIPTO's restriction to rare cells in the adult, its potential for paracrine crosstalk, and its emerging role in inflammation and tissue regeneration-roles it may reprise in tumorigenesis, acting on subsets of tumor cells to foster cancer initiation and progression. We also consider critical gaps in knowledge and resources that stand between the recent, exciting momentum in the CRIPTO field and highly actionable CRIPTO manipulation for cancer therapy and beyond.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

Cripto-1 localizes to dynamic and shed filopodia associated with cellular migration in glioblastoma cells

Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration.

Targeting Nodal and Cripto-1: Perspectives Inside Dual Potential Theranostic Cancer Biomarkers

BACKGROUND

Elucidating the mechanisms of recurrence of embryonic signaling pathways in tumorigenesis has led to the discovery of onco-fetal players which have physiological roles during normal development but result aberrantly re-activated in tumors. In this context, Nodal and Cripto-1 are recognized as onco-developmental factors, which are absent in normal tissues but are overexpressed in several solid tumors where they can serve as theranostic agents.

OBJECTIVE

To collect, review and discuss the most relevant papers related to the involvement of Nodal and Cripto-1 in the development, progression, recurrence and metastasis of several tumors where they are over-expressed, with a particular attention to their occurrence on the surface of the corresponding sub-populations of cancer stem cells (CSC).

RESULTS

We have gathered, rationalized and discussed the most interesting findings extracted from some 370 papers related to the involvement of Cripto-1 and Nodal in all tumor types where they have been detected. Data demonstrate the clear connection between Nodal and Cripto-1 presence and their multiple oncogenic activities across different tumors. We have also reviewed and highlighted the potential of targeting Nodal, Cripto-1 and the complexes that they form on the surface of tumor cells, especially of CSC, as an innovative approach to detect and suppress tumors with molecules that block one or more mechanisms that they regulate.

CONCLUSION

Overall, Nodal and Cripto-1 represent two innovative and effective biomarkers for developing potential theranostic anti-tumor agents that target normal as well as CSC subpopulations and overcome both pharmacological resistance and tumor relapse.

Divergence(s) in nodal signaling between aggressive melanoma and embryonic stem cells

The significant role of the embryonic morphogen Nodal in maintaining the pluripotency of embryonic stem cells is well documented. Interestingly, the recent discovery of Nodal's re-expression in several aggressive and metastatic cancers has highlighted its critical role in self renewal and maintenance of the stem cell-like characteristics of tumor cells, such as melanoma. However, the key TGFβ/Nodal signaling component(s) governing Nodal's effects in metastatic melanoma remain mostly unknown. By employing receptor profiling at the mRNA and protein level(s), we made the novel discovery that embryonic stem cells and metastatic melanoma cells share a similar repertoire of Type I serine/threonine kinase receptors, but diverge in their Type II receptor expression. Ligand:receptor crosslinking and native gel binding assays indicate that metastatic melanoma cells employ the heterodimeric TGFβ receptor I/TGFβ receptor II (TGFβRI/TGFβRII) for signal transduction, whereas embryonic stem cells use the Activin receptors I and II (ACTRI/ACTRII). This unexpected receptor usage by tumor cells was tested by: neutralizing antibody to block its function; and transfecting the dominant negative receptor to compete with the endogenous receptor for ligand binding. Furthermore, a direct biological role for TGFβRII was found to underlie vasculogenic mimicry (VM), an endothelial phenotype contributing to vascular perfusion and associated with the functional plasticity of aggressive melanoma. Collectively, these findings reveal the divergence in Nodal signaling between embryonic stem cells and metastatic melanoma that can impact new therapeutic strategies targeting the re-emergence of embryonic pathways.

Membrane and cytoplasmic marker exchange between malignant neoplastic cells and fibroblasts via intermittent contact: increased tumour cell diversity independent of genetic change

We previously demonstrated that human osteosarcoma cells (SAOS-2) induce contact-dependent apoptosis in endothelium, and expected similar apoptosis in human gingival fibroblasts (h-GF) using SAOS-2 alkaline phosphatase (AP) to identify cells. However, h-GF apoptosis did not occur, despite reduction in AP-negative h-GF number (p < 0.01) and enhancement of this by h-GF TNFα pretreatment (p < 0.01). We suggest that TNFα-enhanced transfer of membrane AP from SAOS-2 to h-GF would explain these data. This idea was investigated using fluorescence prelabelled cells and confocal laser scanning microscopy. Co-cultures of membrane-labelled h-GF (marker-DiO) and SAOS-2 (marker-DiD) generated dual-labelled cells, primarily at the expense of single labelled h-GF (p < 0.001), suggesting predominant membrane transfer from SAOS-2 to h-GF. However, opposite directional transfer predominated when membrane labels were reversed; SAOS-2 further expressed green fluorescent protein (GFP) in cytoplasm and nuclei, and h-GF additionally bore nuclear label (Syto59) (p < 0.001). Cytoplasmic exchange was investigated using h-GF prelabelled with cytoplasmic DDAO-SE and nuclear Syto59, co-cultured with SAOS-2 expressing GFP in cytoplasm and nuclei, and predominant cytoplasmic marker transferred from h-GF to SAOS-2 (p < 0.05). Pretreating h-GF with TNFα increased exchange of membrane markers (p < 0.04) but did not affect either cell surface area profile or circularity. Dual-labelled cells had a morphological phenotype differing from SAOS-2 and h-GF (p < 0.001). Time-lapse microscopy revealed extensive migration of SAOS-2 and cell process contact with h-GF, with the appearance of SAOS-2 indulging in 'cellular sipping' from h-GF. Similar exchange of membrane was seen between h-GF and with other cell lines (melanoma MeIRMu, NM39, WMM175, MM200-B12; osteosarcoma U20S; ovarian carcinoma cells PE01, PE04 and COLO316), while cytoplasmic sharing was also seen in all cell lines other than U20S. We suggest that in some neoplasms, cellular sipping may contribute to phenotypic change and the generation of diverse tumour cell populations independent of genetic change, raising the possibility of a role in tumour progression.