Profiling of Human Neural Crest Chemoattractant Activity as a Replacement of Fetal Bovine Serum for In Vitro Chemotaxis Assays

Fetal bovine serum (FBS) is the only known stimulus for the migration of human neural crest cells (NCCs). Non-animal chemoattractants are desirable for the optimization of chemotaxis as-says to be incorporated in a test battery for reproductive and developmental toxicity. We con-firmed here in an optimized transwell assay that FBS triggers directed migration along a con-centration gradient. The responsible factor was found to be a protein in the 30–100 kDa size range. In a targeted approach, we tested a large panel of serum constituents known to be chem-otactic for NCCs in animal models (e.g., VEGF, PDGF, FGF, SDF-1/CXCL12, ephrins, endothelin, Wnt, BMPs). None of the corresponding human proteins showed any effect in our chemotaxis assays based on human NCCs. We then examined, whether human cells would produce any fac-tor able to trigger NCC migration in a broad screening approach. We found that HepG2 hepa-toma cells produced chemotaxis-triggering activity (CTA). Using chromatographic methods and by employing the NCC chemotaxis test as bioassay, the responsible protein was enriched by up to 5000-fold. We also explored human serum and platelets as a direct source, independent of any cell culture manipulations. A CTA was enriched from platelet lysates several thousand-fold. Its temperature and protease sensitivity suggested also a protein component. The capacity of this factor to trigger chemotaxis was confirmed by single-cell video-tracking analysis of migrating NCCs. The human CTA characterized here may be employed in the future for the setup of assays testing for the disturbance of directed NCC migration by toxicants.

Substrate properties of zebrafish Rtn4b/Nogo and axon regeneration in the zebrafish optic nerve

This study explored why lesioned retinal ganglion cell (RGC) axons regenerate successfully in the zebrafish optic nerve despite the presence of Rtn4b, the homologue of the rat neurite growth inhibitor RTN4-A/Nogo-A. Rat Nogo-A and zebrafish Rtn4b possess characteristic motifs (M1-4) in the Nogo-A-specific region, which contains delta20, the most inhibitory region of rat Nogo-A. To determine whether zebrafish M1-4 is inhibitory as rat M1-4 and Nogo-A delta20, proteins were recombinantly expressed and used as substrates for zebrafish single cell RGCs, mouse hippocampal neurons and goldfish, zebrafish and chick retinal explants. When offered as homogenous substrates, neurites of hippocampal neurons and of zebrafish single cell RGCs were inhibited by zebrafish M1-4, rat M1-4, and Nogo-A delta20. Neurite length increased when zebrafish single cell RGCs were treated with receptor-type-specific antagonists and, respectively, with morpholinos (MO) against S1PR2 and S1PR5a-which represent candidate zebrafish Nogo-A receptors. In a stripe assay, however, where M1-4 lanes alternate with polylysine-(Plys)-only lanes, RGC axons from goldfish, zebrafish, and chick retinal explants avoided rat M1-4 but freely crossed zebrafish M1-4 lanes-suggesting that zebrafish M1-4 is growth permissive and less inhibitory than rat M1-4. Moreover, immunostainings and dot blots of optic nerve and myelin showed that expression of Rtn4b is very low in tissue and myelin at 3-5 days after lesion when axons regenerate. Thus, Rtn4b seems to represent no major obstacle for axon regeneration in vivo because it is less inhibitory for RGC axons from retina explants, and because of its low abundance.

Reggies/flotillins regulate E-cadherin-mediated cell contact formation by affecting EGFR trafficking

In epithelial cells, the reggie/flotillin proteins regulate—in association with PrP—the formation of E-cadherin adherens junctions (AJs) via the EGFR. Reggies control the EGF-mediated phosphorylation and internalization of EGFR. EGF signaling at the plasma membrane induces the macropinocytosis of E-cadherin and thus the formation of AJs.

The reggie/flotillin proteins are implicated in membrane trafficking and, together with the cellular prion protein (PrP), in the recruitment of E-cadherin to cell contact sites. Here, we demonstrate that reggies, as well as PrP down-regulation, in epithelial A431 cells cause overlapping processes and abnormal formation of adherens junctions (AJs). This defect in cell adhesion results from reggie effects on Src tyrosine kinases and epidermal growth factor receptor (EGFR): loss of reggies reduces Src activation and EGFR phosphorylation at residues targeted by Src and c-cbl and leads to increased surface exposure of EGFR by blocking its internalization. The prolonged EGFR signaling at the plasma membrane enhances cell motility and macropinocytosis, by which junction-associated E-cadherin is internalized and recycled back to AJs. Accordingly, blockage of EGFR signaling or macropinocytosis in reggie-deficient cells restores normal AJ formation. Thus, by promoting EGFR internalization, reggies restrict the EGFR signaling involved in E-cadherin macropinocytosis and recycling and regulate AJ formation and dynamics and thereby cell adhesion.

Glycosylphosphatidyl inositol-anchored proteins and fyn kinase assemble in noncaveolar plasma membrane microdomains defined by reggie-1 and -2

Using confocal laser scanning and double immunogold electron microscopy, we demonstrate that reggie-1 and -2 are colocalized in < or =0.1-microm plasma membrane microdomains of neurons and astrocytes. In astrocytes, reggie-1 and -2 do not occur in caveolae but clearly outside these structures. Microscopy and coimmunoprecipitation show that reggie-1 and -2 are associated with fyn kinase and with the glycosylphosphatidyl inositol-anchored proteins Thy-1 and F3 that, when activated by antibody cross-linking, selectively copatch with reggie. Jurkat cells, after cross-linking of Thy-1 or GM1 (with the use of cholera toxin), exhibit substantial colocalization of reggie-1 and -2 with Thy-1, GM1, the T-cell receptor complex and fyn. This, and the accumulation of reggie proteins in detergent-resistant membrane fractions containing F3, Thy-1, and fyn imparts to reggie-1 and -2 properties of raft-associated proteins. It also suggests that reggie-1 and -2 participate in the formation of signal transduction centers. In addition, we find reggie-1 and -2 in endolysosomes. In Jurkat cells, reggie-1 and -2 together with fyn and Thy-1 increase in endolysosomes concurrent with a decrease at the plasma membrane. Thus, reggie-1 and -2 define raft-related microdomain signaling centers in neurons and T cells, and the protein complex involved in signaling becomes subject to degradation.