Podosomes are protrusive structures implicated in macrophage extracellular matrix degradation and three-dimensional migration through cell barriers and the interstitium. Podosome formation and assembly are regulated by cytoskeleton remodeling requiring cytoplasmic tyrosine kinases of the Src and the Abl families. Considering that Abl has been reported to phosphorylate the guanine nucleotide exchange factor Sos1, eliciting its Rac-guanine nucleotide exchange factor activity, and Rac regulates podosome formation in myeloid cells and invadopodia formation in cancer cells, we addressed whether Sos1 is implicated in podosome formation and function in macrophages. We found that ectopically expressed Abl or the Src kinase Fgr phosphorylate Sos1, and the Src kinases Hck and Fgr are required for Abl and Sos1 phosphorylation and Abl/Sos1 interaction in macrophages. Sos1 localizes to podosomes in both murine and human macrophages, and its silencing by small interfering RNA results in disassembly of murine macrophage podosomes and a marked reduction of GTP loading on Rac. Matrix degradative capacity, three-dimensional migration through Matrigel, and transmigration through an endothelial cell monolayer of Sos1-silenced macrophages were inhibited. In addition, Sos1- or Abl-silenced macrophages, or macrophages treated with the selective Abl inhibitor imatinib mesylate had a reduced capability to migrate into breast tumor spheroids, the majority of cells remaining at the margin and the outer layers of the spheroid itself. Because of the established role of Src and Abl kinases to regulate also invadopodia formation in cancer cells, our findings suggest that targeting the Src/Abl/Sos1/Rac pathway may represent a double-edged sword to control both cancer-invasive capacities and cancer-related inflammation.

Plants have emerged as competitive production platforms for pharmaceutical proteins that are required in large quantities. One example is the 65-kDa isoform of human glutamic acid decarboxylase (GAD65), a major autoimmune diabetes autoantigen that has been developed as a vaccine candidate for the primary prevention of diabetes. The expression of GAD65 in plants has been optimized but large-scale purification is hampered by its tendency to associate with membranes. We investigated the potential for large-scale downstream processing by evaluating different combinations of plant-based expression systems and engineered forms of GAD65 in terms of yield, subcellular localization and solubility in detergent-free buffer. We found that a modified version of GAD65 lacking the first 87 amino acids accumulates to high levels in the cytosol and can be extracted in detergent-free buffer. The highest yields of this variant protein were achieved using the MagnICON transient expression system. This combination of truncated GAD65 and the MagnICON system dramatically boosts the production of the recombinant protein and helps to optimize downstream processing for the establishment of a sustainable plant-based production platform for an autoimmune diabetes vaccine candidate.