Identification of cis-acting determinants mediating the unconventional secretion of tau.

The deposition of tau aggregates throughout the brain is a pathological characteristic within a group of neurodegenerative diseases collectively termed tauopathies, which includes Alzheimer's disease. While recent findings suggest the involvement of unconventional secretory pathways driving tau into the extracellular space and mediating the propagation of the disease-associated pathology, many of the mechanistic details governing this process remain elusive. In the current study, we provide an in-depth characterization of the unconventional secretory pathway of tau and identify novel molecular determinants that are required for this process. Here, using Drosophila models of tauopathy, we correlate the hyperphosphorylation and aggregation state of tau with the disease-related neurotoxicity. These newly established systems recapitulate all the previously identified hallmarks of tau secretion, including the contribution of tau hyperphosphorylation as well as the requirement for PI(4,5)P2 triggering the direct translocation of tau. Using a series of cellular assays, we demonstrate that both the sulfated proteoglycans on the cell surface and the correct orientation of the protein at the inner plasma membrane leaflet are critical determinants of this process. Finally, we identify two cysteine residues within the microtubule binding repeat domain as novel cis-elements that are important for both unconventional secretion and trans-cellular propagation of tau.

The Yin and Yang of protein folding.

The study of protein aggregation saw a renaissance in the last decade, when it was discovered that aggregation is the cause of several human diseases, making this field of research one of the most exciting frontiers in science today. Building on knowledge about protein folding energy landscapes, determined using an array of biophysical methods, theory and simulation, new light is now being shed on some of the key questions in protein-misfolding diseases. This review will focus on the mechanisms of protein folding and amyloid fibril formation, concentrating on the role of partially folded states in these processes, the complexity of the free energy landscape, and the potentials for the development of future therapeutic strategies based on a full biophysical description of the combined folding and aggregation free-energy surface.

Association of Bcr-Abl with the proto-oncogene Vav is implicated in activation of the Rac-1 pathway.

Vav is a guanine nucleotide exchange factor for the Rho/Rac family predominantly expressed in hematopoietic cells and implicated in cell proliferation and cytoskeletal organization. The oncogenic tyrosine kinase Bcr-Abl has been shown to activate Rac-1, which is important for Bcr-Abl induced leukemogenesis. Previous studies by Matsuguchi et al. (Matsuguchi, T., Inhorn, R. C., Carlesso, N., Xu, G., Druker, B., and Griffin, J. D. (1995) EMBO J. 14, 257-265) describe enhanced phosphorylation of Vav in Bcr-Abl-expressing Mo7e cells yet fail to demonstrate association of the two proteins. Here, we report the identification of a direct complex between Vav and Bcr-Abl in yeast, in vitro and in vivo. Furthermore, we show tyrosine phosphorylation of Vav by Bcr-Abl. Mutational analysis revealed that the SH2 domain and the C-terminal SH3 domain as well as a tetraproline motif directly adjacent to the N-terminal SH3 domain of Vav are important for establishing this phosphotyrosine dependent interaction. Activation of Rac-1 by Bcr-Abl was abrogated by co-expression of the Vav C terminus encoding the SH3-SH2-SH3 domains as a dominant negative construct. Bcr-Abl transduced primary bone marrow from Vav knock-out mice showed reduced proliferation in a culture cell transformation assay compared with wild-type bone marrow. These results suggest, that Bcr-Abl utilizes Vav as a guanine nucleotide exchange factor to activate Rac-1 in a process that involves a folding mechanism of the Vav C terminus. Given the importance of Rac-1 activation for Bcr-Abl-mediated leukemogenesis, this mechanism may be crucial for the molecular pathogenesis of chronic myeloid leukemia and of importance for other signal transduction pathways leading to the activation of Rac-1.