Cell adhesion-dependent membrane trafficking of a binding partner for the ebolavirus glycoprotein is a determinant of viral entry

Ebolavirus is a hemorrhagic fever virus associated with high mortality. Although much has been learned about the viral lifecycle and pathogenesis, many questions remain about virus entry. We recently showed that binding of the receptor binding region (RBR) of the ebolavirus glycoprotein (GP) and infection by GP pseudovirions increase on cell adhesion independently of mRNA or protein synthesis. One model to explain these observations is that, on cell adhesion, an RBR binding partner translocates from an intracellular vesicle to the cell surface. Here, we provide evidence for this model by showing that suspension 293F cells contain an RBR binding site within a membrane-bound compartment associated with the trans-Golgi network and microtubule-organizing center. Consistently, trafficking of the RBR binding partner to the cell surface depends on microtubules, and the RBR binding partner is internalized when adherent cells are placed in suspension. Based on these observations, we reexamined the claim that lymphocytes, which are critical for ebolavirus pathogenesis, are refractory to infection because they lack an RBR binding partner. We found that both cultured and primary human lymphocytes (in suspension) contain an intracellular pool of an RBR binding partner. Moreover, we identified two adherent primate lymphocytic cell lines that bind RBR at their surface and strikingly, support GP-mediated entry and infection. In summary, our results reveal a mode of determining viral entry by a membrane-trafficking event that translocates an RBR binding partner to the cell surface, and they suggest that this process may be operative in cells important for ebolavirus pathogenesis (e.g., lymphocytes and macrophages)

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients

Regulated transport of the glucose transporter GLUT4.

International audienceIn muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service