It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/β-catenin pathway that is characterized by the activation of certain β-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates β-catenin-independent pathways, including the Wnt/ calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Mycobacterium tuberculosis and Mycobacterium marinum are thought to exert virulence, in part, through their ability to lyse host cell membranes. The type VII secretion system ESX-1 [6-kDa early secretory antigenic target (ESAT-6) secretion system 1] is required for both virulence and host cell membrane lysis. Both activities are attributed to the pore-forming activity of the ESX-1-secreted substrate ESAT-6 because multiple studies have reported that recombinant ESAT-6 lyses eukaryotic membranes. We too find ESX-1 of M. tuberculosis and M. marinum lyses host cell membranes. However, we find that recombinant ESAT-6 does not lyse cell membranes. The lytic activity previously attributed to ESAT-6 is due to residual detergent in the preparations. We report here that ESX-1-dependent cell membrane lysis is contact dependent and accompanied by gross membrane disruptions rather than discrete pores. ESX-1-mediated lysis is also morphologically distinct from the contact-dependent lysis of other bacterial secretion systems. Our findings suggest redirection of research to understand the mechanism of ESX-1-mediated lysis.Mycobacterium tuberculosis | Mycobacterium marinum | ESAT-6 | ESX-1 secretion system | cell membrane lysis

Wnt signaling activates several distinct intracellular pathways, which are important for cell proliferation, differentiation, and polarity. Wnt proteins are secreted molecules that typically signal across the membrane via interaction with the transmembrane receptor Frizzled. Following interaction with Frizzled, the downstream effect of the most widely studied Wnt pathway is stabilization and nuclear translocation of the cytosolic protein, beta-catenin. In this chapter, we discuss two beta-catenin-independent branches of Wnt signaling: 1) Wnt/planar cell polarity (PCP), a Wnt pathway that signals through the small GTPases, Rho and Rac, to promote changes in the actin cytoskeleton, and 2) Wnt/Ca2+, a Wnt pathway that promotes intracellular calcium transients and negatively regulates the Wnt/beta-catenin pathway. Finally, during the course of our discussion, we highlight areas that require future research.

Wnts are secreted ligands that activate several receptor-mediated signal transduction cascades. Homeostatic Wnt signaling through β-catenin is required in adults, because either elevation or attenuation of β-catenin function has been linked to diverse diseases. To contribute to the identification of both protein and pharmacological regulators of this pathway, we describe a combinatorial screen that merged data from a high-throughput screen of known bioactive compounds with an independent focused small interfering RNA screen. Each screen independently revealed Bruton’s tyrosine kinase (BTK) as an inhibitor of Wnt–β-catenin signaling. Loss of BTK function in human colorectal cancer cells, human B cells, zebrafish embryos, and cells derived from X-linked agammaglobulinemia patients with a mutant BTK gene resulted in elevated Wnt–β-catenin signaling, confirming that BTK acts as a negative regulator of this pathway. From affinity purification–mass spectrometry and biochemical binding studies, we found that BTK directly interacts with a nuclear component of Wnt–β-catenin signaling, CDC73. Further, we show that BTK increased the abundance of CDC73 in the absence of stimulation and that CDC73 acted as a repressor of β-catenin-mediated transcription in human colorectal cancer cells and B cells.