A novel interaction linking the FAS-II and phthiocerol dimycocerosate (PDIM) biosynthetic pathways

The fatty acid biosynthesis (FAS-II) pathway in Mycobacterium tuberculosis generates long chain fatty acids that serve as the precursors to mycolic acids, essential components of the mycobacterial cell wall. Enzymes in the FAS-II pathway are thought to form one or more noncovalent multi-enzyme complexes within the cell, and a bacterial two-hybrid screen was used to search for missing components of the pathway and to furnish additional data on interactions involving these enzymes in vivo. Using the FAS-II beta-ketoacyl synthase, KasA, as bait, an extensive bacterial two-hybrid screen of a M. tuberculosis genome fragment library unexpectedly revealed a novel interaction between KasA and PpsB as well as PpsD, two polyketide modules involved in the biosynthesis of the virulence lipid phthiocerol dimycocerosate (PDIM). Sequence analysis revealed that KasA interacts with PpsB and PpsD in the region of the acyl carrier domain of each protein, raising the possibility that lipids could be transferred between the FAS-II and PDIM biosynthetic pathways. Subsequent studies utilizing purified proteins and radiolabeled lipids revealed that fatty acids loaded onto PpsB were transferred to KasA and also incorporated into long chain fatty acids synthesized using a Mycobacterium smegmatis lysate. These data suggest that in addition to producing PDIMs, the growing phthiocerol product can also be shuttled into the FAS-II pathway via KasA as an entry point for further elongation. Interactions between these biosynthetic pathways may exist as a simple means to increase mycobacterial lipid diversity, enhancing functionality and the overall complexity of the cell wall.     

SRC directly phosphorylates Bif-1 and prevents its interaction with Bax and the initiation of anoikis

Bif-1 interacts with Bax and enhances its conformational rearrangement, resulting in apoptosis. However, the molecular mechanism governing the interaction between Bif-1 and Bax is poorly defined. Here we provide evidence that Bif-1 is phosphorylated, an event that can be repressed by apoptotic stimuli. The protein kinase c-Src binds to and directly phosphorylates Bif-1 on tyrosine 80. Moreover, Src phosphorylation of Bif-1 suppresses the interaction between Bif-1 and Bax, resulting in the inhibition of Bax activation during anoikis. Together, these results suggest that phosphorylation of Bif-1 impairs its binding to Bax and represses apoptosis, providing another mechanism by which Src oncogenic signaling can prevent cell death.     

Inflammatory cytokines increase extracellular procathepsin D in permanent and primary endothelial cell cultures

The protease cathepsin D (Cath D) and its proteolytically inactive proform, procathepsin D (ProCath D), turned out to be multifunctional within and outside the cell. Elevated levels of ProCath D occur in malignant tumors and in organs under chronic inflammation. One important source for this increase of ProCath D might be endothelial cells. Here we examined the expression of Cath D in the human endothelial cell line EA.hy 926 and in primary endothelial cells isolated from human umbilical cord veins (HUVEC). After serum-free incubation with or without human interferon-gamma (hIFN-gamma) and/or human tumor necrosis factor-alpha (hTNF-alpha) immature and mature Cath D forms were examined in cell extracts and in cell-conditioned medium concentrates by Western blotting. Lysates of EA.hy 926 cells as well as of HUVEC contained active Cath D as two-chain form, but only negligible amounts of ProCath D and Cath D intermediates. Yet both endothelial cell cultures accumulated ProCath D in their conditioned media in the absence of any stimulus. The treatment with hIFN-gamma and/or hTNF-alpha had little effect on intracellular levels of Cath D, whereas the cytokine stimulation increased the extracellular presence of ProCath D in both endothelial cell cultures. The extracellular increase of ProCath D was not related to induction of apoptosis, as validated by cleaved caspase-3 in cell lysates. Acidification of cytokine-treated media converted ProCath D into Cath D, which was associated with cathepsin-like activity using a fluorogenic substrate-linked assay. We conclude, in vitro, endothelial cells are a cytokine-dependent source for extracellular ProCath D.     

Linking within- and between-host dynamics in the evolutionary epidemiology of infectious diseases

Nested models (also called embedded models) explicitly link dynamical processes that occur at different scales. Recently there has been considerable interest in linking within- and between-host levels of disease dynamics in the study of pathogen evolution. Here we review the extent to which these nested models have increased our understanding of pathogen evolution. We suggest that, although such models have been useful for determining the nature of tradeoffs between epidemiological parameters and for evaluating the consequences of conflicting selection pressures at different scales, the vast majority of previous results could likely have been obtained without the use of nested models per se. Nevertheless, these models have proven very useful through their highlighting of the importance of within-host disease dynamics on pathogen evolution.