Imaging the Molecular Machines That Power Cell Migration

RhoB promotes Salmonella survival by regulating autophagy

Salmonella enterica serovar Typhimurium manipulates cellular Rho GTPases for host cell invasion by effector protein translocation via the Type III Secretion System (T3SS). The two Guanine nucleotide exchange (GEF) mimicking factors SopE and -E2 and the inositol phosphate phosphatase (PiPase) SopB activate the Rho GTPases Rac1, Cdc42 and RhoA, thereby mediating bacterial invasion. S. Typhimurium lacking these three effector proteins are largely invasion-defective. Type III secretion is crucial for both early and later phases of the intracellular life of S. Typhimurium. Here we investigated whether and how the small GTPase RhoB, known to localize on endomembrane vesicles and at the invasion site of S. Typhimurium, contributes to bacterial invasion and to subsequent steps relevant for S. Typhimurium lifestyle. We show that RhoB is significantly upregulated within hours of Salmonella infection. This effect depends on the presence of the bacterial effector SopB, but does not require its phosphatase activity. Our data reveal that SopB and RhoB bind to each other, and that RhoB localizes on early phagosomes of intracellular S. Typhimurium. Whereas both SopB and RhoB promote intracellular survival of Salmonella, RhoB is specifically required for Salmonella-induced upregulation of autophagy. Finally, in the absence of RhoB, vacuolar escape and cytosolic hyper-replication of S. Typhimurium is diminished. Our findings thus uncover a role for RhoB in Salmonella-induced autophagy, which supports intracellular survival of the bacterium and is promoted through a positive feedback loop by the Salmonella effector SopB.

Baculovirus Actin Rearrangement-Inducing Factor 1 Can Remodel the Mammalian Actin Cytoskeleton

The actin rearrangement-inducing factor 1 (Arif-1) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is an early viral protein that manipulates the actin cytoskeleton of host insect cells. Arif-1 is conserved among alphabaculoviruses and is responsible for the accumulation of F-actin at the plasma membrane during the early phase of infection. However, the molecular mechanism underlying Arif-1-induced cortical actin accumulation is still open. Recent studies have demonstrated the formation of invadosome-like structures induced by Arif-1, suggesting a function in systemic virus spread. Here, we addressed whether Arif-1 is able to manipulate the actin cytoskeleton of mammalian cells comparably to insect cells. Strikingly, transient overexpression of Arif-1 in B16-F1 mouse melanoma cells revealed pronounced F-actin remodeling. Actin assembly was increased, and intense membrane ruffling occurred at the expense of substrate-associated lamellipodia. Deletion mutagenesis studies of Arif-1 confirmed that the C-terminal cytoplasmic region was not sufficient to induce F-actin remodeling, supporting that the transmembrane region for Arif-1 function is also required in mammalian cells. The similarities between Arif-1-induced actin remodeling in insect and mammalian cells indicate that Arif-1 function relies on conserved cellular interaction partners and signal transduction pathways, thus providing an experimental tool to elucidate the underlying mechanism. IMPORTANCE Virus-induced changes of the host cell cytoskeleton play a pivotal role in the pathogenesis of viral infections. The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is known for intervening with the regulation of the host actin cytoskeleton in a wide manner throughout the infection cycle. The actin rearrangement-inducing factor 1 (Arif-1) is a viral protein that causes actin rearrangement during the early phase of AcMNPV infection. Here, we performed overexpression studies of Arif-1 in mammalian cells to establish an experimental tool that allows elucidation of the mechanism underlying the Arif-1-induced remodeling of actin dynamics in a well-characterized and genetically accessible system.

The autophagy inducer SMER28 attenuates microtubule dynamics mediating neuroprotection

SMER28 originated from a screen for small molecules that act as modulators of autophagy. SMER28 enhanced the clearance of autophagic substrates such as mutant huntingtin, which was additive to rapamycin-induced autophagy. Thus, SMER28 was established as a positive regulator of autophagy acting independently of the mTOR pathway, increasing autophagosome biosynthesis and attenuating mutant huntingtin-fragment toxicity in cellular- and fruit fly disease models, suggesting therapeutic potential. Despite many previous studies, molecular mechanisms mediating SMER28 activities and its direct targets have remained elusive. Here we analyzed the effects of SMER28 on cells and found that aside from autophagy induction, it significantly stabilizes microtubules and decelerates microtubule dynamics. Moreover, we report that SMER28 displays neurotrophic and neuroprotective effects at the cellular level by inducing neurite outgrowth and protecting from excitotoxin-induced axon degeneration. Finally, we compare the effects of SMER28 with other autophagy-inducing or microtubule-stabilizing drugs: whereas SMER28 and rapamycin both induce autophagy, the latter does not stabilize microtubules, and whereas both SMER28 and epothilone B stabilize microtubules, epothilone B does not stimulate autophagy. Thus, the effect of SMER28 on cells in general and neurons in particular is based on its unique spectrum of bioactivities distinct from other known microtubule-stabilizing or autophagy-inducing drugs.

SMER28 Attenuates PI3K/mTOR Signaling by Direct Inhibition of PI3K p110 Delta

SMER28 (Small molecule enhancer of Rapamycin 28) is an autophagy-inducing compound functioning by a hitherto unknown mechanism. Here, we confirm its autophagy-inducing effect by assessing classical autophagy-related parameters. Interestingly, we also discovered several additional effects of SMER28, including growth retardation and reduced G1 to S phase progression. Most strikingly, SMER28 treatment led to a complete arrest of receptor tyrosine kinase signaling, and, consequently, growth factor-induced cell scattering and dorsal ruffle formation. This coincided with a dramatic reduction in phosphorylation patterns of PI3K downstream effectors. Consistently, SMER28 directly inhibited PI3Kδ and to a lesser extent p110γ. The biological relevance of our observations was underscored by SMER28 interfering with InlB-mediated host cell entry of Listeria monocytogenes, which requires signaling through the prominent receptor tyrosine kinase c-Met. This effect was signaling-specific, since entry of unrelated, gram-negative Salmonella Typhimurium was not inhibited. Lastly, in B cell lymphoma cells, which predominantly depend on tonic signaling through PI3Kδ, apoptosis upon SMER28 treatment is profound in comparison to non-hematopoietic cells. This indicates SMER28 as a possible drug candidate for the treatment of diseases that derive from aberrant PI3Kδ activity.