Snf1/AMP-activated protein kinase activates Arf3p to promote invasive yeast growth via a non-canonical GEF domain

The ARF GTPase regulatory network in collective invasion and metastasis

The ability to remodel and move cellular membranes, and the cargoes regulated by these membranes, allows for specialised functions to occur in distinct regions of the cell in a process known as cellular polarisation. The ability to collectively co-ordinate such polarisation between cells allows for the genesis of multicellularity, such as the formation of organs. During tumourigenesis, the rules for such tissue polarisation become dysregulated, allowing for collective polarity rearrangements that can drive metastasis. In this review, we focus on how membrane trafficking underpins collective cell invasion and metastasis in cancer. We examine this through the lens of the ADP-ribosylation factor (ARF) subfamily of small GTPases, focusing on how the ARF regulatory network - ARF activators, inactivators, effectors, and modifications - controls ARF GTPase function.

AMPK promotes Arf6 activation in a kinase-independent manner upon energy deprivation

AMP-activated protein kinase (AMPK) is a crucial cellular nutrient and energy sensor that maintains energy homeostasis. AMPK also governs cancer cell invasion and migration by regulating gene expression and activating multiple cellular signaling pathways. ADP-ribosylation factor 6 (Arf6) can be activated via nucleotide exchange by guanine-nucleotide-exchange factors (GEFs), and its activation also regulates tumor invasion and migration. By studying GEF-mediated Arf6 activation, we have elucidated that AMPK functions as a noncanonical GEF for Arf6 in a kinase-independent manner. Moreover, by examining the physiological role of the AMPK–Arf6 axis, we have determined that AMPK activates Arf6 upon glucose starvation and 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) treatment. We have further identified the binding motif in the C-terminal regulatory domain of AMPK that is responsible for promoting Arf6 activation and, thus, inducing cell migration and invasion. These findings reveal a noncanonical role of AMPK in which its C-terminal regulatory domain serves as a GEF for Arf6 during glucose deprivation.

Deletion of a small, secreted and cysteine-rich protein Cpl1 leads to increased invasive growth of Cryptococcus neoformans into nutrient agar

Invasive growth of yeast cells into nutrient agar is induced by different stresses and contributes to the survival of yeast cells under several adverse conditions. The mechanism of invasive growth of Saccharomyces cerevisiae has been extensively investigated. However, there is very little information about the mechanism of invasive growth of another human pathogen yeast Cryptococcus neoformans. Here, we report that deletion of a small and secreted cysteine-rich protein Cpl1 in C. neoformans JEC21 leads to increased adhesive and invasive growth into nutrient agar. The increased adhesive and invasive growth does not depend on the only known adhesion protein Cfl1 and its main controller Znf2. Cpl1Δ accumulates significantly higher level of intracellular labile zinc ion, leading to increased glucose uptake, higher level of mitochondrial membrane potential, ATP and Reactive Oxygen Species(ROS) production. Higher level of ROS activates Snf1, leading to invasive growth of Cpl1Δ. Three cysteine residues at the N-terminals of the cysteine-rich domain controls the increased invasive growth under nutrient sufficient conditions. This is the first report that a small and secreted cysteine-rich protein negatively regulates invasive growth of C. neoformans through regulating the intracellular labile zinc ion level. The function of this cysteine-rich domain was systematically investigated by site-directed mutagenensis in C. neoformans. The work contributes to understanding the function of this protein family and the invasive growth mechanism in C. neoformans.

Host Adaptive Immune Status Regulates Expression of the Schistosome AMP-Activated Protein Kinase

Schistosomes exhibit profound developmental adaptations in response to the immune status of their mammalian host, including significant attenuation of parasite growth, development and reproduction in response to deficits in host adaptive immunity. These observations led us to hypothesize that schistosomes regulate the utilization of energy resources in response to immunological conditions within the host. To test this hypothesis, we identified and characterized the Schistosoma mansoni AMP-activated protein kinase (AMPK), a heterotrimeric enzyme complex that is central to regulating energy metabolism at the cellular and organismal level in eukaryotes. We show that expression of the catalytic α subunit is developmentally regulated during the parasite life cycle, with peak expression occurring in adult worms. However, the protein is present and phosphorylated in all life cycle stages examined, suggesting a need for active regulation of energy resources throughout the life cycle. In contrast, transcription of the AMPK α gene is down-regulated in cercariae and schistosomula, suggesting that the protein in these life cycle stages is pre-synthesized in the sporocyst and that expression must be re-initiated once inside the mammalian host. We also show that schistosome AMPK α activity in adult worms is sensitive to changes in the parasite's environment, suggesting a mechanism by which schistosome metabolism may be responsive to host immune factors. Finally, we show that AMPK α expression is significantly down-regulated in parasites isolated from immunodeficient mice, suggesting that modulation of parasite energy metabolism may contribute to the attenuation of schistosome growth and reproduction in immunodeficient hosts. These findings provide insights into the molecular interactions between schistosomes and their vertebrate hosts and suggest that parasite energy metabolism may represent a novel target for anti-schistosome interventions.

Conventional and emerging roles of the energy sensor Snf1/AMPK in Saccharomyces cerevisiae

All proliferating cells need to match metabolism, growth and cell cycle progression with nutrient availability to guarantee cell viability in spite of a changing environment. In yeast, a signaling pathway centered on the effector kinase Snf1 is required to adapt to nutrient limitation and to utilize alternative carbon sources, such as sucrose and ethanol. Snf1 shares evolutionary conserved functions with the AMP-activated Kinase (AMPK) in higher eukaryotes which, activated by energy depletion, stimulates catabolic processes and, at the same time, inhibits anabolism. Although the yeast Snf1 is best known for its role in responding to a number of stress factors, in addition to glucose limitation, new unconventional roles of Snf1 have recently emerged, even in glucose repressing and unstressed conditions. Here, we review and integrate available data on conventional and non-conventional functions of Snf1 to better understand the complexity of cellular physiology which controls energy homeostasis.

In-host microevolution of Aspergillus fumigatus: A phenotypic and genotypic analysis

In order to survive, Aspergillus fumigatus must adapt to specific niche environments. Adaptation to the human host includes modifications facilitating persistent colonisation and the development of azole resistance. The aim of this study is to advance understanding of the genetic and physiological adaptation of A. fumigatus in patients during infection and treatment. Thirteen A. fumigatus strains were isolated from a single chronic granulomatous disease patient suffering from persistent and recurrent invasive aspergillosis over a period of 2 years. All strains had identical microsatellite genotypes and were considered isogenic. Whole genome comparisons identified 248 non-synonymous single nucleotide polymorphisms. These non-synonymous mutations have potential to play a role in in-host adaptation. The first 2 strains isolated were azole susceptible, whereas later isolates were itraconazole, voriconazole and/or posaconazole resistant. Growth assays in the presence and absence of various antifungal stressors highlighted minor changes in growth rate and stress resistance, with exception of one isolate showing a significant growth defect. Poor conidiation was observed in later isolates. In certain drug resistant isolates conidiation was restored in the presence of itraconazole. Differences in virulence were observed as demonstrated in a Galleria mellonella infection model. We conclude that the microevolution of A. fumigatus in this patient has driven the emergence of both Cyp51A-independent and Cyp51A-dependent, azole resistance mechanisms, and additional phenotypes that are likely to have promoted fungal persistence.