Is the model of signal amplification by GPCRs/GEFs activating multiple GTPases relevant to a broad spectrum of heterotrimeric and RAS superfamily GTPases?

Kinetic Model for the Desensitization of G Protein-Coupled Receptor

Desensitization of G-protein-coupled receptors (GPCR) is a general regulatory mechanism adopted by biological organisms against overstimulation of G protein signaling. Although the details of the mechanism are extensively studied, it is not easy to gain an overarching understanding of the process constituted by a multitude of molecular events with vastly differing time scales. To offer a semiquantitative yet predictive understanding of the mechanism, we formulate a kinetic model for the G protein signaling and desensitization by considering essential biochemical steps from ligand binding to receptor internalization. The internalization, followed by receptor depletion from the plasma membrane, attenuates the downstream signal. Together with the kinetic model and its full numerics of the expression derived for the dose-response relation, an approximated form of the expression clarifies the role played by the individual biochemical processes and allows us to identify four distinct regimes for the downregulation that emerge from the balance between phosphorylation, dephosphorylation, and the cellular level of β-arrestin.

Concerted cell and in vivo screen for pancreatic ductal adenocarcinoma (PDA) chemotherapeutics

PDA is a major cause of US cancer-related deaths. Oncogenic Kras presents in 90% of human PDAs. Kras mutations occur early in pre-neoplastic lesions but are insufficient to cause PDA. Other contributing factors early in disease progression include chronic pancreatitis, alterations in epigenetic regulators, and tumor suppressor gene mutation. GPCRs activate heterotrimeric G-proteins that stimulate intracellular calcium and oncogenic Kras signaling, thereby promoting pancreatitis and progression to PDA. By contrast, Rgs proteins inhibit Gi/q-coupled GPCRs to negatively regulate PDA progression. Rgs16::GFP is expressed in response to caerulein-induced acinar cell dedifferentiation, early neoplasia, and throughout PDA progression. In genetically engineered mouse models of PDA, Rgs16::GFP is useful for pre-clinical rapid in vivo validation of novel chemotherapeutics targeting early lesions in patients following successful resection or at high risk for progressing to PDA. Cultured primary PDA cells express Rgs16::GFP in response to cytotoxic drugs. A histone deacetylase inhibitor, TSA, stimulated Rgs16::GFP expression in PDA primary cells, potentiated gemcitabine and JQ1 cytotoxicity in cell culture, and Gem + TSA + JQ1 inhibited tumor initiation and progression in vivo. Here we establish the use of Rgs16::GFP expression for testing drug combinations in cell culture and validation of best candidates in our rapid in vivo screen.

A rapid in vivo screen for pancreatic ductal adenocarcinoma therapeutics

Pancreatic ductal adenocarcinoma (PDA) is the fourth leading cause of cancer-related deaths in the United States, and is projected to be second by 2025. It has the worst survival rate among all major cancers. Two pressing needs for extending life expectancy of affected individuals are the development of new approaches to identify improved therapeutics, addressed herein, and the identification of early markers. PDA advances through a complex series of intercellular and physiological interactions that drive cancer progression in response to organ stress, organ failure, malnutrition, and infiltrating immune and stromal cells. Candidate drugs identified in organ culture or cell-based screens must be validated in preclinical models such as KIC (p48(Cre);LSL-Kras(G12D);Cdkn2a(f/f)) mice, a genetically engineered model of PDA in which large aggressive tumors develop by 4 weeks of age. We report a rapid, systematic and robust in vivo screen for effective drug combinations to treat Kras-dependent PDA. Kras mutations occur early in tumor progression in over 90% of human PDA cases. Protein kinase and G-protein coupled receptor (GPCR) signaling activates Kras. Regulators of G-protein signaling (RGS) proteins are coincidence detectors that can be induced by multiple inputs to feedback-regulate GPCR signaling. We crossed Rgs16::GFP bacterial artificial chromosome (BAC) transgenic mice with KIC mice and show that the Rgs16::GFP transgene is a Kras(G12D)-dependent marker of all stages of PDA, and increases proportionally to tumor burden in KIC mice. RNA sequencing (RNA-Seq) analysis of cultured primary PDA cells reveals characteristics of embryonic progenitors of pancreatic ducts and endocrine cells, and extraordinarily high expression of the receptor tyrosine kinase Axl, an emerging cancer drug target. In proof-of-principle drug screens, we find that weanling KIC mice with PDA treated for 2 weeks with gemcitabine (with or without Abraxane) plus inhibitors of Axl signaling (warfarin and BGB324) have fewer tumor initiation sites and reduced tumor size compared with the standard-of-care treatment. Rgs16::GFP is therefore an in vivo reporter of PDA progression and sensitivity to new chemotherapeutic drug regimens such as Axl-targeted agents. This screening strategy can potentially be applied to identify improved therapeutics for other cancers.

Ypt/Rab GTPases: principles learned from yeast

Ypt/Rab GTPases are key regulators of all membrane trafficking events in eukaryotic cells. They act as molecular switches that attach to membranes via lipid tails to recruit their multiple downstream effectors, which mediate vesicular transport. Originally discovered in yeast as Ypts, they were later shown to be conserved from yeast to humans, where Rabs are relevant to a wide array of diseases. Major principles learned from our past studies in yeast are currently accepted in the Ypt/Rab field including: (i) Ypt/Rabs are not transport-step specific, but are rather compartment specific, (ii) stimulation by nucleotide exchangers, GEFs, is critical to their function, whereas GTP hydrolysis plays a role in their cycling between membranes and the cytoplasm for multiple rounds of action, (iii) they mediate diverse functions ranging from vesicle formation to vesicle fusion and (iv) they act in GTPase cascades to regulate intracellular trafficking pathways. Our recent studies on Ypt1 and Ypt31/Ypt32 and their modular GEF complex TRAPP raise three exciting novel paradigms for Ypt/Rab function: (a) coordination of vesicular transport substeps, (b) integration of individual transport steps into pathways and (c) coordination of different transport pathways. In addition to its amenability to genetic analysis, yeast provides a superior model system for future studies on the role of Ypt/Rabs in traffic coordination due to the smaller proteome that results in a simpler traffic grid. We propose that different types of coordination are important also in human cells for fine-tuning of intracellular trafficking, and that coordination defects could result in disease.