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  • Which other approaches can complement

    2019-07-04

    Which other approaches can complement signaling optogenetics to paint a more complete picture of developmental Erk signaling? For now, the authors have dynamically manipulated Erk signaling but measured the Erk output at steady-state using classic immunofluorescence techniques (Johnson and Toettcher, 2019). Recent studies using live cell biosensors indicate that Erk can display a wide variety of dynamic signaling patterns that fluctuate on minute timescales. Depending on the cellular context, Erk activation can be transient or sustained (Santos et al., 2007), pulsatile (Albeck et al., 2013), oscillating (Shankaran et al., 2009), and even display propagating waves across cell collectives (Aoki et al., 2017). As one example that is relevant to developmental biology, pulsatile Erk activity regulates C. elegans vulval precursor cell fate specification (de la Cova et al., 2017). In this model, kinesin spindle protein that will adopt different fates exhibit different frequencies of Erk activity pulses of fixed amplitude. The Erk pulse frequency is in part regulated by the cell’s location with respect to a point source of EGF, suggesting a mechanism by which a morphogen gradient is interpreted. Using Erk biosensors, it will be therefore important to systematically explore whether distinct, more subtle, dynamic Erk activity patterns occur at different locations within the embryo. With the help of a growing list of biosensors that can be adapted to be spectrally compatible with optogenetic actuators (Regot et al., 2014), one can envision how the production of dynamic Erk signaling output maps in response to precisely timed optogenetic inputs. Previous studies have shown that this is a powerful approach to map the signaling network structure that encodes dynamic Erk signaling states (Ryu et al., 2015). Together, signaling biosensors and optogenetic approaches have the potential to map how dynamic Erk signaling states are encoded by signaling networks and then subsequently decoded into transcriptional states that control different fates at specific embryo locations.
    Introduction The clinical effectiveness of therapeutic strategies targeting oncogenic signaling is often limited by mechanisms of adaptive resistance, in which initial suppression of oncogenic signaling by a drug is incomplete and temporary, followed by signaling reactivation (rebound) in the presence of the drug. Deregulated RAS/RAF/MEK/ERK signaling (extracellular signal-regulated kinase [ERK] signaling) drives growth of a large fraction of human tumors. We and others have shown that relief of negative feedback upon RAF or MEK inhibitor treatment in multiple ERK-dependent tumor contexts, promotes upregulation of various receptor tyrosine kinases (RTKs), which, in turn, activate RAS, resulting in rebound of ERK activity and development of adaptive resistance of the tumor to the inhibitor (Corcoran et al., 2012, Duncan et al., 2012, Karoulia et al., 2016, Lito et al., 2012, Montero-Conde et al., 2013, Prahallad et al., 2012, Sun et al., 2014). The non-receptor protein tyrosine phosphatase SHP2 (PTPN11) mediates signal transduction downstream of various RTKs. It is a core component of a signaling multi-protein complex downstream of activated RTKs, which includes Grb2-associated binder (GAB) 1, GRB2, and other adaptor proteins, that promotes RAS activation by its guanine exchange factor (GEF) SOS (Dance et al., 2008, Grossmann et al., 2010). The development of small-molecule inhibitors of SHP2 provides the opportunity to potentially overcome adaptive resistance by co-targeting both oncogenic signaling and feedback-induced RTK-mediated RAS activation. Recently, SHP2 inhibition and the combination of SHP2 and ALK or MEK inhibitors were shown to have activity in tumors with deregulated ALK (Dardaei et al., 2018) or RAS (Mainardi et al., 2018, Ruess et al., 2018, Wong et al., 2018) signaling, but whether the combined SHP2 and ERK signaling inhibition would be effective in the broader context of ERK-dependent tumors is not known. Thus, we used a recently developed allosteric small-molecule inhibitor of SHP2, SHP099 (Chen et al., 2016, Garcia Fortanet et al., 2016), in an effort to identify molecular determinants of sensitivity and resistance to combined SHP2 and ERK signaling inhibition in ERK-driven tumors.