Multiplexed bioluminescence microscopy via phasor analysis.

Bioluminescence imaging with luciferase-luciferin pairs is a well-established technique for visualizing biological processes across tissues and whole organisms. Applications at the microscale, by contrast, have been hindered by a lack of detection platforms and easily resolved probes. We addressed this limitation by combining bioluminescence with phasor analysis, a method commonly used to distinguish spectrally similar fluorophores. We built a camera-based microscope equipped with special optical filters to directly assign phasor locations to unique luciferase-luciferin pairs. Six bioluminescent reporters were easily resolved in live cells, and the readouts were quantitative and instantaneous. Multiplexed imaging was also performed over extended time periods. Bioluminescent phasor further provided direct measures of resonance energy transfer in single cells, setting the stage for dynamic measures of cellular and molecular features. The merger of bioluminescence with phasor analysis fills a long-standing void in imaging capabilities, and will bolster future efforts to visualize biological events in real time and over multiple length scales.

Membrane Recruitment of the Non-receptor Protein GIV/Girdin (Gα-interacting, Vesicle-associated Protein/Girdin) Is Sufficient for Activating Heterotrimeric G Protein Signaling.

GIV (aka Girdin) is a guanine nucleotide exchange factor that activates heterotrimeric G protein signaling downstream of RTKs and integrins, thereby serving as a platform for signaling cascade cross-talk. GIV is recruited to the cytoplasmic tail of receptors upon stimulation, but the mechanism of activation of its G protein regulatory function is not well understood. Here we used assays in humanized yeast models and G protein activity biosensors in mammalian cells to investigate the role of GIV subcellular compartmentalization in regulating its ability to promote G protein signaling. We found that in unstimulated cells GIV does not co-fractionate with its substrate G protein Gαi3 on cell membranes and that constitutive membrane anchoring of GIV in yeast cells or rapid membrane translocation in mammalian cells via chemically induced dimerization leads to robust G protein activation. We show that membrane recruitment of the GIV "Gα binding and activating" motif alone is sufficient for G protein activation and that it does not require phosphomodification. Furthermore, we engineered a synthetic protein to show that recruitment of the GIV "Gα binding and activating" motif to membranes via association with active RTKs, instead of via chemically induced dimerization, is also sufficient for G protein activation. These results reveal that recruitment of GIV to membranes in close proximity to its substrate G protein is a major mechanism responsible for the activation of its G protein regulatory function.

Evolutionary Conservation of a GPCR-Independent Mechanism of Trimeric G Protein Activation.

Trimeric G protein signaling is a fundamental mechanism of cellular communication in eukaryotes. The core of this mechanism consists of activation of G proteins by the guanine-nucleotide exchange factor (GEF) activity of G protein coupled receptors. However, the duration and amplitude of G protein-mediated signaling are controlled by a complex network of accessory proteins that appeared and diversified during evolution. Among them, nonreceptor proteins with GEF activity are the least characterized. We recently found that proteins of the ccdc88 family possess a Gα-binding and activating (GBA) motif that confers GEF activity and regulates mammalian cell behavior. A sequence similarity-based search revealed that ccdc88 genes are highly conserved across metazoa but the GBA motif is absent in most invertebrates. This prompted us to investigate whether the GBA motif is present in other nonreceptor proteins in invertebrates. An unbiased bioinformatics search in Caenorhabditis elegans identified GBAS-1 (GBA and SPK domain containing-1) as a GBA motif-containing protein with homologs only in closely related worm species. We demonstrate that GBAS-1 has GEF activity for the nematode G protein GOA-1 and that the two proteins are coexpressed in many cells of living worms. Furthermore, we show that GBAS-1 can activate mammalian Gα-subunits and provide structural insights into the evolutionarily conserved determinants of the GBA-G protein interface. These results demonstrate that the GBA motif is a functional GEF module conserved among highly divergent proteins across evolution, indicating that the GBA-Gα binding mode is strongly constrained under selective pressure to mediate receptor-independent G protein activation in metazoans.

Lysyl hydroxylase 2 mediated collagen post-translational modifications and functional outcomes.

Lysyl hydroxylase 2 (LH2) is a member of LH family that catalyzes the hydroxylation of lysine (Lys) residues on collagen, and this particular isozyme has been implicated in various diseases. While its function as a telopeptidyl LH is generally accepted, several fundamental questions remain unanswered: 1. Does LH2 catalyze the hydroxylation of all telopeptidyl Lys residues of collagen? 2. Is LH2 involved in the helical Lys hydroxylation? 3. What are the functional consequences when LH2 is completely absent? To answer these questions, we generated LH2-null MC3T3 cells (LH2KO), and extensively characterized the type I collagen phenotypes in comparison with controls. Cross-link analysis demonstrated that the hydroxylysine-aldehyde (Hylald)-derived cross-links were completely absent from LH2KO collagen with concomitant increases in the Lysald-derived cross-links. Mass spectrometric analysis revealed that, in LH2KO type I collagen, telopeptidyl Lys hydroxylation was completely abolished at all sites while helical Lys hydroxylation was slightly diminished in a site-specific manner. Moreover, di-glycosylated Hyl was diminished at the expense of mono-glycosylated Hyl. LH2KO collagen was highly soluble and digestible, fibril diameters were diminished, and mineralization impaired when compared to controls. Together, these data underscore the critical role of LH2-catalyzed collagen modifications in collagen stability, organization and mineralization in MC3T3 cells.

Activation of the CREB Coactivator CRTC2 by Aberrant Mitogen Signaling promotes oncogenic functions in HPV16 positive head and neck cancer.

Head and neck squamous cell carcinoma (HNSCC) is the 6th most common cancer worldwide and incidence rates are continuing to rise globally. Patients often present with locally advanced disease and a staggering 50% chance of relapse following treatment. Aberrant activation of adaptive response signaling pathways, such as the cAMP/PKA pathway, induce an array of genes associated with known cancer pathways that promote tumorigenesis and drug resistance. We identified the cAMP Regulated Transcription Coactivator 2 (CRTC2) to be overexpressed and constitutively activated in HNSCCs and this confers poor prognosis. CRTCs are regulated through their subcellular localization and we show that CRTC2 is exclusively nuclear in HPV(+) HNSCC, thus constitutively active, due to non-canonical Mitogen-Activated Kinase Kinase 1 (MEKK1)-mediated activation via a MEKK1-p38 signaling axis. Loss-of-function and pharmacologic inhibition experiments decreased CRTC2/CREB transcriptional activity by reducing nuclear CRTC2 via nuclear import inhibition and/or by eviction of CRTC2 from the nucleus. This shift in localization was associated with decreased proliferation, migration, and invasion. Our results suggest that small molecules that inhibit nuclear CRTC2 and p38 activity may provide therapeutic benefit to patients with HPV(+) HNSCC.

Lysyl hydroxylase 2-induced collagen cross-link switching promotes metastasis in head and neck squamous cell carcinomas.

Head and neck squamous cell carcinoma (HNSCC) is the 6th most common cancer worldwide and incidence rates are continuing to rise globally. HNSCC patient prognosis is closely related to the occurrence of tumor metastases, and collagen within the tumor microenvironment (TME) plays a key role in this process. Lysyl hydroxylase 2 (LH2), encoded by the Procollagen-Lysine,2-Oxoglutarate 5-Dioxygenase 2 (PLOD2) gene, catalyzes hydroxylation of telopeptidyl lysine (Lys) residues of fibrillar collagens which then undergo subsequent modifications to form stable intermolecular cross-links that change the biomechanical properties (i.e. quality) of the TME. While LH2-catalyzed collagen modification has been implicated in driving tumor progression and metastasis in diverse cancers, little is known about its role in HNSCC progression. Thus, using gain- and loss-of-function studies, we examined the effects of LH2 expression levels on collagen cross-linking and cell behavior in vitro and in vivo using a tractable bioluminescent imaging-based orthotopic xenograft model. We found that LH2 overexpression dramatically increases HNSCC cell migratory and invasive abilities in vitro and that LH2-driven changes in collagen cross-linking robustly induces metastasis in vivo. Specifically, the amount of LH2-mediated collagen cross-links increased significantly with PLOD2 overexpression, without affecting the total quantity of collagen cross-links. Conversely, LH2 knockdown significantly blunted HNSCC cells invasive capacity in vitro and metastatic potential in vivo. Thus, regardless of the total "quantity" of collagen crosslinks, it is the "quality" of these cross-links that is the key driver of HNSCC tumor metastatic dissemination. These data implicate LH2 as a key regulator of HNSCC tumor invasion and metastasis by modulating collagen cross-link quality and suggest that therapeutic strategies targeting LH2-mediated collagen cross-linking in the TME may be effective in controlling tumor progression and improving disease outcomes.

Synergistic efficacy of combined EGFR and HDAC inhibitors overcomes tolerance to EGFR monotherapy in salivary mucoepidermoid carcinoma.

OBJECTIVES: Mucoepidermoid carcinoma (MEC) is the most common type of salivary gland malignancy. Advanced or high-grade MECs are refractory to chemotherapy, often leading to tumor recurrence/metastasis and abysmal ~35% 5-year survival. Causal links have been established between Epithelial Growth Factor Receptor (EGFR) activation and poor outcome. Herein we investigated the therapeutic efficacy of EGFR inhibition against MEC using in vitro pre-clinical models. MATERIALS AND METHODS: Five human MEC cell lines were used in cell viability, cytotoxicity, apoptosis, cell cycle, 2D-clonogenicity, and 3D-spheroid formation assays following treatment with Erlotinib (EGFR inhibitor), SAHA (Histone Deacetylase inhibitor; HDAC) and CUDC-101 (dual EGFR-HDAC inhibitor). Effects on MEC cancer stem cells were evaluated using flow cytometry. Gene expression and pathway regulation were evaluated via qPCR and Western blot, respectively. RESULTS: MEC cells enter a quiescent, non-proliferative yet rapidly reversible drug tolerant state upon EGFR inhibition. Despite robust suppression of MEC cell proliferation, no discernable apoptosis is detected. Combination of EGFR and HDAC inhibitors exhibits synergistic effects, exerting ~5-fold more potent cell cytotoxicity compared to HDAC or EGFR monotherapy. CUDC-101, a single molecule with dual EGFR-HDAC inhibitor moieties, exerts irreversible and potent cytotoxic activity against MEC cells and blunts MEC cancer stem-cell tumorigenicity. CONCLUSION: MEC cells are intrinsically tolerant to EGFR inhibition. Combining EGFR and HDAC inhibitors exerts synergistic and potent cytotoxic effects, suggesting that EGFR inhibitors still hold significant promise against MEC. Future studies are needed to assess the applicability and efficacy of dual EGFR-HDAC inhibitors for the clinical management of MEC.

Application of bioluminescence resonance energy transfer-based cell tracking approach in bone tissue engineering.

Bioluminescent imaging (BLI) has emerged as a popular in vivo tracking modality in bone regeneration studies stemming from its clear advantages: non-invasive, real-time, and inexpensive. We recently adopted bioluminescence resonance energy transfer (BRET) principle to improve BLI cell tracking and generated the brightest bioluminescent signal known to date, which thus enables more sensitive real-time cell tracking at deep tissue level. In the present study, we brought BRET-based cell tracking strategy into the field of bone tissue engineering for the first time. We labeled rat mesenchymal stem cells (rMSCs) with our in-house BRET-based GpNLuc reporter and evaluated the cell tracking efficacy both in vitro and in vivo. In scaffold-free spheroid 3D culture system, using BRET-based GpNLuc labeling resulted in significantly better correlation to cell numbers than a fluorescence based approach. In scaffold-based 3D culture system, GpNLuc-rMSCs displayed robust bioluminescence signals with minimal background noise. Furthermore, a tight correlation between BLI signal and cell number highlighted the robust reliability of using BRET-based BLI. In calvarial critical sized defect model, robust signal and the consistency in cell survival evaluation collectively supported BRET-based GpNLuc labeling as a reliable approach for non-invasively tracking MSC. In summary, BRET-based GpNLuc labeling is a robust, reliable, and inexpensive real-time cell tracking method, which offers a promising direction for the technological innovation of BLI and even non-invasive tracking systems, in the field of bone tissue engineering.

CRTC1/MAML2 directs a PGC-1α-IGF-1 circuit that confers vulnerability to PPARγ inhibition.

Mucoepidermoid carcinoma (MEC) is a life-threatening salivary gland cancer that is driven primarily by a transcriptional coactivator fusion composed of cyclic AMP-regulated transcriptional coactivator 1 (CRTC1) and mastermind-like 2 (MAML2). The mechanisms by which the chimeric CRTC1/MAML2 (C1/M2) oncoprotein rewires gene expression programs that promote tumorigenesis remain poorly understood. Here, we show that C1/M2 induces transcriptional activation of the non-canonical peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) splice variant PGC-1α4, which regulates peroxisome proliferator-activated receptor gamma (PPARγ)-mediated insulin-like growth factor 1 (IGF-1) expression. This mitogenic transcriptional circuitry is consistent across cell lines and primary tumors. C1/M2-positive tumors exhibit IGF-1 pathway activation, and small-molecule drug screens reveal that tumor cells harboring the fusion gene are selectively sensitive to IGF-1 receptor (IGF-1R) inhibition. Furthermore, this dependence on autocrine regulation of IGF-1 transcription renders MEC cells susceptible to PPARγ inhibition with inverse agonists. These results yield insights into the aberrant coregulatory functions of C1/M2 and identify a specific vulnerability that can be exploited for precision therapy.

Optogenetic activation of heterotrimeric G-proteins by LOV2GIVe, a rationally engineered modular protein.

Heterotrimeric G-proteins are signal transducers involved in mediating the action of many natural extracellular stimuli and many therapeutic agents. Non-invasive approaches to manipulate the activity of G-proteins with high precision are crucial to understand their regulation in space and time. Here, we developed LOV2GIVe, an engineered modular protein that allows the activation of heterotrimeric G-proteins with blue light. This optogenetic construct relies on a versatile design that differs from tools previously developed for similar purposes, that is metazoan opsins, which are light-activated G-protein-coupled receptors (GPCRs). Instead, LOV2GIVe consists of the fusion of a G-protein activating peptide derived from a non-GPCR regulator of G-proteins to a small plant protein domain, such that light uncages the G-protein activating module. Targeting LOV2GIVe to cell membranes allowed for light-dependent activation of Gi proteins in different experimental systems. In summary, LOV2GIVe expands the armamentarium and versatility of tools available to manipulate heterotrimeric G-protein activity.

Engineered BRET-Based Biologic Light Sources Enable Spatiotemporal Control over Diverse Optogenetic Systems.

Light-inducible optogenetic systems offer precise spatiotemporal control over a myriad of biologic processes. Unfortunately, current systems are inherently limited by their dependence on external light sources for their activation. Further, the utility of laser/LED-based illumination strategies are often constrained by the need for invasive surgical procedures to deliver such devices and local heat production, photobleaching and phototoxicity that compromises cell and tissue viability. To overcome these limitations, we developed a novel BRET-activated optogenetics (BEACON) system that employs biologic light to control optogenetic tools. BEACON is driven by self-illuminating bioluminescent-fluorescent proteins that generate "spectrally tuned" biologic light via bioluminescence resonance energy transfer (BRET). Notably, BEACON robustly activates a variety of commonly used optogenetic systems in a spatially restricted fashion, and at physiologically relevant time scales, to levels that are achieved by conventional laser/LED light sources.

A Bioluminescence Resonance Energy Transfer-Based Approach for Determining Antibody-Receptor Occupancy In Vivo.

Elucidating receptor occupancy (RO) of monoclonal antibodies (mAbs) is a crucial step in characterizing the therapeutic efficacy of mAbs. However, the in vivo assessment of RO, particularly within peripheral tissues, is greatly limited by current technologies. In the present study, we developed a bioluminescence resonance energy transfer (BRET)-based system that leverages the large signal:noise ratio and stringent energy donor-acceptor distance dependency to measure antibody RO in a highly selective and temporal fashion. This versatile and minimally invasive system enables longitudinal monitoring of the in vivo antibody-receptor engagement over several days. As a proof of principle, we quantified cetuximab-epidermal growth factor receptor binding kinetics using this system and assessed cetuximab RO in a tumor xenograft model. Incomplete ROs were observed, even at a supratherapeutic dose of 50 mg/kg, indicating that fractional target accessibility is achieved. The BRET-based imaging approach enables quantification of antibody in vivo RO and provides critical information required to optimize therapeutic mAb efficacy.

Identification of NSDHL mutations associated with CHILD syndrome in oral verruciform xanthoma.

OBJECTIVE: The aim of this study was to perform a systematic analysis of the nicotinamide adenine dinucleotide phosphate (NAD[P])-dependent steroid dehydrogenase-like (NSDHL) gene in cases of oral verruciform xanthoma (VX) and to test for the presence of mutations associated with congenital hemidysplasia with ichthyosiform nevus and limb defects (CHILD) syndrome. STUDY DESIGN: DNA was extracted from archived paraffin-embedded tissue of oral VX and control cases. Polymerase chain reaction (PCR) was then used to screen exons 4 and 6 of the NSDHL gene for the presence of 4 known germline mutations associated with CHILD syndrome and 1 somatic mutation previously identified in VX lesions with no known association with CHILD syndrome. RESULTS: Of the 16 oral VX tissue samples, 8 (50%) had known missense mutations associated with CHILD syndrome. Furthermore, 2 of these 8 tissue samples also had an additional missense mutation previously identified in cutaneous VX lesions. No mutations of exons 4 and 6 were found in the 5 negative control tissue samples. CONCLUSIONS: NSDHL gene mutations associated with CHILD syndrome are common in sporadic oral VX cases, suggesting that these mutations confer a greater risk for the development of epithelial barrier defects that promote recurrent oral VX lesions and the potential for direct germline transmission of oral VX susceptibility.

An Immunocompetent Mouse Model of HPV16(+) Head and Neck Squamous Cell Carcinoma.

The incidence of human papilloma virus (HPV)-associated head and neck squamous cell carcinoma (HNSCC) is increasing and implicated in more than 60% of all oropharyngeal carcinomas (OPSCCs). Although whole-genome, transcriptome, and proteome analyses have identified altered signaling pathways in HPV-induced HNSCCs, additional tools are needed to investigate the unique pathobiology of OPSCC. Herein, bioinformatics analyses of human HPV(+) HNSCCs revealed that all tumors express full-length E6 and identified molecular subtypes based on relative E6 and E7 expression levels. To recapitulate the levels, stoichiometric ratios, and anatomic location of E6/E7 expression, we generated a genetically engineered mouse model whereby balanced expression of E6/E7 is directed to the oropharyngeal epithelium. The addition of a mutant PIK3CAE545K allele leads to the rapid development of pre-malignant lesions marked by immune cell accumulation, and a subset of these lesions progress to OPSCC. This mouse provides a faithful immunocompetent model for testing treatments and investigating mechanisms of immunosuppression.

Molecular mechanism of Gαi activation by non-GPCR proteins with a Gα-Binding and Activating motif.

Heterotrimeric G proteins are quintessential signalling switches activated by nucleotide exchange on Gα. Although activation is predominantly carried out by G-protein-coupled receptors (GPCRs), non-receptor guanine-nucleotide exchange factors (GEFs) have emerged as critical signalling molecules and therapeutic targets. Here we characterize the molecular mechanism of G-protein activation by a family of non-receptor GEFs containing a Gα-binding and -activating (GBA) motif. We combine NMR spectroscopy, computational modelling and biochemistry to map changes in Gα caused by binding of GBA proteins with residue-level resolution. We find that the GBA motif binds to the SwitchII/α3 cleft of Gα and induces changes in the G-1/P-loop and G-2 boxes (involved in phosphate binding), but not in the G-4/G-5 boxes (guanine binding). Our findings reveal that G-protein-binding and activation mechanisms are fundamentally different between GBA proteins and GPCRs, and that GEF-mediated perturbation of nucleotide phosphate binding is sufficient for Gα activation.

Dominant-negative Gα subunits are a mechanism of dysregulated heterotrimeric G protein signaling in human disease.

Auriculo-condylar syndrome (ACS), a rare condition that impairs craniofacial development, is caused by mutations in a G protein-coupled receptor (GPCR) signaling pathway. In mice, disruption of signaling by the endothelin type A receptor (ET(A)R), which is mediated by the G protein (heterotrimeric guanine nucleotide-binding protein) subunit Gα(q/11) and subsequently phospholipase C (PLC), impairs neural crest cell differentiation that is required for normal craniofacial development. Some ACS patients have mutations inGNAI3, which encodes Gα(i3), but it is unknown whether this G protein has a role within the ET(A)R pathway. We used a Xenopus model of vertebrate development, in vitro biochemistry, and biosensors of G protein activity in mammalian cells to systematically characterize the phenotype and function of all known ACS-associated Gα(i3) mutants. We found that ACS-associated mutations in GNAI3 produce dominant-negative Gα(i3) mutant proteins that couple to ET(A)R but cannot bind and hydrolyze guanosine triphosphate, resulting in the prevention of endothelin-mediated activation of Gα(q/11) and PLC. Thus, ACS is caused by functionally dominant-negative mutations in a heterotrimeric G protein subunit.