Regulated dynamic subcellular GLUT4 localization revealed by proximal proteome mapping in human muscle cells.

Regulation of glucose transport, which is central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter (also known as SLC2A4) in the plasma membrane (PM) of fat and muscle cells. Physiologic signals [such as activated insulin receptor or AMP-activated protein kinase (AMPK)] increase PM GLUT4. Here, we show that the distribution of GLUT4 between the PM and interior of human muscle cells is dynamically maintained, and that AMPK promotes PM redistribution of GLUT4 by regulating exocytosis and endocytosis. Stimulation of exocytosis by AMPK is mediated by Rab10 and the Rab GTPase-activating protein TBC1D4. APEX2 proximity mapping reveals that GLUT4 traverses both PM-proximal and PM-distal compartments in unstimulated muscle cells, further supporting retention of GLUT4 by a constitutive retrieval mechanism. AMPK-stimulated translocation involves GLUT4 redistribution among the same compartments traversed in unstimulated cells, with a significant recruitment of GLUT4 from the Golgi and trans-Golgi network compartments. Our comprehensive proximal protein mapping provides an integrated, high-density, whole-cell accounting of the localization of GLUT4 at a resolution of ∼20 nm that serves as a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in a physiologically relevant cell type.

GLUT4 dynamic subcellular localization is controlled by AMP kinase activation as revealed by proximal proteome mapping in human muscle cells.

Regulation of glucose transport into muscle and adipocytes, central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter in the plasma membrane ( PM ). Physiologic signals (activated insulin receptor or AMP kinase [ AMPK ]), acutely increase PM GLUT4 to enhance glucose uptake. Here we show in kinetic studies that intracellular GLUT4 is in equilibrium with the PM in unstimulated cultured human skeletal muscle cells, and that AMPK promotes GLUT4 redistribution to the PM by regulating both exocytosis and endocytosis. AMPK-stimulation of exocytosis requires Rab10 and Rab GTPase activating protein TBC1D4, requirements shared with insulin control of GLUT4 in adipocytes. Using APEX2 proximity mapping, we identify, at high-density and high-resolution, the GLUT4 proximal proteome, revealing GLUT4 traverses both PM proximal and distal compartments in unstimulated muscle cells. These data support intracellular retention of GLUT4 in unstimulated muscle cells by a dynamic mechanism dependent on the rates of internalization and recycling. AMPK promoted GLUT4 translocation to the PM involves redistribution of GLUT4 among the same compartments traversed in unstimulated cells, with a significant redistribution of GLUT4 from the PM distal Trans Golgi Network Golgi compartments. The comprehensive proximal protein mapping provides an integrated, whole cell accounting of GLUT4's localization at a resolution of ∼20 nm, a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in physiologically relevant cell type and as such, sheds new light on novel key pathways and molecular components as potential therapeutic approaches to modulate muscle glucose uptake.

GDF15 neutralization restores muscle function and physical performance in a mouse model of cancer cachexia.

Cancer cachexia is a disorder characterized by involuntary weight loss and impaired physical performance. Decline in physical performance of patients with cachexia is associated with poor quality of life, and currently there are no effective pharmacological interventions that restore physical performance. Here we examine the effect of GDF15 neutralization in a mouse model of cancer-induced cachexia (TOV21G) that manifests weight loss and muscle function impairments. With comprehensive assessments, our results demonstrate that cachectic mice treated with the anti-GDF15 antibody mAB2 exhibit body weight gain with near-complete restoration of muscle mass and markedly improved muscle function and physical performance. Mechanistically, the improvements induced by GDF15 neutralization are primarily attributed to increased caloric intake, while altered gene expression in cachectic muscles is restored in caloric-intake-dependent and -independent manners. The findings indicate potential of GDF15 neutralization as an effective therapy to enhance physical performance of patients with cachexia.

Structural basis of the acyl-transfer mechanism of human GPAT1.

Glycerol-3-phosphate acyltransferase (GPAT)1 is a mitochondrial outer membrane protein that catalyzes the first step of de novo glycerolipid biosynthesis. Hepatic expression of GPAT1 is linked to liver fat accumulation and the severity of nonalcoholic fatty liver diseases. Here we present the cryo-EM structures of human GPAT1 in substrate analog-bound and product-bound states. The structures reveal an N-terminal acyltransferase domain that harbors important catalytic motifs and a tightly associated C-terminal domain that is critical for proper protein folding. Unexpectedly, GPAT1 has no transmembrane regions as previously proposed but instead associates with the membrane via an amphipathic surface patch and an N-terminal loop-helix region that contains a mitochondrial-targeting signal. Combined structural, computational and functional studies uncover a hydrophobic pathway within GPAT1 for lipid trafficking. The results presented herein lay a framework for rational inhibitor development for GPAT1.

Growth differentiation factor 15 neutralization does not impact anorexia or survival in lipopolysaccharide-induced inflammation.

Growth differentiation factor 15 (GDF15) causes anorexia and weight loss in animal models, and higher circulating levels are associated with cachexia and reduced survival in cancer and other chronic diseases such as sepsis. To investigate the role of sepsis-induced GDF15, we examined whether GDF15 neutralization via a validated and highly potent monoclonal antibody, mAB2, modulates lipopolysaccharide (LPS)-induced anorexia, weight loss, and mortality in rodents. LPS injection transiently increased circulating GDF15 in wild-type mice, decreased food intake and body weight, and increased illness behavior and mortality at a high dose. GDF15 neutralization with mAB2 did not prevent or exacerbate any of the effects of LPS. Similarly, in GDF15 knockout mice, the LPS effect on appetite and survival was comparable with that observed in wild-type controls. Therefore, effective inhibition of circulating active GDF15 via an antibody or via gene knockout demonstrated that survival in the LPS acute inflammation model was independent of GDF15.

Experiments in the EpiDerm 3D Skin In Vitro Model and Minipigs In Vivo Indicate Comparatively Lower In Vivo Skin Sensitivity of Topically Applied Aneugenic Compounds.

Risk management of in vitro aneugens for topically applied compounds is not clearly defined because there is no validated methodology to accurately measure compound concentration in proliferating stratum basale keratinocytes of the skin. Here, we experimentally tested several known aneugens in the EpiDerm reconstructed human skin in vitro micronucleus assay and compared the results to flow cytometric mechanistic biomarkers (phospho-H3; MPM2, DNA content). We then evaluated similar biomarkers (Ki-67, nuclear area) using immunohistochemistry in skin sections of minipigs following topical exposure the potent aneugens, colchicine, and hesperadin. Data from the EpiDerm model showed positive micronucleus responses for all aneugens tested following topical or direct media dosing with similar sensitivity when adjusted for applied dose. Quantitative benchmark dose-response analysis exhibited increases in the mitotic index biomarkers phospho-H3 and MPM2 for tubulin binders and polyploidy for aurora kinase inhibitors are at least as sensitive as the micronucleus endpoint. By comparison, the aneugens tested did not induce histopathological changes, increases in Ki-67 immunolabeling or nuclear area in skin sections from the in vivo minipig study at doses in significant excess of those eliciting a response in vitro. Results indicate the EpiDerm in vitro micronucleus assay is suitable for the hazard identification of aneugens. The lack of response in the minipig studies indicates that the barrier function of the minipig skin, which is comparable to human skin, protects from the effects of aneugens in vivo. These results provide a basis for conducting additional studies in the future to further refine this understanding.

Lymphangiogenic therapy prevents cardiac dysfunction by ameliorating inflammation and hypertension.

The lymphatic vasculature is involved in the pathogenesis of acute cardiac injuries, but little is known about its role in chronic cardiac dysfunction. Here, we demonstrate that angiotensin II infusion induced cardiac inflammation and fibrosis at 1 week and caused cardiac dysfunction and impaired lymphatic transport at 6 weeks in mice, while co-administration of VEGFCc156s improved these parameters. To identify novel mechanisms underlying this protection, RNA sequencing analysis in distinct cell populations revealed that VEGFCc156s specifically modulated angiotensin II-induced inflammatory responses in cardiac and peripheral lymphatic endothelial cells. Furthermore, telemetry studies showed that while angiotensin II increased blood pressure acutely in all animals, VEGFCc156s-treated animals displayed a delayed systemic reduction in blood pressure independent of alterations in angiotensin II-mediated aortic stiffness. Overall, these results demonstrate that VEGFCc156s had a multifaceted therapeutic effect to prevent angiotensin II-induced cardiac dysfunction by improving cardiac lymphatic function, alleviating fibrosis and inflammation, and ameliorating hypertension.

De novo lipogenesis is essential for platelet production in humans.

Acetyl-CoA carboxylase (ACC) catalyses the first step of de novo lipogenesis (DNL). Pharmacologic inhibition of ACC has been of interest for therapeutic intervention in a wide range of diseases. We demonstrate here that ACC and DNL are essential for platelet production in humans and monkeys, but in not rodents or dogs. During clinical evaluation of a systemically distributed ACC inhibitor, unexpected dose-dependent reductions in platelet count were observed. While platelet count reductions were not observed in rat and dog toxicology studies, subsequent studies in cynomolgus monkeys recapitulated these platelet count reductions with a similar concentration response to that in humans. These studies, along with ex vivo human megakaryocyte maturation studies, demonstrate that platelet lowering is a consequence of DNL inhibition likely to result in impaired megakaryocyte demarcation membrane formation. These observations demonstrate that while DNL is a minor quantitative contributor to global lipid balance in humans, DNL is essential to specific lipid pools of physiological importance.

Pharmacological evaluation of clinically relevant concentrations of (2R,6R)-hydroxynorketamine.

Ketamine is a rapid-onset antidepressant whose efficacy long outlasts its pharmacokinetics. Multiple studies suggest ketamine's antidepressant effects require increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-dependent currents, which have recently been exclusively attributed to its N-methyl-d-aspartate receptor-inactive metabolite (2R,6R)-hydroxynorketamine ((2R,6R)-HNK). To investigate this AMPAR-activation claim further, we estimated and evaluated preclinically and clinically relevant unbound brain HNK concentrations (Cb,u). (2S,6S)-HNK and (2R,6R)-HNK were novelly synthesized, and their neuropharmacokinetic profiles were determined to project relevant Cb,u. Using concentrations (0.01-10 µM) bracketing the pertinent cross-species Cb,u, both compounds' AMPAR modulation was assessed in vitro by electrophysiological recordings and GluA1 surface expression. Neither (2S,6S)-HNK nor (2R,6R)-HNK bound orthosterically to or directly functionally activated AMPARs. (2R,6R)-HNK failed to evoke AMPAR-centric changes in any electrophysiological endpoint from adult rodent hippocampal slices. Conversely, time- and concentration-dependent increases in GluA1 expression occurred only with (2R,6R)-HNK (≥0.1 µM at ≥90 min). The (2R,6R)-HNK concentrations that increased GluA1 expression are consistent with its maximal Cb,u (0.92-4.84 µM) at reportedly efficacious doses of ketamine or (2R,6R)-HNK in mouse depression models, but ≥3-fold above its projected maximal human Cb,u (≤37.8 ±â€¯14.3 nM) following ketamine's clinically antidepressant infusion. These findings provide insight into the observed AMPAR-affecting (2R,6R)-HNK concentrations versus its exposures attained clinically at an antidepressant ketamine dose. To optimize any clinical study with (2R,6R)-HNK to fully assess its translational pharmacology, future preclinical work should test (2R,6R)-HNK concentrations and/or Cb,u of 0.01-0.1 µM to parallel its projected human Cb,u at a clinically antidepressant ketamine dose.

Design and Synthesis of Clinical Candidate PF-06751979: A Potent, Brain Penetrant, ß-Site Amyloid Precursor Protein Cleaving Enzyme 1 (BACE1) Inhibitor Lacking Hypopigmentation.

A major challenge in the development of ß-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitors for the treatment of Alzheimer's disease is the alignment of potency, drug-like properties, and selectivity over related aspartyl proteases such as Cathepsin D (CatD) and BACE2. The potential liabilities of inhibiting BACE2 chronically have only recently begun to emerge as BACE2 impacts the processing of the premelanosome protein (PMEL17) and disrupts melanosome morphology resulting in a depigmentation phenotype. Herein, we describe the identification of clinical candidate PF-06751979 (64), which displays excellent brain penetration, potent in vivo efficacy, and broad selectivity over related aspartyl proteases including BACE2. Chronic dosing of 64 for up to 9 months in dog did not reveal any observation of hair coat color (pigmentation) changes and suggests a key differentiator over current BACE1 inhibitors that are nonselective against BACE2 in later stage clinical development.

α5 nAChR modulation of the prefrontal cortex makes attention resilient.

A loss-of-function polymorphism in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene has been linked to both drug abuse and schizophrenia. The α5 nAChR subunit is strategically positioned in the prefrontal cortex (PFC), where a loss-of-function in this subunit may contribute to cognitive disruptions in both disorders. However, the specific contribution of α5 to PFC-dependent cognitive functions has yet to be illustrated. In the present studies, we used RNA interference to knockdown the α5 nAChR subunit in the PFC of adult rats. We provide evidence that through its contribution to cholinergic modulation of cholinergic modulation of neurons in the PFC, the α5 nAChR plays a specific role in the recovery of attention task performance following distraction. Our combined data reveal the potent ability of this subunit to modulate the PFC and cognitive functions controlled by this brain region that are impaired in disease.

Tipifarnib sensitizes cells to proteasome inhibition by blocking degradation of bortezomib-induced aggresomes.

In this report, we investigated the mechanism responsible for synergistic induction of myeloma cell apoptosis induced by the combination of tipifarnib and bortezomib. Immunofluorescence studies revealed that bortezomib alone resulted in an accumulation of puncta of ubiquitinated proteins that was further enhanced by the addition of tipifarnib. These data suggest inhibition of the degradation of bortezomib-induced aggresomes; and consistent with this possibility, we also observed an increase in p62SQSTM1 in cells treated with the combination. However, autophagy in these cells appears to be normal as LC3BII is present, and autophagic flux appears to be unaffected as demonstrated by the addition of bafilomycin A1. Together, these data demonstrate that tipifarnib synergizes with bortezomib by inducing protein accumulation as a result of the uncoupling of the aggresome and autophagy pathways.

Biomolecule labeling and imaging with a new fluorenyl two-photon fluorescent probe.

Closely involved in the progression of nonlinear bioimaging is the development of optical probes for investigating biological function and activity. Introduction of new fluorescent compounds possessing enhanced nonlinearities is essential for advancing the utility of two-photon absorption (2PA) processes in the biological sciences. Herein, we report the synthesis of fluorene-based fluorophores tailored for multiphoton imaging, incorporating the succinimidyl ester and thioester functionality as reactive linkers for further coupling with a wide variety of biologically relevant molecules. The succinimidyl ester amine reactive probe was conjugated with the cyclic peptide RGDfK and polyclonal antirat IgG protein. Upon conjugation, the basic molecular architecture and photophysical properties of the active 2PA chromophore remain unchanged. Conventional and two-photon fluorescence microscopy (2PFM) imaging of COS-7 and HeLa cells, incubated with either the fluorene-RGD peptide conjugate or the fluorene-IgG conjugate, was demonstrated. The fluorene-IgG conjugate was used to image cell spindles at early mitotic developmental stages.

Excited state intramolecular proton transfer and photophysics of a new fluorenyl two-photon fluorescent probe.

The steady-state photophysical, NMR, and two-photon absorption (2PA) properties of a new fluorene derivative (1) containing the 2-(2'-hydroxyphenyl)benzothiazole (HBT) terminal construct is investigated for use as a fluorescence probe in bioimaging. A comprehensive analysis of the linear spectral properties reveals inter- and intramolecular hydrogen bonding and excited state intramolecular proton transfer (ESIPT) processes in the HBT substituent. A specific electronic model with a double minimum potential energy surface is consistent with the observed spectral properties. The 2PA spectra are obtained using a standard two-photon induced fluorescence method with a femtosecond kHz laser system, affording a maximum 2PA cross section of approximately 600 GM, a sufficiently high value for two-photon fluorescence imaging. No dependence of two-photon absorption efficiency on solvent properties and hydrogen bonding in the HBT substituent is observed. The potential use of this fluorenyl probe in bioimaging is demonstrated via one- and two-photon fluorescence imaging of COS-7 cells.

Amine-reactive fluorene probes: synthesis, optical characterization, bioconjugation, and two-photon fluorescence imaging.

With the increasing demand for confocal and two-photon fluorescence imaging, the availability of reactive probes that possess high two-photon absorptivity, high fluorescence quantum yield, and high photostability is of paramount importance. To address the demand for better-performing probes, we prepared two-photon absorbing amine-reactive fluorenyl-based probes 2-(9,9-bis(2-(2-methoxyethoxy)ethyl)-2-isothiocyanato-9H-fluoren-7-yl)benzothiazole (1) and 2-(4-(2-(9,9-bis(2-(2-ethoxyethoxy)ethyl)-2-isothiocyanato-9H-fluoren-7-yl)vinyl)phenyl)benzothiazole (2), incorporating the isothiocyanate as a reactive linker. Probe design was augmented by integrating high optical nonlinearities, increased hydrophilicity, and coupling with reactive functional groups for specific targeting of biomolecules, assuring a better impact on two-photon fluorescence microscopy (2PFM) imaging. The isothiocyanate (NCS) derivatives were conjugated with cyclic peptide RGDfK and Reelin protein. The study of the chemical and photophysical properties of the new labeling reagents, as well as the conjugates, is described. The conjugates displayed high chemical stability and photostability. The NCS derivatives had low fluorescence quantum yields, while their bioconjugates exhibited high fluorescence quantum yields, essentially "lighting up" after conjugation. Conventional and 2PFM imaging and fluorescence lifetime imaging (FLIM) of HeLa, NT2, and H1299 cells, incubated with two-photon absorbing amine-reactive probe (1), RGDfK-dye conjugate (7), and Reelin-dye conjugate (6), was demonstrated.

2-Deoxyglucose induces Akt phosphorylation via a mechanism independent of LKB1/AMP-activated protein kinase signaling activation or glycolysis inhibition.

The compound 2-deoxyglucose (2-DG) enhances chemotherapy/radiotherapy in cell lines and animal models, prompting two phase I clinical trials with this cancer therapeutic. Although its mechanism of action has not been fully elucidated, it is hypothesized that the molecular basis of 2-DG activity is related to glycolysis inhibition. Here, we report that 2-DG induced Akt phosphorylation at Thr(308) and Ser(473) as early as 15 min post-treatment. These phosphorylation events required phosphatidylinositol-3-kinase activity but were not related to LKB1/AMP-activated protein kinase signaling, the inhibition of glycolysis or epidermal growth factor receptor signaling. The 2-DG-mediated Akt phosphorylation also led to the phosphorylation of Akt downstream targets, such as Foxo3a, GSK3beta, and Chk1. Because the functional consequence of Akt activation includes chemotherapy/radiotherapy resistance, our data suggested that the combination of phosphatidylinositol-3-kinase/Akt inhibitory agents in 2-DG-based chemotherapy/radiotherapy may result in enhanced therapeutic efficacy.

The tumor suppressor LKB1 regulates lung cancer cell polarity by mediating cdc42 recruitment and activity.

The tumor suppressor LKB1 is mutated in 30% of non-small cell lung cancer (NSCLC) tumors and cell lines and is proposed to be a key regulator of epithelial cell polarity; however, how LKB1 regulates cancer cell polarity is not known. The experiments described herein show for the first time that LKB1 is a dynamic, actin-associated protein that rapidly polarizes to the leading edge of motile cancer cells. LKB1 proves to be essential for NSCLC polarity, because LKB1 depletion results in classic cell polarity defects, such as aberrant Golgi positioning, reduced lamellipodia formation, and aberrant morphology. To probe how LKB1 regulates these events, we show that LKB1 colocalizes at the cellular leading edge with two key components of the polarity pathway - the small rho GTPase cdc42 and its downstream binding partner p21-activated kinase (PAK). Importantly, LKB1 functionality is required for cdc42 polarization to the leading edge, maintaining active cdc42 levels, and downstream PAK phosphorylation. To do this, LKB1 interacts only with active form of cdc42 and PAK, but not with inactive cdc42. Taken together, these results show that LKB1 is a critical mediator of the NSCLC polarity program in lung cancer cells through a novel LKB1-cdc42-PAK pathway.

A new water-soluble near-neutral ratiometric fluorescent pH indicator.

Donor-pi-acceptor fluorene derivative 1c is a near-neutral pH indicator whose pKa of approximately 7.0 was determined by both absorption and fluorescence methods. 1c satisfies important criteria for a sensitive ratiomeric fluorescent pH indicator with a distinctive isoemissive point, good dispersion in cell cytosol, and low cytotoxicity. Furthermore, its 2PA cross section of 100 GM in its neutral form suggests its potential in two-photon fluorescence imaging applications.

Farnesyl transferase inhibitors impair chromosomal maintenance in cell lines and human tumors by compromising CENP-E and CENP-F function.

Farnesyl transferase inhibitors (FTI) exhibit anticancer activity as a single agent in preclinical studies and show promise in combination with other therapeutics in clinical trials. Previous studies show that FTIs arrest cancer cells in mitosis; however, the mechanism by which this occurs is unclear. Here, we observed that treatment of various cancer cell lines with the FTI lonafarnib caused mitotic chromosomal alignment defects, leaving cells in a pseudometaphase state, whereby both aligned chromosomes and chromosomes juxtaposed to the spindle poles (termed "lagging chromosomes") were observed in the same cell. To determine how this occurs, we investigated the functionality of two farnesylated mitotic proteins, CENP-E and CENP-F, which mediate chromosomal capture and alignment. The data show that lonafarnib in proliferating cancer cells depletes CENP-E and CENP-F from metaphase but not prometaphase kinetochores. Loss of CENP-E and CENP-F metaphase localization triggered aberrant chromosomal maintenance, causing aligned chromosomes to be prematurely released from the spindle equator and become lagging chromosomes, resulting in a mitotic delay. Furthermore, lonafarnib treatment reduces sister kinetochore tension and activates the BubR1 spindle checkpoint, suggesting that farnesylation of CENP-E and CENP-F is critical for their functionality in maintaining kinetochore-microtubule interactions. Importantly, apparently similar chromosomal alignment defects were observed in head and neck tumors samples from a phase I trial with lonafarnib, providing support that lonafarnib disrupts chromosomal maintenance in human cancers. Lastly, to examine how farnesylation could regulate CENP-E in mediating kinetochore-microtubule attachments, we examined possible docking motifs of a farnesyl group on the outer surface of the microtubule. This analysis revealed three hydrophobic patches on the tubulin dimer for insertion of a farnesyl group, alluding to the possibility of an association between a farnesyl group and the microtubule.