The main activatory and tension-sensitive transitions occur within Prozac sensitive down-states of the potassium selective TREK-2 channel

The two-pore domain potassium selective (K2P) ion-channels TREK-1, TREK-2, and TRAAK essential mechanical stimulation sensors, and TREK-1/2 also targets for the antidepressant Nor-fluoxetine (Prozac). They respond directly to membrane tension by moving from the “down” to “up” conformation, a transition that is associated with a rise in open-probability. However, the mechanosensitive K2P (mK2P) channels can also open while occupying the down conformation, and although these channels are mostly closed, all structural models represent seemingly open conformations. To understand the dynamics between open/closed and up/down states and determine how membrane tension influences transitions between specific conformations, we use a novel method to analyze tension-driven activation of single purified and reconstituted TREK-2 channels. We screen a panel of prospective schemes to find the mechanism that best accounts for specific TREK-2 characteristics as tension-driven activation, suppression by Nor-fluoxetine, and single-channel kinetics.To adequately describe TREK-2 behavior, mechanistic schemes require two separate tension-sensitive transitions, one that occurs between distinct down conformations and one that moves the channel between down and up states. As membrane tension activates TREK-2, it is a transition within the structural down conformations that account for the major increase in open-probability (> 100 fold); the move from down to up further promotes channel opening, but with much lower potency (~3 fold activation).

Abstract MP13: TRPM7 Downregulation Contributes To Cardiovascular Injury And Hypertension Induced By Aldosterone And Salt

TRPM7 has cation channel and kinase properties, is permeable to Mg 2+ , Ca 2+ , and Zn 2+ and is protective in the cardiovascular system. Hyperaldosteronism, which induces hypertension and cardiovascular fibrosis, is associated with Mg 2+ wasting. Here we questioned whether TRPM7 plays a role in aldosterone- induced hypertension and fibrosis and whether it influences cation regulation. Wild-type (WT) and TRPM7-deficient (M7+/Δ) mice were treated with aldosterone (600μg/Kg/day) and/or 1% NaCl (drinking water) (aldo, salt or aldo-salt) for 4 weeks. Blood pressure (BP) was evaluated by tail-cuff. Vessel structure was assessed by pressure myography. Molecular mechanisms were investigated in cardiac fibroblasts (CF) from WT and M7+/Δ mice. Protein expression was assessed by western-blot and histology. M7+/Δ mice exhibited reduced TRPM7 expression (30%) and phosphorylation (62%), levels that were recapitulated in WT aldo-salt mice. M7+/Δ exhibited increased BP by aldo, salt and aldo-salt (135-140mmHg) vs M7+/Δ-veh (117mmHg) (p<0.05), whereas in WT, BP was increased only by aldo-salt (134mmHg). Mesenteric resistance arteries from WT aldo-salt exhibited increased wall/lumen ratio (80%) and reduced internal diameter (15%) whereas vessels from M7+/Δ exhibited thinner walls by reducing cross-sectional area (35%) and increased internal diameter (23%) after aldo-salt. Aldo-salt induced greater collagen deposition in hearts (68%), kidneys (126%) and aortas (45%) from M7+/Δ vs WT. Hearts from M7+/Δ veh exhibited increased TGFβ, IL-11 and IL-6 (1.9-fold), p-Smad3 and p-Stat1 (1.5-fold) whereas in WT these effects were only found after aldo-salt. Cardiac expression of protein phosphatase magnesium-dependent 1A (PPM1A), a Mg 2+ -dependent phosphatase, was reduced (3-fold) only in M7+/Δ mice. M7+/Δ CF showed reduced proliferation (30%) and PPM1A (4-fold) and increased expression of TGFβ, IL-11 and IL-6 (2-3-fold), activation of Stat1 (2-fold), Smad3 (9-fold) and ERK1/2 (8-fold) compared with WT. Mg 2+ supplementation normalized cell proliferation and reduced protein phosphorylation in M7+/Δ CF (p<0.05). Our findings indicate a protective role of TRPM7 in aldosterone-salt induced cardiovascular injury through Mg 2+ -dependent mechanisms.

Modeling predicts that CRISPR-based activators, unlike CRISPR-based repressors, scale well with increasing gRNA competition and dCas9 bottlenecking

Synthetic transcriptional networks built from CRISPR-based repressors (CRISPRi) rely on shared use of a core dCas9 protein. In E. coli, CRISPRi cannot support more than about a dozen simultaneous gRNAs before the fold repression of any individual gRNA drops below 10x. We show with a simple model based on previous characterization of competition in CRISPRi that activation by CRISPR-based activators (CRISPRa) is much less sensitive to dCas9 bottle-necking than CRISPRi. We predict that E. coli should be able to support dozens to hundreds of CRISPRa gRNAs at >10-fold activation.

Improving fold activation of small transcription activating RNAs (STARs) with rational RNA engineering strategies.

Engineered RNAs have become integral components of the synthetic biology and bioengineering toolbox for controlling gene expression. We recently expanded this toolbox by creating small transcription activating RNAs (STARs) that act by disrupting the formation of a target transcriptional terminator hairpin placed upstream of a gene. While STARs are a promising addition to the repertoire of RNA regulators, much work remains to be done to optimize the fold activation of these systems. Here we apply rational RNA engineering strategies to improve the fold activation of two STAR regulators. We demonstrate that a combination of promoter strength tuning and multiple RNA stabilization strategies can improve fold activation from 5.4-fold to 13.4-fold for a STAR regulator derived from the pbuE riboswitch terminator. We then validate the generality of our approach and show that these same strategies improve fold activation from 2.1-fold to 14.6-fold for an unrelated STAR regulator. We also establish that the optimizations preserve the orthogonality of these STARs between themselves and a set of antisense RNA transcriptional repressors, enabling these optimized STARs to be used in more sophisticated circuits. These optimization strategies open the door for creating a generation of additional STARs to use in a broad array of biotechnologies.

Inhibition of Let-7 Maturation By Lin28b Controls Timing of Embryonic and Adult Myeloid Progenitor Phenotypes during Development

Within the mammalian fetal liver (FL) or adult bone marrow (ABM), terminally differentiated neutrophils, monocytes, and red blood cells emerge from a pool of multipotent common myeloid progenitors (CMPs). At steady state, the output of these cells is finally balanced but highly adaptable to the pathophysiologic requirements of the host. In the FL, erythroid output predominates to generate adequate oxygen transport for the metabolic demands of rapid organism growth in a hypoxic intrauterine environment; while granulocytic production comprises a larger balance of myelopoietic output in the ABM to provide innate immunity. The genetic regulators that specify timing of developmental stage-specific granulocytic/erythrocytic output from the myeloid progenitor pool have not been described. Initially defined as regulators of developmental timing in C. elegans, the mammalian Lin28a and Lin28b genes encode RNA binding-proteins capable of contributing to pluripotency induction in fully differentiated cells. Lin28 proteins exert their effects in part through inhibition of the maturation of the let-7 family of microRNAs - of which there are eight forms in mammals - that collectively drive cell terminal differentiation. Therefore, Lin28a/b expression is largely confined to embryonic tissues and is associated with the undifferentiated pluripotent or multipotent state in vivo. Given this role in regulation of multipotent cell function, we assessed the relative roles of theLin28/let-7 axis in developmental timing of myeloid progenitor phenotypes. We first analyzed patterns of let-7 expression during FL and ABM myelopoiesis. We found that several mature let-7 forms accumulated in CMPs during the transition from FL to ABM hematopoiesis (up to 30-fold induction as observed in let-7b in ABM compared to FL), coincident with 5-fold downregulation of the let-7 target and fetal-specific hematopoietic regulator Hmga2, indicating that the presence of mature let-7species may specify ABM-like myelopoiesis. To investigate this concept, we utilized a model of ectopic expression of LIN28B to block maturation of all let-7 species in ABM. Mice with activated LIN28B expression had a FL-like erythroid-dominant hematopoietic phenotype compared to controls (megakaryocyte-erythroid progenitor [MEP]:granulocyte-macrophage progenitor [GMP] ratio 3.9 ± 0.9 vs 0.8 ± 0.1, P = 0.005), along with blunted output of mature neutrophils in the ABM (P = 0.0001). Expression of LIN28B decreased formation of mature let-7 species in CMPs. Taken together, these results suggest that LIN28B expression reprograms ABM CMPs to a FL-like phenotype. To further define the Lin28-let-7 interaction in myelopoiesis, we analyzed the expression patterns of Lin28a and Lin28b as well as precursor let-7 microRNAs and their host genes during myeloid development. We observed up to 30-fold activation of precursor let-7 microRNAs and 4-fold activation of the polycistron encoding let-7b and let-7c2 coincident with 5-fold downregulation of Lin28b in CMPs during the transition from FL to ABM myelopoiesis. These results are consistent with mechanisms regulating levels of mature let-7 microRNAs at both the transcriptional and post-transcriptional level. We next characterized the effects of dysregulation of the Lin28b-let-7 interaction in the FL. Enforced expression of let-7g in mouse embryos drove increased granulocytic output in FL myeloid progenitors in culture (P = 0.001). While ectopic expression of Lin28 has been shown to confer fetal hematopoietic programs in other systems, the impact of loss of Lin28 genes on fetal hematopoiesis has not been reported. Fetal loss of Lin28b skewed the myeloid progenitor pool toward an ABM-like phenotype compared to wild-type animals (MEP:GMP 2.8 ± 0.5 vs. 4± 0.4, P = 0.1), though this phenotype is blunted due to the relative absence of let-7 precursors in the FL compared to the ABM, resulting in deregulated maturation of only a subset of mature let-7 forms. However, Lin28b-null FLs contained increased numbers of mature neutrophils (P = 0.005). Together, these data identify a Lin28/let-7 genetic switch with multiple levels of regulation that controls timing of the developmental myelopoietic phenotype. Disclosures No relevant conflicts of interest to declare.

Novel mechanisms of Na+ retention in obesity: phosphorylation of NKCC2 and regulation of SPAK/OSR1 by AMPK

Enhanced tubular reabsorption of salt is important in the pathogenesis of obesity-related hypertension, but the mechanisms remain poorly defined. To identify changes in the regulation of salt transporters in the kidney, C57BL/6 mice were fed a 40% fat diet [high-fat diet (HFD)] or a 12% fat diet (control diet) for 14 wk. Compared with control diet-fed mice, HFD-fed mice had significantly greater elevations in weight, blood pressure, and serum insulin and leptin levels. When we examined Na+ transporter expression, Na+-K+-2Cl− cotransporter (NKCC2) was unchanged in whole kidney and reduced in the cortex, Na+-Cl− cotransporter (NCC) and α-epithelial Na+ channel (ENaC) and γ-ENaC were unchanged, and β-ENaC was reduced. Phosphorylation of NCC was unaltered. Activating phosphorylation of NKCC2 at S126 was increased 2.5-fold. Activation of STE-20/SPS1-related proline-alanine-rich protein kinase (SPAK)/oxidative stress responsive 1 kinase (OSR1) was increased in kidneys from HFD-fed mice, and enhanced phosphorylation of NKCC2 at T96/T101 was evident in the cortex. Increased activity of NKCC2 in vivo was confirmed with diuretic experiments. HFD-fed mice had reduced activating phosphorylation of AMP-activated protein kinase (AMPK) in the renal cortex. In vitro, activation of AMPK led to a reduction in phospho-SPAK/phospho-OSR1 in AMPK+/+ murine embryonic fibroblasts (MEFs), but no effect was seen in AMPK−/− MEFs, indicating an AMPK-mediated effect. Activation of the with no lysine kinase/SPAK/OSR1 pathway with low-NaCl solution invoked a greater elevation in phospho-SPAK/phospho-OSR1 in AMPK−/− MEFs than in AMPK+/+ MEFs, consistent with a negative regulatory effect of AMPK on SPAK/OSR1 phosphorylation. In conclusion, this study identifies increased phosphorylation of NKCC2 on S126 as a hitherto-unrecognized mediator of enhanced Na+ reabsorption in obesity and identifies a new role for AMPK in regulating the activity of SPAK/OSR1.

Mechanistic Deficits Of a Leukemogenic GATA-2 Mutant

Recent studies have demonstrated a role for the master regulator of hematopoiesis GATA-2 in MonoMAC Syndrome, a human immunodeficiency disorder associated with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Though GATA2 coding region and cis-regulatory element mutations underlie MonoMAC syndrome, many questions remain unanswered regarding how GATA-2 is controlled physiologically and how it is dysregulated in pathological contexts. We dissected how a T354M mutation in the GATA-2 DNA binding zinc finger, which is frequently detected in MonoMAC syndrome and familial MDS/AML, alters GATA-2 activity. The T354M mutation reduced GATA-2 chromatin occupancy, induced GATA-2 hyperphosphorylation, and disrupted GATA-2 subnuclear localization. These molecular phenotypes also characterized an additional familial MDS/AML-associated GATA-2 mutant (Δ355T). T354M hyperphosphorylation and ectopic subnuclear localization were detected in hematopoietic and non-hematopoietic cell lines. We developed a new model system in mouse aortic endothelial (MAE) cells to quantitate GATA-2 activity to regulate endogenous target genes. T354M exhibited significantly reduced activity in this assay (GATA-2: 200-fold activation; T354M: 7.7-fold activation). Mass spectrometric analysis of the phosphorylation states of GATA-2 and T354M revealed that the T354M mutation enhanced phosphorylation at several GATA-2 residues. Analysis of single phosphorylation site mutants indicated that only mutation of S192 (S192A) abolished T354M-induced hyperphosphorylation. The S192A mutation attenuated phosphorylation of sites within wild-type GATA-2 and reduced transactivation activity (50% decrease, p < 0.01). A distinct 60 amino acid (aa) region within the GATA-2 N-terminus was required for T354M hyperphosphorylation and ectopic subnuclear localization. Deletion of this sequence decreased GATA-2 transactivation activity (60 aa deletion: 85% decrease, p < 0.01; 10 aa deletion: 45% decrease, p < 0.05). GATA-1 lacks an analogous subnuclear targeting sequence, and accordingly, a GATA-1(T263M) mutant, which corresponds to the GATA-2(T354M) mutant, localized normally and was not hyperphosphorylated. However, a GATA-1 chimera containing the GATA-2 subnuclear targeting sequence localized to ectopic subnuclear foci in a T263M-dependent manner. The GATA-2 N-terminus endowed GATA-1 with the capacity to induce GATA-2 target genes. By contrast, a GATA-2 chimera containing the GATA-1 N-terminus exhibited normal subnuclear localization. Thus, the leukemogenic T354M mutation utilizes the GATA-2-specific subnuclear targeting sequence to disrupt the normal subnuclear localization pattern, and this disruption is associated with S192-dependent hyperphosphorylation. In addition to its involvement in AML, GATA-2 interfaces with RAS signaling to promote the development of non-small cell lung cancer. We discovered that RAS signaling promotes S192-dependent GATA-2 hyperphosphorylation and ectopic subnuclear localization and propose that GATA-2 is an important component in oncogenic RAS-dependent leukemogenesis, which is being formally tested using innovative mouse models. In summary, dissecting the mechanistic deficits of a leukemogenic GATA-2 mutant revealed unexpected insights into mechanisms underlying physiological GATA-2 function and GATA-2-dependent hematologic pathologies. Disclosures: No relevant conflicts of interest to declare.

Src Kinase Can Activate RUNX1 Activity Via Phosphorylation Of C-Terminal Tyrosines and Activated RUNX1 Stimulates Granulopoiesis

We demonstrated that G-CSF signaling in lineage-negative marrow myeloid progenitors induces C-terminal SHP2 tyrosine phosphorylation more potently than does M-CSF signaling and that SHP2 knockdown in marrow or in the 32Dcl3 cell line reduces Cebpa gene transcription and impairs granulopoiesis.1,2 We also found that Runx1 directly activates Cebpa transcription via promoter elements and via a +37 kb enhancer and that Runx1 gene deletion reduces Cebpa mRNA and impairs granulopoiesis.3 Runx1 is phosphorylated by Src kinase on five tyrosines and dephosphorylated by SHP2, and RUNX1(5F) enhances megakaryopoiesis and rescues CD8 T cell development, whereas RUNX1(5D) is ineffective.4 Mutation to F prevents while change to D or E mimics phosphorylation. These findings suggested that in myeloid cells there exists a G-CSF to SHP2 to RUNX1 to Cebpa activation pathway directing granulopoiesis. We compared wild-type (WT) murine RUNX1b and a series of variants, Y260/375/378/379/386F (5F), Y260F (1F), Y375/378/379/386F (4F), 5D, 1D, and 4D for activation in 293T cells of a reporter with four RUNX1-binding sites linked to the TK promoter and the luciferase cDNA. Expression was compared by Western blotting. WT stimulated the reporter 5-fold on average, relative to empty CMV vector, 5D activated the reporter 15-fold, while 5F was nearly inactive. 4F was also nearly inactive, while 4D had WT activity, as did the 1F and 1D variants. Both 4F and 5F were expressed at or above WT levels; interestingly, 5D, but not 4D or 1D, was expressed at much higher levels than the other variants. A similar pattern was obtained with the MCSFR-Luc reporter. To further query the activity of the four C-terminal tyrosines, we evaluated the activity of Y375/378F (2F*), Y379/386F (2F), 2E, Y375F, Y378F, Y379F, and Y386F. The single residue mutations retained WT activity, whereas both the 2F* and 2F variants demonstrated significant reduction despite expression at or above WT levels. The 2E variant expressed at levels below WT but had increased activity. We then transfected the RUNX1 reporter with activated Src, Runx1, or both – in several experiments Src stimulated the reporter approximately 3-fold, on average, RUNX1 4-fold, and the combination 35-fold. In the same experiments, Src + 5F led to only 6.6-fold activation, and synergy was also lost if the reporter lacked the RUNX1-binding sites. These findings, together with the observation that SHP2 directly activates Src family kinases downstream of G-CSF receptor signaling,5 raises the possibility that in myeloid cells there actually is a G-CSF to SHP2 to Src to Runx1 to Cebpa activation pathway. Consistent with this idea, we find that the 5D variant has reduced interaction with co-expressed HDAC3 relative to WT, as assessed by co-IP, whereas the 5F variant has increased interaction. We previously found that RUNX1-ER stimulates endogenous Cebpa transcription when activated in 32Dcl3 cells. As the ER segment has trans-activating activity, to evaluate RUNX1 variants we utilizing the zinc-inducible MT promoter. Preliminary analysis indicates that both WT RUNX1b and its 5D variant can induce C/EBPa protein and RNA in this context. We previously found that exogenous RUNX1-ER can rescue granulopoiesis when transduced into marrow from Runx1(flox/flox);Mx1-Cre mice subjected 4 weeks earlier to pIpC injections. We now have constructed MIGC retroviral vectors expressing RUNX1, 5F, or 5D and GFP-Cre. These, or the empty MIG or MIGC vectors, were packaged and transduced into marrow isolated from RUNX1(flox/flox) mice. Three days later, lineage-depleted, GFP+ cells were cultured in methylcellulose with IL-3, IL-6, and SCF. Compared with MIG, MIGC reduced the percent CFU-G amongst CFU-G+CFU-M, from 55+/-1% to 10+/-2%; WT RUNX1 or 5D partially rescued granulopoiesis, to 31+/- 8% or 23+/-9% respectively, whereas 5F transduction led to only 4+/-1% CFU-G. In summary, in 293T cells and potentially in myeloid cells Src kinase activates Runx1 and this may favor granulopoiesis. 1. Jack et al. Blood 114:2172, 2009. 2. Zhang and Friedman. Blood 118:2266, 2011. 3. Guo et al. Blood 119:4408, 2012. 4. Huang et al. Genes Dev. 26:1587, 2012. 5. Futami et al. Blood 118:1077, 2011. Disclosures: Cantor: AMGEN: Membership on an entity’s Board of Directors or advisory committees.