An association between vasomotion and oxygen extraction

Vasomotion is defined as a spontaneous local oscillation in vascular tone whose function is unclear but may have a beneficial effect on tissue oxygenation. Optical reflectance spectroscopy and laser Doppler fluximetry provide unique insights into the possible mechanisms of vasomotion in the cutaneous microcirculation through the simultaneous measurement of changes in concentration of oxyhemoglobin ([HbO(2)]), deoxyhemoglobin ([Hb]), and mean blood saturation (S(mb)O(2)) along with blood volume and flux. The effect of vasomotion at frequencies <0.02 Hz attributed to endothelial activity was studied in the dorsal forearm skin of 24 healthy males. Fourier analysis identified periodic fluctuations in S(mb)O(2) in 19 out of 24 subjects, predominantly where skin temperatures were >29.3°C (X(2) = 6.19, P < 0.02). A consistent minimum threshold in S(mb)O(2) (mean: 39.4%, range: 24.0-50.6%) was seen to precede a sudden transient surge in flux, inducing a fast rise in S(mb)O(2). The integral increase in flux correlated with the integral increase in [HbO(2)] (Pearson's correlation r(2) = 0.50, P < 0.001) and with little change in blood volume suggests vasodilation upstream, responding to a low S(mb)O(2) downstream. This transient surge in flux was followed by a sustained period where blood volume and flux remained relatively constant and a steady decrease in [HbO(2)] and equal and opposite increase in [Hb] was considered to provide a measure of oxygen extraction. A measure of this oxygen extraction has been approximated by the mean half-life of the decay in S(mb)O(2) during this period. A comparison of the mean half-life in the 8 normal subjects [body mass index (BMI) <26.0 kg/m(2)] of 12.2 s and the 11 obese subjects (BMI >29.5 kg/m(2)) of 18.8 s was statistically significant (Mann Whitney, P < 0.004). The S(mb)O(2) fluctuated spontaneously in this saw tooth manner by an average of 9.0% (range 4.0-16.2%) from mean S(mb)O(2) values ranging from 30 to 52%. These observations support the hypothesis that red blood cells may act as sensors of local tissue hypoxia, through the oxygenation status of the hemoglobin, and initiate improved local perfusion to the tissue through hypoxic vasodilation.     

Molecular dynamics study of small molecule inhibitors of the Bcl-2 family

We carried out docking and molecular dynamics simulations on ABT-737 and obatoclax, which are inhibitors of the Bcl-2 family of proteins. We modeled the binding mode of ABT-737 with Bcl-x(L) , Bcl-2, and Mcl-1 and examined their dynamical behavior. We found that the binding of the chlorobiphenyl end of ABT-737 was quite stable across all three proteins. However, the phenylpiperazine linker group was dramatically more mobile in Mcl-1 compared to either Bcl-x(L) or Bcl-2. The S-phenyl group at the p4 binding site was well-anchored in Bcl-x(L) and Bcl-2 but was somewhat more mobile in Mcl-1 although the phenyl ring itself on average stayed close to the p4 binding site in Mcl-1. This greater mobility is likely due to the greater openness of the p3 and p4 binding sites on Mcl-1. The calculated binding free energies were consistent with the much weaker binding affinity of ABT-737 for Mcl-1. Obatoclax was predicted to bind at the p1 and p2 binding sites of Mcl-1 and the binding mode was quite stable during the molecular dynamics simulation with Mcl-1 wrapping around the molecule. The modeled binding mode suggests that obatoclax is able to inhibit all three proteins because it makes use of the p1 and p2 binding sites alone, which is a fairly narrow groove in all three proteins unlike the p4 binding site, which is much broader in Mcl-1.     

Allosteric Modulation of a Cannabinoid G Protein-coupled Receptor: BINDING SITE ELUCIDATION AND RELATIONSHIP TO G PROTEIN SIGNALING*

The cannabinoid 1 (CB₁) allosteric modulator, 5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)-ethyl]-amide) ({"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569), has the paradoxical effect of increasing the equilibrium binding of [³H](−)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)phenyl]-4-[3-hydroxylpropyl]cyclohexan-1-ol (CP55,940, an orthosteric agonist) while at the same time decreasing its efficacy (in G protein-mediated signaling). {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 also decreases basal signaling, acting as an inverse agonist for the G protein-mediated signaling pathway. In ligand displacement assays, {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 can displace the CB₁ antagonist/inverse agonist, N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide(SR141716A). The goal of this work was to identify the binding site of {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 at CB₁. To this end, we used computation, synthesis, mutation, and functional studies to identify the {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569-binding site in the CB₁ TMH3-6-7 region. This site is consistent with the results of K3.28¹⁹²A, F3.36²⁰⁰A, W5.43²⁷⁹A, W6.48³⁵⁶A, and F3.25¹⁸⁹A mutation studies, which revealed the {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569-binding site overlaps with our previously determined binding site of SR141716A but extends extracellularly. Additionally, we identified a key electrostatic interaction between the {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 piperidine ring nitrogen and K3.28¹⁹² that is important for {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 to act as an inverse agonist. At this allosteric site, {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 promotes an intermediate conformation of the CB₁ receptor, explaining {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569''s ability to increase equilibrium binding of CP55,940. This site also explains {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569''s ability to antagonize the efficacy of CP55,940 in three complementary ways. 1) {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 sterically blocks movements of the second extracellular loop that have been linked to receptor activation. 2) {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 sterically blocks a key electrostatic interaction between the third extracellular loop residue Lys-373 and D2.63¹⁷⁶. 3) {"type":"entrez-protein","attrs":{"text":"ORG27569","term_id":"1179174593","term_text":"ORG27569"}}ORG27569 packs against TMH6, sterically hindering movements of this helix that have been shown to be important for receptor activation.     

Pathogenic ACVR1R206H activation by Activin A‐induced receptor clustering and autophosphorylation

Fibrodysplasia ossificans progressiva (FOP) and diffuse intrinsic pontine glioma (DIPG) are debilitating diseases that share causal mutations in ACVR1, a TGF‐β family type I receptor. ACVR1^R206H is a frequent mutation in both diseases. Pathogenic signaling via the SMAD1/5 pathway is mediated by Activin A, but how the mutation triggers aberrant signaling is not known. We show that ACVR1 is essential for Activin A‐mediated SMAD1/5 phosphorylation and is activated by two distinct mechanisms. Wild‐type ACVR1 is activated by the Activin type I receptors, ACVR1B/C. In contrast, ACVR1^R206H activation does not require upstream kinases, but is predominantly activated via Activin A‐dependent receptor clustering, which induces its auto‐activation. We use optogenetics and live‐imaging approaches to demonstrate Activin A‐induced receptor clustering and show it requires the type II receptors ACVR2A/B. Our data provide molecular mechanistic insight into the pathogenesis of FOP and DIPG by linking the causal activating genetic mutation to disrupted signaling.     

BCL2-CISD2: An ER complex at the nexus of autophagy and calcium homeostasis?

CISD2, an ER BCL2-associated autophagy regulator also known as NAF-1, is responsible for the human degenerative disorder Wolfram Syndrome 2. In order to interrogate the physiological role of CISD2 we generated and characterized the Cisd2 gene deletion in mice. Cisd2 null mice manifest significant degeneration in skeletal muscle tissues, which is accompanied with augmented autophagy, dysregulated Ca ( 2+) homeostasis and elongated mitochondria. Our findings describe a novel role for BCL2-CISD2 in the homeostatic maintenance of skeletal muscle. It remains to be elucidated how and if the antagonism of the BECN1 autophagy-initiating complex and modulation of ER Ca ( 2+) homeostasis by BCL2-CISD2 are interconnected.     

Mutations in c4orf26, encoding a Peptide with in vitro hydroxyapatite crystal nucleation and growth activity, cause amelogenesis imperfecta

Autozygosity mapping and clonal sequencing of an Omani family identified mutations in the uncharacterized gene, C4orf26, as a cause of recessive hypomineralized amelogenesis imperfecta (AI), a disease in which the formation of tooth enamel fails. Screening of a panel of 57 autosomal-recessive AI-affected families identified eight further families with loss-of-function mutations in C4orf26. C4orf26 encodes a putative extracellular matrix acidic phosphoprotein expressed in the enamel organ. A mineral nucleation assay showed that the protein's phosphorylated C terminus has the capacity to promote nucleation of hydroxyapatite, suggesting a possible function in enamel mineralization during amelogenesis.     

Unique biology of Mcl-1: therapeutic opportunities in cancer

Accumulating evidence suggests that Mcl-1 plays a critical pro-survival role in the development and maintenance of both normal and malignant tissues. Regulation of Mcl-1 expression occurs at multiple levels, allowing for either the rapid induction or elimination of the protein in response to different cellular events. This suggests that Mcl-1 can play an early role in response to signals directing either cell survival or cell death. Deregulation of pathways regulating Mcl-1 that result in its over-expression likely contribute to a cell's inability to properly respond to death signals possibly leading to cell immortalization and tumorigenic conversion. Correspondingly, Mcl-1 has been shown to be up-regulated in numerous hematological and solid tumor malignancies. Moreover, this up-regulation appears to be a factor in the resistance of some cancer types to conventional cancer therapies. Mechanisms that abrogate the pro-survival function of Mcl-1 either by diminishing its levels or inactivating its functional BH3 groove have shown promise for the combinational treatment with existing cancer therapies and as single agents in certain malignancies. Here we review the various pathways that regulate Mcl-1 expression and describe agents that are currently under development to modulate Mcl-1 activity for therapeutic benefit in oncology.     

How telomerase reaches its end: mechanism of telomerase regulation by the telomeric complex

The telomerase enzyme, which synthesizes telomeric DNA repeats, is regulated in cis at individual chromosome ends by the telomeric protein/DNA complex in a manner dependent on telomere repeat-array length. A dynamic interplay between telomerase-inhibiting factors bound at duplex DNA repeats and telomerase-promoting ones bound at single-stranded terminal DNA overhangs appears to modulate telomerase activity and to be directly related to the transient deprotection of telomeres. We discuss recent advances on the mechanism of telomerase regulation at chromosome ends in both yeast and mammalian systems.