PET Imaging of Innate Immune Activation Using 11C Radiotracers Targeting GPR84

Chronic innate immune activation is a key hallmark of many neurological diseases and is known to result in the upregulation of GPR84 in myeloid cells (macrophages, microglia, and monocytes). As such, GPR84 can potentially serve as a sensor of proinflammatory innate immune responses. To assess the utility of GPR84 as an imaging biomarker, we synthesized 11C-MGX-10S and 11C-MGX-11Svia carbon-11 alkylation for use as positron emission tomography (PET) tracers targeting this receptor. In vitro experiments demonstrated significantly higher binding of both radiotracers to hGPR84-HEK293 cells than that of parental control HEK293 cells. Co-incubation with the GPR84 antagonist GLPG1205 reduced the binding of both radiotracers by >90%, demonstrating their high specificity for GPR84 in vitro. In vivo assessment of each radiotracer via PET imaging of healthy mice illustrated the superior brain uptake and pharmacokinetics of 11C-MGX-10S compared to 11C-MGX-11S. Subsequent use of 11C-MGX-10S to image a well-established mouse model of systemic and neuro-inflammation revealed a high PET signal in affected tissues, including the brain, liver, lung, and spleen. In vivo specificity of 11C-MGX-10S for GPR84 was confirmed by the administration of GLPG1205 followed by radiotracer injection. When compared with 11C-DPA-713-an existing radiotracer used to image innate immune activation in clinical research studies-11C-MGX-10S has multiple advantages, including its higher binding signal in inflamed tissues in the CNS and periphery and low background signal in healthy saline-treated subjects. The pronounced uptake of 11C-MGX-10S during inflammation, its high specificity for GPR84, and suitable pharmacokinetics strongly support further investigation of 11C-MGX-10S for imaging GPR84-positive myeloid cells associated with innate immune activation in animal models of inflammatory diseases and human neuropathology.

NO is necessary and sufficient for egg activation at fertilization

The early steps that lead to the rise in calcium and egg activation at fertilization are unknown but of great interest--particularly with the advent of in vitro fertilization techniques for treating male infertility and whole-animal cloning by nuclear transfer. This calcium rise is required for egg activation and the subsequent events of development in eggs of all species. Injection of intact sperm or sperm extracts can activate eggs, suggesting that sperm-derived factors may be involved. Here we show that nitric oxide synthase is present at high concentration and active in sperm after activation by the acrosome reaction. An increase in nitrosation within eggs is evident seconds after insemination and precedes the calcium pulse of fertilization. Microinjection of nitric oxide donors or recombinant nitric oxide synthase recapitulates events of egg activation, whereas prior injection of oxyhaemoglobin, a physiological nitric oxide scavenger, prevents egg activation after fertilization. We conclude that nitric oxide synthase and nitric-oxide-related bioactivity satisfy the primary criteria of an egg activator: they are present in an appropriate place, active at an appropriate time, and are necessary and sufficient for successful fertilization.

Tumor necrosis factor-alpha mediates both apoptotic cell death and cell proliferation in a human hematopoietic cell line dependent on mitotic activity and receptor subtype expression

The TF-1 human erythroleukemic cell line exhibits opposing physiological responses toward tumor necrosis factor-alpha (TNF) treatment, dependent upon the mitotic state of the cells. Mitotically active cells in log growth respond to TNF by rapidly undergoing apoptosis whereas TNF exposure stimulates cellular proliferation in mitotically quiescent cells. The concentration-dependent TNF-induced apoptosis was monitored by cellular metabolic activity and confirmed by both DNA epifluorescence and DNA fragmentation. Moreover, these responses could be detected by measuring extracellular acidification activity, enabling rapid prediction (within approximately 1.5 h of TNF treatment) of the fate of the cell in response to TNF. Growth factor resupplementation of quiescent cells, resulting in reactivation of cell cycling, altered TNF action from a proliferative stimulus to an apoptotic signal. Expression levels of the type II TNF receptor subtype (p75TNFR) were found to correlate with sensitivity to TNF-induced apoptosis. Pretreatment of log growth TF-1 cells with a neutralizing anti-p75TNFR monoclonal antibody inhibited TNF-induced apoptosis by greater than 80%. Studies utilizing TNF receptor subtype-specific TNF mutants and neutralizing antisera implicated p75TNFR in TNF-dependent apoptotic signaling. These data show a bifunctional physiological role for TNF in TF-1 cells that is dependent on mitotic activity and controlled by the p75TNFR.

Signal transduction mediated by the truncated trkB receptor isoforms, trkB.T1 and trkB.T2

The trkB family of transmembrane proteins serves as receptors for BDNF and NT-4/5. The family is composed of a tyrosine kinase-containing isoform as well as several alternatively spliced "truncated receptors" with identical extracellular ligand-binding domains but very small intracellular domains. The two best-characterized truncated trkB receptors, designated as trkB.T1 and trkB.T2, contain intracellular domains of only 23 and 21 amino acids, respectively. Although it is known that the tyrosine kinase isoform (trkB.FL) is capable of initiating BDNF and NT-4/5-induced signal transduction, the functional role or roles of the truncated receptors remain enigmatic. At the same time, the potential importance of the truncated receptors in the development, maintenance, and regeneration of the nervous system has been highlighted by recent developmental and injury paradigm investigations. Here we have used trkB cDNA transfected cell lines to demonstrate that both trkB.T1 and trkB.T2 are capable of mediating BDNF-induced signal transduction. More specifically, BDNF activation of either trkB.T1 or trkB.T2 increases the rate of acidic metabolite release from the cell, a common physiological consequence of many signaling pathways. Further, these trkB.T1- and trkB. T2-mediated changes occur with kinetics distinct from changes mediated by trkB.FL, suggesting the participation of at least some unique rate-limiting component or components. Mutational analysis demonstrates that the isoform-specific sequences within the intracellular domains of each receptor are essential for signaling capability. Finally, inhibitor studies suggest that kinases are likely to be involved in the trkB.T1 and trkB.T2 signaling pathways.

Direct measurement of extracellular proton flux from isolated gastric glands

We used the microphysiometer, a sensitive extracellular pH sensor, to resolve luminal (or apical) H+ secretion and basolateral release of OH- as well as liberation of acidic metabolites in rabbit gastric glands. Stimulation of glands via the adenosine 3',5'-cyclic monophosphate pathway produced a biphasic change in the extracellular acidification rate (EAR): after an initial transient decrease below the unstimulated baseline (-40.9 +/- 3.4%), the EAR increased to a steady-state maximal plateau (+98.1 +/- 5.3%) within 30 min (n = 37). We interpret the biphasic EAR profile as an initial excess of basolaterally released OH- followed by delayed luminal efflux of simultaneously produced H+. The elevated EAR at steady state reflected liberation of metabolic acid attributed to H(+)-K(+)-ATPase enzymatic activity. The presence of H2-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid prevented OH- release and reduced steady-state EAR. Basolateral OH- release and steady-state EAR were also inhibited by the H(+)-K(+)-ATPase inactivators omeprazole and SCH-28080. Inhibition of Na+/H+ exchange did not reduce steady-state EAR and did not affect apical H+ production, as judged by the accumulation of the weak base aminopyrine. Sodium thiocyanate (1 mM), which short circuits intraluminal H+ accumulation, blocked OH- release, demonstrating its dependence on H(+)-OH- separation at the apical membrane. A computerized model was developed to illustrate how the observed biphasic EAR profile would result from a delayed luminal efflux of H+ due to transitory intraluminal compartmentalization.

A metabolic view of receptor activation in cultured cells following cryopreservation

The effect of cryopreservation on agonist-induced receptor activation in mammalian cells was investigated with the Cytosensor microphysiometer, a biosensor that monitors cellular metabolic activity by measuring changes in extracellular pH. In this study, two different cell types--nonadherent TF-1 cells (from a human erythroleukemia patient) and adherent WT3 cells (CHO-K1 cells transfected with the m1 muscarinic acetylcholine receptor)--were cryopreserved by freezing in a disposable cell capsule used in the microphysiometer. The recovery of metabolic activity by TF-1 cells was observed over approximately 1 h following thawing. Responses of the TF-1 cells to granulocyte-macrophage colony-stimulating factor (GM-CSF) and platelet activating factor (PAF) were measured before cryopreservation and 90 min after thawing. The GM-CSF and PAF responses retained 71 +/- 14% and 73 +/- 10% of maximum stimulation, respectively. Post-thaw cholinergic stimulation of WT3 cells was 73 +/- 9% of its level in similarly treated but unfrozen cells. Cryopreservation caused no detectable difference in desensitization of the response due to repeated application of carbachol. These results demonstrate the feasibility of pharmacological studies with cryopreserved cells in the microphysiometer and further suggest that the microphysiometer may be useful in exploring the biological consequences of cryopreservation in the early post-thaw period.

PKC epsilon is involved in granulocyte-macrophage colony-stimulating factor signal transduction: evidence from microphysiometry and antisense oligonucleotide experiments

We have used microphysiometry and antisense methodology to show that the epsilon isoenzyme of protein kinase C (PKC) is involved in the signal transduction pathway of granulocyte-macrophage colony-stimulating factor (GM-CSF) in a human bone marrow cell line, TF-1. These cells require GM-CSF or a related cytokine for proliferation. When the cells are appropriately exposed to GM-CSF, they exhibit a burst of metabolic activity that can be detected on the time scale of minutes in the microphysiometer, a biosensor-based instrument that measures the rate at which cells excrete protons. These cells express PKC alpha and -epsilon, as determined by Western blot analysis. Treatment with isoenzyme-specific antisense oligonucleotides inhibits expression appropriately, but only inhibition of PKC epsilon appreciably diminishes the burst of metabolic activity induced by GM-CSF. Consistent with the involvement of PKC epsilon, GM-CSF appears to activate phospholipase D and does not cause a detectable increase in cytosolic [Ca2+].