Noradrenergic alterations in Parkinson's disease: a combined 11C-yohimbine PET/neuromelanin MRI study

Degeneration of the noradrenergic system is now considered a pathological hallmark of Parkinson's disease, but little is known about its consequences in terms of parkinsonian manifestations. Here, we evaluated two aspects of the noradrenergic system using multimodal in vivo imaging in patients with Parkinson's disease and healthy controls: the pigmented cell bodies of the locus coeruleus with neuromelanin sensitive MRI; and the density of α2-adrenergic receptors (ARs) with PET using 11C-yohimbine. Thirty patients with Parkinson's disease and 30 age- and sex-matched healthy control subjects were included. The characteristics of the patients' symptoms were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS). Patients showed reduced neuromelanin signal intensity in the locus coeruleus compared with controls and diminished 11C-yohimbine binding in widespread cortical regions, including the motor cortex, as well as in the insula, thalamus and putamen. Clinically, locus coeruleus neuronal loss was correlated with motor (bradykinesia, motor fluctuations, tremor) and non-motor (fatigue, apathy, constipation) symptoms. A reduction of α2-AR availability in the thalamus was associated with tremor, while a reduction in the putamen, the insula and the superior temporal gyrus was associated with anxiety. These results highlight a multifaceted alteration of the noradrenergic system in Parkinson's disease since locus coeruleus and α2-AR degeneration were found to be partly uncoupled. These findings raise important issues about noradrenergic dysfunction that may encourage the search for new drugs targeting this system, including α2-ARs, for the treatment of Parkinson's disease.

The PREMISE database of 20 Macaca fascicularis PET/MRI brain images available for research

Non-human primate studies are unique in translational research, especially in neurosciences where neuroimaging approaches are the preferred methods used for cross-species comparative neurosciences. In this regard, neuroimaging database development and sharing are encouraged to increase the number of subjects available to the community, while limiting the number of animals used in research. Here we present a simultaneous positron emission tomography (PET)/magnetic resonance (MR) dataset of 20 Macaca fascicularis images structured according to the Brain Imaging Data Structure standards. This database contains multiple MR imaging sequences (anatomical, diffusion and perfusion imaging notably), as well as PET perfusion and inflammation imaging using respectively [15O]H2O and [11C]PK11195 radiotracers. We describe the pipeline method to assemble baseline data from various cohorts and qualitatively assess all the data using signal-to-noise and contrast-to-noise ratios as well as the median of intensity and the pseudo-noise-equivalent-count rate (dynamic and at maximum) for PET data. Our study provides a detailed example for quality control integration in preclinical and translational PET/MR studies with the aim of increasing reproducibility. The PREMISE database is stored and available through the PRIME-DE consortium repository.

Characterisation of a novel [18F]FDG brain PET database and combination with a second database for optimising detection of focal abnormalities, using focal cortical dysplasia as an example

BACKGROUND

Brain [18F]FDG PET is used clinically mainly in the presurgical evaluation for epilepsy surgery and in the differential diagnosis of neurodegenerative disorders. While scans are usually interpreted visually on an individual basis, comparison against normative cohorts allows statistical assessment of abnormalities and potentially higher sensitivity for detecting abnormalities. Little work has been done on out-of-sample databases (acquired differently to the patient data). Combination of different databases would potentially allow better power and discrimination. We fully characterised an unpublished healthy control brain [18F]FDG PET database (Marseille, n = 60, ages 21-78 years) and compared it to another publicly available database (MRXFDG, n = 37, ages 23-65 years). We measured and then harmonised spatial resolution and global values. A collection of patient scans (n = 34, 13-48 years) with histologically confirmed focal cortical dysplasias (FCDs) obtained on three generations of scanners was used to estimate abnormality detection rates using standard software (statistical parametric mapping, SPM12).

RESULTS

Regional SUVs showed similar patterns, but global values and resolutions were different as expected. Detection rates for the FCDs were 50% for comparison with the Marseille database and 53% for MRXFDG. Simply combining both databases worsened the detection rate to 41%. After harmonisation of spatial resolution, using a full factorial design matrix to accommodate global differences, and leaving out controls older than 60 years, we achieved detection rates of up to 71% for both databases combined. Detection rates were similar across the three scanner types used for patients, and high for patients whose MRI had been normal (n = 10/11).

CONCLUSIONS

As expected, global and regional data characteristics are database specific. However, our work shows the value of increasing database size and suggests ways in which database differences can be overcome. This may inform analysis via traditional statistics or machine learning, and clinical implementation.

Relationships between serotonin availability and frontolimbic response to fearful and threatening faces

Serotonin is a critical neurotransmitter in the regulation of emotional behavior. Although emotion processing is known to engage a corticolimbic circuit, including the amygdala and prefrontal cortex, exactly how this brain system is modulated by serotonin remains unclear. Here, we hypothesized that serotonin modulates variability in excitability and functional connectivity within this circuit. We tested whether this modulation contributes to inter-individual differences in emotion processing. Using a multimodal neuroimaging approach with a simultaneous PET-3T fMRI scanner, we simultaneously acquired BOLD signal while participants viewed emotional faces depicting fear and anger, while also measuring serotonin transporter (SERT) levels, regulating serotonin functions. Individuals with higher activity of the medial amygdala BOLD in response to fearful or angry facial expressions, who were temperamentally more anxious, also exhibited lower SERT availability in the dorsal raphe nucleus (DRN). Moreover, higher connectivity of the medial amygdala with the left dorsolateral prefrontal and the anterior cingulate cortex was associated with lower levels of SERT availability in the DRN. These results demonstrate the association between the serotonin transporter level and emotion processing through changes in functional interactions between the amygdala and the prefrontal areas in healthy humans.

CERMEP-IDB-MRXFDG: a database of 37 normal adult human brain [18F]FDG PET, T1 and FLAIR MRI, and CT images available for research

We present a database of cerebral PET FDG and anatomical MRI for 37 normal adult human subjects (CERMEP-IDB-MRXFDG). Thirty-nine participants underwent static [18F]FDG PET/CT and MRI, resulting in [18F]FDG PET, T1 MPRAGE MRI, FLAIR MRI, and CT images. Two participants were excluded after visual quality control. We describe the acquisition parameters, the image processing pipeline and provide participants' individual demographics (mean age 38 ± 11.5 years, range 23-65, 20 women). Volumetric analysis of the 37 T1 MRIs showed results in line with the literature. A leave-one-out assessment of the 37 FDG images using Statistical Parametric Mapping (SPM) yielded a low number of false positives after exclusion of artefacts. The database is stored in three different formats, following the BIDS common specification: (1) DICOM (data not processed), (2) NIFTI (multimodal images coregistered to PET subject space), (3) NIFTI normalized (images normalized to MNI space). Bona fide researchers can request access to the database via a short form.

Modeling [11C]yohimbine PET human brain kinetics with test-retest reliability, competition sensitivity studies and search for a suitable reference region

Previous work introduced the [¹¹C]yohimbine as a suitable ligand of central α2-adrenoreceptors (α2-ARs) for PET imaging. However, reproducibility of [¹¹C]yohimbine PET measurements in healthy humans estimated with a simplified modeling method with reference region, as well as sensitivity of [¹¹C]yohimbine to noradrenergic competition were not evaluated. The objectives of the present study were therefore to fill this gap.

METHODS

Thirteen healthy humans underwent two [¹¹C]yohimbine 90-minute dynamic scans performed on a PET-MRI scanner. Seven had arterial blood sampling with metabolite assessment and plasmatic yohimbine free fraction evaluation at the first scan to have arterial input function and test appropriate kinetic modeling. The second scan was a simple retest for 6 subjects to evaluate the test-retest reproducibility. For the remaining 7 subjects the second scan was a challenge study with the administration of a single oral dose of 150 µg of clonidine 90 min before the PET scan. Parametric images of α2-ARs distribution volume ratios (DVR) were generated with two non-invasive models: Logan graphical analysis with Reference (LREF) and Simplified Reference Tissue Method (SRTM). Three reference regions (cerebellum white matter (CERWM), frontal white matter (FLWM), and corpus callosum (CC)) were tested.

RESULTS

We showed high test-retest reproducibility of DVR estimation with LREF and SRTM regardless of reference region (CC, CERWM, FLWM). The best fit was obtained with SRTM_CC (r²=0.94). Test-retest showed that the SRTM_CC is highly reproducible (mean ICC>0.7), with a slight bias (-1.8%), whereas SRTM_CERWM had lower bias (-0.1%), and excellent ICC (mean>0.8). Using SRTM_CC, regional changes have been observed after clonidine administration with a significant increase reported in the amygdala and striatum as well as in several posterior cortical areas as revealed with the voxel-based analysis.

CONCLUSION

The results add experimental support for the suitability of [¹¹C]yohimbine PET in the quantitative assessment of α2-ARs occupancy in vivo in the human brain. Trial registration EudraCT 2018-000380-82.

A non-human primate model of stroke reproducing endovascular thrombectomy and allowing long-term imaging and neurological read-outs

Stroke is a devastating disease. Endovascular mechanical thrombectomy is dramatically changing the management of acute ischemic stroke, raising new challenges regarding brain outcome and opening up new avenues for brain protection. In this context, relevant experiment models are required for testing new therapies and addressing important questions about infarct progression despite successful recanalization, reversibility of ischemic lesions, blood-brain barrier disruption and reperfusion damage. Here, we developed a minimally invasive non-human primate model of cerebral ischemia (Macaca fascicularis) based on an endovascular transient occlusion and recanalization of the middle cerebral artery (MCA). We evaluated per-occlusion and post-recanalization impairment on PET-MRI, in addition to acute and chronic neuro-functional assessment. Voxel-based analyses between per-occlusion PET-MRI and day-7 MRI showed two different patterns of lesion evolution: "symptomatic salvaged tissue" (SST) and "asymptomatic infarcted tissue" (AIT). Extended SST was present in all cases. AIT, remote from the area at risk, represented 45% of the final lesion. This model also expresses both worsening of fine motor skills and dysexecutive behavior over the chronic post-stroke period, a result in agreement with cortical-subcortical lesions. We thus fully characterized an original translational model of ischemia-reperfusion damage after stroke, with consistent ischemia time, and thrombus retrieval for effective recanalization.

[18F]F13640, a 5-HT1A Receptor Radiopharmaceutical Sensitive to Brain Serotonin Fluctuations

Introduction

Serotonin is involved in a variety of physiological functions and brain disorders. In this context, efforts have been made to investigate the in vivo fluctuations of this neurotransmitter using positron emission tomography (PET) imaging paradigms. Since serotonin is a full agonist, it binds preferentially to G-protein coupled receptors. In contrast, antagonist PET ligands additionally interact with uncoupled receptors. This could explain the lack of sensitivity to serotonin fluctuations of current 5-HT_1A radiopharmaceuticals which are mainly antagonists and suggests that agonist radiotracers would be more appropriate to measure changes in neurotransmitter release. The present study evaluated the sensitivity to endogenous serotonin release of a recently developed, selective 5-HT_1A receptor PET radiopharmaceutical, the agonist [¹⁸F]F13640 (a.k.a. befiradol or NLX-112).

Materials and Methods

Four cats each underwent three PET scans with [¹⁸F]F13640, i.e., a control PET scan of 90 min, a PET scan preceded 30 min before by an intravenous injection 1 mg/kg of d-fenfluramine, a serotonin releaser (blocking challenge), and a PET scan comprising the intravenous injection of 1 mg/kg of d-fenfluramine 30 min after the radiotracer injection (displacement challenge). Data were analyzed with regions of interest and voxel-based approaches. A lp-ntPET model approach was implemented to determine the dynamic of serotonin release during the challenge study.

Results

D-fenfluramine pretreatment elicited a massive inhibition of [¹⁸F]F13640 labeling in regions known to express 5-HT_1A receptors, e.g., raphe nuclei, hippocampus, thalamus, anterior cingulate cortex, caudate putamen, occipital, frontal and parietal cortices, and gray matter of cerebellum. Administration of d-fenfluramine during PET acquisition indicates changes in occupancy from 10% (thalamus) to 31% (gray matter of cerebellum) even though the dissociation rate of [¹⁸F]F13640 over the 90 min acquisition time was modest. The lp-ntPET simulation succeeded in differentiating the control and challenge conditions.

Conclusion

The present findings demonstrate that labeling of 5-HT_1A receptors with [¹⁸F]F13640 is sensitive to serotonin concentration fluctuations in vivo. Although the data underline the need to perform longer PET scan to ensure accurate measure of displacement, they support clinical development of [¹⁸F]F13640 as a tool to explore experimental paradigms involving physiological or pathological (neurological or neuropsychiatric pathologies) fluctuations of extracellular serotonin.

PET-MRI nanoparticles imaging of blood-brain barrier damage and modulation after stroke reperfusion

In an acute ischaemic stroke, understanding the dynamics of blood-brain barrier injury is of particular importance for the prevention of symptomatic haemorrhagic transformation. However, the available techniques assessing blood-brain barrier permeability are not quantitative and are little used in the context of acute reperfusion therapy. Nanoparticles cross the healthy or impaired blood-brain barrier through combined passive and active processes. Imaging and quantifying their transfer rate could better characterize blood-brain barrier damage and refine the delivery of neuroprotective agents. We previously developed an original endovascular stroke model of acute ischaemic stroke treated by mechanical thrombectomy followed by positron emission tomography-magnetic resonance imaging. Cerebral capillary permeability was quantified for two molecule sizes: small clinical gadolinium Gd-DOTA (<1 nm) and AGuIX^® nanoparticles (∼5 nm) used for brain theranostics. On dynamic contrast-enhanced magnetic resonance imaging, the baseline transfer constant K _trans was 0.94 [0.48, 1.72] and 0.16 [0.08, 0.33] ×10^-3min^-1, respectively, in the normal brain parenchyma, consistent with their respective sizes, and 1.90 [1.23, 3.95] and 2.86 [1.39, 4.52] ×10^-3min^-1 in choroid plexus, confirming higher permeability than brain parenchyma. At early reperfusion, K _trans for both Gd-DOTA and AGuIX^® nanoparticles was significantly higher within the ischaemic area compared to the contralateral hemisphere; 2.23 [1.17, 4.13] and 0.82 [0.46, 1.87] ×10^-3min^-1 for Gd-DOTA and AGuIX^® nanoparticles, respectively. With AGuIX^® nanoparticles, K _trans also increased within the ischaemic growth areas, suggesting added value for AGuIX^®. Finally, K _trans was significantly lower in both the lesion and the choroid plexus in a drug-treated group (ciclosporin A, n = 7) compared to placebo (n = 5). K _trans quantification with AGuIX^® nanoparticles can monitor early blood-brain barrier damage and treatment effect in ischaemic stroke after reperfusion.

Bayesian Estimation of the ntPET Model in Single-Scan Competition PET Studies

This paper proposes an innovative method, named b-ntPET, for solving a competition model in PET. The model is built upon the state-of-the-art method called lp-ntPET. It consists in identifying the parameters of the PET kinetic model relative to a reference region that rule the steady state exchanges, together with the identification of four additional parameters defining a displacement curve caused by an endogenous neurotransmitter discharge, or by a competing injected drug targeting the same receptors as the PET tracer. The resolution process of lp-ntPET is however suboptimal due to the use of discretized basis functions, and is very sensitive to noise, limiting its sensitivity and accuracy. Contrary to the original method, our proposed resolution approach first estimates the probability distribution of the unknown parameters using Markov-Chain Monte-Carlo sampling, distributions from which the estimates are then inferred. In addition, and for increased robustness, the noise level is jointly estimated with the parameters of the model. Finally, the resolution is formulated in a Bayesian framework, allowing the introduction of prior knowledge on the parameters to guide the estimation process toward realistic solutions. The performance of our method was first assessed and compared head-to-head with the reference method lp-ntPET using well-controlled realistic simulated data. The results showed that the b-ntPET method is substantially more robust to noise and much more sensitive and accurate than lp-ntPET. We then applied the model to experimental animal data acquired in pharmacological challenge studies and human data with endogenous releases induced by transcranial direct current stimulation. In the drug challenge experiment on cats using [¹⁸F]MPPF, a serotoninergic 1A antagonist radioligand, b-ntPET measured a dose response associated with the amount of the challenged injected concurrent 5-HT1A agonist, where lp-ntPET failed. In human [¹¹C]raclopride experiment, contrary to lp-ntPET, b-ntPET successfully detected significant endogenous dopamine releases induced by the stimulation. In conclusion, our results showed that the proposed method b-ntPET has similar performance to lp-ntPET for detecting displacements, but with higher resistance to noise and better robustness to various experimental contexts. These improvements lead to the possibility of detecting and characterizing dynamic drug occupancy from a single PET scan more efficiently.

A multi-centre evaluation of eleven clinically feasible brain PET/MRI attenuation correction techniques using a large cohort of patients

AIM

To accurately quantify the radioactivity concentration measured by PET, emission data need to be corrected for photon attenuation; however, the MRI signal cannot easily be converted into attenuation values, making attenuation correction (AC) in PET/MRI challenging. In order to further improve the current vendor-implemented MR-AC methods for absolute quantification, a number of prototype methods have been proposed in the literature. These can be categorized into three types: template/atlas-based, segmentation-based, and reconstruction-based. These proposed methods in general demonstrated improvements compared to vendor-implemented AC, and many studies report deviations in PET uptake after AC of only a few percent from a gold standard CT-AC. Using a unified quantitative evaluation with identical metrics, subject cohort, and common CT-based reference, the aims of this study were to evaluate a selection of novel methods proposed in the literature, and identify the ones suitable for clinical use.

METHODS

In total, 11 AC methods were evaluated: two vendor-implemented (MR-AC_DIXON and MR-AC_UTE), five based on template/atlas information (MR-AC_SEGBONE (Koesters et al., 2016), MR-AC_ONTARIO (Anazodo et al., 2014), MR-AC_BOSTON (Izquierdo-Garcia et al., 2014), MR-AC_UCL (Burgos et al., 2014), and MR-AC_MAXPROB (Merida et al., 2015)), one based on simultaneous reconstruction of attenuation and emission (MR-AC_MLAA (Benoit et al., 2015)), and three based on image-segmentation (MR-AC_MUNICH (Cabello et al., 2015), MR-AC_CAR-RiDR (Juttukonda et al., 2015), and MR-AC_RESOLUTE (Ladefoged et al., 2015)). We selected 359 subjects who were scanned using one of the following radiotracers: [¹⁸F]FDG (210), [¹¹C]PiB (51), and [¹⁸F]florbetapir (98). The comparison to AC with a gold standard CT was performed both globally and regionally, with a special focus on robustness and outlier analysis.

RESULTS

The average performance in PET tracer uptake was within ±5% of CT for all of the proposed methods, with the average±SD global percentage bias in PET FDG uptake for each method being: MR-AC_DIXON (-11.3±3.5)%, MR-AC_UTE (-5.7±2.0)%, MR-AC_ONTARIO (-4.3±3.6)%, MR-AC_MUNICH (3.7±2.1)%, MR-AC_MLAA (-1.9±2.6)%, MR-AC_SEGBONE (-1.7±3.6)%, MR-AC_UCL (0.8±1.2)%, MR-AC_CAR-RiDR (-0.4±1.9)%, MR-AC_MAXPROB (-0.4±1.6)%, MR-AC_BOSTON (-0.3±1.8)%, and MR-AC_RESOLUTE (0.3±1.7)%, ordered by average bias. The overall best performing methods (MR-AC_BOSTON, MR-AC_MAXPROB, MR-AC_RESOLUTE and MR-AC_UCL, ordered alphabetically) showed regional average errors within ±3% of PET with CT-AC in all regions of the brain with FDG, and the same four methods, as well as MR-AC_CAR-RiDR, showed that for 95% of the patients, 95% of brain voxels had an uptake that deviated by less than 15% from the reference. Comparable performance was obtained with PiB and florbetapir.

CONCLUSIONS

All of the proposed novel methods have an average global performance within likely acceptable limits (±5% of CT-based reference), and the main difference among the methods was found in the robustness, outlier analysis, and clinical feasibility. Overall, the best performing methods were MR-ACBOSTON, MR-ACMAXPROB, MR-ACRESOLUTE and MR-ACUCL, ordered alphabetically. These methods all minimized the number of outliers, standard deviation, and average global and local error. The methods MR-ACMUNICH and MR-ACCAR-RiDR were both within acceptable quantitative limits, so these methods should be considered if processing time is a factor. The method MR-ACSEGBONE also demonstrates promising results, and performs well within the likely acceptable quantitative limits. For clinical routine scans where processing time can be a key factor, this vendor-provided solution currently outperforms most methods. With the performance of the methods presented here, it may be concluded that the challenge of improving the accuracy of MR-AC in adult brains with normal anatomy has been solved to a quantitatively acceptable degree, which is smaller than the quantification reproducibility in PET imaging.

Diacylglycerol kinase-ζ regulates mTORC1 and lipogenic metabolism in cancer cells through SREBP-1

Diacylglycerol kinases (DGKs) transform diacylglycerol (DAG) into phosphatidic acid (PA), balancing the levels of these key metabolic and signaling lipids. We previously showed that PA derived from the DGKζ isoform promotes mammalian target of rapamycin complex 1 (mTORC1) activation. This function might be crucial for the growth and survival of cancer cells, especially for those resistant to the allosteric mTOR inhibitor rapamycin. How this positive function of DGKζ coordinates with DAG metabolism and signaling is unknown. In this study, we used a rapamycin-resistant colon cancer cell line as a model to address the role of DGKζ in tumor cells. We found that DGKζ predominated over other PA sources such as DGKα or phospholipase D to activate mTORC1, and that its activity was a component of the rapamycin-induced feedback loops. We show that the DGKζ DAG-consuming function is central to cell homeostasis, as DAG negatively regulates levels of the lipogenic transcription factor SREBP-1. Our findings suggest a model in which simultaneous regulation of DAG and PA levels by DGKζ is integrated with mTOR function to maintain tumor cell homeostasis; we provide new evidence of the crosstalk between mTOR and lipid metabolism that will be advantageous in the design of drug therapies.

Deletion of the acidic-rich domain of the IL-2Rbeta chain increases receptor-associated PI3K activity

Interleukin-2 (IL-2) regulates the proliferation and homeostasis of lymphocytes through the coordinated activation of distinct signaling pathways. Deletion of the acidic-rich domain of the IL-2 receptor beta chain (IL-2Rbeta) prevents association of Src tyrosine kinases to the receptor, as well as IL-2-induced Akt activation. Cells bearing this deletion (BafbetaDeltaA) maintain full proliferation in response to IL-2 both in vivo and in vitro, suggesting that those pathways are dispensable for this important function of IL-2. In this study, we re-examined phosphatidylinositol-3 kinase (PI3K) activation in BafbetaDeltaA cells and found that, in BaF/3 IL-2RbetaDeltaA cells, deletion of the acidic domain induced constitutive activation of the receptor-associated PI3K activity. This, in turn, was responsible for the higher basal Akt activity observed in cells expressing this deletion. Based on these data, and since pharmacological abrogation of PI3K activity prevented IL-2-driven cell proliferation of BafbetaDeltaA cells, we conclude that the PI3K/Akt pathway is still functionally relevant in cells bearing this mutation. Moreover, we show that the PI3K-induced signals are, at least in part, responsible for c-myc expression. In conclusion, we have used this model to better identify those signals that are integral components of the molecular mechanisms responsible for IL-2-regulated cell proliferation.

Cell surface CD28 levels define four CD4+ T cell subsets: abnormal expression in rheumatoid arthritis

CD28 is a costimulatory receptor expressed in most CD4(+) T cells. Despite the long-standing evidence for up- and downregulation of surface CD28 expression in vitro, and the key regulatory role assigned to the upregulation of CD28 counterreceptor [the CD152 (CTLA-4) molecule], in vivo CD28 induction has attracted little attention. We studied CD28 and CD152 expression and function in 33 rheumatoid arthritis (RA) patients, 20 clinically active and 13 inactive, and in 24 healthy donors. Four subsets of CD28(-), CD28(low), CD28(int), and CD28(high) peripheral blood human CD4(+) T cells were defined using three-color flow cytometry. The three CD28(+) subsets displayed a one-, two-, or threefold quantitative difference in their relative number of CD28 antibody binding sites, respectively (P < 0.01). RA patients, whether active or inactive, showed a distinct phenotype when compared to healthy donors: (i) the percentage of CD4(+)CD28(high) cells was increased twofold and the CD4(+)CD28(low) subset was reduced twofold (P < 0.01) and (ii) the CD4(+)CD28(high) cells from RA patients showed an in vivo activated phenotype, CD45RO(+)CD5(high)IL-2Ralpha(+) (P < 0.01). Active RA patients were different from inactive patients. They showed a twofold increase in mean CD28 expression (P < 0.05), whereas each of the CD28(+) subsets in the inactive RA patients showed reduced expression when compared to healthy donors. Notably, both active and inactive RA patients showed abnormal CD28 upregulation when T cells were activated in vitro with CD3 antibodies, but only inactive RA patients showed a hypoproliferative response to TCR/CD3 triggering when compared to healthy donors (P < 0.01). This defective proliferation was normalized by concurrent crosslinking with CD28 antibody. No differences were noted in the expression of CD152 or CD80, a CD28 and CD152 shared ligand. The disregulated in vivo expression of CD28 was related to the RA patients' disease activity and suggests that modulation of CD28 surface levels may be an additional mechanism to finely tune the delicate responsiveness/tolerance balance.

Evidence for the involvement of diacylglycerol kinase in the activation of hypoxia-inducible transcription factor 1 by low oxygen tension

Hypoxia-inducible factor 1 (HIF-1) induces a gene expression program essential for the cellular adaptation to lowered oxygen environments. The intracellular mechanisms by which hypoxia induces HIF-1 remain poorly understood. Here we show that exposure of various cell types to hypoxia raises the intracellular level of phosphatidic acid primarily through the action of diacylglycerol kinase (DGK). Pharmacological inhibition of DGK activity through use of the specific DGK inhibitors and abrogated specifically HIF-1-dependent transcription analyzed with a HIF-1-responsive reporter plasmid. A more detailed analysis revealed that pharmacological inhibition of DGK activity prevented the hypoxia-dependent accumulation of the HIF-1alpha subunit and the subsequent HIF-1-DNA complex formation as well as hypoxia-induced activity of the HIF-1 transactivation domains localized to amino acids 530-582 and 775-826 of the HIF-1alpha subunit. Our results demonstrate for the first time that accumulation of phosphatidic acid through DGK underlines oxygen sensing and provide evidence for the involvement of this lipid kinase in the intracellular signaling that leads to HIF-1 activation.

Changes in the balance between mitogenic and antimitogenic lipid second messengers during proliferation, cell arrest, and apoptosis in T-lymphocytes

Control of lymphocyte cell survival and proliferation is critical for both the immune response and for the prevention of autoimmune and infectious diseases. The actions of interleukin-2, the major T-cell regulatory cytokine, are mediated by the complex network of divergent signalling pathways controlled by its high-affinity receptor. Various studies have indicated that the generation of certain lipid second messengers is an important mechanism in the control of proliferation and cell death. We have examined the relationship between diacylglycerol and ceramide and the levels of the lipids phosphatidylcholine and sphingomyelin, their potential precursors, in the human T-cell line Kit 225 cultured in three distinct conditions to favor apoptosis, cell arrest, and proliferation. Our data show that, in proliferating cells, the ratios of diacylglycerol/ceramide and phosphatidylcholine/sphingomyelin are higher than those found in arrested cells and increase with time in culture. These ratios are rapidly reversed in apoptotic cells. Further experiments reveal that de novo synthesis of both diacylglycerol and phosphatidylcholine is greatest in proliferating cells, whereas sphingomyelin synthase activity is increased in cells undergoing apoptosis. In summary, our results demonstrate for the first time that the ratio of mitogenic/antimitogenic lipids changes dramatically during T-cell proliferation and cell death. These results indicate that lipid second messengers and the enzymes that are responsible for their generation may provide targets for novel therapeutic interventions in the clinical management of immunosuppression and autoimmune disease.

Type Ialpha phosphatidylinositol 4-phosphate 5-kinase is a putative target for increased intracellular phosphatidic acid

Despite the fact that phosphatidic acid (PtdOH) has been implicated as a lipid second messenger for nearly a decade, its intracellular targets have remained unclear. We sought to investigate how an increase in the level of PtdOH could modulate phosphatidylinositol 4-phosphate 5-kinase (PIPkin), an enzyme involved in phosphatidylinositol 4,5-bisphosphate synthesis. Transfection of porcine aortic endothelial (PAE) cells with haemagglutinin (HA)-tagged type Ialpha PIPkin followed by immunofluorescence confocal microscopy revealed the enzyme to be localised to the plasma membrane. When the transfected PAE cells were stimulated with lyso-PtdOH, increased PIPkin activity was found to be associated with HA immunoprecipitates in an in vitro assay. This PIPkin activation was found to be greatly reduced by prior treatment of the cells with 1-butanol, thereby implicating phospholipase D (PLD) as the in vivo generator of PtdOH. In order to determine if the PtdOH-dependent activation of type Ialpha PIPkin was dictated by a specific molecular composition of PtdOH, the wild type murine and porcine alpha isoforms of diacylglycerol kinase (DGK) were individually co-transfected along with type Ialpha PIPkin. Under these conditions an increase in type Ialpha PIPkin lipid kinase activity was found in HA immunoprecipitates in an in vitro assay. No increases in lipid kinase activity were observed when type Ialpha PIPkin was co-transfected with either the human DGKepsilon isoform or a kinase-dead mutant of the murine DGKalpha isoform. These results provide the first direct evidence for the unification of the production of saturated/monounsaturated PtdOH (through two different routes, PLD and DGK) and the in vivo activation of type Ialpha PIPkin by this lipid second messenger.

Involvement of phosphatidylinositol 3-kinase in stromal cell-derived factor-1 alpha-induced lymphocyte polarization and chemotaxis

The role of phosphatidylinositol 3-kinase (PI3-kinase), an important enzyme involved in signal transduction events, has been studied in the polarization and chemotaxis of lymphocytes induced by the chemokine stromal cell-derived factor-1 alpha (SDF-1 alpha). This chemokine was able to directly activate p85/p110 PI3-kinase in whole human PBL and to induce the association of PI3-kinase to the SDF-1 alpha receptor, CXCR4, in a pertussis toxin-sensitive manner. Two unrelated chemical inhibitors of PI3-kinase, wortmannin and Ly294002, prevented ICAM-3 and ERM protein moesin polarization as well as the chemotaxis of PBL in response to SDF-1 alpha. However, they did not interfere with the reorganization of either tubulin or the actin cytoskeleton. Moreover, the transient expression of a dominant negative form of the PI3-kinase 85-kDa regulatory subunit in the constitutively polarized Peer T cell line inhibited ICAM-3 polarization and markedly reduced SDF-1 alpha-induced chemotaxis. Conversely, overexpression of a constitutively activated mutant of the PI3-kinase 110-kDa catalytic subunit in the round-shaped PM-1 T cell line induced ICAM-3 polarization. These results underline the role of PI3-kinase in the regulation of lymphocyte polarization and motility and indicate that PI3-kinase plays a selective role in the regulation of adhesion and ERM proteins redistribution in the plasma membrane of lymphocytes.

Diacylglycerol kinase inhibition prevents IL-2-induced G1 to S transition through a phosphatidylinositol-3 kinase-independent mechanism

Stimulation via IL-2R ligation causes T lymphocytes to transit through the cell cycle. Previous experiments by our group have demonstrated that, in human T cells, IL-2 binding induces phosphatidic acid production through activation of the alpha isoform of diacylglycerol kinase. In this study, using the IL-2-dependent mouse T cell line CTLL-2, we demonstrate that pharmacological inhibition of IL-2-induced diacylglycerol kinase activation is found to block IL-2-induced late G1 to S transition without affecting cell viability. Herein, we demonstrate that diacylglycerol kinase inhibition has a profound effect on the induction of the protooncogenes c-myc, c-fos, and c-raf by IL-2, whereas expression of bcl-2 and bcl-xL are not affected. When the IL-2-regulated cell cycle control checkpoints are examined in detail, we demonstrate that inhibition of diacylglycerol kinase activation prevents IL-2 induction of cyclin D3 without affecting p27 down-regulation. The strict control of cell proliferation exerted by phosphatidic acid through activation of diacylglycerol kinase is independent of other well-characterized IL-2R-derived signals, such as the phosphatidylinositol-3 kinase/Akt pathway, indicating the existence of a different and important mechanism to control cell division.

Interleukin-2 causes an increase in saturated/monounsaturated phosphatidic acid derived from 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol

Phosphatidic acid generation through activation of diacylglycerol kinase alpha has been implicated in interleukin-2-dependent T-lymphocyte proliferation. To investigate this lipid signaling in more detail, we characterized the molecular structures of the diradylglycerols and phosphatidic acids in the murine CTLL-2 T-cell line under both basal and stimulated conditions. In resting cells, 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol subtypes represented 44 and 55% of total diradylglycerol, respectively, and both showed a highly saturated profile containing primarily 16:0 and 18:1 fatty acids. 1-O-Alk-1'-enyl-2-acylglycerol represented 1-2% of total diradylglycerol. Interleukin-2 stimulation did not alter the molecular species profiles, however, it did selectively reduce total 1-O-alkyl-2-acylglycerol by over 50% at 15 min while only causing a 10% drop in 1,2-diacylglycerol. When radiolabeled CTLL-2 cells were challenged with interleukin-2, no change in the cellular content of phosphatidylcholine nor phosphatidylethanolamine was observed thereby ruling out phospholipase C activity as the source of diradylglycerol. In addition, interleukin-2 failed to stimulate de novo synthesis of diradylglycerol. Structural analysis revealed approximately equal amounts of 1,2-diacyl phosphatidic acid and 1-O-alkyl-2-acyl phosphatidic acid under resting conditions, both containing only saturated and monounsaturated fatty acids. After acute (2 and 15 min) interleukin-2 stimulation the total phosphatidic acid mass increased, almost entirely through the formation of 1-O-alkyl-2-acyl species. In vitro assays revealed that both 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol were substrates for 1,2-diacylglycerol kinase alpha, the major isoform in CTLL-2 cells, and that the lipid kinase activity was almost totally inhibited by R59949. In conclusion, this investigation shows that, in CTLL-2 cells, 1,2-diacylglycerol kinase alpha specifically phosphorylates a pre-existing pool of 1-O-alkyl-2-acylglycerol to form the intracellular messenger 1-O-alkyl-2-acyl phosphatidic acid.

An IL-2 receptor beta subdomain that controls Bcl-X(L) expression and cell survival

IL-2 binding to its high-affinity receptor regulates signaling events that control both lymphocyte cell survival and cell cycle progression. Although many studies have examined the mechanisms by which IL-2 regulates cell growth, few studies have dissected the pathways involved in promoting cell survival or the coupling of these pathways to the receptor. In the present study, using the pre-B cell line Baf-B03 transfected with a truncated form of the IL-2 receptor (IL-2R) beta chain, we demonstrate that IL-2-dependent cell survival requires only the N-terminal 350 amino acids of the IL-2Rbeta chain. IL-2-dependent survival of cells expressing the truncated receptor correlates with increases in receptor-associated phosphatidylinositol 3-kinase (PI3K) activity and expression of Bcl-X(L), but not with changes in c-Myc expression or proliferation. Inhibition of the PI3K pathway in these cells, but not in cells expressing the wild-type receptor, has a marked effect on the capacity of IL-2 to prevent cell death and diminishes the Bcl-X(L) response. The requirement for IL-2-induced PI3K activity in suppressing the onset of apoptotic cell death is discussed.

Interleukin-2 stimulates a late increase in phosphatidic acid production in the absence of phospholipase D activation

The signal transduction pathways involving phospholipid metabolism during T-cell proliferation remain partly undefined. Herein we show that interleukin-2 caused a late (> 12 h) rise in the intracellular phosphatidic acid content of CTLL-2 cells which was a consequence of the activation of the enzyme diacylglycerol kinase. No activation of phospholipase D was observed at similar times. Incubation of the cells with a recognized diacylglycerol kinase a isoform inhibitor, R59499, prior to interleukin-2 stimulation was able to block cell cycle entry, diacyglycerol kinase activation and phosphatidic acid accumulation. In contrast, when R59499 was added 3 h after interleukin-2, few or no observable effects on the above three parameters were noticed. These results suggest that the early signaling employed by IL-2 involving the alpha isoform of diacylglycerol kinase is sufficient to control the late increase in phosphatidic acid and that phosphatidic acid is a mitogenic agent in T-cells.

Dual role of ceramide in the control of apoptosis following IL-2 withdrawal

Ceramide is largely known as a lipid second messenger with pleiotropic effects. Increases in ceramide levels have been related to the onset of apoptosis, terminal differentiation, or growth suppression. In this study, addition of exogenous C2-ceramide to CTLL-2 cells is found to block IL-2-induced cell cycle entry, as well as the apoptosis triggered by IL-2 deprivation. The protective effect of C2-ceramide is achieved only in the early stages following cytokine deprivation and is related to the inhibition of bcl-xL degradation and the induction of a G0 arrest of cells. The same treatment over a longer time when, as we demonstrate, ceramide is produced physiologically, enhances cell death by apoptosis. The dual effect of ceramide both in protecting from or inducing apoptosis is discussed further.

Identification and characterization of a new oncogene derived from the regulatory subunit of phosphoinositide 3-kinase

p85/p110 phosphoinositide 3-kinase (PI3K) is a heterodimer composed of a p85-regulatory and a p110-catalytic subunit, which is involved in a variety of cellular responses including cytoskeletal organization, cell survival and proliferation. We describe here the cloning and characterization of p65-PI3K, a mutant of the regulatory subunit of PI3K, which includes the initial 571 residues of the wild type p85alpha-protein linked to a region conserved in the eph tyrosine kinase receptor family. We demonstrate that this mutation, obtained from a transformed cell, unlike previously engineered mutations of the regulatory subunit, induces the constitutive activation of PI3K and contributes to cellular transformation. This report links the PI3K enzyme to mammalian tumor development for the first time.

Identification of insulin-like growth factor-binding protein-1 as a potential physiological substrate for human stromelysin-3

To elucidate the physiological role of human stromelysin-3 (hST-3) in tumor progression and/or wound healing, insulin-like growth factor-binding protein-1 (IGFBP-1) was analyzed as a potential physiological substrate. hST-3 proteolysis generates two fragments of 16 and 9 kDa that react with IGFBP-1 monoclonal antibody, although they do not bind insulin-like growth factor-I (IGF-I) in ligand blot. N-terminal sequencing shows that hST-3 cleaves IGFBP-1 at the His140-Val141 bond located in the IGFBP-1 midregion. We show that IGFBP-1 inhibits IGF-I-induced survival and proliferation of BAF/3 cells, as well as IGF-I-mediated activation of phosphatidylinositol 3-kinase (PI 3-K). Co-incubation of the IGF-I. IGFBP-1 complex with hST-3 restores IGF-I-induced proliferation and PI 3-K kinase activity in these cells. BAF/3 proliferation is significantly increased with the hST-3-treated IGF-I.IGFBP-1 complex compared with that obtained using IGF-I alone. To produce this enhanced proliferation, IGF-I must bind to IGFBP-1 before hST-3 proteolysis, demonstrated using an IGF-I variant that does not bind IGFBP. IGFBP-1 also inhibits IGF-I-induced proliferation of the MCF-7 breast adenocarcinoma, and this inhibition was not seen in hST-3-transfected MCF-7 cells. Such proteolysis may thus play a role in in vivo tumor progression. These results indicate that hST-3 may regulate IGF-I bioavailability by proteolyzing IGFBP, thus favoring cell survival and proliferation.

Intermediate affinity interleukin-2 receptor mediates survival via a phosphatidylinositol 3-kinase-dependent pathway

Peripheral blood T lymphocytes require two signals to enter and progress along the cell cycle from their natural quiescent state. The first activation signal is provided by the stimulation through the T cell receptor, which induces the synthesis of cyclins and the expression of the high affinity interleukin-2 receptor. The second signal, required to enter the S phase, is generated upon binding of interleukin-2 to the high affinity alphabetagamma interleukin-2 receptor. However, resting T cells already express intermediate affinity betagamma interleukin-2 receptors. As shown here, T cell stimulation through intermediate affinity receptors is capable of inducing cell rescue from the apoptosis suffered in the absence of stimulation. Characterization of the signaling pathways utilized by betagamma interleukin-2 receptors in resting T cells, indicated that pp56(lck), but not Jak1 or Jak3, is activated upon receptor triggering. Compelling evidence is presented indicating that phosphatidylinositol 3-kinase associates with the intermediate affinity interleukin-2 receptor and is activated upon interleukin-2 addition. Bcl-xL gene was also found to be induced upon betagamma interleukin-2 receptor stimulation. Finally, pharmacological inhibition of phosphatidylinositol 3-kinase blocked both interleukin-2-mediated bcl-xL induction and cell survival. We conclude that betagamma interleukin-2 receptor mediates T-cell survival via a phosphatidylinositol 3-kinase-dependent pathway, possibly involving pp56(lck) and bcl-xL as upstream and downstream effectors, respectively.

Conformational changes required in the human growth hormone receptor for growth hormone signaling

Growth hormone (GH) plays a significant role in normal growth and development. Signaling to the cell is believed to require growth hormone receptor (GHR) dimerization, which occurs following binding of a single growth hormone molecule to each of two receptors. We have developed human growth hormone receptor-specific monoclonal antibodies, one of which was used here to characterize hormone/receptor interactions. This antibody, GHR05, is directed against the hinge spanning subdomains I and II of the receptor's extracellular region. Antibody binding to the cell surface receptor increases upon receptor binding to growth hormone, but not when it binds a mutant form, hGHG120R, which does not trigger receptor activation. Growth hormone binding thus appears to lead to a conformational change in the receptor epitope recognized by GHR05, giving rise to the active dimer configuration, necessary for signal transduction. Using a chimeric receptor-expressing, growth hormone-dependent murine cell line, we find that GHR05 binds to the receptor in the absence of human GH and delivers a signal leading to cell proliferation. Finally, GHR05 treatment of IM-9 cells, a human cell line expressing a functional human GHR, leads to cell proliferation mediated by the generation of GH-specific signals, including phosphorylation of the JAK2 tyrosine kinase and activation of STAT5.

Differential effect of rapamycin and cyclosporin A in proliferation in a murine T cell line expressing either intermediate or high affinity receptor for IL-2

It has been reported that rapamycin (rap), cyclosporin A (CsA) and FK506 have immunosuppressive effect during the activation process of murine T cells. These drugs were investigated for their suppressive effect on a murine T cell line expressing intermediate (TS1 beta) or high (TS1 alpha beta) affinity IL-2R. Rap and CsA strongly inhibit the IL-2-dependent proliferation of TS1 alpha beta cells while they minimally affect the IL-2-mediated proliferation of TS1 beta cells. FK506 does not have any effect on the IL-2-driven proliferation of either TS1 beta or TS1 alpha beta cells. Simultaneous addition of Rap and CsA or Rap and FK506 inhibit the IL-2-mediated proliferation of TS1 beta and TS1 alpha beta cells and therefore FK506 does not revert the inhibition mediated by Rap in TS1 alpha beta cells. Neither Rap nor CsA affect IL-2R expression and internalization in TS1 alpha beta cells. CsA and Rap strongly inhibit the appearance of DNA binding activity of NF-AT and to a lesser extent NF-kappa B. Rap inhibits IL-2-stimulated phosphatidylinositol 3 (PI3) kinase activity in TS1 alpha beta cells. In TS1 beta cells, Rap activates PI3 kinase on its own, inhibiting the IL-2-stimulated PI3 kinase to a lesser extent. These results suggest that PI3 kinase is a target for Rap action. Our results strongly suggest that we have Rap and CsA sensitive and resistant activation pathways operating in TS1 beta and TS1 alpha beta cells.

Vasopressin-stimulated phosphorylation of rat liver phospholipid methyltransferase in isolated hepatocytes

Addition of vasopressin (1 microM) to isolated rat hepatocytes prelabeled with [32P]phosphate was accompanied by a 250% increase in the phosphorylation of phospholipid methyltransferase. Vasopressin-stimulated phospholipid methyltransferase phosphorylation was time- and dose-dependent. 32P-labeled phospholipid methyltransferase was recovered by immunoprecipitation and SDS-polyacrylamide gel electrophoresis. After electrophoresis, phospholipid methyltransferase was electroeluted from the polyacrylamide gel and subjected to tryptic digestion or HCl hydrolysis. Analysis of 32P-labeled peptides reveals only one site of phosphorylation and the analysis of [32P]phosphoamino acids indicates that phosphoserine is the only labeled amino acid.

Modulation by the ratio S-adenosylmethionine/S-adenosylhomocysteine of cyclic AMP-dependent phosphorylation of the 50 kDa protein of rat liver phospholipid methyltransferase

The present results show that the catalytic subunit of cyclic AMP-dependent protein kinase phosphorylates the 50 kDa protein of rat liver phospholipid methyltransferase at one single site on a serine residue. Phosphorylation of this site is stimulated 2- to 3-fold by S-adenosylmethionine. S-adenosylmethionine-dependent protein phosphorylation is time- and dose-dependent and occurs at physiological concentrations. S-adenosylhomocysteine has no effect on protein phosphorylation but inhibits S-adenosylmethionine-dependent protein phosphorylation. S-Adenosylmethionine/S-adenosylhomocysteine ratios varying from 0 to 5 produce a dose-dependent stimulation of the phosphorylation of the 50 kDa protein. In conclusion, these results show, for the first time, that the ratio S-adenosylmethionine/S-adenosylhomocysteine can modulate phosphorylation of a specific protein.

Phospholipid methyltransferase phosphorylation by intact hepatocytes: effect of glucagon

We have obtained a rabbit antiserum that specifically immunoprecipitates the 50K and 25K proteins of rat liver phospholipid methyltransferase. Exposure of intact rat hepatocytes preincubated with [32P]phosphate to glucagon induces a time-dependent phosphorylation of the 50K protein of phospholipid methyltransferase. The incorporation of 32P into the 50K protein was only on phosphoserine. These data support the concept that the activation of rat liver phospholipid methyltransferase by glucagon is mediated by phosphorylation of the enzyme.

Activation of partially purified rat liver lipid methyltransferase by phosphorylation

Incubation of partially purified rat liver lipid methyltransferase with MgATP and the catalytic subunit of the cyclic AMP dependent protein kinase results in up to 4-fold activation of the methylation reaction. When (gamma-32p) MgATP is included in the assay mixture, the analysis of the phosphoprotein products by electrophoresis shows the incorporation of 32p into a single protein band of about 50K and pI 4.75. It is concluded that rat liver lipid methyltransferase can be converted from a low activity dephosphorylated form to a high activity phosphorylated form.