Behavioral pattern separation is associated with neural and electrodermal correlates of context-dependent fear conditioning

Hippocampus-dependent pattern separation is considered as a relevant factor for context discrimination and might therefore impact the contextual modulation of conditioned fear. However, the association between pattern separation and context-dependent fear conditioning has not been investigated so far. In the current study, 72 healthy female students completed the Mnemonic Similarity Task, a measure of behavioral pattern separation, in addition to a context-dependent fear conditioning paradigm during functional magnetic resonance imaging. The paradigm included fear acquisition in context A and extinction training in context B on a first day, as well as retrieval testing of the fear and extinction memories in the safe context B (extinction recall) and a novel context C (fear renewal) one day later. Main outcome measures comprised skin conductance responses (SCRs) and blood oxygen level-dependent responses in brain regions of the fear and extinction circuit. Regarding retrieval testing, pattern separation did not correlate with extinction recall, but with stronger dorsal anterior cingulate cortex activation and conditioned SCRs (trend) during fear renewal, indicating a stronger retrieval of the fear memory trace. Our findings suggest that behavioral pattern separation ability seems to be important for context-dependent fear modulation, which is impaired in patients with posttraumatic stress disorder.

Cross-paradigm integration shows a common neural basis for aversive and appetitive conditioning

Sharing imaging data and comparing them across different psychological tasks is becoming increasingly possible as the open science movement advances. Such cross-paradigm integration has the potential to identify commonalities in findings that neighboring areas of study thought to be paradigm-specific. However, even the integration of research from closely related paradigms, such as aversive and appetitive classical conditioning is rare - even though qualitative comparisons already hint at how similar the 'fear network' and 'reward network' may be. We aimed to validate these theories by taking a multivariate approach to assess commonalities across paradigms empirically. Specifically, we quantified the similarity of an aversive conditioning pattern derived from meta-analysis to appetitive conditioning fMRI data. We tested pattern expression in three independent appetitive conditioning studies with 29, 76 and 38 participants each. During fMRI scanning, participants in each cohorts performed an appetitive conditioning task in which a CS+ was repeatedly rewarded with money and a CS- was never rewarded. The aversive pattern was highly similar to appetitive CS+ > CS- contrast maps across samples and variations of the appetitive conditioning paradigms. Moreover, the pattern distinguished the CS+ from the CS- with above-chance accuracy in every sample. These findings provide robust empirical evidence for an underlying neural system common to appetitive and aversive learning. We believe that this approach provides a way to empirically integrate the steadily growing body of fMRI findings across paradigms.

Individual cortisol response to acute stress influences neural processing of sexual cues

BACKGROUND AND AIMS

Problematic pornography use can be conceptualized as an impulse control disorder or alternatively as a behavioral addiction. Stress is an important trigger in addiction, but less is known about the neural effect of stress in problematic pornography use. Therefore, we aimed at investigating the effect of stress during the anticipation and viewing of sexually explicit material while considering person characteristics related to potentially being at risk for developing problematic pornography use.

METHODS

In an fMRI study (n = 157 men, age: mean = 25.46, SD = 4.11) we used a sexual incentive delay task. A social stress test was used to induce stress in half of the participants. Salivary cortisol was repeatedly measured and person characteristics were considered moderating the effects of cortisol response.

RESULTS

We found no group differences in the neural responses during the anticipation phase, but a higher reactivity to sexual stimuli in the dACC in the stress group. Acute stress activated a pronounced cortisol response, which positively correlated with neural activations in the reward system (NAcc, dACC) to sexual cues. Further, the individual time spent on pornography use moderated the effect of cortisol in some regions of the reward system (dACC, mOFC).

DISCUSSION AND CONCLUSIONS

Our results suggest that acute stress related increases in cortisol can enhance the incentive value of cues announcing sexual stimuli. This might explain why acute stress is considered a trigger of pornography use and relapse and why individual stress response might be a risk factor for developing a problematic pornography use.

Sexual incentive delay in the scanner: Sexual cue and reward processing, and links to problematic porn consumption and sexual motivation

BACKGROUND AND AIMS

The use of pornography, while unproblematic for the majority, can grow into addiction-like behavior which in its extreme form is labeled as compulsive sexual behavioral disorder in the ICD-11 (WHO, 2018). The aim of this study was to investigate the addiction-specific reactivity to cues in order to better understand underlying mechanisms in the development of this disorder.

METHODS

We have used an optimized Sexual Incentive Delay Task to study brain activity in reward associated brain areas during an anticipation phase (with cues predicting pornographic videos, control videos or no videos) and a corresponding delivery phase in healthy men. Correlations to indicators of problematic pornography use, the time spent on pornography use, and trait sexual motivation were analyzed.

RESULTS

The results of 74 men showed that reward-related brain areas (amygdala, dorsal cingulate cortex, orbitofrontal cortex, nucleus accumbens, thalamus, putamen, caudate nucleus, and insula) were significantly more activated by both the pornographic videos and the pornographic cues than by control videos and control cues, respectively. However, we found no relationship between these activations and indicators of problematic pornography use, time spent on pornography use, or with trait sexual motivation.

DISCUSSION AND CONCLUSIONS

The activity in reward-related brain areas to both visual sexual stimuli as well as cues indicates that optimization of the Sexual Incentive Delay Task was successful. Presumably, associations between reward-related brain activity and indicators for problematic or pathological pornography use might only occur in samples with increased levels and not in a rather healthy sample used in the present study.

Subjective reward value of visual sexual stimuli is coded in human striatum and orbitofrontal cortex

Human neuroimaging research suggests the existence of one core network for the subjective valuation of rewards, including the striatum and orbitofrontal cortex. However, there is little research on the neural representation of subjective reward values of visual sexual stimuli (VSS) and on the role of these subjective valuations in the development of related addictive behaviors. Here, we investigate how neural reactivity to VSS is connected to individual preference using functional magnetic resonance imaging (fMRI). During the fMRI scan, 72 men viewed different VSS film clips. Ratings regarding valence and sexual arousal were collected and used as parametric modulators in the fMRI analysis. Subjects also filled out questionnaires on self-reported symptoms of problematic pornography use (PPU). Firstly, we found that neural reactivity towards VSS clips in the nucleus accumbens, caudate nucleus and orbitofrontal cortex was positively correlated with individual ratings of the respective VSS in all subjects. Second, the strength of the association between neural activity and sexual arousal ratings was positively correlated with self-reported symptoms of PPU. The first result suggests a precise appraisal of VSS according to individual preferences in established reward valuation regions. Secondly, stronger neural differentiation based on preference in participants with more PPU symptoms indicates an increased importance of VSS/preference fit in these individuals. This heightened correspondence between individual liking and neural activity may facilitate PPU development by increased signaling of incentive salience, thus boosting motivation to seek out and respond to these preferred stimuli.

Relationship of sensation seeking with the neural correlates of appetitive conditioning

Previous research has linked sensation seeking with a heightened risk for drug abuse and other risk-taking behavior. As appetitive conditioning presents a model for the etiology and maintenance of addictive behavior, investigating sensation seeking in a classical conditioning paradigm might elucidate possible pathways toward addiction within this model. Furthermore, the theoretical concept underlying sensation seeking proposes a negative relationship between reward processing and sensation seeking in only moderately arousing situations, which has been neglected by previous research.

This study aimed to investigate this inverse relationship in moderately stimulating situations entailing reward processing using functional magnetic resonance imaging. Subjects (N = 38) participated in a classical conditioning paradigm in which a neutral stimulus (CS+) was repeatedly paired with a monetary reward, while another neutral stimulus (CS−) was not.

Imaging results revealed a negative relationship between sensation seeking and neural responses in the insula, amygdala and nucleus accumbens during the early phase and in the dorsal anterior cingulate cortex during the late phase of conditioning. These findings suggest reduced reward learning and consequently diminished processing of outcome expectancy in appetitive conditioning in subjects with high sensation seeking scores. The results are discussed with respect to clinical implications.

Amygdala and nucleus accumbens involvement in appetitive extinction

Extinction of appetitive conditioning is regarded as an important model for the treatment of psychiatric disorders like addiction. However, very few studies have investigated its neural correlates. Therefore, we investigated neural correlates of appetitive extinction in a large human sample including all genders (N = 76, 40 females) to replicate and extend results from a previous study. During differential appetitive conditioning, one stimulus (CS+) was paired with the chance to win a monetary reward, whereas another stimulus (CS−) was not. During appetitive extinction on the next day, neither the CS+ nor the CS− were reinforced. After successful acquisition of appetitive conditioning, the extinction phase elicited significant reductions of valence and arousal ratings toward the CS+ and a significant reduction in skin conductance responses to the CS+ from early to late extinction. On a neural level, early extinction showed significant differential (CS+ − CS−) activation in dACC and hippocampus, whereas involvement of the vACC and caudate nucleus did not replicate. The differential activation of amygdala and nucleus accumbens during late extinction was replicated, with the amygdala displaying significantly higher differential activation during the late phase of extinction as compared to the early phase of extinction. We show discernible signals for reward learning and extinction in subregions of amygdala and nucleus accumbens after extinction learning. This successful replication underlines the role of nucleus accumbens and amygdala in neural models of appetitive extinction in humans that was previously only based on animal findings.

Increased neural reactivity to emotional pictures in men with high hair testosterone concentrations

Testosterone has been linked to alterations in the activity of emotion neurocircuitry including amygdala, orbitofrontal cortex (OFC) and insula and diminished functional amygdala/prefrontal coupling. Such associations have only ever been studied using acute measures of testosterone, thus little is known about respective relationships with long-term testosterone secretion. Here, we examine associations between hair testosterone concentration (HTC), an index of long-term cumulative testosterone levels and neural reactivity during an emotional passive viewing task using functional magnetic resonance imaging (fMRI). Forty-six men viewed negative, positive and neutral pictures in the MRI. HTCs were assessed from 2 cm hair segments. The emotional paradigm elicited neural activation in the amygdala, insula and OFC. HTCs were associated with increased reactivity to negative pictures in the insula and increased reactivity to positive pictures in the OFC. We show an association of long-term testosterone levels with increased emotional reactivity in the brain. These results suggest a heightened emotional vigilance in individuals with high trait testosterone levels.

The relationship between neuroticism and appetitive conditioning

Appetitive conditioning is considered a central mechanism for the vulnerability to psychiatric disorders. However, the investigation of individual differences that are related to altered appetitive learning has been almost neglected so far. The aim of this study was to investigate the link between neuroticism and appetitive conditioning processes. 79 subjects participated in a differential conditioning procedure in which a conditioned stimulus (CS+) was paired with a reward (money) after a fast behavioral response, while a second conditioned stimulus (CS-) was never followed by a reward, irrespective of the behavioral response. As a main result, neuroticism correlated negatively with the underlying neural processes of appetitive conditioning in females, but not in males. In detail, higher levels of neuroticism were associated with decreased neural responses in the left (p = .001) and right amygdala (p = .011), left (p = .063) and right (p = .019) nucleus accumbens, and left (p = .002) and right (p = .021) orbitofrontal cortex (all results are family-wise-error-corrected). The present results support previous findings, which also showed an inverse sex-specific effect in the context of neuroticism and emotional processing in females. In addition, the findings suggest that neuroticism is not solely linked to increased amygdala sensitivity during the processing of negative stimuli but also to decreased neural responses when processing rewarding stimuli. Possible explanations for the sex differences and implications are discussed.

No Sex Difference Found: Cues of Sexual Stimuli Activate the Reward System in both Sexes

Sexually explicit material (SEM) is increasingly used in western societies. One reason for this high usage might be the rewarding property of SEM demonstrated in many brain imaging studies showing an activation of the reward system during the presentation of SEM. It is not yet well understood why women use SEM to a remarkably lesser extent than men. Maybe men react stronger to stimuli - so called SEM cues -, which signal the presentation of SEM and are therefore more vulnerable to use SEM than women. Therefore, the present study aimed at investigating the sex specific neural correlates towards SEM and SEM cues. We were further interested in whether person characteristics as trait sexual motivation, extent of SEM use in the last month, and age at onset of goal-oriented SEM use affect the neural responses to SEM and SEM cues. The trials of the fMRI experiment consisted of an expectation phase with SEM or neutral cues and a presentation phase with SEM or neutral stimuli, respectively. Analyses showed that the reward circuitry was activated by SEM, but also by SEM cues. There were some sex differences in hemodynamic responses to SEM during the presentation phase, but not during the expectation phase to SEM cues in any of the regions of interest. The influence of the investigated person characteristics was only small if existent. The results suggest that sex specific cue processing cannot explain sex differences in the use of SEM.

Failure to Replicate the Association Between Fractional Anisotropy and the Serotonin Transporter Gene (5-HTTLPR, rs25531)

Recent work underlines the importance of alterations in white matter (e.g., measured by fractional anisotropy (FA)) as a neural vulnerability marker for psychiatric disorders. In this context, the uncinate fasciculus (UF), which connects the limbic system with prefrontal areas, has repeatedly been linked to psychiatric disorders, fear processing, and anxiety-related traits. Individual differences in FA may partly be genetically determined. Variation in the promoter region of the serotonin transporter gene (serotonin transporter-linked polymorphic region [5-HTTLPR]) is a particularly promising candidate in this context, which has been linked to psychiatric disorders as well as to limbic and prefrontal reactivity. However, findings on the association between the 5-HTTLPR and FA within the UF-tract have been heterogeneous. The present study investigated this relationship and extended previous work by considering different genetic classification approaches as well as sex effects in a human sample (n = 114). All participants were genotyped for the 5-HTTLPR and the rs25531 polymorphism. As a main result, we did not find any significant relationship between the 5-HTTLPR and FA in the UF-tract although power analyses showed an adequate power. In addition, genotype effects were neither found when different classification approaches were used nor when analyses were carried out in males or females only. The present findings suggest that the association of the 5-HTTLPR and FA seems to be a more labile phenomenon than previously assumed. Possible explanations and limitations are discussed.

Neural correlates of subjective CS/UCS association in appetitive conditioning

Explicit knowledge of conditioned stimulus (CS)/unconditioned stimulus (UCS) associations is proposed as important factor in classical conditioning. However, while previous studies have focused on its roles in fear conditioning, it has been neglected in the context of appetitive conditioning. The present functional magnetic resonance study aimed to investigate neural activation and functional connectivity linked to subjective CS/UCS association in appetitive conditioning. In total, 85 subjects participated in an appetitive acquisition procedure in which a neutral stimulus (CS+) was paired with a monetary reward, while another neutral stimulus (CS-) was never paired with the reward. Directly afterwards, subjective CS/UCS association was assessed by measuring the extent to which the CS+ was thought to be associated with the UCS compared to the CS-. Close relationships were established between subjective CS/UCS association and activations in the primary visual cortex (V1) during the early phase of conditioning and in the striatum during the late conditioning phase. In addition, we observed inverse relationships between subjective CS/UCS association and both V1/ventromedial prefrontal cortex (vmPFC) and striatal/vmPFC connectivity. The results suggest the involvement of decoupling vmPFC connectivity in reward learning in general and the roles of attentional processes in the formation of the subjective CS/UCS association during the early phase and reward prediction during the late phase of appetitive conditioning.

Altered reward learning and hippocampal connectivity following psychosocial stress

Acute stress has a profound influence on learning, as has been demonstrated in verbal learning or fear conditioning. However, its effect on appetitive conditioning is still unclear. Fear conditioning research suggests the possibility of overgeneralization of conditioning to the CS- under acute stress due to its effect on prefrontal and hippocampal processing. In this study, participants (N = 56 males) were subjected to the Trier Social Stress Test or a placebo version. After that, all participants underwent an appetitive conditioning paradigm in the fMRI, in which one neutral cue (CS+) was repeatedly paired with reward, while another (CS-) was not. Importantly, the stress-group revealed overgeneralization of conditioning to the CS- on the behavioral level. On the neural level, stressed participants showed increased connectivity between the hippocampus and amygdala, vACC, and OFC, which maintain specificity of conditioning and also showed reduced differential activation. The results indicate overgeneralization of appetitive conditioning promoted by maladaptive balancing of pattern separation and pattern completion in the hippocampus under acute stress and are discussed with respect to clinical implications.

Neural correlates of appetitive extinction in humans

Appetitive extinction receives attention as an important model for the treatment of psychiatric disorders. However, in humans, its underlying neural correlates remain unknown. To close this gap, we investigated appetitive acquisition and extinction with fMRI in a 2-day monetary incentive delay paradigm. During appetitive conditioning, one stimulus (CS+) was paired with monetary reward, while another stimulus (CS−) was never rewarded. Twenty-four hours later, subjects underwent extinction, in which neither CS was reinforced. Appetitive conditioning elicited stronger skin conductance responses to the CS+ as compared with the CS−. Regarding subjective ratings, the CS+ was rated more pleasant and arousing than the CS− after conditioning. Furthermore, fMRI-results (CS+ − CS−) showed activation of the reward circuitry including amygdala, midbrain and striatal areas. During extinction, conditioned responses were successfully extinguished. In the early phase of extinction, we found a significant activation of the caudate, the hippocampus, the dorsal and ventral anterior cingulate cortex (dACC and vACC). In the late phase, we found significant activation of the nucleus accumbens (NAcc) and the amygdala. Correlational analyses with subjective ratings linked extinction success to the vACC and the NAcc, while associating the dACC with reduced extinction. The results reveal neural correlates of appetitive extinction in humans and extend assumptions from models for human extinction learning.

Don't fear 'fear conditioning': Methodological considerations for the design and analysis of studies on human fear acquisition, extinction, and return of fear

The so-called 'replicability crisis' has sparked methodological discussions in many areas of science in general, and in psychology in particular. This has led to recent endeavours to promote the transparency, rigour, and ultimately, replicability of research. Originating from this zeitgeist, the challenge to discuss critical issues on terminology, design, methods, and analysis considerations in fear conditioning research is taken up by this work, which involved representatives from fourteen of the major human fear conditioning laboratories in Europe. This compendium is intended to provide a basis for the development of a common procedural and terminology framework for the field of human fear conditioning. Whenever possible, we give general recommendations. When this is not feasible, we provide evidence-based guidance for methodological decisions on study design, outcome measures, and analyses. Importantly, this work is also intended to raise awareness and initiate discussions on crucial questions with respect to data collection, processing, statistical analyses, the impact of subtle procedural changes, and data reporting specifically tailored to the research on fear conditioning.

No evidence for blocking the return of fear by disrupting reconsolidation prior to extinction learning

Fear extinction is a central model for the treatment of anxiety disorders. Initial research has reported that the single presentation of a conditioned stimulus prior to extinction learning can permanently block the return of fear. However, only few studies have explored this issue and could not always replicate the findings. The present study examined human fear extinction using a four-day design. On the first day, two neutral stimuli were paired with electrical stimulation (UCS), while a third stimulus (CS-) was not. Twenty-four hours later, one conditioned stimulus (CS+rem) and the CS- were reminded once, 10 min before extinction learning, while the other conditioned stimulus (CS+non-rem) was not presented prior to extinction learning. All stimuli were presented during extinction learning and during two re-extinction sessions (24 h and 6-months after extinction learning) without reinforcement. Blood oxygen level-dependent (BOLD) responses and skin conductance responses (SCRs) to both CS+ and the CS- were explored during acquisition, extinction, and in both re-extinction sessions. Regarding SCRs, the results showed that a single presentation of a conditioned stimulus did not block the return of fear during re-extinction: Fear recovery during re-extinction (24 h and 6-months after extinction learning) was observed for both CS+ compared with the CS- with no difference between CS+rem and CS+non-rem. Regarding BOLD-responses, no significant differences between CS+rem and CS+non-rem were found in region of interest (ROI)-analyses (amygdala, ventromedial prefrontal cortex) during extinction learning and both re-extinction sessions. Whole-brain analyses showed increased BOLD-responses to the CS+non-rem as compared to the CS+rem in several regions (e.g., middle frontal gyrus) during extinction learning and re-extinction (24 h after extinction learning). The present findings suggest that the effect of preventing the return of fear by disrupting reconsolidation seems to be a more labile phenomenon than previously assumed. Possible boundary conditions and implications are discussed.

Altered Appetitive Conditioning and Neural Connectivity in Subjects With Compulsive Sexual Behavior

INTRODUCTION

There has been growing interest in a better understanding of the etiology of compulsive sexual behavior (CSB). It is assumed that facilitated appetitive conditioning might be an important mechanism for the development and maintenance of CSB, but no study thus far has investigated these processes.

AIM

To explore group differences in neural activity associated with appetitive conditioning and connectivity in subjects with CSB and a healthy control group.

METHODS

Two groups (20 subjects with CSB and 20 controls) were exposed to an appetitive conditioning paradigm during a functional magnetic resonance imaging experiment, in which a neutral stimulus (CS+) predicted visual sexual stimuli and a second stimulus (CS-) did not.

MAIN OUTCOME MEASURES

Blood oxygen level-dependent responses and psychophysiologic interaction.

RESULTS

As a main result, we found increased amygdala activity during appetitive conditioning for the CS+ vs the CS- and decreased coupling between the ventral striatum and prefrontal cortex in the CSB vs control group.

CONCLUSION

The findings show that neural correlates of appetitive conditioning and neural connectivity are altered in patients with CSB. The increased amygdala activation might reflect facilitated conditioning processes in patients with CSB. In addition, the observed decreased coupling could be interpreted as a marker for impaired emotion regulation success in this group.

Increased skin conductance responses and neural activity during fear conditioning are associated with a repressive coping style

The investigation of individual differences in coping styles in response to fear conditioning is an important issue for a better understanding of the etiology and treatment of psychiatric disorders. It has been assumed that an avoidant (repressive) coping style is characterized by increased emotion regulation efforts in context of fear stimuli as compared to a more vigilant coping style. However, no study so far has investigated the neural correlates of fear conditioning of repressors and sensitizers. In the present fMRI study, 76 participants were classified as repressors or as sensitizers and were exposed to a fear conditioning paradigm, in which the CS+ predicted electrical stimulation, while another neutral stimulus (CS-) did not. In addition, skin conductance responses (SCRs) were measured continuously. As the main findings, we found increased neural activity in repressors as compared to sensitizers in the ventromedial prefrontal cortex and the anterior cingulate cortex (ACC) during fear conditioning. In addition, elevated activity to the CS+ in amygdala, insula, occipital, and orbitofrontal cortex (OFC) as well as elevated conditioned SCRs were found in repressors. The present results demonstrate increased neural activations in structures linked to emotion down-regulation mechanisms like the ventromedial prefrontal cortex, which may reflect the increased coping effort in repressors. At the same time, repressors showed increased activations in arousal and evaluation-associated structures like the amygdala, the occipital cortex (OCC), and the OFC, which was mirrored in increased SCRs. The present results support recent assumptions about a two-process model of repression postulating a fast vigilant response to fear stimuli, and a second process associated with the down-regulation of emotional responses.

Impact of COMT Val158Met-polymorphism on appetitive conditioning and amygdala/prefrontal effective connectivity

Appetitive conditioning is an important mechanism for the development, maintenance, and treatment of psychiatric disorders like substance abuse. Therefore, it is important to identify genetic variations, which impact appetitive conditioning. It has been suggested that the Val(158) Met-polymorphism in the Catechol-O-Methyl-Transferase (COMT) is associated with the alteration of neural processes of appetitive conditioning due to the central role of the dopaminergic system in reward processing. However, no study has so far investigated the relationship between variations in the COMT Val(158) Met-polymorphism and appetitive conditioning. In this fMRI study, an appetitive conditioning paradigm was applied, in which one neutral stimulus (CS+) predicted appetitive stimuli (UCS) while a second neutral stimulus (CS-) was never paired with the UCS. As a main result, we observed a significant association between the COMT Val(158) Met-genotype and appetitive conditioning: skin conductance responses (SCRs) revealed a significant difference between CS+ and CS- in Val/Val-allele carriers but not in the other genotype groups. Val/Val-allele carriers showed increased hemodynamic responses in the amygdala compared with the Met/Met-allele group in the contrast CS+ > CS-. In addition, psychophysiological-interaction analysis revealed increased effective amygdala/ventromedial prefrontal cortex connectivity in Met/Met-allele carriers. The increased amygdala activity points to facilitated appetitive conditioning in Val/Val-allele carriers while the amygdala/prefrontal connectivity results could be regarded as a marker for altered emotion regulation during conditioning, which potentially impacts appetitive learning sensitivity. The SCRs finding indicates a stronger conditioned response in the Val/Val-allele group and dovetails with the neural differences between the groups. These findings contribute to the current research on COMT in emotional processing.

Functional analysis of three type-2 DGAT homologue genes for triacylglycerol production in the green microalga Chlamydomonas reinhardtii

Photosynthetic organisms like plants and algae can use sunlight to produce lipids as important metabolic compounds. Plant-derived triacylglycerols (TAGs) are valuable for human and animal nutrition because of their high energy content and are becoming increasingly important for the production of renewable biofuels. Acyl-CoA:diacylglycerol acyltransferases (DGATs) have been demonstrated to play an important role in the accumulation of TAG compounds in higher plants. DGAT homologue genes have been identified in the genome of the green alga Chlamydomonas reinhardtii, however their function in vivo is still unknown. In this work, the three most promising type-2 DGAT candidate genes potentially involved in TAG lipid accumulation (CrDGAT2a, b and c) were investigated by constructing overexpression strains. For each of the genes, three strains were identified which showed enhanced mRNA levels of between 1.7 and 29.1 times that of the wild type (wt). Total lipid contents, neutral lipids and fatty acid profiles were determined and showed that an enhanced mRNA expression level of the investigated DGAT genes did not boost the intracellular TAG accumulation or resulted in alterations of the fatty acid profiles compared to wild type during standard growth condition or during nitrogen or sulfur stress conditions. We conclude that biotechnological efforts to enhance cellular TAG amount in microalgae need further insights into the complex network of lipid biosynthesis to identify potential bottlenecks of neutral lipid production.

Light-induced short-term adaptation mechanisms under redox control in the PS II-LHCII supercomplex: LHC II state transitions and PS II repair cycle

Oxygenic photosynthesis takes place in the thylakoid membranes of cyanobacteria, algae and higher plants. While cyanobacteria have adapted to relatively constant environments, higher plants had to evolve mechanisms to adapt to continuous environmental changes. These include changes in light intensity, temperature and availability of water. One of the great challenges in plant cell biology is therefore to determine the regulatory mechanisms employed by higher plants and some algae to adapt to these constant environmental changes. The particular emphasis of this review is the description and characterisation of light-induced redox-controlled processes regulating the photosynthetic reactions, which involves maintaining maximal electron transport flow through the PS II-Cytb6f-PS I-F0F1ATPase electron transport chain and minimising light-induced oxidative damage to PS II which drives the highly oxidising water-splitting reaction. Two of the mechanisms involved in such short-term regulation processes are known as light harvesting complex II (LHC II) state transitions and photosystem II (PS II) repair cycle. They are followed by, and indeed may be a precondition in order to establish, the onset of the subsequent long-term mechanisms of regulation. In particular, the redox control of LHC II state transitions by reversible phosphorylation has been in the focus of many investigations, leading to many new results demonstrating the complexity of thylakoid-associated redox control mechanisms.

Ligation-mediated suppression-PCR as a powerful tool to analyse nuclear gene sequences in the green alga Chlamydomonas reinhardtii

To improve the analysis of unknown flanking DNA sequences adjacent to known sequences in nuclear genomes of photoautotrophic eukaryotic organisms, we established the technique of ligation-mediated suppression-PCR (LMS-PCR) in the green alga Chlamydomonas reinhardtii for (1) walking from a specific nuclear insertion fragment of random knockout mutants into the unknown flanking DNA sequence to identify and analyse disrupted genomic DNA regions and for (2) walking from highly conserved DNA regions derived from known gene iso-forms into flanking DNA sequences to identify new members of protein families. The feasibility of LMS-PCR for these applications was successfully demonstrated in two different approaches. The first resulted in the identification of a genomic DNA fragment flanking a nuclear insertion vector in a random knockout mutant whose phenotype was characterised by its inability to perform functional LHC state transitions. The second approach targeted the cab gene family. An oligonucleotide of a cabII gene, derived from a highly conserved region, was used to identify potential cab gene regions in the nuclear genome of Chlamydomonas. LMS-PCR combined with 3' rapid amplification of cDNA ends (3' RACE) and a PCR-based screening of a cDNA library resulted in the identification of the new cabII gene lhcb4. Both results clearly indicate that LMS-PCR is a powerful tool for the identification of flanking DNA sequences in the nuclear genome of Chlamydomonas reinhardtii.

Three-dimensional structure of Chlamydomonas reinhardtii and Synechococcus elongatus photosystem II complexes allows for comparison of their oxygen-evolving complex organization

Electron microscopy and single-particle analyses have been carried out on negatively stained photosystem II (PSII) complexes isolated from the green alga Chlamydomonas reinhardtii and the thermophilic cyanobacterium Synechococcus elongatus. The analyses have yielded three-dimensional structures at 30-A resolution. Biochemical analysis of the C. reinhardtii particle suggested it to be very similar to the light-harvesting complex II (LHCII).PSII supercomplex of spinach, a conclusion borne out by its three-dimensional structure. Not only was the C. reinhardtii LHCII.PSII supercomplex dimeric and of comparable size and shape to that of spinach, but the structural features for the extrinsic OEC subunits bound to the lumenal surface were also similar thus allowing identification of the PsbO, PsbP, and PsbQ OEC proteins. The particle isolated from S. elongatus was also dimeric and retained its OEC proteins, PsbO, PsbU, and PsbV (cytochrome c(550)), which were again visualized as protrusions on the lumenal surface of the complex. The overall size and shape of the cyanobacterial particle was similar to that of a PSII dimeric core complex isolated from spinach for which higher resolution structural data are known from electron crystallography. By building the higher resolution structural model into the projection maps it has been possible to relate the positioning of the OEC proteins of C. reinhardtii and S. elongatus with the underlying transmembrane helices of other major intrinsic subunits of the core complex, D1, D2, CP47, and CP43 proteins. It is concluded that the PsbO protein is located over the CP47 and D2 side of the reaction center core complex, whereas the PsbP/PsbQ and PsbV/PsbU are positioned over the lumenal surface of the N-terminal region of the D1 protein. However, the mass attributed to PsbV/PsbU seems to bridge across to the PsbO, whereas the PsbP/PsbQ proteins protrude out more from the lumenal surface. Nevertheless, within the resolution and quality of the data, the relative positions of the center of masses for OEC proteins of C. reinhardtii and S. elongatus are similar and consistent with those determined previously for the OEC proteins of spinach.

Phosphatidylglycerol is involved in the dimerization of photosystem II

Photosystem II core dimers (450 kDa) and monomers (230 kDa) consisting of CP47, CP43, the D1 and D2 proteins, the extrinsic 33-kDa subunit, and the low molecular weight polypeptides PsbE, PsbF, PsbH, PsbI, PsbK, PsbL, PsbTc, and PsbW were isolated by sucrose density gradient centrifugation. The photosystem II core dimers were treated with phospholipase A2 (PL-A2), which cuts phosphatidylglycerol (PG) and phosphatidylcholine molecules at the sn-2 position. The PL-A2-treated dimers dissociated into two core monomers and further, yielding a CP47-D1-D2 subcomplex and CP43. Thin layer chromatography showed that photosystem II dimers contained four times more PG than their monomeric counterparts but with similar levels of phosphatidylcholine. Consistent with this was the finding that, compared with monomers, the dimers contained a higher level of trans-hexadecanoic fatty acid (C16:1Delta3tr), which is specific to PG of the thylakoid membrane. Moreover, treatment of dimers with PL-A2 increased the free level of this fatty acid specific to PG compared with untreated dimers. Further evidence that PG is involved in stabilizing the dimeric state of photosystem II comes from reconstitution experiments. Using size exclusion chromatography, it was shown that PG containing C16:1Delta3tr, but not other lipid classes, induced significant dimerization of isolated photosystem II monomers. Moreover, this dimerization was observed by electron crystallography when monomers were reconstituted into thylakoid lipids containing PG. The unit cell parameters, p2 symmetry axis, and projection map of the reconstituted dimer was similar to that observed for two-dimensional crystals of the native dimer.

Mutation of residue threonine-2 of the D2 polypeptide and its effect on photosystem II function in Chlamydomonas reinhardtii

The D2 polypeptide of the photosystem II (PSII) complex in the green alga Chlamydomonas reinhardtii is thought to be reversibly phosphorylated. By analogy to higher plants, the phosphorylation site is likely to be at residue threonine-2 (Thr-2). We have investigated the role of D2 phosphorylation by constructing two mutants in which residue Thr-2 has been replaced by either alanine or serine. Both mutants grew photoautotrophically at wild-type rates, and noninvasive biophysical measurements, including the decay of chlorophyll fluorescence, the peak temperature of thermoluminescence bands, and rates of oxygen evolution, indicate little perturbation to electron transfer through the PSII complex. The susceptibility of mutant PSII to photoinactivation as measured by the light-induced loss of PSII activity in whole cells in the presence of the protein-synthesis inhibitors chloramphenicol or lincomycin was similar to that of wild type. These results indicate that phosphorylation at Thr-2 is not required for PSII function or for protection from photoinactivation. In control experiments the phosphorylation of D2 in wild-type C. reinhardtii was examined by 32P labeling in vivo and in vitro. No evidence for the phosphorylation of D2 in the wild type could be obtained. [14C]Acetate-labeling experiments in the presence of an inhibitor of cytoplasmic protein synthesis also failed to identify phosphorylated (D2.1) and nonphosphorylated (D2.2) forms of D2 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results suggest that the existence of D2 phosphorylation in C. reinhardtii is still in question.

Stabilization of photosystem two dimers by phosphorylation: implication for the regulation of the turnover of D1 protein

A general feature of many membrane protein complexes is that they have oligomeric organisation in vivo. Photosystem II (PSII) is one such example and the possible functional significance of this is explored in this work. Monomeric and dimeric forms of the core complex of PSII have been isolated from non-phosphorylated and phosphorylated thylakoid membranes prepared from spinach. These complexes had the same complement of proteins including, D1 (PsbA), D2 (PsbD), alpha-(PsbE) and beta-(PsbF) subunits of cytochrome b559, CP47 (PsbB), CP43 (PsbC), 33 kDa (PsbO) extrinsic protein and some other smaller subunits, such as PsbH, but did not contain Cab proteins. D1, D2, CP43 and PsbH were the phosphorylated components. Whether phosphorylated or not, the dimeric form of the PSII complex was more stable than the monomeric form. However, when treated with photoinhibitory light the isolated dimers converted to monomers in their non-phosphorylated state but not when phosphorylated. Phosphorylation, however, did not prevent photoinhibition as judged by the loss of oxygen evolving activity. A model is suggested for the role of PSII phosphorylation in controlling the conversion of dimeric PSII to its monomeric form and in this way regulate the rate of degradation of D1 protein during the photoinhibitory repair cycle.

The Role of Phosphatidylglycerol as a Functional Effector and Membrane Anchor of the D1-Core Peptide from Photosystem II-Particles of the Cyanobacterium Oscillatoria chalybea

The intrinsic polypeptide D1, isolated from photosystem (PS) Il-particles of the cyanobacterium Oscillatoria chalybea, was obtained by electroelution and fractionated extraction with organic solvents. Purification was demonstrated by Western blotting and amino acid sequenc­ing. By carrying out D1 -immunization in rabbits a polyclonal monospecific D1-antiserum was obtained.

For the qualitative characterization of D1 as a lipid-binding peptide, the effect of the lipids phosphatidylglycerol (PG), monogalactosyldiacylglyceride (MGDG) and phosphatidylcho­ line (PC) on PSII-oxygen evolution was analysed in reconstitution experiments. In these experiments purified photosystem II (PSII)-particle preparations were treated with the en­zyme phospholipase A2 and supplemented with lipid emulsions. We were able to show that the inhibition of electron transport, as the consequence of this lipase treatment, was only relieved, if phosphatidylglycerol was added to the preparation. A model was proposed, in which phosphatidylglycerol is a functional effector for the optimal conformation of D1 in the PSII core complex. Phosphatidylglycerol molecules are unusually tightly bound to the D1 peptide by hydrophobic interactions. A covalent binding seems not probable. The localisation of phosphatidylglycerol binding sites was found by trypsin treatment of D1 and analysis of the obtained oligopeptides with HPLC and immunoblotting. The binding sites could be confined to the hydrophobic amino acid section between arginine 27 and arginine 225, which is known to be the membrane anchor of D1. This has led us to the conclusion that the phospho­lipid phosphatidylglycerol plays an important role for anchoring the D1-peptide and for its orientation in the thylakoid membrane. Phosphatidylglycerol with its high amount of palmitic acid has in prokaryotic cyanobacteria apparently a role in stabilization and orientation.

The high turn-over of D1 and the spatial separation of the synthesis-and incorporation-site in the membrane, developed during evolution in eukaryotic organisms, might have changed the requirement on the mobility and the orientation of D1 in photosynthetic membranes.

The role of phosphatidylglycerol as a functional effector and membrane anchor of the D1-core peptide from photosystem II-particles of the cyanobacterium Oscillatoria chalybea

The intrinsic polypeptide D1, isolated from photosystem (PS) II-particles of the cyanobacterium Oscillatoria chalybea, was obtained by electroelution and fractionated extraction with organic solvents. Purification was demonstrated by Western blotting and amino acid sequencing. By carrying out D1-immunization in rabbits a polyclonal monospecific D1-antiserum was obtained. For the qualitative characterization of D1 as a lipid-binding peptide, the effect of the lipids phosphatidylglycerol (PG), monogalactosyldiacylglyceride (MGDG) and phosphatidylcholine (PC) on PSII-oxygen evolution was analysed in reconstitution experiments. In these experiments purified photosystem II (PSII)-particle preparations were treated with the enzyme phospholipase A2 and supplemented with lipid emulsions. We were able to show that the inhibition of electron transport, as the consequence of this lipase treatment, was only relieved, if phosphatidylglycerol was added to the preparation. A model was proposed, in which phosphatidylglycerol is a functional effector for the optimal conformation of D1 in the PSII core complex. Phosphatidylglycerol molecules are unusually tightly bound to the D1 peptide by hydrophobic interactions. A covalent binding seems not probable. The localisation of phosphatidylglycerol binding sites was found by trypsin treatment of D1 and analysis of the obtained oligopeptides with HPLC and immunoblotting. The binding sites could be confined to the hydrophobic amino acid section between arginine 27 and arginine 225, which is known to be the membrane anchor of D1. This has led us to the conclusion that the phospholipid phosphatidylglycerol plays an important role for anchoring the D1-peptide and for its orientation in the thylakoid membrane. Phosphatidylglycerol with its high amount of palmitic acid has in prokaryotic cyanobacteria apparently a role in stabilization and orientation. The high turn-over of D1 and the spatial separation of the synthesis- and incorporation-site in the membrane, developed during evolution in eukaryotic organisms, might have changed the requirement on the mobility and the orientation of D1 in photosynthetic membranes.

Phosphatidylglycerol and ß-Carotene Bound onto the D 1-Core Peptide of Photosystem II in the Filamentous Cyanobacterium

Photosystem II-particles from the cyanobacterium Oscillatoria chalybea were isolated by fractionating centrifugation. Purification of these particles was achieved by a 22 hours centrifugation over a linear sucrose density gradient at 217.500xg. The obtained particle fraction exhibited an oxygen evolution activity which corresponded to three times the rate of intact cells and to five times the rate of intact thylakoids. The chlorophyll protein ratio was 1:10 and the ratio manganese/chlorophyll 1:34.

SDS-polyacrylamide gel electrophoresis showed that the photosystem Il-fraction is composed of the core peptides D1 and D2, the chlorophyll-binding peptides CP 43 and CP 47, the extrinsic 33 kDa peptide (manganese stabilizing peptide, MSP) and phycobiliproteins with molecular masses between 16 to 20 kDa. Cyt b559 was not detected in our gel electrophoresis assay. Part of the peptides of the 30 kDa-region (D1, D2, MSP) occurred as aggregates with a molecular mass of 60 to 66 kDa.

The D 1-peptide was isolated from the PS Il-preparation by SDS-gel electrophoresis. The intrinisic peptide reacts in the Western blot procedure with the antiserum to phosphatidylglycerol and with the antiserum to β-carotene. Incubation of the peptide with the antisera to monogalactosyldiglyceride, sulfoquinovosyldiglyceride and zeaxanthine resulted negatively. The binding of phosphatidylglycerol onto the D 1-peptide was confirmed by lipid analysis in HPLC and fatty acid analysis by gas chromatography. Only this lipid, respectively the typical fatty acid mixture of this lipid was detected. The lipid is characterized by the fact that the hexadecenoic acid does not exhibit trans-configuration, as is true for phosphatidylglycerol of higher plants and algae, but occurs in cis-configuration.

With the antibody being directed towards the glycerol-phosphate residue and not towards the fatty acids, it can be concluded from the reaction of the antibodies with the bound lipid that the lipid is bound to the peptide via the fatty acid. The negatively charged phosphatidylglycerol increases the hydrophobicity of the peptide and leads to a negatively charged surface favouring binding of cations like calcium and magnesium. The fact that incubation of this PS Il-fraction with phospholipase inhibits photosynthetic activity by 25% which can be fully restored by addition of phosphatidylglycerol, shows that bound phosphatidylglycerol has a functional role.