- Research article
- Open access
- Published:
Molecular mechanisms underlying the neural correlates of working memory
BMC Biology volume 22, Article number: 238 (2024)
Abstract
Background
Working memory (WM), a core component of executive functions, relies on a dedicated brain system that maintains and stores information in the short term. While extensive neuroimaging research has identified a distributed set of neural substrates relevant to WM, their underlying molecular mechanisms remain enigmatic. This study investigated the neural correlates of WM as well as their underlying molecular mechanisms.
Results
Our voxel-wise analyses of resting-state functional MRI data from 502 healthy young adults showed that better WM performance (higher accuracy and shorter reaction time of the 3-back task) was associated with lower functional connectivity density (FCD) in the left inferior temporal gyrus and higher FCD in the left anterior cingulate cortex. A combination of transcriptome-neuroimaging spatial correlation and the ensemble-based gene category enrichment analysis revealed that the identified neural correlates of WM were associated with expression of diverse gene categories involving important cortical components and their biological processes as well as sodium channels. Cross-region spatial correlation analyses demonstrated significant associations between the neural correlates of WM and a range of neurotransmitters including dopamine, glutamate, serotonin, and acetylcholine.
Conclusions
These findings may help to shed light on the molecular mechanisms underlying the neural correlates of WM.
Background
Working memory (WM), a core component of executive functions [1], refers to temporary storage and manipulation of the information necessary for complex cognitive tasks [2]. WM relies on a dedicated brain system that maintains and stores information in the short term [3]. Considerable effort in the last decades has been directed to investigating such brain system using two different yet complementary neuroimaging approaches, focusing on within-subject effects and between-subject differences respectively. The former examines an individual’s brain activation during WM tasks utilizing functional neuroimaging techniques and the activated brain regions are thought to be responsible for WM processes [4, 5]. The latter explores inter-individual variations in brain structure and function that are linked to inter-individual differences in WM performance by conducting neuroimaging-behavior correlation across subjects [6]. Taking advantage of these approaches, extensive research has identified a distributed set of neural substrates relevant to WM, consistently involving the medial and lateral prefrontal cortex, medial and lateral posterior parietal cortex, and anterior and posterior cingulate cortex [4, 5, 7,8,9,10,11,12,13,14,15]. Nevertheless, the molecular mechanisms (i.e., genetic architecture and neurochemical basis) underlying the neural correlates of WM remain enigmatic.
Resting-state functional magnetic resonance imaging (fMRI) technique has been widely adopted to assess the intrinsic functional architecture of the brain by examining spontaneous fluctuations in the blood-oxygen-level-dependent (BOLD) signal as a potentially important manifestation of spontaneous neuronal activity [16]. Broadly, resting-state fMRI measures can be categorized into local neural activity measures and functional connectivity (FC) measures. The former include amplitude of low-frequency fluctuations (ALFF), fractional amplitude of low-frequency fluctuations (fALFF), and regional homogeneity (ReHo), which describe the local functional features of a single region and cannot depict the relational characteristics between regions. Resting-state FC evaluates inter-regional correlations in spontaneous BOLD signal fluctuations [17] and has shown high reliability [18, 19] and heritability [20, 21]. Resting-state FC measures can be calculated using hypothesis-driven and data-driven approaches. Seed-based FC analysis represents a commonly used hypothesis-driven approach to mapping intrinsic brain connectivity networks [22,23,24]. Due to the fact that seed regions must be specified a priori, this method has lacked an independent view and thus may provide an incomplete picture of whole-brain FC profiling. Although data-driven independent component analysis (ICA) attempts to resolve the dependence on prior knowledge [25], it carries out subjective analysis of physiological signals and noises, which might lead to incorrect models and high residual errors. In contrast, functional connectivity density (FCD) has emerged as a reproducible data-driven, graph-theory method to construct whole-brain FC networks and analyze their nodal degree centrality at the voxel level [26,27,28,29], facilitating a better characterization of brain functional topological organization. Brain areas with higher FCD values are considered more densely interconnected hub regions that are of more importance for neural convergence and global information integration. The FCD method has been employed to identify abnormal functional hubs in neuropsychiatric disorders [30, 31] as well as the neural correlates of human cognitive domains including WM [32,33,34].
The recent introduction of comprehensive whole-brain gene expression atlases, such as the Allen Human Brain Atlas (AHBA) [35, 36], has given rise to the burgeoning field of imaging transcriptomics. Imaging transcriptomics is concerned with the identification of spatial correlations between gene expression patterns and neuroimaging phenotype profiles [37,38,39,40,41,42,43,44,45,46,47,48], commonly followed by further gene category enrichment analysis (GCEA) to determine the biological functions that contribute to such correlations with the use of gene-to-category annotation systems like the gene ontology (GO) [49]. However, traditional GCEA is often biased by gene co-expression and spatial auto-correlation. To address this concern, a flexible ensemble-based null model has recently been developed to enable more valid and interpretable inference of GCEA [50], which allows researchers to better investigate the genetic architecture of neuroimaging phenotypes. In parallel, the progress in nuclear imaging techniques and tracers has made it increasingly feasible to precisely and reliably quantify a set of neurotransmitter receptors and transporters across the whole brain [51,52,53,54,55]. These neurotransmitter atlases have offered us sufficient material to explore the neurochemical basis of brain structure and function [56]. Collectively, the current availability of brain-wide gene expression and neurotransmitter atlases along with the continuing methodological refinement could open new avenues to examine the spatial relations between these atlases and neuroimaging findings, which may yield an updated framework for understanding the potential molecular mechanisms underlying the neural correlates of WM.
Our purposes in the current work were twofold. Initially, we computed FCD using resting-state fMRI data to investigate their associations with WM performance as measured by a 3-back task across a large sample of 502 healthy young adults. Next, we investigated the spatial relations of the identified neural correlates of WM with gene expression and neurotransmitter atlases to examine their potential genetic architecture and neurochemical basis. Schematic representation of the research design and analytical procedure is provided in Fig. 1.
Research design and analytical procedure. Initially, we computed FCD using resting-state fMRI data to investigate their associations with WM performance as measured by a 3-back task across a large sample of 502 healthy young adults. Next, we investigated the spatial relations of the identified neural correlates of WM with gene expression and neurotransmitter atlases to examine their potential genetic architecture and neurochemical basis. Abbreviations: rs-fMRI, resting-state functional magnetic resonance imaging; FCD, functional connectivity density; AHBA, Allen Human Brain Atlas; WM, working memory
Results
Neural correlates of WM
Our voxel-wise analyses revealed significant correlations between WM and FCD across 502 healthy young adults (P < 0.05, cluster-level family-wise error [FWE] corrected). Specifically, there was a significant negative correlation between 3-back task accuracy and FCD in the left inferior temporal gyrus (cluster size = 91 voxels, peak Montreal Neurological Institute [MNI] coordinate: x = -48, y =  − 15, z =  − 36, peak t = -4.54, partial correlation coefficient [pr] =  − 0.232, P < 0.001) (Fig. 2A). In addition, we observed a significant negative correlation between 3-back task reaction time and FCD in the left anterior cingulate cortex (cluster size = 124 voxels, peak MNI coordinate: x =  − 3, y = 30, z = 24, peak t =  − 4.32, pr =  − 0.211, P < 0.001) (Fig. 2B).
Neural correlates of working memory. Correlations of FCD with 3-back task accuracy (A) and reaction time (B) across 502 healthy young adults. Left panel: brain regions with FCD in relation to 3-back task performance. Right panel: scatter plots of the corresponding correlations. Abbreviations: FCD, functional connectivity density; ITG, inferior temporal gyrus; ACC, anterior cingulate cortex; L, left; R, right
Gene categories associated with the neural correlates of WM
A combination of transcriptome-neuroimaging spatial correlation and the ensemble-based GCEA revealed that the neural correlates of WM were spatially associated with gene expression of diverse GO categories (Additional file 1: Table S1). Briefly, the neural correlates of 3-back task accuracy were mainly associated with ensheathment of neurons, axon ensheathment, axonogenesis, myelination, and sodium channel activity (Fig. 3A). The neural correlates of 3-back task reaction time were predominantly associated with neuron differentiation, postsynaptic signal transduction, regulation of neurotransmitter levels, presynapse, GABA-ergic synapse, ion channel complex, and channel activity (Fig. 3B).
Gene categories associated with the neural correlates of working memory. A combination of transcriptome-neuroimaging spatial correlation and the ensemble-based GCEA revealed that the neural correlates of 3-back task accuracy (A) and reaction time (B) were spatially associated with gene expression of diverse GO categories. Abbreviations: GCEA, gene category enrichment analysis; GO, gene ontology; BP, biological process; MF, molecular function; CC, cellular component
Neurotransmitters associated with the neural correlates of WM
Cross-region spatial correlation analyses demonstrated significant associations between the neural correlates of WM and specific neurotransmitters (permutation-based P < 0.05, Bonferroni corrected). Briefly, the neural correlates of 3-back task accuracy were positively associated with dopamine (D2_2: r = 0.267, P = 4 × 10−4) (Fig. 4A and Additional file 2: Table S2). The neural correlates of 3-back task reaction time were positively associated with glutamate (mGluR5_3: r = 0.337, P = 1 × 10−3), and negatively associated with dopamine (D2_2: r =  − 0.294, P = 2 × 10−4), serotonin (SERT_3: r =  − 0.354, P = 2 × 10−4) and acetylcholine (VAChT_3: r =  − 0.235, P = 1.2 × 10−3) (Fig. 4B and Additional file 3: Table S3).
Neurotransmitters associated with the neural correlates of working memory. Cross-region spatial correlations of neurotransmitters with the neural correlates of 3-back task accuracy (A) and reaction time (B). The outermost ring shows the names and maps of 27 neurotransmitter receptors/transporters. The second circle displays the neurotransmitter values across 210 cerebral cortical regions derived from the Human Brainnetome Atlas. The third circle displays the cross-region Pearson’s correlation coefficients between these neurotransmitter maps and the neural correlates of working memory, with the red (blue) color indicating the positive (negative) correlation coefficients and the column height indicating the magnitude of correlation coefficients. The innermost ring displays the permutation-based statistical significance of the spatial correlations, i.e., − log10(P), with the darker color indicating the lower P value; *P < 0.05, Bonferroni corrected. The t maps for the correlations between FCD and working memory performance lie in the center. Abbreviations: 5-HT, 5-hydroxytryptamine; CB1, cannabinoid type 1; D, dopamine; DAT, dopamine transporter; FDOPA, fluorodopa; GABAa, gamma-aminobutyric acid a; MOR, mu opioid receptor; NAT, noradrenaline transporter; SERT, serotonin transporter; VAChT, vesicular acetylcholine transporter; mGluR5, metabotropic glutamate type 5; FCD, functional connectivity density
Sensitivity analysis
To determine the effect of different differential stability (DS, a measure of consistent regional variation across donor brains) threshold selections, we used two other DS cutoff thresholds (top 40% and 60%) during the brain gene expression data processing to obtain normalized expression measures of 4010 and 6016 genes, respectively. By repeating the transcriptome-neuroimaging spatial correlation and the ensemble-based GCEA, we found substantial overlaps between the GO categories identified in the main and sensitivity analyses, with 40% corresponding to Additional File 4: Table S4 and 60% to Additional File 5: Table S5.
Discussion
This study investigated the neural correlates of WM using resting-state fMRI data from a large sample of healthy young adults, as well as their underlying molecular mechanisms using spatial correlations with gene expression and neurotransmitter atlases. Our data showed that better WM performance (higher accuracy and shorter reaction time of the 3-back task) was correlated with lower FCD in the left inferior temporal gyrus and higher FCD in the left anterior cingulate cortex. A combination of transcriptome-neuroimaging spatial correlation and the ensemble-based GCEA revealed that the identified neural correlates of WM were spatially associated with gene expression of diverse GO categories involving important cortical components and their biological processes as well as sodium channels. Cross-region spatial correlation analyses demonstrated significant associations between the neural correlates of WM and a range of neurotransmitters including dopamine, glutamate, serotonin, and acetylcholine. These findings are crucial not only for unraveling the mechanisms underlying WM processes but also for gaining insights into disorders characterized by WM deficits, such as attention deficit hyperactivity disorder, Alzheimer’s disease, and schizophrenia.
We found that 3-back task accuracy and reaction time were negatively correlated with FCD in the inferior temporal gyrus and anterior cingulate cortex respectively, indicating that better WM performance (higher accuracy and shorter reaction time) may rely on lower FCD in the inferior temporal gyrus and higher FCD in the anterior cingulate cortex. Previous functional neuroimaging meta-analyses have suggested WM task-evoked brain activation in a fronto-cingulo-parietal cognitive control network [9, 10]. Our observation of a link between better WM performance and higher FCD in the anterior cingulate cortex is congruent with these prior findings, raising the possibility that increased nodal centrality of the anterior cingulate cortex may reflect its strengthened role in coordinating the fronto-cingulo-parietal cognitive control network in response to WM tasks. It seems counter-intuitive that we observed an association between better WM performance and lower FCD in the inferior temporal gyrus that is outside the fronto-cingulo-parietal network. However, it is likely that brain regions contributing to individual differences in WM are not necessarily those that are directly implicated in WM processes. The inferior temporal gyrus has been frequently known to be involved in multiple cognitive functions including WM [57,58,59,60]. Although speculative, a potential explanation is that lower resting-state intrinsic activity in the inferior temporal gyrus can increase its ability to be recruited according to WM task demands, resulting in better performance. An alternative explanation is that lower FCD in the inferior temporal gyrus may be an epiphenomenon rather than a cause of better WM performance, i.e., more efficient communication and coordination in the fronto-cingulo-parietal network may come at the cost of reduced neural activity in regions outside this network. It is noteworthy that the identified neural correlates of WM were left-lateralized. This hemispheric lateralization is compatible with findings from many prior studies [61,62,63,64,65]. One possible explanation may be that the information being remembered in verbal WM tasks is language-related and thus there is a left-hemisphere dominance in the specific neural processes involved.
A combination of transcriptome-neuroimaging spatial correlation and the ensemble-based GCEA revealed that the neural correlates of WM were spatially associated with gene expression of diverse GO categories involving important cortical components and their biological processes as well as sodium channels. Neurons, axons, and synapses are important cortical components. These cortical components and their biological processes (e.g., ensheathment of neurons and axons, axonogenesis, myelination, neuron differentiation, and postsynaptic signal transduction) have been associated with WM [66,67,68,69,70,71,72,73,74,75,76,77]. Voltage-gated sodium channels are responsible for the generation and propagation of the action potential [78]. Lamotrigine, a use-dependent inhibitor of voltage-gated sodium channels, has been shown to enhance cortical function within the neural circuits subserving WM in patients with bipolar disorder [79], providing indirect evidence for the relation between WM and sodium channels. In addition, the identified GO categories included neurotransmitters, which echoes the following spatial correlation results with neurotransmitter atlases.
Cross-region spatial correlation analyses demonstrated significant associations between the neural correlates of WM and a range of neurotransmitters including dopamine, glutamate, serotonin, and acetylcholine. It is noteworthy that earlier literature has attempted to establish the links between rs-fMRI measures and neurotransmitters using invasive techniques such as positron emission tomography. Benefiting from publicly available neurotransmitter atlases, we could explore such links in a non-invasive way. The involvement of the dopamine system in WM processes is well acknowledged [80,81,82,83,84]. Animal research has shown that mice lacking dopamine receptors exhibit spatial WM deficits [85]. The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the brain [86]. Prior work has demonstrated effects of blocking mGluR5 on dorsolateral prefrontal cortical neuronal firing and WM performance [87]. The serotonin system derives mainly from neurons in the dorsal and ventral raphe nuclei with projections to virtually every brain region that subserves cognition. It is generally accepted that serotonin receptors are engaged in learning and memory, and represent highly favorable molecular targets for cognitive enhancement in disorders [88, 89]. There is solid evidence that acetylcholine receptors play a critical role in facilitating cognitive processes including WM, and cholinergic dysfunction has been associated with cognitive abnormalities in a variety of neurodegenerative and neuropsychiatric diseases [90,91,92,93]. Combined, our findings, taken with the previous reports, support the notion that WM is a complex cognitive function entailing multiple neurotransmitter systems that may work independently or synergistically with each other.
Several limitations are worth mentioning in the present study. First, given that our study sample was a group of educated healthy young adults, these findings might not be representative of the general population. Further investigations in participants with broader age and education ranges are needed to validate our results. Second, we did not perform distortion corrections during the preprocessing of fMRI data, which may have an impact on the BOLD signal. Third, it is not possible to make strong inferences regarding the direction of causality due to the correlative nature of the analyses. Fourth, our transcriptome-neuroimaging spatial correlation analyses only considered the tissue samples in the left cerebral cortex because of limited gene expression data in the right hemisphere and different gene expression profiles between cortical and subcortical regions. The reduced tissue samples along with hemisphere and region selections might introduce potential biases. Finally, the neural correlates of WM were derived from our resting-state fMRI data, while the gene expression and neurotransmitter atlases were obtained from publicly available datasets. Differences across individuals were ignored during the spatial correlation analyses, which may influence our interpretation.
Conclusions
In conclusion, our work demonstrated that better WM performance was correlated with hypoconnectivity in the inferior temporal gyrus and hyperconnectivity in the anterior cingulate cortex. Furthermore, these neural correlates of WM were potentially modulated by specific genetic architecture and neurochemical basis. Our findings may help to shed light on the molecular mechanisms underlying the neural correlates of WM.
Methods
Participants
A total of 502 healthy young adults were recruited by advertisement. All participants met the inclusion criteria of Chinese Han, right-handedness, and within a restricted age range of 18–30 years, which corresponds to a period after the completion of major neurodevelopment and before the onset of neurodegeneration. Exclusion criteria included neuropsychiatric or serious somatic disorders, a history of alcohol or drug abuse, regular smoking (i.e., total number of cigarettes > 20), current medication (e.g., sedative-hypnotics) within a month, pregnancy, MRI contraindications, and a family history of psychiatric illness among first-degree relatives. The MINI-International Neuropsychiatric Interview (M.I.N.I.) and Alcohol Use Disorders Identification Test (AUDIT) were used in the process of excluding participants. This study was conducted in accordance with the Declaration of Helsinki and was approved by the ethics committee of The First Affiliated Hospital of Anhui Medical University (20200094). Written informed consent was obtained from all participants after they had been given a complete description of the study. Detailed data of the sample are listed in Table 1.
Working memory assessment
The letter 3-back task was conducted on a computer to assess WM [9] using E-Prime 2.0 [94]. During the task, each participant viewed a series of letters that were presented sequentially and the presentation time of each letter stimulus was 200 ms with an inter-stimulus interval of 1800 ms. Participants were instructed to press a button on the right with their middle fingers if the letter that appeared on the screen was identical to the one presented 3 letters earlier, and otherwise to press a button on the left with their index fingers. The task consisted of 60 trials. Before the formal test, participants were verbally instructed and had a practice test to ensure that they understood the task. The accuracy and mean reaction time of correct responses were used as the indices of WM performance.
MRI data acquisition
MRI scans were obtained using a 3.0-Tesla MR system (Discovery MR750w, General Electric, Milwaukee, WI, USA) with a 24-channel head coil. Earplugs were used to reduce scanner noise, and tight but comfortable foam padding was used to minimize head motion. High-resolution 3D T1-weighted structural images were acquired by employing a brain volume (BRAVO) sequence with the following parameters: repetition time (TR) = 8.5 ms; echo time (TE) = 3.2 ms; inversion time (TI) = 450 ms; flip angle (FA) = 12°, field of view (FOV) = 256 mm × 256 mm; matrix size = 256 × 256; slice thickness = 1 mm, no gap; 188 sagittal slices; and acquisition time = 296 s. Resting-state BOLD fMRI data were acquired using a gradient-echo single-shot echo planar imaging (GRE-SS-EPI) sequence with the following parameters: TR = 2000 ms; TE = 30 ms; FA = 90°; FOV = 220 mm × 220 mm; matrix size = 64 × 64; slice thickness = 3 mm, slice gap = 1 mm; 35 interleaved axial slices; 185 volumes; and acquisition time = 370 s. Before the scanning, all subjects were instructed to keep their eyes closed, relax, move as little as possible, think of nothing in particular, and not fall asleep during the scans. During and after the scanning, we asked subjects whether they had fallen asleep to confirm that none of them had done so. All MR images were visually inspected to ensure that only images without visible artifacts were included in subsequent analyses.
fMRI data preprocessing
Resting-state BOLD data were preprocessed using Statistical Parametric Mapping (SPM12) [95] and Data Processing & Analysis for Brain Imaging (DPABI) [96, 97]. The first 10 volumes for each participant were discarded to allow the signal to reach equilibrium and the participants to adapt to the scanning noise. The remaining volumes were corrected for the acquisition time delay between slices. Then, realignment was performed to correct the motion between time points. Head motion parameters were computed by estimating the translation in each direction and the angular rotation on each axis for each volume. All participants’ BOLD data were within the defined motion thresholds (i.e., translational or rotational motion parameters less than 2 mm or 2°). We also calculated frame-wise displacement (FD), which indexes the volume-to-volume changes in head position. Several nuisance covariates (the linear drift, the estimated motion parameters based on the Friston-24 model, the spike volumes with FD > 0.5 mm, the white matter signal, and the cerebrospinal fluid signal) were regressed out from the data. Notably, we did not perform global signal regression since it is still a controversial topic in resting-state fMRI analysis [98]. The datasets were then band-pass filtered using a frequency range of 0.01–0.1 Hz. In the normalization step, individual structural images were firstly co-registered with the mean functional images; then the transformed structural images were segmented and normalized to the MNI space using a high-level nonlinear warping algorithm, that is, the diffeomorphic anatomical registration through the exponentiated Lie algebra (DARTEL) technique [99]. Finally, each filtered functional volume was spatially normalized to MNI space using the deformation parameters estimated during the above step and resampled into a 3-mm cubic voxel.
Functional connectivity density analysis
FCD was computed according to the method described by previous studies [26, 31, 100,101,102]. Pearson’s correlation coefficients were calculated between the BOLD time courses of all pairs of voxels and a whole-brain functional connectivity matrix was obtained for each participant. For a given voxel, FCD was defined as the number of functional connections with correlation coefficients above a threshold of 0.25 between that voxel and all other voxels within the whole brain. This threshold was chosen because it effectively filters out noise and weak connections while preserving significant ones, thereby enhancing the accuracy and reliability of the FCD analysis [31, 34, 103,104,105]. Then, we normalized the FCD value of each voxel by dividing it by the global mean FCD value. The resultant FCD maps were spatially smoothed with a 6 mm full-width at half maximum Gaussian kernel.
Correlation between WM and FCD
A voxel-wise approach was used to examine the correlations between WM and FCD across 502 healthy young adults. We used multiple regression model implemented in the SPM12 to identify any voxels in the FCD images that showed significant correlations with 3-back task performance (accuracy and reaction time) while controlling for potential confounders including age, sex, education, and FD. The statistical analysis yielded a t map, representing the correlations between WM and FCD. For the voxel-based analysis, multiple comparison correction was performed using the cluster-level FWE method, resulting in a cluster-defining threshold of P = 0.001 and a corrected cluster significance of P < 0.05.
Brain gene expression data processing
Brain gene expression data were acquired from the AHBA dataset [35, 106], which consists of six human post-mortem brains (Additional file 6: Table S6). The original expression data of more than 20,000 genes at 3702 spatially distinct brain tissue samples were processed following a newly proposed pipeline [37]. First, we updated the probe-to-gene annotations based on the latest information from the National Center for Biotechnology Information (NCBI) by means of the Re-Annotator toolkit [107]. Second, signal intensity filtration was conducted to exclude probes with signal intensity lower than background noise in at least 50% of the samples across all donors. Third, RNA-seq data were used to select the single probe that can represent each gene. Specifically, after excluding genes without RNA-seq measured expression values, we calculated Spearman’s correlations between microarray and RNA-seq measures. We set a threshold of r > 0.2 to select brain-relevant and reliably measured genes in accordance with current guidelines [37] in prior studies [39, 41, 47, 48, 108]. Next, the probe with the highest correlation to RNA-seq data was selected as the representative probe for a gene. Fourth, considering the limited number of tissue samples in the right hemisphere (only two donors) and substantial differences in expression patterns between cortical and subcortical regions, we focused our analysis on the left cerebral cortex [109]. Fifth, scaled robust sigmoid normalization was performed at the within-sample cross-gene and within-gene cross-sample levels to correct for donor-specific effects. Finally, genes with the top 50% highest DS were selected for the subsequent analysis. For one, prior research has reported that genes with higher DS demonstrate more conserved expression patterns and are enriched for brain-related biological functions [110]. For another, gene expression conservation across subjects is a prerequisite for transcriptome-neuroimaging spatial correlation analysis. After these processing procedures, we obtained normalized expression data of 5013 genes for 1280 tissue samples. Since our WM-FCD correlation analysis was performed within a gray matter mask derived from the Human Brainnetome Atlas [111], we further restricted our analyses to the samples within this mask, resulting in a final sample × gene matrix of 623 × 5013.
Correlation with gene expression
We employed transcriptome-neuroimaging spatial correlation and the newly developed ensemble-based GCEA to explore the genetic architecture underlying the neural correlates of WM. Specifically, we drew a spherical region (radius = 3 mm) centered at the MNI coordinate of a given brain tissue sample and extracted the average t-value of voxels within the sphere from the statistical t maps for the WM-FCD correlations. Then, Pearson’s correlation between gene expression and t-values across tissue samples was calculated in a gene-wise manner, yielding 5013 spatial correlation coefficients (henceforth referred to as gene scores). According to the Fulcher et al. study [50], we conducted neuroimaging-spatial ensemble-based GCEA for these gene scores in the following way. First, updated GO term hierarchy and annotation files were obtained from the GO [112] on 11th July 2022. Second, direct gene-to-category annotations were performed for the 5013 AHBA genes, and we restricted our analyses to GO categories with 10–200 annotations. Third, the gene scores were agglomerated at the level of GO categories as a mean score of genes annotated to each GO category. Fourth, 10,000 surrogate maps with spatial autocorrelation matching the t maps were generated using the BrainSMASH package [113], based on the spatial-lag model [114]. Null distributions (i.e., neuroimaging-spatial ensemble-based null model) of mean gene scores for each GO category were generated through spatial correlations between gene expression and the 10,000 spatial autocorrelation-preserving surrogate maps. Finally, statistical significance of a GO category was assessed by comparing the GO category score derived from the real data to the neuroimaging-spatial ensemble-based null. The significance threshold was set at two-sided P < 0.05 (i.e., higher or lower than the null).
Correlation with neurotransmitters
JuSpace is a useful tool allowing for spatial correlation analyses between cross-modal neuroimaging data [115, 116]. To determine the neurochemical basis underlying the neural correlates of WM, we employed JuSpace to examine the spatial correlations of the t maps with nuclear imaging-derived measures covering various neurotransmitter systems including dopamine, serotonin, glutamate, gamma-aminobutyric acid (GABA), acetylcholine, opioid, cannabinoid, noradrenaline, and fluorodopa (Additional file 7: Table S7) [51, 117,118,119,120,121,122,123,124,125,126,127,128,129,130,131]. Specifically, Pearson’s correlation coefficients between the t map and these neurotransmitter maps were computed across 210 cerebral cortical regions derived from the Human Brainnetome Atlas while adjusting for spatial autocorrelation and partial volume with the gray matter probability map. Exact P values were computed using spatial permutation-based null maps with 5000 permutations. Multiple comparisons were corrected using the Bonferroni method and a corrected P < 0.05 was considered significant.
Sensitivity analysis
We chose the genes with the top 50% highest DS to focus our analyses on genes with relatively more conserved expression patterns across six donors in the main analysis. Considering the possible impact of different DS thresholds, we repeated our analysis using two other DS cutoff thresholds (top 40% and 60%).
Data availability
All data generated or analyzed during this study are included in this published article, its supplementary information files, and publicly available repositories. We have submitted the resting-state fMRI data and the code to the Open Science Framework repository, where they are available for free access [132]. Brain gene expression data can be acquired from the AHBA dataset [35, 106] and the data for each brain are now available for download at https://human.brain-map.org/static/download under an open access mandate. Neurotransmitter atlases can be obtained from JuSpace [115, 116].
Abbreviations
- 5-HT:
-
5-Hydroxytryptamine
- ACC:
-
Anterior cingulate cortex
- AHBA:
-
Allen Human Brain Atlas
- ALFF:
-
Amplitude of low-frequency fluctuations
- AUDIT:
-
Alcohol Use Disorders Identification Test
- BOLD:
-
Blood-oxygen-level-dependent
- BP:
-
Biological process
- BRAVO:
-
Brain volume
- CB1:
-
Cannabinoid type 1
- CC:
-
Cellular component
- D:
-
Dopamine
- DARTEL:
-
Diffeomorphic anatomical registration through the exponentiated Lie algebra
- DAT:
-
Dopamine transporter
- DPABI:
-
Data Processing & Analysis for Brain Imaging
- DS:
-
Differential stability
- FA:
-
Flip angle
- FC:
-
Functional connectivity
- FCD:
-
Functional connectivity density
- FD:
-
Frame-wise displacement
- FDOPA:
-
Fluorodopa
- FOV:
-
Field of view
- FWE:
-
Family-wise error
- GABA:
-
Gamma-aminobutyric acid
- GABAa:
-
Gamma-aminobutyric acid a
- GCEA:
-
Gene category enrichment analysis
- GO:
-
Gene ontology
- GRE-SS-EPI:
-
Gradient-echo single-shot echo planar imaging
- ICA:
-
Independent component analysis
- ITG:
-
Inferior temporal gyrus
- L:
-
Left
- M.I.N.I.:
-
MINI-International Neuropsychiatric Interview
- MF:
-
Molecular function
- MNI:
-
Montreal Neurological Institute
- MOR:
-
Mu opioid receptor
- NAT:
-
Noradrenaline transporter
- NCBI:
-
National Center for Biotechnology Information
- PET:
-
Positron emission tomography
- R:
-
Right
- ReHo:
-
Regional homogeneity
- SD:
-
Standard deviation
- SERT:
-
Serotonin transporter
- SPECT:
-
Single photon emission computed tomography
- SPM:
-
Statistical Parametric Mapping
- TE:
-
Echo time
- TI:
-
Inversion time
- TR:
-
Repetition time
- VAChT:
-
Vesicular acetylcholine transporter
- WM:
-
Working memory
- fALFF:
-
Fractional amplitude of low-frequency fluctuations
- fMRI:
-
Resting-state functional magnetic resonance imaging
- mGluR5:
-
Metabotropic glutamate type 5
- mGluRs:
-
Metabotropic glutamate receptors
- pr :
-
Partial correlation coefficient
- rs-fMRI:
-
Resting-state functional magnetic resonance imaging
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Acknowledgements
We thank the Allen Institute for Brain Science founders and staff who supplied the brain expression data. We also thank the subjects who contributed to this study.
Funding
This work was supported by the National Natural Science Foundation of China (grant numbers: 82471952, 82371928, and 82071905), the Anhui Provincial Natural Science Foundation (grant number: 2308085MH277), the Scientific Research Key Project of Anhui Province Universities (grant number: 2022AH051135), the Scientific Research Foundation of Anhui Medical University (grant number: 2022xkj143), and the Postgraduate Innovation Research and Practice Program of Anhui Medical University (grant number: YJS20230012).
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Authors and Affiliations
Contributions
X.X.: conceptualization, methodology, software, formal analysis, investigation, data curation, visualization, writing—original draft. H.Z.: methodology, software, validation, investigation, data curation. Y.S.: methodology, software, validation, investigation, data curation. H.C.: software, validation, formal analysis, data curation. W.Z.: methodology, formal analysis, writing—original draft. J.T.: conceptualization, methodology, validation, resources, supervision, project administration. J.Z.: conceptualization, methodology, validation, resources, writing—review and editing, supervision, project administration. Y.Y.: conceptualization, methodology, validation, resources, writing—review and editing, supervision, funding acquisition. All authors read and approved the final manuscript.
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This study was conducted in accordance with the Declaration of Helsinki and was approved by the ethics committee of The First Affiliated Hospital of Anhui Medical University (20200094). Written informed consent was obtained from all participants after they had been given a complete description of the study.
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Not applicable.
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The authors declare that they have no competing interests.
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Supplementary Information
12915_2024_2039_MOESM1_ESM.xls
Additional file 1: Table S1. Gene categories associated with the neural correlates of working memory. This table presents all gene categories linked to the neural correlates of working memory at a threshold of 0.5. The sheet labeled "Accuracy" features the GO categories related to the neural correlates of 3-back task accuracy, while the "Reaction time" sheet includes the GO categories associated with the neural correlates of 3-back task reaction time. Available in.xls format.
12915_2024_2039_MOESM2_ESM.doc
Additional file 2: Table S2. Spatial associations between the neural correlates of 3-back task accuracy and neurotransmitters. This table outlines associations between the neural correlates of 3-back task accuracy and specific neurotransmitters. Available in.doc format.
12915_2024_2039_MOESM3_ESM.doc
Additional file 3: Table S3. Spatial associations between the neural correlates of 3-back task reaction time and neurotransmitters. This table outlines associations between the neural correlates of 3-back task reaction time and specific neurotransmitters. Available in.doc format.
12915_2024_2039_MOESM4_ESM.xls
Additional file 4: Table S4. Gene categories associated with the neural correlates of working memory at threshold 0.4. This table presents all gene categories linked to the neural correlates of working memory at a threshold of 0.4. The sheet labeled "Accuracy" features the GO categories related to the neural correlates of 3-back task accuracy, while the "Reaction time" sheet includes the GO categories associated with the neural correlates of 3-back task reaction time. Available in.xls format.
12915_2024_2039_MOESM5_ESM.xls
Additional file 5: Table S5. Gene categories associated with the neural correlates of working memory at a threshold of 0.6. This table presents all gene categories linked to the neural correlates of working memory at a threshold of 0.6. The sheet labeled "Accuracy" features the GO categories related to the neural correlates of 3-back task accuracy, while the "Reaction time" sheet includes the GO categories associated with the neural correlates of 3-back task reaction time. Available in.xls format.
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Additional file 6: Table S6. Demographic information of the six adult donors in the AHBA. This table provides the demographic information of six adult donors in the AHBA. Available in.doc format.
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Additional file 7: Table S7. Receptor/transporter maps. This table showcases the foundational information of all neurotransmitter maps provided by JuSpace. Available in.doc format.
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Xu, X., Zhao, H., Song, Y. et al. Molecular mechanisms underlying the neural correlates of working memory. BMC Biol 22, 238 (2024). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12915-024-02039-0
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DOI: https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12915-024-02039-0