Introduction
The hippocampus, as a structure playing a key role in cognitive processes, is known to remain central to the understanding of the Alzheimer’s disease (AD) pathophysiology with sensitivity to the neurofibrillary tangle development and a strong association with progression to AD [
1‐
3]. It is widely recognized that the hippocampus is heterogeneous and can be divided into subregions with different functions and vulnerabilities to neurodegenerative diseases [
4‐
6]. While the hippocampal subregions are thought to exhibit distinct functions, the neural substrate for aberrant functionalities remains elusive.
Several imaging studies have identified and investigated hippocampal subregions with functional MRI (fMRI) in healthy young and aging human brain [
7,
8]. Using in vivo MRI, the hippocampus can be divided into three subregions: cornu ammonis (CA1), CA2/3/dentate gyrus (CA2/3/DG), and subiculum [
9]. Resting-state fMRI studies showed that the disrupted total hippocampal connectivity [
10], right CA1 and left CA2 subregions connectivity [
11], and subiculum network (functional connectivity with frontal and posterior cingulate cortex [PCC] regions) [
12] in mild cognitive impairment (MCI) and AD patients were strongly associated with memory impairment [
5,
13,
14]. In addition to fMRI, the fluorine 18 (
18F) fluorodeoxyglucose (
18F-FDG) PET showed that the glucose metabolism of the left hippocampal body CA2/3 and CA4/DG subregions was significantly lower in the early AD group than in the control group [
15]. In MCI patients, we found that the left hippocampal CA2 functional connectivity measured by resting-state fMRI was associated with decreased dorsal raphe nuclei binding potential measured by [
11C]DASB PET [
16].
The previous structural MRI, fMRI, and
18F-FDG PET studies have reported reduced volume, disrupted intrinsic activity, and hypometabolism of hippocampus and hippocampal subregions in both AD and MCI patients [
11,
12,
15,
17,
18]. However, there is still a lack of systematic examinations of the relationship between AD pathology in the hippocampal subregions and cognitive performance. More importantly, few have related intrinsic activity and metabolism based on the detailed subregional analyses on hippocampal. While hippocampus is widely recognized for its subregions with distinct functions [
6], the localization of pathologies would indicate the specific roles of hippocampal subregions in symptomatology. Examining the relationship between intrinsic activity and metabolism within hippocampal subregions may also provide insight into disease pathogenesis.
Hybrid PET/MRI simultaneously evaluates resting-state brain structure, intrinsic activity, and glucose metabolism, which would provide optimal spatial and temporal registration of both modalities and clarify how neuronal function is impaired and contributes to the mechanisms underlying AD [
19]. We hypothesized that different subregions have various contributions to the functionalities of hippocampus, and we aimed to investigate the aberrant of hippocampal subregions in MCI and AD patients regarding functional connectivity and metabolism using hybrid PET/MRI.
Discussion
To the best of our knowledge, this is the first study to evaluate subregional hippocampal resting-state brain functional connectivity and glucose metabolism in AD study using simultaneous PET/fMRI. Hybrid PET/MRI is capable of simultaneous evaluating resting-state intrinsic activity, glucose metabolism, and gray matter volume, which could provide evidence for better understanding the neurodegenerative mechanisms underlying AD. Our hippocampal subregion-based analysis on simultaneous 18F-FDG PET/fMRI demonstrated that patient groups had significantly reduced functional connectivity and 18F-FDG SUVR in comparison with NC in most of the hippocampal subregions. Specifically, we found a negative correlation between the decreased left CA2/3DG-SMFG connectivity and local 18F-FDG PET hypometabolism in AD patients. In addition, we observed that the right CA1-precuneus connectivity was associated with cognitive impairment in participants with MCI.
Our inter-group subregion-based analysis on fMRI showed that all hippocampal subregions in both patient groups (AD and MCI) had decreased functional connectivity comparing to NC, and CA1 subregion could potentially serve as a major imaging indicator differentiating AD and MCI patients. A recently published study analyzed specific functional connectivity of CA1, subiculum, and CA2/3/4/DG using healthy elderly and their changes in patients with amnestic MCI [
12]. The authors found a significant reduced connectivity within the subiculum network (with frontal cortex and PCC) in amnestic MCI patients. Our current findings are partly in line with this result, showing that the individuals with MCI had reduced connectivity between subiculum and PCC, right MOG, superior occipital gyrus, and cuneus. Besides, our study also demonstrated that the hippocampus, CA1, and CA2/3/DG had reduced connectivity in patient groups compared with NC, mainly in the frontal cortex including SMFG, SFG, precuneus, PCC, etc. Using the whole hippocampus as a seed, we showed that the right whole hippocampus-medial prefrontal cortex, cingulate cortex, right cuneus extending into precuneus, left cuneus, and PCC functional connectivity were disrupted [
18]. More specifically, as shown in Fig.
3a, CA1 was the only subregion exhibiting group differences between AD and MCI patients. The functional connectivity of CA1 decreased in patients with AD mainly in the frontal cortex and precuneus areas, which are the hubs of the default mode network. These results concurred well with a neuroimaging study that showed reduced hippocampal functional connectivity in AD [
33] and also confirmed our earlier study of resting-state fMRI functional connectivity using the total hippocampus as a seed [
34].
Our hippocampal subregional analysis on
18F-FDG PET metabolism showed that both patient groups possessed reduced SUVR in hippocampal subregions (AD < MCI < NC), and the decreased left CA2/3/DG-SMFG connectivity in AD compared to NC group exhibited a negative correlation with the left CA2/3/DG
18F-FDG PET metabolism in AD patients. A recent study of Choi et al. focused on comparing the glucose metabolism of hippocampal subregions in mild-AD patients and healthy controls [
15]. Their results revealed that the reductions in metabolic activity were found varying along the hippocampal axis in early-stage AD patients. When considering the hippocampal body as an entire structure, there was significantly lower glucose metabolism in the AD group than that of control group just in the left CA2/3 and CA4/DG. Apart from the subregions specified by Choi et al., our results showed that all of the hippocampal subregions had reduced metabolism except for the right subiculum, which was supported by our volume results. The subiculum gray matter volume reduction in participants with MCI and AD is not consistent in the previous studies [
35‐
37]. Although we found a trend of subiculum gray matter volume reduction in participants with MCI and AD as compared to healthy controls (
P = 0.5 and
P = 0.1, respectively) which is consistent with previous amnestic MCI study [
37], the reduction of subiculum volume still required further investigation and validation. The novelty of our current study is the relationship between metabolism and strength of hippocampal subregions functional connectivity to remote brain regions in AD patients, which supports the hippocampus disconnection hypothesis, i.e., uncoupling of hippocampus from cortical inputs system may contribute to disinhibition like changes of intrahippocampal activity [
38,
39]. As shown in Fig.
5b, the association between left CA2/3/DG-SMFG connectivity and
18F-FDG PET metabolism revealed a negative correlation and was specific for AD patients, without the presence in neither MCI patients nor NC participants. Contrary to a previous finding suggesting no relationship between functional connectivity and
18F-FDG PET metabolism in the posterior default mode network in the AD group [
40], our results supported a recent evidence from a task fMRI study in humans indicating that greater hippocampal activation in amnestic MCI localized in the CA3/DG region [
37], suggesting similar neural dysfunction. A therapeutic study in amnestic MCI showed that levetiracetam could reduce CA3/DG activation and improve cognitive function [
41], indicating that targeting excess hippocampal activity has therapeutic potential. Overall, by integrating
18F-FDG PET and fMRI data, we found a robust relationship between left CA2/3/DG-SMFG connectivity and local
18F-FDG SUVR in AD patients.
As for the correlation analysis of hippocampal subregions and MMSE, we found that the right CA1-precuneus and right subiculum-SMFG connectivity were positively correlated with cognitive impairment in MCI and AD patients, respectively. A previous animal research study showing that the pattern of CA1 neuron activation indicated cognitive features [
42]. In patients with AD, the CA1 region was severely affected by neuron number and neurofibrillary tangle, which were significantly related to CDR scores [
43]. A recent autopsy study also found that the degree of Lewy body pathology in CA1, but not CA2, predicted pre-mortem episodic memory impairment in patients with Lewy body dementia and Parkinson’s disease [
5,
44]. Their findings suggested that CA1 might be more functionally relevant than CA2 and subiculum regions in memory impairment. These results supported the view that excess hippocampal activation directly contributed to the cognitive decline in prodromal AD [
41,
45].
The partial volume effects on the subregional hippocampal neurodegeneration measurements by PET were minimized by voxelwise PVC. As demonstrated by Fig.
1 and Table
4, the statistical significance levels were improved by PET with PVC in testing the ROI FDG SUVR differences between AD and MCI groups in left and right hippocampus, CA2/3/DG, and right CA1. This is consistent with our previous Alzheimer’s Disease Neuroimaging Initiative studies in small ROIs including amygdala and entorhinal cortex, where the PET spatial resolution of 8-mm FWHM [
28,
29]. In the study, we also performed RBV (region-based voxelwise)-RVC (reblurred Van Cittert iteration) PVC algorithm [
46]. The statistical analysis based on PET with RBV-RVC PVC did not change any conclusions from RVC-based analysis (not shown). Note that the subregional hippocampal neurodegeneration measurements by
18F-FDG PET are consistent with the ones from high-resolution T1-weighted MRI (Figs.
2,
5, and
6). We also confirmed that the subregional hippocampal functional activities from the spatially smoothed fMRI (Gaussian 3D smooth filter with FWHM = 4 mm, see “
Method” section) are separable with 3-T MRI scanners [
16]. We realized that the biological AD definition was recently proposed by the National Institute on Aging and Alzheimer’s Association for the A (Aβ)-T (tau)-N (neuro-degeneration) research framework [
47]. We will have AT(N) CSF or imaging measurements in future study to minimize AD pathology biases.
Table 4
Statistical significance of group difference in subregional hippocampal 18F-FDG SUVRs
Hippocampus | 0.002 | 0.010 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
L Hippocampus | 0.010 | 0.023 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
R Hippocampus | < 0.001 | 0.008 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
CA1 | 0.016 | 0.028 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
L CA1 | 0.171 | 0.110 | < 0.001 | < 0.001 | 0.002 | < 0.001 |
R CA1 | 0.002 | 0.011 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
CA2/3/DG | < 0.001 | 0.005 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
L CA2/3/DG | 0.003 | 0.008 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
R CA2/3/DG | < 0.001 | 0.007 | < 0.001 | < 0.001 | < 0.001 | < 0.001 |
Subiculum | 0.598 | 0.345 | 0.018 | 0.002 | 0.032 | 0.010 |
L Subiculum | 0.483 | 0.246 | 0.004 | < 0.001 | 0.012 | 0.005 |
R Subiculum | 0.971 | 0.575 | 0.1765 | 0.065 | 0.126 | 0.093 |
In summary, the subregional hippocampal level analysis revealed hypometabolism, lower gray matter volume, aberrant functional connectivity, and their relationship in MCI and AD patients. In addition, the right CA1-precauneus connectivity was related to the cognition in MCI patients, and the left CA2/3/DG functional connectivity was correlated to hypometabolism in AD patients. Our findings demonstrate that the associations existed at subregional hippocampal level between the functional connectivity and neurodegeneration measured by simultaneous PET/MRI.
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