Using high-resolution functional MRI in conjunction with computational cortical unfolding techniques, we investigated changes in activity within MTL subregions of cognitively intact, APOE-4 carriers and non-carriers. Analysis of activity during a cognitively challenging, verbal paired associates task revealed significant differences between groups in the left CA2, 3 and dentate gyrus.
It is important to emphasize that these early changes in activity are not accompanied by any memory decline, as evidenced by the scores on the neuropsychological test battery; all subjects were cognitively intact at the time of testing with no measurable memory impairment. These results extend previous findings of widespread increases in cortical activity in cognitively normal APOE-4 carriers compared to non-carriers. In this study, we investigated hippocampal subregions by utilizing a higher resolution technique than previously employed allowing for the detection of activity within separate hippocampal subregions. However, this high-resolution limits our field of view to the medial temporal structures and so we are unable to detect cortical changes in activity within our sample. Since we used the identical verbal paired associates paradigm to that of Bookheimer et al. (2000)
we would expect to find similar cortical patterns of increases in activity in APOE-4 carriers compared to non-carriers. The previous study (Bookheimer et al. 2000
) averaged across all MTL regions (hippocampus, ERC, PRC, PHC) and fusiform gyrus, and saw increased activation in APOE-4 carriers. The previous study used a lower resolution and larger field of view. The signal was likely dominated by extra-hippocampal regions, which may have masked subtle decreases within hippocampal subregions. Furthermore, in the current study due to the limited field of view we did not cover the very posterior medial temporal regions, which may have dominated the signal in the previous study. Thus it is difficult to compare the studies directly. Future studies using both whole-brain and high-resolution methods within the same participants will be needed to investigate the specific balance of activational increases and decreases present within the APOE-4 carriers, who are at risk for future cognitive decline.
Taken together, these results suggest that cognitive compensation wherein subjects use additional extrahippocampal regions to bring their memory-related performance to a normal level during early pathological AD changes may result from decreases in hippocampal CA23DG activity. Investigating the subregional activational changes between the two genetic groups suggests a possible relationship between a reduction in activity within areas first targeted by the disease (hippocampus) and compensatory increase in activity within spared cortical areas.
Due to the small and convoluted nature of the MTL subregions, visualization and localization of activity has proven to be a rather challenging task, making it difficult to detect subtle changes in activity without sufficient resolution. In order to detect such activity changes, previous studies commonly average activity patterns across subjects, thereby increasing signal detection. However, this may result in uncertainty in signal localization, as the hippocampus is increasingly vulnerable to misregistration between subjects. Therefore, previous studies demonstrating an increase in MTL activity within cognitively normal APOE-4 carriers through averaging across subregions may have masked subtle reductions in activity. Additionally, because subregions of the hippocampus serve functionally distinct and even opposite roles within the MTL, averaging across subregions would result in a misrepresentative signal. In fact, in the current study when we average across all hippocampal subregions (CA23DG, CA1 and subiculum) we lose the ability to detect significant group differences in activity. Here we deal with these challenges by using high-resolution functional MRI to detect subtle changes within hippocampal subregions to investigate functional changes associated with genetic risk for AD.
CA3 and dentate gyrus both receive synaptic input from the ERC (Amaral, 1990
). Within the ERC, there is substantial evidence of cell loss and synaptic pathology apparent in even the earliest stages of AD (Gomez-Isla et al., 1996
; Masiliah et al 1994
). Gomez and colleagues (1996)
have shown that single layers show severe atrophy very early in the progression of the disease; compared to controls, subjects with mild AD showed a 60% and 40% reduction in layers II and IV of the ERC, respectively. A recent study utilizing high-resolution MRI found APOE-4 effects on ERC within cognitively normal, APOE-4 carriers (Burggren et al., 2008
). We have replicated these findings in the current study using this novel cortical thickness measurement, which Burggren and colleagues (2008)
have shown to possibly be more sensitive to subtle differences in structure than volume measurements. We find reduced thickness in subiculum and ERC in APOE-4 carriers compared with non-carriers.
The blood-oxygenated-level-dependent (BOLD) signal has been shown to reflect both local field potentials in addition to neural firing rate (Logothetis et al. 2001
). Within the hippocampus a recent study suggested the BOLD signal reflects local field potential (synaptic activity; Ekstrom et al., 2009
). Therefore, since local field potentials contribute to the BOLD measurement, one possible interpretation of our findings could be that the activity increases are reflective of synaptic input from areas projecting to the hippocampus such as the ERC. It is possible that reductions in CA23DG BOLD activity in APOE-4 carriers compared with non-carriers reflect synaptic loss from ERC; however future studies are required to test this hypothesis. Additionally, CA1 region of the hippocampus receives input both from the CA3 pyramidal cells and the entorhinal cortex (Witter et al., 2000) and should thus be similarly affected. We do not see significant group differences within this region, although there is a trend showing decreased activity in APOE-4 carriers compared with non-carriers (, panel B). It may be the case that this particular cognitive task does not engage CA1 to the same extent as CA23DG making it more difficult to detect group differences. Future studies may find specific APOE differences in this region when using a task specific to CA1 (e.g., allocentric spatial encoding; Suthana et al. 2009
We do not detect cortical thickness differences within CA23DG associated with APOE-4. However, it could be that gross structural changes occur later in the progression of the disease and that our functional differences occur prior to them. Animal and autopsy studies show structural differences in post-mortem CA3 and dentate and autopsies studies show structural changes in post-mortem CA3 and dentate gyrus subregions of the hippocampus associated with the APOE-4 allele (Cambon et al., 2000
; Ji et al., 2003
). These structural changes occurring at the level of neurons may be undetectable with the current in vivo resolution of human imaging techniques. However, A recent high-resolution MRI study detected reduced CA3 and dentate gyrus volume in older carriers (mean age: APOE-3 = 71.29; APOE-4 = 69.2) of the APOE-4 allele; however, this effect was not observed in younger carriers (Mueller et al., 2008
). In this study, our groups’ average ages were 60.3 and 61.9 for APOE-3 and APOE-4 respectively. Therefore, it may be that functional changes in CA23DG are detectable prior to significant structural changes measurable with current techniques.
Previous studies have found interaction effects between APOE4 and family history outside of the hippocampus during a face recognition task (Xu et al. 2009). In our study, we did not find an interaction between APOE4 and family history in the CA3DG region of the hippocampus. However, our field of view limited us from reliably investigating other brain regions. Future studies are required to investigate the effects of family history on APOE more extensively.
Previous studies have shown correlations between changes in activity and cognitive decline two years later (Bookheimer et al., 2000
). In our study, there is no way to know which of our subjects, if any, will develop AD symptoms. Therefore, longitudinal studies are required to determine whether effects of APOE-4 on CA23DG prior to symptoms of AD are associated with future cognitive deficits.
With the combination of high-resolution functional MRI and computational cortical unfolding techniques, we have demonstrated the feasibility of examining neural activity in subregions of the hippocampus, thus providing a more complete picture of the pattern of activational changes seen within AD at-risk individuals. We show a marked reduction in activity within the CA2, 3 and dentate gyrus regions of the hippocampus in older asymptomatic carriers of the APOE ε4 allele. These results support the use of these advanced technologies to study older, cognitively intact, genetically at-risk subjects already demonstrating pre-clinical AD changes, measurable by neuroimaging methods and thereby increasing the chance of identifying diseased individuals during the prodromal phase. These methods also hold the promise of identifying risk factors for brain changes reminiscent of AD before the onset of clinical symptoms and of further investigating risk factors beyond the APOE-4 allele. Overall, using advanced neuroimaging technologies in combination with genetics will greatly impact the future of diagnosis and prevention of AD as these methods will serve to create a composite picture of what is normal, or abnormal, for an individual further differentiating disease related pathology from normal aging. Combining activity pattern differences measurable by functional MRI, genetic risk information, and memory-related performance scores, may provide increased predictive power to better prevent and diagnose the onset of AD.