The major finding of the present study is the significant hypertrophy of cell bodies, nuclei, and nucleoli in CA1 neurons of ASYMAD subjects compared with the MCI and age-matched controls group. These results are similar to those observed in previous studies of ASYMAD subjects from the BLSA cohort. These data support the idea that this neuronal hypertrophy is a general phenomenon, not restricted to participants in a single longitudinal study, and is present in both sexes. In addition, our observations indicate a functional and morphologic plasticity and heterogeneity in response to the AD lesions in the human brain even in the ninth or 10th decade of life. In fact, whereas some subjects develop cognitive impairment (MCI or dementia), others remain relatively unscathed (ASYMAD) in presence of significant and comparable AD pathology. This neuronal hypertrophy might be related to or even underlie the capacity of the ASYMAD subjects to maintain the cognition and effective daily living activities preserved, despite the presence of substantial AD lesions.
This observation of diverging clinical outcomes in subjects with comparable AD lesions is consistent with the brain reserve hypothesis already described by various investigators.26
From a functional perspective, the synapses are the critical structures in neurons and their circuits. The enlargement of the neuronal cell body is probably a surrogate for the expansion of the dendritic arbor and postsynaptic structures. This concept is supported by our observation of hypertrophy of cortical neurons in the APPsw/PS1 δE9 transgenic mouse27
and the observation of an increased number of synaptic contacts in the same animal model.28
Another possibility is that the hypertrophic neurons are extending or growing new axonal terminals along with their presynaptic structures. It is plausible that the larger neuronal somata in ASYMAD reflect adaptive changes of neurons and their circuits to compensate for the damage inflicted by neurotoxic effects of Aβ and tau. It is then attractive to postulate that in ASYMAD, the enlargement of nuclei and nucleoli is the expression of enhanced DNA and RNA syntheses necessary to sustain the repair of injured neurons, and their processes and synapses. This latter notion is further supported by 2 recent studies. One study showed that in the frontal cortex of patients with MCI, there is a paradoxical upregulation of presynaptic boutons.29
The other study reported that in the frontal cortex of old-older normal controls with AD pathology, equivalent to our ASYMAD group, synaptophysin, a presynaptic marker, is enhanced.30
An alternative explanation, based on functional neuroimaging studies,31,32
could be that in ASYMAD, some neurons and circuits are irreversibly damaged by AD pathology, and different and compensatory circuits become operative based on different stages of disease. Consequently, the circuitry of the cerebral cortex may remain functional despite the presence of AD pathology.
At the cellular level, there is a positive linear correlation among the size of the nucleolus, nucleus, and cell body in neurons.33
The nucleolar volume appears as a sensitive indicator of the transcriptional and metabolic activities. Our analyses show that the nucleolus was the compartment with the highest percent of increase in volume. Since the nucleolus is a key organelle for the synthesis, assembly, and production of ribosomes, the nucleolar hypertrophy may reflect enhanced transcriptional and biosynthetic activities. Although the molecular mechanisms underlying the observed hypertrophy of neuronal cell bodies, nuclei, and nucleoli in ASYMAD are beyond the scope of this study, we can formulate some possible explanations drawing from previous studies in animals. Studies in transgenic models have shown that the cell body sizes of neurons, their nuclei and nucleoli are plastic structures and could be influenced by overexpression of the APP
gene. Furthermore, neuronal size can also be influenced by exposure to sexual hormones,34
regeneration after axonotomy,35
or brain trauma,36
as well as by learning and enriched environmental experiences.37
One of the main determinants of the cell size is the cell growth, which is modulated by growth factors, i.e., BDNF,38
and by genes and products of the TOR pathway.39
These genes and proteins appear as attractive targets for future studies of ASYMAD.
In terms of the analysis of linguistic ability in this study, it is important to remark that the linguistic scores were available in only 93 out of 678 sisters enrolled in the Nun Study. This explains the availability of these data in only 14 subjects in this study. To circumvent this limitation, we opted to compare the linguistic scores between subjects with or without cognitive deficits. Despite the small number of subjects available, a significant association between higher idea density scores and the absence of cognitive deficits was observed. Although it would have been ideal to examine the linguistic scores in a group composed solely of ASYMAD subjects, this was not possible since only 2 of the ASYMAD subjects had linguistic scores available. Independently of these limitations, it remains a fascinating observation that an intellectual ability measured in the early 20s can predict the likelihood of remaining cognitively normal 5 or 6 decades later, even in the presence of substantial AD pathology. We are aware of the very small size of our samples and the need for a follow-up investigation including larger number of subjects.
It is particularly important to keep in mind that the 4 groups analyzed in this study were clustered based on their clinical states and the severity of NP and NFT. These 2 elements represent insoluble protein aggregates in the brain. We cannot rule out the possibility that although ASYMAD and MCI have comparable NP and NFT scores, they may have different levels of soluble toxin proteins, i.e., Aβ oligomers.
The observation of the 30% frequency of the APOE2
allele in ASYMAD, significantly higher than the MCI and AD group, was unexpected and surprising for its magnitude. Previous studies have shown that the APOE2
allele protects against the development of AD.40
Although the difference in the frequency of APOE2
in ASYMAD was highly significant (p
< 0.012) compared to the combined MCI+AD groups, the interpretation of this difference should be tempered by the small sample size. Nevertheless, the difference suggests that the APOE2
allele has the potential to protect the brain from the neurotoxic effects of Aβ and tau, and may be associated with the neuronal hypertrophy. A final caveat is whether individuals with APOE2
alleles have inherently larger neurons. To answer this question, an analysis of a very large sample of old and young control subjects beyond the scope of this study would be required.