Abstract
The selective loss of von Economo neurons has been linked to the behavioral deficits in behavioral variant frontotemporal dementia (bvFTD) but whether these neurons are affected in bvFTD patients with underlying Alzheimer’s disease (AD) has yet to be established. The present study assesses the von Economo neurons in pathological AD cases clinically diagnosed with either AD or bvFTD. Our results demonstrate no significant loss of von Economo neurons in all pathological AD cases, irrespective of clinical diagnosis or co-existing Lewy body pathology. These results suggest that the behavioral deficits in patients with clinical bvFTD and underlying pathological AD are not driven by the loss of von Economo neurons.
Keywords
INTRODUCTION
Behavioral variant frontotemporal dementia (bvFTD) is a progressive neurodegenerative disorder characterized by marked changes in social behavior and deficits in emotion processing. The anterior cingulate cortex is one of the earliest regions targeted in bvFTD and the selective loss of von Economo neurons in this region has been linked to the aberrant social behavior and affective changes typical of bvFTD [1–4]. Importantly, ∼10% of patients diagnosed with bvFTD have underlying Alzheimer’s disease (AD) [5, 6], but whether the von Economo neurons are also affected in these patients is not known. As such, the present study quantitatively assessed the von Economo neuron density in patients with clinical bvFTD but pathological AD. We hypothesized that significant von Economo neuron loss would be identified in patients with clinical bvFTD only.
Methods
Case selection
Inclusion criteria for the first index group was a clinical diagnosis of bvFTD according to current international consensus criteria [7] and pathological AD without frontotemporal lobar degeneration at autopsy [8]. Nine cases fulfilled these inclusion criteria and were included in this study. Given that all nine cases in this first index group demonstrated co-existing Lewy body inclusions (56% amygdala-only and 44% neocortical Lewy body pathology) [9, 10], a second index group comprised of 9 clinicopathological AD cases with similar age, gender, and Lewy body pathology stage to the first index group was selected. A third index group comprised of 10 clinicopathological AD cases with similar age, gender but no co-existing Lewy body pathology was also included. The final index group was comprised of 15 age- and gender-matched neurologically-normal individuals with no significant neuropathology. Cases were selected from a neuropathological series collected by the Sydney Brain Bank through regional brain donor programs in Sydney, Australia. The brain donor programs hold approval from the Human Research Ethics Committees of The University of New South Wales and comply with the statement on human experimentation issued by the National Health and Medical Research Council of Australia. Patients were diagnosed during life by experienced clinicians using standard clinical diagnostic criteria [11–13] following a medical interview, cognitive testing, and informant history. This research project was approved by the Human Research Ethics Committee of the University of New South Wales.
Assessment of von Economo neurons
Standardized tissue samples of the anterior cingulate cortex posterior to the genu of the corpus callosum were sampled and embedded in paraffin wax using routine procedures. Ten-micrometer serial sections were cut from each block and stained with hematoxylin and alpha-synuclein (mouse; 1 : 500; Cat. No. 610787; BD Biosciences). Von Economo neurons were quantified as previously described [3]. Briefly, von Economo neurons, identified as bipolar elongated neurons with prominent dendrites emerging from apical and basal poles, located in layer V of the sub-area 24b of the anterior cingulate cortex, were counted at x200 magnification using a 10 x 10 eyepiece graticule (500 μm x 500 μm) with standard inclusion and exclusion (upper and right) borders in contiguous, non-overlapping fields. Non-Von Economo neurons in the same area of interest were also quantified as previously described [14], and the Von Economo neurons were normalized to these neurons in the same cortical section. The neuron density measures were standardized to numbers/mm3. Quantitation of neuron populations were performed by two raters blind to case details with an inter-rater and intra-rater variability of < 5%, and no significant differences between raters. Alpha-synuclein stained sections were assessed to determine if Lewy body pathology accumulated in von Economo neurons.
Statistical analysis
Statistical analyses were performed using SPSS (version 24.0; SPSS Inc., Chicago, IL), with a p-value of < 0.05 taken as significant. Group comparisons were assessed using analysis of variance (ANOVA) followed by Bonferonni-corrected post-hoc tests and categorical data was analyzed using Chi-squared analyses.
RESULTS
Demographics (Table 1)
Mean demographic, clinical, and pathological characteristics (± standard deviation) of pathological Alzheimer’s disease (AD) cases with clinical behavioral variant frontotemporal dementia (bvFTD)
PMI, postmortem interval; NFT, neurofibrillary tangles; CERAD, Consortium to Establish a Registry for Alzheimer’s Disease score; *p<0.005 compared to controls.
As designed, there was no significant difference in mean age at death, postmortem interval, or gender between the four index groups. Age of disease onset and disease duration was similar between patient groups. No significant differences in AD pathology (Braak neurofibrillary tangle stage, CERAD score) was identified between patient groups.
Von Economo Neurons
No significant difference in von Economo neuron density was observed in the anterior cingulate cortex between groups (mean±standard error (SE) mm3 in controls: 147,556±21,302; in clinical bvFTD-pathological AD cases: 85±19% of mean control value; in clinicopathological AD cases with Lewy bodies: 63±19% of mean control value; in clinicopathological AD cases without Lewy bodies: 61±18% of mean control value; F(3,39) = 1.340, p = 0.28; Fig. 1A). Von Economo neurons normalized to non-von Economo neurons in the same region also demonstrated no significant difference between groups (mean±standard error (SE) mm3 in controls: 0.18±0.03; in clinical bvFTD-pathological AD cases: 97±23% of mean control value; in clinicopathological AD cases with Lewy bodies: 72±23% of mean control value; in clinicopathological AD cases without Lewy bodies: 59±22% of mean control value; F (3,39) = 0.859, p = 0.470). Lewy body pathology was not observed in the von Economo neurons in any of the present cases (Fig. 1B, C).
A) Mean (± standard error) density of von Economo neurons (as a % mean control) in the anterior cingulate cortex of pathological AD cases with clinical behavioral variant frontotemporal dementia (bvFTD) and clinical Alzheimer’s disease (AD) compared to controls. *p<0.05 compared to controls. B) Alpha-synuclein stained sections demonstrated the absence of Lewy body pathology in von Economo neurons (arrows) of the anterior cingulate.
Correlations
Spearman rank correlations identified no significant relationships between von Economo neuron density, age at death, and disease duration in any of the present cohorts assessed.
DISCUSSION
The present study set out to assess the von Economo neurons in a series of patients with clinical bvFTD found to have pathological AD at autopsy (clinical bvFTD-pathological AD). Co-existing Lewy body pathology was identified in all of the present clinical bvFTD-pathological AD cases and as such, a cohort of clinicopathological AD cases with similar co-existing Lewy body pathology, as well as a cohort of clinicopathological AD cases without co-existing Lewy body pathology, were also assessed. Contrary to anticipated, a significant loss of von Economo neurons was not identified in clinical bvFTD-pathological AD cases. As expected, no significant loss in von Economo neurons was identified in the clinicopathological AD cohorts. The present findings suggest that the behavioral deficits identified in clinical bvFTD-pathological AD cases are not due to a loss in von Economo neurons.
Although unexpected, this is not the first study to demonstrate an absence of significant von Economo neuron loss in patients diagnosed with bvFTD. Instead, these findings converge with a recent study in bvFTD cases with the C9orf72 repeat expansion [4] to demonstrate that the von Economo neurons are not significantly affected in all patients with clinical bvFTD. While this may appear to contest the centrality of the von Economo neurons in the social and affective changes characteristic of bvFTD, clinical assessments have demonstrated significantly milder and more restricted behavioral profiles in bvFTD patients with pathological AD or the C9orf72 repeat expansion [15–18]. Together, these findings suggest that the relatively intact von Economo neurons may be associated with the milder behavioral phenotype in these bvFTD subgroups. In patients with underlying AD, this is supported by the similar patterns of temporoparietal degeneration observed in patients irrespective of whether they have a clinical diagnosis of bvFTD or AD [15].
Social and affective changes are also recognized in AD, with apathy and depression the most commonly observed [19, 20]. Previous studies of von Economo neurons in AD have reported a loss in some [21], but not other AD cohorts [1, 22]. Although a reduction in von Economo neuron density was observed in the present clinicopathological AD cohorts, this was not significant. Neuropsychiatric measures were not available for the present cases. Future studies in larger cohorts longitudinally well-followed with neuropsychiatric measures to autopsy will be needed to determine whether von Economo neurons are associated with specific behavioral symptoms in AD.
In summary, the present study demonstrates no significant loss of von Economo neurons in clinical bvFTD-pathological AD or clinicopathological AD cases, irrespective of co-existing Lewy body pathology. These findings indicate that the behavioral deficits characteristic of clinical bvFTD-pathological AD are not driven by a loss in von Economo neurons.
Footnotes
ACKNOWLEDGMENTS
Tissues were received from the Sydney Brain Bank (SBB), which is supported by the University of New South Wales and Neuroscience Research Australia. We thank Heidi Cartwright for her assistance with preparation of figures. This work was supported by funding to ForeFront, a collaborative research group dedicated to the study of FTD and motor neuron disease, from the National Health and Medical Research Council of Australia (NHMRC) program grant (#1037746) and the Australian Research Council Centre of Excellence in Cognition and its Disorders Memory Node (#CE110001021); RT is supported by an NHMRC-ARC Dementia Research Development Fellowship (#1110369) and GH is supported by a NHMRC Senior Principal Research Fellowship (#1079679).
