Aging-Associated Amyloid-β Plaques and Neuroinflammation in Bottlenose Dolphins (Tursiops truncatus) and Novel Cognitive Health-Supporting Roles of Pentadecanoic Acid (C15:0)

Abstract

There is an urgent need to identify interventions that broadly target aging-related cognitive decline and progression to Alzheimer's disease (AD). Bottlenose dolphins (Tursiops truncatus) have histologic changes similar to AD in humans, and they also develop shared age-associated co-morbidities identified as risk factors for AD in humans, including type 2 diabetes, ferroptosis, and iron overload, which can be driven by nutritional C15:0 deficiency. We hypothesized that (1) dolphins would have amyloid beta (Aβ) plaques and neuroinflammation that paralleled that of humans in relation to age-related progression, quantitative concentration, and brain region; and (2) C15:0 would have dose-dependent activities relevant to protecting cognitive health. Quantitative immunohistochemistry staining was used to assess 68 tissues from archived brains of 19 Navy dolphins to evaluate associations among amyloid beta (Aβ) plaques and neuroinflammation by brain region, sex, and age group. Further, dose-dependent C15:0 activities, using a third-party panel intended to screen for potential AD therapeutics, were evaluated. Similar to humans, dolphins had the highest Aβ plaque density variation in the hippocampus (90th percentile of 4.95 plaques/mm²), where plaque density increased with age (p = 0.05). All measured markers of neuroinflammation were detected, including the highest concentrations of activated microglia (CD68⁺) in the hippocampus (0.46 ± 0.38 cells/mm²). C15:0 was a dose-dependent inhibitor of two targets, fatty acid amide hydrolase (FAAH) (IC₅₀ 2.5 µM, 89% maximum inhibition at 50 µM relative to URB597) and monoamine oxidase B (MAO-B) (IC₅₀ 19.4 µM, 70% maximum inhibition at 50 µM relative to R(-)-Deprenyl). These activities have demonstrated efficacy against Aβ formation and neuroinflammation, including protection of cognitive function in the hippocampus. These findings suggest that, in addition to protecting against AD co-morbidities, C15:0 may play a distinct role in supporting cognitive health, especially at higher concentrations.

Keywords: Alzheimer’s disease; C15:0; amyloid beta plaques; cognitive health; dolphins; fatty acid amide hydrolase (FAAH) inhibitor; monoamine oxidase B (MAO-B) inhibitor; neuroinflammation; neuroprotectant; pentadecanoic acid.

Figure 1

The original image (a) and the use of masking (b) used to identify amyloid beta (Aβ) plaques in the hippocampus of a bottlenose dolphin (Tursiops truncatus), represented in gray.

Figure 2

Box plot of measured Aβ plaque densities in the hippocampus of bottlenose dolphins (Tursiops truncatus) (n = 19) using quantitative immunohistochemical (IHC) staining. The top and bottom bars at the end of the whiskers represent the minimum and maximum data points, respectively, as well as the measurement range; the top and bottom of the box represent the upper quartile Q3 and lower quartile Q1, respectively; the middle bar in the box is the median; the whole box represents the interquartile range; and the diamond represents the mean. The p-value = 0.05 is based on a Kruskal–Wallis test comparing dolphins’ age by high or low Aβ plaque density (defined as > or <0.61 plaques/mm², respectively).

Figure 3

Representative images of IHC staining on FFPE bottlenose dolphin (Tursiops truncatus) brain samples, including positive immunoreactivity visualized with DAB (brown) for (a) IL-6 in the red nucleus, (b) IFN-γ in the hippocampus, (c) TNF-α in the hippocampus, and (d) TGF-β1 in the hippocampus.

Figure 4

Response curve of C15:0 inhibition activities, based on the average of duplicate runs at each concentration, against fatty acid amide hydrolase (FAAH) compared to the positive control (URB597). Data as reported by Eurofins Discovery as part of its Alzheimer’s MoA LeadHunter Panel using standardized enzyme and radioligand binding assays. These data are based on 10 ascending concentrations (3 nM to 50 µM) of pure, free fatty acid C15:0.

Figure 5

Response curve of C15:0 inhibition activities, based on the average of duplicate runs at each concentration, against monoamine oxidase MAO-B compared to the positive control (R(-)-Deprenyl). Data as reported by Eurofins Discovery as part of its Alzheimer’s MoA LeadHunter Panel using standardized enzyme and radioligand binding assays. These data are based on 10 ascending concentrations (3 nM to 50 µM) of pure, free fatty acid C15:0.