Exploring the K isotope composition of Göttingen minipig brain regions, and implications for Alzheimer's disease

Abstract

Natural stable metal isotopes have shown utility in differentiation between healthy and diseased brain states (e.g. Alzheimer's disease, AD). While the AD brain accumulates some metals, it purges others, namely K (accompanied by increased serum K, suggesting brain-blood transferal). Here, K isotope compositions of Göttingen minipig brain regions for two AD models at midlife are reported. Results indicate heavy K isotope enrichment where amyloid beta (Aβ) accumulation is observed, and this enrichment correlates with relative K depletion. These results suggest preferential efflux of isotopically light K+ from the brain, a linkage between brain K concentrations and isotope compositions, and linkage to Aβ (previously shown to purge cellular brain K+). Brain K isotope compositions differ from that for serum and brain K is much more abundant than in serum, suggesting that changes in brain K may transfer a measurable K isotope excursion to serum, thereby generating an early AD biomarker.

Keywords: Alzheimer's disease, brain potassium; isotope geochemistry; isotope metallomics; neurodegeneration, porcine model.

Graphical Abstract

Potassium isotope compositions of minipig brain regions indicate heavy K isotope enrichment coincident with amyloid beta accumulation, suggesting a mechanistic linkage and potential for downstream signal transferal to blood serum (i.e. noninvasive diagnostic potential).

Fig. 1

Anatomical schematic of the Göttingen minipig brain with K isotope ratios—δ⁴¹K—for each region. Values in parenthesis are calculated heavy K isotope enrichment in porcine model M10 (APP/PS1) relative to M9 (PS1 only) (i.e. δ⁴¹K offset between APP/PS1 and PS1 animals, respectively). See Table 1 and Fig. 2 for associated analytical uncertainties.

Fig. 2

Spider plot of K isotope results highlighting the general ∼0.20‰ increase in δ⁴¹K for the younger double-transgenic animal (M10) relative to the older single-transgenic animal (M9) (except for the amygdala). AM—amygdala, BG—basal ganglia, BM—brainstem, CB—cerebellum, CX—cerebral cortex, and HC—hippocampus. Numbers next to data points indicate number of replicate analyses; analytical uncertainties represented by 2σ ± brackets for each sample (determined from replicate analyses).

Fig. 3

Collated isotopic data to date for minipig brain regions from this work and Mahan et al. and Higgins et al. (Note: data for δ⁴¹K are indicative only, as isotopic composition varies considerably both within and between animal models.)

Fig. 4

Plots of δ⁴¹K for the single- and double-transgenic AD model animals in the current work as a function of concentration for biologically relevant elements (biometals and P) and δ⁴⁴Ca (color coded as mentioned earlier). Reproducibility of measurements via Q–ICP–MS was better than 5% (RSD) for all elements, and symbols encompass analytical error; analytical uncertainties for K isotope measurements in Table 2. Statistically significant relations were determined for Mg(*), P(**), Fe(*), and Cu(*). The increasing power of the statistical significance is defined as: P ≤ 0.05 (*); P ≤ 0.01, **; P ≤ 0.001, ***; and P ≤ 0.0001, ****.

Fig. 5

Statistical results for dependence of metal concentrations and δ⁴⁴Ca as a function of δ⁴¹K (unpaired t-tests in GraphPad Prism with 2-tailed P-values).

Fig. 6

Heavy K isotope enrichment (high δ⁴¹K) in the double-transgenic model (M10) relative to the single-transgenic model (M9), expressed as Δ⁴¹K_{double–single} (δ⁴¹K_M10–δ⁴¹K_M9), as a function of the % difference in K concentration in the same reference frame (percentage depletion in M10 relative to M9). All brain regions fall on a discernible trendline (except for the cerebral cortex, CX), suggesting a mechanistic linkage between brain K depletion (associated with AD) and δ⁴¹K. At present, the reason for the excursion of the cerebral cortex off this trendline is unknown.