Study of Aluminum on the Pathology of Alzheimer's Disease: In vitro versus in vivo Evidence

Abstract

We would like to thank Dr Exley for his thoughts [1]. To address his comment regarding data presented in Fig. 1 of our recently published paper [4], which demonstrates that 0.2–20 mM AlCl3 inhibited heparin (10μM)-induced tau (10μM) aggregation, we repeated the heparin-induced tau aggregation experiment using a lower range of AlCl3 (50 nM–2 mM). This concentration range of Al did not show clear inhibition of heparininduced tau aggregation (see Supplementary Figure). Therefore, Dr Exley’s explanation that the excess of Al absorbed heparin and inhibited tau aggregation, may be a possibility. Another possibility as described in our discussion section [4] is that, Al may directly bind to tau, and inhibit heparin-induced tau aggregation. This suggestion of the direct binding of Al to tau is further supported by the presence of Al in neurofibrillary tangles found in Alzheimer’s disease (AD) brains as well as by previous in vitro studies that showed Al-induced tau aggregation in the absence of heparin [2]. In contrast to a previous study [2], our data showed that the addition of Al (50 nM–2 mM) to 10 μM tau produced a 0.05–200 Al/tau molar ratio that failed to evoke tau aggregation with increasing thioflavin T fluorescence. This discrepancy could reflect a difference in the concentration of tau used between the two studies, as Haase and colleagues used 80 μM tau triggered by 3 mM Al. So, while the molar ratio 34.5 of Al (3 mM)/tau (80 μM) may be appropriate for inducing tau aggregation, the same ratio using 10 μM tau may not, indicating a tau-concentration dependency of the aggregation-inducing properties of Al. It seems that a higher concentration of tau, such as 80 μM, is required for Al-induced β-sheet tau aggregation in an in vitro study. Thus, the physiological concentration of tau, which has been reported as being between 1–10 μM in neurons [3], cannot induce fibrilar tau aggregation. In our cellular study [4], Dr. Exley pointed out that N2a cells may not have been exposed to the indicated concentrations (50,100, and 200μM) of Al, in our fresh Al maltorate solution, prepared by mixing an equal volume of AlCl3 solution with Maltol solution (a two fold dilution). Even though exposed to a lower concentration of Al than anticipated (according to Dr Exley’s proposition), these cells clearly exhibited Al-induced SDS-insoluble tau aggregates. However, as suggested from our in vitro tau studies, the aggregates may be amorphous and not fibrilar as shown by AFM. Regarding our animal study [4], independent of the real concentration of the exogenously administrated Al, these animals showed an overall increase (up to ∼ 20 μM) in the total concentration of brain Al in comparison to controls. Acceleration of tau aggregation, however, was not detected by either biochemical or histopathological methods. Rather than tau-based brain pathology, the increased mortality of these animals could have been induced by the peripheral effect of Al on the animal’s physiology and homeostasis. A non-direct role for Al on AD pathogenesis is further supported by Pratico and colleagues (2002). Using an alternative method of Al administration in a different AD model (APP transgenic mouse), they report-