Abstract
Alzheimer disease (AD) is a form of neurodegeneration that develops over the course of multiple decades and as a result of the accumulation of the pathogenic amyloid-β (Aβ) peptide, also known as A4. In late-stage AD, failure of autophagic clearance results in neuronal cell bodies that are almost entirely consumed by autophagic vacuoles (AVs). Previously, we have shown that the potential AD drug latrepirdine (aka Dimebon®), a Russian antihistamine that has shown mixed results in phase II clinical trials in AD, regulates metabolism of the amyloid-β/A4 precursor protein (APP). In two Molecular Psychiatry papers in 2012, we sought to determine the mechanism through which latrepirdine regulates APP metabolism and to determine, using an Alzheimer mouse model, whether latrepirdine provides protection from the toxicity associated with the accumulation of Aβ. In cultured cells, we provided evidence that latrepirdine stimulates MTOR- and ATG5-dependent autophagy, leading to the reduction of intracellular levels of APP metabolites, including Aβ. Consistent with this finding, we found that chronic latrepirdine administration resulted in increased levels of the biomarkers thought to correlate with autophagy activation in the brains of TgCRND8 (APP K670M, N671L, V717F) or wild-type mice, and that treatment was associated with abrogation of behavioral deficit, reduction in Aβ neuropathology, and prevention of autophagic failure among TgCRND8 mice.
Keywords: Alzheimer, therapeutics, latrepirdine, dimebon, synuclein, macroautophagy, amyloid, lysosome, presenilin, neurodegeneration
The clearance of AVs by neurons through the macroautophagy-lysosomal pathway (referred to here as autophagy) is a constitutively active process in the adult brain. During late-stage AD, a progressive slowing of autophagic function occurs, and pathological accumulation of AVs and their substrates is observed, wherein AVs accumulate to replace nearly all of the cytosol in affected neurons. At present, the role that the autophagic defect plays in the pathobiology of AD is unclear. Recent studies of neuronal Aβ accumulation in cases of lysosomal storage diseases suggest that lysosome function is necessary for AV clearance and degradation of Aβ and its precursors.
Cellular studies of the autophagy pathway suggest a role for MTOR-, BECN1-, ATG5-, and ATG7-dependent autophagy in both the generation and degradation of Aβ. These seemingly contradictory findings can be explained via the generation of Aβ from its precursor C99-CTF (C99-C-terminal fragment) through the autophagic pathway as one locus of C99-CTF metabolism. Evidence from our work and other recent studies from independent groups indicates that deletion of the autophagy-essential gene ATG5 results in the accumulation of the Aβ precursor C99-CTF. Deacidification of the lysosome by treatment of cells with either chloroquine (our work) or bafilomycin A1 (other studies) indicates that impaired AV clearance results in hyperaccumulation of both C99-CTF and Aβ. Our time-course experiments in the papers cited here suggest that autophagic turnover is necessary for both the generation of one pool of Aβ, as well as the degradation of pathogenic Aβ aggregates. While increasing autophagic activity may increase Aβ generation, one presumes that this would also cause degradation of some Aβ. The balance between generation and clearance of Aβ would determine whether amyloid accumulation would occur.
Using the TgCRND8 mouse model, we studied the relative autophagic stasis over the course of Aβ neuropathology development. The benefits of this mouse line for this study were 2-fold. First, this mouse line harbors only AD-related mutations in an APP transgene (APP K670M, N671L, V717F), whereas other lines combine AD mutations in APP and PSEN1/PS1 transgenes. This is important because prior work, particularly from the Nixon group, posited an association between AD-related PSEN1 mutations and perturbed AV clearance and lysosomal function. Second, the APP mutations present in the TgCRND8 mouse line result in a rapid onset of neocortical and hippocampal amyloid deposition and behavioral defect by 3 mo of age. These features of the TgCRND8 mouse line allow for the specific attribution of any PSEN1 mutation-related contributions to autophagic and lysosomal pathology.
In one of the two current papers, we provide evidence that 3-mo-old TgCRND8 mice harbor neocortical and hippocampal Aβ plaque pathology, as well as behavioral defect on both cued and contextual memory tasks. However, at this age, no abnormal accumulation of autophagic substrates was observed in the hippocampus or neocortex. By 4 mo of age, the behavioral defect worsens, Aβ pathology has increased substantially, and there is a hyperaccumulation of autophagic substrates including LC3-I, LC3-II, SQSTM1/p62, and SNCA/α-synuclein, suggesting that deposition of insoluble Aβ may be toxic to AV clearance. Interestingly, this result lends some insight into the many reports of the association of SNCA pathology with AD. Though further work is necessary to understand this relationship between Aβ and SNCA deposition and autophagic failure, we proposed that prophylactic or early-stage therapeutic targeting of the autophagic pathway might prevent conversion from functional to defective AV clearance via reduction of the accumulation of these toxic autophagic substrates.
We previously reported increased Aβ secretion following acute treatment of cells or mice with latrepirdine. In mammalian cells, we observed an increase in activation of autophagy following 3–6 h exposure to latrepirdine and a concomitant decrease in intracellular levels of C99-CTF, Aβ, and SQSTM1. We treated TgCRND8 mice for 3 weeks at 2 mo of age, prior to the onset of plaque deposition, and found that latrepirdine stimulated autophagy and reduced intraneuronal Aβ pathology. We further treated TgCRND8 mice for 30 d with latrepirdine beginning at 3 mo of age, an age equivalent to early clinical stage AD when mice have begun depositing Aβ plaques and show behavioral defects. Our results indicate that latrepirdine therapy of TgCRND8 mice at this age enhances autophagy, reduces Aβ deposition, and prevents further behavioral decline. Importantly, we report that chronic latrepirdine therapy also prevents the accumulation of SNCA among TgCRND8 mice, suggesting that latrepirdine might act as a suitable prophylactic or early stage intervention therapy for AD, as well as other synucleinopathies.
Recent publications from Oddo and colleagues suggest that targeting the autophagic pathway as an AD therapeutic strategy is not effective when initiated after the onset of autophagic or lysosomal failure. Our work in the current articles lends some support to this notion, through evidence that autophagic failure occurs as a delayed pathology following the deposition of Aβ in a mouse model with intact murine Psen1. Other groups led by Nixon and Sisodia have reached similar conclusions through studies of the influence of AD-related PSEN1 mutations on lysosomal stasis. Latrepirdine is a drug with a troubled past, but with alluring promise. Latrepirdine was recently reported to possess modest pro-neurogenic activity and here we provide support for investigation of latrepirdine as a scaffold for developing a novel autophagy-regulating drug, which might produce more potent and specific derivatives for future study. For the AD therapeutic development landscape, pro-autophagy interventions might be best considered among the growing list of potential prophylactic or early-stage intervention strategies. Based on the current evidence from Aβ–lowering interventions, it seems highly unlikely that even the best of pro-autophagy therapies will offer meaningful therapeutic benefit in moderate to severe AD.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Footnotes
Previously published online: www.landesbioscience.com/journals/autophagy/article/23487