Pickford et al. (10
) report the effects of reduced expression of beclin 1 (encoded by Becn1
) on various aspects of Aβ pathology in mouse models of AD in which human mutant APP was overexpressed specifically in neurons under the control of the Thy1
). Regardless of whether beclin 1 level is specifically reduced in neurons of AD brains, these findings in animal models may have important implications for our understanding of AD pathogenesis. The authors examined APP+Becn1+/–
mice and found significant increases in extracellular Aβ immunoreactive deposits, number of thioflavin S–positive plaques with globular shapes, and intraneuronal Aβ levels. Moreover, lysosomal abnormalities were more pronounced, and the expression of CD68, a marker of increased phagocytic activity of macrophages and microglia, was higher in APP+Becn1+/–
mice than in APP+Becn1+/+
mice, probably as a result of increased Aβ production.
Intriguingly, beclin 1 deficiency did not seem to affect the extent of synaptic and dendritic degeneration in APP transgenic mice nor calbindin immunoreactivity, a molecular indicator of neurodegeneration (14
), even though the levels of extracellular and intracellular Aβ were significantly higher in APP+Becn1+/–
than in APP+Becn1+/+
). Another interesting observation is that beclin 1 deficiency had age-dependent effects on Aβ level: loss of one copy of Becn1
correlated with increased levels of formic acid–soluble Aβ in 9-month-old APP+
mice but not in 3.5-month-old animals. In human brains, BECN1
mRNA and protein levels in the prefrontal cortex were about 2-fold lower in older than younger people (16
). Therefore, it is conceivable that the more pronounced effect of beclin 1 deficiency in older mice may reflect, at least in part, the lower beclin 1 levels in older animals.
These observations raise the question of how loss of one copy of Becn1
altered Aβ pathology so dramatically in this mouse model of AD (10
). One plausible explanation is reduced autophagy. Indeed, Pickford et al. reported that both the number of microtubule-associated protein 1 light chain 3–positive (LC3-positive) vesicles in cultured primary hippocampal neurons and the ratio of LC3-II to LC3-I in lysates of mouse cortex were lower as a result of beclin 1 deficiency, consistent with a previous report (17
). LC3 is the mammalian homolog of the autophagy-related protein 8 (Atg8), a ubiquitin-like protein conjugated to phosphatidylethanolamine (PE) during autophagosome formation. Therefore, PE-modified LC3 (i.e., LC3-II) is often used as a marker for isolation membranes and autophagosomes (18
). However, considering the potential caveats of using overexpressed GFP-LC3 (18
) and the subtle difference in the LC3-II/LC3-I ratio as shown by Western blot in the present study, it will be helpful to perform additional assays to measure the autophagy flux and activity. Moreover, it will be interesting in the future to determine whether partial reduction of the activities of some Atg
genes, such as Atg7
), also enhances Aβ pathology in APP transgenic mice.
Beclin 1 is a multifunctional protein with many binding partners. For instance, beclin 1 may affect the endosomal/lysosomal pathway (20
). Aβ processing can occur in many subcellular compartments, including the endosomal/lysosomal pathway, which may — through unknown mechanisms — be influenced by beclin 1 deficiency. Also, beclin 1 is well known to interact with Bcl-2 (11
) and aged Becn1+/–
mice exhibit a high incidence of spontaneous tumors (21
). Therefore, aged APP+Becn1+/–
mice may suffer more stress than APP+Becn1+/+
mice. Finally, if the reduced induction of autophagy is indeed the direct underlying mechanism for observed higher levels of Aβ peptides and aggregates, then the present model (10
) appears to contradict previously published work (6
). However, because the Aβ level is affected by both production and clearance (22
), reduced autophagy through beclin 1 deficiency in vivo may inhibit clearance of Aβ, while defects in autophagosome maturation enhance Aβ production. Additional experiments are needed to dissect the precise consequences of manipulating different steps of the autophagy pathway on Aβ pathology and neurodegeneration associated with AD. As the authors discussed thoroughly, the exact molecular and cellular mechanisms linking beclin 1 deficiency and Aβ level remain to be further investigated.