Cole GM, Frautschy SA.
Cat and mouse.
Neuron. 2006 Sep 21;51(6):671-2.
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The study by Mueller-Steiner and colleagues firmly establishes cathepsin B as a biologically relevant Aβ-degrading enzyme. The results add to mounting evidence that the major pathways to the lysosome, endocytosis, and autophagy are important in APP processing for Aβ generation and degradation, and that dysfunction in these pathways early in Alzheimer disease promotes β-amyloidogenesis and neurodegeneration (Nixon and Cataldo, 2006). The authors propose that insufficient CatB activity may contribute to AD, although expression of CatB and other lysosomal hydrolases, including another potential Aβ-degrading cathepsin (CatD), increases in AD and AD models according to their data and earlier work. CatB function could be insufficient, however, if enough of the protease fails to reach Aβ-generating compartments of cells, or its action is impeded once it arrives. This may well be the case in AD brain where autophagy is markedly impaired. Autophagic vacuoles (AVs) accumulate in huge numbers within dystrophic neurites, reflecting their incomplete clearance from neurons. This clearance normally occurs when autophagosomes fuse with lysosomes, and cathepsins efficiently digest autophagic substrates.
We have shown that Aβ is generated in AVs during autophagy (Yu et al., 2005). Autophagy-generated Aβ is normally degraded efficiently in lysosomes, but in AD, this Aβ accumulates within AVs that collect massively in the AD brain. If, as we suspect, autophagosomal-lysosomal fusion, or subsequent steps in autolysosome maturation, are impaired/slowed in this condition, even elevated levels of CatB would be rendered functionally “insufficient.” Elevating CatB levels further, as proposed, might overcome the degradative impasse, although it is worth noting that cathepsin overexpression in some systems promotes cell death after injury. Also, modulating one cathepsin significantly affects the levels of other hydrolases in the lysosome. Strategies to reverse the autophagy block, while more challenging a prospect at present, may have additional merit.
As a sidebar, the data in the report do not formally exclude an "APP secretase" role for CatB. They do show that CatB modulations in vivo have a net effect in favor of degradation of Aβ over any (putative) formation process, but it does not follow from the data that degradation is the only action of CatB on APP metabolism or even the only relevant one. CatD, for example, is reported to have both Aβ-degrading activity and "APP secretase" activity in vitro.
One highlight of this past year in Alzheimer disease research has been the flood of interest and activity around Aβ proteolysis. Several new Aβ-degrading enzymes have been discovered, and others already known have stood tests of increased experimental scrutiny. This report by Sarah Mueller-Steiner and colleagues presents cathepsin B (CatB) as a new member of the Aβ-degrading enzyme family, demonstrating that CatB is capable of degrading fibrillar synthetic Aβ as well as clearing Aβ plaques in vivo.
Previous investigation has linked CatB to Alzheimer disease (AD) by several mechanisms. Extracellular CatB immunoreactivity and enzymatic activity, not present in normal brains, have been demonstrated at the sites of amyloid plaque deposition in human brain (Cataldo et al.). This new paper similarly identifies CatB in association with plaques, specifically enriched at thioflavin S-positive plaques, and demonstrates CatB expression by multiple cell types in the brain. An APP transgenic combined with CatB knockout mouse had an impressive two- to fourfold elevation in plaque deposition, though no change in absolute levels of Aβ as determined by ELISA. The ability of CatB to promote plaque clearance was further demonstrated by viral CatB overexpression in APP transgenic mice.
There has been some controversy over the role of CatB in APP processing, as some studies have found CatB to directly cleave APP, functioning like β-secretase to elevate Aβ levels. By showing that CatB deficiency in vivo produces no change in full-length APP, APP C-terminal fragments, and APPs, Mueller-Steiner and colleagues provide convincing evidence that CatB is not a prominent APP processing enzyme.
In relation to other human CNS diseases, there has been significant interest in CatB for its upregulation in brain tumors (Rempel et al., 1994), likely contributing to metastasis. CatB activity has also been shown to be elevated in the CSF of patients with inflammatory neurologic diseases (Nagai et al., 2000). It is unclear if induction of CatB expression is a protective or toxic response in the disease process, but several studies propose that CatB is required for microglial-mediated neurotoxicity (Gan et al., 2004), and that even CatB by itself is neurotoxic (Kingham et al., 2001). Certainly, more work at the basic research level should be done before promoting CatB as a potential therapeutic.
The findings of this report raise some interesting questions and possibilities. First, why does the significant plaque elevation in APP-overexpressing and CatB-deficient mice not change absolute Aβ levels? Is CatB acting by sequestering the most aggregation-prone Aβ, and does preventing plaque deposition raise levels of soluble oligomers? Second, what mechanism targets secreted CatB to thioflavin S plaques? Is it simply the age of the deposit or the actual structure of the fibrils that matters? Third, this is another example of existing plaques being removed through a proteolytic mechanism. Some evidence suggests that plaques cannot be cleared by the brain after suppressing APP transgene expression (Jankowsky et al., 2005), which may indicate that elevating Aβ clearance mechanisms by proteolytic, transport, antibody, or other means must be enlisted to therapeutically lower Aβ in the brain. Fourth, this finding may increase interest in the analysis of the CatB genetic locus for association to AD, as happened for IDE. Finally, can CatB be used as a therapeutic tool itself? Some of the biological details of CatB’s regulation are known, and if CatB upregulation is not itself toxic, perhaps these data could be exploited as a therapeutic approach to Alzheimer disease.
This is an extensive and very carefully controlled study in which the authors demonstrate that cathepsin B (CatB) can readily degrade Aβ in vivo and in vitro. At pH 6.0, CatB similarly degrades both non-aggregated and aggregated synthetic Aβ1-42, while at pH 7.0, CatB effectively degrades aggregated but not non-aggregated Aβ. At both pH 6.0 and 7.0, CatB acts as a carboxypeptidase, trimming Aβ1-42 first to Aβ1-40 and then Aβ1-38. But CatB can also act as an endopeptidase cleaving Aβ to generate Aβ1-33. In vivo viral expression of CatB caused a substantial decrease in amyloid burden, with lentiviral expression of CatB having an effect comparable to lentiviral NEP, but with the former much more effective at decreasing the number of compacted plaques.
Consistent with this selectivity towards aggregated Aβ, over 70 percent of compacted (thioflavin S-positive) plaques from old (16-20-month) APP transgenic mice co-stain for CatB, and in cell culture, application of aggregated Aβ caused an increase in CatB transcription and activity. Conversely, knockout of CatB in an APP overexpressing background increased amyloid burden as assessed by immunostaining with 3D6, thioflavin S staining, and ELISA of guanidine-HCl-extracted Aβ. Interestingly, while the level of Aβ1-42 was significantly decreased in CatB Kos, the level of total Aβ was not significantly decreased. The latter reflects accumulation of some of the partially degraded Aβ1-42, that is, Aβ1-40, Aβ1-38, and Aβ1-33.
Together, these results indicate that CatB plays an important role in the regulation of aggregated Aβ levels and suggests that a deficit in CatB activity may facilitate Aβ accumulation. However, in Alzheimer disease it is not apparent that there is a deficit in CatB activity. Clearly, this report will prompt further investigations to address this point, and may encourage new studies to look at ways of stimulating CatB activity in vivo. But in regard to the upregulation of CatB, further studies would be well advised to consider what effect newly solubilized Aβ1-33, 1-38, and 1-40 might have on cognition and on vascular deposition. As with everything in life, the utility of upregulating CatB will largely depend on timing. Increased CatB activity would be most beneficial at intervals proceeding profound amyloid deposition, whereas upregulation at later time points could be counter-productive.