. β-Secretase (BACE1) inhibition causes retinal pathology by vascular dysregulation and accumulation of age pigment. EMBO Mol Med. 2012 Sep;4(9):980-91. PubMed.

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  1. BACE1 has undoubtedly been one of the most important targets for the development of new Alzheimer’s disease drugs (1). Early reports suggested that BACE1 knockout mice were phenotypically normal, raising hopes that inhibition of BACE1 may provide a viable therapeutic strategy without toxic side effects (2). However, there are now strong reasons to believe that part of this view was overly optimistic.

    The paper by Cai et al. (3) is an excellent study that provides an important advance in our understanding of the normal function of BACE1. The study shows that BACE1 is highly expressed in retinal pigment epithelium, and that BACE1 knockout mice develop a retinal pathology. While the authors do not definitely establish the molecular mechanism that causes this pathology, some evidence is provided that the VEGF receptor 1 and lysosomal processing by cathepsin D may be involved. The results indicate that a previous report describing a retinal pathology observed in rats treated with a BACE1 inhibitor (LY2811376) was not due to off-target effects (4). The study builds on previous work that has shown a hypomyelination defect in BACE1 knockout mice (5,6). Other studies suggest that voltage-gated sodium channels, P-selectin glycoprotein ligand-1, LRP1, and interleukin-1 receptor II are also affected by BACE1 inhibition (1).

    Does this mean that research on BACE1 as a therapeutic target should be abandoned? Most BACE1 researchers would argue differently. In my opinion, there are several reasons why BACE1 is worthy of further support. Toxic effects may be mild, provided that only partial BACE1 inhibition is needed, and this may be the case if the strategy is to lower Aβ levels rather than to block Aβ production completely. In addition, it may be possible to manage some of the toxic side effects pharmacologically by inhibiting mechanisms that are downstream of BACE1. Finally, it must always be remembered that most drugs have significant side effects, and it should not be surprising that BACE1 inhibitors are no different. As with all drugs, it is necessary to determine whether the beneficial effects outweigh the detrimental effects.

    BACE1 has proven to be a difficult subject for a number of reasons (1). While the present study suggests that more caution is needed in evaluating BACE1's potential as a drug target, ultimately, only clinical trials will decide its suitability.

    References:

    . Is BACE1 a suitable therapeutic target for the treatment of Alzheimer's disease? Current strategies and future directions. Biol Chem. 2010 Aug;391(8):849-59. PubMed.

    . Mice deficient in BACE1, the Alzheimer's beta-secretase, have normal phenotype and abolished beta-amyloid generation. Nat Neurosci. 2001 Mar;4(3):231-2. PubMed.

    . β-Secretase (BACE1) inhibition causes retinal pathology by vascular dysregulation and accumulation of age pigment. EMBO Mol Med. 2012 Sep;4(9):980-91. PubMed.

    . Robust central reduction of amyloid-β in humans with an orally available, non-peptidic β-secretase inhibitor. J Neurosci. 2011 Nov 16;31(46):16507-16. PubMed.

    . Control of peripheral nerve myelination by the beta-secretase BACE1. Science. 2006 Oct 27;314(5799):664-6. PubMed.

    . The beta-secretase enzyme BACE in health and Alzheimer's disease: regulation, cell biology, function, and therapeutic potential. J Neurosci. 2009 Oct 14;29(41):12787-94. PubMed.

  2. This paper by Michael Boulton and colleagues adds a surprising and interesting twist to our understanding of the physiological function and the therapeutic potential of BACE1. The main functional findings and their implications for drug development have been summarized above by David Small. Like many other surprise findings, this study immediately triggers new questions to be answered in future studies. One crucial point is the apparent discrepancy to a previous study that did not observe retinal changes in another BACE1-deficient mouse line (May et al., 2011). Several different BACE1-deficient mouse lines have been generated in the past, which should allow us to determine whether the retinal changes (or the lack thereof) are broadly observed or are a strain-specific feature.

    Another exciting finding is the cleavage of VEGFR1 by BACE1. Future studies will show us whether endogenous VEGFR1 is exclusively cleaved by BACE1 or also by other proteases, similar to what is seen for APP and other BACE1 substrates. A major challenge—not only for VEGFR1, but also other recently identified BACE1 substrates (Kuhn et al., 2012; Zhou et al., 2012)—will be to rescue the phenotypes of BACE1-deficient mice by transgenic expression or addition of the soluble, BACE1-cleaved substrate ectodomains. This will not be an easy task, given that BACE1 cleaves many membrane proteins, and that a specific phenotype in BACE1-deficient mice could be caused by the combined lack of cleavage of several substrates and not just of a single substrate.

    The new study by Boulton and colleagues once again shows us how much more we can still learn about BACE1 function, and that this is an exciting time for BACE1 research.

    References:

    . Robust central reduction of amyloid-β in humans with an orally available, non-peptidic β-secretase inhibitor. J Neurosci. 2011 Nov 16;31(46):16507-16. PubMed.

    . Secretome protein enrichment identifies physiological BACE1 protease substrates in neurons. EMBO J. 2012 Jul 18;31(14):3157-68. PubMed.

    . The neural cell adhesion molecules L1 and CHL1 are cleaved by BACE1 protease in vivo. J Biol Chem. 2012 Jul 27;287(31):25927-40. PubMed.

  3. The paper by Cai et al. is the first description of retinal pathology in both BACE1 and BACE2 knockout (KO) mice. Fundamentally, it will be important to reproduce these data in other labs and in other BACE1 and BACE2 KO mouse lines. Notably, many of the reported phenotypes for BACE1 KO mice are not observed in genetically mixed mouse strains, indicating that they are unlikely to be observed in other species. Indeed, a previous report from May et al. (May et al., 2011) did not observe the same retinal pathology in BACE1 KO mice reported by Cai et al.

    It will also be important to reproduce with other BACE1 inhibitors the in-vitro and in-vivo studies reported by Cai et al. The concentrations of BACE inhibitor IV used by the authors in vitro were significantly higher than the Ki value, and thus, off-target effects cannot be discounted. In addition, the effect of BACE inhibitor IV in the in-vivo choroid neovascularization experiment in adult animals is different than predicted by the BACE1 KO data. It is suggested that BACE1 has distinct roles in the retina at different developmental stages, but this paradox requires further study.

    Most significantly, it is important to establish the reproducibility of these findings following chronic BACE1 inhibitor treatment of mice and other species prior to extrapolation to humans. In studies performed to date at Merck investigating chronic administration of different BACE inhibitors in rodent and non-rodent species, we have not observed retinal changes described in the paper by Cai et al.
      

    References:

    . Robust central reduction of amyloid-β in humans with an orally available, non-peptidic β-secretase inhibitor. J Neurosci. 2011 Nov 16;31(46):16507-16. PubMed.

  4. Thanks for reaching out regarding Lilly’s perspective on this paper. As is always the case with developing innovative medicines, we spend a significant amount of time trying to better understand potential organ effects related to a specific target. In the case of β-secretase inhibitors, we are following their potential impact on the retina very closely. This includes eye assessment monitoring in our Phase 2 study for LY2886721. Unfortunately, we won’t have additional insights to share until we have analyzed the results from this study.

  5. As can be seen in the references below, one hypothesis is that neuron glucose deprivation (neuroglycopenia) could be a trigger of Alzheimer's disease. Robert Vassar and colleagues reported that glucose deprivation increases BACE1 levels and Aβ production in the brains of the Tg2576 transgenic APP mouse model of AD. They identified the molecular mechanism of the BACE1 increase, showing that glucose deprivation induces phosphorylation of the translation initiation factor eIf2α, which in turn increases the translation of BACE1.

    Based on that logic, improving brain energy can indirectly lower or control the overexpression of BACE1. Increasing brain energy with, for example, an association of drugs and supplements that works as an alternative fuel and enhances the glucose metabolization by the neurons, it can lower the excessive eIF2α phosphorylation that, in consequence, will lower the BACE1 levels in a more "physiological" and less toxic way.

    Increasing brain energy with such supplements (that are the "precursor” substances to the development of drugs with a higher efficacy and effectiveness), we have the advantage to treat or prevent not only the BACE1 disorders, but we can also treat the other neuronal and astrocyte disorders that are consequences of brains under energy deprivation.

    There are some (still unexplored) ways of enhancing brain energy, such as giving alternative fuels and/or controlling factors that impair glucose metabolism in the brain. These could be substances that regulate the macrovascular and microvascular system; drugs and supplements that lower oxidative stress, peroxynitrite accumulation, and glycation; or substances that enhance mitochondrial function.

    Supplements such as L-glucuronolactone, L-glucaric acid, L-carnosine, glucosamine sulfate, and medium-chain triglycerides might be worth investigating, and can be the "precursors" to the development of drugs with a higher efficacy to treat brain energy disorders in most neurodegenerative diseases.

    References:

    . Energy inhibition elevates beta-secretase levels and activity and is potentially amyloidogenic in APP transgenic mice: possible early events in Alzheimer's disease pathogenesis. J Neurosci. 2005 Nov 23;25(47):10874-83. PubMed.

    . Phosphorylation of the translation initiation factor eIF2alpha increases BACE1 levels and promotes amyloidogenesis. Neuron. 2008 Dec 26;60(6):988-1009. PubMed.

    . BACE1, the Alzheimer's beta-secretase enzyme, in health and disease. Mol Neurodegener. 2012 Feb 7;7 Suppl 1:L3. PubMed.