Evidence supporting a role for the Mint/X11 proteins as regulators of APP metabolism and Aβ production has been accumulating for 10 years now. Unfortunately, much of the data yield opposing models; Mints appear capable of imparting pro- or anti-amyloidogenic effects. Typically, when this stark a disagreement occurs, the cause is either that we are missing an important part of the puzzle or the process is much more complicated than we envision.
This latest paper from the Sudhof lab (Ho et al., 2008) sought to resolve the issue by performing an extensive (and impressive) array of assays of APP metabolism in APPswe/PS1dE9 mice deleted for each of the three Mint family members. This is a first-rate paper with very high-quality data that address an important question in the field of Alzheimer’s research. Unfortunately, it does not resolve the differences, but it does provide new data that may help focus the search on the source of the differences and the missing pieces. Two key observations are discussed, and we note one concern.
They found that deletion of any one of the three Mints (Mint1-3, aka X11; X11L (X11-like); and X11L2 (X11-like2) delayed the progression of plaques in this mouse model and decreased the products of β-site cleavage of APP and the levels of Aβ40 and Aβ42, particularly at earlier times. This interesting observation is surprising for two reasons. First, the three Mints are differentially expressed in the brain, with Mint1 predominantly in interneurons, Mint2 in pyramidal neurons, and Mint3 ubiquitously expressed. Thus, for each of them to have such strong effects on plaque development and APP metabolism requires either that 1) pathologic Aβ generation occurs in multiple cell types, 2) cells expressing submaximal amounts of Mints may be key to amyloidogenesis, or 3) deletion of one Mint can impact the actions of the others. Indeed, some important data are found in Supplementary Table 2, showing that knockouts of Mint1 or Mint2 significantly decrease (by 55 percent and 32 percent, respectively) the expression of the other, while knocking out Mint3 only lowers Mint2 levels (19 percent decrease). It appears that these values represent changes in Mint expression in the whole brain, so these differences would be magnified in the relevant cell types. The other surprising aspect of these data is that they appear to be completely opposite to two studies from the Suzuki laboratory (Sano et al., 2006; Saito et al., 2008), which reported that knockout of Mint1 or Mint2 led to increased β-site cleavage of APP and increased generation of Aβ peptides. The two laboratories also differ in that the Suzuki lab reports no changes in the levels of Mint1 when Mint2 was deleted and vice versa. Ho et al. (Ho et al., 2008) cites strain differences as a possible source of these differences. While the Suzuki lab clearly states C57BL/6 as the predominant background in their studies, it is difficult to determine the strain background used by Ho et al. If such dramatic differences in the effects of Mints on amyloidogenesis are really the result of strain differences, then it may highlight a key finding that could well prove important in humans. Two other issues are viewed as potentially contributing to these apparent differences, though seem unlikely to explain it all. These are the time points studied (Ho et al. look at three time points of six, nine, and 12 months and noted several differences in effects at different times, while the Suzuki lab focused on 12 months) and the fact that the Ho et al. work is all performed in mice engineered to express the amyloidogenic human APPswe and PS1dE9 mutants. For example, the Ho et al. study found that when Mint3 was knocked out the levels of Aβ in both the cortex and the hippocampus were decreased at three months but increased at the 12-month time point (the same trend and time point observed in Saito et al., 2008).
The concern we note is an issue with both the Sudhof and Suzuki lab publications, and that is the general underappreciation of the potential role of Mint3 in amyloidogenesis. Because Mint1 and Mint2 are expressed in a neuronal-specific fashion, researchers have consistently minimized or ignored the ubiquitously expressed Mint3. However, Mint3 is present in human brain and in an expression pattern reflecting that of Mint2 (Shrivastava-Ranjan et al., 2008) and APP. Ho et al. (Ho et al., 2008) clearly document a strong effect of Mint3 deletion on amyloidogenesis, particularly at the critical early time points, and on Mint2 expression levels. All three Mint proteins share the central and APP-binding PTB domain and C-terminal, dual PDZ domains that also are required for their recruitment to membranes through direct binding to ARF GTPases. They differ in their N-termini, with Mint3 lacking any additional domains, Mint1 and Mint2 having a Munc18 interacting domain (from where the name Munc18-interacting proteins is derived), and only Mint1 having a CASK binding domain, making it the sole member of the Mint family found in the heterotrimeric complex of Mint1/CASK/Velas. The observation in Ho et al. that CASK knockout mice do not share the phenotypes they report for the Mint1 knockout supports the conclusion that Mint1 also has actions independent of that trimeric complex. Similarly, Mint2 is found both at the cell surface and at the Golgi in cultured cells, while Mint3 appears to be predominantly at the Golgi. We have recently shown that Mint2 and Mint3 are present on the post-Golgi vesicular carriers that transport APP from the Golgi toward the cell surface (Shrivastava-Ranjan et al., 2008). Thus, the data are growing that Mint2 and Mint3 are mediators of APP traffic and have all the features of Arf-dependent adaptors that put them squarely in the same sphere as GGAs, putative mediators of SorLa/LR11 and β-secretase traffic.
If one is looking for a molecular model that may explain the confusing and inconsistent data from studies of Mints in APP metabolism, it is sufficient to invoke the field of membrane traffic, an equally confusing and discrepant field. As basic scientists we view Alzheimer disease as a membrane traffic disorder whose molecular mechanisms will only be understood in the context of the normal regulation of sorting and residency of APP and other cargos (e.g., secretases and interactors like LR11) as they move throughout the endomembrane system. We strongly encourage members of the two labs to exchange their valuable animals and pursue these questions in hopes of either identifying a previously unknown piece of this puzzle or unraveling this increasingly complex but critical mechanism.
References:
Ho, A., Liu, X., and Sudhof, T.C. (2008). Deletion of Mint proteins decreases amyloid production in transgenic mouse models of Alzheimer's disease. J Neurosci 28, 14392-14400. Abstract
Saito, Y., Sano, Y., Vassar, R., Gandy, S., Nakaya, T., Yamamoto, T., and Suzuki, T. (2008). X11 Proteins Regulate the Translocation of Amyloid {beta}-Protein Precursor (APP) into Detergent-resistant Membrane and Suppress the Amyloidogenic Cleavage of APP by {beta}-Site-cleaving Enzyme in Brain. The Journal of biological chemistry 283, 35763-35771. Abstract
Sano, Y., Syuzo-Takabatake, A., Nakaya, T., Saito, Y., Tomita, S., Itohara, S., and Suzuki, T. (2006). Enhanced amyloidogenic metabolism of the amyloid beta-protein precursor in the X11L-deficient mouse brain. The Journal of biological chemistry 281, 37853-37860. Abstract
Shrivastava-Ranjan, P., Faundez, V., Fang, G., Rees, H., Lah, J.J., Levey, A.I., and Kahn, R.A. (2008). Mint3/X11{gamma} Is an ADP-Ribosylation Factor-dependent Adaptor that Regulates the Traffic of the Alzheimer's Precursor Protein from the Trans-Golgi Network. Molecular biology of the cell 19, 51-64. Abstract
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