Freitag K, Sterczyk N, Schulz J, Houtman J, Fleck L, Sigrist SJ, Heppner FL, Jendrach M. The autophagy activator Spermidine ameliorates Alzheimer’s disease pathology and neuroinflammation in mice. bioRxiv, December 28, 2020. bioRxiv.

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  1. Immuno-Metabolism of Arginine as a Target for AD

    We were the first to show increased Arginase-1 (Arg-1) expression in AD brains as part of a larger investigation into an alternative immune activation phenotype in early stages of AD that affects arginine metabolism (Colton et al., 2006). Arginine metabolism to polyamines, now referred to as a Polyamine Stress Response (PSR), features prominently in Alzheimer’s and related diseases, but not in healthy individuals (Graham et al., 2015; Mahajan et al., 2020; and others). Unlike others, we employed a mouse model of AD that was engineered to have human-like levels of nitric oxide, which are roughly 100 fold-lower than in mice (Colton et al., 2006; Hoos et al., 2014; Young et al., 2018). We found that these CVN-AD mice had amyloid plaques, neurofibrillary tangles (NFTs), neuronal loss, and learning and memory deficits that increased over time (Colton et al., 2014). 

    Since arginine is a conditionally essential amino acid with limited availability in the brain, we reasoned that arginine would be preferentially metabolized via the polyamine pathway in CVN-AD mice. Thus, we employed difluoromethylornithine (DFMO), a clinically proven inhibitor of the rate-limiting enzyme for polyamine synthesis, ornithine decarboxylase (ODC) (Bailey et al., 2010). We reported that DFMO treatment of CVN-AD mice significantly improved learning and memory performance, while reducing soluble and insoluble Aβ 40/42, amyloid plaques, and CD11c-positive microglial cells (Kan et al., 2015). We also reported that DFMO treatment reduced expression of several enzymes associated with the polyamine pathway (Kan et al., ibid.). Reinforcing our idea that inhibiting the polyamine pathway would be therapeutic in AD, Polis et al. (2018) employed an arginase inhibitor in 3xTg-AD mice and also reported improved behavioral performance and dendritic spine densities. Thus, two different inhibitors of the polyamine pathway in two different mouse models of AD resulted in improved behavioral performance and reductions of Alzheimer’s pathologies. 

    Modification of Graphical Overview from Sandusky-Beltran et al., 2021.

    Other groups took the opposite approach of increasing polyamine levels as a potential therapy for reducing NFTs, which feature prominently in AD brains. Using a tau transgenic mouse, Hunt et al. reported that overexpression of arginase-1 (to increase polyamine levels) resulted in reduced deposition of phospho-tau aggregates, but did not change neuron loss compared to controls (Hunt et al., 2015). This same group now reports that increasing polyamine levels by a genetic strategy to increase ornithine decarboxylase activity resulted in greatly enhanced NFT pathology in a second tau transgenic (Sandusky-Beltran et al., 2019). Thus, their new in vivo data shows that increasing polyamines is linked to an increase in NFTs in a tau transgenic mouse, a result that recapitulates the situation in an AD patient’s brain.

    The idea that unmodified polyamines can inhibit tau aggregation (see image above) and reduce NFTs is not supported by the data presented in Sandusky-Beltran et al. (2021). Specifically, their Figure 3E (below) shows that the percent area of AT8-positive NFT staining of the cortex, CA3, and dentate gyrus regions of the hippocampus are all significantly higher when putrescine and acetyl-spermidine levels were doubled. Interestingly, putrescine is about 20 μM in the PS19 tau mice overexpressing AZIN2, while acetylspermidine is about 3 μM in these same brains. In comparison, the spermidine is about 1000 μM and spermine is about 10 μM in these same brains. Although there is a net increase of about 1.5 μM acetyl-spermidine in AZIN2/PS19 mouse brains, this increase is dwarfed 666-fold when compared to the existing levels of spermidine. So, if unmodified spermidine reduces tau aggregation and deposition, why are AT8-positive NFT structures present in PS19 brains? Going further, Inoue et al. also showed that putrescine, spermidine, acetylspermidine, and acetylspermine levels are all roughly doubled in AD patients compared to control brains (Inoue et al., 2013). AD patients’ brains prominently display NFTs and spermidine is always higher than the other polyamines. These data from AD patients, where NFT pathology contributes to the definition of a person with AD, further support that increased polyamines do not reduce tau aggregation and deposition.

    Therapeutically, the matching data from our and the Polis lab suggest a useful treatment approach where inhibition of the polyamine pathway results in a reduction of Alzheimer’s pathologies and improved behavioral performances.

    PS19 mice were injected with AAV9-AZIN2 virus to overexpress AZIN2 protein and enable increased levels of polyamines in the brains (red squares) or with AAV9-EC-empty cassette as a control (black circles). AZIN2 expressers have significantly more AT8-positive neurofibrillary tangles than found in PS19 mice infected with an empty viral vector.  Thus, the Graphical Overview (above) is incorrect in depicting that unmodified polyamines can block tau seeding and/or aggregation. © American Society for Clinical Investigation. 

    References:

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    Inoue K, Tsutsui H, Akatsu H, Hashizume Y, Matsukawa N, Yamamoto T, Toyo'oka T. Metabolic profiling of Alzheimer's disease brains. Sci Rep. 2013 Aug 6;3:2364. PubMed.

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    Mahajan UV, Varma VR, Griswold ME, Blackshear CT, An Y, Oommen AM, Varma S, Troncoso JC, Pletnikova O, O'Brien R, Hohman TJ, Legido-Quigley C, Thambisetty M. Dysregulation of multiple metabolic networks related to brain transmethylation and polyamine pathways in Alzheimer disease: A targeted metabolomic and transcriptomic study. PLoS Med. 2020 Jan;17(1):e1003012. Epub 2020 Jan 24 PubMed. Correction.

    Polis B, Srikanth KD, Elliott E, Gil-Henn H, Samson AO. L-Norvaline Reverses Cognitive Decline and Synaptic Loss in a Murine Model of Alzheimer's Disease. Neurotherapeutics. 2018 Oct;15(4):1036-1054. PubMed.

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