Trying to follow Alzheimer’s research sometimes feels like walking through an Escher exhibit: the contradictions can border on the absurd.
Take the new findings on SIRT1 and its relation to Alzheimer’s. Research after research shows that SIRT1 apparently protects against Alzheimer’s:
25 July 2010. The sirtuin protein SIRT1 is emerging as an important player in learning and memory, and may have potential as a therapeutic target in Alzheimer disease. Fresh on the heels of a July 11 Nature paper that demonstrated a crucial role for SIRT1 in memory (see ARF related news story on Gao et al., 2010), two new papers add to the growing body of evidence that SIRT1 helps keep brains healthy. In a paper appearing July 21 in the Journal of Neuroscience, researchers led by Valter Longo at the University of Southern California, Los Angeles, show that a SIRT1 knockout mouse has numerous defects in learning and memory. This finding implies that SIRT1 could have a protective role in AD, and indeed, in a July 23 Cell paper, researchers led by Leonard Guarente at the Massachusetts Institute of Technology, Cambridge, report that overexpression of SIRT1 can decrease Aβ production and the number of amyloid plaques in a mouse model of AD.
You’d think, then, that more SIRT1 is better for Alzheimer’s and less is worse. But:
Michán and colleagues also examined a transgenic mouse that overexpressed SIRT1 16-fold in the brain. On this normal mouse background, the authors found that this massive SIRT1 overexpression conferred no improvements in learning or memory, and that synaptic function was unchanged except for a slight increase in neuronal excitability.
And though less is worse, vitamin B3 in the form of niacinamide has been shown to “cure” Alzheimer’s in mice by decreasing the expression of SIRT1: Nicotinamide Restores Cognition in Alzheimer’s Disease Transgenic Mice via a Mechanism Involving Sirtuin Inhibition and Selective Reduction of Thr231-PhosphotauWe evaluated the efficacy of nicotinamide, a competitive inhibitor of the sirtuins or class III NAD+-dependent HDACs in 3xTg-AD mice, and found that it restored cognitive deficits associated with pathology. Nicotinamide selectively reduces a specific phospho-species of tau (Thr231) that is associated with microtubule depolymerization, in a manner similar to inhibition of SirT1. Nicotinamide also dramatically increased acetylated -tubulin, a primary substrate of SirT2, and MAP2c, both of which are linked to increased microtubule stability. .
When asked about this contradiction, Dr. Greene, one of the researchers on this paper says,
You are correct – there are contradictions between the role of Sirt1 in AD. Regardless of these, nicotinamide has good effects in the preclinical models, and has been shown to now be effective for other neurodegenerative diseases as well. Sirt1 may be beneficial at some stages of the disease, and not others – we cannot [reconcile] these differences at this stage, but our research says that nicotinamide is highly effective in preclinical models and that inhibition of Sirt1 plays a role in these effects.
My mind wants to hyperventilate with the contradictions, but then I remember the story of the three blind men describing an elephant and realize the contradiction exists only because we do not yet fully understand.
And that’s what drives research onward.