25 July 2007. Approved therapies for Alzheimer disease—acetylcholinesterase inhibitors and the NMDA antagonist, memantine—primarily tackle disease symptoms. What are badly needed are so-called disease-modifying drugs, ones that attack the underlying pathology and slow or halt disease progression. Not surprisingly, various components of the amyloid-β (Aβ) cascade have been singled out as potential disease-modifying targets, including Aβ itself and the β- and γ-secretases that are needed for its production. Two slightly different approaches to Aβ therapy are described in last week’s PNAS online. Milan Fiala, University of California School of Medicine, Los Angeles, and colleagues report that a curcuminoid boosts the immune system in AD patients and may promote clearance of Aβ, while Amelia Marutle, University of Central Florida, Orlando, and colleagues report that the multi-talented phenserine may help relieve AD-related suppression of neurogenesis.
A Plus-side to Phenserine
Phenserine, an acetylcholinesterase inhibitor, has been in the news before. Its development for AD was stopped in the fall of 2005 when phase 3 clinical trials showed that it had no effect on the primary endpoint, the ADAS-Cog test of cognition (see ARF related news story). However, in addition to blocking acetylcholinesterases, phenserine also reduces levels of Aβ and its precursor protein (APP) in neuronal cells. Though it is not clear how this reduction occurs, it seems specific to the + enantiomer, which does not inhibit esterases. Now, Marutle and colleagues show that in transgenic mouse models of AD, (+)-phenserine reduces APP levels and also stimulates differentiation of human neural stem cell progenitors into neurons. The compound may not only serve as a useful tool for studying neurogenesis in AD and other neurodegenerative disorders, but it may point the way to novel therapeutics.
To see if phenserine can lower levels of Aβ and APP in vivo, Marutle and colleagues administered the + enantiomer to APP23 mice, which produce human APP with the Swedish mutation. When given to 6- to 8-month-old mice, (+)-phenserine reduced levels of APP in the hippocampus by about 38 percent over 2 weeks. There was no effect on APP mRNA levels, suggesting that the drug acts post-transcriptionally. The authors found that levels of glial fibrillary acidic protein (GFAP) were also reduced by the compound. Because elevated GFAP in APP23 mice is due to gliogenesis, they looked to see what effect (+)-phenserine might have on neuronal stem cells. The drug reduced the number of dividing (BrdU-positive) GFAP-positive precursor cells in the hippocampi of APP23 mice that were transplanted with human neuronal stem cells (HNSCs). In contrast, the drug had no effect on the differentiation of HNSCs transplanted into wild-type mice. The results indicate that “(+)-phenserine reduces glial differentiation caused by APP overexpression,” write the authors.
If HNSCs are not differentiating into glia, then what is their fate? One possibility is that they may just be dying out, but Marutle and colleagues detected few HNSC-derived apoptotic cells in the treated animals. Another possibility is that more of the cells are going on to become fully fledged neurons. To test this, the researchers stained mouse brain tissue for human β-III tubulin, a neuronal marker. They found that in the presence of (+)-phenserine there was a significant increase (up to 112 percent) in the number of human neuronal cells in the CA1 and CA2 regions of the hippocampus and also in the motor and somatosensory cortex in APP23 mice. In wild-type mice, the phenserine treatment also induced increases in neurogenesis, but only in the CA1 region of the hippocampus.
Given the latter finding, it seems that (+)-phenserine has other effects on neurogenesis in addition to relieving suppression brought on by overproduction of human APP. “Thus, future studies will be crucial for investigating the specific molecular mechanisms underlying this phenomena [sic], as well as comparative studies for determining the efficacy of various doses of (+)-phenserine,” write the authors.
Currying Favor from the Immune System
Another approach to fighting AD progression is to promote Aβ clearance from the brain. Various active and passive immunotherapies are currently being investigated in this regard (see ARF related news story), but the paper from Fiala and colleagues suggests therapy of a slightly different flavor. They show that curcuminoids, found in turmeric and other spices, stimulate the macrophages and microglia of the innate immune system, which mop up accumulating Aβ and other undesirables.
The authors previously reported that a naturally derived curcuminoid extract spurs macrophages to take up Aβ via phagocytosis (see Zhang et al., 2006). To identify the chemical entity responsible for this stimulation, they separated and tested how individual components of the curcuminoid mixture affect Aβ uptake by human macrophages. They found that bisdesmethoxycurcumin, a minor component, was most potent at stimulating Aβ phagocytosis. The authors confirmed this when they used the chemically synthesized curcumin in the same assays.
It is not exactly clear how bisdesmethoxycurcumin stimulates Aβ phagocytosis, but the authors report that it upregulates macrophage expression of N-acetylglucosaminyltransferase III (GlcNAc-TIII) and Toll-like receptor genes. The latter play a crucial role in the innate immune system because they help cells recognize molecular patterns that are shared by many pathogens. When Fiala and colleagues added Aβ to peripheral blood mononuclear cells (PBMCs) from control volunteers, they found that these genes were upregulated in most cases. In PBMCs from AD patients, however, Aβ downregulated these genes and the curcuminoid helped to correct this. Bisdesmethoxycurcumin increased transcription of the GlcNAc-TIII gene in PBMC samples from four patients, while in the one PBMC sample tested, the curcuminoid also stimulated expression of all 10 Toll-like receptors.
Curcuminoids have been shown to be powerful antioxidants that also help break up Aβ aggregates (see ARF related news story). But this work suggests they have a third mode of action. “Thus, our results may provide an entirely different direction to therapeutic opportunities in AD through the repair of the functional and transcriptional deficits of AD macrophages by curcuminoids,” conclude the authors.—Tom Fagan.
Marutle A, Ohmitsu M, Nilbratt M, Greig NH, Nordberg A, Sugaya K. Modulation of human neural stem cell differentiation in Alzheimer (APP23) transgenic mice by phenserine. PNAS. 2007, July 16. Abstract
Fiala M, Liu P, Espinosa-Jeffrey A, Rosenthal M, Bernard G, Ringman J, Sayre J, Zhang L, Zaghi J, Dejbakhsh S, Chiang B, Hui J, Mahanian M, Baghaee A, Hong P, Cashman J. Innate immunity and transcriptional down-regulation of MGAT-III and Toll-like receptors in Alzheimer’s disease patients are improved by bisdesmethoxycurcumin. PNAS 2007, July 16. Abstract