This is Part 2 of a two-part series. See also Part 1.
15 April 2011. As scientists continue to pursue disease-modifying treatments for Alzheimer’s disease, various forms of oligomers have become the target du jour, and another way to disrupt them is with small molecules. JoAnne McLaurin at the University of Toronto, Ontario, updated AD/PD attendants on scyllo-inositol, which she said is gearing up for a Phase 3 trial after mixed results in Phase 2 (see ARF related news story). Previous studies suggested that scyllo-inositol works by binding Aβ42 oligomers and preventing large aggregates (see ARF related news story). In Barcelona, McLaurin presented new data indicating that the drug binding to Aβ42 promotes autophagy in TgCRND8 mice. Autophagy works poorly in these mice, as seen by the accumulation of large autophagic vesicles. Treatment shrank those enlarged vesicles and increased the amount of Aβ in microglia, suggesting the peptide is being cleared. It also reduced the amount of Aβ in brain blood vessels. McLaurin said her current hypothesis is that scyllo-inositol binds oligomers and helps them get phagocytosed. Autophagy has recently generated intense interest in both mechanistic and therapeutic AD research (e.g., see ARF related news story; ARF news story). Additionally, McLaurin performed microarray studies that showed 275 genes changed expression in the hippocampus after scyllo-inositol treatment. Of these genes, almost half have previously been implicated in AD.
What about other small molecules believed to inhibit aggregation? Christofer Lendel at the Swedish University of Agricultural Sciences, Uppsala, presented the results of testing two such compounds, the dyes Congo red and lacmoid. Both have been reported to interfere with oligomerization and have some similarities to methylene blue, which is currently under investigation as an anti-tau aggregant (see ARF related news story and ARF update). When the researchers looked closer at the biophysics, however, they found that Congo red actually promoted the assembly of Aβ into β-sheets (see Lendel et al., 2010). Lacmoid, on the other hand, preserves the random coil structure of Aβ and prevents its assembly, and thus is a true anti-aggregant. This illustrates the importance of characterizing a molecule’s mechanism of action with structure-based studies, Lendel noted.
Aging and Myelination
Age is the single greatest risk factor for AD, prompting a growing number of AD researchers to study what changes in the aging brain. For example, in monkeys, rats, and mice, levels of a transmembrane protein called Klotho drop with age, said Carmela Abraham of Boston University (see ARF related news story on Chen et al., 2007; Duce et al., 2008). In particular, Klotho levels decline in the brain’s white matter, and this correlates with age-related myelin deterioration, Abraham said. Establishing Klotho’s status as a longevity gene, Klotho knockout mice age prematurely and show hallmarks of human aging, whereas Klotho overexpressors live about one-quarter longer than normal mice (see Kuro-O et al., 1997; Kurosu et al., 2005). APP and presenilin double-transgenic AD mice have little Klotho in their brains, Abraham said. APP fragments have been shown to regulate Klotho expression (see ARF related news story), suggesting a connection between Klotho and AD. Exactly what Klotho does, however, besides suspected roles in insulin signaling and antioxidant activity, is still unclear.
To characterize Klotho’s role in white matter, Abraham’s team added the protein to cultured oligodendrocytes, which are the cells that form myelin. Klotho sped up the rate of the cells’ maturation, Abraham said. She noted that in normal mice, expression levels of Klotho and myelin proteins rise together shortly after birth, suggesting they are closely linked. Additionally, adult Klotho knockout mice have less myelin protein. Electron microscopy reveals that their optic nerve is only about 10 percent myelinated, versus 90 percent in control mice, Abraham said. Young knockouts have normal myelination, however, suggesting that Klotho plays a part in maintaining myelin with age but not in development. The gene might be a therapeutic target in multiple sclerosis (MS), a disease marked by myelin degeneration, Abraham said. She plans to cross MS model mice with mice overexpressing Klotho to see if the protein can promote myelin repair.
Abraham also investigated what lies upstream of Klotho to figure out why its levels decline with age. She found that in the white matter of aging monkeys, the Klotho promoter acquires more methyl groups, which tend to silence genes. Abraham’s team screened 150,000 compounds to find drugs that enhance Klotho expression. The high-throughput screen produced two leading candidates. Both increase Klotho expression in cultured opossum kidney cells and in choroid plexus cells, tissues that normally secrete the protein. These compounds could have promise as treatments for AD, MS, and potentially even in normal aging, Abraham said. It is not yet certain if Klotho levels decline in aging human brains, but since it is decreased in three other species, it is likely to decline in humans as well. However, Abraham told ARF that Klotho is secreted into the cerebrospinal fluid, where she plans to compare its levels in healthy people and those with AD.
Where Do Metals Come In?
Other scientists are taking a different approach to AD treatment, looking at the role of metals in the disease (see ARF story on Top 13 Trends in 2010). Iron has been repeatedly linked to AD, as iron levels in the brain go up with aging (see ARF related news story) and APP has been shown to export iron (see ARF related news story). Moussa Youdim at Technion Israel Institute of Technology in Haifa discussed an iron chelator, M30, which shows therapeutic potential. M30 has a chimeric structure and contains a piece from rasagiline, a PD drug, said Orly Weinreb at Technion. Youdim said M30 has multimodal effects, not just binding iron but also inhibiting monoamine oxidase and affecting the levels of several neurotransmitters such as serotonin, dopamine, and acetylcholine. Ten-month-old APPswe/PS1 mice treated with M30 showed better memory and had less plaque pathology, soluble Aβ, and phosphorylated tau, Youdim said, adding that the drug increased the expression of growth factors and protected the mice from neurodegeneration. Because M30 is lipophilic, it enters the brain well, Youdim said, and is not toxic at effective doses in mice. M30 itself is not a candidate for human development, however, and Youdim said his group is currently developing a more stable variant for Phase 1 studies. M30 may also help Parkinson’s patients (see Gal et al., 2006) and people with amyotrophic lateral sclerosis (ALS). The drug improved survival in a mouse model of ALS, Weinreb said, adding that a Phase 1 trial for ALS patients will start soon. To date, no drug that was reported to extend survival in an ALS mouse model has subsequently worked in human trials (see Alzforum Webinar).
Zinc has also been linked to AD. Zinc is normally released during neurotransmission and affects NMDA receptors involved in long-term potentiation. Researchers at the University of Melbourne found that mice that lack zinc in their synapses develop AD-like symptoms (see ARF related news story). Amyloid plaques sequester zinc and copper, said Rudy Tanzi at Massachusetts General Hospital, Boston, and this could drive some of the synaptic and memory defects of the disease. In addition, zinc and copper promote Aβ aggregation, creating a vicious circle. Tanzi is a co-founder and shareholder in Prana Biotechnology Ltd., Parkville, Australia, a biotech company developing a zinc/copper ligand called PBT2. PBT2 had beneficial effects in AD mice (see ARF related news story) and in a short Phase 2 trial, where it improved some cognitive measures in people with mild AD (see ARF related news story). More people improved on the 250 mg dose than on the 50 mg dose, indicating a dose-dependent response (see Faux et al., 2010). PBT2 was recently shown to increase the number of dendritic spines in the hippocampus of AD mice and restore levels of NMDA receptor subunits to normal levels (see Adlard et al., 2011). After some attempts to secure industry financing for further trials of PBT2, Prana recently announced that the Alzheimer’s Drug Discovery Foundation in New York City will fund a one-year Phase 2 trial to begin in late 2011 in Australia (see press release). The trial will enroll 40 patients who have mild AD and amyloid deposition in the brain as seen by positron emission tomography with Pittsburgh Compound B. Scientists will look for changes in amyloid burden as the primary outcome.—Madolyn Bowman Rogers.
This is Part 2 of a two-part series. See also Part 1.