Available treatments for Alzheimer’s disease offer relatively small symptomatic benefits and more effective treatments are much needed. β-Amyloid has been implicated in the pathogenesis of Alzheimer’s disease and much attention has focussed on inhibiting two enzymes responsible for production of this peptide, the β- and γ-secretases. Although potent inhibitors of both enzymes have been identified, achieving robust activity in animal models and progression to clinical studies has proved more challenging. Using a high-throughput functional genomics screen, scientists at VIB and Galapagos Pharmaceuticals have now identified a constitutively expressed orphan G protein-coupled receptor, GPR3, as another modulator of β-amyloid production. GPR3 is highly expressed in areas of the brain associated with Alzheimer’s disease and levels are elevated in brain tissue from people with sporadic Alzheimer’s disease. The researchers showed that blocking GPR3 prevented accumulation of β-amyloid, both in cell culture experiments and in a mouse model of Alzheimer’s disease.
Since G protein-coupled receptors have been relatively easy to exploit as drug targets, the scientists hope that GPR3 will prove to be a promising target for the treatment of Alzheimer’s disease.
The study is published in the February 13th issue of the journal Science.
There are no medicines that can prevent Alzheimer’s disease and current drugs ease symptoms but do little to stop progression of the disease. There is consequently much current interest in whether lifestyle choices or herbal remedies can prevent or alter the course of Alzheimer’s disease. Two recent reports describe the effect of marijuana-like compounds in elderly rats and extracts of Gingko Biloba in elderly humans.
Previous work has suggested that people who regularly smoked marijuana in the 1960s and 1970s are less likely to develop Alzheimer’s disease in later life. Now scientists at Ohio State University have described details of how constituents of marijuana can combat inflammation and possibly stimulate neurogenesis. The team had found that treatment with WIN-55212-2, a compound with similar pharmacological properties to tetrahydrocannabinol (THC), made a small improvement to the ability of elderly rats to perform memory tests. They then went on to look at how the drug was working, and showed that it lowered inflammation in the hippocampus by acting on the TRPV1 receptor. The team also showed that the action of WIN-55212-2 on CB1 and CB2 receptors led to the generation of new brain cells. The group are continuing to investigate which receptors are most important for reducing inflammation and stimulating neurogenesis. This knowledge could lead to the discovery of new drugs to reduce inflammation and increase the production of new neurons before memory loss becomes apparent.
WIN55212-2 is a full agonist at the CB1 receptor and is not approved as a drug since it elicits cannabis-like effects in humans.
A second report discusses the results of the first randomized, double-blind trial to assess the effectiveness of Ginkgo biloba in reducing the incidence of dementia. Ginkgo is an antioxidant that helps to protect cells in vitro from oxidative damage. Some studies have suggested that ginkgo may also protect against the toxic effects of β-amyloid. Although previous clinical trials of ginkgo leaf extracts have had mixed results, many older people continue to use the herb in the hope of warding off the onset of Alzheimer’s disease. The new study, however, has shown that ginkgo gave no benefit in reducing all-cause dementia or Alzheimer’s dementia. The study involved more than 3000 men and women aged 75 or older who were randomised to placebo or treatment groups. The treated group received 120mg ginkgo twice a day for an average of 6 years. At the end of the study, dementia developed in 246 people taking ginkgo compared with 277 people in the placebo group. The authors note that an effect may have been seen had the study carried on for longer since it is known that there is a significant time lag between initial brain changes and the manifestation of clinical dementia.
The amyloid plaques characteristic of Alzheimer’s disease are made up of small peptides formed by cleavage of amyloid precursor protein (APP). Although the primary function of APP is poorly understood, it is believed to regulate synapse formation and neural plasticity. Synaptic activity of APP involves membrane microdomains containing syntaxin whereas amyloidogenic cleavage occurs primarily in cholesterol-rich lipid raft regions containing one of the proteases needed for APP cleavage, BACE. APP preferentially associates with syntaxin microdomains, but neuronal stimulation causes APP to associate instead with BACE-containing microdomains. It is unclear why this trafficking occurs, but a recently published study in the Journal of Cell Biology showed that movement of APP between the two membrane microdomains is an active process, involving cyclin-dependent kinase 5 (CDK5).
The study also showed that treatment of neurons with the CDK5 inhibitor, roscovitine, which is currently undergoing clinical trials as a treatment for cancer, reduced the association of APP with BACE-rich microdomains, and reduced cleavage.
Alzheimer’s disease is characterised by the presence of neurofibrillary tangles and deposits of amyloid peptides (Aβ) in the brain. N-terminally truncated and pyroglutamate-modified Aβ peptides (Aβ(pE)) are resistant to proteolysis, aggregate more readily than the unmodified peptides, and have been implicated in the initiation of events leading to the development of Alzheimer’s disease. In post mortem examinations, brains from Alzheimer’s patients were found to contain significantly larger amounts of pyroglutamate-modified peptides than brains from aged controls. Aβ3(pE)-42, particularly, is a major component of Aβ deposits in both familial and sporadic Alzheimer’s disease.
A recent letter to the journal Nature identifies glutaminyl cyclase as the enzyme which catalyses the N-terminal pyroglutamate formation in vivo. Glutaminyl cyclise protein and mRNA were found to be up-regulated in Alzheimer’s disease patients and significantly larger concentrations of Aβ3(pE)-42 were also detected. In HEK cells co-expressing amyloid precursor protein (APP) and glutaminyl cyclise, formation of Aβ3(pE)-42 was suppressed by the glutaminyl cyclase inhibitor, PBD150. Different studies looking at the effect of oral dosing of PBD150 to transgenic mice over 3, 6 or 10 months showed dose-dependent decreases in Aβ3(pE)-42 and reduced de novo plaque formation. Transgenic mice treated with PBD150 also showed improved memory as determined by a conditioned fear procedure.
The results suggest that formation of Aβ3(pE)-42 can be reduced by inhibition of glutaminyl cyclise without any effect on previously formed deposits and provide evidence that Aβ3(pE)-42 can act as a seed for aggregation. If inhibition of glutaminyl cyclise is similarly effective in humans, this could provide a new approach to disease-modifying treatment for Alzheimer’s disease.
Amyloid peptide oligomers are believed to contribute to the pathology of Alzheimer’s Disease and much effort has gone into developing inhibitors of the β- and γ-secretases which are key to production of these peptides. An alternative approach to reducing amyloid peptide levels would be to increase their degradation. A number of proteases, including neprilysin, insulin-degrading enzyme and plasmin have been shown to degrade amyloid peptides.
A recent report now describes how increasing plasmin levels by inhibiting plasminogen activator inhibitor-1 (PAI-1) can lead to lowered brain amyloid levels. The PAI-1 inhibitor, PAZ-417, was found to lower brain and plasma amyloid levels and reverse cognitive deficits in mouse models of Alzheimer’s Disease.
PAZ-417 is currently undergoing Phase I clinical studies to investigate safety, tolerability and pharmacokinetics in young and elderly volunteers.
Calpains are calcium-activated cysteine proteases which, when abnormally activated, can initiate degradation of proteins essential for neuronal survival. A report in Journal of Clinical Investigation describes the effect of the selective calpain inhibitor, BDA-410, in a transgenic mouse model of Alzheimer’s Disease.
Calpain inhibition by BDA-410 improved spatial-working memory and associative fear memory in APP/PS1 mice. The authors put forward a hypothesis in which amyloid peptides trigger a cascade leading to calpain activation and, ultimately, synaptic dysfunction and cognitive abnormalities. BDA-410 did not alter levels of amyloid oligomers or plaque load but restored normal phosphorylation levels of transcription factor CREB and normal distribution of synaptic protein, synapsin I. These results suggest that calpain inhibition might provide a new strategy for alleviating memory loss in Alzheimer’s Disease.
BDA-410 has previously been shown to be a potent inhibitor of cysteine proteases of the malaria parasite, Plasmodium falciparum.
It is widely believed that reducing amyloid plaque formation will provide a disease-modifying treatment for Alzheimer’s Disease, either halting or slowing cognitive decline. Amyloid peptides, which aggregate to form neurotoxic plaques, are formed by proteolysis of a precursor protein by the action of two enzymes, β-secretase and γ-secretase. Inhibition of either of these proteases should prevent the formation of amyloid peptides, and much effort has been devoted to the identification of inhibitors. γ-Secretase, particularly, is a promiscuous enzyme and hydrolyses a number of other substrates, including Notch. The Notch signalling pathway is important in many cellular processes and so modulators rather than inhibitors of γ-secretase are preferred.
Relatively early in the search for modulators of γ-secretase, the surprising discovery was made that certain nonsteroidal anti-inflammatory drugs (NSAIDs), including ibuprofen, effectively suppress amyloid peptide production while sparing processing of Notch and other γ-secretase substrates. Further studies identified tarenflurbil (R-flurbiprofen, Flurizan, MPC-7869) as a γ-secretase modulator with the potential to treat Alzheimer’s Disease. A recent report in Nature (Kukar et. al., Nature 2008, 453, 925-929) has now shed new light on how such compounds prevent amyloid formation.
Using photoprobes, the group were able to show that flurbiprofen does not bind to the γ-secretase protein complex but, instead, binds to the amyloid precursor protein.
Disappointingly, Myriad Genetics have recently discontinued development of Flurizan since it failed in a phase III clinical trial in Alzheimer’s patients.
The study by Kukar et. al. does, however, suggest that it may be possible to identify other small molecules that reduce the formation of amyloid peptides by binding to the precursor protein rather than to the γ-secretase complex. It remains to be seen whether similar ‘substrate-binding’ inhibitors of other proteases can be identified.
This week’s news has brought several stories that offer hope to Alzheimer’s patients and their carers. An experimental drug, Rember™, being developed by TauRx Therapeutics has been shown to slow progression of the disease. Rember™ is a new formulation of an old drug, methylene blue. The drug reduces the abnormal tangles of tau protein that are found in Alzheimer’s patients and thought to contribute to the disease. These tangles destroy neurons and their presence is strongly correlated with dementia. The company hopes to carry out further trials and, if these are successful, Rember™ could become available to patients in 2012.
Other researchers have also reported encouraging results for an experimental drug that targets tau tangles. AL-108 (Allon Therapeutics) is derived from an eight amino acid peptide (NAPVSIPQ: “NAP”) synthesized from a naturally occurring neuroprotective brain protein known as activity dependent neuroprotective protein (ADNP). The drug was tested in people with mild cognitive impairment and improved specific memory functions that are relevant in Alzheimer’s Disease.
In separate reports, angiotensin receptor antagonists and statins have both been linked with improved symptoms in Alzheimer’s patients.
Research suggests that people taking angiotensin receptor antagonists to treat hypertension are up to 45% less likely to develop Alzheimer’s Disease or dementia. The research was carried out by scientists at the Boston University School of Medicine and presented at the 2008 International Conference on Alzheimer’s Disease in Chicago. People taking the cholesterol lowering drugs known as statins for 5-7 years were also found to be at reduced risk of developing dementia according to a study published in the July issue of Neurology. (Neurology, Jul 2008; 71: 344 – 350)
A study published in the July 18th issue of The Lancet shows that a drug once used in Russia to treat hayfever has the potential to improve symptoms in dementia patients. The study of 183 patients, tested dimebon (dimebolin) vs placebo in patients with untreated mild-to-moderate dementia. Patients taking dimebon improved over a six month period whilst those taking placebo got worse.
A smaller group of patients who continued taking dimebon for a further six months showed continuing improvement over this period. This ongoing improvement is seen as particularly important since none of the approved drugs for Alzheimer’s Disease has shown increasing improvement over twelve months. Although this was a relatively small study, the initial results are very encouraging and warrant further investigation.
In a separate study, also reported in The Lancet, immunisation against the amyloid-beta peptide was shown to clear amyloid plaques from the brain, but not to prevent the progressive neurodegeneration associated with Alzheimer’s disease.