Triple-transgenic model of Alzheimer's disease with plaques and tangles: intracellular Abeta and synaptic dysfunction. - AlzForum Alzheimer Research Paper of the Week

If this data holds true, it is very good news in the field. Abe Fisher has been working for years to develop highly specific muscarinic (M1) agonists. He is more advanced on that than anyone else, as the major pharmaceutical companies have abandoned that front for AD therapeutics, mostly due to low efficacy and undesirable side effects. There is a rationale for a "good" M1 agonist in AD. First, there is the clear effect of the M1 receptor-driven switch towards a non-amyloidogenic APP metabolism, i.e., stimulation of ADAM secretases. Second, there is the intrinsic cognitive effect of muscarinic agonists. Third, and less well proven, there is the possibility that the muscarinic stimulation favors endogenous production of neurotrophic factors.

View all comments by A. Claudio Cuello

This data confirms our work, done in collaboration with Abraham Fisher, showing that AF267B and two other of his M1 agonists (AF102B, AF150S) all lower CSF and cortical Aβ concentrations in normal rabbits [1]. This confirms many years of in vitro work going back to 1992, when Roger Nitsch showed that M1 receptor activation shifts APP processing into the non-amyloidogenic pathway [2]. We have also demonstrated the opposite effect, in vivo, that decreasing cortical M1 receptor activation by lesioning the nucleus basalis magnocellularis (nbm) results in increased amyloidogenic processing of APP and Aβ deposition [3] and that treatment with AF267B prevents this deposition [4]. The aggregate data suggest a fusion of the cholinergic and amyloid hypotheses: cortical cholinergic deafferentation occurs during preclinical AD [5-7] and leads to Aβ deposition and AD through decreased M1 receptor activation. If this is true, then cholinergic therapy should be preventative, if given early enough. Treatment begun after dementia has been diagnosed is too late, as Aβ deposition has already... Read more

This data confirms our work, done in collaboration with Abraham Fisher, showing that AF267B and two other of his M1 agonists (AF102B, AF150S) all lower CSF and cortical Aβ concentrations in normal rabbits [1]. This confirms many years of in vitro work going back to 1992, when Roger Nitsch showed that M1 receptor activation shifts APP processing into the non-amyloidogenic pathway [2]. We have also demonstrated the opposite effect, in vivo, that decreasing cortical M1 receptor activation by lesioning the nucleus basalis magnocellularis (nbm) results in increased amyloidogenic processing of APP and Aβ deposition [3] and that treatment with AF267B prevents this deposition [4]. The aggregate data suggest a fusion of the cholinergic and amyloid hypotheses: cortical cholinergic deafferentation occurs during preclinical AD [5-7] and leads to Aβ deposition and AD through decreased M1 receptor activation. If this is true, then cholinergic therapy should be preventative, if given early enough. Treatment begun after dementia has been diagnosed is too late, as Aβ deposition has already reached a plateau by this stage and tangle formation has usually proceeded to at least Braak stage IV. The recent ACDS trial results showing that Aricept slows conversion of MCI to AD supports a preventative role for cholinergic therapy in AD. Both muscarinic agonists and acetylcholinesterase inhibitors should now proceed to primary prevention trials.

References:
1. Beach, T.G., Walker, D.G., Potter, P.E., Sue, L.I., and Fisher, A., Reduction of cerebrospinal fluid amyloid β after systemic administration of M1 muscarinic agonists. Brain Res. 2001 Jun 29;905(1-2):220-3. Abstract

2. Beach, T.G., Muscarinic agonists as preventative therapy for Alzheimer's disease. Curr Opin Investig Drugs. 2002 Nov;3(11):1633-6. Review. Abstract

3. Beach, T.G., Potter, P.E., Kuo, Y.M., Emmerling, M.R., Durham, R.A., Webster, S.D., Walker, D.G., Sue, L.I., Scott, S., Layne, K.J., and Roher, A.E., Cholinergic deafferentation of the rabbit cortex: a new animal model of Aβ deposition. Neurosci Lett. 2000 Mar 31;283(1):9-12. Abstract

4. Beach, T.G., Walker, D., Sue, L., Scott, S., Layne, K., Newell, A., Potter, P., Durham, R.A., Emmerling, M., Webster, S.D., Honer, W., Fisher, A., and Roher, A. Immunotoxin lesion of the cholinergic nucleus basalis causes Ab deposition: towards a physiologic animal model of Alzheimer's disease. Curr.Med.Chem. 3, 57-75. 2003.

5. Beach, T.G., Honer, W.G., and Hughes, L.H., Cholinergic fibre loss associated with diffuse plaques in the non- demented elderly: the preclinical stage of Alzheimer's disease? Acta Neuropathol (Berl). 1997 Feb;93(2):146-53. Abstract

6. Beach, T.G., Kuo, Y.M., Spiegel, K., Emmerling, M.R., Sue, L.I., Kokjohn, K., and Roher, A.E., The cholinergic deficit coincides with Aβ deposition at the earliest histopathologic stages of Alzheimer disease. J Neuropathol Exp Neurol. 2000 Apr;59(4):308-13. Abstract

7. Katzman, R., Terry, R., DeTeresa, R., Brown, T., Davies, P., Fuld, P., Renbing, X., and Peck, A., Clinical, pathological, and neurochemical changes in dementia: a subgroup with preserved mental status and numerous neocortical plaques. Ann Neurol. 1988 Feb;23(2):138-44. Abstract

View all comments by Thomas Beach

The triple transgenic animals are a fascinating addition to the tools available. Especially exciting is this new data indicating that these genes have to work together in order to convert mice to an AD model. Obviously, these mice remain an animal model of AD, but are likely a big leap forward from the standard amyloid model mice we used to work with. There might be other animal models that do the same without the need for mutations in three different genes, but to be able to study this in mice will speed up the necessary research enormously.

Intracellular Aβ accumulation sheds light on what might be expected. Intracellular Aβ accumulations had been found in several studies previously, including human brain. However, in transgenic mice this does not appear to be a consistent feature. For a “perfect” model of AD, that’s just strange. If it exists in human AD brains, it should be present in all transgenic mouse models. The conclusion that comes to mind is that the triple transgenic mice enhance important aspects of the pathology that would otherwise be easily missed. Do we... Read more

I think this is among the most important observations shown at the meeting. I had seen Frank LaFerla’s behavior data and antibody reversal earlier in September at a meeting on cognition. It supports and is consistent with the results from most of the mice that cognitive function and Aβ correlate. We have not been able to detect the intracellular Aβ in our Tg2576-based mice, but that may be a technical difference. In any event, it seems very likely that Aβ can cause the memory deficits in the APP-transgenic animals.

This is the first time for the M1 data in vivo to my knowledge. There is a long history of muscarinic cholinergic regulation of APP processing (see Nitsch et al., 1992 or Buxbaum et al., 1992), in addition to the mechanism suggested by Abe. The one control, however, that I know Frank will run if he hasn't already is to evaluate the levels of the transgene mRNAs. Because the transgenes are driven by an autologous promoter (Thy-1,... Read more

The ability of the M1 agonist AF267B to decrease amyloid plaque load, decrease tau phosphorylation, and enhance memory function in the "triple transgenic" mice is indeed encouraging. Selective muscarinic agonists are among the few therapeutic approaches that could help alleviate the symptoms (memory deficits, cognitive dysfunction) of Alzheimer disease and have an impact on the underlying disease process. Over the years, Dr. Fisher has been a strong proponent of using selective muscarinic agonists to treat Alzheimer disease. Although several muscarinic agonists have failed in clinical studies, most of the compounds tested lacked selectivity for M1 receptors or appreciable activity at M1 receptors in the CNS.

We also presented data at the 2004 Society for Neuroscience meeting (1) on the potential neuroprotective effects of a selective M1 agonist CDD-0102. In the studies presented in San Diego, CDD-0102 promoted activation of α-secretase, (as measured by elevated levels of soluble APP-α) and decreased levels of Aβ in HEK 293T cells expressing human M1 receptors, wild-type... Read more

The ability of the M1 agonist AF267B to decrease amyloid plaque load, decrease tau phosphorylation, and enhance memory function in the "triple transgenic" mice is indeed encouraging. Selective muscarinic agonists are among the few therapeutic approaches that could help alleviate the symptoms (memory deficits, cognitive dysfunction) of Alzheimer disease and have an impact on the underlying disease process. Over the years, Dr. Fisher has been a strong proponent of using selective muscarinic agonists to treat Alzheimer disease. Although several muscarinic agonists have failed in clinical studies, most of the compounds tested lacked selectivity for M1 receptors or appreciable activity at M1 receptors in the CNS.

We also presented data at the 2004 Society for Neuroscience meeting (1) on the potential neuroprotective effects of a selective M1 agonist CDD-0102. In the studies presented in San Diego, CDD-0102 promoted activation of α-secretase, (as measured by elevated levels of soluble APP-α) and decreased levels of Aβ in HEK 293T cells expressing human M1 receptors, wild-type APP695 and mutant PS1. In PC12 cells treated with NGF to promote differentiation into a neuronal phenotype, CDD-0102 protected cells from apoptosis and elevated caspase-3 cleavage induced by staurosporine.

CDD-0102 is functionally selective for M1 receptors with minimal activity at other muscarinic receptor subtypes (2,3). It also enhances memory function in rats treated with 192IgG saporin to deplete cortical and hippocampal acetylcholine (4). Further studies are necessary to determine whether CDD-0102 produces similar effects to those observed in transgenic mice for AF267B.

References:
1. Messer WS, Jr, Tang B, Hoss WP, and Ghosh D. Neuroprotective effects of the selective M1 agonist CDD-0102: Stimulation of α-secretase, inhibition of Aβ and prevention of apoptosis. Society for Neuroscience, 29, no. 673.19, 2004.

2. Messer WS Jr, Abuh YF, Liu Y, Periyasamy S, Ngur DO, Edgar MA, El-Assadi AA, Sbeih S, Dunbar PG, Roknich S, Rho T, Fang Z, Ojo B, Zhang H, Huzl JJ 3rd, Nagy PI. Synthesis and biological characterization of 1,4,5,6-tetrahydropyrimidine and 2-amino-3,4,5,6-tetrahydropyridine derivatives as selective m1 agonists. J Med Chem. 1997 Apr 11;40(8):1230-46. 9111297

3. Messer WS, Jr, Abuh YF, Ryan K, Shepherd MA, Schroeder M, Abunada S, and El-Assadi AA. Tetrahydropyrimidine derivatives display functional selectivity for M1 muscarinic receptors in brain. Drug Dev Res 1997;40:171-174.

4. Messer, W.S., Jr., K.A. Bachmann, C. Dockery, A.A. El-Assadi, E. Hassoun, N. Haupt, B. Tang and X. Li. Development of CDD-0102 as a selective M1 agonist for the treatment of Alzheimer’s disease. Drug Dev Res. 2002;57(4):200-213.

View all comments by William Messer

I concur with the other comments: The in-vivo M1 agonist treatment effect on Aß is very encouraging. It has long been clear that different muscarinic receptor subtypes have opposing actions on amyloidogenesis and other physiological processes, and that selective M1 agonists may provide a major step forward from current nonselective cholinergic therapies. M1 is the predominant muscarinic receptor involved in cognition, neuronal excitability, synaptic plasticity and likely regulation of amyloidogenesis.

However, the hypothesis has never been adequately tested since highly selective and potent M1 agonists have been so difficult to develop. Hopefully, Dr. Fisher's persistence in developing M1 agonists will pay off and add a significantly improved therapeutic approach that targets cognition, behavior, and amyloidogenesis. The unanticipated benefits of cholinergic therapies on behavorial problems in AD, including psychosis, have also renewed the interests of big pharma in developing M1 agonists given their potential for schizophrenia (and pain). Hence, we may finally see... Read more

Given the recent interest in the M1 agonist formerly designated AF267B, we thought it would be useful to provide information concerning the basic pharmacologic properties and future development plans for this compound at Neurogenetics, Inc. (licensee) in La Jolla, CA.

Currently designated as NGX267B, this compound is an orally active, rigid analog of acetylcholine. Its pharmacological properties partially mimic the actions of acetylcholine through a stimulation of neurons that are generally spared in the neurodegenerative processes characterizing Alzheimer disease (AD). Consistent with this hypothesis, animal models with predictive utility for the symptomatic treatment of AD have demonstrated efficacy at dosages of NGX267B below those associated with nonspecific effects. NGX267B has also demonstrated disease modification properties involving reduction of β-amyloid and tau deposition in the LaFerla triple transgenic mice, thereby offering insights into mechanisms with long-term clinical implications.

One mechanism of action for NGX267B involves direct stimulation of... Read more

Given the recent interest in the M1 agonist formerly designated AF267B, we thought it would be useful to provide information concerning the basic pharmacologic properties and future development plans for this compound at Neurogenetics, Inc. (licensee) in La Jolla, CA.

Currently designated as NGX267B, this compound is an orally active, rigid analog of acetylcholine. Its pharmacological properties partially mimic the actions of acetylcholine through a stimulation of neurons that are generally spared in the neurodegenerative processes characterizing Alzheimer disease (AD). Consistent with this hypothesis, animal models with predictive utility for the symptomatic treatment of AD have demonstrated efficacy at dosages of NGX267B below those associated with nonspecific effects. NGX267B has also demonstrated disease modification properties involving reduction of β-amyloid and tau deposition in the LaFerla triple transgenic mice, thereby offering insights into mechanisms with long-term clinical implications.

One mechanism of action for NGX267B involves direct stimulation of specific muscarinic (M1) receptors that exist on intact cholinergic neurons within the CNS; therefore, this proposed therapy could complement currently available AD treatments (e.g., acetylcholine esterase inhibitors). It is specific to memory and cognitive behaviors, and largely devoid of adverse effects such as dizziness, salivation, nausea, vomiting, or diarrhea at clinically relevant doses. We consider both monotherapy and concomitant use with existing products plausible at this juncture.

The known biological and pharmaceutical properties of NGX267B should permit an exploration of efficacy and safety across a range of disease severity, co-morbidities, and concomitant medications. For example, no clinically important adverse events have been detected in animals at dosages that enhanced memory and cognitive behaviors; this suggests that the therapeutic index (a measure of the safety margin) may be attractive for the elderly or debilitated patient. The major metabolite of NGX267B has a biological profile similar to that of the parent compound. This makes it less likely that we will experience the disappointing clinical results that occurred in the evaluation of other agents as a result of poor oral absorption or extensive metabolic breakdown into products with less specificity. Finally, an anticipated long duration of action may enhance both efficacy and convenience.

The clinical development program for NGX267B is scheduled to begin approximately mid- to late 2005. Although a variety of cognitive disorders are potential therapeutic targets based upon known pathophysiology, Neurogenetics has identified the symptomatic treatment of mild to moderate Alzheimer disease as an immediate therapeutic goal. The first few clinical studies will explore safety, tolerance, pharmacokinetics, and biodisposition in both asymptomatic young and older volunteers in order to define the permissible clinical dose range for subsequent "proof-of-concept" studies in AD. The influence of dosing frequency on cognitive performance will be evaluated in light of the known neuropsychological effects of cognitive enhancing agents. Additionally, clinical measures such as neuroimaging, computerized neuropsychological test batteries, and cerebrospinal fluid biomarkers may be added as outcome measures to help fully characterize the biological properties of NGX267B before we begin a more traditional dose-ranging paradigm in mild to moderate AD.

View all comments by Michael Murphy
View all comments by Steven Wagner

I concur that the work with the triple transgenic mouse and also the possibility of M1 agonists as therapy are exciting, but I specifically want to comment on a technical issue that is being brought up regarding intraneuronal Aβ, something that the triple transgenic mouse is providing intriguing new insights into. There is no evidence, to my knowledge, that AD mutant mice exist that develop plaques but never show intraneuronal Aβ. One comment mentioned not observing intraneuronal Aβ in a Tg2576-based mouse. I understand the difficulty with convincingly detecting intracellular Aβ. I use, as an analogy, doing a Western blot of brain extract with an anti-Aβ antibody. If you do a short exposure, you can see a faint band for full-length APP but no Aβ band. If you stop there, you can convince yourself that there is no Aβ in brain. But if you expose your gel longer, the APP band will become more pronounced while APP CTFs and Aβ also eventually appear. Similarly with Aβ42 immunohistochemistry, if you do a brief reaction time, you can have a clean image only of plaques. But if you wait... Read more

M1 receptors play a central role in modulating AD-like pathology in transgenic mice
The activation of M1 and M3 muscarinic receptors have long been regarded as a promising approach for AD therapy, because it was shown that they activate the non-amyloidogenic α-secretase pathway. However, the clinical trials did not support their application in clinical practice. The development of new M1 agonists with higher specificity, which can cross the blood-brain barrier, has been the main aim of the research group of Abraham Fisher for several years. The results with such a newly developed M1 agonist obtained in a triple-transgenic mouse model now provide new hope that these compounds will be more successful in clinical trials.

In the paper by Caccamo et al., the M1 agonist AF267B is shown to reduce both the Aβ and tau pathology in the hippocampus and cortex, and to reverse cognitive deficits. Recently my group, in collaboration with the group of Fred van Leuven (Belgium) has shown that overexpression of the α-secretase ADAM-10 prevents amyloid plaque formation and... Read more

M1 receptors play a central role in modulating AD-like pathology in transgenic mice
The activation of M1 and M3 muscarinic receptors have long been regarded as a promising approach for AD therapy, because it was shown that they activate the non-amyloidogenic α-secretase pathway. However, the clinical trials did not support their application in clinical practice. The development of new M1 agonists with higher specificity, which can cross the blood-brain barrier, has been the main aim of the research group of Abraham Fisher for several years. The results with such a newly developed M1 agonist obtained in a triple-transgenic mouse model now provide new hope that these compounds will be more successful in clinical trials.

In the paper by Caccamo et al., the M1 agonist AF267B is shown to reduce both the Aβ and tau pathology in the hippocampus and cortex, and to reverse cognitive deficits. Recently my group, in collaboration with the group of Fred van Leuven (Belgium) has shown that overexpression of the α-secretase ADAM-10 prevents amyloid plaque formation and hippocampal defects in an Alzheimer disease mouse model, and in this way a proof of concept has been provided that up-regulation of the α-secretase activity in vivo could preclude the formation of Aβ and alleviate cognitive deficits (Postina et al., 2004). The paper by the LaFerla group and Fisher now moves an important step further by showing convincingly that small compounds, like the newly developed M1 agonist, can reverse AD-like pathology in transgenic mice.

Concerning the up-regulation of the ADAM-17, I would like to respectfully question the results the authors provided. From our own experiments, we know that in brain homogenates, due to the low abundance of ADAMs, it is very difficult to quantitate their amount without enrichment of these glycoproteins by lectin chromatography. As the authors had not enriched these enzymes and do not show the molecular weights of the ADAM enzymes and also of BACE, these experiments concerning the expression of α- and β-secretase are not definitive just yet. One also has to take into account that activation of the α-secretase may be achieved without increasing its protein level. This has just been described in a paper by Kojro et al., 2006. From my point of view, it would also have been interesting to see whether the muscarinic agonist also increased the amount of secreted APP (sAPPα).

In spite of some open questions concerning the mechanism, the authors now have made an important step forward to causal therapy of AD and have provided evidence that compounds activating the non-amyloidogenic α-secretase pathway might have clinical relevance. This work opens a way to new clinical trials with a new generation of M1 agonists.

References:
Kojro E, Postina R, Buro C, Meiringer C, Gehrig-Burger K, Fahrenholz F. The neuropeptide PACAP promotes the {alpha}-secretase pathway for processing the Alzheimer amyloid precursor protein. FASEB J. 2006 Mar;20(3):512-4. Epub 2006 Jan 9. Abstract

Postina R, Schroeder A, Dewachter I, Bohl J, Schmitt U, Kojro E, Prinzen C, Endres K, Hiemke C, Blessing M, Flamez P, Dequenne A, Godaux E, van Leuven F, Fahrenholz F. A disintegrin-metalloproteinase prevents amyloid plaque formation and hippocampal defects in an Alzheimer disease mouse model. J Clin Invest. 2004 May;113(10):1456-64. Erratum in: J Clin Invest. 2004 Aug;114(4):598. 36919

View all comments by Falk Fahrenholz

I recommend the Primary Papers

Caccamo and colleagues did a great job in demonstrating the potentially important role of M1 receptors in treatment of AD.

One question I have based on my limited knowledge is: Does the receptor agonist have to be specific to the M1 receptor? Would agonist(s) with broader specificity have any side effect(s)? Would M1-specific agonist(s) be more beneficial than donepezil?"

In this respect, I would have used a control group treated with donepezil to demonstrate that M1 agonist(s) would be better medications than aceetylcholine esterase inhibitors. Such a control group would also help to clarify the pathophysiological relevance of the 3XTg mice as an AD model.

View all comments by Takaomi Saido

I recommend the Primary Papers

The news that a specific M1 muscarinic receptor agonist can reverse both cognitive deficits as well as the amyloid and tau pathology in this mouse model is most interesting.

I wonder whether the study by Ganzinelli and colleagues (1) explains why you don't see AD in people with schizophrenia. They report that "circulating antibodies from schizophrenic patients interacting with cerebral M1 muscarinic acetylcholine receptors can act as an inducer of m(1) mAChR-mRNA, and neuronal nitric oxide synthase (nNOS) mRNA gene expression of rat frontal cortex." Might these antibodies be expected to reverse the amyloid pathology in AD?

Martin et al. (2) report lower protein levels of nNOS in the Tg2576 transgenic mouse model. Has anyone used nitroglycerine as a nitric oxide donor in AD and, if so, what have been the results?

References:
1. Ganzinelli S, Borda T, Sterin-Borda L. Regulation of m(1) muscarinic receptors and nNOS mRNA levels by autoantibodies from schizophrenic patients. Neuropharmacology. 2006 Mar;50(3):362-371. Epub 2005 Nov 11. Abstract

2. Martin BL, Tokheim AM, McCarthy PT, Doms BS, Davis AA, Armitage IM. Metallothionein-3 and neuronal nitric oxide synthase levels in brains from the Tg2576 mouse model of Alzheimer's disease. Mol Cell Biochem. 2006 Feb;283(1-2):129-37. Abstract

View all comments by Mary Reid

Reply by Abraham Fisher to Takaomi Saido
In reply to Takaomi Saido’s questions: The progression of Alzheimer disease (AD), associated with loss of the cholinergic neurons and decreases in acetylcholine (ACh), limit the therapeutic potential of the FDA-approved acetylcholinesterase inhibitors (AChEIs) such as donepezil, galantamine, rivastigmine, or tacrine. Postsynaptic M1 muscarinic receptors (M1 mAChR) are predominant in cerebral cortex and hippocampus and have a major role in hippocampal-based learning and memory, particularly for short-term memory, which is impaired in AD. As M1 mAChR are relatively preserved in AD, use of M1 muscarinic agonists in AD treatment is rational.

Unlike AChEIs, M1 muscarinic agonists in theory are independent of ACh levels in the brain, and thus less affected by the extent of degeneration of presynaptic cholinergic terminals. Whilst activation of M1 mAChR is advantageous, stimulation of the other mAChR subtypes leads to side effects. Therefore, the ideal M1 muscarinic agonist should be devoid of M2, M3, and M5 agonistic effects.... Read more

Reply by Abraham Fisher to Takaomi Saido
In reply to Takaomi Saido’s questions: The progression of Alzheimer disease (AD), associated with loss of the cholinergic neurons and decreases in acetylcholine (ACh), limit the therapeutic potential of the FDA-approved acetylcholinesterase inhibitors (AChEIs) such as donepezil, galantamine, rivastigmine, or tacrine. Postsynaptic M1 muscarinic receptors (M1 mAChR) are predominant in cerebral cortex and hippocampus and have a major role in hippocampal-based learning and memory, particularly for short-term memory, which is impaired in AD. As M1 mAChR are relatively preserved in AD, use of M1 muscarinic agonists in AD treatment is rational.

Unlike AChEIs, M1 muscarinic agonists in theory are independent of ACh levels in the brain, and thus less affected by the extent of degeneration of presynaptic cholinergic terminals. Whilst activation of M1 mAChR is advantageous, stimulation of the other mAChR subtypes leads to side effects. Therefore, the ideal M1 muscarinic agonist should be devoid of M2, M3, and M5 agonistic effects. Some muscarinic agonists improved cognition and reduced psychotic episodes in AD patients. However, a scarcity of selective M1 muscarinic agonists has limited the clinical use of muscarinic agonists in AD due to side effects observed at higher doses. The failure of most of the tested agonists was due to the compounds’ inadequate M1 selectivity and poor pharmacokinetics, narrow safety margin, and side effects. AF267B is an M1 selective agonist, is orally available, penetrates the blood-brain barrier, and has a high bioavailability and wide safety margin.

M1 mAChR-mediated activation of α-secretase can increase α-APPs, preventing the formation of Aβ. Activation of M3 mAChR also elevates α-APPs, yet selective M1 agonists are preferable to prevent peripheral M3 mediated-side effects (e.g., gastrointestinal). M2 mAChR and M4 mAChR are ineffective in activating α-secretase as tested by α-APPs release, and the M2 mAChR may even have an inhibitory effect.

The multitude of animal models available complicates comparison among studies from various labs. In this context, the 3xTg-AD model offers the possibility to answer fundamental questions in AD pathology and therapeutic strategies. How validated are the animal models? One validation should include studies of AChEIs, even if such compounds are not expected to cause a disease modification (e.g., reduction in Aβ levels and/or tau hyperphosphorylation). Whether AF267B is more beneficial than donepezil cannot be addressed properly since donepezil was not yet tested in the 3xTg-AD mice. Yet, our study and previous findings indicate that AF267B, unlike donepezil, in addition to treatment of cognitive deficits, can be also regarded as a disease modifier. While few studies reported some beneficial effects of donepezil on behavioral deficits (memory and learning) in Tg mice such as Tg2576, APP23, and AD11, none of these studies reported a decrease in Aβ levels and/or tau hyperphosphorylation.

References:
Dong H, Csernansky CA, Martin MV, Bertchume A, Vallera D, Csernansky JG. Acetylcholinesterase inhibitors ameliorate behavioral deficits in the Tg2576 mouse model of Alzheimer's disease. Psychopharmacology (Berl). 2005 Aug;181(1):145-52. Epub 2005 Oct 15. Abstract

Van Dam D, Abramowski D, Staufenbiel M, De Deyn PP. Symptomatic effect of donepezil, rivastigmine, galantamine and memantine on cognitive deficits in the APP23 model. Psychopharmacology (Berl). 2005 Jun;180(1):177-90. Epub 2005 Jan 15. Abstract

Capsoni S, Giannotta S, Stebel M, Garcia AA, De Rosa R, Villetti G, Imbimbo BP, Pietra C, Cattaneo A. Ganstigmine and donepezil improve neurodegeneration in AD11 antinerve growth factor transgenic mice. Am J Alzheimers Dis Other Demen. 2004 May-Jun;19(3):153-60. Abstract

View all comments by Abraham Fisher

I recommend the Primary Papers

The paper by LaFerla and colleagues adds to considerable evidence suggesting that M1 receptor activation leads to decreased amyloidogenic processing of APP while reduced M1 receptor activation, by means of either pharmacological agents or cholinergic lesion, results in increased Aβ production.

We have done considerable work in this field that readers may find interesting (see attached citations for examples). In particular, we have evaluated three of Abraham Fisher’s M1-selective compounds, including AF267B, and found that all three reduced CSF and cortical levels of Aβ in rabbits.

The present conventional wisdom that cholinergic agents are only palliative and do not affect disease progression is challenged by the collective contrary data that has accumulated over the last 10 years. It is likely that cholinergic therapy would have its greatest potential effects on Aβ deposition and disease progression if it were given as primary prevention, since Aβ deposition has already reached a plateau by the time the clinical diagnosis of Alzheimer disease is made. (1,2)

References:
1. Beach TG, Walker DG, Roher AE, and Potter PE. Anti-Amyloidogenic Activity of Cholinergic Agents. Drug Dev. Res. 56 (2002) 242-247. 2. Beach TG, Walker DG, Potter PE, Sue LI, Fisher A. Reduction of cerebrospinal fluid amyloid beta after systemic administration of M1 muscarinic agonists. Brain Res. 2001 Jun 29;905(1-2):220-3. Abstract

View all comments by Thomas Beach

I recommend the Primary Papers

The results reported by LaFerla and colleagues are very promising. However, before embarking on large-scale clinical studies with selective M1 agonists, certain points, which might have important implications both with respect to the efficacy and safety of these agents, deserve consideration.

While brain muscarinic (M1) receptor density has generally been found to be preserved across all stages of AD, numerous observations from in-vitro studies indicate a loss of the coupling of cortical M1 receptors to G-proteins, which could limit the efficacy of these agents.

Additionally, the precise origin of the M1 receptor-mediated reductions in CSF Aβ levels, which have been reported in pilot clinical studies with this class of compounds in AD, is not known, and this isolated finding may not necessarily reflect brain M1 receptor activation.

Paradoxically, there is also in-vivo evidence that certain central and peripheral responses to nonspecific M1 agonists may be increased in AD patients (1,2) which could potentially influence the safety of these drugs in this population. ... Read more