The number of people living with Alzheimer’s Disease is rapidly increasing with estimates that more than 12.7 million people over the age of 65 will be affected by 2050 in the USA alone. This puts a great burden over the researcher’s shoulders in finding an effective treatment for Alzheimer’s Disease (AD).
The pursuit for a potential cure or therapeutic agent converged scientific efforts toward genetic models that could potentially reveal the AD mechanisms. If that were to happen, researchers can develop a drug that effectively treats Alzheimer’s symptoms or even completely stops disease progression.
But, why would we think that animal models of Alzheimer’s Disease are the key to curing AD?
Well, by now we know that several genes are implicated in Alzheimer’s Disease, although the exact mechanisms behind the disease remain unclear. Creating strings with different genetic mutations allows scientists to examine how these mutations manifest key hallmarks of Alzheimer’s Disease in rodents. AD is characterized by extracellular b-amyloid (Ab) plaques and intracellular neurofibrillary tangles of tau leading to cognitive decline and memory problems. Based on the combination of the specific genetic mutations, researchers can test different drugs and observe the effects.
That being said, this is not an easy task nor a straightforward process. Existing Alzheimer’s mouse models take several forms such as PDAPP, PSEN1, apoE4, and 3xTg. On the other hand, examples of rat models of Alzheimer’s include the intrahippocampal amyloid infusion model and Ab infusion mode.
In this article, we’ll explore the potential of rodent models of Alzheimer’s Disease in drug testing, discuss the process of drug development, potential drug treatments, and the obstacles that are yet to be resolved.
Development of Therapeutic Agents in Alzheimer’s Disease
Transgenic technology, which is extensively used on rodents, provides an opportunity for researchers to reproduce the cause of familial AD and better understand the molecular mechanisms associated with Ab-production. While there’s no scientific consensus about the etiology of AD, the majority of researchers favor the ‘amyloid hypothesis’. Following the understandings from the ‘amyloid hypothesis’, researchers hope they can develop therapeutic agents.
One of the most important contributions of the transgenic mice models is the better understanding of the role of inflammation, oxidative stress, and mitochondrial dysfunction in the pathogenesis of AD. Because of this notion, a lot of efforts have been concentrated on developing an anti-inflammatory treatment as a way to combat the disease.
Nonsteroidal Anti-Inflammatory Drugs for Alzheimer’s Disease
As we’ve seen in the previous paragraph, inflammation and oxidative stress, more specifically neuroinflammation, emerged as an important consideration in Alzheimer’s disease research. This is due to the fact that several studies have linked the development of AD with neuroinflammation where both microglia and astrocytes seem to generate beta-amyloid (Aβ) protein. Further validation for this hypothesis came from studies that show how non-steroidal anti-inflammatory drugs lower the risk for Alzheimer’s Disease.
In one of these studies, Ji-Kyung Choi et al. demonstrated that selective NSAIDs (ibuprofen and possibly celecoxib) protect against the neuronal photology of AD in PSAPP mice models (Swedish mutation – APPTg2576 crossbred with mutant human presenilin-1 (PS-1M146V)). The mechanism behind NSAIDs is that they protect the NAA and glutamate loss (ibuprofen for both, celecoxib only the former). However, the selected NSAIDs don’t protect against the increases in myo-inositol or glutamine, which suggests that there are limitations that have to be considered.
Immunotherapy for Alzheimer’s Disease
Nevertheless, the main wave of enthusiasm for drug development for Alzheimer’s Disease comes from active and passive immunization therapy in PDAPP mice models.
One of the first findings to provide support for the idea of immunotherapy for AD was published in 1999 in Nature by Dale Schenk et al. Even back then, researchers used PDAPP mice models because they express a lot of neuropathological hallmarks of Alzheimer’s Disease in an age-dependent manner. What the authors did in the study is actively immunized mice with beta-amyloid Aβ₄₂. They did this by creating two conditions.
The first group of mice was immunized at 6 weeks of age – before the start of AD-type neuropathology. In contrast to this, the second group of mice was immunized at 11 months – when the amyloid-β deposition was established. The results from this study show that immunization before the onset of the disease prevented β-amyloid-plaque formation. On the other hand, immunization in older mice was less significant, although it still reduced the extent and progression of AD. Since then, there have been many clinical trials for immunization as a potential treatment for Alzheimer’s but immunization with Aβ₄₂ is stopped for now, due to side effects in the CNS.
More recently, in 2004, a study in Neuron was published by Oddo et al., where the authors tried immunization with Aβ₄₂ in a different string of mice models – the 3xTg mice. The results from this study were really promising and again enhanced the efforts for developing an antibody treatment for Alzheimer’s Disease, something we might see in the near future.
What the authors found was that Aβ antibody treatment directed to the hippocampus reduced Aβ intracellular accumulations and extracellular plaques in 3 days, as well as the number of “tangles” in 5 days. One very notable observation was that Aβ deposits emerge before the tau pathology and they’re also cleared first, which gives insights into the hierarchical relationship between Aβ and tau.
PDAPP mice models have also been used for developing immunization with IgG. In 2011, D Morgan published a paper titled Immunotherapy for Alzheimer’s disease. In this paper, Morgan discussed the limitations of active Aβ immunization in human trials due to side effects. According to him, there’s been a lot of creativity in the area of novel vaccine strategies for AD.
Some researchers try to develop B-cell activation and antibody production, with minimal T-cell involvement. However, Morgan gives more merit to three mechanisms of Aβ reduction through anti-Aß immunotherapy – passive immunization. In his study, he demonstrates that passive immunization with bapineuzimab might be a solution in overcoming the side effects of the above-described active immunization. However, the efficacy of this antibody is being questioned.
Research into immunotherapies for Alzheimer’s disease continues even though, as we’ve seen, there are serious limitations and issues that have yet to be resolved. The main reason why research in this direction is so persisting is that the possibility of an effective vaccine has unimaginable implications. We know that in some patients Aβ deposits in the brain can precede symptoms by 20 years and more.
Also, the amyloid hypothesis postulates that these Aβ deposits are the initial event that kick-starts the AD pathology. If this is true then active or passive immunization against Alzheimer’s could eradicate the disease before any functional loss.
Enzyme Inhibitors for Alzheimer’s Disease
Regardless of which hypothesis about the pathogenesis of AD one supports, it’s undeniable that several enzymes play a key role in the pathogenic cascade of AD. What role exactly do these enzymes play and how they interact with each other is still not clear. Nonetheless, this hasn’t stopped researchers from investigating the therapeutic potential of specific enzyme inhibitors in the treatment of Alzheimer’s Disease.
The proponents of the cholinergic hypothesis believe that irreversible deficiency in cholinergic functions leads to a reduction of acetylcholine (a main neurotransmitter in the brain) – initiating the degenerative cascade and Aβ deposits. Following this theory, donepezil (acetylcholinesterase inhibitor) was developed and approved for the treatment of AD. In 2009, Hongxin Dong et al published a paper in Brain Research about the effects of donepezil in the Tg2576 mouse model.
According to the researchers, high doses of donepezil have a measurable effect on the Aβ protein (removing deposits) and prevent synapse loss in the mouse model. However, in humans, since donepezil is an approved drug, researchers have observed only a small effect on cognitive abilities without stopping the progression of the disease.
Gamma-secretase Modulator (GSM)
Another enzyme inhibitor tested on the Tg2576 mouse model is the γ-secretase. In one study from 2010, researchers orally administered GSM (γ-secretase modulator) on a daily basis for a prolonged period of time and observed significant reductions in both diffuse and neuritic plaques. Unfortunately, the clinical trial was discontinued as there were also severe side effects such as an increased risk of skin cancer.
Enthusiasm around drug development for AD has also turned the attention toward β‐secretase inhibitors. Beta is the first protease in the process of producing Aβ deposits. For this reason, the proponents of the amyloid hypothesis (which is the majority of scientists) have investigated the effects of β‐secretase inhibitors on both Tg 2576 and APP mice models.
From the large number of β‐secretase inhibitors being developed, some have actually managed to enter clinical trials and might be approved in the future. The efficacy of these inhibitors is great in reducing Aβ in the brain, CSF, and plasma, but efficacy data from human trials regarding cognitive decline is still lacking.
Glycogen Synthase Kinase-3 (GSK3)
Last but not least, an enzyme inhibitor for the glycogen synthase kinase-3 (GSK3) has been tested on rat models (Aβ infusion models) as a potential therapeutic agent for Alzheimer’s Disease. Some studies have demonstrated that indeed the GSK3 is hyperactive in AD, but using a GSK3 inhibitor might lead to broader adverse effects. In 2011, Ana Martinez et al published a paper in the International Journal of Alzheimer’s Disease investigating the role of GSK3 in the pathogenesis of AD.
The authors believe that this enzyme might be the link between Aβ deposits and tau protein, which is why a small molecule known as tideglusib is currently in Phase-III/IV of clinical trials and might be approved in the future.
Alternative Solutions in Drug Development for Alzheimer’s Disease
While there are several potential drugs for Alzheimer’s Disease in development, none so far has made a breakthrough in the frequency or the severity of the disease. For this reason, some scientists have abandoned the traditional approaches that we’ve discussed so far and turned in a completely different direction.
One such approach is the use of curcumin in the diagnosis, prevention, and treatment of Alzheimer’s disease. Curcumin has a high binding affinity to amyloid-β which is why it’s being used as a diagnostic tool. However, curcumin has protective and preventive effects in many chronic diseases. Plus, many studies have found that it maintains the normal brain structure and function.
How does this translate into treatment for Alzheimer’s Disease? Well, the above-mentioned protective effects of curcumin, as well as its anti-inflammatory effect provide a scientific basis for pursuing curcumin as a potential AD treatment. Several studies have found evidence that curcumin improves the overall memory in patients with AD. Also, curcumin has been tested on an intrahippocampal amyloid infusion rat model and the drug development process has entered clinical trials.
Common Problems with Animal Models
It’s more than obvious that rodent models of Alzheimer’s Disease have contributed and continue to contribute greatly to our understanding of mechanisms underlying AD. Yet, as you may have noticed, most efforts in developing an effective drug treatment for this disease based on animal models have failed. This may be due to severe side effects or no efficacy in human trials. A logical question that follows from this is: Why do animal models of Alzheimer’s Disease fail?
There are several reasons for this. We’ve tried to raise specific issues in the previous sections because a lot of the problems are closely related to the animal model on which the drug is tested – on top of other things. Here are some general considerations that all researchers and those who read scientific papers should keep in mind:
- It may be possible that the animal model is not a good representation of the disease. We’ve seen that there are many different mouse models of AD and today researchers also create new strains by cross-breeding well-established models. This is why the validity of the model is an essential consideration.
- Researchers have to figure out what dose to give the mice as a single intervention and how often it should be repeated. Unfortunately, there are little to no strict guidelines here and a mistake can lead to a negative clinical trial.
- In clinical trials, it can also happen that the intervention based on animal models is not accurately translated. For example, clinical trials on a wrong population, short duration, dosage limitations, etc.
Hopefully, by now you have a clearer idea about the current advancements of drug development in Alzheimer’s Disease. It’s a very challenging task, but with years of research and hard work, scientists are getting close to finding an effective therapeutic agent for the disease. With new and more powerful technologies, we might see a cure or preventative agent for AD sooner than we think.
For now, rodent models of Alzheimer’s Disease provide the most potential, although the models themselves, as well as the experimental designs, need refinement. There are at least two ways in which rodent models can provide new insights – developing new strains or testing new ideas on the well-established strains. These ideas can come in the form of new immunotherapy approaches, anti-inflammatory drugs, or alternative natural remedies turned into potent drugs.
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