Pathophysiology of Neurodegenerative Diseases: New Approaches for Investigation and Recent Advances

Neurodegenerative diseases are heterogeneous conditions characterized by selective dysfunction and progressive loss of neurons, glial cells and their networks in the human brain and spinal cord. As a result, patients exhibit progressive cognitive decline and/or motor dysfunction. Cognitive decline, for example, is a predominant characteristic of Alzheimer’s disease (AD), whereas motor deficits appear in amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD) and Huntington’s disease (HD).

“We currently don’t know how to prevent the neurodegenerative process,” says Dr. Tatiana Rosenstock, neuroscientist and collaborator professor at University of São Paulo, Brazil, and principal scientist at Sygnature Discovery, Nottingham, UK. “In an attempt to stop neurodegeneration, we are trying to understand how and why neurons and other types of cells perish. We believe that this may be the only way to prevent and even cure neurodegenerative diseases.”

These incurable and debilitating conditions are an enormous burden for patients and family members and a substantial medical and public health challenge worldwide. “At present, there is no cure for neurodegeneration, and only a limited number of disease-modifying therapies have been identified,” says Dr. Joana Gil-Mohapel, neuroscientist and assistant teaching professor at the University of Victoria and the University of British Columbia, Canada. “Although the outlook for future patients suffering from neurodegenerative diseases may appear bleak, advancements in the past two decades in our understanding of the etiology surrounding neurodegeneration provides optimism.”

Common mechanisms underlying different neurodegenerative diseases

Despite a vast array of causes of neurodegenerative diseases, research has revealed several common pathways through which neurodegeneration proceeds, including accumulation of insoluble protein aggregates, apoptosis, necrosis, excitotoxicity and neuroinflammation. Mitochondrial dysfunction, downstream oxidative stress and impaired autophagy/lysosomal activity are also important factors in neurodegeneration. Rosenstock’s research focuses on how mitochondria in neurons and astrocytes become dysfunctional during neurodegenerative disease.

“The mitochondria produce energy in the form of ATP, which is responsible for maintaining the homeostasis of the brain and all living cells. If the brain lacks energy, cells start to malfunction and eventually die. Thus, we believe that investigating the role of mitochondria in different disorders may be the best strategy for neuroprotection,” explains Rosenstock. Her research group back in Brazil investigates such mechanisms in neurons and astrocytes.

Impaired brain plasticity also represents a critical pathological mechanism underlying the progressive cognitive and motor deficits observed in neurodegenerative diseases. “Brain plasticity, particularly adult hippocampal neurogenesis, exerts an important role in cognitive function,” explains Gil-Mohapel. Her line of research focuses on how plasticity is affected by aging and certain neurodegenerative conditions, such as HD.  

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Animal models faithfully recapitulate human disease

Gil-Mohapel’s research on neurodegenerative diseases has focused primarily on HD — an autosomal dominant disease caused by mutations in the huntingtin gene. She currently works with YAC128 HD transgenic mice, which express the entire human HD gene containing 128 CAG repeats and show motor abnormalities, cognitive dysfunction and selective neuropathology that mimic the human disease. “We have detected changes in adult hippocampal neurogenesis in these HD mice prior to the onset of motor symptoms and treatment with brain-derived neurotrophic factor (BDNF), a neurotrophin with pro-neurogenic effects, prevented emotional disturbances in early symptomatic YAC128 mice,” explains Gil-Mohapel. “This approach has great translational potential as it can improve the quality of life of patients in early stages of HD,” she concludes.

Rosenstock has also worked with genetic models of HD, including YAC128 mice. She is now working with the ALS SOD1G93A mouse model. “Both HD and ALS models resemble phenotypic and pathological characteristics of the disease, including selective cell death, huntingtin accumulation and decreased levels and activity of the antioxidant enzyme superoxide dismutase 1 (SOD1),” she explains. These models are prime examples of how targeting neurodegeneration-related genes in mice can be valuable to investigate the pathogenesis of neurodegenerative diseases.

The administration of excitotoxins such as quinolinic acid, or mitochondrial inhibitors such as rotenone and 3-nitropropionic acid, reproduces the lesions found in neurodegenerative conditions and also enables the investigation of neurodegenerative disease mechanisms. “This experimental approach allows us to study the correlation between specific disease mechanisms, such as cell death, mitochondrial deregulation and oxidative stress induced by excitotoxins and the development of neurodegenerative processes and behavioral alterations,” says Rosenstock. Lesion models, however, have some pitfalls. “Due to husbandry costs, lesion models primarily use young animals, and the acute insult of administering a toxin does not allow for a progressive pathology and disease progression, as seen in neurodegenerative diseases,” says Gil-Mohapel. “In addition, any potential therapeutic intervention tested in these models is usually administered before or together with the toxin, which does not mimic the time course of most therapeutic interventions,” explains Gil-Mohapel.

A new generation of tools increase the knowledge of neurodegenerative disease pathophysiology

Animal models are powerful tools for understanding the etiology of neurodegenerative diseases. However, they still present limitations and sometimes have poor predictive power for human drug efficacy. These limitations of animals models can be addressed by studying neurodegenerative diseases directly in humans.

“I believe the biggest breakthrough was realizing that some of the mechanisms found in animal models are also found in patient cells, including peripheral blood mononuclear cells or even induced pluripotent stem cell-derived neurons,” says Rosenstock. She explains that “the use of induced pluripotent stem cells ensures that the genetic background of the donor is maintained, that any drug tested will certainly affect human cells and also minimizes the use of animals, following the policy of the animal experimentation called 3Rs — Replacement, Reduction and Refinement.”

Similar to many diseases caused by mutations in a single gene, HD is an attractive candidate for gene therapy. “Specifically, the selective suppression of mutant huntingtin expression is expected to delay the onset and mitigate the severity of HD by preventing the toxic gain of function of the mutated protein without functional loss of its normal counterpart,” says Gil-Mohapel. “Studies using fibroblasts from HD patients combined with gene therapy have also provided promising results, suggesting that allele-specific siRNA-based therapeutics may be clinically feasible and beneficial for a large number of affected individuals,” she explains. Another genetic therapeutic approach currently being considered for HD is the use of the CRISPR-Cas9 system as a gene editing tool. “In the case of HD, preliminary studies using patient-derived HD fibroblasts demonstrated a complete inactivation of mutant huntingtin,” says Gil-Mohapel.

Although induced pluripotent stem cells and other human-based models have allowed substantial progress in neurodegenerative disease modeling and drug screening, these approaches also have some drawbacks. Rosenstock highlights that “they do not mimic an entire organism and therefore we cannot predict that a drug could work in a patient, even if it is effective in cultured cells isolated from humans.” She adds that “induced pluripotent stem cells take 4-8 weeks to produce, which is a long time, and each culture represents just one individual, which is a very restricted outcome.”

New frontiers in candidate therapeutics

As life expectancy increases, there is an urgent and growing need for novel approaches to treat neurogenerative diseases. Driven by the increased knowledge of the mechanisms underlying neurodegeneration, extensive research efforts are being carried out to identify new strategies to modulate previously undruggable targets or to manage diseases at earlier stages.

Growing evidence indicates that HD involves marked changes in BDNF levels and that restoring these levels may be therapeutically beneficial. “Because serum levels of BDNF are thought to reflect its concentration within the brain, serum BDNF may be a useful biomarker for the degenerative process occurring in the HD brain as well as to test the efficacy of therapeutic approaches, particularly those aimed at restoring BDNF-mediated trophic support,” says Gil-Mohapel.

“Other than growth factors, core biomarkers of neurodegenerative pathology are oxidative stress and pro-inflammatory cytokines,” says Rosenstock. “Changes in brain metabolism are also becoming popular biomarkers with the advances of brain imaging studies.” Those biomarkers can be helpful in patient stratification, target engagement and outcome assessment.

“Stem cell therapies have also been considered for neurodegenerative diseases under the assumption that new neurons can replace degenerating ones in the affected brain regions and consequently ameliorate the disease profile”, explains Gil-Mohapel. “Numerous in vitro studies have been conducted to characterize several types of pluripotent stem cells, including embryonic stem cells, mesenchymal cells and neural stem cells, and some offer great promise for new medical treatments for neurodegenerative conditions”. “One of the challenges scientists are facing though, is to find the best cell source for a particular application,” adds Gil-Mohapel.

Investigations of both biomarkers and new treatments for neurodegenerative diseases have proceeded enthusiastically, but disease-modifying therapies still lack. “A critical obstacle that limits the use of some of these novel therapeutic approaches is the limited ability of many compounds to cross the blood-brain barrier,” says Gil-Mohapel. The recent use of nanotechnology and nanoparticles-based therapeutics might be promising to facilitate personalized medicine, although these technologies are still in their infancy.

“With further refinement of all these strategies, a cure for HD and other neurodegenerative conditions may finally be within our reach,” concludes Gil-Mohapel.

About the author

Morgana Moretti, Ph.D., is an active scientist and freelance medical writer with over 12 years of research experience. She holds a doctoral degree in biochemistry, has published dozens of articles in peer-reviewed biomedical literature, and is passionate about sharing her technical knowledge in a way that is relevant and impacts lives.