The Path To Progress Preclinical Studies Of Age-Related Neurodegenerative Diseases: A Perspective On Rodent And HiPSC-Derived Models Part 5

Advantages of hiPSC-Derived Cellular Systems and Disease Models

Improved Disease Modeling and Drug-Discovery Platforms

The potential for hiPSC-based in vitro models to be highly informative for NDDs with a human genetic component investigation is high. 

The relationship between human genetics and memory is a much-discussed issue, especially in modern society, where we focus on personal development and learning. Although our genetic information determines our basic characteristics at birth, memory is different from fixed characteristics such as our height and skin color. Instead, it can be cultivated and improved, and it plays an important role in daily life.

First of all, scientific research shows that memory is not directly related to genetic genes. Although some people are born smarter than others, in the long run, memory can be gradually improved through learning and cultivation. In our learning process from elementary school to university, we need to constantly use and improve our memory to improve our academic performance. For example, we remember problems and solutions through repeated review and practice, which helps to exercise our memory ability.

Secondly, a healthy lifestyle also helps our memory a lot. Physical health has a great impact and effect on memory, such as adequate sleep, a healthy diet, and moderate exercise. We start with our own body and stay healthy through exercise and a healthy lifestyle, which will make our memory clearer and sharper.

Finally, we can also expand our memory ability by constantly broadening our horizons and participating in various new things. For example, we can try to learn a new language and participate in social activities such as travel, and these experiences can create more memories in our minds.

In short, the relationship between human genetics and memory is complex and diverse, and of course, there will be corresponding differences based on personal genetic differences. But we should not ignore what we can do: improving our memory and self-management ability through continuous learning, healthy living, and constantly expanding our horizons is the best way for us to overcome genetic differences. It can be seen that we need to improve our memory, and Cistanche can significantly improve memory because Cistanche is a traditional Chinese medicine with many unique effects, one of which is to improve memory. The efficacy of Cistanche comes from its various active ingredients, including tannic acid, polysaccharides, flavonoid glycosides, etc. These ingredients can promote brain health in many ways.

Click Know to improve short-term memory

The ability to correlate NDD candidate genes to cellular phenotypes, such as RNA and protein expression profiles, morphological changes, and biochemical signatures in hiPSC-derived cells, facilitates studies addressing how the human genetic components of complex NDDs, such as AD and PD, are expressed on a cellular level. 

Moreover, hiPSCs allow the incorporation of the genetic complexity of underlying patient genetic backgrounds in the model, which is often negated (e.g., chemical models) or oversimplified (e.g., monogenetic variant animal models). 

It is well acknowledged that the onset, progression, and severity of neurodegeneration are determined by complex genetic factors and the interplay of several genetic variants with relatively minor effects. 

This innate genetic landscape captured in patient-derived hiPSC models allows us to investigate genetic determinants and biological factors, paired with the ability to generate modular, high-fidelity in vitro NDD models, to build upon our current understanding of disease biology. hiPSC technology currently represents the only available human-based models in which the full genetic landscape of the patients is captured. 

Also, the possibility to apply genome-editing technologies, such as CRISPR-Cas9, using hiPSCs enabled the generation of isogenic models for the evaluation of precise genetic variants. In addition to scalability, hiPSCs are self-renewable. 

Thus, hiPSC-derived models present a forefront promising system for preclinical studies. Moreover, the human genetic diversity captured in patient-derived hiPSC capacity makes them suitable for the evaluation of "personalized medicine" approaches. 

The ability to screen effectively individual reactions to treatments and to identify nonresponders or poor responders before administration in a clinical setting further emphasizes the suitability of hiPSC-derived models for preclinical assessments (Figure 2). 

Collectively, the potential information generated through preclinical studies in hiPSC models is robust, relevant to human subjects, and essential to implement in the design of clinical trials (e.g., stratification of the study subjects) to improve the accuracy of results.

Overcoming Limits of Animal Models

hiPSCs offer a way to overcome many of the issues present in current animal-based models, as previously discussed. First, iPS-derived models provide a human-based platform for evaluating potential therapeutics in the actual genetic landscape of the human disease. 

Second, by deriving cells from patients, many technical issues regarding the best way to model human disease mutations are circumvented because the model contains the unique genetic background of the patient. Third, hiPSC cells are easily accessible to genetic and pharmacological manipulation, enabling high throughput structural and functional assays that assess disease phenotype. 

To this end, we recently developed a hiPSC-derived DA neuronal model obtained from a PD patient with SNCA triplication to assess an all-in-one LV GT aimed at reducing SNCA levels. Our LV intervention successfully reduced SNCA levels and rescued disease-related phenotypes through reactive oxidative species (ROS) generation and preservation of cellular viability.140 

Fourth, compared with the standard rodent model, the generation of in vitro hiPSC-based models can be achieved in a significantly shorter timeline, in the order of 30 days as opposed to animal models that can range widely from months to years. 

Fifth, and in keeping with the previous point, hiPSC-based models are considerably less expensive and energy-consuming than rodent models, particularly when considering ongoing animal housekeeping obligations and infrastructure, regulations, as well as training and compliance requirements. Increasingly economic preclinical disease modeling increases accessibility for research groups to assess putative therapeutics improve translatability and ultimately improve DMT development efficiency.202 

Altogether, these advantages give hiPSC-derived models a distinct advantage in the study of neurodegeneration, and such models have the potential to generate novel discoveries that were inaccessible in previous cell culture and animal disease models. 

The studies reviewed here demonstrate the necessity and value of the incorporation of 2D and 3D hiPSC-based models in NDD mechanistic studies elucidating factors involved in disease causation and pathogenic pathways, as well as in translational and drug-discovery studies, such as therapeutics target identification and validation. 

Together, hiPSC-derived NDD models represent a leap forward in medical research through improved disease modeling that is considerably feasible, with relatively shorter experiments that are cost-effective and perhaps most importantly, accurate and suitable for modeling human diseases.

Figure 2. Schematic of Improved DMT Drug-Development Pipeline for NDDs through Increased Incorporation and/or Substitution of hiPSC-Derived Models Development of any putative DMT begins with the discovery phase, which encompasses both identification of pertinent gene targets and mechanisms of disease. 

Subsequent disease models are devised in the early preclinical phase to explore the cellular disease pathophysiology and validate drug targets, as well as early screening and optimization of early CGT-IPs/DMTs. Progressive development of DMTs, including further model development and target validation, as well as investigation into off-target effects, occurs during the mid-preclinical phase. 

As DMT progresses, further information regarding pharmacokinetic and pharmacodynamic activity is obtained in the late preclinical phase. The increased use of hiPSC-derived models in these phases represents an attractive improvement to the existing drug-development pipeline for several reasons; namely, they are cost-effective, versatile, and most importantly, overcome many of the innate limitations of existing animal models of NDDs. 

At this stage, IND applications may be submitted for putative DMTs, which currently contain 4 critical objectives that must be met with any emerging DMT, particularly CGT-IPs: (1) target selection, (2) lead compound development and optimization, (3) initial and escalating dosing regimen, and (4) establishment of feasibility and ROA. 

Each of these can be addressed accurately and appropriately with the substitution of hiPSC-based models in preclinical studies. DMTs may then receive FDA approval to commence clinical trials. 

The incorporation of hiPSC-based models in an additional phase 1a may be included, which facilitates improved candidate screening in the recruitment phase and identification of potential nonresponders or poor responders based on preclinical genetic studies. 

DMTs with demonstrable safety in phase 1a then progress through phase 1, 2, and 3 clinical trials, after which, those with a significant positive effect receive New Drug Application (NDA) regulatory approval and are released for consumer use with continuing pharmacovigilance to identify any previously unidentified adverse outcomes over time.

Limitations of hiPSC-Derived In Vitro Models Time and Reproducibility

The culturing technologies of hiPSCs emerged over the last decade, and their utility for disease modeling and preclinical assessment of putative DMTs is relatively novel. 

As with any emerging technology, several limitations need to be considered when attempting to accurately recreate the biological characteristics of NDDs. First and foremost, the cultivation of hiPSCs and the generation of accurate and reliable cell models can be expensive, labor-intensive, and time-consuming. On average, it costs $10,000
to USD 25,000 to validate hiPSC lines at a suitable standard for medical research.203 

The processes of reprogramming, differentiation, and maturation are prolonged. Cellular reprogramming of starter tissue into hiPSCs can take at least 20
to 30 days,151 and subsequent cellular differentiation and maturation vary depending on the desired cellular type and method used; for example, generation of mature neurons, astrocytes, and microglia can require anywhere from 6 to 15 weeks,204 4 to 9 weeks,205,206 or 5 to 9 weeks,164,207 respectively. 

As a result, researchers often prefer the use of progenitor cells as stable intermediates for experimentation, as they are broadly representative of the target cell type and can be generated in a faster time frame.204 Thus, the time constraints and costs required for establishing the hiPSC model system may present a concern for many research groups. 

In addition, the time required for the development of key disease-associated cellular phenotypes in hiPSC-derived cells is also a consideration in modeling NDDs. Whereas it takes decades of a patient's life to present clinical and pathological symptoms, disease-related molecular phenotypes in hiPSC-derived models were detected about 2 months post-maturation.208 

Nonetheless, the constraints stem from the limited lifespan of hiPSC-based models in culture that may not be sufficient for developing a complete picture resembling the disease on cellular and tissue levels. 

Strategies aimed at expediting the differentiation and maturation process, such as the incorporation of Notch and g-secretase inhibitors to shorten maturation time,209 and neurogenin-2 (Ngn2) or NeuroD1 overexpression158 have been partially successful and introduced other concerns raised from ectopic expression. 

Other limitations are related to the reproducibility of the systems in repetitive experiments and the inherent model-to-model variability, including setbacks regarding the purity of the hiPSC-derived cultures and the presence of undesirable heterogeneous cellular populations.210 

Thus, it is important to account for heterogeneity in cell populations when using hiPSC-derived models of disease, particularly in DMT screening and efficacy assessment. Moreover, hiPSC cultures have been reported to exhibit genomic instability and are prone to acquire genetic aberrations and mutations during cellular expansion and reprogramming.211 

Care also needs to be taken in the usage of isogenic lines generated by various genome-editing techniques, as potential off-target effects can occur (e.g., unintended clonal variability across isogenic lines and off-target mutagenesis).212,213 Therefore, it is important to periodically evaluate the genomic stability of hiPSCs to ensure rigorous DMT screening.

Challenges in Recapitulating Sporadic NDD Pathophysiology and Cellular Phenotypes

The complexity of NDDs and the lack of comprehensive characterization and mechanistic understanding of their cellular phenotypes pose challenges in the ability to recapitulate them with cellular models, including hiPSC-based systems. 

The causes of NDDs are complex and multifactorial, including polygenic risk factors (i.e., multiple genes and variants), epigenetics marks, aging, sex, and environmental factors, such as oxidative stress triggers, some of which are difficult to induce, specifically in an endless list of various combinations, in a laboratory system. 

Furthermore, the majority of hiPSC-derived AD-PD models are derived from patients with familial mutations that represent only a very small proportion of overall cases.214,215 Since the mechanisms underpinning the familial versus sporadic forms may be distinct, there is a need to establish models derived from hiPSCs and fibroblasts obtained from sAD or sporadic PD patients. 

Toward this goal, initiatives of patients' cell repositories have emerged in recent years; these include Applied StemCell (ASC), California Institute for Regenerative Medicine (CiRA), Cedars Sinai Induced Pluripotent Stem Cell Core, European Bank for Induced Pluripotent Stem Cells (EBiSC), Korean National Stem Cell Bank ( KSCB), National Institute of Aging (NIA), National Institue of Neurological Disorders and Stroke (NINDS), National Institute of General Medical Sciences (NIGMS), and the WiCell Research Institute (WiCell) (Table S2). 

The expected growth of these collections will facilitate the establishment of hiPSC-derived models more suitable for research of the common sAD and sporadic PD. 

Nonetheless, hiPSC models derived from sporadic NDD patients produce appropriate cellular phenotypes characteristic of AD181 and PD216 upon environmental manipulations, such as exposures to neurotoxins and chemical induction of oxidative stress. 

Also, a recent study revealed alterations in mitochondrial protein expression and elevated oxidative stress in hiPSCs generated from sAD patients, despite a marked lack of Ab and tau pathology,217 suggesting that hiPSC models may reveal valuable insight into the nuanced pathophysiology specific to sporadic NDD subtypes. 

Furthermore, a recent study by Meyer et al.218 utilized hiPSCs derived from sAD patients and found that MAPT, encoding the tau protein, was significantly increased in sAD-derived cells compared to healthy controls. 

Thus, despite these challenges, current hiPSC-derived models are excellent surrogates that provide the opportunity to mimic disease pathophysiology and establish gold-standard disease characteristics with the full genetic underpinnings in sporadic patients, which can be harnessed as outcome measures to improve subsequent disease models.

For more information:1950477648nn@gmail.com