Modelling Parkinson's Disease: iPSCs towards Better Understanding of Human Pathology

Ergothioneine-Mediated Neuroprotection of Human iPSC-Derived Dopaminergic Neurons

Cell death involving oxidative stress and mitochondrial dysfunction is a major cause of dopaminergic neuronal loss in the substantia nigra (SN) of Parkinson's disease patients. Ergothioneine (ET), a natural dietary compound, has been shown to have cytoprotective functions, but neuroprotective actions against PD have not been well established. 6-Hydroxydopamine (6-OHDA) is a widely used neurotoxin to simulate the degeneration of dopaminergic (DA) neurons in Parkinson's disease. In this study, we investigated the protective effect of ET on 6-OHDA treated iPSC-derived dopaminergic neurons (iDAs) and further confirmed the protective effects in 6-OHDA-treated human neuroblastoma SH-SY5Y cells. In 6-OHDA-treated cells, decreased mitochondrial membrane potential (ΔΨm), increased mitochondrial reactive oxygen species (mROS), reduced cellular ATP levels, and increased total protein carbonylation levels were observed. 6-OHDA treatment also significantly decreased tyrosine hydroxylase levels. These effects were significantly decreased when ET was present. Verapamil hydrochloride (VHCL), a non-specific inhibitor of the ET transporter OCTN1 abrogated ET's cytoprotective effects, indicative of an intracellular action. These results suggest that ET could be a potential therapeutic for Parkinson's disease.

Upregulated ECM genes and increased synaptic activity in Parkinson's human DA neurons with PINK1/ PRKN mutations

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease. Primary symptoms of PD arise with the loss of dopaminergic (DA) neurons in the Substantia Nigra Pars Compacta, but PD also affects the hippocampus and cortex, usually in its later stage. Approximately 15% of PD cases are familial with a genetic mutation. Two of the most associated genes with autosomal recessive (AR) early-onset familial PD are PINK1 and PRKN. In vitro studies of these genetic mutations are needed to understand the neurophysiological changes in patients' neurons that may contribute to neurodegeneration. In this work, we generated and differentiated DA and hippocampal neurons from human induced pluripotent stem cells (hiPSCs) derived from two patients with a double mutation in their PINK1 and PRKN (one homozygous and one heterozygous) genes and assessed their neurophysiology compared to two healthy controls. We showed that the synaptic activity of PD neurons generated from patients with the PINK1 and PRKN mutations is impaired in the hippocampus and dopaminergic neurons. Mutant dopaminergic neurons had enhanced excitatory post-synaptic activity. In addition, DA neurons with the homozygous mutation of PINK1 exhibited more pronounced electrophysiological differences compared to the control neurons. Signaling network analysis of RNA sequencing results revealed that Focal adhesion and ECM receptor pathway were the top two upregulated pathways in the mutant PD neurons. Our findings reveal that the phenotypes linked to PINK1 and PRKN mutations differ from those from other PD mutations, suggesting a unique interplay between these two mutations that drives different PD mechanisms.

Development of Midbrain Dopaminergic Neurons and the Advantage of Using hiPSCs as a Model System to Study Parkinson's Disease

Midbrain dopaminergic (mDA) neurons are significantly impaired in patients inflicted with Parkinson's disease (PD), subsequently affecting a variety of motor functions. There are four pathways through which dopamine elicits its function, namely, nigrostriatal, mesolimbic, mesocortical and tuberoinfundibular dopamine pathways. SHH and Wnt signalling pathways in association with favourable expression of a variety of genes, promotes the development and differentiation of mDA neurons in the brain. However, there is a knowledge gap regarding the complex signalling pathways involved in development of mDA neurons. hiPSC models have been acclaimed to be effective in generating complex disease phenotypes. These models mimic the microenvironment found in vivo thus ensuring maximum reliability. Further, a variety of therapeutic compounds can be screened using hiPSCs since they can be used to generate neurons that could carry an array of mutations associated with both familial and sporadic PD. Thus, culturing hiPSCs to study gene expression and dysregulation of cellular processes associated with PD can be useful in developing targeted therapies that will be a step towards halting disease progression.

Replacing Animal Testing with Stem Cell-Organoids : Advantages and Limitations

Various groups including animal protection organizations, medical organizations, research centers, and even federal agencies such as the U.S. Food and Drug Administration, are working to minimize animal use in scientific experiments. This movement primarily stems from animal welfare and ethical concerns. However, recent advances in technology and new studies in medicine have contributed to an increase in animal experiments throughout the years. With the rapid increase in animal testing, concerns arise including ethical issues, high cost, complex procedures, and potential inaccuracies.Alternative solutions have recently been investigated to address the problems of animal testing. Some of these technologies are related to stem cell technologies, such as organ-on-a-chip, organoids, and induced pluripotent stem cell models. The aim of the review is to focus on stem cell related methodologies, such as organoids, that can serve as an alternative to animal testing and discuss its advantages and limitations, alongside regulatory considerations.Although stem cell related methodologies has shortcomings, it has potential to replace animal testing. Achieving this requires further research on stem cells, with potential societal and technological benefits.

VPS35 and retromer dysfunction in Parkinson's disease

The vacuolar protein sorting 35 ortholog (VPS35) gene encodes a core component of the retromer complex essential for the endosomal sorting and recycling of transmembrane cargo. Endo-lysosomal pathway deficits are suggested to play a role in the pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). Mutations in VPS35 cause a late-onset, autosomal dominant form of PD, with a single missense mutation (D620N) shown to segregate with disease in PD families. Understanding how the PD-linked D620N mutation causes retromer dysfunction will provide valuable insight into the pathophysiology of PD and may advance the identification of therapeutics. D620N VPS35 can induce LRRK2 hyperactivation and impair endosomal recruitment of the WASH complex but is also linked to mitochondrial and autophagy-lysosomal pathway dysfunction and altered neurotransmitter receptor transport. The clinical similarities between VPS35-linked PD and sporadic PD suggest that defects observed in cellular and animal models with the D620N VPS35 mutation may provide valuable insights into sporadic disease. In this review, we highlight the current knowledge surrounding VPS35 and its role in retromer dysfunction in PD. We provide a critical discussion of the mechanisms implicated in VPS35-mediated neurodegeneration in PD, as well as the interplay between VPS35 and other PD-linked gene products. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.

Secretome Analyses Identify FKBP4 as a GBA1-Associated Protein in CSF and iPS Cells from Parkinson's Disease Patients with GBA1 Mutations

Mutations in the GBA1 gene increase the risk of developing Parkinson's disease (PD). However, most carriers of GBA1 mutations do not develop PD throughout their lives. The mechanisms of how GBA1 mutations contribute to PD pathogenesis remain unclear. Cerebrospinal fluid (CSF) is used for detecting pathological conditions of diseases, providing insights into the molecular mechanisms underlying neurodegenerative disorders. In this study, we utilized the proximity extension assay to examine the levels of metabolism-linked protein in the CSF from 17 PD patients carrying GBA1 mutations (GBA1-PD) and 17 idiopathic PD (iPD). The analysis of CSF secretome in GBA1-PD identified 11 significantly altered proteins, namely FKBP4, THOP1, GLRX, TXNDC5, GAL, SEMA3F, CRKL, APLP1, LRP11, CD164, and NPTXR. To investigate GBA1-associated CSF changes attributed to specific neuronal subtypes responsible for PD, we analyzed the cell culture supernatant from GBA1-PD-induced pluripotent stem cell (iPSC)-derived midbrain dopaminergic (mDA) neurons. The secretome analysis of GBA1-PD iPSC-derived mDA neurons revealed that five differently regulated proteins overlapped with those identified in the CSF analysis: FKBP4, THOP1, GLRX, GAL, and CRKL. Reduced intracellular level of the top hit, FKPB4, was confirmed via Western Blot. In conclusion, our findings identify significantly altered CSF GBA1-PD-associated proteins with FKPB4 being firmly attributed to mDA neurons.

Evaluation of type 1 diabetes mellitus as a risk factor of Parkinson's disease in a Drosophila model

Diabetes mellitus (DM) is a chronic metabolic disease characterized by high blood glucose levels, resulting from insulin dysregulation. Parkinson's disease (PD) is the most common neurodegenerative motor disorder caused by the selective loss of dopaminergic (DA) neurons in the substantia nigra pars compacta. DM and PD are both age-associated diseases that are turning into epidemics worldwide. Previous studies have indicated that type 2 DM might be a risk factor of developing PD. However, scarce information about the link between type 1 DM (T1DM) and PD does exist. In this work, we have generated a Drosophila model of T1DM based on insulin deficiency to evaluate if T1DM could be a risk factor to trigger PD onset. As expected, model flies exhibited T1DM-related phenotypes such as insulin deficiency, increased content of carbohydrates and glycogen, and reduced activity of insulin signaling. Interestingly, our results also demonstrated that T1DM model flies presented locomotor defects as well as reduced levels of tyrosine hydroxylase (a marker of DA neurons) in brains, which are typical PD-related phenotypes. In addition, T1DM model flies showed elevated oxidative stress levels, which could be causative of DA neurodegeneration. Therefore, our results indicate that T1DM might be a risk factor of developing PD, and encourage further studies to shed light into the exact link between both diseases.

Midbrain organoids-development and applications in Parkinson's disease

Human brain development is spatially and temporally complex. Insufficient access to human brain tissue and inadequacy of animal models has limited the study of brain development and neurodegenerative diseases. Recent advancements of brain organoid technology have created novel opportunities to model human-specific neurodevelopment and brain diseases. In this review, we discuss the use of brain organoids to model the midbrain and Parkinson's disease. We critically evaluate the extent of recapitulation of PD pathology by organoids and discuss areas of future development that may lead to the model to become a next-generation, personalized therapeutic strategy for PD and beyond.

Transcription of HOX Genes Is Significantly Increased during Neuronal Differentiation of iPSCs Derived from Patients with Parkinson's Disease

Parkinson's disease (PD) is the most serious movement disorder, but the actual cause of this disease is still unknown. Induced pluripotent stem cell-derived neural cultures from PD patients carry the potential for experimental modeling of underlying molecular events. We analyzed the RNA-seq data of iPSC-derived neural precursor cells (NPCs) and terminally differentiated neurons (TDNs) from healthy donors (HD) and PD patients with mutations in PARK2 published previously. The high level of transcription of HOX family protein-coding genes and lncRNA transcribed from the HOX clusters was revealed in the neural cultures from PD patients, while in HD NPCs and TDNs, the majority of these genes were not expressed or slightly transcribed. The results of this analysis were generally confirmed by qPCR. The HOX paralogs in the 3' clusters were activated more strongly than the genes of the 5' cluster. The abnormal activation of the HOX gene program upon neuronal differentiation in the cells of PD patients raises the possibility that the abnormal expression of these key regulators of neuronal development impacts PD pathology. Further research is needed to investigate this hypothesis.

Activation of Embryonic Gene Transcription in Neural Precursor Cells Derived from the Induced Pluripotent Stem Cells of the Patients with Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases in the world. Despite numerous studies, the causes of this pathology remain completely unknown. This is, among other things, due to the difficulty of obtaining biological material for analysis. Neural cell cultures derived from the induced pluripotent stem cells (IPSCs) provide a great potential for studying molecular events underlying the pathogenesis of PD. This paper presents the results of bioinformatic analysis of the data obtained using RNA-seq technology in the study of neural precursors (NP) derived from IPSCs of the healthy donors and patients with PD carrying various mutations that are commonly associated with familial PD. This analysis showed that the level of transcription of multiple genes actively expressed in the nervous system at the embryonic stage of development was significantly increased in the NP cells obtained from the patients with PD, unlike in the case of healthy donors. Bioinformatic data have been, in general, confirmed using real-time PCR. The obtained data suggest that one of the causes of PD may be the shift of the gene expression pattern in neuronal cells towards embryonic gene expression pattern (termed dematuration).

Lewy Body-like Pathology and Loss of Dopaminergic Neurons in Midbrain Organoids Derived from Familial Parkinson's Disease Patient

Progressive accumulation of α-Synuclein (αSyn) in Lewy bodies (LBs) and loss of dopaminergic (DA) neurons are the hallmark pathological features of Parkinson's disease (PD). Although currently available in vitro and in vivo models have provided crucial information about PD pathogenesis, the mechanistic link between the progressive accumulation of αSyn into LBs and the loss of DA neurons is still unclear. To address this, it is critical to model LB formation and DA neuron loss, the two key neuropathological aspects of PD, in a relevant in vitro system. In this study, we developed a human midbrain-like organoid (hMBO) model of PD. We demonstrated that hMBOs generated from induced pluripotent stem cells (hiPSCs), derived from a familial PD (fPD) patient carrying αSyn gene (SNCA) triplication accumulate pathological αSyn over time. These cytoplasmic inclusions spatially and morphologically resembled diverse stages of LB formation and were composed of key markers of LBs. Importantly, the progressive accumulation of pathological αSyn was paralleled by the loss of DA neurons and elevated apoptosis. The model developed in this study will complement the existing in vitro models of PD and will provide a unique platform to study the spatiotemporal events governing LB formation and their relation with neurodegeneration. Furthermore, this model will also be beneficial for in vitro screening and the development of therapeutic compounds.

The multifaceted role of LRRK2 in Parkinson's disease: From human iPSC to organoids

Parkinson's disease (PD) is the second most common neurodegenerative disease affecting elderly people. Pathogenic mutations in Leucine-Rich Repeat Kinase 2 (LRRK2) are the most common cause of autosomal dominant PD. LRRK2 activity is enhanced in both familial and idiopathic PD, thereby studies on LRRK2-related PD research are essential for understanding PD pathology. Finding an appropriate model to mimic PD pathology is crucial for revealing the molecular mechanisms underlying disease progression, and aiding drug discovery. In the last few years, the use of human-induced pluripotent stem cells (hiPSCs) grew exponentially, especially in studying neurodegenerative diseases like PD, where working with brain neurons and glial cells was mainly possible using postmortem samples. In this review, we will discuss the use of hiPSCs as a model for PD pathology and research on the LRRK2 function in both neuronal and immune cells, together with reviewing the recent advances in 3D organoid models and microfluidics.

Interaction of Alpha Synuclein and Microtubule Organization Is Linked to Impaired Neuritic Integrity in Parkinson’s Patient-Derived Neuronal Cells

Parkinson’s disease (PD) is neuropathologically characterized by the loss of dopaminergic neurons and the deposition of aggregated alpha synuclein (aSyn). Mounting evidence suggests that neuritic degeneration precedes neuronal loss in PD. A possible underlying mechanism could be the interference of aSyn with microtubule organization in the neuritic development, as implied by several studies using cell-free model systems. In this study, we investigate the impact of aSyn on microtubule organization in aSyn overexpressing H4 neuroglioma cells and midbrain dopaminergic neuronal cells (mDANs) generated from PD patient-derived human induced pluripotent stem cells (hiPSCs) carrying an aSyn gene duplication (SNCA^Dupl). An unbiased mass spectrometric analysis reveals a preferential binding of aggregated aSyn conformers to a number of microtubule elements. We confirm the interaction of aSyn with beta tubulin III in H4 and hiPSC-derived mDAN cell model systems, and demonstrate a remarkable redistribution of tubulin isoforms from the soluble to insoluble fraction, accompanied by a significantly increased insoluble aSyn level. Concordantly, SNCA^Dupl mDANs show impaired neuritic phenotypes characterized by perturbations in neurite initiation and outgrowth. In summary, our findings suggest a mechanistic pathway, through which aSyn aggregation interferes with microtubule organization and induces neurite impairments.

Human-Induced Pluripotent Stem Cell-Based Models for Studying Sex-Specific Differences in Neurodegenerative Diseases

The prevalence of neurodegenerative diseases is steadily increasing worldwide, and epidemiological studies strongly suggest that many of the diseases are sex-biased. It has long been suggested that biological sex differences are crucial for neurodegenerative diseases; however, how biological sex affects disease initiation, progression, and severity is not well-understood. Sex is a critical biological variable that should be taken into account in basic research, and this review aims to highlight the utility of human-induced pluripotent stem cells (iPSC)-derived models for studying sex-specific differences in neurodegenerative diseases, with advantages and limitations. In vitro systems utilizing species-specific, renewable, and physiologically relevant cell sources can provide powerful platforms for mechanistic studies, toxicity testings, and drug discovery. Matched healthy, patient-derived, and gene-corrected human iPSCs, from both sexes, can be utilized to generate neuronal and glial cell types affected by specific neurodegenerative diseases to study sex-specific differences in two-dimensional (2D) and three-dimensional (3D) human culture systems. Such relatively simple and well-controlled systems can significantly contribute to the elucidation of molecular mechanisms underlying sex-specific differences, which can yield effective, and potentially sex-based strategies, against neurodegenerative diseases.

Preventing Parkinson's Disease: An Environmental Agenda

Fueled by aging populations and continued environmental contamination, the global burden of Parkinson's disease (PD) is increasing. The disease, or more appropriately diseases, have multiple environmental and genetic influences but no approved disease modifying therapy. Additionally, efforts to prevent this debilitating disease have been limited. As numerous environmental contaminants (e.g., pesticides, metals, industrial chemicals) are implicated in PD, disease prevention is possible. To reduce the burden of PD, we have compiled preclinical and clinical research priorities that highlight both disease prediction and primary prevention. Though not exhaustive, the "PD prevention agenda" builds upon many years of research by our colleagues and proposes next steps through the lens of modifiable risk factors. The agenda identifies ten specific areas of further inquiry and considers the funding and policy changes that will be necessary to help prevent the world's fastest growing brain disease.

Induced Pluripotent Stem Cell-Derived Dopaminergic Neurons from Familial Parkinson's Disease Patients Display α-Synuclein Pathology and Abnormal Mitochondrial Morphology

Accumulation of α-synuclein (α-syn) into Lewy bodies (LBs) and mitochondrial abnormalities are the two cardinal pathobiological features of Parkinson’s disease (PD), which are associated with the loss of dopaminergic neurons. Although α-syn accumulates in many different cellular and mouse models, these models generally lack LB features. Here, we generated midbrain dopaminergic (mDA) neuronal cultures from induced pluripotent stem cells (iPSCs) derived from familial PD (fPD) patients and healthy controls. We show that mDA neuronal cultures from fPD patients with A53T mutation and α-syn gene (SNCA) triplication display pathological α-syn deposits, which spatially and morphologically resemble LBs. Importantly, we did not find any apparent accumulation of pathological α-syn in mDA neuronal culture derived from a healthy donor. Furthermore, we show that there are morphological abnormalities in the mitochondrial network in mDA neuronal cultures from fPD patients. Consequently, these cells were more susceptible to mitochondrial damage compared with healthy donor-derived mDA neuronal cultures. Our results indicate that the iPSC-derived mDA neuronal culture platform can be used to investigate the spatiotemporal appearance of LBs, as well as their composition, architecture, and relationship with mitochondrial abnormalities.

α-synuclein pathogenesis in hiPSC models of Parkinson's disease

α-synuclein is an increasingly prominent player in the pathology of a variety of neurodegenerative conditions. Parkinson’s disease (PD) is a neurodegenerative disorder that affects mainly the dopaminergic (DA) neurons in the substantia nigra of the brain. Typical of PD pathology is the finding of protein aggregations termed ‘Lewy bodies’ in the brain regions affected. α-synuclein is implicated in many disease states including dementia with Lewy bodies (DLB) and Alzheimer’s disease. However, PD is the most common synucleinopathy and continues to be a significant focus of PD research in terms of the α-synuclein Lewy body pathology. Mutations in several genes are associated with PD development including SNCA, which encodes α-synuclein. A variety of model systems have been employed to study α-synuclein physiology and pathophysiology in an attempt to relate more closely to PD pathology. These models include cellular and animal system exploring transgenic technologies, viral vector expression and knockdown approaches, and models to study the potential prion protein-like effects of α-synuclein. The current review focuses on human induced pluripotent stem cell (iPSC) models with a specific focus on mutations or multiplications of the SNCA gene. iPSCs are a rapidly evolving technology with huge promise in the study of normal physiology and disease modeling in vitro. The ability to maintain a patient’s genetic background and replicate similar cell phenotypes make iPSCs a powerful tool in the study of neurological diseases. This review focuses on the current knowledge about α-synuclein physiological function as well as its role in PD pathogenesis based on human iPSC models.

Towards physiologically relevant human pluripotent stem cell (hPSC) models of Parkinson's disease

The derivation of human embryonic stem cells followed by the discovery of induced pluripotent stem cells and leaps in genome editing approaches have continuously fueled enthusiasm for the development of new models of neurodegenerative diseases such as Parkinson's disease (PD). PD is characterized by the relative selective loss of dopaminergic neurons (DNs) in specific areas of substantia nigra pars compacta (SNpc). While degeneration in late stages can be widespread, there is stereotypic early degeneration of these uniquely vulnerable neurons. Various causes of selective vulnerability have been investigated but much remains unclear. Most studies have sought to identify cell autonomous properties of the most vulnerable neurons. However, recent findings from genetic studies and model systems have added to our understanding of non-cell autonomous contributions including regional-specific neuro-immune interactions with astrocytes, resident or damage-activated microglia, neuro-glia cell metabolic interactions, involvement of endothelial cells, and damage to the vascular system. All of these contribute to specific vulnerability and, along with aging and environmental factors, might be integrated in a complex stressor-threshold model of neurodegeneration. In this forward-looking review, we synthesize recent advances in the field of PD modeling using human pluripotent stem cells, with an emphasis on organoid and complex co-culture models of the nigrostriatal niche, with emerging CRISPR applications to edit or perturb expression of causal PD genes and associated risk factors, such as GBA, to understand the impact of these genes on relevant phenotypes.