Emerging regenerative medicine and tissue engineering strategies for Parkinson's disease

Disruption of Dopamine Homeostasis Associated with Alteration of Proteins in Synaptic Vesicles: A Putative Central Mechanism of Parkinson's Disease Pathogenesis

Vestigial dopaminergic cells in PD have selectivity for a sub-class of hypersensitive neurons with the nigrostriatal dopamine (DA) tract. DA is modulated in pre-synaptic nerve terminals to remain stable. To be specific, proteins at DA release sites that have a function of synthesizing and packing DA in cytoplasm, modulating release and reingestion, and changing excitability of neurons, display regional discrepancies that uncover relevancy of the observed sensitivity to neurodegenerative changes. Although the reasons of a majority of PD cases are still indistinct, heredity and environment are known to us to make significant influences. For decades, genetic analysis of PD patients with heredity in family have promoted our comprehension of pathogenesis to a great extent, which reveals correlative mechanisms including oxidative stress, abnormal protein homeostasis and mitochondrial dysfunction. In this review, we review the constitution of presynaptic vesicle related to DA homeostasis and describe the genetic and environmental evidence of presynaptic dysfunction that increase risky possibility of PD concerning intracellular vesicle transmission and their functional outcomes. We summarize alterations in synaptic vesicular proteins with great involvement in the reasons of some DA neurons highly vulnerable to neurodegenerative changes. We generalize different potential targets and therapeutic strategies for different pathogenic mechanisms, providing a reference for further studies of PD treatment in the future. But it remains to be further researched on this recently discovered and converging mechanism of vesicular dynamics and PD, which will provide a more profound comprehension and put up with new therapeutic tactics for PD patients.

General Treatments Promoting Independent Living in Parkinson's Patients and Physical Therapy Approaches for Improving Gait-A Comprehensive Review

This study delves into the multifaceted approaches to treating Parkinson's disease (PD), a neurodegenerative disorder primarily affecting motor function but also manifesting in a variety of symptoms that vary greatly among individuals. The complexity of PD symptoms necessitates a comprehensive treatment strategy that integrates surgical interventions, pharmacotherapy, and physical therapy to tailor to the unique needs of each patient. Surgical options, such as deep brain stimulation (DBS), have been pivotal for patients not responding adequately to medication, offering significant symptom relief. Pharmacotherapy remains a cornerstone of PD management, utilizing drugs like levodopa, dopamine agonists, and others to manage symptoms and, in some cases, slow down disease progression. However, these treatments often lead to complications over time, such as motor fluctuations and dyskinesias, highlighting the need for precise dosage adjustments and sometimes combination therapies to optimize patient outcomes. Physical therapy plays a critical role in addressing the motor symptoms of PD, including bradykinesia, muscle rigidity, tremors, postural instability, and akinesia. PT techniques are tailored to improve mobility, balance, strength, and overall quality of life. Strategies such as gait and balance training, strengthening exercises, stretching, and functional training are employed to mitigate symptoms and enhance functional independence. Specialized approaches like proprioceptive neuromuscular facilitation (PNF), the Bobath concept, and the use of assistive devices are also integral to the rehabilitation process, aimed at improving patients' ability to perform daily activities and reducing the risk of falls. Innovations in technology have introduced robotic-assisted gait training (RAGT) and other assistive devices, offering new possibilities for patient care. These tools provide targeted support and feedback, allowing for more intensive and personalized rehabilitation sessions. Despite these advancements, high costs and accessibility issues remain challenges that need addressing. The inclusion of exercise and activity beyond structured PT sessions is encouraged, with evidence suggesting that regular physical activity can have neuroprotective effects, potentially slowing disease progression. Activities such as treadmill walking, cycling, and aquatic exercises not only improve physical symptoms but also contribute to emotional well-being and social interactions. In conclusion, treating PD requires a holistic approach that combines medical, surgical, and therapeutic strategies. While there is no cure, the goal is to maximize patients' functional abilities and quality of life through personalized treatment plans. This integrated approach, along with ongoing research and development of new therapies, offers hope for improving the management of PD and the lives of those affected by this challenging disease.

Dose-Ranging Effects of the Intracerebral Administration of Atsttrin in Experimental Model of Parkinson's Disease Induced by 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in Mice

Parkinson's disease is one of the most common neurodegenerative disorders characterized by a multitude of motor and non-motor clinical symptoms resulting from the progressive and long-lasting abnormal loss of nigrostriatal dopaminergic neurons. Currently, the available treatments for patients with Parkinson's disease are limited and exert only symptomatic effects, without adequate signs of delaying or stopping the progression of the disease. Atsttrin constitutes the bioengineered protein which ultrastructure is based on the polypeptide chain frame of the progranulin (PGRN), which exerts anti-inflammatory effects through the inhibition of TNFα. The conducted preclinical studies suggest that the therapeutic implementation of Atsttrin may be potentially effective in the treatment of neurodegenerative diseases that are associated with the occurrence of neuroinflammatory processes. The aim of the proposed study was to investigate the effect of direct bilateral intracerebral administration of Atsttrin using stereotactic methods in the preclinical C57BL/6 mouse model of Parkinson's disease inducted by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. The analysis of the dose dependency effects of the increasing doses of Atsttrin has covered a number of parameters and markers regarding neurodegenerative processes and inflammatory responses including IL-1α, TNFα, IL-6, TH, and TG2 mRNA expressions. Accordingly, the evaluation of the changes in the neurochemical profile included DA, DOPAC, 3-MT, HVA, NA, MHPG, 5-HT, and 5-HIAA concentration levels. The intracerebral administration of Atsttrin into the striatum effectively attenuated the neuroinflammatory reaction in evaluated neuroanatomical structures. Furthermore, the partial restoration of monoamine content and its metabolic turnover were observed. In this case, taking into account the previously described pharmacokinetic profile and extrapolated bioavailability as well as the stability characteristics of Atsttrin, an attempt was made to describe as precisely as possible the quantitative and qualitative effects of increasing doses of the compound within the brain tissue microenvironment in the presented preclinical model of the disease. Collectively, this findings demonstrated that the intracerebral administration of Atsttrin may represent a potential novel therapeutic method for the treatment of Parkinson's disease.

Injectable Hydrogels for Nervous Tissue Repair-A Brief Review

The repair of nervous tissue is a critical research field in tissue engineering because of the degenerative process in the injured nervous system. In this review, we summarize the progress of injectable hydrogels using in vitro and in vivo studies for the regeneration and repair of nervous tissue. Traditional treatments have not been favorable for patients, as they are invasive and inefficient; therefore, injectable hydrogels are promising for the treatment of damaged tissue. This review will contribute to a better understanding of injectable hydrogels as potential scaffolds and drug delivery system for neural tissue engineering applications.

Recent approaches in regenerative medicine in the fight against neurodegenerative disease

Neurodegenerative diseases arise due to slow and gradual loss of structure and/or function of neurons and glial cells and cause different degrees of loss of cognition abilities and sensation. The little success in developing effective treatments imposes a high and regressive economic impact on society, patients and their families. In recent years, regenerative medicine has provided a great opportunity to research new innovative strategies with strong potential to treatleva these diseases. These effects are due to the ability of suitable cells and biomaterials to regenerate damaged nerves with differentiated cells, creating an appropriate environment for recovering or preserving existing healthy neurons and glial cells from destruction and damage. Ultimately, a better understanding and thus a further investigation of stem cell technology, tissue engineering, gene therapy, and exosomes allows progress towards practical and effective treatments for neurodegenerative diseases. Therefore, in this review, advances currently being developed in regenerative medicine using animal models and human clinical trials in neurological disorders are summarized.

Bi-directional regulation of AIMP2 and its splice variant on PARP-1-dependent neuronal cell death; Therapeutic implication for Parkinson's disease

BACKGROUND

Parthanatos represents a critical molecular aspect of Parkinson's disease, wherein AIMP2 aberrantly activates PARP-1 through direct physical interaction. Although AIMP2 ought to be a therapeutic target for the disease, regrettably, it is deemed undruggable due to its non-enzymatic nature and predominant localization within the tRNA synthetase multi-complex. Instead, AIMP2 possesses an antagonistic splice variant, designated DX2, which counteracts AIMP2-induced apoptosis in the p53 or inflammatory pathway. Consequently, we examined whether DX2 competes with AIMP2 for PARP-1 activation and is therapeutically effective in Parkinson's disease.

METHODS

The binding affinity of AIMP2 and DX2 to PARP-1 was contrasted through immunoprecipitation. The efficacy of DX2 in neuronal cell death was assessed under 6-OHDA and H2O2 in vitro conditions. Additionally, endosomal and exosomal activity of synaptic vesicles was gauged in AIMP2 or DX2 overexpressed hippocampal primary neurons utilizing optical live imaging with VAMP-vGlut1 probes. To ascertain the role of DX2 in vivo, rotenone-induced behavioral alterations were compared between wild-type and DX2 transgenic animals. A DX2-encoding self-complementary adeno-associated virus (scAAV) was intracranially injected into 6-OHDA induced in vivo animal models, and their mobility was examined. Subsequently, the isolated brain tissues were analyzed.

RESULTS

DX2 translocates into the nucleus upon ROS stress more rapidly than AIMP2. The binding affinity of DX2 to PARP-1 appeared to be more robust compared to that of AIMP2, resulting in the inhibition of PARP-1 induced neuronal cell death. DX2 transgenic animals exhibited neuroprotective behavior in rotenone-induced neuronal damage conditions. Following a single intracranial injection of AAV-DX2, both behavior and mobility were consistently ameliorated in neurodegenerative animal models induced by 6-OHDA.

CONCLUSION

AIMP2 and DX2 are proposed to engage in bidirectional regulation of parthanatos. They physically interact with PARP-1. Notably, DX2's cell survival properties manifest exclusively in the context of abnormal AIMP2 accumulation, devoid of any tumorigenic effects. This suggests that DX2 could represent a distinctive therapeutic target for addressing Parkinson's disease in patients.

Unravelling the Parkinson's puzzle, from medications and surgery to stem cells and genes: a comprehensive review of current and future management strategies

Parkinson's disease (PD) is a neurodegenerative disorder, prevalent in the elderly population. Neuropathological hallmarks of PD include loss of dopaminergic cells in the nigro-striatal pathway and deposition of alpha-synuclein protein in the neurons and synaptic terminals, which lead to a complex presentation of motor and non-motor symptoms. This review focuses on various aspects of PD, from clinical diagnosis to currently accepted treatment options, such as pharmacological management through dopamine replacement and surgical techniques such as deep brain stimulation (DBS). The review discusses in detail the potential of emerging stem cell-based therapies and gene therapies to be adopted as a cure, in contrast to the present symptomatic treatment in PD. The potential sources of stem cells for autologous and allogeneic stem cell therapy have been discussed, along with the progress evaluation of pre-clinical and clinical trials. Even though recent techniques hold great potential to improve the lives of PD patients, we present the importance of addressing the safety, efficacy, ethical, cost, and regulatory concerns before scaling them to clinical use.

Cell Therapy for Parkinson's Disease

Parkinson's Disease (PD) is a neurodegenerative disease characterized by the progressive loss of dopaminergic neurons of the substantia nigra pars compacta with a reduction in dopamine concentration in the striatum. It is a substantial loss of dopaminergic neurons that is responsible for the classic triad of PD symptoms, i.e., resting tremor, muscular rigidity, and bradykinesia. Several current therapies for PD may only offer symptomatic relief and do not address the underlying neurodegeneration of PD. The recent developments in cellular reprogramming have enabled the development of previously unachievable cell therapies and patient-specific modeling of PD through Induced Pluripotent Stem Cells (iPSCs). iPSCs possess the inherent capacity for pluripotency, allowing for their directed differentiation into diverse cell lineages, such as dopaminergic neurons, thus offering a promising avenue for addressing the issue of neurodegeneration within the context of PD. This narrative review provides a comprehensive overview of the effects of dopamine on PD patients, illustrates the versatility of iPSCs and their regenerative abilities, and examines the benefits of using iPSC treatment for PD as opposed to current therapeutic measures. In means of providing a treatment approach that reinforces the long-term survival of the transplanted neurons, the review covers three supplementary avenues to reinforce the potential of iPSCs.

Metal ion mediated aggregation of Alzheimer's disease peptides and proteins in solutions and at surfaces

Although the pathogenesis of Alzheimer's disease (AD) is still unclear, abnormally high concentrations of metal ions, like copper, iron and zinc, were found in senile plaques of AD brain, which inspires extensive studies on the fundamental molecular interactions of metal ions with the pathogenic hallmarks, amyloid-β (Aβ) peptides and tau proteins, respectively forming senile plaques and neurofibrillary tangles (NFTs) in AD brains. Early works concern the concentration effect of the metal ions on Aβ and tau aggregation. Yet, it is obvious that the surrounding environment of the metal ions must also be considered, not just the metal ions as free accessible forms in the solution phase. The most important surrounding environment in vivo is a very large surface area from cell membranes and other macromolecular surfaces. These bio-interfaces make the kinetic pathways of metal ion mediated Aβ and tau aggregation radically different from those in the solution phase. To better understand the role of metal ions in AD peptide and protein aggregation, we summarize and discuss the recent achievements in the research of metal ion mediated Aβ and tau aggregation, particularly the corresponding mechanism differences between the solution phase and the surface environment. The metal ion chelation therapy for AD is also discussed from the point of the surface pool of metal ions.

Cell-therapy for Parkinson's disease: a systematic review and meta-analysis

BACKGROUND

Cell-based strategies focusing on replacement or protection of dopaminergic neurons have been considered as a potential approach to treat Parkinson's disease (PD) for decades. However, despite promising preclinical results, clinical trials on cell-therapy for PD reported mixed outcomes and a thorough synthesis of these findings is lacking. We performed a systematic review and meta-analysis to evaluate cell-therapy for PD patients.

METHODS

We systematically identified all clinical trials investigating cell- or tissue-based therapies for PD published before July 2023. Out of those, studies reporting transplantation of homogenous cells (containing one cell type) were included in meta-analysis. The mean difference or standardized mean difference in quantitative neurological scale scores before and after cell-therapy was analyzed to evaluate treatment effects.

RESULTS

The systematic literature search revealed 106 articles. Eleven studies reporting data from 11 independent trials (210 patients) were eligible for meta-analysis. Disease severity and motor function evaluation indicated beneficial effects of homogenous cell-therapy in the 'off' state at 3-, 6-, 12-, or 24-month follow-ups, and for motor function even after 36 months. Most of the patients were levodopa responders (61.6-100% in different follow-ups). Cell-therapy was also effective in improving the daily living activities in the 'off' state of PD patients. Cells from diverse sources were used and multiple transplantation modes were applied. Autografts did not improve functional outcomes, while allografts exhibited beneficial effects. Encouragingly, both transplantation into basal ganglia and to areas outside the basal ganglia were effective to reduce disease severity. Some trials reported adverse events potentially related to the surgical procedure. One confirmed and four possible cases of graft-induced dyskinesia were reported in two trials included in this meta-analysis.

CONCLUSIONS

This meta-analysis provides preliminary evidence for the beneficial effects of homogenous cell-therapy for PD, potentially to the levodopa responders. Allogeneic cells were superior to autologous cells, and the effective transplantation sites are not limited to the basal ganglia. PROSPERO registration number: CRD42022369760.

Striatonigral distribution of a fluorescent reporter following intracerebral delivery of genome editors

Introduction: Targeted gene editing is proposed as a therapeutic approach for numerous disorders, including neurological diseases. As the brain is organized into neural networks, it is critical to understand how anatomically connected structures are affected by genome editing. For example, neurons in the substantia nigra pars compacta (SNpc) project to the striatum, and the striatum contains neurons that project to the substantia nigra pars reticulata (SNpr). Methods: Here, we report the effect of injecting genome editors into the striatum of Ai14 reporter mice, which have a LoxP-flanked stop cassette that prevents expression of the red fluorescent protein tdTomato. Two weeks following intracerebral delivery of either synthetic nanocapsules (NCs) containing CRISPR ribonucleoprotein targeting the tdTomato stop cassette or adeno-associated virus (AAV) vectors expressing Cre recombinase, the brains were collected, and the presence of tdTomato was assessed in both the striatum and SN. Results: TdTomato expression was observed at the injection site in both the NC- and AAV-treated groups and typically colocalized with the neuronal marker NeuN. In the SN, tdTomato-positive fibers were present in the pars reticulata, and SNpr area expressing tdTomato correlated with the size of the striatal genome edited area. Conclusion: These results demonstrate in vivo anterograde axonal transport of reporter gene protein products to the SNpr following neuronal genome editing in the striatum.

The evolving regulatory landscape in regenerative medicine

Regenerative medicine as a field has emerged as a new component of modern medicine and medical research that encompasses a wide range of products including cellular and acellular therapies. As this new field emerged, regulatory agencies like the Food and Drug Administration (FDA) rapidly adapted existing regulatory frameworks to address the transplantation, gene therapy, cell-based therapeutics, and acellular biologics that fall under the broader regenerative medicine umbrella. Where it has not been possible to modify existing regulation and processes, entirely new frameworks have been generated with the intention of providing flexible, forward-facing systems to regulate this rapidly growing field. This review discusses the current state of FDA regulatory affairs in the context of stem cells and extracellular vesicles by highlighting gaps in the current regulatory system and then discussing where regulatory science in regenerative medicine may be headed based on these gaps and the FDA's historical ability to deal with emerging fields. Lastly, we utilize case studies in stem cell and acellular based treatments to demonstrate how regulatory science has evolved in regenerative medicine and highlight the ongoing clinical efforts and challenges of these therapies.

Polymeric Materials, Advances and Applications in Tissue Engineering: A Review

Tissue Engineering (TE) is an interdisciplinary field that encompasses materials science in combination with biological and engineering sciences. In recent years, an increase in the demand for therapeutic strategies for improving quality of life has necessitated innovative approaches to designing intelligent biomaterials aimed at the regeneration of tissues and organs. Polymeric porous scaffolds play a critical role in TE strategies for providing a favorable environment for tissue restoration and establishing the interaction of the biomaterial with cells and inducing substances. This article reviewed the various polymeric scaffold materials and their production techniques, as well as the basic elements and principles of TE. Several interesting strategies in eight main TE application areas of epithelial, bone, uterine, vascular, nerve, cartilaginous, cardiac, and urinary tissue were included with the aim of learning about current approaches in TE. Different polymer-based medical devices approved for use in clinical trials and a wide variety of polymeric biomaterials are currently available as commercial products. However, there still are obstacles that limit the clinical translation of TE implants for use wide in humans, and much research work is still needed in the field of regenerative medicine.

Neuroprotective potential of plant derived parenchymal stem cells extract on environmental and genetic models of Parkinson disease through attenuation of oxidative stress and neuroinflammation

Parkinson's disease (PD) is a neurodegenerative disorder characterized by both motor and non-motor features. The current treatment regimen for PD are dopamine enhancers which have been reported to worsen the disease prognosis after long term treatment, thus, the need for better treatment options. This study sought to investigate the protective action of Double Stem Cell® (DSC), a blend of stem cells extracts from Swiss apples (Malus Domestica) and Burgundy grapes (Vitis vinifera) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism in mice and genetic model of PD in Drosophila melanogaster. Male albino mice were pretreated with MPTP (4 × 20 mg/kg, i.p., two hourly in 8 h), twelve hours before administration of DSC (8, 40, or 200 mg/kg, p.o.). Thereafter, behavioural, biochemical and immunohistochemical assays were carried out. The impact of vehicle or DSC supplementation on α-synuclein aggregation was evaluated in Drosophila melanogaster using the UAS-Gal4 system, female DDC-Gal4 flies were crossed with male UAS-α-synuclein, the progenies were examined for fecundity, locomotion, memory, and lifespan. MPTP-induced motor deficits in open field test (OFT), working memory impairment (Y-maze test (YMT)) and muscle incoordination (rotarod test) were ameliorated by DSC (8, 40 or 200 mg/kg) through dose-dependent and significant improvements in motor, cognitive and motor coordination. Moreso, MPTP exposure caused significant increase in lipid peroxidation and decrease in antioxidant enzymes activities (glutathione, catalase and superoxide dismutase) in the midbrain which were attenuated by DSC. MPTP-induced expression of microglia (iba-1), astrocytes (glia fibrillary acidic protein; GFAP) as well as degeneration of dopamine neurons (tyrosine hydroxylase positive neurons) in the substantia nigra (SN) were reversed by DSC. Supplementation of flies feed with graded concentration of DSC (0.8, 4 or 20 mg/ml) did not affect fecundity but improved climbing activity and lifespan. Findings from this study showed that Double Stem Cell improved motor and cognitive functions in both mice and Drosophila through attenuation of neurotoxin-induced oxidative stress and neuroinflammation.

Dipeptidyl peptidase 4(DPP4) inhibitors stride up the management of Parkinson's disease

Parkinson's disease (PD) is the 2^nd most common age-related hypokinetic disorder, characterized by dopaminergic degeneration and movement abnormalities. Dopaminergic degeneration in the basal ganglia is primarily seen in PD patients. The therapeutic strategies currently under investigation are to rescue dopaminergic degeneration and promote neuronal regeneration, which could halt disease progression. On the other hand, the therapeutic efficacy of existing drugs used in other disorders has been repurposed in neurodegenerative pathologies. DPP4 inhibitors widely used in treating diabetes have been considered viable target sites and are being tested for efficacy in neurodegenerative pathologies. DPP4 inhibitors have been reported to rescue neuronal degeneration and improve motor functions in various preclinical and clinical PD studies. The current review is focused on the neuroprotective potential, molecular mechanisms and therapeutic potential of DPP4 inhibitors in PD pathology.

Current Status and Future Perspectives on Stem Cell-Based Therapies for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease, affecting 1%-2% of the population over the age of 65. As the population ages, it is anticipated that the burden on society will significantly escalate. Although symptom reduction by currently available pharmacological and/or surgical treatments improves the quality of life of many PD patients, there are no treatments that can slow down, halt, or reverse disease progression. Because the loss of a specific cell type, midbrain dopamine neurons in the substantia nigra, is the main cause of motor dysfunction in PD, it is considered a promising target for cell replacement therapy. Indeed, numerous preclinical and clinical studies using fetal cell transplantation have provided proof of concept that cell replacement therapy may be a viable therapeutic approach for PD. However, the use of human fetal cells remains fraught with controversy due to fundamental ethical, practical, and clinical limitations. Groundbreaking work on human pluripotent stem cells (hPSCs), including human embryonic stem cells and human induced pluripotent stem cells, coupled with extensive basic research in the stem cell field offers promising potential for hPSC-based cell replacement to become a realistic treatment regimen for PD once several major issues can be successfully addressed. In this review, we will discuss the prospects and challenges of hPSC-based cell therapy for PD.

Developmental pathways linked to the vulnerability of adult midbrain dopaminergic neurons to neurodegeneration

The degeneration of dopaminergic and other neurons in the aging brain is considered a process starting well beyond the infantile and juvenile period. In contrast to other dopamine-associated neuropsychiatric disorders, such as schizophrenia and drug addiction, typically diagnosed during adolescence or young adulthood and, thus, thought to be rooted in the developing brain, Parkinson's Disease (PD) is rarely viewed as such. However, evidences have accumulated suggesting that several factors might contribute to an increased vulnerability to death of the dopaminergic neurons at an already very early (developmental) phase in life. Despite the remarkable ability of the brain to compensate such dopamine deficits, the early loss or dysfunction of these neurons might predispose an individual to suffer from PD because the critical threshold of dopamine function will be reached much earlier in life, even if the time-course and strength of naturally occurring and age-dependent dopaminergic cell death is not markedly altered in this individual. Several signaling and transcriptional pathways required for the proper embryonic development of the midbrain dopaminergic neurons, which are the most affected in PD, either continue to be active in the adult mammalian midbrain or are reactivated at the transition to adulthood and under neurotoxic conditions. The persistent activity of these pathways often has neuroprotective functions in adult midbrain dopaminergic neurons, whereas the reactivation of silenced pathways under pathological conditions can promote the survival and even regeneration of these neurons in the lesioned or aging brain. This article summarizes our current knowledge about signaling and transcription factors involved in midbrain dopaminergic neuron development, whose reduced gene dosage or signaling activity are implicated in a lower survival rate of these neurons in the postnatal or aging brain. It also discusses the evidences supporting the neuroprotection of the midbrain dopaminergic system after the external supply or ectopic expression of some of these secreted and nuclear factors in the adult and aging brain. Altogether, the timely monitoring and/or correction of these signaling and transcriptional pathways might be a promising approach to a much earlier diagnosis and/or prevention of PD.

BDNF-modified human umbilical cord mesenchymal stem cells-derived dopaminergic-like neurons improve rotation behavior of Parkinson's disease rats through neuroprotection and anti-neuroinflammation

Parkinson's disease (PD) is a neurodegenerative disease still without any cure. Brain-derived neurotrophic factor (BDNF) has shown therapeutic potential in PD, which is limited by its short half-life and inability to penetrate the blood-brain barrier. Stem cells not only present migration, differentiation and neurotrophy characteristics, but also can be used as delivery vectors for BDNF. This study aimed to investigate the therapeutic effects and possible mechanisms of BDNF-modified human umbilical cord mesenchymal stem cells (hUC-MSCs)-derived dopaminergic (DAergic)-like neurons in the PD rats. Results showed that transplantation of BDNF-modified hUC-MSCs-derived DAergic-like neurons improved the apomorphine induced rotation behavior of PD rats, increased the dopamine concentration and the expression of glial fibrillary acidic protein (GFAP) and ionized calcium-binding adaptor molecule-1 (Iba-1) in the striatum, promoted the expression of tyrosine hydroxylase (TH), nuclear receptor-related factor 1 (Nurr1), pituitary homeobox 3 (Pitx3), BDNF, tyrosine kinase B (TrkB), phosphatidylinositol-3-hydroxykinase (PI3K), phosphorylated protein kinase B (p-Akt), heat shock protein 60 (Hsp60), toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88) and inhibited the neural apoptosis in the substantia nigra (SN) and striatum. Results suggest that BDNF-modified hUC-MSCs-derived DAergic-like neurons improve the rotation of PD rats might through neuroprotection and anti-neuroinflammation by regulating the BDNF-TrkB-PI3K/Akt and Hsp60-TLR4/MyD88 signaling pathways, respectively.

Axonal Tract Reconstruction Using a Tissue-Engineered Nigrostriatal Pathway in a Rat Model of Parkinson's Disease

Parkinson's disease (PD) affects 1-2% of people over 65, causing significant morbidity across a progressive disease course. The classic PD motor deficits are caused by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), resulting in the loss of their long-distance axonal projections that modulate striatal output. While contemporary treatments temporarily alleviate symptoms of this disconnection, there is no approach able to replace the nigrostriatal pathway. We applied microtissue engineering techniques to create a living, implantable tissue-engineered nigrostriatal pathway (TE-NSP) that mimics the architecture and function of the native pathway. TE-NSPs comprise a discrete population of dopaminergic neurons extending long, bundled axonal tracts within the lumen of hydrogel micro-columns. Neurons were isolated from the ventral mesencephalon of transgenic rats selectively expressing the green fluorescent protein in dopaminergic neurons with subsequent fluorescent-activated cell sorting to enrich a population to 60% purity. The lumen extracellular matrix and growth factors were varied to optimize cytoarchitecture and neurite length, while immunocytochemistry and fast-scan cyclic voltammetry (FSCV) revealed that TE-NSP axons released dopamine and integrated with striatal neurons in vitro. Finally, TE-NSPs were implanted to span the nigrostriatal pathway in a rat PD model with a unilateral 6-hydroxydopamine SNpc lesion. Immunohistochemistry and FSCV established that transplanted TE-NSPs survived, maintained their axonal tract projections, extended dopaminergic neurites into host tissue, and released dopamine in the striatum. This work showed proof of concept that TE-NSPs can reconstruct the nigrostriatal pathway, providing motivation for future studies evaluating potential functional benefits and long-term durability of this strategy. This pathway reconstruction strategy may ultimately replace lost neuroarchitecture and alleviate the cause of motor symptoms for PD patients.

New Targets and New Technologies in the Treatment of Parkinson’s Disease: A Narrative Review

Parkinson’s disease (PD) is a progressive neurodegenerative disease, whose main neuropathological finding is pars compacta degeneration due to the accumulation of Lewy bodies and Lewy neurites, and subsequent dopamine depletion. This leads to an increase in the activity of the subthalamic nucleus (STN) and the internal globus pallidus (GPi). Understanding functional anatomy is the key to understanding and developing new targets and new technologies that could potentially improve motor and non-motor symptoms in PD. Currently, the classical targets are insufficient to improve the entire wide spectrum of symptoms in PD (especially non-dopaminergic ones) and none are free of the side effects which are not only associated with the procedure, but with the targets themselves. The objective of this narrative review is to show new targets in DBS surgery as well as new technologies that are under study and have shown promising results to date. The aim is to give an overview of these new targets, as well as their limitations, and describe the current studies in this research field in order to review ongoing research that will probably become effective and routine treatments for PD in the near future.

Advances in Hydrogel-Based Drug Delivery Systems for Parkinson's Disease

Parkinson's disease (PD) is a common central nervous system disorder (CNS) characterized by cell loss in the substantia nigra. Severe loss of dopaminergic neurons and Lewy body formation with α-synuclein inclusions are the main neuropathological features of PD. There's currently no cure for PD, but treatments are available to help relieve the symptoms and maintain quality of life. However, the variety of clinically available therapeutic molecules is mainly limited to treating symptoms rather than halting or reversing disease progression via medical interventions. As an emerging drug carrier, hydrogels loaded with therapeutic agents and cells are attracting attention as an alternative and potentially more effective approach to managing PD. The current work highlights applications of hydrogel-based biomaterials in cell culture and disease modeling as carriers for cells, medicines, and proteins as PD therapeutic models.

Long-Term Evaluation of Intranigral Transplantation of Human iPSC-Derived Dopamine Neurons in a Parkinson's Disease Mouse Model

Parkinson's disease (PD) is a neurodegenerative disorder associated with loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). One strategy for treating PD is transplantation of DA neuroblasts. Significant advances have been made in generating midbrain DA neurons from human pluripotent stem cells. Before these cells can be routinely used in clinical trials, extensive preclinical safety studies are required. One of the main issues to be addressed is the long-term therapeutic effectiveness of these cells. In most transplantation studies using human cells, the maturation of DA neurons has been analyzed over a relatively short period not exceeding 6 months. In present study, we generated midbrain DA neurons from human induced pluripotent stem cells (hiPSCs) and grafted these neurons into the SNpc in an animal model of PD. Graft survival and maturation were analyzed from 1 to 12 months post-transplantation (mpt). We observed long-term survival and functionality of the grafted neurons. However, at 12 mpt, we observed a decrease in the proportion of SNpc DA neuron subtype compared with that at 6 mpt. In addition, at 12 mpt, grafts still contained immature neurons. Our results suggest that longer-term evaluation of the maturation of neurons derived from human stem cells is mandatory for the safe application of cell therapy for PD.

Human neural stem cell-derived extracellular vesicles protect against Parkinson's disease pathologies

Background

Neural stem cells (NSCs) have the ability to generate a variety of functional neural cell types and have a high potential for neuronal cell regeneration and recovery. Thus, they been recognized as the best source of cell therapy for neurodegenerative diseases, such as Parkinson’s disease (PD). Owing to the possibility of paracrine effect-based therapeutic mechanisms and easier clinical accessibility, extracellular vesicles (EVs), which possess very similar bio-functional components from their cellular origin, have emerged as potential alternatives in regenerative medicine.

Material and methods

EVs were isolated from human fibroblast (HFF) and human NSC (F3 cells). The supernatant of the cells was concentrated by a tangential flow filtration (TFF) system. Then, the final EVs were isolated using a total EV isolation kit.

Results

In this study, we demonstrate the potential protective effect of human NSC-derived EVs, showing the prevention of PD pathologies in 6-hydroxydopamine (6-OHDA)-induced in vitro and in vivo mouse models. Human NSC and F3 cell (F3)-derived EVs reduced the intracellular reactive oxygen species (ROS) and associated apoptotic pathways. In addition, F3-derived EVs induced downregulation of pro-inflammatory factors and significantly decreased 6-OHDA-induced dopaminergic neuronal loss in vivo. F3 specific microRNAs (miRNAs) such as hsa-mir-182-5p, hsa-mir-183-5p, hsa-mir-9, and hsa-let-7, which are involved in cell differentiation, neurotrophic function, and immune modulation, were found in F3-derived EVs.

Conclusions

We report that human NSC-derived EVs show an effective neuroprotective property in an in vitro transwell system and in a PD model. The EVs clearly decreased ROS and pro-inflammatory cytokines. Taken together, these results indicate that NSC-derived EVs could potentially help prevent the neuropathology and progression of PD.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01356-2.

Therapeutic Potential of Exosomes Derived from Adipose Tissue-Sourced Mesenchymal Stem Cells in the Treatment of Neural and Retinal Diseases

Therapeutic agents that are able to prevent or attenuate inflammation and ischemia-induced injury of neural and retinal cells could be used for the treatment of neural and retinal diseases. Exosomes derived from adipose tissue-sourced mesenchymal stem cells (AT-MSC-Exos) are extracellular vesicles that contain neurotrophins, immunoregulatory and angio-modulatory factors secreted by their parental cells. AT-MSC-Exos are enriched with bioactive molecules (microRNAs (miRNAs), enzymes, cytokines, chemokines, immunoregulatory, trophic, and growth factors), that alleviate inflammation and promote the survival of injured cells in neural and retinal tissues. Due to the nano-sized dimension and bilayer lipid envelope, AT-MSC-Exos easily bypass blood–brain and blood–retinal barriers and deliver their cargo directly into the target cells. Accordingly, a large number of experimental studies demonstrated the beneficial effects of AT-MSC-Exos in the treatment of neural and retinal diseases. By delivering neurotrophins, AT-MSC-Exos prevent apoptosis of injured neurons and retinal cells and promote neuritogenesis. AT-MSC-Exos alleviate inflammation in the injured brain, spinal cord, and retinas by delivering immunoregulatory factors in immune cells, suppressing their inflammatory properties. AT-MSC-Exos may act as biological mediators that deliver pro-angiogenic miRNAs in endothelial cells, enabling re-vascularization of ischemic neural and retinal tissues. Herewith, we summarized current knowledge about molecular mechanisms which were responsible for the beneficial effects of AT-MSC-Exos in the treatment of neural and retinal diseases, emphasizing their therapeutic potential in neurology and ophthalmology.

Stereotaxic-assisted gene therapy in Alzheimer's and Parkinson's diseases: therapeutic potentials and clinical frontiers

INTRODUCTION

Alzheimer's disease (AD) and Parkinson's disease (PD) are neurodegenerative disorders causing cognitive deficits and motor difficulties in the elderly. Conventional treatments are mainly symptomatic with little ability to halt disease progression. Gene therapies to correct or silence genetic mutations predisposing to AD or PD are currently being developed in preclinical studies and clinical trials, relying mostly on systemic delivery, which reduces their effectiveness. Imaging-guided stereotaxic procedures are used to locally deliver therapeutic cargos to well-defined brain sites, hence raising the question whether stereotaxic-assisted gene therapy has therapeutic potentials.

AREAS COVERED

The authors summarize the studies that investigated the use of gene therapy in PD and AD in animal and clinical studies over the past five years, with a special emphasis on the combinatorial potential with stereotaxic delivery. The advantages, limitations and futuristic challenges of this technique are discussed.

EXPERT OPINION

Robotic stereotaxis combined with intraoperative imaging has revolutionized brain surgeries. While gene therapies are bringing huge innovations to the medical field and new hope to AD and PD patients and medical professionals, the efficient and targeted delivery of such therapies is a bottleneck. We propose that careful application of stereotaxic delivery of gene therapies can improve PD and AD management. [Figure: see text].

The Role of Interstitial Fluid Pressure in Cerebral Porous Biomaterial Integration

Recent advances in biomaterials offer new possibilities for brain tissue reconstruction. Biocompatibility, provision of cell adhesion motives and mechanical properties are among the present main design criteria. We here propose a radically new and potentially major element determining biointegration of porous biomaterials: the favorable effect of interstitial fluid pressure (IFP). The force applied by the lymphatic system through the interstitial fluid pressure on biomaterial integration has mostly been neglected so far. We hypothesize it has the potential to force 3D biointegration of porous biomaterials. In this study, we develop a capillary hydrostatic device to apply controlled in vitro interstitial fluid pressure and study its effect during 3D tissue culture. We find that the IFP is a key player in porous biomaterial tissue integration, at physiological IFP levels, surpassing the known effect of cell adhesion motives. Spontaneous electrical activity indicates that the culture conditions are not harmful for the cells. Our work identifies interstitial fluid pressure at physiological negative values as a potential main driver for tissue integration into porous biomaterials. We anticipate that controlling the IFP level could narrow the gap between in vivo and in vitro and therefore decrease the need for animal screening in biomaterial design.

A Sensor-Based Perspective in Early-Stage Parkinson's Disease: Current State and the Need for Machine Learning Processes

Parkinson's disease (PD) is a progressive neurodegenerative disorder associated with dysfunction of dopaminergic neurons in the brain, lack of dopamine and the formation of abnormal Lewy body protein particles. PD is an idiopathic disease of the nervous system, characterized by motor and nonmotor manifestations without a discrete onset of symptoms until a substantial loss of neurons has already occurred, enabling early diagnosis very challenging. Sensor-based platforms have gained much attention in clinical practice screening various biological signals simultaneously and allowing researchers to quickly receive a huge number of biomarkers for diagnostic and prognostic purposes. The integration of machine learning into medical systems provides the potential for optimization of data collection, disease prediction through classification of symptoms and can strongly support data-driven clinical decisions. This work attempts to examine some of the facts and current situation of sensor-based approaches in PD diagnosis and discusses ensemble techniques using sensor-based data for developing machine learning models for personalized risk prediction. Additionally, a biosensing platform combined with clinical data processing and appropriate software is proposed in order to implement a complete diagnostic system for PD monitoring.

Magnetic resonance imaging of the dopamine system in schizophrenia - A scoping review

For decades, aberrant dopamine transmission has been proposed to play a central role in schizophrenia pathophysiology. These theories are supported by human in vivo molecular imaging studies of dopamine transmission, particularly positron emission tomography. However, there are several downsides to such approaches, for example limited spatial resolution or restriction of the measurement to synaptic processes of dopaminergic neurons. To overcome these limitations and to measure complementary aspects of dopamine transmission, magnetic resonance imaging (MRI)-based approaches investigating the macrostructure, metabolism, and connectivity of dopaminergic nuclei, i.e., substantia nigra pars compacta and ventral tegmental area, can be employed. In this scoping review, we focus on four dopamine MRI methods that have been employed in patients with schizophrenia so far: neuromelanin MRI, which is thought to measure long-term dopamine function in dopaminergic nuclei; morphometric MRI, which is assumed to measure the volume of dopaminergic nuclei; diffusion MRI, which is assumed to measure fiber-based structural connectivity of dopaminergic nuclei; and resting-state blood-oxygenation-level-dependent functional MRI, which is thought to measure functional connectivity of dopaminergic nuclei based on correlated blood oxygenation fluctuations. For each method, we describe the underlying signal, outcome measures, and downsides. We present the current state of research in schizophrenia and compare it to other disorders with either similar (psychotic) symptoms, i.e., bipolar disorder and major depressive disorder, or dopaminergic abnormalities, i.e., substance use disorder and Parkinson's disease. Finally, we discuss overarching issues and outline future research questions.

The Placebo Response in Double-Blind Randomised Trials Evaluating Regenerative Therapies for Parkinson's Disease: A Systematic Review and Meta-Analysis

In the field of stem cell technologies, exciting advances are taking place leading to translational research to develop cell-based therapies which may replace dopamine releasing neurons lost in patients with Parkinson's disease (PD). A major influence on trial design has been the assumption that the use of sham operated comparator groups is required in the implementation of randomised double-blind trials to evaluate the placebo response and effects associated with the surgical implantation of cells. The aim of the present review is to identify the improvements in motor functioning and striatal dopamine release in patients with PD who have undergone sham surgery. Of the nine published trials, there was at the designated endpoints, a pooled average improvement of 4.3 units, with 95% confidence interval of 3.1 to 5.6 on the motor subscale of the Unified Parkinson's Disease Scale in the 'OFF' state. This effect size indicates a moderate degree of improvement in the motor functioning of the patients in the sham surgical arms of the trials. Four of the nine trials reported the results of 18F-Fluorodopa PET scans, indicating no improvements of dopaminergic nigrostriatal neurones following sham surgery. Therefore, while the initial randomised trials relying on the use of sham operated controls were justified on methodological grounds, we suggest that the analysis of the evidence generated by the completed and published trials indicates that placebo controlled trials are not necessary to advance and evaluate the safety and efficacy of emerging regenerative therapies for PD.

Biomaterials enhancing performance of cell and nucleic-acid therapies: An opportunity in the brain

The brain has limited innate ability to promote repair, regeneration and functional recovery following injury, disease, or developmental disorder. Although cell and gene therapies have significant potential in the brain, no single treatment is likely to succeed in isolation. Here we discuss the current state of the art in cell and nucleic-acid-based neurotherapeutics and argue for the development of combination therapies that use biomaterials to help overcome the current limitations of cell and gene therapies alone.

Autophagy-Lysosomal Pathway as Potential Therapeutic Target in Parkinson's Disease

Cellular quality control systems have gained much attention in recent decades. Among these, autophagy is a natural self-preservation mechanism that continuously eliminates toxic cellular components and acts as an anti-ageing process. It is vital for cell survival and to preserve homeostasis. Several cell-type-dependent canonical or non-canonical autophagy pathways have been reported showing varying degrees of selectivity with regard to the substrates targeted. Here, we provide an updated review of the autophagy machinery and discuss the role of various forms of autophagy in neurodegenerative diseases, with a particular focus on Parkinson's disease. We describe recent findings that have led to the proposal of therapeutic strategies targeting autophagy to alter the course of Parkinson's disease progression.

Biofabrication of natural hydrogels for cardiac, neural, and bone Tissue engineering Applications

Natural hydrogels are one of the most promising biomaterials for tissue engineering applications, due to their biocompatibility, biodegradability, and extracellular matrix mimicking ability. To surpass the limitations of conventional fabrication techniques and to recapitulate the complex architecture of native tissue structure, natural hydrogels are being constructed using novel biofabrication strategies, such as textile techniques and three-dimensional bioprinting. These innovative techniques play an enormous role in the development of advanced scaffolds for various tissue engineering applications. The progress, advantages, and shortcomings of the emerging biofabrication techniques are highlighted in this review. Additionally, the novel applications of biofabricated natural hydrogels in cardiac, neural, and bone tissue engineering are discussed as well.

Engineered extracellular vesicles as brain therapeutics

Extracellular vesicles (EVs) are communication channels between different cell types in the brain, between the brain and the periphery and vice-versa, playing a fundamental role in physiology and pathology. The evidence that EVs might be able to cross the blood-brain barrier (BBB) make them very promising candidates as nanocarriers to treat brain pathologies. EVs contain a cocktail of bioactive factors, yet their content and surface can be further engineered to enhance their biological activity, stability and targeting ability. Native and engineered EVs have been reported for the treatment of different brain pathologies, although issues related to their modest accumulation and limited local therapeutic effect in the brain still need to be addressed. In this review, we cover the therapeutic applications of native and bioengineered EVs for brain diseases. We also review recent data about the interaction between EVs and the BBB and discuss the challenges and opportunities in clinical translation of EVs as brain therapeutics.

Restoring lost nigrostriatal fibers in Parkinson's disease based on clinically-inspired design criteria

Parkinson's disease is a neurodegenerative disease affecting around 10 million people worldwide. The death of dopaminergic neurons in the substantia nigra and the axonal fibers that constitute the nigrostriatal pathway leads to a loss of dopamine in the striatum that causes the motor symptoms of this disease. Traditional treatments have focused on reducing symptoms, while therapies with human fetal or stem cell-derived neurons have centered on implanting these cells in the striatum to restore its innervation. An alternative approach is pathway reconstruction, which aims to rebuild the entire structure of neurons and axonal fibers of the nigrostriatal pathway in a way that matches its anatomy and physiology. This type of repair could be more capable of reestablishing the signaling mechanisms that ensure proper dopamine release in the striatum and regulation of other motor circuit regions in the brain. In this manuscript, we conduct a review of the literature related to pathway reconstruction as a treatment for Parkinson's disease, delve into the limitations of these studies, and propose the requisite design criteria to achieve this goal at a human scale. We then present our tissue engineering-based platform to fabricate hydrogel-encased dopaminergic axon tracts in vitro for later implantation into the brain to replace and reconstruct the pathway. These tissue-engineered nigrostriatal pathways (TE-NSPs) can be characterized and optimized for cell number and phenotype, axon growth lengths and rates, and the capacity for synaptic connectivity and dopamine release. We then show original data of advances in creating these constructs matching clinical design criteria using human iPSC-derived dopaminergic neurons and a hyaluronic acid hydrogel. We conclude with a discussion of future steps that are needed to further optimize human-scale TE-NSPs and translate them into clinical products.

TNFα increases tyrosine hydroxylase expression in human monocytes

Most, if not all, peripheral immune cells in humans and animals express tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis. Since TH is typically studied in the context of brain catecholamine signaling, little is known about changes in TH production and function in peripheral immune cells. This knowledge gap is due, in part, to the lack of an adequately sensitive assay to measure TH in immune cells expressing lower TH levels compared to other TH expressing cells. Here, we report the development of a highly sensitive and reproducible Bio-ELISA to quantify picogram levels of TH in multiple model systems. We have applied this assay to monocytes isolated from blood of persons with Parkinson's disease (PD) and to age-matched, healthy controls. Our study unexpectedly revealed that PD patients' monocytes express significantly higher levels of TH protein in peripheral monocytes relative to healthy controls. Tumor necrosis factor (TNFα), a pro-inflammatory cytokine, has also been shown to be increased in the brains and peripheral circulation in human PD, as well as in animal models of PD. Therefore, we investigated a possible connection between higher levels of TH protein and the known increase in circulating TNFα in PD. Monocytes isolated from healthy donors were treated with TNFα or with TNFα in the presence of an inhibitor. Tissue plasminogen activator (TPA) was used as a positive control. We observed that TNFα stimulation increased both the number of TH+ monocytes and the quantity of TH per monocyte, without increasing the total numbers of monocytes. These results revealed that TNFα could potentially modify monocytic TH production and serve a regulatory role in peripheral immune function. The development and application of a highly sensitive assay to quantify TH in both human and animal cells will provide a novel tool for further investigating possible PD immune regulatory pathways between brain and periphery.

Proteomic Characterization of Human Neural Stem Cells and Their Secretome During in vitro Differentiation

Cell therapies represent a promising approach to slow down the progression of currently untreatable neurodegenerative diseases (e.g., Alzheimer's and Parkinson's disease or amyotrophic lateral sclerosis), as well as to support the reconstruction of functional neural circuits after spinal cord injuries. In such therapies, the grafted cells could either functionally integrate into the damaged tissue, partially replacing dead or damaged cells, modulate inflammatory reaction, reduce tissue damage, or support neuronal survival by secretion of cytokines, growth, and trophic factors. Comprehensive characterization of cells and their proliferative potential, differentiation status, and population purity before transplantation is crucial to preventing safety risks, e.g., a tumorous growth due to the proliferation of undifferentiated stem cells. We characterized changes in the proteome and secretome of human neural stem cells (NSCs) during their spontaneous (EGF/FGF2 withdrawal) differentiation and differentiation with trophic support by BDNF/GDNF supplementation. We used LC-MS/MS in SWATH-MS mode for global cellular proteome profiling and quantified almost three thousand cellular proteins. Our analysis identified substantial protein differences in the early stages of NSC differentiation with more than a third of all the proteins regulated (including known neuronal and NSC multipotency markers) and revealed that the BDNF/GDNF support affected more the later stages of the NSC differentiation. Among the pathways identified as activated during both spontaneous and BDNF/GDNF differentiation were the HIF-1 signaling pathway, Wnt signaling pathway, and VEGF signaling pathway. Our follow-up secretome analysis using Luminex multiplex immunoassay revealed significant changes in the secretion of VEGF and IL-6 during NSC differentiation. Our results further demonstrated an increased expression of neuropilin-1 as well as catenin β-1, both known to participate in the regulation of VEGF signaling, and showed that VEGF-A isoform 121 (VEGF121), in particular, induces proliferation and supports survival of differentiating cells.

Cellular senescence in aging and age-related diseases: Implications for neurodegenerative diseases

Aging is the major predictor for developing multiple neurodegenerative diseases, including Alzheimer's disease (AD) other dementias, and Parkinson's disease (PD). Senescent cells, which can drive aging phenotypes, accumulate at etiological sites of many age-related chronic diseases. These cells are resistant to apoptosis and can cause local and systemic dysfunction. Decreasing senescent cell abundance using senolytic drugs, agents that selectively target these cells, alleviates neurodegenerative diseases in preclinical models. In this review, we consider roles of senescent cells in neurodegenerative diseases and potential implications of senolytic agents as an innovative treatment.

Pluripotent Stem Cell Therapies for Parkinson Disease: Present Challenges and Future Opportunities

In Parkinson's disease (PD), there are currently no effective therapies to prevent or slow down disease progression. Cell replacement therapy using human pluripotent stem cell (hPSC)-derived dopamine neurons holds considerable promise. It presents a novel, regenerative strategy, building on the extensive history of fetal tissue grafts and capturing the potential of hPSCs to serve as a scalable and standardized cell source. Progress in establishing protocols for the direct differentiation to midbrain dopamine (mDA) neurons from hPSC have catalyzed the development of cell-based therapies for PD. Consequently, several groups have derived clinical-grade mDA neuron precursors under clinical good manufacture practice condition, which are progressing toward clinical testing in PD patients. Here we will review the current status of the field, discuss the remaining key challenges, and highlight future areas for further improvements of hPSC-based technologies in the clinical translation to PD.