Midbrain Dopaminergic Neuron Development at the Single Cell Level: In vivo and in Stem Cells

Abstract

Parkinson's disease (PD) is a progressive neurodegenerative disorder that predominantly affects dopaminergic (DA) neurons of the substantia nigra. Current treatment options for PD are symptomatic and typically involve the replacement of DA neurotransmission by DA drugs, which relieve the patients of some of their motor symptoms. However, by the time of diagnosis, patients have already lost about 70% of their substantia nigra DA neurons and these drugs offer only temporary relief. Therefore, cell replacement therapy has garnered much interest as a potential treatment option for PD. Early studies using human fetal tissue for transplantation in PD patients provided proof of principle for cell replacement therapy, but they also highlighted the ethical and practical difficulties associated with using human fetal tissue as a cell source. In recent years, advancements in stem cell research have made human pluripotent stem cells (hPSCs) an attractive source of material for cell replacement therapy. Studies on how DA neurons are specified and differentiated in the developing mouse midbrain have allowed us to recapitulate many of the positional and temporal cues needed to generate DA neurons in vitro. However, little is known about the developmental programs that govern human DA neuron development. With the advent of single-cell RNA sequencing (scRNA-seq) and bioinformatics, it has become possible to analyze precious human samples with unprecedented detail and extract valuable high-quality information from large data sets. This technology has allowed the systematic classification of cell types present in the human developing midbrain along with their gene expression patterns. By studying human development in such an unbiased manner, we can begin to elucidate human DA neuron development and determine how much it differs from our knowledge of the rodent brain. Importantly, this molecular description of the function of human cells has become and will increasingly be a reference to define, evaluate, and engineer cell types for PD cell replacement therapy and disease modeling.

The Role of Neurod Genes in Brain Development, Function, and Disease

“…Work over the last two decades has also shown the great influence of the Neurod family in other regions of the posterior nervous system, which include the midbrain, the hindbrain, and the spinal cord. Progenitor cells in the ventral midbrain express high levels of Neurod1 , and the combinatorial expression of Neurod1 with other bHLH factors sub-specifies different neuronal populations emanating from this area, some of which retain Neurod1 expression (Park et al, 2006 ; Arimura et al, 2019 ; Ásgrímsdóttir and Arenas, 2020 ; Poulin et al, 2020 ). Furthermore, Neurod1 and Neurod6 have been recently reported to have a critical function in the development of particular dopaminergic midbrain neurons (Khan et al, 2017 ).…”

A molecular census of midbrain dopaminergic neurons in Parkinson’s disease

“…The identification of regulatory elements that drive the molecular identity of DA neurons can inform differentiation protocols for in vitro studies of dopamine neurons in PD as well as drive the refinement of cellular replacement therapies for PD 28 . To understand the regulatory networks that may drive such transcriptional variation, we used SCENIC (Single-Cell rEgulatory Network Inference and Clustering 29 , to identify 84 regulons (Extended Data Fig.…”

Chemical and Biological Molecules Involved in Differentiation, Maturation, and Survival of Dopaminergic Neurons in Health and Parkinson’s Disease: Physiological Aspects and Clinical Implications

“…In particular, Pax2 is required for the Fibroblast growth factor 8 (Fgf8) production by IsO, while Wnt1 and En1 cooperate with Otx2 and Gbx2 to further refine the expression domain of Fgf8 at the IsO [16,17]. These steps, together with the production of Sonic Hedgehog (Shh) by the floor plate, the other signaling center, are fundamental for the differentiation of progenitor cells into DA progenitor cells [3]: indeed, an orthogonal gradient of Shh and Fgf8 create a cartesian coordinate system that define the positional information for the midbrain DA phenotype induction (Figure 2). In addition, Shh activates the Glioma-associated oncogene 1 (Gli1), that is considered an early marker of midbrain DA progenitors [17].…”

“…Parkinson's disease (PD) is one of the most common neurodegenerative disorders, with an increasing incidence worldwide and a great effort for the health care resources [1,2]. At cellular level, PD is characterized by an irreversible and progressive loss of midbrain dopaminergic (DA) neurons in the substantia nigra pars compacta: this degeneration leads to the dysregulation of the nigrostriatal pathway that causes the manifestation of the clinical motor symptoms associated with PD [3,4].…”

“…Current therapeutic options for PD aim to support the nigrostriatal pathway administrating drugs that are able either to modulate the DA transmission or increase the dopamine level in the brain [3]. Unfortunately, these pharmacological treatments are only able to alleviate the physical symptoms, sometime delaying the disease progression [5], and their efficacy gradually reduces over the time [6].…”