Alpha-synuclein (α-syn) is localized in cellular organelles of most neurons, but many of its physiological functions are only partially understood. α-syn accumulation is associated with Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy as well as other synucleinopathies; however, the exact pathomechanisms that underlie these neurodegenerative diseases remain elusive. In this review, we describe what is known about α-syn function and pathophysiological changes in different cellular structures and organelles, including what is known about its behavior as a prionlike protein. We summarize current knowledge of α-syn and its pathological forms, covering its effect on each organelle, including aggregation and toxicity in different model systems, with special interest on the mitochondria due to its relevance during the apoptotic process of dopaminergic neurons. Moreover, we explore the effect that α-syn exerts by interacting with chromatin remodeling proteins that add or remove histone marks, up-regulate its own expression, and resume the impairment that α-syn induces in vesicular traffic by interacting with the endoplasmic reticulum. We then recapitulate the events that lead to Golgi apparatus fragmentation, caused by the presence of α-syn. Finally, we report the recent findings about the accumulation of α-syn, indirectly produced by the endolysosomal system. In conclusion, many important steps into the understanding of α-syn have been made using in vivo and in vitro models; however, the time is right to start integrating observational studies with mechanistic models of α-syn interactions, in order to look at a more complete picture of the pathophysiological processes underlying α-synucleinopathies.

Dysfunction of cellular homeostasis can lead to misfolding of proteins thus acquiring conformations prone to polymerization into pathological aggregates. This process is associated with several disorders, including neurodegenerative diseases, such as Parkinson’s disease (PD), and endoplasmic reticulum storage disorders (ERSDs), like alpha-1-antitrypsin deficiency (AATD) and hereditary hypofibrinogenemia with hepatic storage (HHHS). Given the shared pathophysiological mechanisms involved in such conditions, it is necessary to deepen our understanding of the basic principles of misfolding and aggregation akin to these diseases which, although heterogeneous in symptomatology, present similarities that could lead to potential mutual treatments. Here, we review: (i) the pathological bases leading to misfolding and aggregation of proteins involved in PD, AATD, and HHHS: alpha-synuclein, alpha-1-antitrypsin, and fibrinogen, respectively, (ii) the evidence linking each protein aggregation to the stress mechanisms occurring in the endoplasmic reticulum (ER) of each pathology, (iii) a comparison of the mechanisms related to dysfunction of proteostasis and regulation of homeostasis between the diseases (such as the unfolded protein response and/or autophagy), (iv) and clinical perspectives regarding possible common treatments focused on improving the defensive responses to protein aggregation for diseases as different as PD, and ERSDs.

Transplantation of immature dopaminergic neurons or neural precursors derived from embryonic stem cells (ESCs) into the substantia nigra pars compacta (SNpc) is a potential therapeutic approach for functional restitution of the nigrostriatal pathway in Parkinson’s disease (PD). However, further studies are needed to understand the effects of the local microenvironment on the transplanted cells to improve survival and specific differentiation in situ. We have previously reported that the adult SNpc sustains a neurogenic microenvironment. Non-neuralized embryoid body cells (EBCs) from mouse ESCs (mESCs) overexpressing the dopaminergic transcription factor Lmx1a gave rise to many tyrosine hydroxylase (Th+) cells in the intact and damaged adult SNpc, although only for a short-term period. Here, we extended our study by transplanting EBCs from genetically engineered naive human ESC (hESC), overexpressing the dopaminergic transcription factors LMX1A, FOXA2, and OTX2 (hESC-LFO), in the SNpc. Unexpectedly, no graft survival was observed in wild-type hESC EBCs transplants, whereas hESC-LFO EBCs showed viability in the SNpc. Interestingly, neural rosettes, a developmental hallmark of neuroepithelial tissue, emerged at 7- and 15- days post-transplantation (dpt) from the hESC-LFO EBCs. Neural rosettes expressed specification dopaminergic markers (Lmx1a, Otx2), which gave rise to several Th+ cells at 30 dpt. Our results suggest that the SNpc enables the robust initiation of neural differentiation of transplanted human EBCs prompted to differentiate toward the midbrain dopaminergic phenotype.

Background: Degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in Parkinson's disease (PD) is responsible for motor and cognitive impairment. Replacing the dopaminergic cell population in the SNpc to restore normal dopamine levels is a potential therapeutic approach. However, improving neuronal integration still requires a reliable cell source for transplantation and a profound understanding of the effects of the local microenvironment on transplanted cells. We have previously shown that embryoid bodies (EBs)-derived cells from mouse embryonic stem cells overexpressing the dopaminergic transcription factor Lmx1a engrafted into SNpc develop tyrosine hydroxylase (TH)-positive phenotype. In the present work, we transplanted EBs-derived cells from genetically engineered human embryonic stem cells (hESCs), overexpressing the dopaminergic transcription factors LMX1A, FOXA2, and OTX2 (hESC-LFO). We determined their potential to differentiate into TH-expressing neurons in the SNpc of an in vivo PD model. Methods: EBs-derived cells from genetically the engineered hESCs-LFO cell line were transplanted, and their neuronal differentiation potential was determined in the SNpc of an in vivo PD model with 6-hydroxy dopamine (6-OHDA). Three rat groups were designed as follows: Untreated (healthy rats), sham (rats administered with saline solution), and 6-OHDA (rats lesioned with 6-OHDA). A one-way ANOVA test was performed for statistical analysis. Results: Neural rosettes, a fundamental developmental hallmark of neuroepithelial tissue, were found at 7 and 15 days post-transplantation (dpt) in ~ 70% of the transplanted brains in all three conditions: Untreated, sham, and 6-OHDA. The majority of the neural rosettes corresponded to the lumen formation stage. In comparison, no graft survival was observed in EB transplants derived from unmodified hESCs. Interestingly, at 30 dpt, hESC-LFO engrafted cells showed neuronal morphology and positive immunolabeling for TH in all the brains exhibiting surviving transplants: 10% 6-OHDA rats, 0% sham, and 100% untreated rats. Conclusions: Overall, our results show that overexpression of LFO factors favors short-term survival while strongly initiating neural differentiation of hESC-derived cells in SNpc surviving grafts by forming neural rosettes and differentiating into TH-positive cells.