Parkinson’s disease (PD) is a complex, age-related neurodegenerative disease with early prominent death and loss of dopaminergic (DA) neurons in the substantia nigra parscompacta (SNpc) 1. The main feature of PD is progressive, extensive dopaminergic neuron loss in the substantia nigra-striatum 2. The main clinical manifestations are static tremor, slow movement and reduction, increased muscle tone, unstable posture. PD is also closely related with various non-motor symptoms, such as cognitive dysfunction, mood and psychotic disorder 3. A considerable number of patients have cognitive impairment, and late stage of PD may have dementia and depression. In the degenerative diseases of the nervous system, the incidence of PD in the 65-year-old population is more than 1%, second only to Alzheimer's disease, and it is the second most common neurodegenerative disorder, bringing an escalating burden on economic terms and quality of life to these patients, their families and society.
The mainstay of Parkinson’s disease managements are drug therapy, surgical treatment and stem cell replacement therapy. Drugs based management strategies are carefully to considerations of the side-effects and tolerance levels of the patient 4. Surgical treatments include thalamicectomy and deep brain stimulation (DBS). Thalamotomy has strong synaptic connections with the thalamus and striatum, but in PD patient's physiological regulations it seems to alter and results in motor function impairments 5. As for DBS, patients have to undergo surgeries again and again, which increase the risk of infection and socioeconomic burdens 6. Although currently available pharmacological or surgical treatments may significantly improve the quality of life of many patients with PD, these are only symptomatic treatments that do not slow or stop the progressive course of the disease. At present, replacement of the progressively degenerated DA neurons through cell transplantation is considered to be the most potential therapy 7. Some data suggested a more immediate and reachable goal of cell transplantation may be neuronal protection 8. Currently, embryonic stem cells (ESCs), neural stem cells (NSCs), induced pluripotent stem cells (iPSCs) and bone marrow mesenchymal stem cells (BMSCs) are available for stem cell replacement therapy 9. The use of ESCs and NSCs existed ethical problems inherent. So much hope is placed on iPSCs which induced human fibroblasts into a source of patient-specific and disease-specific neurons, especially as, in theory, this approach would avoid many of the ethical issues associated with using ESCs 10, 11. Cell replacement therapy is derived from the patient's own cells, avoiding immune rejection. In autologous transplantation, it is necessary to establish iPSCs from each patient, and current technical operations take much time and high cost, so it is difficult to spread to general treatment. In addition, the use of iPSCs with the patient's own genetic factors, the sensitivity of the disease may be high. It also has the risk of introducing cancer cells 12. Specifically, iPSCs often obtained chromosomal abnormalities, with gains or losses of whole chromosomes 13. Different from these stem cells, BMSCs come from patients themselves without ethics dispute. Especially, BMSCs have multi-directional differentiation potential 14, own low risk of tumorigenesis 15 and are rich in source, easy to extract, separate and purify. Many studies in vitro and preclinical strongly proved the therapeutic potential of BMSCs when be applied as a treatment for different pathological conditions 16, 17.
There have been different methods involved in the differentiation of BMSCs, including cell growth factors 18, 19, chemical inducer 20 and lentiviral transduction 21. Currently, in vitro induction of stem cells using growth factors with sonic hedgehog (SHH) and fibroblast growth factors (FGFs), succeeded in inducing adult human BMSCs into DA neurons with 67% of efficiency in 12 days 22, 23. This is the current maximum induction efficiency and the shortest induction time.
Liver X receptors (LXRs) including LXRα and LXRβ are members of the nuclear receptor supergene family of ligand-activated transcription factors 24, and play a key role in the regulation of cholesterol, fatty acid homeostasis 25 and central nervous system (CNS) 26, 27. Deletion of the LXRβ in mice caused hypoplasia in the dentate gyrus, including abnormalities in the formation of progenitor cells and granule cell differentiation 26, 27. Both LXRα and LXRβ are expressed in the epithelial cells of the choroid plexus. It has been found that LXRs played crucial roles in regulation of genes involved in CSF production and structural integrity of choroid plexus 29. LXRs are involved in the processes of myelination and remyelination 30. Activation of LXRs can promote the regeneration and survival of motor neurons 31.
Thus, LXR agonists induce transcriptional activity of LXR target genes, plays therapeutic role in different neurodegeneration animal models32 − 34. Based on our previous work and others have found the induction of cocktail-induced da phenotypes in adult rat BMSCs by using SHH, fibroblast growth factor 8 (FGF8), basic fibroblast growth factor (bFGF) and TO901317 with 87.42% of efficiency in 6 days of period of induction 35. LXR agonist significantly shortened induction time and improved induction efficiency compared to methods which had been reported. But these previous work did not investigate whether induced-cells released dopamine and had the corresponding dopaminergic neurons. We did not sure if the induced cells have a therapeutic effect on Parkinson's disease.
In this study we investigated the effect of TO901317 on the differentiation of hBMSCs into dopamine neurons. We also explored whether induced-cells had dopamine neuronal function and the possible mechanism. Finally, we transplanted the induced-cells into PD rats to observe the therapeutic effect.