Combined cell-based therapy strategies for the treatment of Parkinson’s disease: focus on mesenchymal stromal cells

Abstract

Parkinson’s disease is a neurodegenerative condition characterized by motor impairments caused by the selective loss of dopaminergic neurons in the substantia nigra. Levodopa is an effective and well-tolerated dopamine replacement agent. However, levodopa provides only symptomatic improvements, without affecting the underlying pathology, and is associated with side effects after long-term use. Cell-based replacement is a promising strategy that offers the possibility to replace lost neurons in Parkinson’s disease treatment. Clinical studies of transplantation of human fetal ventral mesencephalic tissue have provided evidence that the grafted dopaminergic neurons can reinnervate the striatum, release dopamine, integrate into the host neural circuits, and improve motor functions. One of the limiting factors for cell therapy in Parkinson’s disease is the low survival rate of grafted dopaminergic cells. Different factors could cause cell death of dopaminergic neurons after grafting such as mechanical trauma, growth factor deprivation, hypoxia, and neuroinflammation. Neurotrophic factors play an essential role in the survival of grafted cells. However, direct, timely, and controllable delivery of neurotrophic factors into the brain faces important limitations. Different types of cells secrete neurotrophic factors constitutively and co-transplantation of these cells with dopaminergic neurons represents a feasible strategy to increase neuronal survival. In this review, we provide a general overview of the pioneering studies on cell transplantation developed in patients and animal models of Parkinson’s disease, with a focus on neurotrophic factor-secreting cells, with a particular interest in mesenchymal stromal cells; that co-implanted with dopaminergic neurons would serve as a strategy to increase cell survival and improve graft outcomes.

Introduction

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the presence of motor manifestations such as tremor, rigidity, bradykinesia, and postural instability and a variety of non-motor symptoms that include sensory abnormalities, autonomic dysfunction, sleep disturbances, fatigue, apathy, and psychiatric disorders (Armstrong and Okun, 2020). PD pathology mainly lies in the degeneration of dopaminergic neurons in the substantia nigra (Poewe et al., 2017). Although there is a wide range of pharmacological treatments for PD, all treatments currently available provide only symptomatic relief. Levodopa is the most potent and effective treatment and remains as the gold standard. However, the beneficial effect of levodopa gets progressively shorter throughout the disease, and prolonged treatment is associated with drug-induced dyskinesia and other disabling side effects that limit its use (Armstrong and Okun, 2020; Lopez-Lopez et al., 2021). With a relatively localized neural degeneration, PD is considered a good target for cell therapy (Fan et al., 2020). Over nearly four decades, different restorative surgical therapies have emerged to replace lost dopaminergic neurons. First clinical trials based on intrastriatal transplantation of human fetal ventral mesencephalic (fVM) tissue showed that grafted cells survive and induce significant, long-lasting improvements in motor symptoms (Lindvall et al., 1990), although possible effects on non-motor symptoms were not explored. However, the low availability of human mesencephalic tissue limited its use. Currently, stem cells represent a promising alternative source for the generation of dopaminergic neurons (Jang et al., 2020; Rahimpour et al., 2022). In addition to important issues such as defining an optimal cell source, better criteria for PD patient selection, improved and standardized protocols, avoiding rejection, and promoting functional integration (Rahimpour et al., 2022), other factors that compromise the survival of grafted dopaminergic neurons, such as mechanical trauma, growth factor deprivation, hypoxia, oxidative stress, and neuroinflammation need to be addressed (Brundin et al., 2000; Moriarty et al., 2017). Low dopaminergic survival can be partially counteracted by neurotrophic factor addition. Although different strategies have been studied, direct, timely, and controllable delivery of neurotrophic factors into the brain faces important challenges (Gantner et al., 2020; Studer and Tabar, 2020). Different types of cells secrete neurotrophic factors constitutively and co-transplantation of these cells with dopaminergic neurons represents a feasible strategy to increase neuronal survival. In this review, we focus on the co-grafting of dopaminergic neurons and neurotrophic factor-secreting cells, with a particular interest in mesenchymal stromal cells (MSCs), as a possible strategy to reduce cell death after grafting, which represents one of the drawbacks faced by cell therapy to make the definitive leap to the clinic (Figure 1).

Figure 1.

Co-grafting of dopaminergic cells with trophic/immunomodulatory cells as a Parkinson’s disease cell therapy.

Co-transplantation strategies are proposed as an approach to improve behavioral deficits, dopamine neuronal survival, and striatal reinnervation. Created with BioRender (https://biorender.com/).

Search Strategy and Selection Criteria

The search strategy and selection criteria were limited to articles published in peer-reviewed journals. A literature review of articles published in English from inception up to April 2022 was conducted by searching in the National Library of Medicine (PubMed) database (https://pubmed.ncbi.nlm.nih.gov/) using the following keywords: fetal ventral mesencephalic tissue, graft, Parkinson’s disease, co-grafts, mesenchymal stromal/stem cells, transplantation, neuroblasts, survival, embryonic stem cells, pluripotent induced stem cells and combination of the above terms. We also revised references of eligible articles.

Historical Overview

In 1975, Seiger and Olson grafted fetal brain tissue into the anterior eye chamber, laying the groundwork for future transplants in the nervous system. Cells derived from fVM tissue, enriched in dopaminergic cells, were the first type of cells transplanted in an animal model of PD. These initial preclinical studies confirmed that dopaminergic grafts were able to survive, extend projections into the host striatum and ameliorate behavioral deficits in dopamine-depleted animals (Bjorklund et al., 1980). Such was the impact of these results that in the early 1980, the first PD patients in the world received intracerebral grafts. However, these patients were grafted with autologous chromaffin cells obtained from the adrenal medulla, which had also been used in animal models of PD, to avoid the practical and ethical issues associated with human fetal tissue (Freed et al., 1981). Although it was initially assumed that adrenal medulla and fVM cells had similar mechanisms of action based on dopamine production, further studies showed that adrenal medulla graft benefits were due to neurotrophic actions that promote the sprouting of striatal dopaminergic fibers and not to dopamine production (Freed et al., 1990). However, the transient and highly variable improvements observed both in animal models and in PD patients, the frequent surgical complications, and the low cell survival observed in post-mortem studies led to the abandonment of the use of adrenal medullary tissue for transplantation (Date et al., 1990). A breakthrough in the field was the use of dissociated cell suspensions derived from fVM tissue, which permitted the targeting of deep structures as well as multiple graft placements (Bjorklund et al., 1983). A crucial step forward in clinical translation was the demonstration that dopaminergic neurons isolated from human embryonic/fetal tissue survive, provide extensive graft-derived striatal innervation, and ameliorate motor deficits after transplantation in preclinical models of PD (Brundin et al., 1986). Based on almost 10 years of transplantation studies in animal models, the first patients receiving grafts of human fVM tissue were implanted in 1987 (Lindvall et al., 1990). This pioneering study demonstrated for the first time that fVM grafted cells can survive in a human brain affected by neurodegenerative disease and could restore dopamine levels, monitored in the striatum using [¹¹C]-raclopride positron emission tomography, for as long as 10 years after surgery (Piccini et al., 1999). Interestingly, post-mortem studies carried out 24 years after grafting showed dense graft-derived reinnervation in the putamen (Hallett et al., 2014). The outcomes of a series of open-label trials showed promising results and generated high expectations in the field, leading the way to two National Institutes of Health-funded, double-blind, randomized, placebo-controlled trials. However, the results were disappointing, with some grafted patients showing normalized dopamine signaling while other participants showed modestly or no recovery compared to patients subjected to sham surgery (Freed et al., 2001). Nevertheless, even more discouraging was the fact that a subset of grafted patients developed graft-induced dyskinesias years later (Hagell et al., 2002), possibly due to the composition of the transplanted cell suspension. Experimental studies using different proportions of dopaminergic and serotonergic neurons in the cell suspension showed that higher proportions of serotonergic neurons, which are usually present in the graft, led to the dyskinesias, since serotonergic terminals could release dopamine as a false transmitter in an unregulated fashion (Politis et al., 2010). Additional evidence suggests that endogenous serotonergic neurons and serotonergic and dopaminergic interactions would contribute to the development of graft-induced dyskinesias (Muñoz et al., 2020). With the knowledge acquired in subsequent research, it is now considered that these studies were carried out prematurely and some of the side effects could have been avoided through careful selection of patients, improving the method of implantation and the composition of the grafted cell suspension, and optimizing the post-transplant immunosuppressive regimes (Bjorklund and Lindvall, 2017). The cumulative reanalysis of previous trials led a European research consortium called TRANSEURO to design a step-by-step optimized open-label trial, which is currently ongoing, with the main objective of developing an efficacious and safe methodology for fVM cell-based therapy for PD (ClinicalTrials.gov identifier NCT01898390) (Barker and TRANSEURO consortium, 2019).

Due to the abovementioned shortage of human fVM tissue for cell-based therapy and advances in stem cell knowledge, many efforts have focused on generating dopaminergic neurons for transplantation from stem cells, which potentially offer an unlimited number of cells. Different stem cell types have been considered as a possible source of neurons for the treatment of PD. However, the best positioned seem to be embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) (Jang et al., 2020). Notable advances in differentiation protocols have led to the achievement of highly pure dopaminergic neurons suitable for clinical trials that showed satisfactory therapeutic effectiveness in animal models of PD and did not cause tumors in these animals (Xiong et al., 2021). The first transplantations using clinically graded hiPSC-derived neurons have already been carried out (Schweitzer et al., 2020). Piao et al. (2021) have recently reported the generation of the first ESC-derived dopaminergic cell product for the treatment of PD, which was approved for a clinical trial in the United States (Takahashi, 2021; Figure 2). There is no doubt that stem cell technologies have the potential to be at the forefront of such PD treatments (Parmar et al., 2020). The results of ongoing and upcoming trials would be highly anticipated since the success of these trials would open up new possibilities for using cell therapy for the treatment of PD (Fan et al., 2020). However, it should be noted that, beyond the motor symptoms, the disease exhibits non-motor symptoms that can be also disabling for the patient, which currently are not targeted by cell therapy and should also be considered in future therapies (Pantcheva et al., 2015).

Figure 2.

Timeline of breakthroughs in cell therapy for Parkinson’s disease.

Relevant contributions in the field of cell therapy are shown in black. Key scientific advances are shown in blue. DA: Dopamine; fVM: fetal ventral mesencephalic; hESCs: human embryonic stem cells; hiPSCs: human induced pluripotent stem cells; MSC: mesenchymal stromal cell; PD: Parkinson’s disease.

Despite the encouraging results obtained, cell-based therapies face important limitations and challenges. In addition to issues concerning safety and reproducibility, a critical aspect is the survival rate of the grafted dopaminergic neurons. Different evidence suggests that only 3–20% of grafted dopaminergic cells survive after the procedure (Brundin et al., 2000; Kim et al., 2020; Li and Li, 2021; Hiller et al., 2022). Triggers that may initiate dopaminergic cell loss in grafts include mechanical trauma, neuroinflammation, poor vascularization, and growth factor deprivation in the host brain (Barker et al., 1996; Brundin et al., 2000; Moriarty et al., 2017). The critical interval during which most dopaminergic cells die is the first few days after transplantation (Sortwell et al., 2000). The idea of utilizing cells with neurotrophic properties to modify the pathologic brain environment or promote neuroprotective effects has been appearing on the horizon for central nervous system disorders. In the following section, we will review the most relevant studies in which combinations of dopaminergic grafts with other cells with supportive properties have been used as a strategy to overcome the poor survival of transplanted neurons (Figure 3).

Figure 3.

Cell-based therapies trialed in patients and co-grafting strategies tested in animal models of PD.

The left-hand-side of the figure shows cell sources used in clinical trials in PD patients and the right-hand-side shows co-grafting strategies tested in animal models of PD. ESCs: Embryonic stem cells; fST: fetal striatal; fVM: fetal ventral mesencephalic; GDNF: glial cell line-derived neurotrophic factor; iPSCs: induced pluripotent stem cells; MSCs: mesenchymal stromal cells; NSCs: neural stem cells; OECs: olfactory ensheathing cells; PD: Parkinson’s disease. Created with BioRender (https://biorender.com/).

Co-Grafting Strategies: a Review

Co-grafting strategies based on the combination of fVM tissue with cells with neurotrophic properties that could promote the survival of transplanted dopaminergic cells represent a therapeutic option to overcome the typical loss of 80–95% of the grafted dopaminergic neurons following transplantation. In the 1980s, the first studies exploring the benefits of peripheral nerve transplantation on the survival of dopaminergic grafts were carried out. Schwann cells, the principal glial cell type in the peripheral nervous system, are involved in neuronal survival and nerve regeneration (Ma et al., 2021). Aguayo et al. (1984) demonstrated that transplanted fVM neurons in 6-hydroxydopamine (6-OHDA)-lesioned rats survived, and extended axons guided by sciatic nerve grafts. Different groups also demonstrated that co-grafts of fVM tissue with peripheral nerves or Schwann cells induced behavioral improvements and promoted axonal growth to the striatum compared to fVM grafts in rat models of PD (van Horne et al., 1991; Timmer et al., 2004). However, other studies did not observe neuroprotective effects after co-transplantation of fVM tissue and saphenous nerve in lesioned monkeys (Collier et al., 1994). Olfactory ensheathing cells (OECs) are another type of peripheral glial cells specialized in supporting continuous olfactory nerve fiber growth throughout adulthood. Co-culture of OECs with nigral cells promoted dopaminergic neurite outgrowth (Denis-Donini and Estenoz, 1988). Implantation of cultured OECs in dopamine-depleted rats after 6-OHDA lesioning was not sufficient to promote tissue repair or functional recovery (Dewar et al., 2007). However, the combination of fVM with OECs enhanced the efficacy of grafted neurons in different rat models of PD (Agrawal et al., 2004; Johansson et al., 2005; Weng et al., 2017). Similar results were obtained when dopaminergic neurons isolated from porcine fVM cells or dopaminergic neurons derived from neural stem cells were co-implanted with OECs (Shukla et al., 2009; Weng et al., 2020).

It is well-established that striatal cells exert both tropic and trophic effects that contribute to providing a target for mesencephalic dopaminergic neurons during the development of the nigrostriatal pathway (Barker et al., 2020). To extend these properties to cell transplantation, several works focused on studying the effects of co-grafting striatal cell suspensions with fVM tissue. In a ground-breaking study carried out by Brundin and collaborators, rat fVM cells were grafted alone or in combination with striatal (i.e., a major dopamine target area) or spinal cord (i.e., a non-target area) cells. The results showed that mixed mesencephalic and striatal suspensions gave rise to a greater area of dense dopaminergic innervation into the host striatum than grafts of fVM alone or fVM combined with spinal cord cells (Brundin et al., 1986). Similar results were obtained in subsequent studies, showing increased dopaminergic survival and more robust, long-lasting functional recovery in denervated rodents and monkeys after nigral-striatal co-grafts (Yurek et al., 1990; Costantini et al., 1994; Emgard-Mattson et al., 1997; Sortwell et al., 1998; Sladek et al., 2008). The effects of co-grafting fVM and striatal tissue in a PD patient were also explored and the results showed improved motor function and a significant increase in the uptake of fluoro-L-DOPA detected by positron emission tomography in the co-grafted putamen (i.e., grafts of fVM and lateral ganglionic eminence tissue) compared to the non-co-grafted side (i.e., a graft of fVM tissue alone) (Lee et al., 2003). The discovery of glial cell line derived-neurotrophic factor (GDNF), which is expressed in the rodent striatum, as the most potent neurotrophin that promotes dopaminergic survival quickly, sparked interest in this factor in neuroprotective and cell therapies for PD. Previous studies showed that GDNF rescues dopaminergic neurons in animal models of PD (Winkler et al., 1996). Different approaches to delivering GDNF into the brain, such as viral delivery, genetically modified cells, nanoparticles, or hydrogels, were used to increase the survival, maturation, and functional integration of transplanted dopaminergic cells (Moriarty et al., 2017; Gantner et al., 2020). Despite its potential, clinical translation of GDNF-related therapies was hampered by safety and efficacy concerns associated with its short half-life, low ability to cross the blood-brain barrier, and side effects derived from long-term delivery. Co-transplantation strategies have been proposed to achieve a sustained release of minimum effective levels of GDNF that could increase its therapeutic benefits. Initial studies showed that co-transplants of fVM and GDNF-secreting clones of a Schwann cell line enhanced the survival and fiber outgrowth from embryonic nigral cells grafted in 6-OHDA-lesioned rats (Wilby et al., 1999). Carotid body glomus cells are highly dopaminergic and produce GDNF, and their neuroprotective and neuroreparative effects have been demonstrated in rodent PD models (Villadiego et al., 2005) and trialed in PD patients (Arjona et al., 2003). Our group in collaboration with Toledo-Aral’s team observed that the survival of dopaminergic neurons, the graft-derived dopaminergic reinnervation, and the performance of the cylinder test were improved in dopamine-depleted rats that received co-grafts of fVM and cell aggregates obtained from the carotid body compared to control groups that received fVM tissue alone (Rodriguez-Pallares et al., 2012). Other studies obtained similar results when different GDNF-producing cells such as fibroblasts, modified neural stem cells or myoblasts were co-grafted with dopaminergic neurons (Ostenfeld et al., 2002; Jingzhong et al., 2005; Deng et al., 2013; Perez-Bouza et al., 2017). Transplantation of expanded neural precursor cells derived from the developing VM was also explored in animal models of PD (Armstrong et al., 2000; Rodriguez-Pallares et al., 2005). Co-grafting VM-derived precursors with cultured astrocytes resulted in almost complete behavioral recovery and a higher number of tyrosine hydroxylase (TH)-positive cells within the graft, with longer fiber length compared to control grafts (Song et al., 2018). Interestingly, astrocytes exhibited region-specific heterogeneity in their secretory profiles, and the beneficial effects on graft survival and function were more evident when VM-derived precursors were co-implanted in the same suspension with VM-derived astrocytes compared to co-transplants including cortex-derived astrocytes. These effects are possibly related to the establishment of a neurotrophic brain environment, as a result of an upregulated expression pattern of neurotrophic factors, trophic extracellular matrix proteins, and antioxidant factors in the striatum after co-transplantation. A reduction in the immune and inflammatory response was also suggested since an increase in the expression of anti-inflammatory markers (interleukin (IL)-1 and IL-10) and the downregulation of proinflammatory markers (Il-1β and inducible nitric oxide synthase) were observed (Song et al., 2018; see for details in Additional Table 1).

Additional Table 1.

Co-grafting-based strategies in Parkinson’s disease

Reference	Co-grafted cells	Animal model	Outcomes
Aguayo et al., 1984	Rat fVM tissue + rat sciatic nerve	6-OHDA Rat (female)	▪ Monoaminergic neurons within the graft extended axons along the nerve bridges
van Horne et al., 1991	Rat fVM tissue + rat sciatic nerve	6-OHDA Rat (male)	▪ Improvement in rotational behavior after co-grafting
Timmer et al., 2004	Rat fVM cells (2-2.5 × 105) + rat SC (2-2.5 × 105) Intrastriatal 8 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Increased survival of DA cells in co-grafts
Collier et al., 1994	Monkey fVM cells + monkey saphenous nerve 1 post-lesion	MPTP Monkey (male)	▪ No neuroprotective effects
Agrawal et al., 2004	Rat fVM cells (2.5 × 105) + rat OECs (2.5 × 105) Intrastriatal 5 wk post-lesion	6-OHDA Rat	▪ Improvement in rotational behavior after co-grafting ▪ Increased survival of grafted DA cells ▪ Restored levels of striatal DA and DOPAC
Johansson et al., 2005	Rat fVM cells (1 × 105) + rat OECs (2 × 105) or rat astrocytes (2 × 105) Intrastriatal 4 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Co-grafts with OECs enhanced DA survival and neurite elongation
Weng et al., 2017	Rat fVM cells (2 × 105) + rat OECs (2 × 105) Intrastriatal 3 wk post-lesion	6-OHDA (MFB) Rat (male)	▪ Co-grafted rats exhibited better motor recovery (rotometer) and enhanced survival of grafted DA cells
Shukla et al., 2009	Rat fVM-derived NSCs (2.5 × 105)+ rat OECs (2.5 × 105) Intrastriatal 4 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Restitution of motor function and increased survival and reinnervation of grafted NSCs
Weng et al., 2020	Porcine fVM cells (2.5 × 105) + rat OECs (2.5 × 105) Intrastriatal 2 wk post-lesion	6-OHDA (MFB) Rat (male)	▪ Better improvement in apomorphine-induced rotational behavior and higher DA survival and less immune response
Brundin et al., 1986	Rat fVM cells (1.2-1.5 × 105) + rat fST cells (1.5-2 × 105) or rat spinal cord cells (1.5-2 × 105) Intrastriatal 3 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Co-grafted animals showed a tendency to improve behavioral effects (rotation test) ▪ Greater area of reinnervation after co-grafts ▪ Similar number of DA cells in co-grafted rats compared to rats grafted with fVM cells alone
Yurek et al., 1990	Rat fVM cells (6-7.5 × 104) + rat fST cells (6-7.5 × 104) (mixed in the same cell suspension or grafted separately) Intrastriatal 1 wk post-lesion	6-OHDA Rat (male)	▪ Larger DA cell body, arborization and improved motor behavior (rotometer) after co-grafting
Costantini et al., 1994	Rat fVM cells (1 × 105) + rat fST cells (1 × 105) Intrastriatal 2-4 months post-lesion	6-OHDA (LV) Rat (3 days old)	▪ Co-transplants produced stronger behavioral effects and dense TH-positive patches within the grafts, without increasing DA survival
Emgard-Mattson et al., 1997	Rat fVM + rat fST cells (lateral and medial ganglionic eminences) (7.9-12.7 × 104 total cells in the suspension) Intrastriatal	6-OHDA (MFB) Rat (female)	▪ Co-grafts containing lateral ganglionic cells showed patches of dense TH-positive fibers ▪ No functional differences were observed after co-grafting (rotometer and staircase tests)
Sortwell et al., 1998	Rat fVM (1.8 × 105) + rat fST cells (1.8 × 105) (mixed or grafted separately) Intrastriatal 4 wk post-lesion	6-OHDA (MFB) Rat (male)	▪ Only rats that received co-grafts placed separately showed significant behavioral recovery and increased DA cell survival ▪ Mixed co-grafts showed enhanced neurite branching and TH-positive rich patches
Sladek et al., 2008	Monkey fVM cells (intranigral) + monkey fST cells (at different points rostrally to the fVM graft, following the nigrostriatal pathway) 2-3 months post-lesion	MPTP Monkey	▪ Striatal grafts contained dense DA axons derived from the fVM graft (target-directed outgrowth)
Lee et al., 2003	Human fVM + human fST (1 lateral ganglionic eminence) Intraputamen	PD patient (male)	▪ Improved motor function and increased L-DOPA uptake
Wilby et al., 1999	Rat fVM cells (3.4 × 106 cells; intranigral) + GDNF-secreting SC line derived from rat sciatic nerve (2.5 × 105 cells; different deposits along the nigrostriatal pathway) 3 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Co-grafts improved DA survival and neurite outgrowth ▪ No additional effects on motor functions
Rodriguez-Pallares et al., 2012	Rat fVM cells (4.5 × 105) + carotid body cells (4.5 × 105) Intrastriatal 6 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Significantly better motor performance (cylinder test) and improved DA survival and graft-derived DA innervation in rats that received co-grafts
Ostenfeld et al., 2002	Rat fVM cells (2.5 × 105) + expanded NPCs obtained from rat fST tissue and modified to produce GDNF (4 × 105) Intrastriatal 3 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Co-grafting increased DA survival ▪ No significant effects on behavioral recovery
JingZhong et al., 2005	Rat embryonic fibroblasts expressing TH + rat embryonic fibroblasts expressing GDNF (total number = 1 × 106) Intrastriatal 2 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Only rats that received co-grafts showed stable and significant behavioral improvements (rotometer) and biochemical recovery (TH expression)
Deng et al., 2013	Rat fVM cells (2.5 × 105) + rat NSCs overexpressing GDNF (2.5 × 105) Intrastriatal 2 wk post-lesion	6-OHDA (MFB) Rat (male)	▪ Co-transplantation significantly reduced apomorphine-induced rotations ▪ Survival and differentiation of NSCs increased after co-grafting.
Perez-Bouza et al., 2017	Cultured rat fVM cells (equivalent to ½ VM) + encapsulated mouse C2C12 myoblasts transfected to produce GDNF Intrastriatal 13 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Higher fiber outgrowth in co-grafted animals ▪ Similar DA survival and graft volume
Song et al., 2018	Mouse or rat fVM-derived NSCs + mouse or rat VM-or cortex-derived astrocytes (ratio 2:1; total number = 4.5 × 105) Intrastriatal	6-OHDA (SN) Rat (female)	▪ Co-grafting with VM-derived astrocytes promoted DA differentiation of NPCs and increased fiber outgrowth
Leveque et al., 2015	Porcine fVM cells (3 × 105)+ rat BM-MSCs Intrastriatal 1 wk post-lesion	6-OHDA (MFB)	▪ MSCs promoted long-term survival of DA neuroblasts and motor recovery
Rodríguez-Pallares et al., 2021	Rat fVM cells (5 × 105) + rat BM-MSCs (2.5 × 104 (low dose) or 2 × 105 (high dose)) Intrastriatal 6 wk post-lesion	6-OHDA (MFB) Rat (female)	▪ Increased survival of DA nigral neurons and striatal terminals after co-grafting fVM cells and low dose of MSCs ▪ High dose of MSCs co-grafted with fVM cells decreased DA survival
Shintani et al., 2007	Rat fVM cells (8 × 104) + mouse CM-derived from BM-MSCs Intrastriatal	6-OHDA (MFB) Rat (male)	▪ Increased survival of grafted DA cells
Clinical trial identifier NCT03684122	Human UC-MSC-derived NSCs (8-12 × 106; intrathecal) + human UC-MSCs (8-12 × 106 (intrathecal) or 4-6 × 107 (intravenous))	PD patients	▪ Clinical trial ongoing

6-OHDA: 6-Hydroxydopamine; BM-MSCs: bone marrow-derived mesenchymal stromal cells; CM: conditioned medium; DA: dopamine; DOPAC: 3,4-dihydroxyphenyl acetic acid; fST: fetal striatal; fVM: fetal ventral mesencephalic; GDNF: glial cell line-derived neurotrophic factor; L-DOPA: L-3,4-dihydroxyphenylalanine; LV: lateral ventricle; MFB: medial forebrain bundle; MPTP: 1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine; NPCs: neural precursor cells; NSCs: neural stem cells; OECs: olfactory ensheathing cells; PD: Parkinson’s disease; SC: Schwann cells; SN: substantia nigra; TH: tyrosine hydroxylase; UC-MSCs: umbilical cord-derived mesenchymal stromal cells; wk: weeks.

As indicated above, the low survival of dopaminergic neurons during and after grafting is probably caused by a combination of factors beyond growth factor deprivation such as apoptosis, neuroinflammation, oxidative stress, hypoxia, and insufficient vascular supply, and immune rejection, suggesting that a pleiotropic therapy would be advantageous. In this regard, MSCs have broad neurotrophic, anti-inflammatory, and immunomodulatory properties, and have been proposed as a promising cell source for cell therapy in PD.

The Potential of Mesenchymal Stromal Cells for Cell Therapy

MSCs are multipotent cells that possess stem cell-like properties including self-renewal and the capacity to differentiate, under appropriate conditions, into several tissues of mesenchymal origin including cartilage, bone, and fat. MSCs were first isolated from bone marrow (BM) and defined as plastic-adherent and colony-forming unit fibroblastic cells. Although BM is the primary source, MSCs can also be isolated from many other sources including adipose tissue, umbilical cord Wharton’s jelly, skin, dental pulp, peripheral blood, and perivascular niches, among others (Andrzejewska et al., 2019; Dabrowska et al., 2021). Interest in MSCs has increased rapidly in recent years due to their widespread availability in the body, high proliferation capacity, few ethical concerns, and low risk of tumor formation. Moreover, their migration and homing abilities (i.e., capacity to preferentially migrate toward damaged tissues), as well as their capacity to regulate the local environment through the release of immunomodulatory and trophic factors, make them an invaluable source for cell therapy (Fricova et al., 2020). MSCs can regulate mechanisms of both innate and adaptive immune responses, through the modulation of cellular responses and secretion of anti-inflammatory mediators. MSCs also participate in angiogenic processes by secreting different growth factors, cytokines, and chemokines. Moreover, the neuroprotective effects of MSCs on damaged or dying nerve cells, as discussed below, are particularly interesting. In addition to direct actions, different studies suggested that MSC-derived products, and particularly MSC-derived extracellular vesicles (MSC-EVs), elicited similar biological effects and have emerged as a powerful regenerative tool (Vilaça-Faria et al., 2019).

Mesenchymal Stromal Cells and Neural Repair

Therapies using MSCs rely on their capacity to modify damaged tissue microenvironments and enhance endogenous neural protection and regeneration. MSCs can secrete into the damaged area neurotrophic and anti-apoptotic factors, and cytokines such as insulin growth factor 1, vascular endothelial growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2, and nerve growth factor, among others. MSCs can also promote neural repair through the production of anti-inflammatory cytokines such as IL-10 and transforming growth factor β and by decreasing pro-inflammatory cytokines such as IL-1β, tumor necrosis factor-alpha, or interferon γ (Fricova et al., 2020). Moreover, there is evidence of the capacity of MSCs to counteract oxidative stress and enhance the antioxidant response (Angeloni et al., 2020). Extracellular matrix molecules produced by MSCs have also been demonstrated to promote neural cell attachment, growth, and axonal extension. Pre-treatment of MSCs under defined conditions (i.e., pre-conditioning) has been shown to modify the secretome of MSCs and improve their efficacy in animal models of different neurological disorders (Baez-Jurado et al., 2019; d’Angelo et al., 2020). Other strategies have been based on the use of genetically engineered MSCs as delivery vehicles to produce specific neurotrophic factors, such as brain-derived neurotrophic factor, or the modification of MSCs to improve their survival and capacity to migrate toward the lesion site, particularly relevant when systemic administrations are performed, or prevent their premature senescence (Ocansey et al., 2020). Significant attention has been paid to the therapeutic potential of MSC-EVs to promote neurite growth and remodeling, enhance neurogenesis and angiogenesis and exert neuroprotective effects in damaged neural tissue. Furthermore, MSC-EVs reduce brain inflammation by inhibiting astrogliosis and switching the polarization of macrophages and microglial cells from pro-inflammatory to pro-repair phenotypes. Treatment with MSC-EVs also reduced the levels of pro-inflammatory cytokines such as tumor necrosis factor-alpha, IL-1β, and IL-6 and increased the levels of anti-inflammatory cytokines IL-4 and IL-10. Overall, MSCs have unique and valuable characteristics for the repair of the nervous system in various diseases including PD (d’Angelo et al., 2020).

Mesenchymal Stromal Cell-Based Therapies for Parkinson’s Disease

Different studies suggest that MSCs can differentiate into neural progenitors from which cells resembling dopamine neurons (i.e., that express the dopaminergic marker TH) can be obtained (Thompson et al., 2019). Neuronal-primed MSCs or dopaminergic cells derived from MSCs have been transplanted in animal models of PD (Delcroix et al., 2011; Park et al., 2012a; Shetty et al., 2013; Wang et al., 2013). However, it remains unclear whether these cells can integrate into the host neural circuits and create new synapses with host neurons. Therefore, most studies have focused on their known ability to produce a wide range of factors. Transplantation of murine BM-MSC into the striatum or the substantia nigra in preclinical models of PD showed significant improvements in behavioral tests and preservation of TH-positive neurons in the substantia nigra and fibers in the striatum (Chen et al., 2017). Dopamine-depleted animals receiving intrastriatal injections of human BM-MSCs grafts also showed neuroprotective effects (Blandini et al., 2010; Cova et al., 2010; Xiong et al., 2013). MSCs obtained from other sources such as adipose tissue or umbilical cord have also been tested with similar results (Schwerk et al., 2015; Bagheri-Mohammadi et al., 2019; Wang et al., 2020; Lian et al., 2021). The mechanisms underlying these improvements have been ascribed to increased levels of GDNF and brain-derived neurotrophic factor, reduction of apoptosis and inflammation, or neurogenic effects (Blandini et al., 2010; Cova et al., 2010; Lian et al., 2021). However, other studies showed modest or no improvement after MSCs grafting (Moloney et al., 2010; Sun et al., 2020). In addition to intracerebral grafting, other routes of administration have also been tested as suitable approaches in PD. Systemic administration of MSCs by intravenous or intracarotid injection induced a significant reduction in amphetamine-induced rotational behavior, preserved TH-positive cells in the substantia nigra and dopaminergic striatal terminals, and reduced astrocytic and microglial response in animal PD models (Chao et al., 2009; Park et al., 2012b; Inden et al., 2013; Cerri et al., 2015; Suzuki et al., 2015; Park and Chang, 2020). Although it has been suggested that systemic administered MSCs do not pass the capillary system of the lungs, other studies showed that MSCs have an intrinsic ability to target the brain and migrate to the lesioned substantia nigra after intravenous administration (Chao et al., 2009; Do et al., 2021). Other works have used intranasal infusion as a non-invasive method of administration. Although some authors reported that MSCs administered intranasally reached the brain parenchyma, migrated to lesioned areas, and survived (Danielyan et al., 2011; Salama et al., 2017), others indicated that MSCs could not be detected in the brain (Chartoff et al., 2011). To address these issues, several modification techniques have been proposed to efficiently guide their migration to target sites or improve focal adhesion, which may be of particular interest to improve their viability within the transplanted environment in co-grafting approaches (Ocansey et al., 2020). Another important aspect is the implantation of a sufficient number of MSCs to guarantee a therapeutic effect. Previous studies suggest that MSCs survive for only a short period after transplantation (Chartoff et al., 2011; Bossolasco et al., 2012). The low immunogenicity of MSCs does not ensure that they are fully immune privileged, and they are likely to provoke immune response despite their immunosuppressive properties (Camp et al., 2009). Different strategies have been tested to increase the survival of the transplanted MSCs such as cell encapsulation, a combination of MSCs with microcarriers that release therapeutic proteins, or co-grafting with OECs (Delcroix et al., 2011; Forouzandeh et al., 2021). Other studies have combined MSC transplantation with other therapeutic strategies such as levodopa treatment or exercise to obtain enhanced improvements (Cucarian et al., 2019; Jalali et al., 2021). Other mechanisms underlying the beneficial effects of MSCs have been proposed. Recent data suggest that MSCs can transfer mitochondria to damaged tissue, which seems particularly relevant in PD therapy due to the proposed central role of damaged mitochondria in dopaminergic neurodegeneration (Liu et al., 2014). Interestingly, it has recently been reported that MSC treatment alleviated locomotor deficits and rescued dopaminergic neurons by inhibiting neuroinflammation in a PD model through regulating intestinal microorganisms by cross talk between the brain and gut (Sun et al., 2022; see for details in Additional Table 2). As PD pathology involves widespread neurodegeneration beyond the nigrostriatal pathway, MSCs transplantation targeting extra basal ganglia structures could also represent a strategy to alleviate the non-motor symptoms of the disease (Pantcheva et al., 2015).

Additional Table 2.

Mesenchymal stromal cell-based strategies in Parkinson’s disease

Reference	Source of MSCs	PD model	MSC survival	DA differentiation	MSC migration	Outcomes
Delcroix et al., 2011	Human BM-MSCs pre- differentiated to DA cells (1.5 × 105 Intrastriatal 2 wk post-lesion	6-OHDA (ST) Rat (female)	Poor survival(1)	No (1)	-	▪ Strong reduction of amphetamine-induced rotations ▪ Neuroprotection of the nigrostriatal pathway ▪ (1)Increased survival and differentiation of MSCs after adhesion to pharmacologically active microcarriers (PAMs)
Park et al., 2012a	Human placental-derived MSCs undifferentiated and differentiated to NPCs (2 × 105 cells/μl)	6-OHDA (MFB) Rat (male) Intrastriatal 30 days post-lesion	Yes	Yes	-	▪ Sprouting of striatal DA fibers
Shetty et al., 2013	Undifferentiated human UC- MSCs or differentiated to DA cells and BM-MSCs and (1 × 106) Intranigral 6w post-lesion	6-OHDA (SN) Rat (male)	Yes	Yes	-	▪ Improvement in rotometer test
Wang et al., 2013	Undifferentiated rat BM-MSCs or differentiated to NPCs under hypoxia or human fetal BM-MSCs (5 × 105) Intrastriatal 4 wk post-lesion	6-OHDA (MFB) Rat (female)	Yes	Yes (under hypoxic conditions)	-	▪ Improvement in rotometer test ▪ Increased striatal DA levels induced by grafting of pre-differentiated MSCs
Chen et al., 2017	Rat BM-MSCs (6 × 104) Intranigral 3 wk post-lesion	6-OHDA (SN) Rat (male)	Yes	No (Differentiation to GFAP-and NSE- positive cells)	Yes	▪ Improvement in rotometer test ▪ Increased number of DA neurons and striatal fiber density
Blandini et al., 2010	Human MSCs (commercial) (1.8 × 105) Intrastriatal 5 days post-lesion	6-OHDA (ST) Rat (male)	Yes	No (GFAP expression)	No	▪ Improvement in rotometer test ▪ Increased survival of DA neurons and striatal fibers ▪ Increased GDNF levels in the SN
Cova et al., 2010	Human MSCs (commercial) (3.2 × 104 or 1.8 × 105 cells) Intrastriatal 5 days post-lesion	6-OHDA (ST) Rat (male)	Yes	No (GFAP expression)	-	▪ Enhanced neurogenesis in the SVZ ▪ Migration of neuroblasts from the SVZ to the ST ▪ Reduced loss of DA neurons and striatal terminals ▪ Secretion of BDNF after grafting
Xiong et al., 2013	Human BM-MSCs (1 × 106) Intrastriatal 2 wk post-lesion	Rotenone (SN) Rat (female)	Yes	Yes (after bFGF exposure)	-	▪ Improvement in rotometer test ▪ Preservation of DA neurons and striatal fibers ▪ Pre-treatment of MSCs with bFGF promoted neural differentiation and enhanced neuroprotective effects
Schwerk et al., 2015	Human AD-MSCs (3 × 105) Intranigral 6 days post-lesion	6-OHDA (MFB) Rat (male)	Yes	No	Yes	▪ Improvement of cognitive but not motor performance ▪ Reduction of DA degeneration ▪ Increased neurogenesis in the SVZ and DG
Bagheri-Mohammadi et al., 2019	Human endometrial-derived MSCs (1 × 104, 5 × 104 or 1 × 105) Intranasal 4 wk post-lesion	6-OHDA (ST) Mouse (male)	Yes	-	Yes (SN)	▪ Improvement in rotometer test ▪ Increase in the number of DA neurons and striatal terminals
Wang et al., 2020	Human UC-MSCs (1 × 107 cells/kg; repeated infusions) Intravenous (tail vein) 5 wk post-lesion	MPTP Mouse (female)	Yes	-	Yes (ST)	▪ Motor improvements (open field and rotarod) ▪ Increased expression of TH in the ST and SN ▪ Reduced expression of apoptotic factors
Lian et al., 2021	Human AD-MSCs (1 × 105) Intrastriatal 4 wk post-lesion	6-OHDA (SN) Mouse (male)	-	-	-	▪ Behavioral improvement (rotarod and open field) ▪ Neuroprotection of DA neurons ▪ Reduction of apoptotic markers
Moloney et al., 2010	Rat BM-MSCs or MSCs modified to produce GDNF (2 × 105) Intrastriatal 4 days pre-grafting	6-OHDA (ST) Rat (male)	Poor survival	-	No	▪ No neuroprotection ▪ No functional recovery ▪ Sprouting of DA fibers around grafted GDNF- transduced MSCs
Sun et al., 2020	Human AD-MSCs or AD-MSCs modified to produce GDNF (5 × 105) Intrastriatal 9 days post-lesion	6-OHDA (ST) Mouse (male)	Yes (GDNF- producing cells)	No (GFAP and Tuj1 expression)	-	▪ Improved performance on behavioral tests ▪ Increased graft survival when GDNF-secreting cells were transplanted ▪ Neuroprotective effects on DA cells
Chao et al., 2009	Mouse BM-MSCs (1 × 105) Intravenous (tail vein) 1-day post-lesion	MPTP Mouse (male)	Yes	No	Yes (SN)	▪ BBB recovery ▪ Reduction of microglial activation ▪ Prevention of DA cell death
Park et al., 2012b	Human BM-MSCs (1 × 106) Intravenous (tail vein) 3 days post-lesion	MPTP Mouse (male)	Yes	No	Yes	▪ Increased neurogenesis in tde SVZ and SN ▪ Increased proliferation in the SN ▪ Reduced loss of DA neurons
Inden et al., 2013	Human BM-MSCs (5 × 105) Intravenous (tail vein) Immediately after lesion	Rotenone Mouse	Yes (few cells)	Yes (TH expression)	Yes (ST)	▪ Significant improvement in behavioral dysfunction (rotarod) ▪ Increased number of DA and NeuN cells in the SN
Cerri et al., 2015	Rat BM-MSCs (1 × 106) Intracarotid 14 days post-lesion	6-OHDA (ST) + BBB disruption Rat (male)	Yes	-	Yes (only after BBB disruption)	▪ Forelimb akinesia is not modified by cell infusion ▪ Reduction in net rotations ▪ No neuroprotective effects
Suzuki et al., 2015	Human BM-MSCs (commercial) (1 × 107) Intravenous (femoral vein) 16 days post-lesion	6-OHDA (ST) Rat (male)	No	-	No	▪ Amelioration in drug-induced rotational behavior ▪ Preservation of DA cells ▪ Inhibition of microglial activation
Park and Chang, 2020	Human MSCs (commercial) (1 × 106; repeated infusions) Intravenous (tail vein)	MPTP Mouse (male)	Very low	-	Yes (ST)	▪ Motor function improved (rotarod) ▪ Neuroprotection of DA cells ▪ Increased levels of GDNF and BDNF in the ST
Danielyan et al., 2011	Rat BM-MSCs (3 or 5 × 105) Intranasal 3-, 7-or 9-days post-lesion	6-OHDA (MFB) Rat (male)	Yes	Yes (TH expression)	Yes (ST, SN and others)	▪ Improvement of motor function ▪ Increased levels of TH in the ST and SN ▪ Reduction in apoptosis markers and pro- inflammatory cytokines
Salama et al., 2017	Mouse BM-MSCs (5 × 105) Intranasal 8 wk post-lesion	Rotenone Mouse	Yes	-	Yes	▪ Improved locomotor performance (parallel rod test, open field, akinesia, tremor) ▪ DA degeneration counteracted
Chartoff et al., 2011	Mouse BM-MSCs (5 × 104) Intranasal 3 wk post-lesion	6-OHDA (ST) Mouse (male)	No	-	No	▪ MSCs did not survive or migrate into the brain after intranasal delivery
Bossolasco et al., 2012	Human BM-MSCs (2 × 106 (intranasal) or 1.8 × 105 (intrastriatal)) 5 days post-lesion	6-OHDA (ST) Rat (male)	Variable (1),(2)	-	-	▪ (1)No signal was detected 60 min after intranasal infusion ▪ (2)Detection 14 days after grafting
Camp et al., 2009	Rat MSCs (commercial) (4 × 105) Intrastriatal Immediately after lesion	6-OHDA (SN) Rat (female)	Yes	-	Yes	▪ MSC administration did not prevent behavioral deficits or DA depletion ▪ Robust cellular immune responses in the ST
Forouzandeh et al., 2021	Neural induced and non-induced human eye conjunctiva-MSCs (encapsulated and non- encapsulated) (3 × 104) Intrastriatal 2 wk post-lesion	6-OHDA (MFB) Rat (male)	Yes	Yes	-	▪ Significant reduction in drug-induced rotations ▪ Increased striatal TH expression
Sun et al., 2022	Human UC-MSCs (1 × 106; repeated infusions) Intranasal 5 days post-lesion	MPTP Mouse (male)	-	-	-	▪ Reduction of motor impairment (pole test and traction test) ▪ Rescue of DA neurons ▪ Inhibition of inflammation ▪ Changes in gut microbiota
Venkataramana et al., 2010(1), 2012(2)	Human BM-MSCs (autologous(1)/allogenic(2)) (1 × 106(1) or 2 × 106 cells/kg(2) body weight) Lateral to the SVZ (bilateral grafts)	PD patients Human	-	-	-	▪ No serious adverse events were observed 36 m after transplantation

6-OHDA: 6-Hydroxydopamine; AD-MSCs; adipose-derived mesenchymal stromal cells; BBB: blood-brain barrier; BDNF: brain derived neurotrophic factor; bFGF: basic fibroblast growth factor; BM-MSCs: bone marrow-derived mesenchymal stromal cells; DA: dopamine; DG: dentate gyrus; GDNF; glial cell line-derived neurotrophic factor; GFAP: glial fibrillary acidic protein; MFB; medial forebrain bundle; MPTP: 1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine; NeuN: neuronal nuclear antigen; NPCs: neural precursor cells; NSE: neuron-specific enolase; PD: Parkinson’s disease; SN: substantia nigra; ST: striatum; SVZ; subventricular zone; TH: tyrosine hydroxylase; Tuj1: β-tubulin III, clone Tuj1; UC-MSCs: umbilical cord-derived mesenchymal stromal cells; wk: weeks.

Delivery of neurotrophic factors by MSCs may also serve as a useful tool for the protection and functional stimulation of grafted dopaminergic cells from any source. In fact, co-grafting of fetal porcine neuroblasts and rat BM-MSCs promoted the survival of neuroblasts that when transplanted alone did not survive in the striatum of immunocompetent (i.e., non-immunosuppressed) denervated rats (Leveque et al., 2015). Interestingly, 50% of transplanted rats showed healthy grafts with TH-positive neurons at 120 days post-transplantation and motor recovery was also observed. Detailed analysis revealed a local control of the host immune response in the striatum that received co-grafts, highlighting the immunosuppressive properties of MSCs and their potential utility for the long-term survival of xenogeneic grafts (Leveque et al., 2015). Our group has recently demonstrated that the combination of murine BM-MSCs and fVM cells in the same suspension significantly improves the survival and reinnervation ability of dopaminergic neurons transplanted in a rat 6-OHDA model of PD. However, it is essential to control tightly the density of the transplanted cells or concentration of MSC-derived products, since these beneficial effects are only observed when fVM tissue is combined with a low number of MSCs (Rodriguez-Pallares et al., 2021; Additional Table 1). Interestingly, a conditioned medium derived from MSC (MSC-CM) containing the entire secretome also enhances the survival of dopaminergic grafts (Shintani et al., 2007), suggesting that the therapeutic effects of MSCs are not only based on direct cell-to-cell interactions but that paracrine signaling seems to be a primary mechanism (Mendes-Pinheiro et al., 2019; Fricova et al., 2020).

As previously indicated, several mechanisms could be responsible for these effects. Our group observed that classical pathways involved in trophic factor-mediated signaling, such as MAPK/ERK and PI3K/Akt pathways, were active in dopaminergic cultures treated with MSC-CM. However, lipid removal decreased the viability and reduced the neuroprotective effects of MSC-CM on pure populations of dopaminergic cells, primary mesencephalic cultures (i.e., neuron-glia mixed), and human iPSC-derived neurons (Parga et al., 2018), suggesting that other factors could also be relevant. We demonstrated for the first time that prostaglandin E2, through prostaglandin E2 receptor 2, was largely responsible for the positive effects of the MSC-CM on dopaminergic survival and the cAMP-PKA pathway, used by many prostaglandins to modulate neuroprotection, was active in dopaminergic cultures (Parga et al., 2018).

Based on these promising results, several groups have conducted clinical trials using autologous or allogenic MSCs isolated from various sources for PD treatment (Fricova et al., 2020). In a pilot, open-label study in 2010, autologous BM-MSCs were transplanted unilaterally into the sublateral ventricular zone in seven PD patients. This preliminary study suggests the safety of this novel therapeutic approach and its potential beneficial effects, since three patients showed improved symptoms (Venkataramana et al., 2010). Two years later, the same research group conducted another open-label study using allogenic BM-MSCs implanted bilaterally into the same area with similar results (Venkataramana et al., 2012). Currently, several clinical trials are in progress with a highly variable setup (ClinicalTrials.gov identifiers NCT02611167; NCT03550183; NCT04506073; NCT04928287; NCT04995081), including a study of co-transplantation of MSCs and MSC-derived neural stem cells in PD patients (ClinicalTrials.gov identifier NCT03684122). The experience gained in previous trials should guide the future directions and emphasize the need for further studies to understand the mechanisms underlying the therapeutic effects of MSCs and standardize experimental designs and protocols to achieve reliable and effective cell-based treatments for PD.

Conclusions and Future Perspectives

Over the decades, cell-based therapies have shown their potential in the treatment of PD. Although prematurely conducted trials in the past have cast doubt on the efficacy of these emerging approaches, human fVM tissue continues to be the gold standard and has undoubtedly paved the way for considering the clinical translation of transplantation of dopaminergic neurons obtained from different sources. The number of healthy dopaminergic cells needed for functional improvements in PD patients is around 40,000–80,000 neurons, which requires readily available cells that contribute to permit the large-scale application of these approaches. Stem cells offer a clear advantage in this regard, and significant progress in obtaining sufficient dopaminergic cells from different expandable sources has been achieved in recent years. However, cell therapy for PD has underlying problems that affect transplanted cells regardless of the source of dopaminergic neurons. Although further studies are necessary, combinations of cell-based strategies, such as the co-transplantation approaches addressed in this review, or combination of cells with pharmacological treatments, deep brain stimulation, or potential new immunological therapies could improve the current results and help to slow or even stop the progression of the disease. Genetic manipulation of the cells may also be another beneficial strategy that could contribute to improving the survival, integration, and functionality of grafted cells. Emerging approaches in neural repair involve the development and application of tissue engineering techniques, such as the use of biomaterial or cellular scaffolds, or patient-specific constructs, that could influence the behavior of native or transplanted cells and promote survival, differentiation, organization, and reconstruction of degenerated circuitry. Further research is necessary to understand the mechanisms underlying the functional recovery of successful transplants and to achieve greater benefits for the millions of patients worldwide affected by PD.

Additional files:

Additional Table 1: Co-grafting-based strategies in Parkinson’s disease.

Additional Table 2: Mesenchymal stromal cell-based strategies in Parkinson’s disease.