A multiple target neural transplantation strategy for Parkinson's disease

Past, present, and future of cell replacement therapy for parkinson's disease: a novel emphasis on host immune responses

Parkinson's disease (PD) stands as the second most common neurodegenerative disorder after Alzheimer's disease, and its prevalence continues to rise with the aging global population. Central to the pathophysiology of PD is the specific degeneration of midbrain dopamine neurons (mDANs) in the substantia nigra. Consequently, cell replacement therapy (CRT) has emerged as a promising treatment approach, initially supported by various open-label clinical studies employing fetal ventral mesencephalic (fVM) cells. Despite the initial favorable results, fVM cell therapy has intrinsic and logistical limitations that hinder its transition to a standard treatment for PD. Recent efforts in the field of cell therapy have shifted its focus towards the utilization of human pluripotent stem cells, including human embryonic stem cells and induced pluripotent stem cells, to surmount existing challenges. However, regardless of the transplantable cell sources (e.g., xenogeneic, allogeneic, or autologous), the poor and variable survival of implanted dopamine cells remains a major obstacle. Emerging evidence highlights the pivotal role of host immune responses following transplantation in influencing the survival of implanted mDANs, underscoring an important area for further research. In this comprehensive review, building upon insights derived from previous fVM transplantation studies, we delve into the functional ramifications of host immune responses on the survival and efficacy of grafted dopamine cells. Furthermore, we explore potential strategic approaches to modulate the host immune response, ultimately aiming for optimal outcomes in future clinical applications of CRT for PD.

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

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease, affecting 1-2% of people over 65. The classic motor symptoms of PD result from selective degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), resulting in a loss of their long axonal projections to the striatum. Current treatment strategies such as dopamine replacement and deep brain stimulation (DBS) can only minimize the symptoms of nigrostriatal degeneration, not directly replace the lost pathway. Regenerative medicine-based solutions are being aggressively pursued with the goal of restoring dopamine levels in the striatum, with several emerging techniques attempting to reconstruct the entire nigrostriatal pathway-a key goal to recreate feedback pathways to ensure proper dopamine regulation. Although many pharmacological, genetic, and optogenetic treatments are being developed, this article focuses on the evolution of transplant therapies for the treatment of PD, including fetal grafts, cell-based implants, and more recent tissue-engineered constructs. Attention is given to cell/tissue sources, efficacy to date, and future challenges that must be overcome to enable robust translation into clinical use. Emerging regenerative medicine therapies are being developed using neurons derived from autologous stem cells, enabling the construction of patient-specific constructs tailored to their particular extent of degeneration. In the upcoming era of restorative neurosurgery, such constructs may directly replace SNpc neurons, restore axon-based dopaminergic inputs to the striatum, and ameliorate motor deficits. These solutions may provide a transformative and scalable solution to permanently replace lost neuroanatomy and improve the lives of millions of people afflicted by PD.

Time perception impairs sensory-motor integration in Parkinson's disease

It is well known that perception and estimation of time are fundamental for the relationship between humans and their environment. However, this temporal information processing is inefficient in patients with Parkinson' disease (PD), resulting in temporal judgment deficits. In general, the pathophysiology of PD has been described as a dysfunction in the basal ganglia, which is a multisensory integration station. Thus, a deficit in the sensorimotor integration process could explain many of the Parkinson symptoms, such as changes in time perception. This physiological distortion may be better understood if we analyze the neurobiological model of interval timing, expressed within the conceptual framework of a traditional information-processing model called "Scalar Expectancy Theory". Therefore, in this review we discuss the pathophysiology and sensorimotor integration process in PD, the theories and neural basic mechanisms involved in temporal processing, and the main clinical findings about the impact of time perception in PD.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

Stem cells as tools in regenerative therapy for retinal degeneration

OBJECTIVE

To describe the use of stem cells (SCs) for regeneration of retinal degenerations. Regenerative medicine intends to provide therapies for severe injuries or chronic diseases where endogenous repair does not sufficiently restore the tissue. Pluripotent SCs, with their capacity to give rise to specialized cells, are the most promising candidates for clinical application. Despite encouraging results, a combination with up-to-date tissue engineering might be critical for ultimate success.

DESIGN

The focus is on the use of SCs for regeneration of retinal degenerations. Cell populations include embryonic, neural, and bone marrow-derived SCs, and engineered grafts will also be described.

RESULTS

Experimental approaches have successfully replaced damaged photoreceptors and retinal pigment epithelium using endogenous and exogenous SCs.

CONCLUSIONS

Stem cells have the potential to significantly impact retinal regeneration. A combination with bioengineering may bear even greater promise. However, ethical and scientific issues have yet to be solved.

Evaluation of surgical techniques for neuronal cell transplantation used in patients with stroke

Transplantation of cultured neuronal cells was performed in two human clinical trials after safety and efficacy was demonstrated in animal models of stroke. The studies tested the utility of human neuronal cellular transplantation into and around the small stroke volume. We developed a stereotactic surgical technique for cell delivery and evaluated that method in 26 patients with basal ganglia region motor stroke. Human neuronal cells (hNT cells; LBS neurons) were delivered frozen then thawed and formulated on the morning of surgery. Patients in a first trial received 2 or 6 million cells in three or nine implants, and in a second trial, 5 or 10 million in 25 implants. A novel cell delivery cannula was designed, manufactured, tested, and used in surgery. Immediate postoperative CT scans and later serial MR scans were used to evaluate the surgical site. Tests on the cell implantation cannula showed that the cells were not damaged and remained viable after injection. All patients underwent uncomplicated surgeries. Cells could be implanted within a 2-h period, maintaining viability of the preparation. Serial evaluations (maximum 5 years) showed no cell-related adverse serologic or imaging-defined effects. One patient had burr hole drainage of an asymptomatic chronic subdural hematoma. Human neuronal cells can be produced in culture and implanted stereotactically into the brains of patients with stroke. Surgical cell delivery did not lead to new neurological deficits, and imaging studies showed no adverse effects. The cannula used allowed precise injection of the clinical cell dose within a time period that maintained cell viability.

Expanded mesencephalic precursors develop into grafts of densely packed dopaminergic neurons that reinnervate the surrounding striatum and induce functional responses in the striatal neurons

The search for alternative sources of dopaminergic cells, other than primary fetal tissue for transplantation in Parkinson's disease has become a major focus of research. Different methodological approaches have led to generation in vitro of cells expressing DA-cell markers, although these cells are frequently unable to survive for a long time in vivo after transplantation and/or induce functional effects in the host brain. In the present study, we grafted cell aggregates treated with antibodies against fibroblast growth factor 4 into dopaminergic-denervated striata in rats. Furthermore, we grafted cell suspensions from primary mesencephalic fetal tissue. Grafts from expanded precursors were able to survive (at least 3 months postgrafting) and most decreased the lesion-induced ipsiversive rotation. In addition, immunolabeling for tyrosine hydroxylase and/or Fos showed that the grafts reinnervated the surrounding striatal tissue with dopaminergic terminals, and induced the expression of Fos in the striatal neurons of the reinnervated area after administration of amphetamine to the host rat. The number of dopaminergic cells in grafts from expanded precursors inducing rotational recovery was usually lower (1,226+/-314) than that in grafts from primary fetal tissue (1,671+/-122), but they were more densely packed in grafts that were of smaller volume and did not have the characteristic central nondopaminergic area observed in grafts from primary fetal tissue. The results suggest that long-term survival and functional integration into the DA-denervated striatum can be achieved with grafts of expanded mesencephalic precursors.

Stem cell therapy for Parkinson's disease

Transplantation of human fetal dopamine (DA) neurons to patients with Parkinson's disease (PD) has given proof of the principle that new neurons can survive for at least a decade, and then functionally integrate and provide significant symptomatic relief. Unfortunately, the ethical, technical, and practical limitations of using fetal DA neurons as the source for cell transplantation in PD, in combination with the development of unwanted grafting-related side effects, have put a halt to the spread of this treatment into clinical practice. Hopefully, recent advances in the fields of stem cell biology and adult neurogenesis research will lead totamen in new exciting ways to better understand and control the biological parameters necessary for achieving safe and successful neuronal replacement in PD patients.

Harnessing endogenous stem cells to treat neurodegenerative disorders of the basal ganglia

New neurons are continuously generated in selective regions of the normal adult mammalian brain. Recent evidence suggests that neural stem or progenitor cells located in the subventricular zone lining of the lateral ventricles can be induced to proliferate and migrate to the adjacent striatum following brain insults or growth factor treatment. This ability to incorporate new cells into the striatum gives the potential of repairing and restoring basal ganglia functions in Parkinson's and Huntington's diseases. The future challenges lay in directing these new cells to adopt the appropriate neuronal phenotypes and to re-establish proper connections and functional circuitry.

Progress in cerebral transplantation of expanded neuronal stem cells

OBJECT

Given the success and limitations of human fetal primary neural tissue transplantation, neuronal stem cells (NSCs) that can be adequately expanded in culture have been the focus of numerous attempts to develop a superior source of replacement cells for restorative neurosurgery. To clarify recent progress toward this goal, the transplantation into the adult brain of NSCs, expanded in vitro before grafting, was reviewed.

METHODS

Neuronal stem cells can be expanded from a variety of sources, including embryos, fetuses, adult bone marrow, and adult brain tissue. Recent investigations of each of these expanded stem cell types have generated a large body of information along with a great number of unanswered questions regarding the ability of these cells to replace damaged neurons. Expanded NSCs offer many advantages over their primary tissue predecessors, but also may exhibit different functional abilities as grafted cells. Because expanded NSCs will most likely ultimately replace primary tissue grafting in clinical trials, this review was undertaken to focus solely on this distinct body of work and to summarize clearly the existing preclinical data regarding the in vivo successes, limits, and unknowns of using each expanded NSC type when transplanted into the adult brain.

CONCLUSIONS

Embryonic stem cell-derived cells have demonstrated appropriate neuronal phenotypes after transplantation into nonneurogenic areas of the adult brain. Understanding the mechanisms responsible for this may lead to similar success with less studied adult neuronal progenitor cells, which offer the potential for autologous NSC transplantation with less risk of tumorigenesis.

Novel aspects of the chemical anatomy of the striatum and its efferents projections

This paper summarizes the results of some of our previous neuroanatomical and immunohistochemical studies on the organization of the striatum and its efferent projections in rodents, monkeys and humans. It also reports recent functional calcium-imaging data obtained in rat brain slices, as well as developmental results gathered with bromodeoxyuridine (BrdU) in monkeys. On one hand, single-axon tracing studies in rats and monkeys have revealed that the majority of striatofugal axon arborizes within most striatal target structures. In humans, SP-positive fibers were found to arborize in the two segments of the globus pallidus, where they were closely apposed to pallidal neurons that expressed the neurokinin-1 receptor (NK-1r). In agreement with such findings, calcium-imaging studies in rats have revealed that pallidal and nigral neurons are both responsive to SP. These findings suggest that the striatofugal projection system is much more widely distributed than previously thought and exerted a multifaceted effect upon its target sites. On the other hand, immunostaining studies in humans have shown the presence of several types of putative dopaminergic neurons intrinsic to the striatum. Furthermore, BrdU labeling experiments in monkeys have demonstrated that new neurons are generated throughout adult life in the striatum of normal monkeys and that their number can be markedly increased by the administration of neuronal growth factors. These findings open new therapeutic avenues for the treatment of neurodegenerative disorders that specifically affect the striatum.