Stem cell strategies for neuroreplacement therapy in Alzheimer's disease

The therapeutic prospects and challenges of human neural stem cells for the treatment of Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder associated with aging. Due to its insidious onset, protracted progression, and unclear pathogenesis, it is considered one of the most obscure and intractable brain disorders, and currently, there are no effective therapies for it. Convincing evidence indicates that the irreversible decline of cognitive abilities in patients coincides with the deterioration and degeneration of neurons and synapses in the AD brain. Human neural stem cells (NSCs) hold the potential to functionally replace lost neurons, reinforce impaired synaptic networks, and repair the damaged AD brain. They have therefore received extensive attention as a possible source of donor cells for cellular replacement therapies for AD. Here, we review the progress in NSC-based transplantation studies in animal models of AD and assess the therapeutic advantages and challenges of human NSCs as donor cells. We then formulate a promising transplantation approach for the treatment of human AD, which would help to explore the disease-modifying cellular therapeutic strategy for the treatment of human AD.

Different Tissue-Derived Stem Cells: A Comparison of Neural Differentiation Capability

BACKGROUND

Stem cells are capable of self-renewal and differentiation into a wide range of cell types with multiple clinical and therapeutic applications. Stem cells are providing hope for many diseases that currently lack effective therapeutic methods, including strokes, Huntington's disease, Alzheimer's and Parkinson's disease. However, the paucity of suitable cell types for cell replacement therapy in patients suffering from neurological disorders has hampered the development of this promising therapeutic approach.

AIM

The innovative aspect of this study has been to evaluate the neural differentiation capability of different tissue-derived stem cells coming from different tissue sources such as bone marrow, umbilical cord blood, human endometrium and amniotic fluid, cultured under the same supplemented media neuro-transcription factor conditions, testing the expression of neural markers such as GFAP, Nestin and Neurofilaments using the immunofluorescence staining assay and some typical clusters of differentiation such as CD34, CD90, CD105 and CD133 by using the cytofluorimetric test assay.

RESULTS

Amniotic fluid derived stem cells showed a more primitive phenotype compared to the differentiating potential demonstrated by the other stem cell sources, representing a realistic possibility in the field of regenerative cell therapy suitable for neurodegenerative diseases.

Directional induction of dopaminergic neurons from neural stem cells using substantia nigra homogenates and basic fibroblast growth factor

To date, complex components of available reagents have been used for directional induction of neural stem cells into dopaminergic neurons, resulting in a poor ability to repeat experiments. This study sought to investigate whether a homogenate of the substantia nigra of adult rats and/or basic fibroblast growth factor could directionally induce neural stem cells derived from the subventricular zone of embryonic rats to differentiate into dopaminergic neurons. Tyrosine hydroxylase-positive cells were observed exclusively after induction with the homogenate supernatant of the substantia nigra from adult rats and basic fibroblast growth factor for 48 hours in vitro. However, in the groups treated with homogenate supernatant or basic fibroblast growth factor alone, tyrosine hydroxylase expression was not observed. Moreover, the content of dopamine in the culture medium of subventricular zone neurons was significantly increased at 48 hours after induction with the homogenate supernatant of the substantia nigra from adult rats and basic fibroblast growth factor. Experimental findings indicate that the homogenate supernatant of the substantia nigra from adult rats and basic fibroblast growth factor could directionally induce neural stem cells derived from the subventricular zone of embryonic rats to differentiate into dopaminergic neurons in the substantia nigra with the ability to secrete dopamine.

Specific protein markers for stem cell cross-talk with neighboring cells in the environment

A stem cell interacts with the neighboring cells in its environment. To maintain a living organism's metabolism, either cell-cell or cell-environment interactions may be significant. Usually, these cells communicate with each other through biological signaling by interactive behaviors of primary proteins or complementary chemicals. The signaling intermediates offer the stem cell's functionality on its metabolism. With the rapid advent of omics technologies, various specific markers by which stem cells cooperate with their surroundings have been discovered and established. In this article, we review several stem cell markers used to communicate with either cancer or immune cells in the human body.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Neural stem cells and Alzheimer's disease: challenges and hope

Alzheimer's disease is characterized by degeneration and dysfunction of synapses and neurons in brain regions critical for learning and memory functions. The endogenous generation of new neurons in certain regions of the mature brain, derived from primitive cells termed neural stem cells, has raised hope that neural stem cells may be recruited for structural brain repair. Stem cell therapy has been suggested as a possible strategy for replacing damaged circuitry and restoring learning and memory abilities in patients with Alzheimer's disease. In this review, we outline the promising investigations that are raising hope, and understanding the challenges behind translating underlying stem cell biology into novel clinical therapeutic potential in Alzheimer's disease.

The effect of amyloidosis-beta and ageing on proliferation of neuronal progenitor cells in APP-transgenic mouse hippocampus and in culture

Stimulation of endogenous neurogenesis and transplantation of neuronal progenitors (NPs) are considered in therapy of neuronal loss associated with ageing and in neurodegenerative diseases with amyloidosis-beta, for example, Alzheimer's disease and Down syndrome. However, the influence of brain environment altered by ageing and deposits of amyloid-beta on proliferation of endogenous and transplanted NPs and their maturation into neurons is not understood. We studied the effect of ageing and development of amyloidosis-beta on proliferation of NPs (1) in the granular layer of dentate gyrus in the hippocampi of APP-transgenic mice (Tg9291) before and after development of amyloidosis-beta, that is, in mice aged 2-4 months and 9-12 months, respectively, and in age-matched controls; and (2) in culture of NPs isolated from brains of control and Tg9291 mice, aged 3 and 9 months. We found that the number of proliferating NPs was reduced in 9-12-months-old mice, in both control and Tg9291, as compared to 2-4-months-old mice. However, the 9-12-months-old Tg9291 mice with amyloid-beta deposits had significantly more proliferating NPs than the age-matched controls. NPs proliferation in culture did not depend on the age, presence of APP-transgene, and amyloidosis-beta in donors. The results indicate that the local brain environment influences proliferation of NPs, and development of amyloidosis-beta in the neurogenic regions attenuates the age-associated reduction of proliferation of NPs. Identification of the responsible mechanisms may be important for development of a successful therapy of neurodegeneration caused by amyloidosis-beta.

Migration of neural precursor cells derived from olfactory bulb in cochlear nucleus exposed to an augmented acoustic environment

The regeneration of the auditory neural system remains a challenge in hearing restoration. Acoustic signals may induce a site-specific cell replacement in the auditory system. This hypothesis was tested with grafted implantation of neural precursor cells (NPCs) along the cochlear nucleus in the adult host followed by an augmented acoustic stimulation. NPCs were obtained from the olfactory bulbs at embryonic day 14-16 and were transplanted into the inside border of cochlear nucleus. The labeled cells survived at least 2 weeks, verified by Hoechst 33342 fluorescence, and by immunostaining for a neuronal marker. In some cases NPCs had migrated directionally to the root of the auditory nerve. This observation demonstrates the survival and migration of NPCs from the olfactory bulb (OB) along the adult auditory nerve in an augmented acoustic environment following implantation.

A cell therapy approach to substitute neural elements in the inner ear

Three different donor tissues were tested for their capacity to survive, integrate and differentiate in the adult inner ear. Surviving embryonic dorsal root ganglion cells were found within the spiral ganglion neuron region and along the auditory nerve fibers. In the presence of exogenous nerve growth factor (NGF), the dorsal root ganglion cells formed extensive growth of neurites that seemed to contact the host neurons. Adult neural stem cells survived relative poorly in the inner ear whereas embryonic stem cells showed a somewhat greater capacity for survival and integration. Overall, the survival rate of implanted tissue was quite low in the cochlea. It is concluded that an inner ear cell therapy approach based on the implantation of exogenous cells will require that important survival factors are identified and supplied. In addition, it is possible that the physical properties of the cochlea, e.g., fluid-filled compartments and very limited space for cell proliferation, are unfavorable, at least in the normal cochlea.

Neprilysin protects human neuronal progenitor cells against impaired development caused by amyloid-β peptide

Transplantation of human neuronal progenitor cells (HNPC) is being considered for neuroreplacement therapy in beta-amyloidosis associated with neuronal loss in Down's syndrome and Alzheimer's disease. However, the influence of amyloid-beta-containing brain environment on the development of HNPCs is unknown. Recently, we demonstrated that amyloid-beta peptide (Abeta) impaired differentiation of HNPCs in culture through oxidative stress. Now we studied the effect of neprilysin, an Abeta-degrading enzyme, on development of neuronal colonies from neurospheres of HNPCs in the presence of Abeta1-40. Neprilysin increased the number of neurospheres that formed colonies of neuron-like cells. This effect of neprilysin was associated with reduced amounts of the monomeric and dimeric Abeta that remained in culture supernatants as well as the Abeta uptaken by differentiating HNPCs. Phosphoramidon, a neprilysin inhibitor, attenuated these effects of neprilysin. In control cultures of HNPCs that grew without exogenous Abeta1-40, the treatment with neprilysin reduced the number of developing colonies. This effect might result from degradation by neprilysin of endogenous Abeta produced and secreted by HNPCs or other peptides that are involved in neuronal development. The results demonstrate that even a partial reduction of extracellular Abeta levels by neprilysin may facilitate development of HNPCs into neurons in an environment overloaded with Abeta. This finding suggests that neprilysin could facilitate neuroreplacement therapy with HNPCs in treatment of neurodegenerative diseases.

Cell Replacement Therapy in the Inner Ear

The mammalian inner ear is vulnerable to genetic disorders and aging, as well as to injuries caused by overstimulation, ototoxic drugs, and viral infections. Due to the poor regeneration of the sensory epithelium and the spiral ganglion neurons in the adult mammalian inner ear, cell replacement therapy strategies have been proposed to compensate for degeneration and loss of sensory and neuronal cells. Transplantation of stem cells and embryonic neurons into the inner ear has revealed that exogenous cells can survive, migrate, differentiate, and extend neuritic projections in the auditory system of adult mammals. These results suggest that cell replacement therapy could provide an effective future treatment alternative for hearing loss and other inner ear disorders.

Amyloid-beta impairs development of neuronal progenitor cells by oxidative mechanisms

Neuronal progenitor cells (NPCs) are being considered for treatment of neurodegenerative diseases associated with beta-amyloidosis: Alzheimer's disease (AD) and Down syndrome (DS). However, the neurotoxic properties of amyloid-beta peptide (Abeta) may impair survival and differentiation of transplanted NPCs. Hence, we studied the influence of Abeta on development of human NPCs--proliferation, migration, formation of colonies of neurons, formation processes--in culture. Pre-fibrillized human Abeta1-40 blocked development of neuronal colonies. NPC development was impaired in the presence of soluble Abeta1-40 (1.75-7 microM), and NPC differentiation into large and small neurons was altered, as demonstrated by morphometry. Antioxidant vitamin E partially abolished these effects, but not the reduced formation of neuronal processes. NPCs cultured with 7 microM Abeta1-40 accumulated Abeta monomers and oligomers and contained higher levels of protein carbonyls and lipid peroxidation products HNE and MDA. We suggest that Abeta1-40 impairs development of NPCs by oxidative damage. Hence, a prerequisite of successful neuroreplacement therapy using NPCs in AD and DS/AD may be removal of amyloid-beta and antioxidative treatment.

Cell therapies for neurological disease – from bench to clinic to bench

The lack of any meaningful regeneration in the adult central nervous system (CNS) subsequent to damage or degeneration stimulated the concept of replacement of the deficient cells by transplantation. Thus, much time and effort has been spent on investigating the potential of cell replacement therapy for repair in a range of conditions of the CNS over the last 25 years. As promising proof of principle basic science results were slowly converted to success in clinical transplantation trials in Parkinson's disease (PD), the future seemed very encouraging for cell therapy. However, the recent randomised, double-blind, placebo-controlled studies of fetal neural transplantation in PD have produced more equivocal results, which has dampened enthusiasm for this approach. However, whilst the translation of cell therapies to the clinic is in limbo, the emergence of stem cells as a source of the replacement tissue has revitalised the laboratory-based studies. This paper attempts to reconcile these disparate views and put forward the authors' view on the future of this form of biological therapy and its implications for related therapies.

Central migration of neuronal tissue and embryonic stem cells following transplantation along the adult auditory nerve

The regeneration of the auditory nerve remains a challenge in restoring hearing. An interesting approach would be to use a cell replacement therapy with the potential to establish connections from the inner ear to the central auditory system. This hypothesis was tested by xenografted (mouse to rat) implantation of embryonic dorsal root ganglion (DRG) neurons and embryonic stem (ES) cells along the auditory nerve in the adult host. DRG neurons were obtained at embryonic day 13-14 in transgenic animals expressing enhanced green fluorescence protein (EGFP). For embryonic stem cells, a tau-GFP ES cell line was used as a donor. The fibers of the auditory nerve in the adult rat were transected through the modiolus at the first cochlear turn, and the biological implants were transplanted into the transection. The transplanted DRG neurons and ES cells survived for a postoperative survival time ranging from 3 to 9 weeks, verified by EGFP/GFP fluorescence, and neurofilament or TUJ1 immunostaining. At 9 weeks following implantation, the implanted DRG neurons were found to have migrated along the auditory nerve in the internal meatus. At the same postoperative time, the ES cells had migrated into the brain stem close to the ventral cochlear nucleus. The results demonstrate not only the survival and migration of xenografted DRG neurons and stem cells along the adult auditory nerve but also the feasibility of a cell replacement therapy in the degenerated auditory system.

Survival and neural differentiation of adult neural stem cells transplanted into the mature inner ear

The cochlear sensory epithelium and spiral ganglion neurons (SGNs) in the adult mammalian inner ear do not regenerate following severe injury. To replace the degenerated SGNs, neural stem cell (NSC) is an attractive alternative for substitution cell therapy. In this study, adult mouse NSCs were transplanted into normal and deafened inner ears of guinea pigs. To more efficiently drive the implanted cells into a neuronal fate, NSCs were also transduced with neurogenin 2 (ngn2) before transplantation. In deafened inner ears and in animals transplanted with ngn2-transduced NSCs, surviving cells expressed the neuronal marker neural class III beta-tubulin. Transplanted cells were found close to the sensory epithelium and adjacent to the SGNs and their peripheral processes. The results illustrate that adult NSCs can survive and differentiate in the injured inner ear. It also demonstrates the feasibility of gene transfer to generate specific progeny for cell replacement therapy in the inner ear.

[Neurogenesis in the adult brain: the demise of a dogma and the advent of new treatments]

Since the early sixties, many concepts concerning neurogenesis have been progressively ruled out. Proof of the persistence of a physiological neurogenesis in adult mammals, including humans, raised the concept of a unique precursor cell giving birth to neurons and glial cells. According to this concept, a real continuum between neuroepithelial cells, radial glia and astrocytes exists from the embryonic period to adult age and generates both neurons and glial cells. Different factors, either secreted in situ or transported by blood, can influence this physiological neurogenesis process. The targets and role of newborn neurons are not clearly understood. In pathological conditions (ischemia, epilepsy, lesions), the physiological neurogenesis process is enhanced; however the significance of this neurogenesis excess (beneficial or deleterious) is not completely known. Advances in understanding the regulation of neurogenesis in these different conditions represent hopes of new therapeutic procedures, not only by improving the control of differentiation and survival of transplanted stem cells, but also by the possibility of modifying the processes of "endogenous neurogenesis".

Is there a future for neural transplantation?

Traditionally neural transplantation has had as its central tenet the replacement of missing neurons that have been lost because of neurodegenerative processes, as exemplified by diseases such as Parkinson disease (PD). However, the effectiveness and widespread application of this approach clinically has been limited, primarily because of the poor donor supply of human fetal neural tissue and the incomplete neurobiological understanding of the circuit reconstruction required to normalize function in these diseases. So, in PD the progress from promising neural transplantation in animal models to proof-of-principle, open-labeled clinical transplants, to randomized, placebo-controlled studies of neural transplantation has not been straightforward. The emergence of previously undescribed adverse effects and lack of significant functional advantage in recent clinical studies has been disappointing and has served to cast a new, and perhaps more realistic, perspective on this treatment approach. In fact, there have been calls by some involved in neural transplantation to return to the drawing board before pressing on with further clinical trials, and the return to basic experimentation. This therefore precipitates the question - is there a future for neural transplantation? It is important to remember that there are a number of possible explanations for the disappointing results from the recent clinical trials in PD, ranging from the mode of transplantation to patient selection. Nevertheless, almost irrespective of these reasons for the current trial results, there have always been significant practical and ethical problems with using human fetal tissue, and so a number of alternative cell sources have been investigated. These alternative sources include stem cells, which are attractive for cell-based therapies because of their potential ease of isolation, propagation and manipulation, and their ability in some cases to migrate to areas of pathology and differentiate into specific and appropriate cell types. Furthermore, the availability of stem cells derived from non-embryonic sources (e.g. adult stem cells derived from the sub-ventricular zone) has removed some of the ethical limitations associated with the use of embryonic human tissue. These potentially beneficial aspects of stem cells means that there is a future for neural transplantation as a means of treating patients with a range of neurological disorders, although whether this will ever translate into a truly effective, widely available therapy remains unknown.

Stem cell therapy for neurodegenerative diseases: the issue of transdifferentiation

In the past few years research on stem cells has exploded as a tool to develop potential therapies to treat incurable neurodegenerative diseases. Stem cell transplantation has been effective in several animal models, but the underlying restorative mechanisms are still unknown. Several events such as cell fusion, neurotrophic factor release, endogenous stem cell proliferation, and transdifferentiation (adult cell acquisition of new unexpected identities) may explain therapeutic success, in addition to replacement of lost cells. This issue needs to be clarified further to maximize the potential for effective therapies. Preliminary stem transplantation trials have already been performed for some neurodegenerative diseases. There is no effective pharmacological treatment for amyotrophic lateral sclerosis, but recent preliminary data both in experimental and clinical settings have targeted it as an ideal candidate disease for the development of stem cell therapy in humans. This review summarizes recent advances gained in stem cell research applied to neurodegenerative diseases with a special emphasis to the criticisms put forward.