Moving from the Dish to the Clinical Practice: A Decade of Lessons and Perspectives from the Pre-Clinical and Clinical Stem Cell Studies for Alzheimer’s Disease

Abstract

To date, there is no definitive treatment for Alzheimer’s disease (AD). The realm of stem cells is very promising in regenerative medicine, particularly neurodegenerative disorders. Various types of stem cells have been used in multiple trials on AD models, trying to find an innovative management of this disease. In this systematic review, we trace the published preclinical and clinical data throughout the last decade, to show how much knowledge we gained so far in this field and the future perspectives of stem cells in AD treatment.

Keywords

Alzheimer’s disease clinical trails neurodegenerative diseases preclinical trials stem cells

INTRODUCTION

Since the development of the cell theory by Mattias Schleiden then Theodor Schwann in 1839, and after Rudulf Virchow popularized the slogan: Omnis cellula e cellula (All cells come from cells) in 1855, cell division has been considered the most fundamental process in the development of living organisms [1]. The cell undergoes diverse processes throughout development that ensures maintenance of organs and tissues integrity [2, 3].

In order to sustain a balance between cell loss and replacement, cells should be capable of self-renewal as well as differentiation. Stem/progenitor cells first described in 1964, lying at the heart of this process as the functional units of regeneration, are clonogenic populations that can give rise to various cell lineages [4, 5]. As their therapeutic potential has already been demonstrated in different settings, stem cells novel applications for clinical and preclinical trials are of great interest, which will hopefully enable the promise of regenerative medicine to be accomplished [6, 7].

Currently, the incurable neurodegenerative diseases reached a staggering prevalence [8]. For example, in the United States, the prevalence of Alzheimer’s disease (AD) is estimated at 4.7 million cases and is expected to continue increasing dramatically [9]. The lack of definitive treatment to such disorders made it imperative for new innovative forms of management, to ameliorate their crippling effects [10].

AD is the most common form of neurodegenerative dementia accounting for about 60% to 70% of cases. AD is associated with several distinct neuropathological features, mainly extracellular amyloid-beta peptide (Aβ) plaques, intracellular neurofibrillary tangles due to deposition of hyperphosphorylated tau protein, and neuron loss with degeneration of the temporal lobe, parietal lobe, and parts of the frontal cortex and cingulate gyrus. Clinically, it shows a progressive impairment of learning, memory, language abilities, disorientation, behavioral problems, body functions decline, and ultimately death. Several hypotheses mentioned risk factors related to AD development such as genetics, neurotransmitters malfunction, and environmental factors, including head trauma and others. Currently, treatment for AD is only symptomatic and there are no disease-modifying therapies [11 –13].

As a proof of concept, stem cells appeared to be promising in the field of AD. The multifactorial complex nature of AD pathogenesis, in addition to the limited efficacy of the current pharmacological and immunological strategies, increased the necessity for novel methods to tackle the disease. This led to the application of various forms of stem cell lines in numerous studies, ranging from basic lab research to preclinical studies and clinical trials, hoping for an innovative stem cell treatment for AD [14].

Due to the actively evolving nature of both stem cell biology and neurogenesis research, we performed a state-of-the-art review tracing preclinical and clinical studies that dealt with stem cells therapeutic role in AD. Here, we aimed to highlight crucial questions as: How far did we progress? What are the challenges we still have? Which cell lines are more promising? Are we ready for a definitive treatment?

METHODS

This is a state-of-the-art review of published data regarding potential use of stem cells in AD treatment.

Inclusion and exclusion criteria

Our inclusion criteria entailed all scientific peer-reviewed original articles (preclinical and clinical trials) published from 2006 until February 2016. As exclusion criteria, we omitted all bench side studies, reviews, editorials, communications, opinions, letters, news, and reports. Also, any study that used concomitant treatment with stem cells or entailed indirect cell use (e.g., cells’ medium, endogenous stem cell population activation or inflammatory mediators derived from the cells) was excluded. This systematic review focused on direct cells application studies only. We used several electronic data-bases (PubMed, Google Scholar, Elsevier ScienceDirect, SpringerLink and PsycINFO) and the keywords “STEM CELLS, NEURODEGENERATIVE DISEASES, ALZHEIMER’S, PRECLINICAL, CLINIAL TRIALS” for search. In addition, we searched the http://www.clinicaltrials.gov website to compare the results of our search with the existing ongoing or finished clinical trials (Table 2). All the results were double checked to ensure they meet the mentioned criteria.

It is should be noted that almost all trials regarding the use of the pluripotent stem cells are bench side studies. Thus, we decided to include some bench side studies in a short paragraph on the topic about pluripotent cells to be more comprehensive regarding all the potentially beneficial stem cells types.

Data handling

Different levels of categorizations were done. We divided the articles into clinical and preclinical, then we further categorized them according to four important parameters (variables): year of publication, AD model (animal or human subjects), type of stem cells used, and route of cells administration. Published articles from the same research group that present different data regarding a research trial are treated as separate articles.

RESULTS

Our search yielded 65 articles published between 2006 and 2016 fulfilling our criteria (Fig. 1), in addition to 13 listed clinical trials on http://clinicaltrials.gov (listed in Table 2). One of the studies compared the use of two separate groups of animals with two different routes of cells injection each with its own control group (4 groups in total), therefore this study was considered as two independent studies. Collectively after adjustment, 66 studies were further analyzed (Table 1). Only one clinical study was found, while the remaining studies were all preclinical ones.

DISCUSSION

Cell types

Lines

Studies have used different types of stem cells for transplantation (Fig. 2). We found that the most used cell types were mesenchymal stem cells (MSCs, 37.9%) and neuronal stem cells (NSCs, 36.4%).

Mesenchymal stem cells. The MSCs are the most studied adult stem cell type since their discovery by Friedenstein and co-workers, especially in age-related disorders [15]. First identified in the bone marrow [16], MSCs are currently being harvested from almost all tissues, such as adipose tissue [17], olfactory bulb [18], placenta [19], cord blood [20], and amniotic fluid [21]. Upon transplantation, MSCs showed the capacity to rapidly home to ischemic sites [22]. They have long proven ability to trigger immune responses via microglia activation in various AD transgenic mice models [23]. In a very elegant study, MSCs have been shown to modulate cytokines and brain inflammation in AβPP/PS1 mice, and authors could prove for the first time the negative correlation between MSC induced microglial activation with high anti-inflammatory cytokines expression on one side and Aβ deposits decrease and tau phosphorylation reduction, mostly in the hippocampus, on the other side [24]. In another debatable study, it was shown that MSCs direct transplantation into the hippocampus can improve cognition in C57BL/6 mice, as well as augmentation of hippocampal neurogenesis, allegedly through Wnt signaling pathway, thus enhancing endogenous repair [25]. MSCs that overexpress the vascular endothelial growth factor was shown to improve neovascularization and the clearance of Aβ, ultimately leading to memory and learning deficits recovery in 2xTg-AD animals [26]. However, a major concern in the study was that in older animals (12 months), the condition was worsened by the accentuated neurodegeneration caused by the AD genotype. Moreover, the robust effect of the VEGF in improving social memory deficit seen in younger animals was attenuated in older animals and no reverse of the impairment of the social recognition memory was accomplished at that age.

In an interesting study set to investigate the potential role of MSCs in treating early AD, high-risk pre-dementia AβPP mice models were treated with MSC from the bone marrow. Two months post injection, a reduction in Aβ deposits and changes in key proteins required for synaptic transmission were detected, suggesting a potential early interventional therapy in prodromal AD by bone marrow-MSC transplantation [27].

In 2015, Kanamaru and colleagues proposed the use of cells other than MSCs that usually takes long culture time for preparation. Therefore, they used bone marrow mononuclear cells (BMMNCs), which do not need such culture time, containing MSCs together with other forms of cells [28]. Still, issues with these cells lines unwanted proliferation or differentiation, and the need for irra-diation or sometimes other toxic preconditioning to achieve proper engraftment [29]. Nevertheless, there are concerns regarding severe side effects resulting from MSCs use, such as imm-unogenicity and potential tumourgenicity [30, 31].

Neuronal stem cells. Notwithstanding the remarkable results obtained with the use of MSCs, research involving neurodegenerative diseases have pointed their efforts to NSCs. Indeed, NSCs transplantation became one of the main lines used for AD research (Fig. 2). The robust migratory capacity of NSCs provided a compelling approach to deliver their therapeutic effect to the brain. The subventricular zone of the lateral ventricles is considered the largest source of NSCs in the adult mammalian brain [32]. NSCs have proved in multiple settings to be safe and effective when transplanted into the hippocampus, improving cognitive functions, synaptic connectivity and neuronal survival in animal models of AD [33]. Genetically modified NSCs expressing the proteolytic enzyme neprilysin that underlies the endogenous degradation of Aβ, led to marked and significant reduction in the pathology of AβPP transgenic mice. A major concern, particularly in the study done by Spencer and colleagues, was the long-term side effects associated with the vector assisted delivery method (lentivirus, adenovirus, or herpes simplex virus) they used to deliver the neprilysin to the target areas. It was also unclear the reason of the adenovirus delivered neprilysin localization at the presynaptic sites, whereas the lentivirus localization was in the cell soma and the endoplasmic reticulum [34 –36]. Transplantation of neuronal precursor cells (NPCs), derived from mouse embryonic stem cells (ESCs), was shown to promote behavioral recovery in a nbM-lesioned rats AD model following commitment to a cholinergic cell phenotype [37]. Unlike other previous studies, it clearly showed an increase in the number of cholinergic neurons after treatment. Still, the question whether NPCs is only a cholinergic survival factor or also contributes to differentiation of cholinergic neurons is yet to be elucidated.

In an in vitro induction study, the combined effect of both nerve growth factor and brain derived neurotropic factor (BDNF) on NSCs differentiation led to increased neurogenesis rate, which may be interesting for future clinical applications, especially for dosage optimization [38]. In addition, NSCs showed significant neuroprotective effect against AD inflammation by suppressing glial and toll-like receptor-4 (TLR4) mediated inflammatory pathway activation, leading to down-regulation of the proinflammatory mediators [interleukin-1 (IL-1), IL-6, tumor necrosis factor alpha (TNFα)] in AβPP/PS1 mouse model, thus ameliorating the cognitive deficits, but with no change in Aβ concentration [39]. It is worth noticing the amelioration of cognitive deficits without difference in Aβ concentration, but associated with the attenuation of the inflammatory response, which warrants further investigation.

Other hypothesis regarding NSCs mechanism of action lies on their ability to enhance mitochondrial biogenesis via increasing the number of mitochondria, and mitochondrial proteins (dynamin related protein 1, mitochondrial fission 1 protein, optic atrophy 1) expression in AβPP/PS1 mice models, thus rescuing the cognitive outcome [40]. These results are of pivotal relevance as mitochondrial bioenergetic deficits usually precede AD pathogenesis, thus adding functional mitochondria through NSC transplantation may be a promising therapeutic strategy. Kim and colleagues showed that NSCs transplantation at the early stages of the disease improved the learning and memory ability and reduced Aβ plaque load and tau hyperphosphorylation in Tg2576 mice. In addition, NSCs transplantation decreased microglial activation [41]. However, it should be mentioned that in advanced stages of the disease, NSCs failed to restore behavioral/cognitive deficits, which correlates well with previous results in older stages of the disease.

As an attempt to identify a suitable line for clinical application, a study used human neural stem cell population (HuCNS-SCs) transplantation in animal models of neurodegeneration. In 3xTg-AD mice, HuCNS-SCs proved to ameliorate context and place dependent learning (tested using the novel subject recognition test). In addition, HuCNS-SCs improved spatial learning and memory impairments (tested with the Morris water maze test, MWM) with proper cell survival and engraftment detected by immunofluorescence and confocal microscopy. The intriguing observation was the improvement in MWM probe trial and novel subject recognition test performance, with no effect on MWM acquisition, suggesting specificity of the processes involved in memory formation and consolidation. Moreover, cognitive improvements were not associated with altered Aβ or tau pathology. Rather, human NSC transplantation seems to improve cognition by enhancing endogenous synaptogenesis [42].

It is important to highlight that despite NSCs and NPCs appear to overlap to a certain degree in terms of gene expression, they can be clearly distinguished based on differences in notch signaling and functional attributes. NPCs collectively describe a mixed population of both NSCs and neural progenitor cells. Neural progenitor cells are generally capable of limited transient self-renewal with generation of one type of cell, in contrast to the highly plastic NSCs that can maintain unlimited lifetime differentiation along the three known cell lineages (multipotent) [43].

Human umbilical cord blood cells (HUCBCs). HUCBCs transplantation has been therapeutically beneficial in many neurodegenerative disorders [44]. Despite acknowledged as clinical waste, HUCBCs provide an alternative source for plenty of MSCs [45]. They have been suggested to modulate the peripheral inflammatory processes, which in turn affect inflammation in the brain parenchyma, and the mobilization of adult stem cells from the bone marrow, leading to extension of the lifespan in AβPP mice [46, 47]. Multiple low doses of HUCBCs were shown to improve cognitive impairment, reduce Aβ-associated neuropathology, and improve the motor skills in long term treated AD mice [48]. Despite the usage of multiple doses to detect the optimum one in those studies, the problem of dose optimization is yet to be solved.

Adipose tissue derived stem cells (ADSCs). ADSCs appeared as a new attractive source for stem cells in regenerative therapy as they are readily abundant, easily accessible, with a detected lower senescence ratio compared to bone marrow-MSCs [49]. In addition, they are known for their ability to differentiate into mesenchymal and non-mesenchymal lineages [50]. One study proposed ADSCs as a safe stem cell source for intravenous (IV) injections, owing to the absence of immune rejections, ethical problems or tumorigenesis. [51]. Long term follow-up (up to 26 months post injection) to determine their potential tumorigenicity in nude mice showed ADSCs to be safe even with high doses.

In AD, intracerebral (IC) transplanted or intravenous (IV) administered ADSCs showed dramatic improvement of impaired memory and neuropathology in AβPP/PS1 mice models, with huge reduction of Aβ peptide deposition with microglial activation, thus suggesting that they have a high therapeutic potential for AD [17, 52]. It was very interesting to show that in cases of IV ADSC delivery, the labeled fluorescence magnetic nanoparticles for in vivo live tracking were demonstrated in all brain regions except the olfactory bulb in Tg2576 model [53]. This study highlights once more the importance of the early detection and intervention in AD, as the early onset injected cells produced major changes in the disease progress and pathology, while late onset treatment was minimally effective.

Hair follicle stem cells = Epidermal neural crest derived stem cells (Epi-NCSCs). A very interesting study assessed the ability of Epi-NCSCs derived from hair follicles, which are the multipotent remnants of ESCs, for neuronal differentiation for the first time. By direct stereotactic transplantation of the cells into the hippocampus of Aβ ₄₀ induced AD rat model, Esmaeilzade and colleagues showed that Epi-NCSC can differentiate into neurons in vivo, offering them the potential for treating neurological diseases with significant advantages over other NSCs sources, as they are readily accessible, with no risks of obtaining NSCs from the brain, and provide a renewable population for autologous transplantation [54]. Yet, further studies are needed to determine the fate of the newly differentiated neurons.

Induced pluripotent stem cells (iPSCs). Since their discovery in 2006 by Yamanka and Takahashi [55], a huge interest focused on induced pluripotent stem cells (iPSCs) for various reasons, such as their high capacity for scaling up and the potential ability to get autologous cells, thereby reducing or eliminating the need for immune-suppression. Recently, xeno-free clinical grade iPSCs have been generated, moving this approach several steps closer to clinical reality [56]. It is worth mentioning that iPSCs exhibit molecular and functional characteristic similar to those of the ESCs, but unlike ESCs, iPSCs raise no major ethical concerns regarding their source. Moreover, they provide autologous cells for cell-based therapies [57]. In spite of the huge potential of iPSCs to reach therapies in neurodegenerative disease, trials on iPSCs are still limited to the basic laboratory boundaries [58, 59].

In summary, while progress has been made last years, the best cell line to be used is still a point of argument. Noteworthy, MSCs and NSCs together accounted for almost 75% of the cases (37.9% and 36.4% respectively). An important question arising from the complex pathological nature of AD is: when to inject the cells? Early injection proved to be more beneficial in protecting and/or reversing the pathology, unlike the late stages management that is not as promising so far.

Cell routing

The best route through which stem cells can reach the target brain tissues for neurogenesis is still a matter of debate despite numerous efforts in elucidating it. It is noteworthy that over 70% of the studies used the IV route (Fig. 2).

It is yet intuitive to assume that to deliver cells to a particular organ, direct implantation techniques should be the best. The safety and tolerability of stereotactic administration was proven by a phase-one clinical study for gene delivery in AD patients [60]. A recent phase-one clinical trial used the same reported successful stereotactic surgery to deliver MSCs intracerebrally, directly into the hippocampus and the precuneus, which was proven to be feasible, safe, and well tolerated [61]. Despite lacking a control group in both studies and the small study sample (10 and 9, respectively), they both proved interventional safety, thus opening the gate for future more controlled wide scale evidence-based clinical trials.

Choosing a cell delivery route for clinical translation is a huge challenge, since the human brain is considerably larger than the mouse brain (the most used animal model for AD). Peripheral delivery of cells and proteins appears to be less effective than intracerebral injections. For instance, a study found that IV delivery route could reduce plasma Aβ, but failed to clear Aβ plaques within the brain [62].

Kanamaru and colleagues rationally debated the need for other less invasive ways of cell delivery, particularly in humans [28]. Based on the theory of blood-brain barrier disruption and leakage in AD patients [63, 64], lots of researchers have constructed their preclinical experiments using the IV route for cell delivery, as an easier and less invasive method. Although Lee and colleagues in 2008 postulated that NSCs are best delivered IV than IC, they also mentioned that in case of MSCs IV injection, most of the injected cells were found in the splenic marginal zone, rather than homing in the brain tissues [65]. This was also consistent with the results of IV injection of cord blood cells that were found all over the body of a murine model [66]. Moreover, Yang and colleagues first reported that the IV injection of BMMNCs is not inferior to the intra-arterial route. Interestingly, Kanamaru and colleagues also reported the inability to detect the IV injected cells in brain tissues after 3 months of transplantation [28]. Such reports illustrate the unpredictable fate of the IV route, and the minimal percentage of localization of the injected cells centrally (in the brain), despite its apparent efficacy, which render this technique debatable.

Direct intra-arterial stem cells transplantation has been used mainly for stroke and ischemic diseases to induce functional recovery in the acute therapeutic window [67, 68]. In 6-hydroxydopamine (6-OHDA)-induced animal model of Parkinson’s disease, MSCs were directly infused in the carotid artery. Systemically injected cells could not efficiently cross the blood-brain barrier to home in the nigrostriatal region unless another agent is given with them facilitating their permeability [69]. Such technique is not employed so far for stem cells delivery in AD. However, preclinical trials used this route in AD models to deliver Monodisperse Iron Oxide Nanoparticles (MIONs) targeting the cerebrovascular amyloid deposits that are found in approximately 90% of patients with AD [70]. The same research group applied it for the delivery of antibody fragments for immunotherapy [71]. Still, this technique is far from optimization for clinical practice.

Other less commonly utilized routes were suggested, even without a clear or convincing justification for their use. For instance, it was tried a direct intra-cardiac injection in a trial to prove the systemic effectiveness as an alternate pathway for cells delivery [20].

A recent study postulated the intranasal (IN) route for cell delivery. In this study, the authors showed that after 7 days of IN application of MSCs, cells were predominantly distributed within the olfactory bulb, hippocampus, cortex, and cerebellum in two different animal models, a Thy1-h[A30P] αS model for Parkinsonism and an AβPP/PS1 model of AD [72]. To achieve functional improvement using such technique, dosage, number of injections and time of treatment should be further investigated for optimization.

Cell interactions

Stem cells’ mechanism of action

Several studies have shown that stem cells trigger anti-inflammatory reactions that activate microglia [73, 74]. The microglia was shown to increase the expression of Aβ degrading enzymes such as insulysin and neprilysin, thus helping in improving AD manifestations [75]. One study postulated that a stem cell-induced neurotrophic factor, namely BDNF, is responsible for the improvement in cognition in 3xTg-AD mice model [76]. It was very interesting in this study to show that the NSCs per se did not induce any direct cognitive beneficial effects via alteration of Aβ or tau pathology, as hypothesized previously in literature. In contrast, NSCs induced elevation in the hippocampal levels of BDNF, leading to increased synaptic density and restoring hippocampal-dependent cognition.

In multiple studies, stem cells were shown to upregulate the anti-inflammatory cytokine IL-10 and to downregulate pro-inflammatory cytokines (TNFα, IL-1, IL-6, IL-8, IL-12) [77 –79], in addition to increase expression of vascular endothelial growth factor that is known to have neuroprotective and neurogenic roles [80].

A more recent explanation to stem cells’ effects is the role of the extracellular vesicles as key components of their paracrine effect. Various types of stem cells release ‘secretomes’. Studies have shown that they deliver bio-active cargo to the neighboring diseased or injured cells, modifying the cellular receptors function, inducing anti-inflammatory, immune modulation, angiogenesis, neurogenesis and synaptogenesis [81 –84]. Moreover, recent reports linked them as mediators to intercellular communication in cancer [85], which warrants more elucidation on their mechanism of action.

In spite of the numerous trials along the past decade to unravel the actual mechanism by which stem cells induce their effect, to date, it is still largely unknown and mostly hypothetical. Furthermore, despite their promise, secretomes clinical application still faces several challenges, including their isolation and production techniques (time consuming ultracentrifugation is the only available technique so far) and heterogeneity.

In vivo stem cell tracking

One of the biggest challenges that face all sorts of stem cells therapies is the lack of reliable in vivo imaging methods to evaluate the biological interactions of transplanted cells [86].

Nanomedicine and nanoparticles-based delivery systems for stem cells are very interesting. Superparamagnetic iron oxide has been used for decades to track stem cells in vivo after in vitro labeling. Nanotechnologies revolutionized the high resolution of in vivo imaging for cell tracking [87]. Magnetic resonance imaging (MRI), positron emission tomography (PET), or single-photon emission computed tomography are used to visualize the labeled stem cells in vivo. MRI imaging has the advantage of high spatial resolution and anatomical information but its major drawback is the limited sensitivity [88 –90]. PET scans are highly sensitive but have low spatial resolution, poor in anatomical delineation, with notable short half-life of the radioisotopes [91, 92]. Fluorescence imaging is another noninvasive imaging method for in vivo tracking. It is also highly sensitive with good resolution at the subcellular level, but suffers from limited penetration power even with the prolonged release dye techniques [93, 94]. Near-infrared fluorescence imaging (NIRFI) using NIR fluorophores improved penetration depth and provided more specific signals, but still has a major drawback of photobleaching [95, 96]. Several multimodal imaging techniques have been developed to overcome the limitations of each single method [97 –99].

Notwithstanding the advances achieved in nanotechnology field, still proper methods of in vivo cell tracking and reporting are far from optimization for reliable use in human subjects.

AD modeling

Preclinical trials use small animal models (mice or rats). Mice models, the most commonly used (72.7%), focused on the overexpression of familial AD associated mutant genes, particularly amyloid precursor protein, presenilin-1, and presenilin-2 [100]. Still, it was shown that the expected outcomes reported by the previous preclinical trials are not consistent with the clinical studies. This was attributed to fundamental discrepancies including the anatomical, pathophysiological and micro-environmental differences in addition to the divergent reporting mechanisms used in humans compared with animals [101, 102].

This problem in particular gave momentum to the uprising iPSCs modeling technologies. Israel and colleagues described the initial iPSCs based in vitro AD model in 2012 focusing on replicating already known familial and sporadic aspects of the disease [103]. Genomic editing enabled the creation of patient specific (isogenic) cell lines. ‘Alzheimer’s in a dish model’ developed by Choi and colleagues marked a cornerstone progress in AD modeling [104, 105]. Based on the rational of the 3D model, Zhang and colleagues developed a 3D-iPSCs derived neuronal model for AD [106]. The importance of 3D-iPSCs model technology stems from their ability to differentiate into multiple neurologic cell types. As these derived cells carry all the information packed in the genome of the patient from where they are derived, they allow disease testing, drug screening and possible treatments modalities, in addition to predicting the possible patient’s interaction with treatments. So far, most iPSC-AD modelling trials have constructed either embryoid body/neurosphere or small molecule-based neuronal differentiation known for glutamatergic cortical forebrain neuron generation. In terms of gene and neuronal marker expression, these cortical neuron cultures comprise a mixture of different cell types of variable maturity levels, such as neurons, astrocytes, oligodendrocytes, and other brain-derived cells. As these diverse cells carry the genomes of patients with AD, most closely resembling early human fetal neurons, expressing AD phenotypes such as increased Aβ ₄₂ production and tau-phosphorylation changes, they are very important for in vitro study models. Very interestingly, compounds that inhibit gamma-secretase activity were effective at reducing Aβ production in AD iPSC-derived neuronal cultures, a concept used in drug screening studies. This will hopefully lead to rational selection of the most successful pharmacological strategy for preventing early AD changes [107].

This new stem cells frontier will enable scientists to genuinely study human-based AD pathology and its response to genetic and pharmacologic manipulation, for more effective future clinical applications.

CONCLUSION AND FUTURE PERSPECTIVES

Cell based therapies offer promising results, but it is clear that they still need more research to optimize lots of hanging problems around them. On one hand, the potential of regeneration of brain tissue using stem cells is sure. On the other hand, we still do not know the best way to deliver these cells to the brain. The complexity of neurodegenerative diseases results in difficulty in optimizing a single direct cell based therapy to reverse all the effects, particularly AD, in which there is no uniform disease progression with multiple levels of pathological changes incorporated along different time points (early, intermediate, and late courses). This is one of the major hurdles that face a potential cell based therapy to such complex disease process. NSCs, claimed to be the best and most effective type for AD management, need proper optimization regarding doses and mode of delivery and whether a concomitant treatment should be administered or not.

In addition, we learned that stem cells are effective, but we are still incapable of clearly identifying their mechanistic effects as well as detect them in vivo after transplantation, which is in our opinion the ultimate hurdle facing their clinical applications. It is paramount to be able to trace your treatment, to detect its dosage level, its toxicity and cellular interactions, not just administer it and applauding an outcome that we are not sure of its cellular level biochemical processes yet.

In iPSCs based therapy, the first landmark clinical trial on humans was launched in 2014 to treat macular degeneration [108, 109]. Even though it started favorably with their first patient, an undefined genomic mutation occurred with the iPSCs of the second patient, which forced them last year to officially suspend the trial till further notice. This brings us back to the point of the real comprehensive understanding of the cellular level based interactions of such cells.

The complex nature of the human brain and its interconnected circuit of neurons make it unlikely for an individual stem cell type or other current management modality to be fully effective in AD treatment single-handedly. Most likely, a future cure will be based on a multimodal approach at different intervals, which will hold the collective benefits of optimized stem cell technology, probably via their medically promising secretomes, in addition to the advances in neurogenesis induction.

DISCLOSURE STATEMENT

Authors’ disclosures available online (http://j-alz.com/manuscript-disclosures/16-0250r2).

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