Neural Stem Cell Therapy for Alzheimer’s Disease: A-State-of-the-Art Review

Abstract

Alzheimer’s disease (AD) is a brain disorder that is more prevalent in developed nations and remains one of most intractable conditions so far. It is characterized by a gradual onset, a prolonged progression, and an unclear pathophysiology. At the present time, there are no effective treatments available for the disease. However, human neural stem cells (hNSCs) have the capacity to substitute lost neurons in a functional manner, strengthen synaptic networks that have been compromised, and repair the damaged brain. Due to the unavailability of restorative therapeutics, there is a significant global burden on the economy. When it comes to the treatment of neurodegenerative diseases, NSCs provide a potentially game-changing approach to treating Alzheimer’s disease. Through the delivery of trophic factors that promote the viability and regeneration of lost neurons in experimental animals suffering from neurodegenerative disorders, these treatments have the potential to facilitate beneficial recuperation. Positive restorative outcomes may be achieved in a variety of ways, including the replacement of lost cells, the combining of cells, the secretion of neurotrophic factors, the formation of endogenous stem cells, and transdifferentiation. Conversely, there are obstacles that need to be overcome before NSC-based treatments can be used in clinical settings. This review article discusses current developments in the use of neural stem cells (NSCs) for the treatment of Alzheimer’s disease (AD). In addition, we highlight the difficulties and opportunities that are involved with the use of neural stem cell transplant treatment for Alzheimer’s disease.

1. Introduction

One of the most serious neurodegenerative disorders is Alzheimer’s disease, which is defined as a progressive loss of neuronal architecture or functions, including neuronal death. A considerable percentage of the elderly population suffers from Alzheimer’s disease (AD) with severe loss of brain function [1]. As a result, improving AD treatment is extremely important. Current estimates suggest that the global population affected by dementia could reach 152.8 million by 2050 [2]. Despite significant advancements [3,4] in pathophysiology, there are currently no treatments available to alleviate the dismal prognosis of AD patients. Traditionally employed treatments, such as N-methyl D-aspartic acid receptor antagonists and cholinesterase inhibitors, are symptomatic; they always have significant adverse effects and lose effectiveness over time [5]. The majority of alternative therapies, apart from medication, have not yet demonstrated appreciable efficacy in the treatment of AD. The precise signaling mechanisms of brain-derived neurotrophic factors are still unknown despite their significance in preserving synaptic plasticity throughout memory and learning. Moreover, insulin therapy does not always demonstrate meaningful therapeutic benefits in real-world settings despite its potential as a safe, brief symptomatic intervention to delay cognitive decline [6]. By modifying brain cell function and neurometabolic pathways, low-level laser therapy may be able to prevent cognitive impairment; however, the best wavelength, dosage, and intensity for a given patient have not yet been established [7]. The use of neural stem cells (NSCs) to replace or repair damaged neurons has gained attention as a promising treatment option for AD due to advancements in stem cell technology. NSCs are the perfect source for cell treatments for AD since they can self-renew and differentiate into several cell types [8]. Additionally, neurotrophic support can be provided by neural stem cells, which release growth factors and other chemicals that support the survival and functionality of already-existing neurons. This may lessen the likelihood that Alzheimer’s disease-related damage and degeneration will affect the remaining neurons [9]. Multiple neurotrophic factors and anti-inflammatory chemicals can be secreted by neural stem cells, offering a multimodal method of action that can promote neuronal survival, lower inflammation, and improve the environment for brain repair. Traditional therapies frequently focus on specific mechanisms or pathways, but multiple facets of the pathophysiology of the disease can be addressed concurrently by stem cell treatment [10]. NSCs can differentiate into neurons, oligodendrocytes, or astrocytes depending on their positioning within CNS and their developmental stage. There is proof that the striatum and hippocampal regions continue to undergo neurogenesis beyond old age in humans [11]. Combining stem cells with NGF (nerve growth factor) has recently been shown to be a helpful tactic for treating AD by halting the death of cells, promoting the development of cholinergic neurons, as well as aiding in the formation of particular neuronal populations. Numerous animal models have been used to study NGF gene therapy, which has resulted in positive clinical studies for AD patients [12].

The relatively new field of stem cell therapy offers hope for improved AD treatment. The degree of functional recovery in the brain’s central nervous system is improved by stem cell therapy. The depleted neural circuitry could be repopulated and rebuilt with the use of foreign stem cells [13]. Because neuroinflammation is a major factor in the growth of injuries and brain damage that ultimately results in cognitive decline, stem cell therapy has the potential to diminish neuroinflammation, which is particularly relevant for patients who acquire AD after aneurysmal subarachnoid hemorrhage [11]. In addition to removing aberrant protein breakdown and neurofibrillary tangles, stem cell treatment can enhance cognition by promoting mitochondrial transport. Neural stem cells can be involved in AD, especially in its early stages. In recent investigations, the assessment of NSCs on AD has been restricted to histological and behavioral observations. Compared to current therapeutic approaches, stem cell transplantation offers a potentially revolutionary means of treating neurodegenerative diseases [14].

Preclinical studies on stem cell therapy for AD should prioritize safety in the future, as well as determine the best ways to source and deliver cells and evaluate how well stem cells work in the degenerate microenvironments of the AD-affected central nervous system [15]. Stem cell-based therapies increase the coherence of previously injured neural networks, improving memory and cognition, even though they cannot adequately replace lost neurons directly. Given the intricate pathophysiology of AD, the ideal course of treatment would involve a variety of approaches, including pharmaceutical methods in addition to external neuron replacement and endogenous synaptogenesis/neurogenesis activation [16]. Neurogenesis and angiogenesis both depend on the integrity of the neurovascular system. Therefore, using vascular progenitor cells in conjunction with NSCs to repair damaged brain arteries and recover damaged neurons may be a more successful way to treat AD than using NSCs alone [17]. Because stem cells may replace damaged neurons and provide neuroprotective and neurorestorative effects, they are likely to play a major role in clinical approaches for the treatment of neurodegenerative diseases in the future. Furthermore, drug administration and revival treatments are now more effective due to recent technological advancements in stem cells utilizing hydrogels and nanoparticles. Therefore, it is anticipated that brain replacement and regenerative therapies will soon be effectively implemented in clinical settings [18].

2. Promise in Other Neurodegenerative Diseases (NDDSs)

A potential therapeutic option for the treatment of numerous neurodegenerative illnesses is the transplantation of NSCs to different brain areas. By producing bioactive substances that control synaptic activity, neuronal excitability, and plasticity, NSCs can contribute to gliogenesis. Moreover, NSCs have the ability to produce and release antagonistic and synergistic chemicals which can activate intracellular NSC regulatory processes like metabolism, transcription factors, and epigenetic reactions. Moreover, NSCs have the ability to integrate into the current circuitry, form synaptic connections with neighboring neurons, and restore the damaged network [19]. Even though neural stem cell research is in its early stages, it has emerged as a promising, interesting, and safe treatment option for neurodegenerative illnesses. The primary aim of stem cell therapy for neurodegenerative disorders is to replicate a neural network that is comparable to the one lost due to the disease and to derive particular neuronal subtypes. Generating environmental enhancement to help host neurons by generating neurotrophic and scavenging toxic substances or constructing auxiliary neural networks around damaged areas is another strategy for treating neurodegenerative diseases [20].

2.1. NSC for Parkinson’s Disease

Deep brain stimulation or medicines that raise dopamine (DA) levels in the damaged dopaminergic neurons are the current alternatives for treating Parkinson’s disease [9]. While the symptoms can be well managed with current drugs, the condition advances over time and results in a major loss of dopaminergic neurons, which cannot be reversed. On the other hand, it has been found that a number of variables, including oxidative stress, mitochondrial dysfunction, protein folding, and ubiquitin-proteasome pathway malfunction, influence the development and course of the illness [21]. Treatment for Parkinson’s disease (PD) is thus complicated due to the underlying pathology of the disease. Researchers have been searching for different ways to replenish DA for the past two decades. By implanting stem cell-derived substitutes for the dopaminergic neurons destroyed due to the disease, scientists are utilizing ESCs, NSCs, and iPSCs as stem cell typologies to stimulate their development into mature dopaminergic cells. In animal models, stem cell therapy has shown some promise in the treatment of Parkinson’s disease [22].

2.2. NSC for Huntington’s Disease

The deprivation of GABAergic inhibitory neurons in the striatum of the forebrain is the hallmark of Huntington’s disease (HD), a fatal neurodegenerative condition with autosomal dominant inheritance. Cortical, brain stem, and hippocampal atrophy also accompany HD. Therapeutic strategies based on stem cells have drawn a lot of interest as potential treatment for HD [23]. The replacement of lost or injured neurons and the alteration of mutant genes carrying enlarged CAG repeats are the two main goals of stem cell treatment for Huntington’s disease [24]. Recent research indicates that NSCs have been the most frequently utilized stem cell type for treating HD. NSCs have been generated and extracted from a variety of sources, including HD patients, somatic cells, and the brain itself [25]. There is strong evidence supporting the transplantation of stem cells or their derivatives in animal models, even if stem cell-based preclinical and clinical trials are still in their early stages. Early stem cell treatments focused on grafting ESC-derived NSCs into HD animals, which showed integration of motor neurons and host circuit development [26]. Additionally, research revealed that dental pulp stem cells could be a viable therapeutic source for HD treatment, resulting in decreased immunological rejection following transplantation [27].

2.3. NSC for Frontotemporal Dementia (FTD)

The second-most prevalent form of neurodegenerative dementia is FTD, an insidious neurodegenerative clinical condition that primarily affects those under age 65 [28]. Development of iPSCs (induced pluripotent stem cells) from skin fibroblasts of FTD patients with R406W and P301L mutations in MAPT investigated tau pathologies and hereditary FTD in vitro in two different investigations. In brief, while stem cell technology for FTD modelling is in its early stages, researchers hope to employ iPSCs as a useful tool by converting them to neural cells which can act as potential therapeutics for neurodegenerative disease to further understand the pathophysiology and create treatment plans [29].

3. Methods and Progress of NSCs in NDDS

Stem cell transplantation has the capacity to undertake several reparative functions in the central nervous system, such as cell replacements and paracrine effects. It appears to be a promising therapeutic option (Figure 1) [30]. Neuronal stem cells are multipotent cells primarily located in the sub-ventricular zone (SVZ) and sub-granular zone of the dentate gyrus of the hippocampus [31,32]. NSCs are differentiated directly from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) or they are collected from brain tissues, genetically reprogrammed from somatic cells using cell signals and morphogens involved in CNS developmental processes [33,34]. NSCs possess a significant capacity for self-renewal and the ability to differentiate into neuronal and glial cells such as astrocytes and oligodendrocytes, making them a plausible therapeutic candidate for the management of NDDs like Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, frontotemporal dementia, etc. [35]. NSCs can modulate the local microenvironment by secreting neurotrophic factors, promoting synaptic plasticity, neuronal survival, and tissue repair [36]. Various approaches to therapy, such as transplantation therapy, stimulating endogenous NSCs, and gene therapy, are being explored. Research has demonstrated that in animal models (

Table 1. Transplantation of NSCs in mice models.

Animal Model	Types of NSC	Sex/Age	Transplantation Surgery	Cerebral	Test/Duration	Mechanism	Observations	Pathological Alterations	Ref.
APP/PS1 Tg mice	hNSC	M/03 months	1.8 × 105 cells/6 µL	Hippocampus	NOR, MWM/4 and 16 weeks	Amyloid phagocytosis, induced microglial activation	Improves cognition	Yes	[37]
	Mice embryos	Both/12 months	1.0 × 105 cells/5 µL, 1 µL/min	Hippocampus	MWM/8 weeks	Synaptogenesis, ↑ long-term Potentiation, neuron expression protein, BDNF	↑ cognition	NO	[38]
	Mice embryo	Both male and female/12 months	1.0 × 105 cells/3 µL, 1 µL/min	Hippocampus	MWM/10 weeks later	Decrease in inflammation-mediated TLR4-glial pathway	↑ cognition	NO	[39]
	GFP-NSCs	M/9 months	1.0 × 105 cells/2 µL/5 min	Hippocampus	MWM/4 weeks later	↑ ChAT activity ↑ Ach concentration	↑ cognition	NO	[40]
3×Tg mice	hCNS-SCs	F/19 months	1.0 × 105 cells/2 µL/injection, 1 µL/min	Hippocampus	MWM and NOR/1 month later	elevate synaptic growth, ↑ endogenous synaptogenesis	↑ spatial learning, memory	No	[41]
	GFP-NSCs mice	NM	1.0 × 105 cells/0.5 µL/min	Hippocampus	Histopathological changes	secrete neprilysin	↑ synaptic density/↓ Aβ	Yes	[42]
	GFP-NSCs mice	NM/18 months	1.0 × 105 cells	Hippocampus	MWM and NOR/1 month later	↑ BDN, ↑ synaptic density	↑ memory, ↑ spatial learning	Yes	[43]
	GFP-NSCs mice	M/12 months	1.0 × 106 cells/µL, 2 µL/10 min	Hippocampus	MWM/2 months later	Neuronal regeneration	↑ memory, ↑ spatial learning	NM	[44]
Tg2576 mice	hNSC	Both/6–9 months	2.5 × 104 cells/hemisphere, 1 µL	Hippocampus	MWM/5 months later	↑ α7 nAChR-expressing astrocytes	↑ endogenous neurogenesis	No	[45]
	Mice embryo	Both/13 months	1.0 × 105 cells/2 µL, 0.4 µL/min	Hippocampus dentate gyrus	Histological examination, MWM/2 months later	↑ synapse formation, endogenous neurogenesis, ↓ inflammatory microglial activation, ↓ Aβ production, ↓ phosphorylated-tau, ↑ Aβ clearance	↑ cognition	Yes	[37]
5 × FAD mice	hCNS-SCs	Both/2 months	1.0 × 105 cells/hemisphere/2 µL/2 min	Hippocampus	MWM and NOR/5 months later	Neuronal regeneration and differentiation	No significant changes in BDNF and cognition	No	[46]
	hiNPCs	M/4 months	1.0 × 105 cells/2 µL/hemisphere, 0.4 µL/min	Hippocampus DG	Y-maze and Barnes maze/5–6 months later	↑ synaptic density, ↑ BDNF, ↑ neuronal regeneration	Improved synaptic network, cognition	Yes	[47]

Aβ: Amyloid β; BDNF: brain-derived neurotrophic factor; ChAT: cholinergic acetyl transferase; F: female; GFP: green fluorescent protein; hiNPCs: human-induced neural progenitor/stem cells; M: male; MWM: Morris water maze; NOR: novel object recognition; ↑: Increase; ↓: Decrease; NM: not mentioned.

) of AD, NSC transplantation alters cognitive behavior by increasing acetylcholine levels [8,37].

4. NSCs’ Effects on Neurogenesis and Synaptogenesis

The hippocampal region has been identified as one of the most significantly damaged regions in AD [48] and plays a vital part in neurogenesis in adults [49]. NSCs within the dentate gyrus (DG) sub granular zone (SGZ) located in the adult hippocampus may continuously produce new neurons. Subsequently, the surviving cells merge with already-existing neural networks, which is one of the pivotal phases in the development of AD and impaired neurogenesis in the adult hippocampus [50]. Memory impairment may result from abnormal neurogenesis, which exacerbates neural sensitivity to AD [51]. The concept of human neurogenesis remains controversial. According to some, human hippocampus neurogenesis, for example, decreases dramatically from childhood to maturity and reaches unnoticed levels in adults [52]. Contrary, another opinion is that human neurogenesis continues as people age [53]. Adult hippocampal neurogenesis (AHN) can be regulated by a number of molecular factors, principally presenilin (PS1), amyloid-β protein precursor (AβPP), and apolipoprotein E (APOE) and its analogs [53]. According to a recent investigation, AHN is prevalent in neurologically unaffected individuals but gradually decreases as AD progresses [48]. The primary supplier of NSCs that develop into granular neurons in the brain of an individual across life is the DG of the hippocampus. Emerging therapeutic modalities in certain neurodegenerative illnesses may be able to benefit from the management and upkeep of the neural stem cell pool [54]. NSCs, which resemble glia and are located in the DG’s SGZ, may grow, self- regenerate, or remain quiescent [49]. For example, in AD, the slow speed of AHN may be accelerated by exciting dormant NSCs to mitigate cognitive deficits [54]. A growing body of research suggests that NSCs are essential for both neurogenesis as well as synaptogenesis. NSCs produce a variety of gene products, including VEGF and neurotrophic factors produced by the brain (BDNF), that exhibit paracrine effects which might enhance cognition and lessen AD-related deficits in neurons [55]. Blurton-Jones et al. examined the impact of NSC transplantation on AD-related cognitive impairment using triple-transgenic mice (3xTg-AD). Their results demonstrated that enhanced cognition was the consequence of a substantial rise in hippocampal synaptic density driven by BDNF rather than a reduction in Aβ and tau pathology. Consequently, transplanted NSCs’ production of neurotrophins and BDNF enhanced synaptic density and corrected cognitive deficits [42]. However, the influence of transplanted NSCs on AHN—that could profit from enhanced synaptic healing or axonal outgrowth—were not examined in this investigation. Parallel to these changes in metabolism, a new investigation using GFP-expressing murine NSCs transplanted into the hippocampi of 12-month-old APP/PS1 mice revealed a spike in the number of active neurons associated with enhanced neurogenesis, as well as synaptogenesis [56]. NSCs not only stimulate synaptic repair to restore functioning neural circuitry, but they also boosted N-acetyl aspartate and glutamate levels, along with synaptophysin and postsynaptic protein-95, upon NSC engraftment [55]. Enhanced neurogenesis, as well as synaptogenesis, were also seen in NSC-engrafted brains in a number of additional preclinical AD investigations. According to Yamasaki et al. [57], injecting hNSCs into 3xTg-AD and CaM/Tet-DTA in mice (a model of hippocampus neuronal loss) enhances formation of synapses and cognitive function [41].

Moreover, two months after transplantation, 12-month-old Tg2576 mice that received hippocampus injection of murine NSCs demonstrated significantly boosted neurogenesis and synaptogenesis [58]. According to another study, administering 6-to 9-month-old Tg2576 mice bilateral hippocampus injections of hNSCs similarly improves neurogenesis and spatial memory [59]. These observations are supported by the observation that engraftment of NSCs led to significant increases in synaptic density in taupathic mice (which overexpress human P301S tau). Elevation in BDNF, nerve growth factor (NGF), and glial-derived neurotrophic factor (GDNF) were seen in engrafted brains [60]. Engrafted NSCs have the potential to improve the AD brain by directly replacing impaired functional neurons and promoting endogenous neurogenesis and synaptogenesis. For instance, in 19-month-old 3xTg-AD mice and 7-month-old CaM/TetDTA mice, hNSCs moved to areas of neurodegeneration and developed into neurons and glial cells [41]. NSC proliferation and differentiation into neurons are stimulated and enhanced by insulin-like growth factor-1 (IGF-1) [61]. According to an investigation, the hippocampus of 3-month-old APP/PS1 mice engrafted with IGF-1-expressing hNSCs showed increased neuroprotection and preferential differentiation of GABAergic cells (which are dysregulated in AD) [62].

5. Functional Recovery Post-NSC Transplantation and Clinical Prospects

Initial studies focus on therapeutic advantages of transplanted neurons in AD-like animals; however, there is a lack of insight into functional recovery mechanisms despite type-specific neuron differentiation in grafted neurospheres [63,64]. Grafted neural stem cells (NSCs) show neuroprotective benefits in Alzheimer’s disease (AD) brains, increasing synaptic quantity and improving cognitive behavior in the hippocampus of AD mice [43]. When grafted NSCs were directly injected into Alzheimer’s disease brains, their neuroprotective effects in halting neuronal degeneration and restoring synapse loss were equivalent [65,66]. Furthermore, in the AD mice’s basal forebrain, transplanted BFCN progenitors generated acetylcholinesterase and released acetylcholine, both of which were necessary for the mice’s cognitive improvement [67]. According to these studies, NSC-derived neurons perform tasks comparable to those of their in vivo counterparts when it comes to acetylcholine biotransformation in the AD brain [67]. Through the release of neurotrophins or neurotransmitters, grafted neural stem cells (NSCs) or derived neurons provide neuroprotective effects that may help patients with Alzheimer’s disease (AD), heal brain damage, and improve cognitive deficits. Research demonstrates that AD rodents have terminal neuronal differentiation and long-term survival, indicating a well-tolerated role in the disordered brain [67,68,69,70]. Furthermore, it was discovered that lesioned AD mice—obtained from transplanted medial ganglionic precursor cells—distinguished and fired an action potential in addition to expressing K+, Na+, and spontaneous postsynaptic currents. These findings suggest that the neuronal stem cells derived from humans have membrane characteristics of fully developed neurons [68]. More significantly, it was shown that most transplanted BFCNs showed excitatory, as well as inhibitory synaptic activity, indicating that they may integrate into the basal forebrain of AD mice’s native cholinergic circuitry system [67]. One month after hippocampus transplantation in AD mice, human iNSCs developed into glutamatergic neurons, according to a follow-up investigation from the same team [47]. These observations strongly suggest that human neural stem cells (NSCs) have been found to enhance synaptic circuits in Alzheimer’s disease (AD), replace damaged neurons, and integrate functionally into neural networks.

Clinically speaking, stem cell-based treatments for AD are still in the early stages of research [71]. Assessing treatment results requires the use of AD animal models, transplantation techniques, and human neural stem cells for transplantation. Creating donor human NSCs that are unique to a certain subtype is crucial for creating treatments that alter disease progression. Treatment for AD requires a variety of human NSC types with unique geographic characteristics and have the potential to evolve into subtype-specific neurons, including BFCNs, GABAergic, and cortical glutamatergic neurons. On the other hand, in the basal forebrain of AD mice, ESC-derived BFCN progenitors mostly produced BFCNs, along with a small amount of GABAergic and glutamatergic neurons [67,68]. Requiring several rounds of neural development before transplantation, human ESC-derived neural stem cells are intricate and diverse. Their use in AD cellular replacement research is thus restricted; precisely, the markers of cell surface CD34−, CD133+, and CD45− and 5E12+ are being used for the growth and isolation of human neural stem cells (NSCs) from embryonic brain tissue in an effort to create homogenous and consistent NSCs [72,73]. Despite their promise in therapy, human neural stem cells produced from fetal brains are deemed unfeasible because of ethical limits and evaluation constraints. In contrast, iNSCs mainly develop into cortical glutamatergic neurons [47,74]. Although they are similar to fetal brain-derived NSCs in terms of survival and differentiation potential, new methods are required to produce subtype-specific human iNSCs, and it is yet unclear how to control their activity in vivo [47], although the findings reveal that in AD brains, ESC-NSCs, human iNSCs, or their derivatives do not exhibit extensive migration or long-distance projection [74]. In order to treat AD-affected brain areas, limitations in grafted human iNSCs in the AD brain may impair therapeutic effectiveness and require more research on region-specific transplantation techniques.

Various delivery routes for injecting neural stem cells (NSCs) into AD animals include intraparenchymal, intranasal, and intraventricular methods. Intraparenchymal injection is currently the best optimal region-specific transplantation strategy, while intranasal delivery is less intrusive, dependable, and programmable as compared to surgical transplantation [75,76,77]. Intranasal administration provides a substitute for introducing hNSCs into Alzheimer’s disease (AD) patients’ brains. However, surgery-based transplantation procedures are not optimal, and further modifications are needed for effective human NSC-based therapy. Subtype specificities are crucial for using human NSCs as specialized donor cells, and appropriate animal models are essential for reliable preclinical studies and translational studies. Because AD mice models are accessible and simple to handle, they are frequently employed to evaluate the therapeutic potential of human NSCs. However, due to species-specific characteristics, these models are unable to effectively mimic the pathogenic symptoms of AD [78,79]. Limited availability of reliable animal models for AD mouse models hinders the establishment of disease-modifying treatment approaches, while primitive non-human Parkinson disease (PD) models offer intriguing possibilities for neuronal replacement treatment strategies [80,81]. The deployment of a customized approach utilizing autologous, iPSC-derived dopaminergic progenitor cells in a 69-year-old patient has successfully launched clinical trials for PD cell replacement treatment [82]. It was evident from the clinical and radiographic evaluations that 18- and 24-months following implantation, PD symptoms had stabilized or improved. Therefore, the creation of models of AD in non-human primates that closely resemble the disease in humans may be useful in the development of disease-modifying cell replacement techniques for improved therapy or long-term symptomatic alleviation in AD patients. It has been demonstrated that soluble Aβ oligomers, or AβOs, cause AD-like symptoms in non-human primate brains. In monkeys, for instance, AβOs may be utilized to create models of AD pathology due to the observation of hyperphosphorylated tau and degeneration of dendritic spines [83,84]. In fact, a great deal of success has been achieved lately. Accordingly, in adult cynomolgus monkeys, multiple injections of AβO in the cerebral parenchyma have been shown to generate enormous Aβ plaques, obvious neurofibrillary tangles, deep neuroinflammation, and specific neurodegeneration very quickly. All of these point to the replication of AD in non-human primates, at least in terms of the traditional neuropathological characteristics of an individual during the initial phases of this ailment [85]. According to these findings, administering AβOs may be able to provide a suitable monkey model for AD, which might be beneficial as a tool for creating curative replacement cells as treatments for AD [86].

6. Lab-to-Clinical Challenges in NSC-Based Therapy

Transplanting neural stem cells (NSCs) offers a promising approach for combating neurodegenerative diseases (NDs). NSCs can be derived from various sources, including embryonic stem cells (ESCs), fetal neural progenitor cells (FNPCs), adult NSCs, and induced pluripotent stem cells (iPSCs) [87]. Adult NSC transplantation has shown favorable outcomes in areas such as immunological compatibility, availability, ethical acceptability, lower tumorigenic risk, and therapeutic efficacy [88]. Research by Lv et al. has explored NSC transplantation for cerebral palsy [89], while Fauser and his team successfully transplanted NSCs from the periventricular midbrain region into the hippocampus [90]. Additionally, scientists have developed various methods and formulations to generate NSCs from ESCs, aiming to address spinal cord injuries (SCIs) and other neurodegenerative conditions [91]. However, ethical issues and potential immune rejection limit the use of ESCs in transplantation. Alternatively, iPSC generated through the reprogramming of somatic cells by stimulating pluripotent genes offer similar pluripotency to ESCs. This approach addresses the ethical concerns associated with ESCs, facilitates autologous transplantation, and reduces the risk of immune rejection [92]. Conversely, FNPCs have shown promise in SCI transplantation, though their clinical application is limited by ethical concerns surrounding fetal tissue use and immune rejection. ESCs and iPSCs possess exceptional differentiation potential. Although adult NSCs are more limited in plasticity, FNPCs exhibit multipotency, although their differentiation capacity is not as extensive as that of ESCs and iPSCs [93]. Regarding immunogenicity, non-autologous ESCs carry a higher risk of immune response and rejection, while autologous iPSCs derived from the patient’s own cells significantly lower this risk, as do autologous NSCs and FNPCs [93]. Induced pluripotent stem cells (iPSCs) and neural stem cells (NSCs) are generally viewed as ethically acceptable options. Regarding availability and scalability, iPSCs are particularly advantageous because they can be derived from any patient, supporting personalized therapeutic approaches. Although adult NSCs are somewhat more limited, they can be harvested from various regions of the adult brain and expanded in vitro. FNPCs, however, are constrained by the restricted availability of fetal tissue. In terms of safety and tumorigenicity, adult NSCs pose a lower risk of gliosis because of their compatibility with host tissue. Conversely, ESCs and iPSCs, despite their high differentiation potential, carry an increased risk of gliosis and tumor formation if not carefully differentiated [94]. Due to their generally lower tumorigenic potential, adult NSCs and FNPCs are often preferred in specific applications [95].

Several factors are critical when selecting a source of NSCs. These include development of unanticipated tumors, biocompatibility issues, unwanted immune responses, irreversibility of therapy, and adventitious agent transmission [96]. Treatment effectiveness can be impacted by variables such as SC type, culture, differentiation, proliferation capability, delivery method, and genetic alteration. Given that stem cells have characteristics in common with cancer cells, such as longevity, resistance to apoptosis, and capacity for reproduction, one possibility is that they will undergo malignant transformation [97]. According to Amariglio et al., gliomagenesis may occur through the transplantation of NSCs in ataxia telangiectasia patients, which is a brain tumor acquired from the donor [98]. In SCID mice receiving human fetal tissue transplants, Shih found that human ESCs can induce primitive, as well as undifferentiated, tumors [99]. Changes in SC features can occasionally be attributed to the intracellular as well as extracellular environments of the in vitro growth. Reprogramming and genetic manipulation of stem cells for therapeutic use may have unfavorable outcomes and associated hazards [100]. Occasionally, the transplanted versatile stem cells may offer an atmosphere that promotes tumor development [101] and an unfavorable immune reaction [102]. In addition to these, a number of additional risk variables are also documented, such as identity, non-homologous usage, undesired transformation, and biological distribution of SCs [103]. A wide variety of SCs are being investigated for therapeutic uses in the field of regeneration medicine, and the foreseeable future of SC research appears bright for a wide range of pathological disorders. Due to the patient uniqueness and ability to provide tailored regenerative therapy, adult SCs and iPSCs offer significant advantages in this field of study. Since the initial human ESCs were derived [104], multiple strains derived from embryos at various stages of their development have been produced [105,106,107,108,109,110]. Though they might not prove appropriate for treating neurological conditions, adult stem cells (SCs) provide promise for disorders caused by genetic inheritance. Management for neurological diseases may be possible with pluripotent stem cells (SCs), such as ESCs and iPSCs, derived from patients; however, clinical studies have not been carried out yet and the most effective delivery method remains to be determined. It is necessary to have efficient angiogenesis and neurogenesis processes, and the SCT method needs to be quantifiable, repeatable, and economical.

It might be essential to alter the environment in the damaged or degenerating area to make it more hospitable to integration and survival in order to accomplish these aims [111]. New neurogenesis factors may be discovered as a result of the quick progress in functional genomics and proteomics, but care must be taken when using these discoveries for clinical treatments [112]. Debates over the placement of neuronal stem cells (NSCs) in the adult hippocampus and their identification in the subependymal and dentate gyrus areas provide particular difficulties for stem cell treatment for the aging brain. But sensible treatments taking these things into account will probably result in major advancements in the use of stem cells for treating the aging brain [113]. Neuro-replacement therapy will undoubtedly become more feasible if we keep learning more about the pathophysiology of AD and support creative research to elucidate the physiological role of neural stem cells in the adult brain [114]. Through the improvement of neuro-regenerative treatment, neural stem cells (NSCs) have the potential to improve Alzheimer’s disease (AD) [115].

For stem cells to be successfully translated into clinical applications, in vivo imaging techniques, such as MRI, SPECT/CT, PET, and BLI, offer valuable tools for tracking transplanted cells in regenerative studies [116]. Hyaluronic acid hydrogels have shown potential in enhancing cell survival post-transplantation. Numerous clinical trials have investigated the safety and feasibility of neural stem cell (NSC) transplantation, with some reports yielding promising results. In a study by Feldman et al., 12 patients with amyotrophic lateral sclerosis (ALS) received intraspinal NSC injections. After 30 months of follow-up, including MRI assessments, standard clinical evaluations, and regular functional assessments, results showed a delayed progression of the disease, suggesting the treatment was safe and effective [117]. In another study, Moviglia et al. [118] transplanted a combination of T-cell vaccines and autologous NSCs into seven ALS patients, extending patient survival from 3.5 to 6 years, with neurological function improvements lasting at least 1 year. Additionally, Mazzini et al. conducted a 60-month follow-up study on 18 ALS patients who received NSC transplants. They did not observe disease progression or serious adverse reactions, further confirming the safety and efficacy of this therapeutic approach [119]. Various NSC transplantation trials for Parkinson’s disease (PD) have also reported positive outcomes with no adverse effects [120]. While these studies highlight the potential of NSC transplantation for treating neurological diseases, questions around its safety and efficacy remain, and further direct clinical data are required to support its widespread use. Although NSC transplantation has shown encouraging therapeutic effects in animal models, its success in humans remains uncertain. Major challenges in NSC transplantation and development include sourcing the cells and scaling up in vitro, as both autologous and allogeneic fetal NSCs face availability and ethical constraints. To bridge the gap between research and therapeutic application, standardized and certified manufacturing processes are essential. Several methods are employed for NSC transplantation, including direct injection into the brain or spinal cord, intravenous transplantation, subarachnoid space transplantation, and direct application to the injury site [121]. Each method has unique therapeutic effects. For example, cells transplanted via veins or the subarachnoid space can disperse through circulation and may settle at injury sites to exert therapeutic benefits. However, the most effective transplantation method has yet to be determined, and there is still no consensus on the optimal approach. Tracking transplanted cells also remains challenging, and further animal studies are required to fully understand the in vivo behavior of NSCs. Hundreds of such studies have been conducted in animal models, yielding encouraging but not conclusive results.

The concentration of transplanted cells significantly impacts cell viability and efficacy. Kim et al. observed that when three different concentrations of MSCs were administered intracerebroventricularly, the lowest concentration produced the most widespread and viable MSCs, suggesting that the therapeutic effect may be mediated by paracrine factor stimulation [122]. While animal studies show positive outcomes, clinical trials using NSCs for human applications are still in the early stages. Clinical data on cellular therapies for Alzheimer’s disease (AD) remain limited. A clinical trial (NCT03117738) investigated the potential of intravenous administration of autologous adipose-derived mesenchymal stem cells (MSCs) over a 15-day period, involving nine procedures; however, the results are yet to be disclosed [123]. In a separate study (NCT01297218), researchers examined the safety and efficacy of stereotactic brain injections of human umbilical cord blood-derived MSCs (hUCB-MSCs) in patients with mild to moderate Alzheimer’s disease (AD) over a span of 24 months [124]. Nine participants underwent a 12-week course of treatment, with no observed dose-dependent toxicity or severe adverse events; only mild symptoms such as headache, dizziness, and transient delirium were reported.

Preclinical evidence showed that administering hUCB-MSCs to the hippocampus of 10-month-old APPswe/PS1dE9 transgenic mice for 40 days resulted in a reduction in Aβ42 levels and amyloid plaque burden, though similar outcomes were not observed in AD patients [125]. An emerging hypothesis suggests that targeting the entorhinal cortex (EC), one of the earliest regions impacted in AD and highly susceptible to neurofibrillary tangle-induced neuronal loss, might enhance treatment effectiveness over hippocampal administration [126]. However, findings from the same trial indicated that patients receiving the treatment experienced a more rapid cognitive decline, with mini-mental state examination (MMSE) scores dropping by nine points over two years, which contrasts sharply with the usual annual decrease of approximately three points in AD progression. Some trials that completed their preliminary phases have progressed to second-phase, extended studies, which aim to gather more comprehensive safety and efficacy data. Notable examples include trials using MSCs derived from a range of sources, such as adipose tissue (NCT04388982, NCT04228666), placenta (NCT02899091), umbilical cord blood (NCT02054208), and bone marrow (NCT03724136, NCT02600130) [127]. These investigations continue to expand the understanding of MSC therapy’s therapeutic potential in AD management. However, it remains unclear if MSCs would exclusively differentiate into neurons or potentially other cell types within a heterogeneous tissue environment. Typically, transplanted stem cells are expected to develop into the intended cell lineage only. One promising delivery route for stem cells targeting the central nervous system (CNS) may be intranasal administration [128].

To be most effective, new therapeutic strategies should target the early stages of Alzheimer’s disease (AD), ideally before significant neurodegeneration and dementia occur. This approach emphasizes that for any stem cell therapy to be successful, treatment should ideally begin at a presymptomatic or early stage, aiming to reduce neurodegeneration and potentially offer neuroprotection. The timing and dosage of cell transplantation significantly impact therapeutic efficacy; however, there is currently no standard guidance on these factors, underscoring the need for further research and experimental trials. More preclinical studies are also needed to clarify stem cells’ mechanisms of action. Therefore, it can be said that NSC transplantation as a replacement therapy is still in its early experimental stages; however, the future looks bright with ongoing exploration and intensified efforts to overcome these obstacles.

7. Conclusions

Use of NSCs as a next-generation therapy against AD needs extensive research and collaborative efforts. They have shown promise in various animal models; however, NSCs as successful therapy will largely depend on stem cell sources, differentiation and integration of NSCs, microenvironment modulation, immunological interactions, safety, and ethical concerns. Thus, NSCs represents a cutting-edge approach in precision medicine and, in the near future, may offer a tailored approach in treating multiple neurodegenerative diseases.