Current status of stem cell therapy for Alzheimer's disease

Hao-Chi Yang，Hai-Ying Zhang，Yun-Xia Zhang， Yuan Fu

1. Public Research Laboratory of Hainan Medical University, Hainan,Haikou 571101, China

2. Anatomy department of Guangdong medical University, Guangdong,Dongguan 523808, China

Keywords:Stem cells Alzheimer's disease Tissue engineering Neurodegenerative diseases

ABSTRACT Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive memory loss and cognitive dysfunction. Currently there is no effective treatment. Given the depth of current stem cell research, researchers have attempted to compensate for neuronal loss in the brains of patients with Alzheimer's disease using stem cell technology. This article will review the pathogenesis of AD, the latest advances in stem cell therapy, and the challenges and prospects of its development.

Aging is one of the most prominent phenomena of this century，some common diseases have sharply increased in the elderly population, such as AD, Parkinson's disease (PD), Huntington's disease (HD), Amyotrophic Lateral Sclerosis (ALS), different types of spinal cerebellum ataxia (Spinocerebellar Ataxias, SCA), etc，the prevalence of AD increases exponentially with age[1]. Alzheimer's disease (AD) generally manifests as progressive memory loss and cognitive impairment, and its main clinical manifestations are dementia. The disease was first reported by Alois Alzheimer in 1907. According to the American Alzheimer's Association report, there are about 50 million people with dementia worldwide, and the cumulative medical cost is about $ 800 billion[2].

AD is a degenerative disease of the central nervous system characterized by gradual loss of memory, learning, reasoning, and ability to do daily activities. The nature of the pathological change is the damage of neurons, and the disorder of the connection between neurons and neurons is interrupted And the deposition of harmful chemicals such as β-amyloid (Amyloid β-protein, Aβ), etc., make the information transmission dysfunction between neurons and present a series of clinical problems. Studies have shown that the clinical manifestations of AD may be due to structural defects in the new hippocampal neurons of the dentate gyrus, which affects normal learning and memory functions[3]. In recent years, with the increase of the average life expectancy of human society and the sharp rise of the aging population, AD will become one of the more serious social burdens. After years of research, scholars have proposed a number of hypotheses about the etiology of AD, but the exact etiology and pathogenesis of AD have not been known so far[4]. Early studies have suggested that the etiology of AD is related to the functional decline of specific neuronal systems in the prefrontal and hippocampus of the brain (such as cholinergic neurons and GABAergic neurons);Other scholars have suggested that the neuropathological characteristics of AD are caused by extracellular Aβ,neuron loss due to deposition, abnormal tangles of intracellular nerve fibers (Neurofibrillar Tangles, NFT), and hyperphosphorylation of Tau protein. In previous studies, the amyloid cascade hypothesis has received the most extensive support and recognition in the field of AD research, but this hypothesis and corresponding pharmacological treatment have not significantly improved the symptoms of AD patients[5]. Central Nervous System (CNS) degenerative diseases have caused a heavy burden on society, and incurable AD remains one of the biggest obstacles in modern medicine. With the advancement of basic medicine and tissue engineering, cell replacement therapy has provided new treatment strategies for neurological diseases. In clinical studies of various neurological diseases (such as stroke, PD, HD, and AD), try stem cell replacement strategies to further understand the molecular mechanisms of neural regeneration and the role of neural regeneration cells[6]. This article summarizes the types of stem cells reported in the current study, and reviews the prospects for future stem cell replacement strategies in the treatment of Alzheimer's disease.

1.Pathogenesis and research status of AD

The main pathological features of AD are: (1) extracellular Aβ plaque deposition; (2) intracellular NFT. The formation of Aβ plaques is caused by abnormal shearing of a series of amyloid peptides. In normal organisms, amyloid precursor protein (APP) is cleaved by α-secretase or β-secretase to produce soluble SAPPα or SAPPβ polypeptides, both of which promote neuron survival and growth. In AD patients, another way is that APP is continuously cleaved by β-secretase and γ-secretase to produce insoluble Aβ40 / 42, which has certain neurotoxicity[7]. Compared to normal individuals, the α-helix and β-sheet structures in these proteins are more abundant in amyloid peptides[8]. On the one hand,compared with normal amyloid peptide alpha helix, abnormal amyloid peptide is rich in β-sheet structure, Aβ stimulates other cells through blood circulation, and then produces additional Aβ, its neurotoxicity can cause the death of a large range of neurons in the central nervous system. On the other hand, intracellular neurofibrillary tangles (NFT) are caused by atypical hyperphosphorylation of Tau protein, which is a type of microtubule-associated protein (MAPs) that plays a role in neurons，it supports the role of cytoskeleton structure and regulates and controls various functions of neurons. Atypical over-activation of protein kinases (PKC and PKA) can also cause excessive phosphorylation of Tau protein in cells, leading to cell apoptosis and neuron loss.

According to the definition of biochemistry and genetics, AD is divided into two types: familial (hereditary) AD (Familial Alzheimer disease (FAD)) and sporadic AD (Sporadic Alzheimer disease (SAD)). The incidence of FAD is relatively low, and its age of onset is more common under 65 years of age[9]. SAD is closely related to the production and metabolism of amyloid precursor protein (APP), presenilin1 (PS1) and presenilin2 (PS2). It is also involved in the regulation of Aβ gene and the pathogenesis of AD is closely related[10]. At the same time, mutations or overexpression of the γ-secretase complex PS1 (located on chromosome 14) or PS2 (located on chromosome 1) or APP (located on chromosome 2) are related to Aβ aggregation and neurodegeneration. Statistical data research shows that the number of elderly people with SAD accounts for about 90% of the total number of AD[11]. Genome-wide association study (GWAS) found that apolipoprotein E (APOE) is the most important related gene in SAD. Triggering Receptor Expressed On Myeloid Cells 2, TREM 20) and a triggering receptor expressed on CD 33 are closely related to Tau protein modification and Aβ microglial phagocytosis[12].

In addition to the above-mentioned pathological changes in AD patients' brains, abnormal production and metabolism of biological macromolecules such as proteins, cholesterol and glucose were also found. Abnormal protein production and metabolism, such as disorders in the ubiquitin-proteasome pathway, can lead to cell death and tangles in the central nervous system. Disorders of ganglioside metabolism can convert non-toxic APP precursors into insoluble toxic substances. In addition, abnormal disruption of glucose metabolism has been shown to cause disorders in the synthesis and modification of Tau protein. Metabolic dysfunction leads to elevated levels of cytokines and reactive oxygen species (ROS) in the brain of AD patients, leading to the occurrence and progressive deterioration of chronic neuroinflammation[13]. Although people have studied AD treatment for a long time, no effective treatment has been found. Currently, the Food and Drug Administration (FDA) has approved the treatment by inhibiting neuronal death-related proteins and restoring the function of the cholinergic neuron system, thereby temporarily delaying the pathogenesis of AD pathogenesis[14]. At present, cocktail therapy has become a new method of AD treatment, but it is not enough to cope with AD patients with a rapidly increasing incidence in an aging society. However, in the strategy of AD treatment, scholars believe that the new treatment method should be effective intervention in the early stages of AD before neuron degradation and dominant dementia occur. The ability of stem cells to differentiate into different types of neurons and glial cells has drawn widespread attention from scholars as a potential treatment for reversing and replacing AD neuron loss. Effective results have been shown in animal model studies. Some studies have shown that injection of neural-like stem cells into the brain of AD animal models has enhanced the synaptic connectivity and neuronal viability of neurons in the brain, and improved the cognitive ability of the AD animal model. However, the application of stem cell therapy in the clinical treatment of AD patients still requires further research to fully understand its safety and effectiveness, but stem cell therapy as a new generation of treatment methods for AD still has potential application value and prospect.

2.Application of stem cells in AD research

Studies have shown that stem cells can establish connections with neural networks in the brain, secrete multiple neurotrophic factors to regulate neuronal synaptic plasticity, and interfere with neurogenesis[14]; And it is possible to increase the level of acetylcholine in the brain, and eventually improve the memory and cognitive ability of AD model animals[12]. The main mode of action of stem cell therapy can be divided into endogenous and exogenous according to the mechanism of action. Traditionally, cell therapies have attempted to replace damaged tissues by cell redifferentiation or directly implanted stem cells[9]. However, current research indicates that stem cell transplantation is not the main source of newborn neurons[13]. In addition, unlike Parkinson's disease, AD is characterized by the death of different types of nerve cells, and this variation precludes the feasibility of transplanting specific mature cell types.

Therefore, the use of paracrine effects to stimulate endogenous repair rather than using cell replacement models has attracted widespread attention from researchers. Research suggests that the nutrition provided by transplanted stem cells can support and improve the microenvironment and promote the survival of damaged residual nerve cells. Using this treatment strategy, the main target for stimulating hippocampal neuronogenesis (to compensate for neurodegeneration) is a key factor in increasing the number of neural stem cells in the brain [3]. Neurogenesis in the hippocampus is thought to play a key role in memory and learning. The paracrine mediators produced after stem cell transplantation are mainly neuroderived neurotrophic factor (BDNF), nerve growth factor (Nerve GrowthFactor, NGF), insulin-like growth factor-1 (IGF-1) and blood vessels Endothelial growth factor (Vascular Endothelial Growth Factor, VEGF) and so on. In addition, the regulation of inflammation is also considered to be another mechanism of action[15]. Scholars found human neural stem cells (Neural Stem Cells, NSCs), mesenchymal stem cells (MSCs), adipose stem cells (ASCs), embryonic stem cells (ECSs) ) and Induced Pluripotent Stem Cells (IPSCs) have unique characteristics and can be used in a variety of ways in stem cell therapy systems[16], which will be introduced one by one below.

2.1Application of NSCs in AD research

In the adult brain, pluripotent neural stem cells (INSCs) are located in the subventricular layer (SVZ) and subgranular layer (SGZ) of the hippocampus in adult mammals[17]. It can differentiate into a variety of cell types, including neurons, astrocytes, and oligodendrocytes. NSCs can be isolated from fetal and postnatal neonatal brain tissue[13],and can be differentiated from ESCs and iPSCs. In AD animal models, transplanted NSCs can differentiate into mature brain cell types. The transplanted cells are widely distributed in the affected tissue (preserving the original characteristics of the cells), and then integrated into the functional neurons of the host brain, allowing the neurons to replace effectively. Because the migration and differentiation of transplanted NSCs are affected by the internal environment of the body, it is unclear whether NSCs can differentiate into specific neural cell types. Some scholars have found that neural stem cells tend to differentiate into non-neuronal glial cell types[18]. The effectiveness of NSCs transplantation on neuron replacement is still inconclusive. Unlike other stem cell transplants, the paracrine effect after NSCs transplantation is more efficient than other cell transplantation[12]. In particular, BDNF secreted by NSCs plays an important role in improving cognitive function in patients with AD[19]. In addition, some scholars have found that NSCs transplantation has neuroprotective, neural regeneration and immune regulation effects. Further research is needed on the tumorigenicity of NSCs transplantation and the mechanism of functional recovery after NSCs transplantation.

At the same time, other stem cells can be induced into NSCs Some scholars have found that INSCs can be induced by fibroblasts, astrocytes and support cells[20]. ESCs-derived NSCs can be differentiated into astrocyte-like cells. However, the survival rate of NSCs in vivo after transplantation is unpredictable[21].Therefore,as an optimized treatment strategy,NSCs can be used as a carrier for carrying therapeutic substances, such as therapeutic agents such as neprilysin (NEP). At present, the use of NSCs as a carrier therapy for drug delivery, rather than neuronal replacement therapy, has attracted widespread attention[22].

2.2Application of MSCs in AD research

MSCs have attracted great attention from many research scholars in their treatment of AD due to their availability, maneuverability, extensive research characteristics, and neuronal differentiation potential[23]. Because it can be administered intravenously, it can penetrate the blood-brain barrier, has low tumorigenicity, and has a small immune response (compared with other cell therapies), which makes MSCs as a cell replacement therapy have certain therapeutic advantages. Scholars have found that transplanted MSCs-derived neurons have a high differentiation rate in vivo. The secreted factors produced by MSCs can stimulate the proliferation, differentiation and neuron growth of neurogenic cells[24]. The anti-inflammatory and immunomodulatory effects of many cytokines are thought to contribute to the recovery of nerve damage[25]. MSCs acquire the ability of immunoregulation by releasing soluble factors (IL-6, IL-10, TGF-β, PGE 2). It can inhibit the function of monocytederived dendritic cells and change the phenotype of natural killer cells. The sources of MSCs are diverse, and adipose tissue is one of the most common sources of MSCs.Adipose tissue-derived MSCs can differentiate into neuron-like cells and astrocytes.They seem to share a stem cell transcription profile with MSCs. Adipose tissuederived MSCs and bone marrow mesenchymal stem cells can secrete multiple neurotrophic factors[26]. In the study of the AD mouse model, it was found that bone marrow mesenchymal stem cells derived from umbilical cord blood can differentiate into neuron-like cells.

2.3Application of ASCs in AD Research

In 2001, Zuk[27] and others discovered that abundant ASCs could be separated from liposuction surgery. The use of autologous stem cells to treat neurodegenerative diseases is highly prospective, but to obtain a large number of neural progenitor cells without harming patients is a problem that researchers need to solve. ASCs have been used in many types of neurodegenerative diseases. The main mechanism of action is to protect damaged or lost neurons by secreting neurotrophic factors and chemokines after ASCs transplantation[28]. In the study of transplanted ASCs, compared with other adult stem cells, ASCs are easy to obtain (obtained in a minimally invasive manner), have no immune rejection[29], and have no tumorigenic properties. ASCs are also commonly used in experimental studies to study diseases such as autoimmune diseases, multiple sclerosis, polymyositis, dermatomyositis, and rheumatoid arthritis. Therefore, cell replacement therapy for ASCs has a wide range of clinical applications[30].

2.4Application of ESCs in AD Research

ESCs are pluripotent stem cells extracted from the inner cell mass of a blastocyst and can produce all types of cells during embryonic development[31]. Because ESCs have strong differentiation ability and direct transplantation have high tumorigenicity, the key technical problem of transplanting ESCs is to strictly monitor the state of cells and maintain the stability of cell differentiation. Some rodent studies have shown that NSCs transplanted from ESCs are tumorigenic, but further experimental results are needed to confirm this[32]. In addition, the difference between NSCs and MSCs is that ESCs have a higher risk of transplant rejection and immune response[33]. Although the brain has natural immune privileges, the human leukocyte antigen (HLA) spectrum of donor cells must be considered during transplantation to avoid immune rejection. To date, human ESCs experiments have successfully produced dopaminergic neurons, spinal motor neurons, and astrocytes[34]. However, the use of human embryonic stem cells (hESCs) in FDA-approved clinical trials is ethically controversial and must be used with caution.

2.5Application of iPSCs in AD Research

IPSCs are a multiple stem cell line reprogrammed from fibroblasts[35]. In iPSC technology, a stem cell with pluripotency similar to hESCs can be obtained by overexpressing four transcription factors (TFs)-Oct 3/4, Sox 2, Klf 4 and c-Myc. In 2006, Shinya Yamanaka's team successfully induced IPSCs through retrovirus-induced overexpression of these four transcription factors[36]. Studies on IPSCs have shown inconsistent results regarding immune responses. Some rodent studies have found that transplanted IPSCs have little immune rejection, while others have found that the major histocompatibility complexes (MHC) of the donor and recipient are incompatible and induce immune rejection[37]. Therefore, the immune rejection of IPSCs needs to be resolved before starting clinical trials.

3.Summary

Alzheimer's disease is a progressive central nervous degenerative disease, and there is currently no effective treatment. Given the regenerative potential of stem cells, there may be great potential in the treatment of various neurodegenerative diseases. Although the mechanism of action of stem cell therapy has not been fully elucidated, many preclinical studies have shown significant results. So far, stem cell technology is still in the development stage, and its rapid development and experimental research progress show that it has direct and indirect potential applications in the treatment of AD. In order for this technology to be successfully transformed in the clinic, further related animal research and clinical experiments are needed. There are many questions about the safety, effectiveness, ethics, and regulatory framework of stem cell therapies. This review provides a brief overview of the etiology and development mechanism of AD and stem cells as a treatment strategy for AD. At the same time, it summarizes and analyzes cell types and the results of some preclinical studies made at present. Prospective prospects provide new ideas for clinical treatment.