**2.1. Embryonic stem cells**

The first evidence that human stem cells, called human embryonic stem cells (hESC), could be derived from a 5-day blastocyst was reported in 1998 by Thomson and colleagues [12]. ES cells have the ability to proliferate indefinitely and are considered pluripotent cells because they can differentiate into all three germ layers (ectoderm, mesoderm, and endoderm) and, thus, they can generate all the differentiated cells of the adult [13, 14]. Despite that they helped stem cell research, they also opened many controversies because ES cells are obtained from blastocyst, killing the fertilized embryo that has the potential to generate a human being [15]. The big ethical issue on the use of hES cells encouraged researchers to understand the pathways underlying the staminality of this kind of cells.

### **2.2. Induced pluripotent stem cells**

The research done with ES cells and the finding of their highly expressed transcription factors, permitted in 2006 to induce mouse's fibroblasts to become pluripotent, by retrovirus-mediated transduction with four transcription factors, i.e., Oct-3/4, Sox2, KLF4, and c-Myc [16]. The following studies allowed to improve the technique, permitting to generate induced pluripotent stem cells (iPSCs) from adult human cells and to reprogram cells from several tissues [15]. Moreover, it is now possible to generate iPS cells by different transduction methods (**Figure 1**), using different viral and nonviral constructs, as well as integrative and non-integrative system approaches [17]. The best methods to reprogram cells are the non-integrative methods, and the four main groups are available: non-integrative viral delivery, episomal delivery, RNA delivery, and protein delivery [18]. The establishment of human iPS cells has led to have an unlimited source of stem cells overcoming the ethical limit of hES cells. Moreover, iPSCs can be reprogrammed from any somatic cell line of the patients providing a way to study diseases' mechanisms potentially for each patient, opening to the so-called personalized medicine (**Figure 1**). Actually, many iPSCs' lines have been generated from patients with neurodegenerative disease, like Alzheimer's disease (AD) [19], Parkinson's disease (PD) [20], amyotrophic lateral sclerosis (ALS) [21], and Huntington's disease (HD) [22].

invasive techniques, like skin biopsy or, more recently, blood withdrawal [3]. These recent findings has led to an outstanding increase in disease mechanisms and drug screening studies involving stem cells, in particular for neurodegenerative diseases because of the impossibility to obtain neural cells from patients. The ability to reprogram patient-specific cells also opens new opportunities for the personalized medicine approach of drug discovery. Moreover, the development of 3D bioprinting provided a useful tool to generate innovative cell cultures, permitting to have a 3D model in which cells can be disposed with a controlled manner and where they can grow in a tissue-like structures [4]. Obviously, 3D bioprinting opened new possibilities in the field of tissue engineering, but it can be helpful also for disease modeling. In fact, the generation of a 3D scaffold that can resemble the human tissues will permit to study neurodegenerative diseases in the so-called brain in dish. Finally, the combination of 3D bioprinting technique with iPSC technology will permit to develop one of the most realistic and reliable in vitro cell cultures, permitting to study organoids with patients' differentiated

Stem cell research is considered one of the most promising areas in cell biology and regenerative medicine due to stem cells' unique properties of self-renewing and differentiation into all types of cells. These cells represent nowadays the main tool in the regenerative medicine field because they permit to generate cells needed for transplantation in several degenerative diseases [1], such as rheumatoid arthritis [5], diabetes mellitus [6], heart failure [7], liver diseases [8], and neurological disorders [9–11]. Moreover, stem cells represent an important tool for modeling human diseases, in particular for diseases that affect cells that cannot be easily collected and cultivated. One of the biggest issues in the study of neurodegenerative diseases is the lack of good cellular models that recapitulate the mechanisms underlying their pathophysiology, and in the last decade, stem cells played a major role in the study of these diseases.

The first evidence that human stem cells, called human embryonic stem cells (hESC), could be derived from a 5-day blastocyst was reported in 1998 by Thomson and colleagues [12]. ES cells have the ability to proliferate indefinitely and are considered pluripotent cells because they can differentiate into all three germ layers (ectoderm, mesoderm, and endoderm) and, thus, they can generate all the differentiated cells of the adult [13, 14]. Despite that they helped stem cell research, they also opened many controversies because ES cells are obtained from blastocyst, killing the fertilized embryo that has the potential to generate a human being [15]. The big ethical issue on the use of hES cells encouraged researchers to understand the pathways

The research done with ES cells and the finding of their highly expressed transcription factors, permitted in 2006 to induce mouse's fibroblasts to become pluripotent, by retrovirus-mediated transduction with four transcription factors, i.e., Oct-3/4, Sox2, KLF4, and c-Myc [16]. The following studies allowed to improve the technique, permitting to generate

cells, leading to a personalized medicine approach in drug testing.

**2. Stem cells**

26 Recent Advances in Neurodegeneration

**2.1. Embryonic stem cells**

underlying the staminality of this kind of cells.

**2.2. Induced pluripotent stem cells**

**Figure 1.** An overview of iPSC technology. Somatic cells can be taken from several sources, like the skin, blood, and urine. There are many reprogramming strategies, and the best ones are the non-integrative strategies. iPSCs can be differentiated into diverse cell lines that can be used for disease modeling, for drug discovery, and for cell replacement therapy (the image was taken from Sharma [23]).

#### *2.2.1. iPSCs in Alzheimer's disease*

Alzheimer's disease (AD) is the most common form of dementia and is characterized by the progressive loss of memory and cognitive functions. The disease leads to a severe form of dementia that causes the death of the patient [24]. The two main hallmarks of the disease are the accumulation of amyloid beta (Aβ) plaques in the extracellular compartment and the aggregation of the tau protein in the intracellular compartment. Only 1–5% of AD cases have a genetic cause, while in the other cases, the real pathogenesis is still unknown [25].

These researches suggest that iPSC-derived neurons from AD patients can help not only to unravel disease's mechanisms but also to screen new treatment and to find new possible drug targets. Moreover, the authors hypothesize that gene correction is a useful tool to generate isogenic controls or to induce AD mutations in healthy controls. Finally, iPSCs can be differentiated into glial cells, e.g., astrocytes, which in recent years gain an important role in the

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Parkinson's disease (PD) is the second most common neurodegenerative disease after AD, with a prevalence of 1% out of the individuals over age 60 years and 4% of the population with an age over 85 years [36]. The most common mutations, found in about 10% of Parkinson's patients, are present in six genes: SNCA, LRRK2, Parkin, PINK1, DJ-1, and ATP13A2 [37].

Usually, iPSCs are differentiated into dopaminergic (DA) neurons to model PD because the disease is characterized by the loss of DA neurons of the *substantia nigra* in the midbrain. Since monogenic mutations cause an idiopathic-like disease, diverse iPSC lines of patients with Parkin and PINK1 mutations (e.g., 2–4 exon deletions of Parkin and PINK1 Q456X) have been developed. It was seen that these cell lines present abnormalities in mitochondrial and dopamine homoeostasis, microtubular stability, and axonal outgrowth, resulting in an optimal model of the disease [38]. For example, many PD cell phenotypes, i.e., mitochondrial dysfunction, elevated α-synuclein, synaptic dysfunction, DA accumulation, and increased oxidative stress and ROS, were found in iPSC-DA neurons of patients carrying mutations in parkin (V324A) and PINK1 (Q456X) genes [39]. The role in neurons' maturation of elevated α-synuclein caused by SNCA gene triplication was investigated in a cellular model obtained from PD-derived iPSCs. The author has claimed that such triplication leads to the impairment of differentiation and maturation of iPSCs [40]. An electrophysiological characterization of control dopaminergic neurons derived from iPSC was provided by Hartfield and colleagues that confirmed that these cells have the physiological hallmarks of dopaminergic neurons previously reported only on rodent slice. These results suggested that such cells can be considered a useful tool for the physiological study of PD [41]. Moreover, several evidences suggest that PD is not only a brain disease but also a gastrointestinal disorder; thus, Son and colleagues differentiated iPSCs carrying an LRRK2 G2019S mutation in both neural and intestinal phenotypes, providing the first evidence that G2019S mutation causes significant changes in gene expression also in the intestinal cells [42]. Interestingly, the pathologic phenotype was reversed in cortical neurons derived from iPSCs of patients mutated in SNCA using a small molecule found by yeast screening, opening new possibilities in drug screening and testing [43]. Finally, iPSCs have allowed an innovative co-culture of microglial cells and cortical neurons, displaying a unique cytokine profile impossible to obtain without iPSCs [44]. iPSCs were proposed to be used for tissue transplantation, and Kikuchi et al. achieved the transplantation of human iPS cell-derived dopaminergic neurons in a primate model of PD treated with MTPT. The authors reported an increase in spontaneous movement of the monkeys, demonstrating for the first time that such

transplantation could be clinically applicable for the treatment of PD patients [45].

pathogenesis of several neurodegenerative diseases.

*2.2.2. iPSCs in Parkinson's disease*

Many groups used and performed several studies on in vitro models with neural and nonneuronal cells derived from iPSCs. For example, higher susceptibility to Aβ1–Aβ42 oligomers was found in neuronal precursors derived from iPSC (iPSCs-NSCs) of a patient with a mutation in the PSEN1 gene (PSEN1-A246E mutation) compared to sporadic AD patient and healthy control [26]. The authors concluded that neurons derived from AD iPSCs could be effective in drug screening, to develop new treatments that protect cells from the toxicity of the Aβ peptides in the AD brain [26]. A similar result was obtained with iPSC-derived neurons of sporadic AD patients and of a patient carrying the pathogenic APP-E693Δ mutation. The study shows that these cell lines produce intracellular Aβ oligomers, resulting in a good cellular model of AD [27]. iPSCs can be used to find new potential biomarkers of the disease, as suggested by Shirotani et al. that developed an innovative method on neurons differentiated from iPSCs [28]. Moreover, induced fAD mutations by genome editing of neurons derived from healthy controls could resemble the pathophysiology of the disease. A decrease in endocytosis and soma-to-axon transcytosis of LDL was found in human neurons with expression of PSEN1ΔE9 induced with genome editing technology. To confirm the potential role of iPSCs in drug discovery, the authors reported that defects were rescued by β-secretase inhibition [29]. Another study reported the generation of an Alzheimer-related protein association network using iPSCs, demonstrating that they can be used as drug screening model and finding a reduction of tau protein after treatment with an inhibitor of γ-secretase [30]. For drug testing, it is important that iPSC-derived neurons are well differentiated, because it was seen that between early and late differentiation stages, cells have different susceptibilities to drugs [31]. Genome editing technology could be used also for mutations' correction, generating an isogenic control. For example, Pires and colleagues reported that A79V-iPSC line in combination of A79V-GC-iPSC line could be used to study pathological cellular phenotypes related to A79V mutation in PSEN [32]. Interestingly, the role of iPSCs in AD research was supported also by analyzing neurons derived from iPSCs of patients with Down syndrome that usually have a high risk of developing AD early. Authors found that such neural cells reproduce AD-like initial cellular hallmark, resulting useful for modeling this variant of AD [33]. Finally, also nonneuronal cells derived from iPSCs could be very useful in disease modeling and drug screening. Many pathological hallmarks were found aberrant in astrocytes derived from iPSCs of fAD and sAD patients suggesting that astrocytic atrophy could be a plausible mechanism for early cognitive impairment and thus opening new therapeutic strategies for AD intervention [34]. Another study reported changes in PSEN1-mutated iPSC-derived astrocytes, revealing the major role of such cells and confirming the importance to implement iPSC technology to support neurodegenerative diseases' study [35].

These researches suggest that iPSC-derived neurons from AD patients can help not only to unravel disease's mechanisms but also to screen new treatment and to find new possible drug targets. Moreover, the authors hypothesize that gene correction is a useful tool to generate isogenic controls or to induce AD mutations in healthy controls. Finally, iPSCs can be differentiated into glial cells, e.g., astrocytes, which in recent years gain an important role in the pathogenesis of several neurodegenerative diseases.

#### *2.2.2. iPSCs in Parkinson's disease*

*2.2.1. iPSCs in Alzheimer's disease*

28 Recent Advances in Neurodegeneration

Alzheimer's disease (AD) is the most common form of dementia and is characterized by the progressive loss of memory and cognitive functions. The disease leads to a severe form of dementia that causes the death of the patient [24]. The two main hallmarks of the disease are the accumulation of amyloid beta (Aβ) plaques in the extracellular compartment and the aggregation of the tau protein in the intracellular compartment. Only 1–5% of AD cases have

Many groups used and performed several studies on in vitro models with neural and nonneuronal cells derived from iPSCs. For example, higher susceptibility to Aβ1–Aβ42 oligomers was found in neuronal precursors derived from iPSC (iPSCs-NSCs) of a patient with a mutation in the PSEN1 gene (PSEN1-A246E mutation) compared to sporadic AD patient and healthy control [26]. The authors concluded that neurons derived from AD iPSCs could be effective in drug screening, to develop new treatments that protect cells from the toxicity of the Aβ peptides in the AD brain [26]. A similar result was obtained with iPSC-derived neurons of sporadic AD patients and of a patient carrying the pathogenic APP-E693Δ mutation. The study shows that these cell lines produce intracellular Aβ oligomers, resulting in a good cellular model of AD [27]. iPSCs can be used to find new potential biomarkers of the disease, as suggested by Shirotani et al. that developed an innovative method on neurons differentiated from iPSCs [28]. Moreover, induced fAD mutations by genome editing of neurons derived from healthy controls could resemble the pathophysiology of the disease. A decrease in endocytosis and soma-to-axon transcytosis of LDL was found in human neurons with expression of PSEN1ΔE9 induced with genome editing technology. To confirm the potential role of iPSCs in drug discovery, the authors reported that defects were rescued by β-secretase inhibition [29]. Another study reported the generation of an Alzheimer-related protein association network using iPSCs, demonstrating that they can be used as drug screening model and finding a reduction of tau protein after treatment with an inhibitor of γ-secretase [30]. For drug testing, it is important that iPSC-derived neurons are well differentiated, because it was seen that between early and late differentiation stages, cells have different susceptibilities to drugs [31]. Genome editing technology could be used also for mutations' correction, generating an isogenic control. For example, Pires and colleagues reported that A79V-iPSC line in combination of A79V-GC-iPSC line could be used to study pathological cellular phenotypes related to A79V mutation in PSEN [32]. Interestingly, the role of iPSCs in AD research was supported also by analyzing neurons derived from iPSCs of patients with Down syndrome that usually have a high risk of developing AD early. Authors found that such neural cells reproduce AD-like initial cellular hallmark, resulting useful for modeling this variant of AD [33]. Finally, also nonneuronal cells derived from iPSCs could be very useful in disease modeling and drug screening. Many pathological hallmarks were found aberrant in astrocytes derived from iPSCs of fAD and sAD patients suggesting that astrocytic atrophy could be a plausible mechanism for early cognitive impairment and thus opening new therapeutic strategies for AD intervention [34]. Another study reported changes in PSEN1-mutated iPSC-derived astrocytes, revealing the major role of such cells and confirming the importance to

a genetic cause, while in the other cases, the real pathogenesis is still unknown [25].

implement iPSC technology to support neurodegenerative diseases' study [35].

Parkinson's disease (PD) is the second most common neurodegenerative disease after AD, with a prevalence of 1% out of the individuals over age 60 years and 4% of the population with an age over 85 years [36]. The most common mutations, found in about 10% of Parkinson's patients, are present in six genes: SNCA, LRRK2, Parkin, PINK1, DJ-1, and ATP13A2 [37].

Usually, iPSCs are differentiated into dopaminergic (DA) neurons to model PD because the disease is characterized by the loss of DA neurons of the *substantia nigra* in the midbrain. Since monogenic mutations cause an idiopathic-like disease, diverse iPSC lines of patients with Parkin and PINK1 mutations (e.g., 2–4 exon deletions of Parkin and PINK1 Q456X) have been developed. It was seen that these cell lines present abnormalities in mitochondrial and dopamine homoeostasis, microtubular stability, and axonal outgrowth, resulting in an optimal model of the disease [38]. For example, many PD cell phenotypes, i.e., mitochondrial dysfunction, elevated α-synuclein, synaptic dysfunction, DA accumulation, and increased oxidative stress and ROS, were found in iPSC-DA neurons of patients carrying mutations in parkin (V324A) and PINK1 (Q456X) genes [39]. The role in neurons' maturation of elevated α-synuclein caused by SNCA gene triplication was investigated in a cellular model obtained from PD-derived iPSCs. The author has claimed that such triplication leads to the impairment of differentiation and maturation of iPSCs [40]. An electrophysiological characterization of control dopaminergic neurons derived from iPSC was provided by Hartfield and colleagues that confirmed that these cells have the physiological hallmarks of dopaminergic neurons previously reported only on rodent slice. These results suggested that such cells can be considered a useful tool for the physiological study of PD [41]. Moreover, several evidences suggest that PD is not only a brain disease but also a gastrointestinal disorder; thus, Son and colleagues differentiated iPSCs carrying an LRRK2 G2019S mutation in both neural and intestinal phenotypes, providing the first evidence that G2019S mutation causes significant changes in gene expression also in the intestinal cells [42]. Interestingly, the pathologic phenotype was reversed in cortical neurons derived from iPSCs of patients mutated in SNCA using a small molecule found by yeast screening, opening new possibilities in drug screening and testing [43]. Finally, iPSCs have allowed an innovative co-culture of microglial cells and cortical neurons, displaying a unique cytokine profile impossible to obtain without iPSCs [44]. iPSCs were proposed to be used for tissue transplantation, and Kikuchi et al. achieved the transplantation of human iPS cell-derived dopaminergic neurons in a primate model of PD treated with MTPT. The authors reported an increase in spontaneous movement of the monkeys, demonstrating for the first time that such transplantation could be clinically applicable for the treatment of PD patients [45].

The studies previously reported hypothesize that iPSC-derived neurons from PD patients can be very useful in the research of PD pathophysiology and to find new therapeutic targets for innovative drugs. Moreover, the possibility to differentiate iPSCs into nonneuronal cells, such as microglial and intestinal cells, will help to unravel the role of immunity response and the gastrointestinal disorder that affect PD patients.

expansion mutation of the trinucleotide (CAG) repeat in the HTT (IT15) gene, encoding a 350-kDa protein called Huntingtin (HTT) [56]. Even though the genetic cause is clear, the mechanisms through which mutant HTT results in the degeneration of some types of neurons are still unclear. Thus, studies on HD models are needed in order to discover treatments.

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31

As the neurodegenerative diseases previously reported, also for HD, neurons differentiated from iPSCs of patients helped to understand the role of mutant HTT gene and the mechanisms that lead to the pathology. For example, early molecular changes in intracellular signaling, expression of oxidative stress proteins, and p53 pathway both in iPSCs and in neurons differentiated from them were reported [57]. Another study reported changes in neuronal development and adult neurogenesis, exploiting the iPSC capacity to model also embryonal development [58]. The generation of iPSCs from a patient that presents an expansion in the HTT gene without any symptom is very intriguing. The generation of iPSCs in an early stage of HD will allow to study the pathological process and the abnormal changes that lead to the pathology [59]. The possibility to differentiate iPSCs into neurons opened the possibility to discover new therapeutic targets, e.g., pre-mRNA trans-splicing modules [60]. Finally, the role of glial cells was investigated in several studies, among these who studied it were Hsiao and colleagues that reported that HD astrocytes provide less pericyte coverage by promoting angiogenesis and reducing the number of pericytes [61]. Finally, in a mouse cell model of HD, many but not all pathological hallmarks of HD were found. This result suggests that nonhu-

man iPSCs must be used carefully when translated into human pathology [62].

role of glial cells in HD pathogenesis.

**3. 3D bioprinting**

The researches previously reported highlight the importance to have a realistic model of the disease to study mechanisms that lead to neurodegeneration and iPSC-derived neurons that represent as a useful tool. They can be used also to perform a study of drug discovery and drug screening, to better understand the effect of chemicals in neurons. Moreover, the possibility to differentiate iPSCs in nonneuronal cells, such as astrocytes, helps to discover the

The term bioprinting was used for the first time in 2009 by Mironov with the release of the first issue of the journal *Biofabrication*, a magazine that took its name from the eponymous term biofabrication. While the term biofabrication is intended to indicate natural processes such as biomineralization, the term bioprinting is defined by Guillemot in 2010 as [63, 64].

The use of computer-aided transfer processes for patterning and assembling living and nonliving materials with a prescribed 2D or 3D organization in order to produce bio-engineered structures serving in regenerative medicine, pharmacokinetic and basic cell biology studies. 3D bioprinting is an emerging technology, used for the manufacture and the generation of artificial tissues and organs [65], adding new approaches to tissue engineering (TE) and regenerative medicine, such as the manufacture of scaffold to support cells, as well as in situ deposition of cell suspensions [63]. Bioprinting technology has allowed to overcome several limits, such as the control of in vitro 3D biological structures and cellular distribution [66].

### *2.2.3. iPSCs in amyotrophic lateral sclerosis*

Amyotrophic lateral sclerosis (ALS) is the most prevalent motor neuron disease and is characterized by the progressive loss of upper and lower motor neurons (MNs), leading to muscle atrophy, paralysis, and finally death usually after 2–5 years from the first diagnosis [21]. Also for ALS the cause is still unknown, but in about 5–10% of cases, several genes are found mutated, among which are SOD1, TARDPB, and FUS [46]. Moreover, in 2013 the GGGGCC-hexanucleotide repeat expansion in C9orf72 locus was found in many familial and sporadic cases of ALS [47].

MNs derived from iPSCs are the most common neural cell type used in ALS involving the use of stem cell differentiation. For example, an increase in oxidative stress and in DNA damage was found in iPSC-derived C9ORF72 MNs, confirming that the reduction of oxidative stress could help to delay patients' death [48]. Moreover, MNs derived from iPSCs with induced mutation in FUS (P525L) were used to investigate the transcriptome and microRNA, finding an alteration of both in pathways with implications for ALS pathogenesis [49]. The role of astrocytes was also investigated in both sporadic and VCP mutant patients, suggesting that in ALS patients, the co-culture between MNs and astrocytes causes alterations in both cell types [50, 51]. Moreover, the genetic correction allowed to study pathways implicated in ALS, like Bhinge and colleagues that found that the activation of AP1 drives neurodegeneration in genetic corrected SOD1 mutant MNs [52]. Small-molecule compounds that regulate IGF-2 expression were found to increase MN resilience, screening the compounds in iPSC-derived MNs [53]. Another example is given by Egawa and colleagues that firstly generated and characterized MNs from iPSCs of patients carrying TDP-43 mutations. They found some pathological hallmark, such as short neurites and abnormal-insoluble TDP-43. Then, they tested trichostatin A, spliceostatin A, garcinol, and anacardic acid and found that the last one, an inhibitor of histone deacetylase, rescued the pathogenic abnormalities like TDP-43 mRNA [54]. All these researches suggest the increasing importance of iPSCs as model for drug screening.

These works suggest that MNs derived from iPSCs of mutated and sporadic ALS patients can be a helpful tool to study both disease mechanisms and drug screening. Several investigations can be done in iPSC-derived MN cellular models, e.g., oxidative stress, DNA damage, and transcriptome. The co-culture between astrocytes and MNs can give information about how they interact with each other and whether this interaction could have a pathophysiologic role in ALS.

#### *2.2.4. iPSCs in Huntington's disease*

Huntington's disease (HD) is characterized by loss of neurons mainly in the caudate nucleus, the putamen, and the cerebral cortex with affection in a later stage of other areas, e.g., the hippocampus and hypothalamus [55]. Despite other neurodegenerative diseases, the cause of HD is well known; in fact it is an autosomal dominant genetic disorder caused by an expansion mutation of the trinucleotide (CAG) repeat in the HTT (IT15) gene, encoding a 350-kDa protein called Huntingtin (HTT) [56]. Even though the genetic cause is clear, the mechanisms through which mutant HTT results in the degeneration of some types of neurons are still unclear. Thus, studies on HD models are needed in order to discover treatments.

As the neurodegenerative diseases previously reported, also for HD, neurons differentiated from iPSCs of patients helped to understand the role of mutant HTT gene and the mechanisms that lead to the pathology. For example, early molecular changes in intracellular signaling, expression of oxidative stress proteins, and p53 pathway both in iPSCs and in neurons differentiated from them were reported [57]. Another study reported changes in neuronal development and adult neurogenesis, exploiting the iPSC capacity to model also embryonal development [58]. The generation of iPSCs from a patient that presents an expansion in the HTT gene without any symptom is very intriguing. The generation of iPSCs in an early stage of HD will allow to study the pathological process and the abnormal changes that lead to the pathology [59]. The possibility to differentiate iPSCs into neurons opened the possibility to discover new therapeutic targets, e.g., pre-mRNA trans-splicing modules [60]. Finally, the role of glial cells was investigated in several studies, among these who studied it were Hsiao and colleagues that reported that HD astrocytes provide less pericyte coverage by promoting angiogenesis and reducing the number of pericytes [61]. Finally, in a mouse cell model of HD, many but not all pathological hallmarks of HD were found. This result suggests that nonhuman iPSCs must be used carefully when translated into human pathology [62].

The researches previously reported highlight the importance to have a realistic model of the disease to study mechanisms that lead to neurodegeneration and iPSC-derived neurons that represent as a useful tool. They can be used also to perform a study of drug discovery and drug screening, to better understand the effect of chemicals in neurons. Moreover, the possibility to differentiate iPSCs in nonneuronal cells, such as astrocytes, helps to discover the role of glial cells in HD pathogenesis.
