**Trophic Factors in the Therapeutic Challenge Against ALS: Current Research Directions**

Anna Sobuś and Bogusław Machaliński

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63428

#### **Abstract**

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212 Update on Amyotrophic Lateral Sclerosis

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Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder, which up to date remains incurable. Multiple experimental approaches toward finding an effective way of reducing ALS progression and improving patients' condition have been proposed but none of them brought significant desired effects. In recent years, studies focused on stem cells (SCs) have proven that not only cells themselves but also trophic factors, which they secrete, may cause positive effects on neural tissue environment. Crucial issues that have to be considered in any study implementing SC's secreted trophic factors are administration route and type of administered cells. Furthermore, the understanding of trophic factor function, secretion manner, and their potential influence on damaged cells may be immensely beneficial. This chapter focuses on recent studies exploiting trophic factors to improve ALS patients and animal ALS models' condition.

**Keywords:** amyotrophic lateral sclerosis, trophic factors, neurotrophins, BDNF, GDNF, VEGF, IGF-1, GLP-1

#### **1. Introduction**

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a fatal neurode‐ generative disease, characterized by progressive loss of motor neuron functions in the spinal cord, cortex, and brainstem [1]. ALS was first described in the 1870s by the French neurolo‐ gist Jean-Marie Charcot. The incidence of ALS ranges from 1 to 4 new cases per year. Typi‐ cal first symptoms, which occur because of gradual loss of neuron functions, are muscle weakness, impaired reflexes, and speech difficulties. Patients in the later stages of the disease

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may also suffer from cognitive impairments, among which aphasia and semantic dementia are the most frequently reported [2]. Eventually, in most cases, weakening of respiratory muscles results in death caused by asphyxiation within 3–5 years after diagnosis. From a pathophysiologic point of view, two forms of ALS are widely described—familial ALS (fALS) and sporadic ALS (sALS). fASL affects approximately 10% of patients, whereas sALS is responsible for the other 90%. Mutations in *SOD1* gene, which encodes Cu/Zn superoxide dismutase 1, are common among patients with fALS, and more than 160 different mutations in *SOD1* have been described to date [2]. However, recent studies have reported potential relationships between other genes and fALS, including those encoding transactive response DNA-binding protein (TARDBP), fused in sarcoma (FUS) and c90RF72 [3]. The precise cause of sALS has not been identified, but environmental factors, lifestyle, and genetic predisposi‐ tions are considered as key factors in the disease development [4]. Evidence has demonstrat‐ ed that especially exposure to tobacco smoke and the quantity of cigarettes smoked are significantly correlated with ALS [5]. Pathophysiological mechanisms that result in neurode‐ generation involve protein aggregation, mitochondrial dysfunction, generation of free radicals, excitotoxicity, disrupted axonal transport, and dysregulation of neurotrophic factor levels [6]. Notably, no effective cure for ALS has been discovered, leaving newly diagnosed patients with no chance for complete recovery. The only remedy approved by the Food and Drug Administration (FDA) is riluzole, an antiglutamatergic agent, which may cause excitotoxicity reduction and, therefore, extend survival by up to few months [7]. Because of the lack of an effective treatment and the relatively late manifestation of first symptoms, often when neuron loss is already in an advanced stage, the search for new ways to manage ALS turns to stem cell (SC)-based therapy. SCs are considered to be a promising tool of modern medicine because of their ability to transform into almost any other cell type and their capability of an infinite divisions number. Two main directions in SC-based therapies for ALS are considered – "structural" cell replacement and humoral neuroprotection via secretion of trophic factors. The main issues that must be considered in all forms of SC-based therapies are the proper administration method, the optimal type of administered cells, and the identifica‐ tion of factors that are crucial for the survival and differentiation of transplanted cells [8]. Recently, the greatest effect evidenced from the studies implementing various types of SCs is their ability to improve neural tissue microenvironment and provide soluble neurotrophic factors rather than structural replacement of lost cells [9]. Accordingly, this review focuses on the role of trophic factors in ALS progression and the chance of implementing therapies that take advantage of their properties.

Two different types of adult SCs are under extensive investigation in the perspective of ALS therapy: mesenchymal stem cells (MSCs) and SCs connected with the organization of nerve tissue. MSCs are an attractive source for potential therapeutic approaches for several reasons. Firstly, MSCs are characterized by great plasticity [10] and can be easily obtained from different sources, including bone marrow [bone marrow stem cells (BMSCs)], adipose tissue [adipocyte stem cells (ASCs)], or umbilical cord blood [11]. Moreover, MSCs can differentiate into cells of all three germ layers (ectoderm, endoderm, and mesoderm) when cultured under specific conditions [12], and their expansion in vitro does not involve any changes in function or chromosome structure, as observed in cells obtained from ALS patients [13]. It has been also found that MSCs can support regeneration of damaged parenchymal cells by scavenging toxic inflammatory cytokines and secreting trophic cytokines that are involved in neuroprotection [14]. Cells connected with nerve tissue organization are also being used in ALS research, including neural progenitor cells (NPCs), astrocyte precursor cells [15], and olfactory en‐ sheathing stem cells (OESCs), which have been recently used in treatment of spinal cord injury [16]. Embryonic SCs have also been extensively investigated, but their implementation raises major clinical and ethical concerns. Although the physiological relevance of pluripotency is of significant importance, induced pluripotent stem cells (iPSCs) have been demonstrated to differentiate into a variety of cell types, including muscle, cardiac muscle, and hepatocyte cells. All of the abovementioned cell types might present different advantages when used in ALS management. Because ALS affects motor neurons at different levels and in various ways, attention is focusing more on restoring neuronal tissue homeostasis than on cell replacement. In addition, conditions in the adult spine do not favor the differentiation of transplanted cells into neural ones. However, it has been observed that the environment in spinal cord fluid from ALS patients stimulates transplanted MSCs to secrete factors that relieve ALS symptoms [17].

may also suffer from cognitive impairments, among which aphasia and semantic dementia are the most frequently reported [2]. Eventually, in most cases, weakening of respiratory muscles results in death caused by asphyxiation within 3–5 years after diagnosis. From a pathophysiologic point of view, two forms of ALS are widely described—familial ALS (fALS) and sporadic ALS (sALS). fASL affects approximately 10% of patients, whereas sALS is responsible for the other 90%. Mutations in *SOD1* gene, which encodes Cu/Zn superoxide dismutase 1, are common among patients with fALS, and more than 160 different mutations in *SOD1* have been described to date [2]. However, recent studies have reported potential relationships between other genes and fALS, including those encoding transactive response DNA-binding protein (TARDBP), fused in sarcoma (FUS) and c90RF72 [3]. The precise cause of sALS has not been identified, but environmental factors, lifestyle, and genetic predisposi‐ tions are considered as key factors in the disease development [4]. Evidence has demonstrat‐ ed that especially exposure to tobacco smoke and the quantity of cigarettes smoked are significantly correlated with ALS [5]. Pathophysiological mechanisms that result in neurode‐ generation involve protein aggregation, mitochondrial dysfunction, generation of free radicals, excitotoxicity, disrupted axonal transport, and dysregulation of neurotrophic factor levels [6]. Notably, no effective cure for ALS has been discovered, leaving newly diagnosed patients with no chance for complete recovery. The only remedy approved by the Food and Drug Administration (FDA) is riluzole, an antiglutamatergic agent, which may cause excitotoxicity reduction and, therefore, extend survival by up to few months [7]. Because of the lack of an effective treatment and the relatively late manifestation of first symptoms, often when neuron loss is already in an advanced stage, the search for new ways to manage ALS turns to stem cell (SC)-based therapy. SCs are considered to be a promising tool of modern medicine because of their ability to transform into almost any other cell type and their capability of an infinite divisions number. Two main directions in SC-based therapies for ALS are considered – "structural" cell replacement and humoral neuroprotection via secretion of trophic factors. The main issues that must be considered in all forms of SC-based therapies are the proper administration method, the optimal type of administered cells, and the identifica‐ tion of factors that are crucial for the survival and differentiation of transplanted cells [8]. Recently, the greatest effect evidenced from the studies implementing various types of SCs is their ability to improve neural tissue microenvironment and provide soluble neurotrophic factors rather than structural replacement of lost cells [9]. Accordingly, this review focuses on the role of trophic factors in ALS progression and the chance of implementing therapies that

Two different types of adult SCs are under extensive investigation in the perspective of ALS therapy: mesenchymal stem cells (MSCs) and SCs connected with the organization of nerve tissue. MSCs are an attractive source for potential therapeutic approaches for several reasons. Firstly, MSCs are characterized by great plasticity [10] and can be easily obtained from different sources, including bone marrow [bone marrow stem cells (BMSCs)], adipose tissue [adipocyte stem cells (ASCs)], or umbilical cord blood [11]. Moreover, MSCs can differentiate into cells of all three germ layers (ectoderm, endoderm, and mesoderm) when cultured under specific conditions [12], and their expansion in vitro does not involve any changes in function or chromosome structure, as observed in cells obtained from ALS patients [13]. It has been also

take advantage of their properties.

214 Update on Amyotrophic Lateral Sclerosis

**Figure 1.** Classification of neurotrophic factors according to their structure and function. NGF, nerve growth factor; BDNF, brain-derived neurotrophic factor; NT-3, neurotrophin-3; NT-4, neurotrophin-4; GDNF, glial-derived neurotro‐ phic factor; NTN, neurturin; ARTN, artemin; PSPN, persephin; EGF, epidermal growth factor; IGF-1, insulin-like growth factor; FGF, fibroblast growth factor; HGF, hepatocyte growth factor; EPO, erythropoietin; VEGF, vascular en‐ dothelial growth factor; CNTF, ciliary neurotrophic factor; TNF, tumor necrosis factor.

These factors are mainly neurotrophins, proteins synthesized and secreted by the brain, spinal cord, and other cells that are dependent on peripheral sensory neurons.

Neurotrophins are responsible for nerve growth and survival, synapse formation, and axonal growth [18]. In general, neurotrophins provide neuroprotection, thus slowing down neuro‐ degeneration. Neurotrophins also prevent oxidative stress and inhibit apoptosis [19]. Neuro‐ trophic factors can be categorized in different ways based on their activities in preventing neuronal cell death [19] or by their structural and functional affiliations (**Figure 1**). Accord‐ ingly, "classic" neurotrophins, ligands of glial-derived neurotrophic factor (GDNF), and neuroprotective cytokines can be distinguished. The group of classic neurotrophins includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) [20]. Furthermore, factors that are connected with more than just neural cells, such as IGF-1, also demonstrate potential as ALS therapies. Growth factors involved in vasculogenesis, such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor, and angiopoietin, might also be correlated with ALS progression.
