Adult stem cells in neural repair: Current options, limitations and perspectives

Abstract

Stem cells represent a promising step for the future of regenerative medicine. As they are able to differentiate into any cell type, tissue or organ, these cells are great candidates for treatments against the worst diseases that defy doctors and researchers around the world. Stem cells can be divided into three main groups: (1) embryonic stem cells; (2) fetal stem cells; and (3) adult stem cells. In terms of their capacity for proliferation, stem cells are also classified as totipotent, pluripotent or multipotent. Adult stem cells, also known as somatic cells, are found in various regions of the adult organism, such as bone marrow, skin, eyes, viscera and brain. They can differentiate into unipotent cells of the residing tissue, generally for the purpose of repair. These cells represent an excellent choice in regenerative medicine, every patient can be a donor of adult stem cells to provide a more customized and efficient therapy against various diseases, in other words, they allow the opportunity of autologous transplantation. But in order to start clinical trials and achieve great results, we need to understand how these cells interact with the host tissue, how they can manipulate or be manipulated by the microenvironment where they will be transplanted and for how long they can maintain their multipotent state to provide a full regeneration.

Key Words: Stem cells; Stem cell therapy; Adult stem cells; Neural stem cells; Bone marrow stem cells; Mesenchymal stem cells; Olfactory ensheathing cells

Core tip: Adult stem cells are useful tools to treat various diseases, but we must first comprehend how they work to use their capacity at maximum. Ageing, inflammation, other stem cells of the host tissue and the co-transplantation with another stem cells type can change their profile and compromise the regeneration process. Having these barriers in mind, several researchers started to look more closely to adult stem cells. In this review we will show some interesting results from experimental and clinical trials at this group of stem cells.

INTRODUCTION

Stem cells represent a promising step for the future of regenerative medicine. As they are able to differentiate into any cell type, tissue or organ, these cells are great candidates for treatments against the worst diseases that defy doctors and researchers around the world.

Stem cells can be divided into three main groups: (1) embryonic stem cells, which are cells of the inner layer of the blastocyst and form the three primary germ layers (ectoderm, mesoderm and endoderm); (2) fetal stem cells, which are found in the embryo and form the various organs of the human body; fetal stem cells include neural crest cells, hematopoietic cells, fetal mesenchymal stem cells, fetal neural stem cells and pancreatic islet progenitors; and (3) adult stem cells, which are found in various regions of the adult organism, such as bone marrow, skin, eyes, viscera and brain (adult stem cells are also known as somatic stem cells).

In terms of their capacity for proliferation, stem cells are also classified as totipotent, pluripotent or multipotent. Totipotent stem cells have a high capacity for differentiation and can form a complete individual, but they lack the capacity for self-renewal. Pluripotent cells, on the other hand, are capable of differentiating into any of the 200 or more kinds of cells that make up the human body. Finally, multipotent cells give rise to cells of a specific tissue, e.g., hematopoietic stem cells give rise to blood cells, while neural stem cells give rise to neurons and glia[1,2]. Another group of stem cells that has been extensively studied in recent years includes induced pluripotent stem cells (IPS). IPS cells are adult differentiated cells that return to their pluripotent state by genetic engineering mechanisms. Some researchers believe these cells have great potential, but there have been numerous reports of extensive epigenetic and transcriptome aberrations with these cells, as well as tumor formation[3,4].

ADULT STEM CELLS

Adult stem cells are multipotent cells that can differentiate into unipotent cells of the residing tissue, generally for the purpose of repair. These cells can be obtained from several different regions of the adult organism, they can be used for autologous transplantation and their great advantage resides in the fact that embryos do not have to be destroyed in order for these cells to be obtained[5,6].

Regardless of the cell type chosen, there is a relative degree of difficulty in extracting viable stem cells from their in vivo niche. There is approximately one hematopoietic stem cell for every 10000 bone marrow cells, and one mesenchymal stem cell for every 10000-100000 cells[7]. For this reason, researchers use substances that can stimulate the growth of these cell populations as well as facilitate the process by which they are obtained. For example, G-CSF (granulocyte stimulating factor) stimulates the production of CD34⁺ hematopoietic stem cells that can be extracted by simple puncture of peripheral blood, which is less invasive than bone marrow aspiration. Other substances that enhance the isolation of adult stem cells are currently under investigation[8].

Another issue of concern is the cell senescence related to the expansion time in vitro. Cell proliferation is decreased mainly due to telomerase activity, and several researchers have focused on telomerase activation in an attempt to increase and maintain population growth in vitro. However, it focusing only on correcting the shortened telomeres is not sufficient, and studies should focus on the role of other factors that participate in telomere maintenance, which may also play an important role in cell differentiation[9].

Senescence of adult organisms has to be considered, given that cell function, potency and replication decrease with time, while mutations may increase. In the case of the nervous system, mutations may result in neuronal loss, abnormal cell function, increased neuronal longevity (resulting in an even greater accumulation of mutations) and decreased regenerative capacity after an injury. These findings have been reported in studies investigating cell proliferation during learning: while young animals showed better performance than older animals, pharmacological stimulation in the older animals led to improved performance, with the newborn neurons integrating into the existent neural circuits[10-12]. Moreover, the aging of stem cell niches may contribute to the aging of stem cells, changing the organism’s regulatory microenvironment, which in turn influences the aging process of other cells. To summarize can be quoted the influence of protein p53 expression levels: when expressed at low levels, it contributes to stem cell metabolic maintenance, but when expressed at high levels, it decreases the number of stem cells by inducing cell death, and the role of energetic metabolism and the production of reactive oxygen species that acts directly in stem cell niches[13].

Another important feature of stem cells is their immunomodulatory power, which allows them to “feel” the environment and change their patterns of migration, interaction and survival. When transplanted into the lesion site, some stem cells interact with the proinflammatory environment, migrate to the site of injury and differentiate into the required cell type in order to effect repair. Moreover, some stem cells play an important role in the proliferation and migration of new cells to the site of injury. This helper profile has been observed in neural stem cells and mesenchymal stem cells. In the case of mesenchymal stem cells, this function can play an important role in changing the macrophage proinflammatory profile into a proregenerating role, once there is no longer an urgent need for a robust immune response[14-16].

Understanding the biology of stem cells and their niches, as well as the mechanisms involved in their multipotent state, are basic criteria to achieve great experimental results and to take the next step towards clinical trials.

MESENCHYMAL STEM CELLS/BONE MARROW STEM CELLS

The bone marrow is the only organ in which two different types of stem cells coexist: the bone marrow stem cell (BMC, also called hematopoietic stem cell) and the mesenchymal stem cell (MSC, also called mesenchymal stromal cell). Bone marrow cells are responsible for generating blood cells, while mesenchymal stromal cells are responsible for supporting hematopoietic progenitors by regulating the niche microenvironment and by facilitating the maturation of blood cells. Mesenchymal cells can also be extracted from adipose tissue, umbilical cord, skin and placenta[17,18].

Experimental studies have shown that MSCs can survive and migrate to the lesion site, prevent astrogliosis and microglial activation, and delay the loss of motoneurons. A comparative study between BMCs and MSCs showed that BMCs are relatively more effective in motor function recovery and have a higher survival rate[18,19]. In clinical trials with BMCs, authors reported motor and sensory improvement, as well as improvement in patients’ quality of life, with some being able to sit again and even get dressed with partial assistance[19-23]. MSCs also showed great results when transplanted with T cells, controlling inflammatory activity in order to create the proper microenvironment for cell transplantation[24]. The immunomodulating effect of MSCs has also been reported in other clinical trials and MSCs have been shown to play an important role as immunomodulators and angiogenic agents in drug resistant patients as well as in patients with cerebral palsy, critical limb ischemia and kidney transplants[25-32]. In another study conducted with 40 patients, bone regeneration was achieved when a great number of CD34⁺ cells were transplanted to the lesion site, 20 patients achieved mature bone regeneration, even in the group that received a low number of cells 4 patients achieved bone regeneration. However, when transplanted together with MSCs, these cells were able to use paracrine and autocrine cross-talk up-regulation to achieve bone regeneration[33].

Despite these positive results, several side effects were reported when MSCs from adipose tissue were administered via the cephalic vein, including chest pain and tightness, mild fever, furuncle on the upper thigh, musculoskeletal pain, painful neck and shoulder, increased sputum, upper respiratory infection, urinary incontinence, urinary tract infection, aggravation of spasticity, neuropathic pain, pain exacerbation, headache, low thyroid stimulating hormone and somnolence[34].

OLFACTORY ENSHEATHING CELLS

Neurogenesis in the olfactory system continues to take place even in the adult. Stem cells proliferate in the subventricular zone of the forebrain, generating neural progenitors that migrate to the olfactory bulb to create new interneurons. If an injury occurs, these neurons are immediately replaced through a surge in neurogenesis. Olfactory ensheathing cells (OECs) surround the axons of the sensory neurons in the olfactory epithelium and form synapses in the olfactory bulb in the brain. Due to their ability to guide the connections between the peripheral nervous system and the central nervous system, as well as their ability to differentiate into non-olfactory cell types, these cells are excellent candidates for cell transplantation[35]. These multipotent cells have been extensively studied in cases of spinal cord injury, and authors have reported that transplants were safe and patients experienced motor and sensory improvement, as well as recovered bladder function and activity of several muscles below the injury level[36,37]. In an amyotrophic lateral sclerosis (ALS) clinical trial conducted with OECs in China, researchers reported that patients experienced no benefits, two patients had severe side effects and one even died following transplantation[38]. One theory contends that OECs should be transplanted together with neural stem cells in order to potentiate the growth of neural processes. OECs have been shown to stimulate axon regeneration by secreting growth factors, axon guidance molecules and basement membrane components. They also aid in tissue repair by effecting structural remodeling and support, modulating the immune system, enhancing neurotrophic and antigenic stimuli and metabolizing toxic macromolecules. Finally, OECs may be transplanted together with growth factor (e.g., bFGF, or basic fibroblast growth factor) to sustain cell survival and proliferation[39-41].

NEURAL STEM CELLS

Neural stem cells were first described by Altman in 1960 and have the potential to differentiate into any cell type in the central nervous system[42-45]. It is known that adult neurogenesis occurs in two brain regions, the subventricular and subgranular zones of the dentate gyrus, and the spinal cord[45].

Neural stem cells have been used to treat several neurologic conditions such as spinal cord injury, ALS, Parkinson’s disease, traumatic brain injury, Huntington’s disease and several demyelinating diseases. The main issues to consider when using neural stem cells are how the transplanted cells will interact with the host microenvironment, how the local immunological response will interfere with and prevent neurorestoration, and how the transplanted neural stem cells will modulate the microenvironment via paracrine and autocrine effects. These mechanisms need to be clarified before moving on to clinical trials. Several investigators have questioned the behavior of transplanted cells and several transplantation strategies have been tested, including co-transplantation with other stem cell types, T cells or neurospheres. Nevertheless, much work needs to be done in order to better comprehend how neural stem cells interact with the host tissue[45-52]. Clinical trials have not shown statistically significant results, Moviglia et al[47] transplanted neural stem cells in seven patients with ALS and only in five patients were observed motor improvements. In their combined protocol the local immunological response were controlled by a T-cell vaccination before the transplants of NSCs, but authors agree that the transplantation of neural stem cells is safe and feasible[46-49] (Table 1).

Table 1 Adult stem cells transplantations: Published clinical trials from the past 4 years.

Ref.	Year	No. of patients	Age range	Type of cell grafted	Adjuvant treatments	Follow-up
Dai et al[53]	2013	20	22-54 yr	MSCs	-	6 mo
García-Santos et al[54]	2013	11	33-61 yr	MSCs	-	12 mo
Tian et al[55]	2013	97	21.1-38.21 yr	MSCs	-	14 d
Frolov et al[56]	2012	20	18-55 yr	HSCs	-	48 mo
Karamouzian et al[20]	2012	31	10-50 yr	BMCs	-	33 mo
Martínez et al[57]	2012	67	49.2 ± 10.31 yr	CD133+	-	12 mo
Mazzini et al[28]	2012	19	20-75 yr	MSCs	-	108 mo
Moviglia et al[47]	2012	7	33-78 yr	NSCs	T-cell vaccine	12 mo
Prasad et al[58]	2012	11	30-70 yr	MNCs	-	52 wk
Brazzini et al[59]	2010	53	38-81 yr	BMCs	-	1-18 mo
Karussis et al[26]	2010	19	53.01 yr	MSCs	-	25 mo
Lee et al[60]	2010	16	64.0 ± 11.61 yr	MSCs	-	5 yr
Venkataramana et al[61]	2010	7	22-62 yr	MSCs	-	12-36 mo

¹The authors only provide mean values. MSCs: Mesenchymal stem cells; HSCs: Hematopoietic stem cells; BMCs: Bone marrow stem cells; NSCs: Neural stem cells; MNCs: Mononuclear cells from bone marrow.

CONCLUSION

Adult stem cell transplantation represents a promising choice of treatment for the field of regenerative medicine, but several aspects must still be clarified before proceeding with clinical trials. More studies are needed to establish how to obtain a large population of adult stem cells and to ensure the safety and viability of the transplants. We must also understand these cells’ mechanisms of interaction and how we can use these mechanisms to achieve full regeneration.

Based on the studies cited here, it is possible to affirm that in the near future we will have effective therapies against various diseases that affect and challenge the medical community and the population at large.