by Dr. Dong Sun
Department of Neurosurgery, School of Medicine, Virginia Commonwealth University, Richmond, Virginia
Traumatic brain injury (TBI) is a major health problem worldwide. Despite improving survival rate after TBI, currently, there is no effective treatment to improve neural structural repair and functional recovery of TBI survivors. Neural regeneration either through stimulating endogenous neural stem cells or through stem cell transplantation has gained increasing attention in the field of neurotrauma research. This article summarizes strategies which have been explored thus far utilizing stem cells to repair and regenerate the injured brain following TBI.
Targeting endogenous neurogenesis to promote neural regeneration following brain trauma
One of the leading discoveries in neuroscience research in recent years is the identification of adult neurogenesis in the mature mammalian brain. It is now well established that new neurons are constantly generated throughout life in the mature mammalian brain in the neurogenic regions of the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus. The adult-generated neurons in the DG play important roles in hippocampal dependent learning and memory functions (Deng et al., 2009; Clelland et al., 2009; Aimone et al., 2014), whereas the SVZ derived new olfactory interneurons are required for the normal functioning of the olfactory bulb network and some selected olfactory behaviors (Moreno et al., 2009; Breton-Provencher et al., 2009; Sakamoto et al., 2014). It is also known that the degree of adult neurogenesis is affected by many factors including pathological conditions of the brain. In the case of TBI, extensive studies have demonstrated that TBI enhances neurogenesis particularly in the DG of the hippocampus in varying types of TBI animal models and also in human (Sun, 2016). The newly generated injury-induced neurons are also capable of becoming mature functional neurons integrating into the existing neural network (Villasana et al., 2015) and are directly associated with spontaneous cognitive functional recovery following TBI (Sun et al., 2007; Sun et al., 2015; Blaiss et al., 2011). These evidences raise the possibility of developing therapies targeting this endogenous neurogenic response to regenerate the injured brain.
Although brain trauma itself can enhance this endogenous neurogenic response, the capacity of this self-repair is rather limited. To achieve the goal of regeneration, many strategies have been explored to further augment this neurogenic response. These strategies include:
- Direct supplementing varying types of growth factors such as basic fibroblast growth factor, epidermal growth factor, or vascular endothelial growth factor (Sun et al., 2009; Sun et al., 2010; Lee and Agoston, 2010; Thau-Zuchman et al., 2010);
- Utilizing small molecules which imitating growth or neurotrophic factors, such as 7,8-dihydroxyflavone, a synthetic neurotrophin TrkB receptor agonist imitates brain-derived neurotrophic factor (BDNF); small-molecule p75NTR signaling modulator, LM11A-31, which is a pharmacologically prepared low molecular weight neuropeptide cerebrolysin etc. (Shi et al., 2013; Chen et al., 2015; Zhao et al., 2015; Zhang et al., 2015);
- Using peptides or FDA approved drugs such as Erythropoietin, Thymosin β4, P7C3 class of aminopropyl carbazole agents, Statins, selective serotonin reuptake inhibitor Imipramine and Fluoxetine (Xiong et al., 2010; Lu et al., 2007b; Han et al., 2011; Blaya et al., 2014; Wang et al., 2011);
- Physical or other radical approaches such as environmental enrichment, physical exercise, transcranial low light laser therapy etc. (Xuan et al., 2014; Gaulke et al., 2005;Piao et al., 2013).
These aforementioned strategies can promote neural regeneration through enhancing neural stem cell proliferation, increasing the survival of newly generated neurons or their neuronal differentiation, as well as through neural protective effect to improve the recovery of functions of the injured animals. These studies have demonstrated the potential of targeting endogenous repair mechanisms for neural regeneration following TBI.
Stem cell transplantation for brain repair and regeneration
Neural transplantation is a prospective therapy for TBI as transplanted cells may differentiate into region-specific cells and integrate into the host tissue to replace the lost cells in the injured brain. Alternatively, transplanted cells could acting as carriers providing trophic support or neurotransmitters to the host tissue to facilitate regeneration. To date, a wide array of cell sources for neural transplantation have been explored for their potential utility for TBI and are listed below.
Embryonic stem cells
Embryonic stem (ES) cells derived from fetal or embryonic brains are highly pluripotent that have unlimited capacity of self-renewal and can give rise to cells of all three primary germ layers. Studies have shown that human ES cells can differentiate, migrate and are capable of making innervations after transplanted into normal or injured brain (Hentze et al., 2007). Thus far in experimental studies, human or mice derived ES cells have been tested as transplantation cell source for TBI treatment in different TBI models with varying results reported. These studies have shown that neural stem cells (NSCs) from human ES cells isolated from fetal brain were capable of survival for an extended period up to 6 weeks, migrating to the contralateral cortex and differentiating into neurons and astrocytes when transplanted into the injured brain following a cortical contusion injury (Wennersten et al., 2004). Transplanted human ES cell-derived NSCs not only can differentiate into mature neurons but also able to release growth factors that aid in the recovery of cognitive functions of the injured host (Gao et al., 2006). NSCs derived from mice fetal brains can survive for extended period of up to 1 year, migrating widely in the injured brain, becoming mature neurons or glial cells. The injured host later showed significant improvement in motor and spatial learning functions (Shear et al., 2004; Riess et al., 2002; Boockvar et al., 2005). Additionally, in vitro modified ES cells either pre-differentiated into mature neurons expressing neurotransmitters or over-expressing growth factors can significantly promote graft survival and neuronal differentiation, also improving the motor and cognitive functional recovery of injured recipients (Becerra et al., 2007; Bakshi et al., 2006; Ma et al., 2012; Blaya et al., 2015).
Adult neural stem cells
It is now well recognized that mature mammalian CNS harbors multipotent stem cells capable of differentiation into a variety of specialized cells throughout life (Lois and Alvarez-Buylla, 1993; Gage et al., 1998). These adult derived neural stem cells can become region-specific cells when transplanted into the normal adult rat brains (Gage et al., 1995; Richardson et al., 2005; Zhang et al., 2003), and can survive for an extended period and become region-specific functional cells in the injured brains in a TBI rodent model (Sun et al., 2011). Neural progenitor cells can also be isolated from various regions of adult human brain from neurosurgical resection tissues (Kukekov et al., 1999; Arsenijevic et al., 2001; Brunet et al., 2002; Brunet et al., 2003; Roy et al., 2000; Nunes et al., 2003; Windrem et al., 2002; Richardson et al., 2006), raised the possibility of using these cells as autologous cell sources for neural transplantation therapies. These cells may be possible to restore the anatomy and function of the injured CNS as shown in a study after grafting adult human derived neural stem/progenitor cells (NS/NPCs) into the demyelinated rat spinal cord (Akiyama et al., 2001). However, due to their adult origin, these cells have less plasticity compared to the ES cells. To date, very few studies have reported the outcome of these adult human derived NS/NPCs in the injured mature CNS. Olstorn and colleagues reported that a small portion (4±1%) of adult human NS/NPCs can survive for 16 weeks after transplantation into the posterior periventricular region in normal adult rats or rats with hippocampal CA1 ischemic injury (Olstorn et al., 2007).
Bone marrow stromal cells
Due to ethical and immunological concerns as well as the risk of tumorigenesis, the translational value of using ES cells for clinic application is limited. Autologous transplantation of NS/NPCs isolated from neurosurgical removed brain tissue from TBI patients is an attractive strategy; however, the success of long term cell survival and functional outcomes of these cells in the treatment of experimental TBI is rather limited. Due to these drawbacks, adult-derived mesenchymal cells, particularly the bone marrow stromal cells (BMSCs), have received more attention.
BMSCs are undifferentiated cells with mixed cell population including stem and progenitor cells. BMSCs have several specificities for neural transplantation:
- can be easily isolated from the mononuclear fraction of bone marrow from patients and be expanded in culture;
- express low level of the major histocompatibility complex antigens (MHC Class II) thus with low antigenicity (Le and Ringden, 2005);
- produce high level of growth factors, cytokines and extracellular matrix molecules that could have potential neurotrophic or neuroprotective effects in the injured brain (Li and Chopp, 2009; Zhang et al., 2013).
The potential of BMSCs for treating TBI have been extensively assessed in experimental TBI models. Cells were delivered either focally to the injured brain, or systemically through intravenous or intra-arterial injections at the acute or sub-acute phase after TBI and significant reduction of neurological deficits including motor and cognitive deficits was reported (Lu et al., 2001; Mahmood et al., 2001; Mahmood et al., 2003). Co-transplantation with biomaterial such as collagen scaffolds, human BMSCs have better graft survival rate and the host has higher degree of tissue regeneration as well as further functional improvement (Lu et al., 2007; Xiong et al., 2009; Guan et al., 2013). The effect of BMSCs in improving sensorimotor function of injured animals was reported even when delivered at two months following TBI (Bonilla et al., 2009). Further studies have demonstrated that the beneficial effort of BMSCs in the injured brain is due primarily to their production of bioactive factors which facilitates the endogenous plasticity and remodeling of the host brain (Li and Chopp, 2009). Thus far, extensively experimental studies have demonstrated the beneficial effects of BMSCs in the injured brain and highlighted the potential of using BMSCs for TBI treatment.
Other potential types of cells and strategies for cell replacement therapy.
Several other type of stem or stem-like cells for TBI application have been explored in recent studies. These cells include human amnion-derived multipotent progenitor cells, human adipose-derived stem cells, human umbilical cord blood and peripheral blood derived MSCs. In experimental TBI studies, these cells have shown neural regenerative effects with significant attenuation of axonal degeneration, improvement of neurological function, and preservation of brain tissue morphology of the injured rats (Chen et al., 2009; Yan et al., 2013; Nichols et al., 2013; Tajiri et al., 2014). Human umbilical cord blood derived MSCs have been tested in a small scale clinic trial for TBI patients, and was reported that patients treated with umbilical cord stem cell had improved neurological function and self-care compared to the control group with no cell transplantation (Wang et al., 2013). Similar to BMSCs, the reported beneficial effect of post-TBI transplantation with these cells is likely due to the neurotrophic effect of the transplanted cells, as direct neuronal differentiation and long term survival were rarely observed.
Recent development of somatic cell reprogramming which generates induced pluripotent stem cells (iPSCs) provides prospects for novel neural replacement strategies. Human iPSCs possess dual properties of unlimited self-renewal and the pluripotent potential to differentiate into multi-lineage cells without ethical concerns. More importantly, patient-specific iPSCs can serve as autologous cell source for transplantation without graft rejection. These unique properties of iPSCs have raised hope that many neurological diseases including TBI might be cured or treated. Thus far rapid progress have been made in the field of reprogramming, however, the optimal source of somatic cells used for applications in neurological disorders has not yet been identified. In TBI models, only two publications explored the feasibility of using iPSCs for post-TBI transplantation, with rather limited information about the function, the survival and integration rate of iPSCs in the injured brain (Dunkerson et al., 2014; Tang et al., 2013).
Conclusion and Perspectives
Extensive studies have shown the prospective of brain repair through enhancing endogenous neurogenesis or through cell replacement strategy using varying types of stem cells. A considerable progress have been made in stem cell based neural regeneration in stroke and neurodegenerative diseases in both experimental and clinical settings. However for neural regeneration of injured brain following TBI, it is still a long way to go from experimental studies to clinics due to the complicity and diversity of brain trauma. To successfully repair and regenerate the injured brain with stem cells, many challenges must be overcome. In the case of endogenous neurogenesis, strategies that can promote long term survival, and guide newly generated cells migrate to the injury site are needed. Whereas in neural transplantation, the fate of transplanted cells is determined by the intrinsic properties of grafted cells and the local environmental cues in the host. To achieve successfully neural transplantation, it is necessary to improve both aspects. These challenges must be overcome in experimental TBI studies before moving forward stem cell therapies for treating the injured brain in clinical studies.
About the Author
Dr. Dong Sun, Dong Sun graduated from Chongqing Medical University in Chongqing, China, with a Medical Doctor degree in 1986 and a Master of Science degree in Radiology from West China Medical University, Chengdu, China in 1989. Following the residency training in Radiology, Dr. Sun became an attending Radiologist for a number of years in the Department of Radiology in 3rd People’s Hospital of Chengdu, Chengdu, China. In 1994, Dr. Sun started to pursue further training as a PhD student in basic Neuroscience research in Experimental Neuropathology at School of Medicine, Southampton University, United Kingdom, and obtained the PhD in 1999. Following graduation, Dr. Sun moved to United States and did two postdoctoral fellowship trainings, first in the Department of Pharmacology at Uniformed Services University of Health Sciences, Maryland, then in the Department of Neurosurgery at Medical College of Virginia, Virginia Commonwealth University (MCV/VCU), Virginia. She subsequently became an Assistant Professor and then tenured Associate Professor in MCV/VCU. Dr. Sun’s research interest is to investigate strategies that can facilitate regeneration of the injured brain following traumatic injury with particular focus on exploring the potential of neural stem cells for brain repair. Using varying types of rodent TBI models, her studies have systematically characterized the endogenous neural stem cell responses in the brain following trauma in animals throughout lifespan and examined the underlying regulatory mechanisms. She have also done extensive work examining the behaviors of neural stem cells in vitro and in vivo following transplantation into the injured brain. Her research also examines the association of aging, neuroinflammation with neurogenesis and cognitive function. Her search is well supported by NIH and other foundation grants.