Open Access

Biomaterial applications in neural therapy and repair

Chinese Neurosurgical Journal20162:34

https://doi.org/10.1186/s41016-016-0057-0

Received: 14 July 2016

Accepted: 12 September 2016

Published: 1 November 2016

Abstract

The use of biomaterials, such as hydrogels, as a scaffold to deliver cells and drugs is becoming increasingly common to treat neurological conditions, including stroke. With a limited intrinsic ability to regenerate after injury, innovative tissue engineering strategies have shown the potential of biomaterials in facilitating neural tissue regeneration and functional recovery. Using biomaterials can not only promote the survival and integration of transplanted cells in the existing circuitry, but also support controlled site specific delivery of therapeutic drugs. This review aims to provide the reader an understanding of the brain tissue microenvironment after injury, biomaterial criteria that support tissue repair, commonly used natural and synthetic biomaterials, benefits of incorporating cells and neurotrophic factors, as well as the potential of endogenous neurogenesis in repairing the injured brain.

Keywords

Biomaterial Hydrogel Microparticles Stroke Cell therapy Drug delivery Brain ECM Neurogenesis Tissue engineering Biocompatibility

Background

Stroke is the leading cause of adult disability affecting nearly 800,000 Americans each year, with ischemic stroke accounting for 80 % of all cases [1]. With the increasing incidence of stroke, and declining mortality, the number of disabled stroke survivors is expected to increase [2]. While patients currently rely on physical therapy to restore motor function after stroke, these improvements are modulated through existing brain circuitry [3], and not through replacing lost cells and tissue with up to 30 % of the stroke patients remaining permanently disabled even with intensive task specific training [4, 5]. In other words, there is a loss of functional tissue after stroke, and thus the physical therapy and rehabilitation is limited to restoring the lost cognitive and physiological functions. The lack of effective treatments for stroke and other neurological diseases can also be explained by the limited regenerative potential of the central nervous system [6].

Regenerative strategies, such as stem cell therapy, have shown limited success in improving behavioral outcomes, but there is no replacement of the lost tissue [7, 8], and hence a large tissue cavity remains in the brains of stroke survivors [9]. There are 2 major limitations with stem cell therapy. Firstly, there is a large scale loss of the transplanted cells in the days following intracerebral implantation, with survival ranging from 1 to 32 % [10]. Similarly, systemic administration of cells in ischemic rats accumulates cells primarily in internal organs instead of the brain [11]. Secondly, intracerebral cell injection requires transplantation into the peri-infarct region, which is considered to be an active site for cerebral reorganization after stroke [12], and multiple injections into this site can potentially damage the tissue even further. One key parameter that has been shown to affect cell survival and neurogenesis is the regulation of inflammatory cytokines. These cytokines have a detrimental effect on the cells transplanted in close proximity to the cavity [13]. The stroke cavity is surrounded by resident immune cells, i.e. microglia, as well as perivascular macrophages who respond to the injury by releasing inflammatory cytokines [14]. In order to modulate the immune response and improve the outcome of cell therapy, an ideal biomaterial will protect the cellular graft from the host response and interact with the activated immune cells in a positive manner.

Injectable biomaterials can be used as a scaffold to fill the stroke cavity and promote interactions between transplanted material and host tissue [1518], deliver drugs or growth factors to the damaged tissue, promote the attachment and engraftment of transplanted cells, and help recruit host cells to repopulate the lost tissue [19]. Protective hydrogels derived from natural and synthetic polymers can incorporate cells, growth factors, and other therapeutics to enhance the microenvironment and provide controlled release of bioactive molecules. This review will briefly address the complex microenvironment after stroke, ideal characteristics of a biomaterial, natural vs synthetic materials, biomaterial based cell and drug delivery, and lastly, explore the potential of endogenous neurogenesis and cell replacement in brain tissue repair.

Stroke microenvironment

In the very early stages of stroke, adenosine triphosphate (ATP) consumption continues despite insufficient synthesis due to reduced glucose and oxygen flow in the tissue. This leads to a drop in total ATP available to the cells and ionic homeostasis. Severe ischemia results in an excess release of the main excitotoxic neurotransmitter glutamate, which promotes a major influx of calcium into the cells and activates the phospholipases and proteases to degrade proteins and membranes in neuronal cells [20]. After the onset of stroke, the core of the infarct results in an area of complete cell death leading to irreversibly damaged tissue [21]. Cerebral blood flow (CBF) is highly compromised in this region with <20 % of baseline blood flow levels [22].

With fast degradation of the damaged host tissue extracellular matrix (ECM), the cavity is filled with extracellular fluid [23], and transplantation of cells in this region would result in severe loss of the injected graft [13, 24]. The brain tissue surrounding the core, or the ischemic penumbra (IP), contains partially dying cells as well as activated microglia, peripheral macrophages and astrocytes. Microglia, the resident immune cells of the brain constantly monitor the microenvironment and respond to the insult with typical macrophagic roles, such as secretion of cytokines, phagocytosis and antigen presentation. Microglia are activated within minutes of the stroke onset and accumulate surrounding the lesion cavity in the IP. The proliferation of these immune cells peaks at 48–72 h after the ischemia and may last for several weeks depending on the extent of damage [25, 26]. While traditionally these cells were considered to be deleterious and neurotoxic by releasing pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-alpha and Interleukin (IL)-1 [2729], it has been shown that activated microglia can maintain and support neuronal survival [30, 31] by releasing anti-inflammatory and neurotrophic factors. Microglia have also been shown to promote neurogenesis by guiding the stem cells to the site of injury [3234].

In addition to traditional immune cells, astrocytes have also been known to express various inflammatory mediators, such as cytokines and chemokines, that mediate the immune response. The astrocytic response after the injury, or reactive gliosis, is characterized by excessive expression of glial fibrillary acidic protein (GFAP), cellular hypertrophy and process extension, creating a glial scar tissue surrounding the lesion [3537]. It is well known that astrocytes are more resistant to oxygen and glucose deprivation [35], which enables them to survive for a prolonged period in the IP where the vasculature is partially maintained [38]. Astrocytes are involved in a number of activities during ischemia, including regulating the blood brain barrier (BBB), CBF regulation, glutamate and ion homeostasis [3842]. Although astrocytes limit axon outgrowth by expressing inhibitory molecules (e.g. proteoglycans) and forming a glial scar [43], they are also known to release extracellular matrix proteins, such as thrombospondins 1 and 2, which have been shown to increase synpatogenesis and axonal sprouting in a stroke brain [44]. In the days to weeks following brain tissue damage, microglia and astrocyte activation shifts its cytokine release profile through the secretion of anti-inflammatory and neurotrophic factors, such as TGF-β, BDNF and NGF [45]. Provision of a scaffold in the tissue cavity can promote host cells to activate endogenous repair processes, such as neurogenesis, and support tissue reconstruction in the stroke damaged brain.

Biomaterials criteria for tissue engineering approach

Biomaterial scaffolds are natural or synthetic 3D polymer networks that provide an appropriate environment for cells to attach, proliferate, and differentiate to facilitate the formation of extracellular matrix (ECM) [46]. It is important to note that the chemical and mechanical properties of a biomaterial determine the fate of transplanted cells, as well as the drug release profile. The extent of cross-linking and rate of degradation are directly affected by the chemical characteristics of the preparation and determine the overall functionality of the biomaterial.

Biocompatible and non-toxic

Biocompatibility of a biomaterial refers to its biological compatibility with the host tissue, as well as all byproducts being non-toxic and avoiding any undesirable effects on the local tissue environment. The long-term biocompatibility of the material with the host brain dictates the effectiveness of the implantation. Most commonly, the number and degree of reactivity of microglia and astrocytes surrounding the biomaterial is used as an indicator of immunorejection [47], or in vivo biocompatibility. The biocompatibility of byproducts from biomaterial degradation must also be considered, as the byproducts are often bioactive, which can influence the surrounding environment. Another factor affecting the biocompatibility of a material is dependent on the method of polymerization. Photopolymerization of a hydrogel, or crosslinking when exposed to light, can lead to formation of free-radicals which are toxic to the encapsulated cells [48, 49], as well as the host cells, which are already under high oxidative stress after the injury. However, polymerization processes that are dependent on changes in temperature or pH produce little to no free radicals and often polymerize at physiological conditions, making these polymers injectable using a minimally invasive procedure.

Biodegradable

For successful integration into neural networks, it is necessary that the chemical properties of the material allow it to degrade over time. Permanent implants could lead to chronic inflammation and sustained activation of glial cells (i.e. a foreign body response) around the implant [50]. When designing a hydrogel, the degradation rate can have effects on both the functionality of the hydrogel, as well as the host response. For example, a slow degrading hydrogel would be preferred in order to support the transplanted cells to develop their own ECM, extend processes, and integrate into neural networks. However, faster degradation could result in a reduced inflammatory response in vivo. Thus, it is important to develop materials that balance the cell supportive nature of hydrogels, as well as the rate of degradation to avoid any additional immune response.

Injectable hydrogels

Since brain injuries, such as stroke, vary in size and shape, an ideal biomaterial will fill the cavity space and form gel to repopulate the lost tissue. Crosslinking of water soluble polymers produces a hydrogel that has excellent nutrient and oxygen permeability, promoting cell survival inside the scaffold [51]. These biomaterials can be formulated to exist in liquid state at room temperature while forming gels in situ, allowing for minimal invasive delivery through small-gauge needles using MRI guidance [52]. For example, collagen, methylcellulose, and agarose are all temperature sensitive polymers and their gelation rates can easily be controlled by adding other natural polymers such as hyaluronic acid [53]. The high water content of hydrogels makes them very biocompatible and promising candidates for tissue engineering applications. It is an important consideration that large volume of hydrogel injection into the lesion cavity or into the peri-infarct area would cause tissue disruption and increased intracerebral pressure [54, 55], and therefore an innovating neurosurgical technique that allows for the drainage of ECF should be employed to avoid additional damage [52].

Gelation and retention

Ideally, hydrogel chemistry facilitates the gelation of the material and minimizes any undesirable diffusion away from the injection site. It is important to avoid any reactions with bioactive molecules in the gel and its precursors, since it can significantly lower the cross-linking density and promote interactions with proteins, leading to an inflammatory response [56]. In order to achieve a complete coverage of the lesion cavity, moderate to rapid gelation of the hydrogel is necessary to avoid permeation into the tissue and support the invasion of host cells into the cavity. Furthermore, mixing of the extracellular fluid (ECF) present in the stroke cavity with the injected hydrogel could change the chemical and mechanical properties and hence influence the extent of gelation and retention. Therefore, it is ideal to drain the ECF before or during the hydrogel injection to avoid mixing and increasing the intracerebral pressure. In experiments where partial diffusion into the host tissue is preferred, such as drug delivery, the hydrogel can be formulated with lower concentrations that support diffusion as well as partial retention in the stroke cavity [52].

Biomaterial stiffness and cell invasion

As mentioned above, the chemical properties of the biomaterial affect the mechanical properties – especially the stiffness of material after polymerization. The compressive modulus measures the stiffness of the biomaterial and can be easily varied by changing the percent composition of its monomers [57], or molecular weight of monomer [58, 59]. The stiffness of the prepared hydrogel is known to affect cell proliferation and differentiation in vivo. If the microtubule compression forces determined by scaffold stiffness are outside the sensitivity range of cells, the cells reinforce by increasing or decreasing actin filament building [60]. Rat neural stem cells (NSCs) grown on soft (<1 kPa) hydrogels differentiated primarily into astrocytes and neurons, however cells cultured on stiffer (>7kPa) gels differentiated into oligodendrocytes [61]. In addition, cells cultured on intermediate stiffness (3.5 kPa) showed the most proliferation. Similarly, mesenchymal stem cells (MSCs) responds differently to varying mechanical properties with cells differentiating to neural like cells on soft gels (0.1–1 kPa), osteogenic cells on stiffer (25–40 kPa), and myogenic cells with intermediate stiffness [62]. It is important to note that an ideal biomaterial will closely resemble the mechanical properties of the host tissue to minimize the contact stresses and an aggravated response from immune cells. Indeed, a concentration-dependent cell infiltration was observed after the injection of ECM hydrogel in ischemic rats [19]. A concentration of 8 mg/mL, with elastic modulus comparable to healthy brain tissue, produced the most significant polarization towards an M2-like macrophage phenotype, as well as the most number of neural progenitors invading the hydrogel. With degradation and loss of cross-links, the compressive modulus of the material decreases and results in loss of mechanical integrity.

Natural biomaterials

Extracellular matrix makes up 20 % of the whole brain tissue volume and plays a role in maintaining key cellular functions [63]. Hydrogels derived from ECM may provide the mechanical properties and signaling molecules to attract host cells into the lesion cavity and obviating the need for exogenous cells [8, 1517]. The decellularized biomaterial contains ECM proteins (e.g. laminin, fibronectin), myelin and growth factors, including VEGF and fibroblast growth factor-2 [64, 65].

ECM harvested from different organ systems, such as the brain, spinal cord, or urinary bladder influence neural stem cell phenotypic fate and the extent of invasion [66]. While the ECM in the peripheral tissue contains high amounts of collagen, fibronectin and laminin, the adult CNS is mainly composed of glycosaminoglycans and proteoglycans [67, 68]. In a comparison study of ECM derived from brain, spinal cord, and urinary bladder, all ECMs increased the number of cells expressing neurites, but only the brain ECM increased neurite length [69]. Injection of urinary bladder derived ECM hydrogel in rodent stroke brain promoted an acute endogenous repair response, with a significant number of neural progenitors invading the hydrogel [19].

In addition to ECM hydrogels, natural polymers like hyaluronic acid (HA) [70, 71], fibrin [72], HA-methylcellulose (HAMC) [73], chitosan [74, 75], and collagen [76] have been used extensively to deliver cells or molecules in the CNS. Collagen is a popular material used in biomedical applications since it is the most abundant protein and main component of peripheral ECM in mammalian tissues. Collagen hydrogels have been used to encapsulate a variety of stem cell types for tissue engineering applications because of their biocompatibility, mechanical strength and immunogenicity [77]. In a rodent model of cerebral ischemia, encapsulation of NSCs in collagen type I hydrogel showed an increase in cell survival (compared to injection of cells alone), formation of synapsis, and facilitated the functional recovery of neural tissue following injury [46]. Another naturally occurring polysaccharide found in CNS and used for hydrogel formation is hyaluronan. It is known to have anti-inflammatory properties and has been shown to promote cell adhesion and survival [78]. Transplantation of a cross-linked hyaluronan and heparin sulfate hydrogel in a mouse models of ischemia significantly promoted the survival of NPCs, and attenuated infiltration of immune cells into the graft compared with the cells delivered in suspension alone [79].

Synthetic biomaterials

Synthetic biomaterials allow precise control over material properties and degradation rates, slowing controlled release of small molecules or drugs into the surrounding tissue.

The commonly used biomaterials for controlled drug delivery are the polymeric agents polylactide (PL), polyglycolide (PG), and the copolymers of lactide and glycolide (PLGA). PLGA particles are loaded with bioactive molecules and are delivered to the site of injury or embedded in hydrogels to further tune the location and rate of delivery. Synthetic biomaterials, unlike natural materials like collagen and Matrigel, are better chemically defined and biologically inert which reduces the variability and the host immune response. In normal untreated animals, injection of PLGA based microspheres evoked an inflammatory response no greater than just the needle tract [80, 81]. There was a peak in astrocyte activation at 1 week post-transplantation and it diminished as the polymer degraded [80, 82, 83]. Like natural biomaterials, byproducts of synthetic materials can also be bioactive and influence the local microenvironment. In a recent study, application of lactic acid (byproduct of PLGA) on cultured slices of developing mouse cerebral cortex supports oligodendrocyte development and myelination [84]. Nanoparticles and gels made from PGA, PLA and PLGA are primarily used for drug delivery since their degradation rate can be controlled by simply adjusting the PL:PG ratio.

Another synthetic polymer known to resist protein absorption and commonly used in biomaterial applications is poly (ethylene glycol) (PEG). Although cells do not directly attach to PEG hydrogels, it is most often mixed with other polymers like HA or gelatin to support cell attachment and migration. Epi-cortical delivery of PEG modified epidermal growth factor (PEG-EGF) significantly increased tissue penetration and endogenous NSC stimulation compared to unbound EGF [85]. PEG based hydrogels are promising in the fields of drug and cell delivery for many reasons, including controlled drug delivery or degradation rate, non-toxicity and biocompatibility.

Incorporating cells and growth factors

With limited endogenous neurogenesis and capacity to regenerate following injury, delivery of exogenous cells and bioactive molecules to the site of injury has shown to modulate the inflammatory response, stimulate endogenous stem cells, and promote neuroprotection and plasticity [86]. These transplanted cells can help in the tissue repair process by directly integrating into the host tissue or by secreting factors that promote neurogenesis [87]. Indeed, human NSCs transplanted in the ischemic parenchyma in rats have shown to release factors, such as VEGF and FGF-2, which are effective in stimulating the endogenous neurogenesis [88, 89]. In addition, transplanted NSCs have shown to induce a downregulation of pro-inflammatory cytokines, such as interleukin-1B and tumor necrosis factor alpha, in ischemic mice’s brains [90], drastically decreasing the microglia driven inflammatory response [91]. Unfortunately, most of the transplanted cells are lost during the acute inflammatory phase [79, 92], and fail to replace the lost connections. However, hydrogels can provide the necessary microenvironment and survival factors to increase the survival and integration of transplanted cells. Matrigel, a commonly used biomaterial derived from a mouse sarcoma with ECM components collagen, entactin, and laminin has been shown to reduce the infarct size after injury, only when used in combination with transplanting cells [93]. With the ability to protect the cells to improve survival and promote neural cell integration, more studies on cell therapy in combination with protective hydrogels would greatly advance neural tissue engineering.

Incorporation of growth factors and other bioactive molecules in a biomaterial allows researchers to deliver site specific factors with a temporal control over the release profile. The release of trophic factors is not only dependent on the chemical and mechanical properties of the material, but also on the method of encapsulation, such as direct loading or covalent binding. Delivery of VEGF via PLGA microspheres recruits endothelial cells into the graft and promote the development of a local neovascular network [94]. Other neurotrophic factors, such as BDNF, GDND and NGF, have been experimented in treating animal models of stroke and have shown the survival, proliferation, and differentiation of transplanted cells. Vascularization of the injected hydrogel is an important consideration to promote cell invasion and integration with the host tissue. Combination tissue engineering strategies involving cells, growth factors, and biomaterials are currently being pursued as a means to enhance cell survival and integration, as well as local delivery of bioactive molecules in the damaged tissue. Integration of cells and growth factors into the biomaterial as a delivery vehicle can provide physical support for the cells and a sustained drug release profile, thereby avoiding the need for multiple injections for drug delivery.

Exploiting endogenous neurogenesis

The ability to repair the brain after injury is hindered by the inability of neurons to undergo mitosis. Despite the limited repair capacity of the CNS [95], some degree of recovery has been observed in an ischemic brain [21]. Under normal conditions, adult neurogenesis occurs only in two specialized niches, the subventricular zone (SVZ) of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus of the hippocampus [96, 97]. Indeed, increase in cell proliferation in rodent SVZ [98], and neurogenesis in an adult brain has been reported following stroke [99]. However, neural progenitor cells in an adult brain have difficulty migrating towards the damaged cortex due to the dense white matter tracts [100]. After researching the mechanisms of neuronal repair in a rodent model of stroke, researchers found that less than 1 % of the lost neurons were replaced by endogenous NSCs [101]. In addition, the pool of NSCs are depleted with age [102], which adds another barrier to tissue repair from endogenous stem cells.

Current strategies are looking into pharmacological means to stimulate the proliferation and migration of NSCs into the areas of tissue damage. Activation of endogenous neurogenesis requires administration of key regulators, such as microRNAs, BNDF or GDNF. These neurotrophic factors, administered or released by transplanted cells, have been known to activate specialized signaling pathways that promote the proliferation of NSCs in lateral ventricle and axonal growth and synaptogenesis [103]. Delivery of betacellulin (BTC), a member of EGF family, into the lateral ventricle induced the expansion of NSCs and neuroblasts, as well as neurogenesis in the olfactory and dentate gyrus [104]. Interestingly, recent studies have shown that endogenous repair mechanisms are not limited to neurogenic niches as glial cells including astrocytes, oligodendrocyte precursors, and pericytes can be reactivated following ischemia and differentiate into neurons [105107].

Conclusions - making it all work

While further development of biomaterials to support the damaged neural environment is still needed, hydrogels use minimally invasive techniques to deliver cells and neurotrophic factors to promote neuroplasticity and angiogenesis, while also promoting the invasion of non-immune cells into the hydrogel. With the ability to control degradation rate and release profile of bioactive drugs, hydrogels provide a promising environment for cell therapy and tissue regeneration.

Exploiting the potential of endogenous neurogenesis to treat brain injury such as stroke has significant advantages over other treatments because it uses the endogenous repair mechanisms to produce functional neurons and participate in the network repair. As the innate capacity to regenerate after injury is limited, stimulating endogenous neurogenesis via exogenous means will only bring us one step closer to our goal of replacing lost neural connections. Combination treatment strategies, including cell and growth factor delivery, stimulating endogenous neurogenesis and rehabilitation could pave the way forward in promoting neuroplasticity and functional recovery after stroke.

Abbreviations

ATP: 

Adenosine triphosphate

BBB: 

Blood brain barrier

BTC: 

Betacellulin

CBF: 

Cerebral blood flow

ECF: 

Extracellular fluid

ECM: 

Extracellular matrix

GFAP: 

Glial fibrillary acidic protein

HA: 

Hyaluronic acid

HAMC: 

Hyaluronic acid-methylcellulose

IL: 

Interleukin

IP: 

Ischemic penumbra

PEG: 

Polyethylene glycol

PEG-EGF: 

Polyethylene glycol modified epidermal growth factor

PG: 

Polyglycolide

PL: 

Polylactide

PLGA: 

Copolymers of lactide and glycolide

SGZ: 

Subgranular zone

SVZ: 

Subventricular zone

TNF: 

Tumor necrosis factor

Declarations

Acknowledgement

Not applicable.

Funding

We gratefully acknowledge funding through a seed grant from the Department of Health of the Commonwealth of Pennsylvania (4100061184) and the National Institute for Neurological Disease and Stroke (R01NS08226), which support our effort on using biomaterials in the treatment of stroke.

Availability of data and materials

This paper is a review article. Referred literature in this paper has been listed in the references part. The datasets supporting the conclusions of this article are available online by searching the PubMed. Some original points in this article come from the laboratory practice in our research centers and the authors’ experiences.

Authors’ contributions

The authors contributed equally to the conceptualization and write-up of this manuscript. Both authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Consent for publication

All authors approved the publication of this manuscript.

Ethics approval and consent to participate

Not applicable.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Bioengineering, University of Pittsburgh
(2)
Department of Radiology, University of Pittsburgh
(3)
McGowan Institute for Regenerative Medicine, University of Pittsburgh
(4)
Centre for the Neural Basis of Cognition, University of Pittsburgh

References

  1. Thrift AG, Dewey HM, Macdonell RA, McNeil JJ, Donnan GA. Incidence of the major stroke subtypes: initial findings from the North East Melbourne stroke incidence study (NEMESIS). Stroke. 2001;32:1732–8.PubMedView ArticleGoogle Scholar
  2. Broderick JP, William M. Feinberg Lecture: stroke therapy in the year 2025: burden, breakthroughs, and barriers to progress. Stroke. 2004;35:205–11.PubMedView ArticleGoogle Scholar
  3. Dalise S, Ambrosio F, Modo M. Brain plasticity and recovery in preclinical models of stroke. Arch Ital Biol. 2014;152:190–215.PubMedGoogle Scholar
  4. Dimyan MA, Cohen LG. Neuroplasticity in the context of motor rehabilitation after stroke. Nat Rev Neurol. 2011;7:76–85.PubMedPubMed CentralView ArticleGoogle Scholar
  5. Lloyd-Jones D, Adams R, Carnethon M, De Simone G, Ferguson TB, Flegal K, et al. Heart disease and stroke statistics--2009 update: a report from the American Heart Association Statistics Committee and Stroke Statistics Subcommittee. Circulation. 2009;119:480–6.PubMedView ArticleGoogle Scholar
  6. Case LC, Tessier-Lavigne M. Regeneration of the adult central nervous system. Curr Biol. 2005;15:R749–53.PubMedView ArticleGoogle Scholar
  7. Smith EJ, Stroemer RP, Gorenkova N, Nakajima M, Crum WR, Tang E, et al. Implantation site and lesion topology determine efficacy of a human neural stem cell line in a rat model of chronic stroke. Stem Cells. 2012;30:785–96.PubMedView ArticleGoogle Scholar
  8. Encarnacion A, Horie N, Keren-Gill H, Bliss TM, Steinberg GK, Shamloo M. Long-term behavioral assessment of function in an experimental model for ischemic stroke. J Neurosci Methods. 2011;196:247–57.PubMedPubMed CentralView ArticleGoogle Scholar
  9. Moreau F, Patel S, Lauzon ML, McCreary CR, Goyal M, Frayne R, et al. Cavitation after acute symptomatic lacunar stroke depends on time, location, and MRI sequence. Stroke. 2012;43:1837–42.PubMedView ArticleGoogle Scholar
  10. Zhang M, Methot D, Poppa V, Fujio Y, Walsh K, Murry CE. Cardiomyocyte grafting for cardiac repair: graft cell death and anti-death strategies. J Mol Cell Cardiol. 2001;33:907–21.PubMedView ArticleGoogle Scholar
  11. Lappalainen RS, Narkilahti S, Huhtala T, Liimatainen T, Suuronen T, Narvanen A, et al. The SPECT imaging shows the accumulation of neural progenitor cells into internal organs after systemic administration in middle cerebral artery occlusion rats. Neurosci Lett. 2008;440:246–50.PubMedView ArticleGoogle Scholar
  12. Mountz JM. Nuclear medicine in the rehabilitative treatment evaluation in stroke recovery. Role of diaschisis resolution and cerebral reorganization. Eura Medicophys. 2007;43:221–39.PubMedGoogle Scholar
  13. Bliss T, Guzman R, Daadi M, Steinberg GK. Cell transplantation therapy for stroke. Stroke. 2007;38:817–26.PubMedView ArticleGoogle Scholar
  14. Bitzer-Quintero OK, Gonzalez-Burgos I. Immune system in the brain: a modulatory role on dendritic spine morphophysiology? Neural Plast. 2012;2012:348642.PubMedPubMed CentralGoogle Scholar
  15. Park KI, Teng YD, Snyder EY. The injured brain interacts reciprocally with neural stem cells supported by scaffolds to reconstitute lost tissue. Nat Biotechnol. 2002;20:1111–7.PubMedView ArticleGoogle Scholar
  16. Bible E, Chau DY, Alexander MR, Price J, Shakesheff KM, Modo M. The support of neural stem cells transplanted into stroke-induced brain cavities by PLGA particles. Biomaterials. 2009;30:2985–94.PubMedView ArticleGoogle Scholar
  17. Bible E, Dell’Acqua F, Solanky B, Balducci A, Crapo PM, Badylak SF, et al. Non-invasive imaging of transplanted human neural stem cells and ECM scaffold remodeling in the stroke-damaged rat brain by (19)F- and diffusion-MRI. Biomaterials. 2012;33:2858–71.PubMedPubMed CentralView ArticleGoogle Scholar
  18. Duncan K, Gonzales-Portillo GS, Acosta SA, Kaneko Y, Borlongan CV, Tajiri N. Stem cell-paved biobridges facilitate stem transplant and host brain cell interactions for stroke therapy. Brain Res. 2015;1623:160–5.PubMedView ArticleGoogle Scholar
  19. Ghuman H, Massensini AR, Donnelly J, Kim SM, Medberry CJ, Badylak SF, et al. ECM hydrogel for the treatment of stroke: Characterization of the host cell infiltrate. Biomaterials. 2016;91:166–81.PubMedView ArticleGoogle Scholar
  20. Lipton P. Ischemic cell death in brain neurons. Physiol Rev. 1999;79:1431–568.PubMedGoogle Scholar
  21. Yuan J. Neuroprotective strategies targeting apoptotic and necrotic cell death for stroke. Apoptosis. 2009;14:469–77.PubMedPubMed CentralView ArticleGoogle Scholar
  22. Lo EH. A new penumbra: transitioning from injury into repair after stroke. Nat Med. 2008;14:497–500.PubMedView ArticleGoogle Scholar
  23. Baeten KM, Akassoglou K. Extracellular matrix and matrix receptors in blood–brain barrier formation and stroke. Dev Neurobiol. 2011;71:1018–39.PubMedPubMed CentralView ArticleGoogle Scholar
  24. Bakshi A, Keck CA, Koshkin VS, LeBold DG, Siman R, Snyder EY, et al. Caspase-mediated cell death predominates following engraftment of neural progenitor cells into traumatically injured rat brain. Brain Res. 2005;1065:8–19.PubMedView ArticleGoogle Scholar
  25. Lalancette-Hebert M, Gowing G, Simard A, Weng YC, Kriz J. Selective ablation of proliferating microglial cells exacerbates ischemic injury in the brain. J Neurosci. 2007;27:2596–605.PubMedView ArticleGoogle Scholar
  26. Denes A, Vidyasagar R, Feng J, Narvainen J, McColl BW, Kauppinen RA, et al. Proliferating resident microglia after focal cerebral ischaemia in mice. J Cereb Blood Flow Metab. 2007;27:1941–53.PubMedView ArticleGoogle Scholar
  27. Barone FC, Arvin B, White RF, Miller A, Webb CL, Willette RN, et al. Tumor necrosis factor-alpha. A mediator of focal ischemic brain injury. Stroke. 1997;28:1233–44.PubMedView ArticleGoogle Scholar
  28. Lambertsen KL, Meldgaard M, Ladeby R, Finsen B. A quantitative study of microglial-macrophage synthesis of tumor necrosis factor during acute and late focal cerebral ischemia in mice. J Cereb Blood Flow Metab. 2005;25:119–35.PubMedView ArticleGoogle Scholar
  29. Minami M, Kuraishi Y, Yabuuchi K, Yamazaki A, Satoh M. Induction of interleukin-1 beta mRNA in rat brain after transient forebrain ischemia. J Neurochem. 1992;58:390–2.PubMedView ArticleGoogle Scholar
  30. Harry GJ, McPherson CA, Wine RN, Atkinson K, Lefebvre d’Hellencourt C. Trimethyltin-induced neurogenesis in the murine hippocampus. Neurotox Res. 2004;5:623–7.PubMedPubMed CentralView ArticleGoogle Scholar
  31. Streit WJ. Microglia as neuroprotective, immunocompetent cells of the CNS. Glia. 2002;40:133–9.PubMedView ArticleGoogle Scholar
  32. Ziv Y, Ron N, Butovsky O, Landa G, Sudai E, Greenberg N, et al. Immune cells contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood. Nat Neurosci. 2006;9:268–75.PubMedView ArticleGoogle Scholar
  33. Thored P, Heldmann U, Gomes-Leal W, Gisler R, Darsalia V, Taneera J, et al. Long-term accumulation of microglia with proneurogenic phenotype concomitant with persistent neurogenesis in adult subventricular zone after stroke. Glia. 2009;57:835–49.PubMedView ArticleGoogle Scholar
  34. Walton NM, Sutter BM, Laywell ED, Levkoff LH, Kearns SM, Marshall 2nd GP, et al. Microglia instruct subventricular zone neurogenesis. Glia. 2006;54:815–25.PubMedView ArticleGoogle Scholar
  35. Panickar KS, Norenberg MD. Astrocytes in cerebral ischemic injury: morphological and general considerations. Glia. 2005;50:287–98.PubMedView ArticleGoogle Scholar
  36. Li H, Zhang N, Sun G, Ding S. Inhibition of the group I mGluRs reduces acute brain damage and improves long-term histological outcomes after photothrombosis-induced ischaemia. ASN Neuro. 2013;5:195–207.PubMedView ArticleGoogle Scholar
  37. Barreto GE, Sun X, Xu L, Giffard RG. Astrocyte proliferation following stroke in the mouse depends on distance from the infarct. PLoS One. 2011;6:e27881.PubMedPubMed CentralView ArticleGoogle Scholar
  38. Chen Y, Swanson RA. Astrocytes and brain injury. J Cereb Blood Flow Metab. 2003;23:137–49.PubMedView ArticleGoogle Scholar
  39. Ransom B, Behar T, Nedergaard M. New roles for astrocytes (stars at last). Trends Neurosci. 2003;26:520–2.PubMedView ArticleGoogle Scholar
  40. Iadecola C, Nedergaard M. Glial regulation of the cerebral microvasculature. Nat Neurosci. 2007;10:1369–76.PubMedView ArticleGoogle Scholar
  41. Kimelberg HK. Astrocytic swelling in cerebral ischemia as a possible cause of injury and target for therapy. Glia. 2005;50:389–97.PubMedView ArticleGoogle Scholar
  42. Anderson CM, Nedergaard M. Astrocyte-mediated control of cerebral microcirculation. Trends Neurosci. 2003;26:340–4. author reply 4–5.PubMedView ArticleGoogle Scholar
  43. Sofroniew MV. Reactive astrocytes in neural repair and protection. Neuroscientist. 2005;11:400–7.PubMedView ArticleGoogle Scholar
  44. Liauw J, Hoang S, Choi M, Eroglu C, Choi M, Sun GH, et al. Thrombospondins 1 and 2 are necessary for synaptic plasticity and functional recovery after stroke. J Cereb Blood Flow Metab. 2008;28:1722–32.PubMedView ArticleGoogle Scholar
  45. Laird MD, Vender JR, Dhandapani KM. Opposing roles for reactive astrocytes following traumatic brain injury. Neurosignals. 2008;16:154–64.PubMedView ArticleGoogle Scholar
  46. Yu H, Cao B, Feng M, Zhou Q, Sun X, Wu S, et al. Combinated transplantation of neural stem cells and collagen type I promote functional recovery after cerebral ischemia in rats. Anat Rec (Hoboken). 2010;293:911–7.View ArticleGoogle Scholar
  47. Fournier E, Passirani C, Montero-Menei CN, Benoit JP. Biocompatibility of implantable synthetic polymeric drug carriers: focus on brain biocompatibility. Biomaterials. 2003;24:3311–31.PubMedView ArticleGoogle Scholar
  48. Lampe KJ, Bjugstad KB, Mahoney MJ. Impact of degradable macromer content in a poly(ethylene glycol) hydrogel on neural cell metabolic activity, redox state, proliferation, and differentiation. Tissue Eng Part A. 2010;16:1857–66.PubMedPubMed CentralView ArticleGoogle Scholar
  49. Williams CG, Malik AN, Kim TK, Manson PN, Elisseeff JH. Variable cytocompatibility of six cell lines with photoinitiators used for polymerizing hydrogels and cell encapsulation. Biomaterials. 2005;26:1211–8.PubMedView ArticleGoogle Scholar
  50. Biran R, Martin DC, Tresco PA. Neuronal cell loss accompanies the brain tissue response to chronically implanted silicon microelectrode arrays. Exp Neurol. 2005;195:115–26.PubMedView ArticleGoogle Scholar
  51. Ta HT, Dass CR, Dunstan DE. Injectable chitosan hydrogels for localised cancer therapy. J Control Release. 2008;126:205–16.PubMedView ArticleGoogle Scholar
  52. Massensini AR, Ghuman H, Saldin LT, Medberry CJ, Keane TJ, Nicholls FJ, et al. Concentration-dependent rheological properties of ECM hydrogel for intracerebral delivery to a stroke cavity. Acta Biomater. 2015;27:116–30.PubMedView ArticleGoogle Scholar
  53. Gupta D, Tator CH, Shoichet MS. Fast-gelling injectable blend of hyaluronan and methylcellulose for intrathecal, localized delivery to the injured spinal cord. Biomaterials. 2006;27:2370–9.PubMedView ArticleGoogle Scholar
  54. Brady ML, Raghavan R, Alexander A, Kubota K, Sillay K, Emborg ME. Pathways of infusate loss during convection-enhanced delivery into the putamen nucleus. Stereotact Funct Neurosurg. 2013;91:69–78.PubMedPubMed CentralView ArticleGoogle Scholar
  55. Valles F, Fiandaca MS, Bringas J, Dickinson P, LeCouteur R, Higgins R, et al. Anatomic compression caused by high-volume convection-enhanced delivery to the brain. Neurosurgery. 2009;65:579–85. discussion 85–6.PubMedPubMed CentralView ArticleGoogle Scholar
  56. Beauchamp Jr RO, St Clair MB, Fennell TR, Clarke DO, Morgan KT, Kari FW. A critical review of the toxicology of glutaraldehyde. Crit Rev Toxicol. 1992;22:143–74.PubMedView ArticleGoogle Scholar
  57. Anseth KS, Metters AT, Bryant SJ, Martens PJ, Elisseeff JH, Bowman CN. In situ forming degradable networks and their application in tissue engineering and drug delivery. J Control Release. 2002;78:199–209.PubMedView ArticleGoogle Scholar
  58. Lin CC, Anseth KS. PEG hydrogels for the controlled release of biomolecules in regenerative medicine. Pharm Res. 2009;26:631–43.PubMedView ArticleGoogle Scholar
  59. Martens PJ, Bryant SJ, Anseth KS. Tailoring the degradation of hydrogels formed from multivinyl poly(ethylene glycol) and poly(vinyl alcohol) macromers for cartilage tissue engineering. Biomacromolecules. 2003;4:283–92.PubMedView ArticleGoogle Scholar
  60. De Santis G, Lennon AB, Boschetti F, Verhegghe B, Verdonck P, Prendergast PJ. How can cells sense the elasticity of a substrate? An analysis using a cell tensegrity model. Eur Cell Mater. 2011;22:202–13.PubMedGoogle Scholar
  61. Leipzig ND, Shoichet MS. The effect of substrate stiffness on adult neural stem cell behavior. Biomaterials. 2009;30:6867–78.PubMedView ArticleGoogle Scholar
  62. Engler AJ, Sen S, Sweeney HL, Discher DE. Matrix elasticity directs stem cell lineage specification. Cell. 2006;126:677–89.PubMedView ArticleGoogle Scholar
  63. Sobel RA. The extracellular matrix in multiple sclerosis lesions. J Neuropathol Exp Neurol. 1998;57:205–17.PubMedView ArticleGoogle Scholar
  64. Guo SZ, Ren XJ, Wu B, Jiang T. Preparation of the acellular scaffold of the spinal cord and the study of biocompatibility. Spinal Cord. 2010;48:576–81.PubMedView ArticleGoogle Scholar
  65. Crapo PM, Medberry CJ, Reing JE, Tottey S, van der Merwe Y, Jones KE, et al. Biologic scaffolds composed of central nervous system extracellular matrix. Biomaterials. 2012;33:3539–47.PubMedPubMed CentralView ArticleGoogle Scholar
  66. Crapo PM, Tottey S, Slivka PF, Badylak SF. Effects of biologic scaffolds on human stem cells and implications for CNS tissue engineering. Tissue Eng Part A. 2014;20:313–23.PubMedView ArticleGoogle Scholar
  67. Novak U, Kaye AH. Extracellular matrix and the brain: components and function. J Clin Neurosci. 2000;7:280–90.PubMedView ArticleGoogle Scholar
  68. Viapiano MS, Matthews RT. From barriers to bridges: chondroitin sulfate proteoglycans in neuropathology. Trends Mol Med. 2006;12:488–96.PubMedView ArticleGoogle Scholar
  69. Medberry CJ, Crapo PM, Siu BF, Carruthers CA, Wolf MT, Nagarkar SP, et al. Hydrogels derived from central nervous system extracellular matrix. Biomaterials. 2013;34:1033–40.PubMedView ArticleGoogle Scholar
  70. Mittapalli RK, Liu X, Adkins CE, Nounou MI, Bohn KA, Terrell TB, et al. Paclitaxel-hyaluronic nanoconjugates prolong overall survival in a preclinical brain metastases of breast cancer model. Mol Cancer Ther. 2013;12:2389–99.PubMedView ArticleGoogle Scholar
  71. Lam J, Lowry WE, Carmichael ST, Segura T. Delivery of iPS-NPCs to the Stroke Cavity within a Hyaluronic Acid Matrix Promotes the Differentiation of Transplanted Cells. Adv Funct Mater. 2014;24:7053–62.PubMedPubMed CentralView ArticleGoogle Scholar
  72. Hyatt AJ, Wang D, Kwok JC, Fawcett JW, Martin KR. Controlled release of chondroitinase ABC from fibrin gel reduces the level of inhibitory glycosaminoglycan chains in lesioned spinal cord. J Control Release. 2010;147:24–9.PubMedView ArticleGoogle Scholar
  73. Ballios BG, Cooke MJ, Donaldson L, Coles BL, Morshead CM, van der Kooy D, et al. A Hyaluronan-Based Injectable Hydrogel Improves the Survival and Integration of Stem Cell Progeny following Transplantation. Stem Cell Rep. 2015;4:1031–45.View ArticleGoogle Scholar
  74. Skop NB, Calderon F, Levison SW, Gandhi CD, Cho CH. Heparin crosslinked chitosan microspheres for the delivery of neural stem cells and growth factors for central nervous system repair. Acta Biomater. 2013;9:6834–43.PubMedView ArticleGoogle Scholar
  75. Wu Y, Wei W, Zhou M, Wang Y, Wu J, Ma G, et al. Thermal-sensitive hydrogel as adjuvant-free vaccine delivery system for H5N1 intranasal immunization. Biomaterials. 2012;33:2351–60.PubMedView ArticleGoogle Scholar
  76. Nakaji-Hirabayashi T, Kato K, Iwata H. In vivo study on the survival of neural stem cells transplanted into the rat brain with a collagen hydrogel that incorporates laminin-derived polypeptides. Bioconjug Chem. 2013;24:1798–804.PubMedView ArticleGoogle Scholar
  77. Cross VL, Zheng Y, Won Choi N, Verbridge SS, Sutermaster BA, Bonassar LJ, et al. Dense type I collagen matrices that support cellular remodeling and microfabrication for studies of tumor angiogenesis and vasculogenesis in vitro. Biomaterials. 2010;31:8596–607.PubMedPubMed CentralView ArticleGoogle Scholar
  78. Jiang D, Liang J, Noble PW. Hyaluronan in tissue injury and repair. Annu Rev Cell Dev Biol. 2007;23:435–61.PubMedView ArticleGoogle Scholar
  79. Zhong J, Chan A, Morad L, Kornblum HI, Fan G, Carmichael ST. Hydrogel matrix to support stem cell survival after brain transplantation in stroke. Neurorehabil Neural Repair. 2010;24:636–44.PubMedPubMed CentralView ArticleGoogle Scholar
  80. Emerich DF, Tracy MA, Ward KL, Figueiredo M, Qian R, Henschel C, et al. Biocompatibility of poly (DL-lactide-co-glycolide) microspheres implanted into the brain. Cell Transplant. 1999;8:47–58.PubMedGoogle Scholar
  81. Zentner GM, Rathi R, Shih C, McRea JC, Seo MH, Oh H, et al. Biodegradable block copolymers for delivery of proteins and water-insoluble drugs. J Control Release. 2001;72:203–15.PubMedView ArticleGoogle Scholar
  82. Gouhier C, Chalon S, Venier-Julienne MC, Bodard S, Benoit J, Besnard J, et al. Neuroprotection of nerve growth factor-loaded microspheres on the D2 dopaminergic receptor positive-striatal neurones in quinolinic acid-lesioned rats: a quantitative autoradiographic assessment with iodobenzamide. Neurosci Lett. 2000;288:71–5.PubMedView ArticleGoogle Scholar
  83. Menei P, Pean JM, Nerriere-Daguin V, Jollivet C, Brachet P, Benoit JP. Intracerebral implantation of NGF-releasing biodegradable microspheres protects striatum against excitotoxic damage. Exp Neurol. 2000;161:259–72.PubMedView ArticleGoogle Scholar
  84. Rinholm JE, Hamilton NB, Kessaris N, Richardson WD, Bergersen LH, Attwell D. Regulation of oligodendrocyte development and myelination by glucose and lactate. J Neurosci. 2011;31:538–48.PubMedPubMed CentralView ArticleGoogle Scholar
  85. Cooke MJ, Wang Y, Morshead CM, Shoichet MS. Controlled epi-cortical delivery of epidermal growth factor for the stimulation of endogenous neural stem cell proliferation in stroke-injured brain. Biomaterials. 2011;32:5688–97.PubMedView ArticleGoogle Scholar
  86. Dibajnia P, Morshead CM. Role of neural precursor cells in promoting repair following stroke. Acta Pharmacol Sin. 2013;34:78–90.PubMedView ArticleGoogle Scholar
  87. Bliss TM, Andres RH, Steinberg GK. Optimizing the success of cell transplantation therapy for stroke. Neurobiol Dis. 2010;37:275–83.PubMedView ArticleGoogle Scholar
  88. Drago D, Cossetti C, Iraci N, Gaude E, Musco G, Bachi A, et al. The stem cell secretome and its role in brain repair. Biochimie. 2013;95:2271–85.PubMedPubMed CentralView ArticleGoogle Scholar
  89. Tureyen K, Vemuganti R, Bowen KK, Sailor KA, Dempsey RJ. EGF and FGF-2 infusion increases post-ischemic neural progenitor cell proliferation in the adult rat brain. Neurosurgery. 2005;57:1254–63. discussion −63.PubMedView ArticleGoogle Scholar
  90. Bacigaluppi M, Pluchino S, Peruzzotti-Jametti L, Kilic E, Kilic U, Salani G, et al. Delayed post-ischaemic neuroprotection following systemic neural stem cell transplantation involves multiple mechanisms. Brain. 2009;132:2239–51.PubMedView ArticleGoogle Scholar
  91. Oki K, Tatarishvili J, Wood J, Koch P, Wattananit S, Mine Y, et al. Human-induced pluripotent stem cells form functional neurons and improve recovery after grafting in stroke-damaged brain. Stem Cells. 2012;30:1120–33.PubMedView ArticleGoogle Scholar
  92. Kelly S, Bliss TM, Shah AK, Sun GH, Ma M, Foo WC, et al. Transplanted human fetal neural stem cells survive, migrate, and differentiate in ischemic rat cerebral cortex. Proc Natl Acad Sci U S A. 2004;101:11839–44.PubMedPubMed CentralView ArticleGoogle Scholar
  93. Jin K, Mao X, Xie L, Galvan V, Lai B, Wang Y, et al. Transplantation of human neural precursor cells in Matrigel scaffolding improves outcome from focal cerebral ischemia after delayed postischemic treatment in rats. J Cereb Blood Flow Metab. 2010;30:534–44.PubMedView ArticleGoogle Scholar
  94. Bible E, Qutachi O, Chau DY, Alexander MR, Shakesheff KM, Modo M. Neo-vascularization of the stroke cavity by implantation of human neural stem cells on VEGF-releasing PLGA microparticles. Biomaterials. 2012;33:7435–46.PubMedPubMed CentralView ArticleGoogle Scholar
  95. Nakagomi T, Molnar Z, Nakano-Doi A, Taguchi A, Saino O, Kubo S, et al. Ischemia-induced neural stem/progenitor cells in the pia mater following cortical infarction. Stem Cells Dev. 2011;20:2037–51.PubMedView ArticleGoogle Scholar
  96. Zhao C, Deng W, Gage FH. Mechanisms and functional implications of adult neurogenesis. Cell. 2008;132:645–60.PubMedView ArticleGoogle Scholar
  97. Ming GL, Song H. Adult neurogenesis in the mammalian central nervous system. Annu Rev Neurosci. 2005;28:223–50.PubMedView ArticleGoogle Scholar
  98. Thored P, Arvidsson A, Cacci E, Ahlenius H, Kallur T, Darsalia V, et al. Persistent production of neurons from adult brain stem cells during recovery after stroke. Stem Cells. 2006;24:739–47.PubMedView ArticleGoogle Scholar
  99. Jin K, Wang X, Xie L, Mao XO, Zhu W, Wang Y, et al. Evidence for stroke-induced neurogenesis in the human brain. Proc Natl Acad Sci U S A. 2006;103:13198–202.PubMedPubMed CentralView ArticleGoogle Scholar
  100. Sundholm-Peters NL, Yang HK, Goings GE, Walker AS, Szele FG. Subventricular zone neuroblasts emigrate toward cortical lesions. J Neuropathol Exp Neurol. 2005;64:1089–100.PubMedView ArticleGoogle Scholar
  101. Arvidsson A, Collin T, Kirik D, Kokaia Z, Lindvall O. Neuronal replacement from endogenous precursors in the adult brain after stroke. Nat Med. 2002;8:963–70.PubMedView ArticleGoogle Scholar
  102. Sanai N, Nguyen T, Ihrie RA, Mirzadeh Z, Tsai HH, Wong M, et al. Corridors of migrating neurons in the human brain and their decline during infancy. Nature. 2011;478:382–6.PubMedPubMed CentralView ArticleGoogle Scholar
  103. Colucci-D’Amato L, Perrone-Capano C, di Porzio U. Chronic activation of ERK and neurodegenerative diseases. Bioessays. 2003;25:1085–95.PubMedView ArticleGoogle Scholar
  104. Gomez-Gaviro MV, Scott CE, Sesay AK, Matheu A, Booth S, Galichet C, et al. Betacellulin promotes cell proliferation in the neural stem cell niche and stimulates neurogenesis. Proc Natl Acad Sci U S A. 2012;109:1317–22.PubMedPubMed CentralView ArticleGoogle Scholar
  105. Nakagomi T, Kubo S, Nakano-Doi A, Sakuma R, Lu S, Narita A, et al. Brain vascular pericytes following ischemia have multipotential stem cell activity to differentiate into neural and vascular lineage cells. Stem Cells. 2015;33:1962–74.PubMedView ArticleGoogle Scholar
  106. Torper O, Ottosson DR, Pereira M, Lau S, Cardoso T, Grealish S, et al. In Vivo Reprogramming of Striatal NG2 Glia into Functional Neurons that Integrate into Local Host Circuitry. Cell Rep. 2015;12:474–81.PubMedPubMed CentralView ArticleGoogle Scholar
  107. Heinrich C, Bergami M, Gascon S, Lepier A, Vigano F, Dimou L, et al. Sox2-mediated conversion of NG2 glia into induced neurons in the injured adult cerebral cortex. Stem Cell Rep. 2014;3:1000–14.View ArticleGoogle Scholar

Copyright

© The Author(s) 2016