Background
Stem cell-based therapies have been performed in various clinical settings, although many lack scientific evidence of their effectiveness [
1]. Among stem cell-based therapies, transplantation of human neural stem/progenitor cells (hNSPCs) is relatively well substantiated by peer-reviewed literatures [
2‐
8]. One reason underlying the relative success of hNSPCs-transplantation is its low occurrence of tumor formation, which is a clear advantage compared with transplantation of embryonic stem cells or their derivatives [
9]. Therefore, we have been examining hNSPCs-transplantation in various preclinical animal models and have shown that hNSPCs-transplantation enhances functional recovery following brain ischemia [
10] and spinal cord injury (SCI) [
11].
Brain ischemia, which is caused by occlusion of a cerebral artery, leads to focal tissue loss and death of multiple neuronal cell types within and around the ischemic region. Patients with brain ischemia exhibit persistent motor, sensory or cognitive impairments, which have devastating effects on their quality of life. Apart from acute thrombolysis, which can be used in only a minority of cases, there is still no effective treatment to promote functional recovery after brain ischemia.
hNSPCs can generate all principle cell types (i.e., neurons, astrocytes and oligodendrocytes) in the brain and therefore have great therapeutic potential in severe neurological diseases, including brain ischemia [
6,
12], which induce death of various cell types [
13,
14]. hNSPCs can be propagated in large quantities for long-term without a notable loss of the ability to proliferate and differentiate [
15]. Therefore, cultured hNSPCs are a promising cell source to treat brain diseases.
We previously showed that transplantation of cultured hNSPCs reduced infarct volume and improved functional prognosis in a rodent model of brain ischemia [
10]. In the damaged brains of the model animals, hNSPCs differentiated into mature neurons within the ischemic region, and some of those new-born neurons were incorporated into the host neural circuitry [
10]. In SCI model mice, grafted hNSPCs differentiated into oligodendrocytes and contributed to re-myelination of host neuronal axons [
5]. Another possible mechanism of the therapeutic effects of hNSPCs-transplantation is their trophic actions. It has been suggested that grafted hNSPCs release molecules which exert neuro-protective roles or reduce inflammation [
10].
Galectin-1(Gal1) is expressed around infarcted tissue after brain ischemia [
10,
16]. Gal1 is a soluble lectin that binds to lactosamine-rich carbohydrate moieties on various molecules [
17]. Although its binding partner in the mammalian brain seems relatively limited [
18], Gal1 is expressed in adult NSCs in the subventricular zone (SVZ) of the lateral ventricles (LV) [
19,
20] and the dentate gyrus (DG) of the hippocampus [
21]. We showed that infusion of human recombinant Gal-1 protein (hGal1) enhanced functional recovery in a rodent model of brain ischemia [
20] but failed to reduce the volume of the infarcted area [
20]. Because hNSPCs-transplantation was effective in reducing infarct volume after brain ischemia [
10], we hypothesized that the combination of hNSPCs-transplantation and continuous delivery of Gal1 at the same time would reduce the volume of the infarcted area and improve functional recovery to a greater extent than hNSPCs-transplantation alone. Indeed, we previously showed that transplantation of hNSPCs overexpressing hGal1 (hGal1-hNSPCs) led to a better functional outcome than transplantation of hNSPCs alone in a non-human primate model of SCI [
22].
In the present study, we analyzed the time course of intrinsic Gal1 expression after brain ischemia. Next, we examined the therapeutic effect of transplantation of hGal1-hNSPCs compared with hNSPCs alone, and found that hGal1-hNSPCs reduced the infarct volume and resulted in better functional recovery after brain ischemia.
Methods
Culture of hNSPCs
This study was carried out in accordance with the principles of the Helsinki Declaration, and the Japan Society of Obstetrics and Gynecology. Approval to use human fetal neural tissues was obtained from the ethical committees of both Osaka National Hospital and Keio University. Written informed consent was obtained from all parents through routine legal terminations performed at Osaka National Hospital.
hNSPCs (oh-NSC-3-fb) were isolated from fetal forebrain tissues (10 gestational weeks [GW]) and propagated using a defined neural progenitor cell basal medium (NPBM; Clonetics)-based medium supplement with human recombinant (hr-) basic fibroblast growth factors-2 (FGF-2, 20 ng/ml; R&D), hr-epidermal growth factor (EGF, 20 ng/ml; R&D), hr-leukemia inhibitory factor (LIF, 10 ng/ml; Chemicon), and GA-100 (5 μg/ml gentamicin sulfate, 5 ng/ml amphotericin B; Clonetics) as described previously [
15,
23].
Lentiviral transduction of hNSPCs
hNSPCs that had undergone more than 10 passages were dissociated into single cells 2 hr before being infected. The concentrated viruses were added to the culture medium to infect the hNSPCs [multiplicity of infection (MOI) = 5]. Two weeks later, neurospheres were formed from the dissociated hNSPCs and were passaged. The efficiency of the transduction was measured by GFP expression with a FACS Calibur (Beckton Dickinson). hNSPCs with an transduction efficiency of greater than 80% were used for transplantation. The third-generation self-inactivating HIV-1-based lentiviral vector pCSII-EF-MCS-IRES2-GFP [
24] was used for the transduction. Two types of lentivirus-transduced hNSPCs were prepared: hGal1-hNSPCs, which were hNSPCs infected with the human Gal1 IRES GFP virus; and hNSPCs, which were hNSPCs infected with the IRES GFP virus.
Animals
Animal experiments were approved by the Animal Experiment Committee of Tokyo Medical and Dental University. Thirty-six male Mongolian gerbils (aged 16-22 weeks and weighing 60-72 g) were housed in groups (3-4 per cage) and maintained on a 14:10-hr light:dark cycle with unlimited access to food and water.
Focal Ischemic Surgery
To induce brain ischemia, animals were anesthetized with 2% isoflurane. The left common carotid artery was occluded with a mini vascular clip for 10 min, after which animals were allowed to recover from anesthesia. During the carotid artery occlusion, stroke symptoms were evaluated using a stroke index (SI)[
25]. Animals manifesting a SI of more than 10 were selected as 'post-ischemic animals' [
25]. In post-ischemic animals, a second 10-min period of ischemia was similarly induced 5 hr later.
Transplantation Surgery
Four days after ischemic surgery, post-ischemic animals were randomly assigned to hNSPCs and hGal1-hNSPCs groups, and the hNSPCs-suspension was transplanted into the caudate nucleus of the lesioned hemisphere. Animals were anesthetized with 2% isoflurane and placed in a stereotaxic frame. A hole was drilled in the left side of the skull to allow the penetration of a 10 μL Hamilton syringe at the following coordinates (mm) relative to bregma: anterior 1.0 mm; lateral 3.0 mm; and ventral 3.0 mm. A 3 μL aliquot of hNSPCs-suspension (50,000 cells) was infused over 2 min, and the syringe was left in place for an additional 2 min to allow diffusion from the tip. All animals received cyclosporine A (10 mg/kg intramuscularly; Wako) 24 hr before transplantation and three times per week for 4 weeks thereafter.
Histological Analyses
At the end of the observation period, animals were anesthetized deeply with diethyl ether, sacrificed, and fixed by perfusion with 4% paraformaldehyde. The post-fixed brains were cut into 50-μm coronal sections using a vibratome. The coronal sections were stained with Nissl for detection and calculation of the ischemic injury area. The area of infarction was measured using the MCID system [
22,
26,
27](InterFocus Imaging), using automatic tiling and area size quantification options at the same exposure time and threshold settings, and then compared between hNSPCs- and hGal1-hNSPCs-transplanted groups. The total infarct area (indirect lesion volume) of the ipsilateral hemisphere was calculated as a percentage of the volume of the contralateral hemisphere, as reported previously [
28].
hNSPCs grafted into the injured brain were identified using an anti-GFP antibody (1:200; MBL), and their phenotypes were examined by immunostaining for the cell-type-specific markers anti-NeuN (1:100; Sigma) and anti-GFAP (1:200; Dako). Grafted NSCs co-labeled with GFP and cell-type-specific markers were detected with a confocal microscope equipped with an argon-krypton laser (LSM510; Zeiss) and a fluorescence microscope (Axioskop 2 Plus; Zeiss).
BrdU labeling
To label S-phase cells, the thymidine analog 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdU) was administered intraperitoneally (50 mg/kg; Sigma). On day 3 after the infarction, two injections (6 h apart) of BrdU were given. 24 hours later, animals were euthanized by transcardiac perfusion. This allowed us to measure the number of cells that incorporated BrdU during a 24-h period and provided an index of the rate of cell birth at a specific time point after ischemia.
Behavioral Tests
Animals were subjected to a series of behavioral tests within 30 days after focal ischemia. The researcher conducting the behavioral testing and scoring was blind to the experimental conditions. All animals were videotaped during behavioral tests. The elevated body swing test (EBST) was used to evaluate asymmetric motor behavior [
29]. Animals were held by the base of the tail and elevated about 10 cm from the tabletop. The direction of body swing, defined as an upper body turn of 10 degrees to either side, was recorded for 1 min during each of the three trials per day. The number of left and right turns was counted, and the percentage of turns made to the side contralateral to the damaged hemisphere (% right-biased swing) was determined. The bilateral asymmetry test (BAT) is a test of unilateral sensory dysfunction [
30]. Two small pieces of adhesive-backed paper dots were used as bilateral tactile stimuli occupying the distal-radial region on the wrist of each forelimb. The time, to a maximum of 3 min, that it took each animal to remove each stimulus from the forelimb (removal time) was recorded in three trials per day. The T-maze spontaneous alternation task is a method to test spatial cognitive function [
31]. Animals were allowed to alternate between the left and right goal arms of a T-shaped maze throughout a 15-trial continuous alternation session. Once they had entered a particular goal arm, a door was lowered to block entry to the opposite arm. The door was reopened only after animals returned to the start arm, thus allowing a new alternation trial to be started. Their behavior was traced with a video-tracking system (PanLab, Barcelona, Spain). The spontaneous alternation rate was calculated as the ratio between the alternating choices and total number of choices.
Statistical Analysis
Unpaired t-tests (for two groups) or repeated measures ANOVAs were used to detect differences between groups in behavioral analyses and histological quantifications.
Discussion
In this study, we showed that Gal1 expression was induced in proliferating reactive astrocytes after brain ischemia and gradually decreased over time. The transplantation of hGal1-hNSPCs after brain ischemia resulted in reduced infarcted volume and better recovery of motor function compared to transplantation of hNSPCs alone. These findings suggest a potential use of hGal1 in combination with transplantation of hNSPCs in the treatment of brain ischemia.
Although transplantation of hGal1-hNSPCs reduced ischemic volume and promoted functional recovery from brain ischemia, the mechanisms underlying these effects are unclear. It is reasonable to consider that the extent of functional recovery is correlated to the size of the infarcted region after brain ischemia as shown by this study and others [
10,
34]. It is possible that the reduced volume of ischemic region was caused by either an increase in the number of transplanted cells after transplantation or a preservation of the otherwise-dying host tissue. Since i) our previous reports showed that hGal1 overexpression neither promoted proliferation or survival nor changed the direction of differentiation (Appendix One) of human neurosphere-derived NSPCs [
22] and ii) we observed no apparent differences in the survival of transplanted cells by overexpression of hGal1, which agrees with previous studies [
22], the reduced volume of the ischemic region could a result from preservation of the host tissue through two possible mechanisms.
First, trophic factors, which may be released from hNSPCs, could preserve the damaged tissue [
13,
35]. Therefore, it is possible that hNSPCs might be induced to express greater amounts and varieties of these trophic factors (e.g., BDNF [
16,
36]), by the over-expression of hGal1. This possibility could be further investigated, for example, by microarray analysis in future experiments. Second, prolonged expression of hGal-1 released from hGal1-hNSPCs might have altered the proliferation and/or migration of reactive astrocytes, thereby preventing further tissue damage after brain ischemia. Considering that i) reactive astrocytes could play a crucial role in preventing the enlargement of the infarcted region after brain ischemia [
34,
37], ii) reactive astrocytes express Gal1 (Figure
1B) [
16], iii) Gal1 regulates the proliferation of reactive astrocytes [
16], iv) soluble Gal1 binds to β1 integrin [
18,
38], which regulates the migration of astrocytes [
39,
40], hGal-1 could influence the proliferation and/or migration of reactive astrocytes after brain ischemia, resulting in a reduction in the size of the ischemic region.
Furthermore, there may be other factors that promote functional improvement independent of the volume of the ischemic region. For these, it is important to consider the function of hGal-1 in promoting neurite outgrowth [
22,
41], possibly through enhancing the binding of neurites to β1 integrin [
18,
38]. After brain ischemia, neurite outgrowth is prevented by inflammatory cytokines and gliosis within and around the ischemic region [
42,
43]. Indeed, after stroke, promotion of neurite outgrowth improved neurological outcome [
44]. Also, in our SCI model, it was suggested that the therapeutic effect of the transplantation of hGal-1-hNSPCs was due to increases in neurite outgrowth [
22,
41]. Thus, the overexpression of hGal1 in hNSPCs might increase neurite outgrowh of graft and/or host neurons in the ischemic brain, which could contribute to better functional recovery. These possibilities should be verified in future studies.
Previously, we showed that infusion of hGal1 protein improved recovery from motor and sensory (but not from cognitive) deficits after brain ischemia [
20]. In this study, we did not see significant differences between the hGal1-hNSPCs-transplanted group and the naïve hNSPCs-transplanted group in the rate of recovery from sensory deficits. This raises the possibility that transplantation of hNSPCs alone is sufficient to promote recovery from sensory deficits caused by ischemia [
20] and is not augmented by hGal1 overexpression. Indeed, in the current study, the hGal1-hNSPCs group showed only marginal improvement in the recovery from sensory deficit compared with hNSPCs group (Figure
3B). This underscores the observation that hGal1 has an additional benefit in the recovery of motor function beyond the effect of hNSPCs transplantation alone.
Considering that Gal1 is expressed endogenously in damaged tissue of various neurological diseases, and application of hGal1 improves symptoms of those diseases [
16,
22,
41,
45‐
47], hGal1 is a promising therapeutic agent. This study provides an alternative method to apply hGal1 the treatment of brain ischemia with transplantation of hNSPCs or alternative cells sources such as Nestin-expressing hair follicle stem cells [
48‐
51].
Acknowledgements and funding
We thank M. Ito, K. Fujita, M. Shiota, and A. Hirayama for secretarial assistance. This work was supported in part by grants from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the project for the realization of regenerative medicine and support for the core institutes for iPS cell research from MEXT to H.O., and Y,K., a Start-up research grant from the Japan Society for the Promotion of Science (JSPS), JSPS Postdoctoral Fellowships for Research Abroad, Naito Memorial Foundation research grant to M.S. and Grant-in-Aid for Encouragement of Young Medical Scientists from Keio University.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
Author contributions: JY, SI, MS, NM, KS and HO designed the research. JY, SI, MS, TK, MH and YK performed the research, JY, SI and MS analyzed data, JY, SI, MS, TK, KS, MN, YT and HO prepared the manuscript. All authors read and approved the final manuscript.