Among mesenchymal stem cells: for the best therapy after ischemic stroke

Owing to the several advantages that they have over other cells, mesenchymal stem cells are among the most promising tools in stem cell therapy after ischemic stroke. Mesenchymal stem cells can be obtained from various sources. Therefore, a wide range of careful comparative studies of these cells is required, from the preclinical stage (in vitro and in vivo) to the clinical stage, in order to develop the best therapy with the most appropriate cell type. An in vivo study in this issue of Stem Cell Research & Therapy compares the therapeutic potential of two cell types, obtained from bone marrow and adipose tissue, in a rodent stroke model. This commentary discusses the significance of comparative studies of mesenchymal stem cells, including the related article in translational research.

Stem cell therapy is one of the most anticipated emerging therapies in the treatment of ischemic stroke. A variety of stem cells or progenitor cells, including neural stem cells, CD34 + cells, and mesenchymal stem cells (MSCs), have been investigated with respect to their therapeutic potential in a number of preclinical studies. Subsequently, clinical studies also have been conducted on several cell types. In the search for an ideal cell delivery, a variety of conditions -such as cell quality (which cell type is used, age, and cell culture), the administration route, cell dose, timing, and cell modifi cation -need to be examined. Which pathological conditions are suitable for each therapy also needs to be evaluated [1][2][3][4]. In this issue of Stem Cell Research & Th erapy, Gutiérrez-Fernández and colleagues [1] look for the best protocol for using MSCs and compare the thera peutic capacity of bone marrowderived MSCs (BM-MSCs) and adipose tissue-derived MSCs (AD-MSCs) as allogenic grafts intra venously administered in rodents in the acute phase with a permanent ischemic stroke model. Th e focal ischemia model is similar to thrombotic or embolic infarction in patients. Th e authors evaluate the treatment eff ects in both the acute phase and the sub acute phase by analyzing neurological function, patho logical changes, and histological brain repair [1].
MSCs are promising therapeutic tools for regenerative medicine. Within the last decade, several clinical trials that employ MSCs have been started, and some of them have shown positive safety results and eff ects [2,4]. MSCs can be obtained from diverse adult tissues (for example, bone marrow, adipose tissue, peripheral blood, and other mesoderm tissues). Furthermore, it is said that MSCs have more advantages in clinical application than other cell types; this includes such advantages as multilineage potential, immunomodulative ability, the secretion of trophic factors for brain tissue, and easier cell preparation with fewer ethical problems [4][5][6]. Th e immunological feature enables a recipient to avoid the risk of rejection without immunosupressants by producing immune tolerance, which means that MSCs can provide easier allogenic cell therapy free from demerits of immune suppressor to patients in the acute phase. An autologous graft has the least risk of side eff ects but, owing to the time required for cell expan sion, is diffi cult to apply in the acute phase. Certainly, a few reports have shown that autologous BM-MSC adminis tration in the subacute or later phase could also be eff ective [7], but given the ability of MSCs to product trophic factors, acute-phase therapy with an allogenic graft needs to be considered.
On the other hand, owing to the diversity of MSC sources, which type of MSC is more favorable is still quite controversial and will need to be clarifi ed. Bone marrow and adipose tissue are thought to be among the most suitable sources for practical use, as both tissues are abundant enough to easily isolate the required number of MSCs. Gutiérrez-Fernández and colleagues [1] demonstrate that the two allogenic MSCs showed equivalent

Abstract
Owing to the several advantages that they have over other cells, mesenchymal stem cells are among the most promising tools in stem cell therapy after ischemic stroke. Mesenchymal stem cells can be obtained from various sources. Therefore, a wide range of careful comparative studies of these cells is required, from the preclinical stage (in vitro and in vivo) to the clinical stage, in order to develop the best therapy with the most appropriate cell type. An in vivo study in this issue of Stem Cell Research & Therapy compares the therapeutic potential of two cell types, obtained from bone marrow and adipose tissue, in a rodent stroke model. This commentary discusses the signifi cance of comparative studies of mesenchymal stem cells, including the related article in translational research. therapeutic benefi ts after permanent ischemia. One report indicates diff erent brain-protective eff ects for BM-MSCs and AD-MSCs in a transient ischemic stroke model [8]. However, if the two MSCs can contribute equally to a cure, an application of MSC therapy would increase by using the two most abundant cell sources.

Mechanism analysis for neural functional recovery after mesenchymal stem cell administration
Th e study by Gutiérrez-Fernández and colleagues [1] shows that, in comparison with the controls, the administration of MSCs signifi cantly improved the neural function, reduced apoptotic cell death, increased cell proliferation, and reduced reactive gliogenesis that causes scar formation.
Several mechanisms are proposed to explain the successful improvement of neural functional recovery through MSC administration: (a) exogenous tissue repair through migration/integration, (b) neuroprotection through the reduction of cell death or immuno modu lation, (c) enhancement of endogenous tissue repair, and (d) plasticity promotion in synapse formation [1,3,5]. Brain tissue consists of a major functional unit (that is, the neurovascular unit) that contains neural and vascular cells. Th us, to achieve a more eff ective tissue reconstruction, both cell lineages need to be considered. Previous studies have shown that MSCs diff erentiate into neurons, glial cells, and vascular endothelial cells, in both in vitro and in vivo conditions [4,5,[7][8][9][10]. However, in the related article and several other reports, there was no graft after the transvenous MSC administration, even though the immunological feature of MSCs can prevent allograft rejection [1,8]. Th erefore, the proposed cure mechanisms would be associated with the trophic factors secreted from MSCs. Correspondingly, the authors observe elevated levels of neurogenesis, synaprogenesis, oligodendro genesis, and angiogenesis markers with increased levels of soluble factors in peri-infarct tissue 14 days after treatment with MSCs [1]. Interestingly, the fi ndings indicate that both MSCs administered just 30 minutes after ischemia can maintain the therapeutic eff ects on the brain repair process even 14 days later. In all comparative parameters, the present study shows no signifi cant diff erence between the group that received BM-MSCs and the one that received AD-MSCs [1], whereas some reports indicate that each set of MSCs has diff erent abilities in terms of secreting trophic factors [8,10]. It is diffi cult to defi ne the perfect integrity of each set of MSCs. In the most popular protocol, MSCs are currently isolated with a few cell surface markers [4,5]. Th e diff erent results are expected to be verifi ed by further fi ndings about cell quality and integrity.
Nevertheless, the multipotency of MSCs would surely be benefi cial if the MSCs were to integrate into the target. Th e previous study by Gutiérrez-Fernández and colleagues [9] and other reports indicate that transarterial administration would deliver MSCs to the peri-infarct area [3], implying the possibility of application to neuroendovascular therapies through a catheter. Th e neural induction potential of MSCs has been extensively examined [5]. Still, further analyses are expected to be conducted on gene regulation mechanisms, including epigenetics while neural diff erentiation is occurring. In this context, an analysis of RNA and microRNA profi les was recently reported [11]. Also, in vitro comparative studies on the molecular biological characteristics have begun to be pub lished, and diff erences among the various MSCs (BM-MSCs, AD-MSCs, and so on) have been detected [10,12]. Th e development of in vitro analysis will contribute to customizing the selection of appropriate graft MSCs in accordance with individual conditions.

The role of in vivo and clinical studies in fi nding an ideal cell delivery
For the best therapy, the maximum therapeutic eff ects should be provided to patients with a protocol that is minimally invasive and provides the least amount of suff ering and risk. Th at is the fi nal goal for translational research. Th e comparative study by Gutiérrez-Fernández and colleagues [1] has demonstrated that both sets of major MSCs -BM-MSCs and AD-MSCs -could become eff ective tools via intravenous administration, which is the least invasive route. Th e authors also have indicated that there is a need to study acute-phase treatment with allogenic grafts by showing the long-term eff ects on brain repair after a single injection of MSCs just after onset [1]. Given the results of several successful clinical trials and the theoretical advantages of MSC therapy, it is hoped that this therapy can become a reliable treatment [3,4,7]. Hence, further comparative studies are desired to assess the protocols in both the preclinical and clinical stages. While the development of an ideal treatment requires huge eff orts and the continual accumulation of major fi ndings, such as the work of Gutiérrez-Fernández and colleagues [1,9], careful studies on multiple aspects will contribute to achieving the best therapy. is grateful to Manfred Biebl and Shin-ichiro Hayashi for editing support and to Shigeru Nakashima (a chair of the Department of Cell Signaling, Gifu University) and Toru Iwama (a chair of the Department of Neurosurgery, Gifu University) for general support.