Mesenchymal stem cells (MSCs) are promising candidates for adult cell therapies in regenerative medicine. To fully exert their potential, efficient homing and migration toward lesion sites play an important role. Local transplantation deposits MSC in spatial proximity to the lesion, but often requires invasive procedures. Systemic administration routes are favored, but require the targeted extravasation of the circulating MSC at the site of injury. Transplanted MSC can indeed leave the blood flow and transmigrate through the endothelial barrier, and reach the lesion site. However, the underlying processes are not completely dissolved yet. Recent in vitro and in vivo research identified some key molecules scattered light on the extravasation mechanism. This review provides a detailed overview over the current knowledge of MSC transendothelial migration. We use the leukocyte extravasation process as a role model to build a comprehensive concept of MSC egress mechanisms from the blood stream and identified relevant similarities as well as important differences between the extravasation mechanisms. Stem Cells 2017;35:1446-1460.

Cortical stem cell transplantation may help replace lost brain cells after stroke and improve the functional outcome. In this study, we transplanted human embryonic stem cell (hESC)-derived neural precursor cells (hNPCs) or vehicle into the cortex of rats after permanent distal middle cerebral artery occlusion (dMCAO) or sham-operation, and followed functional recovery in the cylinder and staircase tests. The hNPCs were examined prior to transplantation, and they expressed neuroectodermal markers but not markers for undifferentiated hESCs or non-neural cells. The rats were housed in either enriched environment or standard cages to examine the effects of additive rehabilitative therapy. In the behavioral tests dMCAO groups showed significant impairments compared with sham group before transplantation. Vehicle groups remained significantly impaired in the cylinder test 1 and 2 months after vehicle injection, whereas hNPC transplanted groups did not differ from the sham group. Rehabilitation or hNPC transplantation had no effect on reaching ability measured in the staircase test, and no differences were found in the cortical infarct volumes. After 2 months we measured cell survival and differentiation in vivo using stereology and confocal microscopy. Housing had no effect on cell survival or differentiation. The majority of the transplanted hNPCs were positive for the neural precursor marker nestin. A portion of transplanted cells expressed neuronal markers 2 months after transplantation, whereas only a few cells co-localized with astroglial or oligodendrocyte markers. In conclusion, hESC-derived neural precursor transplants provided some improvement in sensorimotor function after dMCAO, but did not restore more complicated sensorimotor functions.

IntroductionIntra-arterial cell infusion is an efficient delivery route with which to target organs such as the ischemic brain. However, adverse events including microembolisms and decreased cerebral blood flow were recently reported after intra-arterial cell delivery in rodent models, raising safety concerns. We tested the hypothesis that cell dose, infusion volume, and velocity would be related to the severity of complications after intra-arterial cell delivery.MethodsIn this study, 38 rats were subjected to a sham middle cerebral artery occlusion (sham-MCAO) procedure before being infused with allogeneic bone-marrow mesenchymal stem cells at different cell doses (0 to 1.0 × 106), infusion volumes (0.5 to 1.0 ml), and infusion times (3 to 6 minutes). An additional group (n = 4) was infused with 1.0 × 106 cells labeled with iron oxide for in vivo tracking of cells. Cells were infused through the external carotid artery under laser Doppler flowmetry monitoring 48 hours after sham-MCAO. Magnetic resonance imaging (MRI) was performed 24 hours after cell infusion to reveal cerebral embolisms or hemorrhage. Limb placing, cylinder, and open field tests were conducted to assess sensorimotor functions before the rats were perfused for histology.ResultsA cell dose-related reduction in cerebral blood flow was noted, as well as an increase in embolic events and concomitant lesion size, and sensorimotor impairment. In addition, a low infusion velocity (0.5 ml/6 minutes) was associated with high rate of complications. Lesions on MRI were confirmed with histology and corresponded to necrotic cell loss and blood-brain barrier leakage.ConclusionsParticularly cell dose but also infusion velocity contribute to complications encountered after intra-arterial cell transplantation. This should be considered before planning efficacy studies in rats and, potentially, in patients with stroke.

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Background and Purpose-The present study examined the long-term presence of ␤-amyloid precursor protein (APP) and ␤-amyloid (A␤) accumulation in the rat thalamus after focal cerebral ischemia. Methods-Male Wistar rats were subjected to transient middle cerebral artery occlusion (MCAO) for 2 hours.Sensorimotor outcome was assessed using a tapered/ledged beam-walking task after operation. The distribution of APP and A␤ was examined immunohistochemically at 1 week, 1 month, and 9 months after MCAO. Results-MCAO caused a long-lasting deficit in forelimb and hind limb function assessed using the beam-walking test.Histologic examination revealed a transient increase in APP and A␤ staining in axons in the corpus callosum and in neurons at the border of the ischemic region. APP and A␤ deposits persisted in the thalamic nuclei (ventroposterior lateral and ventroposterior medial nuclei), eventually leading to dense plaque-like deposits by the end of the 9-month follow-up. The deposits were surrounded by an astroglial scar. The deposits were positive for A␤ and N-terminal APP, but not for C-terminal APP. Antibodies against the C-terminal of A␤, ie, A␤42 and A␤40, showed a preferential staining for A␤42. Congo red or thioflavine S did not stain the deposits. Key Words: amyloid Ⅲ cerebral ischemia Ⅲ rats Ⅲ thalamus B eta (␤)-amyloid precursor protein (APP) is a transmembrane protein with a long extracellular N-terminal and short intracellular C-terminal domain. APP is widely expressed in the brain, where its abnormal upregulation can lead to the accumulation of ␤-amyloid (A␤), for example in Down syndrome patients. 1,2,3 A␤ is a hydrophobic self-aggregating peptide consisting of 40 to 42 residues, derived by sequential processing of the ␤-amyloid precursor protein by ␤-secretase and ␥-secretase. 4 A␤ is a major component of senile plaques and is one of the pathologic hallmarks of Alzheimer disease. 5 Brain trauma leads to the accumulation of a number of proteins, mostly as a consequence of the interruption of fast anterograde axonal transport. 6 The accumulated proteins include APP and its proteolytic product A␤, neurofilament proteins, and synuclein proteins. Accumulated proteins usually disappear over time, 7 but APP has been detected for up to 1 year after injury. 8 Cerebral ischemia also leads to a transient upregulation and accumulation of APP. 9,10 For example, APP staining and expression are detected in the subcortical white matter and adjacent to the boundary of the ischemic lesion in the gray matter after transient occlusion of the middle cerebral artery (MCAO). 10 -13 These studies, however, used only short survival periods ranging from days to weeks. The aim of the present study was to assess possible long-term accumulation of APP and A␤ during a 9-month follow-up in rats subjected to transient MCAO. Conclusions-The Materials and Methods AnimalsMale Wistar rats (3 months old, 285 to 325 grams at the beginning of the study; National Laboratory Animal Centre, Kuopio, Finland) were used in the present study. The ani...