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Recovery and Rehabilitation in Stroke
Stem Cells
Olle Lindvall, MD, PhD; Zaal Kokaia, PhD
Abstract—The recent demostration that neurons for transplantation can be generated from stem cells and that the adult
brain produces new neurons in response to stroke has raised hope for the development of a stem cell therapy for patients
affected with this disorder. In this review we propose a road map to the clinic and describe the different scientific tasks
that need to be accomplished to move stem cell– based approaches toward application in stroke patients. (Stroke. 2004;
35[suppl I]:2691-2694.)
Key Words: acute care 䡲 rehabilitation 䡲 stem cells 䡲 stem cell transplantation
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T
he main aim of cell therapy is to restore function in the
diseased human brain by replacing dead neurons with
new neurons through transplantation or stimulation of neurogenesis from endogenous stem/precursor cells. This approach
seems to be more suitable for disorders like Parkinson
disease, in which the main pathology is a relatively selective
degeneration of a specific type of neuron, the nigrostriatal
dopamine neuron. In stroke, occlusion of a cerebral artery
leads to focal ischemia and subsequent damage in a restricted
central nervous system region. To repair the human brain
after stroke may seem unrealistic because of the atrophy and
loss of many different neuron and glial cell types. It could be
argued, however, that reestablishment of even only a fraction
of damaged neuronal circuitries might have significant implications. The clinical trials with intrastriatal transplantation
of human embryonic mesencephalic tissue in patients with
Parkinson disease have provided proof-of-principle that cell
replacement can work in this disorder. The grafted neurons
can survive and reinnervate the striatum,1,2 release dopamine,2 become functionally integrated into host neuronal
circuitries,3 and give rise to clinical benefit.4,5 However, 2 recent
sham surgery-controlled trials showed only modest improvement,6,7 which illustrates that present cell replacement procedures for Parkinson disease patients are far from optimal.
In contrast to Parkinson disease, there is so far no convincing evidence that neuronal replacement can work in stroke
patients. In the only clinical trial reported so far, 12 patients
with stroke affecting the basal ganglia received implants of
neurons generated from the human teratocarcinoma cell line
NTera-2 (NT-2) into the infarcted area.8 This cell line gives
rise to neurons after a complex induction process. The
improvements in some of the patients correlated with increased metabolic activity at the graft site.9 This finding could
be interpreted as graft function, but may also reflect inflammation or increased activity in host neurons. Autopsy in 1
patient revealed a population of grafted cells expressing a
neuronal marker 2 years after surgery.10
In this brief review, we will discuss the prospects of stem
cell therapy to repair the brain after stroke. Stem cells are
immature cells with prolonged self-renewal capacity and,
depending on their origin, the ability to differentiate into
multiple cell types or all cells of the body. Hypothetically,
neurons and other cells useful for brain repair in stroke could
be made from stem cells of four different sources: embryonic
stem cells from the blastocyst, neural stem cells (NSCs) from
the embryonic or adult brain, or stem cells in other tissues, eg,
bone marrow.
Can Cell Therapy Work in Animal Models
of Stroke?
Stem/precursor cells from different sources have been tested
for their ability to reconstruct the forebrain and improve
function after transplantation in animals subjected to stroke
(Table 1). The transplants, including a mouse neuroepithelial
stem cell line, the human NTera-2 cell line, and human bone
marrow cells, have been reported to partly reverse some
behavioral deficits. However, in most cases, the underlying
mechanisms are unclear and there is little evidence for
neuronal replacement. Only few grafted cells have survived
and they have not exhibited the phenotype of the dead
neurons. Moreover, it is unknown if the observed grafted
cells are functional neurons and establish connections with
host neurons.
Despite the poor evidence for significant neuronal replacement in these studies, improvement of various stroke-induced
behavioral deficits has been observed. How is this possible?
Received June 2, 2004; accepted August 5, 2004.
From the Laboratory of Neurogenesis and Cell Therapy, Wallenberg Neuroscience Center (O.L.), and the Laboratory of Neural Stem Cell Biology,
Stem Cell Institute (Z.K.), University Hospital, Lund, Sweden; and the Lund Strategic Research Center for Stem Cell Biology and Cell Therapy (O.L.,
Z.K.), Lund, Sweden.
Correspondence to Dr Olle Lindvall, Section of Restorative Neurology, Wallenberg Neuroscience Center, University Hospital BMC A-11, SE-221 84
Lund, Sweden. E-mail [email protected]
© 2004 Am Heart Association, Inc.
Stroke is available at http://www.strokeaha.org
DOI: 10.1161/01.STR.0000143323.84008.f4
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Stroke
November 2004 (Supplement 1 - New Series)
Properties of Stem/Precursor Cells Grafted in Animal Models of Stroke
Cell Source and Transplantation Approach
Rat SVZ precursors in rat cisterna magna
Graft Survival/Neuronal
Phenotype
Afferent/Efferent
Connections
Effect on Behavioral Deficit
6%/Single cells
?/Single cells
Improved sensorimotor function
Mouse neuroepithelial stem cell line in rat cortex,
striatum or ventricle18
“Good”/30% NeuN⫹
?/?
Improved sensorimotor function or spatial
memory
Human fetal teratocarcinoma cell line (NT-2) in rat
striatum28
“Moderate”/Majority NF⫹,
N-CAM⫹
?/?
Improved passive avoidance task and
asymmetrical motor behaviour
“Good”/Many cells NF⫹
Many/To opposite
hemisphere
?
Rat bone marrow stromal cells systemically or in
penumbra zone in rat or mouse striatum29–32
1–21%/0.02–2% (NeuN⫹,
MAP-2⫹)
?/?
Improved sensorimotor function and
Neurological Severity Score
Human bone marrow stromal cells systemically in
rats33
4%/1–5% (NeuN⫹,
MAP-2⫹)
?/?
Improved sensorimotor function and
Neurological Severity Score
Human umbilical cord blood cells systemically in
rats34
1%/2–3% (NeuN⫹,
MAP-2⫹)
?/?
Improved sensorimotor function and
Neurological Severity Score
27
Immortalized mouse cerebellar precursors on
polymer scaffold in mouse cortex23
? indicates not demonstrated.
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In this context, it should be pointed out that stem cell
transplantation probably can lead to clinically valuable improvements through several mechanisms. First, the tissue
damage per se can stimulate plastic responses or interfere
with neural activity in the host. Second, the transplants can
act as biological minipumps and release a missing transmitter
or secrete growth factors. These factors can stimulate plastic
responses and improve the survival and function of host
neurons. Third, the grafts can restore synaptic transmitter
release by providing a local reinnervation. Fourth, and this is
true neuronal replacement, the grafts can become integrated
into existing neural and synaptic networks, and reestablish
functional afferent and efferent connections.
Recent findings in rodents suggest an alternative approach
to cell therapy in stroke based on self-repair. Stroke leads to
increased generation of neurons from NSCs in the subventricular zone (SVZ), lining the lateral ventricles.11–13 These
immature neurons migrate into the damaged striatum, where
they express markers of striatal medium spiny projection
neurons. Thus, the new neurons seem to differentiate into the
phenotype of most neurons destroyed by the ischemic lesion.
However, because more than 80% of the new neurons die
during the first weeks after stroke, they only replace a small
fraction (about 0.2%) of the mature striatal neurons which
have died.11
Currently, there is a lot of enthusiasm about the therapeutic
potential of endogenous neurogenesis in stroke. However, we
have very little knowledge about the importance of endogenous neurogenesis for brain repair. We know that there is a
neurogenic response after stroke. We know very little how the
various steps of neurogenesis are regulated after stroke or if
the new neurons are functional neurons and become integrated into host neural circuitries. One major problem is that
the majority of the new neurons die after stroke, and very few
survive long-term. Several factors can increase adult neurogenesis by stimulating formation or improving survival of
new neurons, such as FGF-2, EGF, stem cell factor, erythropoietin, BDNF, caspase inhibitors, and antiinflammatory
drugs.14 Recently, it was demonstrated that inflammation is
detrimental for neurogenesis in the dentate gyrus.15,16 Because stroke causes inflammation, these data raise the possibility that inflammation-mediated suppression of neurogenesis plays a role for the ineffective neuroregenerative
response in this condition.
In the ideal transplantation scenario, stem cells implanted
directly into or around the damaged area will differentiate in
situ into those cells which have died. This strategy requires
that the largely unknown developmental mechanisms instructing stem cells to differentiate into specific cell types
will work also in the patient’s brain. The optimum strategy
would probably be to combine transplantation of NSCs close
to the damaged area with stimulation of neurogenesis from
endogenous NSCs. Newly generated neurons are able to
migrate toward the damage11,13,17 and, at least to some extent,
adopt the phenotype of those cells that have died.11,13,18
Available data provide evidence for a neurogenic potential
also in the human brain. Neurogenesis from precursors in the
SVZ has been demonstrated in vivo in humans,19 and precursors capable of forming neurons are found in human subcortical white matter.20
The inadequate blood supply to the ischemic core region
will cause massive loss of newly formed neurons. If cells are
implanted into the penumbra area (region-at-risk), they will
probably be supported by collateral circulation. Adult hippocampal neurogenesis is closely associated with angiogenesis from endothelial cell precursors.21 The creation of such a
vascular niche and the stimulation of vascularization after
stroke will be crucial for the survival of the new neurons.
Administration of vascular endothelial growth factor promotes both neurogenesis in the SVZ and angiogenesis in the
ischemic penumbra region following stroke.22 However, for
efficient repair after stroke it may be necessary to provide the
NSCs with a platform so that they can reform appropriate
brain structure and connections. A recent study in neonatal
mice23 shows that if NSCs are seeded on synthetic extracellular matrix and implanted into the ischemia-damaged area,
then new parenchyma composed of neurons and glia is
formed and becomes vascularized.
Lindvall and Kokaia
Stem Cells in Stroke Recovery and Rehabilitation
2693
stem cell therapies for brain repair after stroke, many basic
issues remain to be solved. We need to move forward with
caution and avoid ill-founded trials of patients. Before clinical trials with stem cell– based approaches are initiated, we
need to know to a much greater extent how to control stem
cell proliferation and differentiation into specific phenotypes,
induce their integration into existing neural and synaptic
circuits, and optimize the functional recovery in animal
models of stroke.
Acknowledgments
Road map for cell replacement therapy toward clinical application in stroke.
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How Can Stem Cell Therapy Be Developed for
Stroke Patients?
In order to develop stem cell therapy toward clinical application in stroke, 3 different tasks should be accomplished:
The first task (Steps 1 and 2 in Figure 1) is to obtain
proof-of-principle that implanted stem cells, or neurons that
are generated from endogenous NSCs, can survive in large
numbers in animals subjected to stroke, migrate to appropriate locations, exhibit morphological and functional properties
of those neurons that have died, and establish afferent and
efferent synaptic interactions with neurons that survived the
insult. Magnetic resonance imaging seems ideal for noninvasive imaging at high spatial and temporal resolution of the
survival, migration and differentiation of grafted cells.17,24
The second task (Step 3 in Figure 1) is to optimize the
behavioral recovery induced by neuronal replacement in
animal models. Strategies to improve survival, differentiation, and integration of endogenous and grafted stem cells
will require detailed knowledge of how these processes are
regulated. The time window after the insult when the generation of new neurons will lead to maximum restitution of
neuronal circuitries and functional recovery should be determined. The third task (Step 4 in Figure 1) is to define which
patients are suitable for stem cell therapy based on findings in
animal models regarding which cell types can be produced
and replaced. The occurrence of striatal neurogenesis after
stroke11,13,25 focuses the interest on patients with basal ganglia infarcts. If stem cells can also generate cortical neurons
and repair axonal damage, patients with lesions in the
cerebral cortex may be included. A strategy for repair of
infarcted white matter was suggested recently by the observation that adult neurospheres injected intravenously or
intraventricularly in mice26 gave rise to cells that migrated to
demyelinated areas, differentiated into oligodendrocyte progenitors, remyelinated axons, and improved function.
Conclusions
Recent progress shows that specific types of neurons and glia
cells suitable for transplantation can be generated from stem
cells in culture. We also see that the adult brain produces new
neurons from its own stem cells in response to stroke.
Although these findings raise hope for the development of
O.L. and Z.K. are supported by grants from the Swedish Research
Council, and the Kock, Söderberg, Craford, and Segerfalk Foundations.
The Lund Stem Cell Center is supported by a Center of Excellence grant
from the Swedish Foundation for Strategic Research.
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Recovery and Rehabilitation in Stroke: Stem Cells
Olle Lindvall and Zaal Kokaia
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Stroke. 2004;35:2691-2694; originally published online September 30, 2004;
doi: 10.1161/01.STR.0000143323.84008.f4
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