Mesenchymal stem cells to treat diabetic neuropathy: a long and strenuous way from bench to the clinic

Abstract

As one of the most common complications of diabetes, diabetic neuropathy often causes foot ulcers and even limb amputations. Inspite of continuous development in antidiabetic drugs, there is still no efficient therapy to cure diabetic neuropathy. Diabetic neuropathy shows declined vascularity in peripheral nerves and lack of angiogenic and neurotrophic factors. Mesenchymal stem cells (MSCs) have been indicated as a novel emerging regenerative therapy for diabetic neuropathy because of their multipotency. We will briefly review the pathogenesis of diabetic neuropathy, characteristic of MSCs, effects of MSC therapies for diabetic neuropathy and its related mechanisms. In order to treat diabetic neuropathy, neurotrophic or angiogenic factors in the form of protein or gene therapy are delivered to diabetic neuropathy, but therapeutic efficiencies are very modest if not ineffective. MSC treatment reverses manifestations of diabetic neuropathy. MSCs have an important role to repair tissue and to lower blood glucose level. MSCs even paracrinely secrete neurotrophic factors, angiogenic factors, cytokines, and immunomodulatory substances to ameliorate diabetic neuropathy. There are still several challenges in the clinical translation of MSC therapy, such as safety, optimal dose of administration, optimal mode of cell delivery, issues of MSC heterogeneity, clinically meaningful engraftment, autologous or allogeneic approach, challenges with cell manufacture, and further mechanisms.

Full text

OPEN
REVIEW ARTICLE
Mesenchymal stem cells to treat diabetic neuropathy:
a long and strenuous way from bench to the clinic
JY Zhou
1
, Z Zhang
1
and GS Qian
2
As one of the most common complications of diabetes, diabetic neuropathy often causes foot ulcers and even limb amputations.
Inspite of continuous development in antidiabetic drugs, there is still no efficient therapy to cure diabetic neuropathy. Diabetic
neuropathy shows declined vascularity in peripheral nerves and lack of angiogenic and neurotrophic factors. Mesenchymal stem
cells (MSCs) have been indicated as a novel emerging regenerative therapy for diabetic neuropathy because of their multipotency.
We will briefly review the pathogenesis of diabetic neuropathy, characteristic of MSCs, effects of MSC therapies for diabetic
neuropathy and its related mechanisms. In order to treat diabetic neuropathy, neurotrophic or angiogenic factors in the form of
protein or gene therapy are delivered to diabetic neuropathy, but therapeutic efficiencies are very modest if not ineffective. MSC
treatment reverses manifestations of diabetic neuropathy. MSCs have an important role to repair tissue and to lower blood glucose
level. MSCs even paracrinely secrete neurotrophic factors, angiogenic factors, cytokines, and immunomodulatory substances to
ameliorate diabetic neuropathy. There are still several challenges in the clinical translation of MSC therapy, such as safety, optimal
dose of administration, optimal mode of cell delivery, issues of MSC heterogeneity, clinically meaningful engraftment, autologous
or allogeneic approach, challenges with cell manufacture, and further mechanisms.
Cell Death Discovery (2016) 2, e16055; doi:10.1038/cddiscovery.2016.55; published online 11 July 2016
FACTS
●Diabetic neuropathy (DN) often causes foot ulcers and even
limb amputations, without efficient therapy.
●DN shows declined vascularity in peripheral nerves and lack of
angiogenic and neurotrophic factors.
●Preclinical and clinical studies indicate that mesenchymal stem
cell (MSC) therapy restores manifestations of DN.
OPEN QUESTIONS
●What is the exact molecular mechanism of MSCs on DN?
●Are there any molecules secreted by MSCs to protect bone
marrow nerve and to maintain bone marrow homeostasis?
●Which challenges would be most difficult in the clinical
translation of MSC therapy?
INTRODUCTION
DN is one of the most frequent complications of diabetes, 66% for
type 1 diabetes and 59% for type 2 diabetes.
1
The pathophysiology
of DN is complicated and not fully elucidated that involves both
vascular and neural components. DN is a systemic and progressive
disorder and its manifestations need many years to develop.
Intervention with tight blood glucose control and treatment with
aldose reductase inhibitor or α-lipoic acid successfully inhibit
the progression of DN,
2,3
but no established curable treatment is
available during the progressive stage. During the past decades, one
of the innovative preclinical study has applied gene therapy or MSC
therapy to DN in animal models,
4
but gene therapy shows weak
result or is ineffective.
MSCs have been believed as a promising regenerative therapy
for DN because of their multipotency and their paracrine secretion
of angiogenic factors and neurotrophic factors. Umbilical cord
blood ex vivo expanded CD34 and umbilical cord matrix MSCs
were well tolerated without adverse effects in a 29-year-old male.
5
MSC therapies offer more benefits than other cell-based therapies.
Practically, as the safety of autologous bone marrow-derived MSCs
(BMSCs) have been documented by variety of clinical trials,
6
it is
highly recommended to use this strategy in a pilot clinical trial for
those who are severely affected by DN. In this review, we will briefly
summarize the pathogenetic mechanisms, effects of MSC treatment,
and challenges from bench to bedside studies of MSCs on DN.
DIABETIC NEUROPATHY
DN is characterized with progressive neuronal loss, demyelination,
and damaged nerve regeneration with ultimately dysfunction of
nerve fibers impairing both the autonomic and somatic divisions
of the nervous system.
7
The pathogenesis of DN is complex but
the same as other complications, hyperglycemia exacerbates its
development. Hyperglycemia damages neurons, Schwann cells,
and endothelial cells of the vasa nervorum in the peripheral
nerves. Hyperglycemia results in oxidative stress, reactive oxygen
species generation, and advance glycation end product produc-
tion, which leads to impairment in sensory, motor, and autonomic
nerve.
8
Several factors involve in the development and progres-
sion of DN (Figure 1).
7–11
1
National Drug Clinical Trial Institution, Xinqiao Hospital, Third Military Medical University, Chongqing 400037, China and
2
Institute of Respiratory Diseases, Xinqiao Hospital, Third
Military Medical University, Chongqing, 400037, China.
Correspondence: GS Qian (qianguisheng@outlook.com)
Received 24 March 2016; revised 23 May 2016; accepted 11 June 2016; Edited by AE Sayan
Citation: Cell Death Discovery (2016) 2, e16055; doi:10.1038/cddiscovery.2016.55
Official journal of the Cell Death Differentiation Association
www.nature.com/cddiscovery

Role of neurotrophic factors in pathogenesis
Except the classical major pathophysiological role of neuro-
trophic factors and vascular supply in DN, the two widely
considered downstream consequences of the cellular mechan-
isms are the loss of neurotrophic support and ischemic
hypoxia. Direct cellular contact is not necessary to provide
neuroprotection.
12–14
Critical in providing a protective micro-
environment, neurotrophic factors are growth factors known to
promote neuron development and survival. They also maintain
functional integrity, promote regeneration, regulate neuronal
plasticity, and aid in the repairing of damaged nerves.
15
The
various protective types of neurotrophic factors affect different
cell populations within the peripheral and central nervous
system.
Deficiency of these neurotrophic factors can cause development
of DN.
16
Diabetes reduces brain-derived nerve factor (BDNF), nerve
growth factor (NGF), and neurotrophin 3 in peripheral nerves by
limiting anterograde and retrograde axonal transport. Intrathecal
delivery of NGF or neurotrophin 3 improves myelinated fiber
innervation in the dermal footpad of diabetic mice, and thus lack
of neurotrophic support affect fiber morphology. Neurotrophic
factors may regulate angiogenesis. BDNF is an essential factor
in maintaining cardiac vessel-wall stability during development.
17
NGF stimulates angiogenesis indirectly by increasing the expres-
sion of (vascular endothelial growth factor (VEGF) and directly
by promoting vascular cell growth. Both neurotrophin 3 (through
binding to TrkC) and leukemia-inhibitory factor serve as inhibitors
of the growth of some endothelial cells.
Role of angiogenic factors in pathogenesis
Many representative growth factors VEGF, insulin-like growth
factor, NGF, BDNF, and fibroblast growth factor-2 (FGF2, also
known as bFGF) have dual effects of being both neurotrophic and
angiogenic. These growth factor levels are decreased in diabetic
animals and are associated with neural function.
18,19
VEGF, a major angiogenic factor, is a potent selective
mitogenic cytokine for endothelial cells. VEGF enhances migra-
tion and proliferation of Schwann cells and promotes axonal
outgrowth and survival of both the neurons and Schwann cells
of superior cervical ganglia and dorsal root ganglia. Insulin-like
growth factors promote neurite outgrowth of neuroblastoma
cells, accelerate regeneration of sensory and motor nerves,
and stimulate Schwann cell mitogenesis and myelination. NGF
provides neuroprotective, repair functions, and directly induces
angiogenesis via promoting survival and differentiation of
sensory and sympathetic neurons.
20
NGF homozygous knockout
mice do not develop proper sympathetic neurons or small neural
crest-derived sensory neurons.
MESENCHYMAL STEM CELLS
MSC classification
MSCs have the capacity of self-renewal and the potential
to differentiate into multiple cell types such as adipocytes,
chondrocytes, and osteoblasts, myocytes, and neurons.
21–24
MSCs can be derived from bone marrow, adipose tissue, nervous
tissue, amniotic fluid, umbilical cord, placenta, menstrual blood,
and dental pulps.
25–31
BMSCs and adipose tissue-derived MSCs
are representativeof this.
32,33
MSCs are a subset of cells that express on their surface CD54/CD102,
CD166, and CD49 as well as CD73 and CD90. They also express
CD44 and CD105, whereas they do not express CD34, CD14,
CD45, CD11a/LFA-1, and CD31, which are surface markers of
hematopoietic cells and/or endothelial cells.
34,35
Although their
differentiation capacity is less than other cell types such as
embryonic stem cells or induced pluripotent stem cells, MSCs
migrate and home to injured sites, acting both by regenerating
tissues and by secreting trophic factors and paracrine mediators.
MSCs have remarkable immunosuppressive properties secreting
cytokines and immunomodulatory substances.
36–41
MSCs secret neurotrophic and angiogenic factors
Delivering neurotrophic or angiogenic factors in the form of
protein or gene for therapy have no significant effect. BMSCs
are effective for reversing various manifestations of experimental
DN.
7
MSCs secrete various cytokines with angiogenic and
neurosupportive effects. MSCs reside in the BM stromal fraction,
which provides the cellular microenvironment supporting hema-
topoiesis. MSCs are adherent and expandable in culture, which
makes it relatively easy to obtain a sufficient number of cells for
MSC therapy.
Human MSCs (hMSCs) produce 84 trophic factors in conditioned
medium and/or cell lysates versus basal medium.
42
Human umbilical
cord blood MSC treatment partially restore the neuronal degenera-
tion and nerve function of femoral nerve.
43
Human umbilical
cord-derived MSCs secrete VEGF, glial cell line-derived neurotrophic
factor (GDNF), and BDNF. Secretion of neurotrophic factors is
demonstrated before, during, and after neuronal differentiation.
Human umbilical cord-derived MSCs and BMSCs both had
measurable amounts of secreted neurotrophic factors. But in vivo
tests did not confirm the secretion of neurotrophic factors and the
antiapoptotic effects seen in vitro.
44
Dental pulp stem cells express various neurotrophic factors,
including BDNF, NGF, and GDNF.
12
The transplantation of cryopre-
served dental pulp stem cells attenuate sciatic nerve blood flow and
sciatic nerve conduction velocity the same as freshly isolated dental
pulp stem cells.
45
Similarly, adipose-derived stem cells differentiated
to glial-like cells also express a range of neurotrophic factors, namely
NGF, BDNF, GDNF, and neurotrophin 4. MSC transplantion into
an animal model of nerve injury show antiapoptosis in the dorsal
root ganglia.
46
Adipose tissue-derived stem cells isolated from the
ischemic limb of diabetic patients have less potent phenotypically
and functionally compared with control normal counterparts
without signs of limb ischemia.
47
Neuroprotective and neuroregenerative mechanisms
The secretion of neurotrophic factors by stem cells provides
neuroprotection and neuroregenerative effects. When trans-
planted into an animal model of Parkinson’s disease, hMSCs
support sustained endogenous proliferation and maturation
of cells in the subventricular zone of rats. Additionally, hMSCs
exert a neuroprotective effect, decreasing the loss of dopami-
nergic neurons and increasing the levels of dopamine in the
animal models of Parkinson’sdisease.
42,48
These effects are
possibly accomplished through decreased caspase-3 activity.
hMSC-treated mice have a lower removal times than that injected
Figure 1. Pathogenesis of diabetic neuropathy.
Mesenchymal stem cells to treat diabetic neuropathy
JY Zhou et al
2
Cell Death Discovery (2016) e16055 Official journal of the Cell Death Differentiation Association

with proteasome inhibitors and no hMSC transplantation.
48
Trans-
planted hMSCs did not differentiate into a neural phenotype
42
and
protected against Purkinje cell loss.
49
These studies demonstrate that
MSCs not only protect against nerve damage but also help regenerate
damaged nerves and restore them to their preinjured state.
The secretion of neurotrophic factors by different populations
of stem cells suggests that no matter the source MSCs have the
ability to decrease and ameliorate the negative effects on injured
nerve fibers, improving the function of the injured nerve. The
release of key neurotrophic factors, along with the neuroprotec-
tive and neuroregenerative effects of stem cells, make them ideal
candidates for arresting and possibly reversing the incapacitating
effects of DN.
MECHANISMS OF MSC TREATMENT FOR DN
MSC therapy may not be a standard treatment option for
all stages of DN because different stages of DN are marked by
different structural or functional changes. At present, MSC therapy
may be applied to those patients who suffer from intractable
symptoms, acute exacerbation, or combined diseases, such as
diabetic foot ulcers or critical limb ischemia. MSC therapies
targeting both vascular and neural elements are advantageous
in treating DN.
50
One recent meta-analysis shows that BMSC
transplantation ameliorates allodynia but not hyperalgesia unless
it is given during the first 4 days after injury.
51
As shown in
Figure 2, stem cells can improve DN through two main pathways.
MSCs improve diabetic glycemic control
MSCs improve glycemic control, accompanied by improved renal
function and regeneration of normal βpancreatic islets.
52
Hypogly-
cemia of MSC transplantation is a direct effect of differentiation to
cells capable of producing insulin (less likely) or an indirect effect of
secretion of immunomodulators, which prevent T cells from eliciting
pancreatic β-cell destruction, or other as yet unknown factors that
influence insulin secretion or action. MSC differentiate into insulin-
producing cells, releasing insulin in a glucose-dependant manner
and improving diabetic symptoms in type 1 diabetic animal.
53,54
These insulin-producing cells express multiple genes related to the
development or function of pancreatic βcells.
54,55
In diabetic NOD
mice, the injection of MSC reduced the capacity of diabetogenic T cells
to infiltrate pancreatic islets thus preventing β-cell destruction.
56
An additional cooperative action of MSCs on co-transplantation with
pancreatic islets results in improved graft morphology and improved
revascularization, indicating that possible trophic factors secreted by
MSCs are aiding islet engraftment.
57
Multiple intravenous infusions of
MSCs resulted in normalization of hyperglycemia, which remained
stable for 9 weeks after infusion, with lower serum levels of insulin and
C-peptide and reversed damaged pancreatic islets to near normal.
58
MSCs secret neurotrophic and angiogenic factors to ameliorate
DN
MSCs offer a novel therapeutic option to treat DN. MSCs modulate
the central nervous system-injured environment and promote
repair as they secrete anti-inflammatory, antiapoptotic molecules,
and trophic factors to support axonal growth, immunomodulation,
angiogenesis, remyelination, and protection from apoptotic cell
death.
59
MSCs are known to support angiogenesis mostly through
a paracrine effect, which augments the microcirculation support-
ing peripheral nerves. This impaired vascular supply has been
implicated in the etiology of DN. Transplanted MSCs not only
directly differentiate into neurons and endothelial cells but also
secrete an increased concentrations of biologically active factors,
such as FGF, VEGF-A, and NGF,
60,61
which are central to nerve and
vascular tissue health. Adipose-derived MSC sheet, which secret
large amounts of several angiogenic growth factors in vitro, both
directly and indirectly accelerate diabetic wound healing.
62
BMSC transplantation increased the expression levels of FGF2
and VEGF, ameliorated sciatic nerve blood flow, prevented
the decreases in the capillary-to-muscle ratio and the neurofila-
ment content, and improved motor nerve conduction velocity
in diabetic animals.
61,63
Despite these benefits, however, motor
nerve conduction velocity and the increase in the levels of NGF
and neurotrophin 3 last for only 4 weeks.
64
Interestingly, when
it comes to neurotrophic factors, these two studies contradict
each other. In one study,
64
levels of NGF and neurotrophin 3, but
not VEGF or FGF2, increase in the animals that received BMSC
transplantation. In another study,
59
however, VEGF and FGF2, but
not NGF and neurotrophin 3, are found to increase in the animals
that received stem cell transplantation. More studies are needed
to understand the effects of MSCs on DN.
MSCs inhibit proinflammation to improve diabetic peripheral
neuropathy
The therapeutic benefit of MSCs in DN is now believed to be by
short-term (hours to days) paracrine and juxtacrine modulation of
immune responses rather than by long-term (days to months)
engraftment of MSCs to the injured site.
10
Subsequent improve-
ments in MSC cell preparations to generate anti-inflammatory
MSC populations resulted in improvements in behavioral assays in
painful diabetic peripheral neuropathy, and mice treated with these
MSCs showed decreased serum concentrations of proinflammatory
cytokines.
10
hMSCs reduce pain-like behaviors (mechanical allodynia
and thermal hyperalgesia) after transplanted in cerebral ventricle.
hMSCs have antinociceptive effect from day 10 after surgery
(6 days after cell injection). hMSCs reduce the mRNA expression
levels of interleukin-1βand neural β-galactosidase overactivation
in prefrontal cortex of spared nerve injury mice.
65
hMSCs reduce
mechanical allodynia and thermal hyperalgesia via tail vein
injection. An antinociceptive effect is measurable from day 11 after
surgery (7 days after cell injection). hMSCs mostly home in the spinal
cord and prefrontal cortex of neuropathic mice. Transplanted hMSCs
downregulate the expression levels of the mouse interleukin-1βand
interleukin-17 and upregulate the expression levels of interleukin-10
and the marker of alternatively activated macrophages CD106 in the
spinal cord of spared nerve injury mice.
63
MSCs improve diabetic cardiac autonomic neuropathy
MSC administration promoted density of sympathetic and para-
sympathetic nerves in the ventricular myocardium of diabetic rats,
increased the ratio of parasympathetic to sympathetic nerve
fibers, and suppressed ventricular arrhythmia inducibility.
66
Figure 2. Mechanisms of the effect of stem cell transplantation on
diabetic neuropathy.
Mesenchymal stem cells to treat diabetic neuropathy
JY Zhou et al
3
Official journal of the Cell Death Differentiation Association Cell Death Discovery (2016) e16055

MSCs regenerate axons and format myelin to ameliorate DN
The injected BMSCs into hindlimb muscles of streptozotocin-
induced diabetic rats restore motor and sensory nerve conduction
velocities to near-normal levels. The injected MSCs are priority and
durably engrafted in the sciatic nerves, and a fraction of the
engrafted MSCs are discriminatively localized near to vasa nervora
of sciatic nerves. MSCs increase the density of vasa nervora and
restores the ultrastructure of myelinated fibers in nerves. MSCs also
upregulate the gene expression of multiple factors participating in
angiogenesis, neural function, and myelination in the MSC-injected
nerves.
67
The nerve grafts that are prepared from poly (3-hydroxy-
butyrate-co-3-hydroxyhexanoate) with oriented nanofiber three-
dimensional surfaces aided to nerve regeneration, either used alone
or with hMSC. Poly (3-hydroxybutyrate-co-3-hydroxyhexanoate)
provided better nerve regeneration when used with hMSCs in
combination than alone and reached the same statistical treatment
effect in functional evaluation and electrophysiological evaluation
when compared with autografting.
68
CHALLENGE AND FUTURE PERSPECTIVE
The application of the use of MSCs to treat DN has been
extensively investigated in preclinical animal models in recent
years and the majority of reports indicate positive effects on DN.
Despite this, there are significant challenges to be met for the
successful clinical translation of these studies from animal model
to the patient’s bedside. Although MSC therapies protect from
neurodegeneration and promote neuroregeneration, there appear
to be many obstacles to be overcome for clinical applications
(Figure 3). These are: (1) optimal dose of administration owing
to limited survival of transplanted cells, (2) safety for risk of
tumor formation, (3) route of transplantation for effectiveness,
(4) autologous or allogeneic approach, impairing potency of MSCs
from diabetes, (5) further mechanisms, and (6) clinical end points
for the efficacy of MSC therapy.
Safety
Even with the reported concerns over possible malignant
transformation above, worldwide clinical studies of both auto-
logous and allogeneic MSC administration have confirmed clinical
safety and initial efficacy. A search of the ClinicalTrials.Gov website
reveals that there are 612 open studies of MSC safety and efficacy
in the treatment of human diseases by the year 2016. In relation
to diabetes, there are 39 open clinical trials using MSCs to treat
type 1 diabetes, type 2 diabetes, or their associated complications.
Another issue that should be overcome for the MSC therapy is
to avoid the risk of tumor formation. Increased tumor formation
was observed in animals owing to the immune-suppressive
effect of MSCs especially with allogeneic transplants,
69
and
in vitro observations of sarcoma after culture of murine MSCs was
reported.
70
But one study indicated a tumor-suppressive activity of
MSCs after preactivation with tumour necrosis factor-α.
71
Frequent
tumor formation in streptozotocin-induced diabetic mice trans-
planted with BMSCs was observed.
72
The high frequency of tumor
formation is accounted for by frequent chromosomal mutation
elicited by repeated passages of BMSCs (only four passages) in the
cell culture system. It may thus be conceivable that fresh BMSCs
without any passage in cell culture should be applied to MSC
therapy to prevent tumor formation.
Dose of administration
The number of cells delivered are very important; however, there
is still a lack of information as to the optimal cell doses that
provide preclinical and clinical efficacy. One study demonstrated
that hMSCs transplanted into animal model generated different
grafts depending on the cell dosing: low numbers of transplanted
hMSCs generated nestin-containing grafts, whereas higher
numbers of transplanted hMSCs generated considerable amounts
of grafts with astroglial markers.
42
The number of cells trans-
planted also raises questions about cell survival; one study
indicated that only 1.7% of total injected hMSCs survived.
48
Despite all of their benefits, much research still needs to be carried
out to understand the homing capabilities of stem cells
48
and the
mechanism of action.
14
The optimal dose for stem cell transplan-
tation needs further characterization prior to being introduced
into clinical trials.
Route of administration
Methods for transportation of MSCs without affecting their
viability and efficacy are important along with issues related
to cryopreservation. Several modes of cell delivery (e.g., topical,
intraocular, and systemic) have been assessed in both preclinical
and clinical studies, and these studies have illustrated the
importance of administration route with the successful outcome.
Systemic delivery is attractive as this may result in benefit for
multiple complications and has the potential to improve glycemic
control. Although an attractive option, the systemic delivery of
MSCs has some barriers, such as homing of these cells to tissues
of interest with high efficiency and clinically meaningful engraft-
ment. More cells are required for injection owing to passive cell
entrapment within non-specific tissues,
73,74
and this can poten-
tially lead to unwanted effects and reduced efficacy of trans-
planted cells. Topical application may be a very relevant alternate
strategy for diabetic foot ulcers but this is approach can be limited
by localized vascular damage as a result of diabetes at the site of
administration. One approach is to implant cells repeatedly to
maintain their effects. At present, the duration of the beneficial
effects of MSC therapy in DN is unknown.
Duration and degree of cell expansion
A major challenge is the large-scale production of MSCs under
GMP conditions and issues of MSC heterogeneity. The duration
and degree of cell expansion and culture has an impact on MSC
morphology, differentiation, viability, and migratory properties.
MSCs not only undergo phenotypic changes in culture and during
passage (size, morphology, and cell surface marker expression)
75
but also lose capacity for functional proliferation and differentia-
tion potential.
75,76
In addition, their ability for cytokine production
is altered.
76
Thus a delicate balance between culture expansion to
gain sufficient numbers of MSCs for therapeutic application and
long-term culture effects needs to be met. Tightly controlling the
microenvironment of MSCs is required. Detailed investigations of
how the microenvironment affects the immunosuppressive effects
of MSCs are still lacking and are required as cell-to-cell contact and
Figure 3. Challenges for clinical application of MSCs to treat diabetic
neuropathy.
Mesenchymal stem cells to treat diabetic neuropathy
JY Zhou et al
4
Cell Death Discovery (2016) e16055 Official journal of the Cell Death Differentiation Association

soluble factors are thought to be the key aspects of MSC-mediated
immunosuppression.
37
Autologous or allogeneic approach
The choice of an autologous or allogeneic approach is an important
consideration as the former may be limited by disease-induced
cell dysfunction and the latter by an immune response to the
transplanted cells. Historical opinions that the immunomodulatory
functions of MSCs results in immune privilege for allogeneic MSC
transplants are being challenged
77–79
with the recommendation
that the antidonor immune responses elicited by allogenic MSCs be
studied in more detail. The limitation of allogenic MSC therapy may
also be related to the gradual decrease in released neurotrophic
factors from transplanted cells that may sustain only 4 weeks or so
after transplantation.
Mechanisms
Despite numerous studies on the transplantation of MSCs in animal
models and patients, insight into the exact mechanisms of action
underlying their beneficial effect remains unclear. Adequate
preclinical animal models are required to accurately represent the
pathological long-term effects of diabetes on the host system. There
are limitations in the current rodent models of DN.
80
There is an
increased need for additional in vitro and in vivo studies to fully
describe in detail the mechanisms of MSC therapy.
CONCLUSION
DN frequently leads to foot ulcers and ultimately limb amputations
without effective clinical therapy. DN is characterized by reduced
vascularity in the peripheral nerves and deficiency in angiogenic
and neurotrophic factors. Only delivering neurotrophic or angio-
genic factors for treatment in the form of protein or gene therapy is
very modest if not ineffective. MSCs have been highlighted as a new
emerging regenerative therapy owing to their multipotency for DN.
MSCs reverse manifestations of DN, repair tissue, and antihypergly-
cemia. MSCs also paracrinely secrete neurotrophic factors, angio-
genic factors, cytokines, and immunomodulatory substances to
ameliorate DN. Challenges in the clinical translation of MSC therapy
include safety, optimal dose of administration, optimal mode of cell
delivery, issues of MSC heterogeneity, clinically meaningful engraft-
ment, autologous or allogeneic approach, challenges with cell
manufacture, and further mechanisms.
ABBREVIATIONS
BDNF, brain-derived nerve factor; BMSC, bone marrow-derived MSC;
DN, diabetic neuropathy; FGF2, fibroblast growth factor-2; GDNF,
glial cell line-derived neurotrophic factor; hMSC, human MSC; MSC,
mesenchymal stem cell; NGF, nerve growth factor; VEGF, vascular
endothelial growth factor.
ACKNOWLEDGEMENTS
This work was supported by a grant from the National Natural Science Foundation of
China (No. 81471040), the Chongqing Natural Science Foundation of China (No.
cstc2015jcyjBX0138), the Natural Science Foundation of Third Military Medical
University (No. 2012XJQ17), and Clinical research projects of Xinqiao Hospital, Third
Military Medical University (No. 2015YLC32).
AUTHOR CONTRIBUTIONS
JY Zhou and GS Qian wrote the manuscript; JY Zhou and Z Zhang prepared the
figures and organized the contents of the manuscript.
COMPETING INTERESTS
The authors declare no conflict of interest.
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