Introduction to embryonic and adult neural stem cells

16 Pages·2016·3.28 MB·English

www.jpnim.com Open Access eISSN: 2281-0692
Journal of Pediatric and Neonatal Individualized Medicine 2016;5(2):e050215
doi: 10.7363/050215
Received: 2015 Jul 30; revised: 2015 Sept 27; rerevised: 2015 Dec 20; accepted: 2016 Mar 01;
published online: 2016 Jul 09
Review
Introduction to embryonic and
adult neural stem cells: from the
metabolic circuits of the niches to
the metabolome
Maria Antonietta Marcialis1, Elisabetta Coni1, Maria Cristina Pintus1,
Alberto Ravarino2, Vassilios Fanos1, Carlo Coni1, Gavino Faa2
1Neonatal Intensive Care Unit, Neonatal Pathology, Puericulture Institute and Neonatal Section, AOU and
University of Cagliari, Italy
2Institute of Pathology, AOU and University of Cagliari, Italy
Proceedings
Proceedings of the 2nd International Course on Perinatal Pathology
(part of the 11th International Workshop on Neonatology · October 26th-31st, 2015)
Cagliari (Italy) · October 31st, 2015
Stem cells: present and future
Guest Editors: Gavino Faa, Vassilios Fanos, Antonio Giordano
Abstract
Metabolomics has provided new insight into the biology that drives
the phenotype of stem cells. During the recent years, metabolic circuits
of embryonic and neural stem cells (NSCs) have been better elucidated.
Many factors contribute to stem cell determination fate: metabolism,
transcriptional signaling, epigenetics, extrinsic mechanisms such as short-
range signals from the niche and humoral signals. The metabolism decides
if a cell proliferates, differentiates or remains quiescent. Embryonic
and adult NSCs share two features: they generate at least one daughter
cell and can differentiate into specialized cells. NSCs use different
pathways depending on their stage of differentiation: glycolysis is highest
in proliferating stem cells and it is essential for stemness. Conversely,
oxidative phosphorylation supports differentiated cells. Moreover, lipid
metabolism maintains proliferation and neurogenesis; indeed, fatty acid
oxidation and fatty acid synthesis represent a component of stem cell fate
regulation.
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progenitor cells: the neuroblasts, producing germ
Keywords
cells and neurons, and the spongioblasts, the
Neural stem cells, metabolomics, metabolism, precursors of glial cells. It was thought that in the
neurogenesis, mitochondria. brain NSCs were absent and that neurons, once
having developed in the brain, had no regenerative
capacity. Scientists were convinced that stem cells
Corresponding author
could only belong to more plastic tissues like
Dr. Maria Antonietta Marcialis, NICU, Neonatal Pathology hematopoietic tissue [4].
and Neonatal Section AOU Cagliari, Cagliari, Italy; email: These dogmas of neurobiology were revisited
[email protected]. in 1961 when Smart and Leblond published the
first article about evidence, in the mouse brain, of
neuro-glial cells in the mitotic phase [5].
How to cite
Those cells were not yet differentiated into
Marcialis MA, Coni E, Pintus MC, Ravarino A, Fanos V, Coni C, neurons and therefore not yet migrated into
Faa G. Introduction to embryonic and adult neural stem cells: from the olfactory bulb. Later, Altman in 1962 and
the metabolic circuits of the niches to the metabolome. J Pediatr Altman and Das in 1965 published the results
Neonat Individual Med. 2016;5(2):e050215. doi: 10.7363/050215. of their studies, according to which, in adult
brains, the birth of new neurons occurred [6, 7].
Consequently, different authors described mitotic
Stem cells
neurons in several species [8-14].
Stem cells are blank cells, unspecialized, In the 2000s it became obvious that, in the
capable of proliferating and regenerating, keeping adult brain, new neurons grow and specialize.
at the same time an undifferentiated state. This However, the rate of neurogenisis was slower
process is called “asymmetric division” because than in the fetal brain and the new neurons
it involves the production not only of new stem were capable of forming new synapses with
cells but also of cells that have the potential to pre-existing cells. It is estimated that the adult
differentiate into specialized cells. The third brain, at hippocampal level, forms about 700 new
feature of stem cells is their paracrine function. neurons a day [15, 16]. This active neurogenesis
Thus, they act as a complex biopharmacy with has an enormous importance for the concept of
different functions: tissue repair, regulation of permanent structural plasticity of the brain and is
cell differentiation and anti-inflammatory action not limited to specific periods of development. By
[1, 2]. 2013, the sites of the niche of NSCs in adult brain
Stem cells can be divided into three groups, were also revealed: adult neurogenesis occurs in
according to their different potential for de- the subventricular zone (SVZ) of the anterolateral
velopment: totipotent cells, which constitute a ventricle, in the hippocampal subgranular zone
zygote, capable of giving rise to a new individual; (SGZ), along the midline near the third and fourth
pluripotent cells (15 days after fertilization), ventricle, in the forebrain, the striatum, amygdala,
capable of producing all types of tissue cells; hypothalamus, at the level of the substantia nigra
and multipotent cells, more differentiated cells and in the subcortical white matter [17-20].
which maintain the self-renewal capacity but,
in addition, can also differentiate into cells that
Embryonic neurogenesis
belong to a specific tissue. Stem cells have been
identified in many organs and tissues of human The embryonic plate is homogeneous in terms
body. Neural stem cells (NSCs) are included in of cellular morphology because it is composed
the multipotent group because they are tripotent of neuro-epithelial cells that develop, after the
cells, since they can proliferate and differentiate formation of the neural tube, into radial glial cells.
into neurons, oligodendrocytes and astrocytes [3]. Radial glial cells are multipotent and become
neurons during the first stage of gestation, while
gliogenesis occurs in the last stage of gestation.
Neural stem cells: a little history
Radial glial cells are bipolar, having an apical
Up to 1960, scientists believed in the dualistic extension in contact with the lateral ventricles and
theory of His, according to which in the primitive a basal process that touches the pial surface. It
neural tube there were two distinct types of seems that this feature is essential in determining
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their function and behavior as they receive signals 36]. Further niches have been found also in the
from several distinct regions of the brain [21, 22]. forebrain, the striatum, amygdala, at the level of
These cells have a high rate of replication, are the substantia nigra, the subcortical white matter
able to repair themselves, form new radial glial and the hypothalamus [37, 38].
cells and differentiate into neural progenitor cells Since the morphology of adult NSCs is
and neurons through the process of asymmetric similar to that of embryonic radial glial cells,
division. Near the birth, glial cells change their they are called radial glia-like cells. The adult
features and generate NSCs that will represent a subventricular region is very similar to SVZ of
useful pool for adult neurogenesis and astrocytes the embryo [33].
and olygodendrocytes [23, 24]. Until few years ago it was thought that,
During intrauterine life, neural and pial pro- differently from embryonic stem cells (ESCs),
liferation is very active and localized in two adult NSCs, did not enter in contact with the liquor.
zones: the ventricular and the subventricular. Since rodent SVZ possesses some similarities
NSCs are localized in the ventricular zone. with humans and are better studied than human
In the human embryo and fetus, distribution, SVZ, they have helped us to understand the
morphology, growth and number of NSCs vary human cellular architecture of this area. In both
widely depending on the gestational age. They are the human and the rodent brain, the progenitor
variously distributed in the cortex, hippocampus, of adult SVZ are radial glia-like cells. However,
striatum, olfactory bulb, near the ventricles in differently from humans, in the murine SVZ it is
the midbrain, cerebellum, spinal corda and retina possible to identify four types of cells:
[25]. During intrauterine brain development 1. NSCs (or B cells), slowly dividing NSCs
there is no evidence of quiescent NSCs, probably showing a non-mobile primary cilium that
because of the high need of neural cells in the reaches the ventricular lumen. B cells have
developing human brain [26]. a long basal process ending near the blood
vessels and another, apical, similar to the
embryonic one dipped in the CSF. This process
Adult neurogenesis
is surrounded by ependymal cells arranged to
Currently, scientists believe that adult NSCs form a pinwheel feature [39].
are the descendents of subventricular radial glial It is thought that this organization is essential
cells; consequently, they arise from embryonic for neurogenesis and that the contact with the
neuroepithelial cells [27, 28]. ventricle may have an important function in
The adult brain is less versatile than the the physiology of the nervous system. B cells
embryonic one. The adult NSCs show several express Glial Fibrillary Acidic Protein (GFAP),
differences from radial cells: most of them are Glutamate Aspartate Transporter (GLAST),
quiescent and, when duplicating, the cell cycle Brain Lipid-Blinding Protein (BLBP) and
is longer than the embryonic one. They live in nestin [40-43].
particular areas, complex and stable (the niches). However, up to now, it does not exist a specific
Finally, they are not so as multipotent as believed marker to identify type B cells.
in the past. Thus, they can only turn into two 2. Intermediate progenitor cells (C), rapidly pro-
specific neuronal types: periglomerular neurons liferating cells that express the Epidermal
in the SVZs and granule neurons in the dentate Growth Factor (EGF) receptor [44].
gyrus. In fact, adult neurogenesis is traditionally 3. Type A cells, neuroblasts expressing markers
located in the SVZ of the anterolateral ventricle, typical of newborn neurons: doublecortin and
characterized by cells that give rise to new Polysialylated-Neural Cell Adhesion Molecule
neurons that migrate into the striatum in the adult (PSA-NCAM).
human brain and olfactory bulb in the murine 4. Type E or ependymal cells, which are post-
brain (through the rostral migratory olfactory mitotic and multiciliated cells. They express
route) [29-33]. S100B protein and CD24 [39, 45].
In the SGZ is located another area of adult It is believed that B cells generate C cells,
neurogenesis, which produces neurons for the which in turn produce ependymal cells. The hu-
dentate gyrus and astrocytes [34]. Recently, man SVZ, compared to the SVZ of rodents, dis-
different authors have described new niches along plays a peculiar cell organization. It is composed
the midline near the third and fourth ventricle [35, of four layers: in the first there are multiciliated
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ependymal cells; the second layer is formed by gut microbioma). Metabolomics presents many
astrocytes and cell bodies interdigitated with advantages in comparison with transcriptomics
expansions of ependymal cells; in the third there and proteomics: it is cheaper, feasible in a large
are B cells, oligodendrocytes-like precursors number of samples, it enables the identification
and ependymal cells; finally, the fourth layer is of endogenous molecules belonging to many
composed by neuronal bodies and myelin tracts metabolic pathways and the whole genome is not
[46-49]. required [52].
The NSCs located in the hippocampus are ca- The level of each metabolite found in the final
pable of differentiating exclusively in neurons of tissue phenotype results from what humans ingest,
the dentate gyrus. Their migration is limited to the metabolize, catabolize or come into contact with.
areas near the site of origin. The radial glia-like The profiles include a collection of peptides,
cells (or cells I) generate granule neurons through lipids, amino acids, nucleic acids, carbohydrates,
a process that includes cells with an increasing organic acids, vitamins, minerals, and exogenous
degree of maturation variable from less mature compounds like food additives, drugs, toxins,
cells (IIa, IIb, type II cells) up to adult neurons. pollutants with a small molecular weight (< 2,000
The “niches” contain, in addition to NSCs, Da [53]).
neurons, astrocytes, axons, the extracellular ma- Thus, the human metabolome consists of a mix
trix and a network of interconnected vessels. of both endogenous and exogenous compounds.
Since the capillaries in the niches are fenestrated, Metabolic profiling is generally applied to bio-
they are called “windows of the brain”. Thus, fluids like urine, cerebrospinal fluid, plasma and
niches may receive, integrate and respond to saliva. Moreover, it is also possible to research
messages that come from outside because they metabolites in extracts of tissues and cell lysis.
are adjacent to blood vessels (10-12 µ) and in Thus, there are two kinds of stem cell sources, the
contact with other neurons and cells such as “in vitro” ones (cellular) and the “ex vitro” ones
microglia and astrocytes. The NSCs of the SVZ (tissue), depending on the origin of the sample.
area, in particular, are associated with projections The profile varies from the metabolomic finger-
of the vascular basal lamina called “fractones”, print, corresponding to intracellular metabolites,
which are rich in extracellular matrix components to the metabolomic footprinting, which includes
and growth factors [50]. secreted metabolites mostly used in microbial
metabolomics [53, 54].
Recently, scientists have discovered how to
Human metabolome
use both intracellular and secreted metabolites
It seems that, in human body, there are about as possible biomarkers of specific cell types
500 different cell types showing a unique cellular such as human pluripotent stem cells and neural
metabolome. Human metabolome consists of progenitor cells [55].
a group of about 41,511 small molecules (me- Metabolites commonly detected by 1H-MRS
tabolites) found by using mass spectrometry or as different peaks are N-acetylaspartate (NAA),
Nuclear Magnetic Resonance (NMR) spectroscopy, creatine, choline, myo-inositol, alanine, lactate,
in a given tissue or fluid, in vitro and 1H-Magnetic glutamine, glutamate and some macromolecular
Resonance Spectroscopy (1H-MRS), which is proteins and lipids [56]. In the last few years,
the correlate of NMR, in vivo. The number of several studies have shown how stem cells use
identified human metabolites is lower compared different metabolic circuits depending on their
with transcriptomes (85,000) and proteomes stage of differentiation [57].
(10,000,000) [50]. Currently, the authors are investigating to de-
Metabolites in a biological system are termine whether very preterm infants have a low
the result of both genetic and environmental density of neural progenitor stem cells and if
influences (epigenetics). Therefore, differently this low density could cause long-term cognitive
from genome and proteome, metabolomic pro- deficits. Stem cells are very plastic and it is also
files do not depend only on our genes. En- possible that differentiated cells could revert back
dogenous metabolites are synthesized by the into stem cells. For this reason, these cells do
enzymes encoded by our genome or by our not have a specific fixed metabolic profile, but a
microfloral genomes and are also related to the profile corresponding to every particular stage of
environment (food, drinks, breathed substances, differentiation.
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Advances in metabolomics have underlined of CNS precursor types. Hypoxia appears to
the role of specific pathways in the fate of stem increase neurogenesis in vitro activating the
cells. Thus, it is the metabolism that decides HIF, which, in turn, regulates the production of
if a cell proliferates, differentiates or remains erythropoietin and VEGF. Oxygen is a source
quiescent. of Reactive Oxygen Species (ROS), which are a
group of highly active, reduced forms of molecular
oxygen such as superoxide and hydroxyl radical.
Metabolic circuits in undifferentiated and dif-
ROS are produced by mitochondrial OXPHOS and
ferentiated stem cells
are normally neutralized by antioxidans. It seems
During the development of the brain, but that endogenous ROS increases self-renewal
also after birth and in adult life, the intracellular and neurogenesis while FOXO are involved in
metabolic reactions have a very rapid kinetics. proliferation, differentiation and cellular survivaI.
Metabolomics allows us to measure metabolite Under stress conditions ROS can increase and
fluxes both within and outside cells in the damage the DNA [62]. In the brain ROS can also
different phases of differentiation. Neurogenesis be generated following a systemic inflammation.
is regulated by many extracellular cues like Excessive levels of ROS can cause mitochondrial
Epidermal, Fibroblast and Insulin-like Growth dysfunction, which consequently inhibits the
Factors. It is thought that, in NSCs, Insulin-like Tricarboxylic Acid cycle (TCA cycle) and the
Growth Factor 1 (IGF1), EGF and Vascular electron transport chain. FOXO protect against
Endothelial Growth Factor (VEGF) stimulate ROS by stimulating antioxidants and DNA repair
the P13K-Akt pathway, the main regulator of enzymes. Interestingly, mitochondrial dysfunction
cellular metabolism. This pathway acts on the and impaired bioenergetics seem to be a common
glucose, glutamine and lipid metabolism by denominator of neurological diseases like autism,
using different effectors like Forkhead tran- depression, bipolar disorder, chronic fatigue syn-
scription factors (FOXO), mammalian Target drome, schizophrenia and Parkinson’s disease
of Rapamycin (mTOR), Hypoxia-Inducible [63]. In addition, in post-mortem brain tissue
Factor 1-alpha (HIF-1-alpha), myelocytomatosis obtained from patients affected by Parkinson’s
oncogene (Myc) and Tumor Suppressor Protein disease, a reduction of the number of SVZ and
(p53). The mTOR and 5’ AMP-activated Pro- SGZ progenitors has been found [64].
tein Kinase (AMPK) regulate the stem cells However, ROS are not only harmful because
energy production by stimulating glycolysis and they play an important role in cell fate decision.
inhibiting oxidative phosphorylation (OXPHOS)
[58]. FOXO factors can elicit the cell cycle arrest,
Metabolic and spectroscopic profiling
resistance to oxidative stress and apoptosis [50,
60]. In particular FOXO3 is a member of a Metabolic and spectroscopic profiling of stem
transcription factor family which regulates NSC cells encompasses the following compounds:
pool. FOXO proteins regulate the stem cells mobile lipids visible on proton NMR (NMR
cycle and the DNA repair [61]. Moreover, FOXO mobile lipids), the choline-containing compounds,
proteins activate genes to combat oxidative stress. the energy metabolism (glycolysis vs. OXPHOS).
Conversely, p53 is necessary for differentiation Maturation markers are absent. NMR mobile
and neurite outgrowth. lipids are associated with apoptosis and necrosis.
They are not stem cell-specific and have been
The role of the oxygen in the neurodevelopment associated with cellular stress. It is believed that
of the stem cells peak of the the fatty acid methylene at 1.28 pp is
a marker of neural progenitor cells [65].
The partial pressure of oxygen in the brain Choline-containing compounds are related to
ranges from 0.55 (4.1 mmHg) in the midbrain to 8% membrane lipid synthesis and cellular prolifera-
(60 mmHg) in the pia, so that the neurogenic stem tion. They decrease with cell differentiation and
cell niches are hypoxic. Probably stem cells have maturation of the postnatal rat brain and human
a selective survival advantage by maintaining an fetal brain [66-68].
undifferentiated state with low oxidative stress. Amino acids play a key role in the discrimina-
It is believed that low O represses neuronal tion of differentiated cells. Alanine, glycine, and
2
differentiation, regulates death and differentiation glutamate are elevated in mesenchimal stem cells.
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Glutamine is quantitatively the most significant ferentiated cells. Although many metabolites
amino acid in plasma. In the brain, glutamine is decreased, lipid concentration increased upon
converted to glutamate by a glutaminase (GLS). differentiation.
Adult neural stem progenitor cells express GLS Takubo et al. in 2013, using metabolome
and glutaminolysis is very important for redox analysis in mice, found a biomarker of HSCs,
balance and glutathione (GSH) biosintesis. the fructose 1,6-bisphosphate, which increases
Transcription factors such as FOXO, Myc and pyruvate levels by activating pyruvate kinase
p53 regulate this pathway through target gene (PK) [78].
activation.
Glutamine can be used as a carbon source
Energy metabolism
after having been converted to glutamate and
then alpha-ketoglutarate, a key component of Mitochondria are specialized organelles of
TCA. Zhang et al. in 2014 demonstrated that bacterial origin that are involved in energy pro-
ESCs rely on glutamine to fuel their proliferative duction, cell signaling, apoptosis and calcium
state [69]. Serine and glycine are synthesized homeostasis [79].
in stem cells from the glycolysis. These amino These semiautonomous organelles are the site
acids are involved in biosynthesis, redox and of the pathways of the intermediate metabolism,
metylation, being precursors of purines, GSH and amino acid biosynthesis, Fatty Acid Oxidation
lipids. Therefore, they can regulate ROS levels (FAO) and steroid metabolism. In addition, they
through GSH. There is a high concentration of are the most important source of endogenous
taurine in the developing brain of rats, while it ROS produced through OXPHOS. ROS lead
decreases during maturation. Taurine stimulates to mutation and alter protein folding and DNA
NSC proliferation and threonine is important for structure. Recent studies have demonstrated
murine ESC propagation. that ROS have not only deleterious effects for
Threonine is an essential amino acid whose the cells but also a physiological role [80]. In
levels increase during differentiation in as- the past, mitochondria were considered the
sociation with an increase in one-carbon me- “energy powerhouse” of cells [81]. It is known
tabolism for purine biosynthesis [68, 70, 71]. that mitochondria play an important role in
Threonine contributes to the synthesis of gly- pluripotency. Recently, scientists have discovered
cine; consequently, it is involved in epigenetic that these organelles are relatively inactive in
regulation. stem cells, while they become very hardworking
The RNA and DNA content is high in ESCs. in differentiated cells. Pluripotent stem cells show
During the development of the brain, the amount immature mitochondria with a globular shape,
of RNA decreases, while the concentration of cristae-poor morphology, decreased OXPHOS
lipid and protein rises. NAA is a metabolite found (with high lactate production) and perinuclear
in mature neurons whose function has not yet localization. All these features indicate a less
been clarified. It is used as a marker of mature active mitochondrial state. Conversely, mito-
neurons together with myo-inositol. Markers chondria of differentiated cells are elongated,
of mature cells are absent in neural progenitor cristae-rich and have an activated TCA cycle [82,
cells and increase during the development of the 83] (Fig. 1). Probably the anaerobic metabolism
fetal brain [72-76]. However, in 2013, Chung typical of pluripotent stem cells is promoted by
et al., using a high-resolution 1H-MRS to study mitochondrial Uncoupling Protein 2 (UCP2)
undifferentiated and differentiated human striatal together with the high levels of hexokinase II and
NSCs, demonstrated that the absolute levels the low levels of pyruvate dehydrogenase [84,
of NAA, the traditional indicator of mature 85]. UCP2 can also suppress ROS production
neurons, were not significantly different between [83]. However, the precise indicator of stem cell
differentiated and undifferentiated cells [77]. differentiation is not the metabolic shift toward
Moreover, differently from previous evidence, OXPHOS, but rather the ratio of glycolysis/
The study by Chung and colleagues revealed that mitochondrial glucose oxidation [69]. Thus, not
the concentration of myo-inositol was almost only can mitochondria mediate catabolic and
50-fold lower with differentiation. In addition, anabolic processes, but they also intervene in the
threonine, glutamate and alanine concentrations regulation and differentiation of stem cells. These
decreased between three- and five-fold in dif- organelles have their own genome, the circular
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Figure 1. Mithocondrial differentiation and reprogramming involves structural and functional shift.
mitochondrial DNA, encoding for 13 essential In totipotent (embryonic) stem cells, glycolysis
protein subunits of I, III, IV and V complexes of is impaired because two different rate-limiting
the respiratory chain [86, 87]. Low mitochondrial enzymes (hexokinase and phosphofructokinase-1)
DNA is associated with more undifferentiated have low activity and ATP synthesis depends
cells and anaerobic metabolism, whereas the in- only on pyruvate analogs and mitochondrial
crease of mitochondrial number and mass is phosphorylation (Fig. 2, Tab. 1). Mitochondria,
related to the differentiation and aerobic metabo- at this stage, are structurally immature. Con-
lism [88-90]. sequently, in these unspecialized cells, the net
oxygen consumption is low. It is believed that
Glucose metabolism differentiation from ESCs into neural progenitors
is stimulated by both eicosanoid pathway and
For each molecule of glucose, 38 ATPs can be fatty acid metabolism. Thus, the inhibition of the
generated, providing energy for protein synthesis, eicosanoid pathway increases the unsaturated fatty
growth and proliferation [91]. Under hypoxia, acids concentration and preserves pluripotency.
ATP can be produced at a rapid rate because Unsaturated fatty acids contain a high number
glycolysis can create a larger percentage of ATP of carbon-carbon double and triple bonds,
than OXPHOS [92, 93]. which makes them susceptible to oxidative reac-
O concentrations can regulate the differen- tions [96].
2
tiation of NSCs. Hypoxia reduces proliferation Pluripotent and multipotent stem cells like
maintaining cell quiescence. Since HIF-1-alfa is NSCs rely on glycolysis regardless of the oxygen
essential to maintain anaerobic glycolysis, it could concentration; for this reason, this process is
reprogram cells to go back to a glycolytic pheno- called aerobic glycolysis (Fig. 3, Tab. 1). This
type typical of undifferentiated cells [94, 95]. switch from mitochondrial phosphorylation to a
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Figure 2. Metabolism of embryonic stem cells (ESCs) depends on pyruvate analogs and mitochondrial phosphorylation
while glycolysis is impaired.
GLUT1 and GLUT3 [97]. From the glucose
Table 1. Finding on undifferentiated and differentiated
stem cell metabolism. catabolism, different biosynthetic pathways start,
involving glycosaminoglycans, proteins and
Lipolysis
Mitochondrial nucleotides. Indeed, amino acid and nucleotide
Type of cell Glycolysis (fatty acid
respiration
oxidation) synthesis increases due to the increase of
Totipotent + ++ +++++ glycolytic intermediates. Nucleotide synthesis is
very important in order to maintain pluripotency
Pluripotent ++++++ + +++
and proliferation. Similarly, NSCs show a high
Multipotent +++++ ++ ++
glycolytic rate that declines during differenti-
Differentiated +++ +++++ +
ation [84].
The metabolism in the quiescent NSCs is
oriented to avoid cellular damage from ROS.
high rate of glycolysis is followed by hyperlactate NSCs live in hypoxic niches and their metabolism
production. Although glycolysis produces only depends on glycolysis (Fig. 4, Tab. 2). Since
two ATPs, in NSCs this pathway is very fast. ROS causes apoptosis, it seems that quiescent
Consequently, it produces a higher number of NSCs prefer a slow-cycling state to avoid being
ATP than mitochondrial OXPHOS. damaged because glycolysis is associated with a
During this state, the cells increase growth and reduction in ROS production. It is believed that
metabolic activity; the glucose uptake and flux GSH is the main cellular antioxidant and the
into the cells arise because of the upregulation enzymes controlling its synthesis, Glutamate-
of the expression of the Glucose Transporters Cysteine Ligase (GCLM) and GSH Synthetase
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Figure 3. Metabolism of pluripotent and multipotent stem cells depends on glycolysis with hyperlactate production.
Figure 4. Metabolism in quiescent neural stem cells (NSCs) depends on glycolysis, which is associated with a low production
of Reactive Oxygen Species (ROS).
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Table 2. Finding on quiescent neural stem cells (NSCs) and neural progenitors metabolism.
Mitochondrial Lipogenesis
Glycolysis ROS FOXO3
respiration (fatty acid synthase)
qNSCs +++++ + + +++++ +
Neural progenitors +++ +++++ ++++ + +++
qNSCs: quiescent neural stem cells; ROS: Reactive Oxygen Species; FOXO3: a member of Forkhead transcription factor family.
(GSS), are elevated in stem cells in order to colytic rates increase at this stage. Anaerobic
prevent oxidative stress. Once neural progenitors glycolysis is also necessary for the cells, which
start to differentiate, the glycolysis rate decreases have to live in the hypoxic environment of the
and becomes more coupled to the Krebs cycle. uterine wall. Following implantation, blood flow
This process is confirmed by a decreased lactate begins in the embryo and glycolysis declines,
production. while oxidative metabolism restarts. Similarly to
Together with the development of mito- their tissue of origin, pluripotent and multipotent
chondria, the metabolism of more differentiated stem cells have immature mitochondria with a
cells depends on mitochondrial oxidation for perinuclear position, reduced oxidative capacity
proliferation (Fig. 5). This preferred metabolic and are dependent on glycolysis. The metabolism
circuit is confirmed by metabolomic studies: many of differentiated cells, similarly to the implanted
of the metabolites identified in differentiated embryo cells, rely on oxidative metabolism
cells were pyrimidine and purine nucleotides, [100].
and tricarboxylic acids involved in the Krebs
cycle. Lipid metabolism
Neural progenitors, which live in a condition
of normoxia, upregulate both glycolysis and As glycolysis in neural stem progenitors
OXPHOS (Fig. 6). It is believed that both fatty cells regulates development, proliferation and
acid synthesis and the increase of ROS resulting differentiation, lipid metabolism maintains pro-
from FOXO3 suppression act as primer of neural liferation and neurogenesis. It is believed that
proliferation. In 2009, Renault and coworkers unsaturated fatty acid preserve pluripotency.
showed that, in mice, FOXO3 regulates the NSC Yanes et al., in 2010, discovered that ESCs have
pool and that its activity is higher in self-renewing a high level of unsaturated lipids and fatty acids
NSCs than in differentiated progeny [61]. while their levels decrease during differentia-
tion [96].
Indeed, omega 6 and omega 3 fatty acids
Analogies between embryonic development
(arachidonic, docosahexanoic and linoleic acid)
and stem cell fate
are typical substrates of pluripotent state.
The study of the developmental embryogenesis Later, Ito et al. in 2012, identified that fatty
has led to the concept of “metabolic centric acid breakdown pathway (FAO) improved the
control” of stem cell differentiation. The oocyte undifferentiated stem cell maintenance [101].
has a high number of mitochondria, which, Conversely, in adult stem cells lipogenesis seems
despite being spherical, small and having few and to be very active (Tab. 1 and Tab. 2). Indeed,
truncated cristae, can easily generate oxidative Knobloch et al. in 2013 demonstrated that adult
ATP to support metabolic requirement [98] neural stem progenitor cells express high levels of
(conversely, women in their forties show senescent Fatty acid synthase (Fasn), the rate-limiting enzyme
mitochondria with abnormal morphology and for lipogenesis. Furthermore, the inhibition of the
declined function). Since the rate of glycolysis is eicosanoid pathway promotes pluripotency and
inhibited due to the reduced function of certain FAO is undetectable in more differentiated cells
enzymes, after fecundation, the single cell embryo [102, 103]. Adult stem cell turnover is regulated
relies much more on oxidative metabolism [99]. by epigenetic modifications like DNA methylation
During morula formation, the mitochondrial [104]. The S-adenosyl methionine cycle is very
content per cell declines, while the number of important in the differentiation from ESCs and
glucose transporters increases; consequently, gly- pluripotent stem cells (PSCs). The metabolites
10/16 Marcialis • Coni • Pintus • Ravarino • Fanos • Coni • Faa