Edited by: Silvia De Marchis, University of Turin, Italy
Reviewed by: Adam C. Puche, University of Maryland, USA; Nader Sanai, Barrow Neurological Institute, USA
*Correspondence: Mirjana Maletic-Savatic and Amanda Sierra, Department of Pediatrics, Baylor College of Medicine, Neurological Research Institute, 1250 Morsund St, MS NR1250.01, Houston, TX 77030, USA. e-mail:
This article was submitted to Frontiers in Neurogenesis, a specialty of Frontiers in Neuroscience.
This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.
Neural stem cells reside in well-defined areas of the adult human brain and are capable of generating new neurons throughout the life span. In rodents, it is well established that the new born neurons are involved in olfaction as well as in certain forms of memory and learning. In humans, the functional relevance of adult human neurogenesis is being investigated, in particular its implication in the etiopathology of a variety of brain disorders. Adult neurogenesis in the human brain was discovered by utilizing methodologies directly imported from the rodent research, such as immunohistological detection of proliferation and cell-type specific biomarkers in postmortem or biopsy tissue. However, in the vast majority of cases, these methods do not support longitudinal studies; thus, the capacity of the putative stem cells to form new neurons under different disease conditions cannot be tested. More recently, new technologies have been specifically developed for the detection and quantification of neural stem cells in the living human brain. These technologies rely on the use of magnetic resonance imaging, available in hospitals worldwide. Although they require further validation in rodents and primates, these new methods hold the potential to test the contribution of adult human neurogenesis to brain function in both health and disease. This review reports on the current knowledge on adult human neurogenesis. We first review the different methods available to assess human neurogenesis, both
The discovery of adult neurogenesis crushed the century-old dogma that no new neurons are formed in the mammalian brain after birth. However, this finding and its acceptance by the scientific community did not happen without hurdles. At the beginning of the last century, based on detailed observations of the brain anatomy reported by Santiago Ramon y Cajal and others, it was established that the human nervous system develops
The field of adult neurogenesis finally took off in the 1990s with the development of new technologies. First, the use of 3H-thymidine, a radioactive nucleotide used to study proliferation when incorporated into the cells during the S phase of the cell cycle, was replaced by its analog, bromodeoxyuridine (BrdU), which could be detected by a specific antibody. Utilization of the BrdU for labeling of newborn cells via immunohistochemistry allowed their further studies by co-labeling with specific neuronal markers (Miller and Nowakowski,
Currently, adult neurogenesis is one of the hot topics in Neuroscience especially because of the new opportunities it may bring for treatments of neurodegenerative diseases, either by harnessing resident progenitors to regenerate the lost tissue (Sohur et al.,
The extent of our knowledge on adult human neurogenesis directly correlates with the type of available techniques that can be applied to human brain tissue research. Several methodologies exist, but each method yields different sensitivity, specificity, and ultimately different units of quantification, thus rendering it difficult to compare different studies. In addition, some methodologies can assess only proliferation (NPCs or total proliferating cells) while others can provide the data on neurogenesis [neuroblasts (NBs, neuronal committed cells) or newborn neurons]. Herein, we review the advantages and disadvantages of methods used to assess adult human neurogenesis both
The majority of the methodologies used to study neurogenesis
Bromodeoxyuridine is widely used in animal models to quantify the number of dividing cells in a tissue and to trace their progeny. When administered systemically, it is incorporated into the DNA during the S phase of the cell cycle and is transmitted to the daughter cells, as long as it is not diluted through many rounds of proliferation (Karpowicz et al.,
The major advantage of BrdU labeling is its sensitivity to detect proliferating cells compared to other immunohistological methods. For instance, neurogenesis in adult rhesus monkeys was only detected using BrdU (Kornack and Rakic,
Despite its wide use, BrdU labeling has some drawbacks. For instance, BrdU does not diffuse freely through the blood–brain barrier (BBB), but rather, it likely uses the deoxythymidine transporters (Spector and Johanson,
Particular cell types and particular stages of the cell cycle of dividing cells can be assessed by specific antibodies. When utilizing these reagents for immunostaining of the human tissue, it is important to take into account antigenicity, which can be affected by the delayed fixation of the postmortem tissue (Boekhoorn et al.,
Antigen | Function | Expression | References |
---|---|---|---|
Ki67 | Unknown | Expression during G1-M | Del Bigio ( |
PCNA | Proliferating cell nuclear antigen, a co-factor of DNA-PolΔ | Synthesized during S phase | Bernier et al. ( |
MCM2 | Minichromosome maintenance protein 2, a DNA helicase | Throughout cell cycle | Jin et al. ( |
Nestin | An intermediate filament | NPC, astrocytes, radial glia, perivascular cells | Arnold and Trojanowski ( |
EGFR | Epidermal growth factor receptor | C cells, A cells | Weickert et al. ( |
GFAP | Glial fibrillary acidic protein, an intermediate filament | Quiescent NPCs, B cells, radial glia, astrocytes | Eriksson et al. ( |
Vimentin | An intermediate filament | Quiescent NPCs, B cells, astrocytes | Arnold and Trojanowski ( |
Musashi | An RNA-binding protein | NPC, astrocytes | Crespel et al. ( |
PSA-NCAM | Polysialylated cell adhesion molecule, involved in cell migration | Migrating neuroblasts | Mikkonen et al. ( |
DCX | Doublecortin, promotes microtubule proliferation | Neuroblasts and neurons | Bedard and Parent ( |
NeuroD | A transcription factor, involved in neuronal commitment | Neuroblasts and neurons | Bedard and Parent ( |
TOAD64 | Turned on after division 64, a membrane associated protein from the TUC4 family involved in axonal growth | Neuroblasts | Jin et al. ( |
NeuN | A splicing factor of the Fox-3 family | Neurons | Eriksson et al. ( |
βIII-Tubulin | A microtubule | Neurons | Arnold and Trojanowski ( |
NSE | Neuron specific enolase | Neurons | Eriksson et al. ( |
Human NPCs have been isolated and cultured
Similar to the 14C-dating used in archeology, this method uses the 14C-content to assess the average age of the cells present in a particular tissue (Spalding et al.,
More recently, methods have been specifically designed to detect neurogenesis in live human brain by means of magnetic resonance imaging (MRI; Figure
The major advantage of MR-based methods is that they are performed in live individuals with no side effects, supporting repeated measures and longitudinal studies. Thus, these methods allow a more controlled experimental design, and variables such as the cause or age of death no longer have to be taken into account. Nonetheless, these methods rely on correlations to indirectly quantify neurogenesis, and extensive validation in both rodents and humans is required to demonstrate that they are specific for neurogenesis. More importantly, it is essential to determine whether the data correlate with the number of NPCs, proliferating NPCs (versus other cell types that proliferate), or newborn neurons. Another major advantage of MRI-based methods is that MRI scanners are widely available in hospitals and research centers worldwide. Thus, these methods could be easily implemented in many labs and offer a unique research opportunity to increase our understanding of the role of adult neurogenesis in humans.
Cerebral blood volume (CBV) can be measured by several methods, one of which is MRI. In MR-based CBV measurements, the contrast agent gadolinium is injected systemically. The chelated gadolinium used is a non-toxic highly lipophobic agent, thus restricted to the intravascular space when the BBB is not challenged (Zaharchuk,
The basis for the CBV studies of neurogenesis is the correlation between angiogenesis and neurogenesis. Increased cortical CBV correlates with angiogenesis in ischemia (Lin et al.,
Here, the MRI modality used is proton magnetic resonance spectroscopy (1H-MRS), which exploits the magnetic properties of different protons to detect an array of small metabolites in the living tissue. Some protons of some metabolites are mobile in the magnetic field, and the relaxation of their spins can be detected by MR. This relaxation is observed as a sinusoid wave of decay in the time domain (free-induction decay, or FID), which is conventionally transformed into a function in the frequency domain (Fourier-fast transform, or FFT; Maletic-Savatic et al.,
This metabolite was initially discovered in rodent NPCs, grown as embryonically derived neurospheres, by high-field nuclear magnetic resonance (NMR; Manganas et al.,
The identity of the 1.28 ppm metabolite remains unknown. Although it most likely contains a lipid component (Manganas et al.,
Nowadays, the consensus is that adult neurogenesis occurs in two main areas of the brain: the subgranular zone (SGZ) of the DG of the hippocampus, where new granule neurons are locally produced and have been associated with learning and memory, and mood disorders; and the subventricular zone (SVZ), from where newborn cells migrate through the rostral migratory stream (RMS) and give rise to neurons in the OB, related to olfaction (Ma et al.,
In rodents, the hippocampal neurogenic cascade starts with the quiescent neuroprogenitors (QNPs; type-1 cells; radial glia), which reside in the SGZ. QNPs proliferate, giving rise to a transient population, the amplifying neuroprogenitors (ANPs; type-2a cells) which in turn proliferate and differentiate into neuronal-committed NBs (type-2b and type-3 cells). Finally, at the end of a 4-week period, the surviving NBs become mature neurons integrated into the circuitry (reviewed by Kempermann et al.,
In humans, adult hippocampal neurogenesis was demonstrated by analysis of postmortem tissue of cancer patients (Eriksson et al.,
Other studies, however, failed to detect NPCs or proliferating cells in the adult hippocampus of epileptic patients using immunohistochemical methods, such as expression of nestin, vimentin, or Ki67 (Arnold and Trojanowski,
In rodents, the stem cells of the SVZ are specialized astrocytes called B cells. These cells proliferate and give rise to C cells, the transient amplifying population of the system (Doetsch et al.,
In humans, proliferating BrdU+ cells were found in the SVZ, but they did not co-localize with either GFAP or NeuN (Eriksson et al.,
Finally, the human OB hosts NPCs, which have been isolated from patients undergoing neurosurgery and grown in culture, where they differentiated into neurons, astrocytes, and oligodendrocytes (Pagano et al.,
The majority of studies on human neurogenesis compare findings in healthy people to those in patients with a variety of neurological diseases. A summary comparing the alteration in neurogenesis in rodent models of disease and human patients is shown in Table
Disease | Area | Rodent data | Human data |
---|---|---|---|
Epilepsy | Hippo. | Acute increase in proliferation | Increase in pNPCs and proliferation in pediatric patients |
Acute increase in neurogenesis | Increase or no change in proliferation in adult patients | ||
Aberrant and ectopic new neurons |
Decrease, no changed or increase in pNBs in adult patients | ||
Huntington's disease | SVZ | Increased or unchanged SVZ proliferation migration of new neurons into the striatum | Increase in proliferation; thicker SVZ increase in pNPCs and pNBs |
Alzheimer's disease | SVZ | Decrease in proliferation |
Decrease in proliferation |
Hippo. | Increase, no change or decrease in proliferation |
No change in proliferation |
|
Parkinson's disease | SVZ | Decrease in proliferation |
Decrease in proliferation |
Hippo. | Decrease in proliferation |
Decrease in pNPCs | |
S. nigra | Induction of neurogenesis | No proliferation or pNPCs | |
Stroke | SVZ | Increase in proliferation |
NR |
Hippo. | Increase in proliferation |
NR | |
Striatum | Induction of neurogenesis from SVZ progenitors | NR | |
Cortex | Induction of neurogenesis from SVZ progenitors | Increase in proliferation |
|
Depression | Hippo. | Antidepressants increase proliferation and neurogenesis | Increase or no changes in proliferation |
Increase in pNPCs in patients treated with antidepressants |
Epilepsy is a common human disease that affects more than 50 million people worldwide (Kuruba et al.,
In rodents, there is abundant literature reporting that hippocampal neurogenesis increases in models of acute seizures, either by administration of glutamate receptors agonists such as kainic acid, or by electrical stimulation of the hippocampus or the piriform cortex. In these models an increment in the proliferation of the hippocampal NPCs results in a concomitant increase in the number of newborn neurons (reviewed by Parent,
Several studies have reported changes in adult neurogenesis in patients with TLE, who were all pharmacoresistant and had to undergo temporal lobectomy, providing the source for studies of neurogenesis. Utilizing Ki67 labeling the initial study showed no evidence of proliferation in the adult epileptic SGZ, since the number of Ki67+ cells was not significantly larger compared to other DG regions (Del Bigio,
Epilepsy also alters SVZ neurogenesis. In rats, pilocarpin-induced status epilepticus produced an increased proliferation of SVZ NPCs as well as an expansion of the RMS, which contained more proliferating cells and NBs (Parent et al.,
Huntington's disease (HD) is caused by expanded CAG repeats in the huntingtin gene, which leads to protein accumulation and neurodegeneration in the striatum, a brain area that lies below the SVZ. In transgenic mice models of HD, in which little striatal neurodegeneration occurs, neurogenesis in the SVZ remains unaltered; whereas in rat models of striatal degeneration an increased SVZ proliferation is observed (reviewed by Curtis et al.,
Studies of HD patients have shown increased intensity of PCNA staining in the SVZ lining of the striatum compared to age-matched controls, suggesting an increase in SVZ proliferation. Additionally, the expression levels of PCNA correlated with the number of CAG repeats in these patients. The proliferating cells expressed markers of glia (GFAP) and neurons (β-III-Tubulin), suggesting the presence of putative B and A cells, respectively (Curtis et al.,
Alzheimer's disease (AD) is characterized by accumulation of β-amyloid and neurofibrillary tangles containing hyperphosphorylated tau protein throughout the cortex and the hippocampus, resulting in progressive dementia (Curtis et al.,
Subventricular zone neurogenesis is also altered in mouse AD-models. For instance, transgenic APP or PSE1 mice as well as wild-type mice infused in the lateral ventricles with βA peptide had reduced SVZ proliferation compared to control mice (Haughey et al.,
The major hallmark of the Parkinson's disease (PD) is the death of dopaminergic neurons in the SN, with consequent impairment of movement control, mood, and motivation (Hoglinger et al.,
Interestingly, in the OB glomerule cell layer there is an increase in the number of newborn as well as total dopaminergic cells, expressing the synthesizing enzyme tyrosine hydroxylase, both in rodents whose nigrostriatal pathway was lesioned with 6-hydroxydopamine (Winner et al.,
A stroke, or cerebrovascular accident, results from occlusion of cerebral arteries leading to decreased local blood flow (ischemia) or from a hemorrhage. In the stroked tissue, two areas of injury can be discriminated: the core infarcted area, where neurons die of necrosis and very little, if any, regeneration is possible; and the penumbra area, which surrounds the infarcted area, is perfused by collateral arteries, and is not irreversibly damaged. Given that the ischemic stroke is the third most frequent cause of mortality in industrialized countries, major scientific efforts have been directed toward discoveries of therapies to facilitate recovery from the insult.
In rodent and non-human primate models of stroke, such as occlusion of the medial cerebral artery occlusion (MCAO), adult neurogenesis is up-regulated both in the SVZ–RMS–OB and the hippocampus (Jin et al.,
This research indicated that harnessing aberrant striatal neurogenesis in stroke may be useful to reduce the neurological deficits in patients (reviewed in Zhang and Chopp,
Depression, or major depressive disorder (MDD), is characterized by anhedonia and the absence of positive affect (Craske et al.,
The hippocampal pathology in MDD has led to the “neurogenesis hypothesis of depression.” Postulated by Drew and Hen (
Stress, or more properly, the failure to adapt to stressful situations, is a shared symptom of depression and other mood disorders, such as anxiety and fear disorders (Craske et al.,
In MDD patients, recent studies have shown alterations in adult hippocampal neurogenesis (Reif et al.,
Finally, there is also sporadic evidence that adult neurogenesis may be altered in other brain diseases. For instance, in
Overall, studies of adult human neurogenesis, even though hampered by limitations of the available methodologies for both
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to thank Juan J.P. Deudero, William T. Choi, and Fatih Semerci for their help during the preparation of this manuscript. This work was supported by the NIH Intellectual and Developmental Disabilities Research Grant (P30HD024064), the McKnight Endowment Fund, the DANA Foundation, and the Farish Foundation (Mirjana Maletic-Savatic).
1H-MRS, proton magnetic resonance spectroscopy; AD, Alzheimer's disease; ANPs, amplifying neuroprogenitors; APP, amyloid precursor protein; BBB, blood–brain barrier; BrdU, bromodeoxyuridine; CBF, cerebral blood flow; CBV, cerebral blood volume; DCX, doublecortin; DG, dentate gyrus; DISC1, disrupted-in-schizophrenia 1; EGFR, epidermal growth factor receptor; FACS, fluorescence-activated cell sorting; FDA, Food and Drug Administration; FFT, Fourier-fast transform; FID, free-induction decay; GFAP, glial fibrillary acidic protein; GFP, green fluorescent protein; HD, Huntington's disease; MCAO, medial cerebral artery occlusion; MCM2, minichromosome maintenance protein; MDD, major depressive disorder; MR, magnetic resonance; MRI, magnetic resonance imaging; NAA,
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