Adult human neurogenesis

I am hoping that this takes over from neuroplasticity as the future of stroke rehab.
This is pretty heavy reading but you should at least know enough about it to ask your medical staff.
https://admin.frontiersin.org/neurogenesis/10.3389/fnins.2011.00047/full
Quite a ways down the most interesting line in here is bolded red
Only a few paragraphs are selected here so go to the complete article at the URL.
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 ex vivo and in vivo and then appraise the changes of adult neurogenesis in human diseases.
Introduction: A Brief History of the Adult Mammalian Neurogenesis Discovery
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 in utero (Colucci-D’Amato et al., 2006). In adult brains, it was thought, no more neurons could be generated, as the brain is grossly incapable of regenerating after damage (for a more detailed historical report see Watts et al., 2005; Whitman and Greer, 2009). This dogma was deeply entrenched in the Neuroscience community, and Altman’s (1962) discovery of newborn cells in well-defined areas of the adult rodent brain was largely ignored. The phenomenon was reexamined in the 1970–1980s, when Michael Kaplan (Kaplan and Hinds, 1977) and Fernando Nottebohm (Goldman and Nottebohm, 1983) demonstrated the presence of newborn cells in the adult brain of mice and canaries, respectively, and showed that these cells had ultrastructural characteristics of neurons. However, such findings could not be repeated in adult rhesus monkeys, where proliferating cells appeared to be glial and endothelial cells and not neurons (Rakic, 1985; Eckenhoff and Rakic, 1988). Thus, neurogenesis seemed to be absent in adult primates (Eckenhoff and Rakic, 1988).

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, 1988). Further, it was shown that neuroprogenitor cells (NPCs), isolated from adult mouse brains, proliferated and differentiated into neurons and astrocytes in vitro (Reynolds and Weiss, 1992). In addition, NPCs labeled with viral vectors were able to migrate and differentiate into neurons in the adult mouse brain (Lois and Alvarez-Buylla, 1993), demonstrating that the adult neurogenesis was functional in rodents. Finally, the existence of adult neurogenesis in humans was firmly established when, in 1998, Gage and colleagues demonstrated for the first time that new neurons were produced in the adult hippocampus (Eriksson et al., 1998).

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., 2006) or by cell transplantation therapies (Goldman and Windrem, 2006). The field is currently on the rise, as shown by the exponential growth of publications with the key words “adult” AND “neurogenesis OR neural stem cells” (PubMed search up to December 31, 2010): a total of 6,437 papers have been published, of which 57% (3,695 papers) was published in the last 5 years (Figure 1). However, only 8% of published papers (530 papers) deal with human data (search including the term “human” in the title), suggesting that the research on adult neurogenesis in humans is still in its infancy. Thus, the actual knowledge on adult human neurogenesis is limited and in many cases, data is directly extrapolated from the rodent literature. Herein, we review the methodologies used to assess adult human neurogenesis and its status in several neuropsychiatric disorders.


Methods to Assess Neurogenesis in vivo
More recently, methods have been specifically designed to detect neurogenesis in live human brain by means of magnetic resonance imaging (MRI; Figure 3). In the MRI scanner, the subjects are exposed to a harmless magnetic field that aligns the magnetic spin of all the protons in the tissue in a low energy configuration; next, the subjects receive radiofrequency electrical stimulation, which excites the spins out of equilibrium. The spins then naturally relax back to their original conformation with time constants T1 (spin–lattice relaxation time, for longitudinal magnetization) and T2 (spin–spin relaxation time, for transversal magnetization; Maletic-Savatic et al., 2008). The difference in relaxation times of different molecules, such as water and fat, is used to generate detailed MRI images of the brain. In addition, further information can be extracted from these constants, and different MR modalities have been adapted to study neurogenesis (Modo and Bulte, 2011).

FIGURE 3

Figure 3. Live methods to assess adult human neurogenesis. These methods are based on magnetic resonance, using MRI scanners available in hospitals worldwide. Because there are no side effects, both healthy and diseased people can be re-scanned throughout aging, before and after exercise, to follow-up the effect of pharmacological interventions, etc. Two main methods have been developed to indirectly quantify adult human neurogenesis using different MR modalities: (A) CBV measurement. The dentate gyrus CBV is a proxy for neurogenesis in physical exercise paradigms. This method is based on the consecutive correlation of neurogenesis–angiogenesis, and angiogenesis–CBV. (A1) High resolution MRI slice of the adult human hippocampus (right panel), showing the different hippocampal subregions (entorhinal cortex, EC, green; dentate gyrus, DG, red; cornu ammonis 1, CA1, light blue; and subiculum, SUB, yellow; central panel) and a typical hippocampal CBV map (warmer colors indicate higher CBV). (A2) Quantification of the mean relative hippocampal CBV (rCBV), before (white bars) and after (black bars) exercise in healthy humans. As in mice, physical exercise resulted in a significant increase in CVB only in the DG (asterisk). (B) Spectroscopy. A lipidic metabolite resonating at 1.28 ppm was identified as a marker of rodent NPCs. (B1) Positioning of the voxel of interest in the cortex and the hippocampus of a healthy person. (B2) Spectroscopic analysis of the metabolite content in the hippocampal voxel using SVD and FFT (small upper insert). Identified metabolites are myoInositol (mI, light blue), choline (Cho, purple), creatine (green), N-acetylaspartate (NAA, dark blue), and the 1.28 ppm metabolite (red). (B3) Quantification of the abundance of the 1.28 ppm metabolite in the cortical (CTX) and hippocampal voxels (LH and RH for left and right hippocampus, respectively), normalized over the amplitude of the creatine peak. The hippocampi had much higher content of the 1.28 ppm metabolite than the cortex. The MRI cutaway is printed from permission from the National High Magnetic Field Lab website (http://www.magnet.fsu.edu/education /tutorials/magnetacademy/mri/). Figures (A1–A2) are reprinted by permission from Pereira et al. (2007), copyright 2007, National Academy of Sciences, U.S.A. Figures (B1–B3) are from Manganas et al. (2007) and are reprinted with permission from AAAS.
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 measurements
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, 2007). Due to its paramagnetic properties, it creates variations in the local magnetic field which lead to decreased T1 signal intensity. These changes can be used to generate maps of the CBV and cerebral blood flow (CBF) by an array of computational methods (reviewed in Zaharchuk, 2007). Among these, the steady-state T1 method is based on the assumption that the MRI signal derives from two separate compartments – intravascular (vessels) and extravascular (brain parenchyma; Lin et al., 1999). When gadolinium is administered, only the T1 signal from the intravascular compartment will decrease, assuming the BBB is intact. Then, the difference between pre-contrast and post-contrast images normalized over a voxel that contains only blood, such as the sagittal sinus, is used to generate the CBV map (Lin et al., 1999). The main advantage of the steady-state T1 method, compared to other methods such as bolus tracking (also called dynamic imaging), is that it renders absolute estimations of the CBV, supporting longitudinal studies of brain perfusion, and has high spatial resolution. This method has been validated through correlation with estimations of gray matter CBV using other imaging modalities. However, it requires longer acquisition time and has lower signal-to-noise ratio than bolus tracking (Lin et al., 1999). Nevertheless, the steady-state T1 method is well-established for determining CBV (Zaharchuk, 2007) and has been recently used by the group of Scott Small to indirectly assess changes in adult human neurogenesis (Pereira et al., 2007).

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., 2002; Seevinck et al., 2010) and gliomas (Aronen et al., 2000; Cha et al., 2003). In turn, angiogenesis occurs in the hippocampal neurogenic niche (Palmer et al., 2000), and both angiogenesis and neurogenesis are elevated in the hippocampus following physical exercise (van Praag et al., 2005; Van der Borght et al., 2009). Thus, because of the correlation of CBV–angiogenesis and angiogenesis–neurogenesis, the CBV might provide an indirect measure of neurogenesis in the adult human hippocampus (Pereira et al., 2007). In fact, the CBV increased selectively in the human dentate gyrus (DG, where NPCs reside) after a 12-week exercise paradigm, and this increase correlated with cognitive performance, such as declarative memory but not delayed recognition (Pereira et al., 2007). As a validation experiment, the authors showed that in running mice, the CBV increased in the DG and not in other hippocampal areas, and this increase correlated with the number of 1- to 3-week-old BrdU+ cells (Pereira et al., 2007). More recently, others have demonstrated increased number of micro-vessels occurring in parallel to increased proliferation (Ki67+ cells) and the number of newborn cells committed to the neuronal lineage (DCX+ cells) after 10 days of running (Van der Borght et al., 2009). However, it remains to be elucidated if the angiogenesis–neurogenesis coupling occurs in conditions other than exercise, which would render the CBV measurements for assessments of human neurogenesis more widely applicable.

Neurogenesis in the Adult Human Hippocampus
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., 2004; Encinas and Enikolopov, 2008).

In humans, adult hippocampal neurogenesis was demonstrated by analysis of postmortem tissue of cancer patients (Eriksson et al., 1998), and changes in it under different conditions such as physical exercise and aging have been observed indirectly using CBV (Pereira et al., 2007) and 1H-MRS (Manganas et al., 2007) in healthy adults in vivo. The presence of functional NPCs in the adult human hippocampus was further demonstrated by culture, expansion, and differentiation of human NPCs in vitro (Kukekov et al., 1999; Roy et al., 2000; Moe et al., 2005). A recent study has shown that the adult human SGZ contains DCX-expressing cells that co-localize both with markers of proliferation (MCM2, Ki67, PCNA) and mature neurons (NeuN, β-III-tubulin), supporting the existence of NBs throughout the human lifespan (Knoth et al., 2010).

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, 1996; Del Bigio, 1999; Blumcke et al., 2001; Seress et al., 2001; Fahrner et al., 2007). This conflicting literature can be explained by different sensitivities of the particular method used in each study. Overall, future work is needed to determine all components of the hippocampal neurogenic niche and the cellular types that comprise human neurogenic cascade.

Relevance of Adult Neurogenesis to Human Disease
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 2. These studies use immunohistochemistry to detect biomarkers of proliferation or specific cell-types, and thus are only able to report differences in proliferation and putative NPCs and NBs (pNPCs, pNBs), but not actual neurogenesis (i.e., formation of new neurons). Thus, we label these detected cells “putative” because none of the studies demonstrated that proliferating cells differentiated into mature, functional neurons. To directly reach the conclusion that neurogenesis is occurring lineage tracing using BrdU or analogs is required.

TABLE 2

Table 2. Adult neurogenesis during disease.
Alzheimer’s Disease
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., 2007a). Some pathological features of AD can be modeled in transgenic mice overexpressing amyloid precursor protein and presenilin 1 (APP/PS1). In these mice, memory impairment and increased hippocampal proliferation and neurogenesis were observed at 9, but not 3 months of age (Yu et al., 2009). However, earlier works showed that 6-month-old APP/PSE1 mice have unaltered proliferation and short-term survival (1–13 days), whereas they have a significant reduction of long-term survival (30–42 days) and differentiation (Verret et al., 2007). In addition, other transgenic mouse models of AD have shown otherwise. For instance, in triple transgenic mice (APP/PSE1/Tau) there is a gradual decrease in SGZ proliferation starting at 6 months of age (Rodriguez et al., 2008). On the other hand, 3-month-old mice expressing mutated APP have increased proliferation (Jin et al., 2004a) although this increase was reverted to control levels in older animals (Lopez-Toledano and Shelanski, 2007). Finally, in 6-week-old transgenic mouse expressing human APP showed decreased proliferation in control housing conditions as well as a decreased 4-week survival in enriched environment conditions (Naumann et al., 2010). In postmortem hippocampal samples from patients with advanced AD, an increased expression of NB proteins (DCX, PSA-NCAM, and NeuroD) compared to age-matched controls was reported, suggesting increased neurogenesis perhaps as a compensatory mechanism to cope with the AD-related cognitive impairment (Jin et al., 2004b). However, a more recent study of presenile AD patients failed to demonstrate increased proliferation in the DG, whereas it showed an increased proliferation (Ki67+ cells) associated with gliogenesis and angiogenesis in other hippocampal regions. Further, the same study attributed changes in DCX immunolabeling to postmortem breakdown (Boekhoorn et al., 2006). Thus, it is clear that more comprehensive studies are needed to clarify the changes in SGZ neurogenesis in AD. Furthermore, the relation between potentially altered neurogenesis and the cognitive impairments observed in AD remains to be elucidated (Lazarov et al., 2010).

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., 2002; Rodriguez et al., 2009; Veeraraghavalu et al., 2010). Decreased proliferation and neuronal differentiation were also observed in cultured NPCs isolated from PSE1 mutant SVZ (Veeraraghavalu et al., 2010) and from APP/PS1 mutant SVZ (Demars et al., 2010). In human AD patients, decreased proliferation (Ki67+ cells) accompanied by a puzzling increase in nestin expression was observed in postmortem sections (Ziabreva et al., 2006). In agreement, cultured embryonic human NPC had decreased proliferation and increased apoptosis when treated with Aβ peptide compared to control NPCs treated with vehicle (Haughey et al., 2002). Thus, there are consistently lower levels in SVZ neurogenesis in AD patients as well as in and rodent AD models, prompting the suggestion that impaired SVZ neurogenesis may have functional consequences in AD (Curtis et al., 2007a). For instance, olfactory deficits significantly predict development of AD in patients with mild cognitive impairment (Devanand et al., 2000), although whether these olfactory deficits are related to decreased SVZ neurogenesis remains unknown.

Stroke/Ischemia
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., 2001; Zhang et al., 2001; Koketsu et al., 2006; Lledo et al., 2006). In addition, stroke also induces ectopic neurogenesis in penumbra areas, such as the striatum, due to atypical migration of SVZ newborn cells (Arvidsson et al., 2002). Cortical neurogenesis in the penumbra area in rodent models of stroke has been found by some (Gu et al., 2000; Jin et al., 2003) but not by others (Arvidsson et al., 2002). Interestingly, the newborn cells differentiated into striatal neurons and acquired the same phenotype of the neurons which had died as a consequence of the stroke, suggesting that neuronal replacement can occur in the stroked striatum (Arvidsson et al., 2002). Although the vast majority of the striatal newborn cells died, possibly due to an unfavorable environment (Arvidsson et al., 2002), stroke-induced striatal neurogenesis seems to have functional consequences in rodents, since it has been shown that the transgenic ablation of the NB protein DCX prevented stroke-induced neurogenesis and worsened the sensorimotor and behavioral deficits after MCAO (Jin et al., 2010).

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, 2009). The patients who suffered the ischemic, middle cerebral artery stroke showed increased proliferation of putative B cells (Ki67, GFAP+ cells) and putative C cells (PSA-NCAM+ cells), in the ipsilateral SVZ compared to the contralateral side of the stroke (Marti-Fabregas et al., 2010). In addition, there were traces of ectopic neurogenesis not in the striatum, but in the cortex. A significant increase in proliferating Ki67+ cells and pNBs (PSA-NCAM+ cells) was found in the cortical penumbra region of ischemic stroke patients compared to age-matched controls (Jin et al., 2006; Macas et al., 2006) as well as in perihematomal regions in patients with intracerebral hemorrhage (Shen et al., 2008). The relevance of this increase in cortical neurogenesis in stroke patients remains to be investigated, but the phenomena certainly raise the hope that neurogenesis might be harnessed as a possible treatment for stroke patients.

Conclusion
Overall, studies of adult human neurogenesis, even though hampered by limitations of the available methodologies for both ex vivo and in vivo assessments, are promising. Development of new antibodies targeted to human antigens will certainly improve immunohistochemical data, but even then, such labeling will provide only putative information. It is in combination with BrdU labeling that the production of new neurons can be assessed and quantified. As more BrdU labeled tissue is generated, the changes in the neurogenic cascade that accompany brain disorders will be elucidated. However, several considerations need to be taken into account when studying postmortem human tissue, in particular the postmortem delay, the cause of death, and the age at the time of death. Thus, the future of adult human neurogenesis research and the prospects of harnessing its potential for treatments of brain disorders will heavily depend on the development and thorough validation of methods for in vivo assessments, as those offer unique opportunity for both cross-sectional and longitudinal studies of the neurogenic niches while they are intact within the living brain tissue.

Received: 20 January 2011; Accepted: 23 March 2011;
Published online: 04 April 2011.