Stem cells: the promises and pitfalls

Adult human neurogenesis: A view from two schools of thought

Are we truly losing neurons as we grow older? If yes, why, and how can the lost neurons be replaced or compensated for? Is so-called adult neurogenesis (ANG) still a controversial process, particularly in the human cerebral cortex? How do adult-born neurons -if proven to exist- contribute to brain functions? Is adult neurogenesis a disease-relevant process, meaning that neural progenitor cells are dormant in adulthood, but they may be reactivated, for example, following stroke? Is the earnest hope to cure neurological diseases justifying the readiness to accept ANG claim uncritically? These are all fundamental issues that have not yet been firmly explained. Although it is completely understandable that some researchers believe that we can add new neurons to our inevitably deteriorating brain, the brain regeneration process still possesses intellectually and experimentally diverting views, as until now, there has been significant confusion about the concept of ANG. This paper is not intended to be an extensively analytical review distilling all findings and conclusions presented in the ANG literature. Instead, it is an attempt to discuss the commonly entertained opinions and then present our reflective insight concerning the current status quo of the field, which might help redirect research questions, avoid marketing an exaggerated hope, and more importantly, save the ever-limited resources, namely, intellectuals' time, facilities, and grants.

Macrophage dynamics are regulated by local macrophage proliferation and monocyte recruitment in injured pancreas

Pancreas injury by partial duct ligation (PDL) activates a healing response, encompassing β-cell neogenesis and proliferation. Macrophages (MΦs) were recently shown to promote β-cell proliferation after PDL, but they remain poorly characterized. We assessed myeloid cell diversity and the factors driving myeloid cell dynamics following acute pancreas injury by PDL. In naive and sham-operated pancreas, the myeloid cell compartment consisted mainly of two distinct tissue-resident MΦ types, designated MHC-II(lo) and MHC-II(hi) MΦs, the latter being predominant. MHC-II(lo) and MHC-II(hi) pancreas MΦs differed at the molecular level, with MHC-II(lo) MΦs being more M2-activated. After PDL, there was an early surge of Ly6C(hi) monocyte infiltration in the pancreas, followed by a transient MHC-II(lo) MΦ peak and ultimately a restoration of the MHC-II(hi) MΦ-dominated steady-state equilibrium. These intricate MΦ dynamics in PDL pancreas depended on monocyte recruitment by C-C chemokine receptor 2 and macrophage-colony stimulating factor receptor as well as on macrophage-colony stimulating factor receptor-dependent local MΦ proliferation. Functionally, MHC-II(lo) MΦs were more angiogenic. We further demonstrated that, at least in C-C chemokine receptor 2-KO mice, tissue MΦs, rather than Ly6C(hi) monocyte-derived MΦs, contributed to β-cell proliferation. Together, our study fully characterizes the MΦ subsets in the pancreas and clarifies the complex dynamics of MΦs after PDL injury.

A comparative study of PKH67, DiI, and BrdU labeling techniques for tracing rat mesenchymal stem cells

Mesenchymal stem cells (MSCs) have generated a great deal of promise as a potential source of cells for cell-based therapies. Various labeling techniques have been developed to trace MSC survival, migration, and behavior in vitro or in vivo. In the present study, we labeled MSCs derived from rat bone marrow (rMSCs) with florescent membrane dyes PKH67 and DiI, and with nuclear labeling using 5 μM BrdU and 10 μM BrdU. The cells were then cultured for 6 d or passaged (1-3 passages). The viability of rMSCs, efficacy of fluorescent expression, and transfer of the dyes were assessed. Intense fluorescence in rMSCs was found immediately after membrane labeling (99.3 ± 1.6% PKH67+ and 98.4 ± 1.7% DiI+) or after 2 d when tracing of nuclei was applied (91.2 ± 4.6% 10 μM BrdU+ and 77.6 ± 4.6% 5 μM BrdU+), which remained high for 6 d. Viability of labeled cells was 91 ± 3.8% PKH67+, 90 ± 1.5% DiI+, 91 ± 0.8% 5 μM BrdU+, and 76.9 ± 0.9% 10 μM BrdU+. The number of labeled rMSCs gradually decreased during the passages, with almost no BrdU+ nuclei left at final passage 3. Direct cocultures of labeled rMSCs (PKH67+ or DiI+) with unlabeled rMSCs revealed almost no dye transfer from donor to unlabeled recipient cells. Our results confirm that labeling of rMSCs with PKH67 or DiI represents a non-toxic, highly stable, and efficient method suitable for steady tracing of cells, while BrdU tracing is more appropriate for temporary labeling due to decreasing signal over time.

An early decrease in cell proliferation after pentylenetetrazole-induced seizures

There are increasing data on the influence of seizures on neurogenesis in the adult brain. However, data on cell proliferation and differentiation during the early stages of kindling are scarce. We have used pentylenetetrazole (PTZ)-induced kindling to investigate the temporal profile of cytogenesis in the germinative zones of adult rat brain. For comparison, we also used a single PTZ-induced generalized tonic-clonic seizure. During kindling development, the density of 5-bromo-2'-deoxyuridine-positive cells demonstrated similar changes in all germinative zones: a dramatic decrease after the first subthreshold PTZ injection, and a gradual increase to the control level following repeated PTZ administration. On the contrary, a single PTZ-induced generalized tonic-clonic seizure was followed by an increase in the number of proliferating cells in both the dentate gyrus and the subventricular zone. These results may indicate the existence of global mechanisms affecting cellular proliferation in adult brain during seizures. Different temporal profiles of neuronal damage and proliferation changes suggest that neurodegeneration is unlikely to be a global proliferation-regulating factor. The data may contribute to better understanding of the initial phase of kindling development and epileptogenesis.

The dark side of BrdU in neural stem cell biology: detrimental effects on cell cycle, differentiation and survival

5-Bromo-2'-deoxyuridin (BrdU) is frequently used in anaylsis of neural stem cell biology, in particular to label and to fate-map dividing cells. However, up to now, only a few studies have addressed the question as to whether BrdU labeling per se affects the cells to be investigated. Here, we focused on the potential impact of BrdU on neurosphere cultures derived from the adult rat brain and on proliferation of progenitors in vivo. In vitro, neurospheres were pulsed for 48 h with BrdU, and cell proliferation, cell cycle, differentiation, survival and adhesion properties were subsequently analyzed. BrdU inhibited the expansion of neural progenitors as assessed by MTS assay and increased the fraction of cells in the G0/G1-phase of the cell cycle. Moreover, BrdU increased cell death and dose-dependently induced adherence of NPCs. Cell adherence was accompanied by a reduced amount of active matrix-metalloproteinase-2 (MMP-2). Furthermore, BrdU repressed neuronal and oligodendroglial differentiation, whereas astroglial fate was not affected. In contrast to the in vitro situation, BrdU apparently did not influence endogenous proliferation of NPCs or neurogenesis in concentrations that are typically used for labeling of neural progenitors in vivo. Our results reveal so far uncharacterized effects of BrdU on adult NPCs. We conclude that, because of its ubiquitous use in stem cell biology, any potential effect of BrdU of NPCs has to be scrutinized prior to interpretation of data.

Adult neurogenesis, neuroinflammation and therapeutic potential of adult neural stem cells

The pathogenesis of neurological diseases and disorders remains mostly unknown. Neuroinflammation has been proposed as a causative factor for neurological diseases. The confirmation that neurogenesis occurs in the adult brain and neural stem cells (NSCs) reside in the adult central nervous system (CNS) of mammals has tremendous implications for our understanding of the physio- and pathology of the nervous system. The generation of newborn neuronal cells in the adult brain is modulated in neurological diseases and during inflammation. This suggests that adult neurogenesis is involved in the pathogenesis of neurological diseases and disorders, particularly during neuroinflammation. In this manuscript, we will review the modulation of adult neurogenesis in neurological diseases and during neuroinflammation. We will discuss the role and contribution of neuroinflammation and adult neurogenesis to neurological diseases and disorders, and for the therapeutic potential of adult NSCs.

Road for understanding cancer stem cells: model cell lines

There is increasing evidence suggesting that stem cells are susceptive to carcinogenesis and, consequently, can be the origin of many cancers. Recently, the neoplastic potential of stem cells has been supported by many groups showing the existence of subpopulations with stem cell characteristics in tumor biopsies such as brain and breast. Evidence supporting the cancer stem cell hypothesis has gained impact due to progress in stem cell biology and development of new models to validate the self-renewal potential of stem cells. Recent evidence on the possible identification of cancer stem cells may offer an opportunity to use these cells as future therapeutic targets. Therefore, model systems in this field have become very important and useful. This review will focus on the state of knowledge on cancer stem cell research, including cell line models for cancer stem cells. The latter will, as models, help us both in the identification and characterization of cancer stem cells and in the further development of therapeutic strategies including tissue engineering.

A comparative framework for understanding the biological principles of adult neurogenesis

Adult neurogenesis has been identified in all vertebrate species examined thus far. However, an evolutionary trend towards a reduction in both the number of proliferation zones and the overall number of newborn cells has been revealed in more recent lineages of vertebrates, such as mammals. Adult neurogenesis, and in particular the characterization of adult neural stem cells in mammals has been the focus of intense research with the goal of developing new cell-based regenerative treatments for neurodegenerative diseases, spinal cord injury, and acute damage due to stroke. Conversely, most other vertebrate classes, which display widespread production of adult neurons, are not typically used as model systems in this context. A more profound understanding of the structural composition and the mechanisms that support proliferation zones in the mature brain have become critical for revealing how adult neural stem cells are maintained in these regions and how they regulate neurogenesis. In this review we argue that comprehensive analyses of adult neurogenesis in various vertebrate and invertebrate species will lead to a more complete understanding of the fundamental biology and evolution of adult neurogenesis and provide a better framework for testing hypotheses regarding the functional significance of this trait.

Marked species and age-dependent differences in cell proliferation and neurogenesis in the hippocampus of wild-living rodents

Variations in the extent of adult neurogenesis and natural and experimental factors controlling it have been described in laboratory animals. The wide range of variation seen even within a species, the mouse, raises the question as to which rates of neurogenesis can be expected in natural populations. Answering this question is important to evaluate the functional significance of adult neurogenesis under natural conditions and to define the factors controlling it. To address this issue, we investigated four species of wild-living rodents and outbred laboratory mice using markers for proliferating cells, Ki-67, and developing neurons, doublecortin and NeuroD. We found about four times as many Ki-67-positive cells per mm3 granule cell layer in two wood mouse species (Muridae; Apodemus spp.) than in bank and pine voles (Arvicolidae; Clethrionomys glareolus and Microtus subterraneus). Laboratory mice show proliferation rates between wood mice and voles. Markers for developing neurons, NeuroD and doublecortin, reflect the findings of proliferation activity. Hippocampal cell proliferation decreases dramatically with age in wild-living species. The onset of the downregulation varies among species. It occurs late in the life span of the yellow-necked wood mouse. In aged animals, the number of proliferating cells per mm3 granule cell layer is reduced to 19% of the adult value. Downregulation occurs early in pine voles, in which cell proliferation in adult animals is reduced to 33% of juvenile values. Proliferation and age-dependent changes along the deep border of the alveus and angular bundle follow those of the dentate gyrus. We conclude that cell proliferation and neurogenesis in the dentate gyrus vary significantly among wild-living rodents, and that they are downregulated with age, but at species-specific time points.

Stem cell therapy for human neurodegenerative disorders-how to make it work

Recent progress shows that neurons suitable for transplantation can be generated from stem cells in culture, and that the adult brain produces new neurons from its own stem cells in response to injury. These findings raise hope for the development of stem cell therapies in human neurodegenerative disorders. Before clinical trials are initiated, we need to know much more about how to control stem cell proliferation and differentiation into specific phenotypes, induce their integration into existing neural and synaptic circuits, and optimize functional recovery in animal models closely resembling the human disease.

Efficacy of doublecortin as a marker to analyse the absolute number anddendritic growth of newly generated neurons in the adult dentate gyrus

Doublecortin (DCX), a microtubule-associated phosphoprotein, has been recently utilized as a marker of newly born neurons in the adult dentate gyrus (DG). Nonetheless, it is unknown whether DCX exclusively labels newly formed neurons, as certain granule cells with the phenotype of differentiated neurons express DCX. We addressed the authenticity of DCX as a marker of new neurons in the adult DG by quantifying cells that are positive for 5'-bromodeoxyuridine (BrdU), DCX and both BrdU and DCX in hippocampal tissues of adult rats treated with daily injections of BrdU for 12 consecutive days. We provide new evidence that neurons visualized with DCX immunostaining in the adult rat DG are new neurons that are predominantly born during the 12 days before euthanasia. This is confirmed by the robust expression of BrdU in 90% of DCX-positive neurons in the DG of animals injected with BrdU for 12 days. Furthermore, DCX expression is specific to newly generated healthy neurons, as virtually all DCX-positive cells express early neuronal antigens but lack antigens specific to glia, undifferentiated cells or apoptotic cells. As DCX expression is also robust in the dendrites, DCX immunocytochemistry of thicker sections facilitates quantification of the dendritic growth in newly born neurons. Thus, both absolute number and dendritic growth of new neurons that are generated in the adult DG over a 12-day period can be quantified reliably with DCX immunostaining. This could be particularly useful for analysing changes in dentate neurogenesis in human hippocampal tissues as a function of ageing or neurodegenerative diseases.

Nonrenewal of neurons in the cerebral neocortex of adult macaque monkeys

The concept that, after developmental periods, neocortical neurons become numerically stable and are normally nonrenewable has been challenged by a report of continuous neurogenesis in the association areas of the cerebral cortex in the adult Macaque monkey. Therefore, we have reexamined this issue in two different Macaque species using the thymidine analog bromodeoxyuridine (BrdU) as an indicator of DNA replication during cell division. We found several BrdU+/NeuN+ (neuronal nuclei) double-labeled cells, but cortical neurons, distinguished readily by their size and cytological and immunohistochemical properties, were not BrdU positive. We examined in detail the frontal cortex, where it is claimed that the largest daily addition of neurons has been made, but did not see migratory streams or any sign of addition of new neurons. Thus, we concluded that, in the normal condition, cortical neurons of adult primates, similar to other mammalian species, are neither supplemented nor renewable.