Therapeutic stem cell plasticity orchestrates tissue plasticity

The neocortex is the evolutionarily newest and most complex part of the mammalian brain. The neurons of the neocortex are born from different neural stem and progenitor cells (for simplicity collectively referred to as progenitors) that reside in proliferative zones during embryonic development. During the neurogenic period, distinct populations of neural progenitors appear, each having specific molecular and cell biological characteristics. These characteristics and the environment of the neural progenitor are responsible for the self‐renewing or neurogenic potential of each neural progenitor type. The fate of daughter cells after neural progenitor division is determined by the mode of division, intrinsic factors such as transcription factors or regulatory ribonucleic acids (RNAs), and by extrinsic signals coming from the surrounding cells. This tight regulation controls the rate of production of neural progenitor types and neurons. The ratios of different progenitor types have a huge impact on the final number of neurons in the adult neocortex, and are partly responsible for vast differences in the brains of different mammals. Key Concepts: Neocortex is a part of the brain characteristic of mammals. Neocortex is a six‐layered structure comprised of post‐mitotic neurons and glial cells. Neocortical neurons are born from neural progenitor cells, mostly during embryonic development. Neural progenitor cells can be classified into different populations based on molecular and cell‐biological features. The molecular and cell‐biological features of neural progenitor cells influence their propensity to self‐renew or produce neurons. Neural progenitor cells are under tight intrinsic and extrinsic control, in order to maintain the equilibrium between proliferative and neurogenic divisions. The relative abundance of different progenitor populations influences the final number of neurons in the adult neocortex.

Cleavage of the Wnt Receptor Ryk Regulates Neuronal Differentiation during Cortical Neurogenesis

Recent studies show that type II transmembrane serine proteases play important roles in diverse cellular activities and pathological processes. Their expression and functions in the central nervous system, however, are largely unexplored. In this study, we show that the expression of one such member, matriptase (MTP), was cell type-restricted and primarily expressed in neural progenitor (NP) cells and neurons. Blocking MTP expression or MTP activity prevented NP cell traverse of reconstituted basement membrane, whereas overexpression of MTP promoted it. The NP cell mobilization induced by either vascular endothelial growth factor or hepatocyte growth factor was also impaired by knocking down MTP expression. MTP acts upstream of matrix metalloproteinase 2 in promoting NP cell mobility. In embryonic stem cell differentiation to neural cells, MTP knockdown had no effect on entry of embryonic stem cells into the neural lineage. High MTP expression or activity, however, shifts the population dynamics from NP cells toward neurons to favor neuronal differentiation. This is the first report to demonstrate the direct involvement of type II transmembrane serine protease in NP cell function.

Neural stem and progenitor cells express a variety of receptors that enable them to sense and react to signals emanating from physiological and pathophysiological conditions in the brain as well as elsewhere in the body. Many of these receptors and were first described in investigations of the immune system, particularly with respect to hematopoietic stem cells. This emerging view of neurobiology has two major implications. First, many phenomena known from the hematopoietic system may actually be generalizable to stem cells from many organ systems, reflecting the cells' progenitor-mediated regenerative potential. Second, regenerative interfaces may exist between diverse organ systems; populations of cells of neuroectodermal and hematopoietic origin may interact to play a crucial role in normal brain physiology, pathology, and repair. An understanding of the origins of signals and the neural progenitors' responses might lead to the development of effective therapeutic strategies to counterbalance acute and chronic neurodegenerative processes. Such strategies may include modifying and modulating cells with regenerative potential in subtle ways. For example, stem cells might be able to detect pathology-associated signals and be used as "interpreters" to mediate drug and other therapeutic interventions. This review has focused on the role of inflammation in brain repair. We propose that resident astroglia and blood-born cells both contribute to an inflammatory signature that is unique to each kind of neuronal degeneration or injury. These cells play a key role in coordinating the neural progenitor cell response to brain injury by exerting direct and indirect environmentally mediated influence on neural progenitor cells. We suggest that investigations of the neural progenitor-immunologic interface will provide valuable data related to the mechanisms by which endogenous and exogenous neural progenitor cells react to brain pathology, ultimately aiding in the design of more effective therapeutic applications of stem cell biology. Such improvements will include: (1) ascertaining the proper timing for implanting exogenous neural progenitor cells in relation to the administration of anti-inflammatory agents; (2) identifying what types of molecules might be administered during injury to enhance the mobilization and differentiation of endogenous and exogenous neural progenitor cells while also inhibiting the detrimental aspects of the inflammatory reaction; (3) divining clues as to which molecules may be required to change the lesioned environment in order to invite the homing of reparative neural progenitor cells.

Translation: Screening for Novel Therapeutics With Disease-Relevant Cell Types Derived from Human Stem Cell Models

The endocannabinoid system is known to regulate neural progenitor (NP) cell proliferation and neurogenesis. In particular, CB(2) cannabinoid receptors have been shown to promote NP proliferation. As CB(2) receptors are not expressed in differentiated neurons, CB(2)-selective agonists are promising candidates to manipulate NP proliferation and indirectly neurogenesis by overcoming the undesired psychoactive effects of neuronal CB(1) cannabinoid receptor activation. Here, by using NP cells, brain organotypic cultures, and in vivo animal models, we investigated the signal transduction mechanism involved in CB(2) receptor-induced NP cell proliferation and neurogenesis. Exposure of hippocampal HiB5 NP cells to the CB(2) receptor-selective agonist HU-308 led to the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin complex 1 (mTORC1) pathway, which, by inhibiting its downstream target p27Kip1, induced NP proliferation. Experiments conducted with the CB(2) receptor-selective antagonist SR144528, inhibitors of the PI3K/Akt/mTORC1 axis, and CB(2) receptor transient-transfection vector further supported that CB(2) receptors control NP cell proliferation via activation of mTORC1 signaling. Likewise, CB(2) receptor engagement induced cell proliferation in an mTORC1-dependent manner both in embryonic cortical slices and in adult hippocampal NPs. Thus, HU-308 increased ribosomal protein S6 phosphorylation and 5-bromo-2'-deoxyuridine incorporation in wild-type but not CB(2) receptor-deficient NPs of the mouse subgranular zone. Moreover, adult hippocampal NP proliferation induced by HU-308 and excitotoxicity was blocked by the mTORC1 inhibitor rapamycin. Altogether, these findings provide a mechanism of action and a rationale for the use of nonpsychotomimetic CB(2) receptor-selective ligands as a novel strategy for the control of NP cell proliferation and neurogenesis.

Age‐related decline in neurogenesis: Old cells or old environment?

Ischemic stroke stimulates neurogenesis in the adult rodent brain. The molecules underlying stroke-induced neurogenesis have not been fully investigated. Using real-time reverse transcription-PCR, we found that stroke substantially up-regulated angiopoietin 2 (ANG2), a proangiogenic gene, expression in subventricular zone neural progenitor cells. Incubation of neural progenitor cells with recombinant human ANG2 significantly increased the number of beta-III tubulin-positive cells, a marker of immature neurons, but did not alter the number of glial fibrillary acidic protein (GFAP)-positive cells, a marker of astrocytes, suggesting that ANG2 promotes neuronal differentiation. Blockage of the ANG2 receptor, Tie2, with small interference RNA (siRNA)-Tie2 attenuated recombinant human ANG2 (rhANG2)-increased beta-III tubulin mRNA levels compared with levels in the progenitor cells transfected with control siRNA. Chromatin immunoprecipitation analysis revealed that CCAAT/enhancer-binding protein (C/EBP beta) up-regulated by rhANG2 bound to beta-III tubulin, which is consistent with published data that there are several C/EBP beta binding sites in the promoter of beta-III tubulin gene. In addition, rhANG2 enhanced migration of neural progenitor cells measured by single neurosphere assay. Blockage of Tie2 with siRNA-Tie2 and a Tie2-neutralizing antibody did not suppress ANG2-enhanced migration. However, inhibition of matrix metalloproteinases with GM6001 blocked ANG2-enhanced migration. Collectively, our data suggest that interaction of ANG2, a proangiogenic factor, with its receptor Tie2 promotes neural progenitor cell differentiation into neuronal lineage cells, whereas ANG2 regulates neural progenitor cell migration through matrix metalloproteinases, which do not require its receptor Tie2.

The effects of neural progenitor and hemopoietic stem cells on C6 glioma cells were studied in in vivo and in vitro experiments. Considerable inhibition of proliferation during co-culturing of glioma cells with neural progenitor cells was revealed by quantitative MTT test and bromodeoxyuridine incorporation test. Labeled neural progenitor and hemopoietic stem cells implanted into the focus of experimental cerebral glioma C6 survive in the brain of experimental animals for at least 7 days, migrate with glioma cells, and accumulate in the peritumoral space. Under these conditions, neural progenitor cells differentiate with the formation of long processes. Morphometric analysis of glioma cells showed that implantation of neural progenitor and hemopoietic stem cells is accompanied by considerable inhibition of the growth of experimental glioma C6 in comparison with the control. The mechanisms of tumor-suppressive effects of neural and hemopoietic stem cells require further investigation.

Visual Activity Regulates Neural Progenitor Cells in Developing Xenopus CNS through Musashi1

The research on human neural progenitor cells holds great potential for the understanding the molecular programs that control differentiation of cells of glial and neuronal lineages and pathogenetic mechanisms of neurological diseases. Stem cell technologies provide also opportunities for pharmaceutical industry to develop new approaches for regenerative medicine. Here we describe the protocol for isolation and maintenance of neural progenitor cells and cortical neurons using human fetal brain tissue. This protocol can be successfully adapted for preparation of rodent neural and oligodendrocyte progenitor cells. While several methods for isolation of neural and oligodendrocyte progenitors from rodent brain tissue have been described, including techniques which use gene transfer and magnetic resonance beads, few methods are focused on derivation of human oligodendrocyte progenitor cells. Development of human culture provides the most physiologically relevant system for investigation of mechanisms which regulate function of oligodendrocyte, specifically of human origin.

The functional significance of adult neural stem and progenitor cells in hippocampal-dependent learning and memory has been well documented. Although adult neural stem and progenitor cells in the subventricular zone are known to migrate to, maintain and reorganize the olfactory bulb, it is less clear whether they are functionally required for other processes. Using a conditional transgenic mouse model, selective ablation of adult neural stem and progenitor cells in the subventricular zone induced a dramatic increase in morbidity and mortality of central nervous system disorders characterized by excitotoxicity-induced cell death accompanied by reactive inflammation, such as 4-aminopyridine-induced epilepsy and ischaemic stroke. To test the role of subventricular zone adult neural stem and progenitor cells in protecting central nervous system tissue from glutamatergic excitotoxicity, neurophysiological recordings of spontaneous excitatory postsynaptic currents from single medium spiny striatal neurons were measured on acute brain slices. Indeed, lipopolysaccharide-stimulated, but not unstimulated, subventricular zone adult neural stem and progenitor cells reverted the increased frequency and duration of spontaneous excitatory postsynaptic currents by secreting the endocannabinod arachidonoyl ethanolamide, a molecule that regulates glutamatergic tone through type 1 cannabinoid receptor (CB(1)) binding. In vivo restoration of cannabinoid levels, either by administration of the type 1 cannabinoid receptor agonist HU210 or the inhibitor of the principal catabolic enzyme fatty acid amide hydrolase, URB597, completely reverted the increased morbidity and mortality of adult neural stem and progenitor cell-ablated mice suffering from epilepsy and ischaemic stroke. Our results provide the first evidence that adult neural stem and progenitor cells located within the subventricular zone exert an 'innate' homeostatic regulatory role by protecting striatal neurons from glutamate-mediated excitotoxicity.

The Temporal Sequence of the Mammalian Neocortical Neurogenetic Program Drives Mediolateral Pattern in the Chick Pallium

The pannexins (Panx1, -2, and -3) are a mammalian family of putative single membrane channels discovered through homology to invertebrate gap junction-forming proteins, the innexins. Because connexin gap junction proteins are known regulators of neural stem and progenitor cell proliferation, migration, and specification, we asked whether pannexins, specifically Panx2, play a similar role in the postnatal hippocampus. We show that Panx2 protein is differentially expressed by multipotential progenitor cells and mature neurons. Both in vivo and in vitro, Type I and IIa stem-like neural progenitor cells express an S-palmitoylated Panx2 species localizing to Golgi and endoplasmic reticulum membranes. Protein expression is down-regulated during neurogenesis in neuronally committed Type IIb and III progenitor cells and immature neurons. Panx2 is re-expressed by neurons following maturation. Protein expressed by mature neurons is not palmitoylated and localizes to the plasma membrane. To assess the impact of Panx2 on neuronal differentiation, we used short hairpin RNA to suppress Panx2 expression in Neuro2a cells. Knockdown significantly accelerated the rate of neuronal differentiation. Neuritic extension and the expression of antigenic markers of mature neurons occurred earlier in stable lines expressing Panx2 short hairpin RNA than in controls. Together, these findings describe an endogenous post-translational regulation of Panx2, specific to early neural progenitor cells, and demonstrate that this expression plays a role in modulating the timing of their commitment to a neuronal lineage.

Although sufficient cholesterol supply is known to be crucial for neurons in the developing mammalian brain, the cholesterol requirement of neural stem and progenitor cells in the embryonic central nervous system has not been addressed. Here we have conditionally ablated the activity of squalene synthase (SQS), a key enzyme for endogenous cholesterol production, in the neural stem and progenitor cells of the ventricular zone (VZ) of the embryonic mouse brain. Mutant embryos exhibited a reduced brain size due to the atrophy of the neuronal layers, and died at birth. Analyses of the E11.5-E15.5 dorsal telencephalon and diencephalon revealed that this atrophy was due to massive apoptosis of newborn neurons, implying that this progeny of the SQS-ablated neural stem and progenitor cells was dependent on endogenous cholesterol biosynthesis for survival. Interestingly, the neural stem and progenitor cells of the VZ, the primary target of SQS inactivation, did not undergo significant apoptosis. Instead, vascular endothelial growth factor (VEGF) expression in these cells was strongly upregulated via a hypoxia-inducible factor-1-independent pathway, and angiogenesis in the VZ was increased. Consistent with an increased supply of lipoproteins to these cells, the level of lipid droplets containing triacylglycerides with unsaturated fatty acyl chains was found to be elevated. Our study establishes a direct link between intracellular cholesterol levels, VEGF expression, and angiogenesis. Moreover, our data reveal a hitherto unknown compensatory process by which the neural stem and progenitor cells of the developing mammalian brain evade the detrimental consequences of impaired endogenous cholesterol biosynthesis.