Showing papers on "Radial glial cell published in 2014"

Molecular regulation of forebrain development and neural stem/progenitor cells

Abstract: The development of the forebrain is dependent on controlled regulation of neural stem/progenitor cells, since the vast majority of all cells in the forebrain are generated by them. The cellular processes influencing forebrain development include cell proliferation, migration, differentiation, and apoptosis, which are regulated in a spatiotemporal manner by genetic programs and interactive protein networks in a given environment. In this thesis, the influence of Netrin1 (NTN1), Human papilloma virus (HPV) E6/E7, and actin-bundling protein with BAIAP2 homology (ABBA) proteins on neural stem/progenitor cells cellular processes was studied and evaluated. In vitro methods were used to uncover the cellular processes regulated and affected by these proteins. In addition, in vivo methods were used to assess their possible impact on forebrain development in mice. NTN1 belongs to a conserved family of laminin-related molecules and it is present in the developing and adult mouse brain. NTN1 has been thought to act as a diffusible long or short-range guidance cue, which influences growing axons and migrating cells in either a chemoattractive or repulsive manner, and NTN1 deficiency in the brain causes defects in axon guidance and cell migration. The main issue of this thesis was to determine the developmental impact of NTN1 in the formation of two forebrain structures, the olfactory bulb and the corpus callosum (CC). Olfactory bulb processes arriving odour information from the nasal cavity and transmit this information forward to various brain regions. During development, the different cell types of olfactory bulb are generated in distinct germinal regions of the brain. Projection neurons are mainly generated by the olfactory bulb stem/progenitor cells whereas majority of the interneurons are produced by the progenitors, which migrate from the forebrain germinal zones into the olfactory bulb via rostral migratory stream (RMS). The origin of non-neural cells, astrocytes and oligodendrocytes, is not as well understood as neurons. We observed that NTN1 has a significant impact on the beginning of stem/progenitor cells migration from the forebrain germinal zones into the olfactory bulb. In more detail, the migration of Ntn1 expressing stem/progenitor cells was delayed, which led to an accumulation of these cells in RMS, and to a substantial reduction of GABAergic interneurons and oligodendrocytes in the olfactory bulb. Thus, the results suggest that NTN1 acts mainly as a detachment/release factor for the Ntn1 expressing stem/progenitor cells in the beginning of their migration and this function is mediated in either cell-autonomous or paracrine manner. CC is the largest axon tract in the forebrain and it integrates motor, sensory and cognitive performances between cerebral hemispheres. In mice, the cerebral hemispheres have to fuse in the midline before the commissural axons can cross the midline and form the CC. This interhemispheric fusion normally includes the removal of leptomeningeal cells found between the hemispheres and disruption of pial basal lamina, which is produced and maintained by the leptomeningeal cells. In Ntn1 deficient mice the leptomeningeal cells were not removed and the pial basal lamina remained intact in the hemispheric midline. Thus, the results suggest that NTN1 is required for the interhemispheric fusion, which precedes midline crossing of commissural axons and is a prerequisite for the formation of the CC. ABBA belongs to the Bin–amphiphysin–Rvs167 (BAR) protein superfamily, which regulates plasma membrane morphology by directly influencing plasma membrane assembly or through rearrangement of the actin cytoskeleton. The functional and expression studies revealed that ABBA participates in the regulation of cell protrusions by enhancing actin dynamics and connecting plasma membrane deformation to actin cytoskeleton. ABBA was present in radial glia-like cells and radial glial cell extensions near meningeal pial basal lamina during CNS development. During early development of brain, interaction between radial glial end-feet and pial basal lamina is required to sustain radial migration and integrity of pial basal lamina. Thus, ABBA may have an important role in formation of radial glial extensions and their connections in CNS. Exquisite balance between proliferation, self-renew and differentiation of stem/progenitor cells is necessary to maintain appropriate tissue homeostasis. HPV16 E6/E7 oncoprotein mediated degradation of p53 and pRb family of proteins seemed to facilitate a defective coupling of proliferation, self-renewal, and differentiation processes inside neural stem/progenitor cells. In addition, these processes also seemed to be susceptible to the environmental signals. Hence, a defective coupling of proliferation, self-renewal, and differentiation processes in neural stem/progenitor cells may lead to an imbalance in normal tissue homeostasis and abnormal growth of tissue in the brain. In summary, even though NTN1, HPV E6/E7, and ABBA proteins affected different cellular processes of neural stem/progenitor cells, all these cellular processes participate in the development of forebrain conformation and, therefore, the future functionality of it. In addition, defects in regulation of neural stem/progenitor cells cellular processes may lead to congenital neural disorders in mice and humans. Thus, these results also increase our overall comprehension of the developmental cellular processes behind the different neural congenital disorders and diseases.

Developmental stage-specific transformation of neural progenitors.

Abstract: Gliomas represent the most common type of primary brain tumor in both children and adults, showing considerable variability in histologic appearance and clinical outcome. The phenotypic differences between types and grades of gliomas have not been explained solely on the grounds of differing oncogenic stimuli, and current evidence suggests that an interaction between the cell of origin, the tumor microenvironment, and specific cancer-causing genetic changes are all important factors in the evolution of central nervous system tumors.1,2 Studies performed in neural stem cells (NSC), a possible candidate for the glioma cell of origin, suggest that some of the variability in glioma biology may be, in part, a reflection of regional differences in the NSCs from which they arise.3,4 However, we don’t know whether the developmental stage of the NSC may also influence its response to oncogenic stimuli. A good candidate in which to address this question is the radial glial cell, as it progresses stepwise through distinct developmental stages and is the neonatal origin of adult subventricular zone (SVZ) neural stem cells.5 As development progresses, neural progenitors, such as radial glial cells, decrease in number, and their proliferation declines to low levels. The remaining neural stem cells become tightly regulated to ensure they do not hyper-proliferate in adult tissues. These control mechanisms are likely imposed during the cell’s progressive restriction in fate potential. Thus, we hypothesized that the susceptibility of these progenitor populations to oncogenic transformation changes as a function of their maturation. To test this, we developed a mouse model that integrates Cre–Lox-mediated, and Tet-regulated expression, to induce expression of activated K-ras into radial glial progenitors at distinct developmental time points. To target radial glial cells, we used the brain lipid binding protein (BLBP) promoter, as it is expressed specifically at the neurogenic and gliogenic stages of radial glial development,6 allowing us to test the transformation potential of this progenitor population across different stages of their development. Taking advantage of the lineage-tracing and inducible characteristics of our model, which allowed us to track the progeny that derived from BLBP+ cells and their response to the oncogenic effects of active K-ras, we identified naturally occurring, developmentally dependent variability in the tumorigenic effects of active K-ras.7 We showed that active K-ras alone was able to induce diffuse malignant gliomas when targeted to embryonic stages, whereas targeting it to late prenatal or postnatal stages did not lead to tumors (Fig. 1). The difference in the transforming capacity of active K-ras between prenatal and postnatal stages suggested to us that early progenitors may be less able to engage tumor suppressor pathways than their more mature counterparts. By sorting BLBP+ cells at defined developmental stages we were able to show that, indeed, the level of cell cycle regulators in these cells varies as a function of age, reflecting the changes in cell cycle kinetics that radial glia undergo during development and mirroring the ability of active K-ras to induce transformation. The biggest changes were observed in ARF, which had a robust increase in expression during late prenatal and postnatal time-points, accompanied by the downregulation of cell cycle progression regulators such as CDK4, cdc25A, and cdc25C. The higher expression of the tumor suppressor ARF at late prenatal and postnatal time points inversely correlated with the ability of K-ras mutations alone to initiate radial glial cell transformation. Thus resistance to oncogenic K-ras may reflect a developmental activation threshold for Ink4a/ARF, which might be related to the basal proliferative rate of cells at different stages in their development. This idea is not new, as it has been shown that expression of Ink4a/ARF increases with age in many tissue specific stem cells, including NSCs.8 However, our analysis was the first lineage-tracing study to show that even though postnatal neural stem cells derive from embryonic radial glial cells, their response to the same oncogenic stimulus is distinct. These distinctions reflect the inherent ability of the cell to engage tumor suppressor pathways in response to an oncogenic stimulus. Interestingly, by deleting p53 in radial glia cells, we were able to overcome the resistance of early postnatal neural progenitors to active K-ras transformation. Thus, it is possible that cancers driven by a single oncogene may in fact derive from earlier precursor populations than their counterparts harboring defects in multiple oncogenic pathways. Figure 1. NSCs in the developing forebrain begin as neuroepithelial cells and transform into radial glial cells, which mature into astrocyte-like cells or type B cells. The tumorigenic effects of K-rasG12D in this population, show developmentally ... Overall, our results highlight the interplay between genetic alterations and the molecular changes that accompany the temporal development of NSCs, and further emphasize the need to view the tumorigenic process of gliomas in the context of normal brain development. The cell context of oncogene expression may determine the phenotype and biologic aggressiveness of the tumor. Thus, the results of genetic or epigenetic alterations leading to brain tumors may be quite different during the course of CNS development, suggesting that unique treatment strategies may be required.