Glial commitment of mesencephalic neural precursor cells expanded as neurospheres precludes their engagement in niche-dependent dopaminergic neurogenesis

Neural precursor cells (NPCs) with high proliferative potential are commonly expanded in vitro as neurospheres. As a population, neurosphere cells show long-term self-renewal capacity and multipotentiality in vitro. These features have led to the assumption that neurosphere cells represent an expansion of the endogenous NPCs residing within the embryonic and adult brain. If this is the case, in principle, bona-fide expansion of endogenous NPCs should not significantly affect their capacity to respond to their original niche of differentiation. To address this issue, we generated primary neurospheres from the dopaminergic niche of the ventral mesencephalon and then transplanted these cells to their original niche within mesencephalic explant cultures. Primary neurosphere cells showed poor capacity to generate dopaminergic neurons in the mesencephalic niche of dopaminergic neurogenesis. Instead, most primary neurosphere cells showed glial commitment as they differentiated into astrocytes in an exclusively neurogenic niche. Subculture of primary cells demonstrated that the neurosphere assay does not amplify niche-responsive dopaminergic progenitors. Further, neurospheres cells were largely unable to acquire the endogenous positional identity within the Nkx6.1(+), Nkx2.2(+), and Pax7(+) domains of mesencephalic explants. Finally, we demonstrate that our observations are not specific for embryonic mesencephalic cells, as NPCs in the adult subventricular zone also showed an intrinsic fate switch from neuronal to glial potential upon neurosphere amplification. Our data suggest that neurosphere formation does not expand the endogenous neurogenic NPCs but rather promotes amplification of gliogenic precursors that do not respond to niche-derived signals of cellular specification and differentiation.

Growth factor deprivation induces an alternative non-apoptotic death mechanism that is inhibited by Bcl2 in cells derived from neural precursor cells

Although apoptosis has been considered the typical mechanism for physiological cell death, presently alternative mechanisms need to be considered. We previously showed that fibroblast growth factor-2 (FGF2) could act as a survival factor for neural precursor cells. To study the death mechanism activated by the absence of this growth factor, we followed the changes in cell morphology and determined cell viability by staining with several dyes after FGF2 removal from mesencephalic neural-progenitor-cell cultures. The changes observed did not correspond to those associated with apoptosis. After 48 h in the absence of FGF2, cells began to develop vacuoles in their cytoplasm, a phenotype that became very obvious 3-5 days later. Double-membrane vacuoles containing cell debris were observed. Vacuolated cells did not stain with either ethidium bromide or trypan Blue, and did not show chromatin condensations. Nonetheless, during the course of culture, vacuolated cells formed aggregates with highly condensed chromatin and detached from the plate. Neural progenitor cells grown in the presence of FGF2 did not display any of those characteristics. The vacuolated phenotype could be reversed by the addition of FGF2. Typical autophagy inhibitors such as 3-MA and LY294002 inhibited vacuole development, whereas a broad-spectrum caspase inhibitor did not. Interestingly, Bcl-2 overexpression retarded vacuole development. In conclusion, we identified a death autophagy-like mechanism activated by the lack of a specific survival factor that can be inhibited by Bcl2. We propose that anti-apoptotic Bcl2 family members are key molecules controlling death activation independently of the cell degeneration mechanism used.

Neural stem cells in development and regenerative medicine

In the last 10 years, enormous interest in neural stem cells has arisen from both basic and medical points of view. The discovery of neurogenesis in the adult brain has opened our imagination to consider novel strategies for the treatment of neurodegenerative diseases. Characterization of neurogenesis during development plays a fundamental role for the rational design of therapeutic procedures. In the present review, we describe recent progress in the characterization of embryo and adult neural stem cells (NSCs). We emphasize studies directed to determine the in vivo and in vitro differentiation potential of different NSC populations and the influence of the surrounding environment on NSC-specific differentiation. From a different perspective, the fact that NSCs and progenitors continuously proliferate and differentiate in some areas of the adult brain force us to ask how this process can be affected in neurodegenerative diseases. We propose that both abnormal cell death activation and decreased natural neuronal regeneration can contribute to the neuronal loss associated with aging, and perhaps even with that occurring in some neurodegenerative diseases. Furthermore, although NSC activation can be useful to treat neurodegenerative diseases, uncontrolled NSC proliferation, survival, and/or differentiation could cause tumorigenesis in the brain. NSC-mediated therapeutic procedures must take into account this latter possibility.

The in vivo positional identity gene expression code is not preserved in neural stem cells grown in culture

Neural stem cell specification depends on antero-posterior (AP) and dorso-ventral (DV) information provided during development. In the present study we identified similar neural stem cell (NSC) populations along the AP axis of the mouse central nervous system: the 'early' NSCs responsive to fibroblast growth factor-2 and the 'late' NSCs responsive to epidermal growth factor (EGF). Gene expression analysis shows that AP and DV transcription factor code is not preserved in NSCs in culture. Neurospheres generated with EGF from different regions showed Emx2, En2 and Krox20 expression beyond their corresponding AP restricted areas (telencephalon, mesencephalon and rhomboencephalon, respectively). Hox genes were rarely expressed. DV markers such as Pax7 and Dbx1 were not expressed in neurosphere cells, whereas Pax6 and Nkx2.1 were highly expressed independently of the NSC source region. In general, this pattern was found under different culture conditions. We propose that signals surrounding NSCs determine their positional identity gene expression code, which may be relevant to establish their definitive fate.

Expression of E6 and E7 papillomavirus oncogenes in the outer root sheath of hair follicles extends the growth phase and bypasses resting at telogen

Hair follicle growth cycle proceeds through a series of stages in which strict control of cell proliferation, differentiation, and cell death occurs. Transgenic mice expressing human papillomavirus type 16 E6/E7 papillomavirus oncogenes in the outer root sheath (ORS) display a fur phenotype characterized by lower hair density and the ability to regenerate hair much faster than wild-type mice. Regenerating hair follicles of transgenic mice show a longer growth phase (anagen), and although bulb regression (catagen) occurs, rest at telogen was not observed. No abnormalities were detected during the first cycle of hair follicle growth, but by the second cycle, initiation of catagen was delayed, and rest at telogen was again not attained, even in the presence of estradiol, a telogen resting signal. In conclusion, expression of E6/E7 in the ORS delays entrance to catagen and makes cells of the ORS insensitive to telogen resting signals bearing to a continuous hair follicle cycling in transgenic mice.

Basic fibroblast growth factor promotes epidermal growth factor responsiveness and survival of mesencephalic neural precursor cells

Epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) induce proliferation of neural precursor cells from several central nervous system regions in vitro. We have previously described two neural precursor cell populations from 13.5 days postcoitium (dpc) mesencephalon, one forming colonies in response to EGF, present in the ventral mesencephalon, and other forming colonies in response to EGF + bFGF, mainly present in the dorsal mesencephalon. In the present work, we show that 13.5 dpc dorsal mesencephalic cells required bFGF only for 1 h to form colonies in response to EGF alone, indicating that these two growth factors act in sequence rather than simultaneously. Absence of bFGF at the beginning of the culture gave rise to very few colonies, even after the addition of EGF + bFGF, suggesting that cells responsive to bFGF were very labile in the primary culture condition. This result is in contrast with cells pretreated with bFGF, which could survive for up to 5 days in the absence of bFGF or EGF, and then were capable of efficiently forming colonies in response to EGF. Basic FGF was also able to support survival of EGF-responsive neural precursors from both ventral and dorsal mesencephalon. The population requiring bFGF to form colonies in response to EGF was identified at different developmental stages (11.5-15.5 dpc), with higher contribution to the total number of neural precursors cells detected (EGF-responsive plus bFGF-responsive) at early stages and in the dorsal region. We show that the differentiation effect of bFGF resulted in the appearance of the mRNA coding for the EGF receptor. Our data suggest that bFGF-responsive neural precursors are the source of EGF-responsive neural precursors.

Random catecholaminergic differentiation of mesencephalic neural precursors

We investigated how several factors influence the catecholaminergic phenotype establishment from embryonic mesencephalic neural precursors in culture. Using a semiquantitative RT-PCR procedure we found no significant effect of several growth factors or conditioned media on tyrosine hydroxylase (TH) mRNA levels. Nevertheless, neural precursor cells expanded by epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) showed the ability to express TH mRNA. Subcultures of EGF expanded neural precursor cells expressed TH mRNA, but not all individual secondary colonies obtained had this characteristic. Preferential dopaminergic differentiation was observed in our culture conditions. Our results suggest that EGF stimulates the proliferation of neural precursor cells that have the potential but differentiate randomly to catecholaminergic cells.

Epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), and basic fibroblast growth factor (bFGF) differentially influence neural precursor cells of mouse embryonic mesencephalon

Growth factors are key elements in the process of neural cell differentiation. We examined the effects of classical mitogens on neural precursor cells, by culturing mouse cells of the embryonic (13.5 days postcoitum) mesencephalon and treating them with epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and transforming growth factor-beta (TGF-beta). Our initial results show that EGF, TGF-alpha, or bFGF, but not NGF or TGF-beta, induced general proliferation of the cultured cells, followed by formation of colonies. Combinations of these three growth factors suggest that most cells with the capacity to form colonies responded to EGF, TGF-alpha, or bFGF. The number of colonies increased significantly when EGF, but not TGF-alpha, was used in combination with bFGF. Furthermore, a population responding only to EGF + bFGF was detected in the dorsal mesencephalon. The colony-forming activity of bFGF was dependent on insulin, but bFGF and insulin cooperation was indirect since we could not observe colony formation in subcultures of cells derived from colonies, even in the presence of insulin. Cells obtained from our colonies displayed neuronal and glial morphology and expressed markers of both neurons and astrocytes; nestin, a marker of neural precursor cells, was also expressed in the majority of colonies. Growth factors also influenced neuronal maturation; the best neurite outgrowth was obtained from cells derived from bFGF-induced colonies cultured in the presence of EGF + bFGF. These data indicate the existence of neural precursor cells in the embryonic mesencephalon that respond differentially to growth factors.