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.