Sonic hedgehog (Shh)/Gli modulates the spatial organization of neuroepithelial cell proliferation in the developing chick optic tectum

Abstract

BackgroundSonic hedgehog (Shh)/Gli pathway plays an important regulatory role on the neuroepithelial cells (NEc) proliferation in the dorsal regions of the developing vertebrate Central Nervous System. The aim of this paper was to analyze the effect of the Shh/Gli signaling pathway activation on the proliferation dynamics and/or the spatial organization of the NEc proliferation activity during early stages of the developing chick optic tectum (OT). In ovo pharmacological gain and loss of hedgehog function approaches were complemented with in vivo electroporation experiments in order to create ectopic sources of either Shh or Gli activator (GliA) proteins in the OT. NEc proliferating activity was analyzed at ED 4/4.5 by recording the spatial co-ordinates of the entire population of mitotic NEc (mNEc) located along OT dorsal-ventral sections. Several space signals (numerical sequences) were derived from the mNEc spatial co-ordinate records and analyzed by different standardized non-linear methods of signal analysis.ResultsIn ovo pharmacologic treatment with cyclopamine resulted in dramatic failure in the OT expansion while the agonist purmorphamine produced the opposite result, a huge expansion of the OT vesicle. Besides, GliA and Shh misexpressions interfere with the formation of the intertectal fissure located along the dorsal midline. This morphogenetic alteration is accompanied by an increase in the mNEc density. There is a gradient in the response of NEcs to Shh and GliA: the increase in mNEc density is maximal near the dorsal regions and decrease towards the OT-tegmental boundary. Biomathematical analyses of the signals derived from the mNEc records show that both Shh and GliA electroporations change the proliferation dynamics and the spatial organization of the mNEc as revealed by the changes in the scaling index estimated by these methods.ConclusionsThe present results show that the Shh/Gli signaling pathway plays a critical role in the OT expansion and modelling. This effect is probably mediated by a differential mitogenic effect that increases the NEc proliferation and modulates the spatial organization of the NEc proliferation activity.

Proliferation of Murine Midbrain Neural Stem Cells Depends upon an Endogenous Sonic Hedgehog (Shh) Source

“…In summary, to our knowledge, this is the first report giving conclusive evidence of a mechanism by which canonical Shh maintains the self-renewal capacity of the NSC population in the tectum, and it is evolutionary conserved as we demonstrated previously in chicken and zebrafish models [32], [33]. More importantly, since we identify an endogenous source of Shh in tectal NSC we suggest that Shh is acting as a niche factor, favoring the microenvironment by potentiating the response of other growth factors such as EGF and FGF-2.…”

Molecular development of fibular reduction in birds and its evolution from dinosaurs

“…To assess the role of IHH signaling in the differentiation of the distal fibula, we treated chicken embryos in ovo with the hedgehog inhibitor Cyclopamine at a late, postmorphogenetic stage just before the onset of IHH production in the zeugopod (HH27, Rapacioli et al. ). This is long after the early role of Sonic Hedgehog (SHH) in patterning the early limb bud (Towers et al.…”

The developing optic tectum: An asymmetrically organized system and the need for a redefinition of the notion of sensitive period

“…+: indicate statistically significant differences (p < 0.01) between GliA and Shh. Modified from Rapacioli et al (2012a). C. Shh and GliA electroporation differentially affect the midbrain morphogenesis.…”

“…Scale bar: 50 μm. Modified from Rapacioli et al (2012a). D. Series of images illustrating the evolution of a Sensitive Developmental Stage in an isotropic and an anisotropic developing system.…”

“…The Shh/Gli pathway establishes the patterning in the ventral neural tube (Jacob and Briscoe, 2003; Cayuso et al, 2006; Cohen et al, 2014) and regulates cell proliferation, cell survival (Bayly et al, 2007) in the developing dorsal midbrain (Agarwala et al, 2001) and a variety of other tissues (Marti et al, 1995; Dahmane et al, 2001; Ingham & McMahon, 2001; Ruiz i Altaba et al, 2002, 2003; Lai et al, 2003). It was demonstrated that the Shh/Gli pathway modulates the mNSc organization along the chick OT d–v axis (Rapacioli et al, 2012a). The Shh/Gli pathway stimulation produces a global increase in the mNSc density.…”