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Fig. 7. Activation of the Shh pathway in dorsal progenitors in the chick neural
tube elicits cell clustering. (A,B) Transfection of Shh (green) elicits
widespread, relatively uniform activation of Nkx2.2 (red) and repression of
Pax7 (red) cell autonomously and non-autonomously. (C) GFP-only transfections
have no effect on endogenous Pax7 or Nkx2.2 (not shown) expression. (D)
Schematic illustrating broad activation of the Shh pathway (indicated by
Nkx2.2 expression) in transfected (via autocrine signaling) and untransfected
(via paracrine signaling) cells. Transfected cell is indicated by a lightening
bolt. (E,F) Cotransfection of Shh with ptc
loop2 (green) induces Nkx2.2
expression (red) only in untransfected cells. In this case Nkx2.2+ cells in
the dorsal (F) but not ventral (F') spinal cord segregate into clusters.
(G) Expression of Pax7 (red) in co-transfected cells (yellow) indicates
autocrine Shh signaling is blocked. (H) Schematic representation of signaling
differences between untransfected and Shh + ptcloop2 co-transfected
cells. Only paracrine Shh signaling occurs. (I-K) Cells transfected with
Gli2N-term or Gli3N-term (green) activators form clusters in the
dorsal spinal cord (J) that do not express Pax7 (red), while ventral cells do
not cluster (not shown). (L) Schematic showing cell-autonomous,
ligand-independent activation of the Shh pathway by transfections of
dominant-activator Gli2 and Gli3 proteins. Untransfected cells retain their
dorsal identity. (M,N) Schematic summarizing results of chick transfection
studies and Shh/Gli pathway in ventral spinal cord progenitor cells. Shh
pathway activation is indicated by blue `+' areas. Blue outlined areas in the
dorsal region represent clusters of cells, while circles indicate individual
cells. In M, arrow thickness indicates relative roles of Gli2 and Gli3
activators in inducing ventral target genes.
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