Karlstrom Lab Research: Pituitary Patterning
Mutants affecting forebrain/pituitary development
We are using a variety of genetic and experimental approaches to
determine
how Hedgehog/Gli (Hh/Gli)mediated cell-cell signaling induces and
patterns the zebrafish forebrain. We are analyzing
several
mutations that affect both Hh/Gli signaling and cell differentiation
in the forebrain. We showed that the detour (dtr) and you-too (yot) loci encode zebrafish Gli1 and
Gli2 (respectively),
hedgehog
responsive transcription factors that are required for formation of a
wide variety of embryonic tissues, including the ventral
forebrain. Mutations in gli2 result in
forebrain
and axon guidance defects. Forebrain defects in dtr(gli1) and yot(gli2) include
mis-specification of ventral diencephalic cell types, specific
loss of of a subset of endocrine secreting cells in the pituitary
gland, and in yot(gli2) the
transdifferentiation
of the pituitary into an ectopic lens (Fig.1).
Fig. 1. Forebrain and axon defects in yout-too
mutants.
Ventral (facial) views of wildtype (left) and yot(gli2) mutant zebrafish
embryos labeled with the ZN-5 antibody to visualize retinal ganglion
cells
and their axons. An ectopic lens often develops at the ventral
midline in
place
of the anterior pituitary (center of right picture, between eyes).
Pituitary Induction and Patterning
The zebrafish is a relatively untapped resource in the study of
endocrine development, and has profound advantages for research into
pituitary induction, patterning and cellular differentiation during
embryogenesis. The zebrafish is genetically tractable, and the physical
and optical accessibility of the embryo makes it possible to observe
the earliest events in pituitary formation. These events occur at
embryonic stages that are extremely difficult to monitor in mammals.
Importantly, pituitary structure and cell-types are remarkably
conserved from fish to humans, making studies in zebrafish relevant to
human development.
Based on our analysis of a series of zebrafish mutations that affect
forebrain patterning and axon guidance (Karlstrom et al, 1996), the
small protein Hedgehog (Hh) has emerged as a critical player in the
early induction of the pituitary placode, as well as in the functional
patterning of the adenohypophysis and the differentiation of a subset
of endocrine cell types (Sbrogna et al, 2003). This role for Hh
is conserved across vertebrates, with defects in human Hh signaling
leading to common congenital defects including holoprosencephaly.
Further, a recent report links aberrant Hh signaling to common
pituitary adenomas in adults. Thus an understanding of how Hh guides
pituitary development is a key issue for human health, and the
zebrafish provides a window into the molecular and cellular mechanisms
involved.
Fig. 2. Conserved organization of the adult
vertebrate pituitary. Left two panels show the location of the
pituitary gland in humans and fish (arrows). The right two panels
show conserved structure within the pituitary (Adapted from Liem et
al). In both species, the adenohypophysis is divided into two
lobes (purple and pink) along the anterior-posterior axis. In
fish, the neurohypophysis (light blue) is positioned dorsally rather
than posteriorly. Within the adenohypophysis, endocrine cell
positions are largely conserved, with PRL and AcTH cells being anterior
(green), GH and TSH secreting cells being medial (yellow), and MSH
secreting cells being posterior (blue).
Fig. 3. shh and A-P patterning. (A) lim3 and
(B) somatolactin expression mark the developing adenohypophysis in 36
hour embryos. (C) nk2.2 is a Hh responsive transcription factor
and is expressed near the source of Hh in the diencephalon. (D)
prolactin cells differentiate in this anterior domain. (E) Model
showing expression of Hh responsive transcription factors and endocrine
cell types in relation to the source of Hh (adapted from Sbrogna
et al., 2003).
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