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Projects
Ret is a transmembrane tyrosine kinase that is
critical for kidney development, and for formation of the
sympathetic, parasympathetic, and enteric nervous system (ENS) as
well as for some aspects of sensory neuron development.
Inactivating Ret mutations in mice and humans cause a failure of
ENS precursor proliferation that leads to intestinal
aganglionosis. The absence of ganglion cells in the bowel causes
intestinal obstruction that can be fatal. To perform it normal
function during development, Ret acts as a growth factor receptor
for four ligands (Glial cell line-derived neurotrophic factor
(GDNF), Neurturin (NRTN), Artemin (ARTN) and Persephin (PSPN).
For these ligands to activate Ret, a
glycosylphosphatidylinositol-linked co-receptor (GFRalpha1,
GFRalpha2, GFRalpha3, or GFRalpha4) must also be present on the
cell surface. Over the past few years, we have demonstrated that
different Ret ligands and co-receptors appear to play distinct
roles in ENS development. GDNF signaling via the GFRalpha1/Ret
complex is critical early in development for neural crest
precursor proliferation and survival. Neurturin signaling via
GFRalpha2 provides trophic support for the majority of enteric
neurons in the mature mouse. Artemin does not appear to play a
significant role in supporting intrinsic innervation to the gut,
but is critical for extrinsic sympathetic innervation to the gut.
Thus the interplay between these Ret ligands controls enteric
neuron cell number, cell size and neuronal fiber density within
the ENS. Changes in the expression patterns for these factors
causes alterations in both ENS anatomy and intestinal function.
Ongoing work in the lab to create and analyze novel mouse models
with altered Ret signaling will more definitively determine how
the ENS may be shaped by specific alterations in Ret signaling
components.
Figure Legend: Whole mount acetylcholinesterase staining of the
bowel shows many of the neurons and neuronal fibers within the
ENS. This method is a much more sensitive way to detect changes
in ENS structure than evaluation of tissue cross-sections.
Comparing (A) to (B) shows the reduced neuronal fiber density in
Neurturin deficient mice (B) compared to wild type animals (A).
In contrast to the relatively mild effect of Neurturin deficiency,
GDNF, GFRalpha1 and Ret deficient animals have more severe
intestinal aganglionosis (Compare (C) to (D)). The aganglionosis
in these animals extends from the proximal duodenum to the end of
the gastrointestinal tract.
Vitamin A metabolites (retinoids) control many
aspects of mammalian development by activating the RAR and RXR
retinoid receptors. These receptors are transcription factors
that regulate gene expression. For many years retinoids have been
known to influence the development of undifferentiated neuronal
precursor cells. More recently it has become clear that retinoids
also influence the expression of Ret and Ret signaling components.
There are ongoing studies in the laboratory to examine the
effects of retinoid deficiency and excess on enteric nervous
system development. These experiments include examining the
effects of retinoids on ENS precursors in primary culture and in
developing mice.
While great progress has been made over the
past few years in identifying molecules that control specific
aspects of enteric nervous system development, there are still
many unanswered questions. One of the primary questions that we
are investigating is the specification of neuronal subsets within
the ENS. This includes identifying molecular markers that specify
enteric neurons within different regions of the gut, and distinct
subpopulations of neurons within a single region of the gut. To
pursue this project we are using modern molecular methods (gene
chip, EST analysis, directed RT-PCR) to identify novel markers of
specific neuronal subpopulations within the ENS. This project has
already led to the identification of a transcription factor whose
expression is largely restricted to colonic enteric neurons during
development and in the mature mouse. This opens new avenues for
investigating how the distinctly different plexi of neurons within
the colon and small bowel are created.
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SCHOOL OF MEDICINE
