Neuro-muscular innervation in neonatal intestinal development

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We look at the development of the neuromuscular layer of the newborn mammalian intestine, which will lengthen many-fold before reaching its adult size. This involves extensive growth of the smooth muscle cells to create the adult form. However, the neurons of the enteric nervous system (ENS), which are all present at birth, must coordinate with this and develop the adult pattern of innervation, as the ENS will eventually regulate the timely contraction and relaxation of the smooth muscle that allows normal digestion. Our goal is to understand how neurons at birth stay alive and adapt to the rapid growth of their target cells, creating the complex, stable relationship seen in the adult.****Current support showed us that a single factor (GDNF; a neurotrophin) is responsible for survival of the ENS neurons after birth, and that GDNF is made by the intestinal smooth muscle cells. Critically, growth of these cells is required for increased GDNF expression that leads to neuron survival and axon outgrowth. We propose that the binding of GDNF to its receptor, RET, causes two outcomes: initially, survival and then axonal extension, and it may be that different levels of GDNF are required for each. We have the cellular and molecular techniques needed to address this, and a unique tissue culture model involving both neonatal intestinal neurons and smooth muscle cells. We will separate the outcomes of GDNF signalling with pharmacological inhibitors that block distinct pathways in the neurons (the only cells expressing GDNF receptors) , and then measure cellular outcomes of survival or the degree of axonal outgrowth. We will then examine the requirements of neurons for GDNF when growth is complete - do very low levels maintain survival? How is this signalling achieved?****Axonal outgrowth seems absent in the adult intestine, but when it arises to facilitate repair, the neurons do not function well, and this is seen elsewhere with nerve damage. We now think that axon outgrowth and nerve function in the neonatal ENS (ie, action potentials and neurotransmitter release) tend not to occur at the same time. Therefore, the structural programme becomes complete as the functional programme gains ascendancy, with synthesis, storage and release of neurotransmitters. Quantitative measurement of the steps involved, such as mRNA and protein, will measure key proteins that belong in either one of these programmes, and this will be correlated with the ability of the neurons to display mature signalling properties. This will help us to understand how the intestine develops in the neonate, a field where little is known.****I will work with trainees, who will mainly be graduate students as well as senior undergraduates. They will gain both technical expertise as well as education in scientific thinking and critical evaluation. To date, our graduates are highly successful, with significant publications in this field, and very good career achievements.**********