The importance of neurotrophin signalling for the functional properties of sensory neurons in the adult peripheral nervous system

How the phenotype of nerve cells is sculptured during development, maintained in adulthood and during normal aging is an area of general neurobiological interest. Knowledge about the signalling pathways that regulate cell survival and maintenance of the neuronal phenotype throughout adult life are therefore important. Growth factors, particularly neurotrophins are an important in this process. Neurotrophins are a group of proteins that are best known for their role in embryonic and early postnatal development when they affect the survival and function of many nerve cells. Different neurotrophins affect different nerve fibres. For example lack of nerve growth factor or its cognate tyrosine kinase receptor TrkA results in a loss of nociceptors, peripheral nerve cells that detect tissue damaging stimuli and signal pain. Mutant animals are insensitive to pain and temperature stimuli. These research findings have subsequently led to the recognition that mutations of genes encoding NGF or TrkA in humans result in congenital analgesia - the absence of pain throughout life. Other neurotrophins, like brain derived neurotrophic factor or neurotrophin 3 support another type of nerve fibres, so called slowly adapting mechanoreceptors and Merkel cells, that have a crucial function in tactile exploratory functions such as localisation of stimuli on the body, detection of the roughness of objects and reading of Braille. The proposed investigations aim to understand the function of neurotrophins in the adult nervous system. Neurotrophins continue to be produced throughout adult life and neurotrophin receptors are found in many nerve cells of the adult. Because of the lack of specific drugs that antagonize or augment the function of neurotrophins it has been difficult to carry out investigations in postnatal animals. Work on transgenic animals has not been possible as mice with the inactivation of genes encoding neurotrophins or their receptors die shortly after birth. However, the availability of novel mutant mice in which the action of neurotrophins or their receptors can be selectively interrupted in adulthood provide a new and exciting window of opportunity to address these question. Given the profound effects of neurotrophin signalling in the embryo, it is expected that neurotrophins continue to exert strong physiological effects in adults. We will use electrophysiological and morphological techniques to examine two well defined systems of sensory nerve fibres, namely cold sensitive thermoreceptors and slowly adapting mechanoreceptors connected to Merkel cells. The results will bear on the understanding how the function of peripheral nerve fibres is maintained throughout life and ageing. It will also provide information whether lack of neurotrophin signalling in adult cause peripheral neuropathies, a common disease in animals and humans. It will provide information of the mechanisms of thermosensation, the afferent signals that control body temperatures and on the nature of tactile sensations.

The proposed investigations will examine the role of three neurotrophins (NGF, BDNF, NT3) and signalling through their cognate high-affinity Trk (A, B and C, respectively) for the function of peripheral sensory neurons in the adult organism. We will use cold sensitive thermoreceptors (NGF-TrkA) and slowly adapting mechanoreceptors (NT3, BDNF, TrkC, TrkB) with their associated Merkel cells (NT3, TrkC) as representatives of unmyelinated and myelinated fibre systems. We will use a genetic approach to interrupt neurotrophin-Trk signalling in mice. One line of experiments will use knockin mice in which a single trk receptor can be specifically blocked for selected periods of time by systemic or local application of synthetic blockers. Another line of experiments will use an inducible cre-loxP strategy to inactivate NT3 or BDNF genes in a temporally and spatially controlled manner in the adult. We will use a skin nerve in vitro preparation to comprehensibly examine the functional consequences of these experimental interventions. Structural changes of primary afferents will be described using fluorescent immunohistochemistry in combination with confocal microscopy of the receptive nerve terminals and Merkels cells in the skin. In addition light microscopy of the the peripheral nerves in plastic sections will aid in the quantitative examination of possible neurodegenerative changes.