Formation of specific nerve connections during development requires the guidance of the growing tip of elongating axons, the growth cone, to its correct target cells. Growth cones respond to a variety of attractive or repulsive guidance molecules present in the environment that are either diffusible or surface-bound by turning towards or away, respectively. The cellular processes by which a growth cone detects different diffusible cues and alters its direction of extension remain to be elucidated. In this study, we aim to understand how a growth cone detects the extracellular gradients of different molecules, relays the directional information intracellularly, and makes appropriate changes in its direction of extension. Our working hypothesis, is as follows: gradients of activation of cell-specific receptors for different diffusible cues, mediated by different second messengers and kinases, all lead to a cytoplasmic gradient in the activation of a common set of proteins in the growth cone. The activated proteins in turn regulate cytoskeletal dynamics at the growth cone asymmetrically to result in an asymmetric modulation of growth cone motility, leading to the turning response. Using neuronal cultures prepared from Xenopus embryos, we will test our hypothesis. Specifically we will determine: (1) the precise role of second messengers Ca and cAMP in encoding the directional information to direct the growth cone extension; (2) the sequence of asymmetric cellular events associated with growth cone motility that are common and essential for turning of growth cones induced by different attractants and repellents; (3) the asymmetric cytoskeletal and membranous events associated with the turning of the growth cone. The scheme underlying these studies is the common mechanisms underlying the turning of growth cones induced by different diffusible molecules. Elucidation of cellular and molecular mechanisms involved in growth cone guidance is critical for our understanding of the development of the nervous systems in normal and pathological conditions. Furthermore, the results from this study will provide important insights on the missing link between various extracellular cues and the regulated nerve growth.
