Biochemical purification of a mammalian slit protein as a positive regulator of sensory axon elongation and branching

Many neurons in both vertebrates and invertebrates innervate multiple targe ts by sprouting secondary axon collaterals (or branches) from a primary axo n shaft. To begin to identify molecular regulators of axon branch initiatio n or extension, we studied the growth of single sensory axons in an in vitr o collagen assay system and identified an activity in extracts of embryonic spinal cord and of postnatal and adult brain that promotes the elongation and formation of extensive branches by these axons. Biochemical purificatio n of the activity from calf brain extracts led to the identification of an amino-terminal fragment of Slit2 as the main active component and to the di scovery of a distinct activity that potentiates its effects. These results indicate that Slit proteins may function as positive regulators of axon col lateral formation during the establishment or remodeling of neural circuits .