It has been recently discovered that an adhesion G-protein-coupled receptor, latrophilin-1, interacts strongly and specifically with a single-pass transmembrane protein, called Lasso (latrophilin-1–associated synaptic surface organiser) or teneurin-2. Both proteins are expressed early in development, both are predominantly neuronal, and their knockouts result in embryonic lethality or nervous system-related defects. However, the biological role of their interaction is unknown.
The thesis focuses on the functions of Lasso and latrophilin-1 in neuronal development, using dissociated hippocampal cell culture as a model of developing neurones. Immunocytochemistry and Western blotting demonstrate that latrophilin-1 resides primarily in axons, growth cones and presynaptic terminals, while Lasso largely localises in the postsynaptic membrane, indicating that in synapses the two proteins form a trans-synaptic pair. Surprisingly, Western Blotting has demonstrated that Lasso is also partially cleaved and released into the extracellular space.
This released Lasso fragment binds to latrophilin-1-expressing cells, suggesting that it could affect axonal guidance by interaction with latrophilin-1 in growth cones. Using microfluidic chambers, a novel paradigm of quantitative assessment of axonal guidance in response to chemoattractant gradient has been developed and characterised. This was employed to demonstrate that soluble Lasso causes axon attraction and fasciculation.
Once the growth cone reaches its Lasso-secreting target, latrophilin-1 binds the membrane-anchored Lasso, forming an inter-cellular link at the site of a future synapse. To elucidate the role of this interaction in synapse formation and/or activity, the latrophilin-1–Lasso complex in developing neuronal cultures was perturbed by the addition of an excess of the soluble latrophilin fragment. The resulting loss by latrophilin-1 of its endogenous ligand inhibits synaptic response to depolarising stimuli.
These observations show that the released Lasso fragment may participate in axonal at- traction and membrane-anchored Lasso may play a role in transmitter exocytosis through its trans-synaptic complex with latrophilin-1. Hypothetical mechanisms and biological relevance of these processes in vivo are proposed.