Role of postsynaptic transmitter receptors in influencing presynaptic neurotransmitter identity

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Retrograde signaling to ensure the security of message transmission over long distances was pioneered by the Greeks and the Chinese in the 3rd century B.C. Signal fires at night and smoke signals by day generated on one tower could be seen by observers on a distant tower, who then lit signal fires or generated smoke signals to acknowledge receipt of the signal. Chains of signal towers reliably transmitted signals over long distances. Inclusion of a mechanism for sending a retrograde acknowledgement signal has been important for the security of modern telecommunication. Most studies of the function of synapses in the brain have focused on the mechanisms involved in the flow of information from the presynaptic cell to the postsynaptic cell. This project tests the hypothesis that retrograde signaling from postsynaptic neurotransmitter receptors stabilizes the expression of the anterograde presynaptic neurotransmitter system in African Clawed Frogs (Xenopus laevis). This project will reveal the role of retrograde information signals in regulating the stability of the signaling system and potentially explain neurological or psychiatric disorders specifically associated with defects in the stability of synaptic signaling. This project will also provide advanced research training for undergraduate students and high school students from groups that are currently underrepresented in science. This project tests the role of postsynaptic transmitters in regulating presynaptic transmitter stability. We will determine whether loss-of-function via focal blockade of endogenous acetylcholine receptors by pancuronium or curare results in gradual loss of choline acetyl-transferase (ChAT) in presynaptic motor neurons (MNs) at the larval Xenopus neuromuscular junction (NMJ). Preliminary results show that this is the case, consistent with destabilization of the endogenous transmitter, acetylcholine. Because these MNs normally express the neurotransmitter gamma-aminobutyric acid (GABA) transiently at earlier stages of development, we will also investigate whether ectopic expression of GABA receptors (GABA A receptors alpha, beta and gamma) in myocytes in a gain-of-function experiment during development leads to stabilization of expression of GABA in presynaptic cholinergic MNs at later stages. Preliminary data show that GABA appears in innervating axon terminals and in cell bodies in the spinal cord and co-expression of the MN markers Hb9, ChAT and Lim3 confirms their primary MN identity. In line with the appearance of glutamic acid decarboxylase 67 (GAD67) and the vesicular GABA transporter (VGAT) in these MNs, GABAergic miniature endplate potentials (mEPPs) are recorded from the myocytes. These results are consistent with stabilization of the previously expressed transmitter, GABA. Investigating the mechanism by which postsynaptic transmitter receptors may regulate presynaptic transmitter stability, we will test the role of molecules thought to be involved in forming transsynaptic bridges (Lrp4 and Lfhpl4), that link postsynaptic receptors to presynaptic stabilizing proteins. Preliminary results knocking down the expression of these molecules recapitulates the loss-of-function experiments and blocks the results of the gain-of-function experiments. Receptor-mediated retrograde signaling at the synapse may be an acknowledgment signal for maintaining transmitter expression. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.