Signaling Pathway Downstream of GABA_A Receptor in the Growth Cone

Abstract

The growth cone is responsible for axonal elongation and pathfinding by responding to various modulators for neurite growth, including neurotransmitters, although the sensor mechanisms are not fully understood. Among neurotransmitters, GABA is most likely to demonstrate activity in vivo because GABA and the GABAA receptor appear even in early stages of CNS development. We investigated the GABAA receptor‐mediated signaling pathway in the growth cone using isolated growth cones (IGCs). Both the GABAA binding site and the benzodiazepine modulatory site were enriched in the growth cone membrane. In the intact IGC, GABA induced picrotoxin‐sensitive Cl− flux (not influx but efflux) and increased the intracellular Ca2+ concentration in a picrotoxin‐ and verapamil‐sensitive manner. Protein kinase C (PKC)‐dependent phosphorylation of two proteins identified as GAP‐43 and MARCKS protein was enhanced in the intact IGC stimulated by GABA, resulting in the release of MARCKS protein and GAP‐43 from the membrane. Collectively, our results suggest the following scheme: activation of the functional GABAA receptor localized in the growth cone membrane → Cl− efflux induction through the GABAA‐associated Cl− channel → Ca2+ influx through an L‐type voltage‐sensitive Ca2+ channel → Ca2+‐dependent phosphorylation of GAP‐43 and MARCKS protein by PKC.

Detection of Structural and Metabolic Changes in Traumatically Injured Hippocampus by Quantitative Differential Proteomics

“…excitatory synaptic transmission; 47 (3) increase AMPA receptor phosphorylation to reduce synaptic depression; 64 and (4) diminish dispersal of GABA receptors from the synapse. 17 This calcineurin compensatory model is consistent with the lack of progression of our TBI model to apoptotic neuronal death, in spite of the massive homeostatic disturbances. It is further supported by the role of cyclosporine A and FK506, which inhibit calcineurins to provide neuroprotection and improve outcomes after TBI.…”

Asymmetric redistribution of GABA receptors during GABA gradient sensing by nerve growth cones analyzed by single quantum dot imaging

“…The asymmetric relocalization was also suppressed by application of the calcium chelator BAPTA-AM (Fig. 2D), emphasizing the role of Ca 2ϩ , a known messenger in GC gradient sensing (23) and in the signaling cascade of GABA (24). These results highlight the role of downstream components of the signaling pathway in the receptor reorganization and points to a cytoskeleton-driven redistribution of GABA A Rs.…”

“…We measured the concentration of intracellular calcium, an important factor in axon pathfinding (23) and in GABA signaling. At early stages of development of the nervous system, activation of GABA A Rs leads to membrane depolarization (28), which may trigger calcium entry through voltage-dependent calcium channels (23,24). Variations of intracellular calcium concentration [Ca 2ϩ ] were detected by measuring the fluorescence ratio F (Fig.…”

GABA Expression Dominates Neuronal Lineage Progression in the Embryonic Rat Neocortex and Facilitates Neurite Outgrowth via GABAAAutoreceptor/Cl−Channels

“…In addition, GABA can both stimulate random motility in CP/SP neurons via GABA B receptor-coupled Ca 2ϩ signals and attenuate their own movement via GABA A receptor/Cl Ϫ channels, as neurons become arranged into primitive layers (Behar et al, 1998). Pharmacological activation of GABA A receptor/Cl Ϫ channels expressed by growth cones fractionated from neurons in the developing cortex increases cytosolic Ca 2ϩ levels via voltage-sensitive L-type Ca 2ϩ channels (Fukura et al, 1996), which can lead to Ca 2ϩ -dependent phosphorylation of specific proteins (Ohbayashi et al, 1998). These latter results suggest a morphogenic role for GABA related to the physiology of neurite outgrowth among embryonic cortical neurons.…”

“…The results of the present study reveal that the autocrine GABAergic circuit operates during the earliest stages of neurite outgrowth before functional synapses form. The intracellular pathway(s) that couples autocrine GABAergic signaling to neurite outgrowth likely involves Ca 2ϩ -dependent activation of (1) protein kinase C with subsequent phosphorylation of GAP-43 and Myristoyl alanine-rich protein kinase C substrate proteins (Fukura et al, 1996) and (2) degradation of spectrin and release of ␣-actinin by calpain (Ohbayashi et al, 1998), which have both been identified as targets of GABAergic signaling in growth cones fractionated from the embryonic and early postnatal rat cortex. The dynamics regarding the complex interplay among the different components at membrane and cytoplasmic levels during neurite outgrowth and how the circuit is developmentally regulated remain to be elucidated.…”