Determining the role of GABAergic signaling in the craniofacial development of larval zebrafish
Beebe, Lindsey Lee
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Although best known as an inhibitory neurotransmitter, intriguing evidence has implicated GABA as a key signaling molecule in craniofacial development in mammals. Glutamate is converted to GABA by an enzyme called glutamic acid decarboxylase (GAD), which exists in two isoforms, GAD67 and GAD65. The GAD1 and GAD2 genes encode these isoforms, respectively. A decrease in GAD activity in the human brain is often associated with epilepsy, schizophrenia and related neurological disorders. In mice and humans, mutations in gad1, but not gad2, result in defects in palate development, and mutations in the Gabrb3 gene, which encodes the β3 subunit of the GABAA receptor, exhibit a comparable phenotype to gad1 mutations. These results suggest that GABA signaling, through the GABAA receptor, can play an important and conserved role in craniofacial development. However, the mechanism of this process is not known and cannot be easily investigated in a mammalian system. In this work, translation-blocking morpholinos against the GAD genes were used to alter expression within the larval zebrafish. While gad2 morphants looked phenotypically normal, gad1 morphant animals exhibited altered cranial structures at 1 and 7 dpf. Yet, both gad1 and gad2 morphants exhibited spontaneous seizure-like neural activity. Through the use of photoactivatable caged-morpholinos, the craniofacial deformities could be bypassed when photolysis was carried out at 24 hpf. Electrophysiological recordings showed that while dark-raised CyHQ-gad1 morphant animals looked phenotypically comparable to wild-type animals, they exhibited abnormal, seizure-like neural activity. These findings support the idea that gad1 exhibits a novel function in craniofacial development, independent of its activity in GABA synthesis.