The JBS-associated E3 Ligase UBR-1 Maintains Synaptic Glutamate Homeostasis and Regulates Neurotransmitter Balance

Johanson-Blizzard Syndrome (JBS) is a rare, sometimes fatal autosomal recessive multi-systemic congenital genetic disorder, characterized by pancreatic exocrine insufficiency and mental retardation. JBS results from mutations in the gene UBR1, an E3 ubiquitin ligase that specifically targets proteins for proteasomal degradation. There are multiple UBR1 homologs in humans; however, mutations in UBR1 alone lead to JBS. Although it is known that loss-of-function mutations in UBR1 cause JBS, the cellular and molecular mechanisms through which UBR1 underlie normal or pathological development remain elusive. In this study, I describe the identification and characterization of UBR-1, the sole UBR1 homologue in the nematode Caenorhabditis elegans (C. elegans). In the first data chapter, I show that UBR-1 functions through a simple premotor interneuron circuit to regulate body bending during backward locomotion. I find that the absence of UBR-1 disrupts glutamate homeostasis, and further identified three components of a UBR-1-regulated signaling pathway - a glutamate metabolic enzyme GOT/GOT-1.2, a vesicular glutamate transporter VGlut3/EAT-4, and a glutamate-gated chloride channel GluCl/AVR-15 - all involved in glutamatergic synaptic transmission in this premotor circuit. These findings suggest that UBR-1 maintains synaptic glutamate homeostasis in this premotor circuit to control locomotion. In the second data chapter, I demonstrate a role of UBR-1-mediated glutamate homeostasis in GABAergic neurons. A simple GABAergic motor circuit controls C. elegans defecation. ubr-1 mutants exhibit defecation defects, coinciding with reduced activity of the GABAergic motor neurons, as well as reduced GABA signaling. I show that the reduced neuronal activity is augmented by removing the glutamate metabolic enzyme GOT-1, whereas reduced GABA signaling is rescued by removing the vesicular glutamate transporter EAT-4, both function in the GABAergic motor neurons. These findings implicate a requirement of UBR-1 in maintaining the balance between inhibitory (GABA) and excitatory (glutamate) neurotransmitters, and of glutamate homeostasis in regulating the activity of GABAergic neurons. Finally, I propose that dysregulation of glutamate homeostasis mediated by the functional loss of UBR1 may underlie systemic and neurodevelopmental defects in JBS patients.

Department dc:contributor.department

Medical Science

Year dc:date.issued

2015

Author dc:creator

• Chitturi, Jyothsna

Advisor dc:contributor.advisor

• Zhen, Mei

• C. elegans
• GOT-1
• JBS
• UBR-1
• VGLUT3

Handle dc:identifier.uri

http://hdl.handle.net/1807/69318

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/69318