Doctor of Philosophy

dissertationThe cellular processes that govern neuronal function are highly complex and tightly regulated in order to perform the elaborate information processing achieved by the brain. This is particularly evident in the trafficking of membrane proteins to and from synapses, which can travel long distances away from the cell body. Regulation of neurotransmitter receptors such as the AMPA-type glutamate receptor (AMPAR), the major excitatory neurotransmitter receptor in the brain, is a crucial mechanism for the modulation of synaptic transmission. Yet, the mechanisms by which AMPARs are transported over long distances are still unclear. We have addressed this question through genetic, cell biological and electrophysiological analysis of the C. elegans AMPAR GLR-1. This dissertation describes the role of long-range transport of AMPARs in the regulation of synaptic strength and provides insights into the cellular mechanisms underlying learning and memory. The pair of interneurons AVA expresses GLR-1 and are part of a welldefined circuit regulating the forward and backward movement of C. elegans in response to sensory inputs. To determine the mechanism for GLR-1 delivery to a synapse, we monitored the real-time trafficking of a fluorescently tagged GLR-1 chimera in AVA. We show that UNC-116, the C. elegans homolog of the vertebrate kinesin-1 (KIF5), is responsible for mediating the rapid, bidirectional transport of GLR-1. This motor-driven transport of GLR-1 modifies synaptic strength by mediating the rapid delivery, removal and redistribution of synaptic AMPARs. In the absence of unc-116, we found that although homomeric GLR-1 AMPARs can still diffuse to and accumulate at proximal synapses, glutamategated currents are decreased due to lack of heteromeric GLR-1/GLR-2-containing AMPARs. Furthermore, we show that transient expression of UNC- 116 can rescue defective glutamatergic signaling in adult unc-116 mutants, demonstrating that motor-dependent transport is ongoing in the adult nervous system and is involved in the regulation of synaptic strength. These data have allowed us to establish a link between motor-dependent transport of AMPARs and the strength of synaptic transmission

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease.

Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease

Wnt signaling regulates acetylcholine receptor translocation and synaptic plasticity in the adult nervous system

The adult nervous system is plastic, allowing us to learn, remember, and forget. Experience-dependent plasticity occurs at synapses--the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt-signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In Caenorhabditis elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/alpha7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/alpha7 to synapses in Wnt-signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/alpha7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors