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Function of Phosphatidylinositol 3-Kinase Class III in the Nervous System

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Date
2010
Author
Zhou, Xiang
Advisor
Wang, Fan
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Abstract

Neurons, with their enormous membrane contents, depend heavily on regulated membrane trafficking processes to maintain their morphology and function. The phosphatidylinositol 3-kinase class III, or PIK3C3, plays a critical role in various membrane trafficking processes including both the endocytic and autophagic pathways. The functions of PIK3C3 in the nervous system in vivo are un-characterized. We reasoned that studying PIK3C3 in neurons would provide us an entry point into understanding the regulations and functions of the neuronal membrane trafficking processes and their roles in neuronal morphogenesis and homeostasis.

We generated a conditional allele of Pik3c3 and first deleted it specifically in the peripheral sensory neurons. Mutant large-diameter myelinated sensory neurons accumulated numerous enlarged vacuoles and ubiquitin-positive aggregates and underwent rapid degeneration. By contrast, Pik3c3-deficient small-diameter unmyelinated neurons accumulated excessive numbers of lysosome-like organelles and degenerated slower than large-diameter neurons. These differential degenerative phenotypes are unlikely caused by a disruption of the autophagy pathway, because inhibiting autophagy alone by conditional deletion of Atg7 results in a completely distinct subcellular phenotypes and very slow degenerations of all sensory neurons. More surprisingly, a noncanonical PIK3C3-independent LC3-positive autophagosome formation pathway was activated in Pik3c3-deficient small-diameter neurons. This work uncovered unexpected differences of the endo-lysosomal systems in different types of neurons and discovered a novel autophagy initiation pathway in vivo in neurons.

To examine the role of PIK3C3 in the central nervous system (CNS), we next deleted Pik3c3 in CNS neural progenitor cells using the Nestin-Cre transgenic line. The resulting conditional knockout mice displayed a severe cortical lamination abnormality caused by defective cortical neuron migration. This finding uncovered a previously under-appreciated role of endocytic trafficking in neural migration, which was further confirmed by electron microscopic analyses of the developing cortex. Moreover, overexpressing the dominant negative forms of Dynamin2 or Rab5, two regulators of endocytosis, caused similar migration defects as Pik3c3-deletion. Mechanistically, Pik3c3-deficient cortical neurons drastically reduced surface Reelin binding sites, and showed significantly decreased levels of Dab1 phosphorylation, despite expressing normal total amount of Reelin receptor ApoER2. This work suggests endocytosis and recycling of Reelin receptors are likely to play an important role in cortical migration regulated by the Reelin signaling pathway.

These studies represent the first in vivo characterization of PIK3C3 functions in mammals, and provide insight into the complexity and functional importance of neuronal endo-lysosomal and autophagic pathways.

Type
Dissertation
Department
Neurobiology
Subject
Neurobiology
Biology, Cell
Biology, Genetics
Autophagy
Endocytic pathway
Neurodegeneration
Neuronal migration
PIK3C3
Vps34
Permalink
https://hdl.handle.net/10161/3008
Citation
Zhou, Xiang (2010). Function of Phosphatidylinositol 3-Kinase Class III in the Nervous System. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/3008.
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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.

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