The Endocytic Protein FCHO-1 is Essential in Preserving the Structural and Functional Integrity of Neurons

Abstract

Neurons are vital for animal physiology and behavior, demanding lifelong structural and functional integrity. This integrity is upheld by intricate endocytic pathways that sustain a multitude of critical neuronal features, such as cell polarity, circuit formation and maintenance, and rapid neurotransmission between neurons and their target cells. However, the precise convergence of these diverse pathways, comprising both unique and shared endocytic proteins, to collectively support the overall health of neurons remains elusive. In this thesis, I investigated the role of FCHO-1, a key endocytic protein, in C. elegans neurons. My studies revealed a remarkable and unexpected facet of FCHO-1, involving the preservation of neuronal structure and function. I showed that fcho-1 null mutant worms exhibited only moderate reductions in synaptic transmission strength and slightly diminished synaptic vesicle abundance — a surprising revelation. However, using electron microscope analysis, I found a distinct phenomenon: numerous abnormal “breaks” on the plasma membranes between neurons, suggestive of compromised neuronal boundaries. To delve deeper into this finding, I introduced a novel PAGEN assay, a fluorescence imaging method enabling quantification of cytoplasmic content exchange among live neurons. Using this method, I demonstrated that compromised neuronal membrane contacts in fcho-1 mutant worms allow for an elevated occurrence of abnormal cytoplasmic content exchange among neurons. Furthermore, I found that accumulation of adhesion molecule SYG-1 is increased in fcho-1 mutant neurons, comparing to wild type neurons. Collectively, these findings reveal FCHO-1’s critical and previously recognized role in maintaining neuron boundaries. This likely occurs through influencing the abundance of adhesion molecules on neuronal membranes, thereby potentially impacting the rigidity and stability of neuron-neuron contacts. In summary, my research uncovers an unexpected facet of the FCHO-1-dependent endocytic pathway — its pivotal role in preserving neuronal structural integrity.