Investigating Mechanisms of Dendritic Pruning in Caenorhabditis elegans Inner Labia 2 Sensory Neurons

Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Cell Biology

First Advisor

Yogev, Shaul

Abstract

During development, the nervous system undergoes vast remodeling events to allow a mature and functional nervous system. These remodeling events can be initiated by developmental programs, activity-dependent mechanisms, or environmental stress. Dysregulated neuronal remodeling may lead to neurological and neurodevelopmental disorders, such as schizophrenia or Down Syndrome, respectively. Notably, neuronal remodeling is not limited to development: in mature nervous systems, environmental stressors such as acute stress can induce reversible atrophy and debranching, highlighting the dynamic capacity of neurons to adapt their morphology in response to environmental cues. Therefore, understanding the molecular mechanisms that govern neuronal remodeling, under developmental programs and in response to environmental stimuli, is critical for elucidating how neurological pathologies arise. In this work, I adapted the nematode Caenorhabditis elegans inner labia 2 (IL2) sensory neurons as a model system to study developmentally-regulated and stress-induced neuronal remodeling. C. elegans enter a reversible, stress-induced developmental diapause known as dauer in response to unfavorable conditions, during which dorsal and ventral IL2 (IL2Q) neurons elaborate complex dendritic arbors that are subsequently pruned upon reproductive development resumption. In contrast to existing model systems such as Drosophila, where remodeling events are permanent, IL2Q neuron remodeling events are reversible, as they are born and morphologically mature during embryogenesis and their remodeling occurs in a differentiated nervous system. This allows the interrogation of the mechanisms that initiate, regulate, and promote dendritic pruning in a controlled manner. I employed a robust method for dauer induction and recovery using temperature-sensitive Daf-c mutants, facilitating synchronized populations for a visual forward genetic screen. Through the screen, I isolated nine mutants with defective IL2Q dendrite pruning, including the novel shy87 allele of sax-1, which encodes a conserved serine/threonine nuclear Dbf2-related (NDR) kinase. Loss of SAX-1 selectively impairs pruning of secondary and tertiary dendrites while sparing quaternary branches. Additionally, SAX-1-directed pruning is cell-autonomous and kinase activity-dependent. In contrast to its reported role of antagonizing neurite outgrowth, my findings suggest a novel role for NDR kinases in directing neurite elimination. How these two functions are coordinated remains to be studied. In this study, I found that SAX-1 works in conjunction with its conserved protein interactors, the large scaffolding protein SAX-2/Furry and the NDR co-activator MOB-2, and genetically interacts with RABI-1/Rabin8, a Rab8 guanine-nucleotide exchange factor (GEF), and RAB-11.2 to regulate branch-specific elimination. Interestingly, RABI-1 and RAB-11.2 only mediate the elimination of secondary branches with SAX-1, but their effect on tertiary branches is minimal. Consistent with the known roles of RABI-1 and RAB-11.2 in regulating membrane dynamics, I found that sax-1 mutants show reduced endocytic events and altered SAX-2 distribution during IL2Q remodeling, suggesting that branch-specific remodeling is coordinated by genetically-encoded programs that regulate membrane trafficking. In summary, these findings provide the first characterization of genetic regulators governing IL2Q dendrite pruning in C. elegans. I identified SAX-1/NDR kinase signaling as a central regulator of IL2Q pruning, coordinating distinct genetic programs for dendrite branch-specific elimination. My work suggests a model that serves as a foundation to interrogate how environmental conditions and physiological cues direct selective dendrite elimination. Furthermore, this work underscores the utility of IL2Q neurons as a model to investigate conserved mechanisms of structural plasticity. Insights gained from this system may inform how neuronal remodeling takes place in higher organisms during chronic stress or pathological conditions.

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