Synapses are the functional connections between neurons and their targets.Synapse degeneration or dysfunction underlie several neurodevelopmental and neurodegenerative disorders ranging from autism to Alzheimer’s disease, making the study of synapse formation and maintenance highly relevant. Various types of synapses exist within the human nervous system, including en passant synapses, which form along the axon, and terminal synapses, which form at the ends of axons. The nervous system of the nematode C. elegans also forms en passant and terminal synapses, which when coupled with a transparent body, a known connectome and ease of genetic manipulation, makes it an ideal model for the study of synaptogenesis. In the first part of my dissertation, I elucidate the cellular mechanisms that regulate a unique synapse rewiring paradigm in C. elegans motor neurons, where pre-exisiting en passant synapses are completely eliminated and new synapses are formed without any alteration in overall axon morphology. My studies have identified a novel role for dynamic microtubules in facilitating this process, through the modulation of synaptic vesicle transport during remodeling. I also provide in vivo evidence for how molecular motors kinesin and dynein, which move towards opposite ends of the microtubule, engage in a tug-of-war to determine the overall direction of motion of synaptic vesicles. In addition, I uncovered an unexpected role for intermediate filament proteins in regulating microtubule dynamics during synapse formation, and a novel kinase that regulates synapse maintenance after remodeling. Finally, I explore how post-translational modifications of tubulin alter long distance synaptic vesicle transport to terminal synapses, using the mechanosensory neurons in C. elegans as a model. Taken together, my dissertation enhances our current understanding of the different pathways regulating synapse dynamics.

The mechanical properties of the extracellular matrix (ECM) have shown to regulate key cellular processes. However, current tools studying cell-ECM biophysical interactions revolve around cell-mediated traction forces, which, as I will show, are not appropriate in natural matrices due to matrix remodeling. I used active microrheology (AMR) to, instead, measure ECM stiffness in order to quantify these interactions in various cell-ECM systems.

In the first system, I evaluated a commonly used 3D cell-culture method in breast cancer research. I show that this model produces a large physical asymmetry in ECM stiffness, which resulted in altered cellular morphology, adhesion-mediated signaling, and phenotype. Importantly, a hallmark result obtained in this culture method was not repeatable once the asymmetry was removed, highlighting the importance of considering biophysical interactions in cell-culture models.

In the second system, my work, in collaboration with Dr. Stephen Weiss, led to the discovery that stem cells are not passive recipients of ECM stiffness signals as previously thought. Rather they can deliberately alter local (pericellular) stiffness with matrix metalloproteinases as a control for cellular functions. In particular, we found that skeletal stem cells competent in their ability to degrade collagen, increased pericellular stiffness via matrix remodeling to activate β1 integrin signaling pathways and thus controlled their own lineage commitment to osteogenic fates. Cells without the ability to degrade their local matrix lost this functionality and were restricted in lineage commitment to adipogenic or chondrogenic fates.

For the third system, I quantified the contributions of cell contractility and matrix metalloproteinases in matrix remodeling for developing a normal mechanical topography in smooth muscle cells. I also provide evidence that it is the distribution of pericellular stiffness rather than a bulk value that instructs cellular behavior. In order to accomplish this task, I automated the AMR system (aAMR) for a tenfold decrease in measurement time. Importantly, aAMR reduces the complexity of AMR to a few mouse clicks, can create stiffness maps over large distances and provides metrics to assess the distribution of stiffness in the pericellular space within the volume of a natural, fibrous hydrogel.

Statement of significance