The Heparan Sulfate Proteoglycan Perlecan Regulates Axonal and Synaptic Stability

Abstract

Heparan sulfate proteoglycans (HSPGs) form essential components of the extracellular matrix (ECM) and basement membrane (BM) and have both structural and signaling roles. Perlecan is a secreted ECM-localized HSPG that contributes to tissue integrity and cell-cell communication. In this thesis, I identify a role for Drosophila Perlecan in the maintenance of larval motoneuron axonal and synaptic stability. In Chapter 1, I discuss known roles for Perlecan and other HSPGs in animal development, with a focus on their functions within the nervous system. In Chapter 2, I describe how loss of Drosophila Perlecan causes alterations in the axonal cytoskeleton and breakage of axons, followed by synaptic retraction of neuromuscular junctions. These phenotypes are not prevented by blocking Wallerian degeneration and are independent of Perlecan’s role in Wingless signaling. Overexpression of Perlecan in motoneurons cannot rescue synaptic retraction phenotypes. Similarly, removing Perlecan specifically from neurons, glia, muscle, fat body, or hemocytes does not cause synaptic retraction, indicating the protein is secreted from multiple cell types and functions non-cell autonomously. Within the peripheral nervous system, Perlecan predominantly localizes to the neural lamella, a specialized ECM surrounding nerve bundles. Loss of Perlecan disrupts neural lamella structure, with reduced ECM thickness observed for the colocalized Viking protein, a Drosophila type IV Collagen homolog. In addition, Viking shows abnormal accumulation and aggregation at sites along the neural lamella that are associated with axonal breakage and exit from their usual boundary within the nerve bundle. Entire nerve bundles degenerate in a temporally coordinated manner across individual hemi-segments during the late stages of larval development. These observations indicate disruption of neural lamella ECM function triggers axonal destabilization and synaptic retraction of motoneurons, revealing a role for Perlecan in axonal and synaptic integrity during nervous system development.