Investigating the Role of DLK Signaling in Synapse Loss Inflammation and Cell Death
Creators: name: Asghari Adib, Elham
Publisher: name: My University
Year: 2022
Subjects: subject: DLK
subject: Synapse loss
subject: inflammation
subject: Structural Plasticity
subject: Molecular, Cellular and Developmental Biology
subject: Science

description: Continued neuronal function throughout the aging brain requires the maintenance of synaptic connections between neurons that form functional circuits. We hypothesize that neurons are equipped with stress response pathways that enable them to sense and respond to defects in their axons, and that important elements of the stress response are controlled by the evolutionarily conserved dileucine zipper kinase (DLK). Signaling downstream of DLK becomes activated in a range of cellular conditions that impair axons and is required for a range of neuronal responses, including the ability to initiate axonal regeneration following peripheral nerve injury (PNI), and neuronal death. The goal of my thesis work has been to understand the effect(s) DLK signaling has on neurons on a cellular level. I focused on two in vivo paradigms of DLK signaling activation: (1) peripheral nerve injury (PNI) in the mouse sciatic nerve, which is expected to induce survival and a regenerative response within injured motoneurons (MNs), and (2) ectopic activation of the Drosophila homolog of DLK in the adult fly brain, which leads to neurodegeneration and early lethality. Chapters 2-4 describe the work and new insights learned from the PNI paradigm. I used a RiboTag approach to profile ribosome-associated transcript changes within motoneurons (MNs) regulated by DLK following PNI (Chapter 2). The distinct subset of DLK-gated genes includes secreted peptides, immune components, and cytokines, but not regeneration-associated genes (RAGs) required for axonal regeneration. I then confirmed that DLK is not required for axonal regeneration and NMJ reinnervation in mouse motoneurons, which contrasts with its essential role in C. elegans. To further understand DLK’s function in MNs, in Chapter 3, I examined the inflammatory response in the spinal cord following PNI, which is associated with a loss of upstream presynaptic inputs from the axotomized MNs. Strikingly, I found that DLK is required for this synaptic loss and for aspects of the microglial response. Following clues from the profiling data, I found that DLK activation in MNs promotes the activation of complement, which is required for synaptic loss. These findings implicated a new function for DLK in stimulating innate immunity and synaptic remodeling. To consider whether this function may be shared in other paradigms of injury and neuronal stress, I compared genomic datasets of DLK-regulated genes across paradigms with my own data in mouse and Drosophila MNs (Chapter 4). These comparisons revealed neuropeptide secretion and signaling as shared targets of DLK regulation. To study the functional relevance and mechanism of new DLK targets, Chapter 5 describes a new paradigm in the adult Drosophila nervous system, which will enable future studies that take advantage of the powerful genetic tools in Drosophila. While DLK was previously known for the cell-autonomous phenotypes it confers upon injured neurons, the cumulative findings from my thesis work have turned our attention to the non-cell-autonomous responses that DLK signaling may trigger following axonal damage. Inflammation and loss of upstream synapses are new roles for DLK, however, shared with previously known roles (axon regeneration and neuron death) an overarching theme of neuronal plasticity. The various forms of structural plasticity gated by DLK may enable a broad range of mechanisms for the nervous system to adapt to damage.