Date on Master's Thesis/Doctoral Dissertation

8-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Pharmacology and Toxicology

Degree Program

Pharmacology and Toxicology, PhD

Committee Chair

Hetman, Michal

Committee Co-Chair (if applicable)

Whittemore, Scott

Committee Member

Cai, Jun

Committee Member

Siskind, Leah

Committee Member

Boakye, Maxwell

Author's Keywords

Oligodendrocyte; spinal cord injury; Proteostasis; Neuroprotection

Abstract

Recent studies demonstrate that neuroprotection strategies targeting the proteostasis network and components of its effector signaling pathways improve cell survival and motor recovery outcomes in several models of neuronal injury and degeneration. However, the individual contributions of these signaling pathways to the pathogenesis of spinal cord injury (SCI), white matter damage, and motor recovery have not yet been determined. Here, I explored the role of the HIF prolyl hydroxylase domain proteins (PHD/EGLN), effectors that can modulate stress responses activated by the proteostasis network, on motor function recovery after SCI. Furthermore, I identified previously unknown candidate mechanisms in an unbiased manner that may regulate oligodendrocyte death and survival in the context of SCI using RiboTag technology. Chapter one presents background information on the SCI pathogenesis, the types of cell death involved in SCI, oligodendrocytes function and death, the proteostasis network as a global target for neuroprotection in SCI, and finally introduces tools that can be used to study OL-specific translatomes after SCI. Chapter two examines the effects of HIF PHD/EGLN inhibition on a mouse model of moderate T9 contusive SCI. Pharmacological inhibition of PHDs using adaptaquin moderately lowers acute induction of activating transcription (Atf4) and C/EBP homologous protein (Chop/Ddit3) mRNAs and prevents the acute decline of oligodendrocyte (OL) lineage mRNAs, but does not improve long-term recovery of hindlimb locomotion or increase chronic white matter sparing. Furthermore, conditional genetic ablation of all three PHD isoenzymes in OLs did not affect Atf4, Chop or OL mRNAs expression levels, locomotor recovery, and white matter sparing after SCI. Chapter three uses RiboTag technology to study the translatome of OLs at different time points after SCI. Using mouse genetics, I tagged ribosomes specifically in OLs, and immuno-purified mRNA associated ribosomal complexes. I identified genes that were upregulated in OLs with a maximum increase at 2 days post-injury (dpi). Genes that may induce oxidative stress (Chac1, Steap3), inhibit survival signaling kinases (Spred3, Spry4, Parvb), and directly contribute to OL death (Runx1) were highly upregulated after SCI. Potential pro-survival genes include Sphk1, Aldh18a1, and Gdnf. My results show that SCI activates multiple signaling pathways that may affect locomotor recovery, cellular homeostasis, and survival. Although PHDs are involved in modulating responses involved in the proteostasis network, PHD inhibition and/or deletion alone is not sufficient to protect white matter after SCI. These observations are in contrast to those from various CNS injury models with primary effects on the grey matter. PHD inhibitors are currently used in 27 clinical trials treating anemia, and currently show potential for neuroprotection in stroke. I show that PHDs may not be suitable targets to improve outcomes in traumatic CNS pathologies that involve acute white matter injury. Finally, I show using RiboTag technology that OLs upregulate genes that may increase oxidative stress and inactivate survival signaling that may collectively contribute to cell death after SCI. Taken together, the work established in this dissertation shows limited involvement of the PHD-ATF4-CHOP pathway in SCI, and identifies novel candidate mediators of OL death/survival after SCI.

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