Investigating the Roles of Poly (ADP-ribose) Polymerase 1 in Cortical Development

Poly (ADP-ribose) polymerase 1 (PARP1) is a ubiquitously expressed enzyme that post-translationally modifies proteins via poly (ADP-ribosylation) (PARylation). PARP1 serves various functions, including DNA damage repair, regulation of cell death pathways, chromatin modification, RNA processing, and transcriptional regulation. Accordingly, mutations in Parp1 or Adprhl2 (encoding the protein ADP-ribosylhydrolase 3, which removes PAR polymers) cause intellectual disability, ataxia, episodic psychosis, neurodegeneration, and developmental delay. Altered PARP1 expression is also associated with numerous neurodegenerative and neuroimmune disorders, including Alzheimer's disease, Parkinson's disease, multiple sclerosis, rheumatoid arthritis, major depressive disorder, and epilepsy. Despite ubiquitous expression and an apparent connection with brain disorders, PARP1's role in neurodevelopment has not been widely studied. Our lab has recently uncovered a novel interaction between PARP1 and the receptor tyrosine kinase ErbB4, which binds its ligand NRG1 to mediate numerous functions during neurodevelopment, including radial migration of excitatory neurons, tangential migration of inhibitory neurons, synaptogenesis, and differentiation. Additionally, ErbB4 has multiple splice forms that confer different signaling modalities. Specifically, the ErbB4-juxtamembrane (JM)-a isoform is cleavable via the enzymes tumor necrosis factor-alpha (TACE) and presenilin/gamma-secretase. Upon NRG1 binding and ErbB4-JMa cleavage, the ErbB4 intracellular domain (E4ICD) is released, which regulates transcription through direct promoter binding. Previous findings have shown that E4ICD complexes with co-factors to repress gliogenesis during early development. Due to PARP1's prominent roles in chromatin modification and transcriptional control, this begs the question as to whether PARP1 is likewise regulating glial gene expression via E4ICD. The aims of this dissertation are two-fold: 1) investigate the role of PARP1 in regulating astrocytic gene expression via E4ICD and 2) further characterize the effect of PARP1 loss on brain development. To explore the role of PARP1-E4ICD in the regulation of astrogenesis, I utilized mouse primary embryonic neural precursor cell (NPC) cultures and transgenic mice with a germline knockout of PARP1, ErbB4, or ErbB4-JMa. I found that NRG1-mediated repression of GFAP expression upon FGF removal from NPC cultures was dependent upon the presence of PARP1, ErbB4, and ErbB4-JMa. Additionally, I showed that PARP1 KO and ErbB4 KO mice overexpress GFAP at birth, indicating the importance of both proteins in vivo. To investigate the effect of PARP1 loss on neurodevelopment more broadly, I analyzed the brain and cortical size of PARP1 KO mice at birth, finding a reduction in brain weight relative to body size, which is associated with a thinner cortex and a reduced cortical surface area. Furthermore, I discovered that PARP1 loss alters early-born neuron migration and increases the density of deeper-layer neurons. To investigate changes in gene expression associated with these findings, I performed RNA-sequencing of the embryonic PARP1 KO cortex. I found that PARP1 loss increases the expression of genes involved in neuronal migration and adhesion, including Reln, which encodes the glycoprotein Reelin. Accordingly, my findings indicate that PARP1 loss increases the abundance of Reelin-expressing cells in the developing (E15.5) and adolescent (P5) mouse brain. I further demonstrated that PARP1 loss, inhibition, or acute knockdown increases Cajal-Retzius cell abundance in vitro, suggesting PARP1 regulates Cajal-Retzius cell development via a cell-autonomous mechanism. Finally, atomic force microscopy showed that NPCs isolated from the PARP1 KO cortex adhere more strongly to the cell adhesion molecule N-cadherin, likely due to excess Reelin. Overall, these findings demonstrate that PARP1 regulates astrogenesis, Cajal-Retzius cell development, and cell adhesion in the developing brain.PHDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169918/1/mmnels_1.pd