Progression through the cell cycle adapts to both internal and environmental stimuli to ensure fidelity of cell division. Endoplasmic reticulum (ER) stress arising from an imbalance between cellular demand for protein folding and ER capacity has been described to cause G1 cell cycle arrest. The molecular components of this ER stress checkpoint have just begun to be uncovered. Although evidence emerge to suggest a prosurvival role for the ER stress-induced cell cycle arrest, the functional significance of this checkpoint in mammalian cells largely remains as an open question. Given the implication of ER stress in multiple human diseases like cancer and neurological disorders, elucidation of the link between ER stress and cell cycle may provide new insights to the pathogenesis and treatment of these diseases. In Chapter 1, I introduce the principles of cell cycle regulation and checkpoint responses, leading to a discussion on the discoveries that support an emerging ER stress checkpoint in eukaryotic cells. In Chapter 2, I investigate the mechanisms underlying cell cycle delay in G1 in response to ER stress in mammalian cells, showing that ER stress reduces the protein expression of Skp2 by downregulating Ufd1, a protein that stabilizes Skp2 through its ability to recruit the deubiquitinating enzyme USP13. This results in an accumulation of p27 that partly contributes to G1 arrest in ER-stressed cells. In Chapter 3, I identify another regulator of the ER stress checkpoint, APC/C-Cdh1, and begin to examine the upstream signals responsible for activating APC/C-Cdh1 under ER stress conditions. In Chapter 4, I provide a summary of the work and discuss the implications of my findings

Leucine Zipper-bearing Kinase (LZK/MAP3K13) is a member of the mixed lineage kinase family with high sequence identity to Dual Leucine Zipper Kinase (DLK/MAP3K12). While DLK is established as a key regulator of axonal responses to injury, the role of LZK in mammalian neurons is poorly understood. By gain- and loss-of-function analyses in neuronal cultures, we identify LZK as a novel positive regulator of axon growth. LZK signals specifically through MKK4 and JNKs among MAP2Ks and MAPKs respectively in neuronal cells, with JNK activity positively regulating LZK protein levels. Neuronal maturation or activity deprivation activates the LZK-MKK4-JNK pathway. LZK and DLK share commonalities in signaling, regulation, and effects on axon extension. Furthermore, LZK-dependent regulation of DLK protein expression and the lack of additive effects on axon growth upon co-manipulation suggest complex functional interaction and cross-regulation between these two kinases. Together, our data support the possibility for two structurally related MAP3Ks to work in concert to mediate axonal responses to external insult or injury in mammalian CNS neurons.

Reactive astrocytes influence post-injury recovery, repair, and pathogenesis of the mammalian CNS. Much of the regulation of astrocyte reactivity, however, remains to be understood. Using genetic loss and gain-of-function analyses in vivo, we show that the conserved MAP3K13 (also known as leucine zipper-bearing kinase [LZK]) promotes astrocyte reactivity and glial scar formation after CNS injury. Inducible LZK gene deletion in astrocytes of adult mice reduced astrogliosis and impaired glial scar formation, resulting in increased lesion size after spinal cord injury. Conversely, LZK overexpression in astrocytes enhanced astrogliosis and reduced lesion size. Remarkably, in the absence of injury, LZK overexpression alone induced widespread astrogliosis in the CNS and upregulated astrogliosis activators pSTAT3 and SOX9. The identification of LZK as a critical cell-intrinsic regulator of astrocyte reactivity expands our understanding of the multicellular response to CNS injury and disease, with broad translational implications for neural repair.