Our lab studies the molecular and cellular mechanisms of cell death using in vitro and in vivo models of neurodegenerative diseases and cancers. The current research topics are i) mitochondrial dysfunction and neurodegeneration; ii) DNA damage- and oxidative stress-induced cell death, especially poly(ADP-ribose) polymerase-1 (PARP-1) dependent cell death (parthanatos) in neurons as well as cancer cells. PARP-1 is a nuclear enzyme and plays an important role in cell death through a caspase-independent manner after ischemia-reperfusion injury, glutamate excitotoxicity and various inflammatory responses, as well as other neurologic and non-neurologic diseases. We recently identified a novel PARP-1 activity associated nuclease (PAAN) as the executioner of parthanatos (Wang Y., et al. Science, 2016). Our studies have made significant contributions in understanding parthanatos in ischemic brain injury.
1. PARP-1 functions and signaling regulation
PARP-1 plays a pivotal role in DNA damage response (Fig. 1). It can be activated either by the DNA alkylating agents abundant in our environment, or the byproducts of the cellular oxidative stress or toxicity. In response to mild DNA damage, PARP-1 facilitates DNA repair process. Blockage of PARP-1 activity sensitizes cancer cells to death. This concept is well supported by PARP inhibitors used in the clinic for cancer therapy. In contrast, in response to severe DNA damage, excessive activation of PARP-1 causes the large DNA fragments and caspase-independent cell death designated parthanatos, which occurs in many organ systems and is widely involved in different neurologic and non-neurologic diseases, including ischemia-reperfusion injury after stroke and myocardial infarction, glutamate excitotoxicity, neurodegenerative diseases, inflammatory injury, reactive oxygen species–induced injury. This type of cell death is profoundly prevented by pharmacological inhibition or genetic deletion of PARP-1 (Fig. 2). Therefore, these PARP-1 studies raise a huge knowledge gap how PARP-1 signaling is regulated in DNA damage or oxidative stress, leading to either DNA repair/cell survival or DNA damage/cell death.
Previous works from the PI have revealed the molecular mechanisms by which PAR triggers apoptosis inducing factor (AIF) release from the mitochondria and translocation to the nucleus, leading to PARP-1 dependent cell death (parthanatos) following brain injury (Fig. 3). Recently we further made an important discovery by identifying macrophage migration inhibitory factor (MIF) as a novel PARP activity associated nuclease (PAAN) and an executioner in parthanatos, which cleaves genomic DNA into large fragments and causes neuron and cancer cell death (Fig. 3). However, many fundamental questions, including 1) how MIF nuclease activity is regulated in response to DNA damage and oxidative stress in neurons and cancer cells; 2) how PARP-1 signaling in DNA damage is regulated and switched between cell death and cell survival in neurons and cancer cells; 3) how to effectively interfere PARP-1 signaling in DNA damage to prevent excess neuron loss but enhance cancer cell death, have not yet been answered. Our goals are to obtain a comprehensive molecular understanding of PARP-1 signaling in DNA damage and to discover how PARP-1 signaling can be manipulated in response to mild or severe DNA damage thereby preventing neuronal cell death but enhancing tumor cell death.
2. Differential regulation of PARP-1 signaling under normoxia and hypoxia/ischemia conditions
PARP-1 plays important roles in ischemic brain injury as well as cancers. Both types of diseases involve unique microenvironment-ischemia or hypoxia, which might significantly alter PARP-1 functions and signaling. In collaboration with Dr. Weibo Luo Lab, we will study how PARP-1 signaling is differential regulated under normoxia and hypoxia/ischemia conditions using ischemic stroke models as well as hypoxic-cancer models.
3. AIF3 functions and regulatory mechanisms in mitochondrial biogenesis and cell death.
Apoptosis-inducing factor (AIF) is an X-chromosome linked mitochondrial flavoprotein serving as a free radical scavenger and plays a vital function in mitochondrial bioenergetics. Previous works from the PI have revealed that AIF release from the mitochondria and translocation to the nucleus mediates PARP-1 dependent cell death (parthanatos) following ischemic brain injury. Spontaneous genetic mutations of AIF have been observed in both human and mouse and provided strong association of Aif gene with the mitochondrial dysfunction and etiology of neuropathy. Recently, we discovered a hitherto unknown AIF splicing isoform, defined as AIF3 distinct to other two known isoforms. Induction of AIF3 causes mitochondrial dysfunction and neuron loss in cortex and hippocampus. Our goals are to 1) obtain a comprehensive understanding of AIF3 functions in mitochondrial bioenergetics; 2) dissect the molecular and cellular mechanisms of AIF3 mediated mitochondrial dysfunction; 3) understand the role of AIF3-meidated mitochondrial dysfunction on neuronal cell death.
4. The role of AIF3 in neurodegeneration of Alzheimer’s disease.
Alzheimer's disease (AD) is a leading cause of dementia, which is characterized by memory loss and thinking problems interfering with daily life. Although the importance of beta amyloid and Tau in AD pathogenesis has been well appreciated, unfortunately no effective treatment is available to prevent dementia progress so far due to the lack of understanding of neurodegeneration in AD. Our Lab has identified AIF3 as a potential new risk factor for neurodegeneration and will study its role and underlying molecular mechanisms of neurodegeneration in AD, with the hope to develop new therapeutic approaches to prevent or delay disease progress.
Currently, our lab is using a combination of tools, including epigenetics, bioinformatics, proteomics and mouse genetics, to understand the role of AIF3, cell signaling and regulation of PARP-1 dependent DNA damage and cell death in human cancers as well as neurologic diseases (Figs. 4-5). Our overall goals are to identify novel therapeutic targets and translate the knowledge to prevent neuron loss but enhance cancer cell death.