The Role of PML in Oxidative Stress and Alzheimer's Disease

About This Grant

Alzheimer’s disease (AD) is a progressive and degenerative disorder of the brain. It is pathologically characterized by the loss of neurons. The key events driving the pathogenesis in AD are not completely understood. MiRNAs are a class of small non-coding RNAs about 22nt long and play a key regulatory role in every cellular process. The first step of miRNA biogenesis is controlled nuclear RNase III enzyme Drosha and its co-factor DGCR8 (Di George Syndrome critical region gene 8), which form the complex microprocessor. It processes primary miRNA transcripts (pri-miRNAs) into a ∼65-80 nucleotide hairpin structure named the precursor miRNAs (pre-miRNAs). Emerging evidence indicates that miRNA biogenesis is subjected to complex regulation. The microprocessor is modulated by recruiting various transient factors to facilitate or attenuate the generation of small subsets of pre-miRNAs under unique conditions. The question of if there are distinct pools of microprocessor that are subjected to unique modulation and regulate a broad spectrum of miRNAs remains open. MiRNAs modulate pathways relevant to the pathogenesis of genetic and sporadic AD. AD is associated with loss of miRNA homeostasis. But the mechanisms responsible for dysregulation of Drosha and miRNA biogenesis in AD remain to be fully illustrated. Promyelocytic leukemia protein (PML) is a well-known tumor suppressor and can function biochemically as a SUMO (small ubiquitin-like modifier) E3 enzyme. It is highly sensitive to oxidative stress. Recent findings show that PML plays a role in the brain and responds to conditions associated with neurodegeneration. However, there are no reports if PML directly regulates miRNA biogenesis and is involved in AD pathogenesis. Our preliminary studies have now identified a new PML- dependent mechanism that potently promotes microprocessor activity. We propose a new concept of SUMOylated Super active Microprocessor (SSaM) and hypothesize that PML SUMOylates Drosha and DGCR8 to form the tricomponent SSaM and promotes robust and broad miRNA biogenesis. Loss of SSaM renders neurons less capable of handling oxidative and AD-related stress and underlies in part the cytopathogenic process in the disease. We will determine biochemically if PML functions as a SUMO E3 to regulate SSaM and miRNA biogenesis in cells; if AD stress targets PML and SSaM to dysregulate miRNA biogenesis and trigger cytopathogenesis in rat primary neurons and in forebrain neurons derived from iPSCs of fAD patients; if inhibition of PML-SSaM axis underlies neurotoxicity in a rat model of AD; and if the levels of PML and SSaM correlate with AD status in postmortem human brains. This study should significantly advance our understanding of the basic process of miRNA biogenesis and provide insights on the loss of miRNA biogenic machinery and homeostasis as a cytopathologic feature of AD.