Survival and Death Regulation of Neurons and Cancer Cells

Apoptosis, or programmed cell death (PCD), is a fundamental biological process that is required for normal development and tissue homeostasis. Complex regulatory pathways control cell death process that is intricately linked to other cellular processes such as cell proliferation, differentiation, and tumorigenesis. Deregulation of apoptosis plays a major role in various diseases including neurodegenerative disorders and cancer. Understanding the molecular mechanism controlling cell survival and death may lead to novel strategies for treatment and prevention of cancer and neurodegenerative diseases, ranging from Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and stroke.

The focus of Dr. Liu's laboratory is to understand the signaling mechanisms that control death and viability of neurons and cancer cells. Our work has established that silencing of E2F responsive genes are required for neuron survival and that apoptotic stimulation leads to activation of cell cycle elements that promote E2F de-repression and neuron death. Our studies have further demonstrated that E2F-responsive apoptotic genes are silenced by E2F4-p130-Suv39H1-HDAC complexes in unstressed neurons and that apoptotic stimulation leads to CDK-dependent phosphorylation of p130, which results in disassembly of the E2F4-p130-Suv39H1-HDAC complexes on promoters of repressed apoptotic genes. Among these de-repressed apoptotic genes are transcription factors B- and C-myb. Elevation of B- and C-myb induces pro-apoptotic Bcl-2 family member Bim, provoking neuron death. One direction of our future work is to identify other key apoptotic genes that are controlled by the E2F de-repression pathway in neurons. Our lab uses interdisciplinary approaches including cellular, molecular, genetic, and biochemical techniques to dissect the apoptotic mechanisms used by neurons and cancer cells and to isolate novel proteins critical for regulation of cell death in those cells. We also use the same techniques to investigate how neural stem cells, such as telencephalic neuroprogenitor cells, are maintained and what promote their differentiation and survival.

The current research in our laboratory are centered on the following three related areas:

1) Involvement of cell cycle machinery in regulation of neuronal cell death. A growing body of evidence indicates that inappropriate activation of the cell cycle machinery mediates neuron apoptotic death during both normal developmental and pathological conditions. We are particularly interested in understanding the involvement of the CDK-Rb-E2F axis, a central cell cycle pathway in regulating cell proliferation in dividing cells, in regulation of neuronal cell death. Published work in this area includes Liu et al., Genes&Dev. 19:719-32, 2005; Liu et al., J. Neurosci. 24:8720-5, 2004; Liu and Greene, Neuron 32:425-38, 2001.

2) Delineation of the signal transduction pathways that implicate ATF5 as a cancer cell-specific survival factor. ATF5 is a bZIP protein of the ATF/CREB family of transcription factors. Recent studies have revealed that survival of cancer cells (including breast, colon, brain, pancreatic, and lung cancers among others), but not non-cancer cells, requires ATF5 function. However, little is known about how ATF5 works (Angelastro et al, Oncogene 25:907-16, 2006; Monaco et al, Int J Cancer 120:1883-90, 2007; Li et al., Mol. Cancer Res. 2009). We are investigating why survival of cancer cells but not normal cells depends on ATF5 function, and to what extent we can exploit this "vulnerability" of cancer cells for their elimination. Using a variety of techniques that include 2-DIGE proteomics, we are making progress in a number of directions that permit a better understanding on the mechanism why interference of ATF5 function leads to death of only cancer cells but not normal cells.

3) Regulation of neural stem cells. ATF5 expression promotes "stemness" of neural stem cells in vitro and in vivo. Down-regulation of ATF5 is required for the progression of neural progenitor cells to neurons and oligodendrocytes (Angelastro et al, J Neurosci. 23:4590-600, 2003; Mason et al, Mol Cell Neurosci. 29:372-80, 2005). We are interested in understanding how ATF5 blocks cell cycle exit and maintains "stemness" of neural stem cells in the developing brain. Using in utero gene transfer technique, we are able to study gene function in vivo during mammalian neurogenesis (see Graphic explanation below). We also want to learn how to control the differentiation process that turns a neural stem cell into a particular type of neuron or glia cell, a process called "directed differentiation".

Our laboratory is making exciting discoveries in a number of frontiers, where both post-doctoral fellows and pre-doctoral students who seek biological research as a component of their career may find excellent training opportunities. As a part of that, one can learn a variety of experimental techniques and use them to address fundamental biological questions. These are techniques that we have successfully used in our previous and current research, including, e.g., gene cloning and mutagenesis, DNA and RNA preparation and quantitative real-time PCR, protein-protein interaction, protein phosphorylation status and functioning analysis, cell culture-related preparation, maintenance and transfection of primary neurons, stem cells and a variety of cell lines, cell proliferation and clonogenic assay, in vitro tumorigenic transformation, in vivo tumorigenesis, biochemical assays such as luciferase and beta-gal reporter assays, kinase and methylation assays, in vitro translation, various survival assays, anti-sense and siRNA gene knock-down, Western blotting, co-immunoprecipitation (IP), subcellular fractionation, ubiquitination analysis, immunohistochemistry (IC), immunofluorescence (IF), electrophoretic mobility shift assay (EMSA), and chromatin immunoprecipitation (ChIP).