Genetic Control of Programmed Cell Death (Apoptosis) in Drosophila

The long-term objective of our research is to gain a comprehensive understanding of the genetic control of apoptosis and cell proliferation, and their connection to human cancer. Apoptosis and cell proliferation are critical for normal development, homeostasis and aging. Inappropriate control is associated with various diseases including cancer and neurodegeneration. We are utilizing the fruit fly Drosophila melanogaster as a genetic model organism. Knowledge obtained in our studies will provide new insights for our understanding of these diseases.

A tutorial in the lab will provide a detailed introduction into modern Drosophila techniques with emphasis on visualizing gene activity and cell death in wild-type and various mutant backgrounds, phenotypic analysis, generating transgenic flies and small scale genetic screens. In addition, students will gain experience in basic molecular biology and protein chemistry. The experiments will be aided by state-of-the-art facilities.

Three major projects are under study in the lab.

1. Genetic screening

We have developed a novel genetic screening method to identify genes involved in cell death control and execution in Drosophila (Figure 1). However, unexpectedly, we also identified genes involved in growth control, signal transduction and tumor suppression. These interesting genes and their role in normal development are currently under intensive study in the lab.

2. Discovery of non-autonomous tumor suppressor genes

We have discovered a novel class of tumor suppressor genes. Normally, cells that lose tumor suppressor genes by genetic inactivation become highly proliferative and resistant to apoptosis, thus promoting tumor formation. However, in our studies, we identified genes which behave differently. If these genes are mutant, it is not the mutant cells which are overgrowing. Instead, the mutant cells influence the behavior of neighboring cells and promote their proliferation and increased apoptotic resistance, causing non-autonomous overgrowth. Thus, these genes qualify as non-autonomous tumor suppressors.

How do non-autonomous tumor suppressor genes work? One class of non-autonomous mutants affects negative regulators of the Hedgehog (Hh) pathway such as patched or PKA, causing deregulated Hh signaling. This deregulated activity promotes non-autonomous proliferation as well as increased apoptosis resistance through up-regulation of Diap1, a potent inhibitor of apoptosis (Figure 2). The non-autonomous control of proliferation and apoptosis by Hh signaling may be needed to generate a supportive micro-environment for tumorigenesis.

3. Apoptosis-induced compensatory proliferation (CP) and its implications for cancer

Before they die, apoptotic cells can secrete cytokines. These cytokines stimulate proliferation of neighboring cells, a process referred to as apoptosis-induced compensatory proliferation (CP). There are two distinct types of apoptosis-induced compensatory proliferation. The first one is triggered when massive apoptosis is induced in proliferation-competent tissue (Figure 3). In extreme cases, this form of CP causes overgrowth which may be relevant for cancer (Figure 4). The second type of CP occurs when apoptosis is induced in post-mitotic tissue (Figure 5).

Understanding the molecular mechanisms of CP is important for cancer research for three reasons. First, apoptosis-induced proliferation resembles inflammation-induced cancer where inflammatory cells secrete cytokines promoting the growth of cancer cells. Second, therapeutic treatment of tumors often seeks to induce apoptosis of cancer cells. However, this may be counter-productive as apoptotic cells can induce proliferation of adjacent cells. In extreme cases, we observe overgrowth of apoptotic tissue. Third, a very important question in cancer research is how transformed cells start proliferating. This is not a trivial question. Cells in a differentiated tissue have exited the cell cycle and rest in a quiescent state. The exact molecular mechanisms by which transformed cells re-enter the cell cycle are largely unknown. The second type of CP may provide answers to this question. When we induce apoptosis in the Drosophila retina which is composed of postmitotic neurons, cone and pigment cells, the neurons produce mitogens which stimulate other cells to re-enter the cell cycle (Figure 5). The characteristics of this cell cycle re-entry are similar to mammalian cells stimulated to re-enter the cell cycle.

Thus, these examples illustrate the importance of CP in for cancer. In genetic screens, we have identified several genes involved in CP. We are analyzing these genes to understand the mechanism of CP and their potential role in cancer.

Significance

Initially, the primary focus of research in the laboratory centered on apoptosis. However, over the years it became clear that in the context of a multi-cellular organism such as Drosophila, apoptosis is intimately linked to cell proliferation and tumor suppression. The discovery of apoptosis-induced compensatory proliferation is an instructive example. Although the cells are dying, they are able to produce cytokines that stimulate proliferation. In extreme cases, this can cause overgrowth. Because induction of apoptosis in tumor cells is a preferred strategy in the clinic, it is important to understand the mechanism of apoptosis-induced proliferation. Likewise, tumor cells need a supportive cellular micro-environment for tumorigenesis. Non-autonomous tumor suppressor genes may generate such a supportive environment. Therefore, it is important to understand the mechanism of action of these genes. The projects in the laboratory are aimed at these questions.