Biology Department

Research Overview

Research Summary

Signal transduction networks governing stress responses, metabolism and aging.

We are interested in signaling systems that regulate the interaction of organisms with their environment. In our work, we take advantage of the wide range of genetic, molecular, and genomic techniques available for our model organism, Drosophila melanogaster. Current work in the lab focuses on the mechanism(s) by which Jun-N-terminal Kinase signaling regulates Insulin signaling in the fly and how this interaction affects stress tolerance, metabolism and aging.

Research Description

Development and physiology of multi-cellular organisms rely on the regulated and cell-specific interpretation of genomic information. Understanding how organisms decode this information to respond appropriately to changes in their environment is a fascinating and challenging pursuit of modern biology. Our work has focused on investigating how signal-mediated changes in gene expression govern cellular functions during development, stress response, and aging of Drosophila melanogaster. As a paradigmatic signaling pathway that relays environmental information to the cell nucleus and that controls distinct cellular actions in a variety of contexts, we study Jun-N-terminal Kinase (JNK) signaling.

JNK belongs to the stress-activated protein kinase subfamily of mitogen-activated protein kinases (MAPKs). Genetic studies in flies and mice have identified evolutionarily conserved requirements for JNK signaling in morphogenetic processes, as well as in cellular stress responses. JNK controls cellular functions by phosphorylating and activating transcription factors. Insight into the downstream genetic programs regulated by these factors thus promises to provide important inroads into understanding JNK's impact on cell physiology.

In pursuit of this goal, we have characterized JNK-induced transcriptome changes in different tissues of Drosophila using Serial Analysis of Gene Expression (SAGE), which we adapted for use in Drosophila (Jasper et al., 2001). SAGE is a method for genome-wide expression profiling ideally suited for the analysis of minute amounts of tissue. The JNK-responsive genes identified in this work could be grouped into a small number of functional classes. One class encodes candidate regulators of cell shape changes and cell adhesion, including Profilin and Integrins. These genes are likely mediators of JNK-induced morphogenetic processes during embryonic development (Jasper et al., 2001).

A second prominent group of JNK-responsive genes identified in our SAGE study encodes proteins with functions in oxidative stress protection and damage repair. This discovery led us to test the role of JNK signaling in promoting the tolerance of the organism against oxidative insults. Activation of JNK by environmental stressors such as reactive oxygen species (ROS) had previously been described in a variety of experimental paradigms. However, the consequences of this activation at the organism level remained poorly understood. Using genetic analysis, we could show that elevated JNK signaling activity renders flies tolerant to oxidants (Wang et al., 2003). Importantly, and in accordance with the free radical theory of aging, we observed a significant increase in longevity in these animals as compared to isogenic wild-type controls. Our work thus introduced JNK signaling as a novel signaling pathway that influences lifespan (Wang et al., 2003).

Figure 1: Lifespan extension by JNK signaling is mediated through Foxo.

Demography of fly populations (males) carrying different genetic mutations. Note that flies heterozygous for puc (these flies have increased JNK activity) have a longer median and maximum lifespan compared to isogenic controls (+/+). This lifespan extension is lost in transheterozygotes of puc with two foxo loss-of-function alleles (foxo 21 and foxo 25), suggesting that foxo is required for lifespan extension by JNK signaling. See Wang et al., 2005 for more details.

In our current work, we attempt to dissect the signaling mechanisms by which the JNK pathway affects longevity. To this end, we have studied potential interactions between JNK and the Insulin/IGF signaling pathway (IIS) in Drosophila. Activation of IIS negatively regulates longevity in worms, flies, and mice. Accordingly, mutant animals with a deficient IIS pathway exhibit increased oxidative stress tolerance and dramatically extended lifespan. A central mediator of this effect is the Forkhead Box O transcription factor Foxo. Foxo is active in situations of low IIS activity, and promotes stress tolerance and longevity in worms and flies. Interestingly, we have found recently that JNK promotes Foxo activity in Drosophila and that foxo gene function is required for the lifespan extension observed in animals with increased JNK activity (see Figure 1). Our studies thus suggest that Foxo integrates nutritional cues (sensed by IIS) and stress signals (via JNK) to regulate lifespan according to environmental conditions. (Wang et al., 2005).

An interesting aspect of the interaction between JNK and Insulin signaling in the fly is that JNK not only influences Foxo activity cell-autonomously, but appears to systemically regulate IIS/Foxo activity through transcriptional repression of the insulin-like peptide Dilp2 in insulin producing cells (IPCs) of the fly brain (Figure 2, Wang et al., 2005).

Figure 2: JNK signaling is active in Insulin Producing Cells (IPCs) of the fly brain.

Xgal staining (A) and immunostaining (B) showing that lacZ is expressed in IPCs in a reporter line that expresses nuclear lacZ in response to JNK activation. IPCs are identified by position in A (arrowhead) and by staining against insulin like peptide 2 (Dilp2) in B.

This mechanism leads us to our current working model for the regulation of stress tolerance and longevity by JNK signaling (Figure 3): cellular stress tolerance can be enhanced locally by cell-autonomous activation of Foxo. Such a mechanism is likely to be required to respond to local insults or to prevent the deterioration of metabolically active tissues, such as the heart or brain. In addition, its function in IPCs allows JNK to adjust the organism's metabolism to environmental challenges on a systemic level.

Current work in the lab attempts to validate this model and to further explore molecular and cellular interactions in the control of longevity, stress tolerance and metabolism of the fly.

Figure 3. Model for the coordination of stress responses with growth control in Drosophila.

In peripheral tissues, oxidative stress causes cell-autonomous activation of JNK signaling. JNK then suppresses IIS and/or activates Foxo by mechanisms that remain to be elucidated. These signaling events result in the activation of Foxo target genes such as l(2)efl and thor which mediate localized stress responses and growth arrest, respectively. JNK/Foxo signaling also promotes systemic changes in growth and metabolism by acting in IPCs, where insulin-like peptides are produced that control IIS activity in peripheral target tissues. JNK activation in IPCs results in a Foxo-dependent decrease of dilp2 expression. By cell-autonomous as well as systemic mechanisms, IIS and JNK signals thus converge on DFoxo to regulate the transcription of genes involved in growth control, stress protection, and longevity.