Original research
The Sterile 20-Like Kinase Tao Controls Tissue Homeostasis by Regulating the Hippo Pathway in Drosophila Adult Midgut

https://doi.org/10.1016/j.jgg.2014.05.007Get rights and content

Abstract

The proliferation and differentiation of adult stem cells must be tightly controlled in order to maintain resident tissue homeostasis. Dysfunction of stem cells is implicated in many human diseases, including cancer. However, the regulation of stem cell proliferation and differentiation is not fully understood. Here we show that the sterile-like 20 kinase, Tao, controls tissue homeostasis by regulating the Hippo pathway in the Drosophila adult midgut. Depletion of Tao in the progenitors leads to rapid intestinal stem cell (ISC) proliferation and midgut homeostasis loss. Meanwhile, we find that the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling activity and cytokine production are significantly increased, resulting in stimulated ISC proliferation. Furthermore, expression of the Hippo pathway downstream targets, Diap1 and bantam, is dramatically increased in Tao knockdown intestines. Consistently, we show that the Yorkie (Yki) acts downstream of Tao to regulate ISC proliferation. Together, our results provide insights into our understanding of the mechanisms of stem cell proliferation and tissue homeostasis control.

Introduction

Adult stem cells are critical to maintaining the normal structure and function of adult tissues (i.e., homeostasis) throughout life. The proliferation of adult stem cells and differentiation of their progeny must be tightly controlled. One of the best examples is the adult intestinal epithelium which is constantly being renewed by intestinal stem cell (ISC) progeny. Deregulation of stem cell proliferation and differentiation could result in the depletion or excessive proliferation of stem cells, eventually leading to diseases such as cancer (Xie and Spradling, 1998, Radtke and Clevers, 2005, Lin, 2008, Morrison and Spradling, 2008). Understanding the regulatory networks will yield insights into the mechanisms of related human diseases.

The Drosophila adult midgut is an attractive model system to study how stem cell proliferation and differentiation are regulated (Casali and Batlle, 2009, Wang and Hou, 2010). Mammalian and Drosophila intestines show marked similarities in terms of development, cellular make-up, and genetic control (Stainier, 2005, Casali and Batlle, 2009, Wang and Hou, 2010). Previous studies have demonstrated that the Drosophila adult midgut is maintained by ISCs (Micchelli and Perrimon, 2006, Ohlstein and Spradling, 2006). ISCs self-renew and produce non-dividing, undifferentiated daughters, termed enteroblasts (EBs), which can be terminally differentiated into either absorptive enterocytes (ECs) or secretory enteroendocrine cells (ee) by differential Notch signaling (Micchelli and Perrimon, 2006, Ohlstein and Spradling, 2006, Ohlstein and Spradling, 2007) (Fig. S1). It has been shown that these ISCs are responsible for midgut homeostasis under normal and regenerative conditions, and that multiple signaling pathways, including the Hippo pathway, are required for ISC proliferation and midgut homeostasis in response to environmental challenges (Amcheslavsky et al., 2009, Jiang et al., 2009, Jiang et al., 2011, Karpowicz et al., 2010, Ren et al., 2010, Staley and Irvine, 2010). The Hippo pathway regulates tissue growth in diverse organisms and has been linked to cancer (Harvey and Tapon, 2007, Zeng and Hong, 2008, Pan, 2010). Previous studies have demonstrated that the Hippo pathway functions in both ISCs and ECs to control ISC proliferation both cell-autonomously and nonautonomously (Karpowicz et al., 2010, Ren et al., 2010, Staley and Irvine, 2010). However, the mechanisms of how it controls ISC proliferation and differentiation are not fully understood.

Tao kinase belongs to the sterile 20-like family, and has been implicated in the regulation of apoptosis, microtubule dynamics, behavior, brain development, and epithelial morphogenesis by promoting Fasciclin 2 endocytosis (Sato et al., 2007, Liu et al., 2010, King et al., 2011, Gomez et al., 2012). Recent studies also demonstrated that Tao can restrict cell proliferation in developing imaginal epithelia by directly phosphorylating the Hippo (Hpo) protein (Boggiano et al., 2011, Poon et al., 2011). However, whether Tao plays any role in the regulation of ISC proliferation and progeny differentiation remains unclear.

In this study, we provide evidence that Tao plays an important role in controlling ISC proliferation and tissue homeostasis. We find that midgut homeostasis is lost due to rapid ISC proliferation in Tao-depleted intestines. We further show that the expression of the Hippo pathway downstream targets is dramatically enhanced in the absence of Tao. Consistently, we find that inactivation of the transcriptional coactivator Yorkie (Yki) effectively suppresses ISC proliferation and tissue homeostasis loss. Collectively, our data uncovers an important mechanism controlling ISC proliferation and midgut homeostasis.

Section snippets

Tao negatively regulates ISC proliferation

In order to identify new regulators of ISC proliferation and differentiation, we carried out an RNA interference (RNAi) screen using the midgut progenitor-specific driver, esgGal4, in the adult posterior midgut (Li et al., 2014). esg is specifically expressed in ISCs and EBs in the midgut (Micchelli and Perrimon, 2006). From this screen, Tao was identified as a candidate controlling ISC proliferation. Single esg+ cells are interspersed in the basal midgut epithelium in control flies (Fig. 1A).

Discussion

The Hippo signaling pathway controls organ size in both Drosophila and mammals (Harvey and Tapon, 2007, Zeng and Hong, 2008, Zhang et al., 2009, Pan, 2010), and its role in stem cell regulation has only been explored recently (Karpowicz et al., 2010, Ren et al., 2010, Staley and Irvine, 2010). However, how Hippo signaling is regulated in restricting ISC proliferation remains largely unknown. Using the Drosophila adult midgut as a model, we uncovered Tao in an RNAi screen to identify additional

Fly lines and husbandry

Flies were maintained on standard media at 25°C. Crosses were raised at 18°C in humidity controlled incubators, or as otherwise noted. Flies hatched in 18°C incubators were picked and transferred to 29°C incubator, unless otherwise specified. Flies were transferred to new vials with fresh food every day, and dissected at time points specified. In all experiments, only the female posterior midgut was analyzed. Information for alleles and transgenes used in this study can be found either in

Acknowledgments

We thank N. Perrimon, S. Bray, S. Hou, H. Sun, G. Baeg, J Jiang, S. Cohen, and V. Riechmann for generous gifts of reagents, the Bloomington Stock Center, Vienna Drosophila RNAi Center (VDRC), TRiP at Harvard Medical School (NIH/NIGMS R01-GM084947) for fly stocks and the Developmental Studies Hybridoma Bank for antibodies. We gratefully acknowledge the comments on the manuscript by Dr. T.Y. Belenkaya and L. Ray. This work was supported by the grants from the National Natural Science Foundation

References (39)

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These authors contributed equally to this work.

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