Mechanical forces have been proposed to modulate organ growth, but a molecular mechanism that links them to growth regulation in vivo has been lacking. We report that increasing tension within the cytoskeleton increases Drosophila wing growth, whereas decreasing cytoskeletal tension decreases wing growth. These changes in growth can be accounted for by changes in the activity of Yorkie, a transcription factor regulated by the Hippo pathway. The influence of myosin activity on Yorkie depends genetically on the Ajuba LIM protein Jub, a negative regulator of Warts within the Hippo pathway. We further show that Jub associates with α-catenin, and that its localization to adherens junctions and association with α-catenin are promoted by cytoskeletal tension. Jub recruits Warts to junctions in a tension-dependent manner. Our observations delineate a mechanism that links cytoskeletal tension to regulation of Hippo pathway activity, providing a molecular understanding of how mechanical forces can modulate organ growth.

When cells undergo apoptosis, they can stimulate the proliferation of nearby cells, a process referred to as compensatory cell proliferation. The stimulation of proliferation in response to tissue damage or removal is also central to epimorphic regeneration. The Hippo signaling pathway has emerged as an important regulator of growth during normal development and oncogenesis from Drosophila to humans. Here we show that induction of apoptosis in the Drosophila wing imaginal disc stimulates activation of the Hippo pathway transcription factor Yorkie in surviving and nearby cells, and that Yorkie is required for the ability of the wing to regenerate after genetic ablation of the wing primordia. Induction of apoptosis activates Yorkie through the Jun kinase pathway, and direct activation of Jun kinase signaling also promotes Yorkie activation in the wing disc. We also show that depletion of neoplastic tumor suppressor genes, including lethal giant larvae and discs large, or activation of aPKC, activates Yorkie through Jun kinase signaling, and that Jun kinase activation is necessary, but not sufficient, for the disruption of apical-basal polarity associated with loss of lethal giant larvae. Our observations identify Jnk signaling as a modulator of Hippo pathway activity in wing imaginal discs, and implicate Yorkie activation in compensatory cell proliferation and disc regeneration.

Mechanical forces affect all the tissues of our bodies. Experiments conducted mainly on cultured cells have established that altering these forces influences cell behaviors, including migration, differentiation, apoptosis, and proliferation [1, 2]. The transcriptional coactivator YAP has been identified as a nuclear relay of mechanical signals, but the molecular mechanisms that lead to YAP activation were not identified [3]. YAP is the main transcriptional effector of the Hippo signaling pathway, a major growth regulatory pathway within metazoa [4], but at least in some instances, the influence of mechanical strain on YAP was reported to be independent of Hippo signaling [5, 6]. Here, we identify a molecular pathway that can promote the proliferation of cultured mammary epithelial cells in response to cyclic or static stretch. These mechanical stimuli are associated with increased activity of the transcriptional coactivator YAP, which is due at least in part to inhibition of Hippo pathway activity. Much of this influence on Hippo signaling can be accounted for by the activation of c-Jun N-terminal kinase (JNK) activity by mechanical strain and subsequent inhibition of Hippo signaling by JNK. LATS1 is a key negative regulator of YAP within the Hippo pathway, and we further show that cyclic stretch is associated with a JNK-dependent increase in binding of a LATS inhibitor, LIMD1, to the LATS1 kinase and that reduction of LIMD1 expression suppresses the activation of YAP by cyclic stretch. Together, these observations establish a pathway for mechanical regulation of cell proliferation via JNK-mediated inhibition of Hippo signaling.