The Hippo signaling pathway is required for salivary gland development and its dysregulation is associated with Sjogren's syndrome

Abstract

Sjogren's syndrome (SS) is a complex autoimmune disease that primarily affects salivary and lacrimal glands and is associated with high morbidity. Although the prevailing dogma is that immune system pathology drives SS, increasing evidence points to structural defects, including defective E-cadherin adhesion, to be involved in its etiology. We have shown that E-cadherin plays pivotal roles in the development of the mouse salivary submandibular gland (SMG) by organizing apical-basal polarity in acinar and ductal progenitors and by signaling survival for differentiating duct cells. Recently, E-cadherin junctions have been shown to interact with effectors of the Hippo signaling pathway, a core pathway regulating organ size, cell proliferation and differentiation. We now show that Hippo signaling is required for SMG branching morphogenesis and is involved in the pathophysiology of SS. During SMG development, a Hippo pathway effector, TAZ, becomes increasingly phosphorylated and associated with E-cadherin and α-catenin, consistent with the activation of Hippo signaling. Inhibition of Lats2, an upstream kinase that promotes TAZ phosphorylation, results in dysmorphogenesis of the SMG and impaired duct formation. SMGs from NOD mice, a mouse model for SS, phenocopy the Lats2-inhibited SMGs and exhibit a reduction in E-cadherin junctional components, including TAZ. Importantly, labial specimens from human SS patients display mislocalization of TAZ from junctional regions to the nucleus, coincident with accumulation of extracellular matrix components, fibronectin and CTGF, known downstream targets of TAZ. Our studies show that Hippo signaling plays a crucial role in SMG branching morphogenesis and provide evidence that defects in this pathway are associated with SS in humans.

Zonula Occludens-1, Occludin and E-cadherin Expression and Organization in Salivary Glands with Sjögren’s Syndrome

“…3 and 4). Our results differ from previous studies, which found that E-cadherin organization was altered in SMG from NOD/ShiLtJ mice at embryonic stages and was lost in hMSG with SS (Enger et al 2013). This difference observed in NOD/ShiLtJ mice could be due to the use of mSMG at embryonic versus post-natal stages (Enger et al 2013).…”

The Hippo pathway effectors TAZ and YAP in development, homeostasis and disease

“…Depletion of LATS2 in ex vivo SMG cultures delocalizes TAZ, resulting in aberrant branching and inhibition of ductal extensions. The defects associated with LATS2 knockdown showed many similarities to phenotypes observed in human Sjogren's syndrome (Enger et al, 2013), an autoimmune disease of the salivary and lacrimal glands. Similar dysregulation of TAZ localization was confirmed in human Sjogren's syndrome tissues, indicating that dysregulated Hippo signaling might be a contributing factor in this disease.…”

“…These changes have recently been shown to rely on accurate Hippo pathway signaling (Enger et al, 2013). In particular, increases in TAZ/YAP levels, along with enhanced phospho-S89-TAZ levels, were found to be associated with the assembly of junctional E-cadherin/α-catenin complexes during SMG branching (Enger et al, 2013). Depletion of LATS2 in ex vivo SMG cultures delocalizes TAZ, resulting in aberrant branching and inhibition of ductal extensions.…”

Cellular and physical mechanisms of branching morphogenesis

“…Inhibiting ROCK, myosin II ATPase, or FAK also stalls branching after cleft initiation (Daley et al, 2009(Daley et al, , 2011. E-cadherin probably also plays a signaling role, as its association with the Hippo pathway effector TAZ (transcriptional co-activator with PDZ-binding motif) regulates branching of salivary explants (Enger et al, 2013).…”