MST are involved in signalling pathways following

MST1/2 are involved in signalling pathways following along the canonical and noncanonical Hippo pathway. The canonical pathway is highly conserved between D. melanogaster and mammals and a master regulator of tissue growth. During development and within the canonical pathway, MST1/2 seem to have redundant roles. In the canonical pathway, activated MST1/2 stimulate large tumour suppressor kinases 1 and 2 (LATS1/2), which phosphorylate and inactivate the transcription factor Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), restricting YAP and TAZ localization to the cytoplasm. In contrast, activated YAP and TAZ move to the nucleus, acting as transcription factors by inducing the expression of various genes controlling growth, cell proliferation, organ size and cell and tissue homeostasis (reviewed in 2, 5) (Figure 1). Functions of MST1/2 other than growth and independent from LATS/YAP/TAZ are mediated via the noncanonical Hippo pathway that is discussed in detail below. Structurally, human MST1 and MST2 consist of 487 amino acids (aa) and 491 aa, respectively. They share 88% similarity and 76% identity in amino CPI-455 sequence and are >95% identical in their catalytic domains. MST1/2 have an N-terminal serine/threonine kinase domain, a coiled coil region, caspase 3 cleavage sites, and a C-terminal SARAH domain [2]. The SARAH domain is a highly conserved dimerisation domain, mediating homo- and heterodimerisation, as well as ligand binding. Activation of MST1 and MST2 requires the phosphorylation at Thr183 (MST1) and Thr180 (MST2). This is facilitated by trans-autophosphorylation within the activation loop by the SARAH domain through the formation of antiparallel dimers (both MST1/2 homo- and heterodimers) [6]. Of note, the binding affinity of MST1/2 heterodimers is ∼6-fold lower than for MST2 homodimers [7]. Both can be cleaved by caspase 3 [326 aa (mice and humans) and 349 aa (humans) for MST1 and 322 aa for MST2] in response to apoptotic stimuli, separating the N-terminal kinase domain (37kDa) and the C-terminal SARAH domain (18kDa). The N-terminal part of MST1/2 has no regulatory inputs (apart from phosphatase-catalysed inactivation) and lacks a nuclear export signal [2]. In general, the regulation of MST1/2 is determined by the composition of MST1/2 complexes, which in turn is defined by the abundance and affinity of various noncatalytic SARAH domain-containing proteins in relation to MST1/2. The relative affinities and tissue specific abundance of SARAH domain-containing proteins in vivo has not yet been determined. Loss of Hippo in D. melanogaster leads to unregulated cell proliferation and tissue overgrowth [4]. Surprisingly and in contrast to the D. melanogaster Hippo mutant, knockout of both mammalian Hippo homologs, Mst1 and Mst2 (), in mice causes embryonic lethality at embryonic day E8.5, showing defects in placental development, vascular patterning, primitive haematopoiesis, and regulation of cell proliferation and survival [8]. Hippo-like phenotypes emerge after tissue-specific MST1/2 deficiency, but only in some tissues. For example, Mst1/2 conditional-deletion in the liver (adreno-cre; and albumin-cre; ) leads to tissue overgrowth and tumourigenesis 9, 10. Conditional deletion of Mst1/2 in the intestine (villin-cre; mice) causes expansion of stem cell and progenitor cell compartments [11]. In contrast, skin-specific deletion of Mst1/2 (K14-cre; caused no abnormalities or hyperplasia [12]. Interestingly, Mst1 and Mst2 single knockout mice are viable and develop normally, revealing that these two kinases act at least in part in a redundant fashion. Unexpectedly, loss of MST1 in humans leads to a severe immunodeficiency syndrome as discussed below 13, 14, 15, 16, 17. A similar phenotype is also seen in mice. In contrast, single knockout mice have normal leukocyte function and no obvious phenotype [8]. Emerging evidence implicates the regulation of a variety of biological processes other than growth and proliferation via the noncanonical Hippo pathway. This is particularly true for leukocytes, where the MST1-dependent noncanonical Hippo pathway seems to play a critical role for various leukocyte functions. This was mostly elucidated by studying Mst1 and Mst2 single- and double-knockout mice and has been complemented by the recent identification of patients with MST1 deficiency.