Hippo Cell Signaling and HS-Proteoglycans Regulate Tissue Form and Function, Age-dependent Maturation, Extracellular Matrix Remodelling and Repair.

This review examined how Hippo cell signaling and HS-proteoglycans (HSPGs) regulate tissue form and function. Despite being a non-weight bearing tissue the brain is regulated by Hippo mechanoresponsive cell signalling pathways during embryonic development. HS-proteoglycans interact with growth factors, morphogens and extracellular matrix components to regulate development and pathology. Pikachurin and Eyes Shut interact with dystroglycan to stabilize the photoreceptor axoneme primary cilium and ribbon synapse facilitating phototransduction and neurotransduction with bipolar retinal neuronal networks in ocular vision, the primary human sense. Another HSPG, Neurexin interacts with structural and adaptor proteins to stabilize synapses and ensure specificity of neural interactions, and aids in synaptic potentiation and plasticity in neurotransduction. HSPGs also stabilize the blood brain barrier and motor neuron basal structures in the neuromuscular junction. Agrin and perlecan localize acetylcholinesterase and its receptors in the neuromuscular junction essential for neuromuscular control. The primary cilium is a mechanosensory hub on neurons, utilized by YAP-TAZ Hippo, Hh, Wnt, TGF-b/BMP receptor tyrosine kinase cell signaling. Members of the glypican HSPG proteoglycan family interact with Smoothened and Patched G-protein coupled receptors on the cilium to regulate Hh and Wnt signaling during neuronal development. Control of glycosyl sulfotransferases and endogenous protease expression by Hippo TAZ YAP represents a mechanism whereby the fine structure of HS-proteoglycans can be potentially modulated spatio-temporally to regulate tissue morphogenesis in a similar manner to how Hippo signaling controls sialyltransferase expression and mediation of cell-cell recognition, dysfunctional sialic acid expression is a feature of many tumors.