Biomaterials Regulate Mechanosensors YAP/TAZ in Stem Cell Growth and Differentiation

Abstract

Tissue-resident stem cells are surrounded by a microenvironment known as 'stem cell niche' which is specific for each stem cell type. This niche comprises of cell-intrinsic and -extrinsic factors like biochemical and biophysical signals, which regulate stem cell characteristics and differentiation. Biochemical signals have been thoroughly studied however, the effect of biophysical signals on stem cell regulation is yet to be completely understood. Biomaterials have aided in addressing this issue since they can provide a defined and tuneable microenvironment resembling in vivo conditions. We review various biomaterials used in many studies which have shown a connection between biomaterial-generated mechanical signals and alteration in stem cell behaviour. Researchers probed to understand the mechanism of mechanotransduction and reported that the signals from the extracellular matrix regulate a transcription factor yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ), which is a downstream-regulator of the Hippo pathway and it transduces the mechanical signals inside the nucleus. We highlight the role of the YAP/TAZ as mechanotransducers in stem cell self-renewal and differentiation in response to substrate stiffness, also the possibility of mechanobiology as the emerging field of regenerative medicines and three-dimensional tissue printing.

Keywords: Biomaterials; Human pluripotent stem cells; Mechanobiology; YAP/TAZ.

Fig. 1

The effect of substrate stiffness on YAP/TAZ. A Stiff substrate activates focal adhesion proteins—talin and vinculin, they bind to and activate integrin, resulting in cell adhesion and spreading. Formation of integrin-talin-vinculin phosphorylates FAK which forms a complex with Src and phosphorylates paxillin, contributing in mechanosignalling and cell spreading. This entire complex creates tension on actinomycin and cytoskeletal filaments which causes stretching of LINC; nuclear envelope protein; ensuring nuclear localization of YAP/TAZ and subsequent gene expression. B On the contrary cells on soft substrate experience less tension, forming an unstable integrin-talin-vinculin complex. Loosely formed FAK-SRC complex does not form tension-dependent stretching of actinomycin, cytoskeletal filaments, and LINC, causing cytoplasmic retention of YAP/TAZ

Fig. 2

The Mammalian Core Hippo Signaling pathway. A When Hippo pathway is ON, YAP and TAZ are cytoplasmic and do not bind to TEAD (DNA binding transcriptional factor). Upstream signals from various cell surface receptors such as GPCR, WNT, TAO Kinases, tight junction (TJ), adhesion junctions (AJ) or mechanical forces (soft substrate, high cell density, small surface area for adhesion etc.) initiates a cascade of phosphorylation reactions of core Hippo pathway proteins. These signals and scaffold protein SAV1 phosphorylate MST1/2. LATS1/2 is phosphorylated by MST1/2 and MAP4K, facilitated by MOB1 and NF2 (also known as Merlin), which subsequently phosphorylates YAP/TAZ at various serine residues (refer Fig. 3). After phosphorylation by LATS1/2-MOB1, YAP/TAZ either binds to 14-3-3 protein resulting in its cytoplasmic retention or binds to angiomotin (AMOT), casein kinase 1 isoform epsilon and delta (CSNK1E/D), β-transducing repeat-containing protein (BTRC), glycogen synthase kinase 3 Beta (GSK3B) or catenin beta (CTNNB) resulting in ubiquitylation and proteasomal-mediated degradation. YAP/TAZ cannot shuttle into the nucleus and bind to one of its DNA binding transcription factors such as TEAD when Hippo pathway is ON. B When Hippo pathway is OFF i.e. in absence of upstream signaling, MST1/2 and LATS1/2 are unphosphorylated and inactive. Unphosphorylated YAP/TAZ is active and is free to move into the nucleus, bind to transcriptional factors, such as TEAD, TP73, ERBB4, EGR1, TBX5, SMAD, or RUNX, and depending upon the binding partner promote specific gene expression

Fig. 3

Schematic representation of A YAP and B TAZ primary protein sequence: Phosphorylation sites on YAP and TAZ by LATS, CSNK1E/D, c-Abl and for proteasomal degradation are highlighted, including phosphorylation of YAP at S381 by LATS1/2 leads to SCF-βTRCP-mediated proteasomal degradation whereas phosphorylation of YAP at S58/62 causes GSK-3β mediated proteasomal degradation. Phosphorylation at Serine 127 (for YAP) and at Serine 89 (for TAZ) by LATS creates a binding site for 14-3-3 thereby retaining YAP/TAZ in the cytoplasm. YAP and TAZ both have a TEAD binding domain, WW domain, CC (coiled-coil region) domain, transactivation domain, and PDZ binding motif; with an additional proline-rich region, WW domain, and SH3 domain