Elsevier

Cellular Signalling

Volume 26, Issue 10, October 2014, Pages 2175-2185
Cellular Signalling

Review
Ser/Thr-phosphoprotein phosphatases in chondrogenesis: neglected components of a two-player game

https://doi.org/10.1016/j.cellsig.2014.06.013Get rights and content

Highlights

  • Signalling pathways in chondroprogenitor cells are still not completely understood.

  • Reversible protein phosphorylation is a key regulator of chondrogenesis.

  • Phosphoprotein phosphatases are important mediators of chondrogenesis.

  • This review discusses phosphatase-mediated pathways during chondrogenesis.

  • Unexplored areas with therapeutic possibilities in osteoarthritis are implicated.

Abstract

Protein phosphorylation plays a determining role in the regulation of chondrogenesis in vitro. While signalling pathways governed by protein kinases including PKA, PKC, and mitogen-activated protein kinases (MAPK) have been mapped in great details, published data relating to the specific role of phosphoprotein phosphatases (PPs) in differentiating chondroprogenitor cells or in mature chondrocytes is relatively sparse. This review discusses the known functions of Ser/Thr-specific PPs in the molecular signalling pathways of chondrogenesis. PPs are clearly equally important as protein kinases to counterbalance the effect of reversible protein phosphorylation. Of the main Ser/Thr PPs, some of the functions of PP1, PP2A and PP2B have been characterised in the context of chondrogenesis. While PP1 and PP2A appear to negatively regulate chondrogenic differentiation and maintenance of chondrocyte phenotype, calcineurin is an important stimulatory mediator during chondrogenesis but becomes inhibitory in mature chondrocytes. Furthermore, PPs are implicated to be mediators during the pathogenesis of osteoarthritis that makes them potential therapeutic targets to be exploited in the close future. Among the many yet unexplored targets of PPs, modulation of plasma membrane ion channel function and participation in mechanotransduction pathways are emerging novel aspects of signalling during chondrogenesis that should be further elucidated. Besides the regulation of cellular ion homeostasis, other potentially significant novel roles for PPs during the regulation of in vitro chondrogenesis are discussed.

Introduction

The formation and maturation of tissues that comprise the musculoskeletal system in the developing embryo is a truly enigmatic process. Formed as early as week 4 in the human embryo, the undifferentiated embryonic connective tissue (mesenchyme) that eventually gives rise to the skeleton develops primarily from components of mesoderm [1]. In addition, cartilaginous parts of the cranium are derivatives of ectomesenchyme of neural crest origin. It is from this mesenchyme that the histogenesis of cartilage commences at condensed sites called chondrification centres that appear starting from week 5 of human embryonic development; mesenchymal cells committed towards the osteochondrogenic lineage differentiate into chondroblasts and start to secrete the specialised components of the cartilage extracellular matrix (ECM).

The transition from elongated mesenchymal cells to ovoid chondroblasts is a highly complex procedure that involves changes in gene expression [2], activation and/or inhibition of various signalling pathways [3], [4], alterations in cytosolic Ca2+ concentration [5], [6], and differentiation stage-dependent expression and function of plasma membrane ion channels [7], [8]. Because of the complexity of programmed alterations necessary for chondrogenic differentiation, the details of this process are still incompletely understood. Besides elucidating the molecular mechanism that drives chondrogenesis, cartilage research is also aimed at providing a better understanding of pathological processes that affect the normal functions of this tissue, causing debilitating disorders. Osteoarthritis (OA) is the most prevalent degenerative joint disease and source of chronic pain for millions of people worldwide. Causes of OA are complex with interplay between mechanical, genetic and lifestyle factors [9]. At the late stages of the disease, OA is characterised by degradation of articular cartilage, synovial inflammation, and osteophyte formation [10]. At present, there are no effective disease modifying drugs available that are specifically targeted to OA; therefore, there is a pressing need to identify and develop novel therapeutic agents to halt or even reverse disease progression [11]. Lack of effective and specific drugs against OA are at least partially a consequence of the fact that the molecular nature of the disease is not adequately known.

This review article focuses on the role of protein phosphorylation, the most widespread mechanism for posttranslational protein modification to modulate cellular behaviour, with special emphasis on current knowledge regarding the involvement of phosphoprotein phosphatases (PPs) in the process of chondrogenesis and the pathogenesis of OA.

Section snippets

Reversible protein phosphorylation is a key regulator of chondrogenesis

Reversible posttranslational modification of proteins provides an extremely efficient means of rapid regulation of protein activity without the immediate need for de novo protein synthesis or targeted protein degradation. Activity of a vast number of cellular proteins is regulated by phosphorylation at select Tyr and/or Ser/Thr residues by Tyr and/or Ser/Thr-specific protein kinases (PK). In particular, approximately one-third of all proteins in a given cell have been shown to be phosphorylated

Ser/Thr-specific phosphoprotein phosphatases (PPs)

For protein phosphorylation to be reversible and adjustable to the actual needs of the organism and/or cell, the actions of both phosphorylation (i.e. PKs) and dephosphorylation (i.e. PPs) are needed to be versatile and exhibit similar levels of complexity. However, in contrast to the high number (more than 500) of PKs, there are only 107 putative Tyr phosphatases, and even less (approx. 30) Ser/Thr-specific PPs encoded in the human genome, albeit some of them have dual specificity [15]. It is

Protein phosphatase 1 (PP1)

Ubiquitously expressed in all eukaryotic cells, PP1 is involved in a host of cellular processes including, but not limited to, cell division, cytoskeletal rearrangement, cellular metabolism, and regulation of ion channel function [34]. Functional PP1 holoenzymes consist of a highly conserved ~ 35 kDa catalytic subunit (C) associated with a regulatory (R) subunit that is responsible for subcellular localisation and determines substrate specificity. While there are only a few genes (PP1cα,

Protein phosphatase 2A (PP2A)

One of the most abundant cytosolic proteins, PP2A catalytic subunits can account for 0.1% of total cellular proteins in certain cell types [33]. Accordingly, PP2A is vital in regulating development, cell cycle, proliferation and cell death, mobility, cytoskeleton dynamics, and a host of intracellular signalling pathways – just like PP1. However, unlike PP1, its regulation is fairly complex. PP2A exists in two forms; a heterodimeric core enzyme and a heterotrimeric holoenzyme. The core enzyme

Protein phosphatase 2B (PP2B, calcineurin, PP3)

Considered as a crucial downstream effector of alterations in cytosolic Ca2 + concentration, calcineurin (CaN, PP2B) has been implicated in various biological processes including immune response, intracellular signalling pathways, and differentiation [59]. The CaN holoenzyme consists of a 60 kDa catalytic subunit (calcineurin A, CNA) which has at least 6 isoforms (Aα1, splice variant Aα2, Aβ1, splice variants Aβ2 and Aβ3, and Aγ), and a 19 kDa regulatory (calcineurin B or CNB) subunit which has

PP4, PP5, and PP6 have not been studied in chondrogenesis

Apart from PP1, PP2A, and PP2B, the potential roles of other phosphoprotein phosphatases (PP4, PP5 and PP6) have not been investigated either in differentiating or mature chondrocytes. PP4 (also known as PPX) is closely related to PP2A in that it also consists of catalytic and regulatory subunits, and it also forms heterodimeric core enzymes and heterotrimeric holoenzymes. An essential phosphatase in all eukaryotic species, PP4 has been reported to be involved in centrosome duplication,

Conclusions and perspectives

Although the number of genes that encode the catalytic subunits of PPs is much smaller compared to the number of genes that encode Ser/Thr PKs, current knowledge relating to the function of PPs is scarce, especially in the field of chondrogenesis. The combinatorial nature of PPs and the resultant complexity could have hindered research progress in this field; indeed, the wide range of different regulatory subunits and other interacting proteins that can associate with PP1 or PP2A catalytic

Conflict of interest statement

The authors wrote this review within the scope of their academic and affiliated research positions. There was no bias or external involvement in this work and the authors declare no competing interests. The authors do not have any commercial relationships that could be construed as biased or inappropriate.

Acknowledgements

C.M. is supported by the European Union through a Marie Curie Intra-European Fellowship for career development (project number: 625746; acronym: CHONDRION; FP7-PEOPLE-2013-IEF). A.M. is the coordinator of the D-BOARD Consortium funded by European Commission Framework 7 program (EU FP7; HEALTH.2012.2.4.5–2, project number 305815, Novel Diagnostics and Biomarkers for Early Identification of Chronic Inflammatory Joint Diseases).

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