Wnt3a and wnt5a as Potential Chondrogenic Stimulators for Nucleus Pulposus Cell Induction: A Comprehensive Review

Abstract

Low back pain remains a highly prevalent pathology engendering a tremendous socioeconomic burden. Low back pain is generally associated with intervertebral disc (IVD) degeneration, a process involving the deterioration of nucleus pulpous (NP) cells and IVD matrix. Scientific interest has directed efforts to restoring cell numbers as a strategy to enable IVD regeneration. Currently, mesenchymal stromal cells (MSCs) are being explored as cell therapy agents, due to their easy accessibility and differentiation potential. For enhancement of MSCs, growth factor supplementation is commonly applied to induce differentiation towards a chondrogenic (NP) cell phenotype. The wnt signaling pathways play a crucial role in chondrogenesis, nonetheless, literature appears to present controversies with regard to wnt3a and wnt5a for the induction of NP cells, chondrocytes, and MSCs. This review aims to summarize the reporting on wnt3a/wnt5a mediated NP cell differentiation, and to elucidate the mechanisms involved in wnt3a and wnt5a mediated chondrogenesis for potential application as cell therapy supplements for IVD regeneration. Our review suggests that wnt3a, subsequently replaced with a chondrogenic stimulating growth factor, can enhance the chondrogenic potential of MSCs in vitro. Contrariwise, wnt5a is suggested to play a role in maintaining cell potency of differentiated NP or chondrogenic cells.

Keywords: Chondrogenesis; Intervertebral disc; Mesenchymal stem cells; Nucleus pulposus; Regeneration; Wnt.

Fig. 1.

Canonical and noncanonical signaling options initiated via wnt3a and the concluded outcome. Wnt3a can bind to Frizzled and low-density lipoprotein receptor-related protein (LRP) 5/6 which leads to the recruitment of Dishevelled (Dsh) and Axin. The formation of the complex of Axin, adenomatous Polyposis Coli (APC), and glycogen synthase kinase-3β (GSK3) is prevented and thus β-catenin is not degraded by this complex (with the help of casein kinase 1α [CK1]) which is canonical signaling. With noncanonical signaling, wnt3a binds to Frizzled without LRP 5/6. Activated Frizzled can lead to the activation of calcium/calmodulin-dependent kinase II (CamKII) and protein kinase C (PKC). Dsh is also able to activate Rac which in turn induces c-Jun N-terminal kinase (JNK). Moreover, Dsh can interact with DAAM1 to activate Rho which in turn activates ROCK. The last noncanonical pathway is through activation of adenylate cyclase (AC), which triggers cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) inhibiting the transcription factor NFATc1. This figure is a hypothetical estimation based on the literature discussed in this paper. Chon, chondrocytes; NPC, nucleus progenitor cell; MSC, mesenchymal stromal cell; PG, proteoglycan.

Fig. 2.

Signaling options initiated via wnt5a and the concluded outcome. Wnt5a can bind to receptor tyrosine kinase-like orphan receptor 2 (ROR2) which leads to the activation of c-Jun N-terminal kinase (JNK). ROR2 is also able to act as a coreceptor with Frizzled. Wnt5a can bind to Frizzled and ROR2 leading to the activation of calcium/calmodulin-dependent kinase II (CamKII) and protein kinase C (PKC). Dishevelled (Dsh) is also able to activate Rac which in turn induces JNK. Moreover, Dsh can interact with DAAM1 to activate Rho which in turn activates ROCK. The last pathway is through activation of adenylate cyclase (AC), which triggers cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) inhibiting the transcription factor NFATc1. This figure is a hypothetical estimation based on the literature discussed in this paper. Chon, chondrocytes; NPC, nucleus progenitor cell; MSC, mesenchymal stromal cell; PG, proteoglycan.

Fig. 3.

A simplified summary of wnt3a effects on different cell types under specific conditions towards potential regeneration of the intervertebral disc, concluded from available literature. (A) Nucleus pulposus cells are reported to not change in their chondrogenic potential, but can start proliferation upon wnt3a stimulation. However, when only one pathway is stimulated in particular, cell senescence can occur. (B) Chondrocytes are determined to start dedifferentiating after wnt3a supplementation. The cells become more potent but also less chondrogenic. (C) Mesenchymal stromal cells are observed to have different reactions upon wnt3a treatment. There appears to be a pattern as proliferation is upregulated after wnt3a supplementation, while chondrogenic differentiation occurs when wnt3a is supplemented and subsequently replaced with other growth factors (GFs).

Fig. 4.

A simplified summary of wnt5a effects on different cell types under specific conditions towards potential regeneration of the intervertebral disc, concluded from available literature. (A) Wnt5a induces redifferentiation of nucleus pulposus (NP) cells, but proliferation is reported to decrease compared to the control. (B) Chondrocytes are determined to start dedifferentiating after wnt5a supplementation. The cells become more potent but also less chondrogenic. (C) In mesenchymal stromal cells, wnt5a is solely capable of inducing chondrogenic differentiation in early stages and inhibits maturation and hypertrophy, maintaining a more potent chondrogenic state.