LGR4, a G Protein-Coupled Receptor With a Systemic Role: From Development to Metabolic Regulation

Abstract

Leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4/GPR48), a member of the GPCR (G protein-coupled receptors) superfamily, subfamily B, is a common intestinal crypt stem cell marker. It binds R-spondins/Norrin as classical ligands and plays a crucial role in Wnt signaling potentiation. Interaction between LGR4 and R-spondins initiates many Wnt-driven developmental processes, e.g., kidney, eye, or reproductive tract formation, as well as intestinal crypt (Paneth) stem cell pool maintenance. Besides the well-described role of LGR4 in development, several novel functions of this receptor have recently been discovered. In this context, LGR4 was indicated to participate in TGFβ and NFκB signaling regulation in hematopoietic precursors and intestinal cells, respectively, and found to be a new, alternative receptor for RANKL (Receptor Activator of NF kappa B Ligand) in bone cells. LGR4 inhibits the process of osteoclast differentiation, by antagonizing the interaction between RANK (Receptor Activator of NF kappa B) and its ligand-RANKL. It is also known to trigger anti-inflammatory responses in different tissues (liver, intestine, cardiac cells, and skin), serve as a sensor of the circadian clock in the liver, regulate adipogenesis and energy expenditure in adipose tissue and skeletal muscles, respectively. The extracellular domain of LGR4 (LGR4-ECD) has emerged as a potential new therapeutic for osteoporosis and cancer. LGR4 integrates different signaling pathways and regulates various cellular processes vital for maintaining whole-body homeostasis. Yet, the role of LGR4 in many cell types (e.g. pancreatic beta cells) and diseases (e.g., diabetes) remains to be elucidated. Considering the broad spectrum of LGR4 actions, this review aims to discuss both canonical and novel roles of LGR4, with emphasis on emerging research directions focused on this receptor.

Keywords: GPCRs (G protein-coupled receptors); LGR4; LGR4-ECD; NFκB - Nuclear Factor κB; RANKL (Receptor Activator for Nuclear Factor k B Ligand); Wnt signaling; inflammation; miRNA34 a/c.

Figure 1

LGR4 gene and protein structure. (A) LGR4 gene contains three main domains: leucine-rich repeats-containing N-terminal domain encoded by exons 1-17, responsible for binding ligands (RSPOs, NORRIN, RANKL); seven-transmembrane (7TM) domain encoded by exon 18, anchoring the receptor within the cell membrane; C-terminal domain encoded by exon 18, responsible for signal transduction. (B) Full-length LGR4 is composed of 951 aa, of which LGR4-ECD comprises aa. 28-528. Within LGR4-ECD aa.28-249 (LRRNT-LRR8) are responsible for RSPO1, whereas aa.80-396 (LRR1-12) for RANKL binding [modified from reference (11)]. (C) 3D structure of LGR4-ECD binding to RSPO1 and RANKL [modified from (12)].

Figure 2

Molecular mechanism of osteoclastogenesis inhibition by LGR4. RANKL (Ligand of Receptor-Activator of NFκB), a common ligand for the two receptors, RANK (Receptor-Activator of NFκB) and LGR4, has opposite effects on osteoclastogenesis. RANKL/RANK interaction (depicted in blue arrows) promotes the process through NFATC dephosphorylation, which allows it to enter the nucleus and stimulate gene expression for osteoclastogenesis. RANKL/LGR4 interaction (depicted in orange), on the other hand, inhibits osteoclastogenesis through activation of Gαq-mediated signaling to stabilize GSK3β and prevent its phosphorylation (inactivation). Activated GSK3β phosphorylates NFATC preventing its entry into the nucleus, thus inhibiting osteoclastogenesis. Excessive stimulation of the RANKL/RANK pathway induces Lgr4 expression as a negative feedback mechanism to finely control the process (11).

Figure 3

Significance of natural and synthetic extracellular domain of LGR4 (LGR4-ECD). Naturally occurring LGR4-ECD controls gonadal development by binding RSPO1 and preventing its participation in Wnt signaling activation in gonadal cells. Synthetic LGR4-ECD by binding RANKL and preventing its interaction with RANK has an anti-osteoporotic effect. Synthetic LGR4-ECD by neutralizing RSPO1 disables its interaction with LGR4, overexpressed by cancer cells, and thus, holds an anti-cancer potential.

Figure 4

Systemic functions of LGR4. LGR4 plays a significant role in embryonic development/stem cell maintenance through its binding to RSPO1 and regulation of the WNT signaling pathway. LGR4 reduces inflammation in multiple tissues through inhibition of NFκB signaling and cytokine production. LGR4 affects metabolism through multiple mechanisms including circadian clock, oxidative and glycolytic pathways regulation.