Selective autophagy bears bone

The endoplasmic reticulum (ER) is a dynamic intracellular network responsible for folding and maturation of organellar and secreted proteins. Selective autophagy of ER (ER‐phagy) is emerging as an essential process that maintains proteostasis in the ER and is regulated by growth conditions. In this issue, Cinque et al (2020) show that fibroblast growth factor 18 (FGF18) specifically activates ER‐phagy through a TFEB/TFE‐dependent transcriptional regulation of the ER‐phagy receptor Fam134b, a process essential for bone ossification and skeletal development.

A new study reveals that FGF18 specifically induces ER‐phagy in chondrocytes, thereby promoting bone ossification and skeletal development in vertebrates.

The ER is the largest cellular organelle that possesses multiple fundamental functions including protein folding, maturation, and trafficking. To maintain its function, it undergoes continuous turnover by the ubiquitin‐proteasome system in a process termed endoplasmic reticulum‐associated protein degradation (ERAD) and by a selective form of autophagy termed ER‐phagy (Chino & Mizushima, 2020). In ER‐phagy, ER fragments are sequestered into autophagosomes which then fuse with lysosomes to degrade their internal content. In mammals, ER‐phagy is mediated at distinct ER subdomains by various autophagy receptors, e.g., Fam134b, Ccpg1, Atl3, Sec62, Tex264, and Rtn3 (Hubner & Dikic, 2020). These receptors are ER membrane proteins that contain LC3 Interacting Region (LIR), through which they bind Atg8 family proteins and recruit ER fragments into autophagosomes. In their study, Cinque et al (2020) show that FGF18 signaling activates ER‐phagy in chondrocytes, mediated by Fam134b receptor, which is directly induced by TFEB/TFE. This signaling pathway allows the ER to respond to metabolic and developmental signals.

It has been reported that autophagy is a crucial developmental process in bone and cartilage, and its dysregulation leads to bone pathogenesis such as Paget disease, osteoarthritis, and mucopolysaccharidoses (Shapiro et al, 2014; Cinque et al, 2015). The growth factor FGF, a regulator of type II collagen (Col2) production in chondrocytes (Moore et al, 2005), was recently characterized by Settembre and coworkers as an essential autophagy mediator acting via FGFR4, JNK, and the autophagy initiation complex VPS34‐Beclin1 (Cinque et al, 2015).

In their current work, Settembre and colleagues systematically outline the entire FGF18 signaling pathway that leads to ER‐phagy activation in chondrocytes. This signaling pathway is initiated by FGF binding to FGFR3/4 receptors leading to mTORC1 inhibition via JNK‐mediated degradation of insulin receptor substrate 1 (IRS1), which in turn promotes TFEB/TFE nuclear translocation followed by specific induction of Fam134b expression and enhanced ER‐phagy. Consistent with its important role in ER‐phagy in this system, Fam134b depletion results in enlargement of ER cisterna and accumulation of cisternal marker CLIMP‐63. Moreover, substitution of the endogenous protein Fam134b with a LIR‐deficient mutant leads to an impaired secretion of angiogenic factors such as VEGFs and CTGF and of matrix remodeling factors such as Mmp13, which are typically produced in an FGF‐dependent manner in hypertrophic chondrocytes. The authors determined the physiological relevance of their findings by knocking down Fam134b in Medaka fish, which abolished FGF‐mediated ER‐phagy and led to accumulation of type II pro‐collagen. This also led to developmental abnormalities mainly restricted to bones such as short body length and aberrant head size. These findings establish a new FGF18 signaling pathway for the regulation of ER‐phagy in chondrocytes during endochondral ossification and skeletal development (Fig 1).

Figure 1. FGF18‐induced ER‐phagy in chondrocytes.

FGF18 signaling or prolonged starvation conditions induce JNK‐dependent degradation of IRS, leading to inhibition of the Akt‐mTORC1 pathway, followed by TFEB/TFE nuclear translocation, and Fam134b activation. The Fam134b promotes ER fragment sequestration into autophagosomes and their consequent lysosomal degradation.

A major effort has been invested to show that the Fam134b promotor is specifically regulated by TFEB/TFE through its DNA‐binding site (CLEAR), and mutation of this site abolished ER‐phagy induction. The authors also found that in chondrocytes lacking TFEB/TFE, FGF18 successfully enhanced autophagy flux but failed to activate ER‐phagy. ER‐phagy was also induced upon prolonged starvation following TFEB dephosphorylation, nuclear translocation, and Fam134b transcriptional induction, suggesting TFEB/TFE‐Fam134b axis as a general ER‐phagy activator. These results underscore TFEB/TFE role as a direct transcriptional activator of Fam134b in chondrocytes and more generally implicate for the first time TFEB/TFE in the regulation of selective autophagy.

Selective autophagy is essential for the maintenance of cellular homeostasis and therefore must be tightly controlled. The link between FGF signaling, TFEB/TFE, and ER‐phagy provides important new insights into regulatory mechanisms of such processes. This work comprehensively delineates the path that leads from FGF18 signaling down to ER‐phagy, while showing its physiological relevance. Still, several questions remain to be investigated: is Fam134b‐mediated ER‐phagy essential for processes other than regulating protein secretion in chondrocytes during skeletogenesis? Are other selective autophagy processes regulated by TFEB/TFE? How redundant are different Fam134b family members? What is the exact role of Fam134b‐dependent ER‐phagy under starvation?

The EMBO Journal (2020) 39: e105965