Suppression of MAPK/JNK-MTORC1 signaling leads to premature loss of organelles and nuclei by autophagy during terminal differentiation of lens fiber cells

Abstract

Although autophagic pathways are essential to developmental processes, many questions still remain regarding the initiation signals that regulate autophagy in the context of differentiation. To address these questions we studied the ocular lens, as the programmed elimination of nuclei and organelles occurs in a precisely regulated spatiotemporal manner to form the organelle-free zone (OFZ), a characteristic essential for vision acuity. Here, we report our discovery that inactivation of MAPK/JNK induces autophagy for formation of the OFZ through its regulation of MTORC1, where MAPK/JNK signaling is required for both MTOR activation and RPTOR/RAPTOR phosphorylation. Autophagy pathway proteins including ULK1, BECN1/Beclin 1, and MAP1LC3B2/LC3B-II were upregulated in the presence of inhibitors to either MAPK/JNK or MTOR, inducing autophagic loss of organelles to form the OFZ. These results reveal that MAPK/JNK is a positive regulator of MTORC1 signaling and its developmentally regulated inactivation provides an inducing signal for the coordinated autophagic removal of nuclei and organelles required for lens function.

Role of the MAPK/cJun NH 2 -terminal kinase signaling pathway in starvation-induced autophagy

“…Indeed, previous studies have demonstrated that MAPK8/9 can be activated in response to TORC1 signaling [35] and that TORC1 activation can require MAPK8/9 signaling [28]. However, we found no requirement for MAPK8/9 upstream (Figure 1) or downstream (Figure 2) of TORC1 during starvation-induced autophagy.…”

“…The requirement of MAPK8 for starvation-induced autophagy [5] may therefore reflect a role for MAPK8 upstream or down-stream of MTOR. Indeed, previous studies have demonstrated that MAPK8/9 can be activated in response to TORC1 signaling [35] and that TORC1 activation can require MAPK8/9 signaling [28]. Alternatively, MAPK8/9 and TORC1 may function in parallel pathways that control autophagy.…”

“…It was therefore possible that MAPK8/9 functions upstream of MTOR in the regulation of autophagy by starvation [28]. To test this hypothesis, we examined TORC1 signaling in response to starvation.…”

Di-retinoid-pyridinium-ethanolamine (A2E) Accumulation and the Maintenance of the Visual Cycle Are Independent of Atg7-mediated Autophagy in the Retinal Pigmented Epithelium

“…In lens development, deficiency of Atg5 or phosphatidylinositol 3-kinase catalytic subunit type (Pik3c3) failed to abolish organelle-free zone (ONF) formation required for lens transparency (50). But more recent data indicate that the mammalian target of rapamycin complex 1 (mTOR-1) signaling is required for proper development of the OFZ in lens (51). These contrasting observations could be explained by differences in gene targeting approaches (choice of exon deletion), cell type (context) dependence, influence of genetic background, and timing of examinations .…”

Chromatin remodeling enzyme Snf2h regulates embryonic lens differentiation and denucleation

“…Lens organelle degradation has recently been linked to autophagy-related processes (Basu et al, 2014) and mitophagy (Costello et al, 2013). We first examined the degradation of mitochondria (visualized by Tomm20 antibodies) and of ER (visualized by PDI antibodies) in control and Snf2h-depleted lenses.…”