Delay of EGF-Stimulated EGFR Degradation in Myotonic Dystrophy Type 1 (DM1)

Abstract

Myotonic dystrophy type 1 (DM1) is an autosomal dominant disease caused by a CTG repeat expansion in the 3′ untranslated region of the dystrophia myotonica protein kinase gene. AKT dephosphorylation and autophagy are associated with DM1. Autophagy has been widely studied in DM1, although the endocytic pathway has not. AKT has a critical role in endocytosis, and its phosphorylation is mediated by the activation of tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR). EGF-activated EGFR triggers the internalization and degradation of ligand–receptor complexes that serve as a PI3K/AKT signaling platform. Here, we used primary fibroblasts from healthy subjects and DM1 patients. DM1-derived fibroblasts showed increased autophagy flux, with enlarged endosomes and lysosomes. Thereafter, cells were stimulated with a high concentration of EGF to promote EGFR internalization and degradation. Interestingly, EGF binding to EGFR was reduced in DM1 cells and EGFR internalization was also slowed during the early steps of endocytosis. However, EGF-activated EGFR enhanced AKT and ERK1/2 phosphorylation levels in the DM1-derived fibroblasts. Therefore, there was a delay in EGF-stimulated EGFR endocytosis in DM1 cells; this alteration might be due to the decrease in the binding of EGF to EGFR, and not to a decrease in AKT phosphorylation.

Mitochondrial Dysfunction in Repeat Expansion Diseases

“…Additionally, Ca 2+ homeostasis is deregulated in these pathologies, with a general increase in Ca 2+ levels in all of them that leads to an exacerbated mitochondrial Ca 2+ uptake, except in C9orf72, where a decrease in mitochondrial Ca 2+ entry has been observed [ 56 , 57 , 58 ]. Mitophagy is also affected, with a decrease in HD and in C9orf72 FTD/ALS, while there is an increase in some autophagy markers in DM1 [ 43 , 51 , 59 , 60 ]. Finally, an increase in intrinsic apoptosis has been observed in all three diseases [ 61 , 62 , 63 ], possibly due to accumulated mitochondrial damage.…”

“…Additionally, the increase of mitochondrial fission (phosphor-DRP1Ser616), autophagic and mitophagic markers (BNIP3 and Parkin) in DM1 models ( Figure 4 ) allow us to think that basal autophagy might be triggered, but it is not enough to conduct the clearance of dysfunctional mitochondria and needs to be enhanced. It would be interesting to study whether mitochondria are being sequestered for degradation or if the degradation process is temporary delayed, as reported in DM1-derived fibroblasts [ 60 ].…”

“…Nevertheless, it would be interesting to address the impact of metformin on autophagy in these disorders. While autophagy induction seems to be positive for HD [ 147 ] and C9orf72-ALS/FTD [ 106 ], DM1 has been associated to abnormally induced basal autophagy [ 60 ]. Due to this, researchers have focused on inhibiting autophagy in DM1 models.…”

“…However, metformin treatment in DM1 fibroblasts was able to induce a partial recovery in mitochondrial metabolism [ 54 ]. Since a delay in the degradation process of proteins has been reported in DM1 [ 60 ], it would be interesting to investigate whether this protective effect of metformin is inherent to autophagy regulation or its autophagy-inducing effect. Moreover, aerobic exercise, another well-known autophagy inducer [ 152 ], has also shown beneficial effects in DM1 patients [ 51 , 55 ].…”