Downregulation of Mitochondrial TSPO Inhibits Mitophagy and Reduces Enucleation during Human Terminal Erythropoiesis.

Martina Moras,Caroline Le Van Kim,Sophie D Lefevre,Sandrina Kinet,Naomi Taylor,Suella Martino,Pedro Gonzalez-Menendez,Jerome Larghero,Mariano A Ostuni,Claude Hattab

doi:10.3390/ijms21239066

Abstract

Translocator protein (TSPO) and voltage dependent anion channels (VDAC) are two proteins forming a macromolecular complex in the outer mitochondrial membrane that is involved in pleiotropic functions. Specifically, these proteins were described to regulate the clearance of damaged mitochondria by selective mitophagy in non-erythroid immortalized cell lines. Although it is well established that erythroblast maturation in mammals depends on organelle clearance, less is known about mechanisms regulating this clearance throughout terminal erythropoiesis. Here, we studied the effect of TSPO1 downregulation and the action of Ro5-4864, a drug ligand known to bind to the TSPO/VDAC complex interface, in ex vivo human terminal erythropoiesis. We found that both treatments delay mitochondrial clearance, a process associated with reduced levels of the PINK1 protein, which is a key protein triggering canonical mitophagy. We also observed that TSPO1 downregulation blocks erythroblast maturation at the orthochromatic stage, decreases the enucleation rate, and increases cell death. Interestingly, TSPO1 downregulation does not modify reactive oxygen species (ROS) production nor intracellular adenosine triphosphate (ATP) levels. Ro5-4864 treatment recapitulates these phenotypes, strongly suggesting an active role of the TSPO/VDAC complex in selective mitophagy throughout human erythropoiesis. The present study links the function of the TSPO/VDAC complex to the PINK1/Parkin-dependent mitophagy induction during terminal erythropoiesis, leading to the proper completion of erythroid maturation.

Highlights

Erythropoiesis is a multistep differentiation process that gives rise to mature erythrocytes
Several studies have shown upon erythroid differentiation, clearance of mitochondria occurs at the reticulocyte stage by mitophagy [9,10,11]
As these studies were mostly performed on mice, it is still unclear the mechanisms occurring during human erythroblast maturation

Summary

Introduction

Erythropoiesis is a multistep differentiation process that gives rise to mature erythrocytes. During the end of the terminal maturation, erythroblasts lose all their organelles, such as the Golgi apparatus, endoplasmic reticulum (ER), ribosomes, and mitochondria (reviewed in [1]). This elimination can occur both by unspecific or by selective macro-autophagy. Activation of autophagy-related proteins (ATG) leads to the formation of the phagophore, a membrane structure originated from ER, that surrounds cytoplasmic cargos forming the autophagosome. This late migrates and fuses to lysosomes, where cargos are degraded [2,3]. Bulk and selective macroautophagy share the same core machinery, this later involves specific proteins exposed at the surface of targeted organelles as a signal to recruit the phagophore [4]

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