Ulk1 plays a critical role in the autophagic clearance of mitochondria and ribosomes during reticulocyte maturation

Abstract

Production of a red blood cell's hemoglobin depends on mitochondrial heme synthesis. However, mature red blood cells are devoid of mitochondria and rely on glycolysis for ATP production. The molecular basis for the selective elimination of mitochondria from mature red blood cells remains controversial. Recent evidence suggests that clearance of both mitochondria and ribosomes, which occurs in reticulocytes following nuclear extrusion, depends on autophagy. Here, we demonstrate that Ulk1, a serine threonine kinase with homology to yeast atg1p, is a critical regulator of mitochondrial and ribosomal clearance during the final stages of erythroid maturation. However, in contrast to the core autophagy genes such as atg5 and atg7, expression of ulk1 is not essential for induction of macroautophagy in response to nutrient deprivation or for survival of newborn mice. Together, these data suggest that the ATG1 homologue, Ulk1, is a component of the selective autophagy machinery that leads to the elimination of organelles in erythroid cells rather that an essential mechanistic component of autophagy. IntroductionErythroid differentiation involves progression through morphologically distinct nucleated precursor stages, from proerythroblast to orthochromatic erythroblasts, prior to enucleation and maturation of the nascent reticulocyte. This process results in a successive reduction in cell volume, massive increase in hemoglobin production and conversion to a purely glycolytic pathway for energy production. Unlike most cells, mature red blood cells have no nucleus or organelles. Following enucleation of orthochromatic erythroblasts, nascent reticulocytes mature over the course of 48 to 72 hours and are cleared of all intracellular organelles, including mitochondria and ribosomes. Approximately 30% of red cell hemoglobin is produced in reticulocytes, and since heme is synthesized in the mitochondria these organelles are among the last to be eliminated (reviewed in Geminard et al 1 ). The programmed clearance of mitochondria that occurs in reticulocytes makes it an ideal system for studying the molecular pathways involved in mitochondrial degradation. Understanding the process by which cells degrade mitochondria is important not only for red cell maturation, but also for other cell types as accumulation of damaged or dysfunctional mitochondria has been implicated in aging and various pathologic processes such as diabetes, cancer, neurodegeneration, and ␣1 antitrypsin disease (reviewed in Pieczenik and Neustadt 2 ).The molecular pathways involved in mitochondrial degradation in reticulocytes are unknown, although several important insights have been made into this process. 15-Lipoxygenase is a lipid-peroxidizing enzyme whose expression peaks in reticulocytes shortly before organelle degradation. 3 It integrates into organelle membranes, disrupts mitochondrial membrane integrity allowing release of proteins from the organelle lumen and proteasome-dependent degradation of lumenal and integral membrane proteins. 4,5 Chemical inh...

Mitochondrial network in the heart

“…It regulates mitochondria number to match the metabolic or developmental demands. In addition, it removes the damaged mitochondria (Tolkovsky et al, 2002;Kundu et al, 2008). Pink1 and Parkin form an axis that regulates mitophagy in mammalian cells.…”

“…Pink1 triggers Parkin translocation from cytosol to mitochondria where Parkin ubiquitinates mitochondrial proteins to form autophagosomes (Geisler et al, 2010). In red blood cells, for example, Nix may recruit isolated membranes to mitochondria (Schweers et al, 2007), a process in which UNC51-like kinase1 (ULK1) and Atg7 are involved in the removal of mitochondria (Kundu et al, 2008;Mortensen et al, 2010).…”

ULK-Atg13-FIP200 Complexes Mediate mTOR Signaling to the Autophagy Machinery

“…We could not determine contribution of each ULK to FIP200 and Atg13 phosphorylation, which may require double knockout or knockdown cells. Given that ULK1 null mice is viable (Kundu et al, 2008), it may be possible that ULK1 and ULK2 have redundant functions. However, based on the mobility shift assay in Figure 6j, it is likely that ULK1 is the major kinase phosphorylating FIP200 and Atg13 in response to rapamycin and nutrient starvation at least in MEF and 293T cells.…”

“…Atg17, which is also required for Atg1 activity, is specifically involved in autophagy but not in the Cvt pathway and is proposed to play a scaffold role (Suzuki et al, 2007). In mammals, two Atg1 homologues ULK1 and ULK2 have been identified and investigated (Chan et al, 2007;Hara et al, 2008;Kundu et al, 2008). Although overexpression of the kinase-dead mutants of ULK1 or ULK2 led to inhibition of autophagy, their roles in the regulation of autophagy have not been clearly defined.…”

The autophagy conundrum in cancer: influence of tumorigenic metabolic reprogramming

“…Autophagy also supports cellular homeostasis in multicellular organisms during fasting. However, in metazoans, autophagy contributes to a diversity of other processes, such as developmental cell death (Berry and Baehrecke, 2007;Denton et al, 2009), clearance of harmful protein aggregates (Renna et al, 2010) and mitochondrial clearance during erythrocyte maturation (Kundu et al, 2008;Sandoval et al, 2008;Novak et al, 2010). By targeting damaged macromolecules and organelles for degradation, autophagy also protects cells from both internal and external insults, including metabolic, oxidative and chemotherapeutic stress.…”