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Mitophagy is a form of autophagy specialized to selectively remove mitochondria. Although the PINK1/Parkin pathway is the best described mitophagy of damaged mitochondria, receptor/mediated mitophagy seems to have a pivotal role in cellular development and specialization. The most studied mitophagy receptor BCL2/adenovirus E1B 19‐kDa‐interacting protein 3‐like (BNIP3L/NIX) is shown to be important for the programmed removal of healthy mitochondria during terminal differentiation of erythrocytes, but its role has been proven in various cell types. Despite recent advances in our understanding of its regulation by phosphorylation and dimerization, there remain numerous questions on how BNIP3L/NIX tightly balances between cellular life and death decisions. This brief review intends to summarize ongoing dilemmas related to BNIP3L/NIX., The inactive form of the BCL2/adenovirus E1B 19‐kDa‐interacting protein 3‐like (BNIP3L/NIX) receptor is found as a monomer phosphorylated on C‐terminal serine 212. Upon mitophagy induction, the receptor is dephosphorylated and this allows BNIP3L/NIX monomers to form the active form of the receptor, BNIP3L/NIX dimer, which is also additionally activated with double phosphorylation near the LIR domain. Dimerization enables stronger recruitment of autophagosomes on selected mitochondria and more efficient mitophagy.

Mitophagy is a conserved intracellular catabolic process responsible for the selective removal of dysfunctional or superfluous mitochondria to maintain mitochondrial quality and need in cells. Here, we examine the mechanisms of receptor-mediated mitophagy activation, with the focus on BNIP3L/NIX mitophagy receptor, proven to be indispensable for selective removal of mitochondria during the terminal differentiation of reticulocytes. The molecular mechanisms of selecting damaged mitochondria from healthy ones are still very obscure. We investigated BNIP3L dimerization as a potentially novel molecular mechanism underlying BNIP3L-dependent mitophagy. Forming stable homodimers, BNIP3L recruits autophagosomes more robustly than its monomeric form. Amino acid substitutions of key transmembrane residues of BNIP3L, BNIP3L(G204A) or BNIP3L(G208V), led to the abolishment of dimer formation, resulting in the lower LC3A-BNIP3L recognition and subsequently lower mitophagy induction. Moreover, we identified the serine 212 as the main amino acid residue at the C-terminal of BNIP3L, which extends to the intermembrane space, responsible for dimerization. In accordance, the phosphomimetic mutation BNIP3L(S212E) leads to a complete loss of BNIP3L dimerization. Thus, the interplay between BNIP3L phosphorylation and dimerization indicates that the combined mechanism of LIR phosphorylation and receptor dimerization is needed for proper BNIP3L-dependent mitophagy initiation and progression. Abbreviations: AMBRA1: autophagy and beclin 1 regulator 1; Baf A1: bafilomycin A(1); BH3: BCL2 homology 3; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2 interacting protein 3 like; CCCP: carbonyl cyanide 3-chlorophenylhydrazone; CoCl(2): cobalt (II) chloride; FKBP8: FKBP prolyl isomerase 8; FUNDC1: FUN14 domain containing 1; GABARAP: GABA type A receptor-associated protein; GST: glutathione S-transferase; IMM: inner mitochondrial membrane; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; OMM: outer mitochondrial membrane; PHB2: prohibitin 2; PI: propidium iodide; PINK1: PTEN induced kinase 1; TM: transmembrane domain; TOMM20: translocase of outer mitochondrial membrane 20

Recent discoveries of autophagic receptors that recognize specific cellular cargo have opened a new chapter in the autophagy field. Selective removal of damaged organelles or protein aggregates is essential for proper cellular homeostasis and survival. Until today, autophagy receptors and their actions have been described in several aspects of selective autophagy including xenophagy, aggrephagy, pexophagy and mitophagy. Receptors have been proven indispensable for the initiation and finalization of specific cargo removal by autophagy. Mitophagy receptors, scAtg32, BNIP3 and BNIP3L/NIX, in LIR-dependent manner, interact with Atg8/LC3/GABARAP family proteins and recruit autophagy machinery to damaged mitochondria. Moreover, it has been shown that BNIP3L/NIX mediates mitochondrial clearance during reticulocyte differentiation. Recent discovery of its homologue BNIP3 enlightened the role of mitophagy receptors in cellular fate, where either survival by mitophagy or death by apoptosis is chosen. Autophagic function of the receptors is regulated by phosphorylation but what the trigger is for the phosphorylation and activation of mitophagy is still obscure. Fine-tuning of mitophagy regulation (and general autophagy) is the major challenge for researchers of our time and their answers will allow us to better understand the role of receptors and autophagy in disease development.