Mitochondrial dysfunction in the erythroid compartment

Abstract

Erythroid mitochondrial retention, a feature associated with impairments in the ubiquitin-proteasome system, is detected in a subset of pediatric lupus patients and is associated with the type I Interferon pathway.

Systemic lupus erythematosus (SLE) is an autoimmune syndrome characterized by dysregulation of the innate and adaptive immune systems, the formation of autoantibodies and immune complexes and, in many patients, a characteristic type I interferon (IFN) response that is generally considered to have pathogenic effects and contribute to multi-organ and vascular damage¹. Evidence also implicates mitochondrial dysfunction, impaired mitophagy and enhanced mitochondrial DNA oxidation and leakage in the induction of autoantigen externalization and modification, immune dysregulation (including the activation of the type I IFN and NLRP3 inflammasome pathways) and organ damage in SLE and other systemic autoimmune diseases ^2,3. These mitochondrial abnormalities have been reported to occur in various innate and adaptive immune cells in SLE, including neutrophils and T cells, and might affect activation status and a variety of cellular functions ^4,5. Mitochondrial dysfunction and release of mitochondrial antigens in SLE has also been described for non-immune blood components such as platelets ⁶. Now, Caielli et al.⁷ report erythroid mitochondrial retention in patients with a subset of childhood-onset SLE (cSLE), triggered by a hypoxia-inducible factor 2α (HIF-2α)-induced metabolic switch that activates the ubiquitin-proteasome system (UPS), resulting in increased type I IFN responses in myeloid cells.

Erythropoiesis is a series of well-coordinated and fine-tuned steps that lead to the production of mature red blood cells (RBCs). A well-established inducer of erythropoiesis is hypoxia, which promotes RBC generation through the induction of HIF, particularly HIF-2α that upregulates erythropoietin transcription and synthesis, leading to bone marrow erythropoiesis ⁸. During erythropoiesis, hematopoietic stem cells generate erythroid progenitors that proliferate and mature into RBCs. Some of these steps require nuclear expulsion and mitochondrial clearance from more immature erythroid forms, which occurs through autophagy-dependent mechanisms (mitophagy). Mitochondria are reprogrammed to drive the differentiation process before being eliminated from RBCs.

Based on previously established systems, Caielli et al.⁷ generated PBMC-derived RBCs in vitro to study mitochondrial removal during erythropoiesis and report a HIF-2α-induced metabolic switch that activates the ubiquitin-proteasome system (UPS), a major intracellular protein degradation complex, and that this phenomenon is needed for mitophagy in RBCs. They present evidence that proteasomal degradation of selected mitochondrial proteins is important in triggering mitophagy in human pro-erythroblasts. The authors found that a subset of patients with cSLE have a defect in RBC mitophagy leading to accumulation of mitochondria-carrying RBCs, in association with higher disease activity, whereas such findings were not evident in 8 patients with juvenile dermatomyositis, another condition characterized by a type I IFN gene signature.

To show that UPS dysfunction can promote aberrant mitophagy, the authors studied RBCs from individuals with chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature (CANDLE) syndrome, a pediatric autoinflammatory condition characterized (in most patients) by biallelic mutations in PSMB8 (the gene encoding the β5i catalytic subunit of the immunoproteasome) that cause abnormal immunoproteasomal function ⁹. Indeed, they found that patients with CANDLE syndrome also had deficient mitochondrial removal in association with impaired UPS activity, suggesting that dysfunction of the immunoproteasome can promote abnormalities in mitophagy during erythropoiesis. Furthermore, when macrophages internalized anti-RBC-opsonized, mitochondria-carrying RBCs, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway was induced, a pathway that senses cytosolic DNA and induces transcription of inflammatory genes, including type I IFNs. This finding is consistent with previous reports that showed that exposure of myeloid cells to mitochondrial antigens/DNA and/or enhanced oxidation of nucleic acids can activate IFN responses through the cGAS-STING pathway ^4,10. Finally, the authors report a correlation between the circulating type I IFN signature and the level of mitochondrial-carrying RBCs in cSLE, suggesting that these cells contribute to the induction of this signature in vivo. These data support previous work indicating that, in addition to plasmacytoid dendritic cells, myeloid cells might be an important source of type I IFNs in SLE ¹¹.

Similar abnormalities in erythroid mitophagy have been reported in conditions such as sickle cell disease, in association with increased levels of circulating mitochondrial DNA, induction of neutrophil extracellular traps (NETs) and the cGAS-STING pathway ¹⁰. Of interest, aberrant NET formation has been proposed to have important pathogenic functions in SLE, in which a subset of neutrophils form NETs enriched in oxidized mitochondrial DNA, also with downstream interferogenic effects through STING-dependent mechanisms ⁴. Whether these mitochondrial-carrying RBCs in SLE also promote NET formation, which could amplify the generation of oxidized nucleic acids and interferogenic responses, is unclear. Furthermore, whether similar mechanisms affect impaired mitophagy in sickle cell disease and SLE, or additional conditions, remains to be identified.

Overall, the the study by Caielli et al.⁷ further highlights that dysregulation of mitochondrial function and mitophagy could have prominent effects in the induction of aberrant type I IFN responses in SLE and that RBCs are another cell type in which mitophagy is dysregulated in this disease, linking it with UPS abnormalities. Despite these provocative findings, some limitations should be highlighted. Whether this model of PBMC-derived RBCs recapitulates what occurs in the lupus human bone marrow requires validation. Whether there are any links between changes in eythropoietin levels in patients with lupus nephritis and the RBC abnormalities detected in these patients is also unclear. Furthermore, the function of HIF-2α dysfunction in the abnormalities described requires further characterization in vivo. The study was conducted using cSLE samples; whether similar abnormalities are relevant to RBCs from adults with SLE is unclear. Importantly, the direct contributions to the IFN signature and immune dysregulation triggered by these mitochondrial-carrying RBCs, when compared to immune cells or platelets, was not studied in vivo and the association remains correlational at this stage.

Moving forward, it will be important to evaluate the drivers of mitochondrial retention in RBCs and of impaired mitophagy in SLE in general. Evidence for aberrant proteasomal degradation has been described previously in SLE ¹² and the interplay of dysregulated proteasome function with ROS and mitochondrial dysfunction in SLE needs to be better characterized. Furthermore, whether specific genetic polymorphisms promote aberrant mitophagy or mitochondrial dysfunction in a subset of patients with SLE should be identified. Importantly, whether patients that exhibit retention of mitochondria in RBCs also have more global mitochondrial dysfunction and enhanced mitochondrial DNA oxidation in other cell types, and overall immune dysregulation and organ damage needs to be analyzed in future studies. Overall, the results from this study emphasize the need to better characterize the mechanisms driving mitochondrial dysfunction in lupus and other systemic autoimmune diseases and suggest that targeting dysregulation of these organelles could have therapeutic benefits.

Figure 1. Deficient mitochondrial removal and association with type I interferon responses.

Defective HIF-2α degradation impairs UPS activation and mitophagy. A percentage of patients with childhood onset SLE display dysregulated erythropoiesis characterized by deficient mitophagy, in association with abnormal regulation of HIF. Mitochondria containing RBCs, when opsonized by antibodies and then engulfed by macrophages, activate the cGAS-STING pathway leading to increased type I IFN synthesis.