Clinical and biological characterization of 20 patients with TANGO2 deficiency indicates novel triggers of metabolic crises and no primary energetic defect.

TANGO2 disease is a severe inherited disorder associating multiple symptoms such as metabolic crises, encephalopathy, cardiac arrhythmias, and hypothyroidism. The mechanism of action of TANGO2 is currently unknown. Here, we describe a cohort of 20 French patients bearing mutations in the TANGO2 gene. We found that the main clinical presentation was the association of neurodevelopmental delay (n = 17), acute metabolic crises (n = 17) and hypothyroidism (n = 12), with a large intrafamilial clinical variability. Metabolic crises included rhabdomyolysis (15/17), neurological symptoms (14/17), and cardiac features (12/17; long QT (n = 10), Brugada pattern (n = 2), cardiac arrhythmia (n = 6)) that required intensive care. We show previously uncharacterized triggers of metabolic crises in TANGO2 patients, such as some anesthetics and possibly l-carnitine. Unexpectedly, plasma acylcarnitines, plasma FGF-21, muscle histology, and mitochondrial spectrometry were mostly normal. Moreover, in patients' primary myoblasts, palmitate and glutamine oxidation rates, and the mitochondrial network were also normal. Finally, we found variable mitochondrial respiration and defective clearance of oxidized DNA upon cycles of starvation and refeeding. We conclude that TANGO2 disease is a life-threatening disease that needs specific cardiac management and anesthesia protocol. Mechanistically, TANGO2 disease is unlikely to originate from a primary mitochondrial defect. Rather, we suggest that mitochondrial defects are secondary to strong extrinsic triggers in TANGO2 deficient patients.

MHC Class I Cross-Presentation: Stage Lights on Sec22b.

Efficient (cross-)presentation of antigens internalized by dendritic cells (DCs) requires vesicular communication between the early secretory and the endocytic/phagocytic pathways, in which the Sec22b protein has been suggested to have a key role. Here, we undertake a critical assessment of two new studies that evaluate the role of Sec22b using gene-targeted mice and come to contradictory conclusions.

Dissociation of ß2-microglobulin determines the surface quality control of major histocompatibility complex class I molecules.

Major histocompatibility complex class I proteins, which present antigenic peptides to cytotoxic T lymphocytes at the surface of all nucleated cells, are endocytosed and destroyed rapidly once their peptide ligand has dissociated. The molecular mechanism of this cellular quality control process, which prevents rebinding of exogenous peptides and thus erroneous immune responses, is unknown. To identify the nature of the decisive step in endocytic sorting of class I molecules and its location, we have followed the removal of optimally and suboptimally peptide-loaded murine H-2K(b) class I proteins from the cell surface. We find that the binding of their light chain, ß2-microglobulin (ß2m), protects them from endocytic destruction. Thus, the extended survival of suboptimally loaded K(b) molecules at 25°C is attributed to decreased dissociation of ß2m. Because all forms of K(b) are constantly internalized but little ß2m-receptive heavy chain is present at the cell surface, it is likely that ß2m dissociation and recognition of the heavy chain for lysosomal degradation take place in an endocytic compartment.

Mycobacterium intracellulare-Related Immune Reconstitution Syndrome in an HIV Patient: A Case Report.

Immune reconstitution syndrome (IRIS) is an uncommon complication seen in patients living with human immunodeficiency virus (HIV) characterized by the decline in a pre-existing condition or a new diagnosis of a masked disease. It is associated with a significant inflammatory component that occurs after starting antiretrovirals, being most frequent in those with severe immunosuppression. Thereby, different types of opportunistic diseases such as non-tuberculous mycobacteria are more frequent in this group of patients, especially those with low lymphocyte counts of below 50 cells/µL. Here, we present the case of an HIV-positive patient who developed lymphadenitis caused by Mycobacterium intracellulare as an unmasked IRIS after initiating treatment for HIV.

Gross anatomy of the craniolateral antebrachial muscles in kinkajou (Potos flavus, Carnivora): Intra- and interspecific variants within the family Procyonidae.

The kinkajou (Potos flavus) is a carnivoran that has special morphological adaptations in its thoracic limbs to climb threes and handling food. This study aimed to describe the craniolateral muscles of the antebrachium in P. flavus. Five specimens were used. The extensor carpi radialis longus and brevis were seen as two separate muscles. The extensor carpi ulnaris muscle can present two bellies with different insertions. An extensor digit III muscle was found. The P. flavus has anatomical variants on the craniolateral antebrachial muscles that were previously reported in other procyonids. Thus, those variants are phylogenetically related within the family Procyonidae, except the doubled bellies of the extensor carpi ulnaris muscle which is exclusive of P. flavus.

FDX2 and ISCU Gene Variations Lead to Rhabdomyolysis With Distinct Severity and Iron Regulation.

BACKGROUND AND OBJECTIVES: To determine common clinical and biological traits in 2 individuals with variants in ISCU and FDX2, displaying severe and recurrent rhabdomyolyses and lactic acidosis. METHODS: We performed a clinical characterization of 2 distinct individuals with biallelic ISCU or FDX2 variants from 2 separate families and a biological characterization with muscle and cells from those patients. RESULTS: The individual with FDX2 variants was clinically more affected than the individual with ISCU variants. Affected FDX2 individual fibroblasts and myoblasts showed reduced oxygen consumption rates and mitochondrial complex I and PDHc activities, associated with high levels of blood FGF21. ISCU individual fibroblasts showed no oxidative phosphorylation deficiency and moderate increase of blood FGF21 levels relative to controls. The severity of the FDX2 individual was not due to dysfunctional autophagy. Iron was excessively accumulated in ISCU-deficient skeletal muscle, which was accompanied by a downregulation of IRP1 and mitoferrin2 genes and an upregulation of frataxin (FXN) gene expression. This excessive iron accumulation was absent from FDX2 affected muscle and could not be correlated with variable gene expression in muscle cells. DISCUSSION: We conclude that FDX2 and ISCU variants result in a similar muscle phenotype, that differ in severity and skeletal muscle iron accumulation. ISCU and FDX2 are not involved in mitochondrial iron influx contrary to frataxin.

The role of MHC class I recycling and Arf6 in cross-presentation by murine dendritic cells.

Cross-presentation by MHC class I molecules (MHC-I) is critical for priming of cytotoxic T cells. Peptides derived from cross-presented antigens can be loaded on MHC-I in the endoplasmic reticulum and in endocytic or phagocytic compartments of murine DCs. However, the origin of MHC-I in the latter compartments is poorly understood. Recently, Rab22-dependent MHC-I recycling through a Rab11+ compartment has been suggested to be implicated in cross-presentation. We have examined the existence of MHC-I recycling and the role of Arf6, described to regulate recycling in nonprofessional antigen presenting cells, in murine DCs. We confirm folded MHC-I accumulation in a juxtanuclear Rab11+ compartment and partially localize Arf6 to this compartment. MHC-I undergo fast recycling, however, both folded and unfolded internalized MHC-I fail to recycle to the Rab11+Arf6+ compartment. Therefore, the source of MHC-I molecules in DC endocytic compartments remains to be identified. Functionally, depletion of Arf6 compromises cross-presentation of immune complexes but not of soluble, phagocytosed or mannose receptor-targeted antigen, suggesting a role of Fc receptor-regulated Arf6 trafficking in cross-presentation of immune complexes.

Impact of the TAP-like transporter in antigen presentation and phagosome maturation.

Cross-presentation is thought to require transport of proteasome-generated peptides by the TAP transporters into MHC class I loading compartments for most antigens. However, a proteasome-dependent but TAP-independent pathway has also been described. Depletion of the pool of recycling cell surface MHC class I molecules available for loading with cross-presented peptides might partly or largely account for the critical role of TAP in cross-presentation of phagocytosed antigens. Here we examined a potential role of the homodimeric lysosomal TAP-like transporter in cross-presentation and in presentation of endogenous peptides by MHC class II molecules. We find that TAP-L is strongly recruited to dendritic cell phagosomes at a late stage, when internalized antigen and MHC class I molecules have been degraded or sorted away from phagosomes. Cross-presentation of a receptor-targeted antigen in vitro and of a phagocytosed antigen in vivo, as well as presentation of a cytosolic antigen by MHC class II molecules, is not affected by TAP-L deficiency. However, accumulation in vitro of a peptide optimally adapted to TAP-L selectivity in purified phagosomes is abolished by TAP-L deficiency. Unexpectedly, we find that TAP-L deficiency accelerates phagosome maturation, as reflected in increased Lamp2b recruitment and enhanced proteolytic degradation of phagocytosed antigen and in vitro transported peptides. Although additional experimentation will be required to definitely conclude on the role of TAP-L in transport of peptides presented by MHC class I and class II molecules, our data suggest that the principal role of TAP-L in dendritic cells may be related to regulation of phagosome maturation.

Endocytic Recycling of MHC Class I Molecules in Non-professional Antigen Presenting and Dendritic Cells.

Major histocompatibility complex class I (MHC I) molecules are glycoproteins that display peptide epitopes at the cell surface of nucleated cells for recognition by CD8+ T cells. Like other cell surface receptors, MHC class I molecules are continuously removed from the surface followed by intracellular degradation or recycling to the cell surface, in a process likely involving active quality control the mechanism of which remains unknown. The molecular players and pathways involved in internalization and recycling have previously been studied in model cell lines such as HeLa. However, dendritic cells (DCs), which rely on a specialized endocytic machinery that confers them the unique ability to "cross"-present antigens acquired by internalization, may use distinct MHC I recycling pathways and quality control mechanisms. By providing MHC I molecules cross-presenting antigens, these pathways may play an important role in one of the key functions of DCs, priming of T cell responses against pathogens and tumors. In this review, we will focus on endocytic recycling of MHC I molecules in various experimental conditions and cell types. We discuss the organization of the recycling pathway in model cell lines compared to DCs, highlighting the differences in the recycling rates and pathways of MHC I molecules between various cell types, and their putative functional consequences. Reviewing the literature, we find that conclusive evidence for significant recycling of MHC I molecules in primary DCs has yet to be demonstrated. We conclude that endocytic trafficking of MHC class I in DCs remains poorly understood and should be further studied because of its likely role in antigen cross-presentation.

Innate Immune Signals Induce Anterograde Endosome Transport Promoting MHC Class I Cross-Presentation.

Both cross-presentation of antigens by dendritic cells, a key pathway triggering T cell immunity and immune tolerance, and survival of several pathogens residing in intracellular vacuoles are intimately linked to delayed maturation of vesicles containing internalized antigens and microbes. However, how early endosome or phagosome identity is maintained is incompletely understood. We show that Toll-like receptor 4 (TLR4) and Fc receptor ligation induces interaction of the GTPase Rab14 with the kinesin KIF16b mediating plus-end-directed microtubule transport of endosomes. As a result, Rab14 recruitment to phagosomes delays their maturation and killing of an internalized pathogen. Enhancing anterograde transport by overexpressing Rab14, promoting the GTP-bound Rab14 state, or inhibiting retrograde transport upregulates cross-presentation. Conversely, reducing Rab14 expression, destabilizing Rab14 endosomes, and inhibiting anterograde microtubule transport by Kif16b knockdown compromise cross-presentation. Therefore, regulation of early endosome trafficking by innate immune signals is a critical parameter in cross-presentation by dendritic cells.