B Cell Endosomal RAB7 Promotes TRAF6 K63 Polyubiquitination and NF-κB Activation for Antibody Class-Switching

Role of canonical and noncanonical autophagy pathways in shaping the life journey of B cells

Autophagy is a regulated intracellular catabolic process by which invading pathogens, damaged organelles, aggregated proteins, and other macromolecules are degraded in lysosomes. It has been widely appreciated that autophagic activity plays an important role in regulating the development, fate determination, and function of cells in the immune system, including B lymphocytes. Autophagy encompasses several distinct pathways that have been linked to B cell homeostasis and function. While B cell presentation of major histocompatibility complex (MHC) class II-restricted cytosolic antigens to T cells involves both macroautophagy and chaperone-mediated autophagy (CMA), plasma cells and memory B cells mainly rely on macroautophagy for their survival. Emerging evidence indicates that core autophagy factors also participate in processes related to yet clearly distinct from classical autophagy. These autophagy-related pathways, referred to as noncanonical autophagy or conjugation of ATG8 to single membranes (CASM), contribute to B cell homeostasis and functions, including MHC class II-restricted antigen presentation to T cells, germinal center formation, plasma cell differentiation, and recall responses. Dysregulation of B cell autophagy has been identified in several autoimmune and autoinflammatory diseases such as systemic lupus erythematosus, rheumatoid arthritis, and inflammatory bowel disease. In this review, we discuss recent advances in understanding the role of canonical and noncanonical autophagy in B cells, including B cell development and maturation, antigen processing and presentation, pathogen-specific antibody responses, cytokine secretion, and autoimmunity. Unraveling the molecular mechanisms of canonical and noncanonical autophagy in B cells will improve our understanding of B cell biology, with implications for the development of autophagy-based immunotherapies.

The Small GTPase Rab7 Regulates Antigen Processing in B Cells in a Possible Interplay with Autophagy Machinery

In B cells, antigen processing and peptide-antigen (pAg) presentation is essential to ignite high-affinity antibody responses with the help of cognate T cells. B cells efficiently internalize and direct specific antigens for processing and loading onto MHCII. This critical step, which enables pAg presentation, occurs in MHCII compartments (MIICs) which possess the enzymatic machinery for pAg loading on MHCII. The intracellular transport systems that guide antigen and maintain this unique compartment remain enigmatic. Here, we probed the possible functional role of two known endosomal proteins, the Rab family small GTPases Rab7 and Rab9, that are both reported to colocalize with internalized antigen. As compared to Rab9, we found Rab7 to exhibit a higher overlap with antigen and MIIC components. Rab7 also showed a higher association with antigen degradation. The inhibition of Rab7 drastically decreased pAg presentation. Additionally, we detected the strong colocalization of perinuclearly clustered and presumably MIIC-associated antigen with autophagy protein LC3. When we pharmacologically inhibited autophagy, pAg presentation was inhibited. Together, our data promote Rab7 as an important regulator of antigen processing and, considering the previously reported functions of Rab7 in autophagy, this also raises the possibility of the involvement of autophagy-related machinery in this process.

RAB3D/MDM2/β-catenin/c-MYC axis exacerbates the malignant behaviors of acute myeloid leukemia cells in vitro and in vivo

RAB3D, a small Ras-like GTPase involved in regulating secretory pathway, plays a cancer-promoting role in several solid tumors. However, its role in leukemogenesis remains unknown yet. Acute myeloid leukemia (AML) is a common acute leukemia with a high mortality. Here, we found the higher expression of RAB3D in bone marrow mononuclear cells derived from AML patients (n = 54) versus healthy participants (n = 20). The following loss- and gain-of-function experiments demonstrated that RAB3D promoted growth, enhanced colony formation and accelerated G1/S transition of U937, THP-1 and KG-1 AML cells. RAB3D silencing inhibited tumorigenesis of AML cells in vivo and delayed AML cells-induced death of mice. Interestingly, the expression of RAB3D is positively correlated with that of an oncogene mouse double minute 2 (MDM2) in bone marrow mononuclear cells of AML patients (r = 0.923, p < 0.001). Intracellular MDM2 was conjugated with more ubiquitins and degraded faster when RAB3D was silenced. A commonly therapeutic target of AML, β-catenin signaling, was activated by RAB3D overexpression, but deactivated after MDM2 was silenced. The RAB3D-induced proliferation acceleration and β-catenin activation were abolished by MDM2 knockdown, implying that RAB3D function by stabilizing MDM2. In addition, c-MYC, a β-catenin downstream effector, was recruited directly to the RAB3D gene promoter (-360/-349 and -136/-125 sites) and induced its transcription. Collectively, this study demonstrates that RAB3D may exacerbate the malignant behaviors of AML cells through forming a positive feedback loop with MDM2/β-catenin/c-MYC signaling. RAB3D might be a novel target of clinical AML treatment.

LUBAC Suppresses IL-21-Induced Apoptosis in CD40-Activated Murine B Cells and Promotes Germinal Center B Cell Survival and the T-Dependent Antibody Response

B cell activation by Tfh cells, i.e., through CD154 engagement of CD40 and IL-21, and survival within GCs are crucial for the T-dependent Ab response. LUBAC, composed of HOIP, SHARPIN, and HOIL-1, catalyzes linear ubiquitination (Linear M1-Ub) to mediate NF-κB activation and cell survival induced by TNF receptor superfamily members, which include CD40. As shown in this study, B cells expressing the Sharpin null mutation cpdm (Sharpin^cpdm) could undergo proliferation, CSR, and SHM in response to immunization by a T-dependent Ag, but were defective in survival within GCs, enrichment of a mutation enhancing the BCR affinity, and production of specific Abs. Sharpin^cpdm B cells stimulated in vitro with CD154 displayed normal proliferation and differentiation, marginally impaired NF-κB activation and survival, but markedly exacerbated death triggered by IL-21. While activating the mitochondria-dependent apoptosis pathway in both Sharpin^+/+ and Sharpin^cpdm B cells, IL-21 induced Sharpin^cpdm B cells to undergo sustained activation of caspase 9 and caspase 8 of the mitochondria-dependent and independent pathway, respectively, and ultimately caspase 3 in effecting apoptosis. These were associated with loss of the caspase 8 inhibitor cFLIP and reduction in cFLIP Linear M1-Ub, which interferes with cFLIP poly-ubiquitination at Lys48 and degradation. Finally, the viability of Sharpin^cpdm B cells was rescued by caspase inhibitors but virtually abrogated – together with Linear M1-Ub and cFLIP levels – by a small molecule HOIP inhibitor. Thus, LUBAC controls the cFLIP expression and inhibits the effects of caspase 8 and IL-21-activated caspase 9, thereby suppressing apoptosis of CD40 and IL-21-activated B cells and promoting GC B cell survival.

TrkA-mediated endocytosis of p75-CTF prevents cholinergic neuron death upon γ-secretase inhibition

The findings shed light into the adverse effects of GSIs observed in the Alzheimer’s field and explain, at least in part, the unexpected worsening in cognition observed in the semagacestat Phase 3 trial.

γ-secretase inhibitors (GSI) were developed to reduce the generation of Aβ peptide to find new Alzheimer’s disease treatments. Clinical trials on Alzheimer’s disease patients, however, showed several side effects that worsened the cognitive symptoms of the treated patients. The observed side effects were partially attributed to Notch signaling. However, the effect on other γ-secretase substrates, such as the p75 neurotrophin receptor (p75NTR) has not been studied in detail. p75NTR is highly expressed in the basal forebrain cholinergic neurons (BFCNs) during all life. Here, we show that GSI treatment induces the oligomerization of p75CTF leading to the cell death of BFCNs, and that this event is dependent on TrkA activity. The oligomerization of p75CTF requires an intact cholesterol recognition sequence (CRAC) and the constitutive binding of TRAF6, which activates the JNK and p38 pathways. Remarkably, TrkA rescues from cell death by a mechanism involving the endocytosis of p75CTF. These results suggest that the inhibition of γ-secretase activity in aged patients, where the expression of TrkA in the BFCNs is already reduced, could accelerate cholinergic dysfunction and promote neurodegeneration.

Phosphoproteomic quantitation and causal analysis reveal pathways in GPVI/ITAM-mediated platelet activation programs

Platelets engage cues of pending vascular injury through coordinated adhesion, secretion, and aggregation responses. These rapid, progressive changes in platelet form and function are orchestrated downstream of specific receptors on the platelet surface and through intracellular signaling mechanisms that remain systematically undefined. This study brings together cell physiological and phosphoproteomics methods to profile signaling mechanisms downstream of the immunotyrosine activation motif (ITAM) platelet collagen receptor GPVI. Peptide tandem mass tag (TMT) labeling, sample multiplexing, synchronous precursor selection (SPS), and triple stage tandem mass spectrometry (MS3) detected >3000 significant (false discovery rate < 0.05) phosphorylation events on >1300 proteins over conditions initiating and progressing GPVI-mediated platelet activation. With literature-guided causal inference tools, >300 site-specific signaling relations were mapped from phosphoproteomics data among key and emerging GPVI effectors (ie, FcRγ, Syk, PLCγ2, PKCδ, DAPP1). Through signaling validation studies and functional screening, other less-characterized targets were also considered within the context of GPVI/ITAM pathways, including Ras/MAPK axis proteins (ie, KSR1, SOS1, STAT1, Hsp27). Highly regulated GPVI/ITAM targets out of context of curated knowledge were also illuminated, including a system of >40 Rab GTPases and associated regulatory proteins, where GPVI-mediated Rab7 S72 phosphorylation and endolysosomal maturation were blocked by TAK1 inhibition. In addition to serving as a model for generating and testing hypotheses from omics datasets, this study puts forth a means to identify hemostatic effectors, biomarkers, and therapeutic targets relevant to thrombosis, vascular inflammation, and other platelet-associated disease states.