B cell-mediated immunity and signal regulation. The roles of MIM/MTSS1 and IRF4

B cells constitute an important part of the adaptive immune system and produce antibodies, which protect organism against infectious agents. Recognition of the antigen by the B cell receptor and the following B cell activation are key steps toward production of protective antibodies. In recent years it has been shown that the actin cytoskeleton is actively involved in the regulation of early events of B cell activation. Therefore, understanding the factors that influence actin cytoskeleton remodeling in B cells is essential for the understanding of B cell-mediated immunity. The focus of this work is on the membrane and actin cytoskeleton regulatory protein, MIM/MTSS1, and on the B cell-specific transcription factor, IRF4. In order to reveal unexplored MIM functions, the gene and protein sequences of MIM were analyzed by using computational tools, bioinformatic resources and online databases. The analysis demonstrates a high overall degree of MIM conservation in vertebrates and we also report on the presence of various short functional motifs. We also show that expression of MIM is downregulated in samples from patients with poor prognosis chronic lymphocytic leukemia (CLL). The role of MIM in B cells was studied by analysis of MIM knock-out (KO) mice. MIM-deficiency results in impaired BCR signaling upon stimulation with surface-bound but not soluble antigen, and elevated reprogramming toward higher oxidative metabolic state upon TLR4/9 stimulation. MIM KO animals also exhibit lower antibody responses against T cell-independent antigen immunization. Finally, the impact of IRF4-deficiency on BCR signaling was examined in an IRF4-KO DT40 cell line. We show that IRF4 deficiency results in disturbed BCR signaling, characterized by impaired Syk, BLNK, MAPK1/2 activation and F-actin engagement, low SHIP phosphatase levels and upregulation of PI3K, PLC��� and Ca2+ signaling. This work is the first comprehensive study of the actin-regulatory protein MIM in B cell-mediated immunity and provides an evolutionary standpoint on the functional significance of MIM protein regions. This study also highlights the fact that IRF4 is not only a downstream effector of BCR signaling but by itself can influence B cell activation via developmental stage-specific expression in B cells

Computational analysis of the evolutionarily conserved Missing In Metastasis/Metastasis Suppressor 1 gene predicts novel interactions, regulatory regions and transcriptional control

Missing in Metastasis (MIM), or Metastasis Suppressor 1 (MTSS1), is a highly conserved protein, which links the plasma membrane to the actin cytoskeleton. MIM has been implicated in various cancers, however, its modes of action remain largely enigmatic. Here, we performed an extensive in silico characterisation of MIM to gain better understanding of its function. We detected previously unappreciated functional motifs including adaptor protein (AP) complex interaction site and a C-helix, pointing to a role in endocytosis and regulation of actin dynamics, respectively. We also identified new functional regions, characterised with phosphorylation sites or distinct hydrophilic properties. Strong negative selection during evolution, yielding high conservation of MIM, has been combined with positive selection at key sites. Interestingly, our analysis of intra-molecular co-evolution revealed potential regulatory hotspots that coincided with reduced potentially pathogenic polymorphisms. We explored databases for the mutations and expression levels of MIM in cancer. Experimentally, we focused on chronic lymphocytic leukaemia (CLL), where MIM showed high overall expression, however, downregulation on poor prognosis samples. Finally, we propose strong conservation of MTSS1 also on the transcriptional level and predict novel transcriptional regulators. Our data highlight important targets for future studies on the role of MIM in different tissues and cancers

B cells rapidly target antigen and surface-derived MHCII into peripheral degradative compartments

In order to mount high-affinity antibody responses, B cells internalise specific antigens and process them into peptides loaded onto MHCII for presentation to T helper cells (T H cells). While the biochemical principles of antigen processing and MHCII loading have been well dissected, how the endosomal vesicle system is wired to enable these specific functions remains much less studied. Here, we performed a systematic microscopy-based analysis of antigen trafficking in B cells to reveal its route to the MHCII peptide-loading compartment (MIIC). Surprisingly, we detected fast targeting of internalised antigen into peripheral acidic compartments that possessed the hallmarks of the MIIC and also showed degradative capacity. In these vesicles, intemalised antigen converged rapidly with membrane-derived MHCII and partially overlapped with cathepsin-S and H2-M, both required for peptide loading. These early compartments appeared heterogenous and atypical as they contained a mixture of both early and late endosomal markers, indicating a specialized endosomal route. Together, our data suggest that, in addition to in the previously reported perinuclear late endosomal MIICs, antigen processing and peptide loading could have already started in these specialized early peripheral acidic vesicles (eMlIC) to support fast peptide-MHCII presentation. This article has an associated First Person interview with the first author of the paper.Peer reviewe

B cells rapidly target antigen and surface-derived MHCII into peripheral degradative compartments

In order to mount high-affinity antibody responses, B cells internalise specific antigens and process them into peptides loaded onto MHCII for presentation to T helper cells (TH cells). While the biochemical principles of antigen processing and MHCII loading have been well dissected, how the endosomal vesicle system is wired to enable these specific functions remains much less studied. Here, we performed a systematic microscopy-based analysis of antigen trafficking in B cells to reveal its route to the MHCII peptide-loading compartment (MIIC). Surprisingly, we detected fast targeting of internalised antigen into peripheral acidic compartments that possessed the hallmarks of the MIIC and also showed degradative capacity. In these vesicles, internalised antigen converged rapidly with membrane-derived MHCII and partially overlapped with cathepsin-S and H2-M, both required for peptide loading. These early compartments appeared heterogenous and atypical as they contained a mixture of both early and late endosomal markers, indicating a specialized endosomal route. Together, our data suggest that, in addition to in the previously reported perinuclear late endosomal MIICs, antigen processing and peptide loading could have already started in these specialized early peripheral acidic vesicles (eMIIC) to support fast peptide–MHCII presentation.</p

For the Jubilee of Professor Andrey Mikhailovich Belavin

The article is dedicated to the anniversary of Andrei Belavin – a renowned Russian archaeologist and expert in Finno-Ugric studies and medieval archeology of the Volga-Kama region. It features the primary milestones of biography and the important events in his scientific life. The authors present an analysis of the main publications by A.M. Belavin and his concepts which had a great impact on the archaeology of the Volga-Kama region

Missing-in-Metastasis/Metastasis Suppressor 1 Regulates B Cell Receptor Signaling, B Cell Metabolic Potential, and T Cell-Independent Immune Responses

Efficient generation of antibodies by B cells is one of the prerequisites of protective immunity. B cell activation by cognate antigens via B cell receptors (BCRs), or pathogen-associated molecules through pattern-recognition receptors, such as Toll-like receptors (TLRs), leads to transcriptional and metabolic changes that ultimately transform B cells into antibody-producing plasma cells or memory cells. BCR signaling and a number of steps downstream of it rely on coordinated action of cellular membranes and the actin cytoskeleton, tightly controlled by concerted action of multiple regulatory proteins, some of them exclusive to B cells. Here, we dissect the role of Missing-In-Metastasis (MIM), or Metastasis suppressor 1 (MTSS1), a cancer-associated membrane and actin cytoskeleton regulating protein, in B cell-mediated immunity by taking advantage of MIM knockout mouse strain. We show undisturbed B cell development and largely normal composition of B cell compartments in the periphery. Interestingly, we found that MIM−/− B cells are defected in BCR signaling in response to surface-bound antigens but, on the other hand, show increased metabolic activity after stimulation with LPS or CpG. In vivo, MIM knockout animals exhibit impaired IgM antibody responses to immunization with T cell-independent antigen. This study provides the first comprehensive characterization of MIM in B cells, demonstrates its regulatory role for B cell-mediated immunity, as well as proposes new functions for MIM in tuning receptor signaling and cellular metabolism, processes, which may also contribute to the poorly understood functions of MIM in cancer.This work was supported by the Academy of Finland (grant ID: 25700, 296684, 307313, and 327378 to PM; 286712 to VŠ), Sigrid Juselius and Jane and Aatos Erkko foundations (to PM), Turku doctoral program in molecular medicine (TuDMM) (to SH-P and MV), and Magnus Ehrnrooth (to AS) and Finnish Cultural (to VŠ) foundations. MF and LC were supported by the Wellcome Trust (212343/Z/18/Z) and EPSRC (EP/S004459/1)