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.

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.

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.

amamutdb.no: A relational database for MAN2B1 allelic variants that compiles genotypes, clinical phenotypes, and biochemical and structural data of mutant MAN2B1 in α-mannosidosis.

α-Mannosidosis is an autosomal recessive lysosomal storage disorder caused by mutations in the MAN2B1 gene, encoding lysosomal α-mannosidase. The disorder is characterized by a range of clinical phenotypes of which the major manifestations are mental impairment, hearing impairment, skeletal changes, and immunodeficiency. Here, we report an α-mannosidosis mutation database, amamutdb.no, which has been constructed as a publicly accessible online resource for recording and analyzing MAN2B1 variants (http://amamutdb.no). Our aim has been to offer structured and relational information on MAN2B1 mutations and genotypes along with associated clinical phenotypes. Classifying missense mutations, as pathogenic or benign, is a challenge. Therefore, they have been given special attention as we have compiled all available data that relate to their biochemical, functional, and structural properties. The α-mannosidosis mutation database is comprehensive and relational in the sense that information can be retrieved and compiled across datasets; hence, it will facilitate diagnostics and increase our understanding of the clinical and molecular aspects of α-mannosidosis. We believe that the amamutdb.no structure and architecture will be applicable for the development of databases for any monogenic disorder.

Molecular control of B cell activation and immunological synapse formation.

B cells form an essential part of the adaptive immune system by producing specific antibodies that can neutralize toxins and target infected or malignant cells for destruction. During B cell activation, a fundamental role is played by a specialized intercellular structure called the immunological synapse (IS). The IS serves as a platform for B cell recognition of foreign, often pathogenic, antigens on the surface of antigen-presenting cells (APC). This recognition is elicited by highly specific B cell receptors (BCR) that subsequently trigger carefully orchestrated intracellular signaling cascades that lead to cell activation. Furthermore, antigen internalization, essential for full B cell activation and differentiation into antibody producing effector cells or memory cells, occurs in the IS. Recent developments especially in various imaging-based methods have considerably advanced our understanding of the molecular control of B cell activation. Interestingly, the cellular cytoskeleton is emerging as a key player at several stages of B cell activation, including the initiation of receptor signaling. Here, we discuss the functions and molecular mechanisms of the IS and highlight the multifaceted role of the actin cytoskeleton in several aspects of B cell activation.

Is the bovine lysosomal phospholipase B-like protein an amidase?

The main function of lysosomal proteins is to degrade cellular macromolecules. We purified a novel lysosomal protein to homogeneity from bovine kidneys. By gene annotation, this protein is defined as a bovine phospholipase B-like protein 1 (bPLBD1) and, to better understand its biological function, we solved its structure at 1.9 Å resolution. We showed that bPLBD1 has uniform noncomplex-type N-glycosylation and that it localized to the lysosome. The first step in lysosomal protein transport, the initiation of mannose-6-phosphorylation by a N-acetylglucosamine-1-phosphotransferase, requires recognition of at least two distinct lysines on the protein surface. We identified candidate lysines by analyzing the structural and sequentially conserved N-glycosylation sites and lysines in bPLBD1 and in the homologous mouse PLBD2. Our model suggests that N408 is the primarily phosphorylated glycan, and K358 a key residue for N-acetylglucosamine-1-phosphotransferase recognition. Two other lysines, K334 and K342, provide the required second site for N-acetylglucosamine-1-phosphotransferase recognition. bPLBD1 is an N-terminal nucleophile (Ntn) hydrolase. By comparison with other Ntn-hydrolases, we conclude that the acyl moiety of PLBD1 substrate must be small to fit the putative binding pocket, whereas the space for the rest of the substrate is a large open cleft. Finally, as all the known substrates of Ntn-hydrolases have amide bonds, we suggest that bPLBD1 may be an amidase or peptidase instead of lipase, explaining the difficulty in finding a good substrate for any members of the PLBD family.

Systematic structure-activity study on potential chaperone lead compounds for acid α-glucosidase.

Acid α-glucosidase (GAA) is a lysosomal enzyme and a pharmacological target for Pompe disease, an inherited lysosomal storage disorder (LSD). An emerging treatment for LSDs is the use of pharmacological chaperones, small molecules that enhance total cellular activity of the target lysosomal protein. We have systematically studied thirteen inhibitors, which provide good lead compounds for the development of GAA chaperones. We have verified binding on GAA at low and neutral pH, mapping the range of pH during transport to lysosomes. These ligands inhibit GAA competitively and reversibly, and a few of the compounds show higher molecular stabilisation capacity than would be expected from their binding affinity. These molecules also increase lysosomal localisation of GAA variants in cells. In order to understand the specific molecular mechanism of the interactions, we docked the compounds to a homology model of the human GAA. Three factors contribute to the tightness of binding. Firstly, well-positioned hydroxy groups are essential to orient the ligand and make the binding specific. Secondly, the open nature of the GAA active site allows both large and small ligands to bind. The third and most important binding determinant is the positive charge on the ligand, which is neutralised by Asp 518 or Asp 616 on GAA. Our study creates a firm basis for the design of drugs to treat Pompe disease, as it provides a comparable study of the ligand properties. Our analysis suggests a useful drug design framework for specific pharmacological chaperones for human GAA.

Molecular and cellular characterization of novel {alpha}-mannosidosis mutations.

α-Mannosidosis is a lysosomal storage disorder caused by mutations in the MAN2B1 gene. The clinical presentation of α-mannosidosis is variable, but typically includes mental retardation, skeletal abnormalities and immune deficiency. In order to understand the molecular aetiology of α-mannosidosis, we describe here the subcellular localization and intracellular processing of 35 MAN2B1 variants, including 29 novel missense mutations. In addition, we have analysed the impact of the individual mutations on the three-dimensional structure of the human MAN2B1. We categorize the MAN2B1 missense mutations into four different groups based on their intracellular processing, transport and secretion in cell culture. Impaired transport to the lysosomes is a frequent cause of pathogenicity and correlates with a lack of protein processing (groups 1 and 3). Mutant MAN2B1 proteins that find their way to the lysosomes are processed, but less efficiently than the wild-types (groups 2 and 4). The described four categories of missense mutations likely represent different pathogenic mechanisms. We demonstrate that the severity of individual mutations cannot be determined based only on their position in the sequence. Pathogenic mutations cluster into amino acids which have an important role on the domain interface (arginines) or on the folding of the enzyme (prolines, glycines, cysteines). Tolerated mutations generally include surface mutations and changes without drastic alteration of residue volume. The expression system and structural details presented here provide opportunities for the development of pharmacological therapy by screening or design of small molecules that might assist MAN2B1 folding and hence, transport and activity.

Overexpression of Man2C1 leads to protein underglycosylation and upregulation of endoplasmic reticulum-associated degradation pathway.

Unfolded glycoproteins retained in the endoplasmic reticulum (ER) are degraded via the ER-associated degradation (ERAD) pathway. These proteins are subsequently transported to the cytosol and degraded by the proteasomal complex. Although the sequential events of ERAD are well described, its regulation remains poorly understood. The cytosolic mannosidase, Man2C1, plays an essential role in the catabolism of cytosolic free oligomannosides, which are released from the degraded proteins. We have investigated the impact of Man2C1 overexpression on protein glycosylation and the ERAD process. We demonstrated that overexpression of Man2C1 led to modifications of the cytosolic pool of free oligomannosides and resulted in accumulation of small Man(2-4)GlcNAc(1) glycans in the cytosol. We further correlated this accumulation with incomplete protein glycosylation and truncated lipid-linked glycosylation precursors, which yields an increase in N-glycoprotein en route to the ERAD. We propose a model in which high mannose levels in the cytosol interfere with glucose metabolism and compromise N-glycan synthesis in the ER. Our results show a clear link between the intracellular mannose-6-phosphate level and synthesis of the lipid-linked precursors for protein glycosylation. Disturbance in these pathways interferes with protein glycosylation and upregulated ERAD. Our findings support a new concept that regulation of Man2C1 expression is essential for maintaining efficient protein N-glycosylation.

Characterization and subcellular localization of human neutral class II alpha-mannosidase [corrected].

A glycosyl hydrolase family 38 enzyme, neutral alpha-mannosidase, has been proposed to be involved in hydrolysis of cytosolic free oligosaccharides originating either from ER-misfolded glycoproteins or the N-glycosylation process. Although this enzyme has been isolated from the cytosol, it has also been linked to the ER by subcellular fractionations. We have studied the subcellular localization of neutral alpha-mannosidase by immunofluorescence microscopy and characterized the human recombinant enzyme with natural substrates to elucidate the biological function of this enzyme. Immunofluorescence microscopy showed neutral alpha-mannosidase to be absent from the ER, lysosomes, and autophagosomes, and being granularly distributed in the cytosol. In experiments with fluorescent recovery after photo bleaching, neutral alpha-mannosidase had slower than expected two-phased diffusion in the cytosol. This result together with the granular appearance in immunostaining suggests that portion of the neutral alpha-mannosidase pool is somehow complexed. The purified recombinant enzyme is a tetramer and has a neutral pH optimum for activity. It hydrolyzed Man(9)GlcNAc to Man(5)GlcNAc in the presence of Fe(2+), Co(2+), and Mn(2+), and uniquely to neutral alpha-mannosidases from other organisms, the human enzyme was more activated by Fe(2+) than Co(2+). Without activating cations the main reaction product was Man(8)GlcNAc, and Cu(2+) completely inhibited neutral alpha-mannosidase. Our findings from enzyme-substrate characterizations and subcellular localization studies support the suggested role for neutral alpha-mannosidase in hydrolysis of soluble cytosolic oligomannosides.