Mutations within the NH2-terminal transmembrane domain of membrane immunoglobulin (Ig) M alters Ig alpha and Ig beta association and signal transduction

SCHOOL model and new targeting strategies

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.

Immune cell signaling: a novel mechanistic model reveals new therapeutic targets

Multichain immune recognition receptors (MIRRs) represent a family of surface receptors that is expressed on different cells and that transduces extracellular signals, leading to many biological responses. The most intriguing common structural feature of MIRR family members is that the extracellular recognition domains and the intracellular signaling domains are located on separate subunits. It is not clear how extracellular ligand binding triggers MIRRs and initiates intracellular signal-transduction processes. In this article, I suggest that the structural similarity of the MIRRs provides the basis for the similarity in the mechanisms of MIRR-mediated transmembrane signaling. This hypothesis assumes that the therapeutic strategies learned from a novel mechanistic model of MIRR-mediated signal transduction, the signaling chain homo-oligomerization model, are generalized for this entire family and have important implications for the treatment of many disorders that are mediated by immune cells, including HIV.

Ig-independent Ig beta expression on the surface of B lymphocytes after B cell receptor aggregation

In order for humoral immune responses to develop, B cells must be able to recognize, bind, and internalize Ags. These functions are performed by the BCR, which is also responsible for initiating and transducing activation signals necessary for B cell proliferation and differentiation. We have examined surface expression patterns of individual components of the BCR following anti-Ig- and Ag-induced aggregation. Specifically, the localization and expression levels of the Ag-binding component, surface Ig (sIg), and the Igbeta component of the Igalpha/Igbeta signaling unit were investigated to determine their individual participation in the internalization and signal transduction. Using primary murine B cells, we found that while >95% of the sIg is internalized following anti-Ig-induced aggregation, 20-30% of Igbeta remains on the surface. These results suggest that sIg and Igbeta may function independently following the initial stages of signal transduction.

Ligand-dependent and -independent processes in B-cell-receptor-mediated signaling

The B cell antigen receptor (BCR) is a protein complex expressed on the surface of immature and mature B cells. After ligand-induced aggregation, this complex generates signals that lead to a variety of biological outcomes, including survival, proliferation and differentiation. During B cell development intermediate forms of the BCR are expressed on the surface. The composition of these pro- and preBCR complexes reflects the ordered assembly of the BCR complex and they exist to generate signals for positive selection at defined developmental checkpoints. Because these receptors lack the ability to bind conventional ligands, the pro- and preBCR have been postulated to signal via ligand-independent processes. This ligand-independent or constitutive signal may also play a role in the survival of peripheral mature B cells. Here we discuss the evidence for ligand-independent functions for the BCR and postulate how it may be regulated and linked to biological processes associated with B cell development and survival.

Hybrid Membrane IgM with the Transmembrane Region of I-Aα Facilitates Enhanced Presentation of Distinct Epitopes to T Cells

The role of B cell Ag receptors (membrane Ig or mIg) in the efficient Ag presentation to T cells, including the requirement of mIgM-associated Igα/Igβ, remains unclear. We report here that mIgM, substituted with greater than two-thirds of the NH2-terminal Aα transmembrane (TM) regions of the MHC class II molecule, are capable of mediating the efficient presentation of specific Ag to some (Group 1) but not all (Group 2) T cell hybridomas. In contrast, the generation of epitopes recognized by the Group 2 hybridomas can be mediated only by the wild-type mIgM. Tyrosine phosphorylation appears to be necessary for the enhanced Ag presentation to Group 2 hybridomas, while it does not for Group 1 hybridomas. In addition, differential sensitivity of Ag processing to leupeptin, different duration required for epitope generation/presentation, as well as the involvement of distinct epitopes for stimulation of these groups of T cell hybridomas were observed. These results suggest that transport of the mIgM/Ag complexes to an endocytic compartment(s) for generation of certain T cell epitopes may be mediated by the N-terminal TM sequence of mIgM, independent of Igα/Igβ association. This function can be replaced by two-thirds of the NH2-terminal TM region of Aα chain of class II molecules.

Identification of the sites of interaction between lymphocyte phosphatase-associated phosphoprotein (LPAP) and CD45

Human lymphocyte phosphatase-associated phospho-protein (LPAP) is a phosphoprotein of unknown function that noncovalently associates with CD45 in lymphocytes. In CD45-deficient human T cells, LPAP protein is synthesized at normal levels but is more rapidly degraded than in wild-type cells. Expression of CD45 cDNA rescues LPAP protein expression. This strongly suggests that LPAP is protected from degradation through its interaction with CD45. We have mapped the sites of interaction between LPAP and CD45 employing chimeric CD45 molecules and LPAP deletion mutants. Our data demonstrate that the interaction between LPAP and CD45 is mediated via the transmembrane regions of both molecules. In addition, the intracytoplasmic amino acids adjacent to the transmembrane region of LPAP may influence its binding to CD45.