The signal transduction mechanism of immunoreceptors and its clinical application has always been a frontier hotspot in biology and medicine, which can help us understand the fundamental immune principles and develop innovative immunotherapies.
Current immunotherapies are essentially based on the signal regulation of immunoreceptors, such as immune checkpoint blockade therapy, CAR-T and TCR-T cell therapies, etc. Immunoreceptors are all transmembrane proteins, and the existence of juxta-membrane positively charged residues in transmembrane proteins is a known conclusion in classic biochemistry textbooks. However, the signal mechanism, physiological and pathological functions, and application prospects of these sequences have not been systematically studied.
A perspective article published in Nature Reviews Immunology, researchers led by Prof. XU Chenqi and HE Xing from the Center for Excellence in Molecular Cell Science (Shanghai Institute of Biochemistry and Cell Biology) of the Chinese Academy of Sciences, along with SHI Xiaoshan from the Institute of Synthetic Biology, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences. The article focuses on a general signaling motif—the basic-residue-rich sequence (BRS), defines the BRS motif, summarizes its juxta-membrane signal transduction mechanisms, discusses the relation between BRS mutations in immunoreceptors and human diseases, and explores the potential of BRS in innovative immunotherapies.
The article first systematically analyzed the sequence characteristics of human single-transmembrane proteins and found that 70% of human single-transmembrane proteins carry BRS in the intracellular juxta-membrane region. BRS is usually 10 amino acids long, carries two or more net positive charges, and is often located in the intracellular juxta-membrane region, but can also be distributed in the distal-membrane position. To date, BRS in various immunoreceptors such as antigen receptors (T cell receptors, B cell receptors), co-stimulatory receptors, co-inhibitory receptors, NK cell receptors, Fc receptors, and cytokine receptors has been experimentally reported.
Based on existing research reports, this article summarized the juxta-membrane electrostatic regulation network theory mediated by BRS. BRS can interact electrostatically with negatively charged or π-electron-carrying lipid and protein molecules in the cell membrane and juxta-membrane region, and environmental factors can further regulate these interactions, forming a spatiotemporally dynamic juxta-membrane electrostatic network.
Current known network members include: BRS, acidic phospholipids (such as PS, PI(4,5)P2), steroid molecules (such as cholesterol, hydroxycholesterol), cations (such as calcium ions), and membrane proteins or peripheral membrane proteins carrying negative charges or π-electrons rich regions (such as LCK, p85, LAG3, PLCγ1), etc. Through the juxta-membrane electrostatic network, BRS regulates the phosphorylation, ubiquitylation, liquid-liquid phase separation, and mechanical signal transduction of immunoreceptors, covering the life cycle of immunoreceptors from rest, triggering, signal amplification, signal decay, and degradation.
Next, the article systematically organized the relevance between BRS mutations and diseases, as well as the prospects for the translational application of BRS. Through systematic analysis of the UniProt database, more than 100 disease-related BRS mutations have been recorded, but the specific pathogenic mechanisms of most of them still need to be studied. The prospects for the translational application of BRS include the signal regulation of BRS in natural immunoreceptors and the design of synthetic immunoreceptors using BRS.
The steroid metabolite 7α-hydroxycholesterol can loosen the packing of cell membrane lipid molecules, helping the BRS of the TCR signaling subunit CD3ε to bind stronger with the membrane. Thus, in the preparation of TCR-T cells, it inhibits the basal phosphorylation signal of TCR, increases the proportion of memory cells, and improves the longevity of immunotherapy.
On the other hand, the E-CAR molecule formed by adding the CD3ε signaling region to the second-generation CAR molecule has better signal transduction ability, where BRS can mediate the formation of liquid-liquid phase separation through cation-π bonds, helping cells form more mature and efficient immune synapses, thereby enhancing the antigen sensitivity and longevity of E-CAR-T cells. In addition to CAR, BRS is also crucial for the efficient signal transduction of another synthetic receptor, SNIPR.
Finally, the article points out that the human proteome has a rich library of BRS, which potentially has a broad application. However, at the current stage, the understanding and application of BRS signal transduction mechanisms are still very limited, and there are a series of important issues that need to be resolved. For example, can BRS be divided into subclasses? Do different subclasses have different signaling patterns? How do various BRS mutations lead to human diseases? How to rationally manipulate BRS signals or rationally design synthetic receptors containing BRS? The answers to these questions will greatly enhance our understanding of the immune system and help the development of immunotherapies.
Reference:https://www.nature.com/articles/s41577-024-01105-6
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