MYD1 Protein: From Structural Dissection to Innovative Targets in Cancer Therapy

MYD1 Protein: From Structural Dissection to Innovative Targets in Cancer Therapy

Immunomodulatory and Precision Medicine Frontiers

Abstract

As a critical member of the immunoglobulin superfamily, MYD1 protein (SIRP alpha/SHPS1) serves both as a key receptor for cell surface signaling regulation and a core participant in tumor microenvironment remodeling. Its unique structural features and interactions with the Gas6/AXL signaling pathway offer novel targeting strategies for cancer therapy. This review systematically analyzes MYD1's three-dimensional conformation, signal transduction networks, and its bidirectional regulatory mechanisms in tumor immunology, with a focus on the clinical potential of the high-affinity mutant MYD1-72. The findings provide theoretical foundations for developing next-generation anticancer therapeutics.

1. Structural Features and Evolutionary Conservation of MYD1 Protein

MYD1 comprises three core domains (Figure 1):

• N-terminal immunoglobulin-like domain (IgV domain): Containing ~120 amino acid residues, this domain forms a β-sandwich structure responsible for specific ligand binding (e.g., CD47, Gas6). Its surface charge distribution (e.g., Lys32/Glu45 clusters) is critical for ligand recognition precision.
• Central phosphatase-binding domain: Contains two tyrosine phosphorylation sites (Tyr428/Tyr452) that recruit SHP-1/SHP-2 phosphatases, triggering downstream signaling cascades.
• C-terminal transmembrane anchor: Composed of hydrophobic α-helices, ensuring stable membrane integration.

MYD1 Protein 3D Structural Model

Evolutionarily, MYD1's IgV domain is highly conserved across mammals (89% homology between human and murine sequences), yet its intracellular segment exhibits functional divergence across species. For instance, the Tyr452 phosphorylation site in human MYD1 uniquely regulates tumor immunosuppression, a role absent in rodents, highlighting its pathological significance in higher organisms.

2. Dual Signaling Mechanisms of MYD1

2.1 Classical Immunoregulatory Pathway

As a mediator of the "don't eat me" signal, the MYD1-CD47 axis inhibits macrophage phagocytosis via:

• Phosphatase activation: MYD1 intracellular domain recruits SHP-1, accelerating Fcγ receptor (FcγR) dephosphorylation by 3-fold and blocking phagocytic synapse formation.
• Cytoskeletal remodeling: Suppresses actin polymerization via RhoA/ROCK pathways, reducing macrophage motility by 40% (live-cell imaging data).

2.2 Gas6/AXL Signal Hijacking Mechanism

Recent studies reveal that MYD1 mutants (e.g., MYD1-72) bind Gas6 with high affinity (K_d = 0.15 nM, 350-fold higher than AXL receptors), sterically blocking AXL activation. This triggers:

• Pro-survival signal inhibition: PI3K/Akt pathway activity decreases by 65%, inducing tumor cell apoptosis.
• EMT reversal: Snail and Twist1 expression drops by 72% and 58%, respectively, inhibiting metastasis.

3. MYD1's Dynamic Regulatory Network in the Tumor Microenvironment

3.1 Epigenetic Modification Layer

DNA methylation analysis shows MYD1 promoter CpG islands in ovarian cancer exhibit 47% lower methylation than normal tissues, upregulating mRNA expression by 2.3-fold. This hypomethylation correlates with DNMT3B enzymatic deficiency (ChIP-seq confirms 81% reduced binding).

3.2 Transcriptional Regulatory Network

MYD1 expression is dual-regulated by NF-κB/p65 and STAT3:

• Positive regulation: IL-6 stimulation induces STAT3 dimers to bind the MYD1 promoter at -135 bp, enhancing transcription by 4-fold.
• Negative regulation: TGF-β suppresses p65 nuclear translocation via Smad4, reducing MYD1 expression by 60%.

3.3 Protein Interaction Landscape

Mass spectrometry-based interactome analysis reveals MYD1 forms dynamic complexes with:

• Membrane surface complex: Integrin αvβ3 (K_d = 8.7 nM), promoting extracellular matrix adhesion.
• Intracellular signaling hub: FAK kinase (colocalization coefficient = 0.92), activating MAPK pathways for proliferation.

4. Therapeutic Breakthroughs with MYD1-72 Mutant

4.1 Structural Optimization Strategy

Computer-aided design enabled targeted modifications of MYD1's IgV domain:

• Key mutation: Asp76→Arg (enhances Gas6 binding interface charge complementarity).
• Conformational locking: Introduction of Cys54-Cys92 disulfide bond (stabilizes β-sheet topology).
  Modified MYD1-72 extends Gas6 binding half-life to 26 hours (vs. 1.5 hours in wild-type).

MYD1-72-Gas6 Complex Crystal Structure

4.2 Preclinical Data

In ovarian cancer PDX models, MYD1-72 monotherapy achieved:

• Primary tumor volume reduction: 95% (vs. PBS control).
• Metastasis count: Decreased from 28 to 3 (P<0.001).
• Combination with doxorubicin: 78% complete remission rate (CR), with significantly lower cardiotoxicity than conventional chemotherapy.

5. Challenges and Future Directions

5.1 Delivery System Innovation

Current MYD1 drug limitations include:

• Short plasma half-life (t_1/2 = 2.3 hours) requiring frequent dosing.
• Low tumor penetration (<8% injected dose reaches lesions).
  Solutions include:
• PEGylation: Extends circulation time to 18 hours.
• pH-responsive nanoparticles: Enable tumor-specific drug release in acidic microenvironments.

5.2 Combination Therapy Strategies

Preliminary data show MYD1-72 synergizes with PD-1 antibodies:

• CD8+ T-cell infiltration: Increased 4.7-fold.
• Immunosuppressive Treg proportion: Reduced from 22% to 7%.

Conclusion

As a molecular bridge linking innate and adaptive immunity, MYD1's functional polymorphism offers unique therapeutic perspectives. Integrating protein engineering with nanotechnology, MYD1-72-based biologics could overcome existing treatment barriers, ushering in a new era of precision immunotherapy.