Recombinant Human Platelet endothelial cell adhesion molecule (PECAM1)

What is the basic structure of human PECAM1?

PECAM1 is a 130-kDa glycoprotein member of the immunoglobulin (Ig) superfamily. It consists of six extracellular Ig-like homolog domains (IgD), a 19-residue transmembrane domain, and a 118 amino acid cytoplasmic tail containing motifs that mediate interactions with cytosolic signaling molecules . The protein contains immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in its cytoplasmic region that can recruit signaling proteins through Src-homology domain 2 interactions .

PECAM1 is heavily glycosylated with nine potential glycosylation sites distributed across its six IgL domains, with three specifically located on IgL1-2 (N52 and N84 on IgL1, and N151 on IgL2) . These glycosylation patterns can influence PECAM1's functional properties and homophilic binding capabilities.

Where is PECAM1 expressed in human tissues?

PECAM1 is widely expressed across multiple cell types in the vascular and immune systems. It is found on platelets, certain T cells, monocytes, neutrophils, and is particularly concentrated at the intercellular junctions of confluent endothelial cell monolayers . In endothelial cells, PECAM1 is the most abundant component of cell-cell junctions, making it a critical molecule for maintaining vascular integrity .

The differential expression of PECAM1 across these cell types suggests cell-specific functions and regulation mechanisms that researchers should consider when designing experiments targeting particular physiological processes.

What are the primary functions of PECAM1 in vascular biology?

PECAM1 serves multiple critical functions in vascular biology:

• Regulation of platelet function: Cross-linking PECAM1 inhibits the aggregation and secretion of platelets in response to collagen and GPVI-selective agonists, suggesting a role in regulating thrombosis .

• Maintenance of endothelial junctional integrity: Through homophilic interactions between adjacent cells, PECAM1 helps maintain the barrier function of endothelial monolayers, which is crucial for controlling vascular permeability .

• Leukocyte transendothelial migration: PECAM1 supports the process by which leukocytes cross the endothelial barrier during inflammation .

• Angiogenesis: PECAM1 plays a significant role in the formation of new blood vessels, as demonstrated by its importance in endothelial tube formation assays .

• Mechanosensation: PECAM1 functions as part of mechanosensing complexes that detect and respond to fluid shear stress from blood flow .

These diverse functions make PECAM1 a central molecule in understanding vascular homeostasis and pathological conditions.

How does PECAM1 participate in cellular signaling pathways?

PECAM1 mediates signaling through its cytoplasmic ITIMs, which become phosphorylated upon cross-linking of the receptor. This phosphorylation enables the recruitment of signaling proteins, particularly the tyrosine phosphatases SHP-1 and SHP-2 . The signaling cascade initiated by PECAM1 can:

• Inhibit tyrosine phosphorylation stimulated by various agonists such as convulxin and thrombin in platelets .

• Suppress the mobilization of calcium from intracellular stores, affecting downstream cellular responses .

• Trigger ERK1/2 downstream signal pathways via SHP-2 recruitment in naïve T-cells, leading to cell survival gene transcription and enhanced tolerance .

Methodologically, researchers investigating PECAM1 signaling should consider both direct effects on ITIM phosphorylation and downstream consequences on calcium mobilization and protein phosphorylation patterns using phospho-specific antibodies and calcium flux assays.

What is known about PECAM1 homophilic interactions at the molecular level?

PECAM1 homophilic interactions (binding between PECAM1 molecules on adjacent cells) have been extensively characterized at the molecular level through crystallographic studies. The crystal structure of PECAM1 IgD1 and IgD2 revealed several key interfaces that stabilize these interactions:

• A large 1608 Ų interface between adjacent IgD1 molecules .

• A rigid intrachain interface between IgD1 and IgD2 that governs IgD1 orientation .

• A 550 Ų buried surface between IgD1 and IgD2 .

• A smaller interface between adjacent IgD2 domains of uncertain significance .

Functional validation studies using single amino acid substitutions have confirmed that residues within the IgD1-IgD1 and IgD1-IgD2 interfaces are critical for PECAM1 homophilic binding, while those in the IgD2-IgD2 interface appear less important . These substitutions disrupted PECAM1's ability to:

• Localize at endothelial cell-cell borders

• Mediate the formation of endothelial tubes

• Restore endothelial barrier integrity

This structural information provides crucial insights for researchers designing targeted mutations or therapeutic interventions aimed at modulating PECAM1 function.

How does PECAM1 interact with PIEZO1 and what is the functional significance?

Recent research has uncovered a novel interaction between PECAM1 and the mechanosensitive ion channel PIEZO1. This interaction represents an important intersection between two prominent mechanisms for sensing shear stress in endothelium:

• Physical interaction: PECAM1 interacts directly with PIEZO1 and directs it to cell-cell junctions. This localization is critical for proper mechanosensing in endothelial cells .

• Structural requirements: The extracellular N-terminus of PECAM1 is critical for this interaction, though the C-terminal intracellular domain (previously linked to shear stress responses) also contributes .

• Dynamic regulation: While PECAM1-PIEZO1 interaction appears constitutive, cadherin-5 (CDH5) also drives PIEZO1 to junctions in a shear stress-dependent manner, suggesting complex regulatory mechanisms .

• Functional consequences: PIEZO1 is required for Ca²⁺-dependent formation of adherens junctions and associated cytoskeleton, suggesting it provides force-dependent Ca²⁺ entry for junctional remodeling .

Researchers investigating mechanotransduction should consider this interaction when designing experiments, as modulating either PECAM1 or PIEZO1 will likely affect the function of both proteins in endothelial responses to mechanical forces.

What are effective approaches for studying PECAM1 homophilic binding?

To effectively study PECAM1 homophilic interactions, researchers can employ several complementary approaches:

When designing these experiments, researchers should consider using appropriate expression systems, such as human-derived HEK293 cells, to mimic the native glycosylation patterns of PECAM1 .

What methods can be used to study PECAM1's role in platelet function?

To investigate PECAM1's role in platelet function, researchers can employ several methodological approaches:

When interpreting results, researchers should consider that PECAM1's inhibitory effects on thrombin-mediated aggregation are generally less pronounced than its effects on GPVI-mediated responses, suggesting pathway-specific regulation .

How can PECAM1's role in endothelial cell function be assessed experimentally?

Several experimental approaches can be used to assess PECAM1's role in endothelial cell function:

• Endothelial tube formation assay: This Matrigel-based assay serves as an in vitro measure of angiogenesis. Comparing tube formation between wild-type and PECAM1-mutant endothelial cells provides insights into PECAM1's role in vascular morphogenesis . Previous studies using blocking antibodies against PECAM1 IgD1 and IgD2 have demonstrated PECAM1's importance in this process .

• Barrier integrity assays: Techniques such as ECIS (Electric Cell-substrate Impedance Sensing) or TEER (Transendothelial Electrical Resistance) measurements can quantify PECAM1's contribution to maintaining endothelial barrier function under both basal conditions and following inflammatory challenges .

• Shear stress response studies: Flow chamber experiments coupled with live cell imaging can assess how PECAM1 mutations affect endothelial alignment and remodeling in response to fluid shear stress .

• Leukocyte transmigration assays: Transwell systems with endothelial monolayers allow quantification of how PECAM1 variants affect the rate and route of leukocyte migration across the endothelial barrier .

• PECAM1/PIEZO1 colocalization studies: Immunofluorescence microscopy using tagged PIEZO1 and PECAM1 can reveal their spatial relationship at cell junctions and how this changes with shear stress or other stimuli .

These approaches can be combined with genetic manipulation (CRISPR/Cas9 editing, siRNA knockdown, or expression of mutant forms) to comprehensively assess PECAM1's functional roles.

What is PECAM1's role in cancer metastasis and how can it be targeted therapeutically?

PECAM1 plays a complex role in cancer progression, particularly in advanced metastatic stages:

• Metastasis promotion: The tumor microenvironment (TME), functioning in part through PECAM1, drives advanced metastatic progression and is essential for progression through its preterminal end stage .

• Cell-specific effects: Studies using PECAM1-knockout and chimeric mice revealed that vascular endothelial cell (VEC) PECAM1, rather than tumor cell PECAM1, mediates these metastasis-promoting effects .

• Therapeutic targeting: Anti-PECAM1 monoclonal antibody (mAb) therapy has shown promising results in suppressing:

  • End-stage metastatic progression

  • Tumor-induced cachexia in tumor-bearing mice

  • Proliferation (but not angiogenesis or apoptosis) within advanced tumor metastases

• Mechanism of action: Anti-PECAM1 mAb appears to inhibit the proliferation of PECAM1-negative tumor cells by altering the concentrations of secreted factors, as demonstrated in modified 3D coculture assays .

• Broad applicability: Because its antimetastatic effects are mediated by binding to VEC rather than to tumor cells, anti-PECAM1 mAb therapy may be effective against multiple tumor types .

This research suggests that PECAM1-targeted therapeutic approaches represent a promising TME-targeted strategy to suppress end-stage metastatic progression—a typically treatment-refractory clinical entity. Researchers designing such therapies should focus on targeting the vascular compartment rather than tumor cells directly.

How do mutations in key PECAM1 interfaces affect its function in endothelial cells?

Research into PECAM1 structure-function relationships has revealed that mutations in specific interfaces have differential effects on endothelial cell functions:

The functional consequences of these mutations include:

• Junctional localization: Mutations in IgD1-IgD1 and IgD1-IgD2 interfaces prevent PECAM1 from properly localizing to endothelial cell-cell borders .

• Tube formation: These mutations significantly impair the ability of endothelial cells to form organized tube structures in Matrigel, a critical aspect of angiogenesis .

• Barrier integrity: Mutant PECAM1 molecules fail to restore endothelial barrier integrity, highlighting the importance of these specific interfaces in maintaining vascular permeability control .

These findings validate the physiological relevance of the interactions observed in crystal structures and provide a mechanistic understanding of how PECAM1 homophilic binding supports endothelial function. Researchers can use this information to develop more targeted approaches to modulate specific PECAM1 functions in vascular diseases.

How does PECAM1 function in mechanosensing and what are the implications for vascular disease?

PECAM1 plays a critical role in endothelial mechanosensing through several mechanisms:

• Mechanosensory complex: PECAM1 has traditionally been viewed as part of a mechanosensory complex or "triad" with VE-cadherin (CDH5) and VEGFR2 .

• PIEZO1 interaction: Recent research revealed that PECAM1 physically interacts with the mechanosensitive ion channel PIEZO1 and directs it to cell-cell junctions . This interaction requires PECAM1's extracellular N-terminus, with additional contributions from its C-terminal intracellular domain .

• Junctional remodeling: PIEZO1, directed by PECAM1 to cell junctions, provides force-dependent Ca²⁺ entry required for adherens junction formation and cytoskeletal remodeling .

• Differential interactions: While PECAM1-PIEZO1 interaction appears stable, CDH5 interacts with PIEZO1 in a shear stress-dependent manner. Interestingly, PIEZO1 does not interact with VEGFR2, suggesting a revision of the traditional mechanosensory triad model .

The implications for vascular disease research are significant:

• Atherosclerosis: Disturbed flow patterns at vessel bifurcations contribute to atherosclerosis development. Understanding how PECAM1-PIEZO1 interactions respond to different flow patterns may provide insights into early atherogenesis.

• Vascular permeability: As PECAM1-PIEZO1 interactions affect junctional integrity, targeting this complex could help control vascular leakage in inflammatory conditions.

• Mechanotherapeutics: The detailed understanding of PECAM1's mechanosensing role opens possibilities for developing therapeutic approaches that modulate endothelial responses to mechanical forces.

Researchers investigating vascular mechanobiology should consider both PECAM1's direct role and its regulatory effect on PIEZO1 localization and function when designing experiments and interpreting results.

What expression systems are optimal for producing functional recombinant human PECAM1?

The choice of expression system significantly impacts the structure and function of recombinant PECAM1, particularly due to its complex glycosylation pattern:

• Mammalian expression systems: Human-derived cell lines such as HEK293 are preferred for producing PECAM1 with glycosylation patterns that closely mimic native expression . This is particularly important when studying:

  • Homophilic binding properties

  • Interactions with other proteins

  • Structure-function relationships

• Insect cell systems: While frequently used for recombinant protein production, insect cells may produce PECAM1 with altered glycosylation patterns that could affect its functional properties .

• Glycosylation considerations: When designing recombinant PECAM1 constructs, researchers should pay careful attention to the nine potential glycosylation sites, particularly N52 and N84 on IgL1 and N151 on IgL2, which may influence protein folding and binding properties .

• Purification strategies: For structural studies, affinity tags followed by size exclusion chromatography have been successfully employed to obtain pure PECAM1 fragments suitable for crystallization or SAXS analysis .

The experimental objectives should guide expression system selection, with mammalian systems being particularly important when studying interactions that may be influenced by post-translational modifications.

How can PECAM1 function be effectively manipulated in experimental settings?

Researchers can manipulate PECAM1 function using several approaches:

• Antibody-based methods:

  • Function-blocking antibodies targeting specific PECAM1 domains (particularly IgD1 and IgD2) have been used to inhibit homophilic interactions .

  • Anti-PECAM1 monoclonal antibodies can suppress metastatic progression in animal models by binding to vascular endothelial PECAM1 .

• Genetic manipulation:

  • CRISPR/Cas9-mediated knockout or knockin of PECAM1 mutations in cell lines or animal models.

  • Expression of dominant-negative mutants with specific alterations in key interfaces:

    • Mutations in the IgD1-IgD1 and IgD1-IgD2 interfaces (but not IgD2-IgD2) disrupt homophilic binding .

    • Hydrophobic residues (L74, I112, F188, I190) can be mutated to charged residues (E) to disrupt intermolecular binding .

• Chimeric approaches:

  • Creation of chimeric mice with PECAM1 knockout in specific cell types has revealed the importance of vascular endothelial cell PECAM1 in processes like cancer metastasis .

• Domain-specific constructs:

  • Expression of isolated PECAM1 domains or domain combinations can help dissect their specific contributions to PECAM1 function .

These approaches can be combined with the functional assays described earlier (platelet aggregation, endothelial tube formation, barrier integrity measurements) to comprehensively assess how specific manipulations affect PECAM1-dependent processes.

How is PECAM1 involved in the immune response and what are the implications for immunotherapy?

PECAM1's role in immune regulation has significant implications for immunotherapy development:

• T cell regulation: PECAM1 homophilic interactions between naïve T-cells and antigen-presenting cells lead to phosphorylation of ITIMs, triggering ERK1/2 signaling via SHP-2 recruitment. This results in transcription of cell survival genes and enhanced T-cell tolerance .

• Trans-homophilic interactions: PECAM1's ability to engage in homophilic binding between different cell types (trans-homophilic) provides a mechanism for immune cell-endothelial communication that could be therapeutically targeted .

• Pathogen interactions: PECAM1 can interact with pathogen proteins such as Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP-1), potentially accelerating infected red blood cell aggregation in blood vessels . This suggests PECAM1 might be involved in pathogen immune evasion strategies.

Future research directions might explore:

• How modulating PECAM1 function affects T cell responses to cancer immunotherapy

• Whether PECAM1-targeted approaches could enhance immune cell trafficking to tumors

• The potential to block pathogen-PECAM1 interactions as a therapeutic strategy for infectious diseases

What are the current challenges in translating PECAM1 research into clinical applications?

Despite promising preclinical findings, several challenges remain in translating PECAM1 research into clinical applications:

• Functional complexity: PECAM1's diverse functions in different cell types present challenges for therapeutic targeting. Inhibiting PECAM1 might have beneficial effects on cancer metastasis but potentially detrimental effects on vascular integrity or inflammation.

• Specificity of targeting: Developing therapeutics that target specific PECAM1 functions or cell-type-specific PECAM1 interactions without affecting others remains technically challenging.

• Delivery challenges: For anti-PECAM1 antibody therapies targeting vascular endothelial PECAM1, ensuring sufficient delivery to the tumor microenvironment while minimizing off-target effects requires optimization.

• Biomarker development: Establishing reliable biomarkers to identify patients most likely to benefit from PECAM1-targeted therapies and to monitor treatment efficacy is an important area for future research.

• Combination strategies: Determining how PECAM1-targeted approaches might synergize with existing cancer therapies or anti-angiogenic treatments requires systematic investigation.

Addressing these challenges will require collaborative efforts between structural biologists, cell biologists, immunologists, and clinical researchers to fully realize the therapeutic potential of PECAM1-targeted interventions.

How do PECAM1's interactions with other proteins inform our understanding of vascular homeostasis?

PECAM1's interactions with various protein partners provide crucial insights into vascular homeostasis:

• PIEZO1 interaction: The recently discovered interaction between PECAM1 and the mechanosensitive ion channel PIEZO1 suggests a more integrated model of mechanosensing than previously understood . This partnership appears critical for:

  • Directing PIEZO1 to cell-cell junctions

  • Facilitating force-dependent calcium entry

  • Supporting junctional remodeling in response to mechanical forces

• Cadherin-5 (CDH5) dynamics: While PECAM1-PIEZO1 interaction appears constitutive, CDH5 interacts with PIEZO1 in a shear stress-dependent manner . This suggests a complex regulatory network that fine-tunes endothelial responses to varying mechanical environments.

• SHP-1 and SHP-2 recruitment: PECAM1's ITIMs recruit these phosphatases, linking mechanical sensing to biochemical signaling cascades that regulate cellular functions .

These interactions collectively suggest that vascular homeostasis depends on a sophisticated protein network centered around PECAM1 that integrates mechanical forces, cell-cell adhesion, and intracellular signaling. Future research should explore how these interactions are dysregulated in vascular diseases and whether targeting specific interaction interfaces could provide therapeutic benefits.

Understanding these protein-protein interactions at the molecular level may lead to more precise interventions that modulate specific aspects of PECAM1 function while preserving others, potentially reducing side effects compared to global PECAM1 inhibition.