DDR2 Human

What is DDR2 and what is its molecular structure?

DDR2 is a 130 kDa type I transmembrane glycoprotein belonging to the discoidin-like domain-containing subfamily of receptor tyrosine kinases. The mature human DDR2 protein consists of:

• A 378 amino acid extracellular domain (ECD) containing the discoidin-like domain

• A 22 amino acid transmembrane segment

• A 434 amino acid cytoplasmic domain that includes the kinase domain

The recombinant form typically encompasses amino acids Gln24-Arg399 and may include a C-terminal 6-His tag for purification purposes . The discoidin-like domain mediates DDR2's interactions with various collagens, particularly collagens I, III, and X, with collagens II and V serving as less efficacious ligands .

How is DDR2 expressed in different human tissues?

DDR2 demonstrates a tissue-specific expression pattern that differs from its related receptor DDR1. While DDR1 is predominantly expressed in epithelial cells, DDR2 is primarily found in cells of connective tissues that originate from embryonic mesoderm .

Expression profile of DDR2:

• High expression levels: skeletal muscle, heart muscle, kidney, and lung

• Developmental expression: found in the developing nervous system

• Cellular distribution: predominantly in mesenchymal cells rather than epithelial cells

It's important to note that DDR2 protein expression does not necessarily correlate with its phosphorylation levels, suggesting tissue-specific regulation of its activity. The highest levels of DDR2 phosphorylation have been detected in lung, ovary, and skin tissues .

What are the primary physiological functions of DDR2?

DDR2 functions as an extracellular matrix sensor that modulates several critical cellular processes:

• Cell proliferation: Essential for chondrocyte and fibroblast proliferation, affecting bone growth and wound healing

• Matrix remodeling: Regulates the expression of matrix metalloproteinases (MMPs), including MMP-1, MMP-2, MMP-8, and MMP-13

• Collagen interaction: Selectively recognizes the triple helical structure of collagen and can inhibit collagen fibrillogenesis

• Tissue development: Plays crucial roles in embryonic development, particularly in bone formation

• Wound healing: Contributes to dermal fibroblast function during tissue repair processes

Knockout studies have shown that DDR2-deficient mice exhibit dwarfism and shortening of long bones due to reduced chondrocyte proliferation, highlighting its importance in skeletal development .

What experimental models are available for studying DDR2 function?

Several experimental models have been employed to investigate DDR2 function:

In vivo models:

• DDR2 knockout mice: Generated by deleting the K1 exon of the kinase domain, spanning amino acids 578 to 619. These mice display dwarfism with 12-15% reduction in long bone length

• Atherosclerosis models: Both ApoE-/- mice and rabbit models have been used to study DDR2 in atherosclerotic plaques

In vitro models:

• Cell lines: Various fibroblast cell lines, vascular smooth muscle cells (VSMCs), and chondrocytes

• siRNA knockdown approaches: Used to temporarily reduce DDR2 expression in cultured cells

• Recombinant protein studies: Using purified DDR2 extracellular domain to study collagen binding

When selecting a model, researchers should consider the specific aspect of DDR2 biology they aim to investigate, as different models may highlight different functional aspects of this receptor.

How can DDR2 activity be measured in experimental settings?

DDR2 activity can be assessed through multiple complementary approaches:

Receptor phosphorylation:

• Immunoprecipitation followed by western blotting with phosphotyrosine antibodies

• Tissue extracts can be analyzed to determine tissue-specific phosphorylation patterns

Functional readouts:

• Cell proliferation assays: Particularly useful in fibroblasts and chondrocytes

• Migration assays: Important for wound healing models

• MMP expression and activity assays: Using qPCR for mRNA levels and zymography for MMP activity

Receptor-ligand interaction:

• Binding assays with recombinant DDR2 and various collagen types

• Assessment of collagen fibrillogenesis in the presence of DDR2

For accurate assessment, it's recommended to combine multiple approaches, as DDR2 activation has unusually slow kinetics compared to other receptor tyrosine kinases.

What is the role of DDR2 in atherosclerotic plaque development?

DDR2 has been identified as a potentially significant factor in atherosclerotic plaque development and stability:

• Expression pattern: DDR2 is abundantly present in human atherosclerotic plaques, distributed primarily around fatty and necrotic cores

• Temporal distribution: In rabbit models, DDR2 appears in early-stage lesions along the lower edge, and in middle-stage lesions, it becomes diffusely distributed throughout the plaque

• Cellular localization: Interestingly, DDR2 does not completely overlap with macrophages or VSMCs but tends to localize around collagen fibers

Research has shown that oxidized low-density lipoprotein (ox-LDL) upregulates DDR2 expression in VSMCs, suggesting DDR2 responsiveness to proatherosclerotic conditions. Experimental knockdown of DDR2 in VSMCs inhibits migration, proliferation, and collagen I-induced expression of MMPs, particularly MMP-2 .

This suggests DDR2 may contribute to plaque vulnerability by promoting extracellular matrix degradation, a hypothesis that warrants further investigation.

How does DDR2 regulate matrix metalloproteinases (MMPs) and what are the implications?

DDR2 serves as a critical regulator of multiple MMPs in a tissue-specific manner:

The regulation of MMPs by DDR2 has significant implications:

• In wound healing: Promotes ECM remodeling necessary for tissue repair

• In atherosclerosis: May contribute to plaque instability through matrix degradation

• In arthritis: Excessive MMP production may promote cartilage destruction

• In cancer: May facilitate tumor cell invasion through ECM degradation

The DDR2-MMP axis represents a potential therapeutic target, particularly in diseases characterized by dysregulated ECM turnover such as fibrosis, arthritis, and metastatic cancer.

What experimental approaches are used to study DDR2 in disease models?

Researchers employ multiple approaches to investigate DDR2's role in disease:

Genetic manipulation techniques:

• Knockout mouse models: Complete DDR2 deletion to study developmental and physiological impacts

• Conditional knockout models: Tissue-specific DDR2 deletion

• siRNA knockdown: Used in cell culture and some in vivo models to temporarily reduce DDR2 expression

Functional assays:

• Wound healing models: To assess proliferation and migration in skin repair

• Atherosclerotic plaque models: Using high-fat diets in susceptible animals

• Fibrosis models: Studying DDR2's role in organ fibrosis

• Arthritis models: Examining cartilage degradation mechanisms

Analytical methods:

• Immunohistochemistry: For localization of DDR2 in tissue sections

• Co-localization studies: To determine DDR2's spatial relationship with other molecules

• Phosphoproteomic analysis: To identify downstream signaling pathways

When designing experiments, researchers should consider both acute and chronic effects of DDR2 manipulation, as its roles in development versus adult tissue homeostasis may differ.

How does DDR2 interact with other cellular receptors and signaling pathways?

DDR2 does not function in isolation but participates in complex signaling networks:

• Interaction with Shc and Src: Upon collagen binding and autophosphorylation, DDR2 promotes associations with these signaling molecules

• Extracellular interactions: The extracellular domain (ECD) of DDR2 exists as a noncovalent dimer in solution, and this dimerization enhances collagen binding

• Crosstalk with growth factor receptors: DDR2-mediated MMP expression may regulate the availability of growth factors, allowing crosstalk with other signaling systems

• Potential integrin interactions: While not fully characterized, DDR2 may coordinate with integrins in cell-matrix adhesion and signaling

The complex signaling network involving DDR2 requires sophisticated experimental approaches, including interactome analysis and phosphoproteomic profiling, to fully elucidate.

What are the main challenges in studying DDR2 function in human tissues?

Researchers face several significant challenges when investigating DDR2:

• Slow activation kinetics: Unlike most receptor tyrosine kinases that activate within minutes, DDR2 activation by collagen is unusually slow, making temporal studies difficult

• Tissue heterogeneity: DDR2 expression varies across tissues and even within the same tissue type, requiring careful sample preparation

• Functional redundancy: Potential overlap in function with DDR1 and other collagen receptors can complicate interpretation of results

• Isoform diversity: Multiple isoforms may exist with potentially distinct functions

• Post-translational modifications: These can alter DDR2 function in ways that may be tissue-specific

To address these challenges, researchers should employ multiple complementary approaches and carefully control for variability in experimental systems.

What promising therapeutic targets exist in the DDR2 signaling pathway?

The involvement of DDR2 in multiple pathological processes suggests several potential therapeutic approaches:

• Kinase inhibition: Targeting the kinase domain of DDR2 to block downstream signaling

• Blocking collagen-DDR2 interaction: Disrupting the binding of collagens to the discoidin domain

• Targeting DDR2-induced MMPs: Inhibiting the specific MMPs that are upregulated by DDR2 activation

• Modulating DDR2 expression: Approaches to downregulate DDR2 in diseases where it is overexpressed

Therapeutic development must consider the physiological roles of DDR2 in tissue homeostasis and repair to avoid unintended consequences of DDR2 inhibition.