CD62L (L-selectin), a member of the selectin family of adhesion receptors (alongside CD62E and CD62P), is a transmembrane glycoprotein with a molecular weight ranging from 74-105 kDa, depending on cell type and glycosylation status. Structurally, it consists of an N-terminal C-type lectin domain critical for carbohydrate-mediated ligand binding, an epidermal growth factor (EGF)-like domain, two complement-binding protein-like short consensus repeat (SCR) domains, a spacer region, a transmembrane domain, and a short cytoplasmic tail. This multidomain architecture enables dynamic interactions with ligands such as GlyCAM-1, CD34, and MAdCAM-1 through calcium-dependent recognition of sialylated Lewis X moieties. Constitutively expressed on most leukocytes including naive T cells, neutrophils, monocytes, and eosinophils, CD62L mediates leukocyte tethering and rolling along vascular endothelia during inflammatory responses. It plays essential roles in lymphocyte homing to peripheral lymph nodes via high endothelial venules (HEVs) and facilitates neutrophil recruitment to inflamed tissues. Activation-induced proteolytic cleavage of its extracellular domain generates soluble CD62L, a regulatory mechanism that modulates leukocyte adhesion dynamics. While broadly expressed in circulating leukocytes, CD62L expression is notably absent on memory T cell subsets, reflecting its role in distinguishing naive and effector immune cell populations.
PTK7 (Protein Tyrosine Kinase 7), also known as colon carcinoma kinase 4 (CCK4), is an evolutionarily conserved transmembrane receptor belonging to the pseudokinase family of receptor tyrosine kinases (RTKs). Structurally, it comprises an extracellular domain with seven immunoglobulin-like loops and two fibronectin type III repeats, a transmembrane helix, and an intracellular pseudokinase domain lacking catalytic activity due to critical residue substitutions in its ATP-binding pocket. This unique architecture enables PTK7 to function as a co-receptor in Wnt/β-catenin signaling pathways through interactions with Wnt ligands and Frizzled receptors, mediating planar cell polarity during embryonic development and tissue morphogenesis. Biochemically, PTK7 orchestrates cell migration and invasion through cytoskeletal reorganization via RAC1 and JNK activation. Emerging evidence highlights its dual role in oncology: while acting as a tumor suppressor in acute myeloid leukemia by modulating cell differentiation, it paradoxically promotes metastasis in solid tumors like colorectal and triple-negative breast cancers through matrix metalloproteinase activation and epithelial-mesenchymal transition.