Lacritin - Signaling

Signaling

One particularly remarkable feature about lacritin is its cell targeting specificity. Lacritin targets a restricted group of epithelial cells (including human corneal epithelia), and not fibroblastic, glioma or lymphoblastic cells. New studies suggest that the cell surface proteoglycan syndecan-1 is partly responsible. Syndecan-1 binds many growth factors through its heparan sulfate side-chains. It also binds lacritin, but heparan sulfate interferes with lacritin binding. Since syndecans are always decorated with heparan sulfate, this means that heparanase must be available to partially or completely cleave off heparan sulfate, allowing lacritin to bind. Indeed, siRNA studies without and with rescue demonstrated that heparanase regulates lacritin function.

Biotinylated cell surface proteins from a lacritin-responsive cell were incubated with lacritin under conditions of physiological salt. Those that bound lacritin were sequenced by mass spectrometry. Few bound. The most prominent was syndecan-1 (SDC1). In confirmatory pull-down assays, binding was not shared with family members syndecan-2 or syndecan-4, indicating that the protein core (and not the negatively charged heparan sulfate side-chains) was the main site of binding. Further analysis narrowed the site to syndecan-1's N-terminal 51 amino acids. Syndecan-1 is widely expressed on epithelial and other cell types. How is this mechanism cell-specific? The answer appears to be the restricted availability of active epithelial heparanase (HPSE) that serves as an 'off-on' switch for lacritin binding. This novel mechanism appears at first glance to be poor for ocular health, since heparanase release from invading lymphocytes in the corneal stroma is inflammatory. Yet heparanase is a normal secretory product of the corneal epithelium. Likely it is largely latent and activation may be local.

Lacritin mitogenic signaling follows two pathways:

• Gα_i or Gα_o → PKCα/PLCγ2 → Ca2+ → calcineurin → NFATC1
• Gα_i or Gα_o → PKCα/PLCγ2/PLD1 → mTOR

Rapid dephosphorylation of PKCα causes it to transiently move from the cytoplasm to the area of the Golgi apparatus and peripheral nucleus. Here, it forms a complex with PKCα and PLCγ2 from which downstream mTOR and NFAT signaling is initiated.

The upstream Gα_i or Gα_o signaling suggests the involvement of a G-protein-coupled receptor (GPCR). A candidate GPCR is under study. Syndecan-1 likely serves as a co-receptor. Binding lacritin may improve its GPCR affinity.