SUN-LB130 Identification of IGSF1 Ligands

Immunoglobulin superfamily, member 1 (IGSF1), is an X-linked, type 1 transmembrane glycoprotein that is highly expressed in the anterior pituitary gland and testes. Mutations in the IGSF1 gene cause congenital central hypothyroidism, variable hypoprolactinemia, growth hormone dysregulation, and macroorchidism. Igsf1 knockout mice exhibit reduced pituitary TRH receptor (Trhr1) expression with an associated impairment in TRH-stimulated TSH secretion. The mechanism through which IGSF1 loss leads to reductions in Trhr1 levels is unresolved, at least in part because IGSF1’s cellular functions are unknown. The mature form of the IGSF1 protein consists of seven extracellular Ig loops, a single transmembrane domain containing a positively charged arginine, and a short intracellular carboxy-tail devoid of known functional motifs. Recently, IGSF1 was argued to be a member of the leukocyte receptor cluster (LRC) family. LRC proteins act as cell surface receptors for soluble or membrane-bound proteins. We therefore hypothesized that IGSF1 is a cell surface receptor for a presently undescribed ligand that regulates Trhr1 expression in pituitary thyrotrope cells. To identify candidate IGSF1 ligands, we implemented a new ligand trapping method, Ecto-Fc MS. We fused the extracellular (Ecto) domain of IGSF1 to the fragment crystallizable (Fc) region of human IgG, creating an Ecto-Fc fusion protein. Secreted IGSF1-Fc was purified and used as a ligand trap for bait proteins extracted from rat testes. The protein complexes were affinity purified with protein A beads, trypsin digested into peptides, subjected to orthogonal high-pH fractionation, and identified by tandem LC-MS/MS. More than 700 proteins were enriched in IGSF1-Fc preparations compared to an Fc-only negative control. Several secreted ligands and plasma-membrane proteins were identified, many of which are also expressed in pituitary thyrotrope cells. Identifying the ligand or ligands will enable us to determine IGSF1 function, and may lead to the discovery of novel causes of central hypothyroidism and macroorchidism.