FcRn Binds and Transports Albumin

描述（由申请人提供）：我们测试了一个被遗忘了40年的假设，即Brambell受体，mhc相关的IgG Fc受体（FcRn），保护白蛋白免受细胞内分解代谢降解，就像它保护IgG一样，延长了两种分子的半衰期。我们的初步研究证实了这一假设的三个预测，表明白蛋白在低pH下与FcRn结合，就像IgG一样，在与IgG结合位点不同的位置形成三分子复合物。初步实验表明白蛋白与天然受体直接结合；SPR实验显示KD在fM范围内。此外，我们发现fcrn缺陷（KO）小鼠比WT小鼠更快地降解白蛋白，并且fcrn缺陷小鼠的血清白蛋白浓度约为WT对照组的一半。这些数据推断，单位时间内每个IgG分子保护的白蛋白分子不被降解的数量要多出许多倍，并且FcRn可能以某种方式参与了白蛋白的生物合成。

DESCRIPTION (provided by applicant): We tested a forgotten 40-year old hypothesis that the Brambell receptor, the MHC-related Fc receptor for IgG (FcRn), protects albumin from intracellular catabolic degradation just is it does IgG, prolonging the half-lives of both molecules. Our preliminary studies affirmed three predictions of this hypothesis, showing that albumin binds FcRn at low pH, just as does IgG, at a site distinct from the IgG binding site, forming a trimolecular complex. Preliminary experiments indicate direct binding of albumin to the native receptor; SPR experiments show KD in fM range. Further, we found that FcRn-deficient (KO) mice degrade albumin more rapidly than WT mice, and that the serum albumin concentration in FcRn-deficient mice is about half that of WT controls. These data infer that many fold more albumin molecules are protected from degradation per IgG molecule per unit time, and that FcRn may somehow be involved in albumin biosynthesis. Our specific aims are two fold. First, we will characterize the molecular interaction of albumin with FcRn by determining affinity, kinetic rate constants, specificity, and stoichiometry of interaction. Second, we will test six predictions or extensions of the hypothesis; namely, that native FcRn from tissue will bind albumin, that albumin may be transported from mother to offspring, that FcRn mediates trans-endothelial flux of albumin, that FcRn participates in hepatocyte production of albumin, that albumin transport across cell in vitro is FcRn mediated, and that a family deficient in albumin and IgG because of accelerated degradation rates of both molecules have a faulty FcRn gene. We apply a broad range of methods encompassing many disciplines. In the first aim, mostly biochemical, we use surface plasmon resonance, radioligand binding assays, size exclusion chromatography, isothermal titration calorimetry, and affinity chromatography. The second aim, which includes several physiological goals, employs pharmacokinetic analysis, in vivo transport studies in KO mice, tissue culture, immunofluorescence, and molecular biology. The implications of these studies extend far on a broad range of health and disease topics. These include the homeostatic regulation of serum albumin concentration; the flux of albumin across endothelium between plasma and extravascular compartments arid across other tissue barriers; the mechanism of hepatocyte albumin production; the placental transport of proteins; and the evolution of the MHC family of proteins.