A new method for purification of human plasma retinol-binding protein and transthyretin

Retinol binding protein IV purified from Escherichia coli using intein-mediated cleavage as a suitable replacement for serum sources

Retinol binding protein IV (RBP) functions as the principal carrier of retinol (Vitamin A) in the blood, where RBP circulates bound to another serum protein, transthyretin. Isolation of pure RBP from the transthyretin complex in human serum can be difficult, but expression of RBP in recombinant systems can circumvent these purification issues. Human recombinant RBP has previously been successfully expressed and purified from E. coli, but recovery of active protein typically requires extensive processing steps, such as denaturing and refolding, and complex purification steps, such as multi-modal chromatography. Furthermore, these methods produce recombinant proteins, often tagged, that display different functional and structural characteristics across systems. In this work, we optimized downstream processing by use of an intein-based expression system in E. coli to produce tag-free, human recombinant RBP (rRBP) with intact native amino termini at yields of up to ~15 mg/L off column. The novel method requires solubilization of inclusion bodies and subsequent oxidative refolding in the presence of retinol, but importantly allows for one-step chromatographic purification that yields high purity rRBP with no N-terminal Met or other tag. Previously reported purification methods typically require two or more chromatographic separation steps to recover tag-free rRBP. Given the interest in mechanistic understanding of RBP transport of retinol in health and disease, we characterized our purified product extensively to confirm rRBP is both structurally and functionally a suitable replacement for serum-derived RBP.

Retinol-Binding Protein Interferes with Transthyretin-Mediated β-Amyloid Aggregation Inhibition

β-Amyloid (Aβ) aggregation is causally linked to Alzheimer's disease. On the basis of in vitro and transgenic animal studies, transthyretin (TTR) is hypothesized to provide neuroprotection against Aβ toxicity by binding to Aβ and inhibiting its aggregation. TTR is a homotetrameric protein that circulates in blood and cerebrospinal fluid; its normal physiological role is as a carrier for thyroxine and retinol-binding protein (RBP). RBP forms a complex with retinol, and the holoprotein (hRBP) binds with high affinity to TTR. In this study, the role of TTR ligands in TTR-mediated inhibition of Aβ aggregation was investigated. hRBP strongly reduced the ability of TTR to inhibit Aβ aggregation. The effect was not due to competition between Aβ and hRBP for binding to TTR, as Aβ bound equally well to TTR-hRBP complexes and TTR. hRBP is known to stabilize the TTR tetrameric structure. We show that Aβ partially destabilizes TTR and that hRBP counteracts this destabilization. Taken together, our results support a mechanism wherein TTR-mediated inhibition of Aβ aggregation requires not only TTR-Aβ binding but also destabilization of TTR quaternary structure.

Comparison of capillary electrophoresis and capillary liquid chromatography coupled to mass spectrometry for the analysis of transthyretin in human serum

Capillary electrophoresis and capillary liquid chromatography coupled to mass spectrometry (CE-MS and CapLC-MS, respectively) are nowadays very suitable techniques for the separation and characterization of intact proteins in biological fluids. In this paper, we compare the performance of both techniques for the analysis of transthyretin (TTR), which is a homotetrameric protein (relative molecular mass (Mr) ∼56,000) involved in different types of amyloidosis. Furthermore, it is also presented a novel sample pretreatment based on immunoprecipitation (IP) using Protein A Ultrarapid Agarose™ (UAPA) magnetic beads (MBs) to purify TTR from serum samples. This novel IP based on MBs allowed the detection of TTR monomeric proteoforms that were not possible to analyze by conventional IP in solution. In addition, UAPA MBs provided many other desirable advantages including higher selectivity and minimal unspecific binding of other proteins. CE-MS and CapLC-MS were applied to analyze serum samples from healthy controls and familial amyloidotic polyneuropathy type I (FAP-I) patients, who suffered from the most common hereditary systemic amyloidosis. Both techniques allowed detecting the same TTR proteoforms, including the mutant TTR (Met 30) variant (variation in relative molecular mass (ΔMr) was +32.07, from wild-type TTR). Migration/retention times and relative quantitation of the different proteoforms were similar and reproducible in both cases, but the limits of detection (LODs) achieved by CE-MS were slightly lower (2-2.5-fold). Some other differences were also found on separation selectivity (migration orders and separation of antibody), peak efficiency, total analysis time, calibration ranges and experimental Mr accuracy.

Biochemical Basis for Dominant Inheritance, Variable Penetrance, and Maternal Effects in RBP4 Congenital Eye Disease

Gestational vitamin A (retinol) deficiency poses a risk for ocular birth defects and blindness. We identified missense mutations in RBP4, encoding serum retinol binding protein, in three families with eye malformations of differing severity, including bilateral anophthalmia. The mutant phenotypes exhibit dominant inheritance, but incomplete penetrance. Maternal transmission significantly increases the probability of phenotypic expression. RBP normally delivers retinol from hepatic stores to peripheral tissues, including the placenta and fetal eye. The disease mutations greatly reduce retinol binding to RBP, yet paradoxically increase the affinity of RBP for its cell surface receptor, STRA6. By occupying STRA6 nonproductively, the dominant-negative proteins disrupt vitamin A delivery from wild-type proteins within the fetus, but also, in the case of maternal transmission, at the placenta. These findings establish a previously uncharacterized mode of maternal inheritance, distinct from imprinting and oocyte-derived mRNA, and define a group of hereditary disorders plausibly modulated by dietary vitamin A.

Exome analysis identified a novel mutation in the RBP4 gene in a consanguineous pedigree with retinal dystrophy and developmental abnormalities

Retinitis Pigmentosa (RP) is a common form of retinal degeneration characterized by photoreceptor degeneration and retinal pigment epithelium (RPE) atrophy causing loss of visual field and acuities. Exome sequencing identified a novel homozygous splice site variant (c.111+1G>A) in the gene encoding retinol binding protein 4 (RBP4). This change segregated with early onset, progressive, and severe autosomal recessive retinitis pigmentosa (arRP) in an eight member consanguineous pedigree of European ancestry. Additionally, one patient exhibited developmental abnormalities including patent ductus arteriosus and chorioretinal and iris colobomas. The second patient developed acne from young age and extending into the 5^th decade. Both patients had undetectable levels of RBP4 in the serum suggesting that this mutation led to either mRNA or protein instability resulting in a null phenotype. In addition, the patients exhibited severe vitamin A deficiency, and diminished serum retinol levels. Circulating transthyretin levels were normal. This study identifies the RBP4 splice site change as the cause of RP in this pedigree. The presence of developmental abnormalities and severe acne in patients with retinal degeneration may indicate the involvement of genes that regulate vitamin A absorption, transport and metabolism.

Interactions amongst plasma retinol-binding protein, transthyretin and their ligands: implications in vitamin A homeostasis and transthyretin amyloidosis

Retinol transport complex consisting of retinol-binding protein (RBP) and transthyretin (TTR) is involved in the transport of retinol (vitamin A) and thyroxine (T(4)) in the human plasma. RBP is a 21-kDa single polypeptide chain protein, synthesized in the liver, which binds and transports retinol to the target organs. The circulating RBP binds to another protein called TTR, a 55-kDa homotetrameric T(4) transport protein. Such protein-protein complex formation is thought to prevent glomerular filtration of low molecular mass RBP. Misfolding and aggregation of TTR is implicated in amyloid disorders such as familial amyloid polyneuropathy (FAP) and senile systemic amyloidosis (SSA). Recent observations suggest that both RBP and T(4), the physiological ligands of TTR, prevent its misfolding and amyloid fibril formation, suggesting yet another structure-function relationship to this protein-protein complex. TTR2, a poorly characterized protein, was also found bound to RBP in human and pig plasma but its significance remains to be understood. Furthermore, knockout models of both RBP and TTR unequivocally demonstrated the importance of this protein-protein complex in retinoid transport. Thus, interactions amongst multiple components of retinol transport play critical roles in vitamin A homeostasis and TTR amyloidosis.