Additional N-glycosylation at Asn(13) rescues the human LHbeta-subunit from disulfide-linked aggregation

Hidden Relationships between N-Glycosylation and Disulfide Bonds in Individual Proteins

N-Glycosylation (NG) and disulfide bonds (DBs) are two prevalent co/post-translational modifications (PTMs) that are often conserved and coexist in membrane and secreted proteins involved in a large number of diseases. Both in the past and in recent times, the enzymes and chaperones regulating these PTMs have been constantly discovered to directly interact with each other or colocalize in the ER. However, beyond a few model proteins, how such cooperation affects N-glycan modification and disulfide bonding at selective sites in individual proteins is largely unknown. Here, we reviewed the literature to discover the current status in understanding the relationships between NG and DBs in individual proteins. Our results showed that more than 2700 human proteins carry both PTMs, and fewer than 2% of them have been investigated in the associations between NG and DBs. We summarized both these proteins with the reported relationships in the two PTMs and the tools used to discover the relationships. We hope that, by exposing this largely understudied field, more investigations can be encouraged to unveil the hidden relationships of NG and DBs in the majority of membranes and secreted proteins for pathophysiological understanding and biotherapeutic development.

Two Hormones for One Receptor: Evolution, Biochemistry, Actions, and Pathophysiology of LH and hCG

LH and chorionic gonadotropin (CG) are glycoproteins fundamental to sexual development and reproduction. Because they act on the same receptor (LHCGR), the general consensus has been that LH and human CG (hCG) are equivalent. However, separate evolution of LHβ and hCGβ subunits occurred in primates, resulting in two molecules sharing ~85% identity and regulating different physiological events. Pituitary, pulsatile LH production results in an ~90-minute half-life molecule targeting the gonads to regulate gametogenesis and androgen synthesis. Trophoblast hCG, the "pregnancy hormone," exists in several isoforms and glycosylation variants with long half-lives (hours) and angiogenic potential and acts on luteinized ovarian cells as progestational. The different molecular features of LH and hCG lead to hormone-specific LHCGR binding and intracellular signaling cascades. In ovarian cells, LH action is preferentially exerted through kinases, phosphorylated extracellular-regulated kinase 1/2 (pERK1/2) and phosphorylated AKT (also known as protein kinase B), resulting in irreplaceable proliferative/antiapoptotic signals and partial agonism on progesterone production in vitro. In contrast, hCG displays notable cAMP/protein kinase A (PKA)-mediated steroidogenic and proapoptotic potential, which is masked by estrogen action in vivo. In vitro data have been confirmed by a large data set from assisted reproduction, because the steroidogenic potential of hCG positively affects the number of retrieved oocytes, and LH affects the pregnancy rate (per oocyte number). Leydig cell in vitro exposure to hCG results in qualitatively similar cAMP/PKA and pERK1/2 activation compared with LH and testosterone. The supposed equivalence of LH and hCG has been disproved by such data, highlighting their sex-specific functions and thus deeming it an oversight caused by incomplete understanding of clinical data.

Identification of human IgG1 variant with enhanced FcRn binding and without increased binding to rheumatoid factor autoantibody

Various studies have demonstrated that Fc engineering to enhance neonatal Fc receptor (FcRn) binding is effective for elongating half-life or increasing cellular uptake of IgG. A previous study has shown that a N434H mutation to enhance FcRn binding resulted in increased binding to rheumatoid factor (RF) autoantibody, which is not desirable for therapeutic use in autoimmune disease. In this study, we first showed that all the existing Fc variants with enhanced FcRn binding also show increased RF binding, and then identified specific mutations that could be introduced to those Fc variants to reduce the RF binding. Furthermore, we generated novel Fc variants that do not increase RF binding and show half-lives of 45 d in cynomolgus monkey, which is longer than those of previously reported Fc variants. In addition, we generated novel Fc variants with antigen sweeping activity that do not increase RF binding. We expect that these novel Fc variants will be useful as antibody therapeutics against autoimmune diseases.

The recombinant equine LHβ subunit combines divergent intracellular traits of human LHβ and CGβ subunits

The pituitary LHβ and placental CGβ subunits are products of different genes in primates. The major structural difference between the two subunits is in the carboxy-terminal region, where the short carboxyl sequence of hLHβ is replaced by a longer O-glycosylated carboxy-terminal peptide in hCGβ. In association with this structural deviation, there are marked differences in the secretion kinetics and polarized routing of the two subunits. In equids, however, the CGβ and LHβ subunits are products of the same gene expressed in the placenta and pituitary (LHβ), and both contain a carboxy-terminal peptide. This unusual expression pattern intrigued us and led to our study of eLHβ subunit secretion by transfected Chinese hamster ovary and Madin-Darby canine kidney cells. In continuous labeling and pulse-chase experiments, the secretion of the eLHβ subunit from the transfected Chinese hamster ovary cells was inefficient (medium recovery of 16%-25%) and slow (t1/2 > 6.5 hours). This indicated that, the secretion of the eLHβ subunit resembles that of hLHβ rather than hCGβ. In Madin-Darby canine kidney cells grown on Transwell filters, the eLHβ subunit was preferentially secreted from the apical side, similar to the hCGβ subunit secretory route (∼65% of the total protein secreted). Taken together, these data suggested that secretion of the eLHβ subunit integrates features of both hLHβ and hCGβ subunits. We propose that the evolution of this intracellular behavior may fulfill the physiological demands for biosynthesis of the LH and CG β-subunits in the pituitary and placenta, respectively.

Interrelationship of steric stabilization and self-crowding of a glycosylated protein

In the eukaryotic cell, protein glycosylation takes place in the crowded environment of the endoplasmatic reticulum. With the purpose of elucidating the impact of high concentration on the interactions of glycoproteins, we have conducted a series of small-angle x-ray scattering experiments on the heavily glycosylated enzyme Peniophora lycii phytase (Phy) and its deglycosylated counterpart (dgPhy). The small-angle x-ray scattering data were analyzed using an individual numerical form factor for each of the two glycoforms combined with two structure factors, a hard sphere and a screened coulomb potential structure factor, respectively, as determined by ab initio analysis. Based on this data analysis, three main conclusions could be drawn. First, at comparable protein concentrations (mg/ml), the relative excluded volume of Phy was approximately 75% higher than that of dgPhy, showing that the glycans significantly increase excluded-volume interactions. Second, the relative excluded volume of dgPhy increased with concentration, as expected; however, the opposite effect was observed for Phy, where the relative excluded volume decreased in response to increasing protein concentration. Third, a clear difference in the effect of salinity on the excluded-volume interactions was observed between the two glycol forms. Although the relative excluded volume of dgPhy decreased with increasing ionic strength, the relative excluded volume of Phy was basically insensitive to increased salinity. We suggest that protrusion forces from the glycans contribute to steric stabilization of the protein, and that glycosylation helps to sustain repulsive electrostatic interactions under crowded conditions. In combination, this aids in stabilizing high concentrations of glycosylated proteins.

Altered kinetics of pituitary response to gonadotropin-releasing hormone in women with variant luteinizing hormone: correlation with ovulatory disorders

OBJECTIVE

The LH response of pituitary gland to gonadotropin-releasing hormone (GnRH) stimulation is not well defined in patients with mutant beta-subunit (Trp(8) to Arg(8) and Ile(15) to Thr(15)). Here we compared the relative activities and dynamics of LH secretion in patients with wild-type and variant LH following injection of GnRH.

METHODS

A GnRH stimulation test was performed in 33 patients with ovulatory disorders (patient group) and 29 women with normal ovulatory cycles (control group) heterozygous for the variant LHbeta allele. Blood samples were obtained up to 120 min after GnRH injection. Serum LH response was determined by comparing the results of LH immunoassays using a monoclonal antibody that recognizes wild-type LH only with those of another assay using a polyclonal antibody that recognizes equally both variant and wild-type LH (total LH). The ratio of variant LH to total LH (LH ratio) was used to determine the serum LH status.

RESULTS

The LH ratio in the control group showed the peak 15 min after GnRH injection, while that in the patient group showed the peaks 30-60 min after injection. The LH ratio in the patient group at 120 min after injection was significantly lower than that in the control group. The percent increases in LH ratio in both groups showed the peak 15 min after injection. The patient group had significantly lower changes of LH ratio at 15, 60, 90 and 120 min after GnRH injection compared with that in the control group.

CONCLUSION

Differences in circulatory kinetics of the two types of LH may explain the differences in LH function between patients with ovulatory disorders and women with normal ovulatory cycles.

Effect of additional N-glycosylation signal in the N-terminal region on intracellular function of the human gonadotropin alpha-subunit

hCG, LH, FSH, and TSH are a family of heterodimeric glycoprotein hormones that contain a common alpha-subunit, but differ in their hormone-specific beta-subunits. The alpha-subunit has two N-glycosylation sites at Asn52 and Asn78. To obtain more information on the relationship between the structure and function of the alpha-subunit, we introduced a novel N-glycosylation site in the N-terminal region by mutating Asp3 and Gln5 into Asn and Thr, respectively. Glycosylation mutants were expressed alone or with hCGbeta-subunit in Chinese hamster ovary cells. New N-linked oligosaccharides were efficiently added to the wild-type and mutant alpha-subunits lacking N-glycan at Asn52 (alpha deltaAsn1), Asn78 (alpha deltaAsn2), and both (alpha deltaAsn(1 + 2)). The new sugar chain did not affect secretion and assembly except that 1) it increased the intracellular degradation of alpha deltaAsn(1 + 2), and 2) it augmented the assembly of alpha deltaAsn1 with hCGbeta-subunit. Amino acid changes generated the attachment of O-glycosylation in free alpha-subunit but not in assembled form. These data indicate that the newly introduced N-glycosylation consensus sequence is functional, and that the N-terminal region of the alpha-subunit is flexible and can be modified without affecting the intracellular function. Furthermore, amino acid sequences in the N-terminus are involved in the O-glycosylation in free alpha-subunit.

A common genetic variant of luteinizing hormone; relation to normal and aberrant pituitary-gonadal function

Mutations of the luteinizing hormone (LH) subunit genes are extremely rare. Only one polymorphic LHbeta gene variant makes an exception. In 1992, an immunologically anomalous form of LH was found in a healthy woman, and it was subsequently found to be caused by two point mutations leading to two amino acid substitutions in the LHbeta subunit. Of the two point mutations, Trp(8)Arg and Ile(15)Thr, the first one is mainly responsible for the altered immunoreactivity and the latter one introduces an extra glycosylation site into Asn(13) of the mutated LHbeta peptide. The frequency of this variant LHbeta allele differs widely between ethnic groups, being most common in aboriginal Australians (carrier frequency >50%; allelic frequency 28.3%) and totally lacking from Kotas of Southern India. Functional differences have been detected when wild-type LH and variant LH have been compared. Variant LH possesses increased in vitro bioactivity, whereas its half-life in circulation is shorter in comparison to wild-type LH. Also the regulation of the variant LHbeta gene differs due to additional changes in its promoter sequence. Correlations of occurrence of variant LH with various clinical conditions involving LH function suggest that it represents a biologically less active form of LH and may be related to borderline suppression of gonadal function, including subfertility. In this article, we will review the current information about the differences observed in structure and functions between the wild-type and variant LH, as well as their possible pathophysiological correlations.