S111N mutation in the helical domain of human Gs(alpha) reduces its GDP/GTP exchange rate

Genotype-phenotype correlations in pseudohypoparathyroidism type 1a patients: a systemic review

BACKGROUND

Pseudohypoparathyroidism type 1a (PHP1a) is a rare endocrine disease caused by partial defects of the α subunit of the stimulatory Guanosin triphosphate (GTP) binding protein (Gsα) resulting from maternal GNAS gene variation. The clinical manifestations are related to PTH resistance (hypocalcemia, hyperphosphatemia, and elevated serum intact PTH) in the presence or absence of multihormone resistance, and Albright's hereditary osteodystrophy (AHO).

OBJECTIVES

To summarize the molecular genetics results and clinical characteristics as well as to explore the correlations between them.

METHODS

Articles pertaining to PHP1a until May, 31, 2021 were reviewed and 527 patients with genetic diagnosis were included in the data analysis. The clinical characteristics and molecular genetics results of these patients were analyzed and compared to explore the correlations between them.

RESULTS

A total of 258 GNAS rare variants (RVs) were identified in 527 patients. The RVs were most commonly found in exons 1 and 7 (17.6% each), with frameshift (36.8%), and missense (31.3%) being the main types of RVs. The median age of onset was 5.0 years old. The most common clinical manifestations were elevation of PTH (86.7%) and AHO (87.5%). Thyroid stimulating hormone resistance was the most common hormone resistance (75.5%) other than PTH resistance. Patients with missense and in-frame RVs had lower incidence rates of the round face (P = .001) and subcutaneous ossifications (P < .001) than those with loss-of-function (non-sense, frameshift, splicing site variants, and large deletions) variants.

CONCLUSIONS

This study revealed the correlation between loss-of-function RVs with round faces and subcutaneous ossifications in PHP 1a patients. Further exploration of genotype-phenotype correlations through more standardized and prospective studies with long-term follow-up is necessary.

What is the role of the helical domain of Gsalpha in the GTPase reaction?

Structural analysis of Gsalpha shows that it is composed of two domains: the ras-like domain (RD) that is conserved in all members of the GTPase superfamily and is homologous to the monomeric G-proteins (e.g., p21ras) and an alpha-helical domain (HD) that is unique to heterotrimeric G-proteins. Little is known about the function of the HD. Recent experiments by Bourne and co-workers, who expressed both the RD and the HD of Gsalpha separately and found that GTP hydrolysis is very slow if only recombinant RD is present but is accelerated when the HD is added, suggest that the HD serves as an intrinsic GTPase-activating protein (GAP). In this work, the GTP hydrolysis in Gsalpha was studied. The results obtained by calculating catalytic effects with and without the HD provide evidence for the role of the HD as a GAP. It is demonstrated that a major part of the catalysis is obtained because of an allosteric influence of the HD on the RD. Structural as well as energetic considerations suggest that the HD confines the RD to a more compact conformation, pushing the phosphate into an orientation where it is further stabilized, thus lowering the overall reaction barrier. The resemblance between the behavior of rasGAP and the HD suggests that the conclusion may be a general conclusion, applicable for all of the G-protein members.

Early manifestation of calcinosis cutis in pseudohypoparathyroidism type Ia associated with a novel mutation in the GNAS gene

OBJECTIVE

To clarify the molecular defect for the clinical finding of congenital hypothyroidism combined with the manifestation of calcinosis cutis in infancy.

CASE REPORT

The male patient presented with moderately elevated blood thyrotropin levels at neonatal screening combined with slightly decreased plasma thyroxine and tri-iodothyronine concentrations, necessitating thyroid hormone substitution 2 weeks after birth. At the age of 7 months calcinosis cutis was seen and the patient underwent further investigation. Typical features of Albright's hereditary osteodystrophy (AHO), including round face, obesity and delayed psychomotor development, were found.

METHODS AND RESULTS

Laboratory investigation revealed a resistance to parathyroid hormone (PTH) with highly elevated PTH levels and a reduction in adenylyl cyclase-stimulating protein (Gsalpha) activity leading to the diagnosis of pseudohypoparathyroidism type Ia (PHP Ia). A novel heterozygous mutation (c364T > G in exon 5, leading to the amino acid substitution Ile-106 --> Ser) was detected in the GNAS gene of the patient. This mutation was not found in the patient's parents, both of whom showed normal Gsalpha protein activity in erythrocytes and no features of AHO. A de novo mutation is therefore likely.

CONCLUSIONS

Subcutaneous calcifications in infancy should prompt the clinician to a thorough search for an underlying disease. The possibility of AHO and PHP Ia should be considered in children with hypothyroidism and calcinosis cutis. Systematic reviews regarding the frequency of calcinosis in AHO are warranted.

Mutation of the highly conserved Arg165 and Glu168 residues of human Gsalpha disrupts the alphaD-alphaE loop and enhances basal GDP/GTP exchange rate

G protein signalling regulates a wide range of cellular processes such as motility, differentiation, secretion, neurotransmission, and cell division. G proteins consist of three subunits organized as a Galpha monomer associated with a Gbetagamma heterodimer. Structural studies have shown that Galpha subunits are constituted by two domains: a Ras-like domain, also called the GTPase domain (GTPaseD), and an helical domain (HD), which is unique to heterotrimeric G-proteins. The HD display significantly higher primary structure diversity than the GTPaseD. Regardless of this diversity, there are small regions of the HD which show high degree of identity with residues that are 100% conserved. One of such regions is the alpha helixD-alpha helixE loop (alphaD-alphaE) in the HD, which contains the consensus aminoacid sequence R*-[RSA]-[RSAN]-E*-[YF]-[QH]-L in all mammalian Galpha subunits. Interestingly, the highly conserved arginine (R*) and glutamic acid (E*) residues form a salt bridge that stabilizes the alphaD-alphaE loop, that is localized in the top of the cleft formed between the GTPaseD and HD. Because the guanine nucleotide binding site is deeply buried in this cleft and those interdomain interactions are playing an important role in regulating the basal GDP/GTP nucleotide exchange rate of Galpha subunits, we studied the role of these highly conserved R and E residues in Galpha function. In the present study, we mutated the human Gsalpha R165 and E168 residues to alanine (A), thus generating the R165--> A, E168--> A, and R165/E168--> A mutants. We expressed these human Gsalpha (hGsalpha) mutants in bacteria as histidine tagged proteins, purified them by niquel-agarose chromatography and studied their nucleotide exchange properties. We show that the double R165/E168--> A mutant exhibited a fivefold increased GTP binding kinetics, a higher GDP dissociation rate, and an augmented capacity to activate adenylyl cyclase. Structure analysis showed that disruption of the salt bridge between R165 and E168 by the introduced mutations, caused important structural changes in the HD at the alphaD-alphaE loop (residues 160-175) and in the GTPaseD at a region required for Gsalpha activation by the receptor (residues 308-315). In addition, other two GTPaseD regions that surround the GTP binding site were also affected.

GTP gamma S increases Nav1.8 current in small-diameter dorsal root ganglia neurons

Tetrodotoxin-resistant (TTX-R) sodium current in small-size dorsal root ganglia (DRG) neurons is upregulated by prostaglandin E(2) and serotonin through a protein kinase A (PKA)/protein kinase (PKC) pathway, suggesting G protein modulation of one or more TTX-R channels in these neurons. Recently, GTP(gammaS), a hydrolysis-resistant analogue of GTP, was shown to increase the persistent current produced by the TTX-R Na(v)1.9. In this study, we investigated the modulation of another TTX-R channel, Na(v)1.8, by GTP(gammaS) in small-diameter DRG neurons from rats using whole-cell voltage clamp recordings. Because it has been suggested that fluoride, often used in intracellular recording solutions, may bind to trace amounts of aluminum and activate G proteins, we recorded Na(v)1.8 currents with and without intracellular fluoride, and with the addition of deferoxamine, an aluminum chelator, to prevent fluoride-aluminum binding. Our results show that GTP(gammaS) (100 micro M) caused a significant increase in Na(v)1.8 current (67%) with a chloride-based intracellular solution. Although the inclusion of fluoride instead of chloride in the pipette solution increased the Na(v)1.8 current by 177%, GTP(gammaS) further increased Na(v)1.8 current by 67% under these conditions. While the effect of GTP(gammaS) was prevented by pretreatment with H7 (100 micro M), a non-selective PKA/PKC inhibitor, the fluoride-induced increase in Na(v)1.8 current was not sensitive to H7 (100 micro M), or to inclusion of deferoxamine (1 mM) in the intracellular solution. We conclude that G protein activation by GTP(gammaS) increases Na(v)1.8 current through a PKA/PKC mechanism and that addition of fluoride to the pipette solution further enhances the current, but is not a confounding variable in the study of Na(v)1.8 channel modulation by G proteins independent of a PKA/PKC pathway or binding to aluminum.