TRPM6 N-Terminal CaM- and S100A1-Binding Domains

Recent advances on the structure and the function relationships of the TRPV4 ion channel

The members of the superfamily of Transient Receptor Potential (TRP) ion channels are physiologically important molecules that have been studied for many years and are still being intensively researched. Among the vanilloid TRP subfamily, the TRPV4 ion channel is an interesting protein due to its involvement in several essential physiological processes and in the development of various diseases. As in other proteins, changes in its function that lead to the development of pathological states, have been closely associated with modification of its regulation by different molecules, but also by the appearance of mutations which affect the structure and gating of the channel. In the last few years, some structures for the TRPV4 channel have been solved. Due to the importance of this protein in physiology, here we discuss the recent progress in determining the structure of the TRPV4 channel, which has been achieved in three species of animals (Xenopus tropicalis, Mus musculus, and Homo sapiens), highlighting conserved features as well as key differences among them and emphasizing the binding sites for some ligands that play crucial roles in its regulation.

Interaction of Calmodulin with TRPM: An Initiator of Channel Modulation

Transient receptor potential melastatin (TRPM) channels, a subfamily of the TRP superfamily, constitute a diverse group of ion channels involved in mediating crucial cellular processes like calcium homeostasis. These channels exhibit complex regulation, and one of the key regulatory mechanisms involves their interaction with calmodulin (CaM), a cytosol ubiquitous calcium-binding protein. The association between TRPM channels and CaM relies on the presence of specific CaM-binding domains in the channel structure. Upon CaM binding, the channel undergoes direct and/or allosteric structural changes and triggers down- or up-stream signaling pathways. According to current knowledge, ion channel members TRPM2, TRPM3, TRPM4, and TRPM6 are directly modulated by CaM, resulting in their activation or inhibition. This review specifically focuses on the interplay between TRPM channels and CaM and summarizes the current known effects of CaM interactions and modulations on TRPM channels in cellular physiology.

Involvement of Oxidative Stress in Protective Cardiac Functions of Calprotectin

Calprotectin (CLP) belonging to the S-100 protein family is a heterodimeric complex (S100A8/S100A9) formed by two binding proteins. Upon cell activation, CLP stored in neutrophils is released extracellularly in response to inflammatory stimuli and acts as damage-associated molecular patterns (DAMPs). S100A8 and S100A9 possess both anti-inflammatory and anti-bacterial properties. The complex is a ligand of the toll-like receptor 4 (TLR4) and receptor for advanced glycation end (RAGE). At sites of infection and inflammation, CLP is a target for oxidation due to its co-localization with neutrophil-derived oxidants. In the heart, oxidative stress (OS) responses and S100 proteins are closely related and intimately linked through pathophysiological processes. Our review summarizes the roles of S100A8, S100A9 and CLP in the inflammation in relationship with vascular OS, and we examine the importance of CLP for the mechanisms driving in the protection of myocardium. Recent evidence interpreting CLP as a critical modulator during the inflammatory response has identified this alarmin as an interesting drug target.

TRPM5 Channel Binds Calcium-Binding Proteins Calmodulin and S100A1

Melastatin transient receptor potential (TRPM) channels belong to one of the most significant subgroups of the transient receptor potential (TRP) channel family. Here, we studied the TRPM5 member, the receptor exposed to calcium-mediated activation, resulting in taste transduction. It is known that most TRP channels are highly modulated through interactions with extracellular and intracellular agents. The binding sites for these ligands are usually located at the intracellular N- and C-termini of the TRP channels, and they can demonstrate the character of an intrinsically disordered protein (IDP), which allows such a region to bind various types of molecules. We explored the N-termini of TRPM5 and found the intracellular regions for calcium-binding proteins (CBPs) the calmodulin (CaM) and calcium-binding protein S1 (S100A1) by in vitro binding assays. Furthermore, molecular docking and molecular dynamics simulations (MDs) of the discovered complexes confirmed their known common binding interface patterns and the uniqueness of the basic residues present in the TRPM binding regions for CaM/S100A1.

Case Report: A Novel Non-Canonical Splice Site Variant (c.1638+7T>C) in TRPM6 Cause Primary Homagnesemia With Secondary Hocalcemia

OBJECTIVE

Primary hypomagnesemia with secondary hypocalcemia (HSH) is caused by loss-of-function mutations in the TRPM6 gene encoding the epithelial magnesium channel. It is characterized by hypomagnesemia and secondary hypocalcemia associated with neurological symptoms. Here, we aimed to investigate the genetic defects of the TRPM6 gene found in a girl from China.

METHODS

The genomic DNA of the proband and the parents was extracted for whole-exome sequencing. Sanger sequencing was further performed to validate the candidate variants. Subsequently, the TRPM6 gene deletion was verified by quantitative PCR (qPCR) experiment. The effect of the variant on mRNA splicing was analyzed through a minigene splice assay and reverse transcription PCR (RT-PCR) in vitro.

RESULTS

The proband presented with the symptoms of generalized seizures, tetany, and muscle spasms, which were refractory to anticonvulsant treatment. Phenotypic data indicated that the patient had hypomagnesemia, poor parathyroid hormone response, and resultant hypocalcemia. The trio whole-exome sequencing identified that the proband carried compound heterozygous variants in the TRPM6 gene, a paternally derived exon 6 deletion, and a maternally derived splicing variant (c.1638+7T>C) in exon 14. The minigene splice assay confirmed that the c.1638+7T>C variant resulted in exon 14 skipping, which caused the alteration of TRPM6 mRNA splicing.

CONCLUSION

Our results support that the compound heterozygous variants in TRPM6 are responsible for HSH in this patient. A novel pathogenic splicing variant (c.1638+7T>C) in the intron 14 disturbs the normal TRPM6 mRNA splicing, suggesting that the non-classical splice variant plays a critical role in HSH. This variant is essential for future effective genetic diagnosis.

TRPM7 N-terminal region forms complexes with calcium binding proteins CaM and S100A1

Transient receptor potential melastatin 7 (TRPM7) represents melastatin TRP channel with two significant functions, cation permeability and kinase activity. TRPM7 is widely expressed among tissues and is therefore involved in a variety of cellular functions representing mainly Mg²⁺ homeostasis, cellular Ca²⁺ flickering, and the regulation of DNA transcription by a cleaved kinase domain translocated to the nucleus. TRPM7 participates in several important biological processes in the nervous and cardiovascular systems. Together with the necessary function of the TRPM7 in these tissues and its recently analyzed overall structure, this channel requires further studies leading to the development of potential therapeutic targets. Here we present the first study investigating the N-termini of TRPM7 with binding regions for important intracellular modulators calmodulin (CaM) and calcium-binding protein S1 (S100A1) using in vitro and in silico approaches. Molecular simulations of the discovered complexes reveal their potential binding interfaces with common interaction patterns and the important role of basic residues present in the N-terminal binding region of TRPM.

A functional study for verifying the pathogenicity of a TRPM6 variant of uncertain significance: A novel non-canonical splicing-site variant in primary hypomagnesemia with secondary hypocalcemia

INTRODUCTION

Primary hypomagnesemia with secondary hypocalcemia (HSH) is a rare autosomal recessive disease caused by biallelic variants in TRPM6 gene.

METHODS

In this study, we reported a Chinese patient diagnosed with HSH in Tianjin Children's Hospital. Detailed clinical examination and laboratory test were performed and whole exome sequencing (WES) was applied to detect the pathogenic gene in the proband. The suspected compound heterozygous variant in TRPM6 gene was verified by Sanger sequencing and quantitative real-time PCR technology. Minigene assay was performed to verify the function of the variant suspected to affect splicing process.

RESULTS

The patient presented with seizure and markedly decreased levels of serum magnesium and calcium. WES combined with functional study diagnosed a pediatric patient with HSH caused by a compound heterozygous variant in TRPM6 gene, containing a novel non-canonical splicing-site variant c.5058-26A > G and a heterozygous deletion in exons 27-33 (chr9q21.13: 77357467-77376734).

CONCLUSIONS

The compound heterozygous variant in TRPM6 gene is the pathogenic cause of the proband. The combined application of WES and functional study contribute to validating the effect of an uncertain genetic variant on splicing, improving the pathogenicity evidence and identifying the etiology of the disease. It is helpful for early diagnosis and treatment of HSH.

Intrinsically disordered protein domain of human ameloblastin in synthetic fusion with calmodulin increases calmodulin stability and modulates its function

Constantly increasing attention to bioengineered proteins has led to the rapid development of new functional targets. Here we present the biophysical and functional characteristics of the newly designed CaM/AMBN-Ct fusion protein. The two-domain artificial target consists of calmodulin (CaM) and ameloblastin C-terminus (AMBN-Ct). CaM as a well-characterized calcium ions (Ca²⁺) binding protein offers plenty of options in terms of Ca²⁺ detection in biomedicine and biotechnologies. Highly negatively charged AMBN-Ct belongs to intrinsically disordered proteins (IDPs). CaM/AMBN-Ct was designed to open new ways of communication synergies between the domains with potential functional improvement. The character and function of CaM/AMBN-Ct were explored by biophysical and molecular modelling methods. Experimental studies have revealed increased stability and preserved CaM/AMBN-Ct function. The results of molecular dynamic simulations (MDs) outlined different interface patterns between the domains with potential allosteric communication within the fusion.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.