Biophysical and structural studies of the human calcium- and integrin-binding protein family: understanding their functional similarities and differences

Decreased expressions of calcium- and integrin-binding protein 4 in the testes of patients with azoosperma and in the ejaculated spermatozoa of patients with abnormal semen parameters

OBJECTIVE

Oligoasthenozoospermia (OAZS), asthenozoospermia (AZS), and azoospermia are the major causes of male infertility with no effective therapeutic treatment. To determine the expression and distribution of calcium- and integrin-binding protein 4 (CIB4) in the patients with azoospermia and OAZS, it is important to explore novel targets and treatment strategies for patients with abnormal semen parameters.

DESIGN

A small piece of testicular tissue was obtained by multi-site testicular biopsy from each of a total of 16 patients with azoospermia revealed in two groups, i.e., nonobstructive azoospermia (NOA; n = 8) and obstructive azoospermia (OA; n = 8). Sperm samples were collected from 24 infertile men categorized in two groups, OAZS (n = 11) and AZS (n = 13). The control group contained a total of 20 fertile men. Computer-assisted semen analysis (CASA) was performed to assess the sperm motility. Quantitative real-time PCR (RT-qPCR) and Western blot analyses were performed to examine the expression of CIB4.

RESULTS

The results of RT-qPCR and Western blotting analyses revealed the decreased expression levels of CIB4 mRNA and CIB4 protein in both the sperm of the OAZS patients and the testis tissue of NOA patients. The results of linear regression analysis of CIB4 mRNA expression with the clinical features of participants showed that the CIB4 mRNA level was positively correlated with sperm progressive motility (r = 0.397, P < 0.01) and sperm viability (r = 0.364, P < 0.05).

CONCLUSIONS

We conclude that decreased expression level of CIB4 in human testis and sperm, may be a contributor or a downstream indicator for some cases of azoospermia, especially OA and OAZS. The high expression level of CIB4 mRNA in patients with normal semen parameters suggests that CIB4 in human-ejaculated spermatozoa would potentially contribute to the diagnosis and treatment of male infertility caused by deficient sperm motility and count. These results provide novel insights into the molecular mechanisms underlying the male infertility and the clinical importance of CIB4 in the treatment of infertility.

The Diverse Functions of the Calcium- and Integrin-Binding Protein Family

The calcium- and integrin-binding protein (CIB) family, comprising four evolutionarily conserved members (CIB1, CIB2, CIB3, and CIB4), is characterized by canonical EF-hand motifs. The functions of CIBs in the inner ear have been investigated, although further research is still necessary to gain a comprehensive understanding of them. Among the CIB family members, CIB2 is essential for auditory function. CIB3 and CIB2 jointly participate in the regulation of balance. Beyond their sensory roles, CIBs exhibit multifunctionality through calcium-dependent interactions with diverse molecular partners, contributing to the pathogenesis of various conditions, including neurological disorders, cardiovascular diseases, cancer, and male infertility. In this review, we discuss the conserved structure of the CIB family, highlighting its contributions to various biological functions. We also summarize the distribution and function of the CIB family, emphasizing the pivotal roles of CIB2 and CIB3 in hearing and balance.

Mechano-electrical transduction components TMC1-CIB2 undergo a Ca2+-induced conformational change linked to hearing loss

TMC1, a unique causative gene associated with deafness, exhibits variants with autosomal dominant and recessive inheritance patterns. TMC1 codes for the transmembrane channel-like protein 1 (TMC1), a key component of the mechano-electrical transduction (MET) machinery for hearing. However, the molecular mechanism of Ca2+ regulation in MET remains unclear. Calcium and integrin-binding protein 2 (CIB2), another MET component associated with deafness, can bind with Ca2+. Our study shows that TMC1-CIB2 complex undergoes a Ca2+-induced conformational change. We identified a vertebrate-specific binding site on TMC1 that interacts with apo CIB2, linked with hearing loss. Using an ex vivo mouse organotypic cochlea model, we demonstrated that disruption of the calcium-binding site of CIB2 perturbs the MET channel conductivity. After systematically analyzing the hearing loss variants, we observed dominant mutations of TMC1 cluster around the putative ion pore or at the binding interfaces with CIB2. These findings elucidate the molecular mechanisms underlying TMC1-linked hearing loss.

Differentially Expressed Genes in Cardiomyocytes of the First Camelized Mouse Model, Nrapc.255ins78 Mouse

Background/Objectives: The first camelized mouse model (Nrapc.255ins78) was developed to investigate the mechanisms underlying camels' adaptation to extreme environments. Previous studies demonstrated that these mice exhibit a cold-resistant phenotype, characterized by increased expression of inflammatory cytokine-related genes in the heart under cold stress. Nebulin-related anchoring protein (NRAP) plays a critical role in organizing myofibrils during cardiomyocyte development. This study builds on prior research by analyzing the heart transcriptomes of Nrapc.255ins78 mice under non-stress conditions to explore the origins of inflammatory cytokine responses during cold exposure. Methods: RNA sequencing was performed on the hearts of 12-week-old male and female Nrapc.255ins78 and wild-type control mice. Results: Differential expression analysis identified 25 genes, including 12 associated with cell cycle and division, all consistently downregulated in Nrapc.255ins78. Notably, the calcium and integrin-binding protein gene (Cib3) was significantly upregulated (FDR < 0.05; p < 0.001). Conclusions: These differentially expressed genes suggest altered calcium dynamics in cardiomyocytes and mechanisms for maintaining homeostasis, supporting the hypothesis that inflammatory cytokines during cold exposure may represent an adaptive response. These findings provide valuable insights into the genetic mechanisms of temperature adaptation in camels and highlight potential pathways for enhancing stress resistance in other mammals.

Complexes of vertebrate TMC1/2 and CIB2/3 proteins form hair-cell mechanotransduction cation channels

Calcium and integrin-binding protein 2 (CIB2) and CIB3 bind to transmembrane channel-like 1 (TMC1) and TMC2, the pore-forming subunits of the inner-ear mechano-electrical transduction (MET) apparatus. These interactions have been proposed to be functionally relevant across mechanosensory organs and vertebrate species. Here, we show that both CIB2 and CIB3 can form heteromeric complexes with TMC1 and TMC2 and are integral for MET function in mouse cochlea and vestibular end organs as well as in zebrafish inner ear and lateral line. Our AlphaFold 2 models suggest that vertebrate CIB proteins can simultaneously interact with at least two cytoplasmic domains of TMC1 and TMC2 as validated using nuclear magnetic resonance spectroscopy of TMC1 fragments interacting with CIB2 and CIB3. Molecular dynamics simulations of TMC1/2 complexes with CIB2/3 predict that TMCs are structurally stabilized by CIB proteins to form cation channels. Overall, our work demonstrates that intact CIB2/3 and TMC1/2 complexes are integral to hair-cell MET function in vertebrate mechanosensory epithelia.

Structural dynamics of calcium and integrin-binding protein 2 (CIB2) reveal uncommon flexibility and heterogeneous calcium and magnesium loading

Calcium- and Integrin-Binding protein 2 (CIB2) is a widely expressed protein with an uncertain biological role. Two of its four EF-hand motifs bind Mg(II) and/or Ca(II), thus triggering conformational changes. Although previous studies suggested that CIB2 preferentially binds Mg(II) over Ca(II) under physiological conditions, an atomic level characterization of CIB2 in the presence of both cations was lacking. Based on a combination of solution NMR, exhaustive molecular dynamics simulations and isothermal titration and differential scanning calorimetry, we characterized the interaction of CIB2 with both Ca(II) and Mg(II) ions and elucidated the protein regions involved in the interaction with the α7B integrin target. Analysis of experimental amide nitrogen relaxation rates shows that the EF4 motif exhibits high mobility regardless of the specific bound metal ion and demonstrates that the Mg(II)- and Ca(II)-bound state of CIB2 is relatively floppy, with pico-nanosecond motions induced in a region involved in target recognition. Overall, our data indicate a preferential, thermodynamically stable but structurally flexible state for CIB2, in which a Mg(II) ion is bound to EF3 and a Ca(II) ion to EF4. These results unveil the role of metal binding events in CIB2 and offer new insights into the dynamic regulation of target recognition.

Complexes of vertebrate TMC1/2 and CIB2/3 proteins form hair-cell mechanotransduction cation channels

Calcium and integrin-binding protein 2 (CIB2) and CIB3 bind to transmembrane channel-like 1 (TMC1) and TMC2, the pore-forming subunits of the inner-ear mechano-electrical transduction (MET) apparatus. These interactions have been proposed to be functionally relevant across mechanosensory organs and vertebrate species. Here we show that both CIB2 and CIB3 can form heteromeric complexes with TMC1 and TMC2 and are integral for MET function in mouse cochlea and vestibular end organs as well as in zebrafish inner ear and lateral line. Our AlphaFold 2 models suggest that vertebrate CIB proteins can simultaneously interact with at least two cytoplasmic domains of TMC1 and TMC2 as validated using nuclear magnetic resonance spectroscopy of TMC1 fragments interacting with CIB2 and CIB3. Molecular dynamics simulations of TMC1/2 complexes with CIB2/3 predict that TMCs are structurally stabilized by CIB proteins to form cation channels. Overall, our work demonstrates that intact CIB2/3 and TMC1/2 complexes are integral to hair-cell MET function in vertebrate mechanosensory epithelia.

Unnatural Amino Acids for Biological Spectroscopy and Microscopy

Due to advances in methods for site-specific incorporation of unnatural amino acids (UAAs) into proteins, a large number of UAAs with tailored chemical and/or physical properties have been developed and used in a wide array of biological applications. In particular, UAAs with specific spectroscopic characteristics can be used as external reporters to produce additional signals, hence increasing the information content obtainable in protein spectroscopic and/or imaging measurements. In this Review, we summarize the progress in the past two decades in the development of such UAAs and their applications in biological spectroscopy and microscopy, with a focus on UAAs that can be used as site-specific vibrational, fluorescence, electron paramagnetic resonance (EPR), or nuclear magnetic resonance (NMR) probes. Wherever applicable, we also discuss future directions.

Calcium- and Integrin-Binding Protein 2 (CIB2) in Physiology and Disease: Bright and Dark Sides

Calcium- and integrin-binding protein 2 (CIB2) is a small EF-hand protein capable of binding Mg²⁺ and Ca²⁺ ions. While its biological function remains largely unclear, an increasing number of studies have shown that CIB2 is an essential component of the mechano-transduction machinery that operates in cochlear hair cells. Mutations in the gene encoding CIB2 have been associated with non-syndromic deafness. In addition to playing an important role in the physiology of hearing, CIB2 has been implicated in a multitude of very different processes, ranging from integrin signaling in platelets and skeletal muscle to autophagy, suggesting extensive functional plasticity. In this review, we summarize the current understanding of biochemical and biophysical properties of CIB2 and the biological roles that have been proposed for the protein in a variety of processes. We also highlight the many molecular aspects that remain unclarified and deserve further investigation.

TMT-based quantitative proteomic analysis of antitumor mechanism of Sporisorium reilianum polysaccharide WM-NP-60 against HCT116 cells

Sporisorium reilianum is an active edible and medicinal phytopathogenic fungus. Our study indicated that the S. reilianum polysaccharide WM-NP-60 could inhibit the growth of HCT116 cells in a dose-dependent manner. In addition, WM-NP-60 could trigger the cell cycle of HCT116 arrest at the G₁ phase and induce its apoptosis. In order to explore the anti-tumor mechanism of WM-NP-60, TMT-based quantitative proteomic analysis was used. Results indicated that 369 differentially expressed proteins including 240 up-regulated and 129 down-regulated proteins in WM-NP-60 treated HCT116 cells compared with normal HCT116 cells. Furthermore, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that 192 pathways were enriched containing 15 metabolic pathways with significant difference (P < 0.05). The levels of mRNA and protein up-regulated TGFβR1, P107, DP1 and down-regulated THBS1 related to TGF-β signaling pathway were verified with qRT-PCR and Western Blot (WB). These findings will provide theoretical basis for the important role of fungal polysaccharides in the field of tumor treatment.

Usher Syndrome: Genetics and Molecular Links of Hearing Loss and Directions for Therapy

Usher syndrome (USH) is an autosomal recessive (AR) disorder that permanently and severely affects the senses of hearing, vision, and balance. Three clinically distinct types of USH have been identified, decreasing in severity from Type 1 to 3, with symptoms of sensorineural hearing loss (SNHL), retinitis pigmentosa (RP), and vestibular dysfunction. There are currently nine confirmed and two suspected USH-causative genes, and a further three candidate loci have been mapped. The proteins encoded by these genes form complexes that play critical roles in the development and maintenance of cellular structures within the inner ear and retina, which have minimal capacity for repair or regeneration. In the cochlea, stereocilia are located on the apical surface of inner ear hair cells (HC) and are responsible for transducing mechanical stimuli from sound pressure waves into chemical signals. These signals are then detected by the auditory nerve fibers, transmitted to the brain and interpreted as sound. Disease-causing mutations in USH genes can destabilize the tip links that bind the stereocilia to each other, and cause defects in protein trafficking and stereocilia bundle morphology, thereby inhibiting mechanosensory transduction. This review summarizes the current knowledge on Usher syndrome with a particular emphasis on mutations in USH genes, USH protein structures, and functional analyses in animal models. Currently, there is no cure for USH. However, the genetic therapies that are rapidly developing will benefit from this compilation of detailed genetic information to identify the most effective strategies for restoring functional USH proteins.

Genome-Wide Meta-Analyses of FTND and TTFC Phenotypes

INTRODUCTION

FTND (Fagerstrӧm test for nicotine dependence) and TTFC (time to smoke first cigarette in the morning) are common measures of nicotine dependence (ND). However, genome-wide meta-analysis for these phenotypes has not been reported.

METHODS

Genome-wide meta-analyses for FTND (N = 19,431) and TTFC (N = 18,567) phenotypes were conducted for adult smokers of European ancestry from 14 independent cohorts.

RESULTS

We found that SORBS2 on 4q35 (p = 4.05 × 10-8), BG182718 on 11q22 (p = 1.02 × 10-8), and AA333164 on 14q21 (p = 4.11 × 10-9) were associated with TTFC phenotype. We attempted replication of leading candidates with independent samples (FTND, N = 7010 and TTFC, N = 10 061), however, due to limited power of the replication samples, the replication of these new loci did not reach significance. In gene-based analyses, COPB2 was found associated with FTND phenotype, and TFCP2L1, RELN, and INO80C were associated with TTFC phenotype. In pathway and network analyses, we found that the interconnected interactions among the endocytosis, regulation of actin cytoskeleton, axon guidance, MAPK signaling, and chemokine signaling pathways were involved in ND.

CONCLUSIONS

Our analyses identified several promising candidates for both FTND and TTFC phenotypes, and further verification of these candidates was necessary. Candidates supported by both FTND and TTFC (CHRNA4, THSD7B, RBFOX1, and ZNF804A) were associated with addiction to alcohol, cocaine, and heroin, and were associated with autism and schizophrenia. We also identified novel pathways involved in cigarette smoking. The pathway interactions highlighted the importance of receptor recycling and internalization in ND.

IMPLICATIONS

Understanding the genetic architecture of cigarette smoking and ND is critical to develop effective prevention and treatment. Our study identified novel candidates and biological pathways involved in FTND and TTFC phenotypes, and this will facilitate further investigation of these candidates and pathways.

CIB4 is essential for the haploid phase of spermatogenesis in mice†

Spermatogenesis is a complex developmental process that involves the proliferation of diploid cells, meiotic division, and haploid differentiation. Many genes are shown to be essential for male fertility using knockout (KO) mice; however, there still remain genes to be analyzed to elucidate their molecular mechanism and their roles in spermatogenesis. Calcium- and integrin-binding protein 1 (CIB1) is a ubiquitously expressed protein that possesses three paralogs: CIB2, CIB3, and CIB4. It is reported that Cib1 KO male mice are sterile due to impaired haploid differentiation. In this study, we discovered that Cib4 is expressed strongly in mouse and human testis and begins expression during the haploid phase of spermatogenesis in mice. To analyze the function of CIB4 in vivo, we generated Cib4 KO mice using the CRISPR/Cas9 system. Cib4 KO male mice are sterile due to impaired haploid differentiation, phenocopying Cib1 KO male mice. Spermatogenic cells isolated from seminiferous tubules demonstrate an essential function of CIB4 in the formation of the apical region of the sperm head. Further analysis of CIB4 function may shed light on the etiology of male infertility caused by spermatogenesis defects, and CIB4 could be a target for male contraceptives because of its dominant expression in the testis.

Oligomeric state, hydrodynamic properties and target recognition of human Calcium and Integrin Binding protein 2 (CIB2)

Calcium- and Integrin-Binding protein 2 (CIB2) is a small and ubiquitously expressed protein with largely unknown biological function but ascertained role in hearing physiology and disease. Recent studies found that CIB2 binds Ca2+ with moderate affinity and dimerizes under conditions mimicking the physiological ones. Here we provided new lines of evidence on CIB2 oligomeric state and the mechanism of interaction with the α7B integrin target. Based on a combination of native mass spectrometry, chemical cross-linking/mass spectrometry, analytical gel filtration, dynamic light scattering and molecular dynamics simulations we conclude that CIB2 is monomeric under all tested conditions and presents uncommon hydrodynamic properties, most likely due to the high content of hydrophobic solvent accessible surface. Surface plasmon resonance shows that the interaction with α7B occurs with relatively low affinity and is limited to the cytosolic region proximal to the membrane, being kinetically favored in the presence of physiological Mg2+ and in the absence of Ca2+. Although CIB2 binds to an α7B peptide in a 1:1 stoichiometry, the formation of the complex might induce binding of another CIB2 molecule.

Preferential Binding of Mg2+ Over Ca2+ to CIB2 Triggers an Allosteric Switch Impaired in Usher Syndrome Type 1J

Calcium and integrin binding protein 2 (CIB2) shares with the other members of the CIB family the ability to bind Ca²⁺ and Mg²⁺ via two functional EF-hand motifs, namely EF3 and EF4. As a cation sensor, CIB2 is able to switch to a conformation likely associated with specific biological functions yet to be clarified. Recent findings demonstrate the involvement of CIB2 in hearing physiology and a single, conservative point mutation (p.E64D) has been related to Usher Syndrome type 1J (USH1J) and non-syndromic hearing loss. We present an exhaustive biochemical and biophysical characterization of human wild type (WT) and E64D CIB2. We found that CIB2 does not possibly work as a calcium sensor under physiological conditions, its affinity for Ca²⁺ (K_d^app = 0.5 mM) being too low for detecting normal intracellular levels. Instead, CIB2 displays a significantly high affinity for Mg²⁺ (K_d^app = 290 μM), and it is probably Mg²⁺ -bound under physiological conditions. At odds with the homologous protein CIB1, CIB2 forms a non-covalent dimer under conditions that mimic the physiological ones, and as such it interacts with its physiological target α7B integrin. NMR spectroscopy revealed a long-range allosteric communication between the residue E64, located at the N-terminal domain, and the metal cation binding site EF3, located at the C-terminal domain. The conservative E64D mutation breaks up such inter-domain communication resulting in the impaired ability of CIB2 to switch to its Mg²⁺-bound form. The ability to bind the target integrin peptide was substantially conserved for E64D CIB2, thus suggesting that the molecular defect associated with USH1J resides in its inability to sense Mg²⁺ and adopt the required conformation.

Regulation of protein-coding gene and long noncoding RNA pairs in liver of conventional and germ-free mice following oral PBDE exposure

Gut microbiome communicates with the host liver to modify hepatic xenobiotic biotransformation and nutrient homeostasis. Polybrominated diphenyl ethers (PBDEs) are persistent environmental contaminants that are detected in fatty food, household dust, and human breast milk at worrisome levels. Recently, long noncoding RNAs (lncRNAs) have been recognized as novel biomarkers for toxicological responses and may regulate the transcriptional/translational output of protein-coding genes (PCGs). However, very little is known regarding to what extent the interactions between PBDEs and gut microbiome modulate hepatic lncRNAs and PCGs, and what critical signaling pathways are impacted at the transcriptomic scale. In this study, we performed RNA-Seq in livers of nine-week-old male conventional (CV) and germ-free (GF) mice orally exposed to the most prevalent PBDE congeners BDE-47 and BDE-99 (100 μmol/kg once daily for 4-days; vehicle: corn oil, 10 ml/kg), and unveiled key molecular pathways and PCG-lncRNA pairs targeted by PBDE-gut microbiome interactions. Lack of gut microbiome profoundly altered the PBDE-mediated transcriptomic response in liver, with the most prominent effect observed in BDE-99-exposed GF mice. The top pathways up-regulated by PBDEs were related to xenobiotic metabolism, whereas the top pathways down-regulated by PBDEs were in lipid metabolism and protein synthesis in both enterotypes. Genomic annotation of the differentially regulated lncRNAs revealed that majority of these lncRNAs overlapped with introns and 3'-UTRs of PCGs. Lack of gut microbiome profoundly increased the percentage of PBDE-regulated lncRNAs mapped to the 3'-UTRs of PCGs, suggesting the potential involvement of lncRNAs in increasing the translational efficiency of PCGs by preventing miRNA-3'-UTR binding, as a compensatory mechanism following toxic exposure to PBDEs. Pathway analysis of PCGs paired with lncRNAs revealed that in CV mice, BDE-47 regulated nucleic acid and retinol metabolism, as well as circadian rhythm; whereas BDE-99 regulated fatty acid metabolism. In GF mice, BDE-47 differentially regulated 19 lncRNA-PCG pairs that were associated with glutathione conjugation and transcriptional regulation. In contrast, BDE-99 up-regulated the xenobiotic-metabolizing Cyp3a genes, but down-regulated the fatty acid-metabolizing Cyp4 genes. Taken together, the present study reveals common and unique lncRNAs and PCG targets of PBDEs in mouse liver, and is among the first to show that lack of gut microbiome sensitizes the liver to toxic exposure of BDE-99 but not BDE-47. Therefore, lncRNAs may serve as specific biomarkers that differentiate various PBDE congeners as well as environmental chemical-mediated dysbiosis. Coordinate regulation of PCG-lncRNA pairs may serve as a more efficient molecular mechanism to combat against xenobiotic insult, and especially during dysbiosis-induced increase in the internal dose of toxicants.

Variants in CIB2 cause DFNB48 and not USH1J

The genetic, mutational and phenotypic spectrum of deafness-causing genes shows great diversity and pleiotropy. The best examples are the group of genes, which when mutated can either cause non-syndromic hearing loss (NSHL) or the most common dual sensory impairment, Usher syndrome (USH). Variants in the CIB2 gene have been previously reported to cause hearing loss at the DFNB48 locus and deaf-blindness at the USH1J locus. In this study, we characterize the phenotypic spectrum in a multiethnic cohort with autosomal recessive non-syndromic hearing loss (ARNSHL) due to variants in the CIB2 gene. Of the 6 families we ascertained, 3 segregated novel loss-of-function (LOF) variants, 2 families segregated missense variants (1 novel) and 1 family segregated a previously reported pathogenic variant in trans with a frameshift variant. This report is the first to show that biallelic LOF variants in CIB2 cause ARNSHL and not USH. In the era of precision medicine, providing the correct diagnosis (NSHL vs USH) is essential for patient care as it impacts potential intervention and prevention options for patients. Here, we provide evidence disqualifying CIB2 as an USH-causing gene.

Molecular analysis of CIB4 gene and protein in Kermani sheep

The human calcium- and integrin-binding protein (CIB) family is composed of CIB1, CIB2, CIB3, and CIB4 proteins and the CIB4 gene affects fertility. Kermani sheep is one of the most important breeds of Iranian sheep breeds. The aim of this study was to analyze for the first time molecular characteristics of the CIB4 gene and protein in Kermani sheep. Different tissues were collected from the Kermani sheep and real time PCR was performed. The PCR products were sequenced, comparative analyses of the nucleotide sequences were performed, a phylogenetic tree was constructed, and different characteristics of CIB4 proteins were predicted. Real time PCR results showed that the CIB4 gene is expressed only in testis of Kermani sheep. The cDNA nucleotide sequence was identical with small tail Han sheep, cattle, goat, camel, horse, dog, mouse and human, respectively 100, 99, 99, 98, 98, 96, 96, and 96%. Hence, it can be suggested that the CIB4 gene plays a role in male fertility. Based on the phylogenetic analysis, sheep CIB4 gene has a close relationship with goat and cattle first, and then with camel and whale. Although we demonstrated that CIB4 is a testis-specific gene, expressed only in the testis and it interacts with other proteins, the mechanisms by which CIB4 expression is regulated need to be elucidated.

CIB2 Negatively Regulates Oncogenic Signaling in Ovarian Cancer via Sphingosine Kinase 1

Sphingosine kinase 1 (SK1) is a key regulator of the cellular balance between proapoptotic and prosurvival sphingolipids. Oncogenic signaling by SK1 relies on its localization to the plasma membrane, which is mediated by the calcium and integrin binding protein CIB1 via its Ca²⁺-myristoyl switch function. Here we show that another member of the CIB family, CIB2, plays a surprisingly opposite role to CIB1 in the regulation of SK1 signaling. CIB2 bound SK1 on the same site as CIB1, yet it lacks the Ca²⁺-myristoyl switch function. As a result, CIB2 blocked translocation of SK1 to the plasma membrane and inhibited its subsequent signaling, which included sensitization to TNFα-induced apoptosis and inhibition of Ras-induced neoplastic transformation. CIB2 was significantly downregulated in ovarian cancer and low CIB2 expression was associated with poor prognosis in ovarian cancer patients. Notably, reintroduction of CIB2 in ovarian cancer cells blocked plasma membrane localization of endogenous SK1, reduced in vitro neoplastic growth and tumor growth in mice, and suppressed cell motility and invasiveness both in vitro and in vivo Consistent with the in vitro synergistic effects between the SK1-specific inhibitor SK1-I and standard chemotherapeutics, expression of CIB2 also sensitized ovarian cancer cells to carboplatin. Together, these findings identify CIB2 as a novel endogenous suppressor of SK1 signaling and potential prognostic marker and demonstrate the therapeutic potential of SK1 in this gynecologic malignancy. Cancer Res; 77(18); 4823-34. ©2017 AACR.

CIB1 and CIB2 are HIV-1 helper factors involved in viral entry

HIV-1 relies on the host-cell machinery to accomplish its replication cycle, and characterization of these helper factors contributes to a better understanding of HIV-host interactions and can identify potential novel antiviral targets. Here we explored the contribution of CIB2, previously identified by RNAi screening as a potential helper factor, and its homolog, CIB1. Knockdown of either CIB1 or CIB2 strongly impaired viral replication in Jurkat cells and in primary CD4+ T-lymphocytes, identifying these proteins as non-redundant helper factors. Knockdown of CIB1 and CIB2 impaired envelope-mediated viral entry for both for X4- and R5-tropic HIV-1, and both cell-free and cell-associated entry pathways were affected. In contrast, the level of CIB1 and CIB2 expression did not influence cell viability, cell proliferation, receptor-independent viral binding to the cell surface, or later steps in the viral replication cycle. CIB1 and CIB2 knockdown was found to reduce the expression of surface molecules implicated in HIV-1 infection, including CXCR4, CCR5 and integrin α4β7, suggesting at least one mechanism through which these proteins promote viral infection. Thus, this study identifies CIB1 and CIB2 as host helper factors for HIV-1 replication that are required for optimal receptor-mediated viral entry.

A Novel C-Terminal CIB2 (Calcium and Integrin Binding Protein 2) Mutation Associated with Non-Syndromic Hearing Loss in a Hispanic Family

Hearing loss is a complex disorder caused by both genetic and environmental factors. Previously, mutations in CIB2 have been identified as a common cause of genetic hearing loss in Pakistani and Turkish populations. Here we report a novel (c.556C>T; p.(Arg186Trp)) transition mutation in the CIB2 gene identified through whole exome sequencing (WES) in a Caribbean Hispanic family with non-syndromic hearing loss. CIB2 belongs to the family of calcium-and integrin-binding (CIB) proteins. The carboxy-termini of CIB proteins are associated with calcium binding and intracellular signaling. The p.(Arg186Trp) mutation is localized within predicted type II PDZ binding ligand at the carboxy terminus. Our ex vivo studies revealed that the mutation did not alter the interactions of CIB2 with Whirlin, nor its targeting to the tips of hair cell stereocilia. However, we found that the mutation disrupts inhibition of ATP-induced Ca2+ responses by CIB2 in a heterologous expression system. Our findings support p.(Arg186Trp) mutation as a cause for hearing loss in this Hispanic family. In addition, it further highlights the necessity of the calcium binding property of CIB2 for normal hearing.

Identification of novel integrin binding partners for calcium and integrin binding protein 1 (CIB1): structural and thermodynamic basis of CIB1 promiscuity

The short cytoplasmic tails of the α- and β-chains of integrin adhesion receptors regulate integrin activation and cell signaling. Significantly less is known about proteins that bind to α-integrin cytoplasmic tails (CTs) as opposed to β-CTs to regulate integrins. Calcium and integrin binding protein 1 (CIB1) was previously identified as an αIIb binding partner that inhibits agonist-induced activation of the platelet-specific integrin, αIIbβ3. A sequence alignment of all α-integrin CTs revealed that key residues in the CIB1 binding site of αIIb are well-conserved, and was used to delineate a consensus binding site (I/L-x-x-x-L/M-W/Y-K-x-G-F-F). Because the CIB1 binding site of αIIb is conserved in all α-integrins and CIB1 expression is ubiquitous, we asked if CIB1 could interact with other α-integrin CTs. We predicted that multiple α-integrin CTs were capable of binding to the same hydrophobic binding pocket on CIB1 with docking models generated by all-atom replica exchange discrete molecular dynamics. After demonstrating novel in vivo interactions between CIB1 and other whole integrin complexes with co-immunoprecipitations, we validated the modeled predictions with solid-phase competitive binding assays, which showed that other α-integrin CTs compete with the αIIb CT for binding to CIB1 in vitro. Isothermal titration calorimetry measurements indicated that this binding is driven by hydrophobic interactions and depends on residues in the CIB1 consensus binding site. These new mechanistic details of CIB1-integrin binding imply that CIB1 could bind to all integrin complexes and act as a broad regulator of integrin function.