FKBP8 Enhances Protein Stability of the CLC-1 Chloride Channel at the Plasma Membrane

GPX4 inhibits apoptosis of thyroid cancer cells through regulating the FKBP8/Bcl-2 axis

GPX4 has attracted much attention as a key molecule of cell ferroptosis, but its role in cell apoptosis is rarely reported, and its role in apoptosis of thyroid cancer (TC) cell has not been reported. The analysis of TCGA database showed that both GPX4 and FKBP8 were highly expressed in TC tumor tissues; The expression of GPX4 and FKBP8 were positively correlated. The immunohistochemical analysis further confirmed that GPX4 and FKBP8 were highly expressed in TC tumor tissues. In addition, the high expression of GPX4 and FKBP8 were both significantly correlated with the poor prognosis of TC. Silencing GPX4 significantly inhibited the proliferation, induced apoptosis of TC cells, and reduced tumor growth in mice. The co-immunoprecipitation assay revealed a physical interaction between GPX4 and FKBP8 observed in the TC cells. Knockdown of FKBP8 significantly inhibited the proliferation and induced apoptosis of TC cells. Rescue experiments suggested that knockdown of FKBP8 could reverse the strengthens of cell proliferation and apoptosis and the higher expression of FKBP8 and Bcl-2 caused by overexpression of GPX4. Our results suggest that the GPX4/FKBP8/Bcl-2 axis promotes TC development by inhibiting TC cell apoptosis, which provides potential molecular targets for TC therapeutic strategies.

Chaperone activity of niflumic acid on ClC-1 chloride channel mutants causing myotonia congenita

Myotonia congenita (MC) is an inherited rare disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations in the skeletal muscle chloride channel ClC-1, important for the stabilization of resting membrane potential and for the repolarization phase of action potentials. Thanks to in vitro functional studies, the molecular mechanisms by which ClC-1 mutations alter chloride ion influx into the cell have been in part clarified, classifying them in “gating-defective” or “expression-defective” mutations. To date, the treatment of MC is only palliative because no direct ClC-1 activator is available. An ideal drug should be one which is able to correct biophysical defects of ClC-1 in the case of gating-defective mutations or a drug capable to recover ClC-1 protein expression on the plasma membrane for trafficking-defective ones. In this study, we tested the ability of niflumic acid (NFA), a commercial nonsteroidal anti-inflammatory drug, to act as a pharmacological chaperone on trafficking-defective MC mutants (A531V, V947E). Wild-type (WT) or MC mutant ClC-1 channels were expressed in HEK293 cells and whole-cell chloride currents were recorded with the patch-clamp technique before and after NFA incubation. Membrane biotinylation assays and western blot were performed to support electrophysiological results. A531V and V947E mutations caused a decrease in chloride current density due to a reduction of ClC-1 total protein level and channel expression on the plasma membrane. The treatment of A531V and V947E-transfected cells with 50 µM NFA restored chloride currents, reaching levels similar to those of WT. Furthermore, no significant difference was observed in voltage dependence, suggesting that NFA increased protein membrane expression without altering the function of ClC-1. Indeed, biochemical experiments confirmed that V947E total protein expression and its plasma membrane distribution were recovered after NFA incubation, reaching protein levels similar to WT. Thus, the use of NFA as a pharmacological chaperone in trafficking defective ClC-1 channel mutations could represent a good strategy in the treatment of MC. Because of the favorable safety profile of this drug, our study may easily open the way for confirmatory human pilot studies aimed at verifying the antimyotonic activity of NFA in selected patients carrying specific ClC-1 channel mutations.

Regulation of ClC-2 Chloride Channel Proteostasis by Molecular Chaperones: Correction of Leukodystrophy-Associated Defect

The ClC-2 channel plays a critical role in maintaining ion homeostasis in the brain and the testis. Loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the white matter disease leukodystrophy. Clcn2-deficient mice display neuronal myelin vacuolation and testicular degeneration. Leukodystrophy-causing ClC-2 mutant channels are associated with anomalous proteostasis manifesting enhanced endoplasmic reticulum (ER)-associated degradation. The molecular nature of the ER quality control system for ClC-2 protein remains elusive. In mouse testicular tissues and Leydig cells, we demonstrated that endogenous ClC-2 co-existed in the same protein complex with the molecular chaperones heat shock protein 90β (Hsp90β) and heat shock cognate protein (Hsc70), as well as the associated co-chaperones Hsp70/Hsp90 organizing protein (HOP), activator of Hsp90 ATPase homolog 1 (Aha1), and FK506-binding protein 8 (FKBP8). Further biochemical analyses revealed that the Hsp90β-Hsc70 chaperone/co-chaperone system promoted mouse and human ClC-2 protein biogenesis. FKBP8 additionally facilitated membrane trafficking of ClC-2 channels. Interestingly, treatment with the Hsp90-targeting small molecule 17-allylamino-17-demethoxygeldanamycin (17-AAG) substantially boosted ClC-2 protein expression. Also, 17-AAG effectively increased both total and cell surface protein levels of leukodystrophy-causing loss-of-function ClC-2 mutant channels. Our findings highlight the therapeutic potential of 17-AAG in correcting anomalous ClC-2 proteostasis associated with leukodystrophy.

Changes in Expression and Cellular Localization of Rat Skeletal Muscle ClC-1 Chloride Channel in Relation to Age, Myofiber Phenotype and PKC Modulation

The ClC-1 chloride channel 1 is important for muscle function as it stabilizes resting membrane potential and helps to repolarize the membrane after action potentials. We investigated the contribution of ClC-1 to adaptation of skeletal muscles to needs induced by the different stages of life. We analyzed the ClC-1 gene and protein expression as well as mRNA levels of protein kinase C (PKC) alpha and theta involved in ClC-1 modulation, in soleus (SOL) and extensor digitorum longus (EDL) muscles of rats in all stage of life. The cellular localization of ClC-1 in relation to age was also investigated. Our data show that during muscle development ClC-1 expression differs according to phenotype. In fast-twitch EDL muscles ClC-1 expression increased 10-fold starting at 7 days up to 8 months of life. Conversely, in slow-twitch SOL muscles ClC-1 expression remained constant until 33 days of life and subsequently increased fivefold to reach the adult value. Aging induced a downregulation of gene and protein ClC-1 expression in both muscle types analyzed. The mRNA of PKC-theta revealed the same trend as ClC-1 except in old age, whereas the mRNA of PKC-alpha increased only after 2 months of age. Also, we found that the ClC-1 is localized in both membrane and cytoplasm, in fibers of 12-day-old rats, becoming perfectly localized on the membrane in 2-month-old rats. This study could represent a point of comparison helpful for the identification of accurate pharmacological strategies for all the pathological situations in which ClC-1 protein is altered.

Skeletal muscle ClC-1 chloride channels in health and diseases

In 1970, the study of the pathomechanisms underlying myotonia in muscle fibers isolated from myotonic goats highlighted the importance of chloride conductance for skeletal muscle function; 20 years later, the human ClC-1 chloride channel has been cloned; last year, the crystal structure of human protein has been solved. Over the years, the efforts of many researchers led to significant advances in acknowledging the role of ClC-1 in skeletal muscle physiology and the mechanisms through which ClC-1 dysfunctions lead to impaired muscle function. The wide spectrum of pathophysiological conditions associated with modification of ClC-1 activity, either as the primary cause, such as in myotonia congenita, or as a secondary adaptive mechanism in other neuromuscular diseases, supports the idea that ClC-1 is relevant to preserve not only for skeletal muscle excitability, but also for skeletal muscle adaptation to physiological or harmful events. Improving this understanding could open promising avenues toward the development of selective and safe drugs targeting ClC-1, with the aim to restore normal muscle function. This review summarizes the most relevant research on ClC-1 channel physiology, associated diseases, and pharmacology.

Defective Gating and Proteostasis of Human ClC-1 Chloride Channel: Molecular Pathophysiology of Myotonia Congenita

The voltage-dependent ClC-1 chloride channel, whose open probability increases with membrane potential depolarization, belongs to the superfamily of CLC channels/transporters. ClC-1 is almost exclusively expressed in skeletal muscles and is essential for stabilizing the excitability of muscle membranes. Elucidation of the molecular structures of human ClC-1 and several CLC homologs provides important insight to the gating and ion permeation mechanisms of this chloride channel. Mutations in the human CLCN1 gene, which encodes the ClC-1 channel, are associated with a hereditary skeletal muscle disease, myotonia congenita. Most disease-causing CLCN1 mutations lead to loss-of-function phenotypes in the ClC-1 channel and thus increase membrane excitability in skeletal muscles, consequently manifesting as delayed relaxations following voluntary muscle contractions in myotonic subjects. The inheritance pattern of myotonia congenita can be autosomal dominant (Thomsen type) or recessive (Becker type). To date over 200 myotonia-associated ClC-1 mutations have been identified, which are scattered throughout the entire protein sequence. The dominant inheritance pattern of some myotonia mutations may be explained by a dominant-negative effect on ClC-1 channel gating. For many other myotonia mutations, however, no clear relationship can be established between the inheritance pattern and the location of the mutation in the ClC-1 protein. Emerging evidence indicates that the effects of some mutations may entail impaired ClC-1 protein homeostasis (proteostasis). Proteostasis of membrane proteins comprises of biogenesis at the endoplasmic reticulum (ER), trafficking to the surface membrane, and protein turn-over at the plasma membrane. Maintenance of proteostasis requires the coordination of a wide variety of different molecular chaperones and protein quality control factors. A number of regulatory molecules have recently been shown to contribute to post-translational modifications of ClC-1 and play critical roles in the ER quality control, membrane trafficking, and peripheral quality control of this chloride channel. Further illumination of the mechanisms of ClC-1 proteostasis network will enhance our understanding of the molecular pathophysiology of myotonia congenita, and may also bring to light novel therapeutic targets for skeletal muscle dysfunction caused by myotonia and other pathological conditions.

A 3-mRNA-based prognostic signature of survival in oral squamous cell carcinoma

Background

Oral squamous cell carcinoma (OSCC) is the most common type of head and neck squamous cell carcinoma with an unsatisfactory prognosis. The aim of this study was to identify potential prognostic mRNA biomarkers of OSCC based on analysis of The Cancer Genome Atlas (TCGA).

Methods

Expression profiles and clinical data of OSCC patients were collected from TCGA database. Univariate Cox analysis and the least absolute shrinkage and selection operator Cox (LASSO Cox) regression were used to primarily screen prognostic biomarkers. Then multivariate Cox analysis was performed to build a prognostic model based on the selected prognostic mRNAs. Nomograms were generated to predict the individual’s overall survival at 3 and 5 years. The model performance was assessed by the time-dependent receiver operating characteristic (ROC) curve and calibration plot in both training cohort and validation cohort (GSE41613 from NCBI GEO databases). In addition, machine learning was used to assess the importance of risk factors of OSCC. Finally, in order to explore the potential mechanisms of OSCC, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was completed.

Results

Three mRNAs (CLEC3B, C6 and CLCN1) were finally identified as a prognostic biomarker pattern. The risk score was imputed as: (−0.38602 × expression level of CLEC3B) + (−0.20632 × expression level of CLCN1) + (0.31541 × expression level of C6). In the TCGA training cohort, the area under the curve (AUC) was 0.705 and 0.711 for 3- and 5-year survival, respectively. In the validation cohort, AUC was 0.718 and 0.717 for 3- and 5-year survival. A satisfactory agreement between predictive values and observation values was demonstrated by the calibration curve in the probabilities of 3- and 5- year survival in both cohorts. Furthermore, machine learning identified the 3-mRNA signature as the most important risk factor to survival of OSCC. Neuroactive ligand-receptor interaction was most enriched mostly in KEGG pathway analysis.

Conclusion

A 3-mRNA signature (CLEC3B, C6 and CLCN1) successfully predicted the survival of OSCC patients in both training and test cohort. In addition, this signature was an independent and the most important risk factor of OSCC.