A novel nonsense mutation in the MIP gene linked to congenital posterior polar cataracts in a Chinese family

Lens Aquaporin-5 Inserts Into Bovine Fiber Cell Plasma Membranes Via Unconventional Protein Secretion

Purpose

To spatially map aquaporin-5 (AQP5) expression in the bovine lens, molecularly characterize cytoplasmic AQP5-containing vesicles in the outer cortex, and elucidate AQP5 membrane trafficking mechanisms.

Methods

Immunofluorescence was performed on bovine lens cryosections using AQP5, TOMM20, COX IV, calnexin, LC3B, Sec22β, LIMP-2, and connexin 50 antibodies and the membrane dye CM-DiI. AQP5 plasma membrane insertion was defined via line expression profile analysis. Transmission electron microscopy (TEM) was performed on bovine lens sections to examine cytoplasmic organelle morphology and subcellular localization in cortical fiber cells. Bovine lenses were treated with 10-nM bafilomycin A1 or 0.1% dimethyl sulfoxide vehicle control for 24 hours in ex vivo culture to determine changes in AQP5 plasma membrane expression.

Results

Immunofluorescence analysis revealed cytoplasmic AQP5 expression in lens epithelial cells and differentiating fiber cells. In the lens cortex, complete AQP5 plasma membrane insertion occurs at r/a = 0.951 ± 0.005. AQP5-containing cytoplasmic vesicles are spheroidal in morphology with linear extensions, express TOMM20, and contain LC3B and LIMP-2, but not Sec22β, as fiber cells mature. TEM analysis revealed complex vesicular assemblies with congruent subcellular localization to AQP5-containing cytoplasmic vesicles. AQP5-containing cytoplasmic vesicles appear to dock with the plasma membrane. Bafilomycin A1 treatment reduced AQP5 plasma membrane expression by 27%.

Conclusions

AQP5 localizes to spheroidal, linear cytoplasmic vesicles in the differentiating bovine lens fiber cells. During fiber cell differentiation, these vesicles incorporate LC3B and presumably fuse with LIMP-2–positive lysosomes. Our data suggest that AQP5 to the plasma membrane through lysosome-associated unconventional protein secretion, a novel mechanism of AQP5 trafficking.

A novel missense mutation in the gene encoding major intrinsic protein (MIP) in a Giant panda with unilateral cataract formation

Background

Cataracts are defects of the lens that cause progressive visual impairment and ultimately blindness in many vertebrate species. Most cataracts are age-related, but up to one third have an underlying genetic cause. Cataracts are common in captive zoo animals, but it is often unclear whether these are congenital or acquired (age-related) lesions.

Results

Here we used a functional candidate gene screening approach to identify mutations associated with cataracts in a captive giant panda (Ailuropoda melanoleuca). We screened 11 genes often associated with human cataracts and identified a novel missense mutation (c.686G > A) in the MIP gene encoding major intrinsic protein. This is expressed in the lens and normally accumulates in the plasma membrane of lens fiber cells, where it plays an important role in fluid transport and cell adhesion. The mutation causes the replacement of serine with asparagine (p.S229N) in the C-terminal tail of the protein, and modeling predicts that the mutation induces conformational changes that may interfere with lens permeability and cell–cell interactions.

Conclusion

The c.686G > A mutation was found in a captive giant panda with a unilateral cataract but not in 18 controls from diverse regions in China, suggesting it is most likely a genuine disease-associated mutation rather than a single-nucleotide polymorphism. The mutation could therefore serve as a new genetic marker to predict the risk of congenital cataracts in captive giant pandas.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07386-8.

The relationship between major intrinsic protein genes and cataract

BACKGROUND

Genetic factors play an essential role in the development of cataracts, and the major intrinsic protein (MIP) gene is a type of causative genes. Our study aims to discuss the current research progress of MIP genes responsible for cataractogenesis in DNA and protein levels, which is essential in achieving a response to the molecular deficiencies and pathophysiologic features of cataract.

METHODS

We developed a search strategy using a combination of the words "Cataract", "Mutation", "MIP gene", and "AQP0" to identify all articles from PubMed, Web of Science, Scopus, and Google Scholar up to December 2019. To find more articles and to ensure that databases were thoroughly searched, the reference lists of selected items were also reviewed.

RESULTS

A total of 29 MIP gene mutations causing congenital cataract were obtained by searching these databases and analyzing the results of genetic mutation pathogenicity prediction software tools; most of them caused amino acid codon changes in the H4, H5, H6, C-TIDs, and loop C in the structure of the MIP protein. However, there was no clear causality between lens morphology, phenotypes, and genotypes. The genotype TC in polymorphism c.-4T > C and haplotype CCG of rs2269348, c.-4T > C, and rs74641138 in MIP may attach an additional genetic risk factor for age-related cataract.

CONCLUSION

These single-base mutations and single nucleotide polymorphisms might be importantly involved in the pathogenesis of congenital cataract and age-related cataract, respectively. This review provides a significant reference for clinical trials and theoretical studies.

Genetic modifiers of rodent animal models: the role in cataractogenesis

Visual impairment leads to a decrease in quality of life. Cataract is the most commonly observed ocular disease in humans that causes vision disorders. The risk factors associated with cataract development include aging, infections, eye injuries, environmental causes, such as radiation and exposure to ultraviolet rays in sunlight, and genetic mutations. Additionally, several cataract patients display phenotypic heterogeneity, suggesting the role of genetic modifiers in the modulation of severity and onset time of cataractogenesis. However, the genetic modifiers associated with cataract have not been identified in humans yet. In contrast, the identification and mapping of genetic modifiers have been successfully carried out in mice and rats. In this review, we focus on the genetic modifiers of cataract in the rodent models.

Identification of a novel MIP frameshift mutation associated with congenital cataract in a Chinese family by whole-exome sequencing and functional analysis

Purpose

To detect the underlying pathogenesis of congenital cataract in a four-generation Chinese family.

Methods

Whole-exome sequencing (WES) of family members (III:4, IV:4, and IV:6) was performed. Sanger sequencing and bioinformatics analysis were subsequently conducted. Full-length WT-MIP or K228fs-MIP fused to HA markers at the N-terminal was transfected into HeLa cells. Next, quantitative real-time PCR, western blotting and immunofluorescence confocal laser scanning were performed.

Results

The age of onset for nonsyndromic cataracts in male patients was by 1-year old, earlier than for female patients, who exhibited onset at adulthood. A novel c.682_683delAA (p.K228fs230X) mutation in main intrinsic protein (MIP) cosegregated with the cataract phenotype. The instability index and unfolded states for truncated MIP were predicted to increase by bioinformatics analysis. The mRNA transcription level of K228fs-MIP was reduced compared with that of WT-MIP, and K228fs-MIP protein expression was also lower than that of WT-MIP. Immunofluorescence images showed that WT-MIP principally localized to the plasma membrane, whereas the mutant protein was trapped in the cytoplasm.

Conclusions

Our study generated genetic and primary functional evidence for a novel c.682_683delAA mutation in MIP that expands the variant spectrum of MIP and help us better understand the molecular basis of cataract.

The Role of Aquaporins in Ocular Lens Homeostasis

Abstract: Aquaporins (AQPs), by playing essential roles in the maintenance of ocular lens homeostasis, contribute to the establishment and maintenance of the overall optical properties of the lens over many decades of life. Three aquaporins, AQP0, AQP1 and AQP5, each with distinctly different functional properties, are abundantly and differentially expressed in the different regions of the ocular lens. Furthermore, the diversity of AQP functionality is increased in the absence of protein turnover by age-related modifications to lens AQPs that are proposed to alter AQP function in the different regions of the lens. These regional differences in AQP functionality are proposed to contribute to the generation and directionality of the lens internal microcirculation; a system of circulating ionic and fluid fluxes that delivers nutrients to and removes wastes from the lens faster than could be achieved by passive diffusion alone. In this review, we present how regional differences in lens AQP isoforms potentially contribute to this microcirculation system by highlighting current areas of investigation and emphasizing areas where future work is required.

Aquaporin 0 Modulates Lens Gap Junctions in the Presence of Lens-Specific Beaded Filament Proteins

Purpose

The objective of this study was to understand the molecular and physiologic mechanisms behind the lens cataract differences in Aquaporin 0-knockout-Heterozygous (AQP0-Htz) mice developed in C57 and FVB (lacks beaded filaments [BFs]) strains.

Methods

Lens transparency was studied using dark field light microscopy. Water permeability (P_f) was measured in fiber cell membrane vesicles. Western blotting/immunostaining was performed to verify expression of BF proteins and connexins. Microelectrode-based intact lens intracellular impedance was measured to determine gap junction (GJ) coupling resistance. Lens intracellular hydrostatic pressure (HP) was determined using a microelectrode/manometer system.

Results

Lens opacity and spherical aberration were more distinct in AQP0-Htz lenses from FVB than C57 strains. In either background, compared to wild type (WT), AQP0-Htz lenses showed decreased P_f (approximately 50%), which was restored by transgenic expression of AQP1 (TgAQP1/AQP0-Htz), but the opacities and differences between FVB and C57 persisted. Western blotting revealed no change in connexin expression levels. However, in C57 AQP0-Htz and TgAQP1/AQP0-Htz lenses, GJ coupling resistance decreased approximately 2.8-fold and the HP gradient decreased approximately 1.9-fold. Increased P_f in TgAQP1/AQP0-Htz did not alter GJ coupling resistance or HP.

Conclusions

In C57 AQP0-Htz lenses, GJ coupling resistance decreased. HP reduction was smaller than the coupling resistance reduction, a reflection of an increase in fluid circulation, which is one reason for the less severe cataract in C57 than FVB. Overall, our results suggest that AQP0 modulates GJs in the presence of BF proteins to maintain lens transparency and homeostasis.

Acetazolamide Suppresses Multi-Drug Resistance-Related Protein 1 and P-Glycoprotein Expression by Inhibiting Aquaporins Expression in a Mesial Temporal Epilepsy Rat Model

Background

Mesial temporal epilepsy (MTLE) is the most common type of focal epilepsy in adults, and is often drug-resistant. This study investigated the effects of aquaporins (AQP) inhibitor on multi-drug-resistant protein expression in an MTLE rat model.

Material/Methods

The MTLE rat model was established by injecting pilocarpine into rats. The MTLE rats were divided into an MTLE-6 h group, an MTLE-12 h group, and an MTLE-24 h group, together with a normal saline group (NS), to examine the AQP4 expression by using Western blot assay and immunohistochemistry assay. The other 18 MTLE model rats were used to observe the effects of the AQP4 inhibitor, acetazolamide, on the multi-drug-resistant protein 1 (MRP1) and P-glycoprotein (Pgp) by using Western blot and immunohistochemistry assays, respectively.

Results

AQP4 expression was enhanced in hippocampal tissues of MTLE model rats compared to NS rats (P<0.05). More positively stained AQP4 was discovered in hippocampal tissues of MTLE model rats. AQP4 inhibitor significantly decreased multi-drug-resistant protein MRP1 and Pgp expression in the AQP4 inhibitor Interfere group and the AQP4 inhibitor Therapy group compared to the TMLE model group (P<0.05).

Conclusions

The present findings confirm that the AQP4 inhibitor, acetazolamide, effectively inhibits the multi-drug-resistant protein, MRP1, and Pgp, in the MTLE rat model.

A novel missense mutation of Mip causes semi-dominant cataracts in the Nat mouse

Major intrinsic protein of lens fiber (MIP) is one of the proteins essential for maintaining lens transparency while also contributing to dominant cataracts in humans. The Nodai cataract (Nat) mice harbor a spontaneous mutation in Mip and develop early-onset nuclear cataracts. The Nat mutation is a c.631G>A mutation (Mip^Nat), resulting in a glycine-to-arginine substitution (p.Gly211Arg) in the sixth transmembrane domain. The Mip^Nat/Nat homozygotes exhibit congenital cataracts caused by the degeneration of lens fiber cells. MIP normally localizes to the lens fiber cell membranes. However, the Mip^Nat/Nat mice were found to lack an organelle-free zone, and the MIP was mislocalized to the nuclear membrane and perinuclear region. Furthermore, the Mip^Nat/+ mice exhibited milder cataracts than Mip^Nat/Nat mice due to the slight degeneration of the lens fiber cells. Although there were no differences in the localization of MIP to the membranes of lens fiber cells in Mip^Nat/+ mice compared to that in wild-type mice, the protein levels of MIP were significantly reduced in the eyes. These findings suggest that cataractogenesis in Mip^Nat mutants are caused by defects in MIP expression. Overall, the Mip^Nat mice offer a novel model to better understand the phenotypes and mechanisms for the development of cataracts in patients that carry missense mutations in MIP.

Inherited Congenital Cataract: A Guide to Suspect the Genetic Etiology in the Cataract Genesis

Cataracts are the principal cause of treatable blindness worldwide. Inherited congenital cataract (CC) shows all types of inheritance patterns in a syndromic and nonsyndromic form. There are more than 100 genes associated with cataract with a predominance of autosomal dominant inheritance. A cataract is defined as an opacity of the lens producing a variation of the refractive index of the lens. This variation derives from modifications in the lens structure resulting in light scattering, frequently a consequence of a significant concentration of high-molecular-weight protein aggregates. The aim of this review is to introduce a guide to identify the gene involved in inherited CC. Due to the manifold clinical and genetic heterogeneity, we discarded the cataract phenotype as a cardinal sign; a 4-group classification with the genes implicated in inherited CC is proposed. We consider that this classification will assist in identifying the probable gene involved in inherited CC.

Identification of a novel missense mutation of MIP in a Chinese family with congenital cataracts by target region capture sequencing

Congenital cataract is both clinically diverse and genetically heterogeneous. To investigate the underlying genetic defect in three-generations of a Chinese family with autosomal dominant congenital cataracts, we recruited family members who underwent comprehensive ophthalmic examinations. A heterozygous missense mutation c.634G > C (p.G212R) substitution was identified in the MIP gene through target region capture sequencing. The prediction results of PolyPhen-2 and SIFT indicated that this mutation was likely to damage the structure and function of MIP. Confocal microscopy images showed that the intensity of the green fluorescent signal revealed much weaker signal from the mutant compared to the wild-type MIP. The expressed G212R-MIP was diminished and almost exclusively cytoplasmic in the HeLa cells; whereas the WT-MIP was stable dispersed throughout the cytoplasm, and it appeared to be in the membrane structure. Western blot analysis indicated that the protein expression level of the mutant form of MIP was remarkably reduced compared with that of the wild type, however, the mRNA levels of the wild-type and mutant cells were comparable. In conclusion, our study presented genetic and functional evidence for a novel MIP mutation of G212R, which leads to congenital progressive cortical punctate with or without Y suture.

MALDI Imaging Mass Spectrometry Spatially Maps Age-Related Deamidation and Truncation of Human Lens Aquaporin-0

PURPOSE

To spatially map human lens Aquaporin-0 (AQP0) protein modifications, including lipidation, truncation, and deamidation, from birth through middle age using matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS).

METHODS

Human lens sections were water-washed to facilitate detection of membrane protein AQP0. We acquired MALDI images from eight human lenses ranging in age from 2 months to 63 years. In situ tryptic digestion was used to generate peptides of AQP0 and peptide images were acquired on a 15T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Peptide extracts were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and database searched to identify peptides observed in MALDI imaging experiments.

RESULTS

Unmodified, truncated, and fatty acid-acylated forms of AQP0 were detected in protein imaging experiments. Full-length AQP0 was fatty acid acylated in the core and cortex of young (2- and 4-month) lenses. Acylated and unmodified AQP0 were C-terminally truncated in older lens cores. Deamidated tryptic peptides (+0.9847 Da) were mass resolved from unmodified peptides by FTICR MS. Peptide images revealed differential localization of un-, singly-, and doubly-deamidated AQP0 C-terminal peptide (239-263). Deamidation was present at 4 months and increases with age. Liquid chromatography-MS/MS results indicated N246 undergoes deamidation more rapidly than N259.

CONCLUSIONS

Results indicated AQP0 fatty acid acylation and deamidation occur during early development. Progressive age-related AQP0 processing, including deamidation and truncation, was mapped in human lenses as a function of age. The localization of these modified AQP0 forms suggests where AQP0 functions may change throughout lens development and aging.

Regulatory effect of miRNA 320a on expression of aquaporin 4 in brain tissue of epileptic rats

OBJECTIVE

To study the expression of miRNA 320a in the brain tissue of epileptic rats and analyze its effect on the expression of aquaporin 4 (AQP4).

METHODS

All rats were performed with the intraperitoneal injection of lithium chloride (3 mmol/kg) and then the intraperitoneal injection of pilocarpine (30 mg/kg) 24 h later (injected twice) to prepare the epileptic model of Wistar rats. Rats in the control group were injected with the equal volume of normal saline. According to the Racine scale, rats with over stage 3 of epilepsy were chosen and the brain tissue was separated quickly and then stored at -80 °C. The immunohistochemistry was used to detect the expression of aquaporin in the brain tissue of epileptic model and the Real-time PCR was employed to determine the difference in the expression of miRNA 320a and AQP4 in the brain tissue of rats between the epileptic model group and control group. Five 5-day neonatal Wistar rats were chosen to collect the cerebral cortex and their primary astrocytes were separated and cultured. They were transfected with miRNA mimic and imitated to the endogenous miRNA 320a to up-regulate the expression of miRNA 320a.

RESULTS

In the model group, the expression of AQP4 was significantly higher than the control group (P < 0.01). However, the expression of miRNA 320a in the model group was lower than control group (P < 0.05), which was negatively correlated to AQP4. In the primary astrocytes, the transfection of miRNA 320a mimic could significantly reduce the expression of AQP4, while its inhibitor could up-regulate the expression of AQP4, which indicated that miRNA 320a could reduce the expression of AQP4.

CONCLUSIONS

In the primary astrocytes of rats, the miRNA 320a could inhibit the expression of AQP4 and after adding the inhibitor of miRNA 320a, the expression of AQP4 was up-regulated.