A cell wall-localized cytokinin/purine riboside nucleosidase is involved in apoplastic cytokinin metabolism in Oryza sativa

In the final step of cytokinin biosynthesis, the main pathway is the elimination of a ribose-phosphate moiety from the cytokinin nucleotide precursor by phosphoribohydrolase, an enzyme encoded by a gene named LONELY GUY (LOG). This reaction accounts for most of the cytokinin supply needed for regulating plant growth and development. In contrast, the LOG-independent pathway, in which dephosphorylation and deribosylation sequentially occur, is also thought to play a role in cytokinin biosynthesis, but the gene entity and physiological contribution have been elusive. In this study, we profiled the phytohormone content of chromosome segment substitution lines of Oryza sativa and searched for genes affecting the endogenous levels of cytokinin ribosides by quantitative trait loci analysis. Our approach identified a gene encoding an enzyme that catalyzes the deribosylation of cytokinin nucleoside precursors and other purine nucleosides. The cytokinin/purine riboside nucleosidase 1 (CPN1) we identified is a cell wall-localized protein. Loss-of-function mutations (cpn1) were created by inserting a Tos17-retrotransposon that altered the cytokinin composition in seedling shoots and leaf apoplastic fluid. The cpn1 mutation also abolished cytokinin riboside nucleosidase activity in leaf extracts and attenuated the trans-zeatin riboside-responsive expression of cytokinin marker genes. Grain yield of the mutants declined due to altered panicle morphology under field-grown conditions. These results suggest that the cell wall-localized LOG-independent cytokinin activating pathway catalyzed by CPN1 plays a role in cytokinin control of rice growth. Our finding broadens our spatial perspective of the cytokinin metabolic system.

Syndromic cardiac disorder is associated with a non-coding deletion that induces a 3D chromatin remodeling and PITX2 expression dysregulation

 

In a first family (family#1), we identified 53 members of whom 17 present a syndromic cardiac disorder characterized by electrical disorders (sinus node dysfunction, atrial fibrillation...) and developmental defects (atrial septal defect, valvopathy, left ventricle non-compaction...) following an autosomal dominant model. Among the affected family members, 6 are implanted with a pacemaker and one experienced a sudden death at 43yo. Despite a strong linkage pointing to the 4q25 region, exome sequencing failed to identify causal variant. Interestingly, 6 additional non-related families presenting the same phenotype have been also identified.

Our aims are to identity the causal mutation and the molecular mechanism underlying this complex cardiac syndrome.

Genetic study has been performed using whole genome sequencing (WGS). Based on transgenic mouse strains, we assessed the impact of Family#1 mutation on the phenotype and on gene expression. Then, we generated human cardiomyocytes derived iPS cells (CM-iPS) isogenic models to evaluate the epigenome (CUT&RUN and ATAC-seq), transcriptome (RNA-seq) and topological associated domain (TAD) remodelling (Hi-C).

By WGS we uncovered a deletion of 15kb in a gene desert area on 4q25, segregating in all affected relatives of Family#1. The 6 other families present overlapping deletions. Mouse model recapitulates the cardiac phenotype and exhibit a dysregulation of Pitx2 expression in cardiac specific compartments. Based on human CM-iPS models, epigenetic data highlight among the 15kb deletion a unique open region containing a CTCF binding site, crucial for delimiting TAD boundaries. Hi-C assay reveals the fusion of 2 TADs and highlights new interactions between PITX2 and atrial specific regulatory elements.

We identified a deletion located within a gene desert area associated with a complex cardiac disorder. The CTCF binding site contained in the deletion seems key in the TAD border. The TAD remodelling leads to new (regulatory) interactions and expression dysregulation of PITX2. We describe a new molecular mechanism implying a yet unidentified non-coding regulatory element of PITX2 and responsible for a complex electrical and developmental cardiac syndrome.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Nantes UniversitéFrench national reserch agency (ANR)

Massive calcification around large joints in a patient subsequently diagnosed with adult-onset hypophosphatasia

We report a 64-year-old Japanese woman with a history of progressive loss of motor function and painful swelling of large joints. At the age of 54, profound calcification appeared around the shoulder and hip joints, which did not heal after repeated surgical resections. Iliac bone biopsy revealed osteomalacic changes. Laboratory data showed low serum alkaline phosphatase (ALP) activity and a high urine phosphoethanolamine (PEA) concentration with normal serum calcium, phosphate, and fibroblast growth factor 23 (FGF23) levels. Subsequent genetic analysis of the ALPL gene confirmed the diagnosis of hypophosphatasia (HPP) with the identification of a heterozygous single nucleotide deletion, c.1559delT (p.Leu520ArgfsX86). We started a mineral-targeted enzyme replacement therapy, asfotase alfa (AA), to treat the patient's musculoskeletal symptoms. A follow-up bone biopsy after 12 months of AA treatment showed improvement of osteomalacia. Calcified deposits around the large joints were unchanged radiographically. To our knowledge, this is the first report of a patient with an adult-onset HPP who presented with profound calcification around multiple joints. Nonspecific clinical signs and symptoms in patients with adult-onset HPP often result in delayed diagnosis or misdiagnosis. We propose that bone biopsy and genetic analysis should be considered along with laboratory analysis for all patients with ectopic calcification around joints of unknown etiology for accurate diagnosis and better treatment.

Novel variant of the glycerol-3-phosphate dehydrogenase-1 Like (GPD1-L) gene in Japanese Brugada syndrome patients

Background

The incidence of Brugada syndrome (BrS) varies among racial groups. Several studies reported Glycerol-3-Phosphate Dehydrogenase 1-Like (GPD1-L) gene is associated with BrS. However, most of these studies were reported from Western countries, so the evidence about GPD1-L mutation is limited especially among Asian BrS patients. This study aimed to search for rare variants in GPD1-L among Japanese BrS patients and to investigate the pathogenicity.

Method

We performed whole-exome sequencing for patients with Brugada type 1 ECG pattern from Japanese multicenter BrS cohort consisting of SCN5A-negative BrS probands (n=288) and controls (n=372). We conducted patch-clamp study in human embryonic kidney (HEK) 293 cells cotransfected with the wild-type sodium channel (SCN5A) and wild-type or mutant GPD1-L expression plasmid.

Results

We identified a rare variant in GPD1-L, p.D262N (c.784g>a) in 2 of 288 BrS probands, which was not identified in 372 controls. The minor allele frequency of the variant is 0.0014% in the Genome Aggregation Database. One proband was a 49-year-old man and the other was 34-year-old man who both developed a ventricular fibrillation. ECGs of both probands showed Brugada Type 1 pattern after administration of the pilsicainide. In functional study, coexpression of D262N GPD1-L with SCN5A in HEK293 cells significantly reduced inward sodium currents compared with wild-type GPD1-L. Additionally, inward sodium currents with D262N were similar to those with A280V GPD1-L, which was associated with BrS in previous reports (Figure). Also, several pathogenicity prediction programs, such as SIFT (score: 0.031) and PolyPhen2 (score: 0.937) predicted deleterious effects of GPD1-L D262N.

Conclusion

We identified a rare variant in GPD1-L at the rate of 0.7% in Japanese BrS patients without SCN5A mutations. GPD1-L, p.D262N reduces inward sodium currents and may be a novel susceptible variant for BrS in the Japanese population.

Funding Acknowledgement

Type of funding sources: None. Figure 1. Current–voltage curve

Primary bone adult T cell lymphoma with multiple skeletal lesions and debilitating painful osteolysis: a case report

There have been only a limited number of reports on primary adult T cell lymphoma/leukemia (ATL) in the bone. This is a case report of a 75-year-old patient initially reporting multiple bone pains that were attributed to osteolytic ATL. The patient developed spontaneous chest/back pain and visited a local hospital. Laboratory tests showed high levels of alkaline phosphatase (ALP), and computed tomography (CT) revealed skeletal lesions with osteolysis. Although multiple myeloma was initially suspected, the results of bone marrow aspiration and bone biopsy were inconsistent. After he was referred to our hospital, mild hypercalcemia (10.4 mg/dL) with low-normal intact parathyroid hormone (PTH) (27 pg/mL), low parathyroid hormone-related protein (PTHrP), and elevated 1,25-dihydroxy vitamin D (1,25OH₂D) levels (136 pg/mL) narrowed the differential diagnosis down to lymphomatous and granulomatous diseases, and then, the high serum soluble IL-2 receptor (3,450 U/mL) and the flower cells recognized in the peripheral blood sample suggested the involvement of ATL. Finally, the reevaluation of the iliac bone biopsy sample led us to the histological diagnosis of ATL infiltration in the bone. The subsequent two courses of chemotherapy in addition to denosumab resulted in an objective partial metabolic response indicated in 18-fluorine-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT). Although very rare, the bone involvement of ATL could be used for the differential diagnosis for local osteolytic bone pain in addition to multiple myeloma and metastatic bone diseases.

Nitrogen Nutrition Promotes Rhizome Bud Outgrowth via Regulation of Cytokinin Biosynthesis Genes and an Oryza longistaminata Ortholog of FINE CULM 1

Oryza longistaminata, a wild rice, can propagate vegetatively via rhizome formation and, thereby, expand its territory through horizontal growth of branched rhizomes. The structural features of rhizomes are similar to those of aerial stems; however, the physiological roles of the two organs are different. Nitrogen nutrition is presumed to be linked to the vegetative propagation activity of rhizomes, but the regulation of rhizome growth in response to nitrogen nutrition and the underlying biological processes have not been well characterized. In this study, we analyzed rhizome axillary bud growth in response to nitrogen nutrition and examined the involvement of cytokinin-mediated regulation in the promotion of bud outgrowth in O. longistaminata. Our results showed that nitrogen nutrition sufficiency promoted rhizome bud outgrowth to form secondary rhizomes. In early stages of the response to nitrogen application, glutamine accumulated rapidly, two cytokinin biosynthesis genes, isopentenyltransferase, and CYP735A, were up-regulated with accompanying cytokinin accumulation, and expression of an ortholog of FINE CULM1, a negative regulator of axillary bud outgrowth, was severely repressed in rhizomes. These results suggest that, despite differences in physiological roles of these organs, the nitrogen-dependent outgrowth of rhizome axillary buds in O. longistaminata is regulated by a mechanism similar to that of shoot axillary buds in O. sativa. Our findings provide a clue for understanding how branched rhizome growth is regulated to enhance nutrient acquisition strategies.

Preclinical proof-of-concept study: antisense-mediated knockdown of CALM as a therapeutic strategy for calmodulinopathy

Background

Calmodulin (CaM) is a ubiquitous Ca2+ sensor molecule encoded by three distinct calmodulin genes, CALM1–3, and has an important role for cardiac ion channel function. Recently, heterozygous missense mutations in CALM genes were reported to cause a new category of life-threatening genetic arrhythmias such as long-QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT), which is called as “calmodulinopathy”. The patients with calmodulinopathy show poor prognosis and there is no effective treatment for them.

Purpose

Considering the dominant-negative effect of mutant calmodulin proteins produced by heterozygous missense mutations in CALMs, we aimed to prove the concept of antisense-based therapy to treat calmodulinopathy using human iPS cell-derived cardiomyocyte (hiPSC-CM) model.

Methods

We designed multiple locked nucleic acid (LNA) gapmer-antisense oligonucleotides (ASOs) targeting CALM2 and analyzed the silencing efficiency and toxicity in cultured cells to select the most potent ASO. Using CMs differentiated from hiPSCs which were generated form a 12-year-old boy with LQTS carrying a heterozygous CALM2-N98S mutation, CALM2 expression and action potentials (APs) were analyzed to evaluate the efficacy of ASOs.

Results

We identified several ASOs which reduced CALM2 expression without affecting cell viability in human cultured cells (HepG2) (ASO 50 nM, n=2; Figure 1A). Considering further experiments in vivo mouse model, we investigated the CALM2 silencing activity in mouse cultured cells (3T3-L1) without transfection (free-uptake) (ASO 1 μM, n=2; †ASOs have homologous sequence between human and mouse; Figure B). After free-uptake CALM2 silencing analysis in 3T3-L1 cells, we identified that ASO #2 has the most potent CALM2 silencing activity and low cytotoxicity (Figure 1B). ASO #2 effectively reduced CALM2 expression even in hiPSC-CMs (ASO(−): n=3, lipofection: n=4, free-uptake: n=3; P<0.05; Figure 1C). In action potential recordings, we demonstrated that ASO #2 ameliorated prolonged AP durations (APD90) in N98S-hiPSC-CMs at 0.5 Hz pacing (ASO(−): 666±123 ms (n=7), lipofection: 329±21 ms (n=8), free-uptake: 388±34 ms (n=12); P<0.05; Figure 1D).

Conclusion

Our results using patient-derived hiPSC-CM model suggest that ASO-based therapy might be a promising strategy for the treatment of calmodulinopathy.

Figure 1

Funding Acknowledgement

Type of funding source: Private company. Main funding source(s): Nissan Chemical Corporation

Molecular mechanism of a new cardiac syndrome associated with a regulatory element deletion of chromosome 4q25

Introduction

We identified a large family of 53 members of whom 22 present a new cardiac syndrome characterized by electrical disorders and developmental defects following an autosomal dominant model. Among the affected family members 6 are implanted with a pacemaker, 2 experienced syncope and one a sudden death at 43yo. Linkage analysis points, with high confidence, to the chromosome 4q25 region. This region is associated with the Ankyrin syndrome (mutation in ANK2) sharing partly the electrical defects observed in the affected family members. No mutation was found in the coding region of the 4q25 region as well as in the coding and non-coding part of the ANK2.

Objective

Our aims are first to identity the responsible mutation present in this family and understand the molecular mechanisms leading to this new syndrome.

Method

Whole genome sequencing (WGS) has been employed to identify genetic variants responsible for this syndrome. ChIP-seq and ATAC-seq were used for functional annotation and genome editing (CRISPR-Cas9) to generate iPS cellular models.

Results

By WGS we uncovered a deleted region of 15kb in a gene desert area on 4q25, segregating in all affected relatives. Five other families (3 French and 2 Japanese) presenting the same phenotype show overlapping deletions. We generated human cardiac epigenetic data and identified among the 15kb deleted region a unique active enhancer region within the presence of a transcript factor CTCF binding site. Isogenic cell lines where the 15kb and the CTCF binding have been deleted are under investigation.

Conclusion

We identified a new cardiac syndrome and for the first time a mutation located within a gene desert area leading to severe and complex cardiac disorders. We demonstrated the presence of a likely gene regulatory element. Experiments are ongoing to characterize the molecular mechanisms and consequence of the deletion on gene expression.

Funding Acknowledgement

Type of funding source: Public Institution(s). Main funding source(s): Pays de la loire - Etoiles montantes

Systematic Evaluation of KCNQ1 variant using ACMG/AMP Guidelines and Risk Stratification in Long QT Syndrome Type 1

Background

Mutation/variant-site specific risk stratification in long-QT syndrome type 1 (LQT1) has been well investigated, but it is still challenging to adopt current enormous genomic information to clinical aspects caused by each mutation/variant. We assessed a novel variant-specific risk stratification in LQT1 patients.

Methods

We classified a pathogenicity of 142 KCNQ1 variants among 927 LQT1 patients (536 probands and 391 family members) based on the American College of Medical Genetics and Genomics (ACMG) and Association for Molecular Pathology (AMP) guidelines and evaluated whether the ACMG/AMP-based classification was associated with arrhythmic risk in LQT1 patients.

Results

Among 142 KCNQ1 variants, 60 (42.3%), 58 (40.8%), and 24 (16.9%) variants were classified into pathogenic (P), likely pathogenic (LP), and variant of unknown significance (VUS), respectively. The ACMG/AMP guideline-based classification was significantly associated with syncopal events (particularly those during exercise) and LQT risk score (Schwartz score) in overall population. On the other hand, arrhythmic risk was completely different between probands and families even in the same variants. The baseline QTc interval and variant location could stratify the risk in family members but not in probands, however, the ACMG/AMP-based KCNQ1 variant classification stratified the risk in LQT1 probands as well as family members. Multivariate analysis showed that proband (HR=2.52; 95% CI: 1.93–3.30; p<0.0001), longer QTc interval (≥500ms) (HR=1.41; 95% CI: 1.11–1.79; p<0.0001), variants at membrane spanning (MS) (vs. those at N/C terminus) (HR=1.40; 95% CI: 1.07–1.85; p=0.02), C-loop (vs. N/C terminus) (HR=1.58; 95% CI: 1.11–2.24; p=0.01), and P variants [(vs. LP) (HR=1.71; 95% CI: 1.33–2.23; p<0.0001), (vs. VUS) (HR=1.96; 95% CI: 1.19–3.46; p=0.007)] were significantly associated with syncopal events. A clinical score (0–4) based on the proband, QTc (≥500ms), variant location (MS or C-loop) and P variant by the ACMG/AMP guidelines allowed identification of patients more likely to have arrhythmic events (Figure A and B).

Conclusion

Comprehensive evaluation of clinical findings and pathogenicity of KCNQ1 variants based on the ACMG/AMP-based evaluation may stratify arrhythmic risk of congenital long-QT syndrome type 1.

Figure 1

Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): Health Science Research Grant from the Ministry of Health,Labor and Welfare of Japan for Clinical Research on Measures for Intractable Diseases (H24-033, H26-040, H27-032) and a research grant from the Japan Agency for Medical Research and Development (AMED) (15km0305015h0101, 16ek0210073h0001)

Flowering time control in rice by introducing Arabidopsis clock-associated PSEUDO-RESPONSE REGULATOR 5

Plants flower under appropriate day-length conditions by integrating temporal information provided by the circadian clock with light and dark information from the environment. A sub-group of plant specific circadian clock-associated PSEUDO-RESPONSE REGULATOR (PRR) genes (PRR7/PRR3 sub-group) controls flowering time both in long-day and short-day plants; however, flowering control by the other two PRR gene sub-groups has been reported only in Arabidopsis thaliana (Arabidopsis), a model long-day plant. Here, we show that an Arabidopsis PRR9/PRR5 sub-group gene can control flowering time (heading date) in rice, a short-day plant. Although PRR5 promotes flowering in Arabidopsis, transgenic rice overexpressing Arabidopsis PRR5 caused late flowering. Such transgenic rice plants produced significantly higher biomass, but not grain yield, due to the late flowering. Concomitantly, expression of Hd3a, a rice florigen gene, was reduced in the transgenic rice.Abbreviations: CCT: CONSTANS, CONSTANS-LIKE, and TOC1; HD: HEADING DATE; LHY: LATE ELONGATED HYPOCOTYL; Ppd: photoperiod; PR: pseudo-receiver; PRR: PSEUDO-RESPONSE REGULATOR; TOC1: TIMING OF CAB EXPRESSION 1; ZTL: ZEITLUPE.

299 The prevalence and role of SCN10A variants in Han Chinese patients with Brugada syndrome: the SADS-TW BrS registry

On Behalf

SADS-TW BrS registry

Background

Brugada syndrome (BrS) is an inheritable arrhythmic disease responsible for sudden cardiac death. Information on the prevalence and role of SCN10A variants in BrS is limited and equivocal.

Purpose

We aimed to investigate the prevalence and role of SCN10A variants in BrS in Han Chinese.

Methods

From 2000 to 2017, we prospectively and consecutively enrolled 176 unrelated BrS patients from the Han Chinese population in Taiwan (the SADS-TW BrS registry). Thirty-four BrS-related genes were screened by next-generation sequencing, using Taiwan Biobank as the population reference. The pathogenicity was evaluated by literature review and in silico analyses, including the SKAT-O algorithm.

Results

The SKAT-O algorithm showed that rare variants of SCN10A, but not common variants, were significantly different between BrS patients and healthy controls in the additive and dominant models (p-value <0.001), suggesting that rare SCN10A variants may play a role in BrS. Six likely pathogenic SCN10A variants were found in 6 patients and were compared to 25 pathogenic or likely pathogenic SCN5A variants found in 29 patients. The patients with likely pathogenic SCN10A variants tended to exhibit sudden death in older age and have a shorter QRS interval than those carrying pathogenic or likely pathogenic SCN5A variants or no variants in either gene (p = 0.06, 0.07, respectively). Collectively, the prevalence of likely pathogenic SCN10A variants was 3.4% in Han Chinese patients with BrS in Taiwan.

Conclusions

SCN10A likely pathogenic variants were present in 3.4% of Han Chinese BrS patients. Rare SCN10A variants may play a role in BrS, and may have impact on clinical and electrocardiographic manifestations.

Table 1. Patient Nucleotide Amino acid TWB gnomAD_EA REVEL CADD PHRED SIFT Polyphen-2 GERP++ 1 c.5789A > T p.D1930V 0.001318 0.0008700 0.479 24.5 Damaging Possibly damaging 4.22 2 c.2341G > A p.G781R 0 0.00005301 0.866 33 Damaging Probably damaging 4.83 3 c.5587C > T p.R1863W 0.000502 0 0.832 27.8 Damaging Probably damaging 1.97 4 c.2161C > T p.P721S 0.000989 0.0009016 0.933 28.5 Damaging Probably damaging 4.19 5 c.3749G > A p.R1250Q 0 0 0.907 31 Damaging Probably damaging 4.23 6 c.1825A > T p.R609W 0.000659 0.0001591 0.811 32 Damaging Probably damaging 4.28 Clinical and predicted functional characteristics of 6 likely pathogenic SCN10A variants. EA = East Asian; GERP = Genomic Evolutionary Rate Profiling; TWB = Taiwan Biobank. Transcript: NM_006514.3.

Abstract 299 Figure. Location of the SCN10A variants

Nitrogen-dependent regulation of de novo cytokinin biosynthesis in rice: the role of glutamine metabolism as an additional signal

Cytokinin activity in plants is closely related to nitrogen availability, and an Arabidopsis gene for adenosine phosphate-isopentenyltransferase (IPT), IPT3, is regulated by inorganic nitrogen sources in a nitrate-specific manner. In this study, we have identified another regulatory system of cytokinin de novo biosynthesis in response to nitrogen status. In rice, OsIPT4, OsIPT5, OsIPT7 and OsIPT8 were up-regulated in response to exogenously applied nitrate and ammonium, with accompanying accumulation of cytokinins. Pre-treatment of roots with l-methionine sulfoximine, a potent inhibitor of glutamine synthetase, abolished the nitrate- and ammonium-dependent induction of OsIPT4 and OsIPT5, while glutamine application induced their expression. Thus, neither nitrate nor ammonium, but glutamine or a related metabolite, is essential for the induction of these IPT genes in rice. On the other hand, glutamine-dependent induction of IPT3 occurs in Arabidopsis, at least to some extent. In transgenic lines repressing the expression of OsIPT4, which is the dominant IPT in rice roots, the nitrogen-dependent increase of cytokinin in the xylem sap was significantly reduced, and seedling shoot growth was retarded despite sufficient nitrogen. We conclude that plants possess multiple regulation systems for nitrogen-dependent cytokinin biosynthesis to modulate growth in response to nitrogen availability.

UniVIO: a multiple omics database with hormonome and transcriptome data from rice

Plant hormones play important roles as signaling molecules in the regulation of growth and development by controlling the expression of downstream genes. Since the hormone signaling system represents a complex network involving functional cross-talk through the mutual regulation of signaling and metabolism, a comprehensive and integrative analysis of plant hormone concentrations and gene expression is important for a deeper understanding of hormone actions. We have developed a database named Uniformed Viewer for Integrated Omics (UniVIO: http://univio.psc.riken.jp/), which displays hormone-metabolome (hormonome) and transcriptome data in a single formatted (uniformed) heat map. At the present time, hormonome and transcriptome data obtained from 14 organ parts of rice plants at the reproductive stage and seedling shoots of three gibberellin signaling mutants are included in the database. The hormone concentration and gene expression data can be searched by substance name, probe ID, gene locus ID or gene description. A correlation search function has been implemented to enable users to obtain information of correlated substance accumulation and gene expression. In the correlation search, calculation method, range of correlation coefficient and plant samples can be selected freely.

Cytokinin activity of cis-zeatin and phenotypic alterations induced by overexpression of putative cis-Zeatin-O-glucosyltransferase in rice

cis-Zeatin (cZ) is generally regarded as a cytokinin with little or no activity, compared with the highly active trans-zeatin (tZ). Although recent studies suggested possible roles for cZ, its physiological significance remains unclear. In our studies with rice (Oryza sativa), cZ inhibited seminal root elongation and up-regulated cytokinin-inducible genes, and its activities were comparable to those of tZ. Tracer experiments showed that exogenously supplied cZ-riboside was mainly converted into cZ derivatives but scarcely into tZ derivatives, indicating that isomerizations of cZ derivatives into tZ derivatives are a minor pathway in rice cytokinin metabolism. We identified three putative cZ-O-glucosyltransferases (cZOGT1, cZOGT2, and cZOGT3) in rice. The cZOGTs preferentially catalyzed O-glucosylation of cZ and cZ-riboside rather than tZ and tZ-riboside in vitro. Transgenic rice lines ectopically overexpressing the cZOGT1 and cZOGT2 genes exhibited short-shoot phenotypes, delay of leaf senescence, and decrease in crown root number, while cZOGT3 overexpressor lines did not show shortened shoots. These results propose that cZ activity has a physiological impact on the growth and development of rice.

DNA compaction in a crowded environment with negatively charged proteins

We studied the conformational properties of DNA in a salt solution of the strongly charged protein bovine serum albumin. DNA is compacted when a suitable amount of bovine serum albumin is added to the solution due to a crowding effect and strong electrostatic repulsion between DNA and bovine serum albumin, both of which carry negative charges. However, DNA undergoes an unfolding transition with an increase in the salt concentration. This observation contradicts the current understanding of polymer- and salt-induced condensation, ψ condensation. We propose a simple theoretical model by taking into account the competition between the translational entropy of ions and electrostatic interaction.

Overexpression of a type-A response regulator alters rice morphology and cytokinin metabolism

Genome-wide analyses of rice (Oryza sativa L.) cytokinin (CK)-responsive genes using the Affymetrix GeneChip(R) rice genome array were conducted to define the spectrum of genes subject to regulation by CK in monocotyledonous plants. Application of trans-zeatin modulated the expression of a wide variety of genes including those involved in hormone signaling and metabolism, transcriptional regulation, macronutrient transport and protein synthesis. To understand further the function of CK in rice plants, we examined the effects of in planta manipulation of a putative CK signaling factor on morphology, CK metabolism and expression of CK-responsive genes. Overexpression of the CK-inducible type-A response regulator OsRR6 abolished shoot regeneration, suggesting that OsRR6 acts as a negative regulator of CK signaling. Transgenic lines overexpressing OsRR6 (OsRR6-ox) had dwarf phenotypes with poorly developed root systems and panicles. Increased content of trans-zeatin-type CKs in OsRR6-ox lines indicates that homeostatic control of CK levels is regulated by OsRR6 signaling. Expression of genes encoding CK oxidase/dehydrogenase decreased in OsRR6-ox plants, possibly accounting for elevated CK levels in transgenic lines. Expression of a number of stress response genes was also altered in OsRR6-ox plants.

Moderate increase of mean daily temperature adversely affects fruit set of Lycopersicon esculentum by disrupting specific physiological processes in male reproductive development

BACKGROUND AND AIMS

Global warming is gaining significance as a threat to natural and managed ecosystems since temperature is one of the major environmental factors affecting plant productivity. Hence, the effects of moderate temperature increase on the growth and development of the tomato plant (Lycopersicon esculentum) were investigated.

METHODS

Plants were grown at 32/26 degrees C as a moderately elevated temperature stress (METS) treatment or at 28/22 degrees C (day/night temperatures) as a control with natural light conditions. Vegetative growth and reproductive development as well as sugar content and metabolism, proline content and translocation in the androecium were investigated.

KEY RESULTS

METS did not cause a significant change in biomass, the number of flowers, or the number of pollen grains produced, but there was a significant decrease in the number of fruit set, pollen viability and the number of pollen grains released. Glucose and fructose contents in the androecium (i.e. all stamens from one flower) were generally higher in the control than METS, but sucrose was higher in METS. Coincidently, the mRNA transcript abundance of acid invertase in the androecium was decreased by METS. Proline contents in the androecium were almost the same in the control and METS, while the mRNA transcript level of proline transporter 1, which expresses specifically at the surface of microspores, was significantly decreased by METS.

CONCLUSIONS

The research indicated that failure of tomato fruit set under a moderately increased temperature above optimal is due to the disruption of sugar metabolism and proline translocation during the narrow window of male reproductive development.

Functional characterization and expression analysis of a gene, OsENT2, encoding an equilibrative nucleoside transporter in rice suggest a function in cytokinin transport

We identified four genes for potential equilibrative nucleoside transporters (ENTs) from rice (Oryza sativa; designated OsENT1 through OsENT4). Growth analysis of budding yeast (Saccharomyces cerevisiae) cells expressing OsENTs showed that OsENT2 transported adenosine and uridine with high affinity (adenosine, K(m) = 3.0 microm; uridine, K(m) = 0.7 microm). Purine or pyrimidine nucleosides and 2'-deoxynucleosides strongly inhibited adenosine transport via OsENT2, suggesting that OsENT2 possesses broad substrate specificity. OsENT2-mediated adenosine transport was resistant to the typical inhibitors of mammalian ENTs, nitrobenzylmercaptopurine ribonucleoside, dilazep, and dipyridamole. The transport activity was maximal at pH 5.0 and decreased slightly at lower as well as higher pH. In competition experiments with various cytokinins, adenosine transport by OsENT2 was inhibited by isopentenyladenine riboside (iPR). Direct measurements with radiolabeled cytokinins demonstrated that OsENT2 mediated uptake of iPR (K(m) = 32 microm) and trans-zeatin riboside (K(m) = 660 microm), suggesting that OsENT2 participates in iPR transport in planta. In mature plants, OsENT2 was predominantly expressed in roots. The OsENT2 promoter drove the expression of the beta-glucuronidase reporter gene in the scutellum during germination and in vascular tissues in germinated plants, suggesting a participation of OsENT2 in the retrieval of endosperm-derived nucleosides by the germinating embryo and in the long-distance transport of nucleosides in growing plants, respectively.

The complete nucleotide sequence and multipartite organization of the tobacco mitochondrial genome: comparative analysis of mitochondrial genomes in higher plants

Tobacco is a valuable model system for investigating the origin of mitochondrial DNA (mtDNA) in amphidiploid plants and studying the genetic interaction between mitochondria and chloroplasts in the various functions of the plant cell. As a first step, we have determined the complete mtDNA sequence of Nicotiana tabacum. The mtDNA of N. tabacum can be assumed to be a master circle (MC) of 430,597 bp. Sequence comparison of a large number of clones revealed that there are four classes of boundaries derived from homologous recombination, which leads to a multipartite organization with two MCs and six subgenomic circles. The mtDNA of N. tabacum contains 36 protein-coding genes, three ribosomal RNA genes and 21 tRNA genes. Among the first class, we identified the genes rps1 and psirps14, which had previously been thought to be absent in tobacco mtDNA on the basis of Southern analysis. Tobacco mtDNA was compared with those of Arabidopsis thaliana, Beta vulgaris, Oryza sativa and Brassica napus. Since repeated sequences show no homology to each other among the five angiosperms, it can be supposed that these were independently acquired by each species during the evolution of angiosperms. The gene order and the sequences of intergenic spacers in mtDNA also differ widely among the five angiosperms, indicating multiple reorganizations of genome structure during the evolution of higher plants. Among the conserved genes, the same potential conserved nonanucleotide-motif-type promoter could only be postulated for rrn18-rrn5 in four of the dicotyledonous plants, suggesting that a coding sequence does not necessarily move with the promoter upon reorganization of the mitochondrial genome.

A cardiac sodium channel mutation identified in Brugada syndrome associated with atrial standstill

Mutations in the cardiac Na+ channel gene SCN5A are responsible for multiple lethal ventricular arrhythmias including Brugada syndrome and congenital long QT syndrome. Here we report a case of Brugada syndrome with ST elevation in the right precordial and inferior leads accompanied by atrial standstill and spontaneous ventricular fibrillation. Atrial standstill and J wave elevation were provoked by procainamide. Genetic analysis revealed a missense mutation (R367H) in SCN5A. The resultant mutant Na+ channel was nonfunctional when expressed heterologously in Xenopus oocytes. Our study suggests that genetic defects in SCN5A may be associated with atrial standstill in combination with ventricular arrhythmias.

Cold induces shifts of voltage dependence in mutant SCN4A, causing hypokalemic periodic paralysis

BACKGROUND

The authors reported a mutation, P1158S, of the human skeletal muscle sodium channel gene (SCN4A) in a family with cold-induced hypokalemic periodic paralysis (hypoKPP) and myotonia.

OBJECTIVE

To identify mechanisms of temperature dependency in this channelopathy.

METHODS

Using the amphotericin B perforated patch clamp method, sodium currents were recorded at 22 and 32 degrees C from the wild-type (WT) and P1158S mutant SCN4A expressed in tsA201 cells. Computer simulation was performed, incorporating the gating parameters of the P1158S mutant SCN4A.

RESULTS

P1158S mutant SCN4A exhibited hyperpolarizing shifts in voltage dependence of both activation and inactivation curves at a cold temperature and a slower rate of inactivation than the WT. Computer simulation reproduced the abnormal skeletal muscle electrical activities of both paralysis at a low potassium concentration in the cold and myotonia at a normal potassium concentration.

CONCLUSIONS

Both paralysis and myotonia are attributable to the biophysical properties of the SCN4A mutation associated with hypoKPP. This is the first report of an SCN4A mutation that exhibits temperature-dependent shifts of voltage dependence in sodium channel gating.

Cloning of a functional splice variant of L-type calcium channel beta 2 subunit from rat heart

L-type Ca(2+) channels are heteromultimeric and finely tuned by auxiliary subunits in different tissues and regions. Among auxiliary subunits, beta subunit has been shown to play important roles in many functional aspects of Ca(2+) channel. Rat heart was reported to specifically express beta(2a) subunit. However, the slow inactivation rates of Ca(2+) currents recorded from recombinant Ca(2+) channels with the beta(2a) subunit, and the reported inability to detect beta(2a) subunit in rabbit heart by reverse transcription-PCR analysis raise the possibility of the existence of other beta subunits. We cloned a splice variant of beta(2) subunit from rat heart, using rapid amplification of cDNA 5' ends. The splice variant is highly similar to human beta(2c) subunit that was cloned from human ventricle. Northern blot analysis detected the rat beta(2c) subunit abundantly in rat heart and brain. The deduced amino acid sequence of the beta(2c) subunit was different from that of the beta(2a) subunit only in the N-terminal region. When the beta(2c) subunit was expressed along with alpha(1c) and alpha(2)delta subunits in baby hamster kidney cells, the inactivation rates were comparable with those from native cardiac myocytes, although those with the beta(2a) subunit were slow. Taken together, these observations suggest that the beta(2c) subunit is a functional beta(2) subunit expressed in heart and that the short N-terminal region plays a major role in modifying inactivation kinetics.

Enhanced Na(+) channel intermediate inactivation in Brugada syndrome

Brugada syndrome is an inherited cardiac disease that causes sudden death related to idiopathic ventricular fibrillation in a structurally normal heart. The disease is characterized by ST-segment elevation in the right precordial ECG leads and is frequently accompanied by an apparent right bundle-branch block. The biophysical properties of the SCN5A mutation T1620M associated with Brugada syndrome were examined for defects in intermediate inactivation (I:(M)), a gating process in Na(+) channels with kinetic features intermediate between fast and slow inactivation. Cultured mammalian cells expressing T1620M Na(+) channels in the presence of the human beta(1) subunit exhibit enhanced intermediate inactivation at both 22 degrees C and 32 degrees C compared with wild-type recombinant human heart Na(+) channels (WT-hH1). Our findings support the hypothesis that Brugada syndrome is caused, in part, by functionally reduced Na(+) current in the myocardium due to an increased proportion of Na(+) channels that enter the I:(M) state. This phenomenon may contribute significantly to arrhythmogenesis in patients with Brugada syndrome. The full text of this article is available at http://www.circresaha.org.

Block of sodium channels by divalent mercury: role of specific cysteinyl residues in the P-loop region

Divalent mercury (Hg(2+)) blocked human skeletal Na(+) channels (hSkM1) in a stable dose-dependent manner (K(d) = 0.96 microM) in the absence of reducing agent. Dithiothreitol (DTT) significantly prevented Hg(2+) block of hSkM1, and Hg(2+) block was also readily reversed by DTT. Both thimerosal and 2,2'-dithiodipyridine had little effect on hSkM1; however, pretreatment with thimerosal attenuated Hg(2+) block of hSkM1. Y401C+E758C rat skeletal muscle Na(+) channels (mu1) that form a disulfide bond spontaneously between two cysteines at the 401 and 758 positions showed a significantly lower sensitivity to Hg(2+) (K(d) = 18 microM). However, Y401C+E758C mu1 after reduction with DTT had a significantly higher sensitivity to Hg(2+) (K(d) = 0.36 microM) than wild-type hSkM1. Mutants C753Amu1 (K(d) = 8.47 microM) or C1521A mu1 (K(d) = 8.63 microM) exhibited significantly lower sensitivity to Hg(2+) than did wild-type hSkM1, suggesting that these two conserved cysteinyl residues of the P-loop region may play an important role in the Hg(2+) block of the hSkM1 isoform. The heart Na(+) channel (hH1) was significantly more sensitive to low-dose Hg(2+) (K(d) = 0.43 microM) than was hSkM1. The C373Y hH1 mutant exhibited higher resistance (K(d) = 1.12 microM) to Hg(2+) than did wild-type hH1. In summary, Hg(2+) probably inhibits the muscle Na(+) channels at more than one cysteinyl residue in the Na(+) channel P-loop region. Hg(2+) exhibits a lower K(d) value (<1. 23 microM) for inhibition by forming a sulfur-Hg-sulfur bridge, as compared to reaction at a single cysteinyl residue with a higher K(d) value (>8.47 microM) by forming sulfur-Hg(+) covalently. The heart Na(+) channel isoform with more than two cysteinyl residues in the P-loop region exhibits an extremely high sensitivity (K(d) < 0. 43 microM) to Hg(+), accounting for heart-specific high sensitivity to the divalent mercury.

A novel SCN5A mutation associated with idiopathic ventricular fibrillation without typical ECG findings of Brugada syndrome

Mutations in the human cardiac Na+ channel alpha subunit gene (SCN5A) are responsible for Brugada syndrome, an idiopathic ventricular fibrillation (IVF) subgroup characterized by right bundle branch block and ST elevation on an electrocardiogram (ECG). However, the molecular basis of IVF in subgroups lacking these ECG findings has not been elucidated. We performed genetic screenings of Japanese IVF patients and found a novel SCN5A missense mutation (S1710L) in one symptomatic IVF patient that did not exhibit the typical Brugada ECG. Heterologously expressed S1710L channels showed marked acceleration in the current decay together with a large hyperpolarizing shift of steady-state inactivation and depolarizing shift of activation. These findings suggest that SCN5A is one of the responsible genes for IVF patients who do not show typical ECG manifestations of the Brugada syndrome.

A case of mixed connective tissue disease (MCTD) complicated with MPO-ANCA-related necrotizing glomerulonephritis

Renal diseases of mixed connective tissue disease (MCTD) are not unusual. Although most of them are SLE-like renal impairment with immune complex deposits, systemic sclerosis- (SSc) like renal impairments with intimal thickening of interlobular arteries or arterioles are also encountered. Several cases of SSc complicated with MPO-ANCA-related necrotizing glomerulonephritis (nGN) are reported. Here we report a case which developed MPO-ANCA-related nGN 16 years after the diagnosis of MCTD. She exhibited pauci-immune focal nGN and significantly high titer of MPO-ANCA. She was successfully treated with prednisolone and cyclophosphamide. We believe this is the first case in which MPO-ANCA-related nGN was demonstrated in a patient with MCTD.

Selective block of late currents in the DeltaKPQ Na(+) channel mutant by pilsicainide and lidocaine with distinct mechanisms

The congenital long QT syndrome is an inherited disorder characterized by a delay in cardiac repolarization, leading to lethal cardiac arrhythmias such as torsade de pointes. One form of this disease involves mutations in the voltage-dependent cardiac Na(+) channel, which includes an in-frame deletion of three amino acids (Lys-1505, Pro-1506, and Gln-1507; DeltaKPQ). The potential for selective suppression of the mutant was examined by heterologous expression of DeltaKPQ-Na(+) channels in Chinese hamster fibroblast cells via single-channel recording. In a single-channel cell-attached patch study, DeltaKPQ-Na(+) channels yielded currents that peaked at approximately 1 ms after voltage steps to 0 mV with aberrant late currents, which were composed of burst and isolated openings. The affinity of certain anesthetics (pilsicainide and lidocaine) to the late currents of the mutant channels was examined. It was revealed that 1) pilsicainide (1 microM), an open channel blocker of voltage-dependent Na(+) channels, remarkably decreased the late currents primarily by the shortening of burst duration without suppressing the initial peak current; and 2) lidocaine (1 microM), an inactivated channel blocker, decreased the late currents primarily by the suppression of isolated channel openings. Because the late currents in DeltaKPQ mutants are mainly composed of the burst openings, we conclude that pilsicainide is capable of selectively blocking the late currents in the mutant Na(+) channels that show dominant abnormal burst openings such as in DeltaKPQ mutants.

Cardiac Na(+) channel dysfunction in Brugada syndrome is aggravated by beta(1)-subunit

BACKGROUND

Mutations in the gene encoding the human cardiac Na(+) channel alpha-subunit (hH1) are responsible for chromosome 3-linked congenital long-QT syndrome (LQT3) and idiopathic ventricular fibrillation (IVF). An auxiliary beta(1)-subunit, widely expressed in excitable tissues, shifts the voltage dependence of steady-state inactivation toward more negative potentials and restores normal gating kinetics of brain and skeletal muscle Na(+) channels expressed in Xenopus oocytes but has little if any functional effect on the cardiac isoform. Here, we characterize the altered effects of a human beta(1)-subunit (hbeta(1)) on the heterologously expressed hH1 mutation (T1620M) previously associated with IVF.

METHODS AND RESULTS

When expressed alone in Xenopus oocytes, T1620M exhibited no persistent currents, in contrast to the LQT3 mutant channels, but the midpoint of steady-state inactivation (V(1/2)) was significantly shifted toward more positive potentials than for wild-type hH1. Coexpression of hbeta(1) did not significantly alter current decay or recovery from inactivation of wild-type hH1; however, it further shifted the V(1/2) and accelerated the recovery from inactivation of T1620M. Oocyte macropatch analysis revealed that the activation kinetics of T1620M were normal.

CONCLUSIONS

It is suggested that coexpression of hbeta(1) exposes a more severe functional defect that results in a greater overlap in the relationship between channel inactivation and activation (window current) in T1620M, which is proposed to be a potential pathophysiological mechanism of IVF in vivo. One possible explanation for our finding is an altered alpha-/beta(1)-subunit association in the mutant.

Bone marrow transplantation attenuates murine IgA nephropathy: role of a stem cell disorder

BACKGROUND

The pathogenesis of IgA nephropathy is still obscure. The aim of this study was to investigate whether the fundamental pathogenesis of IgA nephropathy lies in bone marrow stem cells (BMCs).

METHODS

We used donors of two different strains for bone marrow transplantation (BMT) into mice with a high content of serum IgA (ddY strain, HIGA mice), a murine model of IgA nephropathy. One group (B6-->HIGA, N = 5) received BMCs of C57BL/6j (B6) mice, and the other (HIGA-->HIGA, N = 8) were reconstituted with BMCs of HIGA mice.

RESULTS

Twenty-six weeks after BMT, in B6-->HIGA mice, mesangial deposits of IgA and C3 were statistically milder than those in HIGA-->HIGA mice. Light microscopic observations disclosed that glomerular sclerosis and mesangial matrix expansion in B6-->HIGA mice were decreased compared with those in HIGA-->HIGA mice. These B6-->HIGA mice also excreted less urinary albumin than HIGA-->HIGA mice. Furthermore, serum levels of IgA in B6-->HIGA mice were markedly lower than those in HIGA-->HIGA mice. Size analysis of serum IgA revealed that macromolecular IgA were notably lower in B6-->HIGA mice than in HIGA-->HIGA mice.

CONCLUSIONS

Our results suggest that qualitative and quantitative changes of serum IgA are determined at the level of stem cells, and that BMT from normal donors can attenuate glomerular lesions in HIGA mice. This approach may offer a new avenue to study the pathogenesis of IgA nephropathy.

ATP/ADP switches the higher-order structure of DNA in the presence of spermidine

In the living cellular environment, DNAs exist in a compact state in the presence of a polyamine, such as spermidine. We found that the hydrolysis of ATP into ADP induces the folding of elongated DNAs, by the single-chain observation of individual T4 DNA molecules. This result is discussed in relation to the possible role of ATP as a regulatory factor in genetic activity, in addition to its well-established role as an energy source.

Structural determinants of slow inactivation in human cardiac and skeletal muscle sodium channels

Skeletal and heart muscle excitability is based upon the pool of available sodium channels as determined by both fast and slow inactivation. Slow inactivation in hH1 sodium channels significantly differs from slow inactivation in hSkM1. The beta(1)-subunit modulates fast inactivation in human skeletal sodium channels (hSkM1) but has little effect on fast inactivation in human cardiac sodium channels (hH1). The role of the beta(1)-subunit in sodium channel slow inactivation is still unknown. We used the macropatch technique on Xenopus oocytes to study hSkM1 and hH1 slow inactivation with and without beta(1)-subunit coexpression. Our results indicate that the beta(1)-subunit is partly responsible for differences in steady-state slow inactivation between hSkM1 and hH1 channels. We also studied a sodium channel chimera, in which P-loops from each domain in hSkM1 sodium channels were replaced with corresponding regions from hH1. Our results show that these chimeras exhibit hH1-like properties of steady-state slow inactivation. These data suggest that P-loops are structural determinants of sodium channel slow inactivation, and that the beta(1)-subunit modulates slow inactivation in hSkM1 but not hH1. Changes in slow inactivation time constants in sodium channels coexpressed with the beta(1)-subunit indicate possible interactions among the beta(1)-subunit, P-loops, and the slow inactivation gate in sodium channels.

Effects of amlodipine on native cardiac Na+ channels and cloned alpha-subunits of cardiac Na+ channels

The inhibitory effects of amlodipine besilate (CAS 11470-99-6) on the native Na+ current (INa) and cloned human cardiac Na+ channel alpha subunit (hH1) were studied by whole cell patch clamp techniques. Amlodipine produced tonic block of INa in a concentration- and holding potential (HP)-dependent manner with hyperpolarization of H infinity. Amlodipine produced phasic blockade of INa, which was dependent on HP and pulse duration. Amlodipine produced tonic blockade of hH1 in a concentration-dependent manner with 1 : 1 stoichiometry, and phasic blockade of hH1 which was dependent on the pulse duration. Amlodipine blocked INa in a voltage- and frequency-dependent manner via affinity to the resting as well as inactivated conformations of the alpha subunit.

A de novo missense mutation of human cardiac Na+ channel exhibiting novel molecular mechanisms of long QT syndrome

Mutations in a human cardiac Na+ channel gene (SCN5A) are responsible for chromosome 3-linked congenital long QT syndrome (LQT3). Here we characterized a de novo missense mutation (R1623Q, S4 segment of domain 4) identified in an infant Japanese girl with a severe form of LQT3. When expressed in oocytes, mutant Na+ channels exhibited only minor abnormalities in channel activation, but in contrast to three previously characterized LQT3 mutations, had significantly delayed macroscopic inactivation. Single channel analysis revealed that R1623Q channels have significantly prolonged open times with bursting behavior, suggesting a novel mechanism of pathophysiology in Na+ channel-linked long QT syndrome.

Pharmacological targeting of long QT mutant sodium channels

The congenital long QT syndrome (LQTS) is an inherited disorder characterized by a delay in cardiac cellular repolarization leading to cardiac arrhythmias and sudden death often in young people. One form of the disease (LQT3) involves mutations in the voltage-gated cardiac sodium channel. The potential for targeted suppression of the LQT defect was explored by heterologous expression of mutant channels in cultured human cells. Kinetic and steady state analysis revealed an enhanced apparent affinity for the predominantly charged, primary amine compound, mexiletine. The affinity of the mutant channels in the inactivated state was similar to the wild type (WT) channels (IC50 approximately 15-20 microM), but the late-opening channels were inhibited at significantly lower concentrations (IC50 = 2-3 microM) causing a preferential suppression of the late openings. The targeting of the defective behavior of the mutant channels has important implications for therapeutic intervention in this disease. The results provide insights for the selective suppression of the mutant phenotype by very low concentrations of drug and indicate that mexiletine equally suppresses the defect in all three known LQT3 mutants.

Molecular determinants of beta 1 subunit-induced gating modulation in voltage-dependent Na+ channels

Recombinant brain, skeletal muscle, and heart voltage-gated Na+ channel alpha subunits differ in their functional responses to an accessory beta 1 subunit when coexpressed in Xenopus oocytes. We exploited the distinct beta 1 subunit responses observed for the human heart (hH1) and human skeletal muscle (hSkM1) isoforms to identify determinants of this response. Chimeric alpha subunits were constructed by exchanging the S5-S6 interhelical loops of each domain between hH1 and hSkM1 and then examined for effects on inactivation induced by coexpressed beta 1 subunit in oocytes. Substitution of single S5-S6 loops in either domain 1 (D1/S5-S6) or domain 4 (D4/S5-S6) of hSkM1 by the corresponding segments of hH1 produced channels that exhibited an attenuated response to coexpressed beta 1 subunit. Substitutions of both D1/S5-S6 and D4/S5-S6 in hSkM1 by the corresponding loops from hH1 completely abolished the effects of the beta 1 subunit on inactivation. The reciprocal chimera in which both D1/S5-S6 and D4/S5-S6 from hSkM1 were transplanted into hH1 exhibited significant beta 1 responsiveness (accelerated inactivation). The region within D4/S5-S6 that conferred beta 1 responsiveness was determined to reside primarily within an extracellular loop between the putative pore-forming segment SS2 and D4/S6. Gating modulation was also demonstrated using a chimeric beta subunit consisting of the extracellular domains of beta 1 and the transmembrane and C-terminal domains of the rat brain beta 2 subunit. These results suggest that the D1/S5-S6 and D4/S5-S6 loops in the alpha subunit and the extracellular domain of the beta 1 subunit are important determinants of the beta 1 subunit-induced gating modulation in Na+ channels.

Multiple domains contribute to the distinct inactivation properties of human heart and skeletal muscle Na+ channels

Voltage-gated Na+ channels are essential for the normal electrical excitability of neuronal and striated muscle membranes. Distinct isoforms of the Na+ channel alpha-subunit have been identified by molecular cloning, and their functional attributes have been defined by heterologous expression coupled with electrophysiological recording. Two closely related Na+ channel alpha-subunit isoforms, hH1 (human heart) and hSkM1 (human skeletal muscle), exhibit differences in their inactivation properties and in their response to the coexpressed beta 1-subunit. To localize regions that contribute to inactivation and to beta 1-subunit response, we have exploited these functional differences by studying chimeric channels composed of segments from both hH1 and hSkM1. Chimeras in which one or more of the cytoplasmic interdomain regions (ID1-2, ID2-3, and ID3-4) were exchanged between hH1 and hSkM1 exhibit inactivation properties identical with the background channel isoform, suggesting that these regions are not sufficient to cause gating differences. In contrast, inactivation properties of chimeras composed of approximately equal halves of the two channel isoforms were intermediate between hH1 and hSkM1. Furthermore, the response to the coexpressed beta 1-subunit was dependent on structures located in the carboxy-terminal half of the alpha-subunit, although domains D3, D4, and the carboxy terminal are not singularly responsible for this effect. These data indicate that inactivation differences between hH1 and hSkM1 are determined by multiple alpha-subunit domains.

Molecular mechanism for an inherited cardiac arrhythmia

In the congenital long-QT syndrome, prolongation of the cardiac action potential occurs by an unknown mechanism and predisposes individuals to syncope and sudden death as a result of ventricular arrhythmias. Genetic heterogeneity has been demonstrated for autosomal dominant long-QT syndrome by the identification of multiple distinct loci, and associated mutations in two candidate genes have recently been reported. One form of hereditary long QT (LQT3) has been linked to a mutation in the gene encoding the human heart voltage-gated sodium-channel alpha-subunit (SCN5A on chromosome 3p21). Here we characterize this mutation using heterologous expression of recombinant human heart sodium channels. Mutant channels show a sustained inward current during membrane depolarization. Single-channel recordings indicate that mutant channels fluctuate between normal and non-inactivating gating modes. Persistent inward sodium current explains prolongation of cardiac action potentials, and provides a molecular mechanism for this form of congenital long-QT syndrome.

Genomic organization and chromosomal assignment of the human voltage-gated Na+ channel beta 1 subunit gene (SCN1B)

Voltage-gated sodium (Na+) channels are essential for the generation and propagation of action potentials in striated muscle and neuronal tissues. Biochemically, Na+ channels consist of a large alpha subunit and one or two smaller beta subunits. The alpha subunit alone can exhibit all of the functional attributes of a voltage-gated Na+ channel, but requires a beta 1 subunit for normal inactivation kinetics. While genetic mutations in the skeletal muscle Na+ channel alpha-subunit gene can cause human disease, it is not known whether hereditary defects in the beta 1 subunit underlie any inherited syndromes. To help explore this further, we have carried out an analysis of the detailed structure of the human beta 1 subunit gene (SCN1B) including the delineation of intron-exon boundaries by genomic DNA cloning and sequence analysis. The complete coding region of SCN1B is found in approximately 9.0 kb of genomic DNA and consists of five exons (72 to 749 bp) and four introns (90 bp to 5.5 kb). Using a 15.9-kb genomic SCN1B clone, we assigned the gene to the long arm of chromosome 19 (19q13.1-q13.2) by fluorescence in situ hybridization. An intragenic polymorphic (TTA)n repeat that is positioned between two tandem Alu repetitive sequences was also characterized. The (TTA)n repeat exhibits 5 distinct alleles and a heterozygosity index of 0.59. This information should be useful in evaluating SCN1B as a candidate gene for hereditary disorders affecting membrane excitability.

Co-regulated expression of glomerular 12/15-lipoxygenase and interleukin-4 mRNAs in rat nephrotoxic nephritis

Arachidonate 12- and 15-lipoxygenase (LO) products are generated in experimental glomerulonephritis. 15-S-HETE (a 15-LO product) and lipoxins (interaction products between 5-LO and either 12-LO or 15-LO) counteract the proinflammatory actions of leukotrienes. IL-4 has been shown to up-regulate 15-LO gene expression in human leukocytes. Based on homology with human 15-LO, we cloned a 0.76 kbp fragment of a rat LO cDNA from leukocytes stimulated by interleukin-4 (IL-4). The deduced amino acid sequence shows 71.0% and 60.1% homology to human 15-LO and 12-LO, respectively, and 100% homology to a recently cloned "leukocyte type" rat 12-lipoxygenase enzyme, which possesses significant 15-lipoxygenase activity (heretofore referred to as "12/15-LO"). A deletion mutant was utilized to generate internal standard cRNA in quantitative PCR assays. Glomerular 12/15-LO mRNA increased significantly over controls 24 and 48 hours after NTS injection, then decreased at 72 hours. RNA from NTS glomeruli contained higher levels of 12/15-LO mRNA than that from unstimulated peripheral leukocytes, suggesting that 12/15-LO transcription is up-regulated locally in native and/or infiltrating glomerular cells. Glomerular IL-4 mRNA increased markedly 16 hours post-NTS, and was then reduced, suggesting a potential role for T cell-derived IL-4 in directing the expression of 12/15-LO during glomerulonephritis. This represents the first demonstration of tandem regulated in vivo gene expression for a lymphokine (IL-4) and a lipoxygenase, both of which promote counter-inflammatory influences in immune complex-mediated injury.

Voltage-gated Na+ channel beta 1 subunit mRNA expressed in adult human skeletal muscle, heart, and brain is encoded by a single gene

Voltage-gated Na+ channels are heteromeric proteins consisting of alpha and beta subunits. Although alpha subunits alone are sufficient to encode functional channels, beta 1 subunits appear to modulate the kinetics of inactivation. We have used a cross-species reverse transcriptase polymerase chain reaction approach to isolate cDNAs encoding a Na+ channel beta 1 subunit from human heart and skeletal muscle. The deduced amino acid sequence of the human beta 1 subunit exhibits 96% identity with the rat brain beta 1 subunit. Human beta 1 mRNA transcripts are abundantly expressed in skeletal muscle, heart, and brain. Genomic Southern blot hybridization experiments suggest that a single gene located on chromosome 19 encodes the human beta 1 subunit that is expressed in all three of these tissues. Co-expression of the human beta 1 subunit with the recombinant human skeletal muscle alpha subunit (hSkM1) in Xenopus oocytes results in Na+ currents that inactivate rapidly. In contrast, the human beta 1 subunit has no effect on the function of the tetrodotoxin-insensitive human heart Na+ channel (hH1). These findings indicate that the human beta 1 subunit is widely expressed but does not functionally modify all Na+ channel isoforms.

Expression of the sodium channel beta 1 subunit in rat skeletal muscle is selectively associated with the tetrodotoxin-sensitive alpha subunit isoform

Transcripts homologous to the rat brain sodium channel beta subunit (beta 1) are prominently expressed in both innervated and denervated adult skeletal muscle and in heart, but not in neonatal skeletal or cardiac muscle. Regulation of beta 1 mRNA expression closely parallels that of SkM1 alpha during development, after denervation in adult muscle, and in primary muscle culture, but does not follow SkM2 expression under any condition examined. In oocytes, beta 1 interacts functionally with SkM1 to modulate the abnormally slow inactivation kinetics observed with this alpha subunit expressed alone. We conclude that a common beta 1 subunit is expressed in skeletal muscle, heart, and brain and that in skeletal muscle, this subunit is specifically associated with the SkM1, rather than the SkM2, sodium channel isoform.