CFTR as a cAMP-dependent regulator of sodium channels

Cystic fibrosis transmembrane regulator (CFTR), the gene product that is mutated in cystic fibrosis (CF) patients, has a well-recognized function as a cyclic adenosine 3',5'-monophosphate (cAMP)-regulated chloride channel, but this property does not account for the abnormally high basal rate and cAMP sensitivity of sodium ion absorption in CF airway epithelia. Expression of complementary DNAs for rat epithelial Na+ channel (rENaC) alone in Madin Darby canine kidney (MDCK) epithelial cells generated large amiloride-sensitive sodium currents that were stimulated by cAMP, whereas coexpression of human CFTR with rENaC generated smaller basal sodium currents that were inhibited by cAMP. Parallel studies that measured regulation of sodium permeability in fibroblasts showed similar results. In CF airway epithelia, the absence of this second function of CFTR as a cAMP-dependent regulator likely accounts for abnormal sodium transport.

Uniqueness of primate forelimb posture during quadrupedal locomotion

Among the characteristics that are thought to set primate quadrupedal locomotion apart from that of nonprimate mammals are a more protracted limb posture and larger limb angular excursion. However, kinematic aspects of primate or nonprimate quadrupedal locomotion have been documented in only a handful of species, and more widely for the hind than the forelimb. This study presents data on arm (humerus) and forelimb posture during walking for 102 species of mammals, including 53 nonhuman primates and 49 nonprimate mammals. The results demonstrate that primates uniformly display a more protracted arm and forelimb at hand touchdown of a step than nearly all other mammals. Although primates tend to end a step with a less retracted humerus, their total humeral or forelimb angular excursion exceeds that of other mammals. It is suggested that these features are components of functional adaptations to locomotion in an arboreal habitat, using clawless, grasping extremities.

Analysis of subunit assembly of the Na-K-ATPase

The Na-K-ATPase, or sodium pump, is comprised of two subunits, alpha and beta. Each subunit spans the lipid bilayer of the cell membrane. This review summarizes our efforts to determine how the two subunits interact to form the functional ion transporter. Our major approach has been to observe the potential for subunit assembly when one or both subunits are truncated or present as chimeras that retain only a limited region of the Na-K-ATPase. DNAs encoding these altered subunit forms of the avian Na-K-ATPase are expressed in mammalian cells. Monoclonal antibodies specific for the avian beta-subunit are then used to purify newly synthesized avian beta-subunits, and the presence of accompanying alpha-subunits indicates that subunit assembly has occurred. The ectodomain of the beta-subunit (approximately residues 62-304) is sufficient for assembly with the alpha-subunit, and a COOH-terminal truncation of the beta-subunit that lacks aminoacyl residues beyond 162 will assemble inefficiently. A maximum of 26 aminoacyl residues of the alpha-subunit are necessary for robust assembly with the beta-subunit, when this sequence replaces the COOH-terminal half of the loop between membrane spans 7 and 8 in the SERCA1 Ca-ATPase. This region of the Ca-ATPase faces the lumen of the endoplasmic reticulum. These findings encourage study of other related questions, including whether there is preferential assembly of certain subunit isoforms and how various P-type ATPases are targeted to their appropriate subcellular compartments.

Novel treatments for obesity and osteoporosis: targeting apoptotic pathways in adipocytes

Obesity and osteoporosis have grave consequences for human health, quality of life, and even the efficiency of the labor force and economy. However, these pathologies share a common cell progenitor, revealing a surprising target for drug research and development. Recent findings show that high adipocyte count in bone marrow is directly related to bone loss, as fat cells replace osteoblasts (or bone-forming cells). The objective of this review is to examine the importance of adipocyte apoptosis in the treatment of obesity and/or osteoporosis, with special emphasis on natural products as promising leads for drug development. We have induced in vivo adipocyte apoptosis, using leptin, ciliary neurotrophic factor (CNTF), beta adrenergic agonists and conjugated linoleic acid (CLA) in rodents. The results of leptin treatments on rats are suppressed food intake, reduced body weight, reduced body fat, adipocyte apoptosis, and elevated energy expenditure. Further, leptin treatment of leptin-deficient (ob/ob) mice increases endosteal bone formation and bone mineral density. Adipocyte apoptosis has also been induced in vitro using tumor necrosis factor-alpha (TNF-alpha), (-)-epigallocatechin gallate (EGCG) from Camellia sinensis and ajoene, from Allium sativum. Natural products have potential for inducing apoptosis of adipose tissue, inhibiting bone marrow adipogenesis and increasing the expression of osteogenic factors in bone, thereby yielding effective treatments for obesity and osteoporosis.

Insulin Resistance Negatively Influences the Muscle-Dependent IGF-1-Bone Mass Relationship in Premenarcheal Girls

CONTEXT

IGF-1 promotes bone growth directly and indirectly through its effects on skeletal muscle. Insulin and IGF-1 share a common cellular signaling process; thus, insulin resistance may influence the IGF-1-muscle-bone relationship.

OBJECTIVE

We sought to determine the effect of insulin resistance on the muscle-dependent relationship between IGF-1 and bone mass in premenarcheal girls.

DESIGN, SETTING, AND PARTICIPANTS

This was a cross-sectional study conducted at a university research center involving 147 girls ages 9 to 11 years.

MAIN OUTCOME MEASURES

Glucose, insulin, and IGF-1 were measured from fasting blood samples. Homeostasis model assessment of insulin resistance (HOMA-IR) was calculated from glucose and insulin. Fat-free soft tissue (FFST) mass and bone mineral content (BMC) were measured by dual-energy x-ray absorptiometry. Our primary outcome was BMC/height.

RESULTS

In our path model, IGF-1 predicted FFST mass (b = 0.018; P = .001), which in turn predicted BMC/height (b = 0.960; P < .001). IGF-1 predicted BMC/height (b = 0.001; P = .002), but not after accounting for the mediator of this relationship, FFST mass. The HOMA-IR by IGF-1 interaction negatively predicted FFST mass (b = -0.044; P = .034). HOMA-IR had a significant and negative effect on the muscle-dependent relationship between IGF-1 and BMC/height (b = -0.151; P = .047).

CONCLUSIONS

Lean body mass is an important intermediary factor in the IGF-1-bone relationship. For this reason, bone development may be compromised indirectly via suboptimal IGF-1-dependent muscle development in insulin-resistant children.

Structural analysis and expression of a chromosomal gene encoding an avian Na+/K(+)-ATPase beta 1-subunit

Chicken chromosomal DNA encoding the Na+/K(+)-ATPase beta 1-subunit was cloned and characterized. Its exon-intron structure is identical to mammalian (human and rat) beta 1-subunit genes. The transcription initiation site, TATA box, and an ATTGG (antisense CCAAT) sequence follow approximately 1 kilobase of GC-rich 5' upstream sequence that contains many consensus sequences for transcription factors whose relative positions are conserved between human and chicken genes. When this beta 1-subunit gene was stably incorporated into mouse L cells and C2C12 cells, the avian beta 1-subunit was expressed under the control of the its own promoter.

26 amino acids of an extracellular domain of the Na,K-ATPase alpha-subunit are sufficient for assembly with the Na,K-ATPase beta-subunit

Chimeric cDNAs encoding a sarcoplasmic/endoplasmic reticulum Ca-ATPase (SERCA1) and regions of the Na,K-ATPase alpha-subunit were constructed to seek the minimal region of the alpha-subunit sufficient for assembly with the Na,K-ATPase beta-subunit. cDNAs encoding a chimera and the chicken beta-subunit were coexpressed in mammalian cells and assembly was assayed by immune precipitation of the chimeric subunit with a monoclonal antibody to the chicken beta-subunit. A chimera containing 26 amino acyl residues of the Na,K-ATPase alpha 1-subunit (NDVEDSYGQQWTFEQRKIVEFTCHTA) (Asn894 to Ala919) that replaced the corresponding avian SERCA1 Ca-ATPase amino acyl residues (Thr871 to Thr898) was able to assemble with the chicken beta-subunit. This alpha-subunit region is predicted to be extracellular, located between membrane-spanning domains 7 and 8 (H7-H8). Chimeras that assembled with full-length beta-subunit also assembled with a beta-subunit chimera that retained only the ectodomain of the chicken beta 1-subunit. These results suggest that the Na,K-ATPase alpha-subunit has the same topology in the membrane as the sarcoplasmic reticulum Ca-ATPase, probably with 10 membrane-spanning domains, and that the aminoacyl residues between membrane domains H7 and H8 are involved in assembly with the beta-subunit in the extracellular/lumenal space.

Assembly of the extracellular domain of the Na,K-ATPase beta subunit with the alpha subunit. Analysis of beta subunit chimeras and carboxyl-terminal deletions

The role of the extracellular domain of the Na,K-ATPase beta subunit in assembly with the alpha subunit was investigated. A chimeric protein consisting of the extracellular domain of the beta subunit fused with the transmembrane and cytoplasmic domains of dipeptidyl peptidase IV assembles with the alpha subunit. An inverse chimera consisting of the cytoplasmic and transmembrane domains of the beta subunit fused with the extracellular domain of dipeptidyl peptidase IV does not assemble with the alpha subunit. The assembly data from these chimeras demonstrate that the extracellular domain of the beta subunit is both necessary and sufficient for assembly with the alpha subunit. Deletions of up to 146 extracellular amino acids from the carboxyl terminus of the beta subunit appear to result in misfolding of the subunit, but do allow reduced assembly with the alpha subunit. Together, the assembly data from chimeras and carboxyl-terminal deletions have identified a 96-residue extracellular domain which contains sequences involved in subunit assembly. While the chimeric subunits properly localize to the plasma membrane, deletion of as few as 4 amino acids from the carboxyl terminus impairs the ability of the beta subunit to be transported to the plasma membrane.

Repeat modules and N-linked glycans define structure and antigenicity of a critical enterotoxigenic E. coli adhesin

Enterotoxigenic Escherichia coli (ETEC) cause hundreds of millions of cases of infectious diarrhea annually, predominantly in children from low-middle income regions. Notably, in children, as well as human volunteers challenged with ETEC, diarrheal severity is significantly increased severity in blood group A (bgA) individuals. EtpA, is a secreted glycoprotein adhesin that functions as a blood group A lectin to promote critical interactions between ETEC and blood group A glycans on intestinal epithelia for effective bacterial adhesion and toxin delivery. EtpA is highly immunogenic resulting in robust antibody responses following natural infection and experimental challenge of human volunteers with ETEC. To understand how EtpA directs ETEC-blood group A interactions and stimulates adaptive immunity, we mutated EtpA, mapped its glycosylation by mass-spectrometry (MS), isolated polyclonal (pAbs) and monoclonal antibodies (mAbs) from vaccinated mice and ETEC-infected human volunteers, and determined structures of antibody-EtpA complexes by cryo-electron microscopy. Both bgA and mAbs that inhibited EtpA-bgA interactions and ETEC adhesion, bound to the C-terminal repeat domain highlighting this region as crucial for ETEC pathogen-host interaction. MS analysis uncovered extensive and heterogeneous N-linked glycosylation of EtpA and cryo-EM structures revealed that mAbs directly engage these unique glycan containing epitopes. Finally, electron microscopy-based polyclonal epitope mapping revealed antibodies targeting numerous distinct epitopes on N and C-terminal domains, suggesting that EtpA vaccination generates responses against neutralizing and decoy regions of the molecule. Collectively, we anticipate that these data will inform our general understanding of pathogen-host glycan interactions and adaptive immunity relevant to rational vaccine subunit design.