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T W White

Publications and source records attributed to T W White.

At least 37 records · Page 2Linked to original sources
PubMed1998

Targeted ablation of connexin50 in mice results in microphthalmia and zonular pulverulent cataracts.

In the ocular lens, gap junctional communication is a key component of homeostatic mechanisms preventing cataract formation. Gap junctions in rodent lens fibers contain two known intercellular channel-forming proteins, connexin50 (Cx50) and Cx46. Since targeted ablation of Cx46 has been shown to cause senile-type nuclear opacities, it appears that Cx50 alone cannot meet homeostatic requirements. To determine if lens pathology arises from a reduction in levels of communication or the loss of a connexin-specific function, we have generated mice with a targeted deletion of the Cx50 gene. Cx50-null mice exhibited microphthalmia and nuclear cataracts. At postnatal day 14 (P14), Cx50-knockout eyes weighed 32% less than controls, whereas lens mass was reduced by 46%. Cx50-knockout lenses also developed zonular pulverulent cataracts, and lens abnormalities were detected by P7. Deletion of Cx50 did not alter the amounts or distributions of Cx46 or Cx43, a component of lens epithelial junctions. In addition, intercellular passage of tracers revealed the persistence of communication between all cell types in the Cx50-knockout lens. These results demonstrate that Cx50 is required not only for maintenance of lens transparency but also for normal eye growth. Furthermore, these data indicate that unique functional properties of both Cx46 and Cx50 are required for proper lens development.

Animals↗
PubMed1998

Cloning and expression of two related connexins from the perch retina define a distinct subgroup of the connexin family.

We have cloned cDNAs for two closely related connexins (Cx), Cx35 and Cx34.7, from a perch retinal cDNA library. Sequencing of PCR products from genomic DNA revealed that both connexins have an intron 71 bp after the translation initiation site; in Cx35, the intron is 900 bp in length, whereas in Cx34.7 it is approximately 20 kb. Southern blots of genomic DNA suggest that the two connexins represent independent single copy genes. In Northern blots, Cx35 and Cx34.7 transcripts were detected in retina and brain; Cx34.7 also showed a weak signal in smooth muscle (gut) RNA. Antibodies against Cx35 labeled a 30 kDa band on a Western blot of retinal membranes, and in histological sections, the pattern of antibody recognition was consistent with labeling of bipolar cells and unidentified processes in the inner plexiform and nerve fiber layers. When expressed in Xenopus oocytes, Cx35 and Cx34.7 formed homotypic gap junctions, but the junctional conductance between paired oocytes expressing Cx35 was 10-fold greater than that recorded for gap junctional channels formed by Cx34.7. The homotypic gap-junctional channels were closed in a voltage-dependent manner but with relatively weak voltage sensitivity. Heterotypic gap junctions formed by Cx35 and Cx34.7 displayed junctional conductances similar to those of Cx34.7 homotypic pairs and showed a slightly asymmetric current-voltage relationship; the side expressing Cx35 exhibited a higher sensitivity to transjunctional potentials. An analysis of the sequence and gene structure of the connexin family revealed that perch Cx35 and Cx34.7, skate Cx35, and mouse Cx36 constitute a novel gamma subgroup.

Animals↗
PubMed1998

Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma.

Mutations in GJB2 encoding the gap junction protein connexin-26 (Cx26) have been established as the basis of autosomal recessive non-syndromic hearing loss. The involvement of GJB2 in autosomal dominant deafness has also been proposed, although the putative mutation identified in one family with both deafness and palmoplantar keratoderma has recently been suggested to be merely a non-disease associated polymorphism. We have observed a similar phenotype in an Egyptian family that segregated with a heterozygous missense mutation of GJB2, leading to a non-conservative amino acid substitution (R75W). The deleterious dominant-negative effect of R75W on gap channel function was subsequently demonstrated in the paired oocyte expression system. Not only was R75W alone incapable of inducing electrical conductance between adjacent cells, but it almost completely suppressed the activity of co-expressed wildtype protein. The Cx26 mutant W77R, which has been implicated in autosomal recessive deafness, also failed to form functional gap channels by itself but did not significantly interfere with the function of wildtype Cx26. These data provide compelling evidence for the serious functional consequences of Cx26 mutations in dominant and recessive deafness.

Amino Acid Sequence↗
PubMed1998

Influence of intravenous L-carnitine administration in sheep preceding an oral urea drench.

Two experiments were conducted to investigate the effect of i.v. administration of L-carnitine on selected metabolites in sheep and to determine the feasibility of using L-carnitine to ameliorate the deleterious effects of hyperammonemia in sheep. In Exp. 1, i.v. L-carnitine solutions were administered at three levels in a replicated Latin square: 0 (CONT), 6.36 (CAR 1), and 12.72 (CAR 2) mmol L-carnitine/kg x (75) BW using Suffolk ewes (n = 6; average BW 75+/-3 kg). Plasma L-carnitine concentration was increased (P<.05) by treatment (51.9 vs 102.3, and 96.4 micromol/L in CONT, CAR 1, and CAR 2, respectively). Plasma glucose concentration was elevated (P<.05) in CAR 2 and CAR 1. Plasma NEFA concentration was highest (P<.05) in CAR 2. Area under the response curve for glucose was greater (P<.02) in CAR 2. In Exp. 2, Suffolk ewes (n = 16; average BW 48+/-2 kg) were used in a randomized complete block design with a 2x2 factorial treatment arrangement to determine the effects of i.v. L-carnitine administration during an oral urea load test (OULT). L-Carnitine (0 and 6.36 mmol/kg x (75) BW) was administered i.v. at 30 min, and an oral urea drench (50% wt/vol; 0 and 300 mg/kg BW) was administered at 60 min. Plasma L-carnitine was increased (P<.0001) by i.v. L-carnitine. Plasma ammonia N was highest (P<.0001) in the UREA treatment compared with the CONT, CARN, and CARN + UREA treatments (148 vs 95, 101, and 108 micromol/L, respectively). Intravenous L-carnitine administration influenced plasma glucose and NEFA concentrations in sheep and, when administered 30 min preceding an OULT, prevented the development of subclinical hyperammonemia in sheep.

Administration, Oral↗
PubMed1998

Effects of fish meal and sodium bentonite on daily gain, wool growth, carcass characteristics, and ruminal and blood characteristics of lambs fed concentrate diets.

We evaluated the effects of replacing some soybean meal (SBM) protein with fish meal (FM) protein in diets adequate and slightly deficient in CP, with or without .75% sodium bentonite (NaB) on performance and ruminal and blood metabolites of individually fed Suffolk lambs. Diets were based on corn, SBM, and cottonseed hulls. In Exp. 1, five lambs were assigned to each of the three dietary treatments (11% CP with 3% FM, 13% CP with 0 or 3% FM). Lambs fed diets that contained 11% CP with 3% FM or 13% CP with 0% FM had similar DMI and ADG. Gain and feed efficiency were slightly improved (P = .18) by the 13% CP diet with 3% FM. In Exp. 2, 32 lambs were assigned to four dietary treatments (13.5% CP of DM) in a 2 x 2 factorial arrangement (0 or 3% FM, and 0 or .75% NaB on an as-fed basis). The DMI and ADG were increased (P < .05) by FM and NaB supplementation. Interactions (P < .05) revealed that NaB increased DMI, ADG, gain per feed (g/kg of DMI), and plasma urea N concentration in the absence of FM but not in the presence of FM in the diet. Neither FM nor NaB influenced (P = .25) wool growth. Total ruminal VFA were increased (P < .06) by FM and NaB. Differences in mineral content of phalanx bone, liver, and kidney were small and may be related to the mineral content of diets and the effect of NaB on mineral solubilities. Similar DMI and ADG of lambs fed FM and NaB separately and in combination suggest that their beneficial effect is not additive.

Animal Feed↗
PubMed1996

Connections with connexins: the molecular basis of direct intercellular signaling.

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Amino Acid Sequence↗
PubMed1996

The cellular Internet: on-line with connexins.

Most cells communicate with their immediate neighbors through the exchange of cytosolic molecules such as ions, second messengers and small metabolites. This activity is made possible by clusters of intercellular channels called gap junctions, which connect adjacent cells. In terms of molecular architecture, intercellular channels consist of two channels, called connexons, which interact to span the plasma membranes of two adjacent cells and directly join the cytoplasm of one cell to another. Connexons are made of structural proteins named connexins, which compose a multigene family. Connexin channels participate in the regulation of signaling between developing and differentiated cell types, and recently there have been some unexpected findings. First, unique ionic- and size-selectivities are determined by each connexin; second, the establishment of intercellular communication is defined by the expression of compatible connexins; third, the discovery of connexin mutations associated with human diseases and the study of knockout mice have illustrated the vital role of cell-cell communication in a diverse array of tissue functions.

Amino Acid Sequence↗
PubMed1996

Multiple connexin proteins in single intercellular channels: connexin compatibility and functional consequences.

In vertebrates, the protein subunits of intercellular channels found in gap junctions are encoded by a family of genes called connexins. These channels span two plasma membranes and result from the association of two half channels, or connexons, which are hexameric assemblies of connexins. Physiological analysis of channel formation and gating has revealed unique patterns of connexin-connexin interaction, and uncovered novel functional characteristics of channels containing more than one type of connexin protein. Structure-function studies have further demonstrated that unique domains within connexins participate in the regulation of different functional properties of intercellular channels. Thus, gap junctional channels can contain more than one connexin, and this structural heterogeneity has functional consequences in vitro. Moreover, emerging evidence for the existence of intercellular channels containing multiple connexins in native tissues suggests that the functional diversity generated by connexin-connexin interaction could contribute to complex communication patterns that have been observed in vivo.

Animals↗
PubMed1996

Dominant inhibition of intercellular communication by two chimeric connexins.

1. The physiological significance of communication through gap junction channels has been difficult to assess because channel activity cannot be experimentally modulated in a specific manner. To address this problem we have constructed chimeric connexins that function as dominant-negative inhibitors of intercellular channel activity.

Animals↗
PubMed1996

Seasonal effects of supplemental fat or undegradable protein on the growth and metabolism of Holstein calves.

One summer (heat stress) and one winter (neutral conditions) trial were conducted to assess the effects of varied amounts of RUP and fat on the growth and metabolic responses of calves. In each trial, 190-kg Holstein calves (n = 45) were fed a basal diet of corn and cottonseed hulls supplemented with soybean meal (control) or supplemented with raw or roasted whole soybeans or the basal ration plus prilled hydrogenated tallow either with or without a mixture of feather meal and blood meal. Prilled tallow was more detrimental to fiber digestion and growth performance than were whole soybeans, especially during summer. During winter, calves fed a high (49%) percentage of the dietary CP as RUP had similar BW gain and efficiency of feed usage as did calves fed a moderate percentage (39%), but, during summer, calves fed the high percentage of RUP had greater BW gain and efficiency than did calves fed moderate percentages of RUP. Ruminal concentrations of VFA were parallel to digestibilities of fiber in the total tract. Ruminal concentrations of ammonia or blood concentrations of urea were not consistent with the concentrations of RDP fed. Concentrations of glucose, NEFA, insulin, and thyroid hormones in the blood were primarily affected by season. Data suggested that a higher percentage and AA quality of RUP improved the growth of calves during summer but not during winter.

Ammonia↗
PubMed1995

Intercellular channels in teleosts: functional characterization of two connexins from Atlantic croaker.

Gap junction channels, composed of protein subunits termed connexins, are believed to play a critical role in the process of oocyte differentiation and maturation. We have used the paired Xenopus oocyte assay to characterize functionally two connexin genes, connexin-32.2 and connexin-32.7, recently cloned from the ovary of the Atlantic croaker (Micropogonia undulatus), a species that has emerged as a useful model to study the process of maturation of the ovarian follicle. We have found that, while both connexin proteins were expressed at comparable levels in Xenopus oocytes, only one, connexin-32.2, was functionally competent to induce the formation of intercellular channels. Connexin-32.2 channels exhibited voltage-dependent closure that was similar to, but distinct from that of previously characterized mammalian connexins. In addition, the silent connexin-32.7 was unable to functionally interact with connexin-32.2, either in heterotypic channels or as dominant negative inhibitor. Because connexin-32.2 expression is strikingly regulated during oocyte maturation, these data provide further evidence for a role of intercellular channels in the control of oocyte-follicular cell interactions.

Animals↗
PubMed1995

Changes in connexin expression and distribution during chick lens development.

Gap junctions are composed of the connexins, a family of proteins which have been shown to be the structural and functional building blocks of gap junctional intercellular channels. In the chick lens, three members of the connexin (Cx) family have been characterized: Cx 43 has been shown to be a component of interepithelial cell gap junctions, and Cx 45.6 and Cx 56 have been shown to be part of fiber-to-fiber junctions. Early in chick development, gap junctional communication between differentiating lens fibers in the chick loses its sensitivity to blockade by elevated carbon dioxide (CO2). The lens epithelial cells, however, remain CO2-sensitive throughout development. A possible explanation for the change in CO2 sensitivity is that the lens fibers express differentiation-specific connexins with different physiological properties. Using specific antibodies for each of the three lens connexins, we show here that the lens fibers began to express Cx 45.6, and Cx 56 at the developmental stage when lens fibers acquired CO2-insensitivity. However, electron microscopic immunocytochemistry revealed that both Cx 45.6 and Cx 56 were found in interepithelial cell gap junctions, in addition to Cx 43. Conductances between paired Xenopus oocytes injected with Cx 43, Cx 45.6 and Cx 56 mRNAs revealed that all three connexins were CO2-sensitive in this expression system. Taken together, these data ruled out the possibility that the change in CO2 sensitivity observed in vivo could be explained on the basis of the distribution of connexins alone; other parameters of cellular context, such as post-translational processing, must be involved in the observed developmental changes in physiology.

Animals↗
PubMed1995

Functional analysis of selective interactions among rodent connexins.

One consequence of the diversity in gap junction structural proteins is that cells expressing different connexins may come into contact and form intercellular channels that are mixed in connexin content. We have systematically examined the ability of adjacent cells expressing different connexins to communicate, and found that all connexins exhibit specificity in their interactions. Two extreme examples of selectivity were observed. Connexin40 (Cx40) was highly restricted in its ability to make heterotypic channels, functionally interacting with Cx37, but failing to do so when paired with Cx26, Cx32, Cx43, Cx46, and Cx50. In contrast, Cx46 interacted well with all connexins tested except Cx40. To explore the molecular basis of connexin compatibility and voltage gating, we utilized a chimera consisting of Cx32 from the N-terminus to the second transmembrane domain, fused to Cx43 from the middle cytoplasmic loop to the C-terminus. The chimeric connexin behaved like Cx43 with regard to selectivity and like Cx32 with regard to voltage dependence. Taken together, these results demonstrate that the second but not the first extracellular domain affects compatibility, whereas voltage gating is strongly influenced by sequences between the N-terminus and the second transmembrane domain.

Animals↗
PubMed1994

Characterization of a cell surface glycoprotein associated with maturation of rat spermatozoa.

The principal galactose oxidase/NaB[3H]4-labeled membrane protein of rat caudal epididymal spermatozoa was isolated by hydrophobic interaction chromatography. The protein is released from the membrane by the action of phosphatidylinositol specific phospholipase C, and thereby its properties are transformed from those of a protein anchored to the hydrophobic membrane to those of a hydrophilic solution protein. Because it is the only membrane-associated protein released by the enzyme which did not absorb to a propylaspartate resin, a simple, single step purification procedure was devised. Although the amino terminus of the protein is blocked to Edman degradation, the majority of the protein structure was determined from a series of tryptic peptides and from limited acid hydrolysis. Approximately 65% of the protein mass is carbohydrate which is primarily attached through O-glycosidic bonds to the 18 threonines. The molecular weight of the glycoprotein was estimated to be 16,600, considerably smaller than the M(r) = 26,000 to 37,000 previously determined by gel electrophoresis. The anomalous electrophoretic behavior is undoubtedly due to the large percentage of carbohydrate. The distribution of carbohydrate on the protein side chains suggests the protein may form a positively charged, specialized scaffolding for the presentation of the carbohydrate moieties. Because the appearance of the ability to label the protein with galactose oxidase is correlated with sperm maturation in the epididymis, the glycoprotein structures may be an important component in the fertilization process. The combination of linkage by glycosylphosphatidylinositol and low molecular weight mucin-like structure indicates this may be a member of a new class of membrane proteins.

Amino Acid Sequence↗
PubMed1994

Rat epididymis-specific sperm maturation antigens. I. Evidence that the 26 kD 4E9 antigen found on rat caudal epididymal sperm tail is derived from a protein secreted by the epididymis.

Monoclonal antibody 4E9, which was raised against a partially purified detergent extract of rat caudal epididymal sperm, recognizes the tail of sperm from the cauda, but not from caput epididymidis, as well as epithelial cells in a restricted region of the distal caput/corpus epididymidis and proteins in epididymal fluid from corpus and cauda epididymidis. The antigen is apparently a glycoprotein, since it is retained on a Ricinus communis agglutinin I lectin column. Epididymal fluid antigens have apparent M(rs) of 38-26 kD, whereas the membrane-associated form of the molecule has an M(r) of 26 kD. Immunocytochemical data and Western immunoblot data suggest that the membrane antigen is derived from the fluid antigen, which, in turn, is secreted by the epididymal epithelium. Characterization of the membrane antigen indicates that it is tightly associated with the sperm surface, behaving as though it is an integral membrane protein. The antigen persists on ejaculated sperm.

Animals↗