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N Chaudhari

Publications and source records attributed to N Chaudhari.

14 recordsLinked to original sources
PubMed1992

A single nucleotide deletion in the skeletal muscle-specific calcium channel transcript of muscular dysgenesis (mdg) mice.

The skeletal muscle-specific dihydropyridine-sensitive calcium channel is a critical component of excitation-contraction coupling in skeletal muscle. A recessive mutation in mice, muscular dysgenesis (mdg), has previously been described as resulting in defective excitation-contraction coupling. Although the channel-forming subunit (alpha 1) of the skeletal calcium channel is not detectable immunologically, specific mRNA of normal size is present in dysgenic muscle. cDNA for this calcium channel alpha 1 subunit has now been cloned from dysgenic muscle and sequenced in its entirety. A single nucleotide deletion occurs at nucleotide 4010 of the cDNA, resulting in a shift of the translational reading frame. The mutation has been confirmed by direct sequencing of PCR products from homozygous mutant and normal muscle. The mutant polypeptide is predicted to contain the first three repeating domains, five of the normal six transmembrane helices of the last repeating domain, and an altered and truncated C terminus. The mature mRNA encoding the dysgenic alpha 1 subunit appears to be labile. It is possible that premature termination of translation renders the mutant mRNA subject to degradation by nucleases. This work resolves a long-standing controversy on the nature of the primary genetic defect in muscular dysgenesis.

Amino Acid Sequence↗
PubMed1989

Restoration of normal function in genetically defective myotubes by spontaneous fusion with fibroblasts.

Muscular dysgenesis in mice is a genetic disease of skeletal muscle caused by the recessive mutation mdg. Muscle fibres in affected mice are paralysed because of the failure of excitation-contraction coupling. Unlike normal myotubes in primary culture, dysgenic myotubes do not contract, either spontaneously or in response to electrical stimulation. The deficiency results from mutation of the gene for the skeletal muscle dihydropyridine receptor, an essential sarcolemmal component both of excitation-contraction coupling and of the slow calcium-ion channel. It has recently been shown that the addition of fibroblasts from normal (but not dysgenic) mice to cultures of dysgenic myotubes can restore spontaneous contractions in a small fraction of these myotubes, but the mechanism for this 'rescue' was not determined. In principle, if fibroblast nuclei were able to incorporate into myotubes, such nuclei could then supply the missing muscle-specific gene product. We have now investigated this possibility using nuclear, cytoplasmic and plasmalemmal markers. We report that the rescue to contractile ability in genetically paralysed dysgenic muscle is mediated by the previously unrecognized ability of fibroblasts to fuse spontaneously with developing myotubes.

Animals↗
PubMed1989

Specific absence of the alpha 1 subunit of the dihydropyridine receptor in mice with muscular dysgenesis.

Muscular dysgenesis is a lethal mutation in mice that results in a complete absence of skeletal muscle contraction due to the failure of depolarization of the transverse tubular membrane to trigger calcium release from the sarcoplasmic reticulum. In order to determine whether the defect in muscular dysgenesis leads to a specific loss of one of the components of excitation-contraction coupling or to a generalized loss of all components of excitation-contraction coupling, we have analyzed skeletal muscle from control and dysgenic mice for the sarcoplasmic reticulum and transverse tubular proteins which are believe to function in excitation-contraction coupling. We report that the proteins involved in sarcoplasmic reticulum calcium transport, storage, and release [Ca2+ + Mg2+)-ATPase, calsequestrin, and calcium release channel) are present in dysgenic muscle. Also present in dysgenic muscle is the 175/150-kDa glycoprotein subunit (alpha 2) of the dihydropyridine receptor. However, the 170-kDa dihydropyridine binding subunit (alpha 1) of the dihydropyridine receptor is absent in dysgenic muscle. These results suggest that the specific absence of the alpha 1 subunit of the dihydropyridine receptor is responsible for the defects in muscular dysgenesis and that the alpha 1 subunit of the dihydropyridine receptor is essential for excitation-contraction coupling in skeletal muscle.

Animals↗
PubMed1989

The muscular dysgenesis mutation in mice leads to arrest of the genetic program for muscle differentiation.

Muscular dysgenesis (mdg) is a mutation in mice which causes the failure of excitation-contraction coupling in skeletal muscle. Although the sarcolemma, the sarcoplasmic reticulum, and the contractile apparatus all maintain nearly normal function, sarcolemmal depolarization fails to cause calcium release from the sarcoplasmic reticulum. Recently, the primary genetic defect in this mutation was shown to be located in the structural gene for the dihydropyridine receptor. We have examined the developmental expression from Fetal Day 15 onward, in normal and mutant muscle, of several unidentified genes as well as genes which are known markers of muscle differentiation. We find that the majority of mRNA sequences are found at similar concentrations in normal and dysgenic muscles at birth. Many differentiation-related genes also are expressed at normal levels early during myogenesis in mutant mice. However, as late fetal development progresses in dysgenic muscle, the mRNA concentrations for these genes fail to undergo the rapid rise which is characteristic of normal muscle. Several additional, unidentified genes, which normally would be down-regulated during development, remain expressed at a high level in dysgenic muscle. Thus, the primary absence of a functional dihydropyridine receptor appears to prevent the changes in gene expression which are necessary for maturation of skeletal muscle.

Actins↗
PubMed1988

Comparison of pulsatile subcutaneous gonadotropin-releasing hormone and exogenous gonadotropins in the treatment of men with isolated hypogonadotropic hypogonadism.

Eight men with isolated hypogonadotropic hypogonadism were treated with pulsatile gonadotropin-releasing hormone (GnRH) after maximal testicular growth and function had already been achieved with human chorionic gonadotropin (hCG) and human menopausal gonadotropin (hMG). Only four subjects could normalize plasma testosterone (T) levels (group A). After 18 months of GnRH therapy, testicular size of group A increased by 53% (P less than 0.01) over that previously attained with exogenous gonadotropins. However, despite further testicular growth, two men who were previously azoospermic on hCG/hMG remained so on GnRH. In the other two patients, total sperm count increased minimally. Thus, pulsatile gonadotropin levels achieved with GnRH are more effective in stimulating testicular growth, but not necessarily sperm output, than are stable gonadotropin concentrations obtained with hCG/hMG.

Adult↗
PubMed1985

Brain "identifier sequence" is not restricted to brain: similar abundance in nuclear RNA of other organs.

A repeated 82 base pair sequence in genomic DNA of the rat was previously proposed as being a control element governing brain (neuron) specific genetic expression. This intronic sequence, termed the brain "identifier" (ID), is complementary to small RNA species localized in brain cytoplasm, and it was thought to be represented specifically in RNA produced by brain nuclei in vitro. The RNA blot analyses of total nuclear and polyadenylated heterogeneous nuclear RNA described in the present report show that this ID sequence is also present in the liver and kidney in abundances similar to those in the brain. This repeated sequence is not, therefore, restricted to transcripts produced in the brain as suggested from previous transcriptional "runoff" experiments. Measurements on rat and mouse nuclear RNA indicate that the abundance of ID sequence transcript is roughly proportional to the number of copies of this repeat in the respective genomes. This suggests a rather random genomic location and transcription of this sequence. From these results it seems improbable that the ID sequence functions as a transcriptional-level control element in genes expressed specifically in the brain.

Animals↗
PubMed1983

Genetic expression in the developing brain.

The adult mouse brain contains complex populations of polyadenylated [poly(A)+] and nonpolyadenylated [poly(A)-] messenger RNA's (mRNA's). These mRNA's are separate sequence populations, similar in complexity, and in combination are equivalent to approximately 150,000 different mRNA sequences, of average length. Essentially all of the "adult" poly(A)+ mRNA's are present in the brain at birth. In contrast, most of the poly(A)- mRNA's are absent. Brain poly(A)- mRNA's begin to appear soon after birth, but the full adult complement is not reached until young adulthood. This suggests that these poly(A)- mRNA's specify proteins required for the biological capabilities of the brain that emerge during the course of postnatal development.

Animals↗
PubMed1979

Characterization of long and short repetitive sequences in the sea urchin genome.

Long and short repetitive sequences were purified from the DNA of Paracentrotus lividus under conditions designed to optimize the yield of complete, end to end sequences. Double-stranded long repeat DNA prepared in this manner ranged in length from approximately 3000 to 15 000 nucleotide pairs with average sizes of approximately 6000 base pairs. In the electron microscope, long repeat DNA was observed to possess continuous sequences that often appeared to be terminated by one or more loops and/or fold backs. Long repeat DNA sequences, resheared to 300 base pairs, were found to have an average melting point identical to that for sheared native DNA. Thus, the reassociated duplexes of long repetitive DNA seem to possess very few mismatched base pairs. Reassociation kinetic analyses indicate that the majority of the long repeat sequences are reiterated only 4--7 times per haploid amount of DNA. Melt-reassociation analyses of short repetitive DNA, at several criteria, support the previously held concept that these sequences belong the sets or families of sequences which are inexact copies of one another. Our studies also support hypotheses suggesting that short repetitive sequences belong to families which may have arisen via distinct salttatory events. The relationships between long and short repetitive DNA sequences are considered with respect to widely held concepts of their sequence organization, evolution, and possible functions within eucaryotic genomes. A model for the possible organization of short repeats within long repetitive DNA sequences is also presented.

Animals↗
PubMed1979

The evolution of the long and short repetitive DNA sequences in sea urchins.

The rates of evolution of purified long and short repetitive DNA sequences were examined by hybridisation analysis between the DNAs from several species of sea urchins. We find that the rates of nucleotide substitution are very comparable within mutually retained sequences for the two classes of repetitive DNA. The loss of hybridisable sequences between species also occurs at similar rates among both the short and long repetitive DNA sequences. Between species that separated less than 50 million years ago, hybridisable short repetitive sequences are lost all through the spectrum of reiteration frequencies. The long repeats contain a few sequences which are highly conserved within all of the species examined, and which amount to approximately 1% of the total genome. The short repetitive class, on the other hand, does not seem to contain any such highly conserved elements. The long repetitive sequences internally appear to contain short 'units' of reiteration, which may comprise families within the long repetitive class. We find no evidence to indicate that the majority of long and short repetitive sequences evolve by different mechanisms or at different rates.

Animals↗
PubMed1979

Internal organization of long repetitive DNA sequences in sea urchin genomes.

In keeping with earlier reports, we have found that reassociated long repeat DNA from sea urchins is thermostable, indicating the absence of evolutionarily diverged families of repeated sequences. However, we found that when fragments of radiolabeled long repeat DNA were denatured and reassociated with intact long repeat driver DNA, then sheared to 350 basepairs and assayed for thermal stability, the level of mismatch found in the duplexes varied inversely with the length of the starting fragments. This effect was shown to be due directly to the physical size of the molecules involved in reassociation. These results are consistent with, and support a model for, long repeat DNA in which short units of repetition are arranged in precise arrays. The significance of this arrangement of sequence units within long repeat DNA is discussed.

Animals↗