Does the circadian pacemaker act through cyclic AMP to drive the melatonin rhythm in chick pineal cells?

Cyclic AMP is a key regulator of melatonin production in the chick pineal gland. Agents that raise cyclic AMP levels (such as forskolin), or cyclic AMP analogues (such as 8-bromocyclic AMP), increase melatonin synthesis and release, whereas agents that lower cyclic AMP levels (including light) decrease melatonin synthesis and release. A circadian oscillator in these cells also raises and lowers melatonin output. We have been investigating the relationships between cyclic AMP and the circadian pacemaker in the regulation of melatonin production. In the chick pineal (unlike certain neuronal systems), the weight of the evidence indicates that cyclic AMP is not on an entrainment pathway to the circadian pacemaker. Instead, cyclic AMP appears to act downstream from the pacemaker. The pacemaker might itself act directly through cyclic AMP, regulating melatonin content by raising and lowering cyclic AMP levels. If this were the case, and if the effects of cyclic AMP levels on melatonin output are saturable (as they must be), then, in the face of such saturating levels of cyclic AMP, the pacemaker should no longer raise or lower melatonin output. To test this prediction, maximally effective concentrations of forskolin and 8-bromocyclic AMP were determined. Both agents markedly increased melatonin output. After 36 hr, cells were refractory to additional stimulation of melatonin output by addition of both agents together, or by higher concentrations of forskolin (although cyclic AMP levels could still be raised further). Nonetheless, the circadian pacemaker continued to raise and lower melatonin output: The rhythm persisted in the face of saturating levels of cyclic AMP. It is therefore suggested that the circadian pacemaker in chick pineal cells acts with, not through, cyclic AMP to regulate melatonin synthesis. Cyclic AMP and the pacemaker act synergistically to regulate serotonin N-acetyltransferase activity and the melatonin rhythm, with cyclic AMP mediating acute effects and amplitude regulation.     

Definitive localization of Becker muscular dystrophy in Xp by linkage to a cluster of DNA polymorphisms (DXS43 and DXS9)

Summary A study of linkage between Becker muscular dystrophy and four X chromosome-specific DNA polymorphisms in 17 kindreds has indicated that this gene is located in Xp, as already anticipated by single pedigree analysis. In particular the DXS43 and DXS9 loci, identified by probes D2 and RC8, respectively, are closely linked to each other and are both located at approximately 15 cM from the Becker locus. These linkage data, together with the previously established linkage between Becker and the DXS7 locus identified by probe L 1.28, indicate that the Becker gene is located in the same region where Duchenne has been mapped and also yield information about relative genetic distances among different DNA polymorphisms of the X chromosome.     

Different mosaicism frequencies for proximal and distal Duchenne muscular dystrophy (DMD) mutations indicate difference in etiology and recurrence risk.

In about 65% of the cases of Duchenne muscular dystrophy (DMD) a partial gene deletion or duplication in the dystrophin gene can be detected. These mutations are clustered at two hot spots: 30% at the hot spot in the proximal part of the gene and about 70% at a more distal hot spot. Unexpectedly we observed a higher frequency of proximal gene rearrangements among proved "germ line" mosaic cases. Of the 24 mosaic cases we are aware of, 19 (79%) have a proximal mutation, while only 5 (21%) have a distal mutation. This finding indicates that the mutations at the two hot spots in the dystrophin gene differ in origin. Independent support for the different mosaicism frequency was found by comparing the mutation spectra observed in isolated cases of DMD and familial cases of DMD. In a large two-center study of 473 patients from Brazil and the Netherlands, we detected a significant difference in the deletion distribution of isolated (proximal:distal ratio 1:3) and familial cases (ratio 1:1). We conclude from these data that proximal deletions most likely occur early in embryonic development, causing them to have a higher chance of becoming familial, while distal deletions occur later and have a higher chance of causing only isolated cases. Finally, our findings have important consequences for the calculation of recurrence-risk estimates according to the site of the deletion: a "proximal" new mutant has an increased recurrence risk of approximately 30%, and a "distal" new mutant has a decreased recurrence risk of approximately 4%.     

Dissimilarity between prostaglandin E1 and nitric oxide donors as potentiators of plasma exudation in the rabbit skin in vivo

The ability of prostaglandin E₁ (PGE₁) and nitric oxide (NO) donor compounds such as sodium nitroprusside (SNP), glyceryl trinitrate (GTN), and 3-morpholino-sydnonimine (SIN-1) to modulate the histamine- and bradykinin-induced increase in microvascular permeability have been investigated in rabbit skin. The effect of the NO synthesis inhibitor N^ω-nitro- -arginine methyl ester ( -NAME) on the plasma exudation induced by histamine and bradykinin was also studied. Local edema formation was evaluated using [¹²⁵I]human serum albumin. New Zealand white rabbits received an intravenous injection of [¹²⁵I]human albumin followed immediately by the intradermal injection of edematogenic agents into the shaved dorsolateral skin. PGE₁ (0.1 nmol/site) significantly potentiated both histamine- and bradykinin-induced edema. In contrast, SNP (0.4–400 nmol/site), SIN-1 (0.4–400 nmol/site), and GTN (0.4–40 nmol/site) did not affect the edematogenic response induced by either histamine or bradykinin. GTN (0.4–40 nmol/site) also had no effect on the increase in plasma exudation induced by histamine and bradykinin in the presence of PGE₁. -NAME (50–400 nmol/site), but not its enantiomer -NAME, dose-dependently reduced the edema formation induced by a combination of either histamine or bradykinin with PGE₁. This inhibition was significantly reversed by SNP (4–400 nmol/site) and by high doses (2.5 μmol/site) of -arginine (but not by -arginine). Our results thus demonstrate that PGE₁, but not nitrovasodilators, can actually potentiate histamine- and bradykinin-induced edema in rabbit skin. This discrepancy cannot be explained by the lack of vasodilator activity of the nitrovasodilators since these were able to reverse the -NAME-induced inhibition of the edema provoked by histamine. Rather, this difference most likely reflects the ability of PGE₁ to modulate vascular permeability by mechanism(s) other than an increase in arterial flow.     

Estimate of the intrafamilial correlation for serum creatine kinase and pyruvate kinase in females at risk for Duchenne and Becker muscular dystrophies.

In order to verify the possibility of nonrandom X-inactivation in females heterozygous for Duchenne (DMD) and Becker (BMD) muscular dystrophies, intrafamilial correlations and the heritabilities for serum creatine kinase (CK) and pyruvate kinase (PK) were estimated in a large sample of females belonging to families with affected patients. The results of the present investigation suggest that the apparent intrafamilial correlations for serum CK reported in previous studies in DMD families are not related with the presence of the DMD/BMD gene. Our data do not seem to support the hypothesis of a gene leading to a preferential inactivation of the X-chromosome in females at risk for the dystrophin gene.     

Effects of reproductive compensation and genetic drift on X-linked lethals

A revival of interest in Haldane's equilibrium theory for X-linked lethals has been stimulated by the introduction of accurate tests for the detection of female heterozygotes in Lesch-Nyhan disease. Application of these tests appears to indicate an excess of familial cases. This excess can be attributed to ascertainment bias, a difference in female and male mutation rates, genetic drift, and reproductive compensation. Reproductive compensation will be particularly effective in increasing the proportion of familial cases if (1) birth control is widespread; (2) selection against affected males acts in utero; (3) affected sons show symptoms at an early age; and (4) sons are more highly valued than daughters. We demonstrate how only a few generations of reproductive compensation are sufficient to achieve an approximate equilibrium between selection and mutation showing a high proportion of familial cases. We also discuss the random fluctuations around equilibrium caused by genetic drift.     

Red-green color vision impairment in Duchenne muscular dystrophy

The present study evaluated the color vision of 44 patients with Duchenne muscular dystrophy (DMD) (mean age 14.8 years; SD 4.9) who were submitted to a battery of four different color tests: Cambridge Colour Test (CCT), Neitz Anomaloscope, Ishihara, and American Optical Hardy-Rand-Rittler (AO H-R-R). Patients were divided into two groups according to the region of deletion in the dystrophin gene: upstream of exon 30 (n=12) and downstream of exon 30 (n=32). The control group was composed of 70 age-matched healthy male subjects with no ophthalmological complaints. Of the patients with DMD, 47% (21/44) had a red-green color vision defect in the CCT, confirmed by the Neitz Anomaloscope with statistical agreement (P<.001). The Ishihara and the AO H-R-R had a lower capacity to detect color defects--5% and 7%, respectively, with no statistical similarity between the results of these two tests nor between CCT and Anomaloscope results (P>.05). Of the patients with deletion downstream of exon 30, 66% had a red-green color defect. No color defect was found in the patients with deletion upstream of exon 30. A negative correlation between the color thresholds and age was found for the controls and patients with DMD, suggesting a nonprogressive color defect. The percentage (66%) of patients with a red-green defect was significantly higher than the expected <10% for the normal male population (P<.001). In contrast, patients with DMD with deletion upstream of exon 30 had normal color vision. This color defect might be partially explained by a retina impairment related to dystrophin isoform Dp260.     

Mutation in the Scyl1 gene encoding amino-terminal kinase-like protein causes a recessive form of spinocerebellar neurodegeneration

Here, we show that the murine neurodegenerative disease mdf (autosomal recessive mouse mutant 'muscle deficient') is caused by a loss-of-function mutation in Scyl1, disrupting the expression of N-terminal kinase-like protein, an evolutionarily conserved putative component of the nucleocytoplasmic transport machinery. Scyl1 is prominently expressed in neurons, and enriched at central nervous system synapses and neuromuscular junctions. We show that the pathology of mdf comprises cerebellar atrophy, Purkinje cell loss and optic nerve atrophy, and therefore defines a new animal model for neurodegenerative diseases with cerebellar involvement in humans.