Circadian rhythms in Neurospora crassa: a mutation affecting temperature compensation.

The circadian rhythm of conidiation (spore formation) in Neurospora crassa is known to be temperature compensated, that is, the period is only slightly affected by the incubation temperature. Thus, the Q10 (the relative rate enhancement corresponding to a 10 degrees C rise in temperature) of the rhythm of the bd csp strain from 14 to 30 degrees C was 1.1, whereas the Q10 of the uncompensated growth rate in the same interval was 2.4. A mutation at the cel locus resulted in loss of the temperature-compensation property in cultures grown below 22 degrees C. The Q10 of the rhythm below 22 degrees C was 2.2, and periods of about 40 hr were observed. In contrast, the Q10 of the rhythm above 22 degrees C was 1.1, with circadian periods of 18-21 hr. Thus, cel displayed a threshold temperature or "break point" for the temperature compensation of its rhythm. Supplementation of cel strains, which require fatty acids, with unsaturated or short-chain fatty acids raised the threshold temperature to 26 degrees C, whereas supplementation with long-chain saturated fatty acids lowered it to 18 degrees C. These data suggest a role for fatty acids, as liquid components or as cellular metabolites, in the mechanism of temperature compensation.     

Circadian rhythms in Neurospora crassa: lipid deficiencies restore robust rhythmicity to null frequency and white-collar mutants

The conidiation rhythm in the fungus Neurospora crassa is a model system for investigating the genetics of circadian clocks. Null mutants at the frq (frequency) locus (frq(9) and frq(10)) make no functional frq gene products and are arrhythmic under standard conditions. The white-collar strains (wc-1 and wc-2) are insensitive to most effects of light, and are also arrhythmic. All three genes are proposed to be central components of the circadian oscillator. We have been investigating two mutants, cel (chain-elongation) and chol-1 (choline-requirer), which are defective in lipid synthesis and affect the period and temperature compensation of the rhythm. We have constructed the double mutant strains chol-1 frq(9), chol-1 frq(10), chol-1 wc-1, chol-1 wc-2, cel frq(9), cel frq(10), and cel wc-2. We find that these double mutant strains are robustly rhythmic when assayed under lipid-deficient conditions, indicating that free-running rhythmicity does not require the frq, wc-1, or wc-2 gene products. The rhythms in the double mutant strains are similar to the cel and chol-1 parents, except that they are less sensitive to light. This suggests that the frq, wc-1, and wc-2 gene products may be components of a pathway that normally supplies input to a core oscillator to transduce light signals and sustain rhythmicity. This pathway can be bypassed when lipid metabolism is altered.     

Circadian rhythms in dynamic electrocardiography

A group of ten healthy subjects aged 32-58 years was studied by dynamic electrocardiography. An electrocardiogram (ECG) was recorded on CM 5 lead for 14 seconds every 15 minutes during the 24 hours. All the subjects were under a similar nyctohemeral schedule, sleeping between 23.30 and 06.45, and all followed their spontaneous diet without any restriction. "Mean Cosinor" analysis of the data revealed statistically significant circadian rhythms of heart rate (HR), R and T-wave voltages, duration of the QT interval and ST segment displacement, but not of the duration of the corrected QT (QTc).     

Circadian rhythms in cardiovascular disease

Circadian rhythms have long been recognized to occur in many biologic phenomena, including secretion of hormones and activities of the autonomic nervous system. More recently, circadian rhythms have also been noted to occur in the incidences of certain cardiac and cerebrovascular events, including transient myocardial ischemia, myocardial infarction, sudden cardiac death, and stroke. The pathophysiology and the mechanisms underlying these variations are the focus of much investigation. The effects of different drug treatments on these circadian rhythms are also being studied. This information should enable better treatment strategies to be planned for patients who have either silent or symptomatic episodes of transient myocardial ischemia and potentially to prevent the occurrence of sudden, catastrophic cardiac events.     

Circadian rhythms in the fetus

Throughout gestation, the close relationship between mothers and their progeny ensures adequate development and a successful transition to postnatal life. By living inside the maternal compartment, the fetus is inevitably exposed to rhythms of the maternal internal milieu such as temperature; rhythms originated by maternal food intake and maternal melatonin, one of the few maternal hormones that cross the placenta unaltered. The fetus, immature by adult standards, is however perfectly fit to accomplish the dual functions of living in the uterine environment and developing the necessary tools to “mature” for the next step, i.e. to be a competent newborn. In the fetal physiological context, organ function differs from the same organ’s function in the newborn and adult. This may also extend to the developing circadian system. The information reviewed here suggests that the fetal circadian system is organized differently from that of the adult. Moreover, the fetal circadian rhythm is not just present simply as the initial immature expression of a mechanism that has function in the postnatal animal only. We propose that the fetal suprachiasmatic nucleus (SCN) of the hypothalamus and fetal organs are peripheral maternal circadian oscillators, entrained by different maternal signals. Conceptually, the arrangement produces internal temporal order during fetal life, inside the maternal compartment. Following birth, it will allow for postnatal integration of the scattered fetal circadian clocks into an adult-like circadian system commanded by the SCN.     

Circadian rhythms in cardiovascular diseases--arrhythmias

Circadian patterns have been observed for variety of cardiovascular disorders, including cardiac arrhythmias, sudden cardiac death, cerebrovascular events, episodes of stable angina, unstable angina and acute myocardial infarction. The morning predominance of these events has been well documented in a number of large population studies. This fact provides the stimulus for better understanding our own chronobiology and the periodicity of circadian rhythms which contribute to this predominance of adverse events in the morning hours. We now understand that a number of important physiological parameters such as heart rate, blood pressure, vascular reactivity, cardiac contractility, and various hemostatic factors all demonstrate a circadian pattern similar to that described for cardiovascular disorders. Several recent studies have also emphasize the importance of changes in posture, time of awakening, physical activity, mental stress as potential triggers. Circadian patterns have been observed for several supraventricular arrhythmias, premature ventricular beats, ventricular tachycardias including sudden cardiac death. The chronobiological therapeutic systems can help to prevent life threatening events.     

Circadian rhythms and fertility

Circadian rhythms impact on a wide range of physiological systems and this impact extends to fertility, such that disruptions to timing systems can impact upon reproductive capacity. This is highlighted most obviously in mutant mouse models whereby deletion or mutation of single genes results not only in disrupted circadian rhythmicity, but also compromised male and female reproductive function. In this review, we discuss the presence of circadian clocks in female and male reproductive tissues and the role these clocks play in the generation of oestrus cycles, ovulation, sperm generation, implantation and the maintenance of pregnancy. Given the increased incidence of shiftwork and international travel which disrupt circadian rhythmicity, and the increasing prevalence of reproductive technologies whereby early embryo development occurs without external time cues, it is important for us to consider the role of circadian rhythms in fertility.     

Circadian rhythms and arthritis

The clinical symptoms of rheumatoid arthritis (RA) show a circadian variation; joint stiffness and pain are more prominent in the early morning. An altered functioning of the hypothalamic-pituitary-adrenal axis (cortisol) and of the pineal gland (melatonin) seems to be important factors in the perpetuation and clinical circadian symptoms of RA. Consistently, human proinflammatory cytokine production exhibits a diurnal rhythmicity with peak levels during the night and early morning, at a time when plasma cortisol (anti-inflammatory) is lowest and melatonin (proinflammatory) is highest. Sex hormones also seem to be involved in circadian rhythms of RA symptoms. Increased pain intensity and sleep disturbances are observed during the luteal phase in patients who have RA, when estrogen (and progesterone) levels would be higher than in the follicular phase. The occurrence of circadian rhythms of the inflammatory reaction suggest important implications for scheduling activities of daily living, for measurements in clinical trials, and possibly for the time at which antirheumatic drugs--including corticosteroids and nonsteroidal anti-inflammatory drugs--are administered.     

Repressible cation-phosphate symporters in Neurospora crassa.

The filamentous fungus Neurospora crassa possesses two nonhomologous high-affinity phosphate permeases, PHO-4 and PHO-5. We have isolated separate null mutants of these permeases, allowing us to study the remaining active transporter in vivo in terms of phosphate uptake and sensitivity to inhibitors. The specificity for the cotransported cation differs for PHO-4 and PHO-5, suggesting that these permeases employ different mechanisms for phosphate translocation. Phosphate uptake by PHO-4 is stimulated 85-fold by the addition of Na+, which supports the idea that PHO-4 is a Na(+)-phosphate symporter. PHO-5 is unaffected by Na+ concentration but is much more sensitive to elevated pH than is PHO-4. Presumably, PHO-5 is a H(+)-phosphate symporter. Na(+)-coupled symport is usually associated with animal cells. The finding of such a system in a filamentous fungus is in harmony with the idea that the fungal and animal kingdoms are more closely related to each other than either is to the plant kingdom.     

Circadian rhythms and the liver

This narrative review briefly describes the mammalian circadian timing system, the specific features of the liver clock, also by comparison with other peripheral clocks, the role of the liver clock in the preparation of food intake, and its relationship with energy metabolism. It then goes on to provide a chronobiological perspective of the pathophysiology and management of several types of liver disease, with a particular focus on metabolic-associated fatty liver disease (MAFLD), decompensated cirrhosis and liver transplantation. Finally, it provides some insight into the potential contribution of circadian principles and circadian hygiene practices in preventing MAFLD, improving the prognosis of advanced liver disease and modulating liver transplantation outcomes.     

Circadian rhythms in cardiovascular diseases--ischemic heart disease

Several key advances in understanding of pathophysiology now provide the opportunity to develop improved treatment and prevention strategies. First, the importance o mechanism of plaque rupture and thrombosis in onset of myocardial infarction. Second, there has been demonstrated, that plaques that lead to acute occlusion often have only a mild degree of stenosis. A third advance that has stimulated the field has been the recognition that time of onset of cardiac events is not random but instead shows a circadian pattern of onset. In the decade since the 1985 observation by Muller a spol. that the frequency of onset of myocardial infarction peaks at 9 a.m., numerous publications have supported this observation not only for myocardial infarction, but also for sudden cardiac death, transient myocardial ischemia, and stroke. Refinement of these epidemiological observations has led, first, to the conclusion that the morning peak in disease onset is due in part to the physical and mental stressors associated with morning awakening and activity and, second, that stressors such as heavy physical activity and anger can trigger acute cardiovascular events.     

Interconvertible Forms of Glycogen Phosphorylase in Neurospora crassa

Glycogen phosphorylase in extracts of N. crassa mycelia has two interconvertible forms: one active (a form) and the other inactive (b form) in the absence of 5'-AMP. The conversion of the b to the a form requires ATP-Mg(2+) and proceeds at higher rate in the presence of 3',5'-cyclic AMP.