Role of lipids in the Neurospora crassa membrane. I. Influence of fatty acid composition on membrane lipid phase transitions

Temperature compensation and membrane composition in Neurospora crassa

The link between temperature compensation of the circadian rhythm and temperature-induced adjustment of membrane composition in Neurospora crassa is briefly reviewed. In common with most organisms, Neurospora responds to changes in growth temperature by adjusting its lipid composition, primarily by increasing the degree of unsaturation of its fatty acids at low temperature. This may result in maintenance of either membrane fluidity or phase transition behavior over a range of temperatures. In Neurospora, there are three mutations (frq, cel, and chol-1) that affect temperature compensation of the circadian rhythm; cel and chol-1 are defective in lipid synthesis, and frq interacts with the other two in double-mutant strains. This suggests that lipid metabolism may play a role in temperature compensation of the rhythm, and that the FRQ gene product may also be involved in membrane function, either in regulating lipid composition or as a sensor responding to changes in lipid composition.

Temperature-induced changes in fatty acid composition of myelinated and non-myelinated axon phospholipids

The olfactory (non-myelinated) and trigeminal (myelinated) nerve axons of garfish show changes in phospholipid fatty acid composition when these fish are acclimated to temperatures ranging from 11 to 35 degrees C. Myelinated and non-myelinated nerve axons show similar changes in the percent saturated, percent 16-carbon, percent 18-carbon, and percent 20-carbon-and-greater unsaturated fatty acids. The observed changes in phospholipid fatty acid composition fit a linear regression model suggesting a gradual change in axonal phospholipid fatty acid composition with temperature. The temperature-induced changes in garfish nerve phospholipid fatty acid composition are consistent with the general observation of increased saturated fatty acid residues in plasma membrane phospholipids of organisms acclimated to higher environmental temperatures. The garfish data are similar to data previously obtained for goldfish tissues and Tetrahymena.

Role of lipids in the Neurospora crassa membrane: IV. Biochemical and electrophysiological changes caused by growth on phytanic acid

Neurospora crassa strain cel, which is deficient in fatty acid synthesis, was grown with phytanic acid supplementation. The temperature dependence of membrane potential is increased by growth on phytanic acid. A temperature change of 40 degrees C produces a change of 184 mV in phytanic acid-grown cells as compared to a 50 mV change for cel grown on palmitic acid or wild-type. Membrane resistance (measured as DC input resistance) of phytanic acid-grown cells did not differ from cel grown on palmitic acid or wild-type. Lipid analysis of cel grown on phytanic acid revealed approximately 7 mole percent phytanic acid incorporation into phospholipids, no change in phospholipid base composition, a reduction of ergosterol content from 80 to 30 percent, and the induction of beta sitosterol, a sterol not usually present in Neurospora. beta sitosterol accounted for approximately 60 percent of the sterol present. Incorporation of 7 mole percent phytanic acid into phospholipids lowers the phase transition temperature by approximately 5 degrees C, and decreases the heat content of the phase transition (delta H) slightly. Results are discussed in relation to Refsum's disease, a human neurological disorder associated with high plasma levels of phytanic acid. It is proposed that high intracellular phytanic acid concentration induces novel sterol synthesis and that the incorporation of the novel sterol into the membrane is responsible for the increased temperature sensitivity of membrane potential. The excitable membrane deficits observed in patients with Refsum's disease may be explained by such a mechanism.

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.

Effects of temperature acclimation on Neurospora phospholipids. Fatty acid desaturation appears to be a key element in modifying phospholipid fluid properties

Experiments were conducted on the effect of growth temperature on phospholipids of Neurospora. Strains grown at high (37 degrees C) and low (15 degrees C) temperatures show large differences in the proportions of phospholipid fatty acid alpha-linolenate (18 : 3) which can vary by 10-fold over this temperature range. Changes in the phospholipid base composition are less dramatic; the most significant is an increase in phosphatidylethanolamines at low temperatures accompanied by a concomitant decrease in phosphatidylcholine. It appears that phospholipid fatty acid desaturation is closely regulated with respect to growth temperature. Over the 37 to 15 degrees C growth temperature range there appear to be at least two desaturase systems in Neurospora which are under different controls. Production of 18 : 1 and 18 : 2 species appears to occur at high levels over the entire temperature range, whereas the production of 18 : 3 seems to be inversely related to growth temperature. Shifting 37 degrees C-acclimated cultures to 15 degrees C produces a growth lag period of approximately 3 h, during which the level of 18 : 3 increases markedly. Differential scanning calorimetry of phospholipids from 37 degrees C cells shows a phase transition at -22 degrees C while lipids from 15 degrees C cultures exhibit a phase transition with reduced enthalpy at about -41 degrees C. The data are consistent with the idea that phospholipid composition in Neurospora is under strict control and suggest that membrane fluidity is regulated with respect to growth temperature through changes in membrane lipid composition.

Correlation between fluidity and fatty acid composition of phospholipid species in Tetrahymena pyriformis during temperature acclimation

The correlation between the fluidity of phospholipids and their fatty acid composition was studied by spin label technique and gas-liquid chromatography for three major phospholipid species in Tetrahymena pyriformis during temperature acclimation. The fluidity of 2-aminoethylphosphonolipid increased within the first 10 h of the cold-acclimation when the content of gamma-linolenic acid in 2-aminoethylphosphonolipid was highest, and it then decreased up to 24 h. On the other hand, the fluidities of phosphatidylethanolamine and phosphatidylcholine showed a gradual decrease up to 24 h after the temperature shift, although gamma-linolenic acid contents were highest at 10 h after the temperature shift. Thus the fluidity changes of these two phospholipids were interpreted as resulting from the altered content of other fatty acids in addition to gamma-linolenic acid, since the gamma-linolenic acid content was smaller than that of 2-aminoethylphosphonolipid. The results suggest that the content of gamma-linolenic acid in 2-aminoethylphosphonolipid plays a role in regulating the thermal adaptation process.

Role of lipids in the Neurospora crassa membrane. II. Membrane potential and resistance studies; the effect of altered fatty acid composition on the electrical properties of the cell membrane

The effect of doubling the saturated fatty acid content on the electrophysiology of Neurospora crassa membranes was studied. Intracellular membrane input resistance (Rm) and potential (Em) were measured for wild-type (w/t) and cel- (Tween 40) organisms as a function of temperature. Over the 0 to 40 degrees C temperature range studied, mean Em values of both w/t and cel- (Tw 40) organisms increased from -160 to -210 mV. This difference is greater than that expected from Nernst potential considerations, indicating an active component of Em. This active component is insensitive to a doubling of the saturated fatty acid content. Rm exhibits a temperature dependence and hysteresis. Averaged data indicate an increase in Rm with decreased temperature. The slope of the temperature dependence varies among individual hyphae. Above 17.5 degrees C cel- (Tw 40) hyphae averaged greater than 70% higher values of Rm than w/t. Below 17.5 degrees C w/t Rm data divided into low and high temperature dependence groups, while cel- data exhibited a low temperature dependence. The results are discussed in relation to gel-liquid crystal phase transitions, membrane fluidity, and the contribution of fatty acid structure to membrane electrical properties.