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Title: PAQR-2 Regulates Fatty Acid Desaturation during Cold Adaptation in C. elegans}
Article:
Abstract:
C. elegans PAQR-2 is homologous to the insulin-sensitizing adiponectin receptors in mammals, and essential for adaptation to growth at 15°C, a low but usually acceptable temperature for this organism. By screening for novel paqr-2 suppressors, we identified mutations in genes involved in phosphatidylcholine synthesis( cept-1, pcyt-1 and sams-1) and fatty acid metabolism( ech-7, hacd-1, mdt-15, nhr-49 and sbp-1). We then show genetic evidence that paqr-2, phosphatidylcholines, sbp-1 and Δ9-desaturases form a cold adaptation pathway that regulates the increase in unsaturated fatty acids necessary to retain membrane fluidity at low temperatures. This model is supported by the observations that the paqr-2 suppressors normalize the levels of saturated fatty acids, and that low concentrations of detergents that increase membrane fluidity can rescue the paqr-2 mutant.
Introduction:
To function over a range of temperatures, poikilotherms must make adaptive adjustments to their physiology[1]–[4]. For example, adaptation to cold in Drosophila correlates with changes in the phospholipid composition of cellular membranes, favoring a higher abundance of shorter and/or unsaturated fatty acids to maintain membrane fluidity[5]–[8]. Another aspect of cold adaptation involves the activation of stress responses, including the inaptly named heat shock proteins[4],[9],[10]. On the whole however, little is known about the regulatory pathways, from receptors to effectors, that coordinate the well-documented physiological changes that poikilotherms make as they adapt to cold temperatures. The C. elegans paqr-2 mutant is unable to adapt to growth at 15°C, which is close to the lowest temperature at which this species can be propagated[11],[12]. paqr-2 encodes a protein with seven transmembrane domains homologous to the mammalian adiponectin receptors, AdipoR1 and AdipoR2, that are insuling-sensitizing metabolic regulators of which the signaling pathways are not well understood[1]–[3],[13]–[15]. Like its mammalian homologs, paqr-2 is important for metabolic regulation: the paqr-2 mutant shows an abnormal fatty acid( FA) composition, an excess of lipid droplets when combined with a paqr-1 mutation, and is synthetic lethal with loss-of-function( lof) mutations in genes that promote FA turnover such as sbp-1 and nhr-49[5],[12], which are C. elegans homologs of SREBP and HNF4[16]–[19]. One important limitation of our earlier work on paqr-2 is that we used a candidate gene approach, mostly inspired from previous studies in mice, to try and identify genes that may interact with paqr-2. While often successful, such an approach makes it impossible to discover unexpected interactions. Here, we used an unbiased forward genetic approach in which we screened randomly mutagenized worm populations to isolate and then characterize mutations that suppress the cold adaptation defect of the paqr-2 mutant. This is a classical investigative strategy for which C. elegans is eminently suited[20]. Using this approach, we isolated nine paqr-2 suppressors and establish a novel regulatory pathway connecting paqr-2, phosphatidylcholine( PC) synthesis and the regulation of FA desaturation to increase membrane fluidity during cold adaptation.
Discussion:
We have shown that mutations in either of two metabolic pathways can suppress the paqr-2 phenotypes: 1) mutations that reduce phosphatidylcholine synthesis; and 2) mutations that affect FA metabolism. Mutations in either pathway suppress the paqr-2 phenotypes by augmenting FA desaturation either directly( gof alleles of nhr-49 or mdt-15, or sbp-1 overexpression), or indirectly by reducing PC levels hence upregulating sbp-1( lof alleles of sams-1, pcyt-1 or cept-1), or by increasing the exposure of FA to Δ9 desaturases via increased release from TAGs( lof allele of aak-2) or inhibition of β-oxidation( lof alleles of ech-7 or hacd-1). Increasing the relative abundance of unsaturated FAs in biological membranes increases their fluidity[1]–[3], and this is likely the essential effect of paqr-2 during cold adaptation. Several evidences suggest that the pathway outlined in Figure 4 is indeed regulated by paqr-2, rather than being a parallel pathway important for cold adaptation. Firstly, all nine paqr-2 suppressors isolated in our non-biased screen were part of that pathway. Secondly, all nine suppressors also suppressed a very specific paqr-2 morphological phenotype that was not screened for, namely the withered tail tip. Thirdly, the convergence of the identified pathway onto Δ9-desaturases is consistent with their established importance during cold adaptation[5],[7]. Fourthly, both the cold adaptation and tail phenotypes of paqr-2 were corrected by the use of low concentrations of detergents expected to mimic the effects of increasing fatty acid desaturation on membrane fluidity. The present study therefore suggests that paqr-2 is a transmembrane protein essential for the regulation of Δ9 desaturases during cold adaptation via the modulation of PC levels, hence sbp-1. However, the direct point of interaction between PAQR-2 and its target pathway is not immediately apparent, but one interesting hypothesis is that PAQR-2 acts on FA metabolism by regulating PC abundance via an associated phospholipase activity. Indeed, PAQR-2 belongs to a large and diverse protein family of membrane hydrolases that includes Per1, a yeast GPI-phospholipase A2[9]. Others have previously correlated an increase in unsaturated FAs with adaptation to 15°C growth in C. elegans[11], and with cold adaptation in many other organisms[1]–[3]. A role for Δ9 desaturases for growth at low temperatures in C. elegans has also been documented previously, and it is particularly striking that fat-6;fat-7 double mutants exhibit a temperature sensitivity similar to the paqr-2 mutant, growing well at 20°C and poorly at 15°C[5]. Here we add a novel level of regulation by linking paqr-2, PCs, sbp-1 and Δ9 desaturases into a regulatory axis controlling the increase in unsaturated FAs during cold adaptation. At present we do not know whether regulating the activity of Δ9 desaturases is the only essential function of paqr-2 during cold adaptation. Our finding that paqr-2 dependent increases in FA unsaturation is an essential component of the adaptation to 15°C in C. elegans may seem to conflict with Murray et al. who previously estimated that FA unsaturation changes contribute only about 15% of the cold adaptation response[16]. However, it is important to keep in mind that Murray et al. were concerned with survival of C. elegans at the acutely lethal temperature of 0°C while we are concerned here with adaptive changes at a low but physiologically acceptable temperature. Could the paqr-2 homologs play a role during cold adaptation even in homeotherms? Increased serum adiponectin levels have been observed in rats kept at 4°C for 24 hours, with adiponectin mRNA levels becoming elevated in brown adipose tissue( BAT)[30]. Also, human subjects wearing a 10°C liquid-conditioned suit for two hours have shown a near-doubling of circulating adiponectin levels[31]. Furthermore, the AdipoRs are expressed in temperature-sensitive neurons in the brain[32], and adiponectin itself has sequence homology with hibernation-associated plasma proteins in Asian chipmunks and woodchucks[30],[33], which again suggests that it may be part of a protein family that functions during cold adaptation in mammals.
Cold-blooded organisms such as insects , fish or worms must make physiological adjustments when the temperature in their environment decreases . One essential adaptive measure is to increase the fluidity of the cellular membranes that are made of fatty molecules and would tend to harden at low temperatures , just as butter would . In our study we identify genes that are regulated by PAQR-2 , a membrane protein that we show to be essential for adjusting the membrane fluidity during cold adaptation in the nematode C . elegans . Interestingly , the genes influenced by PAQR-2 are all involved in fatty acid metabolism . We speculate that the human homologs of PAQR-2 , which are receptors for the hormone adiponectin , may have similar functions .