Aminotransferase and the Production of Alanine During Hyperosmotic Stress in Paramecium calkinsi

When Paramecium calkinsi encounter hyperosmotic stress, intracellular free alanine increases. In vivo assays indicate that the reaction catalyzed by alanine aminotransferase contributes to the build up of alanine in response to hyperosmotic shock. ¹⁴C-pyruvate is converted to ¹⁴C-alanine in cells grown axenically at 200 mosm. When shifted to 600 mosm, the rate of conversion of pyruvate to alanine increases, and conversion at either 200 or 600 mosm is blocked by 1 mM aminooxyacetic acid (AOA), an inhibitor of aminotransferase. Intracellular free alanine increase is partially inhibited by AOA, and AOA prevents cells living in fresh water from acclimating to higher salinities, an indication that the increase in intracellular alanine is physiologically significant.     

Origin and Morphogenetic Movements of the Pores of the Contractile Vacuoles in Paramecium qurelia

The position of the contractile vacuoles in non-dividing Paramecium aurelia, as judged by that of the pores, is relatively constant both laterally and longitudinally, as is the distance between them. Some variability of the distance between the pores persists from the recent fission among the smaller animals of normal shape. At the onset of fission two new vacuoles appear, one anterior to each of the old vacuoles; all the vacuoles must undergo relocation because their mean positions now are somewhat removed from that in the non-dividing animals. The pores of the front daughter are too close together: the anterior pore is too far back, and the posterior is too far forward; likewise the pores of the hind daughter are too close together: the anterior too far forward as well as the posterior. The relocation does not begin until shortly before separation of the daughters and is practically complete by the time the daughters have assumed their normal length/width ratio after separation. A neuroneme connects the pore to one of the basal granules adjacent to its place of origin, now far removed.
Occasionally more than one new pore appears at the usual time and place during very early fission; these are very close together and usually serve a single vacuole. However, most of them fail to survive the relocation following this and the next fission, so they are found in the posterior position very rarely. But, in some individuals they may survive and serve a single or separate vacuoles. In a certain clone of Paramecium aurelia multiple pores and vacuoles are rather frequent; these are, of course, the result of the tendency to produce multiple pores and vacuoles at the time of fission.     

The response of contractile and non-contractile vacuoles of Paramecium calkinsi to widely varying salinities.

Paramecium calkinsi from tidal marshes survive a wide salinity range. Fluid output of contractile vacuoles of these cells decreased as salinity of the medium to which they were acclimated increased, and both pulse rate and vacuole volume were used to regulate output. When cells were first exposed to more dilute medium, contractile vacuoles greatly increased volume so that fluid output increased even though pulse rate decreased. In cells shifted to a more concentrated medium, contractile vacuole output decreased by decreasing pulse rate. The contractile vacuole is surrounded by a set of collecting structures which change form as the salinity changes. Distensible ampullae are found in media of low salinity and collecting canals are found in media of high salinity. When cells are shifted from high salinity to low, the number of ampullae increases and the number of canals decreases. When cells are shifted from low salinity to high, the number of ampullae decreases and the number of canals decreases. Other non-contracting vacuoles also appear in response to a hypoosmotic shock. These include vacuoles within the cell as well as "blisters" on the surface. The number and frequency of blisters increases with the size of the hypoosmotic shock. They detach from cells without resulting in any visible loss of cytoplasm. Non-contractile vacuoles may play a role in sequestering and removing excess water that the contractile vacuoles cannot handle.     

Host and symbiont intraspecific variability: The case of Paramecium calkinsi and “Candidatus Trichorickettsia mobilis”

Newly isolated strains of the ciliate Paramecium calkinsi and their cytoplasmic bacterial endosymbionts were characterized by a multidisciplinary approach, including live observation, ultrastructural investigation, and molecular analysis. Despite morphological resemblance, the characterized P. calkinsi strains showed a significant molecular divergence compared to conspecifics, possibly hinting for a cryptic speciation. The endosymbionts were clearly found to be affiliated to the species “Candidatus Trichorickettsia mobilis” (Rickettsiales, Rickettsiaceae), currently encompassing only bacteria retrieved in an obligate intracellular association with other ciliates. However, a relatively high degree of intraspecific divergence was observed as well, thus it was possible to split “Candidatus Trichorickettsia” into three subspecies, one of which represented so far only by the newly characterized endosymbionts of P. calkinsi. Other features distinguished the members of each different subspecies. In particular, the endosymbionts of P. calkinsi resided in the cytoplasm and possessed numerous peritrichous flagella, although no motility was evidenced, whereas their conspecifics in other hosts were either cytoplasmic and devoid of flagella, or macronuclear, displaying flagellar-driven motility. Moreover, contrarily to previously analyzed “Candidatus Trichorickettsia” hosts, infected P. calkinsi cells frequently became amicronucleate and demonstrated abnormal cell division, eventually leading to decline of the laboratory culture.     

Photosynthetic Response of Carrots to Varying Irradiances

Response to irradiance of leaf net photosynthetic rates (P _N) of four carrot cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) were examined in a controlled environment. Gas exchange measurements were conducted at photosynthetic active radiation (PAR) from 100 to 1 000 μmol m⁻² s⁻¹ at 20 °C and 350 μmol (CO₂) mol⁻¹(air). The values of P _N were fitted to a rectangular hyperbolic nonlinear regression model. P _N for all cultivars increased similarly with increasing PAR but Cascade and Oranza generally had higher P _N than CC. None of the cultivars reached saturation at 1 000 μmol m⁻² s⁻¹. The predicted P _N at saturation (P _Nmax) for Cascade, CC, Oranza, and RCC were 19.78, 16.40, 19.79, and 18.11 μmol (CO₂) m⁻² s⁻¹, respectively. The compensation irradiance (I _c) occurred at 54 μmol m⁻² s⁻¹ for Cascade, 36 μmol m⁻² s⁻¹ for CC, 45 μmol m⁻² s⁻¹ for Oranza, and 25 μmol m⁻² s⁻¹ for RCC. The quantum yield among the cultivars ranged between 0.057–0.033 mol(CO₂) mol⁻¹(PAR) and did not differ. Dark respiration varied from 2.66 μmol m⁻² s⁻¹ for Cascade to 0.85 μmol m⁻² s⁻¹ for RCC. As P _N increased with PAR, intercellular CO₂ decreased in a non-linear manner. Increasing PAR increased stomatal conductance and transpiration rate to a peak between 600 and 800 μmol m⁻² s⁻¹ followed by a steep decline resulting in sharp increases in water use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Functional Aqp1 Gene Product Localizes on The Contractile Vacuole Complex in Paramecium multimicronucleatum

In a ciliate Paramecium, the presence of water channels on the membrane of contractile vacuole has long been predicted by both morphological and physiological data, however, to date either the biochemical or the molecular biological data have not been provided. In the present study, to examine the presence of aquaporin in Paramecium, we carried out RT-PCR with degenerated primers designed based on the ParameciumDB, and an aquaporin cDNA (aquaporin 1, aqp1) with a full-length ORF encoding 251 amino acids was obtained from Paramecium multimicronucleatum by using RACE. The deduced amino acid sequence of AQP1 had NPA-NPG motifs, and the prediction of protein secondary structure by CNR5000 and hydropathy plot showed the presence of six putative transmembrane domains and five connecting loops. Phylogenetic analysis results showed that the amino acid sequence of AQP1 was close to that of the Super-aquaporin group. The AQP1-GFP fusion protein clearly demonstrated the subcellular localization of AQP1 on the contractile vacuole complex, except for the decorated spongiome membrane. The functional analyses of aqp1 were done by RNA interference-based gene silencing, using an established feeding method. The aqp1 was found to be crucial for the total fluid output of the cell, the function of contractile vacuole membranes.     

甲壳动物横纹肌的收缩蛋白质与调节机制

甲壳动物横纹肌肌原纤维的肌丝陈列,收缩蛋白质和收缩的Ca²⁺依赖性调节机制与脊椎动物横纹肌有不少差异.脊椎动物横纹肌、甲壳动物快肌与慢肌的粗丝与细丝的数量比依次为1:2,1:3和1:6,肌丝阵列各异.甲壳动物粗肌丝由肌球蛋白和副肌球蛋白组成,其分子装配与脊椎动物不同.细肌丝含有肌动蛋白、原肌球蛋白和肌钙蛋白,肌钙蛋白-T分子量较高,肌钙蛋白-C仅1个Ca²⁺结合位点.甲壳动物横纹肌兼有细肌丝调节与粗肌丝调节.     

Contractile vacuoles in cells of a fresh water sponge,Spongilla lacustris

Summary Forty or more independently functioning contractile vacuoles (CVs) occupy the central region of fresh water sponge pinacocytes. Each CV undergoes a cycle of enlargement by fusion, movement, shape change, rounding up, and emptying over the course of 5–30 min. Diameter at discharge varies between 1 and 13 m. CVs in all cell types are associated with submicroscopic coated vesicles. Filled CVs are bounded by an unmodified trilaminar membrane, but vacuoles with excess membrane frequently show coated evaginations. These evaginations are thought to pinch off as coated vesicles, providing an avenue for membrane recycling in the CV system.Supported by NIH grants AS-T01-GM-0723 and GM-23708-CBY     

Acid Phosphatase in the Digestive Vacuoles and Lysosomes of Paramecium caudatum: A Timed Study1

Enzyme cytochemistry was used to determine when acid phosphatase (AcPase) becomes associated with the digestive vacuoles (DVs) of axenically grown Paramecium caudatum that were pulsed with latex beads for 2–3 min. When cells were incubated in the Gomori medium, AcPase was not observed in the discoidal vesicles, the acidosomes, and the newly released DVs up to 3 min old or in most DVs 3–6 min old. The number of AcPase-positive DVs increased to 56% when DVs were 12–18 min old. Similar results were obtained using the napthol AS-TR phosphate-hexaotized rosanilin method at the light microscopic level where hundreds of DVs were scored though the maximal level of positive DVs obtained by this method was lower. In addition to DVs of specific ages, AcPase was found in ER, in some Golgi vesicles, and small vesicles similar in diameter to Golgi vesicles which may represent primary lysosomes in this ciliate. Larger vesicles abundant near the DV-II were only partially filled with reaction product. These vesicles, which could be identified by their paracrystalline sheets and a prominent glycocalyx lining the luminal surface of their membranes, fit the definition for secondary lysosomes. These results, which indicate that lysosomes fuse with DVs only after they have attained a certain age, suggest the existence of specific recognition factors on the membranes of secondary lysosomes as well as DV-II.     

The Cell Adhesion Molecule DdCAD-1 in Dictyostelium Is Targeted to the Cell Surface by a Nonclassical Transport Pathway Involving Contractile Vacuoles

DdCAD-1 is a 24-kD Ca²⁺-dependent cell– cell adhesion molecule that is expressed soon after the initiation of development in Dictyostelium cells. DdCAD-1 is present on the cell surface as well as in the cytosol. However, the deduced amino acid sequence of DdCAD-1 lacks a hydrophobic signal peptide or any predicted transmembrane domain, suggesting that it may be presented on the cell surface via a nonclassical transport mechanism. Here we report that DdCAD-1 is transported to the cell surface via contractile vacuoles, which are normally involved in osmoregulation. Immunofluorescence microscopy and subcellular fractionation revealed a preferential association of DdCAD-1 with contractile vacuoles. Proteolytic treatment of isolated contractile vacuoles degraded vacuole-associated calmodulin but not DdCAD-1, demonstrating that DdCAD-1 was present in the lumen. The use of hyperosmotic conditions that suppress contractile vacuole activity led to a dramatic decrease in DdCAD-1 accumulation on the cell surface and the absence of cell cohesiveness. Shifting cells back to a hypotonic condition after hypertonic treatments induced a rapid increase in DdCAD-1–positive contractile vacuoles, followed by the accumulation of DdCAD-1 on the cell membrane. 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, a specific inhibitor of vacuolar-type H⁺-ATPase and thus of the activity of contractile vacuoles, also inhibited the accumulation of DdCAD-1 on the cell surface. Furthermore, an in vitro reconstitution system was established, and isolated contractile vacuoles were shown to import soluble DdCAD-1 into their lumen in an ATP-stimulated manner. Taken together, these data provide the first evidence for a nonclassical protein transport mechanism that uses contractile vacuoles to target a soluble cytosolic protein to the cell surface.The cellular slime mold Dictyostelium discoideum transits from the solitary amoeboid state to an organized multicellular structure during development. This process is initiated in cells upon the depletion of nutrients, leading to the expression of many developmentally regulated genes and the chemotactic migration of cells in response to extracellular cAMP. Cells stream in concentric rings and/or spirals toward aggregation centers, giving rise to multicellular entities called pseudoplasmodia or slugs. The migrating slugs eventually culminate in the formation of fruiting bodies consisting of primarily spores and stalk cells (for review see Loomis, 1975).Multicellularity during development is maintained by the expression of cell–cell adhesion molecules, which fall into two broad categories based on their sensitivity to EDTA (for reviews see Gerisch, 1980; Siu et al., 1988; Siu, 1990; Fontana, 1995; Bozzaro and Ponte, 1995). There are two types of EDTA-sensitive cell adhesion sites. The EDTA/EGTA-sensitive cell adhesion sites, also known as contact sites B, are mediated by the Ca²⁺-dependent cell adhesion molecule gp24/DdCAD-1 (Knecht et al., 1987; Brar and Siu, 1993), while the EDTA-sensitive/EGTA- resistant sites are probably mediated by a Mg²⁺-dependent cell adhesion molecule (Fontana, 1993). The molecular nature of the latter sites is not yet known. Both types of adhesion sites are responsible for cell–cell interactions in the early stages of development. Coinciding with the aggregation stage is the rapid accumulation of the cell adhesion molecule gp80, which mediates the EDTA-resistant cell adhesion sites or contact sites A (Muller and Gerisch, 1978; Siu et al., 1985; Kamboj et al., 1988, 1989). In postaggregation stages, the EDTA-resistant adhesion sites are mediated by the membrane glycoprotein gp150 (Geltosky et al., 1979; Siu et al., 1983; Gao et al., 1992).DdCAD-1 is expressed by cells soon after the initiation of development (Knecht et al., 1987). Antibodies raised against gel-purified DdCAD-1 specifically inhibit the EDTA/EGTA-sensitive cell–cell adhesion sites and block development (Loomis, 1988). We have purified DdCAD-1 to homogeneity and demonstrated that labeled soluble DdCAD-1 binds to cells in an EDTA/EGTA-sensitive manner (Brar and Siu, 1993). Binding of DdCAD-1 to cells is prevented when cells are precoated with anti– DdCAD-1 antibodies, consistent with a homophilic mode of interaction. In addition, binding of DdCAD-1 to cells inhibits cell reassociation, indicating that it contains only a single cell binding site.Recent cloning of the DdCAD-1 cDNA predicts a protein of 23,924 daltons (Wong et al., 1996). The deduced amino acid sequence of DdCAD-1 shows significant sequence similarities with members of the cadherin family, and it contains a Ca²⁺-binding motif residing in the carboxy-terminal region. Indeed, Ca²⁺ overlay experiments have shown that DdCAD-1 is a Ca²⁺-binding protein with multiple binding sites (Brar and Siu, 1993; Wong et al., 1996). It is therefore conceivable that DdCAD-1 is a primitive member of the cadherin superfamily and it may mediate cell–cell adhesion in a manner similar to that of cadherins (Shapiro et al., 1995; Nagar et al., 1996). Another novel feature of the predicted sequence is that it lacks an amino-terminal hydrophobic signal peptide or a transmembrane domain, suggesting that DdCAD-1 is a soluble protein. Consistent with this observation, both subcellular fractionation and immunofluorescence microscopy have revealed a predominant cytoplasmic localization of DdCAD-1, indicating that 60–80% of DdCAD-1 is soluble (Brar and Siu, 1993; Sesaki and Siu, 1996). However, IgG binding and capping experiments clearly demonstrate that a substantial amount of DdCAD-1 is present on the cell surface (Brar and Siu, 1993; Wong et al., 1996). Interestingly, DdCAD-1 undergoes rapid translocation from the cytoplasm to the plasma membrane in the preaggregation stage of development (Sesaki and Siu, 1996), and then it becomes concentrated on filopodial structures and in cell– cell contact regions. These observations thus raise the question of how DdCAD-1 is transported and anchored to the cell surface.In this report we present morphological and biochemical evidence that DdCAD-1 is transported to the cell surface from the cytosol via contractile vacuoles, which is known so far to function exclusively in osmoregulation in cells. Furthermore, we show that isolated contractile vacuoles selectively take up soluble DdCAD-1 into their lumen in a cell-free system. Our results demonstrate, for the first time, a protein targeting function for contractile vacuoles and a novel nonclassical protein transport mechanism.     

The Physiological and Biochemical Response of Standardbred Horses to Exercise of Varying Speed and Duration

LINDHOLM, ARNE and BENGT SALTIN: The physiological and biochemical response of standardbred horses to exercise of varying speed and duration. Acta vet. scand. 1974, 15, 310–324. — Welltrained standardbred horses were studied to examine the metabolic response to excercise of various speeds and duration. Comparisons between interval (400, 700, 1,000 and 2,000 m) and continuous trotting (1 hr., 2 hrs.) and racing were made. Muscle and rectal temperatures were recorded before and immediately after each work bout. Heart rate was linearly related to trotting speed, and maximal heart rate (240 beats × min.−1) was achieved when trotting at least 700 m at close to maximal speed (12.0–12.5 m×sec.−1). Biopsy specimens from the gluteus medius muscle and venous blood were obtained before and after each work bout. Muscle and blood lactate values were markedly increased first at speeds close to maximal speed (11.4–12.5 m×sec.−1). Trotting 6×700 m at 12.5 m×sec.−1 produced as high muscle and blood lactate values as 23.7 and 19.0 mmol×kg−1 wet weight and l−1, respectively. Corresponding values after a race were about 15 mmol×kg−1 (muscle) and l−1 (blood). Glycogen utilization was related to work intensity and was most pronounced during the first work bouts. At a speed of 12 m×sec.−1 and trotting 2000 m, there was a glycogen utilization of near 12 mmol glucose units × kg−1 × min.−1 wet muscle. It is concluded that interval training over a distance of 700–1000 m repeated 4–6 times with a trotting speed close to maximal speed (11.4–12.5 m×sec.−1) appears to be optimal. ATP; CP; blood lactate; glycogen utilization; heart rate; horse skeletal muscle; muscle lactate; racing training.     

The Contractile Vacuole of Amoeba proteus. III. Vacuolar Response to Phagocytosis1

The reaction of the contractile vacuole of Amoeba proteus to single and multiple phagocytosis under controlled conditions has been studied. Fluid intake into the cytoplasm from the phagosomes induces secretion by the contractile vacuole of equivalent excess volumes:. Vacuolar response is rapid (200 sec) and may be initiated by increases of protoplasmic hydration of as little as 1%. Cytoplasmic uptake of fluid from the phagosome can occur against an osmotic gradient; thus some form of active transport is implied.     

Isolation and Characterization of Paramecium Mutants Defective in Their Response to Magnesium

Four mutant strains of Paramecium tetraurelia with a reduced ability to respond behaviorally to Mg(2+) have been isolated. Voltage-clamp analyses showed that their Mg(2+) insensitivity is associated with a reduced Ca(2+) -dependent Mg(2+) current. The four mutants, which have been dubbed ``eccentric,' result from recessive mutations in two unlinked loci, xntA and xntB. Further analysis of xntA(1) showed it to be unlinked to any of the behavioral mutants of P. tetraurelia described previously, but it is allelic to d4-521, a ``K(+)-resistant' strain, and d4-596, a ``Ba(2+)-shy' mutant. The varied pleiotropic effects of xntA(1), which include increased resistance to Ni(2+) and Zn(2+) poisoning, suggest that the locus encodes a central regulator of cell function in Paramecium.     

棒叶落地生根对干旱与复水的生理响应

为探讨棒叶落地生根(Kalanchoe tubiflora)耐旱的机制,在干旱与复水条件下,对其叶片的一些生理生化指标进行了测定。结果表明,随干旱时间延长,棒叶落地生根叶片中O2-·生成速率增大,H2O2含量升高,导致脂质过氧化产物MDA含量增高;同时SOD活性升高,CAT活性降低;可溶性糖与脯氨酸含量增加,但复水后这些指标均恢复到干旱前的水平。这说明棒叶落地生根能够耐受干旱环境是通过积累渗透调节物质,提高活性氧的清除能力,从而减少氧化胁迫造成的伤害。     

Hyperosmotic volume regulation in the gills of the ribbed mussel, Geukensia demissa: rapid accumulation of betaine and alanine

The content of betaine and alanine in gills of the ribbed mussel Geukensia demissa increases rapidly following transfer of the tissues from 250 to 1000 mOsm seawater (SW). Increases in alanine, proline and glycine account for most of the increase in the amino acid pool. The betaine content increases from 45 to 150 μmol/g dry weight within 12 h. Transfer of isolated gills from 250 to 1000 mOsm SW results in a temporary cessation of all ciliary activity. Within 20–40 min following transfer, ciliary activity has recovered. Recovery of ciliary activity precedes recovery of tissue hydration. The uric acid content of gills is unchanged by exposure to hyperosmotic media, suggesting that uric acid is not a store of nitrogen for alanine synthesis from pyruvate. In other organisms, the accumulation of betaine in response to hyperosmotic stress is a slow (days to weeks) process that probably involves changes in gene expression. The rapid, large increases in betaine reported here suggest that gene expression is not a factor in volume recovery by euryhaline bivalve tissues exposed to acute hyperosmotic stress.     

Paramecium GPI proteins: variability of expression and localization

In Paramecium primaurelia, the two major classes of cell surface proteins, the surface antigen (SAg) and the surface GPI proteins (SGPs), are linked to the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor. In the present study, we have characterized the expression of the SGPs in several geographical strains of P. primaurelia and P. tetraurelia at different temperatures, 23 degrees C and 32 degrees C. The identification of the expressed SGPs was performed on purified cilia, by establishing the SGP SDS-PAGE profiles under four different conditions: with or without their anchoring lipid, cleaved with a Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC), and either in a reduced or in an unreduced state. This screening revealed the existence of specific sets of ciliary SGPs, as a function of temperature and the geographical origin of the strains. The SGPs the most abundant at 23 degrees C and 32 degrees C displayed a rapid turnover. We also looked for the presence of PI-PLC releasable proteins in purified cortices. In addition to the SAg and SGPs, the cortical fraction was shown to contain other PI-PLC releasable proteins, not found in the ciliary fraction, thus localized exclusively in the interciliary region.     

Studies on Feeding and Digestion in Protozoa. III. Acid Phosphatase Activity in Food Vacuoles of Paramecium multimicronucleatum

SYNOPSIS. Acid phosphatase activity was studied in total mounts and sections of agnotobiotic Paramecium multimicronucleatum by the alpha-naphthyl phosphate-hexazotized rosanilin method. Timing was achieved by India ink marking of food vacuoles. Enzyme activity is present in small endoplasmic granules and in the greatest part of food vacuoles. Following an inactive stage (stage I) of an average length of 5 min the activity appears at the periphery of the vacuole, in most cases in the form of granules (stage II). A high activity level (stage III) is attained within 1 1/2 min and maintained for the most part of the vacuolar cycle. The activity disappears only in the latest vacuoles before egestion (stage IV). The appearance of activity is not concurrent with but succeeding to the maximum of vacuolar acidity as ascertained by feeding Congo red stained killed yeast cells. On the basis of these results the food vacuoles may be looked upon as belonging to the lysosomes sensu lato.     

Response of caudal neurosecretory cells of Salvelinus fontinalis to variations in the ionic composition of the environment

Summary The response of caudal neurosecretory cells in Salvelinus fontinalis to exposure to media in which one ion (Na, K, Mg, Ca and Cl) was either selectively enriched or depleted was evaluated by morphometric criteria. Morphological changes indicating stimulation of synthetic activity were observed: exposure to sea water with low potassium concentration was the most efficacious in inducing the increase in the average diameter of caudal cells and the number of nucleoli for both cells with lobed and with nonlobed nuclei. The proportion of cells with lobed nuclei was also increased. To a lesser degree and in decreasing order, experimental milieu with modified magnesium contents, either fresh water enriched in this ion or seawater depleted of the same ion, and fresh water enriched in calcium also resulted in significant increases in cell diameter and the number of nucleoli.The two cytological cell types, viz cells with non-lobed nuclei or cells with lobed nuclei, are interpreted as being linked to different levels of neurosecretory activity.Supported by NSERC (Canada)