CD26 expression determines lung metastasis in mutant F344 rats: involvement of NK cell function and soluble CD26

The association between CD26 expression, tumor cell adhesion, metastasis, and natural killer (NK) cell function was investigated in a CD26 mutant Fischer 344 (F344/DuCrj) substrain from Japanese breeders (F344JAP) in comparison with wild-type F344 substrains from US (F344/Crl) and Hannover (HAN; F344/Ztm) breeders. F344JAP rats lack the dipeptidyl peptidase IV activity of CD26 and show a reduced cell surface expression of the mutated CD26 glycoprotein. In vivo adhesion of vital dye-labeled MADB106 tumor cells, tumor colonization, CD26 enzymatic activity, and CD26 immunoreactivity in lungs and soluble CD26-like protein expression in serum were markedly reduced in F344JAP rats. These findings demonstrate that CD26 protein expression exerts a key role in lung metastasis. In addition, NK cell cytotoxicity against MADB106 cells was diminished in the mutant F344 substrain, suggesting that CD26 enzymatic activity sustains NK cytotoxicity. Interestingly, tumor cells lacked CD26 immunoreactivity in vitro, but displayed CD26 immunoreactivity in situ after in vivo inoculation as well as after incubation with rat serum, indicating that soluble CD26-like protein assembles in tumor cells during in vivo passage, which may interact with the process of tumor adhesion and metastasis. Overall, these findings indicate that altered expression and function of a single enzyme-the CD26 protein--can drastically change the outcome of metastatic disease.     

Endomucin is expressed in embryonic dorsal aorta and is able to inhibit cell adhesion

Glucagon-like peptide-1 (GLP-1) is an incretin, which induces glucose-dependent insulin secretion. GLP-1 is rapidly degraded by dipeptidyl peptidase IV (DPPIV) after its release. We investigated whether DPPIV-deficient F344/DuCrj rats show improved glucose tolerance when compared with DPPIV-positive F344/Jcl rats. Oral glucose tolerance test indicated improved glucose tolerance in F344/DuCrj rats, but blood glucose levels of the two strains were almost the same 120 min after the glucose bolus. Valine-pyrrolidide, a DPPIV inhibitor, had no effect on the glucose tolerance of F344/DuCrj rats, but improved that of F344/Jcl rats. Enhanced insulin secretion and high plasma active GLP-1 levels were detected in an intraduodenal glucose tolerance test. Glucose tolerance is improved in DPPIV-deficient F344/DuCrj rats via enhanced insulin release mediated by high active GLP-1 levels. Our results suggest that DPPIV inhibition is a rational strategy to treat diabetic patients by improving glucose tolerance with low risk of hypoglycemia.     

Enhanced secretion of glucagon-like peptide 1 by biguanide compounds

Metformin was reported to increase plasma active glucagon-like peptide-1 (GLP-1) in humans. There are two possible mechanisms for this effect: (1) metformin inhibits dipeptidyl peptidase IV (DPPIV), an enzyme degrading GLP-1, and (2) metformin enhances GLP-1 secretion. To elucidate the mechanism(s), we examined (1) IC(50) of metformin for DPPIV inhibition, (2) plasma active GLP-1 changes after oral biguanide (metformin, phenformin, and buformin) treatment in fasting DPPIV-deficient F344/DuCrj rats, and (3) plasma intact GLP-1 excursions after oral administration of metformin and/or valine-pyrrolidide, a DPPIV inhibitor, in fasting DPPIV-positive F344/Jcl rats. Our in vitro assay showed that metformin at up to 30mM has no inhibitory activity towards porcine or rat DPPIV. Metformin treatment (30, 100, and 300mg/kg) increased plasma active GLP-1 levels dose-dependently in DPPIV-deficient F344/DuCrj rats (approximately 1.6-fold at 3 and 5h after administration of 300mg/kg). This treatment had no effect on blood glucose levels. Similarly, phenformin and buformin (30 and 100mg/kg) elevated plasma intact GLP-1 levels in F344/DuCrj rats. In DPPIV-positive F344/Jcl rats, coadministration of metformin (300mg/kg) and valine-pyrrolidide (30mg/kg) resulted in elevation of plasma active GLP-1, but neither metformin nor valine-pyrrolidide treatment alone had any effect. These findings suggest that metformin has no direct inhibitory effect on DPPIV activity and that metformin and the other biguanides enhance GLP-1 secretion, without altering glucose metabolism. Combination therapy with metformin and a DPPIV inhibitor should be useful for the treatment of diabetes.     

Improvement of high fat-diet-induced insulin resistance in dipeptidyl peptidase IV-deficient Fischer rats

F344/DuCrj rats are genetically deficient in dipeptidyl peptidase IV (DPPIV). This enzyme degrades glucagon-like peptide-1 (GLP-1), which induces glucose-dependent insulin secretion. Glucose tolerance of F344/DuCrj rats is improved as a result of enhanced insulin release induced by high levels of plasma GLP-1. In this study, we fed F344/DuCrj rats and DPPIV-positive F344/Jcl rats, aged five weeks, on a high-fat (HF) diet to examine the effect of DPPIV deficiency on food intake and insulin resistance. F344/Jcl rats gained significantly more body weight and consumed significantly more food than F344/DuCrj rats from Week 4 on either control or HF diet. Glucose excursion in the oral glucose tolerance test (OGTT) was improved in F344/DuCrj rats fed on the control or HF diet at all times examined, compared with F344/Jcl rats. Homeostasis model assessment (HOMA) insulin resistance values of F344/DuCrj and F344/Jcl rats fed on HF diet were higher than those of animals fed on control diet up to Week 6. However, HOMA insulin resistance values of F344/DuCrj rats fed on HF diet became significantly lower than those of F344/Jcl rats on HF diet during Weeks 8-10. The area under the insulin curve in the OGTT at Week 10 showed that the insulin resistance of HF-diet-fed F344/DuCrj rats was greatly ameliorated. Plasma active GLP-1 concentrations of F344/DuCrj rats in the fed state were significantly higher than those of F344/Jcl rats. These observations suggest that DPPIV deficiency results in improved glucose tolerance and ameliorated insulin resistance owing to enhanced insulin release and inhibition of food intake as a result of high active GLP-1 levels.     

Inhibition of CD26/dipeptidyl peptidase IV enhances CCL11/eotaxin-mediated recruitment of eosinophils in vivo

Chemokines mediate the recruitment of leukocytes to the sites of inflammation. N-terminal truncation of chemokines by the protease dipeptidyl peptidase IV (DPPIV) potentially restricts their activity during inflammatory processes such as allergic reactions, but direct evidence in vivo is very rare. After demonstrating that N-terminal truncation of the chemokine CCL11/eotaxin by DPPIV results in a loss of CCR3-mediated intracellular calcium mobilization and CCR3 internalization in human eosinophils, we focused on the in vivo role of CCL11 and provide direct evidence for specific kinetic and rate-determining effects by DPPIV-like enzymatic activity on CCL11-mediated responses of eosinophils. Namely, it is demonstrated that i.v. administration of CCL11 in wild-type F344 rats leads to mobilization of eosinophils into the blood, peaking at 30 min. This mobilization is significantly increased in DPPIV-deficient F344 rats. Intradermal administration of CCL11 is followed by a dose-dependent recruitment of eosinophils into the skin and is significantly more effective in DPPIV-deficient F344 mutants as well as after pharmacological inhibition of DPPIV. Interestingly, CCL11 application leads to an up-regulation of DPPIV, which is not associated with negative feedback inhibition via DPPIV-cleaved CCL11((3-74)). These findings demonstrate regulatory effects of DPPIV for the recruitment of eosinophils. Furthermore, they illustrate that inhibitors of DPPIV have the potential to interfere with chemokine-mediated effects in vivo including but not limited to allergy.     

Comparison of characteristics between F344 and Slc:Wistar rats--Slc:Wistar rats cannot be distinguished from the F344 strain

With respect to F344/DuCrj and Slc: Wistar rats, both widely used in Japan, it was found that there is a close similarity in the changes of body weights and survival rates, and in the organ distribution and incidence of spontaneous tumors. To examine the degree of homozygosity between F344 and Slc: Wistar strains, tumor transplantation and skin grafting were performed. The bladder carcinomas that originated from F344/DuCrj rats grew subcutaneously in the other F344 strains and Slc: Wistar rats, but did not grow in the other Wistar-derived strains. The skin grafts between F344/DuCrj or F344/NSlc and Slc: Wistar rats were accepted, but those between F344/DuCrj or Slc: Wistar and the other Wistar-derived strains were rejected. These results suggest that Slc: Wistar rats cannot be distinguished genetically from the F344 strain of rats.     

An active-site mutation (Gly633-->Arg) of dipeptidyl peptidase IV causes its retention and rapid degradation in the endoplasmic reticulum.

Dipeptidyl peptidase IV (DPPIV), a serine protease expressed on the cell surface, is deficient in a Fischer rat substrain. Northern blot analysis showed no difference in the size and amount of DPPIV mRNA between normal (344/NC) and deficient (344/CRJ) rats. Cloning and sequencing of DPPIV cDNAs revealed a G to A transition at nucleotide 1897 in the cDNA sequence of 344/CRJ, which leads to substitution of Gly633-->Arg in the active-site sequence Gly629-Trp-Ser-Tyr-Gly633 determined for the wild-type DPPIV [Ogata, S., Misumi, Y., Takami, N., Oda, K., & Ikehara, Y. (1992) Biochemistry 31, 2582-2587]. Pulse-chase experiments with hepatocytes showed that the wild-type DPPIV was initially synthesized as a 103-kDa form with high-mannose-type oligosaccharides, which was processed to a mature form of 109 kDa with the complex type during intracellular transport. In contrast, the mutant DPPIV, although being synthesized as the 103-kDa form, was rapidly degraded without being processed to the mature form. Site-directed mutagenesis of the wild-type and mutant cDNAs and their transfection/expression in COS-1 cells confirmed that the single substitution of Gly633-->Arg is sufficient to cause the rapid intracellular degradation of DPPIV. Immunoelectron-microscopic observations showed that the mutant DPPIV was detectable only in the endoplasmic reticulum (ER), in contrast to the distribution of the wild-type DPPIV in the Golgi complex and on the cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression and spatial heterogeneity of dipeptidyl peptidases in endothelial cells of conduct vessels and capillaries

Dipeptidyl peptidase IV (DPPIV)/CD26 is by far the most extensively studied member of the prolyl oligopeptidase family of serine proteases. The discovery of the related enzymes DPP8 and DPP9 necessitates a (re-)evaluation of the DPPIV-like enzymatic activity in cells and organs. In this study, we aimed (1) to investigate the expression of the individual dipeptidyl peptidases in different types of endothelial cells (ECs) and (2) to reconsider published data in relation to our findings. Examination of DPP expression in rat primary ECs of aortic, endocardial and cardiac microvascular origin revealed the presence of DPPIV-like activity in all cell lysates. More than half of this activity could be attributed to DPP8/9. Western blot analysis revealed an abundance of the DPP8 protein as compared to DPP9. The expression of DPPIV and DPP8 was significantly higher in the cardiac microvascular endothelium than in the other ECs, suggesting a more pronounced role of these DPPs in the microvasculature. In situ, DPP activity in ventricular microvasculature was completely inhibited by sitagliptin, indicating that DPPIV is the predominant DPPIV-like enzyme in this organ. By contrast, immunohistochemical studies indicated DPP9 as the predominant DPP in human carotid artery ECs. In conclusion, our results support a highly regulated expression of individual DPPs in ECs, with a spatial heterogeneity in the cardiovascular tree.     

Cryopreservation of spermatozoa from closed colonies, and inbred, spontaneous mutant, and transgenic strains of rats

We attempted to cryopreserve spermatozoa from closed colonies (Jcl:SD and Jcl:Wistar), and inbred (BN/Crj, F3441 DuCrj, LEW/Crj, Long-Evans and WKY/NCrj), mutant (Zitter [WTC.ZI-zi] and Tremor [TRM]), transgenic (human A-transferase [A], and green fluorescent protein [GFP]) strains of rats. Rat epididymal spermatozoa suspended in cryopreservation solution (23% egg yolk, 8% lactose monohydrate, and 0.7% Equex Stm, pH 7.4, adjusted with 10% Tris [hydroxymethy] aminomethane) were frozen and stored at -196 degrees C. After thawing at 37 degrees C, the spermatozoa were instilled into the tip of each uterine horn of the recipients. A total of five recipient females for each strain were inseminated with cryopreserved spermatozoa, and normal live offspring of all strains (Jcl:SD: 11, Jcl:Wistar: 13, BN/Crj: 9, F344/DuCrj: 28, LEW/Crj: 4, Long-Evans: 6, WKY/NCrj: 8, TRM: 24, WTC.ZI-zi: 27, A: 30 and GFP: 20) were obtained.     

The LEXF: A new set of rat recombinant inbred strains between LE/Stm and F344

A new set of rat RI strains consisting of 11 independent strains and 13 of their substrains was established by inbreeding F₂ rats between F344/DuCrj and LE/Stm. The strain distribution pattern was examined for 66 microsatellite loci, 8 biochemical genetic markers, 2 histocompatibility loci, and 2 coat color genes. A rat salivary protein gene Spe1 was newly mapped on Chr 1. Received: 13 August 1996 / Accepted: 23 December 1996     

Morphometric comparison of the rat mandible in f344 substrains, f344/du and f344/n.

The shape of the mandible was compared by morphometric methods to ascertain the genetic differences between two substrains of F344 rats, F344/DuCrlCrlj and F344/NSlc. Since these two substrains are clearly different in survival and the incidence of age associated disorders; thus, some genetic differences are suggested to be present between them. Although previous microsatellite analysis did not detect any differences between the two F344 substrains, the present study clearly detected interesting differences in the mandible morphology. At 2 months of age, the F344/Du mandible was characterized by a larger size, especially in length, than the F344/N mandible. The shape of the mandible seemed to be more variable in F344/N. This clear substrain difference suggests the importance of the substrain recognition in F344 rats, especially in experimental usage.     

Establishment and characteristics of four sub-strains of F344 rats reared on various low protein and low energy diets.

Four sub-strains, reared by sib-mating and having for their origin the F344/DuCrj strain of rats, were established by feeding with different levels of low protein and low energy diets, and their characteristics investigated. The amounts of crude protein (CP) and digestible energy (DE) in the four diets were 17.6%-3.0 kcal, 10.5%-2.5 kcal, 8.4%-2.0 kcal, and 10.5%-2.5 kcal, respectively, and the four sub-strains established here were provisionally designated as F344/Tig1, F344/Tig2, F344/Tig3 and F344/Tig4, respectively. Intakes of nitrogen-corrected metabolizable energy (MEn) did not differ, and a large intake of digestible crude protein (DCP) was observed in F344/Tig1 rats. The body weight of rats provided with lower-nutrient diets showed a tendency to decrease until the F2 generation, but no change among the generations was seen subsequently, and the same compiled differences in protein content were maintained. Similar transitions were observed in the lifetime rearing test. Though F344/Tig3 rats, which were reared on minimum nutrients, showed a tendency to delayed puberty, we were easily able to breed four pairs in every generation using procedures similar to those used for other strains of rats. There were no differences among the F344/Tig1 to -3 strains of rats in body length, digestive tract length, or organ weight per body weight, and all the rats had a normal range of biochemical values. But the F344/Tig4 showed a high glutamic-oxaloacetic transaminase (GOT), and a tendency to decreased liver function and a shorter lifespan. These sub-strains of F344 rats clarified differences in fatty acid compositions, such as docosahexaenoic acid (DHA) in serum, liver and the brain. The rats were intended to be useful animal models for the study of nutritional environments and their influence on the memory and learning.     

Dose-dependent recruitment of CD25+ and CD26+ T cells in a novel F344 rat model of asthma

The ovalbumin (OVA)-induced airway inflammation in rats is a commonly used model to explore the pathobiology of asthma. However, its susceptibility varies greatly between rat strains, and presently Brown Norway (BN) rats are preferentially used. Since recruitment of T cells to the lungs depends on the CD26 (dipeptidyl peptidase IV, DPPIV) expression, Fischer 344 strain (F344) rats are a highly relevant rat strain, in particular because CD26-deficient substrains are available. To establish a F344 rat model of asthma, we challenged F344 rats using different doses of aerosolized antigen (0%, 1%, 2.5%, 5%, and 7.5% OVA) and compared these effects with intratracheal instillation of OVA (1.5 mg/0.3 ml). Asthmoid responsiveness was determined by analysis of early airway responsiveness (EAR), antigen-specific IgE levels, as well as airway inflammation including the composition of T cell subpopulations in the bronchoalveolar lavage (BAL) and lung tissue with special respect to the T cell activation markers CD25 and CD26. Even low allergen doses caused allergen-specific EAR and increases of antigen-specific IgE levels. However, EAR and IgE levels did not increase dose dependently. Higher concentrations of OVA led to a dose-dependent increase of several immunological markers of allergic asthma including an influx of eosinophils, T cells, and dendritic cells. Interestingly, a dose-dependent increase of CD4(+)/CD25(+)/CD26(+) T cells was found in the lungs. Summarizing, we established a novel F344 rat model of aerosolized OVA-induced asthma. Thereby, we found a dose-dependent recruitment of cellular markers of allergic asthma including the activated CD4(+)/CD25(+)/CD26(+) T cell subpopulation, which has not been described in asthma yet.     

Cholangiocyte marker-positive and -negative fetal liver cells differ significantly in their ability to regenerate the livers of adult rats exposed to retrorsine

We have used monoclonal antibodies against cell-surface developmental epitopes in combination with micromagnetic beads to isolate phenotypically defined subpopulations of cholangiocyte marker-positive fetal liver epithelial cells (CMP-FLEC). Differentiation potential was evaluated by injecting cell isolates from dipeptidyl peptidase IV (DPPIV) positive (DPPIV+) Fischer donor rats into the spleen of partially hepatectomized, DPPIV negative (DPPIV-) Fischer host rats exposed to retrorsine. At various time points, liver tissue was harvested and cells in DPPIV+ colonies were phenotyped by immunofluorescence and histochemical protocols. Functional differentiation and liver replacement were determined by comparing donor and host hepatocyte protein expression patterns and DPPIV enzyme activity in extracts from livers of host rats receiving CMP-FLEC. Our results showed that bipotentiality was retained during differentiation and maturation of CMP-FLEC, indicating that the acquisition of ductal morphology and phenotype were not indicative of lineage commitment. CMP-FLEC transplanted into the adult rat liver lost ductal and gained hepatocyte markers, and acquired protein expression patterns in 2D gels with a close similarity (>75% spot match) to host hepatocytes but differing significantly from the transplanted CMP-FLEC cell isolate (<25% spot match). The average size of donor hepatocyte colonies increased with time so that by 1 year, up to 70% of the host rat liver was replaced by CMP-FLEC derived DPPIV+ hepatocytes. Depletion of CMP-FLEC from fetal liver isolates resulted in a marked decrease in adult liver colonization, suggesting that a high percentage of the hepatocyte colonies in animals receiving total fetal liver isolates are derived from CMP-FLEC.     

Inconsistent susceptibility to autoimmunity in inbred LEW rats is due to genetic crossbreeding involving segregation of the arthritis-regulating gene Ncf1

We recently identified a single-nucleotide polymorphism in the Ncf1 gene, a component of the NADPH oxidase complex, to be the cause of one of the strongest identified loci for arthritis severity in rats. This polymorphism was found to be naturally occurring in a collection of inbred rat strains as well as in wild rats. Among the inbred strains we found that different LEW substrains (LEW/Ztm and LEW/Mol), originating from different breeders, showed an allelic discrepancy in Ncf1, suggesting an impact on arthritis susceptibility between these substrains. In fact, the LEW/Mol strain was completely resistant to pristane-induced arthritis, in contrast to the LEW/Ztm strain, which was susceptible. Moreover, the LEW/Mol strain had higher production of radical oxygen species in peripheral blood leukocytes, a phenomenon most likely regulated by the polymorphisms in the Ncf1 gene. However, the phenotypic difference between LEW/Mol and LEW/Ztm is most likely a combination of several genes, of which Ncf1 is suggested to be the major regulating gene. This has also been confirmed by previous linkage analyses involving the LEW/Ztm strain which shows that a QTL on chromosome 12, most likely caused by polymorphism of Ncf1, is the major regulatory gene but that other loci are contributing. That more genes are likely to contribute was shown by a complete genome comparison of the LEW/Ztm and the LEW/Mol rat strains that uncovered an introduction of approximately 37% non-LEW genome into the LEW/Mol strain, which probably was caused by past crossbreeding. Therefore, the LEW/Mol should be regarded as a recombinant inbred strain.     

Hippocampal 8-[3H]Hydroxy-2-(Di-n-Propylamino)Tetralin Binding Site Densities, Serotonin Receptor (5-HT1A) Messenger Ribonucleic Acid Abundance, and Serotonin Levels Parallel the Activity of the Hypothalamopituitary-Adrenal Axis in Rat

We have previously demonstrated that susceptibility of the Lewis rat to inflammatory disease, compared with the relatively resistant Fischer F344/N rat, is related to a hyporesponsive hypothalamopituitary-adrenal axis to inflammatory and other stress mediators. Because serotonin (5-HT) and the 5-HT1A receptor are important stimulators of this axis, we have investigated the levels of 8-[3H]-hydroxy-2,3-(di-n-propylamino)tetralin binding sites, 5-HT1A mRNA, 5-HT, and 5-hydroxyindoleacetic acid in various brain regions of Lewis, outbred Harlan Sprague Dawley, and Fischer F344/N rats. Lewis rats expressed significantly fewer hippocampal and frontal cortical 8-[3H]-hydroxy-2,3-(di-n-propylamino)tetralin binding sites and less 5-HT1A mRNA than Harlan Sprague Dawley and Fischer F344/N rats. Adrenalectomy increased the number of 8-[3H]hydroxy-2,3-(di-n-propylamino)tetralin binding sites and 5-HT1A mRNA expression in the hippocampus of all three strains. Levels of hippocampal 5-HT in Fischer F344/N rats were significantly greater than levels detected in the same regions from Lewis and Harlan Sprague Dawley rats. Hypothalamic 5-HT and 5-hydroxyindoleacetic acid levels in Harlan Sprague Dawley rats were higher than the same area from the other two strains. Adrenalectomy increased the levels of 5-hydroxyindoleacetic acid in the hypothalamus of all three strains. We conclude that hippocampal 5-HT1A receptor densities and 5-HT levels in the rat parallel the activity and responsiveness of the hypothalamopituitary-adrenal axis.     

Regulation of expression and function of dipeptidyl peptidase 4 (DP4), DP8/9, and DP10 in allergic responses of the lung in rats.

The expression of dipeptidyl peptidase 4 (DP4, CD26) affects T-cell recruitment to lungs in an experimental rat asthma model. Furthermore, the gene of the structural homologous DP10 represents a susceptibility locus for asthma in humans, and the functional homologous DP8/9 are expressed in human leukocytes. Thus, although several mechanisms may account for a role of DP4-like peptidases in asthma, detailed information on their anatomical sites of expression and function in lungs is lacking. Therefore, bronchi and lung parenchyma were evaluated using immunohistochemistry and histochemical/enzymatic activity assays, as well as quantitative real-time PCR for this family of peptidases in naïve and asthmatic rat lungs derived from wild-type F344 and DP4-deficient F344 rat strains. Surprisingly, results show not only that the induction of experimental asthma increases DP4 enzymatic activity in the bronchoalveolar lavage fluid and parenchyma, but also that DP8/9 enzymatic activity is regulated and, as well as the expression of DP10, primarily found in the bronchial epithelium of the airways. This is the first report showing a differential and site-specific DP4-like expression and function in the lungs, suggesting a pathophysiologically significant role in asthma. (J Histochem Cytochem 56:147–155, 2008)     

Slc:Wistar outbred rats show close genetic similarity with F344 inbred rats

Although Slc:Wistar rats are used widely in biomedical research as outbred rats, close similarities in growth curves, survival rates, and immunological and biochemical phenotypes have been reported between Slc:Wistar and F344 inbred rats. We reported previously that nine genetic variations that were fixed in Slc:Wistar rats had identical genotypes in F344 rats. Here, we examined the genetic characteristics of Slc:Wistar rats using 27 simple-sequence length polymorphism (SSLP) markers and compared them with other Wistar stocks available in Japan and with some F344 strains. Among 27 SSLP loci, 23 (85%) were fixed in the Slc:Wistar rats, which was the highest among the other Wistar stocks. The 23 fixed loci shared identical genotypes with corresponding loci in F344 rats. Further, the predominant allele types in the unfixed loci had allele frequencies as high as 80%, and these alleles were identical in the F344 rats. When the nine genetic variations reported previously are added, a total of 32 (89%) out of the 36 loci examined were fixed and identical in the Slc:Wistar and F344 rat genomes. These findings indicate the low genetic variation in Slc:Wistar rats and the high genetic similarity between the Slc:Wistar and F344 inbred rats. This study demonstrates the importance of characterizing outbred rats and the need to pay ample attention to the genetic characteristics the Slc:Wistar rats for their proper use.     

Comparison of Fat Storage between Fischer 344 and Obesity‐Resistant Lou/C Rats Fed Different Diets**

Objective: We aimed to characterize further the Lou/C (LOU) and Fischer 344 (F344) rat strains for nutritional traits to validate their use as contrasting strains for molecular genetic studies. Research Methods and Procedures: Five batches of LOU and F344 rats were used to measure caloric intake, weight gain, and body composition when fed a chow diet, a self‐selection diet (together with the study of preferences for macronutrients), hypercaloric diets, and a chow diet in a cold environment. Results: Despite a higher caloric intake when fed a chow diet, LOU rats showed a lower weight gain, final body weight, and percentage of fat tissue, together with a higher percentage of carcass weight, than F344 rats. When fed a self‐selection diet, LOU males ingested less protein and more fat than F344 males, and the reverse was observed for females. In this condition, feed efficiency was reduced in LOU but increased in F344 rats compared with the chow diet. Diet‐induced obesity was observed in F344 rats but not in LOU rats fed hypercaloric diets. In a cold environment, both LOU and F344 rats displayed an increased percentage of brown adipose tissue compared with control groups, together with a higher caloric intake. Discussion: The study shows robust nutritional differences between the LOU rat, a lean strain with a low feed efficiency and resistant to diet‐induced obesity, and the contrasting F344 rat strain. It also shows the interest in these strains for studying the genetic components of resistance to obesity.