von Willebrand factor binds specifically to sulfated glycolipids

The human plasma glycoprotein Factor VIII/von Willebrand factor (vWF) binds specifically and with high affinity to sulfatides (galactosylceramide-I3-sulfate). vWF does not bind to gangliosides, neutral glycolipids, phospholipids, or cholesterol 3-sulfate. Although the largest oligomers of vWF bind preferentially to sulfatides, vWF monomers and dimers also bind but with reduced affinity. vWF binding is inhibited at high ionic strength or low pH, by some sulfated polysaccharides and by antibodies to vWF. Binding of vWF to sulfatides is probably responsible for its agglutination of aldehyde-fixed erythrocytes and may play a role in vWF-induced platelet adhesion or platelet aggregation.     

Altered processing of integrin receptors during keratinocyte activation

We used monoclonal antibodies against specific integrin subunits to examine the role of integrin receptors in keratinocyte activation. We found that before activation, beta 1 subunits in keratinocytes showed a diffuse distribution, whereas after activation, keratinocytes organized beta 1 receptors into marginal adhesion plaques. In immunoprecipitation experiments with antibodies against beta 1 integrin subunits, we found mostly immature subunits synthesized in keratinocytes freshly harvested from skin. Moreover, integrin receptor complexes immunoprecipitated from these cells by monoclonal antibodies against alpha 2, alpha 3, or alpha 5 subunits contained only immature beta 1 subunits. With keratinocytes cultured 4-7 days, anti-beta 1 antibodies immunoprecipitated mostly mature beta 1 subunits, and integrin complexes immunoprecipitated from cultured cells by anti-alpha subunit antibodies contained mostly mature beta 1 subunits. Antibodies directed against beta 1 subunits also inhibited keratinocyte migration. Based on these results, we suggest that up-regulation of migration by activated keratinocytes depends on changes in processing of pre-beta 1 subunits to mature beta 1 subunits. We also studied the distribution of integrin subunits in skin and on keratinocytes migrating out of skin explants. Whereas beta 1, alpha 2, and alpha 3 subunits were detected in keratinocytes in skin and migrating out of explants, alpha 5 subunits were observed only in migrating cells.     

Lesions in gshA (Encoding gamma-L-glutamyl-L-cysteine synthetase) prevent aerobic synthesis of thiamine in Salmonella enterica serovar typhimurium LT2

Thiamine pyrophosphate is an essential cofactor that is synthesized de novo in Salmonella enterica serovar Typhimurium and other bacteria. In addition to genes encoding enzymes in the biosynthetic pathway, mutations in other metabolic loci have been shown to prevent thiamine synthesis. The latter loci identify the integration of the thiamine biosynthetic pathway with other metabolic processes and can be uncovered when thiamine biosynthesis is challenged. Mutations in gshA, encoding gamma-L-glutamyl-L-cysteine synthetase, prevent the synthesis of glutathione, the major free thiol in the cell, and are shown here to result in a thiamine auxotrophy in some of the strains tested, including S. enterica LT2. Phenotypic characterization of the gshA mutants indicated they were similar enough to apbC and apbE mutants to warrant the definition of a class of mutants unified by (i) a requirement for both the hydroxymethyl pyrimidine (HMP) and thiazole (THZ) moiety of thiamine, (ii) the ability of L-tryosine to satisfy the THZ requirement, (iii) suppression of the thiamine requirement by anaerobic growth, and (iv) suppression by a second-site mutation at a single locus. Genetic data indicated that a defective ThiH generates the THZ requirement in these strains, and we suggest this defect is due to a reduced ability to repair a critical [Fe-S] cluster.     

Modularity of the Mtr respiratory pathway of Shewanella oneidensis strain MR‐1

Four distinct pathways predicted to facilitate electron flow for respiration of externally located substrates are encoded in the genome of Shewanella oneidensis strain MR‐1. Although the pathways share a suite of similar proteins, the activity of only two of these pathways has been described. Respiration of extracellular substrates requires a mechanism to facilitate electron transfer from the quinone pool in the cytoplasmic membrane to terminal reductase enzymes located on the outer leaflet of the outer membrane. The four pathways share MtrA paralogues, a periplasmic electron carrier cytochrome, and terminal reductases similar to MtrC for reduction of metals, flavins and electrodes or to DmsAB for reduction of dimethyl sulphoxide (DMSO). The promiscuity of respiratory electron transfer reactions catalysed by these pathways has made studying strains lacking single proteins difficult. Here, we present a comprehensive analysis of MtrA and MtrC paralogues in S. oneidensis to define the roles of these proteins in respiration of insoluble iron oxide, soluble iron citrate, flavins and DMSO. We present evidence that some periplasmic electron carrier components and terminal reductases in these pathways can provide partial compensation in the absence of the primary component, a phenomenon described as modularity, and discuss biochemical and evolutionary implications.     

Enhancement of Survival and Electricity Production in an Engineered Bacterium by Light-Driven Proton Pumping

Microorganisms can use complex photosystems or light-dependent proton pumps to generate membrane potential and/or reduce electron carriers to support growth. The discovery that proteorhodopsin is a light-dependent proton pump that can be expressed readily in recombinant bacteria enables development of new strategies to probe microbial physiology and to engineer microbes with new light-driven properties. Here, we describe functional expression of proteorhodopsin and light-induced changes in membrane potential in the bacterium Shewanella oneidensis strain MR-1. We report that there were significant increases in electrical current generation during illumination of electrochemical chambers containing S. oneidensis expressing proteorhodopsin. We present evidence that an engineered strain is able to consume lactate at an increased rate when it is illuminated, which is consistent with the hypothesis that proteorhodopsin activity enhances lactate uptake by increasing the proton motive force. Our results demonstrate that there is coupling of a light-driven process to electricity generation in a nonphotosynthetic engineered bacterium. Expression of proteorhodopsin also preserved the viability of the bacterium under nutrient-limited conditions, providing evidence that fulfillment of basic energy needs of organisms may explain the widespread distribution of proteorhodopsin in marine environments.Classic experiments in microbial bioenergetics used light-driven reactions from halobacterial bacteriorhodopsin or the photosynthetic reaction center to provide a temporary driving force for understanding transport and chemiosmotic coupling (6, 7, 19, 35). However, light-driven reactions have not been used in metabolic engineering to alter microbial physiology and production of chemicals. The recent discovery of proteorhodopsin (PR) in ocean microorganisms and the ease with which this membrane protein can be functionally expressed by recombinant bacteria have made possible many engineering strategies previously not available (1, 16). In this paper, we describe progress toward the goal of integrating light-driven reactions with biocatalysis.In contrast to the situation for established industrial microorganisms, such as Escherichia coli, our current understanding of less-studied algal and phototrophic bacteria may limit metabolic engineering strategies which require genetic manipulation. Metabolic engineering strategies using photosynthetic bacteria have focused largely on methods to increase hydrogen production, and improvements rely mainly on engineering of nitrogenase and hydrogenase to produce H₂. Algae appear to be suited to large-scale cultivation for lipid production, but so far little has been done to engineer these organisms (36). In principle, platform microbial hosts capable of producing a diverse range of products could be boosted by addition of light-driven processes from phototrophic metabolism.To demonstrate the feasibility of transferring a light-driven process into a nonphotosynthetic bacterium, we chose to study proteorhodopsin (PR) first because it is one of the simplest mechanisms for harnessing the energy from light. The proteorhodopsins are a group of transmembrane proteins that use the light-induced isomerization of retinal, the oxidative cleavage product of the carotenoid β-carotene, either to initiate signaling pathways or to catalyze the transfer of ions across cell membranes (8). PR was discovered by metagenomic analysis of marine samples (1) and is related to the well-studied bacteriorhodopsin of archaea (33) and rhodopsin (34), a eukaryotic light-sensing protein. The membrane potential generated by light-driven proton pumping by PR has been confirmed to drive ATP synthesis in a heterologous system (25). However, bacteria expressing heterologous PR were shown not to benefit from this pumping activity, as no significant increases in growth rates were observed (9). This led to the suggestion that PR may benefit the organism only under starvation conditions. In agreement with this hypothesis, Gomez-Consarnau et al. (10) have reported that the light-dependent growth rates of a marine flavobacterium that has a native PR are increased only when the organism is cultured under energy-limited conditions.Studies of both native and recombinant systems in which rhodopsins are expressed have generated light-dependent membrane potentials. In membrane vesicles isolated from a native host, the light-dependent membrane potential generated by bacteriorhodopsin provides the driving force for ATP synthesis (35) and uptake of leucine and glutamate (20, 22). More recently, studies of recombinant systems have coupled the membrane potential to other transport processes. In one example, the membrane potential-dependent export of specific toxic molecules increased when E. coli cells expressing both an archaeal rhodopsin and a specific efflux pump were exposed to light (17). In another experiment, starved E. coli cells expressing PR increased the swimming motion of their flagella when they were illuminated (44). Based upon measurements of flagellar motion as a function of light intensity and azide concentration, the proton motive force generated by PR was estimated to be −0.2 V, a value similar to the value for aerobic respiration in E. coli (42).As a nonphotosynthetic host for recombinant PR expression, we chose the dissimilatory metal-reducing bacterium Shewanella oneidensis strain MR-1, which is genetically tractable for engineering and is able to use a variety of terminal electron acceptors, including insoluble metal oxides (11, 30). Key to the ability of this bacterium to reduce metal oxides is a multicomponent extracellular respiratory pathway that transports electrons from menaquinol to cytochromes in the outer membrane. This pathway is composed of a cytoplasmic membrane tetraheme protein (CymA), a periplasmic decaheme protein (MtrA), an integral outer membrane protein (MtrB), and a decaheme lipoprotein (MtrC) that is associated with MtrB (14, 37, 40). The ability of S. oneidensis to reduce extracellular metal oxides has made it possible to harvest electrons from this organism by coupling it to an electrode which serves as the electron acceptor (21). The electron flow to the outer surface allows respiration rates to be measured directly by electrochemistry.In the current work, we introduced PR into an electricity-generating bacterium, S. oneidensis strain MR-1, and demonstrated that there was integration of a light-driven process into the metabolism of a previously nonphotosynthetic organism that resulted in a useful output. We show here that PR allows cells to survive for extended periods in stationary phase and that the presence of light results in an increase in electricity generation. A possible physiological model to explain these effects is discussed.     

Structure,Function, and Insights into the Biosynthesis of a Head-to-Head Hydrocarbon in Shewanella oneidensis Strain MR-1

A polyolefinic hydrocarbon was found in nonpolar extracts of Shewanella oneidensis MR-1 and identified as 3,6,9,12,15,19,22,25,28-hentriacontanonaene (compound I) by mass spectrometry, chemical modification, and nuclear magnetic resonance spectroscopy. Compound I was shown to be the product of a head-to-head fatty acid condensation biosynthetic pathway dependent on genes denoted as ole (for olefin biosynthesis). Four ole genes were present in S. oneidensis MR-1. Deletion of the entire oleABCD gene cluster led to the complete absence of nonpolar extractable products. Deletion of the oleC gene alone generated a strain that lacked compound I but produced a structurally analogous ketone. Complementation of the oleC gene eliminated formation of the ketone and restored the biosynthesis of compound I. A recombinant S. oneidensis strain containing oleA from Stenotrophomonas maltophilia strain R551-3 produced at least 17 related long-chain compounds in addition to compound I, 13 of which were identified as ketones. A potential role for OleA in head-to-head condensation was proposed. It was further proposed that long-chain polyunsaturated compounds aid in adapting cells to a rapid drop in temperature, based on three observations. In S. oneidensis wild-type cells, the cellular concentration of polyunsaturated compounds increased significantly with decreasing growth temperature. Second, the oleABCD deletion strain showed a significantly longer lag phase than the wild-type strain when shifted to a lower temperature. Lastly, compound I has been identified in a significant number of bacteria isolated from cold environments.Currently, there is industrial interest in nongaseous microbial hydrocarbons for specialty chemical applications and, more recently, as high-energy biofuels (20, 27, 34). Microbes produce hydrocarbons of different types, for example, aliphatic isoprenoid compounds (20) and alkanes from fatty aldehyde decarbonylation (10). Fatty aldehyde decarbonylation is not well understood but offers a clean route to diesel fuels from fatty acids.Certain microbes also make a distinctly different class of long-chain hydrocarbons, generally C₂₅ to C₃₃ in chain length, that contain a double bond near the middle of the chain (1, 3, 5, 15, 30, 31, 33, 34). These long-chain olefinic hydrocarbons are thought to derive from processes different than isoprene condensation and decarbonylation mechanisms. This class of hydrocarbons has been shown by carbon-14-labeling studies (2) to derive from fatty acids. The process, described in 1929 by Channon and Chibnall (9), has become known as head-to-head hydrocarbon biosynthesis. Albro and Ditmar (3) defined the head-to-head condensation as coupling of the head (C₁) and the α-carbon (C₂) of two fatty acids with decarboxylation, a reaction that should not be confused with an acyloin-like carboxyl carbon-to-carboxyl carbon coupling. Products of the head-to-head mechanism have been identified in Gram-positive bacteria such as Micrococcus luteus (29, 30) and Arthrobacter aurescens (13) and in Gram-negative bacteria such as Stenotrophomonas maltophilia (28). Micrococcus and Arthrobacter strains produce fatty acids that are methyl branched terminally and subterminally (8, 29, 30). The long-chain olefinic hydrocarbons from those strains similarly contain a mixture of terminal and subterminal methyl group branching (2, 13, 31).Albro and Ditmar (3, 4) acquired direct evidence for the head-to-head mechanism occurring in microbial whole organisms and cell extracts. In cell extracts, it was shown that one of the fatty acid carboxyl groups is lost as carbon dioxide, with the remaining carbon atoms being retained in the resultant hydrocarbon (4). The hydrocarbons contain a double bond at the point of condensation. More recently, Beller et al. described the genes encoding head-to-head fatty acid condensation pathway enzymes from Micrococcus luteus, which are known as ole genes for the olefin products formed (5). Three genes from Micrococcus luteus were shown to confer on Escherichia coli the ability to make long-chain olefinic hydrocarbons. Two recent patent applications by L. Friedman et al. (18 September 2008, WO2008/113041; 4 December 2008, WO2008/147781) also described a three- or four-gene cluster as being involved in head-to-head hydrocarbon biosynthesis to make olefins. The patent applications identified homologs to ole genes in different bacteria, including strains of Shewanella.Bacteria of the genus Shewanella have been heavily studied over the last decade because they are widespread and have the ability to use a startling variety of electron acceptors for respiration (11). There are more than 20 completed genome sequences for Shewanella strains. The model system for studying Shewanella is S. oneidensis MR-1. The genome sequencing of S. oneidensis MR-1 was reported in 2002 (16), and the organism has been shown to be highly amenable to genetic manipulation (11).The present study used Shewanella oneidensis strain MR-1 as a model system to investigate hydrocarbon biosynthetic genes and the possible biological function of the proteins they encode. The hydrocarbon produced by the Ole proteins in S. oneidensis MR-1 was found to be very different from hydrocarbons previously identified as deriving from a head-to-head condensation mechanism (28, 29, 32). The product was identified here as 3,6,9,12,15,19,22,25,28-hentriacontanonaene by chemical modification studies, mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. Previously, a similar polyolefin had been identified in many Antarctic bacteria (22). Cloning of a heterologous oleA gene into S. oneidensis MR-1 was found to produce a completely different set of products. A hydrocarbon deletion mutant showed a distinctly longer growth lag than wild-type cells when shifted to a lower temperature, suggesting that the ole genes in S. oneidensis MR-1 may aid the cells in adapting to a sudden drop in temperature.     

Aerobic training increases the expression of adiponectin receptor genes in the peripheral blood mononuclear cells of young men

Little is known about the effect of exercise training on the expression of adiponectin receptor genes in peripheral blood mononuclear cells (PBMCs). In this study, we investigated the effects of aerobic training on the expression of AdipoR1 and AidpoR2 mRNAs in PBMCs, whole body insulin sensitivity, and circulating adiponectins in men. Thirty young men were randomly assigned to either a control (n=15) or an exercise (n=15) group. Subjects assigned to the exercise group underwent a 12-week jogging and/or running programme on a motor-driven treadmill at an intensity of 60%-75% of the age-based maximum heart rate with duration of 40 minutes per session and a frequency of 5 days per week. Two-way mixed ANOVA with repeated measures was used to test any significant time-by-group interaction effects for the measured variables at p=0.05. We found significant time-by-group interaction effects for waist circumference (p=0.001), VO_2max (p<0.001), fasting insulin (p=0.016), homeostasis model assessment for insulin resistance (HOMA-IR) (p=0.010), area under the curve (AUC) for insulin response during the 75-g oral glucose tolerance test (p=0.002), high-molecular weight (HMW) adiponectin (p=0.016), and the PBMC mRNA levels of AdipoR1 (p<0.001) and AdipoR2 (p=0.001). The exercise group had significantly increased mRNA levels of AdipoR1 and AdipoR2 in PBMCs, along with increased whole body insulin sensitivity and HMW adiponectin, decreased waist circumference, and increased VO_2max compared with the control group. In summary, the current findings suggest that exercise training modulates the expression of AdipoR1 and AdipoR2 mRNAs in PBMCs, implying that manipulation of the expression of these genes could be a potential surrogate for lifestyle intervention-mediated improvements of whole body insulin sensitivity and glucose homeostasis.     

The Mtr Respiratory Pathway Is Essential for Reducing Flavins and Electrodes in Shewanella oneidensis

The Mtr respiratory pathway of Shewanella oneidensis strain MR-1 is required to effectively respire both soluble and insoluble forms of oxidized iron. Flavins (riboflavin and flavin mononucleotide) recently have been shown to be excreted by MR-1 and facilitate the reduction of insoluble substrates. Other Shewanella species tested accumulated flavins in supernatants to an extent similar to that of MR-1, suggesting that flavin secretion is a general trait of the species. External flavins have been proposed to act as both a soluble electron shuttle and a metal chelator; however, at biologically relevant concentrations, our results suggest that external flavins primarily act as electron shuttles for MR-1. Using deletion mutants lacking various Mtr-associated proteins, we demonstrate that the Mtr extracellular respiratory pathway is essential for the reduction of flavins and that decaheme cytochromes found on the outer surface of the cell (MtrC and OmcA) are required for the majority of this activity. Given the involvement of external flavins in the reduction of electrodes, we monitored current production by Mtr respiratory pathway mutants in three-electrode bioreactors under controlled flavin concentrations. While mutants lacking MtrC were able to reduce flavins at 50% of the rate of the wild type in cell suspension assays, these strains were unable to grow into productive electrode-reducing biofilms. The analysis of mutants lacking OmcA suggests a role for this protein in both electron transfer to electrodes and attachment to surfaces. The parallel phenotypes of Mtr mutants in flavin and electrode reduction blur the distinction between direct contact and the redox shuttling strategies of insoluble substrate reduction by MR-1.Shewanella oneidensis strain MR-1 (MR-1) is a facultative anaerobe capable of respiring a variety of substrates, including various metals and metal oxides, a phenotype that is important for bioremediation and metal cycling in natural environments (22, 53). At near-neutral pH, Fe(III) and Mn(IV) often are present as insoluble oxide minerals. Dissimilatory metal-reducing bacteria such as MR-1 have developed pathways to transfer electrons from the interior of the cell to these external terminal electron acceptors. In some bacteria, these pathways also can transfer electrons to electrodes, which can be harnessed for renewable energy and remote biosensor applications (23, 26, 27). Beyond increasing our understanding of this unusual process, applying anaerobic microbial extracellular respiration to new technologies requires a thorough understanding of the molecular dynamics and cellular physiology of electron source utilization (substrate oxidation) and the reduction of insoluble terminal electron acceptor(s). There are four proposed mechanisms to explain how insoluble substrates are reduced by Shewanella: (i) direct contact, (ii) electron shuttling, (iii) chelation, and (iv) electrically conductive appendages (reviewed in reference 18). We will focus on the first three strategies here.Flavins recently have been discovered to accelerate the reduction of both iron oxide minerals (51) and electrodes (30) by MR-1. Riboflavin (vitamin B₂) is a precursor for the biosynthesis of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) (13). Riboflavin and FMN both can be observed to build up in the supernatant of anaerobic and aerobically grown cultures of MR-1 (30, 51). However, the mechanism by which flavins enhance the rate of iron oxide mineral or electrode reduction is unknown, although recent work is consistent with a critical role for these compounds in mediating solid Fe(III) reduction by MR-1 (42). Since soluble (chelated) Fe(III) is reduced faster than insoluble Fe(III) by MR-1 (6), one possible explanation for the enhancement of insoluble iron reduction by flavins is increased available soluble iron via chelation (1, 2, 30). Flavins also may be utilized as redox-active compounds to traffic electrons between extracellular reductases on the surface of the cell and insoluble substrates (30, 51), a process termed electron shuttling (18, 39, 41). The chelation of the terminal electron acceptor during electrode reduction is not relevant when the anode is composed of graphite. Therefore, electron shuttling likely is responsible for the flavin enhancement of current production on poised-potential electrodes (30). However, it is unclear if the chelation of metals by flavins influences insoluble metal reduction by S. oneidensis (30).The Mtr pathway is required for the reduction of metals and electrodes (5, 6, 9, 17). Five primary protein components have been identified in this pathway: OmcA, MtrC, MtrA, MtrB, and CymA (47). Current models of electron transfer in MR-1 assume that electrons from carbon source oxidation are passed via the menaquinone pool to the inner membrane-anchored c-type cytochrome CymA (19, 31). These electrons then are transferred to a periplasmic c-type cytochrome, MtrA, and eventually to outer membrane (OM)-anchored c-type cytochromes MtrC and OmcA, which interact with an integral OM scaffolding protein, MtrB (32, 33, 43). These OM cytochromes then can reduce various substrates, including iron oxides and electrodes (8, 9, 12, 36, 47). Since the Mtr system is required by MR-1 to reduce many different substrates (18), it also could be capable of reducing extracellular flavins. Indeed, electron transfer to carbon electrodes is impaired in strains lacking Mtr pathway components (9, 17), which may be explained simply by a decreased ability to reduce extracellular flavins. The observation that Mtr mutants produce less current on electrodes than the wild type could be due to (i) less current generated per cell (either direct reduction or flavin mediated), (ii) decreased attachment to the electrode surface, (iii) differences in external flavin concentrations, or (iv) a combination of these three possibilities. Determining the specific activity (current produced per unit of attached biomass) of Mtr mutants on electrodes under conditions where flavin levels were controlled would allow for differentiation between these possibilities. To date, this kind of analysis has not been reported.The results presented here extend our knowledge of how S. oneidensis catalyzes the reduction of insoluble substrates. Experiments using a model iron chelator and electron shuttle are consistent with electron shuttling being the primary mechanism by which flavins enhance insoluble iron oxide reduction rates. Moreover, we demonstrate that MR-1 reduces extracellular flavins at physiologically relevant rates and that the Mtr pathway accounts for at least 95% of this activity. The specific activities of various mutant strains lacking Mtr pathway components on poised-potential electrodes also are reported. Our data suggest that MtrC is responsible for most of the electron transfer to carbon electrodes, while OmcA is involved in attachment and has a lesser role in electron transfer. These observations could have broader implications regarding the role of OmcA in the reduction of soluble and insoluble substrates (8, 9, 36).     

Construction and elementary mode analysis of a metabolic model for Shewanella oneidensis MR-1

A stoichiometric model describing the central metabolism of Shewanella oneidensis MR-1 wild-type and derivative strains was developed and used in elementary mode analysis (EMA). Shewanella oneidensis MR-1 can anaerobically respire a diverse pool of electron acceptors, and may be applied in several biotechnology settings, including bioremediation of toxic metals, electricity generation in microbial fuel cells, and whole-cell biocatalysis. The metabolic model presented here was adapted and verified by comparing the growth phenotypes of 13 single- and 1 double-knockout strains, while considering respiration via aerobic, anaerobic fumarate, and anaerobic metal reduction (Mtr) pathways, and utilizing acetate, n-acetylglucosamine (NAG), or lactate as carbon sources. The gene ppc, which encodes phosphoenolpyruvate carboxylase (Ppc), was determined to be necessary for aerobic growth on NAG and lactate, while not essential for growth on acetate. This suggests that Ppc is the only active anaplerotic enzyme when cultivated on lactate and NAG. The application of regulatory and substrate limitations to EMA has enabled creation of metabolic models that better reflect biological conditions, and significantly reduce the solution space for each condition, facilitating rapid strain optimization. This wild-type model can be easily adapted to include utilization of different carbon sources or secretion of different metabolic products, and allows the prediction of single- and multiple-knockout strains that are expected to operate under defined conditions with increased efficiency when compared to wild type cells.     

Genetic and environmental heterogeneity of residual variance of weight traits in Nellore beef cattle

Background

Many studies have provided evidence of the existence of genetic heterogeneity of environmental variance, suggesting that it could be exploited to improve robustness and uniformity of livestock by selection. However, little is known about the perspectives of such a selection strategy in beef cattle.

Methods

A two-step approach was applied to study the genetic heterogeneity of residual variance of weight gain from birth to weaning and long-yearling weight in a Nellore beef cattle population. First, an animal model was fitted to the data and second, the influence of additive and environmental effects on the residual variance of these traits was investigated with different models, in which the log squared estimated residuals for each phenotypic record were analyzed using the restricted maximum likelihood method. Monte Carlo simulation was performed to assess the reliability of variance component estimates from the second step and the accuracy of estimated breeding values for residual variation.

Results

The results suggest that both genetic and environmental factors have an effect on the residual variance of weight gain from birth to weaning and long-yearling in Nellore beef cattle and that uniformity of these traits could be improved by selecting for lower residual variance, when considering a large amount of information to predict genetic merit for this criterion. Simulations suggested that using the two-step approach would lead to biased estimates of variance components, such that more adequate methods are needed to study the genetic heterogeneity of residual variance in beef cattle.