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Determination of life-span in Caenorbabditis elegans by four clock genes; Genetic analysis of the roles of daf-28 and age-1 in regulating Caenorbabditis elegans dauer formation; A genetic pathway conferring life extension and resistance to UV stress in Caenorbabditis elegans
A ‘digest’ of somerecentpapersof interest in the pri17.q jaumal~. Si~a~ng by theWiKitreceptor tvrosinek&se is negatkly regulated fn viuoby the proteintyrosine phosphatase Shpl R,F. PAULSON, S. VESELY,
K.A. SlMlNOVlTCH
AND A. RERNSTEIN N%t. f&net. 13,3G7-315 Amity& of how different mutations
interact can lead to an understanding of the role that mutant genes normally play in a pathway. Cell signalling has been genetically analysed extensively in rlrosophila and Caenorhnbdftis e&arts, and this paper describes an elegant analysis using the same approach in mice. The receptor tyrosine
kinase Kit is essenrial for the formation of germ ceils, melanocytes and haematopoietic cells among others. The W” mutation in Kit reduces cell signailing and mice homoxygous for the mutation are sterile, unpigmented, severely anaernkz and have a mast cell deficiency. Another mutation, motbeaten (me), has the opposite effect on mast cells: resulting in overprotiferation. This locus encodes a haematopoietic protein tyrosine phosphatase, Shpl When the double mutant, homozygous for Wand me, was examined the mast cells were restored to normal levels the two mu~tions cancel each other out. Shpl could be a negative regulator of Kit signalling, acting in opposition by
removing the phosphates from molecules phospho~Iated in the Kit pathway. Reduction in Rit signaliing can be rescued by a reduction in an opposing phosphatase. Although activation of Kit by ligand binding results in rapid association with Shpi, among other proteins, the substrate of Shpl does not seem to be Kit itself. Fhospho~~tion levels of Kit in response to signalllfflg are unaltered in memutants. However, the phosphorylation of She is increased. She is pan of the pathway that functions to activate MAP kinase; in memutants MAP kinase activity is increased Kit signals, at least in pan, through the MAP k&se pathway, and Shpi acts on She to negatively regulate signalliig. .4
Estimateof the genomicmutationrate deleteriousto overallfitnessin E. coli T.T.
KIROTA AND M. LYNCH
Nature 381,694-696 Small-effect deleterious mutations have been blamed for everything from the evolution of sex to the extinction of rare species. But the charges have been made with little knowledge of how ofren such mutations really occur. The problem, even in a haploid population, is one of detection - leave a bacterial culture to grow and lineages with a small disadvantage will quickly fall by the wayside,
Determination of life-spanin CaenorbabdlrTs elegars by fourclockgenes It.IAKOWSK, Scie?xe
ANP)S. H~MI
272, 1010-1013
Geneticanalysisof the rolesof daf28 andage-1in regulating CuenorbabdlllJelegans dauerformation E.P.M-NE, T. INOUE AND J.H. THOMA4 Gendics 143,1193-l205 Ageneticpathwayconferringlife extensionandresistanceto W stressfn ~e~orbubd~f~s efeguns s. ~~1
AND IX. JOHNSON
Genelics 143,1207-1218 A
mutation in the Caenorbabditls e/egans gene age-l, isolated by M.R. Wass in 1983,cduses worms to live 50% longer than normal. Thii record lifespan extension, long unchalleng~. has now
overtaken by their fitter brethren. Ribota and Lynch started 50 Erchercbirl cofi lines from one single colony and sought to capture deleterious mutations by imposing a single-cell bottleneck every day (simply by picking and streaking a single colony per line each morning% at each bottleneck the line becomes monomo~hic, f&g even disadvantageous m~atio~. The limes were maintained in this manner for 300 days, at which time the changes of fimw were measured, fitness being defmed as exponential growth rate in particularcultune conditions. Mean fitness of the lines decreased linearly over time while the
overall variance of fitness increased liiearly. From the rates of change of these two quantities the hum value of the frequency of ikness-reducing mutations (0.00017 mutations per genome per generation) and the maximum average fmctional loss of fitness per mutation (0.012) were estimated. As expected, the rate of deleterious mutation is much higher (roughly a Zion-fold higher) than the rate of advantageous mutations measured in the same cells by others. But it is intriguing to note that the rate of deleterious mutations per cell generation per megabass is within an order of rnagnitude of that seen in Bvsopbifa. b
been hopelessly outclassed by new aging mutants. Aging mutations appear to fall into two classes. The first, studied by lakowski and Hekimi, identities four clock genes. These mutations, which slow the rate of biological processes on time scales from fractions of a second to days, also slow agmg. Clock mutants live long slow lives, with mean lifespans up to twice that of wild type. The sec. ond class of aging mutations appear to activate a natural mechanism of lifespan extension. A worm that encounters unfavourable conditions early in larval development becomes a developmentally arrested dauer larva until conditions improve. Dauer larvae do not age - even after months of dauer arrest, their adult Iifespan is the normal two weeks. ~u~~on~i~~ve murations cause worms to become dauer larvae under normal conditions. Mutations in a few dauer-constkutive genes can extend adult lifespan even without causing daucr formation. Unlike the clock genes. these rtf (dauer-iormation) genes sknv aging speclficaily,
without affecting the rate of other processes. Malone etal..and Mumkami and Johnson present evidence that many different mutations that extend lifespan may ali act through thll rnechan&m. Based on complementation and mapping data, Malone et a/. suggest ti age-l is identical to the dauer-constitutive gene dafi23. Murakami and Johnson present evidence that lifespan extension by mutations in age-l, daf-2 and spe-26aII cause worms to be resistant to IJV irradiation and all act bough a common gene daf--36 Lakowski and Hekimi present two pieces of evidence that clock mutations and dauer mutations extend lifespan by different mechanisms. First, daf-ibis not necessary for the extension of lifespan by clock mu~tions. tHowever, this result is disputed by‘Munkami andjohnson.) Second, the adult lifespan of a daf-2; c/k-l double mutant is longer than that of either single mutant, and almost six times as long as wild type. If we could do this to ourselves, we wou!d !tve for p5 400 years.
TIG
%PTFMfER
1996 VOL. 12 No. 9
343
K.A. SlMlNOVlTCH
AND A. RERNSTEIN N%t. f&net. 13,3G7-315 Amity& of how different mutations
interact can lead to an understanding of the role that mutant genes normally play in a pathway. Cell signalling has been genetically analysed extensively in rlrosophila and Caenorhnbdftis e&arts, and this paper describes an elegant analysis using the same approach in mice. The receptor tyrosine
kinase Kit is essenrial for the formation of germ ceils, melanocytes and haematopoietic cells among others. The W” mutation in Kit reduces cell signailing and mice homoxygous for the mutation are sterile, unpigmented, severely anaernkz and have a mast cell deficiency. Another mutation, motbeaten (me), has the opposite effect on mast cells: resulting in overprotiferation. This locus encodes a haematopoietic protein tyrosine phosphatase, Shpl When the double mutant, homozygous for Wand me, was examined the mast cells were restored to normal levels the two mu~tions cancel each other out. Shpl could be a negative regulator of Kit signalling, acting in opposition by
removing the phosphates from molecules phospho~Iated in the Kit pathway. Reduction in Rit signaliing can be rescued by a reduction in an opposing phosphatase. Although activation of Kit by ligand binding results in rapid association with Shpi, among other proteins, the substrate of Shpl does not seem to be Kit itself. Fhospho~~tion levels of Kit in response to signalllfflg are unaltered in memutants. However, the phosphorylation of She is increased. She is pan of the pathway that functions to activate MAP kinase; in memutants MAP kinase activity is increased Kit signals, at least in pan, through the MAP k&se pathway, and Shpi acts on She to negatively regulate signalliig. .4
Estimateof the genomicmutationrate deleteriousto overallfitnessin E. coli T.T.
KIROTA AND M. LYNCH
Nature 381,694-696 Small-effect deleterious mutations have been blamed for everything from the evolution of sex to the extinction of rare species. But the charges have been made with little knowledge of how ofren such mutations really occur. The problem, even in a haploid population, is one of detection - leave a bacterial culture to grow and lineages with a small disadvantage will quickly fall by the wayside,
Determination of life-spanin CaenorbabdlrTs elegars by fourclockgenes It.IAKOWSK, Scie?xe
ANP)S. H~MI
272, 1010-1013
Geneticanalysisof the rolesof daf28 andage-1in regulating CuenorbabdlllJelegans dauerformation E.P.M-NE, T. INOUE AND J.H. THOMA4 Gendics 143,1193-l205 Ageneticpathwayconferringlife extensionandresistanceto W stressfn ~e~orbubd~f~s efeguns s. ~~1
AND IX. JOHNSON
Genelics 143,1207-1218 A
mutation in the Caenorbabditls e/egans gene age-l, isolated by M.R. Wass in 1983,cduses worms to live 50% longer than normal. Thii record lifespan extension, long unchalleng~. has now
overtaken by their fitter brethren. Ribota and Lynch started 50 Erchercbirl cofi lines from one single colony and sought to capture deleterious mutations by imposing a single-cell bottleneck every day (simply by picking and streaking a single colony per line each morning% at each bottleneck the line becomes monomo~hic, f&g even disadvantageous m~atio~. The limes were maintained in this manner for 300 days, at which time the changes of fimw were measured, fitness being defmed as exponential growth rate in particularcultune conditions. Mean fitness of the lines decreased linearly over time while the
overall variance of fitness increased liiearly. From the rates of change of these two quantities the hum value of the frequency of ikness-reducing mutations (0.00017 mutations per genome per generation) and the maximum average fmctional loss of fitness per mutation (0.012) were estimated. As expected, the rate of deleterious mutation is much higher (roughly a Zion-fold higher) than the rate of advantageous mutations measured in the same cells by others. But it is intriguing to note that the rate of deleterious mutations per cell generation per megabass is within an order of rnagnitude of that seen in Bvsopbifa. b
been hopelessly outclassed by new aging mutants. Aging mutations appear to fall into two classes. The first, studied by lakowski and Hekimi, identities four clock genes. These mutations, which slow the rate of biological processes on time scales from fractions of a second to days, also slow agmg. Clock mutants live long slow lives, with mean lifespans up to twice that of wild type. The sec. ond class of aging mutations appear to activate a natural mechanism of lifespan extension. A worm that encounters unfavourable conditions early in larval development becomes a developmentally arrested dauer larva until conditions improve. Dauer larvae do not age - even after months of dauer arrest, their adult Iifespan is the normal two weeks. ~u~~on~i~~ve murations cause worms to become dauer larvae under normal conditions. Mutations in a few dauer-constkutive genes can extend adult lifespan even without causing daucr formation. Unlike the clock genes. these rtf (dauer-iormation) genes sknv aging speclficaily,
without affecting the rate of other processes. Malone etal..and Mumkami and Johnson present evidence that many different mutations that extend lifespan may ali act through thll rnechan&m. Based on complementation and mapping data, Malone et a/. suggest ti age-l is identical to the dauer-constitutive gene dafi23. Murakami and Johnson present evidence that lifespan extension by mutations in age-l, daf-2 and spe-26aII cause worms to be resistant to IJV irradiation and all act bough a common gene daf--36 Lakowski and Hekimi present two pieces of evidence that clock mutations and dauer mutations extend lifespan by different mechanisms. First, daf-ibis not necessary for the extension of lifespan by clock mu~tions. tHowever, this result is disputed by‘Munkami andjohnson.) Second, the adult lifespan of a daf-2; c/k-l double mutant is longer than that of either single mutant, and almost six times as long as wild type. If we could do this to ourselves, we wou!d !tve for p5 400 years.
TIG
%PTFMfER
1996 VOL. 12 No. 9
343