El pasado colonial y la independencia colombiana en los libros de texto escolar

It seems straightforward to assign much of the ability of the Drosophila insulin and TOR signalling pathways to stimulate growth to their ability to promote protein synthesis via activation of dS6K and inactivation of d4E-BP. However, there is a

significant problem with this explanation: other mutations that affect translation do not

have the same mutant phenotypes as dTOR and D rosophila insulin/PI3K pathway

mutants.

A class of dominant mutations exist in Drosophila that are collectively known as

the M inutes. Many of these mutations are within ribosomal genes, so would be

expected to impair the rate of translation when present (Lambertsson, 1998).

Drosophila that are homozygous for Minute mutations die early in development.

Heterozygous animals have a characteristic dominant phenotype, which includes developmental delay during the larval period, and long slender bristles in the adult fly. In contrast to the loss of function mutation of Drosophila insulin/PI3K pathway components, the vast majority of Minute mutations do not have any effects on final body size. This is probably because the extended developmental time compensates for

any reductions in growth rate in Minute heterozygotes. Although weak mutations in the

Drosophila insulin/PI3K signalling pathway do cause some developmental delay, it

appears that this delay is not sufficient to compensate for the reduced growth rates induced, and the resulting adult flies are therefore reduced in size.

Chapter 1: Introduction

Likewise the clonal phenotype of Minute mutations is not comparable to that of

Drosophila insulin/PI3K pathway mutations. Although if the whole animal is made up

of M inute heterozygous cells these cells can survive, slow-growing M in u te

heterozygous cells present in clones that are surrounded by wild type tissue are rapidly eliminated. This effect has been ascribed to a process known as cell competition, where healthy cells can proliferate at the expense of genetically weaker cells (Morata and Ripoll, 1975). This observation can be contrasted with the clonal phenotypes of loss of function mutations in the Drosophila insulin/PI3K pathway, in which slow growing mutant clones are able to survive, even if mutations are strong. However the clones are smaller than any wild-type twin spot clones, and consist of small cells (Neufeld et al., 1998).

How can we reconcile the observations that reducing rates of translation doesn’t seem to lead to overall changes in disc size but reducing insulin/PI3K signalling does? Also, why do Minute heterozygous cells that survive in an entirely mutant animal disappear when they are in competition with wild type cells, whereas heterozygous

Drosophila insulin/PI3K mutant clones do not disappear in this way? One possibility is

that while mutations in the Minute genes lead to a reduced rate of global protein synthesis. D pi 10-mediated post-translational modifications of dS6K or d4E-BP affect

the translation rate of only a subset of RNAs. As I have discussed earlier (in section 1.2.2.4) the phosphorylation of the ribosomal protein S6 by S6K enhances the

translation of mRNAs that contain a 5" terminal oligopyrimidine tract, many of which encode ribosomal proteins or translation elongation factors (Dufner and Thomas, 1999). In addition, different mRNAs display different abilities to compete for binding of the initiation complex that contains eIF4E. So modulating the availability of eIF4E by controlling 4E-BP phosphorylation could also alter the pattern of mRNA translation as opposed to simply altering its rate.

Although this may not be a complete explanation, it is clear that dS6K and

eIF4E are able to alter growth of the fruit fly in a manner that is not achieved by mutations in the Minute genes. The phenotype of ds6k mutant flies has already been described in section 1.3.4. Studies in Drosophila examining the effects of d4E-BPl over-expression, and deIF4E phosphorylation on growth have also recently been carried out, and will be described below.

The Drosophila 4E-BP, also known as Thor, was originally characterised as

Chapter 1: Introduction

although interactions of d4E-BP with Drosophila eIF4E (deIF4E) have been seen in S2

cells, the consensus eIF4E-binding site in d4E-BP is not completely conserved. This raises the question of whether d4E-BP is as important as its mammalian counterpart in translational regulation. The strength of d4E-BP-deIF4E binding could be enhanced by reverting the binding site sequence to the consensus, in a mutant protein known as 4E-BP^^ (Miron et al., 2001). Over-expression of this mutant protein led to cell autonomous reductions in cell size. Moreover, over-expression of 4E -B P^ in the eye disc was able to reduce the overgrowth phenotype associated with over-expression of dAkt, and its over-expression in the wing could enhance the size reductions associated with expression of dominant negative D pi 10 (Miron et al., 2001).

As expression of wild-type d4E-BP had none of the above effects, these results alone do not demonstrate that d4E-BP normally plays a role in Drosophila grow th regulation. However, the data does clearly point to such a role for the initiation factor, deIF4E in imaginai disc growth regulation, and this is corroborated by the observation

that homozygous d e IF 4 E mutants arrest growth during larval development.

Mammalian eIF4E is phosphorylated in response to growth factors, on serine 209. The homologous residue in deIF4E is serine 251. Flies whose deIF4E gene is mutated so that it encodes a non-phosphorylatable protein, in which serine 251 is mutated to alanine, have a reduced viability. Any escapers are small, but remain appropriately proportioned. It is interesting to note that, in contrast to ds6K mutants, where only cell size is affected, both the size and the number of cells in deIF4E^^^^^ flies are affected.

Although the differences in patterns of translation caused by Minute mutations, and mutations in the Drosophila insulin/PI3K pathway may cause the differences in observed phenotype, an alternative explanation is possible: Insulin/PI3K signalling may affect more than just translation rates. For example, other biosynthetic pathways may be modulated by the levels of insulin/PI3K signalling present in a cell.