The role of insulin signalling in the endocrine stress response in Drosophila melanogaster: A mini-review.

The endocrine stress response in Drosophila includes catecholamines, juvenile hormone (JH), 20-hydroxyecdysone (20E) and the insulin/insulin-like growth factor signalling pathway (IIS). Several changes in the IIS and hormonal status that occur under unfavourable conditions are universal and do not depend on the nature of stress exposure. The reviewed studies on the impact of different element of the Drosophila IIS, such as insulin-like receptor, the homologue of its substrate, CHICO, the transcription factor dFOXO and insulin like peptide 6, on the hormonal status suggest that the IIS controls catecholamine metabolism indirectly via JH, and there is a feedback loop in the interaction of JH and IIS. Moreover, at least one of the ways in which the IIS is involved in the control of stress resistance is mediated through JH/dopamine signalling.

Insulin-like peptide DILP6 regulates juvenile hormone and dopamine metabolism in Drosophila females.

Insulin-like peptide DILP6 is a component of the insulin/insulin-like growth factor signalling pathway of Drosophila. Juvenile hormone (JH) and dopamine (DA) are involved in the stress response and in the control of reproduction. In this study, we investigate whether DILP6 regulates the JH and DA levels by studying the effect of a strong hypomorphic mutation dilp641 on JH and DA metabolism in D. melanogaster females. We show that DILP6 regulates JH and DA metabolism: the mutation dilp641 results in a reduction in JH-hydrolysing activity and an increase in the activities of DA synthesis enzymes (alkaline phosphatase (ALP) and tyrosine hydroxylase (TH)). In the mutant females, we also found increased fecundity in addition to the intensity of the response (stress reactivity) of ALP and TH to heat stress. As we showed previously, this suggests an increased level of JH synthesis. We confirm this suggestion by treating the mutant females with the JH inhibitor, precocene, which restors the activity and stress reactivity of ALP and TH as well as fecundity to levels similar to those in the control flies. The data suggest a feedback system in the interaction between JH and DILP6 in which DILP6 negatively regulates the JH titre via an increase in the hormone degradation and a decrease in its synthesis.

Mutation in the Drosophila insulin-like receptor substrate, chico, affects the neuroendocrine stress-reaction development.

It is shown for the first time that the insulin receptor substrate gene chico controls the functioning of the systems of metabolism of dopamine and juvenile hormone in Drosophila melanogaster females under normal conditions and in thermal stress.

[Gene dilp6 regulates octopamine metabolism in Drosophila melanogaster].

The effect of strong hypomorphic mutation of the insulin-like protein gene (dilp6) on metabolism of octopamine (one of the main biogenic amines in insects) was studied in Drosophila melanogaster males and females. The activity of tyrosine decarboxylase (the key enzyme of octopamine synthesis) and the activity of octopamine-dependent N-acetyltransferase (the enzyme of its degradation) were measured. It was demonstrated that the activity of both studied enzymes is decreased under normal conditions in the dilp6 41 mutants (as we previously demonstrated, this is correlated with an increased level of octopamine). It was also found that hypomorphic mutation of the dilp6 gene decreases the intensity of tyrosine decarboxylase response to heat stress. Thus, it was demonstrated for the first time that insulin-like DILP6 protein in drosophila influences the level of octopamine (regulating the activity of the enzyme degrading octopamine).

The insulin-like receptor gene expression in the tissues synthesizing gonadotropic hormones at sexual maturation of Drosophila melanogaster females].

The insulin/insulin-like growth factor signaling pathway is involved in the regulation of the synthesis of insect gonadotropic hormones, juvenile (JH) and 20-hydroxyecdysone (20E). We carried out the immunohistochemical analysis of the insulin receptor (InR) expression in the corpus allatum (the JH-producing gland) and in the ovarian follicular cells (a site for the synthesis of 20E precursor, ecdysone) in the process of sexual maturation of D. melanogaster females and examined the influence of exogenous JH on the InR expression in these tissues. For the first time, it was demonstrated that InR was expressed in follicular cells and that its expression in corpus allatum and follicular cells of Drosophila females was stage-specific, i.e., the expression intensity in young females greatly exceeded that in mature individuals. We also found a negative feedback loop in the regulation of JH levels by the insulin signaling pathway in Drosophila adults: the experimental increase in the JH titers in young females dramatically reduced the InR expression intensity in corpus allatum and follicular cells.

[dFOXO Transcription Factor Regulates Juvenile Hormone Metabolism in Drosophila melanogaster Females].

dFOXO transcription factor is a component of the insulin/insulin-like growth factor signaling pathway in Drosophila. Juvenile hormone negatively regulates dFOXO gene expression. In the present work, the effect of hypomorphic dFOXO mutation on juvenile hormone metabolism under normal and stressing conditions and on D. melanogaster female resistance to thermal stress was studied. It was demonstrated that dFOXO mutation in D. melanogaster females induces (1) an increase in the level of juvenile hormone degradation and in the intensity of the response of the juvenile hormone metabolism system to thermal stress and (2) a decrease in thermal stress resistance. These parameters are indicators of the level of juvenile hormone synthesis and indicate its decrease in females with decreased dFOXO expression. Thus, the presence of feedback in the regulation of dFOXO gene expression by juvenile hormone was established for the first time.

[Knockdown of InR gene in ventral nephrocytes promotes resistance to toxic stress in Drosophila melanogaster females].

Hemolymph filtration in insects is performed by nephrocytes, additional cells of the circulatory system that are not connected to Malpighian vessels. Drosophila has two types of nephrocytes: the ventral ("garland"), which are situated around the connection site of the esophagus and proventriculus, and the pericardial, which are localized around the heart. In this study, we examined the role of the of insulin-like receptor (InR)gene in regulation of the function of ventral nephrocytes (VNC) in D. melanogaster females. Immunofluorescent analysis of female VNC with anti-InR antibodies revealed for the first time that the InR gene is expressed in VNC cells. To determine whether a change in the level of InR expression has an effect on VNC function in Drosophila females, we implemented an antisense suppressor of the InR gene, together with a driver that is expressed specifically in VNC. VNC function was evaluated by survival of the females exposed to toxic stress (treatment with AgNO3). This study has shown for the first time that suppression of InR expression in VNC leads to a rise in the survival of flies under conditions of toxic stress.

[InR gene expression and octopamine metabolism in Drosophila melanogaster females].

The influence of suppression of the expression of the Drosophila insulin-like receptor gene (InR) in corpus allatum (the gland-synthesizing juvenile hormone) on octopamine and juvenile hormone metabolism and on the development of the stress-reaction in Drosophila melanogaster females was studied. It was demonstrated that the suppression of InR gene expression in corpus allatum induces in D. melanogaster females an increase in the activity of the enzyme that limits the rate of octopamine synthesis (tyrosine decarboxylase), as well as in the level of juvenile hormone degradation and the intensity of the response of octopamine and juvenile hormone metabolism systems to heat stress. It was mentioned that a decrease in InR gene expression in corpus allatum does not influence the activity of OA-dependent N-acetyltransferase (the enzyme that degrades octopamine). It was established that the influence of suppression of the InR gene expression in corpus allatum on octopamine metabolism is mediated by juvenile hormone, since the processing of flies by exogenous juvenile hormone restores the activity oftyrosine decarboxylase in flies with decreased InR expression in corpus allatum up to the normal level.

[P-M hybrid dysgenesis affects juvenile hormone metabolism in Drosophila melanogaster females].

This paper studies the metabolism of the juvenile hormone, which affects gonads functioning in Drosophila melanogasterfemales under P-M hybrid dysgenesis. It is shown that dysgenic females grown at 29°C have increased levels of the juvenile hormone (its degradation and stress reactivity are reduced), which apparently is a compensatory response to ovarian hypoplasia.

Role of D1- and D2-like receptors in age-specific regulation of juvenile hormone and 20-hydroxyecdysone levels by dopamine in Drosophila.

In Drosophila females, the precursor of 20-hydroxyecdysone (20E), ecdysone, is synthesized in the ovary follicular cells. Juvenile hormone (JH) is synthesized de novo in the specialized endocrine gland, corpus allatum (CA); JH degradation is carried out by the enzymes synthesized in the fat body (FB). Earlier we have shown that in Drosophila females, dopamine up- or down-regulates 20E and JH levels depending on the developmental stage. The present study focuses on the role of D1- and D2-like receptors in the age-specific regulation of 20E and JH metabolism by dopamine. We show that in Drosophila melanogaster females D1- and D2-like receptor genes (DopR and DD2R accordingly) are expressed in CA and FB, but not in follicular cells. The level of DopR expression in CA is much higher in the young than in the mature females, whereas DD2R expression is much higher in the mature than in the young females. The DopR and DD2R expression patterns in FB are opposite to those observed in CA. In addition, we obtained data suggesting that the age-specific changes of DopR and DD2R expression in FB are under the control of 20E.

Decrease in juvenile hormone level as a result of genetic ablation of the corpus allatum cells affects the synthesis and metabolism of stress related hormones in Drosophila.

Previous studies have shown that juvenile hormone (JH) regulates dopamine (DA) and octopamine (OA) content in Drosophila, and we have shown the influence of an increase in JH level on DA and OA metabolism in young females of Drosophila virilis and Drosophila melanogaster. Here we investigate the effects of genetic ablation of a subset of cells in the Corpusallatum (CA, endocrine gland synthesizing JH) on the DA levels and activities of alkaline phosphatase (ALP), tyrosine hydroxylase (TH), DA-dependent arylalkylamine N-acetyltransferase (DAT) and tyrosine decarboxylase (TDC) in young D. melanogaster females under normal conditions and upon heat stress (38°Ð¡). We show that ablation of СА cells causes: (1) a decrease in ALP, TH and DAT activities, (2) an increase in DA level and (3) an increase in TDC activity in young females. The CA ablation was also found to modulate ALP, TH and TDC responses to heat stress. Mechanisms of regulation of DA and OA levels by JH in Drosophila females are discussed.

Downregulation of the dopamine D2-like receptor in corpus allatum affects juvenile hormone synthesis in Drosophila melanogaster females.

In Drosophila, juvenile hormone (JH) is synthesized de novo in the specialized endocrine gland, corpusallatum (CA). Dopamine (DA) controls JH levels by either stimulating or inhibiting its synthesis and degradation depending on the developmental stage. The present study focuses on the role of D2-like receptors in the regulation of JH synthesis by dopamine. We show that D2-like receptors (DD2R) are expressed in CA cells of Drosophila melanogaster females. In addition, the level of JH production was analyzed in D. melanogaster females with decreased DD2R expression in CA (vs. corresponding control flies) by assessing multiple indices of JH synthesis (JH-hydrolyzing activity and stress reactivity of the system of JH metabolism, activity and stress reactivity of the alkaline phosphatase, activity and stress reactivity of the tyrosine decarboxylase). The differential value obtained for each index suggests increased JH production in female flies that downregulate DD2R. Based on these findings, we postulate that the DA inhibiting effect on the JH synthesis in D. melanogaster is mediated at least in part via D2-like receptors.

Mechanisms of age-specific regulation of dopamine metabolism by juvenile hormone and 20-hydroxyecdysone in Drosophila females.

20-hydroxyecdysone (20E) and the juvenile hormone (JH) have an age-specific effect on total dopamine (DA) content in Drosophila (Gruntenko and Rauschenbach 2008). Earlier we studied the mechanism of influence of 20E and JH on DA metabolism in young females (Rauschenbach et al. in J Insect Physiol 53:587-591, 2007a: Arch Insect Biochem Physiol 65:95-102, 2008a; Gruntenko et al. in Arch Insect Biochem Physiol 72:263-269, 2009). Here we investigate the effects of 20E and JH on the activities of the alkaline phosphatase (ALP), tyrosine hydroxylase (TH) and DA-dependent arylalkylamine N-acetyltransferase (AANAT) in mature females of wild type D. virilis under normal conditions and under heat stress (38°C). 20E feeding of the flies led to a substantial decrease in ALP and TH activities and to an increase in AANAT activity in mature females. JH application resulted in an increasing of ALP and TH activities, but did not influence AANAT activity in mature females. A rise in JH and 20E levels was found to change ALP and TH stress reactivities. Mechanisms of age-specific regulation of DA level by 20E and JH in Drosophila females are discussed.