Ecdysteroid receptors located in the central nervous system of an insect

Control of ecdysteroidogenesis in prothoracic glands of insects: a review

The very first step in the study of the endocrine control of insect molting was taken in 1922. Stefan Kopec characterized a factor in the brain of the gypsy moth, Lymantria dispar which appeared to be essential for metamorphosis. This factor was later identified as the neuropeptide prothoracicotropic hormone (PTTH), the first discovery of a series of factors involved in the regulation of ecdysteroid biosynthesis in insects. It is now accepted that PTTH is the most important regulator of prothoracic gland (PG) ecdysteroidogenesis. The periodic increases in ecdysteroid titer necessary for insect development can basically be explained by the episodic activation of the PGs by PTTH. However, since the characterization of the prothoracicostatic hormone (PTSH), it has become clear that in addition to 'tropic factors', also 'static factors', which are responsible for the 'fine-tuning' of the hemolymph ecdysteroid titer, are at play. Many of these regulatory factors are peptides originating from the brain, but also other, extracerebral factors both of peptidic and non-peptidic nature are able to affect PG ecdysteroidogenesis, such as the 'classic' insect hormones, juvenile hormone (JH) and the molting hormone (20E) itself. The complex secretory pattern of ecdysteroids as observed in vivo is the result of the delicate balance and interplay between these ecdysiotropic and ecdysiostatic factors.

Neural and hormonal regulation of growth of corpora allata in the cockroach, Diploptera punctata

DNA synthesis and mitosis in the corpora allata (CA) of adult Diploptera punctata males were investigated with total cell count after 5'-bromo-2'-deoxyuridine immunodetection and colchicine arrestment both in vivo and in vitro. The CA exhibited a single wave of DNA synthesis followed by cell division during the first 4 days after the imaginal ecdysis. A second mitotic wave was experimentally induced after the nervous connections between the CA and the brain were severed on day 4. Spontaneous mitosis was abolished in cockroaches treated with a juvenile hormone (JH) analog. This inhibitory regulation in vivo appeared to act through brain neurosecretory cells since in the denervated CA mitotic activity was unaffected by JH treatment. An in vitro system supporting growth of the corpus allatum was established to study direct hormonal effects. By using continuous bromodeoxyuridine labeling in vitro for 6 days, we showed that DNA synthesis of corpus allatum cells was unaffected by direct contact with JH. In contrast, 20-hydroxyecdysone exerted direct mitogenic action on allatal cells. These and previous results suggest that CA cells alternate between JH synthesis and a proliferative state in which they divide in a self-renewing fashion to yield differentiated progeny. We propose that in newly enclosed adult Diploptera punctata males, low JH titer and high ecdysteroid titer promote mitosis in CA cells. As the ecdysteroid titer declines, JH produced by the CA acts on brain neurosecretory cells which dispatch inhibitory signals through nerves to prevent continuous proliferation of CA cells.

An ecdysiostatin from flies

A bioassay for compounds with ecdysiostatic activity ('ecdysiostatins') was developed in order to prove the existence of an ecdysiostatin in blowfly larvae. The factor eluted by HPLC like Neb-TMOF (trypsin modulating oostatic factor), a hexapeptide that inhibits ecdysone biosynthesis. The ecdysiostatic activity of Neb-TMOF is specific, related peptides were less active or inactive. TMOF inhibits ecdysone biosynthesis rather than its secretion. It increases the cAMP level of ring glands. Effectors that elevate the intracellular cAMP can mimic the inhibitory effect of the ecdysiostatin. This suggests that cAMP may control steroidogenesis in fly larvae by shutting ecdysone biosynthesis. Thus, the hexapeptide acts as a prothoracicostatic hormone (PTSH) that controls ecdysone biosynthesis.

How schistosomes profit from the stress responses they elicit in their hosts

Results obtained with the model Trichobilharzia ocellata-Lymnaea stagnalis have confirmed the hypothesis that the physiological effects evoked by schistosomes in their snail host--castration and giant growth--are brought about by them interfering with the neuroendocrine systems (NES) regulating the physiological processes concerned. As soon as differentiating cercariae are present in the daughter sporocysts a factor can be detected in the haemolymph of the snail host, called schistosomin, which acts both at the central and the peripheral parts of the NES involved in regulation of reproduction and growth. Schistosomin appears to be a host-derived factor, which is probably released by cells of the internal defence system, the haemocytes, and by connective tissue cells, the telo-glial cells. It meets the criteria of having a cytokine-like function although its molecular structure does not show sequence homology with any of the vertebrate-type cytokines identified to date. Its cytokine nature explains why schistosomin can interfere with different neuroendocrine regulatory systems both at the central and peripheral--target--level, namely after binding to its own receptor. Schistosomin is probably not only responsible for the effects exerted by the parasite on female reproduction but also for those on male reproduction and on growth so that energy and space become available for the continuous production of cercariae. The nature of the humoral cercarial factor, which induces schistosomin release, is as yet unknown. Based on its hydrophobic character and on the fact that it can pass through the wall of the daughter sporocyst, it is supposed to be a diffusible molecule or a protonephridial excretion product. It does not seem to be a vertebrate-type steroid, an ecdysteroid or an eicosanoid. Results obtained in vitro have indicated that schistosomin might have a suppressive effect on haemocyte activity. Plasma from snails 5-6 weeks post-exposure showed a tendency to inhibit phagocytic activity of haemocytes from non-infected snails, that is preparatory to the escape and migration of cercariae. Once shedding has started this effect of schistosomin is overrruled by a strong activation of haemocyte activity coinciding with the tissue damage that the cercariae cause in the host. The cercariae escape from being attacked by masking their surface coat with host molecules. As the physiological effects caused by schistosomes resemble those observed during stress in mammals, experiments were carried out to find out whether schistosomin is also released in non-parasitized snails during stress resulting in an inhibiting effect on reproduction.(ABSTRACT TRUNCATED AT 400 WORDS)

Ecdysteroid binding sites in gastrolith forming tissue and stomach during the molting cycle of crayfish Procambarus clarkii

The distribution of ecdysteroid binding sites in the stomach and gastrolith disc tissue of crayfish (Procambarus clarkii) was examined in relation to the molting stage by thaw-mount autoradiography. The radiolabeled hormone analogue ponasterone A (25-deoxy-20-hydroxyecdysone) was used. Ecdysteroid binding sites were demonstrated only in certain molting stages, the small gastrolith period and the aftermolt stage. In gastrolith epithelium, ponasterone A binding sites first appeared in the cytoplasm, and then in the nuclei and cytoplasm. In the stomach epithelium, many nuclear binding sites were detectable during the period of gastrolith secretion. These periodical changes in specific ponasterone A binding when correlated with the molting stages clearly show that ecdysteroids may function as an initiator for gastrolith formation and reabsorption. The findings also suggest that ecdysteroids control calcium transport in the stomach epithelium. The time-related and functional differences of cytoplasmic and nuclear concentration of ecdysteroid receptors indicate the presence of cytoplasmic and nuclear receptors associated with specific actions.

Developmental regulation of ecdysteroid receptors in the nervous system of Manduca sexta

The technique of steroid hormone autoradiography has been used to study the cellular distribution of ecdysteroid binding sites in the ventral nervous system of the tobacco hornworm moth, Manduca sexta. The ligand was 26-[125I]iodoponasterone. Tissue was examined from the subesophageal ganglia, thoracic ganglia, and abdominal ganglia of larvae at two times during the larval-pupal transient: the 2nd day of wandering and the prepupal stage. The patterns of neuronal binding seen were compared with those found in earlier autoradiographic studies of hormone binding in tissue sampled on the 1st day of wandering, in the pharate adult, and in the 4-day-old moth (Fahrbach and Truman, '89). The pattern of binding was reproducible but dependent upon developmental stage: whereas only a subset of neurons exhibited nuclear accumulation of radiolabeled ecdysteroids on the 1st day of wandering, less than 24 hours later nearly every neuron in the ventral nervous system was labeled. A limited pattern of binding, however, was seen again in the prepupal nervous system. Thus, the insect nervous system is able to use a single hormone both as a general cue for metamorphic development and as a single targeted to stage-specific subsets of neurons by alternating periods of ubiquitous expression of receptor with periods during which the capacity to bind the steroid hormone is highly restricted.

Developmental changes in the presence of ecdysteroid receptors in the central nervous system of third instar larvae of Sarcophaga bullata

Radiolabeled ponasterone A, a high affinity ligand for ecdysteroid receptors which agonises the effects of 20-hydroxyecdysone, was used in combination with thaw-mount autoradiography to study the stage-specific presence of ecdysteroid receptors in the central nervous system of Sarcophaga bullata. In third instar larvae, nuclear high affinity binding of tritiated or iodinated ponasterone A occurs in the same target cells and both radioligands were displaced by an 100-fold excess of unlabeled ponasterone A or an 500-fold excess of 20-hydroxyecdysone. Target neurons for ponasterone A appear first in the third instar larvae on day 4.0 (early wandering stage) where many cells of the perineurium, ring gland, lateral neurosecretory cells in the brain and certain neurons in abdominal ganglia exhibit nuclear high-affinity binding for ponasterone A. At day 5.5 after larviposition, less binding is present in the perineurium but many neurons, including certain neurosecretory cells in the pars intercerebralis, pars lateralis, tritocerebrum, and neurosecretory cell groups 7, 8, 9, and 10 of the dipteran suboesophageal and abdominal ganglia show increased nuclear ecdysteroid binding. At this stage nuclear binding also occurs in the ring gland except in the central corpus allatum and for the first time in the neurons of the inner optic lobes. The results show that ecdysteroid receptors are present in distinct cerebral neurons and that their expression or ecdysteroid-binding capability is under developmental control.

Uptake, distribution and binding of vertebrate and invertebrate steroid hormones and time-dependence of ponasterone A binding in Calliphora vicina. Comparisons among cholesterol, corticosterone, cortisol, dexamethasone, 5 alpha-dihydrotestosterone, 1,25-dihydroxyvitamin D3, ecdysone, estradiol-17 beta, ponasterone A, progesterone, and testosterone

The presence of specific binding sites for radiolabelled vertebrate-type and arthropod-type steroid hormones was investigated in several organs including salivary gland, and central nervous system of third instar Calliphora vicina larvae by thaw-mount autoradiography. Ponasterone A, a 20-hydroxyecdysone agonist and 20-hydroxyecdysone are the only steroids which bind to nuclear high affinity binding sites. These binding sites are DNA associated while nucleoli show no tracer binding. Ecdysone, an endogenous 20-hydroxyecdysone precursor, is taken up by target cells but no significant nuclear binding occurs. 1,25-Dihydroxyvitamin D3 concentrates in cytoplasm only and its uptake is highest compared to all other steroids. Progesterone and testosterone show weak accumulation in the cytoplasm, while for cholesterol, corticosterone, cortisol, dexamethasone, dihydrotestosterone and estradiol-17 beta, no noticeable uptake occurs. For ponasterone A, a clear time dependence of uptake and intracellular distribution is visible, suggesting the existence and involvement of specific ecdysteroid uptake and transport mechanisms. These results suggest the presence of binding sites for various mammalian steroids in insects. Whether vertebrate steroid hormones or metabolites of them play a role in insects or whether the uptake and binding is based on chemical similarities alone without biological significance remains to be further investigated.

Ecdysteroid receptors in the central nervous system of Manduca sexta: their changes in distribution and quantity during larval-pupal development

Ecdysteroids act initially by binding to nuclear and possibly also extranuclear receptors. The presence and expression of these receptors in the insect brain was investigated in the present study as a means of defining these neurons involved in ecdysteroid-regulated processes at different developmental stages. Early in the fifth larval stadium of Manduca sexta, when endogenous ecdysteroid levels are low, receptors for ecdysteroids in cerebral neurons are either absent or present at low levels. Receptors can be reliably detected only on day 0 and are not found again until day 3.5, at the beginning of the commitment peak in the ecdysteroid titer, when they occur in a small stage-specific population of cells. At this time, ecdysteroid receptors are found mainly in nuclei but are also observed at low levels in cytoplasm. By day 4.8, ecdysteroid receptors are exclusively nuclear, and the number of target cells has increased dramatically in several brain regions, including those with known neurosecretory cell groups. This population and organization of ecdysteroid target cells is constant up to day 6, after which time the number of target neurons declines. By day 7.8, only 10% of the number of labelled neurons seen on days 4.8-6.8 remain in peripheral areas. In the pupal brains, ecdysteroid receptors reappear in a new population of neurons. The results indicate changes in the genomic regulation of a varying neuron population by ecdysteroids during fifth stadium development.

Ecdysteroid receptors of the blowfly Calliphora vicina. Characterization of binding to nonspecific DNA

Ecdysteroids, the molting hormones of arthropods, act like vertebrate steroid hormones by binding to an intracellular receptor protein. We have recently isolated a protein from nuclei of blowfly larvae which has satisfied the requirements of an ecdysteroid receptor. The receptor was partially purified and its ecdysteroid-binding properties were characterized. The availability of receptor preparations which have been stabilized by partial purification now enables us to study the general DNA-binding properties of ecdysteroid receptors. DNA-binding characteristics of ecdysteroid receptors were studied with calf thymus DNA. Affinity for DNA was observed both in the presence and in the absence of steroid ligand but the ligand clearly enhanced binding of receptors to DNA. Receptor preparations contained a heterogeneous mixture of receptors; up to 25% of DNA-binding receptors, and nonbinding forms of ecdysteroid receptors. The ability to bind to DNA was subject to inactivation which was not affected by partial purification, but which could be decelerated by dilution of the receptor preparation. Thus, dilution resulted in a spurious activation of DNA binding. A genuine activation, which would have led to an increase in the percentage of the DNA-binding form of the ecdysteroid receptor complex, was not observed.