In vitro synthesis, release and uptake of storage proteins by the fat body of Manduca sexta: putative hormonal control

Presynaptic contact and activity opposingly regulate postsynaptic dendrite outgrowth

The organization of neural circuits determines nervous system function. Variability can arise during neural circuit development (e.g. neurite morphology, axon/dendrite position). To ensure robust nervous system function, mechanisms must exist to accommodate variation in neurite positioning during circuit formation. Previously, we developed a model system in the Drosophila ventral nerve cord to conditionally induce positional variability of a proprioceptive sensory axon terminal, and used this model to show that when we altered the presynaptic position of the sensory neuron, its major postsynaptic interneuron partner modified its dendritic arbor to match the presynaptic contact, resulting in functional synaptic input (Sales et al., 2019). Here, we investigate the cellular mechanisms by which the interneuron dendrites detect and match variation in presynaptic partner location and input strength. We manipulate the presynaptic sensory neuron by (a) ablation; (b) silencing or activation; or (c) altering its location in the neuropil. From these experiments we conclude that there are two opposing mechanisms used to establish functional connectivity in the face of presynaptic variability: presynaptic contact stimulates dendrite outgrowth locally, whereas presynaptic activity inhibits postsynaptic dendrite outgrowth globally. These mechanisms are only active during an early larval critical period for structural plasticity. Collectively, our data provide new insights into dendrite development, identifying mechanisms that allow dendrites to flexibly respond to developmental variability in presynaptic location and input strength.

Larval fat body cells die during the early pupal stage in the frame of metamorphosis remodelation in Bombyx mori

In holometabolus insects, morphology of the larval fat body is remodeled during metamorphosis. In higher Diptera, remodeling of the fat body is achieved by cell death of larval fat body cells and differentiation of the adult fat body from primordial cells. However, little is known about remodeling of the fat body at pupal metamorphosis in Lepidoptera. In this study, we found that cell death of the larval fat body in Bombyx mori occurs at shortly after pupation. About 30% of the fat body cells underwent cell death on days 1 and 2 after pupation. The cell death involved genomic DNA fragmentation, a characteristic of apoptosis. Surgical manipulation and in vitro culture of fat body cells revealed that 20-hydroxyecdysone and juvenile hormone had no effect on either initiation or progression of cell death. During cell death, a large increase in activity of caspase-3, a key enzyme of cell death, was observed. Western blot analysis of the active form of caspase-3-like protein revealed that the length of caspase-3 of B. mori was much larger than that of caspase-3 in other species. The results suggest that larval fat body cells of B. mori are removed through cell death, which is mediated by a caspase probably categorized in a novel family.

Hormonal regulation of the expression of two storage proteins in the larval fat body of the greater wax moth (Galleria mellonella)

During larval development of the greater wax moth, Galleria mellonella, genes of storage proteins LHP76 and LHP82 are tissue- and stage-specifically expressed. In this study, hormonal regulation of this expression has been investigated in vivo. Messenger RNAs of the juvenile hormone (JH-suppressible) Lhp82 gene are present only during the feeding period of the final larval instar, suggesting that a high level of JH during earlier stages prevents its expression and that a small rise in JH titer observed on day 8 of the final larval instar is responsible for the rapid shut-off of its transcription. Application of 1micro g of JH analog (fenoxycarb) specifically inhibits expression of Lhp82, whereas Lhp76 mRNAs remain at the same level. 20-hydroxyecdysone (20HE) does not exert any inhibitory effects on transcription of Lhp genes when injected in a dose of 0.5 or 1.5 micro g per individual, regardless of larval age. However, the same dose of 20HE significantly lowers the rate of LHPs synthesis within the fat body and completely blocks secretion of LHPs into the hemolymph. Therefore, we propose that 20HE inhibits the synthesis of storage proteins and their secretion without altering the level of mRNAs.

Site of synthesis and phylogenetic distribution of a hemolymph trophic factor of the tobacco hornworm, Manduca sexta

Identification of fifth instar larval Manduca sexta fat body and epidermis as sites of synthesis of a hemolymph protein (hemolymph trophic factor or HTF) was achieved using in vitro 3H-leucine incorporation into protein and subsequent immunoprecipitation of tissue homogenates. Fat body is the primary site of HTF synthesis with a maximal rate on Day 1; epidermis is a secondary site with peak synthesis on Day 0. In vitro radiolabelling followed by TCA precipitation of general protein of fat body and epidermal homogenates suggest that fat body actively elaborates protein on Days 0-5 with peak rates on Days 1 and 4, while epidermis is active on Days 0-5 with a peak rate on Day 3. Based on Anti-HTF ELISA estimates, HTF [500 to 1000 micrograms/ml] was found in the hemolymph of representatives of the insect orders Blattodea, Hemiptera, Orthoptera, and Lepidoptera and in the class Crustacea, but not in the class Merostomata. These studies suggest a possible fundamental role for HTF among modern arthropods in cuticular deposition involving both epidermis and fat body. The physiological role of HTF is undetermined.