Studies of programmed salivary gland regression during larval-pupal transformation in Chironomus thummi. I. Acid hydrolase activity

Autophagy in animal development

Macroautophagy (autophagy) delivers intracellular constituents to the lysosome to promote catabolism. During development in multiple organisms, autophagy mediates various cellular processes, including survival during starvation, programmed cell death, phagocytosis, organelle elimination, and miRNA regulation. Our current understanding of autophagy has been enhanced by developmental biology research during the last quarter of a century. Through experiments that focus on animal development, fundamental mechanisms that control autophagy and that contribute to disease were elucidated. Studies in embryos revealed specific autophagy molecules that mediate the removal of paternally derived mitochondria, and identified autophagy components that clear protein aggregates during development. Importantly, defects in mtDNA inheritance, or removal of paternal mtDNA via mitochondrial autophagy, can contribute to mitochondrial-associated disease. In addition, impairment of the clearance of protein aggregates by autophagy underlies neurodegenerative diseases. Experiments in multiple organisms also reveal conserved mechanisms of tissue remodeling that rely on the cooperation between autophagy and apoptosis to clear cell corpses, and defects in autophagy and apoptotic cell clearance can contribute to inflammation and autoimmunity. Here we provide an overview of key developmental processes that are mediated by autophagy in multiple animals.

Analysis of a lung defect in autophagy-deficient mouse strains

Yeast Atg1 initiates autophagy in response to nutrient limitation. The Ulk gene family encompasses the mammalian orthologs of yeast ATG1. We created mice deficient for both Ulk1 and Ulk2 and found that the mice die within 24 h of birth. When found alive, pups exhibited signs of respiratory distress. Histological sections of lungs of the Ulk1/2 DKO pups showed reduced airspaces with thickened septae. A similar defect was seen in Atg5-deficient pups as both Ulk1/2 DKO and Atg5 KO lungs show numerous glycogen-laden alveolar type II cells by electron microscopy, PAS staining, and increased levels of glycogen in lung homogenates. No abnormalities were noted in expression of genes encoding surfactant proteins but the ability to incorporate exogenous choline into phosphatidylcholine, the major phospholipid component of surfactant, was increased in comparison to controls. Despite this, there was a trend for total phospholipid levels in lung tissue to be lower in Ulk1/2 DKO and Atg5 KO compared with controls. Autophagy was abundant in lung epithelial cells from wild-type mice, but lacking in Atg5 KO and Ulk1/2 DKO mice at P1. Analysis of the autophagy signaling pathway showed the existence of a negative feedback loop between the ULK1 and 2 and MTORC1 and 2, in lung tissue. In the absence of autophagy, alveolar epithelial cells are unable to mobilize internal glycogen stores independently of surfactant maturation. Together, the data suggested that autophagy plays a vital role in lung structural maturation in support of perinatal adaptation to air breathing.

The role of autophagy in mammalian development: cell makeover rather than cell death

Autophagy is important for the degradation of bulk cytoplasm, long-lived proteins, and entire organelles. In lower eukaryotes, autophagy functions as a cell death mechanism or as a stress response during development. However, autophagy's significance in vertebrate development, and the role (if any) of vertebrate-specific factors in its regulation, remains unexplained. Through careful analysis of the current autophagy gene mutant mouse models, we propose that in mammals, autophagy may be involved in specific cytosolic rearrangements needed for proliferation, death, and differentiation during embryogenesis and postnatal development. Thus, autophagy is a process of cytosolic "renovation," crucial in cell fate decisions.

Programmed cell death in the larval salivary glands of Apis mellifera (Hymenoptera, Apidae)

The morphological and histochemical features of degeneration in honeybee (Apis mellifera) salivary glands were investigated in 5th instar larvae and in the pre-pupal period. The distribution and activity patterns of acid phosphatase enzyme were also analysed. As a routine,the larval salivary glands were fixed and processed for light microscopy and transmission electron microscopy.Tissue sections were subsequently stained with haematoxylin -eosin,bromophenol blue,silver,or a variant of the critical electrolyte concentration (CEC) method.Ultrathin sections were contrasted with uranyl acetate and lead citrate.Glands were processed for the histochemical and cytochemical localization of acid phosphatase,as well as biochemical assay to detect its activity pattern. Acid phosphatase activity was histochemically detected in all the salivary glands analysed.The cytochemical results showed acid phosphatase in vesicles, Golgi apparatus and lysosomes during the secretory phase and,additionally, in autophagic structures and luminal secretion during the degenerative phase. These findings were in agreement with the biochemical assay. At the end of the 5th instar, the glandular cells had a vacuolated cytoplasm and pyknotic nuclei, and epithelial cells were shed into the glandular lumen.The transition phase from the 5th instar to the pre-pupal period was characterized by intense vacuolation of the basal cytoplasm and release of parts of the cytoplasm into the lumen by apical blebbing; these blebs contained cytoplasmic RNA, rough endoplasmic reticule and, occasionally, nuclear material. In the pre-pupal phase, the glandular epithelium showed progressive degeneration so that at the end of this phase only nuclei and remnants of the cytoplasm were observed.The nuclei were pyknotic,with peripheral chromatin and blebs. The gland remained in the haemolymph and was recycled during metamorphosis. The programmed cell death in this gland represented a morphological form intermediate between apoptosis and autophagy.

The sources of acid hydrolases for photoreceptor membrane degradation in a grapsid crab

Dawn photoreceptor breakdown in the crab Leptograpsus variegatus is analysed at the ultrastructural level. Coated vesicles derived from microvilli are assembled as mutlivesicular bodies (mvbs), which degrade to multilamellar bodies (mls) and are lysed. Cytochemical markers for hydrolases were a fluoride-inhibited beta-glycerophosphatase and a fluoride-insensitive p-nitrophenyl phosphatse, with indistinguishable distributions when localised at pH 5.0. These enzymes are injected into the secondary lysomes from two sources: (i) immediately after dawn Golgi bodies are highly active, and differentiate a transtubular network, from which tubules and vesicles detach, and can be seen fusing with mvbs and mlbs. (ii) Saccules derived from the rough endoplasmic reticulum (RER) provide a second source and are most often seen in association with late mlbs. Both kinds of primary lysosome rare give AcPh-positive responses when free in the cytosol, but are seen to do so as they make contact with their secondary lysosomal targets. Lipid droplets and lipofuscin bodies are interpreted as the residual products of breakdown. These results are discussed in relation to previous findings on photoreceptor membrane breakdown in a dinopid spider. Attention is drawn to the implied diversity of organisation of lysosomal compartments in receptors which internalise membranes of similar compositions.

Autophagy in dedifferentiating newt iris epithelial cells in vitro

The ultrastructure of dedifferentiating iris epithelial cells of adult newts was studied in vitro, under the conditions which allow subsequent conversion into lens cells. The dense population of melanosomes which characterize the normal differentiated cells are progressively lost before the cells convert into lens cells. The loss of melanosomes is accompanied by the appearance of a layer of material of intermediate density surrounding the melanosomes. Subsequently vacuoles of various sizes appear in which melanosomes, ribosomes, and multivesicular bodies are sequestered. Further observations suggest that these affected organelles are destined for exocytosis. The cytochemical test for acid phosphatase is positive in the immediate vicinity of melanosomes, in the melanosome clusters, and in the multivesicular bodies. On the other hand, normal iris epithelial cells in situ are negative to the acid phosphatase test. The results are interpreted to indicate that autophagy and exocytosis of melanosomes and other organelles occur during the dedifferentiation of iris epithelial cells.