PROTEIN-SYNTHESIS, DNA-DEGRADATION, AND MORPHOLOGICAL-CHANGES DURING PROGRAMMED CELL-DEATH IN LABIAL GLANDS OF MANDUCA-SEXTA

Labial glands of the tobacco hornworm Manduca sexta (Lepidoptera: Sphi ngiidae), homologues of Drosophila salivary glands, undergo programmed cell death (PCD) in a 4-day period during larva-to-pupa metamorphosis . The programmed death of the labial gland was examined by electron mi croscopy and measurement of protein synthesis as well as measurement o f DNA synthesis, end-labeling of single strand breaks, and pulsed-fiel d gel electrophoresis. One of the earliest changes observed is a sharp drop in synthesis of most proteins, coup ed with synthesis of a glyci ne-rich protein, reminiscent of silk-like proteins. From a morphologic al standpoint, during the earliest phases the most prominent changes a re the formation of small autophagic vacuoles containing ribosomes and an apparent focal dissolution of the membranes of the endoplasmic ret iculum, whereas later changes include differing destruction at the lum enal and basal surfaces of the cell and erosion of the basement membra ne. By the fourth day of metamorphosis, individual cells become rapidl y vacuolated in a cell-independent manner.In the vacuolated cells on d ay 3, chromatin begins to coalesce. it is at this period that unequivo cal nucleosomal ladders are seen and end-labeling in situ or electroph oretic techniques document single or double-strand breaks, respectivel y DNA synthesis ceases shortly after the molt to the fifth instar, as detected by incorporation of tritiated thymidine and weak TUNEL labeli ng. Large size fragments of DNA are seen shortly after DNA synthesis c eases and thence throughout the instar, raising the possibility of pot ential limitations built into the cells before their final collapse. ( C) 1997 Wiley-Liss, Inc.