Laurie G. Paavola

Cellular Mechanisms Involved in Luteolysis

Regression of the mammalian corpus luteum is commonly thought to involve autophagy (cellular self-digestion) or heterophagy (removal of cells by macrophages), or both. However, few studies have focused on these mechanisms, and their actual importance in luteolysis thus remains uncertain. Moreover, studies dealing with these topics have often come to differing conclusions; e.g., van Lennep and Madden (1) reported that autophagy was probably only a minor component in the overall process of involution in humans, whereas it was proposed in a study on sheep that autophagy played a substantial role in the destruction of luteal tissue (2).

THE ROLE OF LIPOPROTEINS IN STEROIDOGENESIS BY HUMAN LUTEINIZED GRANULOSA CELLS IN CULTURE

Progesterone secretion by primary cultures of luteinized human granulosa cells was markedly reduced when the cells were incubated in lipoprotein-deficient medium. Addition of LDL, but not HDL3, to cells cultured in lipoprotein-deficient medium stimulated progestin secretion. The effects of LDL were dose-dependent and saturable (Km = 5.5 micrograms LDL protein/ml). LDL also stimulated [3H]-oleate incorporation into cellular sterol esters, with half maximal stimulation occurring at LDL concentrations of 10 micrograms protein/ml. The cultured cells bound and internalized [125I]-LDL in a dose dependent and saturable manner (Km = 5-10 micrograms LDL protein/ml). [125I]-LDL uptake was specific in that unlabeled LDL, but not unlabeled HDL3, competed with labeled LDL for uptake [125I]-HDL3 was also taken up by the cells, but by a lower affinity mechanism. We conclude that luteinized human granulosa cells utilize LDL-carried cholesterol for progestin synthesis, and that LDL is taken up via a specific, high affinity process.

Role of the Chloroplast Envelope in Thylakoid Biogenesis

Thylakoid biogenesis in Chlamydomonas reinhardtii y-1 occurred at a linear rate when degreened cells incubated at 38°C were exposed to light. Photosystem 2 (PS2) activity, which was negligible in degreened cells, increased in parallel with chlorophyll (Chl). Kinetic parameters suggest that these PS2 units were larger and more efficient in light capture than those made at 25° C. Membranes made during the initial minutes of greening emanated from the chloroplast envelope. Kinetics of accumulation of chlorophyll a/b-binding (Cab) proteins implied that these proteins were degraded immediately upon import into the plastid unless Chl was available. Molecular modeling studies suggest that Chl is required to convert these proteins from an extended, protease-sensitive conformation into a compact, folded complex. A protease was partially purified that may be involved in degradation of the uncomplexed Cab proteins. These studies support the hypothesis that assembly of thylakoid membranes in this organism occurs in association with the chloroplast envelope.

A Program of Cell Death and Extracellular Matrix Degradation in Fetal Membranes Prior to Labor

Membranes surround the fetus to retain amniotic fluid and prevent infectious agents ascending the reproductive tract from entering the amniotic compartment. The fetal membranes also have important transport and endocrine functions during pregnancy in addition to their structural roles (1). The fetal membranes usually rupture during labor and it has generally been held that the breaking of the membranes is the consequence of physical forces associated with uterine contraction. However, recent observations suggest that structural changes occur in the membranes before labor. Thus, biochemical changes, as well as physical forces, are involved in membrane rupture.