PRLP

PRLP

SRC

The mammalian prion protein (PrPC) is a cellular protein of unknown function, an altered isoform of which (PrPSc) is a component of the infectious particle (prion) thought to be responsible for spongiform encephalopathies in humans and animals. We report here the isolation of a cDNA that encodes a chicken protein that is homologous to PrPC. This chicken prion-like protein (ch-PrLP) is identical to the mouse PrP at 33% of its amino acid positions, including an uninterrupted stretch of 24 identical residues, and it displays the same structural domains. In addition, ch-PrLP, like its mammalian counterpart, is attached to the cell surface by a glycosyl-phosphatidylinositol anchor. We find that ch-PrLP is the major protein in preparations of an acetylcholine receptor-inducing activity that has been purified greater than 10(6)-fold from brain on the basis of its ability to stimulate synthesis of nicotinic receptors by cultured myotubes. The ch-PrLP gene is expressed in the spinal cord and brain as early as embryonic day 6; and in the spinal cord, the protein appears to be concentrated in motor neurons. Our results therefore raise the possibility that prion proteins serve normally to regulate the chemoreceptor number at the neuromuscular junction and perhaps in the central nervous system as well.

PMID: 1715573 [PubMed - indexed for MEDLINE]

Ovarian follicle development is dependent on growth factors that stimulate cell proliferation and act as survival factors to prevent apoptosis of follicle cells. We examined the mechanism of the protective effect of IGF-I against Fas ligand (FasL)-induced apoptosis of granulosa cells and its relationship to cell proliferation. IGF-I activated both the phosphoinositide 3'-OH kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathways. Experiments using specific inhibitors of these pathways showed that protection by IGF-I was mediated by the PI3K pathway and not the MAPK pathway. Recombinant adenoviruses were used to test whether the downstream target of PI3K activation, Akt kinase, was required for protection against apoptosis. Expression of dominant negative Akt (dnAkt) prevented protection by IGF-I while expression of constitutively active Akt (myrAkt) mimicked the effect of IGF-I. Treatment with IGF-I, or expression of myrAkt, increased progression from G0/G1 to S phase of the cell cycle while expression of dnAkt inhibited G0/G1 to S phase progression and prevented the stimulatory effect of IGF-I. We tested whether cell cycle progression was required for protection from apoptosis using the cyclin dependent kinase-2 inhibitor roscovitine, which blocks cells at the G1/S transition. Roscovitine prevented the protective effect of IGF-I and myrAkt expression against apoptosis. Therefore, activation of Akt is not sufficient to protect granulosa cells from apoptosis in the absence of cell cycle progression. In summary, IGF-I protects granulosa cells from apoptosis by activation of the PI3K/Akt pathway. This protective effect can only occur when progression from G1 to S phase of the cell cycle regulated by the PI3K/Akt pathway is unperturbed.