A novel AP180-related protein in vesicles that concentrate at acetylcholine receptor clusters

Cell proliferation in the brains of NMDAR NR1 transgenic mice

We have used genetically engineered NMDA receptor NR1+/- mice in which the gene for the NR1 subunit was modified in such a way that these mice express only 50% of the NR1 subunit. The NR1 subunit is necessary for NMDA receptor channel function. We investigated the effects of reduced NMDA receptor function on cell proliferation in the hippocampus and the amygdala of the adult mouse brain. Transgenic (NR1+/-) and wild-type (NR1+/+) mice were injected with BrdU. We collected brain sections cutting through the rostro-caudal extension of the entire hippocampus of the NR1+/- and NR1+/+ (wild-type) mice. The phenotype of BrdU-positive cells was identified by double labeling with antibodies to neuronal or glial markers. Our results show that the NR1+/- mice, which express the NMDAR NR1 subunit at a low level, have a significant (p<0.01) increase in the number of BrdU-positive cells in the dentate gyrus and the amygdala compared to NR1+/+ mice. Some of these dividing cells express the neuronal marker NeuN. Our results indicate that low expression of the NR1 subunit significantly increases cell proliferation and neurogenesis, suggesting that low NMDARs activity contributes to the increase in cell proliferation in the adult brain.

Potential role for a novel AP180-related protein during endocytosis in MDCK cells

Clathrin assembly protein, AP180, was originally identified as a brain-specific protein localized to the presynaptic junction. AP180 acts to limit vesicle size and maintain a pool of releasable synaptic vesicles during rapid recycling. In this study, we show that polarized epithelial Madin-Darby canine kidney (MDCK) cells express two AP180-related proteins: the ubiquitously expressed 62-kDa clathrin assembly lymphoid myeloid leukemia (CALM, AP180-2) protein and a novel high-molecular-weight homolog that we have named AP180-3. Sequence analysis of AP180-3 expressed in MDCK cells shows high homology to AP180 from rat brain. AP180-3 contains conserved motifs found in brain-specific AP180, including the epsin NH2-terminal homology (ENTH) domain, the binding site for the α-subunit of AP-2, and DLL repeats. Our studies show that AP180-3 from MDCK cells forms complexes with AP-2 and clathrin and that membrane recruitment of these complexes is modulated by phosphorylation. We demonstrate by immunohistochemistry that AP180-3 is localized to cytoplasmic vesicles in MDCK cells and is also present in tubule epithelial cells from mouse kidney. We observed by immunodetection that a high-molecular-weight AP180-related protein is expressed in numerous cells in addition to MDCK cells.

Differential responses of human neuroblastoma and glioblastoma to apoptosis

Staurosporine, a protein kinase and etoposide, a topoisomerase II inhibitor, are known to enhance apoptosis. The differential effects of these agents on T98G glioblastoma and SK-N-SH neuroblastoma, cell lines both derived from human tumors, have not been determined. We assessed cellular viability, DNA fragmentation and laddering, chromatin condensation, and Poly(ADP-ribose) polymerase (PARP) cleavage induced by these agents at a series of concentrations and times. In addition, to gain an understanding of the mechanism by which these agents work, we measured Protein Kinase C (PKC) activity. Staurosporine induced significant alterations in all apoptotic parameters tested in both cell lines. Etoposide induced apoptotic alterations similar to those caused by staurosporine in neuroblastoma but produced no detectable apoptotic changes in glioblastoma cells. Etoposide induced membrane but not cytosolic PKC activity in neuroblastoma but had no effect on PKC activity in glioblastoma. Our results show that the induction of apoptosis is cell type dependent. PKC activity appears to be crucial in the initiation of apoptosis.

Poly (ADP-ribose) polymerase induction is an early signal of apoptosis in human neuroblastoma

Poly (ADP-ribose) polymerase (PARP) is an abundant chromatin associated protein important in DNA repair, maintenance of chromosomal stability and programmed cell death. Here we report that an increase in caspase 3-activity and cleavage of PARP serves as an early execution phase signal in human neuroblastoma. Human neuroblastoma SK-N-SH cells were exposed to a protein kinase inhibitor, staurosporine, or a topoisomerase II inhibitor, etoposide, at various concentrations and time points. Cells exposed to staurosporine (0.1 microM) for 30 min showed an increase in caspase 3-activity and by 1 h an increase in PARP 116-kDa band and an 85-kDa cleavage product, which further increased in density with time after treatment. Quantitative analysis for condensed chromatin material using bisbenzimide, and DNA fragmentation enzyme immunoassays showed a significant increase in apoptosis 5 h after staurosporine treatment. This was further confirmed with a Klenow fragment of DNA polymerase I assay which primarily detects single-stranded DNA breaks. A significant decrease in mitochondrial metabolism occurred within 8-12 h after treatment. Studies using Trypan Blue exclusion, and lactic dehydrogenase (LDH) release revealed a significant increase in membrane permeability 8 h after staurosporine (0.1 microM) or etoposide (10 microM) treatments. Cleavage of lamin B1, a protein important in maintaining the nuclear envelope integrity was observed 12 h after staurosporine treatment. Our results show that activation of caspase 3 followed by PARP cleavage occur at much earlier time point than any other morphological or biochemical parameters of apoptosis or cytotoxicity.