Apolipoprotein E receptor 2 interactions with the N-methyl-D-aspartate receptor

In our previous studies we showed that apoE treatment of neurons activated ERK 1/2 signaling, and activation was blocked by treatment with inhibitors of the low density lipoprotein receptor family, the N-methyl-d-aspartate (NMDA) receptor antagonist MK 801, and calcium channel blockers. We hypothesized an interaction between the low density lipoprotein receptor family members and the NMDA receptor. In the present study, we confirmed through co-immunoprecipitation experiments an interaction between the apoE receptor, ApoEr2, and NMDAR1 through their extracellular domains. We also found that the PDZ1 domain of PSD95, a postsynaptic scaffolding protein, interacted with the C terminus of ApoEr2 via an alternatively spliced, intracellular exon. This interaction between ApoEr2 and PSD95 in neurons was modulated by NMDA receptor activation and an ApoEr2 ligand. We also found that the PDZ2 domain of PSD95 interacted with the NR2A and NR2B subunits of NMDA receptors. Full-length PSD95 increased cell surface levels of ApoEr2 and its cleavage, resulting in increases in secreted ApoEr2 and C-terminal fragments of ApoEr2. These studies suggest that ApoEr2 can form a multiprotein complex with NMDA receptor subunits and PSD95.     

Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.     

Isolation and characterization of phosphatases from vascular smooth muscle

Actomyosin preparations of the carotid arteries of cattle contain a soluble phosphatase activity, which can be removed from the contractile proteins by repeated washings. This enzyme activity is lowered by high ionic strength, potassium fluoride, zinc acetate, ammonium molybdate, and vanadate. Magnesium ions enhance the enzyme activity. The phosphatase activity shows a maximum between pH 5.5 and 6.0 and a plateau of pH 7-9. By means of gel filtration on Sepharose 6B the phosphate activity is separated into three peaks, which are characterized with respect to their inhibition by potassium fluoride, ammonium molybate, and vanadate and their dependence on pH.     

Modulation of juxtamembrane cleavage ("shedding") of angiotensin-converting enzyme by stalk glycosylation: evidence for an alternative shedding protease.

The role of juxtamembrane stalk glycosylation in modulating stalk cleavage and shedding of membrane proteins remains unresolved, despite reports that proteins expressed in glycosylation-deficient cells undergo accelerated proteolysis. We have constructed stalk glycosylation mutants of angiotensin-converting enzyme (ACE), a type I ectoprotein that is vigorously shed when expressed in Chinese hamster ovary cells. Surprisingly, stalk glycosylation did not significantly inhibit release. Introduction of an N-linked glycan directly adjacent to the native stalk cleavage site resulted in a 13-residue, proximal displacement of the cleavage site, from the Arg-626/Ser-627 to the Phe-640/Leu-641 bond. Substitution of the wild-type stalk with a Ser-/Thr-rich sequence known to be heavily O-glycosylated produced a mutant (ACE-JGL) in which this chimeric stalk was partially O-glycosylated; incomplete glycosylation may have been due to membrane proximity. Relative to levels of cell-associated ACE-JGL, rates of basal, unstimulated release of ACE-JGL were enhanced compared with wild-type ACE. ACE-JGL was cleaved at an Ala/Thr bond, 14 residues from the membrane. Notably, phorbol ester stimulation and TAPI (a peptide hydroxamate) inhibition of release-universal characteristics of regulated ectodomain shedding-were significantly blunted for ACE-JGL, as was a formerly undescribed transient stimulation of ACE release by 3, 4-dichloroisocoumarin. These data indicate that (1) stalk glycosylation modulates but does not inhibit ectodomain shedding; and (2) a Ser-/Thr-rich, O-glycosylated stalk directs cleavage, at least in part, by an alternative shedding protease, which may resemble an activity recently described in TNF-alpha convertase null cells [Buxbaum, J. D., et al. (1998) J. Biol. Chem. 273, 27765-27767].