Regulated subcellular distribution of the NR1 subunit of the NMDA receptor

NMDA (N-methyl-D-aspartate) receptors are selectively localized at the postsynaptic membrane of excitatory synapses in the mammalian brain. The molecular mechanisms underlying this localization were investigated by expressing the NR1 subunit of the NMDA receptor in fibroblasts. NR1 splice variants containing the first COOH-terminal exon cassette (NR1A and NR1D) were located in discrete, receptor-rich domains associated with the plasma membrane. NR1 splice variants lacking this exon cassette (NR1C and NR1E) were distributed throughout the cell, with large amounts of NR1 protein present in the cell interior. Insertion of this exon cassette into the COOH-terminus of the GluR1 AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor was sufficient to cause GluR1 to be localized to discrete, receptor-rich domains. Furthermore, protein kinase C phosphorylation of specific serines within this exon disrupted the receptor-rich domains. These results demonstrate that amino acid sequences contained within the NR1 molecule serve to localize this receptor subunit to discrete membrane domains in a manner that is regulated by alternative splicing and protein phosphorylation.

NGF-stimulated retrograde transport of trkA in the mammalian nervous system

The present study was designed to clarify the in vivo function of trkA as an NGF receptor in mammalian neurons. Using the rat sciatic nerve as a model system, we examined whether trkA is retrogradely transported and whether transport is influenced by physiological manipulations. Following nerve ligation, trkA protein accumulates distal to the ligation site as shown by Western blot analysis. The distally accumulating trkA species were tyrosine phosphorylated. The trkA retrograde transport and phosphorylation were enhanced by injecting an excess of NGF in the footpad and were abolished by blocking endogenous NGF with specific antibodies. These results provide evidence that, upon NGF binding, trkA is internalized and retrogradely transported in a phosphorylated state, possibly together with the neurotrophin. Furthermore, our results suggest that trkA is a primary retrograde NGF signal in mammalian neurons in vivo.

Positron emission tomographic imaging of the dopamine transporter with 11C-WIN 35,428 reveals marked declines in mild Parkinson's disease

Parkinson's Disease (PD) is characterized by a selective loss of nigrostriatal dopaminergic neurons that results in a marked reduction of dopaminergic nerve terminals in the striatum. Recently, 11C-WIN 35,428, a cocaine analogue that specifically labels the dopamine transporter, was developed and can be used to label dopaminergic nerve terminals in vivo by positron emission tomography. In healthy control subjects, binding of 11C-WIN 35,428 is highest in the striatum. In addition, 2 symmetrical focal areas of low binding were observed in the midbrain. The cerebellum functioned as an appropriate region for nonspecific binding. The binding of 11C-WIN 35,428 in patients with PD (Hoehn-Yahr II) was compared with that in healthy control subjects by using the (region-cerebellum)/cerebellum ratio for data acquired 34 to 82 minutes after injection. In control subjects, this ratio varied, at approximately 2, in the striatum. In patients with PD, binding in the posterior putamen was reduced by 78%, whereas the anterior putamen and the caudate nucleus showed a reduction of 59 and 39%, respectively. The reduction in 11C-WIN 35,428 binding was highest in the midbrain (84%). The high specific/nonspecific binding ratio and the pronounced reduction in binding in mild PD may permit detection of even earlier stages of PD or presymptomatic individuals with dopaminergic cell loss.