Is the intrasomal phase of fast axonal transport driven by oscillations of intracellular calcium?

Comparative effects of sodium pyrithione evoked intracellular calcium elevation in rodent and primate ventral horn motor neurons

Oral administration of sodium pyrithione (NaP) causes hindlimb weakness in rodents, but not in primates. Previous work using Aplysia neurons has demonstrated that NaP produces a persistent influx of Ca(2+) ions across the plasma membrane. To determine whether this also occurs in mammalian neurons and whether this could underlie the inter-species difference between rodents and primates, we have tested the effects of NaP on intracellular Ca(2+) levels ([Ca(2+)](i)) in rat and monkey motor neurons in vitro. Motor neurons present in spinal cord slices from rhesus monkey embryos (E37 and 56) and from rat E16 were dissected and cultured on glass coverslips. Following 2 weeks (rhesus) or 2-3 days (rat) in culture, neurons were loaded with fura-PE3/AM, and examined for [Ca(2+)](i) changes in response to NaP. Rhesus motor neurons were identified by immunostaining for Islet-1 (MN specific antigen) and neuron specific enolase (NSE). Motor neurons from both species exhibited dose-dependent NaP-evoked increases in [Ca(2+)](i) However, the dose-response curve for the Rhesus motor neurons was significantly shifted to the right of the rat dose-response curve, whereas the overall amplitude of the Ca(2+) rise was similar in both species. As shown previously for the Aplysia neurons, the action of NaP is attenuated by SKF 96365, an inhibitor of store-operated calcium entry. In contrast the action of NaP is unaffected by nifedipine and tetrodotoxin, blockers of voltage-dependent Ca(2+) and Na(+) channels, respectively, or by ouabain, an inhibitor of the plasma membrane Na(+)/K(+) ATPase. Our results indicate that the NaP-induced increase in [Ca(2+)](i) is conserved across species and suggest that the toxicological sensitivity of rodent over primate to pyrithione could be due to the enhanced sensitivity of rodent motor neurons to NaP-evoked intracellular Ca(2+) elevation.

The dynamic and motile properties of intermediate filaments

For many years, cytoplasmic intermediate filaments (IFs) were considered to be stable cytoskeletal elements contributing primarily to the maintenance of the structural and mechanical integrity of cells. However, recent studies of living cells have revealed that IFs and their precursors possess a remarkably wide array of dynamic and motile properties. These properties are in large part due to interactions with molecular motors such as conventional kinesin, cytoplasmic dynein, and myosin. The association between IFs and motors appears to account for much of the well-documented molecular cross talk between IFs and the other major cytoskeletal elements, microtubules, and actin-containing microfilaments. Furthermore, the associations with molecular motors are also responsible for the high-speed, targeted delivery of nonfilamentous IF protein cargo to specific regions of the cytoplasm where they polymerize into IFs. This review considers the functional implications of the motile properties of IFs and discusses the potential relationships between malfunctions in these motile activities and human diseases.

Regulation of angiotensin II-induced neuromodulation by MARCKS in brain neurons

Angiotensin II (Ang II) exerts chronic stimulatory actions on tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), and the norepinephrine transporter (NET), in part, by influencing the transcription of their genes. These neuromodulatory actions of Ang II involve Ras-Raf-MAP kinase signal transduction pathways (Lu, D., H. Yang, and M.K. Raizada. 1997. J. Cell Biol. 135:1609-1617). In this study, we present evidence to demonstrate participation of another signaling pathway in these neuronal actions of Ang II. It involves activation of protein kinase C (PKC)beta subtype and phosphorylation and redistribution of myristoylated alanine-rich C kinase substrate (MARCKS) in neurites. Ang II caused a dramatic redistribution of MARCKS from neuronal varicosities to neurites. This was accompanied by a time-dependent stimulation of its phosphorylation, that was mediated by the angiotensin type 1 receptor subtype (AT1). Incubation of neurons with PKCbeta subtype specific antisense oligonucleotide (AON) significantly attenuated both redistribution and phosphorylation of MARCKS. Furthermore, depletion of MARCKS by MARCKS-AON treatment of neurons resulted in a significant decrease in Ang II-stimulated accumulation of TH and DbetaH immunoreactivities and [3H]NE uptake activity in synaptosomes. In contrast, mRNA levels of TH, DbetaH, and NET were not influenced by MARKS-AON treatment. MARCKS pep148-165, which contains PKC phosphorylation sites, inhibited Ang II stimulation of MARCKS phosphorylation and reduced the amount of TH, DbetaH, and [3H]NE uptake in neuronal synaptosomes. These observations demonstrate that phosphorylation of MARCKS by PKCbeta and its redistribution from varicosities to neurites is important in Ang II-induced synaptic accumulation of TH, DbetaH, and NE. They suggest that a coordinated stimulation of transcription of TH, DbetaH, and NET, mediated by Ras-Raf-MAP kinase followed by their transport mediated by PKCbeta-MARCKS pathway are key in persistent stimulation of Ang II's neuromodulatory actions.

Spatial and temporal aspects of Ca2+ oscillations in Xenopus laevis melanotrope cells

Spatio-temporal aspects of Ca2+ signaling in melanotrope cells of Xenopus laevis have been studied with confocal laser-scanning microscopy. In the whole-frame scanning mode, two major intracellular Ca2+ compartments, the cytoplasm and the nucleus, were visualized. The basal [Ca2+] in the nucleus appeared to be lower than that in the cytoplasm and Ca2+ oscillations seemed to arise synchronously in both compartments. The N-type channel blocker omega-conotoxin eliminated oscillations in both regions, indicating a strong coupling between the two compartments with respect to Ca2+ dynamics. Line-scanning mode, which gives higher time resolution, revealed that the rise phase of a Ca2+ oscillation is not a continuous process but consists of 3 or 4 discrete steps. Each step can be seen as a Ca(2+)-wave starting at the cell membrane and going through the cytoplasm at a speed of 33.3 +/- 4.3 microns/s. Before the Ca(2+)-wave enters the nucleus, a delay of 120.0 +/- 24.1 ms occurred. In the nucleus, the speed of a wave was 80.0 +/- 3.0 microns/s. Treatment with the Ca(2+)-ATPase inhibitor thapsigargin (1 MicroM) almost completely eliminated the apparent difference in the basal [Ca2+] in the cytoplasm and the nucleus, reduced the delay of a Ca(2+)-wave before entering the nucleus to 79.8 +/- 8.7 ms, and diminished the nuclear wave speed to 35.0 +/- 4.9 microns/s. These results indicate that a cytoplasmic thapsigargin-sensitive ATPase near the nuclear envelope is involved in buffering Ca2+ before the Ca2+ wave enters the nucleus. After sensitizing IP3 receptors by thimerosal (10 microM) the speed of the cytoplasmic Ca(2+)-wave was increased to 70.3 +/- 3.6 microns/s, suggesting that IP3 receptors may be involved in the propagation of the cytoplasmic Ca2+ wave. Our results indicate that in melanotropes the generation and propagation of Ca2+ oscillations is a complex event involving influx of Ca2+ through N-type Ca2+ channels, propagation of the cytoplasmic Ca2+ wave through mobilization of intracellular stores and a regulated Ca2+ entry into the nucleus. We propose that Ca(2+)-binding proteins may act as a Ca2+ store for propagation of the wave in the nucleus.