Anti-beta 2 subunit antisense oligonucleotides modulate the surface expression of the alpha 1 subunit of N-type omega-CTX sensitive Ca2+ channels in IMR 32 human neuroblastoma cells

In vivo spectrometric calcium flux recordings of intrinsic Caudate-Putamen cells and transplanted IMR-32 neuroblastoma cells using miniature fiber optrodes in anesthetized and awake rats and monkeys

A method is described to enable the recording of transient intracellular calcium changes in deep brain structures in anesthetized and awake animals using a fluorescent indicator combined with in vivo optical detection methods. Optrodes were fabricated using a bifurcated fiber-optic cable with an attached infusion guide cannula. After intracranial implantation of an optrode, animals were prepared in the following manner, (1) rats (intra-striatal) and monkeys (intra-putamen) were infused with the fluorescent calcium indicator, Oregon Green, to load intrinsic cells; or (2) rats were intra-striatally transplanted with a slurry of dye-loaded IMR-32 neuroblastoma cells via pipette ejection. Excitation light from an argon-ion laser was launched through the optrode and passed into the tissue. The resulting calcium-induced fluorescence signals were captured by the optrode, then detected and processed by externalized photomultiplier- and CCD-based spectrometer electronics. In approximately 25% of all intrinsic cell recordings, the baseline fluorescence intensity was relatively stable over time whereas in the remainder, large amplitude oscillations were observed with a frequency in the range of 0.5-2 Hz. These Ca(2+) transients were inhibited by local infusion of 10 microM omega-conotoxin MVIIC and 1 microM TTX. Extracellular electrophysiological recordings that were made adjacent to the optrode tip revealed that the Ca(2+) oscillations were in phase with the burst firing of striatal neurons. This suggested that the optical signals had a neuronal origin, most likely from medium spiny neurons. Baseline fluorescence intensity increased during infusion of high [K(+)](o), the calcium ionophore, A-23187, or during temporary bilateral carotid artery occlusion. Monkey (Saimiri sciureus) putamen recordings also affirmed the presence of similar calcium-related transients in a non-human primate. In the transplant preparations, the IMR-32 cells displayed a stable, non-oscillating baseline fluorescence. They were similarly responsive to high [K(+)](o) challenge and appeared viable for at least several hours. Similar optical recording approaches might be applied to monitor other fluorescent, chemiluminescent or bioluminescent events from almost any brain structure. Moreover, transplanted transfected cells expressing a single specific receptor or ion-channel protein may effectively serve as biosensing elements for the measurement of extracellular neurochemical signaling.

Differential expression and association of calcium channel subunits in development and disease

Voltage-gated calcium channels (VDCC) are essential to neuronal maturation and differentiation. It is believed that important signaling information is encoded by VDCC-mediated calcium influx that has both spatial and temporal components. VDCC are multimeric complexes comprised of a pore-forming alpha1 subunit and auxiliary beta and alpha2/delta subunits. Changes in the fractional contribution of distinct calcium conductances to the total calcium current have been noted in developing and differentiating neurons. These changes are anticipated to reflect the differential expression and localization of the pore-forming alpha1 subunits. However, as in vitro studies have established that beta regulates the channel properties and targeting of alpha1, attention has been directed toward the developmental expression and assembly of beta isoforms. Recently, changes in the beta component of the omega-conotoxin GVIA (CTX)-sensitive N-type VDCC have indicated differential assembly of alpha1B with beta in postnatal rat brain. In addition, unique properties of beta4 have been noted with respect to its temporal pattern of expression and incorporation into N-type VDCC complexes. Therefore, the expression and assembly of specific alpha1/beta complexes may reflect an elaborate cellular strategy for regulating VDCC diversity. The importance of these developmental findings is bolstered by a recent study which identified mutations in the beta4 as the molecular defect in the mutant epileptic mouse (lethargic; lh/lh). As beta4 is normally expressed in both forebrain and cerebellum, one may consider the impact of the loss of beta4 upon VDCC assembly and activity. The importance of the beta1b and beta4 isoforms to calcium channel maturation and assembly is discussed.

Beta1B subunit of voltage-dependent Ca2+ channels is predominant isoform expressed in human neuroblastoma cell line IMR32

Human neuroblastoma cells (IMR32) respond to treatment with either dibutyryl-cAMP or nerve factor by acquiring a neuronal phenotype which is accompanied by a marked increase in the density of neuronal (N-type) VDCC currents. Using IMR32 cells as a model for neuronal differentiation, we were interested in examining possible changes in the level of expression of the alpha1B subunit of N-type calcium channels as well as beta subunit isoforms. Upon differentiation with dibutyryl-cAMP and 5-bromo-2-deoxyuridine for 16 days, we observed a dramatic increase in alpha1B protein which initiated between day 8 and 10. Day 10 evidenced maximal expression of alpha1B protein, which was followed by an interval of relatively constant expression of alpha1B (day 12 to day 16). Monitoring beta subunit expression using a pan specific anti-beta antibody (Ab CW20), we observed an increase in expression of a single 82 kDa beta subunit. The predominant 82 kDa beta subunit expressed throughout the course of differentiation was identified as the beta1b isoform using a panel of beta subunit specific antibodies. Of significance, neither the beta2 nor beta3 isoforms were detected in full differentiated IMR32 cells. Contrary to a previous report on the absence of neurotypic expression of VDCC beta subunits in a second model for in vitro differentiation, NGF-treated rat pheochromocytoma cells (PC12 cells) [1], we report the regulated expression of the beta1b protein in differentiated IMR32 cells suggesting a cell specific function for this beta subunit which parallels the acquisition of the neuronal phenotype. The restrictive expression of the beta1b in IMR32 cells may reflect a cell-type specific function that extends beyond its role as an auxiliary subunit of VDCC complexes.

Recent status of the antisense oligonucleotide approaches in oncology

Antisense oligonucleotides designed to complement a region of a particular messenger RNA may inhibit gene expression potentially through sequence-specific hybridization. Their inhibiting effect has been shown in a variety of in vitro and in vivo models in oncology, whereas much rarer clinical trials have been carried out. Rigorous demonstration of in vitro and in vivo specific effects upon their targets is mandatory before their use as drugs in cancer therapy.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.