Protease treatment of cerebellar purkinje cells renders omega-agatoxin IVA-sensitive Ca2+ channels insensitive to inhibition by omega-conotoxin GVIA

Relationship between Ca2+ and cAMP as second messengers in ACTH-induced cortisol production in bovine adrenal fasciculata cells

In adrenal fasciculata cells stimulated by ACTH, Ca2+ and cAMP play indispensable roles as second messengers in cortisol production. However, whether their second messengers cooperatively or independently participate in steroid production remains unclear. We focused on the roles of Ca2+ and cAMP in cortisol production in bovine adrenal fasciculata cells stimulated by ACTH for a relatively short period (1 h). Incubation of the cells with 100 pM ACTH in Ca2+-containing (normal) medium for 1 h increased cortisol production without affecting cAMP content. In contrast, treatment of the cells with the peptide at a higher concentration (1 nM) significantly augmented both cortisol production and cAMP content. However, ACTH did not increase either of them in the Ca2+-free medium. ACTH rapidly increased the intracellular free Ca2+ concentration ([Ca2+]i) in the normal medium, but did not influence [Ca2+]i in the Ca2+-free medium, indicating that ACTH caused Ca2+ influx into the cells. ACTH-induced Ca2+ influx and cortisol production were suppressed by a voltage-sensitive L-type Ca2+ channel blocker but not by a T-type, N-type, or P-type Ca2+ channel blocker. In contrast, dibutyryl cAMP, a cell-permeable cAMP analog, greatly enhanced cortisol production in the normal or Ca2+-free medium and slowly caused Ca2+ influx into the cells. These results strongly suggest that Ca2+, as a second messenger, is more critical than cAMP for cortisol production. However, both second messengers jointly participate in the production in adrenal fasciculata cells stimulated by ACTH.

Short-Term Facilitation at a Detonator Synapse Requires the Distinct Contribution of Multiple Types of Voltage-Gated Calcium Channels

Neuronal calcium elevations are shaped by several key parameters, including the properties, density, and the spatial location of voltage-gated calcium channels (VGCCs). These features allow presynaptic terminals to translate complex firing frequencies and tune the amount of neurotransmitter released. Although synchronous neurotransmitter release relies on both P/Q- and N-type VGCCs at hippocampal mossy fiber-CA3 synapses, the specific contribution of VGCCs to calcium dynamics, neurotransmitter release, and short-term facilitation remains unknown. Here, we used random-access two-photon calcium imaging together with electrophysiology in acute mouse hippocampal slices to dissect the roles of P/Q- and N-type VGCCs. Our results show that N-type VGCCs control glutamate release at a limited number of release sites through highly localized Ca²⁺ elevations and support short-term facilitation by enhancing multivesicular release. In contrast, Ca²⁺ entry via P/Q-type VGCCs promotes the recruitment of additional release sites through spatially homogeneous Ca²⁺ elevations. Altogether, our results highlight the specialized contribution of P/Q- and N-types VGCCs to neurotransmitter release.SIGNIFICANCE STATEMENT In presynaptic terminals, neurotransmitter release is dynamically regulated by the transient opening of different types of voltage-gated calcium channels. Hippocampal giant mossy fiber terminals display extensive short-term facilitation during repetitive activity, with a large several fold postsynaptic response increase. Though, how giant mossy fiber terminals leverage distinct types of voltage-gated calcium channels to mediate short-term facilitation remains unexplored. Here, we find that P/Q- and N-type VGCCs generate different spatial patterns of calcium elevations in giant mossy fiber terminals and support short-term facilitation through specific participation in two mechanisms. Whereas N-type VGCCs contribute only to the synchronization of multivesicular release, P/Q-type VGCCs act through microdomain signaling to recruit additional release sites.

Membrane depolarization regulates intracellular RANKL transport in non-excitable osteoblasts

Parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (VD3) are important factors in Ca(2+) homeostasis, and promote osteoclastogenesis by modulating receptor activator of nuclear factor kappa-B ligand (RANKL) mRNA expression. However, their contribution to RANKL intracellular transport (RANKLiT), including the trigger for RANKL lysosomal vesicle (RANKL-lv) fusion to the cell membrane, is unclear. In neurons, depolarization of membrane potential increases the intracellular Ca(2+) level ([Ca(2+)]i) and promotes neurotransmitter release via fusion of the synaptic vesicles to the cell membrane. To determine whether membrane depolarization also regulates cellular processes such as RANKLiT in MC3T3-E1 osteoblasts (OBs), we generated a light-sensitive OB cell line and developed a system for altering their membrane potential via delivery of a blue light stimulus. In the membrane fraction of RANKL-overexpressing OBs, PTH and VD3 increased the membrane-bound RANKL (mbRANKL) level at 10 min after application without affecting the mRNA expression level, and depolarized the cell membrane while transiently increasing [Ca(2+)]i. In our novel OB line stably expressing the channelrhodopsin-wide receiver, blue light-induced depolarization increased the mbRANKL level, which was reversed by treatment of blockers for L-type voltage-gated Ca(2+) channels and Ca(2+) release from the endoplasmic reticulum. In co-cultures of osteoclast precursor-like RAW264.7 cells and light-sensitive OBs overexpressing RANKL, light stimulation induced an increase in tartrate-resistant acid phosphatase activity and promoted osteoclast differentiation. These results indicate that depolarization of the cell membrane is a trigger for RANKL-lv fusion to the membrane and that membrane potential contributes to the function of OBs. In addition, the non-genomic action of VD3-induced RANKL-lv fusion included the membrane-bound VD3 receptor (1,25D3-MARRS receptor). Elucidating the mechanism of RANKLiT regulation by PTH and VD3 will be useful for the development of drugs to prevent bone loss in osteoporosis and other bone diseases.

Contributions of T-type voltage-gated calcium channels to postsynaptic calcium signaling within Purkinje neurons

Low threshold voltage-gated T-type calcium channels have long been implicated in the electrical excitability and calcium signaling of cerebellar Purkinje neurons although the molecular composition, localization, and modulation of T-type channels within Purkinje cells have only recently been addressed. The specific functional roles that T-type channels play in local synaptic integration within Purkinje spines are also currently being unraveled. Overall, Purkinje neurons represent a powerful model system to explore the potential roles of postsynaptic T-type channels throughout the nervous system. In this review, we present an overview of T-type calcium channel biophysical, pharmacological, and physiological characteristics that provides a foundation for understanding T-type channels within Purkinje neurons. We also describe the biophysical properties of T-type channels in context of other voltage-gated calcium channel currents found within Purkinje cells. The data thus far suggest that one specific T-type isoform, Ca_v3.1, is highly expressed within Purkinje spines and both physically and functionally couples to mGluR1 and other effectors within putative signaling microdomains. Finally, we discuss how the selective potentiation of Ca_v3.1 channels via activation of mGluR1 by parallel fiber inputs affects local synaptic integration and how this interaction may relate to the overall excitability of Purkinje neuron dendrites.