Calcium density increment of a neuron

Calcium density increment of a neuron

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I learned that, as the spike in a neuron occurs, the calcium-ion concentration increases due to opening of the voltage-gated calcium channels.

Due to location of these channels, I wonder whether the calcium concentration only increases at the tip of an axon, or throughout the cell.

Calcium in biology

Calcium ions (Ca 2+ ) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, [1] [2] where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization. Many enzymes require calcium ions as a cofactor, including several of the coagulation factors. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.

Plasma calcium levels in mammals are tightly regulated, [1] [2] with bone acting as the major mineral storage site. Calcium ions, Ca 2+ , are released from bone into the bloodstream under controlled conditions. Calcium is transported through the bloodstream as dissolved ions or bound to proteins such as serum albumin. Parathyroid hormone secreted by the parathyroid gland regulates the resorption of Ca 2+ from bone, reabsorption in the kidney back into circulation, and increases in the activation of vitamin D3 to calcitriol. Calcitriol, the active form of vitamin D3, promotes absorption of calcium from the intestines and bones. Calcitonin secreted from the parafollicular cells of the thyroid gland also affects calcium levels by opposing parathyroid hormone however, its physiological significance in humans is dubious.

Intracellular calcium is stored in organelles which repetitively release and then reaccumulate Ca 2+ ions in response to specific cellular events: storage sites include mitochondria and the endoplasmic reticulum. [3]

Characteristic concentrations of calcium in model organisms are: in E. coli 3mM (bound), 100nM (free), in budding yeast 2mM (bound), in mammalian cell 10-100nM (free) and in blood plasma 2mM. [4]

Ion Transport: Calcium Channels☆

Naomi Niisato , Yoshinori Marunaka , in Reference Module in Biomedical Sciences , 2020

Voltage-Gated Ca 2 + Channels

Ca 2 + channels at the plasma membrane are characterized, in general, as voltage-gated Ca 2 + channels ( Table 2 and Fig. 1 ) ( Isom et al., 1994 Catterall et al., 2003 Belkacemi et al., 2005 ). This means that the voltage-gated Ca 2 + channel is activated by membrane depolarization i.e., open probability of the Ca 2 + channel is increased by depolarization of the plasma membrane ( Moczydlowski, 2003a ). The voltage-gated Ca 2 + channels are classified as L, T, N P/Q, and R types based on the current profiles through the channels ( Catterall et al., 2003 ). These types of channels are, in general, located in skeletal muscles, heart muscles, and neurons. The voltage-gated Ca 2 + channel belongs to the super family of voltage-gated cation channel including Na + , K + , and Ca 2 + channels. The voltage-gated cation channels have the sequence of the S4 segments for generating pores for ion permeation across the membrane, which is called as α1 subunit. The voltage-gated Ca 2 + channel is composed of a pseudo-oligomeric α1 subunit, as well as an extracellular α2 subunit, a cytoplasmic β subunit, and membrane-spanning γ and δ subunits. α1 subunit consists of four internally homologous repeats, domains I, II, III, and IV. Each domain contains an S1 through S6 transmembrane motif. Thus, α1 subunit has totally 24 transmembrane domains. Mammalian α1 subunits of the L, T, N, P/Q, and R-types Ca 2 + channels are encoded by distinct genes α1S, α1C, α1D, and α1F genes for the L-type Ca 2 + channel, α1G, α1H, and α1I genes for the T-type Ca 2 + channel, α1B gene for the N-type Ca 2 + channel, α1A gene for P/Q-type Ca 2 + channel, and α1E gene for the R-type Ca 2 + channel. In respiratory tissues, an L-type Ca 2 + channel expressed in airway smooth muscles ( Ge et al., 2013 Gui et al., 2011 ), vascular smooth muscle ( Wan et al., 2013 De Proost et al., 2007 ), epithelial cells ( De Proost et al., 2007 ) and neuroepithelial bodies ( De Proost et al., 2007 ): a T-type Ca 2 + channels in vascular smooth muscle ( Wan et al., 2013 ), and bronchial smooth muscle and vascular endothelial cells ( De Proost et al., 2007 ): N- and P/Q-type Ca 2 + channels in inspiratory neurons ( Koch et al., 2013 ): an R-type Ca 2 + channel in Clara-like cells ( De Proost et al., 2007 ).

Table 2 . Properties and classification of voltage-gated Ca 2 + channels.

Channel type of propertyKineticsVoltage activationPharmacologyLocationFunctionGenes
LLong durationHigh threshold (&gt−30 mV)Blocked by DHPsHeart, skeletal muscle, vascular smooth muscle, neuroendocrineLink membrane depolarization to cytosolic Ca 2 + signalingαS,
TTransientLow threshold (&lt−30 mV)Blocked by DHPs to a less extentSinoatrial node of heart,
brain neurons
Repetitive firing of action potentialsαG,
NIntermediate-long durationHigh threshold (&gt−30 mV)Insensitive to DHPs,
Blocked by ω-conotoxin GVIA
presynaptic terminals, dendrites,
cell bodies of neurons
Exocytotic neurotransmitter releaseαB
P/QIntermediate-long durationHigh threshold (&gt−30 mV)Insensitive to DHPs,
Blocked by ω-agatoxin IVA
cerebellar Purkinje, granule cells, and cell bodies of central neuronsExocytotic neurotransmitter releaseαA
RIntermediateHigh threshold (&gt−30 mV)Insensitive to DHPs,
Blocked by ω-agatoxin IIIA
cerebellar granule cells, neuronsExocytotic neurotransmitter releaseαE

Fig. 1 . A membrane folding model of voltage-gated Ca 2 + channel. A voltage-gated cation channel has the sequence of the S4 segments generating pore for ion permeation across the membrane, which is called as α1 subunit. A voltage-gated Ca 2 + channel is composed of a pseudo-oligomeric α1 subunit, as well as an extracellular α2 subunit, a cytoplasmic β subunit, and membrane-spanning γ and δ subunits. α1 subunit consists of four internally homologous repeats, domains I, II, III, and IV, and this α1 subunit forms an ion-permeable pore. Each domain contains an S1 through S6 transmembrane motif. Thus, α1 subunit has totally 24 transmembrane domains.

Modified from Isom LL, De Jongh KS, Catterall WA (1994) Auxiliary subunits of voltage-gated ion channels. Neuron 12:1183–1194, with permission of re-use.

L-type Ca 2 + channel: The L-type Ca 2 + channel is activated when the plasma membrane depolarizes (>−30 mV high threshold) ( Striessnig et al., 2004 Belkacemi et al., 2005 ). Further, once the L-type Ca 2 + channel is activated, the channel maintains its activity during a long time period. The L-type Ca 2 + channel is blocked by 1,4-dihyropyridines (1,4-DHP) such as nitrendipine via binding of 1,4-DHP to the channel, while Bay K 8644, an analog of 1,4-DHP, activates the channel by locking the channel to stay at an open state. The L-type Ca 2 + channel is located in heart muscles, skeletal muscles, smooth muscles, neurons, uterus, and neuroendocrine cells. The L-type Ca 2 + channels are classified as CaV1.1, CaV1.2, CaV1.3, and CaV1.4. The L-type Ca 2 + channel is activated by protein kinase A via an increase of its open probability, and plays a role in excitation-contraction coupling. The L-type Ca 2 + channel also contributes to maintenance of basal Ca 2 + concentration in airway smooth muscles and ERK1/2 phosphorylates the Ser496 in β2-subunit or Ser1928 in α1-subunit, switching the L-type Ca 2 + channel between open and closed states, respectively.

T-type Ca 2 + channel: The T-type Ca 2 + channel is activated when the plasma membrane depolarizes (<−30 mV low threshold), and is also inactivated over a more negative voltage range ( Belkacemi et al., 2005 ). When the T-type Ca 2 + channel is activated, the channel maintains its activity during a short time period (transient duration). These characteristics of T-type Ca 2 + channel indicate that the channel functions at the initial time of action potential and produces repetitive firing ( Moczydlowski, 2003b ). The T-type Ca 2 + channel is located in sinoatrial (SA) node of heart and brain neurons. Compared with the L-type Ca 2 + channel, the T-type Ca 2 + channel is specifically blocked by mibefradil, efonidipine and Ni 2 + . The T-type Ca 2 + channels are classified as CaV3.1, CaV3.2, and CaV3.3 based on the amino-acids sequence.

N-type Ca 2 + channel: The N-type Ca 2 + channel is activated when the plasma membrane depolarizes (>−30 mV high threshold) like the L-type Ca 2 + channel ( Belkacemi et al., 2005 ). When the N-type Ca 2 + channel is activated, the channel maintains its activity during a relatively long time period (intermediate-long duration). The N-type Ca 2 + channel is insensitive to 1,4-dihyropyridines, and is blocked by ω-conotoxin GVIA. This type of Ca 2 + channel is located in presynaptic terminals, dendrites, and neuronal bodies, involving in exocytotic neurotransmitter release. The N-type Ca 2 + channel is classified as CaV2.2.

P/Q-type Ca 2 + channel: The P/Q-type Ca 2 + channel is activated when the plasma membrane depolarizes (>−30 mV high threshold) ( Belkacemi et al., 2005 ). When the P/Q-type Ca 2 + channel is activated, the channel maintains its activity during a relatively long time period (intermediate-long duration). The P/Q-type Ca 2 + channel is insensitive to 1,4-dihyropyridines, and is blocked by ω-agatoxin IVA. This type of Ca 2 + channel is located in cerebellar Purkinje, granule cells, and cell bodies of central neurons. The P/Q-type Ca 2 + channel plays a role in exocytotic neurotransmitter release. The P/Q-type Ca 2 + channel is classified as CaV2.1 based on the amino-acids sequence.

R-type Ca 2 + channel: The R-type Ca 2 + channel is activated when the plasma membrane depolarizes (>−30 mV high threshold) like the L-type Ca 2 + channel ( Belkacemi et al., 2005 ). When the R-type Ca 2 + channel is activated, the channel maintains its activity during a relatively long time period (intermediate duration), but less time than T, N and P/Q-type Ca 2 + channels. The N-type Ca 2 + channel is insensitive to 1,4-dihyropyridines, and is blocked by ω-agatoxin IIIA. This type of Ca 2 + channel is located in cerebellar granule cells and neurons, and plays a role in exocytotic neurotransmitter release like the N-type Ca 2 + channel. The R-type Ca 2 + channel is classified into CaV2.3 based on the amino-acids sequence.

Neuronal calcium signaling: function and dysfunction

Calcium (Ca(2+)) is an universal second messenger that regulates the most important activities of all eukaryotic cells. It is of critical importance to neurons as it participates in the transmission of the depolarizing signal and contributes to synaptic activity. Neurons have thus developed extensive and intricate Ca(2+) signaling pathways to couple the Ca(2+) signal to their biochemical machinery. Ca(2+) influx into neurons occurs through plasma membrane receptors and voltage-dependent ion channels. The release of Ca(2+) from the intracellular stores, such as the endoplasmic reticulum, by intracellular channels also contributes to the elevation of cytosolic Ca(2+). Inside the cell, Ca(2+) is controlled by the buffering action of cytosolic Ca(2+)-binding proteins and by its uptake and release by mitochondria. The uptake of Ca(2+) in the mitochondrial matrix stimulates the citric acid cycle, thus enhancing ATP production and the removal of Ca(2+) from the cytosol by the ATP-driven pumps in the endoplasmic reticulum and the plasma membrane. A Na(+)/Ca(2+) exchanger in the plasma membrane also participates in the control of neuronal Ca(2+). The impaired ability of neurons to maintain an adequate energy level may impact Ca(2+) signaling: this occurs during aging and in neurodegenerative disease processes. The focus of this review is on neuronal Ca(2+) signaling and its involvement in synaptic signaling processes, neuronal energy metabolism, and neurotransmission. The contribution of altered Ca(2+) signaling in the most important neurological disorders will then be considered.

What is the role of calcium in neurotransmitter release?

The shape of the calcium channel protein allows only calcium ions to pass through the channel. There the calcium ions interact with the neurotransmitter containing vesicles (membrane-bound containers) causing them to fuse with the cell membrane, and release the neurotransmitters into the synaptic cleft.

Likewise, what does calcium do in an action potential? Action potentials open voltage-sensitive calcium channels in excitable cells, leading to an influx of calcium ions. Calcium ions may control, among others, cell excitability, neurotransmitter release, or gene transcription.

Similarly, you may ask, what is the role of calcium at the synapse?

One important role of calcium ions at a chemical synapse is to a. act as a transmitter substance. facilitate the binding of the transmitter substance with receptor molecules in the post-synaptic membrane.

How is a neurotransmitter released?

Molecules of neurotransmitters are stored in small "packages" called vesicles (see the picture on the right). Neurotransmitters are released from the axon terminal when their vesicles "fuse" with the membrane of the axon terminal, spilling the neurotransmitter into the synaptic cleft.

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A focused approach to imaging neural activity in the brain

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When neurons fire an electrical impulse, they also experience a surge of calcium ions. By measuring those surges, researchers can indirectly monitor neuron activity, helping them to study the role of individual neurons in many different brain functions.

One drawback to this technique is the crosstalk generated by the axons and dendrites that extend from neighboring neurons, which makes it harder to get a distinctive signal from the neuron being studied. MIT engineers have now developed a way to overcome that issue, by creating calcium indicators, or sensors, that accumulate only in the body of a neuron.

“People are using calcium indicators for monitoring neural activity in many parts of the brain,” says Edward Boyden, the Y. Eva Tan Professor in Neurotechnology and a professor of biological engineering and of brain and cognitive sciences at MIT. “Now they can get better results, obtaining more accurate neural recordings that are less contaminated by crosstalk.”

To achieve this, the researchers fused a commonly used calcium indicator called GCaMP to a short peptide that targets it to the cell body. The new molecule, which the researchers call SomaGCaMP, can be easily incorporated into existing workflows for calcium imaging, the researchers say.

Boyden is the senior author of the study, which appears today in Neuron. The paper’s lead authors are Research Scientist Or Shemesh, postdoc Changyang Linghu, and former postdoc Kiryl Piatkevich.

Molecular focus

The GCaMP calcium indicator consists of a fluorescent protein attached to a calcium-binding protein called calmodulin, and a calmodulin-binding protein called M13 peptide. GCaMP fluoresces when it binds to calcium ions in the brain, allowing researchers to indirectly measure neuron activity.

“Calcium is easy to image, because it goes from a very low concentration inside the cell to a very high concentration when a neuron is active,” says Boyden, who is also a member of MIT’s McGovern Institute for Brain Research, Media Lab, and Koch Institute for Integrative Cancer Research.

The simplest way to detect these fluorescent signals is with a type of imaging called one-photon microscopy. This is a relatively inexpensive technique that can image large brain samples at high speed, but the downside is that it picks up crosstalk between neighboring neurons. GCaMP goes into all parts of a neuron, so signals from the axons of one neuron can appear as if they are coming from the cell body of a neighbor, making the signal less accurate.

A more expensive technique called two-photon microscopy can partly overcome this by focusing light very narrowly onto individual neurons, but this approach requires specialized equipment and is also slower.

Boyden’s lab decided to take a different approach, by modifying the indicator itself, rather than the imaging equipment.

“We thought, rather than optically focusing light, what if we molecularly focused the indicator?” he says. “A lot of people use hardware, such as two-photon microscopes, to clean up the imaging. We’re trying to build a molecular version of what other people do with hardware.”

In a related paper that was published last year, Boyden and his colleagues used a similar approach to reduce crosstalk between fluorescent probes that directly image neurons’ membrane voltage. In parallel, they decided to try a similar approach with calcium imaging, which is a much more widely used technique.

To target GCaMP exclusively to cell bodies of neurons, the researchers tried fusing GCaMP to many different proteins. They explored two types of candidates — naturally occurring proteins that are known to accumulate in the cell body, and human-designed peptides — working with MIT biology Professor Amy Keating, who is also an author of the paper. These synthetic proteins are coiled-coil proteins, which have a distinctive structure in which multiple helices of the proteins coil together.

Less crosstalk

The researchers screened about 30 candidates in neurons grown in lab dishes, and then chose two — one artificial coiled-coil and one naturally occurring peptide — to test in animals. Working with Misha Ahrens, who studies zebrafish at the Janelia Research Campus, they found that both proteins offered significant improvements over the original version of GCaMP. The signal-to-noise ratio — a measure of the strength of the signal compared to background activity — went up, and activity between adjacent neurons showed reduced correlation.

In studies of mice, performed in the lab of Xue Han at Boston University, the researchers also found that the new indicators reduced the correlations between activity of neighboring neurons. Additional studies using a miniature microscope (called a microendoscope), performed in the lab of Kay Tye at the Salk Institute for Biological Studies, revealed a significant increase in signal-to-noise ratio with the new indicators.

“Our new indicator makes the signals more accurate. This suggests that the signals that people are measuring with regular GCaMP could include crosstalk,” Boyden says. “There’s the possibility of artifactual synchrony between the cells.”

In all of the animal studies, they found that the artificial, coiled-coil protein produced a brighter signal than the naturally occurring peptide that they tested. Boyden says it’s unclear why the coiled-coil proteins work so well, but one possibility is that they bind to each other, making them less likely to travel very far within the cell.

Boyden hopes to use the new molecules to try to image the entire brains of small animals such as worms and fish, and his lab is also making the new indicators available to any researchers who want to use them.

“It should be very easy to implement, and in fact many groups are already using it,” Boyden says. “They can just use the regular microscopes that they already are using for calcium imaging, but instead of using the regular GCaMP molecule, they can substitute our new version.”

The research was primarily funded by the National Institute of Mental Health and the National Institute of Drug Abuse, as well as a Director’s Pioneer Award from the National Institutes of Health, and by Lisa Yang, John Doerr, the HHMI-Simons Faculty Scholars Program, and the Human Frontier Science Program.

Calcium Intake and Health

There are striking inequities in calcium intake between rich and poor populations. Appropriate calcium intake has shown many health benefits, such as reduction of hypertensive disorders of pregnancy, lower blood pressure particularly among young people, prevention of osteoporosis and colorectal adenomas, lower cholesterol values, and lower blood pressure in the progeny of mothers taking sufficient calcium during pregnancy. Studies have refuted some calcium supplementation side effects like damage to the iron status, formation of renal stones and myocardial infarction in older people. Attention should be given to bone resorption in post-partum women after calcium supplementation withdrawal. Mechanisms linking low calcium intake and blood pressure are mediated by parathyroid hormone raise that increases intracellular calcium in vascular smooth muscle cells leading to vasoconstriction. At the population level, an increase of around 400-500 mg/day could reduce the differences in calcium intake between high- and middle-low-income countries. The fortification of food and water seems a possible strategy to reach this goal.

Keywords: calcium calcium intake fortification health hypertensive disorders.

Neurobiology of Psychiatric Disorders

Jiang-Zhou Yu , Mark M. Rasenick , in Handbook of Clinical Neurology , 2012

Intracellular calcium channels

Intracellular calcium stores release calcium in response to specific receptors on their surface. These receptors function as channels to allow the release of calcium from those intracellular stores. The molecules responsible for this fall into two basic and similar groups: one is called the inosotol 1,4,5‐trisphosphate (IP3) receptors and the other is known as ryanodine receptors. IP3 receptors bind IP3 as well as ATP, and have the capability for regulation by phosphorylation. IP3 activates the IP3 receptor and allows calcium to be released from the intracellular stores into the cell.

Ryanodine receptors were first isolated in skeletal muscle, but appear to enjoy a wide tissue distribution as well, particularly within excitable cells. They allow for a rapid release of calcium from intracellular stores in response to cyclic adenosine diphosphate ribose and perhaps other intracellular messengers. Several subclasses of both IP3 receptors and ryanodine receptors are known to exist.

In addition to releasing calcium, the intracellular calcium stores must have the capability of rapidly concentrating calcium. This is important because, if calcium is a diffusible messenger which is going to deliver a really discrete message, then there must be the capability of rapidly taking calcium up. A variety of calcium ATPases are found on the plasma membrane, and these calcium ATPases are capable of exporting calcium from the cell into the extracellular milieu quite rapidly. These calcium ATPases are activated by calcium and calmodulin and, in the presence of these compounds, increase significantly their sensitivity to calcium and their ability to pump it out of the cell. The calcium ATPases located on the endoplasmic reticulum or the sarcoplasmic reticulum likewise are able to transport calcium rapidly back into these cellular storage sites for calcium. The primary difference between these calcium pumps and those found on the plasma membrane is that the regulation by calcium calmodulin is not known to occur in microsomal calcium ATPases. Figure 2.13 shows intraneuronal mechanisms for maintaining and modulating calcium concentration.

Fig. 2.13 . Calcium signaling in neuron. Mechanisms for calcium entry, regulation of cellular calcium concentration and intracellular calcium-responsive proteins are demonstrated. Several calcium-binding proteins are omitted for the sake of simplicity.


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