T-type calcium channel

2018-03-09T21:23:53Z

2600:1700:8EF0:7A50:8142:F73F:8714:C491: /* Fast-acting */

'''T-type calcium channels''' are low-voltage activated calcium channels that open during membrane [[depolarization]]. These channels aid in mediating calcium influx into cells after an action potential or depolarizing signal. The entry of calcium into various cells has many different physiological responses associated with it. Within cardiac and smooth muscle cells voltage-gated calcium channel activation initiates contraction directly by allowing the cytosolic concentration to increase. Not only are T-type calcium channels known to be present within cardiac and smooth muscle, but also are present in many neuronal cells within the central nervous system. Different experimental studies within the 1970s allowed for the distinction of T-type calcium channels (transient opening calcium channels) from the already well-known [[L-type calcium channels]] (Long-Lasting calcium channels). The new T-type channels were much different from the L-type calcium channels due to their ability to be activated by more negative membrane potentials, had small single channel conductance, and also were unresponsive to calcium antagonist drugs that were present.<ref name=Catterall>{{cite journal | vauthors = Catterall WA | title = Voltage-gated calcium channels | journal = Cold Spring Harbor Perspectives in Biology | volume = 3 | issue = 8 | pages = a003947 | date = August 2011 | pmid = 21746798 | doi = 10.1101/cshperspect.a003947 | pmc=3140680}}</ref> These distinct calcium channels are generally located within the brain, peripheral nervous system, heart, smooth muscle, bone, and endocrine system.<ref name=Medandlife>{{cite journal | vauthors = Iftinca MC | title = Neuronal T-type calcium channels: what's new? Iftinca: T-type channel regulation | journal = Journal of Medicine and Life | volume = 4 | issue = 2 | pages = 126–38 | year = 2011 | pmid = 21776294 | pmc = 3124264 | doi = | url = }}</ref>

The distinct structures of T-type calcium channels are what allow them to conduct in these manners, consisting of a primary α1 subunit. The α1 subunit of T-type channels is the primary subunit that forms the pore of the channel, and allows for entry of calcium.

T-type calcium channels function to control the pace-making activity of the SA Node within the heart and relay rapid action potentials within the [[thalamus]]. These channels allow for continuous rhythmic bursts that control the SA Node of the heart.<ref name=Perez-Reyes>{{cite journal | vauthors = Perez-Reyes E | title = Molecular physiology of low-voltage-activated t-type calcium channels | journal = Physiological Reviews | volume = 83 | issue = 1 | pages = 117–61 | date = January 2003 | pmid = 12506128 | doi = 10.1152/physrev.00018.2002 }}</ref>

Pharmacological evidence of T-type calcium channels suggest that they play a role in several forms [[cancer]],<ref name="Dziegilewski">{{cite journal|date=April 2014|title=T-type calcium channels blockers as new tools in cancer therapies|journal=Pflügers Archiv|volume=466|issue=4|pages=801–10|doi=10.1007/s00424-014-1444-z|pmid=24449277|vauthors=Dziegielewska B, Gray LS, Dziegielewski J}}</ref> [[absence epilepsy]],<ref name="Nelson">{{cite journal|year=2006|title=The role of T-type calcium channels in epilepsy and pain|journal=Current Pharmaceutical Design|volume=12|issue=18|pages=2189–97|doi=10.2174/138161206777585184|pmid=16787249|vauthors=Nelson MT, Todorovic SM, Perez-Reyes E}}</ref> [[pain]],<ref name="Todorovic">{{cite journal|date=April 2014|title=Targeting of CaV3.2 T-type calcium channels in peripheral sensory neurons for the treatment of painful diabetic neuropathy|journal=Pflügers Archiv|volume=466|issue=4|pages=701–6|doi=10.1007/s00424-014-1452-z|pmid=24482063|vauthors=Todorovic SM, Jevtovic-Todorovic V}}</ref> and [[Parkinson's disease]].<ref name=":0">{{Cite journal|last=Bermejo|first=Pedro Emilio|last2=Anciones|first2=Buenaventura|date=2009-09-01|title=Review: A review of the use of zonisamide in Parkinson’s disease|url=http://tan.sagepub.com/content/2/5/313|journal=Therapeutic Advances in Neurological Disorders|language=en|volume=2|issue=5|pages=313–317|doi=10.1177/1756285609338501|issn=1756-2856|pmc=3002602|pmid=21180621}}</ref> Further research is continuously occurring to better understand these distinct channels, as well as create drugs to select for these channels.

{{infobox protein
|Name=[[CACNA1G|Calcium channel, voltage-dependent, T-type, alpha 1G subunit]]
|caption=
|image=
|width=
|HGNCid=1394
|Symbol=CACNA1G
|AltSymbols=
|IUPHAR_id = yes
|OMIM=604065
|RefSeq=NM_018896
|UniProt=O43497
|PDB=
|ECnumber=
|Chromosome=17
|Arm=q
|Band=22
|LocusSupplementaryData=
}}
{{infobox protein
|Name= [[CACNA1H|Calcium channel, voltage-dependent, T-type, alpha 1H subunit]]
|caption=
|image=
|width=
|HGNCid=1395
|Symbol=CACNA1H
|AltSymbols=
|IUPHAR_id = yes
|EntrezGene=8912
|OMIM=607904
|RefSeq=NM_001005407
|UniProt=O95180
|PDB=
|ECnumber=
|Chromosome=16
|Arm=p
|Band=13.3
|LocusSupplementaryData=
}}
{{infobox protein
|Name=[[CACNA1I|Calcium channel, voltage-dependent, T-type, alpha 1I subunit]]
|caption=
|image=
|width=
|HGNCid=1396
|Symbol=CACNA1I
|AltSymbols=
|IUPHAR_id = yes
|EntrezGene=8911
|OMIM=608230
|RefSeq=NM_001003406
|UniProt=Q9P0X4
|PDB=
|ECnumber=
|Chromosome=22
|Arm=q
|Band=13.1
|LocusSupplementaryData=
}}

== Function ==

Like any other channel in a cell membrane, the primary function of the T-type voltage gated calcium channel is to allow passage of ions, in this case calcium, through the membrane when the channel is activated. When membrane depolarization occurs in a cell membrane where these channels are embedded, they open and allow calcium to enter the cell, which leads to several different cellular events depending on where in the body the cell is found. As a member of the Cav3 subfamily of voltage-gated calcium channels, the function of the T-type channel is important for the repetitive firing of action potentials in cells with rhythmic firing patterns such as cardiac muscle cells and neurons in the thalamus of the brain.<ref name="Catterall"/> T-type calcium channels are activated in the same range as [[Voltage-gated ion channel|voltage-gated]] [[sodium channels]], which is at about -55 mV. Because of this very negative value at which these channels are active, there is a large driving force for calcium going into the cell. The T-type channel is regulated by both [[dopamine]] and other neurotransmitters, which inhibit T-type currents. Additionally, in certain cells [[angiotensin]] II enhances the activation of T-type channels.<ref name="Catterall"/>

=== Heart ===

This is important in the aforementioned depolarization events in the pace-making activity of the [[Sinoatrial node|sinoatrial (SA) Node]] in the heart and in the neuron relays of the thalamus so that quick transmission of action potentials can occur. This is very important for the heart when stimulated by the [[sympathetic nervous system]] that causes the heart rate to increase, in that not only does the T-type calcium channel provide an extra depolarization punch in addition to the voltage gated sodium channels to cause a stronger depolarization, but it also helps provide a quicker depolarization of the cardiac cells.<ref name="Catterall"/><ref name="Perez-Reyes"/>

=== Fast-acting ===

Another important facet of the T-type voltage gated calcium channel is its fast voltage-dependent inactivation compared to that of other calcium channels. Therefore, while they help provide stronger and quicker depolarization of cardiac muscle cells and thalamus nerve cells, T-type channels also allow for more frequent depolarization events. This is very important in the heart in the simple fact that the heart is better apt to increase its rate of firing when stimulated by the sympathetic nervous system innervating its tissues. Although all of these functions of the T-type voltage gated calcium channel are important, quite possibly the most important of its functions is its ability to generate potentials that allow for rhythmic bursts of action potentials in cardiac cells of the [[sinoatrial node]] of the heart and in the [[thalamus]] of the brain.<ref name="Catterall"/> Because the T-type channels are voltage dependent, hyperpolarization of the cell past its inactivation voltage will close the channels throughout the SA node, and allow for another depolarizing event to occur. The voltage dependency of the T-type channel contributes to the rhythmic beating of the heart.<ref name="Perez-Reyes"/>

== Structure ==

[[Voltage-gated calcium channels]] are made up of several subunits. The α1 subunit is the primary subunit that forms the transmembrane pore of the channel.<ref name="Catterall"/> The α1 subunit also determines the type of calcium channel. The β, α2δ, and γ subunits, present in only some types of calcium channels, are auxiliary subunits that play secondary roles in the channel.<ref name="Medandlife"/>

=== α1 Subunit ===

The α1 subunit of T-type calcium channels is similar in structure to the α subunits of [[potassium ion|K+]](potassium ion) channels, [[sodium ion|Na+]](sodium ion) channels, and other [[calcium ion|Ca2+]](calcium ion) channels. The α1 subunit is composed of four domains (I-IV), with each domain containing 6 transmembrane segments (S1-S6). The hydrophobic loops between the S5 and S6 segments of each domain form the pore of the channel.<ref name="Catterall"/><ref name="Perez-Reyes"/> The S4 segment contains a high quantity of positively charged residues and functions as the voltage sensor of the channel opening or closing based on the membrane potential.<ref name="Perez-Reyes"/> The exact method by which the S4 segment controls the opening and closing of the channel is currently unknown.

=== Auxiliary subunits ===

The β, α2δ, and γ subunits are auxiliary subunits that affect channel properties in some calcium channels. The α2δ subunit is a dimer with an extracellular α2 portion linked to a transmembrane δ portion. The β subunit is an intracellular membrane protein. The α2δ and β subunits have an effect on the conductance and kinetics of the channel.<ref name=NeurontoBrain>{{cite book | first1 = John G. | last1 = Nicholls | first2 = A. Robert | last2 = Martin | first3 = Paul A. | last3 = Fuchs | first4 = David A. | last4 = Brown | first5 = Mathew E. | last5 = Diamond | first6 = David A. | last6 = Weisblat| name-list-format = vanc | title=From neuron to brain|year=2012|publisher=Sinauer Associates|location=Sunderland, Mass.|isbn=9780878936090|pages=87–88|edition=5th}}</ref> The γ subunit is a membrane protein that has an effect on the voltage sensitivity of the channel.<ref name="NeurontoBrain"/> Current evidence shows that isolated T-type α1 subunits have similar behavior to natural T-type channels, suggesting that the β, α2δ, and γ subunits are absent from T-type calcium channels and the channels are made up of only an α1 subunit.<ref name="Perez-Reyes"/>

== Variation ==

There are three known types of T-type calcium channels, each associated with a specific α1 subunit.
{| class="wikitable"
|-
! Designation !! α1 Subunit !! Gene
|-
| [[CACNA1G|Cav3.1]] || α1G || ({{Gene|CACNA1G}})
|-
| [[CACNA1H|Cav3.2]] || α1H || ({{Gene|CACNA1H}})
|-
| [[CACNA1I|Cav3.3]] || α1I || ({{Gene|CACNA1I}})
|}

== Pathology ==

When these channels are not functioning correctly, or are absent from their usual domains, several issues can result.

=== Cancer ===
T-type Calcium channels are expressed in different human cancers such as breast, colon, prostate, [[insulinoma]], [[retinoblastoma]], [[leukemia]], ovarian, and [[melanoma]], and they also play key roles in proliferation, survival, and the regulation of [[cell cycle progression]] in these forms of cancer . This was demonstrated through studies that showed that down regulating T-type channel isoforms, or just blocking the T-type calcium channels caused [[cytostatic]] effects in cancer cells such as [[gliomas]], breast, melanomas, and ovarian, esophageal, and colorectal cancers .
Some of the most notorious forms of cancer tumors contain [[cancer stem cells]] (CSC), which makes them particularly resistant to any cancer therapy . Furthermore, there is evidence that suggests that the presence of the CSC in human tumors may be associated with the expression of T-type calcium channels in the tumors.<ref name="Todorovic" />

=== Epilepsy ===

The major disease that involves the T-type calcium channel is absence epilepsy. This disease is caused by mutations of T-type calcium channel itself. When an individual has this disease, they will move in and out of a sleep-like state, even during normal activities.<ref name="Catterall" /> Experiments on the Genetic Absence Epilepsy Rat of Strasbourg ([[GAERS]]) suggested that absence epilepsy in the rat was linked to T-type channel protein expression.<ref name=Nelson /> In fact, neurons isolated from the [[reticular nucleus of the thalamus]] of the GAERS showed 55% greater T-type currents, and these currents were attributed to an increase in the Cav3.2 mRNA, according to Tally et al.<ref name=Nelson /> suggesting that T-type protein expression was up regulated in the GAERS. Further experiments on the GAERS showed that, indeed, the expression of T-type calcium channels play a key role in seizures caused by absence epilepsy in the GAERS.<ref name=Nelson /> Also, other evidence suggest that T-type calcium channel expression is not only up regulated in absence epilepsy, but also in other forms of epilepsy as well.<ref name=Nelson />

=== Pain ===

The Cav3.2 isoform of T-type calcium channels has been found to involve in [[pain]] in animal models with acute pain<ref name="pmid16939637">{{cite journal | vauthors = Choi S, Na HS, Kim J, Lee J, Lee S, Kim D, Park J, Chen CC, Campbell KP, Shin HS | title = Attenuated pain responses in mice lacking Ca(V)3.2 T-type channels | journal = Genes, Brain, and Behavior | volume = 6 | issue = 5 | pages = 425–31 | year = 2007 | pmid = 16939637 | doi = 10.1111/j.1601-183X.2006.00268.x }}</ref> and chronic pain: [[neuropathic pain]]<ref name=Dziegilewski /><ref name="pmid26785151">{{cite journal | vauthors = Bourinet E, Francois A, Laffray S | title = T-type calcium channels in neuropathic pain | journal = Pain | volume = 157 Suppl 1 | issue = | pages = S15–22 | year = 2016 | pmid = 26785151 | doi = 10.1097/j.pain.0000000000000469 }}</ref> (PDN), [[inflammatory pain]]<ref name="pmid24447516">{{cite journal | vauthors = Kerckhove N, Mallet C, François A, Boudes M, Chemin J, Voets T, Bourinet E, Alloui A, Eschalier A | title = Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol | journal = Pain | volume = 155 | issue = 4 | pages = 764–72 | year = 2014 | pmid = 24447516 | doi = 10.1016/j.pain.2014.01.015 }}</ref> and [[visceral pain]].<ref name="pmid21690417">{{cite journal | vauthors = Marger F, Gelot A, Alloui A, Matricon J, Ferrer JF, Barrère C, Pizzoccaro A, Muller E, Nargeot J, Snutch TP, Eschalier A, Bourinet E, Ardid D | title = T-type calcium channels contribute to colonic hypersensitivity in a rat model of irritable bowel syndrome | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 27 | pages = 11268–73 | year = 2011 | pmid = 21690417 | pmc = 3131334 | doi = 10.1073/pnas.1100869108 }}</ref>

=== Parkinson's disease ===
Increased neuronal bursting occurs throughout the central motor system in both human forms and animals models of Parkinson's disease.<ref>{{Cite journal|last=Rubin|first=Jonathan E.|last2=McIntyre|first2=Cameron C.|last3=Turner|first3=Robert S.|last4=Wichmann|first4=Thomas|date=2012-07-01|title=Basal ganglia activity patterns in parkinsonism and computational modeling of their downstream effects|url=http://onlinelibrary.wiley.com/doi/10.1111/j.1460-9568.2012.08108.x/abstract|journal=European Journal of Neuroscience|language=en|volume=36|issue=2|pages=2213–2228|doi=10.1111/j.1460-9568.2012.08108.x|issn=1460-9568|pmc=3400124|pmid=22805066}}</ref> T-type calcium channels are highly expressed in [[basal ganglia]] structures as well as neurons in the motor areas of the thalamus and are thought to contribute to normal and pathological bursting by means of low-threshold spiking.<ref name=":1">{{Cite journal|last=Devergnas|first=Annaelle|last2=Chen|first2=Erdong|last3=Ma|first3=Yuxian|last4=Hamada|first4=Ikuma|last5=Pittard|first5=Damien|last6=Kammermeier|first6=Stefan|last7=Mullin|first7=Ariana P.|last8=Faundez|first8=Victor|last9=Lindsley|first9=Craig W.|date=2016-01-01|title=Anatomical localization of Cav3.1 calcium channels and electrophysiological effects of T-type calcium channel blockade in the motor thalamus of MPTP-treated monkeys|url=http://jn.physiology.org/content/115/1/470|journal=Journal of Neurophysiology|language=en|volume=115|issue=1|pages=470–485|doi=10.1152/jn.00858.2015|issn=0022-3077|pmc=4760490|pmid=26538609}}</ref> Basal ganglia recipient neurons in the thalamus are particularly interesting because they are directly inhibited by the basal ganglia output.<ref>{{Cite journal|last=Albin|first=Roger L.|last2=Young|first2=Anne B.|last3=Penney|first3=John B.|date=1989-01-01|title=The functional anatomy of basal ganglia disorders|url=http://www.sciencedirect.com/science/article/pii/016622368990074X|journal=Trends in Neurosciences|volume=12|issue=10|pages=366–375|doi=10.1016/0166-2236(89)90074-X}}</ref> Consistent with the standard rate model of the basal ganglia, the increased firing in basal ganglia output structures observed in Parkinson's disease would exaggerate the inhibitory tone in thalamocortical neurons. This may provide the necessary hyperpolarization to de-inactivate T-type calcium channels, which can result in rebound spiking. In normal behavior, bursting likely plays a role in increasing the likelihood of [[Neurotransmission|synaptic transmission]], initiating state changes between rest and movement, and might signal [[Neuroplasticity|neural plasticity]] due to the intracellular cascades brought on by the rapid influx of calcium.<ref>{{Cite journal|last=Bosch-Bouju|first=Clémentine|last2=Hyland|first2=Brian I.|last3=Parr-Brownlie|first3=Louise C.|date=2013-01-01|title=Motor thalamus integration of cortical, cerebellar and basal ganglia information: implications for normal and parkinsonian conditions|url=http://journal.frontiersin.org/article/10.3389/fncom.2013.00163/full|journal=Frontiers in Computational Neuroscience|volume=7|pages=163|doi=10.3389/fncom.2013.00163|pmc=3822295|pmid=24273509}}</ref> While these roles are not mutually exclusive, most attractive is the hypothesis that persistent bursting promotes a motor state resistant to change, potentially explaining the akinetic symptoms of Parkinson's disease.<ref>{{Cite journal|last=Leventhal|first=Daniel K.|last2=Gage|first2=Gregory J.|last3=Schmidt|first3=Robert|last4=Pettibone|first4=Jeffrey R.|last5=Case|first5=Alaina C.|last6=Berke|first6=Joshua D.|title=Basal Ganglia Beta Oscillations Accompany Cue Utilization|url=http://linkinghub.elsevier.com/retrieve/pii/S0896627312000360|journal=Neuron|volume=73|issue=3|pages=523–536|doi=10.1016/j.neuron.2011.11.032|pmc=3463873|pmid=22325204}}</ref>

== As a drug target ==

[[Calcium channel blockers]] (CCB) such as [[mibefradil]] can also block L-type calcium channels, other enzymes, as well as other channels.<ref name=Dziegilewski /> Consequently, research is still being conducted to design highly selective drugs that can target T-type calcium channels alone.<ref name=Dziegilewski />

=== Cancer ===
Furthermore, since T-type calcium channels are involved in proliferation, survival and cell cycle progression of these cells, they are potential targets for anticancer therapy.<ref name="Dziegilewski" /> Like mentioned above, blockage or down regulation of the T-type calcium channels causes [[cytostasis]] in tumors; but this blockage or down regulation of the T-channels may also induce [[cytotoxic effects]]. Consequently, it is not yet clear what the benefits or disadvantages of targeting T-type calcium channels in anticancer therapy are.<ref name="Dziegilewski" /> On the other hand, a combined therapy involving administration of a T-type channel [[antagonist]] followed by cytotoxic therapy is currently in its [[clinical trial]] phase.<ref name="Dziegilewski" />

=== Painful Diabetic Neuropathy (PDN) ===
In addition, drugs used for treating PDN are associated with serious side effects and target specifically the CaV3.2 isoform (responsible for development of neuropathic pain in PDN) could reduce side effects.<ref name="Todorovic" /> As a result, research to improve or design new drugs is currently on-going.<ref name="Todorovic" />

=== Parkinson's disease ===
T-type calcium channels represent an alternative approach to Parkinson's disease treatment as their primary influence is not concerning the central [[Dopamine|dopaminergic system]]. For example, they offer great potential in reducing side effects of dopamine replacement therapy, such as [[levodopa-induced dyskinesia]]. The co-administration of T-type calcium channel blockers with standard Parkinson's disease medications is most popular in Japan, and several clinical studies have shown significant efficacy.<ref name=":0" /> However, most of these drugs are experimental and operate in a non-specific manner, potentially influencing sodium channel kinetics as well as dopamine synthesis. Novel T-type calcium channel inhibitors have recently been discovered which more selectively target the CaV3.3 channel sub-type expressed in central motor neurons, showing robust modulation in a rodent and primate models of Parkinson's disease.<ref name=":1" /><ref>{{Cite journal|last=Xiang|first=Zixiu|last2=Thompson|first2=Analisa D.|last3=Brogan|first3=John T.|last4=Schulte|first4=Michael L.|last5=Melancon|first5=Bruce J.|last6=Mi|first6=Debbie|last7=Lewis|first7=L. Michelle|last8=Zou|first8=Bende|last9=Yang|first9=Liya|date=2011-12-21|title=The Discovery and Characterization of ML218: A Novel, Centrally Active T-Type Calcium Channel Inhibitor with Robust Effects in STN Neurons and in a Rodent Model of Parkinson’s Disease|url=https://dx.doi.org/10.1021/cn200090z|journal=ACS Chemical Neuroscience|volume=2|issue=12|pages=730–742|doi=10.1021/cn200090z|pmc=3285241|pmid=22368764}}</ref>

== References ==
{{reflist|33em}}

[[Category:Calcium channels]]

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T-type calcium channel