User:ItnosoyI/sandbox

Sources

1. The L-type calcium channel in the heart: the beat goes on

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1297268/

2. L-Type Calcium Channels: The Low Down

https://www.physiology.org/doi/full/10.1152/jn.00486.2004

3. Voltage-gated calcium channels: Their discovery, function and importance as drug targets

https://journals.sagepub.com/doi/full/10.1177/2398212818794805

4. L-Type Calcium Channels: Structure and Functions

https://www.intechopen.com/books/ion-channels-in-health-and-sickness/l-type-calcium-channels-structure-and-functions

5. Switching off calcium-dependent inactivation in l-type calcium channels by an autoinhibitory domain

https://www.pnas.org/content/103/42/15657

6. L-type Ca2+ channels in heart and brain

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3968275/

7. Voltage-gated calcium channels: Introduction

https://www.guidetopharmacology.org/GRAC/FamilyIntroductionForward?familyId=80

8. L-type calcium channel targeting and local signalling in cardiac myocytes

https://academic.oup.com/cardiovascres/article/98/2/177/277825

History
(4) In 1953, Paul Fatt and Bernard Katz discovered voltage gated calcium channels in crustacean muscle. The channels exhibited different activation voltages and calcium conducting properties and were thus separated into High Voltage Activating channels (HVA) and Low Voltage Activating channels (LVA). After further experimentation, it was found that HAV channels would open to 1,4-dihydropyridine (DHPs). (3) Using DHPs, they found that HVA channels were specific to certain tissues and reacted differently which lead to further categorization of the HVA channels into L-type, P-type, and N-type. L-type calcium channels were peptide sequenced and was found that there were 4 kinds of L-type calcium channels α1S (Skeletal Muscle), α1C (Cardiac),  α1 D (found in the brain), and α1F (found in the retina). In 2000, after more research was done on α1 subunits in voltage-gated calcium channels, a new nomenclature was used that called L-type calcium channels CaV1 with its subunits being called CaV1.1, CaV1.2, CaV1.3, and CaV1.4. (6) Research about each CaV1 subunit continues to learn more about their structure, function, and pharmaceutical applications.

Structure
(1)(4)L-type Calcium Channels contain 5 different subunits, the α1(170–240 kDa), α2(150kDa), δ(17-25 kDa), β(50-78 kDa), and γ(32 kDa) subunits. (1)The α2, δ, and β subunits are non-covalently bonded to the α1 subunit and modulate ion trafficking and biophysical properties of the α1 subunit. The α2 and δ subunits are in the extracellular space while the β and γ subunits are located in the cytosolic space.

(7)The α1 subunit is a heterotetramer that has four transmembrane regions, known as Domain I-IV, that cross the plasma six times as α-helices, being called S0-S6 (S0 and S1 together cross the membrane once). The α1 subunit as a whole contains the voltage sensing domain, the conduction pore, and gating apparatus. (what article says). To sense the cell’s voltage, the S1-S3 helices contain many negatively charged amino acids amino acids while S4 helices contain mostly positively charged amino acids with a P-loop connecting the S4 to S5 helices. After the S1-6 domains, there are six C domains that consist of two EF-hand motifs (C1-2 and C3-4) and a Pre-IQ domain (C5) and IQ domain (C6). There are also two EF-hand motifs on the N-terminus. Both the N and C terminus are in the cytosolic space with the C-terminus being much longer than the N-terminus.

(6)(4) The β subunit is known to have four isoforms (β1-β4) to regulate the channel's functions and is connected to α1 through the α1 I and II linker in the cytosol at the β α1-binding pocket (ABP). Each isofrom contains a src homology 3 domain (SH3) and a guanylate-kinase like domian (GK) that are seperated by a HOOK domain, and three unstructured regions.

The α2 and δ subunits are connected together by disulfide bonds (sometimes known as the α2δ subunit) and interact with α1. they have 4 know isoforms called α2δ-1 to α2δ-2 and contain contain a von Willebrand A (VWA) domain and a Cache domain. The α2 region is in the extracellular space while the δ region is in the cell membrane and have been seen to be anchored with a glycosylphosphatidylinositol (GPI) anchor.

The γ subunit has eight isoforms (γ1-γ8) and is connected to the α1 subunit and has only been found in muscle cells in the CaV1.1 and CaV1.2 channels. NOt much is known about the γ subunit, but it has been linked to interactions in hydrophobic fores.

Mechanism
Opening of the pore in L-type calcium channels takes place in the α1 subunit. When the membrane depolarizes, the S4 helix moves through the S4 and S5 linkers to the cytoplasmic ends of the S5 and S6 helices. This opens the activation gate which is formed by the inner side of the S6 helices in the α1 subunit.

(5) The most predominant way of autoinhibition of L-type calcium channels is with the Ca+2/Cam complex. (4) As the pore opens and causes an influx of Calcium, calcium binds to calmodulin and then interacts with the loop that connects the adjacent EF-hand motifs and causes a conformational change in the EF-hand motif so it interacts with the pore to cause quick inhibition in the channel. It is still debated on where and how the pore and EF-hand interact. Hydrophobic pockets in the Ca+2/Cam complex will also bind to three sections of the QI domain known as the “aromatic anchors” (5). (5) The CA+2/Cam complex has a high affinity towards L-type calcium channels, allowing it to get blocked even when there are low amounts of calcium present in the cell.(5) The pore eventually closes as the cell repolarizes and causes a conformational change in the channel to put it in the closed conformation.

Inhibition and Modulation
Drugs that inhibit L-type calcium channels are, (given in wiki page). (2)One of the most recognized characteristics of the L-type calcium channel is it’s unique sensitivity to 1,4-dihydropyridines (DHPs). (3) Unlike other voltage gated calcium channels, L-type calcium channels are resistant to ⍵-CT X (GVIA) and ⍵-AG A (IVA) inhibitory drugs.

A well observed form of modulation is due to alternative splicing. (6)A common form of modulation from alternative splicing is the C-terminal modulator (CTM). It has a positively charged α-helix on the C-terminal called the DCRD and a negatively charged helix right after the IQ motif (CaM interaction site). The two helices can form a structure that bind competitively with CaM to reduce the open-state probability and lower calcium-dependent inhibition (CDI).

(3)Alternative splicing is also seen on the β subunits to create different isoforms to give channels different properties due to palymination to (6) RNA editing. Other forms of modulation on the β subunit include increasing or decreasing of the subunit’s expression. This is due to the fact that β subunits increase the open-probability of the channel, activity in the plasma membrane, and antagonize the ubiquitination of the channel.

(1)L-type calcium channels are also modulated by G protein-coupled receptors and the adrenergic nervous system. (6)PKA activated by a G protein-coupled receptors cascade can phosphorylate L-type calcium channels, after channels form a signaling complex with AKAPs, to increase calcium current through the channel, increasing the open-state probability, and an accelerated recovery period. Activated PLC from G protein-coupled receptors can breakdown polyphosphoinositides to decrease the channels calcium current by 20%-30%.

(3) The adrenergic nervous system has been seen to modulate L-type calcium channels by cleaving the C-terminal fragment when the β-adrenergic receptor is stimulated to increase activation of the channels.