Cav1.1

Cav1.1, encoded by the gene cacna1s, is a calcium channel, voltage-dependent, L type, alpha 1S subunit. It is primarily expressed in skeletal muscle cells and is responsible for excitation-contraction coupling. Mutations of the channel are the cause of muscle pathophysiologies.

In humans, cacna1s, the gene which encodes Cav1.1, is composed of 44 exons located on chromosome 1 (1q32.1). [2360]

There exist multiple Cav1.1 transcript variants across species as a result of the alternative splicing of cacna1s. [2361]

However only the canonical “adult” and the “neonatal/embryonic” variants are expressed at relevant levels. This neonatal variant lacks the in frame exon 29. [2362]

The human Cav1.1 protein is composed of 1873 amino acid (aa) and has a molecular weight of 212 Kda.

There exists a number of protein isoforms that arise from the translation of the aforementioned transcript variants:

• The neonatal Cav1.1e isoform excludes exon 29, which encodes 19 residues. This corresponds to the short segment connecting helices S3 and S4 of the fourth conserved repeat. The functional consequences of this splicing event result in a faster current activation, larger current amplitude, and hyperpolarizing voltage-dependence of activation of Cav1.1e relative to CaV1.1a (adult). [2362] [2361]
• Cav1.1e is thought to help support EC coupling in muscle cells [1227]

Like most mammalian proteins, Cav1.1 is subject to a series of post translational modifications (PTM).

Multiple phosphorylation sites were identified in Cav1.1. In the proximal C-terminal domain, these are serine 1575 (S1575) and threonine 1579 (T1579). In vitro phosphorylation experiments revealed that CaV1.1-S1575 is the target for both cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase II, whereas CaV1.1-T1579 is a target for casein kinase 2. [2363]

Lika most Cav channels, Cav1.1 is made up of a single protein composed of 4 homologous domains (DI-DIV). Each domain is made up of 6 transmembrane subunits (S1-S6) connected by extracellular loops. S1-4 form the voltage sensing domain (VSD) whereas the S5-6 form the pore module (PM). The S4 subunit of each domain contains a series of positively charged residues.

When membrane depolarization occurs, these charged residues cause the movement of the S4 subunit. This translocation of the S4 helices causes further conformational rearrangements within CaV1.1 that activate the receptor and open the channel pore. Negatively charged glutamate (E) and aspartate (D) residues present in the S2 and S3 helices interact with the positively charged, voltage-sensing residues in S4 [2361] Structural resolution of the ion channel highlighted a number of Cav1.1 specific features:

• Cav1.1 is ∼170 Å in height and 100 Å in width [2364]
• The extracellular segments linking the S5-S6 helices, known as the pore loop or P-loop, each contain highly conserved glutamate residues called the E-E-E-E motif. This tetra-acidic structure is thought to facilitate selective passage of Ca2 + and other divalent cations such as Ba2 +, Mg2 +, Mn2 + and Sr2 + [2362][2361]
• The SII–SIII linker is indispensable for skeletal-type EC coupling [2362]
• The SI–SII loop is the site for interaction with the β1a subunit, which supports membrane trafficking and is required for the tetradic arrangement of CaV1.1 within triad junctions [2362]
• The γ subunits interact with VSDIV, whereas the cytosolic β subunits interact with VSDII of α1. [2364]

Cav1.1, along with other L-type channels, has 3 broadly-defined gating modes [2362]:

• Mode 0: the closed state of the channel
• Mode 1: brief ~1 ms opening
• Mode 2: longer opening duration due to strong depolarisation or interaction with other actors

Each of CaV1.1's four voltage sensors play a different role in the kinetics of the channel: one (VSD-I) is crucial for CaV1.1 activation, while the others (VSD-II, VSD-III, VSD-IV) trigger calcium release more rapidly. [2365]

Single channel unitary conductance

The single channel unitary conductance of Cav1.1 has not yet been determined.

Models

There are currently no genetic ion channel models available

Cellular and Tissue

Cav1.1 is predominantly expressed in the skeletal muscles as the protein contains a unique sequence in its SII-SIII linker which couples the channels to ryanodine receptor (RyR1) [2366]

Developmental

qRT-PCR analysis showed that Cav1.1e is present at low levels in mature muscle but composed up to 80% of all Cav1.1 isoforms in myotubes, wild-type myotubes. [2362]

Within the skeletal muscles, Cav1.1 is present at the triad junctions formed by the plasma membrane of the transverse-tubule network (T-tubules) and the terminal cisternae of the Sarcoplasmic reticulum (SR). Within the junctional membrane, individual Cav1.1 proteins are clustered in groups of four, with every “tetrad” separated by a RyR1 homotetramer [2362] [2361]

Excitation-contraction coupling (ECC)

Cav1.1 is a key player in the process of excitation-contraction (EC) coupling of skeletal muscles.
Despite allowing for calcium entry, Cav1.1 primarily functions as a voltage sensor, detecting depolarisation of muscle cells.
Upon depolarisation, Cav1.1’s structural conformation shifts causing changes to the other proteins it interacts with, namely the ryanodine receptors (RyR1). Activation of RyRs is responsible for the release of Ca2+ from intracellular stores such as the sarcoplasmic reticulum, triggering muscle contraction via a Ca2 + entry-independent, conformational coupling mechanism. [2362] [2367]

Channelopathies

Muscle impairment

Given Cav1.1’s crucial voltage-sensing role in excitation-contraction coupling, impairment of Cav1.1’s function is often the cause of congenital muscle pathophysiologies [2362]. These include:

• Hypokalemic periodic paralysis (HypoKPP) [2368]
• Malignant hyperthermia (26238698)
• Normokalemic periodic paralysis (NormoPP) [2367]

CACN1S knock-out models further highlight the role of Cav1.1 in muscle contraction as Cacna1s KO mice were paralyzed and died at birth due to inability to breathe. (37468485) In humans, the decline in muscle function with aging, is a direct result of a reduction of Cav1.1 expression, as well as a reduction its β1a subunit. [2369]

Most muscular channelopathies are a consequence of loss of function mutations. Though no current evidence for gain of function mutations exists, such mutations are thought to also potentially cause pathologies, given how tightly the process of calcium influx during EC coupling is regulated during development [2367]

Developmental splicing errors

As previously mentioned, there exists an adult (Cav1.1a) and a neonatal(Cav1.1e) isoform of Cav1.1. The latter is characterized by its exclusion exon 29, resulting in a protein with faster activation and higher current amplitude. Aberrant expression of Cav1.1e in adult tissue is known to cause Myotonic Dystrophy Type 1, distinguished by muscle weakness, due to pathologically enhanced calcium influx. [2367]

Cancer

Though not a direct cause for cancer, CACNA1S was shown to have particularly low expression in lung and kidney cancer. The gene could serve as a treatment target for these specific types of cancer. (28781648)

Auxiliary subunit

Cav1.1 channels typically exist as multi-subunit complexes comprised of the main pore-forming α1 subunit, as previously described, and auxiliary subunits α2δ-1, β1a, and the γ1 subunit [2367]

• α2δ-1 shapes the slow activation kinetics of L-type calcium currents but has no role in EC coupling. α2δ-1 knockout mice are viable and show no apparent motor defects, suggesting that it is not vital for triggering of muscle contraction [2362] [2367]
• β1a promotes trafficking to the plasma membrane, facilitates the assembly of tetrads, and is essential for the formation of a functional channel complex. β1a is essential for muscle contraction. [2369] Skeletal-type EC coupling is absent in muscle cells genetically null for β1a [2362] and β1a knockout in mice results in paralyzed muscles and perinatal death, a phenotype similar to that of the dysgenic (CaV1.1-null) mice [2367]
• γ1 causes a depolarizing shift of channel inactivation [2362]. Absence of γ results in a right shift in Cav1.1 inactivation [2370]. γ1 is not essential for muscle function, as γ1 knockout mice are viable and normal. However, it does modulates the voltage dependence of inactivation of both CaV1.1 L-type currents and EC coupling [2367]

Other proteins and compounds

STAC3 (SH3 and cysteine-rich domain 3) is one of three members of a family of scaffold proteins containing C1 and tandem SH3 domains. The STAC3 isoform is specifically and exclusively expressed in skeletal muscle where it colocalizes with CaV1.1 and RyR1 in the triad junctions. Knockout of STAC3 in mice and fish results in a failure of EC coupling, without any impairment to muscle excitability. These results suggest that STAC3 is critical for efficient functional expression of CaV1.1 channels and for its role as a chaperone for proper membrane expression. [2367] STAC3 is present alongside SH3 and cysteine-rich domain-containing protein 3 is a 1:1:1 stoichiometry with the Cav1.1 a1 subunit. [2361] As explained in, Function, Cav1.1 interacts with ryanodine receptors (RyR1), to induce a SR internal calcium release, in turn triggering muscle contraction. However, this unique conformational excitation-contraction coupling is bidirectional. RyR1 mutations and other functional changes to the protein are known to also impact the activity of Cav1.1. [2362]

JP45 is a membrane protein interacting with Cav1.1 and the sarcoplasmic reticulum Ca2+ storage protein calsequestrin (CASQ1). JP45 and CASQ1 were shown to strengthen skeletal muscle contraction by modulating Cav1.1 channel activity [2371].

Cav1.1 lacks calcium-dependent inactivation (CDI). This is in contrast to other high voltage-activated Cavs, such as CaV1.2, CaV1.3, and CaV2.1 which required the anchoring of calmodulin (CaM) to a conserved IQ motif in their carboxyl-termini [2362]

Contributors: Katherine Johnston