Absence of the β subunit ( cchb1 ) of the skeletal muscle dihydropyridine receptor alters expression of the α ₁ subunit and eliminates excitation-contraction coupling

Abstract

The multisubunit (␣ 1S , ␣ 2 ͞␦, ␤ 1 , and ␥) skeletal muscle dihydropyridine receptor transduces transverse tubule membrane depolarization into release of Ca 2؉ from the sarcoplasmic reticulum, and also acts as an L-type Ca 2؉ channel. The ␣ 1S subunit contains the voltage sensor and channel pore, the kinetics of which are modified by the other subunits. To determine the role of the ␤ 1 subunit in channel activity and excitation-contraction coupling we have used gene targeting to inactivate the ␤ 1 gene. ␤ 1 -null mice die at birth from asphyxia. Electrical stimulation of ␤ 1 -null muscle fails to induce twitches, however, contractures are induced by caffeine. In isolated ␤ 1 -null myotubes, action potentials are normal, but fail to elicit a Ca 2؉ transient. L-type Ca 2؉ current is decreased 10-to 20-fold in the ␤ 1 -null cells compared with littermate controls. Immunohistochemistry of cultured myotubes shows that not only is the ␤ 1 subunit absent, but the amount of ␣ 1S in the membrane also is undetectable. In contrast, the ␤ 1 subunit is localized appropriately in dysgenic, mdg͞mdg, (␣ 1S -null) cells. Therefore, the ␤ 1 subunit may not only play an important role in the transport͞insertion of the ␣ 1S subunit into the membrane, but may be vital for the targeting of the muscle dihydropyridine receptor complex to the transverse tubule͞sarcoplasmic reticulum junction.

Defects in calcium homeostasis and mitochondria can be reversed in Pompe disease

“…37 The Cacnb1 knockout mice die at birth from asphyxia. 64 The mechanism of CACNB1 upregulation in Pompe muscle remains unclear. It is likely that it is not the storage per se, but rather the changes in the signaling pathway linked to the lysosomes that are responsible for the finding.…”

The β 1a subunit is essential for the assembly of dihydropyridine-receptor arrays in skeletal muscle

“…In contrast, ␣ 1S and ␤ 1a subunit null-mutant mice display a lack of EC coupling and, thus, lethal muscle paralysis (8,9). Although failure of EC coupling is obvious in the ␣ 1S -null (dysgenic) mouse muscle, which lacks the voltage-sensor, the molecular mechanism leading to the complete lack of EC coupling in the ␤ 1a -null mice is not fully understood (8). Myotubes from ␤ 1a -null mice show reduced L-type Ca 2ϩ currents, charge movements, and 1,4-dihydropyridine binding (10).…”

“…Using molecular biology, protein biochemistry, and immunocytochemistry techniques, we show that the zebrafish mutant relaxed lacks the DHPR ␤ 1a subunit. In contrast to previous studies (8,11), we can use this novel model organism to demonstrate that the ␣ 1S is correctly targeted into skeletal muscle triad junctions in the absence of ␤ 1a . However, DHPRs lacked the skeletal muscle-specific arrangement in tetrads and displayed substantially reduced charge movement.…”

“…Targeted deletions of the ␣ 2 ␦-1 and ␥ subunits do not critically interfere with EC coupling function (6,7). In contrast, ␣ 1S and ␤ 1a subunit null-mutant mice display a lack of EC coupling and, thus, lethal muscle paralysis (8,9). Although failure of EC coupling is obvious in the ␣ 1S -null (dysgenic) mouse muscle, which lacks the voltage-sensor, the molecular mechanism leading to the complete lack of EC coupling in the ␤ 1a -null mice is not fully understood (8).…”

“…Myotubes from ␤ 1a -null mice show reduced L-type Ca 2ϩ currents, charge movements, and 1,4-dihydropyridine binding (10). The staining intensity for ␣ 1S in immunocytochemistry was initially described to be below detectability (8), but later it was found to be significantly higher than that of dysgenic myotubes (11). Therefore, it remained unclear whether the loss of EC coupling in ␤ 1a -null myotubes was caused by decreased membrane expression of DHPRs, or by the lack of a specific contribution of the ␤ subunit to the EC coupling mechanism (10,12).…”