Skeletal muscle excitation–contraction coupling is independent of a conserved heptad repeat motif in the C-terminus of the DHPRβ_1a subunit

Abstract

In skeletal muscle excitation–contraction (EC) coupling the sarcolemmal L-type Ca²⁺ channel or 1,4-dihydropyridine receptor (DHPR) transduces the membrane depolarization signal to the sarcoplasmic Ca²⁺ release channel RyR1 via protein–protein interaction. While it is evident that the pore-forming and voltage-sensing DHPRα_1S subunit is essential for this process, the intracellular DHPRβ_1a subunit was also shown to be indispensable. We previously found that the β_1a subunit is essential to target the DHPR into groups of four (tetrads) opposite the RyR1 homotetramers, a prerequisite for skeletal muscle EC coupling. Earlier, a unique hydrophobic heptad repeat motif (L⋯V⋯V) in the C-terminus of β_1a was postulated by others to be essential for skeletal muscle EC coupling, as substitution of these residues with alanines resulted in 80% reduction of RyR1 Ca²⁺ release. Therefore, we wanted to address the question if the proposed β_1a heptad repeat motif could be an active element of the DHPR–RyR1 signal transduction mechanism or already contributes at the ultrastructural level i.e. DHPR tetrad arrangement. Surprisingly, our experiments revealed full tetrad formation and an almost complete restoration of EC coupling in β₁-null zebrafish relaxed larvae and isolated myotubes upon expression of a β_1a-specific heptad repeat mutant (LVV to AAA) and thus contradict the earlier results.

1. Introduction

Excitation–contraction (EC) coupling is understood as the signal transduction process linking membrane depolarization to the contraction of a muscle cell. In skeletal muscle this involves a direct crosstalk between two Ca²⁺ channels, the plasmalemmal high voltage-activated skeletal muscle Ca²⁺ channel or 1,4-dihydropyridine receptor (DHPR) and the intracellular Ca²⁺ release channel, the ryanodine receptor type-1 (RyR1) in the sarcoplasmic reticulum (SR) membrane. Membrane depolarization induces conformational changes in the voltage-sensing DHPRα_1S which are transduced to RyR1 via protein–protein interaction [1,2]. This leads to opening of RyR1 without the need of DHPR Ca²⁺ influx [3]. Both the channels are targeted into the muscle triad junctions where clusters of DHPR in the sarcolemma co-localize with clusters of RyR1 in closely juxtaposed SR membrane [4,5]. Ultrastructurally, DHPRs, visible as “particles” in freeze-fracture images, are arranged in groups of four (tetrads) to communicate with every other RyR1 homotetramer and thus are geometrically arranged in orthogonal arrays following the RyR1-arrays [6,7].

The skeletal muscle DHPR is a hetero-multimeric protein complex consisting of the central α_1S subunit and the auxiliary subunits β_1a, α₂δ-1, and γ₁ [8]. According to the current model, the pore-forming and voltage-sensing α_1S subunit transduces the opening signal to RyR1, essentially via the intracellular loop connecting homologous repeats II and III [9,10]. The intracellular β_1a subunit was shown to have multiple roles in targeting and modulating the central α_1S subunit [11,12]. The lack of β_1a is incompatible with skeletal muscle EC coupling, and leads to perinatal lethality in β₁-null mice due to respiratory paralysis [13] or to an immotile larval phenotype in the β₁-null zebrafish mutant relaxed [14,15]. As shown in zebrafish relaxed the absence of β_1a specifically leads to, (i) reduction of α_1S membrane targeting, (ii) severe reduction in α_1S charge movement, and (iii) complete absence of the ultrastructural arrangement of DHPRs into tetrads in orthogonal arrays, a prerequisite for skeletal muscle EC coupling [14].

Despite the inability to ventilate their gills, relaxed larvae are able to survive for few days due to oxygen and metabolite diffusion via the skin [16]. This in combination with the possibility of zygote injection into the externally developing embryos makes zebrafish mutant relaxed an excellent in vivo expression system. In a reconstitution study we showed that the expression of β_1a in the relaxed system restored the motile phenotype and that DHPR tetrad formation is an exclusive property of β_1a [17]. Thus, specific structural elements important for the formation of tetrads must exist in β_1a.

Earlier studies, with β_1a/β_2a chimeras and truncation mutants expressed in murine β₁-null myotubes, restricted the domain of β_1a that is essential for skeletal muscle EC coupling to its C-terminus [18–20]. More precisely, a β_1a-specific C-terminal hydrophobic heptad repeat motif (L478, V485, V492) was proposed to control the EC coupling activity [22]. A β_1a mutant with the heptad repeat LVV exchanged to alanines could only reconstitute 20% of intracellular Ca²⁺ transients and also induced a significant positive shift in their voltage-dependence. Leucine heptad repeat motifs are well known to mediate protein–protein interactions [21]. On this basis it was hypothesized that the heptad repeat of β_1a is involved in direct interaction with RyR1 [20,22] and thus plays an active role in the signal transduction from DHPR to RyR1 via β_1a.

Moreover, since we showed that β_1a is specifically required for tetrad formation [17] which is a prerequisite for EC coupling, it is also possible that already the arrangement of DHPRs into tetrads is dependent on this β_1a-specific heptad repeat motif. Therefore, we wanted to address this question by comparing the ultrastructural arrangement of DHPR with functional recordings of EC coupling. If, despite the lack of the LVV motif, tetrads but no intact EC coupling could be restored, the hypothesis of β_1a as a direct signal transducer – as an additional β_1a function – would be supported. On the other hand, if tetrads were not formed, the conclusion would be that the LVV motif is simply involved in the initial scaffolding process which ultrastructurally enables the interaction of DHPR and RyR1 by placing them in the appropriate relative configuration.

We took advantage of the β_1a-null zebrafish relaxed expression system that allows us to directly compare the effects observed in vitro to an intact in vivo muscle system. Mutant constructs from zebrafish β_1a (zf-β_1aAAA) and from a mammalian (rabbit) β_1a (rb-β_1aAAA), in which the conserved hydrophobic heptad repeat motif LVV was exchanged to alanines, were expressed in isolated relaxed myotubes and entire larvae. Our results with β_1aAAA show that knock out of the LVV motif did not interfere with correct targeting of DHPR into tetrads. Furthermore, and to our surprise, heptad repeat mutants were able to restore robust intracellular Ca²⁺ transients in relaxed myotubes and a fully motile phenotype in relaxed larvae, thus illustrating restoration of proper DHPR–RyR1 coupling. In contrast to the earlier proposals, our results indicate that the β_1a-specific C-terminal heptad repeat motif LVV is not a critical determinant of skeletal muscle EC coupling, because it is neither necessary for tetrad formation nor for DHPR–RyR1 signal transduction.

2. Materials and methods

Experimental procedures were essentially the same as described earlier in detail [17] and thus only a concise summary is given.

2.1. Zebrafish embryos

Rearing and breeding of zebrafish, heterozygous for the β₁-null mutation relaxed (red^ts25) was performed according to the established procedures [23,24]. Freshly spawned eggs were directly used for zygote RNA microinjection and/or raised at 28 °C to be used for experiments.

2.2. Expression plasmids

The cDNAs of the β subunits and mutants were N-terminally in-frame fused to GFP cDNA and cloned into expression vector pCI-neo (Promega). Nucleotide numbers (nt) are given in parenthesis. All sequences generated and modified by PCR were checked for integrity by sequence analysis (Eurofins MWG Operon, Martinsried, Germany).

2.2.1. zf-β_1aAAA

Fusion PCR was used to generate LVV/AAA substitutions (L478A, V485A, V492A) with zf-β_1a cDNA (GenBank AY952462) in pCI-neo as template. The sense primer was used for T to C transition which created triplet codons GCC (nt 1452–1454) and GCG (nt 1473–1475) both coding for alanine instead of valine. With the antisense primer, triplets CTG (nt 1431–1433) and GTC (nt 1452–1454) were mutated to GCG and GCC respectively, both coding for alanine instead of leucine and valine. To gain the final construct zf-β_1aAAA, the PCR-generated EarI–XbaI fusion fragment (nt 879–1803) was co-ligated with fragment EcoRV–EarI from zf-β_1a (nt −748 to 879) into the EcoRV/XbaI (nt −748 to1803) cleaved zf-β_1a clone.

2.2.2. rb-β_1aAAA

The LVV/AAA substitutions (L478A, V485A, V492A) were created by using the fusion PCR technique with rb-β_1a cDNA (GenBank NM_001082279) in pCI-neo as template. The sense primer substituted GTC (1452–1454) and GTG (1473–1475) to GCC and GCG respectively, both coding for alanine instead of valine. The antisense primer was used to replace CTG (1431–1433) and GTC (1452–1454) with GCG and GCC respectively, both coding for alanine instead of leucine and valine. The resulting BstXI–XbaI (nt 834–1801) fusion product was ligated together with fragment EcoRV–BstXI from rb-β_1a (nt −763 to 834) into the EcoRV/XbaI (nt −763 to 801) cut rb-β_1a to generate the final construct rb-β_1aAAA.

2.2.3. GFP

GFP alone was cloned into expression vector pCI-neo for standardizing experimental conditions [17].

2.3. Zygote injection of in vitro synthesized RNA

All β subunits and mutants were linearized with restriction enzyme XbaI, but GFP with NotI. Purified and linearized DNA templates were used for in vitro transcription followed by phenol/chloroform extraction and ethanol precipitation. RNA pellets were resuspended in RNAse-free water, fidelity checked on an agarose gel under denaturing conditions and the aliquots were stored at −80 °C until use. Eggs from heterozygous parental zebrafish in one-cell stage were injected within 20 min after spawning. Approximately 2.6 ng of RNA, containing 0.1% phenol red as an injection volume tracer [24] was injected per egg. The GFP fluorescence of 8-h-old healthy injected embryos was quantified using a photomultiplier system. Only proper developing injected embryos with a mean fluorescence signal exceeding 40% above uninjected control embryos were considered for further experiments.

2.4. Identification of rescued larvae

Differentiation of motility-restored homozygous relaxed larvae, used in motion analysis experiments, from the injected “normal” siblings (i.e. heterozygous and wild-type, WT) was done by keeping all injected larvae in isolation up to 5 days. During this period a gradual fallback to the paralyzed phenotype due to degradation of the injected β-RNAs and translated proteins was observed for restored relaxed but not normal larvae. Genotype confirmation of the relaxed phenotype was done by RFLP. In the case when larval tails were used for freeze-fracture electron microscopy, motility-restored relaxed larvae had to be identified immediately by RFLP test on the larval heads. For this, genomic DNA was extracted as described previously [17] and was used as PCR template to amplify a 459 bp fragment containing the relaxed mutation. Restriction enzyme digest of the PCR product with BsrI cleaved the 459 bp fragment into 279 and 180 bp fragments, only in case of WT but not in relaxed alleles.

2.5. Freeze-fracture electron microscopy

27–30 h post-fertilization (hpf) injected motile larvae were decapitated and the tails were fixed with 6% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.2; both Sigma) for 30 min at room temperature. Samples were stored in 3% glutaraldehyde at 4 °C. Tails were infiltrated in 30% glycerol in water and fractured in the double replica holder, shadowed with platinum at 45 °C, and replicated with carbon in a BFA 400 Balzers freeze-fracture unit (Balzers SpA). Replicas were analyzed in an electron microscope (model 410; Philips). Only myotubes with peripheral couplings between the surface membrane and the SR were analyzed.

2.6. Digital motion analysis

27–30 hpf larvae were dechorionated using pronase and transferred into 24-well plates to keep individuals identifiable for several days. 2-min video sequences were recorded with a Sony CCD AVC-D7CE b/w camera, converted into stacks of differential images [25] and 3 × 3 median filtered to eliminate pixel noise. The mean luminance of every image reflecting larval movement was quantified by automated counting of total dynamic pixels per image and plotted against time. On this processed signal, peak detection was performed. For quantifying the larval ‘movement extent’, the cumulative dynamic pixels per peak were calculated, the mean value for all larval movements of each experimental group determined and standardized to the values obtained from normal larvae.

2.7. Primary culture of zebrafish myotubes

25–28 hpf homozygous relaxed larvae were identified by their inability to move despite tactile stimulation. Motile normal siblings were used for control experiments. 100–150 larvae were dechorionated using Pronase (Protease, Type XIV, Sigma) [23], anesthetized and decapitated and the tails digested with collagenase type I (Sigma) to dissociate muscle cells. The cells were transfected with 2 μg of plasmid cDNA using the AMAXA™ rat neonatal cardiomyocyte nucleofector kit (AMAXA Biosystems, Köln, Germany) according to the manufacturer's manual. Myocytes were cultured at 28 °C for 4–6 days for electrophysiological experiments.

2.8. Whole cell patch clamp

Immobilization-resistant intramembrane charge movement as well as intracellular Ca²⁺ transients were recorded from transfected (GFP-positive) myotubes. Charge movement is a measure of functional membrane expression of the L-type Ca²⁺ channel complex [26]. Borosilicate glass patch pipettes (Harvard Instruments) had resistances of 3.5–5 MΩ when back-filled with internal solution containing 100 mM CsAspartate, 10 mM HEPES, 0.5 mM CsEGTA, 3 mM MgATP, and 0.2 mM Fluo-4 (pH 7.4 with CsOH). The bath solution consisted of 10 mM Ca(OH)₂, 100 mM l-aspartate, and 10 mM HEPES (pH 7.4 with tetraethylammonium hydroxide). Contractions of myotubes were blocked by adding 100 μM of the myosin-II blocker N-benzyl-p-toluene sulfonamide (Sigma) to the bath solution. Leak currents were subtracted by a P/4 prepulse protocol and the test pulses were preceded by a 1-s prepulse to −30 mV to inactivate endogenous T-type currents [26]. Total charge movement was calculated by integrating the ON-component of gating currents (Q_on). 0.2 mM Fluo-4 was added to the patch pipette solution to measure intracellular Ca²⁺ release. The average fluorescence of a 10-ms time period in the plateau phase of the 200-ms test pulse was normalized to the resting fluorescence and expressed as ΔF/F₀.The voltage dependence of Q_on and ΔF/F₀ was fitted according to the Boltzmann distribution:

$$A=\frac{A_{\text{max}}}{1+\text{exp}[-(V-V_{1/2})/k]}$$

where A is Q_on or ΔF/F₀, V_1/2 is the potential at which A = A_max/2, and k is a slope factor. Data were analyzed using ClampFit 10.0 (Axon Instruments) and SigmaPlot 10.0 and 11.0 (SPSS Science, Chicago, IL) software.

2.9. Statistics

Statistical significance from experimental approaches was assessed using unpaired Student's t test or one-way analysis of variance (ANOVA), as appropriate. Data are reported as mean ± SE. p values < 0.05 were considered significant.

3. Results and discussion

3.1. Conservation of the heptad repeat motif in the C-terminus of the DHPRβ_1a subunit

Voltage-gated Ca²⁺ channel β subunits are structurally organized into two conserved domains, namely the SH3 (Src homology 3) and GK (guanylate kinase) domains, which are flanked by three variable regions: the N-terminus, a HOOK region, and the C-terminus [27–30] (Fig. 1A). Amino acid sequence alignment of C-termini of different β_1a subunits revealed the conservation of the previously described [22] β_1a-specific heptad repeat motif (LVV) from fish to human (Fig. 1B). Murine β₁-null myotubes expressing a β_1a mutant with the heptad repeat motif LVV exchanged to AAA were described to have a 5-fold reduction in intracellular Ca²⁺ release [22]. To address the question whether this reduction is due to the lack of signal transduction from DHPR to RyR1 via β_1a as proposed [20,22] or is already due to a targeting problem on the ultrastructural level, i.e. the lack of tetrad formation, we studied the heptad repeat mutation in β₁-null zebrafish relaxed larvae and isolated myotubes. To validate species-independence of the results we used β_1aAAA mutants derived from the native zebrafish β_1a as well as from a mammalian (rabbit) β_1a. Zebrafish and rabbit β_1a subunits share 76% all-over amino acid identity and 68% in their C-termini.

Fig. 1.

A conserved leucine-valine heptad repeat motif in the DHPRβ_1a C-terminus. (A) Cartoon of the domain organization of the β_1a subunit based on crystal structure models [28–30]. (B) Sequence alignment of β_1a C-termini from different vertebrate classes, from fish to human, showed the conservation of the β_1a-specific leucine-valine heptad repeat motif (boxed). To test for the contribution of the heptad repeat motif in skeletal muscle EC coupling, the LVV residues (boxed) were substituted by AAA (β_1aAAA) in zebrafish and rabbit β_1a subunits to be expressed in the zebrafish β₁-null mutant relaxed system. Sequence for human (Homo sapiens) β_1a, anole lizard (Anolis carolinensis) β_1a, and Xenopus (Xenopus tropicalis) β_1a were extracted from genomic assemblies at http://www.ensembl.org using a BLAST search with zf-β_1a or rb-β_1a cDNA sequences. GeneBank accession numbers for all other sequences used are: mouse (Mus musculus) β_1a, NM_031173; rabbit (Oryctolagus cuniculus) β_1a, NM_001082279 and zebrafish (Danio rerio) β_1a, AY952462.

3.2. Intact ultrastructural organization of DHPRs in tetrads in relaxed larvae expressing β_1aAAA

To test for a possible ultrastructural mistargeting of the channel complex as a consequence of the AAA mutation, we injected the RNA coding for rb-β_1aAAA into relaxed zygotes and analyzed the orientation of DHPR particles opposite the RyR1 in 27–30 hpf larval tail muscles by freeze-fracture electron microscopy (Fig. 2). While freeze-fracture replicas of untransfected relaxed larvae showed arbitrarily arranged DHPR particles within all clusters (Fig. 2, middle panel, from an archive of 109 images from 14 fractured fish tails), replicas of relaxed larvae expressing rb-β_1aAAA revealed a correct arrangement of DHPR particles in tetrads organized in orthogonal arrays (Fig. 2, right panel, from 43 images from 3 fish tails) comparable to myotubes from normal larvae (Fig. 2, left panel, from an archive of 151 images from 8 fish tails). Thus, rb-β_1aAAA is able to restore the correct ultrastructural arrangement of DHPRs in tetrads, essential for skeletal muscle EC coupling. At this point our results, showing full restoration of tetrad formation in combination with the severely reduced EC coupling postulated earlier [22], would apparently suggest an additional function of β_1a, i.e. as an active signal-transmitting element in the EC coupling process.

Fig. 2.

Intact DHPR tetrad formation with β_1aAAA mutant expressed in relaxed myotubes. Freeze-fracture replicas of tail muscle tissue of normal zebrafish larvae (left panel) revealed the arrangement of DHPR particles in tetrads (indicated by red dots) organized in orthogonal arrays. In contrast, the β₁-null mutant relaxed (middle panel) lacks DHPR tetrad formation. Comparable to normal larvae, relaxed larvae zygote-injected with in vitro synthesized rb-β_1aAAA RNA, displayed correct assembly of DHPR particles in tetradic arrays (right panel). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3.3. Full motility restoration in relaxed larvae expressing β_1aAAA

Surprisingly, relaxed larvae that were zygote-injected with either zf- or rb-β_1aAAA mutant RNA displayed vigorous movements (Movie S1 and S2 in Supplementary Material). To further test whether the larval motility gives an indication of the reduced Ca²⁺ release as reported [20,22], a detailed movement analysis was performed (Fig. 3). 2-min videos of individual larva were recorded, converted into differential images and total number of dynamic pixels per frame were plotted against time (Fig. 3A). Interestingly, relaxed larvae expressing β_1aAAA mutant revealed movement profiles similar to relaxed larvae expressing WT β_1a, with double-peaks representing larval muscle contraction and relaxation [17]. Movement extent was quantified and the values obtained for rescued relaxed larvae were standardized to uninjected normal larvae (100 ± 3%, n = 114). The movement extents of relaxed larvae expressing either zf-β_1aAAA (104 ± 4%, n = 30) or rb-β_1aAAA (97 ± 2%, n = 65) were indistinguishable (p > 0.05) from uninjected normal larvae and from WT β_1a expressing larvae, with 97 ± 4%, n = 49, for zf-β_1a and 95 ± 5%, n = 21, for rb-β_1a (Fig. 3B).

Fig. 3.

Full restoration of motility in relaxed larvae zygote-injected with β_1aAAA mutant RNA. (A) Representative plots of total dynamic pixels per frame of 2-min video recordings of spontaneous larval movements. Relaxed larvae zygote-injected with either rb-β_1a (left) or rb-β_1aAAA RNA (right) revealed a similar movement profile. (B) To quantify larval movement extent, mean values of cumulative dynamic pixels per movement for each experimental group were calculated and standardized to those of normal larvae. Relaxed larvae zygote-injected with either zf-β_1aAAA or rb-β_1aAAA mutant RNA showed full recovery of larval movement extent, indistinguishable (p > 0.05) from normal larvae or from relaxed larvae injected with either zf-β_1a or rb-β_1a. One-way ANOVA revealed overall non-significance (p = 0.62, F_(4,274) = 0.66). Uninjected relaxed larvae never showed any motility (nd, not detectable).

3.4. Proper intracellular Ca²⁺ release in relaxed myotubes expressing β_1aAAA

The above result was quite unexpected, because a perfect restoration of motility in larvae zygote-injected with β_1aAAA mutant is not consistent with the 80% reduction in Ca²⁺ release as described from β_1aAAA expressing murine β_1a-null myotubes [20,22]. Therefore, we analyzed relaxed myotubes transfected with WT β_1a and β_1aAAA mutants from zebrafish and rabbit by patch clamp (Fig. 4). Unlike other vertebrates, skeletal muscle of higher teleost fish – and thus zebrafish – have no DHPR inward Ca²⁺ currents [31] and thus, a possible contamination of intracellular Ca²⁺ release recordings by extracellular Ca²⁺ influx is impossible so that our data shows pure skeletal-muscle-type EC coupling. Contrary to the earlier results of the Coronado laboratory but in agreement with our motility restoration data, both β_1aAAA mutants were able to restore pronounced intracellular Ca²⁺ release in response to 200-ms test pulses to potentials of −50 to +70 mV (Fig. 4A). In order to correct for different expression levels, indicated by small differences in Q_max (Table 1), ΔF/F₀ values were normalized to Q_max (Fig. 4B). Values of ΔF/F₀/Q_max were identical (p > 0.05) between zf-β_1aAAA (0.14 ± 0.02 mV; n = 14) and zf-β_1a (0.16 ± 0.03; n = 9) but slightly smaller in rb-β_1aAAA (0.14 ± 0.01; n = 41) versus rb-β_1a (0.18 ± 0.02 mV; n = 23; p = 0.044). Both β_1aAAA mutants induced a small shift in the voltage-dependence of intracellular Ca²⁺ transients towards more positive potentials, that was slightly significant (p = 0.036) for zf-β_1aAAA (8.94 ± 2.32 mV; n = 14) compared to zf-β_1a (−0.06 ± 3.49 mV; n = 9), but highly significant (p = 0.003) for rb-β_1aAAA (7.56 ± 0.97 mV; n = 41) in comparison to rb-β_1a (1.69 ± 1.81 mV; n = 23).

Fig. 4.

The β_1aAAA mutation has only a minor effect on intracellular Ca²⁺ transients. (A) Representative intracellular Fluo-4 Ca²⁺ recordings from relaxed myotubes expressing zebrafish and rabbit WT β_1a (upper panel) or β_1aAAA mutant subunits (lower panel). Pronounced intracellular Ca²⁺ transients in response to 200-ms test pulses with similar kinetics were recorded with all constructs. Differences in transient size with β_1aAAA can be explained by slight differences in expression levels as indicated by differences in intramembrane charge movement recordings (insets). (B) Voltage dependence curves of ΔF/F₀ were corrected for differences in expression levels by normalizing ΔF/F₀ to Q_max. Transients were slightly shifted towards more positive potentials for the β_1aAAA constructs.

Table 1.

	Qmax (nC/μF)	V1/2 − Q (mV)	(ΔF/F0)max	V1/2 − (ΔF/F0)max (mV)
rb-β1a	11.07 ± 0.77 (n = 22)	−4.49 ± 0.86	1.96 ± 0.17 (n = 23)	1.68 ± 1.81
rb-β1aAAA	8.80 ± 0.59 (n = 35)	−2.29 ± 0.64	1.25 ± 0.11 (n = 41)	7.80 ± 0.93
zf-β1a	11.13 ± 1.72 (n = 12)	−3.97 ± 2.19	1.75 ± 0.36 (n = 9)	−0.06 ± 3.49
zf-β1aAAA	10.93 ± 1.00 (n = 18)	−3.39 ± 1.06	1.45 ± 0.18 (n = 14)	8.91 ± 2.32

Values of maximal intramembrane charge movement (Q_max), maximal intracellular Ca²⁺ release ((ΔF/F₀)_max) and the corresponding half-maximal activation potentials, recorded from relaxed myotubes transfected with either β_1a or β_1aAAA mutant subunits from zebrafish and rabbit.

In comparison to the earlier study by the Coronado laboratory, the effects of the LVV/AAA mutation on size and voltage dependence of intracellular Ca²⁺ transients observed in our experiments (Table 1) are even minor than the effects that were observed with their control mutant D5ALAc, where 3 amino acids out of step to the LVV motif were exchanged to alanines. D5ALAc yielded 23% reduced (ΔF/F₀)_max values with a 24 mV right-shift in voltage dependence upon 200-ms test pulses [22]. Thus the LVV residues of the heptad repeat motif, like the residues exchanged in the D5ALAc mutant, are rather functionally irrelevant. Rather, the β_1a C-terminus is an optimized structure in itself and consequently any change in the C-terminus affects the general folding and hence protein functioning. However, as shown in the in vivo motility data, these minor changes are not sufficient to show quantifiable effects on the extent of spontaneous larval movements which mirrors the skeletal muscle EC coupling.

4. Conclusion

Our results do not support an essential role of the β_1a-specific hydrophobic C-terminal heptad repeat LVV in skeletal muscle EC coupling as both structural (tetrad formation) and functional interactions are restored with β_1aAAA in reconstitution studies. Intracellular Ca²⁺ transients resulting from activation of RyR1 by the DHPR are very little affected by the β_1a heptad repeat mutation, and these minor changes do not influence spontaneous muscle activity. Relaxed larvae expressing β_1aAAA mutants displayed motility indistinguishable from those injected with WT β_1a. The reasons underlying the discrepancy between our results and those from the earlier study [20,22] are not clear. Besides putative differences in the expression systems, a second and perhaps more likely possibility for this incongruency might be crucial differences between the clones employed. To elucidate this dissonance we intended to include analyzes from the original murine β_1a heptad repeat mutant clone D5ALA [22]. Unfortunately we were unsuccessful due to the disappearance of the original clone (Dr. Timothy J. Kamp and Dr. David Sheridan, personal communications, 2008). According to our novel finding that the β_1a heptad repeat motif is not the key determinant of DHPR–RyR1 coupling, doors are reopened for further in-depth structural–functional studies on the role of the DHPRβ_1a subunit in skeletal muscle EC coupling.

Conflict of interest

None.