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Received 9 Jul 2012 | Accepted 14 Jan 2013 | Published 26 Feb 2013

Barbara Mosca1, Osvaldo Delbono2, Maria Laura Messi2, Leda Bergamelli1, Zhong-Min Wang2, Mirko Vukcevic3, Ruben Lopez3, Susan Treves1,3, Miyuki Nishi4, Hiroshi Takeshima4, Cecilia Paolini5, Marta Martini6, Giorgio Rispoli6, Feliciano Protasi5 & Francesco Zorzato1,3

Muscle strength declines with age in part due to a decline of Ca2 release from sarcoplasmic reticulum calcium stores. Skeletal muscle dihydropyridine receptors (Cav1.1) initiate muscle contraction by activating ryanodine receptors in the sarcoplasmic reticulum. Cav1.1 channel activity is enhanced by a retrograde stimulatory signal delivered by the ryanodine receptor.

JP45 is a membrane protein interacting with Cav1.1 and the sarcoplasmic reticulum Ca2 storage protein calsequestrin (CASQ1). Here we show that JP45 and CASQ1 strengthen skeletal muscle contraction by modulating Cav1.1 channel activity. Using muscle bres from

JP45 and CASQ1 double knockout mice, we demonstrate that Ca2 transients evoked by tetanic stimulation are the result of massive Ca2 inux due to enhanced Cav1.1 channel activity, which restores muscle strength in JP45/CASQ1 double knockout mice. We envision that JP45 and CASQ1 may be candidate targets for the development of new therapeutic strategies against decay of skeletal muscle strength caused by a decrease in sarcoplasmic reticulum Ca2 content.

1 Department of Experimental and Diagnostic Medicine, General Pathology section, University of Ferrara, Via Borsari 46, Ferrara 44121, Italy. 2 Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA. 3 Department of Biomedicine and

Anesthesiology, Basel University Hospital, Hebelstrasse 20, Basel 4031, Switzerland. 4 Department of Biological Chemistry, Graduate School of

Pharmacological Sciences, Kyoto University, Kyoto 606-8501, Japan. 5 CeSI-Center for Research on Ageing & DNI-Department of Neuroscience and Imaging,

University Gabriele d Annunzioof Chieti, Via Colle dellAra, 66100 Chieti, Italy. 6 Department of Biology and Evolution, Physiology and Biophysics University of

Ferrara, Via Borsari 46, Ferrara 44121, Italy. Correspondence and requests for materials should be addressed to F.Z. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 4:1541 | DOI: 10.1038/ncomms2496 | http://www.nature.com/naturecommunications

DOI: 10.1038/ncomms2496

Enhanced dihydropyridine receptor calcium channel activity restores muscle strength in JP45/CASQ1 double knockout mice

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496

Activation of skeletal muscle contraction is initiated by the propagation of the action potential deep into the muscle bre by means of the transverse tubular system (T

system)13. T-tubule depolarization causes massive release of Ca2 from the sarcoplasmic reticulum (SR) throughout the entire length of the muscle bre by a process called excitation-contraction coupling1,2. Loss of muscle function has been recognized as a debilitating and life-threatening condition not only in the elderly, but also in cachexia in cancer patients and in all those clinical conditions associated with prolonged bed rest4,5. The decay of muscle strength is caused by several factors, including a decrease of releasable calcium from the skeletal muscle SR calcium store6,7.

EC coupling is operated by a macromolecular complex comprising the a1-subunit of the voltage-dependent L-type

Ca2 channel (dihydropyridine receptor, DHPR, Cav1.1), the ryanodine receptor (RyR) and calsequestrin (CASQ1)8, in the contact region between the T system and the SR membrane3. Cav1.1 acts as a voltage sensor and generates orthograde signals that cause opening of the RyR whereby Ca2 is released from the

SR into the myoplasm, leading to activation of the contractile proteins1. Analysis of voltage-dependent calcium currents in RyR1 knockout (KO) muscle cells was fundamental to clarify the signalling mechanisms between the RyR and Cav1.1. It is now accepted that the RyR1 not only receives an orthograde signal from Cav1.1, but also generates a retrograde signal which is important for the activation of Cav1.1 channel activity9,10. The mechanism by which the retrograde Ca2 current enhancement is modulated and its exact physiological role remain elusive.

Although calcium inux across the sarcolemma is thought to be non relevant for muscle contraction11, it has been proposed that it may have a role in replenishing the SR during sustained muscle contractions12. Two different modes of calcium inux in skeletal muscle have been described: (i) calcium inux via Cav1.1 associated with prolonged membrane depolarization is referred to as excitation-coupled calcium entry (ECCE)10, (ii) calcium inux via STIM1 and OraI1 stimulated by internal store depletion is referred to as store-operated calcium entry (SOCE)1315. However, the analysis of macroscopic calcium currents in adult muscle bres recorded under voltage clamp condition16 suggests that both voltage-dependent calcium release and/or SR calcium depletion are not sufcient to activate inward calcium currents and thus challenged the physiological relevance of ECCE and SOCE in adult mammalian bres. Nevertheless, calcium inux in muscle cells deserves further investigation, because of the potential impact of calcium inux in determining releasable SR calcium content17, a crucial factor for proper sustained muscle force development 6,7,18.

Calcium inux in skeletal muscle is affected by drugs19,20 and by accessory proteins such as junctophyllin and mitsugumin-29, two proteins localized in the membrane compartment which form the junction between T tubules and SR12,21. T tubules-SR junction membranes encompasses also JP45, a developmentally regulated 45-kDa transmembrane protein that interacts via its luminal carboxy-terminal domain with CASQ1, the major calcium storage protein of SR, and with its amino terminal domain with Cav1.122. Ablation of calsequestrin 1 in skeletal muscle bres results in a decrease of total calcium

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Figure 1 | Ca2 transients and Mn2 quenching in FDB bres from single and DKO mice. FDB bres were loaded with the fast low-afnity Ca2 dye

Mag-Fluo-4AM 21,22. (a) Ca2 transients were triggered by supramaximal eld stimulation with single pulses of 0.5 ms duration. Continuous lines: Ca2

transients recordings in an external solution containing 1.8 mM CaCl2. Overall ANOVA P-valueo0.0001; multicomparison Dunnets ANOVA post test

shows difference between of the peak calcium values: WT versus DKO Po0.01, WT versus JP45 KO Po0.01; WT versus CASQ1 KO Po0.01; dotted grey

lines: Ca2 transients recordings in an external solution containing 100 mM La3. Overall ANOVA P-valueo0.0001; multicomparison Dunnets ANOVA

post test shows difference between of the peak calcium values: WT versus DKO Po0.01, WT versus JP45 KO Po0.01, WT versus CASQ1 KO Po0.01.

(b) Recording of Ca2 transients upon stimulation with repetitive pulses at 100 Hz for 300 ms duration. Black and grey lines show recordings in the

presence of 1.8 mM Ca2 and 100 mM La3 in the external solution, respectively. In the presence of La3, the overall ANOVA P-value iso0.0001;

multicomparison Dunnets ANOVA post test shows differences: WT versus DKO Po0.01, WT versus JP45 KO Po 0.05, WT versus CASQ1 KO P o0.01.

(c) Mn2 quenching of Fura-2 uorescence. Black lines: Mn2 inux was triggered by repetitive pulses at 100 Hz (arrow) for 300 ms duration. Overall

ANOVA P-valueo0.0001; multicomparison Dunnets ANOVA post test shows differences: WT versus DKO Po0.01. Dotted grey lines: Mn2 quenching

recordings in the presence of 50 mM nifedipine in the external solution.

2 NATURE COMMUNICATIONS | 4:1541 | DOI: 10.1038/ncomms2496 | http://www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496 ARTICLE

C 2C 12myotubes

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Figure 2 | Expression of calcium entry proteins in fast and slow muscles from DKO mice. (a) Real-time reverse transcription (RT) PCR from soleus and EDL of wild-type and DKO mice. Real-time RT PCR was carried out as previously described41. Briey, total RNA was extracted using TRIzol reagent from Soleus or EDL muscles after frozen tissue homogenization, and Cav1.2, TRPC3, OraI1 and STIM1 gene expression was evaluated by quantitative real-time PCR. Boxes represent the mean (s.e.m.) fold change compared with values obtained from WT for each gene of interest. Gene expression levels were normalized to the expression of the Tata box-binding protein (TBP) and desmin, whose expression was equal between wild-type and KO animals. Pooled data are results carried out on muscles from 5 to 7 different mice. (b) Expression of D29 Cav1.1 isoform mRNA in EDL and Soleus from 1-month-old

WT and DKO mice. The expression of neonatal D29 Cav1.1 isoforms allele (lower band) is present in mRNA from C2C12 myotubes. The D29 Cav1.1

isoforms could be absent or below the detection limit of PCR in EDL. Soleus displays a faint band corresponding to the D29 exon Cav1.1 transcript.

release from the SR, which leads to impaired muscle performance and contractile activation2326. Chronic depletion of JP45 induces a decrease of muscle strength in 3-month-old JP45 KO mice27. The decay in strength is apparently not linked to atrophy, but to defects in the EC-coupling machinery caused by alteration of the functional expression of DHPR Cav1.1 in the T-tubular network.

In the present study, we tested the hypothesis that the skeletal muscle Cav1.1 channel activity is not only regulated by the RyR, but additionally by the JP45/CASQ1 complex. We generated double JP45 and CASQ1 KO (DKO) and compared their Cav1.1-mediated Ca2 signals to those observed in WT and each single

JP45 and CASQ1 KO mice. Our results show that in DKO mice, tetanic stimulation of skeletal muscle bres causes massive Ca2 inux due to enhanced Cav1.1 channel activity and this Ca2 inux restores muscle strength. By using this animal model, we have unveiled a signalling pathway which may be an important target for drugs against the loss of skeletal muscle strength caused by decrease of the SR calcium content.

ResultsCalcium transient in exor digitorum brevis bres. We compared Ca2 homoeostasis in WT, single JP45 and CSQ1 KO and

DKO using the ratiometric Ca2 indicator Indo-1 and found that the resting calcium concentration was increased in DKO1.150.23* (n 48) bres compared with WT (0.920.10,

n 40), JP45 null (1.030.18, n 42) and CASQ1 null

(0.950.11) bres (F405/F480 indo-1 ratio values are means.d.; *WT versus DKO Po0.001 two tailed MannWhitney). In the presence of 1.8 mM Ca2 in the extracellular solution, the peak intracellular Ca2 transients measured with the low-afnity calcium indicator MagFluo4 were 1.0090.18 (n 47),

0.670.16 (n 49), 0.800.25 (n 40) and 0.820.25 (n 47)

for WT, DKO, JP45 KO and CASQ1 KO, respectively (DF/Fo values are expressed as mean s.d., Fig. 1 top panels). The signicant decrease of the peak Ca2 transient in DKO bres is not due to a lower membrane density of RyRs (Supplementary

Fig. S1). The half-time of the decay of the Ca2 transients in exor digitorum brevis (FDB) bres from DKO mice was signicantly slower compared with WT (3.70.7, n 47 and

4.81.9 ms, n 49 in WT and DKO bres, respectively. Analysis

of variance (ANOVA) P-valueso0.0001; multicomparison Dunnets post test WT versus DKO Po0.01).

Enhanced ECCE in FDB bres from DKO mice. The slower decay of the calcium transients in FBD bres from DKO mice is not due to a decrease of calcium uptake in to the SR by the Ca2 pump because we did not observe a reduction of the SERCA1 and

SERCA2 expression in DKO mice (Supplementary Fig. S1). The increase of the half-time of the decay of the calcium transient is rather linked to an inux of extracellular Ca2 , as in the presence of La3 , a non-specic calcium channel blocker10, the difference in the half-time for the decay of calcium transient between WT and DKO bres disappears (3.10.7 and 3.31.1 ms for WT and DKO, respectively). The effect of La3 was much more evident upon stimulation of FDB bres with repetitive action potentials. In the presence of La3 , the Ca2 transient amplitude of tetanic stimulation was highest in WT bres(1.310.28; n 37) compared with that of DKO (0.620.14*,

n 34), JP45 KO (1.050.30y, n 26) and CASQ1

(0.700.15*n 24) (DF/Fo values are mean s.d., *Po0.01,

yPo0.05, (multicomparison Dunnets ANOVA post test, Fig. 1). In CASQ1 null bres, the summation of Ca2 transient peaks was dramatically different compared with WT, JP45 null and

DKO bres. After the few initial peaks, which display an amplitude 50% lower compared with WT, the fused Ca2 transients in CASQ1 KO bres rapidly decayed to basal levels.

This event reects depletion of Ca2 stores due to the ablation of the Ca2 storage protein2325. However, at variance with CASQ1 null bres, the double KO (DKO) bres exhibit a sustained Ca2 transient which persisted for the entire duration of the repetitive stimulation in the presence of calcium in the extracellular solution (Fig. 1 middle panels, arrow Ca2 ).

Depolarization of DKO bres causes massive Ca2 inux. The sustained calcium transients evoked by tetanic stimulation in

DKO bres is caused by massive Ca2 inux, as in the presence of 100 mM La3 in the external solution (Fig. 1 middle panel, arrow La3 ), the Ca2 transient curve overlapped with that of

CASQ1 KO bres. These data unambiguously demonstrate that ablation of JP45 in a CASQ1 null background unveils a robust Ca2 inux component coupled to membrane depolarization.

NATURE COMMUNICATIONS | 4:1541 | DOI: 10.1038/ncomms2496 | http://www.nature.com/naturecommunications

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496

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DKO (n 15) muscle bres. Values are means.e.m. error bars show s.e.m. (b) Normalized values to maximal charge movement. Values are means.e.m.

error bars show s.e.m. (cd) Representative charge movement traces recorded in FDB bres voltage-clamped in the whole-cell conguration of the patch clamp. Holding potential: 80 mV. Command pulses of 25 ms duration evoked currents from 80 to 80 mV. Selected traces correspond to the steepest

part of the curve. Notice the larger amplitude and 10 mV shift of the curve toward more negative potentials in DKO compared with WT mice. (e) Calcium currentmembrane voltage relationship. Command pulses of 400 ms duration evoked currents from 80 to 80 mV. Notice the larger current amplitude for

DKO (n 19) compared with WT (n 29) mice. Values are means.e.m. (fg) Representative calcium currents recorded at the indicated membrane

voltages. Fitting curves, their respective equations and best tting parameter values are described in Tables 1 and 2.

This conclusion is also supported by fura-2 manganese quenching experiments performed in FDB bres (Fig. 1 lower panels). At an excitation wavelength of 360 nm, the Ca2 independent isosbestic wavelength of fura-2, Mn2 entry quenches fura-2 uorescence28. We measured the extent of the uorescence quenching at the end of a 300-ms-long train of pulses at 100 Hz. We found that manganese quenching of fura-2 uorescence was

0.110.039 (n 16), 0.0340.021 (n 17),

0.0340.011 (n 19), 0.0270.019 (n 35) for DKO, WT,

JP45 KO and CASQ1 KO bres, respectively (DF/Fo values are means.d.). The threefold increase of fura-2 uorescence quenching by Mn2 in DKO bres was abolished by the addition of 50 mM nifedipine (Fig. 1 lower panels grey line), a specic inhibitor of Cav1.110. Skeletal muscle membrane encompasses several molecules that can mediate calcium inux, including the neonatal splice variant (D29) of Cav1.129, TRPC315 and OraI1/Stim114, Cav1.2. We investigated the expression levels of other known calcium inux channels, and we found no changes both in fast and slow DKO bres (Fig. 2). Altogether, these data support the conclusion that the increase of excitation-

coupled Mn2 entry is mediated by an enhancement of calcium currents through adult form Cav1.110. The next set of experiments was designed to evaluate the changes in the functional properties of the Cav1.1.

Increase of Cav1.1 channel activity in DKO bres. The T-tubular system is the membrane compartment richest in Cav1.130 and high-resolution electron microscopy shows that volume and surface of the T-tubular system in FDB bres from 1-month-old DKO and WT mice is not different (Supplementary Fig. S2). Thus, the threefold increase of excitation-coupled Mn2 entry is not fully explained by changes of T-tubular membrane extensions or by a small increase of the Cav1.1 membrane density (Supplementary Fig. S1), but rather could be consistent with a modication of the channel activity of Cav1.1. To examine this possibility, we investigated the capacitive and Ca2 currents of

Cav1.1 in FDB bres from DKO and WT mice by using the whole-cell conguration of the patch-clamp technique. Intact FDB bres from DKO mice show 10 mV shift to more negative

4 NATURE COMMUNICATIONS | 4:1541 | DOI: 10.1038/ncomms2496 | http://www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496 ARTICLE

Table 1 | Best-tting parameters describing the voltage-dependence of charge movement.

Qmax (nC lF 1) VQ1/2 (mV) K WT (n 13) 308 142.2 161.5

DKO 3511 241.7 141.9

(n 15) (P 0.724) (P 0.001) (P 0.426)

Data points were tted to a Boltzmann equation of the form:Qon Qmax/[1 exp(V Vm)/K], where Qmax is the maximum charge, Vm is the

membrane potential, VQ is the charge movement half-activation potential and K is the steepness of the curve as described in ref. 27. The number of FDB bres from three to four mice is between parentheses. Results are expressed as the means.e.m. Statistical signicance was assessed using Students t-test. The a-level was set at P 0.05.

Table 2 | Best-tting parameters describing the voltage-dependence of calcium current.

Gmax (nS/nF) V1/2 (mV) Vr (mV) z WT (n 29) 908 4.72.5 623.1 3.90.6

DKO 13212 3.81.9 644.4 4.00.8

(n 29) (P 0.005) (P 0.775) (P 0.712) (P 0.921)

Data points were tted to the following equation: ICa Gmax (V Vm)/{1 exp[zF(V V)/

RT]}, where Gmax is the maximum conductance, V is the membrane potential, Vr is the reversal potential, V is the half-activation potential, z is the effective valence, F is the Faraday constant, R is the gas constant and T is the absolute temperature (296 K) as described in ref. 43. The number of FDB bres from three to four mice is between parentheses. Results are expressed as the means.e.m. Statistical signicance was assessed using Students t-test. The a-level was set at P 0.05.

potential of the half maximal gating charge, and small nonsignicant increase (15%) in maximal gating charge, compared with control (Fig. 3b and Table 1). The small increase of maximal gating charges do not account for the B45% increase of in peak

Ca2 current density, which was found in DKO FDB bres (Fig. 3e). Half-maximal Ca2 currents in wild-type (WT) and

DKO mice were observed at very similar membrane potentials (Table 2). These results demonstrate that enhanced calcium inux is accounted for an increase of the Cav1.1 channel activity.

Skeletal muscle performance is restored in DKO mice. To investigate physiological relevance of the enhanced Cav1.1 channel activity in DKO mice, we measured in vivo muscle performance by assessing spontaneous motor activity (Fig. 4a). Three weeks of training improved skeletal muscle performance in both WT and KO mice, however, the total running distance in JP45 null and CASQ1 null mice was B40 km lower compared with WT and DKO mice (139.72.90 km, 118.12.35 km versus 175.32.96 km, 175.54.23 km, respectively). The enhanced Cav1.1 channel activity restored muscle performance in DKO mice despite a low degree of atrophy of fast bres (Supplementary Fig. S3), and may be a signature of improved muscle strength31 mediated by massive Ca2 inux via Cav1.110. To investigate this, we studied the mechanical properties of intact extensor digitorum longus (EDL) (Fig. 4b). EDL from WT and DKO were stimulated with a train of tetani at 0.27 Hz. The onset maximal tetanic force normalized per muscle cross-sectional area of EDL from WT and DKO were not signicantly different (327.86 117.53 mN versus 364.6476.22 mN, respectively; means.d. n 9), whereas the time course of force development of EDL

from DKO was dramatically different compared to those of WT. In DKO mice, the rst train of repetitive pulses of 350 ms duration at 100 Hz caused an initial increase of isometric force and then rapidly decayed to B20% of the onset value at the end of the repetitive pulses stimulation (Fig. 4b, arrow middle panel rst). The reduction of force development during trains of action

potentials is most likely indicative of poor calcium buffering capacity of SR caused by the ablation of CASQ1, as a similar inability to sustain muscle contraction has been observed in muscles from CASQ1 KO mice24 but not in the EDL muscles from JP45 KO mice27, which have a normal CASQ1 expression level. Ablation of JP45 in a CASQ1 KO background has a remarkable effect on the peak tension after repetitive tetanic trains at 0.27 Hz. We observed that in the EDL from the DKO mice the isometric tension developed at the end of the last pulse of the train increased up to 300% of the initial value while a modest decrease was evident in WT mice (Fig. 4b arrow middle panel last; lower panels). On the basis of the data reported in Figs. 1 and 3, we reasoned that the ablation of JP45 in the CASQ1 KO background supports a strong calcium inux component mediated by Cav1.1 channel activity which leads to (i)

accumulation of intracellular calcium and to (ii) the improvement of the peak force development after trains of tetanic stimulation. We tested this possibility by examining the effect of La3 on the dynamics of force development of EDL from DKO mice during trains of tetani. As expected incubation of EDL with an external solution containing 100 mM La3 blocked calcium inux, and the increase of isometric force at the end of each train of pulses at 100 Hz in DKO and had no effect on WT muscles (Fig. 4b, lower panels). The effect of La3 in muscles from DKO mice was reversed by re-exposing EDLs to a bathing solution containing 1.8 mM CaCl2 (Fig. 5).

DiscussionHere we investigated the role of the JP45/CASQ1 complex on the modulation of Cav1.1 function by analysing the functional properties of skeletal muscle bres from JP45/CASQ1 DKO mice. Our results show that in DKO mice, calcium transients induced by repetitive action potential are supported by massive calcium inux from the extracellular environment. The massive increase of calcium inux is consistent with an enhancement of the Cav1.1 channel activity because: (1) it is inhibited by nifedipine, a blocker of Cav1.1; (2) it does not correlate with an increase in the expression of other know calcium inux channels such as OraI1, TRPC3 and the neonatal isoform of Cav1.1; (3) is associated with a 45% increase of the Cav1.1 peak calcium current density in intact single FDB bres. This massive calcium inux via Cav1.1 restores the development of in vitro muscle force of

EDL from DKO mice. The maintenance of muscle force in vitro is paralleled by the recovery of muscle performance of DKO mice in vivo. The characterization of the JP45/CASQ1 DKO animal model supports the conclusion that JP45/CASQ1 complex may be a genetically encoded modulator of the Cav1.1 channel activity.

The Cav1.1 complex has a dual function: it acts as (i) voltage sensor which activates, via a mechanical coupling, the RyR, and as(ii) a slow activating voltage-dependent calcium channel. Calcium inux via Cav1.1 channel activity was considered not important for skeletal muscle EC coupling, as it was shown that skeletal muscles can contract for hours in extracellular medium containing very low (sub nM) calcium concentrations11. The idea that skeletal muscle EC coupling is independent from the inux of extracellular calcium was conrmed later by pharmacological and genetic manipulation of Cav1.1 function3235. However, in evaluating the functional signicance of the Cav1.1 channel activity, one can not dismiss results showing that the inux of extracellular calcium in involved in the development of muscle contraction in amphibian and mammalian muscle bres36,37. This apparent discrepancy might be ascribed to different experimental models and conditions that were used to probe the importance of Cav1.1 channel activity in EC coupling. In this study we exploited the JP45/CASQ1 DKO mouse model to

NATURE COMMUNICATIONS | 4:1541 | DOI: 10.1038/ncomms2496 | http://www.nature.com/naturecommunications

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496

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Figure 4 | Skeletal muscle performance of WT and DKO mice. (a) In vivo evaluation of muscle strength. Spontaneous activity in 46 week-old WT, DKO, JP45 KO and CASQ1 KO mice individually housed in cages equipped with a running wheel. Data points are expressed as means.e.m.; n 10 to 13 mice.

Overall ANOVA P-valueo0.0001; multicomparison Dunnets ANOVA post test shows difference: WT versus DKO P40.05; WT versus JP45 KO Po0.05;

WT versus CASQ1 KO Po0.01. (b) Evaluation of tetanic force of intact EDL. Top and middle row of panels: EDLs were triggered by eld stimulation in

bathing solution containing 1.8 mM CaCl2 with a train of repetitive pulses (100 Hz, 350 ms duration) at 0.27 Hz (left panels). Time course of force

development of the rst and last tetani of the trace displayed in the left panels (middle and right panels). Lower row of panels: after repetitive train stimulation the EDL muscles from control (left panel) and DKO (right panel) mice were incubated for 10 min in a bathing solution containing 100 mM La3 and were then stimulated with repetitive trains of pulses at 0.27 Hz. Data point represents the force developed at the end of 350 ms duration repetitive pulse stimulation (values are mean s.d. n 8, * Po0.05 MannWhitney). The increase the force developed at the end of tetanic stimulation in EDL from

KO mice was abolished by 100 mM La3 a blocker of the calcium inux into muscle bres (compare DKO Ca2 versus DKO La3 ).

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investigate to role of Cav1.1 channel activity during EC coupling and its modulation by JP45/CASQ1 complex. Although we are aware that the JP45/CASQ1 KO mouse model may not recapitulate the physiological setting present in mammalian muscle bres expressing normal levels of both JP45 and CASQ1, our data provide a strong case as to the potential physiological signicance of the Cav1.1 channel activity during EC coupling, at least in mutant muscle bres. The robust calcium inux via Cav1.1 channel activity which was observed during repetitive action potential in JP45/CASQ1 DKO mice results in better contractile function in vitro and, most importantly, in vivo. Such

an effect on muscle contractile function could reect (i) an indirect global adaptive cellular response to the chronic ablation of two important SR proteins, or (ii) result from the lack of specic regulatory mechanism operated by the JP45/CASQ1 complex on the Cav1.1 channel activity. Although we cannot exclude any of the two possibilities, we are condent that our results clearly indicate the physiological importance of the dual activity of Cav1.1 during EC coupling of the DKO muscle bres.

In this mouse model Cav1.1 clearly operates not only as the voltage sensor that activates RyR (Fig. 2), (ii) but also as calcium inux channel which contribute to maintain an adequate level of

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496 ARTICLE

DKO

1.8 mM Ca2+ 100 uM La3+ 1.8 mM Ca2+ 2502001501005005 s 20 s 40 s 60 s

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250 200 150 100

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Figure 5 | Tetanic contraction of EDL from WT and JP45/CASQ1 double KO mice. EDL from wild-type and DKO were rst incubated in a Tyrodes solution containing 1.8 mM CaCl2. Maximal tetanic force was triggered by eld stimulation 28 with a train of repetitive pulses (100 Hz of 350 ms duration) at 0.27 Hz. Tetani at 5, 20, 40 and 60 s are shown. The same stimulation protocols was applied to muscle which were incubated for 10 min in a bathing solution containing 100 mM La3 (traces WT La3 and DKO La3 ). The increase the force developed at the end of tetanic stimulation in EDL from KO

mice was abolished by 100 mM La3 a blocker of the calcium inux into muscle bres (compare DKO left panel with middle panel). The effect of La3

was reversed by 10 min long exposure of EDL muscles to a bathing solution containing Ca2 (right panels).

releasable SR calcium content during sustained muscle activity evoked by tetanic stimulation (Fig. 3). These data imply that the JP45/CASQ1 complex modulate Cav1.1 channel activity also in normal bres, in particular, it may operate in those conditions which causes a decit SR calcium load.

On the basis of these data we propose that JP45/CASQ1 complex is genetically encoded modulator of the Cav1.1. channel activity, and that it constitutes a potential target to devise novel therapeutic strategies against the decline of skeletal muscle strength linked to decreased SR calcium content6,7,12.

Methods

Generation JP45 CASQ1 DKO mice. JP45 KO was obtained as described by Delbono et al.27. CASQ1 KO mice were obtained as described by Paolini et al.26. DKO mice were generated by crossing to each other established JP45 KO and CASQ1 KO lines backcrossed in C57BL6J (Supplementary Fig. S4).

Morphology. Immunohistochemistry of EDL and Soleus was carried out as described by Delbono et al.38. EDL and soleus muscles were embedded in OCT, snap-frozen in isopentane, cryosectioned at the mid-belly region (10 mm) and mounted on coverslips for immunostaining. For staining mounted sections were air dried, treated with PBS containing 1% bovine serum albumin (BSA) and 2% horse serum for 30 min and incubated overnight at 48 C with a PBS solution containing0.01% Triton X100, 1% BSA, 2% horse serum, 0.5% mg ml 1 anti-mouse slow myosin heavy chain (MAB 1628, Millipore, Billerica, MA), 2 mg ml 1 anti rat alaminin (MAB1914, Millipore). Sections were then washed with PBS for 15 min four times, and incubated at room temperature for 40 min with a PBS containing Alexa Fluor 488 anti-mouse IgG Ab (2 mg ml 1) and Cy3 anti rat IgG Ab (0.5 mg ml 1).

After incubating in the secondary Ab, sections were washed with PBS for 15 min four times, dehydrated with ethanol and mounted using a glycerol medium. Fluorescence images were imaged using a Leica DM5000B uorescence microscope and analysed with Analysis software package from Soft Imaging System, Muenster, Germany. Image analysis of muscle sections was performed in four steps: (1) determination of the muscle bre boundaries, (2) determination of the muscle bre cross-sectional area, (3) calculation of the per cent of muscle bres positive for anti MHC I Ab and (4) determination of the per cent of muscle bres negative for anti MHC I Ab staining. The muscle bre cross-sectional area was determined using the minimal Ferets diameter (the minimum distance of parallel tangents at opposing borders of the muscle bre). High-resolution electron microscopy was carried out as described by Paolini et al.26 Volume and surface of the transverse tubule (TT)

network (see Table in Supplementary. Fig. 4) were determined using the well-established stereology point and intersection counting techniques39,40 in EM micrographs taken at 14,000 of magnication. (a) Measurement of relative bre

volume occupied by TT. After covering the images with an orthogonal array of dots at a spacing of 0.20 mm, the ratio between numbers of dots falling in the TT lumen and the total number of dots covering the whole image represent the relative volume of bre occupied by the TT. (b) Measurement of TT surface area to volume. The images were covered with two sets of grid lines separated by a distance of 0.24 mm and intersecting at right angles. The frequency of intersections between the membrane of interest (TT proles) and the grid lines was counted. The ratio of TT surface area to volume was obtained from the formula C/2dP test, where C is the number of intersections, d is the spacing between the grid lines, and P test is the number of grid intersections in the test area.

Gene expression analysis. Expression of neonatal D29 isoform of Cav1.1 was detected by semiquantitative RT-PCR41. Total RNA was extracted from homogenized mouse muscle tissues EDL and SOL, and cultured C2C12 myotubes using TRIzol reagent. Eight hundred nanograms of RNA were rst reverse transcribed into cDNA; the Cav1.1 cDNA was amplied by PCR using primers29, which span exons 2734: forward 50-AGTCGGAGCAGATGAACCAC-30 and reverse 50-ATGGCCTTGAACTCATCCAG-30. The PCR amplication conditions were 95 C for 5 min, followed by 37 cycles of 94 C for 40 s, 51 C for 40 s and 70 C for 1 min, followed by a 5-min extension at 72 C. The RT-PCR products encoding the adult and neonatal(Cav1.1D29) isoforms are 790-bp long (upper band) and (lower band) 733 bp long, respectively.

Analysis of total SR and muscle strength assessment. Total SR membranes were prepared22 starting from a 20% skeletal muscle total homogenate; this was sedimented at 3,000gmax for 10 min and the resulting supernatant was centrifuged

at 15,000gmax for 20 min to remove the myobrillar protein components. The

15,000gmax supernatant was then centrifuged for 60 min at 100,000gmax to isolate the total SR (microsomal) pellet. SDS-polyacrylamide electrophoresis and western blot of total SR proteins were carried out as described by Anderson et al.22 Blots were probed with a polyclonal primary Ab followed by peroxidase-conjugated secondary antibodies. The immunopositive bands were visualized by chemiluminescence using the Super Signal West Dura kit from Thermo Scientic. Densitometry of the immunopositive bands was carried out by using BioRad GelDoc 2000. [3H]-PN200-110 and [3H]-Ryanodine binding was carried out according to Anderson et al.42 Briey, total SR membranes were incubated for 1 h in the dark in a solution containing 50 mM TrisHCl pH 7.5, 10 mM CaCl2 plus protease inhibitor cocktail (ROCHE cat. no. 05892953001), 0.055 nM PN200-100 and ( )-[5-methyl-3H]. The samples were then ltered through Whatman glass

microbre GF/B lters by Millipore manifold ltering apparatus, rinsed three times

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2496

with 5 ml of solution containing 200 mM choline chloride, 20 mM Tris-HCl pH 7.5, and the radioactivity retained on the lters was determined by liquid scintillation counting. [3H]-ryanodine binding was carried out by incubating total SR membranes for 1216 h at room temperature with 20 mM HEPES pH 7.4, 1 M NaCl, 5 mM AMP, 20 mM CaCl2, 0.0510 nM [3H]-ryanodine and protease inhibitor cocktail (ROCHE cat. no. 05892953001). Membrane bound [3H]-ryanodine was determined by scintillation counting as described above. Nonspecic binding was evaluated in the presence of 1 mM unlabelled nifedipine and 10 mM unlabelled ryanodine, respectively.Curve tting was performed using Graph Pad Prism 4 software package. Skeletal muscle performance and mechanical properties of EDL were analysed as described by Debono et al.27 Briey, animals were individually housed in cages equipped with a running wheel carrying a magnet. Wheel revolutions were registered by a reed sensor connected to an I-7053D Digital-Input module (Spectra AG, Egg, Switzerland), and the revolution counters were read by a standard laptop computer via an I-7520 RS-485toRS-232 interface converter (Spectra AG, Egg, Switzerland). Digitized signals were processed by the mouse running software developed at Santhera Pharmaceuticals, Liestal, Switzerland. To test force in vitro, EDL muscles were dissected and mounted into a muscle testing set-up (Heidelberg-Scientic Instruments, Heidelberg, Germany). Muscle force was digitized at 4 kHz using an AD Instruments converter and stimulated with 15 V pulses for 0.5 ms. EDL tetanus was recorded in response to 400 ms pulses at 10120 Hz. Specic force was normalized to the muscle cross-sectional area (CSA) wet weight (mg)/length (mm) 1.06

(density mg mm 3).

Cell electrophysiology recordings and optical recording. FDB bres from WT and DKO mice were enzymatically dissociated, plated and recorded following published procedures43,44. The composition of the pipette solution was (mM): 140 Cs-aspartate; 5 Mg-aspartate2, 10 Cs2EGTA (ethylene glycol-bis(a-aminoethyl ether)-N,N,N0N0-tetraacetic acid), 10 HEPES (N-[2-hydroxyethyl]piperazine-N0-[2-ethanesulfonic acid]), pH was adjusted to 7.4 with CsOH. The external solution contained (mM): 145 TEA (tetraethylammonium hydroxide)-Cl, 10 CaCl2, 10 HEPES and 0.001 tetrodotoxin. Solution pH was adjusted to 7.4 with TEA.OH. For charge movement recording, calcium current was blocked with the addition of0.5 Cd2 plus 0.3 La3 to the external solution43. Peak Ca2 currents were normalized to membrane capacitance and expressed as Amperes per Farad, whereas intramembrane charge movements were calculated as the integral ofthe current in response to depolarizing pulses and expressed per membrane capacitance as Coulombs per Farad. Fura-2 Mn2 quenching in intact FDB bres was carried out as previoulsy described19,28. Calcium transients were measured by using the low-afnity fuorescent calcium indicator MagFluo445,46. Briey, changes in the [Ca2 ]i induced by supramaximal eld stimulation were monitored in FDB bres loaded with Mag-Fluo-4/AM in Tyrodes buffer. All experiments were carried out at room temperature (2022 C) in the presence of 50 mM N-benzyl-ptoluenesulfonamide (BTS) (Tocris) to minimize movement artefacts. Measurements were carried out with a Nikon ECLIPSE TE2000-U inverted uorescent microscope equipped with a 20 magnication objective. Fluorescent

signals were capture by a photomultiplier connected to a Nikon Photometer P101 amplier. Calcium transients were analysed by ADinstrument Chart5 and Origin.6 programs. Changes in uorescence were calculated as DF/F (Fmax Frest)/

(Frest). Resting calcium was measured with Indo1 loaded FDB bres45.

Statistical analysis. We used GraphPad Prims 4.0 software package to perform curve tting and statistical analysis.

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Acknowledgements

This work was supported by funds from Swiss Muscle foundation, A.F.M., S.N.F and

Department of Biomedicine University Hospital Basel. This study was also supported by

Research Grant no. GGP08153 from the Italian Telethon ONLUS Foundation to F.P. and

grants from the NIH/NIA (AG13934 and AG15820) to O.D.

Author contributions

B.M. and L.B. developed and monitored mouse colony, collected and analysed calcium

measurements data; M.V. collected and analysed expression data; R.L. collected and

analysed force measurements data; M.T. and H.T. generated mice model; C.P. and F.P

provided CASQ1 KO mice and performed structural analysis; O.D., M.L.M., Z-.M.W.,

M.M. and G.R. collected and analysed electrophysiological data; S.T. collected and

analysed biochemical data; and F.Z. took care of conception and design of the experi

ments, collection and analysis of biochemical, histochemistry and physiological data, and

drafting of the manuscript.

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How to cite this article: Mosca, B. et al. Enhanced dihydropyridine receptor calcium

channel activity restores muscle strength in JP45/CASQ1 double knockout mice. Nat.

Commun. 4:1541 doi: 10.1038/ncomms2496 (2013).

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Abstract

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Muscle strength declines with age in part due to a decline of Ca²⁺ release from sarcoplasmic reticulum calcium stores. Skeletal muscle dihydropyridine receptors (Ca_v 1.1) initiate muscle contraction by activating ryanodine receptors in the sarcoplasmic reticulum. Ca_v 1.1 channel activity is enhanced by a retrograde stimulatory signal delivered by the ryanodine receptor. JP45 is a membrane protein interacting with Ca_v 1.1 and the sarcoplasmic reticulum Ca²⁺ storage protein calsequestrin (CASQ1). Here we show that JP45 and CASQ1 strengthen skeletal muscle contraction by modulating Ca_v 1.1 channel activity. Using muscle fibres from JP45 and CASQ1 double knockout mice, we demonstrate that Ca²⁺ transients evoked by tetanic stimulation are the result of massive Ca²⁺ influx due to enhanced Ca_v 1.1 channel activity, which restores muscle strength in JP45/CASQ1 double knockout mice. We envision that JP45 and CASQ1 may be candidate targets for the development of new therapeutic strategies against decay of skeletal muscle strength caused by a decrease in sarcoplasmic reticulum Ca²⁺ content.

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Title

Enhanced dihydropyridine receptor calcium channel activity restores muscle strength in JP45/CASQ1 double knockout mice

Author

Mosca, Barbara; Delbono, Osvaldo; Laura Messi, Maria; Bergamelli, Leda; Wang, Zhong-min; Vukcevic, Mirko; Lopez, Ruben; Treves, Susan; Nishi, Miyuki; Takeshima, Hiroshi; Paolini, Cecilia; Martini, Marta; Rispoli, Giorgio; Protasi, Feliciano; Zorzato, Francesco

Pages

1541

Publication year

2013

Publication date

Feb 2013

Publisher

Nature Publishing Group

e-ISSN

20411723

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1038/ncomms2496

ProQuest document ID

1313235433

Enhanced dihydropyridine receptor calcium channel activity restores muscle strength in JP45/CASQ1 double knockout mice

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