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Because it is based on general principles, the quantitative framework that we propose should help understand diseases other than those due to primary defects of the couplon. Specifically, diseases of the SERCA pump and muscular dystrophies that involve increased leak across the plasmalemma will share some of the aspects of the UPP, and in the neuromuscular ailment ALS, alterations in Ca2+ handling by mitochondria (which we recently described15) will bring to bear a process similar to the hyper-coupled pathogenetic pathway. 2. The first direct probe of CICR as disease mechanism. CICR, which results in sparks at individual couplons, does not work in normal EC coupling of mammals16,91 but is turned on in artificial conditions5 and diseases that alter the “top” of the couplon (DHPR and RyR, shown below). We will test directly the presence of CICR. Normal couplon operation results in Ca2+ transients of 6 to 15 µM in the triadic gap. We will apply “artificial Ca2+ sparks” reaching up to 8 µM17, thereby providing an effective test. The presence of CICR in cells that normally lack it will constitute a powerful driver of downstream changes and pathogenesis. Spontaneous sparks and waves are likely to occur, with consequent store depletion, promotion of SOCE, increase in [Ca2+]c and the ensuing cascade of HPP. CICR should help explain localized structural damage, characteristic in diseases with cores. 3. Solutions to mysteries of pathogenesis. SR diseases provide multiple conundrums. Gaps in description, including whether or not [Ca2+]c is changed and [Ca2+]SR is depleted, will be clarified for the mutants at hand. Deeper mysteries include why the “pure” MH phenotype (represented in our study by the R163C) is subclinical, until disrupted catastrophically upon exposure to triggers, while complex MH/CCD syndromes (e.g. the Y522S) include structure changes, detected as early as 2 months, later leading to cores that progress stereotypically18. A related problem is the spatially delimited, discrete nature of the lesions, which suggests a non-linear noxogenic mechanism. A third aspect of the same conundrum is the genesis of the CCD diseases associated with RyR loss of function. Avila19 explains the florid MH/CCD as a matter of degree, a stronger gain of function than in MH cases. This theory fails to explain why cores are prominent in EC uncoupling cases, like I4898T in humans20. Evidently, the genesis of structural lesions is not a linear process, whereby more gain of RyR function causes more damage. Three ingredients will interfere: one is Ca2+ mediated feedback; another is mitochondria (which contribute to early pathogenesis18); the 3rd is mechanics. Indeed, after unloading (e.g. by tenotomy), cores develop in most cells of the soleus within 7 days21,22. All 3 effects will make the generation of lesions local, spatially and temporally discrete. Our measurements will provide information on [Ca2+]c (a driver of pathogenesis, e.g. via proteases), [Ca2+]SR (which determines local release flux) and [Ca2+]m, (whose impact we have discussed18). The study will also provide spatially resolved images of [Ca2+]c transients, allowing us to identify local perturbations (like spontaneous sparks) and reveal susceptibility to CICR, which may arise upon local alterations of mechanical5 or metabolic origin5,15. 4. Establish roles of calsequestrin in health and disease. Casq, Jn and Tr constitute the deeper reaches of the couplon (fig 1, 4). Their roles were discussed in a symposium organized by S Györke and the PI 23-32. Mutations in Casq2 are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT) and sudden death, a linkage interpreted as consequence of either release of RyR2 from allosteric inhibition by Casq226 or loss of SR buffering29. No human diseases are yet linked to Casq1 mutations. But, Casq1 is decreased in thyroid autoimmunity with myopathy33; SNPs in non-coding regions of its gene are associated with type-2 diabetes34,35 and its expression is reduced in diabetic patients and rat models36.

A concept emerges of functional and pathogenetic symmetry between Casq1 and 2. Indeed, Casq1- and 1-2-null mice have MH phenotype27. This was explained by a finding of our lab; RyR turn off when [Ca2+]SR falls below 150 µM8. Thus, as described for Casq2 in the heart, Casq1 is a [Ca2+]SR-sensitive ‘valve’, which closes RyRs as [Ca2+]SR falls. Control works both ways: inhibition when [Ca2+]SR decreases; promotion, especially clear in the heart, when [Ca2+]SR increases. We call this luminal Ca-dependent action, LCDA. We will fully characterize LCDA by comparing Ca2+ movements in Wt and the Casq1-null model. We will separate buffering from gating effects of Casq, studying the Tr/Jn null, which contains Casq1 but lacks the physical connection necessary for gating effects on the RyR. In sum, our proposed research will clarify the roles of Casq1, understand MH caused outside the RyR and help decide among alternative pathogenetic schemes in CPVT. 5. Understand the genesis of muscle weakness in disease. A common sign of disease is muscle weakness, which often presents as easier fatiguability. A major cell-level cause of muscle fatigue is altered Ca2+ movement9. Muscle weakness is a differential sign of CCD, not shared with “pure” MH. We will quantify Ca2+ movements, in rested conditions and in the fatigue produced by trains of action potentials (AP). MH is subclinical, but is fatiguability normal? Is “weakness” in CCD of the Un-coupled sort (the UPP) synonymous with faster fatiguability? [Ca2+]SR should be higher in the UPP; therefore, if fatigue is due to SR depletion one expects its onset to be slower! One study of Casq-null mice found decreased40 fatiguability, while another found it increased41. No comparable study exists for CCD. Our study will answer all these questions.

6. Answer an intractable question of normal EC coupling. As drawn in figs 1, 3 and 4, half of the channels in a couplon engage V sensors with precise stoichiometry, we call them V. The others do not have overlaying DHPRs, we call them C. How (and whether) C channels activate is among the most persistent mysteries of EC coupling. We will tackle it using mutant RyRs as tools. With transfection techniques pioneered by Vergara64 and established for RyR by our lab42 we will combine mutant and native RyRs into heteromeric channels, and gather functional data in muscle expressing such combinations. This will be contrasted with numerical couplon simulations1,43 that assume specific mechanisms of activation. Our working hypothesis (H4) is that C channels are allosterically activated by the surrounding V channels (see figs 3, 4).

B. Innovation. In early work. With mentor Martin Schneider, the PI defined the stoichiometry of the first Ca2+ dyes (ApIII and Arsenazo), demonstrated for the first time the buffering action of Ca2+ dyes, developed the 2-Vaseline gap and, with Werner Melzer, the “removal” method to calculate Ca2+ flux. In earlier cycles of this grant, identified the DHPR as Vm sensor of EC coupling3; demonstrated interconversion of charge as the mechanism of Vm-dependent inactivation84; provided the 1st evidence of positive feedback between Ca2+ release and the Vm sensor44; introduced the 1st allosteric model of EC coupling, superseding the “toilet plunger”45; proposed dual control of Ca release and with N Shirokova provided evidence for it46,47; provided with L Blatter the 1st images of Ca2+ sparks in skeletal muscle48; introduced the 1st calculation of flux in sparks49 and with M Fill compared it to unitary RyR current to conclude that sparks are produced by multiple channels50; showed that sparks are non-physiologic in the mammal16 due to an inhibition of RyR by DHPR51. With Stern and Pizarro introduced the couplon concept and implemented it as a model1,43. Described “embers” and used them to evaluate unitary channel flux in vivo52. Invented SEER and applied it to measure [Ca2+]SR in the frog and myotubes53, providing what is until now the only image of the depletion events associated with skeletal sparks (we named them “skraps”54). SEER later found other applications57-59. During the present period. Introduced expression of exogenous RyRs in adult muscle42, developed hybrid SNAP-Indo sensors for various organelles including nuclei55; developed biosensor mt11-YC3.6 for mitochondria56, SEER of carboxy-SNARF for pH, applying it to basophils57 and neutrophils58; introduced imaging of SOCE by SEER of mag-indo59; created D4cpv-Casq for imaging [Ca2+]SR in mice60; provided the first quantification of variability of biosensor performance60; with L Blatter introduced imaging of Ca2+ events in 4 dimensions, to identify sparks that are in focus61, used J Zhou’s records of Ca2+ flux in lesioned areas of cells with ALS15 to provide the first quantification of the flux of Ca2+ removal by mitochondria in skeletal muscle56. Innovations for the next funding period are (I) technological and (II) conceptual: I) SEER of Di-8-ANEPPS for dynamic imaging of membrane voltage (fig 16); use of 2-photon “artificial Ca2+ sparks” as probes of CICR (fig 13); novel troponin-based biosensors for chronic monitoring of calcium stores in the whole animal (fig 9); engineered mice with muscle-specific expression of improved biosensors, for both cytosol and SR. Imaging in 4D to monitor individual couplons (fig 22); use of D4cpv-Casq as a “two-edge sword” that measures [Ca2+]SR and restores Casq function (fig 19); use of fura-2 in 2P-confocal Ca2+ imaging. II) Conceptual innovations include: use of the Cell Boundary Theorem63, CBT, to derive the features of pathogenetic pathways; quantification of fatigue-causing Ca2+ movements in disease; the idea of CICR as driver of pathogenesis; deriving composition of heteromeric RyRs from local fluorescence and using heteromeric RyRs to probe mechanism on altered couplons.

C. Approach. C.1. Progress Report and Preliminary Data. Hypotheses and specific aims of the present period. The general aim was to understand cell-wide Ca2+ release in terms of control at the single channel level. The Aims were to test 4 hypotheses: H1. CICR and CDI are the main interaction mechanisms among channels in situ. H1 was upheld, with the demonstration of both CICR17 as mechanism of activation and CDI of termination of Ca2+ sparks62. H2. RyR1 is activated by Vm, RyR3 by CICR, both are subjected to CDI. H2 was upheld; its validity conclusively shown by transient expression of RyR3 in mouse muscle42. The method64 allows for the acute expression of any protein. We showed how to combine it with V clamp, confocal imaging and regional recording of Ca flux. We found that (1) expression of RyR3 (or tagged RyR1) was segmental, with density decaying by 1 or 2 orders of magnitude away from transcribing nuclei. The regions with high expression of RyR3 responded to Vm pulses with sparks, and the rest of the cell conserved the non-event response characteristic of the mammal. The peak of release flux was differentially increased in the expressing area42. In all, the results affirm that sparks, which involve CICR, and the larger peak of flux in the frog, represent flux through RyR3 channels. The results mean that mammals, with their one-isoform RyR endowment, do not normally produce CICR. Further work upheld the role of CDI, both in frog muscle62, and in the mammal8. H3 Local control by Ca is conditioned by the local geometry and the presence of the DHPR. H3 was also upheld.

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6 shows mCa2+]SR (noteporaneous

we will use texpress it

ns are subjertinsried) “Ts discussed

The 3rd is neously or

in the 3 (steady and mito

nts of EC te cellular e of these ation and for these

ms 1 and 2 utant with . ethods: (i) dye, used nin-based on and in ct cells, an form). We e of TnXX leted with ill require

cibly than expresses es and the d lasers at 2P) mode, 1P 360).

g V clamp mulus, V the “top” gulators. evolution

erived for ult in the

wed by a onstrated refore be

on of mr YS.

ntially, in of Casq

measures e scales) with the

the biosensoconstitutivelect to endog

TnSR”, a bioin Griesbec

F

Fi

0

20

40

60

0.0

0.4

0.8

1.2

or in crossiny and homo

genous interosensor simik’s letter.

Fig 5. [Ca2+]c &

Fig 6. [Ca2+]SR i

ig 7. Permeabi

0

0

0

0

200

0 2000

4

8

2

[Caamou

nt, m

M

Pe

Bupow

W

A

B

300

ngs with miceozygously inrference8, weilar to TnXX

& mr in YS cells

n Wt & YS

lity & buffering

time, ms

400 60

III

a2+]SR, μM

ermeability(s‐1)

ufferwer, B

Wt

YS

400

e n e

X,

s.

.

s

00

PermoverP derise provCa2+

plotsexam(I) aconscellu

Tcorrewhic(slopinjecdestbounaberphenEC cto a mutacan imag

Tconssolid(greyrestiincre

Fdecathe ttheremin

Bmeagenecapais prshou

Wweakin thPITFincrewas with indestres[Ca2

Tis apstrikias w

meability, Pr driving forcecays from aand remain

vides the bes+buffering ps amount rmples. The and decreasstant (fig 177

ular level of tThe intriguinelate in B. Ach coincides pe is verticacted with Caabilization ond Ca2+. Arrantly high notype of thicoupling un0.5 s train

ant. The AP be used wit

ging” and figThe trains astitutively exd trace), at ty or pink). ng and teta

eases duringFig 10 A demay in [Ca2+]Stransient asse is further drest. Because B suring B in eral feature acity in MH aredicted unduld delay theWe will subjekness, a sige YS and IT

FALLS. An oease in [Ca2

observed inthe also imppendent messes the inh+]SR, which sThe observappropriate, sing whole-bo

well.

P. The analye, [Ca2+]SR.

an initial peans elevated st quantificatpower of threleased, Δ[(minus) slopes sharply 76). We havethe [Ca2+] deng propertieAt the time owith the “no

al; 7B). Wasq-silencingof the state oA similar de

P and reles mutant. nder physioof APs, at 6frequency isthout substa16, below.

are repeatedxpressing Tnt=3 min of sThe biosensnic [Ca2+]c.

g the experimmonstrates [R that recovsociated witdepletion an

decays withall 3 modelsof what we and MH+CCder the simpe decay in B ect all muta

gnature of CT than in the observation l2+]SR as Ca2

n repeated pplausible breeasurement herent strengserve as cheations with Yo is expectaody episode

ysis will con Fig 7 contin

ak (published(7A). P is

tion of the prhe SR, B, is[Ca]T,SR, vs pe is B. Typnear its ende preliminariependence oes observedof the “humpotch” in [Ca2

We reported g RNAi28 anof Casq, reserangement ease of the

olgical stim60 Hz, mons chosen beantial alterat

d every 2 snXX (rhod sitimulation (fsor respondThe measu

ment. Both mCa2+]SR in a ers with τ ~th the train nd reduction

h [Ca2+]SR, its we will testcall the HP

CD diseasesplest hypothe

and the onsnts to studie

CCD, reflects“pure MH” Rlike the notc2+ is being rpulses and, eak in the risusing the cygth in combecks of eachS cells clear

ation of extraes. I expect

ntinue with thnues the anad in 8). In thproportiona

rimary anoms calculated

[Ca2+]SR spically B is hd, as the SRily concludeof Casq bindd in the YSp” in flux (fig2+]SR (fig 6),

a similar snd suggesteulting in intrcould be pYS, contrib

muli. Fig 8 shitored by F

ecause direction of the A

s to elicit fagnal in blac

fatigued, a ts slowly, bu

ure by rhod monitors muWt cell unde1.5 s. By 1 retains its aof the trans

t will be lowt (our H1) w

PP, contribut. The uncoeses to havset of fatiguees as in figss a pattern oRC mouse. ch in fig 6 sereleased!). Bequally imp

se of [Ca2+]c,ytosolic monbining measu other . rly need conaordinary evt the cellular

he calculatioalysis of thee YS P is h

al to channemaly in the 3d as d[Ca]T,S

imultaneoushigh at the bR depletes (d that this [Cing in vitro93

S have a ng 5) and P (fB becomes singularity i

ed that it refra-SR releasprecipitated buting to the

hows the reof rhod-1.

ct imaging ofAP time cou

atigue. Figck, TnXX ratransient red

ut provides rdoes not re

ust be used fer 0.5 s trainmin of thes

amplitude. Asient. [Ca2+]S

w in fatigue whether decr

ing to the loupled IT mo

ve higher [Cae. 9 & 10, askof fatigue on

eems implauBut it is reaportantly, is s derived from

nitor. This arures of both

nfirmation. Wvents. MH “s

correlate to

on of P —rae examples. igher, has ael popen. Themutants of A

SR /d[Ca2+]SR

sly measurebeginning of (II). In the Ca2+] depen3. notable fig 7A), infinite n mice flects a se of its by the e florid

sponse The respon

f Vm demonsurse. See s

g 9 shows 2tio in red). duced by 30reliable steaeturn to initiafor a quantitns every 2 se stimuli the

At 3 min, whSR recovers

as well. Byreased B is aow functionaodel, insteada2+]SR, which

king whethenset differen

sible (it is anl, because synchronousm an entirelyrgument alsoh [Ca2+]c and

While cautionseizures” are

o be dramatic

Fig

[Ca2+

] c, μ M

, fro

m rh

od

atio of flux In the Wt

a large 2nd erefore, P Aim 1. R Fig 7B ed in the the pulse Casq-null, B

ndence of B

nse is highestrated that 6section titled

2 measures Shown ar

0-40%) and aady values, ral values, beatively corre

s. The first te accumulatehen fatigue iand force re

y a al d, h

er nt

n it s y o d

n e c

9. TnXX improv

Fig 10. A) s trains of s

0 500

0.2

0.3

0.4

B is much lois the expre

r and more 60 Hz is the

d “A Pitfall s

s of [Ca2+]cre the first tafter 10 minreporting fulecause its cect descriptioransient proed depletions establisheebounds slig

Fig 8. Activatio

ves measuremen

[Ca2+]SR and Bstimuli, applied

0 200

2

4

6F/F0

tim

Y

W

1000

rhrhrhTTT

ower, and isession at the

fused in thee highest thasolved by Vm

in a mouserain (resting

n of recoveryl recovery o

concentrationon. oduces ~30%n is 10%, bued (panel B)ghtly after 10

on by AP trains

nt of [Ca2+]c.

B) force upon 0d every 2 s.

400

me, ms

Y522S

Wild type

time, ms1500 2000

0.

0.

0.

0.

0.

1.hod, restinghod, fatiguedhod, recoverednXX, restingnXX, fatiguednXX, recovered

s e

e at m

e g, y

of n

% ut ), 0

s.

0.5

60

[Ca2+

] c, μ M

, fro

m T

nXX

.0

.2

.4

.6

.8

.0

INNOVATION. There is no precedent for measures of [Ca2+]SR in disease. The sole precedent in fatigue has the important virtue of using a living mouse9, but is marred by movement and sensitivity issues. Innovation extends to the biosensor mice and the “designer” TnSR version of TnXX, entirely new approaches to measuring [Ca2+]SR . P and B (in Wt and disease; at rest, in fatigue and in depletion) will be provided for the first time. Previously “permeability” was calculated, by us73 and others, as the ratio of flux over [Ca]T,SR, which assumed that [Ca2+]SR and [Ca]T,SR are proportional. This is wrong, as it requires constancy of B. Entry flux mi and SOCE. To complete the quantitative picture we will evaluate two Ca2+ fluxes, small, but significant if altered over long periods. mi will be measured in two ways: by SEER of mag-indo trapped in the t system, a technique developed in the present funding period59, and by release flux analyis of Ca2+ transients elicited by sudden changes in [Ca2+]e, a technique illustrated in fig 11 (with Prof. P Bolaños, IVIC, Caracas). FDB cells were in a flow chamber, adherent or held by blunt pipettes. [Ca2+]c transients were measured with fura-2 upon depletion by Thaps or high K exposure in 0 [Ca2+]e. Note entry roughly proportional to [Ca2+]e, fully blocked by 2-APB and insensitive to DHP. Peak influx, calculated73 from the Ca2+ transient of cell A in 2 mM [Ca2+]e, is 2-20 µM/s or 0.1-1 % of the sustained Vm-operated SR release flux. This estimate, which depends on the assumed rate of removal into organelles and buffers, is roughly consistent with that obtained by ratio imaging of mag-indo in t tubules59. Mitochondrial Ca2+ transients. We will measure [Ca2+]m and mm (fig 2) globally and locally. These measures will be critical to look for a mitochondrial role as we test H3. The novel mt11-YC3.6 sensor will be used56. Hypothesis Testing. H1. The goal of the measurements is to learn about pathogenesis. The study is designed as a test of three hypotheses. H1 proposes that a common set of mechanisms leading to altered properties (a pathogenetic pathway) will be unleashed by gain-of-function alterations of the couplon, regardless of where they take place. These cover the RC and YS models of Aim 1, as well as the Casq-lacking model of Aim 2. Logically, loss-of-function defects —the IT mutation, and diseases that course with loss of RyRs— should exhibit essentially opposite features in their pathway. Main features of these pathways (fig 12) are derived with the Cell Boundary Theorem63, CBT, as described immediately for the HPP: the primary defect is greater P (due to higher popen) at rest or under stimuli. Other functions being constant, the CBT and mass action dictate that steady [Ca2+]SR, [Ca]T,SR, releasable Ca2+, Δ[Ca]T,SR, and B will decrease. If SOCE is graded with [Ca2+]SR

78, it must increase and so will net mi. A 2nd application of the CBT then predicts increased [Ca2+]c, [Ca2+]m, ms, and saturation of cytosolic buffers (fig 2). The features of UPP are derived nearly symmetrically.

Some of these predictions have already found confirmation in the literature. For instance, increases in [Ca2+]c are reported in many MH mutations; the CBT requires that they be accompanied by increase in net mi—SOCE or other influx must be potentiated, and/or PMCA or other efflux inhibited. Indeed, an increase in SOCE has been confirmed in various instances of MH and MH-like changes67,68. Note also that data in YS mice (figs 5-8) are substantially consistent with the predictions of H1. PITFALLS; ALTERNATIVE HYPOTHESES. Measuring [Ca2+]SR requires electroporation. MH mice are susceptible to “seizing” during anesthesia. In preliminary work, however, Y522S mice did not present this problem. Serendipitously, YS mice are also very sensitive to body temperature79, which drops with anesthesia, so that they tolerate the treatment when it is performed at 22° and nothing is done to keep them warm.

Biosensors for [Ca2+]SR and [Ca2+]m are introduced by transfection; cytosolic dyes by incubation with the AM form. Both treatments cause injury. To avoid the acute introduction of sensors we will use the TnXX mouse, already available, for imaging [Ca2+]c, the D4cpv-ΔAsp mouse, under development, for imaging [Ca2+]SR and we will engineer a mouse for constitutive expression in muscle of TnSR, when that sensor becomes available. Biosensor +/+ mice will be crossed with the mutants and the (also homozygous) Casq-nulls, to obtain mutant mice that express the biosensor. While developing these doubly-engineered mice we will continue using acute transfection of sensors and AM dyes.

Fig 11. [Ca2+]c by fura-2. FDB cell exposed to various [Ca2+]e plus high K and channel blockers.

Fig 12. Main features of the HPP and the UPP

time, s400 1200 1600 2000 2400 2800

SKF

80

µM

SKF

80

µM 2-AP

B 80

µM

K

TG 10 µM

K

Thaps

[Ca2+]e, mM 55 20

highK

2-AP

B

SKF

CELL A

1000 1200 1400 1600 1800 2000

Nif 20 µMNIFEDIPINE

55 00

CELL B

0.0

0.1

0.2

0.3

0.4

0.5

0.6

[Ca2+

] c, μ

M

mito

mi, SOCE

mr

mu [Ca2+]c

[Ca2+]m

mmbuffersCasq

[Ca2+]SR[Ca]T,SR HPP

P

B

mito

mi, SOCE

mr

mu [Ca2+]c

[Ca2+]m

mmbuffersCasq

[Ca2+]SR[Ca]T,SRUPP

B

P

Wthe Not SOCwhetsmais nolargehumconschanAim micesenstentaindudetamemscanthe cthe cstartthe amarkredumouconcmouthe fcells

Bwe ftriggB). Pbe a

Wwill c

TconsmonCICRdone

Cfull monMUTmuta

Ithe din thIn figcagedye)the cirregHam

What if somunderlying the CBT—it

CE, or B, arther or not ll changes inonlinear, noe). Other ps and notc

sidering highnges in the c

1.2. Test He42,92. MH sitivity to Caatively reporce CICR weils in 17,77). T

mbrane with nner acquirecell, so that cell boundats in the framarrow, just aked in the cucing the amse cells do

centrations ose, loweringfrog, but do

s17,77. But, just as wfound CICR ers with pro

Preliminarily ctivated by a

We will probconclude thaThe initial fisistent with dths, a stageR’s incidence for the otheCICR shouldpanoply of itored.

TATION-SPEations, differf, as proposdegree of Rye YS. We fo

g 15A a meme and Ca2+ d are found cell diffuses

gular plasmamilton has fo

me predictionmechanistict is a theorere sensitive

removal byn [Ca2+]c, (out recruited uunexpected

ches in the ehly nonlineacalsequestrinH2: CICR bemutant chaa79. Muscle rted in aginge apply artifiThe fiber is solution co

es images wno light or hry (marked

me marked bat the start color table. mplitude of o not respof caffeine g CT to ~0.5oes not con

we were reain the YS m

opagated wawe estimate

action potenbe all mutantat CICR contnding of CIdisease stage of early cce as driver er models, ad hasten stoHPP. Loc

ECIFIC STUrential aspecsed19, the pryR gain of fuound intriguimbrane permdye. Areas near nuclei.in, showing

alemmal peround indicati

ns of H1 arc assumptioem—but, forto a certainy mitochondur measuremunless incre

observatioevolution of ar processen network orecomes actinnels are hof an ALS

g mice80. Heicial sparks in a dual containing fluohile the otheeavy photo-by a white

by an arrow.of CICR. TThreshold [the stimulus

pond to trigand 4-cmc

5 µM. We contribute to n

aching this nmutant (fig 1aves (A) ane their CT at ntials. ts in this watributes to EICR was in ges describecores, and a

of pathogeapproximatelre depletion

cal damage

UDIES. Whcts will be exrimary differeunction, a ming local altemeabilized Y

of high fluo B, C) a la

g that these meabilizatioions of ER s

e disprovedns will be rr instance, wn range of [dria is sensments53 showeases in [Cans (includinvariables) ws—like coor a CICR-fueive upon gahypersensitivS model feance we expas in Fig 13nfocal scann

o and NDBFer sends IR products hit line). A pro

. Trigger [Cahe value, 0Ca2+] (CT), s, is ~0.3 µggers up to

allow CICRonclude thatormal EC c

negative con14). YS respnd localized

~2 µM; this

ay and if CICC coupling ia 6-wk old

ed for the YSat 6 monthsnesis will inly age-match, promoting of sarcome

hile the samxplored. ence betwee

more severeerations in thYS cell is immorescence (rarge artificial

areas of hign but to an stress in MH

d? Then revised. whether Ca2+]SR; sitive to w that it a2+]c are ng dips, will force perative tra

eled “explosiain-of-functiove to halothatures CICRect an incre3, which illusner (Zeiss LF-EGTA90, alight in 2P rthe cell. Th

opagated resa2+] is measu.7 µM in figwhich is fou

µM in the froo 8 µM17,77

R operationCICR oper

coupling in

nclusion in tponded to asparks (arris greater th

CR is found,n response d YS mouseS18, at 2 mons, when unscrease withhed comparSOCE, whic

eric structur

me battery

en MH/CCDphenotype ihe first cellsmersed in sorevealing upl spark applgh [dye] areorganellar c

H. Capture

Figansitions in

on”. Aim 3 on mutationshane and c

R in certaineased operatstrates a typ

LIVE-Duo) eqa Mg2+-insenregimen, to ge artificial spsponse ured at

g 13, is und by og. Wt 7. Low in the

rates in mouse

the Wt, artificial rows in han in the fro

, we will detto action poe. We will nths, when estructured ch age. Althorisons will bech will increare and orga

of measure

D and MH is is expected

s examined. olution with

ptake of the lied outside

e not due to change. S.of a water-

g 13. IR flash cau

Fig

the array owill address

s. CICR is caffeine, an

conditions1

tion of CICRpical responquilibrated th

nsitive cage generate anpark is visibl

og, but withi

termine CT. otentials. repeat the early lesionsores are deugh a simila

e made with ase steady anelles sho

ements will

uses artificial sp

14. Y522S. A,

Fig 15. AreasC is at comp

of RyRs, pos these possnot operativindication o

5, and CICR in the MH nse in the frohrough its peof high effic

n “artificial sple in the top

in a range w

If CT is low

measuremes are detecteetected. Wear staging hall three. [Ca2+]c and uld ensue,

be applied

ark, then CICR.

waves; B, osc

s of high [dye] pressed fluores

olymerizationsibilities. ve in normaof increasedR has beenmodels. To

og (technicaermeabilizedciency. Onepark” outsiderow, outside

where it could

w enough, we

ents at agesed, then at 4

e expect thahas not been

[Ca2+]m —theand will be

to all three

[Ca] scale, µM

cillatory sparks.

in Y522S cell. cence scale.

n

al d n o al d e e e

d

e

s 4 at n

e e

e

solubtransbetwand to remotransilizatwith

BprimIf thedoesAim spontest predpreliuse showA PITcausVm inboth16B provexcitand insidthan

Aor secausrelevfunctresufunct2.1.1[Ca2

manthe f[Ca2

releashowiv, v,

Bpossexcitand predrepre[Ca2

PITFwher

ble dye sitional re

ween roughSR may be

orgaodeling sient permtion assocER stress.

Because ary defect ise predictionss not decrea

1.3. Test Hntaneous Cawe will com

dicts spontanminary confour 5-LIVE s

w how repeaTFALL, SOLse of functionn plasmalem spectra shishows fluor

vided by J Vted by Ex2) stability. I

de the cell (n 60 Hz.

AAltering the eparating it ses MH-like vant to undetions of thelting in gaintion due to c1. The Cas+]SR is low8, agement in following res+]SR decays asable Ca2+ wn in fig 17 a, vi and vii aBased on tsibilities for tatory at hig(3) a grade

dicted in fig esenting the+]SR lowered

FALLS. (Vareas Paolini

in egions h ER e due anellar

and meab-ciated

its s low P the s in fig 12 arase in spite oH311 (local ca2+ release kbine imagingneous Ca2+ firmation of slit scanner,

ated activity fLVED BY Vmnal loss priomma and t tft53. Fig 16 srescence EmVergara) anby (Em1 excf driven at not shown).

Aim 2. Modecouplon (figfrom the codiseases26.

erstanding pe proteins an-of-function components q1-null. Aa form of luWt vs Casq

sults: (i) in thmore rapidlyis reduced

and preliminre all justifiehe inhibitorycontrol by C

gh [Ca2+], (2ed effect, m

18 by the e “naked” Ryd and restingriability). Ou84 reported n

Fig 1range

IT mouse ise not upheld

of a less conchanges in Akills mitochog of Ca2+ trarelease, repthis hypothe which operfrom the sam

m IMAGING. or to the Ca2+

tubules by Sshows measm1 excited bd Em2 excite

cited by Ex1)higher freq Therefore

els with alteg 1) at the inouplon by d We will qu

pathogenesiat the “botto

unleash theof the coup

As stated beuminal Ca-deq1-null micehe Casq-nully and complby 20%82 an

narily commud by the lossy effects of

Casq at high2) a threshoildly excitatodifference

yR. Thus, ing [Ca2+]c incrur first studno changes

6. A, Both spee Em1 excited by

s expected tod we will testductive RyRAP-activated

ondria locallyansients andpeatedly at tesis. For derates for longme couplon wIn studies u

+-linked evenSEER of Di-sures in a cey lights Ex1ed by Ex2 (), providing quency APs we use stim

erations of cntra-SR levedeleting Tr/Juantify alters in both m

om” of the ce HPP pathwlon other tha

efore, we foependent ac. This entai, release fluxetely8 and (ind (v) a fatigunicated in76

s of buffer; if Casq1 at h [Ca2+]SR (bld effect, neory at rest. between then case 3, theased.

dy8 found nin [Ca2+]c in

ctra of Di-8-ANEy Ex1 (blue) or E

o enter the t alternative

R, or that SOd mr cause y, thus explad mitochondthe same id

etecting spong periods of will be ident

using trains onts of interes-8-ANEPPSell upon a 6(blue, specred). C, Sa signal of deteriorate

mulation freq

calsequestrel (changingJn) causes Mred function

muscle and hcouplon. Agway. This aan the RyR.ound that Cction, LCDA.ils all measux lacks the qiii) P does nogued pattern6, B is severi and iii evin

low [Ca2+]blue in fig 18eutral at res Effects of e blue curve null would

no significan the Casq1-

EPPS shift uponEm2 by Ex2 (red

uncoupled passumption

OCE does noshear force

aining the earial Δψ at suentified countaneous evtime withou

tified. of APs at phst here. To c. SEER enh0 Hz train oified in A; sEER dividesoptimal sen

e, especiallyquency not g

rin. or deleting

MH-like funcin the Casq

heart. It wgain the woim tests wh

asq mediate. We will chaures in the Tquasi-steadyot fall upon ln installs morely reducedce a gating SR we envi8): (1) a 2-w

sting and higdeleting Ca

ves and curvd have leak

nt change i-2-null, both

n depolarizationd). C, SEER rati

pathogeneticns. For examot respond aes, leading tarly lesions ub-sarcomeruplon or couvents, whichut affecting th

hysiologic ratcontrol for Ahances sensof APs. spectra s (Em2

nsitivity y deep greater

Casq, ctional alterq- and Tr/Jnill also help

orking hypotether this is

es inhibitionaracterize LTable. Somey phase thatlong-lasting ore quickly i and (vii) lacrole of Casqsion three way effect, gh [Ca2+]SR asq can be rve 4 (red) increased,

in [Ca2+]SR consistent

n (J Vergara). Bo (red/blue); ave

Fig 1

0.0

0.4

0.8

1.2

c pathway (Umple, that leabove normato lesions indescribed in

ric resolutionplons. Fig 1

h can be scahe cell61. W

tes, AP deteAP integrity wsitivity of mo

rations27. In n-null in det

p understandthesis is H1s also true u

n of Ca2+ reCDA by come are alreadt follows initidepolarizatiin pulse traicks Ca2+-depq, lost in the

B, fluorescence erage AP in inse

17. B is reduce

Fig 18. Mode

0 200 400

[Ca2+]SR, μM

amou

nt, m

M

Wt

Csq‐null

UPP, fig 12)ak (P at rest

al [Ca2+]SR. n CCD; locan18). For then, as in15. H314B providesarce, we wi

With fig 22 we

erioration is awe will imageonitors when

the heart, itail. This isd the contro1: alterationsupon gain-of

elease whenmparing Ca2

dy done, withal peak82, (iion8; (iv) totans83. (vi) Aspendence. inull.

during AP trainet.

ed in the null.

ls of LCDA.

600 800

). t)

al e 3 s ll e

a e n

it s

ol s f-

n + h )

al s i,

. In

with P Alhighsens

Bon thwill cundePITFprovrequhandeffec2.1.2gatineffecunpushouof thThe immedime3, wbe mand at hi2.2. by exhotlywill bby C

Qµmoreleared).INNOminimwill medge

(Devbasiccoupadvain ad100-abouThis The by aof Rtetrameamodcalcuchan

curves 1 or len instead fer [Ca2+]SR in

sor that is unBy analogy whe Casq-nulcompare theer patch clamFALLS. Thevides a view uires permead in (i) the Sct of increase2. The Tr/Jnng effects octs i, and iv ublished); thuld not be chhis model (C

hypothesis ediate vicinensionality”, ill make mo

made withoutherefore lagh Vm, with The relatioxtension CP

y debated qube attributab

Casq, unchanQuantified bol/liter of fibease flux upo. Similar expOVATION. Smize the bufmaximize bued sword”81.

Aim 3. veloped withc EC couplplons in a cances of the dult mouse -fold along thut this techn

implies thatassortment n anonymou

RyR tetrameamers, whicsurements el”1 adaptedulations parnges in Ca2+

2 (which prfound increan the null76. nder developwith cardiacll couplon (c

e response omp, and (ii) aese two expe

of excitabilitabilization an

SR is loaded ed [Ca2+]SR cn null. Willf Casq1 arethrough vii (

herefore the hanged unle

C Perez); thisproposes th

nity of RyR a physico-cre specific qt the elaborack the putatconserved [Cnship betw

PVT in Casquestions26,29.ble to loss of nged in the nby D4cpv fluer (fig 19, topon high exprperiments wSince its inceffering proviuffering. To

Heteromerih MD Stern).ing mechancontrolled mlast period.

muscle and he fiber (shoique: the det the mutantis independ

us Editor of rs, mixing W

ch will be kof Ca2+ mad to the gertial substitut

flux, similar

edict no effeased [Ca2+]c Measuring

pment will rec cells26 we ecurve #4 in of Wt and Caartificial Ca2+

eriments havty at conditiond explores via increase

can only be l be subjecte mediated (defined in 2

buffering css [Ca2+]SR cs will serve hat the one-

channels, chemical proquantitative ate numericstive functionaCa2+]SR and

ween MH an2 mutations. We will prf gating contnull, will be iuorescence,p). This Casression in C

will test the reeption, we kded by the Chighlight its

c channels. Here we “nisms we wmanner. This

(1) We42 shfound that

own in fig 20ensity of totat RyR protom

dent, i.e. prothe Lefebvr

Wt and mutknown on aanagement ometry andtion of mutar to those ob

ect of the de(unpublishe[Ca2+]SR is d

educe variabexpect an exfig 18). As t

asq-nulls at e+ sparks in pve different vons closer toCa2+-sensit

ed Ca2+ in thisolated withted to the saby Tr and/o

2.1.1), but nocapacity provchanges. Cto test the idimensionaleffectively

ocess with knpredictions s. Indeed, Cality in this n[Ca]T,SR, willd calseque) results from

rovide an anrol of RyRs, dentified as , exogenousq retains its

Casq-null celelationship bknew that theCasq moiety

dual purspo

and heteroturn the tabill use the ms approach howed how density d of

0). (2) Thenal RyR1 doesmers assortportional to e paper69. Uant protomeaverage, ca(Table) and properties

ated protombserved with

eletion in thed) and we c

difficult. Thebility. xcitatory effetest of this helevated [Capermeabilizevirtues and do physiologictivity—an unhe pipette orh (ii) (as [Caame set of or Jn. Accoot ii or iii. Cvided by Caasq, howevemportance ol polymers odeliver Ca2

nown formalbased on thCa2+ wires wnull. Thus, el be taken asstrin. Whem inadequatswer. If MH-normally exthe controlle

s [Casq] in s buffer proplls, including

between locae novel tool

y, we built D4ose, we call

ogeneous coles”. To tesmutant RyRis made poto express ef expressionn Lefebvre es not change with Wt in d. This is sUsing theseers in a conalculate the d (e) compa

of mouse mmers—assum

the YS (figs

e rested statecurrently finde Tn-based S

ect of [Ca2+]hypothesis wa2+]SR to (i) Ved cells. drawbacks: c, whereas

nphysiologic r external so

a2+]SR stays emeasureme

ording to theCasq1 is redasq will be er, is less coof Casq locaof Casq (cal+ by “diffusl properties hese propertwill lack theirequality of ts evidence ather the MHte buffering -like gain of

xerted by Caer. Such co cells expre

perties81 becg a quasi-steal [Casq] and

D4cpv-Cas4cpv-ΔAsp, led this bios

ouplons st hypotheseRs to disrupossible by texogenous Rn varies by 1et al69 demoe in spite of tetramers, w

shown belowe advances wntrolled way

compositioare with demuscle. Th

med to alter s 5-7).

e). d a SR

]SR we Vm,

(i) (ii) means of eolution, whicelevated afteents, to test e hypothesiuced by ~20largely consoncentrated ation and thlled “Ca2+ wsion enhanc(references ties. But qur nucleation he initial (peagainst the v

H susceptibilor loss of ga

f function is fasq. If no gaontrol shouldessing D4cpause it restoeady stage d susceptibi

sq would hava deletion v

sensor fused

es on pt the three RyRs 10 to nstrated wit

f major chanwithout chanw by developwe will (a) cy, (b) from ton of coupletailed simuhe approachallosteric e

Fig 19. Hig

Fig 20.

excitation. Och also raiseer [Ca2+]c is the hypothe

s we expec0% in this nserved andin the termine vectorial hires”86), whic

cement by in 28). Mod

ualitative prepoint at the

eak) value ovectorial hypity of the Ca

ating effects found in theain is found, d be graded wpv-Casq maores the typiafter the pelity of channve dual applvariant. Hered to native C

h mutant Rynges in the enging their nping formalischange the lothe local colons, (d) prlations by th is promis

energies—ca

gh [D4cpv-Casq

. Variable expres

On the othee [Ca2+]c. Thereduced53).esis that the

ct to still findull (C Pereztotal SR Canal cisternaehypothesis86

ch reach thereduction o

deling, in Aimedictions can

RyR mouthf flux elicited

pothesis. asq-null (andon RyR, are

e Tr/Jn-null, iCa bufferingwith [Casq]. ay reach 20ical shape oeak82 (fig 19nels to open.lications. Toe instead weCasq a “two

yR a key facexogenous dnumbers. (3sm proposedocal makeup

omposition oroduce locathe “couploning: in initia

auses abrup

q] restores flux81.

ssion of RyRs.

r e e d z, a e 6. e of m n

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o-

ct d. ) d p

of al n al pt

3.1. muta3.1.2obtacondrepre

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calculateed vs d in

y good fit witsistent with ortment. It the maximu0.4 of [Wt].

ater d we roach 3.1.2. ant plasmidcle will extenUsing Eq 1

local denchiometry is ed vs d in 2 (1) subject muscle or Iplete set of also apply afor the preseeating stimuhe responseer-responsivods of time aages of a caLocal and spAllosteric ae of alloster

Wt cells exprin mid-pote

meation, not allosteric faod-Wyman-

raction amonace in 23Bulates the das 30% smalls because tach other” vecouplon m

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ouplons. with GFP (a

arlier work, west d. Ac

gCa=1.0); 2:2cal [mutant] =[(1-d)4+4d×ed thus

fig 21A. Thth data in 69

independealso show

um d reache To generatdevised th Transfectin into mutand the data we can kno

nsity of te(namely, th1B). HavingFDB cells oT-transfectemeasureme

artificial sparence of CIC

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actor 1/f, wChangeux94

ng the channB, generateata roughly, ler for the IThe mutant pery well.

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ent, Qm, kineot operate icouplon, figplon model,

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Fig 21. A, i

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ct adult WtData in69 sugT mice with tetramers wgCa=0.12 whrtment is inde×0.12 + [(1-

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panel) a square grid of alternating channels. Then, at bottom, we gave each channel the 23 degree rotation of the actual couplon89. A notable topological consequence ensued: C channels moved away from and lost all contact with classmates; the V class also separated, but all inter-class contacts remained. The consequences for information transfer seem obvious. The DHPR “masters” now have directly controlled “minions” (V) that cannot conspire among themselves but exert tight, 3-way control on C-class “slaves”. This hyerarchy will apply to allosteric transmission but not to CICR. Besides, CICR is largely discarded for the mammal42. In fact, the allosteric hyerarchy helps understand why there is no CICR in the Wt, even though the channels are individually activatable by Ca2+. (A 3rd alternative, that the C class is idle, is almost unthinkable.)

The numerics that we developed to explore allosteric interactions of RyR2 in cardiac EC coupling43 will be adapted to the mammalian skeletal case. All the action will be allosteric, except for putative Ca2+ binding sites mediating Ca2+ inactivation or—in the MH mutants— CICR. In the Wt all RyR1 have the same gating scheme, as do all DHPR. The transition rates among RyR1 and DHPR states are modulated by allosteric interaction energies, in thermodynamic detailed balance. Each C RyR has 3 all-V nearest neighbors, while each V RyR has, as nearest neighbors, 3 C RyR and a tetrad of DHPRs. In its fullest generality, the entire couplon could be thought of as one giant molecule with innumerable conformations, but, as in the cardiac model43, we will assume that the interaction consists of additive, pairwise, allosteric interaction energies εj at the points of contact, which depend on the states of the two contacting channels. By thermodynamics, the ratio of forward and backward transitions between two states of a channel is multiplied by ( )exp /j kTεΔ∑ of the change in εj between the two states, given the current states of its neighbors. In this way, the state of any channel is communicated to the gating of its neighbors, providing a path for activation of the uncoupled RyR. In the simple example in section 3.2, 1/f is the exponential of the allosteric energy between “activated” neighbor protomers in a tetramer.

Based on independent assortment, one can calculate the average makeup of neighbors of each RyR, which will depend on d. Thus in fig 21C the curve in blue is the fraction of heteromeric IT channels in a transfected Wt for which neighbors have full flux (stoichiometry 4:0 or 3:1), whereas in black is the fraction with no flux. Slight changes apply when an IT +/wt mouse is transfected. When the YS mutant plasmid is expressed, the gating parameters, rather than the current, will be changed in the model as a function of d.

Flux, computed for the specific neighborhood, will be confronted with flux measured locally, under a wide variety of protocols. The variety will be indeed large. In addition to changes in couplon composition, we will have the whole repertoire of challenges, including artificial sparks, conditioning pulses, variations in starter SR load; changes in Casq connection or its deletion, and many others.

In the original couplon model, the SR lumen was treated as a single global compartment. This may still be adequate, since intra-SR gradients during EC coupling are very small54. However, the current version can treat JSR pools individually at each couplon, refilling from a common FSR. This may be useful for modeling release sites imaged individually (as in fig 22). As in the previous models, Ca2+ diffusion and buffering within the junctional cleft will be treated dynamically, by reaction-diffusion partial differential equations coarsely discretized on a 10 nm grid. Simulation of cell-wide responses requires thousands of stochastic runs. To reach acceptable runtimes the model has now been parallelized on a large MPI cluster. PITFALLS. There is no sure single test for allosteric activation. However, there are only a limited number of possible mechanisms by which the numeric model will reproduce sudden dips in flux and permeability, as those already seen in the YS (figs 5-7). One is CICR, but Ca2+-mediated effects are abolished with enough BAPTA, a technique that we know (e.g. 8) and will continue to use. Breaks can also be due to sudden changes in B predicted to occur upon aggregation changes in Casq, but this mechanism can be multiply disabled in the experimental context of this project. Most importantly, allosteric coupling should feature telltale nonlinearities and discontinuities in its dependence on the local concentration on altered protomers and/or tetramers. The ability to change these concentrations is, by design, the most characteristic feature of the present approach. A restatement of purpose. Every renewal seems justified to its proponent. In this case, however, we see a truly novel path ahead. Thanks to advances in the last funding periods we think we understand the basics of EC coupling, and know how to measure Ca2+ and its movements in the compartments that matter. We want to use this knowledge to understand diseases where altered Ca2+ movements are the culprit. In every cycle of this grant we held the prospect of applications (to fatigue, disease and aging) as justification for basic research. Now the promise can be realized. Because we can also use mutant channels to modify the couplon in a deliberate manner, we can address stubborn problems of basic EC coupling. We thus believe to have reached a balance between translational aspects and the questions on basic function that always guided our work.