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doi: 10.1152/physiol.00008.201025:250-259, 2010. ;Physiology

Yemima Berman and Kathryn N. NorthMuscle Metabolism

-Actinin-3 inA Gene for Speed: The Emerging Role of

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A Gene for Speed: The Emerging Role of-Actinin-3 in Muscle Metabolism

A common polymorphism (R577X) in the ACTN3 gene results in complete defi-

ciency of -actinin-3 protein in 16% of humans worldwide. The presence of

-actinin-3 protein is associated with improved sprint/power performance in

athletes and the general population. Despite this, there is evidence that the nullgenotype XX has been acted on by recent positive selection, likely due to its

emerging role in the regulation of muscle metabolism. -Actinin-3 deficiency

reduces the activity of glycogen phosphorylase and results in a fundamental

shift toward more oxidative pathways of energy utilization.

Yemima Berman1,2,3 andKathryn N. North1,

1Institute for Neuroscience and Muscle Research, The ChildrenHospital at Westmead, Westmead; 2Faculty of Medicine

University of Sydney, Sydney; and 3Department of ClinicGenetics, Royal North Shore Hospital, St. Leonards, Austral

[email protected]

Deficiency of the fast-fiber skeletal muscle protein

-actinin-3 is common in the general population

due to a polymorphic-null allele in the ACTN3

gene. Numerous independent studies have estab-

lished that the absence of-actinin-3 is detrimen-

tal to sprint and power performance in athletesand in the general population (1, 25, 55, 63, 66).

The sarcomeric -actinins have long been consid-

ered to be primarily structural proteins. However,

recent data suggest that -actinin-3 plays a signif-

icant role in the regulation of muscle metabolism.

-Actinin-3 deficiency results in a shift in the char-

acteristics of fast glycolytic muscle fibers toward

those of slow muscle fibers with oxidative metab-

olism (48, 49, 62). This review examines the emerg-

ing role of -actinin-3 in regulation of skeletal

muscle metabolism.

The -Actinin Family of Proteins

The -actinins are a family of actin-binding proteins

that have been identified in a diverse range of organ-

isms, suggesting an ancient origin (3, 8, 33, 50). The

-actinin protein structure is comprised of three do-

mains; an NH2-terminal actin-binding domain, a

central rod domain containing four internal repeated

122-amino acid motifs, and a COOH-terminal region

containing two EF-hand calcium binding motifs. The

four repetitive motifs found in -actinin share ho-

mology with spectrin, suggesting a common evolu-

tionary origin of the -actinin proteins and thespectrin family of actin binding cytoskeletal proteins,

of which dystrophin is a member (13, 75). There is

marked evolutionary conservation of the -actinin

genes across species, particularly within the NH2-

terminal actin-binding domain (9).

There are four -actinin genes in humans,

ACTN1ACTN4 (9, 85). ACTN1 and ACTN4 contain

functional calcium-sensitive EF hands, whereas

the skeletal muscle or sarcomeric -actinins, en-

coded by ACTN2 and ACTN3, have EF hands that

are not calcium sensitive (15). In humans, -acti-

nin-2 is expressed in the heart, in all skeletal mus-

cle fibers, and in the brain, whereas -actinin-3 is

expressed only in fast glycolytic skeletal muscle

fibers, is not present in cardiac muscle, and has

low levels of expression in the brain (50). These two

proteins diverged from one another following a

gene duplication event over 300 million years ago

(mya), but have retained considerable sequence

similarity (43). Human -actinin-2 and -actinin-3

are 79% identical and 91% similar at the amino

acid level (9, 42).

The sarcomeres are repeating units that con-

stitute the contractile apparatus of the muscle

fiber (myofibril) and are comprised of actin-con-

taining thin filaments and thick filaments con-

taining myosin (19). The thin filaments are

anchored to electron-dense bands known as Z-lines, in perpendicular orientation to the myofi-

brils. The ordered alignment of the Z-lines in

adjacent myofibrils enables co-coordinated con-

tractions between myofibrils and allows trans-

mission of contractions to the costameres at

which the Z-line is linke d to the muscle mem-

brane. The sarcomeric -actinins are major com-

ponents of the Z-line and historically have been

thought to have a primarily structural role in

skeletal muscle (10, 11). In addition to actin, they

bind to many of the Z-line-associated proteins

including myotilin, myopalladin, Z-band alterna-

tively spliced PDZ motif protein (ZASP), filamin-,

actinin-, and telethonin-binding protein of the

Z-disc (FATZ), and titin (1, 5, 7, 27, 28, 40, 65).

The -actinins can form antiparallel dimers with

themselves or other -actinins, allowing cross

linking of actin and titin filaments from neigh-

boring sarcomeres and are thought to play a

significant role in maintenance of the structural

integrity of the Z-line of skeletal muscle (911,

16, 47, 70).

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-Actinin-3 Deficiency is Commonin the General Population

In 1999, we described a common single-base trans-

version (CT) in exon 16 of the ACTN3 gene that

converts an arginine residue (R) to a stop codon (X)

at amino acid position 577 (56). Approximately

16% of the world population is completely defi-

cient in -actinin-3 protein due to homozygosity

for the R577X stop codon (ACTN3 577XX genotype)(48). There is variation in frequency of the R577X

allele in different ethnic groups, with allele fre-

quencies of 0.55 in Europeans, 0.52 in Asian pop-

ulations, and 0.09 in Africans (49). -Actinin-3

deficiency does not result in muscle disease, sug-

gesting that it is not essential for baseline muscle

function, and that the closely related isoform,

-actinin-2, can at least partially compensate for

its absence at the Z-line in fast muscle fibers. How-

ever, ACTN3 has been highly conserved during

vertebrate evolution, suggesting that the sarco-

meric -actinins are not completely functionally

redundant and that ACTN3 has evolved to perform

specific functions in fast fibers (47, 50).

We have studied genetic variation around the

ACTN3 R577X polymorphism across a wide range of

populations using DNA available through the Inter-

national HapMap project (48). Low levels of genetic

variation and an unusually long region of complete

homozygosity in the region surrounding the 577X

allele suggest a recent and rapid expansion in the

frequency of this allele amongst Eurasian population

due to positive selection. Thus, although ACTN3 ap-

pears have been conserved through early evolu-

tion(300 mya), there is now recent positive selection

(1533 thousand years ago) favoring the nonfunc-

tional ACTN3 allele. This suggests that both states,

-actinin-3 deficiency and -actinin-3 expression,

may confer benefits to muscle function that have

been acted on through natural selection.

-Actinin-3 Deficiency ImprovesHuman Sprint Performance

In 2003 in collaboration with the Australian Insti-

tute of Sport, we showed that ACTN3 genotype is

strongly associated with elite athlete status (83).

There is a striking and highly significant reductionin the frequency of-actinin-3-deficient individu-

als among sprint/power athletes compared with

controls (FIGURE 1). The association between the

ACTN3 genotype and sprint performance has been

replicated in a number of studies in populations of

varied ethnicity, including European, American,

and Israeli athletes (1, 25, 55, 63, 66). A meta-analysis

of existing published data has given a P value of

0.5 1011 of the effect of ACTN3 genotype on

sprint performance (46). Although -actinin-3

deficiency is associated with poorer muscle strength

and sprint performance, loss of-actinin-3 appears to

be beneficial in certain circumstances, with the fre-

quency of the XX-null genotype higher in endurance

athletes than in controls in some studies (26, 83).

Similar ACTN3 genotype associations have also

been demonstrated in nonathletes, with deficiency of

-actinin-3 associated with significantly slower 40-m

sprint times in Greek adolescent males (51), lower

isometric maximal voluntary muscle contractions(20), and lower knee extensor shortening and length-

ening peak torques in untrained adult women and

men (20, 51, 77, 78). In summary, the large number of

human studies that have been performed to date

show that the ACTN3 R577X polymorphism repre-

sents an important genetic factor associated with

variations in muscle performance in humans, with

the presence of-actinin-3 associated with improved

sprint and power performance.

Understanding the Role of-Actinin-3 in Muscle Performance:

The Actn3 KO Mouse

To better understand the mechanisms underlying

the effect of-actinin-3 on skeletal muscle perfor-

mance and the factors that might contribute to-

ward positive selection for the X allele during

recent human evolution, we generated a knockout

mouse (Actn3KO) completely deficient for -acti-

nin-3 at the protein and mRNA level (49). Similar to

humans, wild-type (WT) mice express -actinin-3

predominantly in fast fibers. Unlike humans, -ac-

tinin-2 is usually expressed predominantly in type

1 and type IIa fibers in postnatal mouse muscle. In

the Actn3 KO, -actinin-2 is upregulated and ex-

pressed in all fibers, mimicking the pattern of expres-

sion seen in ACTN3 577XX humans. Like humans,

-actinin-3 deficiency in the Actn3 KO mouse does

not result in overt muscle disease. The Actn3 KO

mice appear normal and have similar activity levels

to WT mice on open-field testing (49).

-Actinin-3 Deficiency ReducesMuscle Mass

Actn3 KO mice weigh slightly less than WT mice,

with lower muscle mass seen in all muscles nor-

mally expressing -actinin-3 (48). The heart andslow-twitch soleus muscle (located in the lower

hindlimb underlying the gastrocnemius) do not

normally express -actinin-3 and so provide an

internal negative control. There was no difference

in the size of the heart between WT and Actn3 KO

mice. Unlike the other muscles analyzed, we saw a

trend toward an increase in size in the soleus. This

may reflect hypertrophy of the soleus to compen-

sate for reduced strength in the surrounding mus-

cles. The increase in size of the soleus also argues

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against an overall runt effect among the Actn3

KOs. Rather, it suggests that the reduction in mus-

cle mass in the presence of-actinin-3 deficiency

is specific to the muscles in which it is normally

expressed. As further evidence of a role for -acti-

nin-3 in muscle size, -actinin-3 deficiency has

also been associated with reduced muscle mass in

Japanese and American women (24, 77, 78, 86).

The reduction in muscle mass that we see in the

Actn3KO appears to be due to a reduced diameterof the type 2B, fast glycolytic fibers that normally

express -actinin-3. We see no change in the num-

ber of muscle fibers or significant alteration in fiber

type as defined by myosin heavy chain isoform.

Rather, we see that the type 2B muscle fibers have

a cross-sectional area that is 34% smaller than the

2B fibers found in WT littermates. Similarly, actinin-3

deficiency has been shown to reduce the total muscle

cross-sectional area occupied by fast, glycolytic (type

2X) fibers in ACTN3 577XX humans (77).

-Actinin-3 Deficiency Reduces

Muscle Strength

Grip strength is significantly lower in Actn3 KO

mice (67%) compared with WT mice, although

still within the normal range overall (48). This con-

firms that the Actn3KO mice are modeling normal

variation rather than weakness as a manifestation

of muscle disease (FIGURE 1). Human studies have

also shown reduced muscle strength in XX individ-

uals. In a group of elite male road cyclists (n 46),

individuals with XX-genotypes were found to have

less peak power output and less power to tolerate

high submaximal workloads compared with RR ge-

notypes (35). Reduced peak torque values werealso seen among XX women in a large cohort of

women across a broad span of age range (848

women aged 2290 yr) (78).

-Actinin-3 Deficiency Results in ImprovedEndurance Capacity

Intriguingly, we found that Actn3 KO mice have an

increased capacity to run longer distances (FIGURE 1)

(49). Using a modified intrinsicexercisetest where mice

are run to exhaustion, Actn3 KO mice were able to run

on average 33% further than WT mice. This data is

consistent with the findings of our original human

association study in which we found a trend to-

ward an increase in the frequency of XX individuals

among endurance athletes, reaching significance

FIGURE 1. -Actinin-3 is associated with altered muscle performanceA: ACTN3 R577X genotype frequencies in controls and elite sprint and endurance athletes. The frequency of the 577X(-actinin-3 deficient) genotype is significantly lower in the total power athlete group (6%) than in controls (18%) andsignificantly higher in female endurance athletes (29%) than in female controls. B: Actn3 KO mice have improved enduance performance. Actn3 KO mice run farther before exhaustion in an intrinsic exercise capacity test. C: Actn3 KO micdisplay reduced grip strength compared with wild-type (WT) mice.D: Actn3 KO mice have reduced muscle mass. Meamuscle mass of triceps (TRIC), tibialis anterior (TA), gastrocnemius (GST), quadriceps (QUAD), and spinalis thoracis (SPexcised from male 8-wk-old mice. Values are means 95% CI. Significant difference: *P 0.05; **P 0.01; ***P0.001. Figure was adapted from Refs. 48, 49, 83.

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mice compared with 1.0% in WT. In ACTN3 577XX

humans, there is also an increase in glycogen com-

pared with 577RR and 577RX individuals, who ex-

press -actinin-3.

Glycogen metabolism is the key pathway for en-

ergy production during high-intensity activity, and

depletion of glycogen results in muscle fatigue.

Glycogen metabolism is controlled by complex

feedback mechanisms (38). Glycogen synthesis is

controlled by the delivery of glucose to the cell(glucose transport) and the enzyme glycogen syn-

thase. Glycogen utilization (glycogenolysis) is cat-

alyzed by glycogen phosphorylase.

Glycogen synthase and glycogen synthase activ-

ity levels are increased (by 100% and 50% com-

pared with WT) in Actn3KO mouse muscle (62).

However, when corrected for total glycogen syn-

thase levels, the percentage activity of glycogen

synthase is not increased. Glycogen synthase activ-

ity is regulated by a number of factors including

phosphorylation, activation by glucose 6-phos-

phate, insulin, and exercise. Existing evidence sug-

gests that, when glycogen content is high, strongfeedback decreases glycogen synthase activity, mak-

ing glycogen synthase the rate-limiting step in glyco-

genesis (38). Elevation of total glycogen synthase and

active glycogen synthase in the presence of ele-

vated glycogen in the Actn3 KO mouse suggests

there may be some additional feedback mecha-

nism in the presence of-actinin-3 deficiency that

combats the expected reduction in glycogen syn-

thase activity in this state.

-Actinin-3 Deficiency Results in ReducedGlycogen Phosphorylase Activity in Muscle

The key enzyme in glycogenolysis, glycogen phos-phorylase (GPh), is significantly reduced in Actn3

KO mouse muscle (62). Enzyme quantitation

shows activity in the Actn3 KO mouse muscle is

26% compared with 53% in WT mice. An interac-

tion between glycogen phosphorylase and sarco-

meric -actinins has previously been reported (18).

By confocal microscopy, we have also shown that

-actinin-3 and glycogen phosphorylase colocalize

at the Z-line.

A reduced capacity to break down glycogen for

energy would likely be disadvantageous to sprint

athletes, who rely on endogenous fuels such as

muscle glycogen to rapidly produce energy forcontraction. Reduced availability of glucose may,

in turn, result in a compensatory shift toward aero-

bic metabolism, as observed in Actn3 KO mice.

Such changes could be advantageous to endurance

athletes, allowing them to preferentially use other

fuels such as fatty acids for energy generation.

At the electron microscopic (EM) level, Actn3 KO

muscle type 2B fibers contain concentric ring-like

structures surrounded by and filled by glycogen

particles (FIGURE 3C). These structures also stain

with antibodies to glycogen phosphorylase and Z-

line proteins desmin and myotilin (62) (FIGURE 4).

Glycogen is typically stored near the contractile

apparatus of muscle. However, when glycogen

stores located at the contractile apparatus are filled

up, further glycogen synthesis can occur in other

regions of the cell (54). The glycogen accumula-

tions that we see in Actn3 KO mouse muscle may

reflect increased glycogen storage and loosening of

the association between glycogen and the myofi-

bril. Alternately, -actinin-3 deficiency may desta-

bilize complexes usually reliant on -actinin-3

homodimers or heterodimers for structural integ-

rity, leading to displacement of glycogen from its

usual location within the muscle fiber (43).Utilizing global proteomic analysis, we found

differential expression of phosphorylated forms of

GPh in Actn3 KO muscle compared with WT (62).

Since phosphorylation is one of the methods by

which GPh activity is regulated, we hypothesise

that -actinin-3 plays a role in regulation of GPh

activity by altering its posttranslational phospho-

rylation and that -actinin-3 deficiency results in

decreased activity of GPh due to altered phospho-

rylation. The reduction in GPh activity may explain

FIGURE 2.Actn3 KO mice display a shift toward more oxidative pathwaysof metabolismEnzyme analyses show increased expression of enzymes involved in the glycolysis path-way (PFK, HK, and GAPDH) in the Actn3 KO mice. The anaerobic conversion of pyru-vate to lactate by lactate dehydrogenase (LDH) is reduced, whereas the activity ofenzymes involved in mitochondrial oxidative metabolism [citrate synthase (CS), succi-nate dehydrogenase (SDH), cytochrome c oxidase (CCO)], and fatty acid oxidation[3-hydroxyacyl-CoA dehydrogenase (BHAD), and medium chain acyl-CoA dehydroge-nase (MCAD)] are increased in Actn3 KO mice. Glycogen content is increased in Actn3KO mice, and glycogen phosphorylase activity is decreased. Hexokinase activity (HK) isalso increased in Actn3Kos, whereas glucose-6-phosphate (G6P) and Phsosphofructoki-nase (PFK) are not. NADH tetrazoleum reductase (NADH-TR) levels are increased inActn3KOs by staining.

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the observed increase in muscle glycogen content

and decrease the capacity of muscle to use glyco-

gen as a fuel. This, in turn, could explain the switch

in preferred fuel source, from anaerobic metabo-

lism toward more oxidative metabolism as seen in

Actn3 KO mice.

-Actinin-3 Deficiency:A Pretrained State for

Improved Endurance and PoorerSprint Performance?

The improved endurance performance in Actn3

KO mice and in ACTN3 577XX humans, and the

shift in muscle metabolism toward a slow oxidative

phenotype with increased glycogen content in

Actn3 KO mice are all consistent with -actinin-3-

deficient muscle being pretrained for endurance

performance. It has long been argued that there is

an evolutionary trade-off between sprint and en-

durance performance, as well as a functional

trade-off between the effects of endurance and

resistance training (34). Sprint and resistancetraining utilize exercise of short duration and high

intensity, resulting in muscle hypertrophy, with

increased fiber cross-sectional area, protein con-

tent, and an increased ability to generate force (1,

14, 21, 73). Endurance training (in which the length

of exercise is increased and intensity is reduced)

induces a shift in skeletal muscle metabolism to-

ward a more oxidative form of metabolism.

Oxidative metabolism produces a longer lasting

and more stable supply of ATP, making oxidative

fibers more fatigue resistant. Endurance training

results in reduced fast-fiber cross-sectional area,

increased mitochondrial mass, increased oxidative

enzymes, and reduced glycolytic enzymes (37, 60,

71, 74). Training for both strength and enduranceappears to limit the amount of strength gains an

individual can make, suggesting that endurance

training may somehow limit skeletal muscle

growth (36).

It is possible, therefore, that improved endur-

ance capacity in the presence of-actinin-3 defi-

ciency may result in a limitation in explosive/

power abilities. Unfortunately, there are not

adequate or reliable methods by which to test

sprint capacity in mice. However, the presence of

reduced grip strength in Actn3 KO mice compared

with WT does suggest a reduction in explosive

power.Exercise training improves utilization of fat as an

energy source and reduces the rate at which gly-

cogen is utilized, thereby delaying glycogen deple-

tion. Similar to our Actn3 KO mouse, exercise

training results in higher glycogen stores. Also

FIGURE 3. Actn3 KO mice have increased glycogen content and reduced glycogen phosphorylase activity in skeletal muscleA: representative PAS staining images of male 8-wk-old mouse quadriceps muscle cross sections demonstrate glycogen levels are higher in Actn3KO muscle compared with WT. B: glycogen assays on lower hind leg muscles tibialis anterior (TA), extensor digitorum longus (EDL), soleus (SOL),and gastrocnemius (GST), quadriceps (QUAD), and spinalis thoracis (SPN) from female 8-wk-old mice. C: glycogen synthase and glycogen synthaseactivity are increased in KO muscle, but percentage activity of glycogen synthase is not increased. Glycogen phosphorylase activity and percentageactivity are reduced in the Actn3KO. Values are means 95% CI. Significant difference: *P 0.05; **P 0.01; ***P 0.001. Figure adapted fromRef. 62.

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similar to our Actn3 KO mice, endurance training

in humans has been shown to result in increased

hexokinase activity and reduced lactate dehydro-

genase in muscle (6, 41, 52, 69). A reduction in LDH

can result in poorer sprint performance since LDH

is needed to convert pyruvate, the final product of

glycolysis, to lactate when oxygen is absent or in

short supply.

How Does -Actinin-3 AlterSkeletal Muscle Metabolism?

Historically, the sarcomeric -actinins have been

considered primarily structural proteins, but we

have mounting evidence that the principle role of

-actinin-3 is an effect on muscle metabolism and

that it has evolved specialized expression in fast

muscle fibers because of an important role in the

regulation of energy metabolism.

We have shown that -actinin-3 colocalizes with

and increases glycogen phosphorylase activity, but

the precise molecular mechanisms involved are yet

to be determined. There is strong evidence to sug-

gest that glycogen levels play a role in regulating

how fuel is utilized in muscle. Increased muscle

glycogen has been shown to increase carbohydrate

oxidation during exercise (4, 12, 22, 79). When gly-cogen is depleted, skeletal muscle may also oxidize

free fatty acids to produce ATP and preserve mus-

cle glycogen (61). If muscle glycogen is low before

exercise, there is a shift toward decreased carbo-

hydrate oxidation and increased lipid oxidation.

Patients with McArdles disease lack functional

glycogen phosphorylase in muscle and cannot

break down glycogen stores. These patients suffer

from exercise intolerance and a shift toward lipid

utilization for fuel (44). Interestingly, the ACTN3

577XX genotype is associated with improved mus-

cle performance in these patients (45). The mech-

anism by which -actinin-3 deficiency improvesexercise tolerance in these patients is unknown.

Given that most patients with McArdles disease

have no functional muscle glycogen phosphorylase

and are therefore unable to utilize glycogen stores,

it is unlikely that increased muscle glycogen would

improve exercise capacity in these patients. It is

possible that the increased fatty acid oxidation ca-

pacity seen in Actn3 KO mice could improve fuel

utilization in the absence of glycogenolysis, how-

ever, this functional link is yet to be tested.

Interestingly, sprint training also increases the

ability for rapid glycogen breakdown (glycogenol-

ysis) during shirt bursts of maximal or submaximal

activity. It is interesting to speculate whether re-

duced glycogen phosphorylase activity associated

with -actinin-3 deficiency might reduce the abil-

ity to utilize glycogen during sprint activity.

We are in the process of trying to unravel the

pathway of events that lead to the metabolic phe-

notype in Actn3 KO mice. We have examined the

time course of appearance of the structural and

metabolic phenotypes in Actn3 KO muscle. The

reduction of glycogen phosphorylase activity

higher muscle glycogen content, and increased

glycolytic and mitochondrial enzymes occur con-currently at 4 wk postnatally. These metabolic

changes are preceded by upregulation of -acti-

nin-2 and interacting proteins at the Z-line, sug-

gesting that structural alterations may lie upstream

of the metabolic changes. Since -actinin-2 and -3

and glycogen phosphorylase are colocated at the

Z-line, loss of -actinin-3 may alter the three-di-

mensional conformation of the Z-line, which in

turn could alter the availability of glycogen phos-

phorylase for phosphorylation and activation. Al-

FIGURE 4. Actn3KOs have intramuscular accumulations ofglycogen that stain for GPh and MYOTAntibodies to myotilin (MYOT; top) and GPh (second panel) used on a sin-gle muscle section demonstrate that these proteins co-localise in cytoplas-mic inclusions in the KO muscle. Electron microscopy (EM) showsconcentric ring-like structures in Actn3 KO highlighted with black asterisks.An asterisk has been used on each of the WT and KO images to allow indi-vidual fibers to be readily compared between images. Scale bars are 50m wide. Images are from Ref. 62 and used with the permission ofHumMol Genet.

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ternately, the structural and metabolic effects of

-actinin-3 deficiency may be due to independent

and unrelated actions of the -actinin-3 protein.

A review of the known interaction partners of the

sarcomeric -actinins provide tantalizing hints at

alternate possible mechanisms underlying the ef-

fects of-actinin-3 on metabolism. In addition to

their structural cross-linking functions at the Z-

line, the -sarcomeric -actinins interact with a

number of signaling molecules. -Actinin-2 and -3interact with the calsarcin family of proteins,

which, through their interaction with calcineurin,

are involved in muscle fiber-type determination

and regulation of expression of fiber type-specific

genes (17, 27, 30, 31, 72). -Actinin-2 has been

shown to interact with membrane-bound signaling

proteins such as the NMDA glutamate receptor,

Kv1.4 and Kv1.5 potassium channels, and cardiac

L-type calcium channels (23, 64, 80, 81).

The -actinins also bind to the soluble signaling

factors phosphoinositol 3-kinase (PI3K) and phos-

phoinositol-4,5-bisphosphate (PIP2), and G-pro-

tein-coupled receptor kinase (29, 32, 68). PIP2 actsas a substrate for enzymes as well as promoting the

recruitment of proteins to the plasma membrane and

subsequent activation of signaling cascades. In the

presence of -actinin-3 deficiency, any alteration of

total -actinin levels or differential binding between

-actinin-2 and -actinin-3 could affect regulation of

one or many of these important signaling pathways.

Conclusion

Over one billion people worldwide are deficient in

-actinin-3, and there is increasing evidence tosuggest that ACTN3 genotype is an important ge-

netic variant that influences the metabolic func-

tion of human muscle. -Actinin-3 deficiency

results in a fundamental shift in metabolism away

from the anaerobic pathway toward the oxidative

pathways of muscle metabolism, which provides an

explanation for the association between-actinin-3 de-

ficiency, poorer sprint and power performance, and

enhanced endurance performance. The increase in

metabolic efficiency of -actinin-3-deficient muscle

could also provide an explanation for the adaptive ben-

efit of the 577X allele during recent human evolution.

The next challenge will be to dissect the molecularmechanisms underlying this metabolic phenotype and

explore whether ACTN3 genotype influences glucose

homeostasis and adaptive responses to diet in the

modern world.

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