Summary accordion , a zebrafish behavioral mutant , has a muscle relaxation defect due to a mutation in the ATPase Ca 2 + pump SERCA 1

Several factors have made the zebrafish an attractive vertebrate model for the study of motor development. First, forward genetics can be applied to zebrafish to identify genes that are essential for proper behaviors (Granato et al., 1996). Second, embryos are transparent, allowing the visualization of dynamic events, such as Ca 2+ transients and axonal outgrowth in live fish (Fetcho and O’Malley, 1997; Lewis and Eisen, 2003). Third, electrophysiological techniques can be used to analyze the physiology of embryonic neurons and muscles to dissect embryonic behaviors (Drapeau et al., 2002). Fourth, zebrafish embryos exhibit readily assayable and well-characterized behaviors (Eaton and Farley, 1973; Saint-Amant and Drapeau, 1998). Zebrafish embryos display three stereotyped behaviors by 36 hours post-fertilization (hpf) (Saint-Amant and Drapeau, 1998). The earliest behavior consists of repetitive spontaneous alternating coiling of the tail, much like a metronome. This simple slow coiling behavior is independent of stimulation and starts at 17 hpf, reaches a peak frequency of 1 Hz at 19 hpf, and declines to less than 0.1 Hz by 26 hpf. After 21 hpf, embryos start to respond to mechanosensory stimulation with coils that are stronger than spontaneous coils. Typically embryos respond with two alternating coils. By 26 hpf mechanosensory stimulation initiates swimming episodes. The frequency of muscle contractions during swimming increases from 7 Hz at 26 hpf to 30 Hz at 36 hpf, the latter being similar to the frequency of swimming of adult zebrafish (Buss and Drapeau, 2001). The production of any touch response can be divided into several steps from sensory perception to muscle activation (reviewed by Roberts, 2000; Berchtold et al., 2000). In zebrafish, two types of mechanosensory neurons sense touch stimuli. Head and yolk stimulation acts through trigeminal neurons, whereas tail stimulation activates Rohon-Beard neurons (Drapeau et al., 2002). Once triggered by sensory input, interneuronal networks located in the hindbrain and spinal cord produce the appropriate motor rhythm (Fetcho, 1992). Motoneurons are activated by these central networks, release acetylcholine at neuromuscular junctions (NMJs) and depolarize the muscle membrane (Buss and Drapeau, 2002; Li et al., 2003). In muscle, depolarization is sensed by voltagedependent L-type Ca 2+ channels, such as dihydropyridine receptors, and activates ryanodine receptors in the sarcoplasmic reticulum (SR) membrane to release Ca 2+ from the SR to the cytosol (Fill and Copello, 2002). Increased cytosolic Ca2+ activates troponin C that initiates actin/myosin sliding, thus contracting the muscles (Gordon et al., 2000). Cytosolic Ca2+ levels are rapidly decreased by sarco(endo)plasmic reticulum Ca 2+-ATPase 1 (SERCA1), a Ca2+ pump expressed in skeletal muscle, which enables relaxation (McLennan et al., 1997). When wild-type zebrafish embryos are touched at 24 hours post-fertilization (hpf), they typically perform two rapid alternating coils of the tail. By contrast, accordion(acc) mutants fail to coil their tails normally but contract the bilateral trunk muscles simultaneously to shorten the trunk, resulting in a pronounced dorsal bend. Electrophysiological recordings from muscles showed that the output from the central nervous system is normal in mutants, suggesting a defect in muscles is responsible. In fact, relaxation in acc muscle is significantly slower than normal. In vivo imaging of muscle Ca2+ transients revealed that cytosolic Ca2+ decay was significantly slower in acc muscle. Thus, it appears that the mutant behavior is caused by a muscle relaxation defect due to the impairment of Ca2+ re-uptake. Indeed, accmutants carry a mutation in atp2a1gene that encodes the sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1), a Ca 2+ pump found in the muscle sarcoplasmic reticulum (SR) that is responsible for pumping Ca2+ from the cytosol back to the SR. As SERCA1 mutations in humans lead to Brody disease, an exerciseinduced muscle relaxation disorder, zebrafish accordion mutants could be a useful animal model for this condition.