Objective      To determine the effect of down-regulation of matrix metallopeptidase 21 (MMP21) on the development of zebrafish embryos. Methods      The expression of MMP21 gene was down-regulated by injecting morpholino antisense oligonucleotides (MO) in zebrafish embryos at 1～2 cell stage. The effectiveness of MMP21-MO was detected by RT-PCR at 10 somite stage. Four different concentration gradients of MMP21-MO injection groups

that is

0.5

0.75

1 and 1.5 mmol/L

and 2 control groups (Con-MO injection group and wild-type group) were set up in this study. The development of zebrafish embryos was observed with stereoscopic microscopy

and the deaths and malformations in different groups were observed. Whole-mount in situ hybridization was used to examine the expression of cardiac myosin light chain 2 (cmlc2) at 24 and 48 hours post fertilization (hpf). Cardiac morphology of the living embryos was observed at 72 hpf for cardiac looping

and embryonic heart rate at 24

48 and 72 hpf were counted and the ventricular contraction fraction (VSF) in each group at 72 hpf were measured to identify the effect of down-regulation of MMP21 on cardiac function in zebrafish. Results      MMP21-MO microinjection effectively knockdown the expression level of MMP21 in a dose-dependent manner. The rate of malformation was the highest and the mortality was low in the group of 1 mmol/L MMP21-MO. The main manifestations of malformation were edema of the anterior region of the heart

abnormal cardiac looping

weakened heart beat and slow heart rate. The results of whole-mount in situ hybridization showed abnormal phenotypes of cmlc2 at 24 and 48 hpf

which suggesting abnormal cardiac jogging and cardiac looping. At 72 hpf

zebrafish cardiac morphology also showed abnormal cardiac looping in the MMP21-MO injection group. Compared with the control groups

the heart rate was slowed down

and the VSF was decreased in the MMP21-MO injection group. Conclusion      Knockdown of MMP21 gene results in abnormal cardiac looping and cardiac dysfunction. MMP21 may play an important role in the cardiac looping.