The current report demonstrates a modified technique for intramyocardial injection of viral vectors for cardiac gene manipulation, which was modified from a method for myocardial infarction induction by Gao et al.13 Currently, in vivo characterization of specific gene functions most often involve the generation of knockout or transgenic mice3,14,15,16,17, which is expensive, time-consuming, and labor-intensive. Alternatively, delivery of gene vectors or siRNA by systemic or local injection is also widely practiced for gene manipulation in cardiovascular research4,5,7. In particular, intramyocardial injection cannot be substituted for cardiac gene manipulation under certain circumstances: when site directed injection is required (e.g., border zone injection in the myocardial infarction model)11; when duration-restricted gene manipulation is required (e.g., adenovirus injection). Here, we showed that the modified intramyocardial injection method is simple, time-saving, and highly efficient.
Critical Steps Within the Protocol and Troubleshooting:
For successful operation of this protocol, several critical steps should be noted. Before aspirating virus, the air within the Hamilton syringe and the attached needle must be evacuated, otherwise the air injected into the myocardium may cause topical cardiac injury or even death. To further avoid this issue, the ready-to-use Hamilton syringe filled with an adequate virus volume should not be placed with the plunger end downward, because this may spontaneously aspirate air by the gravity of the metal plunger. Anesthesia should be carefully monitored, as deep anesthesia may delay post-operation recovery, and severely deep anesthesia may cause death. After adequate anesthesia, the heart should not be externalized out of the chest cavity by force, since this may result in severe lung injury. Indeed, proper heart externalization requires only a gentle push of the heart, and any resistance may indicate pushing toward the improper direction.
Limitations of the Technique:
The elimination of intubation in this technique, which reduces the time needed for the procedure, suggests that the intramyocardial injection procedure should be finished within a relatively limited time window to avoid death. According to our experience, the heart should not be externalized for more than 30 s, because this increases the death rate and slows post-operative recovery. Therefore, the use of intubation is recommended for the first attempts of the protocol described here. Another limitation is the uneven distribution of viral vectors delivered by this method, which also exists in conventional methods of intramyocardial injection7. Moreover, the method described here is more useful for cardiac-specific gene manipulation in uninjured hearts, which can be followed by the establishment of different cardiac disease models18,19,20,21,22,23. However, the use of the current method in delivering agents to injured hearts such as infarcted hearts may be limited, because these mice may not tolerate the procedure.
Significance with Respect to Existing Methods:
Conventional intramyocardial injection requires intubation and mechanical ventilation24, and makes it difficult to locate the injection site due to the fast mouse heartbeat. These issues significantly prolong the operation time13, thus increasing variations posed by the time delay. The modified technique presented here is quick and allows precise injection site location by manually securing the externalized heart; overall, significantly potentiating subsequent study. Moreover, the elimination of intubation and mechanical ventilation make the modified method accessible to almost any laboratory.