The renin-angiotensin system (RAS) is an important regulator of blood pressure homeostasis, in which the protease renin cleaves the angiotensinogen into the angiotensin I (Ang I), followed by further cleaving of the Ang I by the angiotensin-converting enzyme (ACE) into angiotensin II (Ang II), and thereby resulting in blood vessel contraction and hypertension.1 In hypertensive cardiovascular disease, the leading cause of death worldwide, pharmacological inhibition of the ACE and Ang II receptors has been demonstrated to be beneficial for ameliorating heart impairment.2 The discovery of the ACE2, the ACE homologue, has introduced further complexities into the canonical RAS signal cascade, as the major biologically active product of ACE2 will hydrolyze Ang II to form Ang 1–7, and thereby counterbalancE the ACE activity.3,4 It was thus believed that the primary function of the ACE2 is to regulate the blood pressure homeostasis (reviewed in Refs. 56.–7).
Recent biochemical studies revealed that the ACE2 may modulate the RAS, and thus impact the blood pressure regulation in vitro.4 Several animal models have been used to explore the biological functions of the ACE2 in vivo,8,9 and genetic inactivation of the ACE2 has been shown to impair the cardiac functions in mice.8,9 Nevertheless, mutation of angiotensin-converting enzyme-related (ACER) gene, a Drosophila ACE2 homolog, was proven to result in a severe defect during heart morphogenesis.8 Other than its function in heart development, ACER also plays an important role in regulating sleeping behavior, as flies lacking ACER generally experience reduced night-time sleep and exhibit greater sleep fragmentation.10
As outlined above, the mammalian ACE2 regulates cardiac contractility mainly, whereas Drosophila ACER regulates heart development during embryogenesis.8 Apart from this distinction, ACER also likely regulates the heart physiology in adult flies as it is expressed in the heart of Drosophila during development.11 However, the issue of whether ACER regulates the physiological functions of adult flies has not been explored in depth. Here, we took advantage of a noninvasive optical coherence tomography (OCT) imaging method to assess whether ACER modulates cardiac functions in living adult flies.
OCT, introduced in 1991, is a powerful tool for obtaining noncontact and noninvasive tomographic images of biological tissues.12 In essence, OCT entails combining a broadband light source and a Michelson interferometer with a short coherence gate. The differences in the intensities of the backscattered light from the tissue are then analyzed to generate structural imaging. With the ability to image up to 3 mm in depth and to achieve better than 15 μm in axial resolution, OCT fills the niche between ultrasound and confocal microscopies.12 OCT had previously been successfully utilized to obtain in vivo images of the living heart in Drosophila.1318.104.22.168.18.–19 Moreover, combining the frequency domain OCT system with the recently developed frequency swept source OCT (SS-OCT) further improves the detection sensitivity, and thereby makes rapid B-mode, M-mode, and Doppler OCT imaging of Drosophila possible.1516.17.18.–19
In this article, we applied SS-OCT and a novel algorithm19 to evaluate the cardiac functions of Drosophila. Cardiac parameters, including the HR, end-diastolic diameter (EDD), end-systolic diameter (ESD), and percent fractional shortening (%FS), were automatically calculated from a large number of heartbeat M-mode OCT records. We found that the aforementioned contractile parameters declined with age in wild-type Drosophila. The age-dependent physiological functions of the heart were significantly reduced in ACER knock-down flies. Additionally, down-regulation of ACER increased the stress-induced heart failure rates and decreased the lifespans in flies, suggesting that ACER is essential for both the heart physiology and the longevity of Drosophila.
Fly Stocks and Generation of Transgenic Flies
Two twi-24B-gal4 lines, a mesoderm and a heart driver,20 were obtained from the Bloomington Stock Center. All fly stocks and genetic crosses were maintained on standard yeast-glucose medium at 25°C. To generate the ACER RNA interference construct, UAS-ACER-RNAi and ACER-cDNA (LD28328) were obtained from J. L Juang of the National Health Research Institutes of Taiwan. A 523-bp DNA fragment was PCR amplified using a pair of primers, 5′-CGCGTCTAGAGTGCTGGAGGCGCGTAGGTTC-3′ and 5′-CGCGTCTAGAGTCGGCATAGGAGCGGTGACC-3′ (with the XbaI site underlined). The amplified DNA fragment was digested with XbaI, and then cloned first onto the AvrII site. Subsequently, the same DNA fragment was subcloned onto the NheI site of a pWIZ vector as described in Ref. 21. The orientation of the DNA construct was confirmed by restriction enzyme digestion. To generate transgenic flies, the standard germ-line transformation procedure using as the parental line was performed.
RT-PCR was used to determine the silencing efficiency of the transgenic flies carrying the UAS-ACER RNAi construct. Three flies carrying transgene and were driven by twi-24B-gal4. PCR was performed using the ACER-specific primers, ACER 2-1-3′ CACAAACGGCTTCTCCGGAT and ACER 2-5′ AACTGGCTTGGTATTG, as well as rps17 primers, 5′-CGAACCAAGACGGTGAAGAAG-3′ and 5′-CCATAGAGGTAGTTCAACGTCC-3′, to assess the expression of the transgene. Phenotypic analysis of all the transgenic lines exhibited similar cardiac phenotypes. Several transgenic lines were obtained for ACER-Ri9, which exhibited the strongest silencing efficiency and was thus used for phenotypic analysis (Fig. 1).
SS-OCT Measurements and Quantification of Heartbeat Parameters
To acquire the contractile parameters of adult flies, similar protocols were adopted.13,19 Briefly, in each experiment, two-dimensional (2-D) OCT images were first obtained in the longitudinal direction to identify the dorsal midline of the A1–A3 abdominal segments of the Drosophila. Then, the OCT image orientation was rotated by 90 deg, such that the conical chamber (CC) of the heart was acquired in the transverse plane. Measurements were always made in the same location in the abdominal segment. A group of 20 flies was examined with an SS-OCT system (OCM1300SS, Thorlabs Inc., Newton, New Jersey). The median wavelength of the SS-OCT system was 1310 nm with an axial resolution of in tissue, a total power of 10 mW, and an A-scan rate of 16 kHz. Since heart function of Drosophila decelerates greatly between 5 and 7 weeks of age, to assess the age-dependent heartbeat parameters, 1-, 3-, and 7-week old flies were used. It has been previously reported that anesthetic treatment induces cardiac arrhythmias in mice, and that carbon dioxide or ether affects heart performances in Drosophila.2223.–24 To sidestep the aforementioned issues, during the image assessment, the Drosophila were first anesthetized with triethyl amine, and then gently immobilized on wax gel with the dorsal side facing the OCT probe. The flies were subsequently allowed to return to a fully awake state on the bench for 20 min before being subjected to OCT scanning.
Figure 2 is a representative video depicting a transverse 2-D OCT image of the CC in the heart during diastole and systole in 1-week old wild-type Drosophila. By acquiring in-depth scans at the midline of the CC overtime without scanning, one can obtain an M-mode image of the vertical movements of the edges of the heart (-axis) overtime (-axis). M-mode OCT images can provide the contraction pattern which gives us an objective assessment of heart wall motion.
We then employed a method we had previously proposed19 for the rapid analysis of the beat-to-beat contraction–relaxation parameters of the heart in Drosophila by using semiautomatic cardiac chamber segmentation in B-mode OCT images based on the random walker algorithm. Random walker25 is a semiautomatic segmentation method based on graph theory that can provide a unique solution that is robust to weak/noisy object boundaries such as the cardiac tube of Drosophila. Further details and proofs can be found in Refs. 19 and 26. Briefly, after contrast enhancement of the image, we created an initial set of seed points on the first frame. In the next frame, the cross-sections of the heart tube were automatically segmented in a total of 2000 frames, and the size of the inner margin was represented by the area for each Drosophila. Then, depending on the histogram distribution of the changing area during each heartbeat cycle, various contractile parameters, including the HR, EDD, ESD, and %FS, were determined accordingly (see Ref. 19 for details). The FS was calculated as , which represents the extent of the changes in the cardiac diameter during systole; this can provide an estimate of the contractility of the heart tube.
To monitor the stress-induced cardiac performance of adult flies, external heart pacing stress protocol was adopted and modified.14 For each genotype, 100 flies were anesthetized with FlyNap® (Carolina Biological Supply Company, Burlington, North Carolina) for 3 min and 30 s and paced with a standard square wave stimulator at 40 V and 6 Hz for 30 s. In heart failure, we meant that either cardiac arrest or fibrillation was observed via the phase difference microscope within 2 min. The failure rate is defined as the number of flies with heart failure divided by the total number of flies.
A group of 10 newly enclosed flies were collected and cultured with standard media in a vial at 25°C. Viable flies were scored and transferred to a new vial every 5 days. The log-rank test was performed to determine the differences in the lifespan of the flies.
Down-Regulation of ACER Impaired Contractile Properties of Drosophila Heart
Previous studies indicated that ACER is critical for the heart morphogenesis of Drosophila.8 Nevertheless, the function of ACER in heart physiology has not been precisely determined. To address this question, we have utilized RNA interference to knock-down the ACER expression. The expression of the silencing construct was driven by the mesodermal- and cardial-specific gal4 driver, twi-24B-gal4, using a UAS/gal4 system.27 Quantitative RT-PCR showed that the knock-down efficiency of double-stranded ACER RNAi construct in late-fly embryos was moderate. In comparison with the case of the control twi-24B-gal4 flies, the relative expression levels of ACER mRNA was 45% in ACER knock-down flies, twi-24B-gal4>ACER-Ri9 (Fig. 1).
Having successfully knock-downed the expression of ACER, we next employed SS-OCT imaging to acquire various heartbeat parameters. As can be seen from Fig. 2, a transverse image of the conical CC of an adult Drosophila underneath the dorsal midline of the A1–A3 abdominal segments was obtained using SS-OCT. The cardiac chamber contracted rhythmically with occasional irregular pulsing activities (see Video 1). Since the flies did not possess red blood cells in their circulatory fluids, the heart chambers showed up as darker pixels under OCT. During heart contractions, the darker pixels, corresponding to the edges of the heart tube, crossed over to become lighter background pixels.
We found that ACER knock-down flies exhibited significantly enlarged systolic chamber dimensions with markedly impaired systolic functions as demonstrated in the B-mode images (see Fig. 3, Video 2). The representative M-mode OCT images of both wild-type control and ACER knock-down flies in 1, 3, and 7-week old flies are also shown in Fig. 4. The results for ACER knock-down Drosophila at different ages conclusively indicated impaired systolic functions with enlarged diastolic and systolic diameters as compared with wild-type flies.
To better assess the age-dependent contractile properties of Drosophila, parameters including the HR, EDD, ESD, and FS were computed from wild-type and ACER-Ri9 flies during aging. Both wild-type and ACER-Ri9 flies exhibited an age-dependent decline in the HR [Fig. 5(a)]. Nevertheless, the decline in HR of ACER-Ri9 flies was consistently slower than that of wild-type flies. The differences in the HR between wild-type and ACER-Ri9 flies were more evident than in aged animals [Fig. 5(a)]. The average HR of wild-type and ACER-Ri9 flies were 261.86 and 242.02 bpm, respectively. In young wild-type flies, the ESD was not significantly altered. However, in 7-week old wild-type flies, ESD decreased [Fig. 5(b)]. Unlike the case of wild-type flies, the ESD of ACER knock-down flies increased with age [Fig. 5(b)]. The EDD of the wild-type flies was indistinguishable from that of the ACER-Ri9 flies [Fig. 5(c)]. However, the EDD of both wild-type and ACER-Ri9 flies decreased at the age of 7 weeks, and the EDD of the ACER-Ri9 flies was significantly larger than that of the wild-type flies [Fig. 5(c)]. Moreover, we found that the FS of the wild-type flies did not decline with age [Fig. 5(d)]. In contrast, the FS of ACER-Ri9 flies declined consistently with age and was significantly decreased compared with the wild-type flies [Fig. 5(d)]. Since the FS represents the extent of the changes in the cardiac chamber during systole and can be used as an index of cardiac contractibility, our results suggested that the down-regulation of ACER impaired the contractile properties of the Drosophila heart.
Impact of Down-Regulation of ACER on Cardiac Performance of Adult Flies
As shown above, silencing the expression of ACER reduced the contractile properties of the Drosophila heart (Fig. 5), and it is very likely that the deficit in ACER may also affect the heart performance in flies. To study this issue, a stress protocol was employed to test the stress-induced heart functions of the ACER knock-down flies. We found that down-regulating the ACER expression increased the likelihood of heart failure during electrical cardiac pacing in ACER knock-down flies in the ages of 1 week (chi-squared test, ) and 7 weeks (chi-squared test, ) (Fig. 6). Since down-regulation of ACER affected both the contractibility and the performance of the adult heart in Drosophila, we conclude that ACER is essential for the cardiac functions in Drosophila (Figs. 5 and 6).
Necessity of ACER for the Longevity of Drosophila
It has been shown that modulation of the gene expression in the heart can alter the longevity of Drosophila.28 As shown above, cardiac-specific down-regulation of ACER did indeed exacerbate the age-dependent cardiac functions of Drosophila. We posit that the down-regulation of ACER directly impacts the longevity of the flies. In a survivorship assay, we found that the mean lifespans of control twi-24B-gal4 and twi-24B-gal4>ACER flies were 66.85 and 54.90 days, respectively (Fig. 7). Statistical analyses revealed that the differences in the survivorships of the wild-type control and ACER knock-down flies were significant (log-rank test, ) indicating that the activity of ACER is essential for the survival of the flies.
Discussions and Conclusion
The present study was the first investigation of in vivo functional changes in the heart of ACER knock-down flies. It has been reported that the disruption of the ACER results in a severe and a lethal defect during heart morphogenesis in Drosophila.8 Nevertheless, genetic analysis of the same ACER-mutant allele revealed that there existed a second mutation on the secondary chromosome, in which case it was likely to contribute to the embryonic lethality of the ACER-mutant allele (unpublished observation by M.T. Su). To sidestep this complicated issue and to better ascertain whether ACER plays a physiological role in adult animals, we adopted the RNA-interference approach. Our data indicated that ACER knock-down flies exhibited age-dependent changes in both the heart beating and the contractility of the heart tube, and lifespan were reduced when ACER was down-regulated suggesting that ACER is essential for both adult heart physiology and longevity of Drosophila.
Although we have shown that the HR of wild-type control flies is reduced in an age-dependent manner [Fig. 5(a)], our results were less significant when compared with other studies.28 Since the heartbeat of Drosophila was both myogenic and neurogenic, it was affected by temperature, aging, and genetic background, as well as anesthetic agent.23,29 To reduce the variability of heartbeat, decapitated flies were used as previously reported.29,30 As we have used OCT to obtain the heartbeat parameters from live flies in this study, it was expected that variability of heartbeat was increased, which may reduce the statistical significance. However, these age-dependent cardiac functions declined significantly when ACER was down-regulated. On the other hand, due to differences in size between flies, large variations existed in both EDD and ESD within the tested groups; however, use of %FS can eliminate the effect of body size.
Although we have shown that down-regulation of ACER affects the age-dependent heart functions and the longevity, the mechanism by which ACER deficiency leads to the above phenotypes was not clear. It has been reported that the expression levels of NADPH oxidase and reactive oxygen species (ROS) formation were significantly higher in ACE2 KO mice.31,32 Since oxidative stress was elevated in congestive heart failure patients and correlates well with the development and progression of cardiovascular diseases (reviewed in Ref. 33), it will be interesting to study in the future whether ROS levels also increase in the heart of ACER knock-down flies.
This work was supported by grants from the National Science Council (Grant No. NSC 96-2628-M-003-001-MY3, NSC 100-2325-B-003-002, and NSC 101-2311-B-003-002) and National Health Research Institute (NHRI-EX91-9109SC) of Taiwan.