Description

The C. elegans intestine is a major site of the generation and storage of chemical energy. Adenosine triphosphate (ATP) is a major molecule involved in energy transfer, hence regulation of intracellular ATP levels is a critical aspect of metabolism. Recently, ATP sensor proteins have been used to indirectly measure ATP levels in living cells (Zhang et al., 2018). These sensors are derived by inserting the ε subunit of the bacterial F₀F₁-ATP synthase between two fluorescent protein domains, allowing detection of protein bound to ATP by Förster Resonance Energy Transfer, or FRET (Imamura et al., 2009). Others have recently demonstrated use of FRET-based ATP sensors in the C. elegans pharynx and muscle (Tsuyama et al., 2013; Wang et al., 2019). Here we test the intestinal expression of a similar FRET-based ATP sensor derived from the green/red fluorochrome pair Clover/mApple (Figure 1A) (Mendelsohn et al., 2018). When the protein is bound to ATP, the two fluorochromes are brought into proximity, permitting emission from Clover to excite mApple (Imamura et al., 2009; Mendelsohn et al., 2018).

We made transgenic animals expressing the sensor under the control of the pept-1 promoter, which in arrays drove strongest expression in the anterior intestinal ring, int1 (Mendelsohn et al., 2018; Nehrke, 2003). We measured FRET by comparing red signal in image pairs as described in Figure 1B. Individual adult animals were imaged twice in the same focal plane. In the first image, mApple was detected by a TRITC filter set to reveal the sensor location independent of ATP levels. A second image was obtained using a filter set to excite Clover and detect red FRET emission from mApple. We devised an automated image analysis pipeline in Python to rapidly compute average ratios of FRET:mApple signal for each image pair and combine the data across multiple animals.

We tested three genetic conditions to evaluate whether the sensor produces similar relative ATP levels as previously determined by others (Fig. 1C). Knockdown of the nuclear-encoded ATP synthase subunit gene atp-3 by RNAi results in a decrease of 60-80%, and knockdown of daf-2, an increase of 100-200% in biochemical ATP levels in whole animals, depending on their age (Dillin et al., 2002). In contrast, mutation of daf-16 produces little or no change in ATP levels (Braeckman et al., 1999). Mutation of daf-2 produced a modest increase in muscle ATP levels after several days, measured by a similar FRET-based ATP sensor, ATeam (Wang et al., 2019). As shown in Fig. 1C, our results were qualitatively similar. However, the differences seen with the sensor, while statistically significant, are of smaller magnitude, i.e. only a 40% decrease in atp-3(RNAi) and a 50% increase in daf-2(RNAi). Conservatively, therefore, an in vivo ATP biosensor is best used to compare relative free ATP levels across genotypes and conditions, and not to directly infer actual biochemical changes.

Our results suggest that even with a modest low-cost fluorescence setup, an in vivo ATP FRET reporter can be a useful way to measure aspects of metabolic state in the C. elegans intestine. Improvements in expression of the reporter and the use of confocal microscopy are likely to further increase the usefulness of this system.