Our understanding of Parkinson's disease (PD) has been revolutionized by the discovery of disease-causing genetic mutations. The most common of these is the G2019S mutation in the LRRK2 kinase gene, which leads to increased kinase activity. However, the link between increased kinase activity and PD is unclear. Previously, we showed that dopaminergic expression of the human LRRK2-G2019S transgene in flies led to an activity-dependent loss of vision in older animals and we hypothesized that this may have been preceded by a failure to regulate neuronal activity correctly in younger animals. To test this hypothesis, we used a sensitive measure of visual function based on frequency-tagged steady-state visually evoked potentials. Spectral analysis allowed us to identify signals from multiple levels of the fly visual system and wild-type visual response curves were qualitatively similar to those from human cortex. Dopaminergic expression of hLRRK2-G2019S increased contrast sensitivity throughout the retinal network. To test whether this was due to increased kinase activity, we fed Drosophila with kinase inhibitors targeted at LRRK2. Contrast sensitivity in both day 1 and day 14 flies was normalized by a novel LRRK2 kinase inhibitor ‘BMPPB-32’. Biochemical and cellular assays suggested that BMPPB-32 would be a more specific kinase inhibitor than LRRK2-IN-1. We confirmed this in vivo, finding that dLRRK− null flies show large off-target effects with LRRK2-IN-1 but not BMPPB-32. Our data link the increased Kinase activity of the G2019S-LRRK2 mutation to neuronal dysfunction and demonstrate the power of the Drosophila visual system in assaying the neurological effects of genetic diseases and therapies.
Although the primary deficits in Parkinson's disease (PD) are related to rigidity, postural instability, bradykinesia and tremor, a wide variety of visual issues have also been reported—ranging from abnormal light adaptation to visual hallucinations (1). The discovery that dopamine plays an active part in signal regulation in the human retina (2,3), and that retinal dopamine is reduced in PD (4), means that some of the visual consequences of PD may originate in the retina—the earliest and most fundamental stage of visual processing.
One problem with assaying visual deficits in human Parkinson's patients is that this is a heterogeneous disease with multiple genetic and environmental origins. A powerful complement to this approach is to dissect the complex neural deficits using the genetically tractable model organism, Drosophila. Although flies are only distantly related to humans in evolutionary terms, many of the neuronal circuits in the vertebrate and fly eye appear to be analogous. This was first noted by Ramon y Cajal using silver staining (5) and it has been confirmed by more modern cytochemical tools (6). Crucially, both vertebrates and flies have dopaminergic neurons in the visual system (7,8) and dopaminergic circuits modulate fly vision (9,10). It is therefore possible that PD-associated pathogenic mutations may have similar effects on very early visual processing in both flies and humans.
Here, we focus on a single nucleotide polymorphism in the LRRK2 gene (G2019S). This is carried by a relatively large proportion of human PD patients, 3–40%, depending on ethnicity (11), making it the most common genetic cause of PD. Nonetheless, the role of the LRRK2 protein in the pathogenesis of PD is unclear (12). Manipulations of LRRK2 in mouse have not generated robust neuronal phenotypes (13), with the most marked responses suggesting abnormal kidney function (14). However, expressing a range of LRRK2 transgenes in the fly has revealed mitochondrial and synaptic phenotypes (15–17). Recently, our group (9) showed a highly selective response to LRRK2-G2019S expression in fly dopaminergic neurons: a loss of visual response and degeneration of the retina in old flies. This was not seen with the expression of wild-type hLRRK2 or other hLRRK2 mutations.
What leads to this loss of visual function? We showed that the degeneration was accelerated by increased neuronal activity (9) and hypothesized that young G2019S flies could be shown to have amplified neuronal response, if only a more sensitive assay were available. In human visual electrophysiology, the steady-state visual evoked potential (SSVEP) method is a sensitive technique that is often used to measure neuronal response amplitudes in both adults and more challenging populations such as infants. In the SSVEP assay, responses to flickering patterns are routinely recorded by an array of electrodes and their signals used to compute the visual sensitivity. Sensitivity is high because responses to many hundreds of stimulus events are averaged together and out-of-band noise is eliminated from the analysis. In this article, we develop an SSVEP assay for Drosophila retina and show it is sensitive enough to demonstrate that one-day-old flies expressing LRRK2-G2019S in their dopaminergic neurons already have abnormal visual neurotransmission. A similar approach has been used to examine the processing of visual signals in the fly brain (18–20).
The discovery (in biochemical assays) that the G2019S mutation increases the kinase activity of LRRK2 (21) has led to the development of several potential kinase inhibitors that may provide the basis for novel therapeutic approaches. These could potentially be of great value, as the current symptomatic PD treatment by L-DOPA is time-limited and does not delay disease progression. We therefore used our novel SSVEP assay to do a ‘first in vivo’ test of two kinase inhibitors targeted at LRRK2. One inhibitor ‘BMPPB-32’ is a new LRRK2 reference compound characterized by high specificity and selectivity to the LRRK2 kinase domain (see Supplementary Material, Material and Methods). The other compound, ‘LRRK2-IN-1’, is a leading LRRK2 kinase inhibition reagent in biochemical assays, first described in ref. (22). We find that both compounds ameliorate the abnormal visual phenotypes associated with the G2019S mutation.
Finally, the ideal therapeutic compound should be specific for the kinase domain of LRRK2, and not affect other kinases. Achieving this selectivity is possible but challenging because of the more or less conserved nature of the ATP-binding pocket in vertebrate kinases. To test our compounds for non-specific kinase effects, we applied them to the dLRRK− null fly and used the sensitive SSVEP assay to show that LRRK2-IN-1 has stronger off-target effects than BMPPB-32. Our SSVEP approach therefore highlights the possibilities of the Drosophila visual system as an effective model for genetic and therapeutic analysis of visual responses related to human vision.
Contrast-driven SSVEP responses in Drosophila
We recorded the response of the fly visual system to a pre-programmed sequence of frequency-tagged flickering stimuli (Fig. 1). This type of stimulus is commonly used in electrophysiological experiments because it generates frequency- and phase-locked response components with a very high signal-to-noise ratio (19,20,24–28). Our first goal is to understand how the response relates to neural activity in each of the components of the visual pathway (Fig. 1F). In Drosophila, the response is mediated by the retinal photoreceptors which synapse onto the second-order neurons. Some second-order neurons, the amacrine cells, mediate lateral interactions in the network, but the main lamina neurons project to the third-order cells in the medulla. Since the retinal, lamina and medulla neurons are effectively electrically linked in series (29), all of these neurons may contribute to the waveform. The responses from these different stages can, in theory, be disambiguated by the different components they generate in the frequency domain, and this can be confirmed by genetic dissection of the retinal network.
In SSVEP experiments, each frequency-tagged stimulus component with a fundamental frequency of F generates a set of peaks at multiples of F. For example, in Figure 1D, the responses to the input components at 12 and 15 Hz are evident at 12, 15, 24 and 30 Hz. These are coded in green. In this article, we adopt the nomenclature used in previous papers (27,30) when referring to frequency components: each component is identified by the code [harmonic]F[input] where [input] indexes the different input categories (1 = probe, 2 = mask) and [harmonic] indicates the multiple of each input fundamental. For example, the component corresponding to the second harmonic of the ‘probe’ component is 2F1, while the third harmonic of the ‘mask’ component would be 3F2.
The largest responses to the inputs are typically found at the first and second harmonics: ‘1F’ and ‘2F’. For the first input frequency component that we call the ‘probe’, these frequencies correspond to 12 and 24 Hz (‘1F1’ and ‘2F1’).
Signals that respect the polarity of the input can be thought of as weighted Fourier components of the square wave that we use to drive the photoreceptors. These include the first harmonic (1F), and other ‘odd multiple’ harmonics at 3F, 5F, 7F and so on. They must result from an asymmetric response to the light onset and offset and it is most likely that their primary source is the sustained photoreceptor depolarization event elicited by the onset of the periodic light pulse. ‘Even’ harmonics (2F, 4F, 6F, …) on the other hand must be generated by ‘symmetrical’ response to either the onset or the offset of light and we therefore propose that their origin lies in the responses of the second-order lamina cells which generate the transient responses that accompany the on- and off-transients of traditional electroretinograms. Finally, responses at low-order sums and differences of the input frequencies must arise from non-linear combinations of the inputs (31,32). These responses (e.g. 2F1+2F2) are most likely to arise from interactions of the lamina and medulla neurons, where non-linear operations such as rectification or divisive normalization at higher levels of visual processing may occur. It is also possible that they derive from a non-linear transducer stage in the retina.
Isolation of photoreceptor and neuronal responses
To test these hypotheses about the physiological origins of the harmonic signature in SSVEP responses, we used genetic knockouts to isolate individual processing stages. The ort−/− knockout mutation inactivates the histamine A receptors on the second-order lamina neurons, thereby preventing photoreceptor → lamina and photoreceptor → amacrine cell signalling (33). The effect of this mutation is very clear in an SSVEP analysis (Fig. 2), where we compare wild-type (wt) and ort−/− flies. The 1F1 components in wild-type flies (Fig. 2Ai) are moderately reduced in this ort mutant (Fig. 2Aii and iii): at the high end of the contrast range, they are approximately half the wild-type. This reduction in the 1F1 responses may result from the fact that we have abolished the feedback loop from the laminar neurons and amacrine cells that normally regulates photoreceptor function (34).
However, the 2F1 (Fig. 2Bi) responses are almost completely eliminated in the ort−/− flies (Fig. 2Bii and iii)—as would be expected if they were a result of transient synaptic firing. The 2F1 response in these ort mutants is only evident at high contrast, where it is ∼20% of the wild-type, though significantly (one-tail t-test, P < 0.05) above the system noise (Fig. 2Biii). This residual response could arise from a number of sources. Most likely, it derives from the small on- and off-transients in the polarization/depolarization cycle inherent to the photoreceptors themselves [as shown in intracellular photoreceptor recordings, e.g. (35)]. It is also possible that transmission can occur through other, indirect routes (e.g. other types of histamine receptors or gap junctions). A last possibility that this ort mutation does not block photoreceptor output completely seems unlikely based on previous recordings (33). The data from Figure 2 indicate that a significant fraction of the second harmonic response originates from neuronal signalling mechanisms and synaptic transmission rather than from the photoreceptors.
Finally, the fourth-order intermodulation term 2F1+2F2 is entirely abolished, with the level of the signal similar to the system noise (Fig. 2C). This suggests that the 2F1+2F2 signalling is entirely neuronal.
Thus, the 2F1 and 2F1+2F2 components depend on synaptic transmitter release by the photoreceptors and so these components indicate signalling downstream of the sensory neurons. This is encouraging because it allows us to make direct comparisons between neuronal data in flies and vertebrates (particularly humans).
Contrast response functions in Drosophila
We designed our experiments to deliver stimuli similar to those used in earlier human and animal work. These stimuli sweep through different contrast levels, allowing us to measure and analyse population-level contrast versus response functions (CRFs). The measurement of these CRFs is important. Sensory neurons must adjust their sensitivity in order to ensure that they signal changes in the environment efficiently. To achieve this, they scale their sensitivity by a factor related to the average amplitude of the local spatiotemporal input. This ‘normalization’ appears to be a canonical computation that is also found across different parts of the central nervous system and in a wide variety of different organisms (36). When this multiplicative scaling is applied to the input stage, it results in a stereotypical rightward shift of the logarithmic input versus output function of the neuron that has been observed in a wide variety of experiments (25,27,37–41). In addition, some systems also demonstrate ‘response gain’: a scaling of the outputs rather than the inputs, resulting in a downward compression of the response versus contrast curve. It has been observed that both types of gain control can result from a single type of multiplicative input modulation with differences between contrast and response gain control arising from differences in the size and specificity of the gain pool (42).
In the Drosophila experiments described here, we therefore swept the temporal contrast of the ‘probe’ stimuli through a range of values (0–69%) and measured responses to these stimuli both in isolation, and in the presence of a 30% contrast mask at a different temporal frequency. Performing the experiments using ‘frequency tagging’ in this manner allows us to measure the effects of stimulus and mask contrast in isolation even though they are superimposed in the stimulus. If the Drosophila nervous system exhibits the type of gain control found in other organisms, we expect to see a reduction in the probe response when the constant mask contrast is presented at the same time.
Response versus contrast
Robust CRFs from control flies (white eyed, +, day 1) are shown in Figure 3. Unmasked responses (shown in grey) from both the first (1F1) and second (2F1) harmonic components of the swept probe (Fig. 3Ai and ii, respectively) increase monotonically with contrast and we can measure reliable signal amplitudes over the entire non-zero contrast range. This high signal-to-noise ratio is a feature of SSVEP recordings and derives from the fact that hundreds of instances of each stimulus condition are averaged together in a phase-sensitive manner and out-of-band noise can be rejected entirely. Fitting a three-parameter hyperbolic ratio function, as in human studies (25,43), provides a smooth curve that passes through the mean data for both the 1F1 and 2F1 data points (Fig. 3B).
Masking and intermodulation
When a mask contrast is applied at a different frequency (F2), the response to the swept input probe changes. The grey curves show the responses when the mask is absent and are therefore a baseline; the red curves show the response measured in the presence of a constant contrast (30%) mask as the probe contrast increases from 0 to 69% (Fig. 3). Responses at 1F1, 2F1, 2F2+2F1 and 2F2 are shown in Figure 3Ai–iv, respectively, and the fitted hyperbolic ratio curves for 1F1 and 2F1 are shown in Figure 3B. The parameters of the hyperbolic fits (Fig. 3C) show that the nature of these changes is different at the 1F1 and 2F1 component frequencies: in the 1F1 response (Fig. 3Bi), our fitting procedures indicate a reduction in the both c50 (the semisaturation constant) and Rmax (the maximum response). In comparison, the change in the 2F1 component is best modelled as a large change in Rmax with no significant change in c50 (Fig. 3Cii). Masking at the neuronal stage therefore manifests as almost a pure response gain change in this phenotype.
The frequency spectrum obtained from a two-input experiment also contains significant power at sums and differences of the input frequencies. In our data (Fig. 3Aiii), these ‘intermodulation’ responses are maximal around the point at which the mask and probe contrast are equal (30% contrast)—a phenomenon also seen and modelled in our human data (30). When the second input frequency is not present, the response at this intermodulation term is, naturally, flat and gives an estimate of baseline noise levels.
Finally, it is instructive to examine the response at the second harmonic of the mask frequency 2F2 as the probe contrast increases (Fig. 3Aiv). In all our experiments, we see the mask response remaining relatively constant until the probe contrast matches it. The mask response then declines relatively steeply as the probe contrast increases further. Again, this ‘winner-take-all’ normalization is a feature of both human EEG and vertebrate single unit experiments (25,30).
Aberrant responses in the TH>G2019S model of PD at day 1
We have shown that the SSVEP technique generates CRFs that strongly resemble those of humans, that the 1F1 component represents photoreceptor signalling and that the 2F1 and 2F1+2F2 represent separate stages of the neuronal response. Our hypothesis is that the degeneration seen in old flies expressing LRRK2-G2019S was due to hyperactivity at an early age. Therefore, we next ask if the SSVEP technique is powerful enough to reveal small changes due to dopaminergic expression of the PD-related mutation G2019S in the youngest flies, when they are 1 day old. We used the tyrosine hydoxylase (TH) GAL4 (44) to drive expression of either normal human hLRRK2 or hLRRK2-G2019S and compared these with a cross between the GAL4 and wild-type flies (TH/+) that do not express a human transgene. Data are displayed in three figures: the raw data in Figure 4, the fitted curves in Figure 5 and the phase data in Figure 6. Statistical analysis is given in Figure 8 and Table 1.
|Fig||Parameter||TH>G2019S versus TH/+ (no drugs)||TH>G2019S|
|LRRK2-IN-1 versus no drug||BMPPB-32 versus no drug|
|8D||2F1 c50||***||***||P = 0.06|
|Fig||Parameter||TH>G2019S versus TH/+ (no drugs)||TH>G2019S|
|LRRK2-IN-1 versus no drug||BMPPB-32 versus no drug|
|8D||2F1 c50||***||***||P = 0.06|
*P < 0.05, **P < 0.01, ***P < 0.001.
We find that neuronal responses in 1-day-old TH>hLRRK2 flies are largely similar to those in the TH/+ controls (Fig. 4). However, we observe significant changes in the visual responses of the TH>G2019S flies at this very early stage.
Most strikingly, the contrast sensitivity of the TH>G2019S flies is increased dramatically. This results in a shift of the unmasked CRFs relative to the TH>hLRRK2 and TH/+. Because of this shift, the peak neuronal response (2F1 and 2F2) is larger than that seen in controls. The shift in the input/output curve induced by masking is also much larger (Fig. 4B). TH>G2019S flies also show a much larger fourth-order intermodulation term (2F1+2F2, Fig. 4D), though the peak of the response still occurs when the contrast of the F2 stimulus component is ∼30% of F1 contrast.
The increased sensitivity of TH>G2019S flies could result from a change in either the Rmax or c50 parameter. Estimates of these parameters will also provide the basis for statistical comparison of genotypes and, later in the article, of drugs. We therefore performed bootstrapped fits of the hyperbolic ratio function to the TH>G2019S, TH>hLRRK2 and TH/+ data. These fits show that the dopaminergic expression of the PD-associated G2019S genetic variant affects both c50 and Rmax for 1F1 and 2F1 (Fig. 5). The masked Rmax value for TH>G2019S is larger than for the controls (TH>hLRRK2 and TH/+) (P < 0.01), but the unmasked values of Rmax are similar in all three genotypes. This suggests that the masked, but not the unmasked Rmax parameter at 2F1 distinguishes between the phenotypes—most likely because many of the unmasked curves simply fail to saturate at the contrast levels that we are using, making estimates of maximum response level unreliable.
The 1F1 c50 of both the unmasked and masked components is reduced to ∼50% in TH>G2019S, compared with the control flies. The variance is also smaller suggesting consistent differences between the TH>G2019S and control flies. Similarly, the unmasked 2F1 c50 in TH>G2019S is reliably smaller than in the TH>hLRRK2 and TH/+ controls. As the 2F1 component originates from the second-order retinal neurons, we have used this parameter in our statistical tests, and find the difference between the unmasked TH>G2019S and TH/+ is highly significant at P < 0.01.
A second useful index of neuronal activity is the temporal phase of the SSVEP response (Fig. 6). A common feature of CRFs measured in both humans and animals is that response phase advances with increasing stimulus contrast. This phase advance is predicted by simple biophysical models of gain control that treat sensory neurons as leaky integrators with a contrast-dependent membrane impedance (28,45) Although the phase of an SSVEP signal is related to temporal lags in the signal transduction pathway, its interpretation is complicated by the circular nature of the signal (phase advances and phase lags cannot be disambiguated) and also by the fact that changes in the shape of a complex evoked waveform can generate different phase lags in its constituent frequency components.
It is clear that significant phase shifts between the TH>G2019S and TH/+ responses are present at certain frequencies. In Figure 6, we plot the phases and amplitudes of three representative frequency components (1F1, 2F1 and 2F2) near their maximum amplitudes. For consistency, only data from the masked conditions (which contain both F1 and F2 components) are plotted. Responses at 1F1 differ by just <1/6 of a full stimulus period (equivalent to 1/6 * 1/F1 of a second or approximately 13 ms) with the TH>G2019S responses leading those from the TH/+ controls (Fig. 6A). Responses at 2F1 differ by a little more than a quarter of a cycle, yielding a phase lag for the control animals of ∼12 ms (Fig. 6B). Similarly, we observe a phase shift of slightly more than 1/4 of a cycle in the 2F2 responses. Because a single cycle of the 2F2 frequency (30 Hz) is 1/30 s, this phase shift corresponds to a temporal difference of ∼10 ms, but this estimate is less reliable because of the high variance of the G2019S data at this frequency and the relatively small magnitude of the control 2F2 response. These data suggest that the reduced response latency of visual signals in the TH>G2019S flies originates in the photoreceptors and is inherited by the second-order laminar neurons.
In summary, we find that expression of LRRK2-G2019S in dopaminergic neurons has an effect on both the photoreceptor response (F1), and the neuronal signalling of both the second-order lamina neurons (2F1) and of the neurons which generate the 2F1+2F2 signal (which we suggest is the lamina/medulla system). Modelling suggests that the predominant effect is an increase in both maximum masked response amplitude (Rmax) and unmasked response sensitivity (c50) in the TH>LRRK2-G2019S animals associated with a small reduction in response latency. In old flies, SSVEP analysis shows a reduction in visual gain in the TH>G2019S flies, as exemplified by the reduced 2F1 Rmax (Supplementary Material, Fig. S4). These both confirm our previous data collected using traditional flash electroretinograms (9), and show that SSVEP detects both increased and decreased gain in the TH>G2019S flies at young and old ages respectively.
The changes in visual response sensitivity that we observe in flies carrying the TH>G2019S mutation appear to be a biomarker for this mutation. Biomarkers such as these are interesting because they allow us to assay the effect of potential therapeutic compounds targeted at the LRRK2 protein. The G2019S mutation lies within the kinase domain of this protein, rendering it constitutively active. We therefore asked whether the biomarker we have identified in flies could be used to test a novel class of candidate PD drugs that are specifically targeted at the human LRRK2 kinase domain.
Kinase inhibitors restore wild-type CRFs
Previous work has identified LRRK2-IN-1 (22) as a useful reference compound for LRRK2 kinase inhibition. We have now synthesized BMPPB-32, a novel LRRK2 inhibitor identified from the patent literature (Fig. 7A). Biochemical and cell-based assays show potent inhibition of kinase activity of both wild-type and G2019S variants of hLRRK2. In a biochemical Lantha-Screen® assay (Fig. 7B), BMPPB-32 inhibited hLRRK2 and G2019S with apparent Ki's of 1.5 and 6 nm, respectively. In HEK293 cells, apparent IC50 values of BMPPB-32 on LRRK2 WT, the overactive variant G2019S and the LRRK2-IN-1 inhibition-resistant mutant A2016T were 34, 94 and 64 nm, respectively (Fig. 7C). Profiling using large kinase panels suggests that BMPPB-32 is a selective LRRK2 inhibitor at physiologically relevant ATP concentrations, with good selectively for LRRK2 (Supplementary Material, Figs S1–S3, Results, Table S1). We therefore applied 2.5µM BMPPB-32 throughout larval life, finding that 1-day-old flies contained 37 ng BMPPB-32 pr. g fly tissue. (Supplementary Material, Results). We then measured the effects on the SSVEP responses on 1-day-old adult flies which had been treated either with BMPPB-32 or LRRK2-IN-1.
Figure 8A shows the effects of these two kinase inhibitors on the 2F1 components of the frequency-tagged responses in TH>G2019S flies, in comparison with the wild-type (TH/+) and untreated TH>G2019S. Whereas, as noted above, the TH>G2019S has a much steeper CRF, with greater masking, both kinase inhibitors restored the TH>G2019S CRF to a shape similar to that of the control flies. Masking also appears similar to control levels. Both LRRK2-IN-1 and BMPPB-32 affect the complete TH>G2019S phenotype (Fig. 8B, statistics in Table 1), rescuing the photoreceptor (1F1) and neural response (2F1+2F2). We also examined the effect of the kinase inhibitors on the phase of the 1F1 responses, and found that both drugs reverted the phase change of the TH>G2019S flies (Fig. 8C).
We noted previously that the most salient effect of the LRRK2-G2019S mutation was to increase the sensitivity of the unmasked 1F1 and 2F1 responses and reduce the maximum amplitude of the masked response. The sensitivity change corresponds to a reduction in the semisaturation constant (c50) when the curves are fitted with a hyperbolic ratio function, while the amplitude change corresponds to an increase in the Rmax parameter. In Figure 8D, we plot these parameters (c50 for the unmasked 2F1 component, Rmax for the masked 2F1) for the four phenotypes listed above. The reduction in c50 in the TH>G2019S animals is highly significant, while no significant difference is observed between the control (TH/+) and LRRK1-IN-1 drug-treated animals, indicating that this kinase inhibitor is effective in restoring wild-type neuronal sensitivity. Similar effects are measured for the Rmax parameter. The BMPPB-32 kinase inhibitor has qualitatively similar effects to LRRK2-IN-1 with a statistically significant restoration of Rmax and a near-significant increase in c50. BMPPB-32 also reverts the loss of visual neuronal gain seen in 14-day-old TH>G2019S flies (Supplementary Material, Fig. S4).
One problem in any drug assay is off-target effects. For example, an inhibitor targeted at the kinase domain of one protein may bind to other proteins that have similar kinase domain structure thereby rendering its use as a therapeutic agent problematic. To examine the potential off-target effects of the two compounds tested above, we measured SSVEPs in dLRRK− flies, in which the homologous dLRRK gene has been knocked out. CRFs in these flies resemble those of the TH/+ controls (compare Fig. 9Ai and ii). The probe (2F1) and mask (2F2) responses in the dLRRK− flies resemble those of the outcross controls, although the variance is slightly greater. When LRRK2-IN-1 is applied, a severe change in phenotype is seen, with an increase in amplitude of both signals (Fig. 9A and Biii) and strong masking shift (Fig. 9Biii). This may indicate that LRRK2-IN-1 is binding to other kinases, perhaps those related to (mammalian) MAPK7 or DCLK2 (22)—see also Supplementary Material, Figures S1–S3 and Results. However with BMPPB-32, there is no significant change in the CRFs in the dLRRK− knockout (Fig. 9A and Biv), suggesting this compound has no major off-target effects affecting the SSVEP. This is consistent with the clean kinase selectivity profile of BMPPB-32 in mammalian cells (Supplementary Material, Figs S1–S3 and Results).
We have developed and applied the SSVEP technique to Drosophila, measuring contrast-driven responses at multiple visual processing stages. Neuronal responses measured in this manner show an intriguing functional homology with those measured in vertebrates.
Using these SSVEP measurements, we show that the presence of a PD-related human transgene affects visual processing and that such changes are reversible in vivo by drug treatment.
In most respects, our SSVEP data from Drosophila mimic those observed in other species. Our stimulation paradigms were adapted directly from those used in humans (25,27,30,46) and cats (25,28). As in these papers, we found that responses at the probe frequency increased with contrast and exhibited multiplicative masking in the presence of a second, constant contrast component. The response at the mask frequency exhibited a non-linear ‘winner-take-all’ behaviour—remaining relatively constant until the point at which the mask and probe contrasts were equal, then decaying rapidly (25). We also observed robust second (1F1+2F1) and fourth (2F1+2F2) order non-linear intermodulation terms that peaked when the probe and mask were equal in contrast (30). These results suggest that many of the computations involved in luminance and contrast processing are conserved between the early stages of the mammalian and Drosophila visual systems.
We also note some differences between our results and those from other organisms. While we observe strong masking of the neuronal responses, this is best described by response gain control (a reduction in the maximum firing rate) rather than the contrast-gain control (a decrease in sensitivity) as observed in other organisms (25,27,30). The reason for this is unclear. One explanation may be that our response curves do not saturate—possibly because we are using a zero-dimensional stimulus that drives spatially tuned neurons only weakly. In addition, our light source was monochromatic and short wavelength which may have slowed the reconversion of metarhodopsin to rhodopsin in the phototransduction cascade, thereby reducing the overall temporal sensitivity of the preparation (47–49). Without a saturating response, it is difficult to distinguish changes in response gain from changes in contrast gain unambiguously and it is possible that both effects are present in our data. Finally, our stimulus drove responses across much of the visual field exciting essentially all neurons in the visual system to some degree. It is possible that masking in Drosophila depends on the spatial configuration and extent of stimuli as it does in the human visual system (50,51) and that altering the relative sizes of the excitatory and inhibitory stimulus components can lead to a range of contrast response functions that encompass both response- and contrast-gain effects (42).
Effects of the PD-related LRRK2-G2019S
We find that expressing LRRK2-G2019S in the dopaminergic neurons affects neural signalling in the retinal network at day 1, and that this is manifest as an increase in sensitivity, masking, intermodulation and shorter latency. These characteristics suggest that visual neurons may be depolarized, and unable to maintain their resting membrane potential because they cannot synthesize enough ATP to pump cations across the plasma membrane fast enough. Knockout of another PD-related gene, parkin, also results in membrane depolarization (52). Although losses of photoreceptor function and disorganization in the regular lattice of ommatidia have been seen when LRRK2 and other PD-related genes (e.g. α-synuclein) were manipulated (9,53,54), the reason for this neurodegeneration was unknown. Hindle et al. (9) showed that increasing neuronal activity accelerated the loss of photoreceptor function, which was associated with mitochondrial decay and possible problems with ATP synthesis. Now our data provide support for the idea that extra (unregulated) neuronal activity is an early step towards neuronal degeneration.
In Drosophila larvae, the rate of transmitter release at the neuromuscular junction was slowed by LRRK2-G2019S expression (16,17). Our data do not allow us to separate changes in neuronal excitability from changes in transmitter release. However, we note that exogenous dopamine slows the speed of the isolated photoreceptor response (10), while visual response negatively correlates with dopamine levels in the brain (55). Thus, if LRRK2-G2019S expression in the dopaminergic neurons reduces the release of dopamine, we would expect a faster visual response. This is commensurate with our phase data, which showed a decrease in latency of 10–13 ms at the photoreceptors. A similar change in latency was inherited by the second-order neurons.
However, there is a major difference between the previous neuromuscular junction experiments (16,17); and our work in the ways in which LRRK2-G2019S was expressed. Their recordings were made from synapses at which the transgene was expressed both pre- and post-synaptically, but our biggest difference comes from neurons (in the lamina, medulla) in which LRRK2-G2019S was not expressed. Rather the lamina and medulla neurons surround the dopaminergic neurons in which the G2019S transgene was expressed. An impact on surrounding neurons of expressing G2019S suggests non-autonomous actions, as proposed by Braak et al. (56) and reported from human and animal studies of another PD-related gene, α-synuclein (57–59).
In-vivo testing of kinase function of LRRK2
A consistent feature of LRRK2-G2019S in biochemical assays is elevated kinase activity compared with wild-type hLRRK2 (21). Our day 1 visual data also show a marked difference between TH>hLRRK2 and TH>G2019S, which is reduced by kinase inhibitors LRRK2-IN-1 and BMPPB-32, suggesting that the kinase function is crucial in the whole organism as well as in biochemical and cell culture assays. The use of dLRRK− flies, in which no LRRK homolog is present, discriminates the two inhibitors, BMPPB-32 and LRRK2-IN-1, with the latter providing marked off-target effects.
A wide range of kinase inhibitors are being developed for inhibition of LRRK2 (60,61), but little testing has been done in the whole organism. Two kinase inhibitors, GW5074 and sorafenib, have been tested in Drosophila lifespan and climbing assays, and provided some extension of lifespan (62). Their median lifespan was 12–13 weeks, making our fly assay much faster (<14 days treatment, <1 h recording/and analysis per fly).
Our SSVEP assay therefore provides a robust and stable platform, with quantitative outputs that gives insights into basic neuronal processing, mechanisms of gain control, suggesting homology in retinal function between flies and mammals, and can be used for assessing the effectiveness and selectivity of drug development candidates.
MATERIALS AND METHODS
Fly stocks and genetics
Stock vials of Drosophila melanogaster were kept on yeast-sucrose-agar fly food (63). The GAL4/UAS system was used for targeted gene expression using the UAS-constructs UAS-hLRRK2 or UAS-G2019S (64), and TH-GAL4 (44), to achieve expression in the dopaminergic neurons. For controls, we crossed TH-GAL4 with a white-eyed wild-type (w1118, herein +). These fly crosses were used for direct comparison with our previous paper (9). The dLRRK− line, also known as e03680 (65,66), is a complete knockout of the fly homolog to LRRK2. In developing the technique, we also used the histamine receptor A null ort−/− (ortUS6096, also known as ort5) strain (33). As these flies have white eyes, we compared their visual response with w1118 flies. Experimental flies were kept in a 12 h light on:12 h off constant temperature room (25°C), and allowed to lay eggs on instant fly food (Carolina Biological Supply). After 2 days, the adult flies were removed. Vials were inspected daily, females collected and the visual response was tested within 10–18 h of eclosion.
Female flies were aspirated out of their vial into a shortened pipette tip. Once the head emerged, each fly was secured with a small droplet of nail polish (Creative Nail Design) (Fig. 1A) and the fly was allowed to recover in the dark for ∼10 min. The fly was illuminated by a blue light from an LED. The intensity was controlled by a sequence generator encoded in Matlab. In some parts of the sequence, we delivered a single square wave, flickering about the mean illumination at a frequency of 12 Hz. This stimulus we called the ‘probe’. In other parts of the sequence, we delivered a wave made up of the sum of two square waves, one at 12 Hz and the other at 15 Hz. In this case we refer to the second, 15 Hz component as the ‘mask’. We used square waves to reflect the stimuli used in our earlier drosophila ERG experiments (9) and SSVEP experiments with human subjects (30). We placed a single electrode on the surface of the Drosophila eye and recorded a single waveform in response to each stimulus sequence. Each stimulus sweep gave a signal well above the system noise, so that a phase-locked response is evident (Fig. 1C). These time series were analysed in the frequency domain using a Fourier transform from which the scalar amplitude (Fig. 1D) and phase, or temporal ‘lag’ (Fig. 1E) of individual frequency components was extracted (67). Full details of the apparatus and recording method are presented in Supplemental Material.
We tested the effect on fly vision of two potential LRRK2 kinase inhibitors. One, BMPPB-32 is novel and fully characterized in the Supplementary Material. There we report its synthesis, biochemical and cellular properties, role as a kinase inhibitor and uptake to the CNS. The second inhibitor is LRRK2-in-1 (Division of Signal Transduction Therapy (DSTT) Unit at the University of Dundee). Stock solutions in ethanol were kept at −20°C until the day of use. The solution was diluted into de-ionized water, and mixed with the Instant Fly Food compound, making the final concentration 2.5 μm. Adults (8–10 female, 3–5 male flies) were allowed to lay eggs on the food for 2 days and then removed. Larvae were allowed to develop into pupae, and the emerging adults collected between 4 and 18 h.
This work was part-funded by the Wellcome Trust (ref: 097829/Z/11/A). Funding to pay the Open Access publication charges for this article was provided by the York Open Access Fund.
We are grateful to Wanli Smith, Serge Birman and Roger Hardie for gifts of flies, and to Sean Sweeney, Heidi Baseler and John Sparrow for their comments on draft manuscripts.
Conflict of Interest statement. K.V.C., G.P.M. and M.H. are full-time employees at H. Lundbeck A/S. F.A., C.J.H.E., O.M.N. and A.R.W. declared no funding received from any company; Lundbeck supplied compounds for testing gratis.