Why?

Hockberger et al (1999) mentions that blue light up to 470 nm can have some phototoxic effect on mammalian cells, from 2 to 6 J/cm². In our experiments, we have observed a much higher cell death rate in the cultures exposed for 24h to blue light than in the control. 24h at 20 mW/cm² is actually more than 1700 J/cm². To stay on the safe side, and give the cells 6 J/cm² during 30 minutes would mean an average of 32 W/m².

• How much activation can we get with this irradiance?
• Will we have to push it up?
• What lighting pattern will give best results?

Other papers, like Marchesini et al (1989) say that 500 W/m² at 488 nm for 5 minutes does have more a beneficial effect than phototoxic. But we will need to lit the cells probably for hours to clearly see that the reporter production switched on.

What?

To optimize the number of experiments to perform, we have built a simple model to predict the proportion of photoproduct to be expected at every time point when a sample with LovTAP-VP16 when it's illuminated with a time varying light input.

How?

Photoactivation

When light with the appropriate wavelength goes through a solution with photoactive molecules, a number of photons will be absorbed and transfer their energy to these molecules. In the case of LovTAP-VP16, this energy will favor the conformational change into the “active” state. To simplify the model, we will suppose that the protein will be active once it has received enough energy.

The molar absorptivity, also know as extinction coefficient, ε, is directly related to the absorption cross section, σ, through the formula:

If ε is in L mol⁻¹cm⁻¹ we get σ in m². From it we can calculate the number of photons absorbed by a material per unit of distance traveled by the light, using the expression:

where N is the number of photons, m is the number of absorbing molecules per unit volume and x is distance. We can write m in terms of the amount of molecules in the differential volume dV:

and, being dV=Adx, with A the area seen by the light:

Now, the quantum yield of the LOV2 domain, Q, is defined as the number of photons of a particular wavelength required to trigger the photoactivation of one protein. We can use it to relate dN with dM_phot, the variation of the amount of photoproduct:

substituting in the previous expression and defining F as the photon flux:

Activation parameters

Fig. 1: Comparison of the activation dynamics produced by the model and the experiments by Kasahara

From Kasahara et al (2002) (Table I) we took the values of ε = 14000 L mol⁻¹cm⁻¹ and Q = 0.34 for the LOV2 domain in phot1 in Arabidopsis and rize. They are very similar to the LOV2 domain in phot1 from Avena sativa, according to Salomon et al (2000). Kasahara used a 446 nm light source, at 80 µmol m⁻²s⁻¹, what is 22 mW/m². Using those parameters provides the same dynamics they observed in their experiments for the first 10 s at 4ºC.

To adapt this parameters to the wavelength of the LEDs we are using, the molar absorption has to be modified. Absorbance at 470 nm, the wavelength of the LEDs we use, is some 75% (Kasahara et al (2002), Fig. 1) of the peak absorbance at 450 nm. Since the extinction coefficient, or molar absorption, is proportional to the absorbance, we can expect ε to be between 8000 and 10000 L mol⁻¹cm⁻¹.

Deactivation

Since the ground state, or dark state, is more stable, active LovTAP-VP16 proteins will spontaneously release a photon and go back to the dark state. This rate is parametrized with the half life of the photoproduct.

Deactivation parameters

For 4ºC, Kasahara et al (2002) don't provide the value for the half life of the photoproduct, but just the proportion of photoproduct in the equilibrium: 0.91. The half life that allows for this is τ_1/2 = 200 s. To verify whether this setup makes sense, we compared it to the experimental results presented in Salomon et al (2000). The only experiment they did with monochromatic light was at 445 nm and 55 µmol m⁻²s⁻¹, or 15 mW/m². In ice, they measured a k_ppf/k_reg of 17, being k_reg = ln(2)/τ_1/2 and k_ppf = F Q σ. Using the previously selected parameters, our model yields exactly 17 for that value.

In both references it's said that the half life at room temperature is around 30 s. That would mean the half life being divided by 3.33 every 10ºC. It would then be reasonable to think that at physiological temperature, more than 10ºC over room temperature, the half life would drop to around 9 s.

Interestingly, fig. 3 shows that, at 24ºC, our model predicts a 95% of photoactivation at 180 W/m², quite in line with the 200 W/m² saturation irradiance (470 nm) at room temperature mentioned by Strickland et al (2008) for LovTAP. This suggests that our model, built with parameters from the isolated phot1 LOV2 domain, might also provide useful information to predict the photodynamics of LovTAP, what gives us some hope of having it work for LovTAP-VP16.

Results

Fig. 2: Time evolution of the proportion of photoproduct with 4 different lightning cycles.

Fig. 3: Photoproduct in the equilibrium (after 200 s) for different irradiance values at 4ºC, 24ºC and 37ºC.

Let's try to answer the questions proposed in the beginning:

• How much activation can we get with this irradiance?

We said we could go up to 32 W/m², or 3.2 mW/cm², with no damage to the cells. Unfortunately, according to the model we will get only around 45% activation (see fig. 3).

• Will we have to push it up?

Depending on the difference of DNA affinity of LovTAP-VP16 it might be interesting to experiment with irradiance values over this, to get a higher photoconversion. Pushing it up to 200 W/m² we should be getting a 85% photoactivated LovTAP-VP16. To understand the effect the photoactivation level might have on the expression, we implemented a simple mass-action model.

• What lighting pattern will give best results?

Until now we have talked about average (in time) irradiance values but, with our current bioreactor setup, we can only achive this by pulse modulation. As seen in fig. 2, the pulse width doesn't affect to the average photoactivation. But cycles of several seconds involve minimal intant photoproduct proportion of under 25% (as in fig. 2 for 25% duty cycle and 20 s cycle length). If a LovTAP-VP16 dimer requires several seconds to bind DNA and recruit all the transcription machinery (as suggested by Darzacq et al), it might be beneficial not to use cycle lengths near the half life of the LovTAP-VP16 photoproduct, or else the recruitment might tend to be aborted at the photoactivation minima.