Team:Waterloo/Math Book/sRNA
From 2014.igem.org
Math Book: Silencing RNA (sRNA)
- Overview
- Model Formation
- Model Reduction & Steady State Behaviour
- Parameters
- Results
- Sensitivity Analysis
- Conclusion
The ordinary differential equation model for small ribonucleic acid (sRNA) gene silencing was formulated for the purpose of:
- Model Formation
- Model Reductions and Steady State Behaviour
- Parameters
- Results
- Sensitivity Analysis
- Conclusion
Model Formation
Inspiration for the model came from the metabolic pathway reported in the literature by Abia in 2007 [17]. In the network, sRNA binds to by Hfq, a chaperone protein which increases the binding rate between sRNA and its target mRNA substantially. Once bound, the Hfq-sRNA-mRNA complex is broken down by a degradosome, a specialized quaternary structure in sRNA-regulated gene expression.
At least, this is how the pathway works in E. coli. A major difficulty is that Hfq in S. aures doesn’t seem to play any major physiological role [18]. To make matters more difficult, the existence of a chaperone protein for sRNA in S. aures has yet to be discovered [19]. Additionally, the proteins that make up the degradosome in E. coli are not present in S. aures..
Our solution to these problems was to simple provide Staphylococcus aures the Hfq present in E. coli. In this way, a model of sRNA gene-regulation could be implemented to aid with laboratory design, and respond to the purposes of the model. Since Hfq would need to be expressed in the target cell, the reaction network took the form of Figure X.
Applying the usual mass action to the reaction network in Figure X, we arrive at the model equations:
This model somewhat lends itself well to the idea of simplification, primarily between all of the complexes. In particular, we will apply a quasi-steady state assumption to the movement between all of the complexes. Recall the dynamics of the complexes seen in the network diagram, we can simplify this a little by first considering the dynamics of the complexes as a single, isolated system, showcased in the figure below.
If we assume no source and no degradation of H (for now), the above system is modeled as:
One simplifying assumption would be if, rather than a Michaelis-Menton term, we had a mass-action term. We don't need to make this assumption, but it makes the rest of the calculations a whole lot easier (as in, will fit between the margins easier). Consider the first-order approximation to the Michaelis-Menton Term:
With this approximation (which is valid for small H ms - something we can look into), the model reduces nicely. In particular the flux term is approximated as:
The significance of this flux is that it represents the steady state breakdown rate of mRNA and sRNA. What we would really like is this flux term as a function of m and s so that we don't have to keep track of the entire Hfq-complex pathway in our model.
Since we are assuming that flux through this pathway happens much faster than transcription and translation, let's also consider the total amount of Hfq to be constant and given by HT = H + Hs + Hms, then we apply the Quasi-Steady State Assumption to our model:
Written more compactly:
The Least Squares Solution to this equation is:
Which means that the flux term is approximated as:
Which means that the flux term is approximated as:
Where K = k3/(k1k2) and note that k3HT plays a similar (identical, actually) role to Vmax in the Michaelis-Menton formula. Note, however, that J(0,0)is not defined. Observe that:
So a more precise definition of J is:
With all of these simplifying assumptions, the model reduces to:
Here, we've lumped all of the Hfq protein complexes (and, indeed, Hfq) into one species. Notice that the last two differential equations are independent of the rest. Even better - they're Linear! And even better, it's a 2x2 system! Focusing on the last two equations, and rewriting them in vector-matrix form:
Focusing on the homogeneous part, notice that the matrix is lower triangular. Thus, the eigenvalues appear on the diagonal, λ1 = - βH; λ2=-βh. The corresponding eigenvectors are:
and thus, the fundamental matrix is:
With the Fundamental Matrix, we can apply Variation of Parameters to get the general solution for HT. Since the transcription for Hfq is under the control of our wonderful experimenters, we could chose to have αh be some arbitrary function of time. Regardless, the general solution is:
Where h(0), and HT(0) are initial conditions on h and HT.
This looks pretty crazy, but it gives us a way of solving for HT, written explicitly:
The input is how αh varies with time. If we take the initial conditions to be zero for each we get:
At any rate, the point here is that we can solve for exactly how HT varies with time. We can also see that if our forcing term is constant, then we'll settle down to a steady state concentration of HT. Additionally, our original model then reduces to three equations:
Notice also that the last equation doesn't really influence the dynamics of the first two. Additionally, if HT is constant, then it's possible to observe a phase-portrait of the system. It's also possible to look at possible steady states. In other words, we can actually do some analysis on this model without physically being supercomputers.
Parameters
We identified parameters in the literature. The identified parameters and their sources are given in the table below.
sRNA Parameters from Literature
Parameter | Value | Description | Reference |
αm | 1/600 (nM*s)-1 | transcription mRNA | Fender et al. |
αs | 1/600 (nM*s)-1 | transcription sRNA | Fender et al. |
αM | 1/600 (nM*s)-1 | translation of the mRNA | Fender et al. |
k-1 | 0.7*10-4s-1 | dissociation constant of Hs to Hfq and sRNA | Fender et al. |
k1 | 106 s-1 | association constant of Hfq and sRNA | Fender et al. |
k2 | 3.5*106 (M*s) | association constant of Hs and mRNA | Fender et al. |
k3 | 0.7*10-4 s-1 | dissociation constant of Hms, assuming that the Hfq-sRNA binding to the MicC region is independent of the sRNA, mRNA binding. Therefore, k-1=k3 | Fender et al. |
Sensitivity Analysis
To get a better handle on the dynamics of the system we ran a local sensitivity analysis. This determined what parameters the sRNA system is most sensitive to. The flux control coefficients for the sRNA system can be seen in the figure to the right.
The most notable of these are the flux control coefficients for the following parameters: β M(the degradation rate of YFP), αm (the transcription rate of the YFP mRNA transcript). These large flux control coefficient values demonstrate the system is highly sensitive to changes in these variables. So if we were able to influence these rates we would be able to dramatically change the level of flux through the system. In order for the sRNA system to be most efficient, we would want the flux of YFP through the system to be as low as possible. The most direct way to affect this would be to alter these values. Since we want to use this sRNA system to control a protein whose degradation rate and transcription rate we could not alter, we need to turn our gaze to some of the other factors at play.
Some other notable flux control coefficients are αS, αM, K-1. These rates are respectively the rate of transcription of sRNA, the rate of translation of YFP, and the rate of dissociation of Hfq-sRNa to Hfq and sRNA. These rates have the least impact on the system and are not good targets for optimization of sRNA.