Tooltip on top First of all , the Model is an essential part of any project that aids in understanding parts function and kinetics and predicts their results depending on primary experimented parametric values for their specific differential equations. On the other hand, the project design may affect model results by integrating new parts that have a significant effect on the project.Moreover, these results help you in adding new conditions for fitting the new design. [5].
For our project this year, we have constructed 4 main models using Ordinary differential equations (ODEs). Three of them describe parts’ function and activity, while the last model describes cells’ behavior and their interaction based on assumptions that we have conducted by the aid of experimental literature. These models are:
Take care while exploring our models 1 and 2 from the dimensions of our graphs considering the time frame and the population scale level , as they show a sequence in their actions and values for getting the most satisfactory prediction for our project results.
We simulate the kinetics of Vascular endothelial growth factor (VEGF) and its receptors by ODEs[1]. The receptor is composed of the 2 different chains: VEGFR-1 and VEGFR-2. Firstly, the ligand binds to VEGFR2 because the binding affinity of VEGFR-2 to its ligand is higher compared to that of VEGFR-1[2]. Eventually, this binding dimerizes the receptor chains leading to activation of the internal domain parts which consists of tobacco etch virus (TEV) protease. This protease cleaves at its own cleavage sites on the internal domain chains to release the dCas-9 system.
This model is concised into 7 equations that describe the binding kinetics of VEGF initially to VEGFR-2, then binding of this (VEGF - VEGFR-2) complex to VEGFR-1 to form (VEGFR2-VEGF-VEGFR1) complex. Sequentially, the complex initiates the dimerization of the receptor’s chains to activate the TEV protease that releases the dCas-9 system.
This system is composed of 7 main population:
R2 | Vascular endothelial growth factor receptor 2 (VEGFR2) |
R2A | Complex of VEGF and VEGFR2 |
R1 | Vascular endothelial growth factor receptor 1 (VEGFR1) |
R2AR1 | Complex of (VEGF-VEGFR2) and VEGFR1 |
D | Dimerization process of VEGFR chains |
TEV | Tobacco etch virus protease |
C | dCas-9 system |
Description | Value | Units | Reference | |
---|---|---|---|---|
K1 | Rate of binding between vascular endothelial growth factor (VEGF) to VEGFR2. | 0.9 | M−1 s−1 | [3] |
K2 | Rate of binding between (VEGF-VEGFR2) complex to (VEGFR1). | 4.4 | M−1 s−1 | [3] |
K3 | Dimerization rate of VEGFR chains. | 2.1 | cm2 mol−1 s− 1 | [3] |
K5 | Cleavage rate of TEV protease to release dCas9 system. | 1 | units | [4] |
K6 | Rate of activation of Cas9 system. | 0.001 | s−1 | [-] |
RD1 | Rate of dissociation of (VEGF-VEGFR2) complex. | 1 | s−1 | [3] |
RD2 | Rate of dissociation of (VEGF-VEGFR1) complex. | 0.00132 | s−1 | [3] |
d1 | Rate of degradation of VEGFR2. | 0.1 | s−1 | [3] |
d2 | Rate of degradation of (VEGF-VEGFR2) complex | 0.1 | s−1 | [3] |
d3 | Rate of degradation of VEGFR1. | 0.09 | s−1 | [3] |
d4 | Rate of degradation of (VEGFR2-VEGF-VEGFR1) complex. | 0.5 | s−1 | [3] |
d5 | Degradation rate of TEV protease activator. | 0.01 | s−1 | [4] |
d6 | Degradation rate of the dCas9 system | 0.05 | s−1 | [5] |
The first equation describes free VEGF receptor -2 (R2) which decreases upon:
The second equation describes (VEGF-VEGFR2) complex (R2A) which decreases upon:
The third equation describes free receptors-1 (R1) which decrease upon:
The fourth equation describes (VEGFR2-VEGF-VEGFR1) complex (R2AR1) which decrease upon:
The fifth equation describes the receptor dimerization process at rate (K3) as in graph(2). This process vanishes by binding complex (R2AR1) degradation at rate (d4).
Thesixth equation describes TEV protease activation (as in graph (2) ) upon dimerization of VEGFR chains at a rate of (K3). This binding results in an increase of free TEV concentration. However, this concentration is decreased by normal TEV protease degradation rate at (d5).
The seventh equationseventh equation describes TEV protease activation sequelae in which the free dCas-9 system concentration increases ( as in graph (3) ) in response to the protease action on cleavage specific sites at the receptor’s internal domain (TCS1 and TCS2) at rate of (K5). The activation of the dCas-9 system depends on the integration of the dcas-9 N-terminal and C-terminal at rate (K6). Nevertheless, dCas9 concentration decreases by the normal dCas-9 system degradation rate (d6).
In order to choose the suitable VEGFR to reach the full activity of our internal domain parts, we noticed that TEV protease activity depends on the dimerization rate of the receptor’s chains.Dependently, this dimerization rate depends on the receptor-ligand binding state; thus, we held a comparison between the 2 forms of the receptor:
Homodimer receptor ( VEGFR1- VEGFR1) | Heterodimer receptor ( VEGFR2- VEGFR1) |
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From the previous table, we can conclude that the binding affinity of the heterodimer receptor is higher and more stable than the homodimer receptor as in graphs(2,5). Sequentially, the dimerization level in the heterodimer receptor is higher which reflects higher activation of the receptor internal domain including TEV protease and dCas9- system as in graphs(3,6).
In this model, we simulate the kinetics and the sequel of activity of the internal domain upon dimerization of the 2 separate domains of vascular endothelial growth factor receptor (VEGFR), activating Tobacco Etch Virus (TEV) protease which releases both N-terminal and C-terminal of dCas9 system and leading to their binding and activation [1]. The dCas9 system is uploaded with 3 different transcription activators (VP46, GAL4 and CMV trans-enhancer) to show a satisfactory level of YAP-1 [2], resulting in optimum proliferation and differentiation of stem cells.[2]
In other words, this system consists of 5 populations. Briefly, our system started by releasing and activating the dCas9 system, uploaded with 3 different transcription activators (VP46, GAL4 and CMV trans-enhancer) which induces the YAP-1 transcription.
In addition to the population mentioned in model (1), the activation level of the different transcription activator and YAP-1 transcription used in that model based on the results of model (1). So the activation of the external domain till production YAP-1 is sequential .
C | dCas system |
VP | VP64 transcription activator |
GAL | GAL4 transcription activator |
CMV | CMV trans-enhancer |
YAP | Yes associated proteins-1 |
Description | Value | Units | Reference | |
---|---|---|---|---|
K7 | Rate of activation of CMV trans-enhancer for transcription of YAP. | 5.4 | M−1 s−1 | [3] |
K8 | Rate of activation of VP64 transcription activator for transcription of YAP. | 1.8 | M−1 s−1 | [4] |
K9 | Rate of activation of GAL4 transcription activator for transcription of YAP. | 1.3 | cm2 mol−1 s− 1 | [3] |
d7 | Natural degradation rate of CMV trans-enhancer. | 0.15 | units | [-] |
d8 | Natural degradation rate of VP64 transcription. | 0.2 | s−1 | [-] |
d9 | Natural degradation rate of GAL4 transcription activator. | 0.05 | s−1 | [-] |
d10 | Rate of dissociation of (VEGF-VEGFR1) complex. | 3.3 | s−1 | [5] |
The first equation escribes the cytomegalovirus (CMV) trans-enhancer activation. This CMV trans-enhancer depends on the activation of the dCas-9 system, yielding the optimum level of YAP-1 at the rate (K7). Moreover, its activity is decreased by the normal CMV trans-enhancer degradation rate (d7).
The second equation describes the activation level of VP64. When the dcas-9 system is activated , VP-64 is turned on leading to release of an optimum level of YAP-1 at rate (K8). This activation is decreased by normal VP-64 degradation rate (d8).
The third equation describes GAL4 activation level. Upon activation of dCas-9 system, GAL4 is stimulated, meanwhile releasing optimum level of YAP-1 at rate (K9), respectively. In contrast, this activation is decreased by normal GAL-4 degradation rate (d9)
The last equation in that model describes YAP-1 transcription level that determined by activation rates of CMV trans-enhancer , VP64 and GAL4 transcription activators at rate (K7,K8 and K9) respectively. In contrast, this activation is decreased by normal GAL-4 degradation rate (d10).
These equations represent the kinetics of binding of VRGF to its receptor and the dimerization process for activating the internal domain parts till releasing of the dCas-9 system (d4).
Graph (7) Illustrates the relation between activation levels of different transcriptional activators (CMV trans-enhancer , VP64 and GAL4) for transcription for YAP-1 (d4).
In order to choose the most satisfactory level of YAP-1[2] production for proliferation and regeneration of the stem cells, we held a comparison between 3 different transcription activators ( CMV trans-enhancer , VP64 and GAL4) showing different levels of YAP-1 based on parameter values supported by literature excremental date [3] as shown in the following table. This directed us to choose the integrated design of these 3 different transcription activators as this integrated design reaches the satisfactory level of YAP-1 in small time (20 unit time).
CMV trans-enhancer only which shows high expression level of YAP-1, but not the optimum level, reaching 12800 at 65 time units | VP64 transaction activator only which shows very low expression level of YAP-1 , reaching 1050 at 55 time units. |
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GAL4 transcription activator that shows low expression level of YAP-1, reaching 2000 at 75 time units. | Three transcription activator Integrated design which shows the optimum level of YAP-1, reaching 16000 at 65 time units. |
As there are different types of transcription activator that can be used, as mentioned in the previous table :
This model gave an advantage to integrate and compare different transcription activators (CMV trans-enhancer, VP64 and GAL4) by ODEs based on our parameter values. Thus, this model is considered to be modular.
Future teams can use these designed ODEs for testing the same or different transcription activators’ activity for protein production and estimating parameter values to fit their design.
In order to make these ODEs and the model accessible for Igem teams, we have made a user-friendly online interface tool to allow the users to edit the parameter values for their aimed transcription activator.
In this model, we simulate the kinetics and activity of our MMP-9 specific mRNA switch, which is activated upon binding of matrix metalloproteinase 9 (MMP9) to its MMP9-nanobody through changing into circular form [1](as shown in figure ()) for initiating transcription of YAP-1 in the targeted cells.[1].We prevent the switch’s basal activity through adding hammerhead ribozyme (HHR)[1] to our design.[2]. In presence of HHR. the switch will not circularize except in the existence of MMP-9, forbidding the poly A tail to bind with the mRNA cap. Indeed, the HHR undergoes self-cleavage which cleaves the following poly A tail , allowing proper switch on-off state transition, respectively. However, in absence of HHR, no cleavage will happen which means the presence of poly A tail, permitting switch’s basal activity[1].
In other words, this system consists of 6 populations. Simply, the MMP-9 (MMP) binds to MMP9-nanobody (RA) to form a binding complex (CO). This combination promotes switch circulation (CF) in presence of poly A tail (PA) that maintains its stability and HHR to control basal activity of poly A tail for production of the only desired amount of (YAP).
MMP | Matrix metalloproteinase9 |
RA | Free MMP9-nanobody |
CO | Binding complex of MMP-9 and RA |
CF | Circular form of the switch |
PA | Poly A tail |
HHR | Hammerhead ribozyme |
Description | Value | Units | Reference | |
---|---|---|---|---|
K11 | Rate of binding between MMP-9 and MMP-9 nanobody. | 0.9 | M−1 s−1 | [2] |
K12 | Rate activation of poly A tail to initiate and maintain the stability of the switch. | 4.4 | M−1 s−1 | [1] |
K13 | Cleavage rate of HHR to separate poly A tail. | 2.1 | cm2 mol−1 s− 1 | [3] |
K14 | Rate of circulation of the switch. | 1 | units | [-] |
K15 | Activation rate of MS2 aptamer. | 0.001 | s−1 | [1] |
d11 | Degradation rate of MMP-9. | 1 | s−1 | [-] |
d12 | Degradation rate of MMP-9 nanobody. | 0.00132 | s−1 | [-] |
d13 | Degradation rate of the binding complex. | 0.1 | s−1 | [-] |
d14 | Degradation rate of the circulating form of the switch. | 0.1 | s−1 | [-] |
d10 | Degradation rate of YAP. | 3.3 | s−1 | [4] |
The first equation describes the decrease in free MMP-9 level. This is due to:
The second equation describes the decrease in the number of free MMP-9 nanobodies (RA) . This is due to:
The third equation describes the number of binding complexes that have been obtained from MMP-9 binding to MMP-9 nanobody (RA) at rate (K11). In addition to the normal degradation rate of these binding complexes at rate (d13).
The fourth equation describes the switch’s ability to circularize which happens once the MMP-9 binds to MMP-9 nanobody (RA) at rate (K11). By the aid of poly A tail at rate (K12) for maintaining the switch stability before the circulation. Once circulation is initiated:
The fifth equation describes MS2 aptamer activity that increases upon binding of free MMP-9 nanobodies to MMP-9 at rate (K11) and maintains the switch stability that happens after the switch circulation at rate (K15). In addition to the normal degradation of the switch circular form at rate (d14) and degradation of MMP-9 at rate (d11).
Thesixth equation describes the HHR function in decreasing the poly A tail activity before circulation. Through cleaving specific cleavage sites before PA at rate (K13). In addition to the normal degradation of the switch circular form at rate (d14).
The seventh equation describes the YAP-1 translation which depends on:
To sum up, the activation of our conditioned switch through presence of MMP-9 results in changing the switch into its circular form for transcription of the normal amount of YAP-1 in the targeted cells [5].
The first design of the switch did not include the hammerhead ribozyme (HHR) part, so we modeled the parts as it was. After further searching, we found that presence of poly A tails increases basal activity of the switch (which impair the on and off state) that can initiate transcription of YAP-1 in absence of MMP-9 to activate it which is supported by literature experimental data[1]. so we have changed our design to add an HHR part for safety conditioned production of YAP-1, as shown in the next figures with and without basal activity of the switch.
With HHR | Without HHR |
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The comparison between presence and absence of HHR part shows:
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Description | Value | Units | Reference | |
---|---|---|---|---|
K1 | Rate of expression of external domain (CCP1) Syn-Notch receptor | 0.001 | day-1 × cell-1 | [1] |
K2 | Reciprocal rate of degradation of MSCs | 15 | Days-1 | [2] |
K3 | Rate of formation on Syn-Notch receptor on MSCs | 0.033 | Days-1 | [1] |
K4 | Rate of the binding state between S and B | 0.000411 | day-1 × cell-1 | [3] |
K5 | Reciprocal rate of degradation of autoreactive B-cell | 30 | Days | [4] |
K8 | Rate of dissociation of the binding state | 0.001 | day-1 | [-] |