Difference between revisions of "Team:NTU-LIHPAO-Taiwan/Modeling/Pathways"

Pathways

Bacteria Distribution

After orally ingested, certain amounts of modified probiotic L. casei can survive under acidic condition when passing through the stomach. Owning to gastric acid, bile acid and digestive enzymes, the presence of these bacteria in the intestinal tract only last for a limited time, and then are washed out in faeces.[1] During the process of transition and temporary colonization, CPP-PYY complexes are produced and exert the following effect.

In the first part of Modeling, bacteria distribution in the small intestine was investigated. Data from Y. K. Lee et al. showed bacteria number against time adhered on the duodenum, jejunum, and ileum in mouse after orogastric intubation of 10⁹ L. casei, which were listed in Table 1.[2] The ratio (%) of bacteria number to total ingested number were also indicated.

By summing up all the data, we constructed the continuous L. casei cell number and ratio distribution against time with curve fitting in MATLAB, as shown in Figure 1 and Figure 2. However, experimental data from suicide mechanism should be incorporated to complete the simulation for the survival of L. casei in the small intestine, here we hypothesized 4 days for bacteria survival. Therefore, the time-changing tendency could be utilized for further calculation.

CPP-PYY Penetration Efficiency

Knowing the L. casei cell number variation against time, we proceeded to simulate the penetration process for the cell penetrating peptide, TAT with PYY. Although the mechanism for TAT translocation depends on the delivery cargo and its sequence, most papers concluded that it is mediated via an as yet uncharacterized pinocytosis/endocytosis related mechanism, which is also receptor-independent.[3]

From the experimental results of CPP-PYY complex production rate, here we took 10^-10 μg/min for example, we hypothesized that one-fourth of the products secreted from the cell will be distributed in the direction toward the small intestine epithelial cells, not being digested by the intestine fluid. Liang JF et al. had provided the effective permeability (P_eff) of insulin-TAT conjugates by in vitro intestinal absorption assay on cultured Caco-2 cell monolayer, which was calculated based on the following equation.[4]

where J_ss is the flux, dC_R/dt is the change in conjugate concentration in the receiver chamber at steady-state, V_R is the volume of receiver buffer, A is the cross sectional area of the exposed tissue, and C_D is the conjugate concentration in the donor chamber. The result from Liang JF et al. was applied for modeling, where the value of P_eff was presented as 1.62×10^-5 (cm/s). Moreover, the results further implied that the product seemed to be through a transcytosis-like mechanism, which confirmed that our product can penetrate through the epithelial cells.[4]

The parameters for calculation were obtained from literatures, where L. casei cell size range = 0.7-1.1 x 2.0-4.0 micrometer[5], and the mean total mucosal surface of the small intestine interior averages 32 m² in recent research.[6] One-tenth of the bacteria height multiplied the mucosa surface area were taken as the donor chamber volume, where one-fourth of the CPP-PYY complexes without being digested were oriented toward the epithelial cells. Finally, the flux of the complexes through villi was achieved, giving us the result of mass rate per unit volume. Figure 3 demonstrates the amount of penetrating CPP-PYY complexes though villi against time.

Adsorption Into Bloodstream

The inside wall of the small intestine is lined with villi that contain blood capillaries to carry away the absorbed molecules for circulation. The large surface area contributed by these villi allows absorption to happen quickly and efficiently.

To evaluate the efficiency of the adsorption, we incorporated “Fick’s Law of Binary Diffusion”, where the diffusion coefficient was given by S R Chary and R K Jain.[7] Taken in calculation for our simulation, the measured interstitial fluid diffusion coefficient (D_AB) of bovine serum albumin in tissues was 5.8 x 10^-7 cm²/s as reference.

where J_A is the mass flux of CPP-PYY complex, ρ is the density of villus cavity, and ω_A stands for the mass fraction of the complex. The direction of diffusion was considered from the epithelial cells toward capillaries, signified as y direction, and the distance in between was viewed as 4 μm referred to literature.[8] Furthermore, the mass fraction was set “zero” in the capillaries owning to the rapid absorption, giving the value of gradient as “one” for the following estimation. The result of this effect of binary diffusion is illustrated in Figure 4.

Thrombin Cleavage Efficiency

After CPP-PYY complexes are transported across intestinal cells, they enter capillaries with the help of CPP transmembrane protein and diffusion. However, to suppress appetite, CPP-PYY complexes should be cleaved. Therefore, in our design, we utilized a linker which has a thrombin-favor cleavage site and thrombin is ubiquitous in blood vessel.

From the literature by J. Y. Chang, the percentage of polypeptide cleaved by thrombin is applied to CPP-PYY complexes.[9] The amino acid sequence of CPP-PYY complexes is LEAGCKNFFPRSFTSCGSLE, and we sought to cut through Arg-Ser(R-S) site. Therefore, data of Arg-Thr(R-T) site in salmon calcitonin was taken for reference, because Ser and Thr are both polar and uncharged residue. The figure of time to percentage of Arg-Thr cleavage is shown below. However, due to the fluid motion in circulation, we supposed that nearly 100% of the complexes would be cleaved during 30 minutes.

Healthin Drug Effect

Steinert RE et al. had administrated the intragastric load of a mixed liquid meal on subjects for observing the plasma concentration of active PYY after meal, as shown in Figure 6.[10] The physiological PYY concentration in plasma is approximately 45 pg/ml, and it reaches maximum at 113 pg/ml after 30 minutes of food intake. To prevent the potential side effect of PYY resistance, as well as exert the optimal appetite suppresion effect at the same time, we sought to control the PYY concentration at the level of 100 pg/ml (0.1 μg/L) after taking Healthin.

As the blood volume is about 78 ml per kilogram, we considered total average blood volume of 5 liter in human body for calculation.[11] The final concentration of PYY against time is presented in Figure 7, which exerts the optimal effect after one day of ingestion followed by gradual decrease during 4 days.