Cotransport model and experimental design

Preliminary experiments were carried out to further investigate the accumulative transporter behaviour in comparison to the model, as presented in Appendix B. The vesicle uptakes experiments were designed using the model to systematically consider different levels of driving forces due to either the electrical potential difference or the sodium gradient across the membrane. However, while the model made logical predictions of the behaviour, a more thorough experiment must be carried out in the future as the pilot results (n = 1) did not entirely agree with the model predictions and what would be expected based on physical considerations. This indicated the sensitivity of the experimental protocol and complexity of the cotransport system as there are many contributing factors that can affect the uptake results. Hence, a well-defined experimental protocol is needed to improve the model validation.

4.5

Conclusion

A model for an accumulative amino acid transporter was developed, to mechanistically integrate the effects of the chemical and electrical potential driving forces related to the cotransport of sodium ions. This cotransport model was investigated in detail, including the effect of different assumptions depending on the charge of the transport proteins. However, it was suggested by the simulation results that the ultimate level of accumulation at steady state is essentially indifferent to the transporter charge assumption, since this is determined by the electrochemical potential differences. This study also acted as a significant improvement on a previous study where the accumulative transporter was modelled as simple Michaelis-Menten type behaviour. Finally, together with the exchanger/facilitative model from the previous chapter, the cotransport model representing the accumulative transporters will be used in the integrated study of the placental amino acid transport system in the next chapter.

Integrated model of placental

amino acid transport system

5.1

Introduction

The placenta is important for fetal development as it mediates the transfer of all essential nutrients, such as amino acids, required by the fetus. Insufficiency of amino acids during pregnancy as a result of impaired placental transport mechanism is associated with poor fetal growth, which can lead to chronic disease in adult life [2, 4, 35, 72, 101]. While currently there are no treatments for intrauterine growth restriction, a better understanding of the placental transport system could potentially contribute to advancement in treatment strategies for intervention and prevention. The transport of amino acids from the maternal blood through the placenta to the fetal circulation is a complex process and difficult to understand intuitively. This is because amino acids do not simply diffuse through the placental membranes, but instead their transfer is mediated by transport proteins or transporters, which have different passive or active transport mechanisms and substrate specificities. Chapter 2 provides further background on each transporter mechanism. Furthermore, there are over twenty amino acids (nine of which are essential), which can either inhibit or promote each other’s transport. Additionally, the transport system is influenced by physiological factors including placental blood flow, metabolism, and placental morphology [5, 102]. Hence, given this inherent complexity a systematic approach using mathematical modelling is necessary to help describe the transport system.

While many studies of amino acid transport across epithelia have focussed on individual transporters, the integrated study of the interactions between multiple transporters is limited [19, 73]. We have previously introduced an integrated model of the placental amino acid transport system, applied to the uptake and exchange of serine and alanine [16]. However, a systematic analysis of the transport system as a whole is required,

including more mechanistic transporter models [5, 103], which have been explored extensively in Chapter 3 and Chapter 4. This chapter will present for the first time such a systematic fully-integrated approach, with the aim of understanding how the composition of amino acids and transporter activity can drive the overall net transfer of all amino acids to the fetus. This modelling framework represents an important novel contribution to the field by describing the placenta as a whole system, allowing us to capture for the first time the fundamental interactions between different transporters and amino acids transferred across the placenta.

Ultimately, this knowledge can lead to treatment that is administered to the mother and designed to stimulate the transport of specific amino acids as required. The treatment can be in a form of an intravenous supply of amino acids to the mother or gene therapy to regulate the placental transporters [104]. Without such mechanistic insights into how placental amino acid transport functions as a system, these treatments are deemed difficult, if not impossible. Moreover, this systematic model can also provide predictive information on transport behaviour in response to certain pathological conditions, which can help clinicians understand disease mechanisms better. Finally, this type of modelling framework could also be applied to other epithelial transport systems, such as in kidney, intestine, and brain

The chapter will start by explaining the placental amino acid transport system and its model implementation. This is followed by the results using simplified model situations to explain step by step the processes of how amino acids are transferred to the fetus across each placental membrane individually. Subsequently, both placental membranes are combined, producing a complete representation of how amino acids get across the placenta to the fetus. Sensitivity analyses of model parameters are presented to understand the transport system as a whole and how this affects different classes of amino acids. Lastly, an example of the impact of a certain genetic condition with elevated phenylalanine levels on the amino acid transport system is explored by the model.