Integrative feedback and robustness in a lipid biosynthetic network

Abstract

The homeostatic control of membrane lipid composition appears to be of central importance for cell functioning and survival. However, while lipid biosynthetic reaction networks have been mapped in detail, the underlying control architecture which underpins these networks remains elusive. A key problem in determining the control architectures of lipid biosynthetic pathways, and the mechanisms through which control is achieved, is that the compositional complexity of lipid membranes makes it difficult to determine which membrane parameter is under homeostatic control. Recently, we reported that membrane stored elastic energy provides a physical feedback signal which modulates the activity in vitro of CTP:phosphocholine cytidylyltransferase (CCT), an extrinsic membrane enzyme which catalyses a key step in the synthesis of phosphatidylcholine lipids in the Kennedy pathway (Kennedy 1953 J. Am. Chem. Soc. 75, 249-250). We postulate that stored elastic energy may be the main property of membranes that is under homeostatic control. Here we report the results of simulations based on this postulate, which reveal a possible control architecture for lipid biosynthesis networks in vivo.

Figure 1

Model for CCT regulation. (a) Monolayer with optimum and large spontaneous curvature. (b) Monolayer constrained to lie flat in a bilayer; large stored elastic energy indicated by darker shading. (c) CCT α-helices partition into the membrane, relieving stored elastic energy.

Figure 2

Feedback hypothesis: the relationship between the lipid composition of a membrane, and therefore its stored elastic energy, and the activity of the enzymes of lipid biosynthesis would complete a feedback loop and ensure homeostatic control of the torque tension.

Figure 3

The location of the substrates and products of the CCT and CPT catalysed reactions (Jackowski & Baburina 2002). In our simulations, CDPcho, formed at the membrane, is modelled as a variable species. The concentrations of cho and pcho, which are external to the membrane, are clamped.

Figure 4

Model network diagram. Note that the drain reactions present on each species have been omitted for clarity.

Figure 5

Sensitivity analysis for (i) the torque parameter and (ii) PC concentration for three feedback configurations. (a) No feedback. (b) Normal feedback (equation (2.5)) at CCT. (c) Feedback at multiple (eight) reactions: normal feedback (equation (2.5)) applied to CCT (R5), g3p-AcT (R7), CPT (R9), PACT (R36) and inverse feedback (equation (3.1)) applied to the four PLC mediated reactions (R12, R14, R15 and R40). The plots show how the quantities change as each reaction rate is altered (from the values found for the TSS). The torque parameter is stabilized by feedback; however, the concentrations are not constrained by the feedback, rather they change to maintain the torque parameter. All reaction numbers refer to the reaction list provided in table 1; the enzyme labels used in the key are detailed in tables 4 and 5.

Figure 6

Time-course data for a twofold rate increase (at time point zero) in the source reactions R5 (CCT), R7 (g3p-AcT) and R8 (FA source reaction). The plots contrast the changes in the lipid concentrations and the torque parameter for the network without integrative feedback with a network with the same multi-point feedback configuration outlined in figure 5.

Figure 7

Effect of feedback control at ECT. (a) Normal feedback at CCT. (b) Normal feedback at CCT and ECT. (c) Normal feedback at CCT and inverse feedback at ECT. Keys are the same as given in figure 5.

Figure 8

Subsystem model for comparison of changes when the rate of the reaction mediated by (a) CCT or (b) ECT is increased: (i) the effect on the subsystem components and (ii) the concentration changes for the subsystem's lipids (solid arrows) and the effect of each change on the torque parameter (dotted arrows) (the arrows indicate the increases or decreases in the quantities but do not reflect the magnitude of the changes, the shading indicates type II character with type I/0 lipids shown unshaded).