An approach for dynamical network reconstruction of simple network motifs
© Nakatsui et al.; licensee BioMed Central Ltd. 2013
Published: 13 December 2013
One of the most important projects in the post-genome-era is the systemic identification of biological network. The almost of studies for network identification focused on the improvement of computational efficiency in large-scale network inference of complex system with cyclic relations and few attempted have been done for answering practical problem occurred in real biological systems. In this study, we focused to evaluate inferring performance of our previously proposed method for inferring biological network on simple network motifs.
We evaluated the network inferring accuracy and efficiency of our previously proposed network inferring algorithm, by using 6 kinds of repeated appearance of highly significant network motifs in the regulatory network of E. coli proposed by Shen-Orr et al and Herrgård et al, and 2 kinds of network motif in S. cerevisiae proposed by Lee et. al. As a result, our method could reconstruct about 40% of interactions in network motif from time-series data set. Moreover the introduction of time-series data of one-factor disrupted model could remarkably improved the performance of network inference.
The results of network inference examination of E. coli network motif shows that our network inferring algorithm was able to apply to typical topology of biological network. A continuous examination of inferring well established network motif in biology would strengthen the applicability of our algorithm to the realistic biological network.
The investigation of network dynamics in biology is a major issue in systems and synthetic biology. Recent advances in high-throughput technologies for comprehensive observation of cells produce a lot of data for analyzing dynamics of complex system such as gene regulatory networks and metabolic pathways. Time-series with dynamic behavior are one of such data involving enormous amount of information regarding the regulation of biological network in vivo. However, as such information is entirely implicit, it requires the development of adequate analytic and computational methods to reconstruct biological systems. The key in developing such computational methods is to build a reliable mathematical model for analyzing biological networks, and to explore parameter values in the model within vast searching space. Tominaga et al. and Maki et al. have developed a novel method [1, 2] inferring conceptual biological networks by the combination of a dynamical network model called S-system  with a traditional parameter estimation based on simple genetic algorithms [4, 5]. The S-system is based on an ordinary differential equation, in which the temporal (time-dependent) dynamic process of system components are characterized by power-law formalism. The S-system is suitable for conceptual modeling and describing complex systems with a loop or a cyclic interaction because the dynamic behavior of the network can be easily obtained by numerical integration and customized . The values of interrelated coefficients in the formalism are directly or indirectly related to the regulation mechanism in the network model. The inferred network structure from the inference of parameters provides one of the best candidates for the biological network structure. However, S-system requires a large number of parameters that must be estimated to identify dynamical biological networks; the number of estimated parameters is 2n(n + 1) (where n is the number of system components).
We previously proposed efficient procedures for inferring biological network based on experimentally observed time-series data of mRNA or metabolites [7–10] using S-system and real-coded genetic algorithms (RCGAs)  with a combination of uni-modal normal distribution crossover(UNDX)  and minimal generation gap(MGG) . Other groups have also developed several methods to optimize parameters using S-system [14–19], Beside of S-system modeling, a lot of network reconstruction algorithms from time-series have been developed [20–27]. However, most of the works focused on the improvement of computational efficiency in large-scale network inference of complex systems with cyclic relations and few attempts have been done for answering practical problems occurred in real biological systems. Herrgård et. al., Shen-Orr et. al., and Lee et. al. proposed that the gene regulatory network in Escherichia coli or Saccharomyces cerevisiae identified by experimental studies is composed of the limited number of network motif; each motif has simple form of relationships between transcription factors and genes [28, 29]. Little attention has been paid to evaluate the performance of network inference for such simple network motifs with dynamical modeling, S-system. In this paper, in order to evaluate the inferring performance of our previously proposed network inferring algorithms, we applied our algorithm to 8 kinds of simple form of network motifs proposed by Shen-Orr et. al. , Herrgård et. al. , and Lee et. al.  Shen-Orr et. al. and Herrgård et. al. suggested repeated appearances of highly significant motifs. Lee et. al suggested network motifs based on genome-wide location data.
Results and discussion
Results of network identification
However, the lower values of precision were often observed in our previous works applied to other types of networks, so that we have already developed a method to remove the false-positive interactions inferred by parallel computing [7, 8]. Even though we can apply our previously proposed method to improve the precision values, our aim here is to see how both precision and recall values can be improved by altering the information content of time-series data.
We applied our previously proposed algorithm to the network motifs proposed by Herrgård and Shen-Orr. As a result, the efficiency (recall) of our method exhibited relatively high in most of network motifs. In particular, in the Regulatory Interactions (RI) model, we reconstructed about 68% of interactions in the model. Interestingly, the performance of network inference for complex regulatory network including cyclic interactions (AR and ML) was better than that for simple network analyzed in this study. It is likely that the abundant information related to dynamic behavior contained in time-series data for complex regulatory network constrains the degree of freedom S-system modeling, for this reason, the false-positive or false-negative interactions for complex network are reduced.
In order to examine how to improve both the accuracy and efficiency, we attempted to infer the network candidates based on 5 time-series data including time-series for one-factor disrupted model. In this situation, the performance of inferring accuracy and efficiency remarkably increased. This result suggests that the inferring performance can be improved by adding other kinds of time-series data.
Note that the present performance is examined by a set of data generated from arbitrary given parameter values. We should test the performance of our method for various structures of networks with different parameters as well as for observed data. From practical point of view, there have been various kinds of data accumulated under different experimental conditions. The differential information content of such data is expected to further improve the performance of our method. A continuous examination of inferring well-established network motifs in biology would strengthen the applicability of our algorithm to the realistic biological network including gene regulatory networks or metabolic pathways.
In order to evaluate the applicability of our inferring algorithm, we prepared 8 kinds of artificial network models, Regulatory Interaction (RI), Regulator Module (RM), Target Module (TM), Feed-Forward (FF), Single Input Module (SIM), Dense Overlapping Regulation (DOR), Autoregulation (AR), and Multicomponent-Loop (ML). Each network model contains a significant network motif in the regulatory network of Escherichia coli proposed by Shen-Orr et. al. and Herrgård et al [28, 29], and that of Saccharomyces cerevisiae proposed by Lee et. al . We modified the 8 network motifs to network models consisting of 4 nodes (X1, X2, X3, and X4) without a loss of each network topology. Figure 1 shows each network structure analyzed in this paper.
Subsequently, we prepared artificial time-series data containing 40 sampling points for each network motif by the numerical integration . The reference time-series data of 8 network models are shown in Figure 2.
where n is the number of system components (genes or metabolites) in the investigating network, X i is the experimentally observed response (gene expression level for gene expression network, or concentration of metabolites for metabolic pathway's investigation), α i and β i are apparent positive rate constant, and g ij and h ij are interrelated coefficients between X i s.
The first term on the right-hand side of Eq. (1) corresponds to the synthetic process of X i , and the second term expresses the degradation process of X i . The value of g ij (h ij ) express the interactive effects of X j to the synthetic process (degradation process) of X i . The value of g ij (h ij ) also determine the structure of the interactions between X i and X j . When the value of g ij (h ij ) is positive, X j induces a synthetic process (degradation process) of X i . On the other hand, when g ij (h ij ) is negative, X j suppresses the synthetic process (degradation process) of X i . When the value of g ij (h ij ) is zero, then there are no effects of X j on the synthetic (degradation) process of X j .
The biological network can be inferred by estimating α i , β i , and h ij in the S-system formula. A representation of S-system parameters to be estimated is shown in Figure 1.
Real-coded genetic algorithms
The S-system is a formalism of ordinary non-linear differential equations, and thus the system can easily be solved numerically by using numerical integration algorithm customized specifically for this formalism . However, when an adequate time-course of relevant state variable is given, a set of parameter values α i , β i , g ij , and h ij , in many cases, will not be uniquely determined, because it is highly possible that the other set of parameter values will also show a similar time-course. Therefore, even if one set of parameter values that could explain the observed time-course is obtained, this set is still one of the best candidates that explain the observed time-courses. Our strategy is to explore and exploit these candidates within the immense huge searching space of parameter values.
where D is the total number of data-sets that experimentally observed under the different kind of experimental conditions such as disruption of genes or inhibition of kinase activities, N is the total number of experimentally observable state variables and T is the total number of sampling points over time in one experimental conditions. The computational task is to find out a set of parameter values that minimizes the objective function E. We have developed the efficient computational technique based on real-coded genetic algorithms (RCGAs) as a nonlinear numerical optimization method which is much less likely to be stranded in local minima. This technique is based on the combination of the operator called uni-modal normal distribution crossover (UNDX)  with the alternation of generation model called minimal generation gap (MGG) model . Furthermore, in order to find the skeletal structure (small-size system) of the S-system formalism that explain the experimentally observed response, some of the parameters (g ij and h ij ), absolute values of which are less than a given threshold value are to be removed (reset to zero) during optimization procedures.
Evaluation of identified network
where FN i is the number of false-negative interactions in i-th network candidates. Both precision and recall values are defined between 0.0 to 1.0, and the best value of precision and recall are 1.0.
This work was partly supported by the commission for Development of Artificial Gene Synthesis Technology for Creating Innovative Biomaterial from the Ministry of Economy, Trade and Industry (METI), Japan. This work was also partly supported by JSPS KAKENHI Grant Number 23700358, Grait-in-Aid for Young Scientists (B), "Development of high accurate method for parameter estimation by the combination of numerical optimization and symbolic computation".
Publication of this supplement was supported by the commision for Development of Artificial Gene Synthesis Technology for Creating Innovative Biomaterial from the Ministry of Economy, Trade and Industry (METI), Japan.
This article has been published as part of BMC Systems Biology Volume 7 Supplement 6, 2013: Selected articles from the 24th International Conference on Genome Informatics (GIW2013). The full contents of the supplement are available online at http://www.biomedcentral.com/bmcsystbiol/supplements/7/S6.
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