Deep learning has emerged as a powerful methodology for predicting a variety of complex biological phenomena. However, its utility for biological discovery has so far been limited, given that generic deep neural networks provide little insight into the biological mechanisms that underlie a successful prediction. Here we demonstrate deep learning on biological networks, where every node has a molecular equivalent (such as a protein or gene) and every edge has a mechanistic interpretation (e.g., a regulatory interaction along a signaling pathway).With knowledge-primed neural networks (KPNNs), we exploit the ability of deep learning algorithms to assign meaningful weights to multi-layered networks for interpretable deep learning. We introduce three methodological advances in the learning algorithm that enhance interpretability of the learnt KPNNs: Stabilizing node weights in the presence of redundancy, enhancing the quantitative interpretability of node weights, and controlling for the uneven connectivity inherent to biological networks. We demonstrate the power of our approach on two single-cell RNA-seq datasets, predicting T cell receptor stimulation in a standardized in vitro model and inferring cell type in Human Cell Atlas reference data comprising 483,084 immune cells.In summary, we introduce KPNNs as a method that combines the predictive power of deep learning with the interpretability of biological networks. While demonstrated here on single-cell sequencing data, this method is broadly relevant to other research areas where prior domain knowledge can be represented as networks.To overcome lack of interpretability as a key limitation of deep learning in biology and medicine, here we explore the concept -and demonstrate the feasibility -of deep learning on application-specific biological networks rather than on generic ANNs. By applying deep learning directly to biological networks (such as signaling pathways and gene-regulatory networks), this approach coerces deep learning algorithms to stick closely to the regulatory mechanisms that are relevant in cells, thereby making the learned models interpretable.We developed knowledge-primed neural networks (KPNNs) as a framework for interpretable deep learning on biological networks (applied here to signaling pathways and gene-regulatory networks), and we introduce three modifications to generic deep learning that enhance interpretability: (i) Repeated network training with random deletion of hidden nodes (a technique known as dropout 40 ), which yields robust results in the presence of network redundancy; (ii) dropout on input data in order to enhance quantitative interpretability of node weights; (iii) training on control inputs to normalize for the uneven connectivity of biological networks. To validate our method for interpretable deep learning on biological networks, we applied KPNNs to single-cell RNA-seq data for T cell receptor stimulation 41 and to a reference catalog of immune cells from the Human Cell Atlas 42 .
RESULTS
KPNNs enable deep learning on biolo...