Ribosome Profiling and Single-Cell RNA Sequencing Reveal the Unfolded Protein Response as a Central Regulator of Pigeon Lactation

Pigeons and other birds generate a milk-like secretion in their crop sacs to nourish their offspring, yet the detailed processes involved are not fully elucidated. This study examines crop sacs from 225-day-old unpaired, non-lactating male pigeons (MN) alongside males that initiated lactation on the first day after incubation (ML). Through RNA sequencing, ribosome profiling, and single-cell transcriptome sequencing (scRNA-seq), we observed a significant upregulation of genes linked to ribosome assembly and protein synthesis in ML samples compared to MN. Our scRNA-seq analysis identified 12 distinct cell types and 22 clusters, highlighting that secretory epithelial cells (SECs) express high levels of plasma cell markers like IGLL1 and MZB1. Additionally, RNA FISH and IgY assays confirmed that SECs play a crucial role in producing endogenous IgY during lactation. We propose that signaling from fibroblast-derived BAFF signal activates SECs, mimicking B cell transformation and enhancing protein production through the unfolded protein response (UPR). This research sheds light on the cellular dynamics of pigeon milk production and contributes to a greater comprehension of avian biology.