RNA-sequencing of the human milk fat layer during colostrum, transitional, and mature stages of lactation
Human milk fat globules, by enveloping cell contents during their secretion into milk, are a rich source of mammary cell RNA. Here, we pair this non-invasive mRNA source with RNA sequencing technology to probe the milk fat layer transcriptome during three stages of lactation: colostral, transitional, and mature milk production. We find that transcriptional profiles cluster not by postpartum day, but by milk Na:K ratio, indicating that women sampled during the same postpartum time frame could be at markedly different stages of gene expression. Each stage of lactation is characterized by a dynamic range (105-fold) in transcript abundances not previously observed with microarray technology. We discovered that transcripts for isoferritins and cathepsins are strikingly abundant during colostrum production, highlighting the potential importance of these proteins for neonatal health. Two transcripts, encoding ß-casein (CSN2) and a-lactalbumin (LALBA), make up 45% of the total pool of mRNA in mature lactation. Genes significantly expressed across all stages of lactation are associated with making, modifying, transporting, and packaging milk proteins. Stage-specific transcripts are associated with immune defense during the colostral stage, up-regulation of the machinery needed for milk protein synthesis during the transitional stage, and the production of lipids during mature lactation. We observed strong modulation of key genes involved in lactose synthesis and insulin signaling. In particular, PTPRF may serve as a biomarker linking insulin resistance with insufficient milk supply. This study provides the methodology and reference data set to enable future targeted research on the physiological contributors to sub-optimal lactation in humans. Overall design: Milk fat mRNA profiles were generated from Day 2 and mature milk samples obtained from lactating mothers