Genome wide transcriptional changes underlie gradual and recurrent adaptation to protein malnutrition in zebrafish

Lysosome-rich enterocytes (LREs) are a specialized population of intestinal cells that mediate the uptake and absorption of dietary proteins in fish and neonatal mammals. Loss of LRE function causes inhibited growth and poor survival due to protein malnutrition. Previously, we reported that in zebrafish loss of Plasmolipin (pllp), an endosomal membrane protein highly expressed in LREs, impairs LRE differentiation and dietary protein absorption, resulting in marked reduction in survival rates. Raising pllp homozygous mutants surviving to adulthood and in-crossing them for multiple generations resulted in a recovery of survival and growth, even when fed a low protein diet. To uncover mechanisms underlying this phenomenon, we compared the older adapted pllp allele with genetically related wild type (WT) fish and a newly generated pllp mutant allele. Using transcriptome profiling and quantitative protein absorption assays, we found that the adapted pllp mutants exhibit upregulation of LRE endocytic components associated with a capacity for protein absorption that exceeds that of WT. This hyperactivation of LRE endocytic and absorptive activity is accompanied by transcriptional regulation of immune genes that may contribute to the enhanced survival of pllp mutants in the face of increased exposure to environmental antigens. Genetic analyses indicate that these divergent phenotypes emerge gradually over generations and can recur in an independently generated pllp allele upon inbreeding and natural selection. Overall, our study illustrates that genome-wide transcriptional changes are associated with stable improvements in intestinal function and organismal survival in response to protein malnutrition.