Identification of a Novel Candidate Regulator of HSC Aging.

It is well documented that both quantitative and qualitative changes in the murine hematopoietic stem cell (HSC) population occur with age. In mice, the effect of aging on stem cells is highly strain-specific, thus suggesting genetic regulation plays a role in HSC aging. We have previously mapped a quantitative trait locus (QTL) to murine Chr 2 that is associated with the variation in frequency of HSCs between aged B6 and D2 mice. In C57BL/6 (B6) mice the HSC population steadily increases with age, whereas in DBA/2 mice, this population declines. A QTL regulating the natural variation in lifespan between the two strains was mapped to the same location on mouse Chr 2, thus leading to the hypothesis that stem cell function affects longevity. B6 alleles, associated with expansion of the stem cell pool, are also associated with a ~50% increase in lifespan. Using a congenic mouse model, in which D2 alleles in the QTL interval were introgressed onto a B6 background, genome wide gene expression analyses were performed using sorted lineage negative hematopoietic cells, which are enriched for primitive stem and progenitor cells. Three variables were examined using Affymetrix M430 arrays:

• the effect of strain--congenic versus background;

• the effect of age--2 months versus 22 months; and

• the effects of 2 Gy of radiation because previous studies indicated that congenic animals were highly sensitive to the effects of mild radiation compared to B6 background animals.

Extensive analysis of the expression arrays pointed to a single strong candidate, the gene encoding ribosome binding protein 1 (Rrbp1). Real-time PCR was used to validate the differential expression of Rrbp1 in lineage negative, Sca-1+, c-kit+ (LSK) cells, a population highly enriched for stem and progenitor cells. Further analysis revealed the presence eight non-synonymous, coding single nucleotide polymorphisms (SNPs), and at least one of them because of its location and nature may significantly alter protein structure and function. The Rrbp1 gene consists of 23 exons in mouse and is highly conserved among mammalian species including mouse, human, and canine. The Rrbp1 protein is present on the surface of the rough endoplasmic reticulum where it tethers ribosomes to the membrane, stabilizes mRNA transcripts, and mediates translocation of nascent proteins destined for the cell secretory pathway. It is well established that the interaction of HSCs with microenvironmental niches in the bone marrow is crucial for their maintenance and self-renewal, and that this interaction is mediated in part by the molecular repertoires displayed on the cell surfaces of both HSCs and niche stromal cells. Therefore, we hypothesize that age and strain specific variation in Rrbp1, through its role in the secretory pathway, affects the molecular repertoire at the cell surface of the HSC, thus altering the way stem cells interact with their niches. This altered microenvironmental interaction could have profound effects on fundamental properties relevant to stem cell aging such as pluripotency, self-renewal, and senescence.