Naked Mole-Rat Neonatal HSPCs Are Homing Deficient Due to Overexpression of the CXCL12/CXCR7 Axis
Authors: Emmrich Stephan , Gorbunova Vera 
Published: December 2024

Naked mole-rats (NMR) emerged as a powerful model to study healthy aging. NMRs are mouse-sized rodents that have maximum lifespan up to 40 years in captivity, and are resistant to the majority of age-related diseases. However, the developmental properties of NMR stem and progenitor compartments are poorly understood. Previously we compiled the first complete cell atlas of hematopoiesis and the immune system in naked mole-rats, identifying several neotenic features, wherein fetal characteristics of hematopoiesis are preserved in adulthood.
In the previous studies we defined the most primitive hematopoietic stem and progenitor cell (HSPC) compartment in NMRs as marked by Lineage-Thy1.1+CD34+ (LTC), as these cells showed highest colony formation and replating capacity in vitro and highest engraftment and multi-lineage potential in vivo, when compared to other Lineage- cell populations. Strikingly, bone marrow (BM) frequencies of NMR LTCs, mouse LIN-Kit+Sca-1+ (LSK) and human LIN-CD34+CD38lo are in a close range of 0.1-0.3% of total BM cells.
Here we report a high frequency BM HSPC population, which we termed neoLTC, exclusively found in neonatal animals. Colony formation in semisolid cultures and short term xenotranplantations functionally confirmed presence of HSPCs in this population at similar abundances than in adult LTCs. To characterize transcriptomic properties of neoLTCs, we performed population RNA-Seq and integrated them with an RNA-Seq profile collection of adult and neonate human and mouse HSCs as well as adult NMR LTCs. Remarkably, while their immunophenotype is identical to adult NMR HSPCs, neoLTCs showed a unique transcriptomic profile devoid of many canonical HSPC genes. Using the human hematopoietic stem cell (HSC) ontogeny map (PMID 35418685) to pinpoint functional gene subsets which were differentially expressed, we found that neoLTCs underexpressed 9/15 HSC transcription factors and 5/6 HSC signature genes, whereas they overexpressed 5/7 HSC quiescence genes, 9/11 NMR HSPC genes and 10/22 nascent HSC genes. Moreover, neoLTCs produced multiple secreted matrix components related to neonatal BM niche formation, convergently neoLTCs overexpressed all mesenchymal stem cell (MSC) subset genes from the HSC ontogeny map. However, when compared with freshly isolated NMR MSCs from our most recent publication, neoLTCs clustered separately from MSCs and LTCs, and had substantially more common genesets with LTCs than MSCs.
The most striking finding was overexpression of both CXCL12 and its alternative receptor CXCR7, thereby abrogating neoLTC homing capacity, which we confirmed by intra-femural xenotransplantations. Captive NMR colony queens may have a litter survival rate ranging between 20-90%, depending on the individual queen specimen, colony size, non-breeder subordinate age and other factors. Interestingly, the neoLTC expression signature resurfaced in whole marrow isolates from moribund (but not in healthy) neonates, which were marked by up to 10-fold overabundance of neoLTCs, accumulation of a direct neoLTC-derived precursor and depleted marrow granulocytes. We thus hypothesize that the neoLTC cell population protects newborns during the critical transition at birth and shortly beyond by boosting a supply of innate immune cells if needed. In summary, our results identify a transient cell stage during postnatal hematopoiesis unique to naked mole-rats.