Clonal Compensation Between Hematopoietic Stem Cells upon Differentiation Deficiency
Authors: Lu Rong , Nguyen Lisa 
Published: October 2019

Abstract
In most organ systems, regeneration is a coordinated effort that involves many stem cells, but little is known about how individual stem cells compensate for the functional deficiencies of other stem cells. Functional coordination between stem cells is critically important during disease progression and treatment when a subset of HSCs fail or become malignant. We hypothesize that individual HSCs heterogeneously compensate for specific deficiencies, as recent work from our group and others suggest that HSCs heterogeneously supply blood. To test this hypothesis, we tracked mouse HSCs in vivo using a single-cell tracking technology that we had previously developed.
We found that individual HSCs heterogeneously compensate for the lymphopoiesis deficiencies of other HSCs by increasing individual clonal expansion and altering lineage bias. Clonal expansion refers to the increase in clonal progenies. Lineage bias refers to the preferential production of specific blood cell types. This compensation rescues the overall blood supply and influences blood cell types outside of the deficient lineages in distinct patterns. We identified the molecular regulators and signaling pathways associated with this form of HSC coordination using RNA sequencing. Specifically, the STAT3 pathway and NF-B signalingwere activated, and PTEN signaling was inhibited in HSCs during the compensation process.
To investigate the dynamics of HSC coordination, we employed a genetically modified mouse model that expresses simian diphtheria toxin (DT) receptor under the control of the CD11b promoter. Monocytes derived from this mouse line can be ablated upon DT administration. We co-transplanted HSCs derived from normal and the genetically modified mice, then conditionally ablated the monocyte population repeatedly, and tracked the temporal responses of individual normal HSCs. Our time-course analysis revealed that a distinct subset of HSC clones produced rapid and persistent responses to the blood perturbations. These clones had not been highly active in the affected lineages prior to the perturbation. We identified several significant temporal profiles that indicate a remarkable heterogeneity in the responses of HSCs to blood system changes. Together, these data suggest that HSC differentiation is coordinated in a deterministic manner during compensation and is independent of the normal differentiation program.
Our findings suggest that stem cells interact with each other and form a coordinated cellular network that is robust enough to withstand minor functional disruptions. Individual HSCs distinctly adapt their differentiation program to compensate for deficient HSCs and specifically overproduce undersupplied cell types. The heterogeneity in the compensation activities of individual HSC clones may be essential for maintaining robustness in blood regeneration and suggests that stem cell coordination is a complex process. A better understanding of the clonal level differences in individual HSCs is critically important for identifying the pathogenesis of blood diseases. Exploiting the innate compensation capacity of stem cell networks may improve the diagnosis and treatment of many diseases. For example, the identification of the molecular regulators and pathways involved in HSC compensation can help develop new therapeutic treatments that enhance the innate compensation capacity of stem cells.

Disclosures
No relevant conflicts of interest to declare.