Dopamine motivates stem cells for reward

In this issue of Blood, Liu et al describe how dopamine produced by sympathetic nerves in the bone marrow (BM) niche directly controls hematopoietic stem and progenitor cells (HSPCs) via D2 subfamily dopamine receptors.¹

The BM niche is a highly complex microenvironment of many different coordinating cell types that support HSPCs throughout life.² Substantial evidence has demonstrated that the sympathetic nervous system innervates the BM microenvironment and regulates hematopoiesis.³ Trafficking of HSPCs in and out of the BM even follows our circadian rhythms, with more HSPCs in circulation during sleep.⁴ In this context, adrenergic peripheral nerves control HSPCs indirectly by contact with BM stromal cells. More recently, it was shown that HSPCs are directly regulated by the neurotransmitter γ-aminobutyric acid (GABA) via cell surface receptors, but the source of GABA was predominantly from B cells and not nerves.⁵ In the study described in Blood, Liu et al demonstrate that direct regulation of HSPCs can also occur by cell surface dopamine receptors and that dopamine-producing nerves are near HSPCs in the niche.

Dopamine signaling is critical in the central nervous system. It regulates behavioral responses related to reward, and dopamine deficiency can lead to neurodegenerative disease.⁶ There are 5 dopamine receptors that are classified as either D₁ (Drd1 and -5) or D₂ type (Drd2, -3, and -4).⁷ After showing that dopamine levels are high in BM, the authors identified the dopamine receptors expressed on BM cells. They found Drd2 and Drd3 were expressed on hematopoietic stem cells (HSCs) and HSPCs but not on differentiated hematopoietic cells. They used Drd2 and Drd3 knockout mouse models to determine if loss of these dopamine receptors produced a hematopoietic phenotype. Single Drd2 and Drd3 knockouts and a Drd2/3 double knockout all had a significant decrease in HSC and HSPC numbers. HSCs isolated from Drd2/3 double-knockout BM and transplanted into primary and then secondary recipients showed poor reconstitution. These transplantation experiments demonstrated that Drd2/3 function in HSCs is cell autonomous and independent of global loss of function, which could cause other secondary phenotypes in the BM.

To resolve the source of dopamine production in the BM, Liu et al looked for expression of tyrosine hydroxylase (TH), which is necessary for dopamine synthesis. TH was found in BM nerve fibers but not in other stromal cell types or hematopoietic cells. Interestingly, analysis of HSC distribution in the BM showed enrichment near TH⁺ nerves. Ablation of TH⁺ nerves using genetic tools and diphtheria toxin reduced dopamine levels in the BM and led to a significant decrease in HSCs and HSPCs. The supportive function of TH⁺ nerves in the BM niche was further confirmed by the lower transplantation efficiency of wild-type lineage-negative cells when transplanted into TH⁺ nerve–ablated recipients.

An interesting aspect of studying neuroreceptors in the hematopoietic system is that there are already many small-molecule modulators available for functional studies. Some of these small molecules are US Food and Drug Administration–approved drugs that have the potential of being repurposed for use in the hematopoietic system, as was shown for the GABA B receptor agonist baclofen in a preclinical umbilical cord blood transplantation model.⁵ In this study, Liu et al used carbidopa, a drug that inhibits the enzyme that converts L-DOPA to dopamine, to interrogate the function of dopamine only in tissues peripheral to the nervous system, because the drug does not cross the blood-brain barrier. Consistently, carbidopa treatment phenocopied Drd2/3 double-knockout mice and resulted in reduction of HSCs and HSPCs. This was also true for treatment of mice with the high-affinity D₂-type dopamine receptor antagonist haloperidol. Conversely, treatment of donor HSPCs with the D2-type receptor agonist 7OH-DPAT stimulated proliferation of wild-type but not Drd2/3 double-knockout donor cells. The agonist 7-OHDPAT was also able to rescue transplantation in irradiated recipients with reduced TH⁺ cells. Together these results demonstrate that small-molecule agonists and antagonists of the D2-type receptor can have both positive and negative effects on HSPC proliferation and transplantation, respectively.

Next, Liu et al sought to understand the mechanism of dopamine regulation of HSCs and HSPCs. Drd2/3 double-knockout HSPCs, as well as haloperidol-treated HSPCs, had significant cell cycle defects. Looking for a potential pathway related to these defects in cell cycle and proliferation, they found that ERK/MAPK signaling was lower in Drd2/3 double-knockout HSPCs. The authors speculated it was unlikely these changes in ERK/MAPK signaling occurred directly downstream of the G protein–coupled D₂-type dopamine receptors. Instead, they found synergism between D₂-type dopamine receptors and the stem cell factor receptor c-Kit. The authors then identified another candidate for crosstalk between D₂-type dopamine receptors and ERK signaling: the SFK lymphocyte protein tyrosine kinase (Lck). Not only is Lck expressed in HSPCs, but its messenger RNA levels go up and down with D₂-type dopamine receptor agonism or antagonism, respectively. Most compelling was that the authors could partially rescue loss of Drd2/3 function in double-knockout HSPCs by overexpression of Lck. These data begin to outline a framework for D₂-type dopamine receptor signaling in HSPCs via regulation of Lck and ERK and possible synergism with c-Kit signaling.

After extensive past studies that showed an important role for adrenergic signaling in the BM niche,³ and more recently GABA signaling,⁵ it is fascinating that this new role for dopamine has emerged. The finding that HSCs are in close proximity to TH⁺ nerves was unexpected, because electron microscopy data suggested that nerves in the BM were in contact only with stromal cells.³ It will be important to learn more about how dopamine signaling is used during steady-state hematopoiesis and during stress. Of particular interest will be how dopamine signaling plays out in the BM niche during aging, when changes in TH⁺ nerves are observed.^8,9 No doubt the motivation to find out more about dopamine signaling in the BM niche will yield significant rewards.