A Single Copy of Large Tumor Suppressor 1 or 2 Is Sufficient for Normal Hematopoiesis
Hematopoietic stem cells (HSCs) are crucial as they can self-renew and differentiate into all blood cell subsets, maintaining hematopoietic homeostasis. The Hippo tumor suppressor pathway, with Large tumor suppressor 1 (LATS1) and Large tumor suppressor 2 (LATS2) as key mediators, is important for cell cycle regulation, genome integrity, etc. However, their role in the hematopoietic system was unclear.
Expression of Lats1 and Lats2 in Hematopoietic Cells
Lats1 and Lats2 were widely expressed in hematopoietic cells. Higher expression was seen in primitive hematopoietic cells like hematopoietic progenitor cells (HPCs: Lin-Sca-1-c-Kit+) compared to mature cells such as CD4+ or CD8+ T cells and B220+ B cells. Also, Lats1 had higher expression than Lats2 in hematopoietic cells.
Generation of Lats1 and Lats2 Knockout Mice
Using the CRISPR/Cas9 system, Lats1 or Lats2 knockout mice were generated. Two knockout lines for each gene were obtained. Lats1 or Lats2 homozygous mutants led to early embryonic lethality, while heterozygous mice were viable and fertile. The expression level of Lats1 and Lats2 was halved in the bone marrow (BM) of heterozygotes.
Normal Hematopoiesis in Heterozygotes
In 8 – 12 week-old mice, no differences were found in numbers of circulating white blood cells, neutrophils, lymphocytes, monocytes, platelets, red blood cells, and hemoglobin concentration between Lats1 or Lats2 heterozygotes and their littermates. The total number of BM cells was also similar. Flow cytometry analyses showed no obvious differences in the frequency or total number of early HPCs (HPC, CMP, GMP, MEP, LMPP, and LSK) or HSCs (LT-HSCs and CD34 negative LSK [CD34-LSK]).
Depletion of Lats1 or Lats2 and Overall Survival
Treating Lats1 or Lats2 heterozygous and control mice with 5-FU (which kills rapidly proliferating cells and replenishes the hematopoietic system) showed no significant differences in overall survival between heterozygotes and wild-type control mice. This indicated that the depletion of Lats1 or Lats2 did not affect the long-term self-renewal of HSCs.
Discussion
Hippo signaling, with Lats1 and Lats2 as key mediators, is not essential for hematopoiesis or HSC function in the context of this study. Lats1 and Lats2 are not haploinsufficiency genes. The downstream effectors of Hippo signaling (Yap1 and Taz) may not be essential in normal hematopoiesis. The Hippo signaling pathway may not be as crucial in the hematopoietic system (a liquid environment) as in solid organs. Also, the role of Lats1 and Lats2 may be context-dependent. Future studies could focus on protein levels, kinase activity levels, and the role in more extreme hematopoietic stress conditions.
This study, conducted by Zhi-Gang Li, Xue-Mei Fu, Cheng-Yan Chai, et al. from Southwest Hospital, Third Military Medical University (Army Medical University), and Wuhan University of Science and Technology, provides valuable insights into the role of Lats1 and Lats2 in hematopoiesis. It was published in the Chinese Medical Journal in 2020. The research was supported by multiple grants.
doi:10.1097/CM9.0000000000000934
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