Cytokine-induced megakaryocytic differentiation is regulated by genome-wide loss of a uSTAT transcriptional program

Area of Science:

• Cellular and Molecular Biology
• Developmental Biology
• Hematopoiesis

Background:

• Metazoan development relies on transcriptional networks responding to extracellular signals like cytokines.
• The Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway is a conserved signaling module activated by cytokines.
• The precise mechanisms by which STAT activation influences lineage-specific transcriptional programs remain incompletely understood.

Purpose of the Study:

• To elucidate how STAT5 activation modulates megakaryocytic transcriptional programs.
• To investigate the role of tyrosine-unphosphorylated STAT5 (uSTAT5) in regulating megakaryopoiesis.
• To understand the dual transcriptional outcomes of STAT5 phosphorylation in response to thrombopoietin (TPO).

Main Methods:

• Chromatin immunoprecipitation sequencing (ChIP-seq) to map STAT5 and CTCF genome-wide.
• Analysis of gene expression changes in response to TPO stimulation.
• Co-localization studies of STAT5, CTCF, and ERG at target gene loci.

Main Results:

• In the absence of TPO, uSTAT5 resides in the nucleus, colocalizes with CTCF, and actively represses megakaryocytic gene expression.
• TPO-induced STAT5 phosphorylation causes its genome-wide relocation to specific DNA binding sites.
• This relocation leads to the release of uSTAT5-mediated repression and the activation of pSTAT5-driven programs regulating apoptosis and proliferation.
• uSTAT5-mediated repression is achieved by restricting the access of the transcription factor ERG to its target genes.

Conclusions:

• STAT5 exhibits a dual role in megakaryopoiesis, acting as a repressor in its unphosphorylated state and an activator upon phosphorylation.
• uSTAT5 functions as a transcriptional repressor by limiting the binding of key lineage-specific transcription factors like ERG.
• This study reveals a novel mechanism of cytokine-mediated differentiation through the dynamic regulation of STAT5's transcriptional activity and genomic localization.