Dynamic Regulation of a Ribosome Rescue Pathway in Erythroid Cells and Platelets

Abstract

Summary Protein synthesis continues in platelets and maturing reticulocytes, although these blood cells lack nuclei and do not make new mRNA or ribosomes. Here, we analyze translation in primary human cells from both anucleate lineages by ribosome profiling and uncover dramatic accumulation of post-termination, unrecycled ribosomes in the 3´UTRs of mRNAs. We demonstrate that these ribosomes accumulate as a result of the natural loss of the ribosome recycling factor ABCE1 during terminal differentiation. Induction of ribosome rescue factors PELO and HBS1L is required to support protein synthesis when ABCE1 levels fall. including for hemoglobin production during blood cell development. Our observations suggest that this distinctive loss of ABCE1 in anucleate blood lineages could sensitize them to defects in ribosome homeostasis, perhaps explaining in part why genetic defects in the fundamental process of ribosome production (“ribosomopathies”) often affect hematopoiesis specifically.

Ribosomes in platelets protect the messenger

“…5 Transgenic overexpression of PELO in PLPs significantly decreased the half-life of the majority of PLP mRNAs. Given that PELO expression is low in platelets, 7 these data suggest that megakaryocytes invest low levels of PELO into platelets as a mechanism for preserving mRNA levels in anucleate platelets that are incapable of transcribing new mRNA. In addition to promoting mRNA decay, the release of stalled ribosomes by PELO frees the ribosomes for additional rounds of translation.…”

“…In a recent report, Mills et al found that ribosomes occupy more than 6000 mRNAs in platelets. 7 In their study, they found that ribosomal footprint levels generally correlate with protein levels in platelets. 8 They also found that thrombin increases ribosomal occupancy on mRNAs, consistent with previous work demonstrating that bulk translation increases in activated platelets.…”

“…The Mills et al study group previously found that ribosomes accumulate on the 39UTR of certain transcripts in platelets and in PLPs, which have naturally low expression of the ribosomal rescue and surveillance factor PELO. 7 Conversely, overexpression of PELO decreases ribosomal tracking into the 39UTR. 7 Because PELO mediates mRNA degradation when ribosomes enter the 39UTR, 10 the authors examined roles for PELO in mRNA decay in PLPs harvested from a megakaryocytic cell line.…”

“…7 Conversely, overexpression of PELO decreases ribosomal tracking into the 39UTR. 7 Because PELO mediates mRNA degradation when ribosomes enter the 39UTR, 10 the authors examined roles for PELO in mRNA decay in PLPs harvested from a megakaryocytic cell line. By using RNA-Seq, they demonstrated that the half-life of mRNAs in cultured PLPs is 5.7 hours, a result that coincides with previous measurements of ex vivo mRNA decay in platelets.…”

“…Consistent with this possibility, reduced PELO expression in erythrocytes, another anucleate cytoplast, is associated with decreased protein synthesis. 7 Because suppressed translation may accompany increased mRNA stability, the net effect of reduced PELO on individual or total protein production in platelets remains to be determined. Ribosome profiling of platelets and platelet-like particles (PLPs) demonstrates a role for PELO in mRNA decay.…”