A tail with a leading role in megakaryocytes: the glycoprotein Ib

Comment on Kanaji et al, page 3161

Megakaryocytes possess a unique internal programming that allows an evolving response to a cytokine, thrombopoietin (TPO), including cellular expansion followed by polyploidization and maturation. The study of this lineage has been greatly aided by analysis of genetically modified mice and/or models of related human syndromes.

The Bernard-Soulier syndrome is associated with abnormal bone marrow megakaryocytes with poorly developed demarcation membranes, giant circulating platelets, and a reduced platelet count (coined as macrothrombocytopenia). The genetic basis of this syndrome has long been recognized as caused by mutations that impair expression of a multisubunit receptor, the glycoprotein (GP) Ib-IX complex. The mouse phenotype mimicking the human Bernard-Soulier syndrome can be salvaged by expression of a human GP Ibα transgene.¹ FIG1 

In this issue of Blood, Kanaji and colleagues describe a series of elegant experiments that explore the importance of the cytoplasmic tail of GP Ib in rescuing mice from the Bernard-Soulier–like phenotypes. Using the megakaryocyte-specific platelet factor 4 promoter,²  these investigators generated a transgenic mouse line expressing onto a GP Ibα knock-out background¹  a variant human GP Ibα subunit lacking the 6 terminal residues (605-610) on the cytoplasmic tail. This system was compared with the rescue of the GP Ib null phenotype produced by a wild-type human GP Ibα allele. The cytoplasmic tail was chosen as a target mainly because it has been previously shown to be critical for binding to the signal transduction protein 14-3-3ξ.³  The latter has been implicated in various processes that have been described by others to affect megakaryocyte proliferation and/or ploidy (eg, Drachman et al⁴ ). 14-3-3ξ influences intracellular signaling pathways (eg, Raf, MLK, MEKK, phosphatidylinositol-3 [PI-3] kinase, IRS-1), cell cycling (eg, Cdc25, Wee1, CDK2, centrosome), apoptosis (eg, BAD, ASK-1), and the regulation of transcription (eg, FKHRL1, DAF-16, p53, TAZ, TLX-2, histone deacetylase).⁵ 

The phenotypes in the new transgenic models generated by Kanaji et al illustrate an involvement of the GP Ibα cytoplasmic tail in thrombopoietin-mediated events, including megakaryocyte proliferation, polyploidization, and the expression of apoptotic markers in maturing megakaryocytes. Furthermore, this study demonstrates an increase in thrombopoietin-mediated Akt phosphorylation in the truncated variant, leading the authors to hypothesize that a GP Ibα/14-3-3ξ/PI-3 kinase complex is involved in regulating thrombopoietin-mediated responses (see figure).

A hypothesis is presented whereby the cytoplasmic tail of GP Ibα sequesters signaling proteins, such as 14-3-3ξ and PI3K, and down-regulates the Akt-dependent pathway. The authors speculate that in the truncated GP Ibα variant, a shift in the PI3K/Akt axis results in increased Akt activation and downstream consequences of increased endomitosis and accumulation of a greater percentage of high ploidy megakaryocytes. Although this is an appealing contention, it awaits further analyses to demonstrate a GP Ibα/14-3-3ξ/PI-3 kinase complex in vivo. It will also be of great interest to examine whether excess (via overexpression) of full-length versus a cytoplasmic tail portion of GP Ib results in greater sequestration of this complex and hindrance of thrombopoietin-related signaling.

Finally, while this study demonstrates that the molecular basis of the macrothrombocytopenia is linked to an absence of the cytoplasmic tail of the GP Ibα subunit of the GP Ib-IX complex, the authors rightfully point out that a role for the extracytoplasmic domains of the complex cannot be excluded.