Of mice, men, and megakaryocytes: CK1α governs thrombopoiesis Free

In this issue of Blood, by uncovering CK1α as a master regulator of megakaryocyte cytoskeletal dynamics, Kollotzek et al¹ illuminate a critical pathway governing platelet production with therapeutic and mechanistic implications.

Platelet biogenesis (also called thrombopoiesis) relies on a precisely choreographed cytoskeletal program governing maturation, polarization, and proplatelet formation (PPF) by megakaryocytes (MKs). In their comprehensive study, Kollotzek et al from the laboratory of Oliver Borst, unravel a critical role for casein kinase 1α (CK1α) as a central regulator of these processes in both murine and human systems (see figure). Using a newly generated PF4-Cre–driven, MK-specific Csnk1a1 knockout mouse, the authors convincingly demonstrate that CK1α deficiency leads to macrothrombocytopenia characterized by a ∼30% reduction in circulating platelets and an increased mean platelet volume, which can indicate impaired megakaryocyte (MK) differentiation. Importantly, platelet clearance rates were only mildly accelerated, suggesting defective production rather than increased destruction as the primary cause of thrombocytopenia.

Histologically, CK1α-deficient bone marrow (BM) MKs showed hyperplasia with reduced nuclear lobulation, diminished von Willebrand factor expression, and skewed DNA ploidy toward more immature 4N MKs, all classical hallmarks of arrested maturation. Moreover, MKs lacking CK1α exhibited a decreased ability to contact BM sinusoids and form proplatelets both in vitro and in vivo, as elegantly visualized by 2-photon intravital microscopy. These defects culminated in an increased rate of ectopic MK fragmentation, indicating a failure to execute coordinated PPF at sinusoidal interfaces. Mechanistically, this study reveals that CK1α deficiency disrupts MK cytoskeletal architecture, characterized by reduced F-actin polymerization, defective podosome formation, and impaired microtubule formation. CK1α-deficient MKs displayed a reduction in detyrosinated and acetylated tubulin, indicators of microtubule stability, and an increase in phosphorylated TPPP, a LIMK-regulated factor that inhibits tubulin polymerization.

Another key mechanistic insight is the nuclear retention of the cell cycle regulator p21 in CK1α-deficient MKs, coupled with elevated levels of the tumor suppressor p53.² Pharmacological inhibition of p21 with UC2288 was sufficient to rescue both MK ploidy and PPF. This finding implicates the p53/p21 axis and its downstream effectors, particularly the ROCK/LIMK/cofilin pathway, as a critical conduit through which CK1α regulates actin cytoskeletal dynamics (see figure).³ CK1α-deficient MKs displayed increased phospho-cofilin and phospho-LIMK levels consistent with reduced actin turnover and cytoskeletal rigidity. The authors extended their analysis to human cells by employing CRISPR/Cas9–mediated deletion of Csnk1a1 in human CD34⁺ progenitor cells. These studies also demonstrated reduced MK size, impaired PPF, cytoskeletal disruption, and upregulation of the p53/p21 signaling pathway, consistent with the findings from their murine studies. Taken together, these studies highlight the crucial role that CK1α plays in regulating platelet production in both murine and human megakaryocytes (MKs).

This work expands our current understanding of thrombopoiesis by placing CK1α at the crossroads of signaling, cytoskeletal regulation, and nuclear maturation in MKs. CK1α has garnered interest as a therapeutic target in hematologic malignancies, particularly in del(5q) myelodysplastic syndromes (MDS), where haploinsufficiency of CSNK1A1 sensitizes malignant clones to lenalidomide-induced degradation of CK1α.^4-6 However, the frequent thrombocytopenia observed in these patients with MDS has long lacked a unifying mechanistic explanation. The present study fills some of these knowledge gaps by demonstrating that CK1α deficiency directly impairs MK maturation, rather than simply suppressing MK proliferation.

Although the current study makes significant mechanistic strides, several key questions remain unanswered. First, to what extent is the observed phenotype attributable to defects in cytoskeletal maturation vs impaired early lineage commitment? The use of the PF4-Cre promoter, which is activated in mature megakaryocytes but, in some studies, may not be perfectly restricted to MKs and platelets, supports a late-stage role. However, potential effects on megakaryocyte-erythroid progenitors cannot be entirely excluded.⁷ Second, is p21 retention the principal driver of cytoskeletal disarray or one of several parallel pathways affected by the loss of CK1α in MKs? The ability of p21 inhibition to rescue some defects is compelling. Nevertheless, CK1α is known to regulate a host of other substrates, including components of the Wnt and NF-κB pathways.^8,9 Future studies exploring whether these pathways also modulate MK maturation will undoubtedly enrich our understanding of this mechanistic landscape.

Moreover, the dual regulation of actin and tubulin by CK1α positions it as a critical orchestrator of MK cytoskeletal remodeling. Whether CK1α directly phosphorylates cytoskeletal regulators or exerts its effects via upstream modulation (eg, Rho GTPases) remains to be deciphered.¹⁰ The observed microtubule defects raise the possibility of direct regulation of microtubule-associated proteins beyond p21-mediated LIMK activation, which remains to be elucidated. Clinically, these mechanistic findings raise a potential therapeutic concern that should also be taken into consideration. As CK1α inhibitors advance into trials for MDS and acute leukemias, their antineoplastic efficacy may have the side effect of compromising platelet production, leading to thrombocytopenia and increased bleeding risk, especially in patients with existing cytopenias. Transient p21 inhibition or co-administration of agents that stabilize cytoskeletal dynamics, to preserve platelet production without compromising their antineoplastic efficacy, may be considered as alternative therapeutic strategies in the future.

In conclusion, the study by Kollotzek et al expands our understanding of thrombopoiesis by identifying CK1α as a crucial gatekeeper that regulates cytoskeletal dynamics in MKs. It may justify a reexamination of CK1α-targeted therapies, not only as precision weapons against hematologic malignancies but also as potential disruptors of terminal megakaryocyte (MK) maturation and consequent platelet production. Future investigations dissecting the broader signaling milieu surrounding CK1α will be essential to harness its therapeutic potential safely and effectively.

Conflict-of-interest disclosure: The authors declare no competing financial interests.