Hereditary xerocytosis is a rare red blood cell disease related to gain-of-function mutations in the FAM38A gene, encoding PIEZO1, in 90% of cases; PIEZO1 is a broadly expressed mechano-transducer that plays a major role in many cell systems and tissues that respond to mechanical stress. In erythrocytes, PIEZO1 adapts the intracellular ionic content and cell hydration status to the mechanical constraints induced by the environment. Until recently, the pathophysiology of hereditary xerocytosis was mainly believed to be based on the "PIEZO1-Gardos channel axis" in erythrocytes, according to which PIEZO1-activating mutations induce a calcium influx that secondarily activates the Gardos channel, leading to potassium and water efflux and subsequently to red blood cell dehydration. However, recent studies have demonstrated additional roles for PIEZO1 during early erythropoiesis and reticulocyte maturation, as well as roles in other tissues and cells such as lymphatic vessels, hepatocytes, macrophages and platelets that may affect the pathophysiology of the disease. These findings, presented and discussed in this review, broaden our understanding of hereditary xerocytosis beyond that of primarily being a red blood cell disease and identify potential therapeutic targets. | INTRODUCTIONThe PIEZO proteins are a family of mechano-transducers first described in a neuron derived cell line in 2010. 1 The two members are PIEZO1, encoded by FAM38A on chromosome 16, and PIEZO2, encoded by FAM38B on chromosome 18. They are expressed in many tissues. 1,2 The structure of PIEZO1 has been recently elucidated. A first description of the murine protein revealed its three-dimensional structure, consisting of a three-bladed homotrimeric transmembrane helix completed by an extracellular cap. An intra-cytoplasmic domain, split into three parts, called "beams," extends from the transmembrane domain. The C-terminal end of each monomer has an extracellular domain (ECD), which forms the cap, and an intracellular domain (CTD), which appears to be connected to the paddles by the beams and closes the inner ion pore (Figure 1). The assembly provides a hydrophilic central transmembrane duct bordered by six helices that allows the passage of ions. Thus PIEZO1 constitutes a mechanosensitive ion channel due to the flexibility of the three blades. 1 Deformation of the plasma membrane subsequent to mechanical stimulation induces rotation of the PIEZO1 blades, transmitting such deformation to the ECD and CTD domains, allowing the ion channel to open using a lever-like mechanism. 2 When a compressive force is applied parallel to the axis of the ion channel, it induces lateral membrane tension, resulting in flattening and widening of PIEZO1, which causes the pore to open and allows the passage of ions. Such deformations are reversible, and PIEZO1 returns rapidly to its original conformation. [3][4][5] After opening, PIEZO1 acts as a passive channel for mono/divalent cations depending on their extracellular-intracellular Nicolas Jankovsky and Alexis Caulier...
Among histone deacetylases, HDAC6 is unusual in its cytoplasmic localization. Its inhibition leads to hyperacetylation of non‐histone proteins, inhibiting cell cycle, proliferation and apoptosis. Ricolinostat (ACY‐1215) is a selective inhibitor of the histone deacetylase HDAC6 with proven efficacy in the treatment of malignant diseases, but anaemia is one of the most frequent side effects. We investigated here the underlying mechanisms of this erythroid toxicity. We first confirmed that HDAC6 was strongly expressed at both RNA and protein levels in CD34 + ‐cells‐derived erythroid progenitors. ACY‐1215 exposure on CD34 + ‐cells driven in vitro towards the erythroid lineage led to a decreased cell count, an increased apoptotic rate and a delayed erythroid differentiation with accumulation of weakly hemoglobinized immature erythroblasts. This was accompanied by drastic changes in the transcriptomic profile of primary cells as shown by RNAseq. In erythroid cells, ACY‐1215 and shRNA‐mediated HDAC6 knockdown inhibited the EPO‐dependent JAK2 phosphorylation. Using acetylome, we identified 14‐3‐3ζ, known to interact directly with the JAK2 negative regulator LNK, as a potential HDAC6 target in erythroid cells. We confirmed that 14‐3‐3ζ was hyperacetylated after ACY‐1215 exposure, which decreased the 14‐3‐3ζ/LNK interaction while increased LNK ability to interact with JAK2. Thus, in addition to its previously described role in the enucleation of mouse fetal liver erythroblasts, we identified here a new mechanism of HDAC6‐dependent control of erythropoiesis through 14‐3‐3ζ acetylation level, LNK availability and finally JAK2 activation in response to EPO, which is crucial downstream of EPO‐R activation for human erythroid cell survival, proliferation and differentiation.
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