Biology subjects

Hmidan, H.

Publications and source records attributed to Hmidan, H..

3 recordsLinked to original sources

bioRxiv2024

H3.3 deposition counteracts the replication-dependent enrichment of H3.1 at chromocenters in embryonic stem cells

Chromocenters in mouse cells are membrane-less nuclear compartments that represent typical heterochromatin stably maintained during the cell cycle. Here, we explore how histone H3 variants, replicative H3.1/H3.2 or replacement H3.3, mark these domains during the cell cycle. In mouse embryonic stem cells (ESCs), neuronal precursor cells (NPCs) as well as immortalized 3T3 cells, we find a strong and distinct H3.1 enrichment at chromocenters, with some variation in ESCs. Mechanistically, this H3.1 selective enrichment depends on the DNA Synthesis Coupled (DSC) deposition pathway operating in S phase. Yet, this selective enrichment is challenged when we target H3.3 deposition through the DNA Synthesis Independent (DSI) deposition pathway mediated by HIRA. Altering the H3.1/H3.3 equilibrium at chromocenters in ESCs affects its heterochromatin properties leading to mitotic defects. We thus reveal opposing mechanisms for H3.1 and H3.3 deposition with different enforcement according to cell cycle and potency which determine their ratio at chromocenters and are critical for genome stability and cell survival.

cell biology↗

bioRxiv2023

PAK1 and NF2/Merlin jointly drive myelination by remodeling actin cytoskeleton in oligodendrocytes

In the central nervous system (CNS), myelin formation by oligodendrocytes (OLs) relies on actin dynamics. Actin polymerization supports the ensheathment step, when the OL process contacts the axon, while a drastic shift to actin depolymerization is required to enable the following step of wrapping and expansion of myelin membranes. The molecular mechanisms triggering this switch, essential for proper myelination, have yet to be elucidated. Here, we identify P21-activated kinase 1 (PAK1) as a major regulator of actin depolymerization in OLs. We show that PAK1 accumulates in OLs in a kinase inhibited form, triggering actin disassembly and, consequently, myelin expansion. Remarkably, we identify NF2/Merlin as an endogenous inhibitor of PAK1 by proteomics analysis of its binding partners. We found that Nf2 knockdown in OLs results in PAK1 activation and impairs myelin formation, and that pharmacological inhibition of PAK1 in Nf2-knockdown OLs rescues these defects. Moreover, we demonstrate that modulating PAK1 activity in OLs controls myelin expansion and provide compelling evidence indicating that specific Pak1 loss-of-function in oligodendroglia stimulates the thickening of myelin sheaths in vivo. Overall, our data indicate that PAK1-NF2/Merlin duo plays a key role in actin cytoskeleton remodeling in OLs, required for proper myelin formation. These findings have broad mechanistic and therapeutic implications for demyelinating diseases and neurodevelopmental disorders. SignificanceRemodeling actin cytoskeleton plays a crucial role in myelin formation by oligodendrocytes (OLs). Recent studies have shown that expansion and wrapping of myelin membranes around axons depends on actin depolymerization. However, the molecular mechanisms triggering this key step in myelination are not fully elucidated. Using genetic and pharmacological tools as well as proteomics analyses, we found that PAK1 (P21 Activated Kinase 1) kinase activity is maintained inhibited by NF2/Merlin in OLs to allow actin depolymerization and, consequently, myelin membrane expansion. Pak1 loss-of-function in OLs leads to an increase in myelin thickness in the white matter of adult mice, confirming the role of PAK1 inactivation in myelin membrane expansion.

neuroscience↗

bioRxiv2022

Transient regulation of focal adhesion via Tensin3 is required for nascent oligodendrocyte differentiation

The differentiation of oligodendroglia from oligodendrocyte precursor cells (OPCs) to complex and extensive myelinating oligodendrocytes (OLs) is a multistep process that involves largescale morphological changes with significant strain on the cytoskeleton. While key chromatin and transcriptional regulators of differentiation have been identified, their target genes responsible for the morphological changes occurring during OL myelination are still largely unknown. Here, we show that the regulator of focal adhesion, Tensin3 (Tns3), is a direct target gene of Olig2, Chd7, and Chd8, transcriptional regulators of OL differentiation. Tns3 is transiently upregulated and localized to cell processes of immature OLs, together with integrin-{beta}1, a key mediator of survival at this transient stage. Constitutive Tns3 loss-of-function leads to reduced viability in mouse and humans, with surviving knockout mice still expressing Tns3 in oligodendroglia. Acute deletion of Tns3 in vivo, either in postnatal neural stem cells (NSCs) or in OPCs, leads to a two-fold reduction in OL numbers. We find that the transient upregulation of Tns3 is required to protect differentiating OPCs and immature OLs from cell death by preventing the upregulation of p53, a key regulator of apoptosis. Altogether, our findings reveal a specific time window during which transcriptional upregulation of Tns3 in immature OLs is required for OL differentiation likely by mediating integrin-{beta}1 survival signaling to the actin cytoskeleton as OL undergo the large morphological changes required for their terminal differentiation.

neuroscience↗