FHOD1 interaction with nesprin-2G mediates TAN line formation and nuclear movement

Abstract

Active positioning of the nucleus is integral to division, migration, and differentiation of mammalian cells1. Fibroblasts polarizing for migration orient their centrosomes by actin-dependent nuclear movement2. This nuclear movement depends on nesprin-2 giant (N2G), a large, actin-binding outer nuclear membrane component of transmembrane actin-associated (TAN) lines that couple nuclei to moving actin cables3. Here, we identify the diaphanous formin FHOD1 as an interaction partner of N2G. Silencing FHOD1 expression or expression of fragments containing binding sites of N2G or FHOD1 disrupted nuclear movement and centrosome orientation in polarizing fibroblasts. Unexpectedly, silencing of FHOD1 expression did not affect the formation or rearward flow of dorsal actin cables required for nuclear positioning. Rather, N2G-FHOD1 interaction provided a second connection to actin cables essential for TAN line formation and thus nuclear movement. These results reveal a unique function for a formin in coupling an organelle to actin filaments for translocation and suggest that TAN lines require multi-point attachments to actin cables to resist the large forces necessary to move the nucleus.

Direct force probe reveals the mechanics of nuclear homeostasis in the mammalian cell

“…Because F-actin and microtubule motors are known to exert forces on the nuclear surface (2,5,6,18,(25)(26)(27)(28), it is surprising that they do not contribute to mechanical homeostasis of the nucleus. It seems that these cytoskeletal networks have the primary role of generating active forces through motor activity on the nuclear surface to position it.…”

“…It is known that the migrating cell moves the nucleus by transferring cytoskeletal forces through connections between the cytoskeleton and the nuclear surface (1,2). Even in a stationary cell, the nuclear shape and central position are stably maintained in mechanical homeostasis at defined locations in the cell, despite the fact that the dynamic cytoskeleton continues to generate constantly fluctuating forces on the nucleus (3,4).…”

“…It is known that the migrating cell moves the nucleus by transferring cytoskeletal forces through connections between the cytoskeleton and the nuclear surface (1, 2). Even in a stationary cell, the nuclear shape and central position are stably maintained in mechanical homeostasis at defined locations in the cell, despite the fact that the dynamic cytoskeleton continues to generate constantly fluctuating forces on the nucleus (3, 4).The source of fluctuating forces on the nucleus includes nuclear-embedded microtubule motors such as dynein and kinesin (5-7) and actomyosin forces that push on and pull the nucleus (2,8,9). The nucleus is also exposed to extracellular forces, such as those applied to adhesion receptors, which can be transmitted through the cytoskeleton onto the nuclear surface (10-12).…”

“…The source of fluctuating forces on the nucleus includes nuclear-embedded microtubule motors such as dynein and kinesin (5-7) and actomyosin forces that push on and pull the nucleus (2,8,9). The nucleus is also exposed to extracellular forces, such as those applied to adhesion receptors, which can be transmitted through the cytoskeleton onto the nuclear surface (10-12).…”

Nuclear migration events throughout development

“…On the cytoplasmic side of TAN lines, the formin FHOD1 plays a structural role. FHOD1 is not necessary for the formation of the actin cables or their retrograde movement but, instead, acts as a second connection between nesprin-2G and actin thus reinforcing the coupling of the nucleus to actin cables (Kutscheidt et al, 2014). On the basis of these data, we have a solid understanding of the molecular mechanism and players, including actin cables and lamins, that connect nuclei to retrograde flow.…”