STEF/TIAM2-mediated Rac1 activity at the nuclear envelope regulates the perinuclear actin cap

Abstract

The perinuclear actin cap is an important cytoskeletal structure that regulates nuclear morphology and re-orientation during front-rear polarisation. The mechanisms regulating the actin cap are currently poorly understood. Here, we demonstrate that STEF/TIAM2, a Rac1 selective guanine nucleotide exchange factor, localises at the nuclear envelope, co-localising with the key perinuclear proteins Nesprin-2G and Non-muscle myosin IIB (NMMIIB), where it regulates perinuclear Rac1 activity. We show that STEF depletion reduces apical perinuclear actin cables (a phenotype rescued by targeting active Rac1 to the nuclear envelope), increases nuclear height and impairs nuclear re-orientation. STEF down-regulation also reduces perinuclear pMLC and decreases myosin-generated tension at the nuclear envelope, suggesting that STEF-mediated Rac1 activity regulates NMMIIB activity to promote stabilisation of the perinuclear actin cap. Finally, STEF depletion decreases nuclear stiffness and reduces expression of TAZ-regulated genes, indicating an alteration in mechanosensing pathways as a consequence of disruption of the actin cap.

Actin-Based Cell Protrusion in a 3D Matrix

“…Actin regulators associated with the nuclear envelope are able to change the characteristics of existing actin filaments to support nuclear movement and force transduction. The Rac GEF STEF/TIAM2 localises to the nuclear envelope, and controls perinuclear Rac activity to regulate actin dynamics and contractility at this subcellular region [119] . Furthermore, the actin-bundling activity of FHOD1 [120] and fascin [121] can support the formation of thick actin fibres associated with the nucleus.…”

Ctdnep1 and Eps8L2 regulate dorsal actin cables for nuclear positioning during cell migration

“…In the last years, perinuclear actin organization and function have been revealed important for nuclear positioning and cell migration [27,32,62]. Actin organization around the nucleus is crucial for dendritic cells to migrate in 3D environments [62].…”

“…The perinuclear actin cap comprises actin bundles on the dorsal side of the nucleus that are attached to focal adhesions. Furthermore, perinuclear actin cap is involved in nuclear shape and mechanotransduction but not in nuclear movement [25,27–32].…”

Actin chromobody imaging reveals sub-organellar actin dynamics

“…SEM imaging of non-transfected cells treated with saponin and cytoskeleton stabilizing buffer, we observed a similar network of actin fibers accumulating on the nucleus, validating the localization pattern observed in our AC-ER fluorescence data (Figure 3g). A perinuclear actin cap structure has been previously reported and is known to have important roles in multiple cellular processes [22][23][24][25][26][27] . However, as with other actin structures, the actin filaments specifically associated with the nucleus have been difficult to visualize with pan-actin probes due to the overwhelming signal from surrounding actin stress fibers and cortical actin.…”