Reviewer #1 (Public review):
Summary:
This is an interesting study describing intensity changes of lamellipodial Arp2/3 complex incorporation dependent on the substratum the cells are spreading on (PLL vs fibronectin), but also on manipulation of either contractility or osmotic pressure or even external mechanical load exerted onto cells, e.g., by increasing medium viscosity. The authors use quite fancy cell systems for their studies, first of all, a CRISPR-engineered fibroblast cell line in which both endogenous loci of the Arp2/3 complex subunit Arpc2 are tagged with mScarlet, but at the same time, conditionally removable using tamoxifen. These lines, optionally also harboring Pxn-GFP and Lifeact-miRFP670, have previously been described by the authors (Chandra et al, 2022, PMID: 34861242). In addition, they use cells allowing local photoactivation of Rac signalling through a Tiam1 activation module combined with Halo-tagged Arpc2, apparently stably co-expressed in tamoxifen-treated Arpc2-KO fibroblasts. These cells may or may not have been published previously.
Overall, the study provides convincing evidence that Arp2/3 complex accumulation in the lamellipodium negatively correlates with its width and perhaps the mechanical load these actin networks are exposed to at the leading edge membrane, shown initially through allowing cells to spread on substrates in which the formation of integrin-based adhesions is poor (PLL) or stimulated (through fibronectin). In the latter case, lamellipodia are comparably narrow, perhaps reasonably well clutched, and thus feel sufficient counter-force at the leading edge membrane to build a dense, Arp2/3-dependent actin network. Albeit interesting and important to show as the authors did, these results are not entirely surprising given the literature published on actin remodeling in cells in conditions similar to those used by the authors (i.e., on PLL). Thus, the results should be better embedded into the context of this previous literature to more precisely reveal which aspects are new and interesting and which ones are more or less intuitive and expected.
However, the authors also show yet another result, which is quite spectacular indeed, revealing dramatic local protrusion of a Rac-dependent lamellipodium on PLL only in the presence of methylcellulose, but not on PLL alone. Although the authors cannot fully explain the mechanisms causing these results, they are thought-provoking and will certainly stimulate future, relevant research on this topic and new insights. Altogether, I think this is an interesting study that can be shared rapidly, given that the authors provide more experimental detail and transparency concerning their used cell model systems. Aside from a few other suggestions for amendments and corrections, I would also recommend citing classical literature that has provided the basis for the interpretation of the results shown here, as specified below.
Specific criticism and comments:
(1) I feel the paper is interesting for actin remodeling and Arp2/3 complex aficionados, but quite difficult to read and to understand in places for non-experts in the field, so I think the text requires more detailed explanation of specific terms, model systems used, and overall correction of either grammatical or semantic errors, or colloquial language.
(2) In general, I think the characterization of Arp2/3 complex incorporation into the lamellipodia of cells spreading on PLL versus FN is interesting, as it has not been done previously in such a systematic fashion to my knowledge. However, I think the authors could emphasize better how this relates to previously established structural features of actin filament networks, published on PLL. So more than 3 decades ago, Hotchin & Hall published clear evidence that starved fibroblasts can only form focal complexes or adhesions downstream of PDGF or LPA-stimulation if seeded on FN, but not on PLL (see Figure 1 in PMID: 8557752). Around the same time, Flinn and Ridley showed this virtual absence of classical, Rac-dependent focal complexes to be accompanied by the formation of beautiful, broad lamellipodia (see Fig. 1A in PMID: 8743960), which only formed in the absence of excess RhoA activity and thus contractility by the way (see also below). A few years later, Small et al summarized all these phenotypes in a comprehensive review and also showed that cells on PLL (similar to the rapidly migrating keratocytes) combined large, flat lamellipodia with tiny, nascent adhesions scattered throughout these structures (see Figure 2 in PMID: 10047522). These authors also noted that the sole inhibitor-mediated reduction of contractility could switch FN-phenotypes with narrow, ruffling lamellipodia and peripheral focal complexes back to a PLL-type phenotype of broad lamellipodia (see Figure 1 in PMID: 10047522). In the following decade then, different labs (Verkhovsky, Bershadsky, Vavylonis, Watanabe et al) showed beautiful phase contrast or fluorescence movies illustrating that the broad lamellipodial phenotype of cells plated on PLL was accompanied by low frequency membrane ruffling and instead a rapid, continuous rearward flow of continuously assembling actin filament networks, partly also directly shown with actin networks labeled with both LifeAct and Arp2/3 complex subunits (see e.g. PMIDs 18800171 and 22500749). In Alexandrova et al, 2008 (PMID 18800171), authors showed that the formation of adhesions in spreading cells triggers the transition from fast to slow flow (which is of course relevant to the current study and conclusions), whereas Ryan et al, 2012 (PMID 22500749) already established the broad incorporation of actin and Arp2/3 complex into the very broad lamellipodia formed on PLL by Xenopus fibroblasts and the rapid flow of both components from distal to proximal lamellipodial regions. None of these seminal studies has been cited, although they are highly relevant for the interpretation and conclusions of the results presented. I would strongly recommend specifically referring to these studies, as this will actually support the conclusions and interpretations drawn.
(3) On the subject of literature, on the second page of the intro, end of 2nd paragraph, the authors describe Rac signaling to Arp2/3 complex through WRC considered essential for Arp2/3-mediated actin assembly at lamellipodial leading edges, but aside from one of their own papers cite none of the seminal studies by Insall, Scita, Stradal, Rottner, Bogdan labs having published seminal aspects on this pathway.
Considering the rapid F-actin flow in lamellipodia, obviously accompanied by admittedly sparse but continuous Arp2/3 complex incorporation, it is not so surprising that the latter will be obligatory here, and also the accumulation of its prominent activator WRC, as well as the branch stabilizer cortactin. Thus, the data described on page 3 of the Results section could also be framed in the context of all this previously published knowledge, providing a more comprehensive and realistic view of the relevance and novelty of the described data.
(4) In the abstract, the authors state in the context of the force-feedback mechanism established in vitro for the formation of Arp2/3 complex-dependent actin networks that "this phenomenon has not been explored through the examination of real-time responses of endogenous actin networks in cells". In my view, this is not correct, as in their prominent Cell paper, the Sixt laboratory has done exactly that (Mueller et al, 2017, PMID: 18800171). Although Mueller et al have not looked at Arp2/3 complex dynamics as far as I recall, they have still connected the extent and hence intensity of actin networks at the leading edges of keratocyte lamellipodia with the forces exerted onto them, including direct experimental manipulation of those forces. Although the study has been cited in an independent context, this point should be made clear, and the corresponding sentence in the abstract should be amended.
(5) One point that struck me a little bit was the authors' detailed description of cell spreading on PLL and the quite strong variability of Arp2/3 incorporation dependent on the timing after spreading (as for instance the very strong and quite narrow Arp2/3 leading edge intensity at 2 hours post-seeding in Figure 3S2D). In the authors' view, they have worked with a very clean system, as they emphasized to even have eliminated the FN-locus in their cells, excluding the secretion of endogenous FN (PMID: 34861242), but how about ECM components potentially present in serum, such as, for instance, vitronectin? Indeed, it looks like the authors have done all experiments in the presence of 10% serum as far as I can see, although most of the classical PLL-experiments mentioned above have been performed with starved cells in the absence of serum. I think it would generate a more complete picture of the phenotypes and results as compared to the literature if the authors performed a subset of the key experiments on PLL without serum. I don't think the starving of cells as such is important and could be counteracted by simply lamellipodia-inducing growth factors adding into the spreading medium, traditionally perhaps PDGF or EGF (dependent on the receptor distribution of those fibroblasts), but the absence of serum would have two advantages: it would not only exclude any potential impact of serum-containing ECM components, but also alleviate the hyperstimulation of the Rho-pathway through LPA-bound BSA, the major serum-protein, which has previously been shown to counteract the "undisturbed" formation of PLL-type lamellipodia (see Figure 1B in Flinn & Ridley, PMID: 8743960).
(6) Regarding the scanning EM-images shown in the Supplement, currently called Figure 3S2A and -B (in the text erroneously termed Figures 3S1A and-B, see above). I am not sure how representative these individual EM-images of the cell plated on PLL are, given the data of rapid rearward flow of actin and Arp2/3 complex subunits, at least at early stages of spreading. Again, the classical literature on PLL-type lamellipodia and, in particular, previously published movies of such lamellipodia suggest broad lamellipodia with few ruffles, and the opposite with cells plated on FN. So in this context, the scanning EM-data shown on both PLL and FN do neither fit the authors' own data very well nor the literature, and I would recommend making sure that the individual cells selected were (i) correctly annotated and (ii) representative of a specific time point of spreading actually fitting the previously described data.
(7) It also surprised me to see that the authors describe the spreading process on PLL to actually be much slower than on FN (see Figure 3S2C - in the text Figure 3S1C). It is tempting to speculate that this might change if plating the cells in serum-free medium, as traditionally, full spreading and lamellipodia formation downstream of PDGF-stimulation (at least in 3T3 fibroblasts) is described to occur in the range of 10-30 minutes at maximum, and not several hours as shown here. This point could also be considered, or at least discussed.
(8) The movies are of very high quality and beautiful to look at, but it would help the reader to get a bit more information in the legends (like the meaning of the time-stamps, which will display elapsed time in minutes:seconds I assume, but this info is missing from the legends as far as I can see. Also, it would help the reader to better mark in the movies when a specific treatment kicks in. For instance, in movie 10, the legend states treatment starts at 10:00 (minutes:seconds?), but it would help very much if the authors could paste the term "blebbistatin" directly into the movie, beginning with the frame of treatment start.