cells Article SUN-MKL1 Crosstalk Regulates Nuclear Deformation and Fast Motility of Breast Carcinoma Cells in Fibrillar ECM Microenvironment Ved P. Sharma 1,2, * , James Williams 3 , Edison Leung 1 , Joe Sanders 3 , Robert Eddy 1 , James Castracane 3 , Maja H. Oktay 1,2,4,5 , David Entenberg 1,2,4 and John S. Condeelis 1,2,4,6, *   Citation: Sharma, V.P.; Williams, J.; Leung, E.; Sanders, J.; Eddy, R.; Castracane, J.; Oktay, M.H.; Entenberg, D.; Condeelis, J.S. SUN-MKL1 Crosstalk Regulates Nuclear Deformation and Fast Motility of Breast Carcinoma Cells in Fibrillar ECM Microenvironment. Cells 2021, 10, 1549. https://doi.org/ 10.3390/cells10061549 Academic Editor: Hava Gil-Henn Received: 9 June 2021 Accepted: 16 June 2021 Published: 19 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA; edison.leung@gmail.com (E.L.); robert.eddy@einsteinmed.org (R.E.); moktay@montefiore.org (M.H.O.); david.entenberg@einsteinmed.org (D.E.) 2 Gruss Lipper Biophotonics Center, Albert Einstein College of Medicine, Bronx, NY 10461, USA 3 Colleges of Nanoscale Science and Engineering, SUNY Polytechnic Institute, Albany, NY 12203, USA; jkwilliams333@gmail.com (J.W.); jmsanders905@gmail.com (J.S.); jim.castracane@gmail.com (J.C.) 4 Integrated Imaging Program, Albert Einstein College of Medicine, Bronx, NY 10461, USA 5 Department of Pathology, Albert Einstein College of Medicine, Bronx, NY 10461, USA 6 Department of Surgery, Albert Einstein College of Medicine, Bronx, NY 10461, USA * Correspondence: ved.sharma@einsteinmed.org (V.P.S.); john.condeelis@einsteinmed.org (J.S.C.) Abstract: Aligned collagen fibers provide topography for the rapid migration of single tumor cells (streaming migration) to invade the surrounding stroma, move within tumor nests towards blood vessels to intravasate and form distant metastases. Mechanisms of tumor cell motility have been studied extensively in the 2D context, but the mechanistic understanding of rapid single tumor cell motility in the in vivo context is still lacking. Here, we show that streaming tumor cells in vivo use collagen fibers with diameters below 3 μm. Employing 1D migration assays with matching in vivo fiber dimensions, we found a dependence of tumor cell motility on 1D substrate width, with cells moving the fastest and the most persistently on the narrowest 1D fibers (700 nm–2.5 μm). Interestingly, we also observed nuclear deformation in the absence of restricting extracellular matrix pores during high speed carcinoma cell migration in 1D, similar to the nuclear deformation observed in tumor cells in vivo. Further, we found that actomyosin machinery is aligned along the 1D axis and actomyosin contractility synchronously regulates cell motility and nuclear deformation. To further investigate the link between cell speed and nuclear deformation, we focused on the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex proteins and SRF-MKL1 signaling, key regulators of mechanotransduction, actomyosin contractility and actin-based cell motility. Analysis of The Cancer Genome Atlas dataset showed a dramatic decrease in the LINC complex proteins SUN1 and SUN2 in primary tumor compared to the normal tissue. Disruption of LINC complex by SUN1 + 2 KD led to multi-lobular elongated nuclei, increased tumor cell motility and concomitant increase in F-actin, without affecting Lamin proteins. Mechanistically, we found that MKL1, an effector of changes in cellular G-actin to F-actin ratio, is required for increased 1D motility seen in SUN1 + 2 KD cells. Thus, we demonstrate a previously unrecognized crosstalk between SUN proteins and MKL1 transcription factor in modulating nuclear shape and carcinoma cell motility in an in vivo relevant 1D microenvironment. Keywords: breast cancer invasion and metastasis; in vivo tumor cell motility; nuclear deformation; LINC complex; SRF-MKL1 signaling 1. Introduction The tumor microenvironment (TME) plays an essential role in breast cancer invasion and metastasis [1–5]. TME consists of ECM, stromal cells (e.g., cancer associated fibroblasts, adipocytes) and immune cells (e.g., macrophages, neutrophils, etc.) [6–8], and drives breast Cells 2021, 10, 1549. https://doi.org/10.3390/cells10061549 https://www.mdpi.com/journal/cells