What are cytoskeletal cages?
Cytoskeletal components can assemble into cage-like structures within the cell. These cages serve to segregate intracellular regions or organelles, and can protect their contents from mechanical stress.
At the level of the whole cell, the cell cortex is a meshwork of actin filaments and myosin motor proteins that lies just underneath the plasma membrane. Extensive crosslinking of actin filaments can form a cage-like barrier between the ectoplasm (the outer, non-granulated layer of the cytoplasm) and the endoplasm (the inner, dense region of the cytoplasm that contains the organelles). On a smaller scale, localized external forces at the cell periphery lead to formin-dependent assembly of a perinuclear actin ring or cage that surrounds and protects the nucleus [1]. Myosin VI can initiate the formation of actin cages that encapsulate damaged mitochondria to ensure their isolation and eventual removal from the mitochondrial homeostasis network [2].
Vimentin intermediate filaments have been reported to form a cage around melanosome pigment granules which can impede their transport [3], and a similar cage is assembled to surround lipid droplets during adipose conversion [4], [5]. Although it is not commonly thought of as a cytoskeletal cage, the lamin network that lies underneath the nuclear membrane provides three-dimensional structure and support to the nucleus, and directs chromosome organization.
During mitosis, microtubules assemble to form a spindle apparatus that encages the duplicated chromosomes and subsequently pulls them apart to create two sets of chromosomes for each daughter cell. This mitotic spindle contains three types of microtubules; astral microtubules that connect the spindle to the cell cortex, kinetochore microtubules that attach individual chromosomes to the spindle, and interpolar microtubules that connect the two halves of the spindle.
Septins predominantly assemble into ring-like structures that act as a scaffold for other cytoskeletal elements during cytokinesis. However, septin rings can further assemble into cage-like structures that are targeted to bacteria in order to label them for autophagy [6], [7].
References
- Shao X, Li Q, Mogilner A, Bershadsky AD, and Shivashankar GV. Mechanical stimulation induces formin-dependent assembly of a perinuclear actin rim. Proc. Natl. Acad. Sci. U.S.A. 2015; 112(20):E2595-601. [PMID: 25941386]
- Kruppa AJ, Kishi-Itakura C, Masters TA, Rorbach JE, Grice GL, Kendrick-Jones J, Nathan JA, Minczuk M, and Buss F. Myosin VI-Dependent Actin Cages Encapsulate Parkin-Positive Damaged Mitochondria. Dev. Cell 2018; 44(4):484-499.e6. [PMID: 29398621]
- Chang L, Barlan K, Chou Y, Grin B, Lakonishok M, Serpinskaya AS, Shumaker DK, Herrmann H, Gelfand VI, and Goldman RD. The dynamic properties of intermediate filaments during organelle transport. J. Cell. Sci. 2009; 122(Pt 16):2914-23. [PMID: 19638410]
- Franke WW, Hergt M, and Grund C. Rearrangement of the vimentin cytoskeleton during adipose conversion: formation of an intermediate filament cage around lipid globules. Cell 1987; 49(1):131-41. [PMID: 3548999]
- Heid H, Rickelt S, Zimbelmann R, Winter S, Schumacher H, Dörflinger Y, Kuhn C, and Franke WW. On the formation of lipid droplets in human adipocytes: the organization of the perilipin-vimentin cortex. PLoS ONE 2014; 9(2):e90386. [PMID: 24587346]
- Mostowy S, Bonazzi M, Hamon MA, Tham TN, Mallet A, Lelek M, Gouin E, Demangel C, Brosch R, Zimmer C, Sartori A, Kinoshita M, Lecuit M, and Cossart P. Entrapment of intracytosolic bacteria by septin cage-like structures. Cell Host Microbe 2010; 8(5):433-44. [PMID: 21075354]
- Sirianni A, Krokowski S, Lobato-Márquez D, Buranyi S, Pfanzelter J, Galea D, Willis A, Culley S, Henriques R, Larrouy-Maumus G, Hollinshead M, Sancho-Shimizu V, Way M, and Mostowy S. Mitochondria mediate septin cage assembly to promote autophagy of Shigella. EMBO Rep. 2016; 17(7):1029-43. [PMID: 27259462]
