LINC in Sperm Head Formation Figure 7. Model illustrating LINC complex polarization during sperm head formation. Two novel spermiogenesis-specific LINC complexes polarize to opposite nuclear poles where they appear to connect the nucleus/acrosome to surrounding manchette microtubules (MT) and F-actin, respectively. The anterior polarizing non-nuclear LINC complex (red) consists of Sun1g and Nesprin3, both mainly located within the outer acrosomal membrane (OAM). Interacting with plectin Sun1g/Nesprin3 LINC complexes could anchor to anterior actin filaments present at membranous junction of spermatids and Sertoli cells. The posterior polarizing LINC complex (green) is formed by Sun3 and Nesprin1. Interaction of Nesprin1 with microtubules (i.e. via dynein/dynactin and/or kinesin II) would enable the linkage of Sun3/Nesprin1 LINC complexes to the posterior manchette. Thus, both poles of differentiating spermatid nuclei would be connected to cytoskeletal elements by two distinctive LINC complexes. doi:10.1371/journal.pone.0012072.g007 unpublished data), they actually do not interact, at least in the given cellular context. Similarly, via its SUN domain Sun3 has the potential to directly interact with the KASH domain of Nesprin3, but this alternative to form a LINC complex appears excluded during spermiogenesis. Hence, our data show for the first time that, even though diverse SUN and KASH populations might be present within the same cell, they do not generally share their LINC complex partners. Rather, they can distinguish between the partners to form accurately defined LINC complexes which could be assigned for distinct tasks. This appears at least to be true for postmeiotic germ cells: two different but well-defined LINC complexes polarize to opposing poles and hence link to different cytoskeletal structures (Figure 7; see below). However, how SUN proteins can take a decision to which KASH partner they actually bind and how this differential binding is regulated, remains an interesting and important quest for the future. Previous studies have reported that structural integrity of LINC complexes as well as the general amount of their components are critical parameters for maintaining nuclear shape. In particular, Olins and colleagues [30] documented that granulocytes, a class of highly deformable white blood cells having exceptional shaped nuclei (i.e. highly lobulated), are characterized by a remarkable paucity of LINC complex components. This specific modulation during granulocyte differentiation could directly affect nucleo- cytoskeletal linkage and in turn is suggested to be an evolutionary adaptation to facilitate nuclear and cellular deformability [30]. Evidence for a direct influence of SUN domain proteins on nuclear morphology came from a study which showed that in Dictyostelium the expression of an N-terminally truncated Sun1 led to its mislocalization and thence to severe nuclear deformations [31]. Likewise, depletion of or mutations in nesprins have dramatic consequences on general nuclear architecture and shape [32,55] and affected nuclei show serious deformations [56]. Thus, the amounts as well as the formation of functional LINC complexes are in fact critical for nuclear morphology and shape. However, given that not only decreased levels of LINC complex components or their mislocalization can alter nuclear shape but also target-oriented redistribution of SUN and KASH proteins, it appears quite conceivable that LINC complexes are also critical for mediating sperm head formation. The Sun3/Nesprin1 LINC complex at the posterior sperm head pole The presence of Sun1g/Nesprin3 anterior as well as Sun3/ Nesprin1 posterior remarkably resembles the distribution of spermatid-specific cytoskeletal structures. Our data obtained by PLoS ONE | www.plosone.org 9 August 2010 | Volume 5 | Issue 8 | e12072