LINC Complexes Mediate the Positioning of Cone Photoreceptor Nuclei in Mouse Retina

Abstract

It has long been observed that many neuronal types position their nuclei within restricted cytoplasmic boundaries. A striking example is the apical localization of cone photoreceptors nuclei at the outer edge of the outer nuclear layer of mammalian retinas. Yet, little is known about how such nuclear spatial confinement is achieved and further maintained. Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) consist of evolutionary-conserved macromolecular assemblies that span the nuclear envelope to connect the nucleus with the peripheral cytoskeleton. Here, we applied a new transgenic strategy to disrupt LINC complexes either in cones or rods. In adult cones, we observed a drastic nuclear mislocalization on the basal side of the ONL that affected cone terminals overall architecture. We further provide evidence that this phenotype may stem from the inability of cone precursor nuclei to migrate towards the apical side of the outer nuclear layer during early postnatal retinal development. By contrast, disruption of LINC complexes within rod photoreceptors, whose nuclei are scattered across the outer nuclear layer, had no effect on the positioning of their nuclei thereby emphasizing differential requirements for LINC complexes by different neuronal types. We further show that Sun1, a component of LINC complexes, but not A-type lamins, which interact with LINC complexes at the nuclear envelope, participate in cone nuclei positioning. This study provides key mechanistic aspects underlying the well-known spatial confinement of cone nuclei as well as a new mouse model to evaluate the pathological relevance of nuclear mispositioning.

Nuclear migration events throughout development

“…Klarsicht, its SUN partner Klaroid, lamin, dynactin and dynein work together to move nuclei in the model pseudostratified epithelium of Drosophila eye discs (Fan and Ready, 1997;Kracklauer et al, 2007;Patterson et al, 2004). Likewise, in the developing mouse neocortex or retina, mutations in the LINC complex components disrupt interkinetic nuclear migration (Razafsky et al, 2012;Zhang et al, 2009). As nesprin-2 interacts with both dynein and kinesin-1, this suggests that KASHdomain proteins function here by coordinating microtubule motors at the surface of migrating nuclei ).…”

“…KASH-domain proteins might also be involved in kinesin-mediated basal movements during G1/S phase. However, nuclear positioning is not completely disrupted by mutations within LINC complexes in developing eye epithelia of fly, zebrafish or mouse (Kracklauer et al, 2007;Patterson et al, 2004;Razafsky et al, 2012;Tsujikawa et al, 2007;Yu et al, 2011), suggesting that other mechanisms are sufficient for at least some nuclear migration events in these tissues. Furthermore, it is worth noting that, in other systems discussed below, LINC complexes appear dispensable for interkinetic nuclear migration.…”

Migration, Integration and Maturation of Photoreceptor Precursors Following Transplantation in the Mouse Retina

“…While the apically directed migration appears to be an active process and requires LINC complex proteins [50,51], it is not yet clear whether the basally directed movements involve migration or whether the cell bodies are simply pushed basally by the birth of new cells at the apical surface. Similarly, nothing is known regarding whether and how immature rods migrate in development.…”

Developmental regulation of linkers of the nucleoskeleton to the cytoskeleton during mouse postnatal retinogenesis

“…[25][26][27][28][29] Evidence from several species suggests that an evolutionaryconserved "molecular axis" consisting of lamins/Sun proteins/ nesprins/cytoplasmic cytoskeletal elements regulates nuclear movement and anchorage. 13,21 For example, in D. melanogaster, the migration of photoreceptor precursor nuclei is similarly altered by the mutation of LaminDm 0 (B-type lamin), klaroid (SUN), Klarsicht (KASH) or the glued subunit of dynactin. [30][31][32][33] Likewise in zebrafish, either the overexpression of p50/dynamitin, which dissociates the dynactin complex, or the overexpression of the KASH domain, which interferes with endogenous LINC complex assembly in a dominant-negative manner, lead to mispositioning of photoreceptor nuclei.…”