LKB1 coordinates neurite remodeling to drive synapse layer emergence in the outer retina

Abstract

Structural changes in pre and postsynaptic neurons that accompany synapse formation often temporally and spatially overlap. Thus, it has been difficult to resolve which processes drive patterned connectivity. To overcome this, we use the laminated outer murine retina. We identify the serine/threonine kinase LKB1 as a key driver of synapse layer emergence. The absence of LKB1 in the retina caused a marked mislocalization and delay in synapse layer formation. In parallel, LKB1 modulated postsynaptic horizontal cell refinement and presynaptic photoreceptor axon growth. Mislocalized horizontal cell processes contacted aberrant cone axons in LKB1 mutants. These defects coincided with altered synapse protein organization, and horizontal cell neurites were misdirected to ectopic synapse protein regions. Together, these data suggest that LKB1 instructs the timing and location of connectivity in the outer retina via coordinate regulation of pre and postsynaptic neuron structure and the localization of synapse-associated proteins.

LKB1 and AMPK instruct cone nuclear position to modify visual function

“…First, we show that deletion of the LKB1 target AMPK causes nuclear translocation defects similar to that observed in LKB1 mutant mice. This is notable, because we have shown that AMPK is dispensable for other early features of OPL organization ( Burger et al, 2020 ). AMPK could participate in nuclear translocation through at least two mechanisms.…”

“…We asked whether there was a cell-intrinsic requirement for LKB1 in cones by selectively deleting Lkb1 from these cells using a cone-specific Cre line ( Figure 2A ; HRGP-Cre ; [ Le et al, 2004 ], hereinafter referred to as LKB1 CONE ). Because this Cre becomes active at P8 ( Le et al,2004 ; Dilan et al, 2018 ; Razafsky et al, 2012 ), this line enables us to assess the role of LKB1 specifically during nuclear translocation and bypass roles for this molecule in other aspects of retina development ( Burger et al, 2020 ). Deletion of Lkb1 in cones alone induced nuclear translocation defects similar to that observed in whole retina deletion ( Figure 2B ).…”

“…Expansion microscopy was performed as previously described ( Asano et al, 2018 ; Burger et al, 2020 ). In brief, tissue samples were prepared for immunohistochemistry as described above.…”

“…LKB1 regulates 14 AMPK family kinases, AMPKα1/α2, SAD-A/-B, NUAK1/2, SIK1–3, MARK1–4, and SNRK ( Lizcano et al, 2004 ; Jaleel et al, 2005 ). LKB1 was a good candidate for our studies because it plays cell-specific roles in the polarization of several neuron types, and we recently showed that this property extends to the outer retina, where it coordinates neurite remodeling to drive synapse layer emergence ( Kuwako and Okano, 2018 ; Shelly et al, 2007 ; Barnes et al, 2007 ; Courchet et al, 2013 ; Burger et al, 2020 ). The LKB1 pathway has also been implicated in centrosome positioning, a key step in neuron migration ( Asada et al, 2007 ; Asada and Sanada, 2010 ).…”

A Picture Worth a Thousand Molecules—Integrative Technologies for Mapping Subcellular Molecular Organization and Plasticity in Developing Circuits

“…For ease-of-use and increased application in diverse biological contexts, ExM labeling methods have been simplified (Chozinski et al, 2016;Tillberg et al, 2016) and optimized protocols have been established for applications in model systems important for developmental analysis including zebrafish (Freifeld et al, 2017), C. elegans (Yu et al, 2020), and Drosophila (Jiang et al, 2018;Gao et al, 2019a; Figures 5D-F). ExM has revealed subtle defects in the developing fly (Menon et al, 2019) and mouse visual system (Burger et al, 2020) in genetic loss-of-function deletion experiments impacting synaptogenesis.…”