Development and Plasticity of Outer Retinal Circuitry Following Genetic Removal of Horizontal Cells

Abstract

The present study examined the consequences of eliminating horizontal cells from the outer retina during embryogenesis upon the organization and assembly of the outer plexiform layer (OPL). Retinal horizontal cells exhibit a migration defect in Lim1-conditional knock-out (Lim1-CKO) mice and become mispositioned in the inner retina before birth, redirecting their dendrites into the inner plexiform layer. The resultant (mature) OPL, developing in the absence of horizontal cells, shows a retraction of rod spherules into the outer nuclear layer and a sprouting of rod bipolar cell dendrites to reach ectopic ribbon-protein puncta. Cone pedicles and the dendrites of type 7 cone bipolar cells retain their characteristic stratification and colocalization within the collapsed OPL, although both are atrophic and the spatial distribution of the pedicles is disrupted. Developmental analysis of Lim1-CKO retina reveals that components of the rod and cone pathways initially co-assemble within their normal strata in the OPL, indicating that horizontal cells are not required for the correct targeting of photoreceptor terminals or bipolar cell dendrites. As the rod spherules begin to retract during the second postnatal week, rod bipolar cells initially show no signs of ectopic growth, sprouting only subsequently and continuing to do so well after the eighth postnatal week. These results demonstrate the critical yet distinctive roles for horizontal cells on the rod and cone pathways and highlight a unique and as-yet-unrecognized maintenance function of an inhibitory interneuron that is not required for the initial targeting and co-stratification of other components in the circuit.

Onecut1 and Onecut2 redundantly regulate early retinal cell fates during development

“…The collapse of the OPL in the DKO retinas was consistent with previous findings that development of the OPL is dependent on HCs (14,16). Loss of HCs leads to loss of ribbon synapses and flat contacts between photoreceptors and bipolar cells (14,16).…”

“…Loss of HCs leads to loss of ribbon synapses and flat contacts between photoreceptors and bipolar cells (14,16). To further characterize the defects in the OPL in the DKO retina, we used fluor-conjugated peanut agglutinin (PNA) to stain the cone flat contacts with bipolar cells.…”

Sip1 regulates the generation of the inner nuclear layer retinal cell lineages in mammals

“…The massive reduction in the plexiform layers observed in Sip1 mutants is likely to reflect the role of Sip1 in controlling the numbers of INL cells, as a similar collapse of plexiform layers was observed in Foxn4 and Lim1 mutant retinae in which HCs and ACs are affected (Keeley et al, 2013;Li et al, 2004). The reduction in b-wave amplitudes observed in our ERG recordings is consistent with the collapse of the OPL and the reduction in the number of BPs.…”

Neuronal remodeling in retinal circuit assembly, disassembly, and reassembly

“…In animals with impaired photoreceptor transmission such as the cacna1f mutant, nob2 [42, 73] and the CNGA3/CNGB1 double-knockout mutant [74], horizontal cell and RBC processes initially target the OPL correctly, but later sprout (Figure 4A). Intriguingly, CBCs appear less susceptible to sprouting compared to RBCs, except when horizontal cells are ablated in adult mouse retina [75, 76]. Although it is not yet understood why dendritic sprouting is more commonly observed for RBCs compared to CBCs, this finding suggests that the repair of rod and cone pathways may require distinct strategies.…”

“…Horizontal cell processes sprout but bipolar cell dendrites remain laminated in a mutant in which horizontal cells lack a synaptic adhesion molecule receptor, NGL-2 (Figure 3B) [82]. Also, RBCs are found to send ectopic processes into the ONL, contacting retracted rod photoreceptor axons when some horizontal cells fail to reach the OPL [76] or are ablated in adult retina [75]. Thus, the sprouting of neurites from different second-order neuron populations appears to be independently regulated during retinal disassembly.…”