Microglial involvement in experience-driven pruning of miswired retinal projections in Ten-m3 deficient mice

Abstract

Precise neural connectivity is vital for brain function. The template for this is set-up during development through a spatiotemporal array of genetically determined cues. Neural activity, in part regulated by experience, also plays an important role in establishing neural connectivity though. Of note, environmental enrichment (EE) – a protocol that enhances social, physical, and cognitive experience - can drive partial repair of the miswired visual pathway of Ten-m3 knock-out (KO) mice. Remarkably, six weeks of EE from birth drove pruning of the most miswired retinogeniculate terminals. This was underway at postnatal day (P)26/27, but a mechanism was lacking. Microglia, the brain’s immune cells, have established roles in structural plasticity. The aim of this thesis was to determine whether microglia are involved in the EE-driven corrective pruning occurring in Ten-m3 KO mice. It was hypothesised EE might drive microglia to engulf the most miswired terminals in a defined postnatal window. Results supported this hypothesis. EE-driven, localised microglial-reactivity was reliably observed at the corrective pruning site of P25 Ten-m3 KO mice with no comparable effect in wildtypes. This followed a clearly defined time course - commencing between P18 and P21, peaking at P25 and ceased by P30 - and was accompanied by evidence of targeted microglial engulfment of the most miswired terminals. Both effects temporally aligned with the EE-driven corrective pruning. An attempt to confirm causal microglial involvement was made by administering daily minocycline injections from P18-24 with EE from birth. While the EE-driven corrective pruning, microglial reactivity and engulfment in Ten-m3 KO mice all appeared blocked with minocycline, the vehicle injections had the same effect, suggesting the injection protocol itself was countering EEs beneficial effect. Thus, while this thesis provides strong evidence for microglial involvement, the effect of minocycline remains inconclusive.