Synapse Loss in Cortex of Agrin-Deficient Mice after Genetic Rescue of Perinatal Death

Abstract

Agrin-deficient mice die at birth because of aberrant development of the neuromuscular junctions. Here, we examined the role of agrin at brain synapses. We show that agrin is associated with excitatory but not inhibitory synapses in the cerebral cortex. Most importantly, we examined the brains of agrin-deficient mice whose perinatal death was prevented by the selective expression of agrin in motor neurons. We find that the number of presynaptic and postsynaptic specializations is strongly reduced in the cortex of 5-to 7-week-old mice. Consistent with a reduction in the number of synapses, the frequency of miniature postsynaptic currents was greatly decreased. In accordance with the synaptic localization of agrin to excitatory synapses, changes in the frequency were only detected for excitatory but not inhibitory synapses. Moreover, we find that the muscle-specific receptor tyrosine kinase MuSK, which is known to be an essential component of agrin-induced signaling at the neuromuscular junction, is also localized to a subset of excitatory synapses. Finally, some components of the mitogen-activated protein (MAP) kinase pathway, which has been shown to be activated by agrin in cultured neurons, are deregulated in agrin-deficient mice. In summary, our results provide strong evidence that agrin plays an important role in the formation and/or the maintenance of excitatory synapses in the brain, and we provide evidence that this function involves MAP kinase signaling.

The Process-inducing Activity of Transmembrane Agrin Requires Follistatin-like Domains

“…The function of the filopodialike processes in the developing CNS is unknown, but they might represent initial stages during the formation of excitatory spine synapses (24). Consistent with this idea, mice that lack CNS agrin develop 30% fewer glutamatergic synapses (11).…”

“…Consistent with this hypothesis, inhibition of TM-agrin synthesis markedly reduced the number of synapses in hippocampal neurons (29). In addition, mice that lack agrin in the CNS have a 30% reduction in the number of excitatory synapses and spines in the cerebral cortex (11).…”

“…Although neurons from mice with a targeted deletion of the agrn gene form synaptic specializations in vitro and in vivo (7,8), the acute suppression of agrin expression or function by antisense oligonucleotides or antibodies influences the formation and function of interneuronal synapses (9,10). Likewise, brains of agrin-deficient mice, whose perinatal death was prevented by the re-expression of agrin in motor neurons, have a severely reduced number of pre-and postsynaptic specializations as well as functional deficits at excitatory synapses in the CNS 3 (11). Although these data are consistent with a role of agrin during CNS synaptogenesis, the precise function of agrin during CNS development remains unclear.…”

Neuronal LRP4 regulates synapse formation in the developing CNS

“…This hypothesis is further strengthened by our observation that agrin accumulated at contact sites between dendrites of neurons with increased LRP4 expression and axons overexpressing agrin, whereas agrin did not aggregate at contact sites with neurons in which LRP4 expression was reduced. Interestingly, neurons in the brain of agrin −/− mice, in which the perinatal lethality has been rescued by motoneuron-specific expression of agrin, also develop fewer synapses and shorter dendrites (Ksiazek et al, 2007).…”

Heparan Sulfate Proteoglycans

“…Agrn Agrin Null allele: embryonic lethal; reduced number, size, and density of postsynaptic acetylcholine receptor aggregates in muscles; abnormal intramuscular nerve branching and presynaptic differentiation (Gautam et al 1996(Gautam et al ,1999; smaller brains (Serpinskaya et al 1999); abnormal development of interneuronal synapses (Gingras et al 2007); increased resistance to excitotoxic injury (Hilgenberg et al 2002); reduced number of cortical presynaptic and postsynaptic specializations (Ksiazek et al 2007). Floxed allele: Inactivation in podocytes does not affect glomerular charge selectivity or glomerular architecture (Harvey et al 2007).…”