Enhanced synaptic connectivity and epilepsy in C1q knockout mice

Abstract

Excessive CNS synapses are eliminated during development to establish mature patterns of neuronal connectivity. A complement cascade protein, C1q, is involved in this process. Mice deficient in C1q fail to refine retinogeniculate connections resulting in excessive retinal innervation of lateral geniculate neurons. We hypothesized that C1q knockout (KO) mice would exhibit defects in neocortical synapse elimination resulting in enhanced excitatory synaptic connectivity and epileptiform activity. We recorded spontaneous and evoked field potential activity in neocortical slices and obtained video-EEG recordings from implanted C1q KO and wild-type (WT) mice. We also used laser scanning photostimulation of caged glutamate and whole cell recordings to map excitatory and inhibitory synaptic connectivity. Spontaneous and evoked epileptiform field potentials occurred at multiple sites in neocortical slices from C1q KO, but not WT mice. Laser mapping experiments in C1q KO slices showed that the proportion of glutamate uncaging sites from which excitatory postsynaptic currents (EPSCs) could be evoked ("hotspot ratio") increased significantly in layer IV and layer V, although EPSC amplitudes were unaltered. Density of axonal boutons was significantly increased in layer V pyramidal neurons of C1q KO mice. Implanted KO mice had frequent behavioral seizures consisting of behavioral arrest associated with bihemispheric spikes and slow wave activity lasting from 5 to 30 s. Results indicate that epileptogenesis in C1q KO mice is related to a genetically determined failure to prune excessive excitatory synapses during development. complement cascade | seizure | synaptic pruning | development | axonal boutons E arly brain development is marked by an exuberant outgrowth of axons that innervate multiple targets. These projections are later refined through axonal pruning, a developmental regulatory process by which excessive connectivity is eliminated and mature patterns of neuronal innervation become established (1). Cortical synaptic pruning events, such as selective branch elimination for segregating layer V cortical neuronal projections to subcortical targets (1), and refinement of interhemispheric (2) and intracortical (3) are important for the development of precisely wired connections and normal information processing. The activation of a number of genes and proteins is crucial for the process of synaptic pruning in the mammalian brain (4). Recently, molecules ordinarily associated with immune and inflammatory processes have been shown to play nonimmune roles in synaptic pruning (5). C1q and C3 genes of the classical complement cascade are necessary for synapse elimination in the retino-geniculate pathway during development (6). Activation of C1q protein by astrocytes leads to triggering of a protease cascade, deposition of C3 and opsonization or "tagging" of synapses that are likely eliminated by microglia. Lateral geniculate relay neurons in C1q knockout (KO) mice deficient in these proteins retain multiple synaptic contacts f...

The role of the immune system in central nervous system plasticity after acute injury

“…It is known from post-natal studies that complement proteins mediate microglia-dependent pruning of synapses within neuronal circuits. As such, C1q-deficient mice display abnormal neocortical excitatory synaptic connectivity, as well as enhanced epileptiform activity (Chu et al, 2010). These evidences suggest a possible reactivation of developmental pathways also in adult CNS disorders (Stevens et al, 2007).…”

Experimental focal neocortical epilepsy is associated with reduced white matter volume growth: results from multiparametric MRI analysis

“…Synaptic, neuronal and axonal pruning is an essential step during brain development, starting early after birth and completed around the time of sexual maturation [for review see: (Luo and O'Leary 2005)] and has been found in rats (Andersen and Teicher 2004) as well as in humans (Glantz et al 2007). Failure to prune excessive excitatory synapses during development has been linked to epileptogenesis in a C1q knockout mice model (Chu et al 2010), but it is unknown whether a pruning dysregulation plays a role in the tetanus toxin epilepsy model.…”

Functional Complement C1q Abnormality Leads to Impaired Immune Complexes and Apoptotic Cell Clearance

“…Recently, C1q was found to participate in synapses remodeling and in the clearance of damaged CNS neurons after neuronal injury and neurodegenerative disorders (36)(37)(38). Again, both C1q and C3 binding to damaged neurons is required for efficient silent phagocytosis of apoptotic cells (16).…”