Identification of Neuronal Pentraxins as Synaptic Binding Partners of C1q and the Involvement of NP1 in Synaptic Pruning in Adult Mice

Abstract

Elements of the immune system particularly that of innate immunity, play important roles beyond their traditional tasks in host defense, including manifold roles in the nervous system. Complement-mediated synaptic pruning is essential in the developing and healthy functioning brain and becomes aberrant in neurodegenerative disorders. C1q, component of the classical complement pathway, plays a central role in tagging synapses for elimination; however, the underlying molecular mechanisms and interaction partners are mostly unknown. Neuronal pentraxins (NPs) are involved in synapse formation and plasticity, moreover, NP1 contributes to cell death and neurodegeneration under adverse conditions. Here, we investigated the potential interaction between C1q and NPs, and its role in microglial phagocytosis of synapses in adult mice. We verified in vitro that NPs interact with C1q, as well as activate the complement system. Flow cytometry, immunostaining and co-immunoprecipitation showed that synapse-bound C1q colocalizes and interacts with NPs. High-resolution confocal microscopy revealed that microglia-surrounded C1q-tagged synapses are NP1 positive. We have also observed the synaptic occurrence of C4 suggesting that activation of the classical pathway cannot be ruled out in synaptic plasticity in healthy adult animals. In summary, our results indicate that NPs play a regulatory role in the synaptic function of C1q. Whether this role can be intensified upon pathological conditions, such as in Alzheimer’s disease, is to be disclosed.

Neuronal pentraxins as biomarkers of synaptic activity: from physiological functions to pathological changes in neurodegeneration

“…3b-4). The recently described role of NPTX1 in synaptic pruning, by activation of the complement-microglia axis, could be intensified upon neurodegeneration (Kovács et al 2020) (Fig. 3b-5).…”

“…Moreover, there is evidence that NPTXs are involved in intracellular processes such as mitochondrial dynamics and trafficking, and neuronal apoptosis (Clayton et al 2012;Rahim et al 2013;Tseng and Bixby 2011). In addition, recent studies concluded a role of NPTXs in synaptic pruning by activation of the classical complement cascade (Kovács et al 2020) (Fig. 3a).…”

“…Moreover, NPTX1 negatively regulates excitatory synapse density through interaction with a subunit of slowrectifying-gated Kv7 potassium channel (Figueiro-Silva et al 2015). NPTX1 is enriched in the synaptic plasma membrane and also detected in mitochondria (in accordance with its role in mitochondrial apoptosis) (Kovács et al 2020). New data suggested astrocyte-secreted glypican-4 (GPC4) to induce NPTX1 release from pre-synaptic terminals through The NPTX1 release from the pre-synaptic terminal into the synaptic cleft may be modulated by the astrocyte-secreted glypican-4 (GPC4) (1), whereas NPTX2 is synthesize and release in response to neuronal activity or BNDF (2).…”

“…NPTXs modulate synaptic function and plasticity (Xu et al 2003), however their potential role in the immune system has been poorly described in scientific literature and is open to debate. A new study published in 2021, confirmed a role of NPTXs in synaptic pruning by activation of the complement cascade (Kovács et al 2020). Accordingly, NPTXs may perform Fig.…”

Granulocyte Colony Stimulating Factor causes cerebellar deficits and anxiety in a mouse model of CSF-1 receptor-related leukoencephalopathy

“…In contrast, the expression of genes encoding other components of the complement cascade ( C3), complement receptors (Itgam, Itgax, Itgb2 ), of. neuronal proteins that mediate the synaptic deposition of C1q ( Nptx1, Nptx2) (35) and of Trem2 , a microglial receptor involved in synapse removal (36) was unchanged (Supplementary Fig. 1).…”