Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Abstract

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

Figure 1.. Model circuits for studying activity-dependent synaptic pruning.

The classic models for studying synaptic pruning in the mammalian CNS include developmental remodeling at the retinogeniculate circuit (a), cerebellum (b), and cortical synapses (c). Within the retinogeniculate circuit (a), retinal ganglion cell (RGC) axonal arbors initially form synapses with relay neurons in overlapping territories within the lateral geniculate nucleus (LGN) (far left panel in a). Prior to eye opening (Birth-P10), spontaneous neuronal activity in the retina results in pruning of RGC synapses (dotted red and blue lines) and eye-specific segregation so that each eye occupies a discrete territory prior to eye opening (middle panel in a)^–,. Relay neuron synapses undergo further remodeling after eye opening in response to light exposure from P20-P30 when relay neuron spines are pruned (far right panel, green)^–. In the cerebellum (b), Purkinje cell somas are initially innervated by multiple climbing fiber inputs (far left panel in b). In the first stage of pruning (2^nd panel in b), the weaker climbing fiber inputs at the soma are pruned, while the strongest somatic input translocates to the Purkinje dendrites. In the second stage of pruning (3^rd panel in b), remaining climbing fiber inputs to the soma are pruned, while the single climbing fiber input at the Purkinje dendritic arbor is maintained^,. In the cortex (c), changes in sensory experience such as monocular deprivation or whisker manipulation (right panel in c) results in pruning of thalamocortical presynaptic terminals (red, dotted lines) and cortical dendritic spines (blue) in response to the changes in neuronal activity^–,,–.

Figure 2.. Summary of glial cell engulfment mechanisms regulating synaptic pruning.

Multiple glial engulfment pathways have been identified to regulate synaptic pruning (a). A mechanism by which microglia prune synapses is through CX3CR1-CX3CL1-ADAM10 signaling (b). Neuron-expressed CX3CL1 is cleaved from the membrane by the protease ADAM10 which initiates signaling to its microglial receptor CX3CR1. CX3CR1 signaling instructs microglial synapse engulfment by a, yet-to-be identified mechanism. Another mechanism involved in microglial synapse pruning is the complement signaling cascade. C1q induces the formation of the C3 convertase through complement factors C2 and C4. C3 convertase then cleaves and activates C3, which then directs microglia to engulf synapses via CR3 (c)^,. In contrast, CD47 binding to its receptor SIRPα on microglia inhibits engulfment of another subset of synapses. Similarly, the complement inhibitor SRPX2 binds C1q at the membrane in order to prevent C1q-mediated synapse engulfment and pruning. Astrocytes also play roles in synapse pruning through the phagocytic receptors MEGF10 and MerTK. MerTK may directly bind externalized PtdSer at the synaptic membrane (d). Further, astrocyte-microglia crosstalk has been implicated in the pruning process (e). For example, astrocyte production of TGFβ directs C1q production by RGCs to influence microglial pruning. Also, astrocytic IL33 expression signals to its microglial receptor IL1RL1 and instructs microglial synapse engulfment and pruning via a, yet-to-be-identified, downstream mechanism. Cluster of differentiation 47 (CD47), signal regulatory protein α (SIPRα), sushi repeat containing protein X-linked 2 (SRPX2), transforming growth factor β (TGFβ), multiple epidermal growth factor-like domains protein 10 (MEGF10), Mer proto-oncognene tyrosine kinase (MerTK), phosphatidylserine (PtdSer), a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10), C-X3-C motif chemokine ligand 1 (CX3CL1), C-X3-C motif chemokine receptor 1 (CX3CR1), interleukin 33 (IL33), interleukin 1 receptor like 1 (IL1RL1), complement receptor 3 (CR3), complement component 3 (C3), complement component 1q (C1q), complement component 4 (C4), complement component 2 (C2).

Figure 3.. Local “death of a synapse.”

During synaptic pruning, mitochondrial molecules in the dendrite activate Caspase 3 which drives AMPA receptor internalization from the postsynaptic membrane and LTD^,. Caspase 2 also impacts AMPAR internalization although though a separate pathway. Activity-dependent changes at the synapse, for example via LTD, may also influence mitochondrial processes and compartmentalized cell death molecules in order to regulate pruning (dotted lines). In some, but not all cases, caspase signaling leads to externalization of to the outer membrane at synapses. This exposed PtdSer can subsequently bind molecules such as C1q, GPR56, and TREM2 to induce pruning^,,. α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid (AMPA), phosphatidylserine (PtdSer), G protein coupled receptor 56 (GPR56), triggering receptor expressed on myeloid cells (TREM2), complement component 1q (C1q).

Figure 4.. Synaptic pruning and neurodevelopmental disorders.

A summary of published findings depicting development changes in synapse density in the brains of neurotypical individuals compared to patients with autism spectrum disorders (ASD)^, or schizophrenia (SZ)^,– (based on figure from). The developmental periods in early life and adolescence when synaptic pruning primarily occurs are highlighted in yellow. Key molecules implicated in aberrant pruning during these two periods in ASD (mTOR, TSC^,, PTEN, FMRP^,,,–,, eIF4E^,) and SZ (C4, MHC class I^,) are also indicated. Environmental risk factors which may, in concert, affect pruning are also indicated^–, such as maternal infection and diet in utero and pollution, infection, and stress later in development. Genetic factors, which likely interact with environment factors, are present throughout life (signified by the double helix). Mammalian target of rapamycin (mTOR), tuberous sclerosis (TSC), phosphatase and tensin homolog (PTEN), fragile X mental retardation protein (FMRP), eukaryotic translation initiation factor 4E (eIF4E), major histocompatibility complex class I (MHC I), complement 4 (C4).