Activity-Dependent Synapse Refinement: From Mechanisms to Molecules.

The refinement of immature neuronal networks into efficient mature ones is critical to nervous system development and function. This process of synapse refinement is driven by the neuronal activity-dependent competition of converging synaptic inputs, resulting in the elimination of weak inputs and the stabilization of strong ones. Neuronal activity, whether in the form of spontaneous activity or experience-evoked activity, is known to drive synapse refinement in numerous brain regions. More recent studies are now revealing the manner and mechanisms by which neuronal activity is detected and converted into molecular signals that appropriately regulate the elimination of weaker synapses and stabilization of stronger ones. Here, we highlight how spontaneous activity and evoked activity instruct neuronal activity-dependent competition during synapse refinement. We then focus on how neuronal activity is transformed into the molecular cues that determine and execute synapse refinement. A comprehensive understanding of the mechanisms underlying synapse refinement can lead to novel therapeutic strategies in neuropsychiatric diseases characterized by aberrant synaptic function.

The molecular signals that regulate activity-dependent synapse refinement in the brain.

The formation of appropriate synaptic connections is critical for the proper functioning of the brain. Early in development, neurons form a surplus of immature synapses. To establish efficient, functional neural networks, neurons selectively stabilize active synapses and eliminate less active ones. This process is known as activity-dependent synapse refinement. Defects in this process have been implicated in neuropsychiatric disorders such as schizophrenia and autism. Here we review the manner and mechanisms by which synapse elimination is regulated through activity-dependent competition. We propose a theoretical framework for the molecular mechanisms of synapse refinement, in which three types of signals regulate the refinement. We then describe the identity of these signals and discuss how multiple molecular signals interact to achieve appropriate synapse refinement in the brain.

In utero intraocular AAV injection for early gene expression in the developing rodent retina.

The visual system is the best system to study activity-dependent sensory circuit development. The connections from the retina to the dorsal lateral geniculate nucleus, the retinogeniculate connections, undergo extensive remodeling during early postnatal life. Thus, techniques that allow the expression of transgenes early in the developing retina are essential to study visual system development. Here, we describe a protocol to express genes-of-interest in the developing mouse retina via in utero intraocular adeno-associated virus injections. For complete details on the use and execution of this protocol, please refer to Yasuda et al. (2021).

An activity-dependent determinant of synapse elimination in the mammalian brain.

To establish functional neural circuits in the brain, synaptic connections are refined by neural activity during development, where active connections are maintained and inactive ones are eliminated. However, the molecular signals that regulate synapse refinement remain to be elucidated. When we inactivate a subset of neurons in the mouse cingulate cortex, their callosal connections are eliminated through activity-dependent competition. Using this system, we identify JAK2 tyrosine kinase as a key regulator of inactive synapse elimination. We show that JAK2 is necessary and sufficient for elimination of inactive connections; JAK2 is activated at inactive synapses in response to signals from other active synapses; STAT1, a substrate of JAK2, mediates inactive synapse elimination; JAK2 signaling is critical for physiological refinement of synapses during normal development; and JAK2 regulates synapse refinement in multiple brain regions. We propose that JAK2 is an activity-dependent switch that serves as a determinant of inactive synapse elimination.

Tyrosine phosphorylation of the transmembrane protein SIRPα: Sensing synaptic activity and regulating ectodomain cleavage for synapse maturation.

Synapse maturation is a neural activity-dependent process during brain development, in which active synapses preferentially undergo maturation to establish efficient neural circuits in the brain. Defects in this process are implicated in various neuropsychiatric disorders. We have previously reported that a postsynaptic transmembrane protein, signal regulatory protein-α (SIRPα), plays an important role in activity-dependently directing synapse maturation. In the presence of synaptic activity, the ectodomain of SIRPα is cleaved and released and then acts as a retrograde signal to induce presynaptic maturation. However, how SIRPα detects synaptic activity to promote its ectodomain cleavage and synapse maturation is unknown. Here, we show that activity-dependent tyrosine phosphorylation of SIRPα is critical for SIRPα cleavage and synapse maturation. We found that during synapse maturation and in response to neural activity, SIRPα is highly phosphorylated on its tyrosine residues in the hippocampus, a structure critical for learning and memory. Tyrosine phosphorylation of SIRPα was necessary for SIRPα cleavage and presynaptic maturation, as indicated by the fact that a phosphorylation-deficient SIRPα variant underwent much less cleavage and could not drive presynaptic maturation. However, SIRPα phosphorylation did not affect its synaptic localization. Finally, we show that inhibitors of the Src and JAK kinase family suppress neural activity-dependent SIRPα phosphorylation and cleavage. Together, our results indicate that SIRPα phosphorylation serves as a mechanism for detecting synaptic activity and linking it to the ectodomain cleavage of SIRPα, which in turn drives synapse maturation in an activity-dependent manner.

Activity-dependent proteolytic cleavage of cell adhesion molecules regulates excitatory synaptic development and function.

Activity-dependent remodeling of neuronal connections is critical to nervous system development and function. These processes rely on the ability of synapses to detect neuronal activity and translate it into the appropriate molecular signals. One way to convert neuronal activity into downstream signaling is the proteolytic cleavage of cell adhesion molecules (CAMs). Here we review studies demonstrating the mechanisms by which proteolytic processing of CAMs direct the structural and functional remodeling of excitatory glutamatergic synapses during development and plasticity. Specifically, we examine how extracellular proteolytic cleavage of CAMs switches on or off molecular signals to 1) permit, drive, or restrict synaptic maturation during development and 2) strengthen or weaken synapses during adult plasticity. We will also examine emerging studies linking improper activity-dependent proteolytic processing of CAMs to neurological disorders such as schizophrenia, brain tumors, and Alzheimer's disease. Together these findings suggest that the regulation of activity-dependent proteolytic cleavage of CAMs is vital to proper brain development and lifelong function.