# How do neurons in the brain decide to refine their synaptic connections in vivo?

> **NIH NIH R01** · BOSTON CHILDREN'S HOSPITAL · 2020 · $652,027

## Abstract

Formation of functional neural circuits is critical for proper functioning of the brain. To establish the most
efficient synaptic circuits, synaptic connections must be refined by neural activity during development. In this
proposal, we will determine the molecules and manner by which functional circuits are established by neural
activity, focusing on the limbic system (including the hippocampus and cingulate cortex), which is implicated in
emotional processing, memory formation and social behavior. Using a mouse genetic system in which
restricted populations of hippocampal neurons can be conditionally inactivated, we found that hippocampal
axons are refined through activity-dependent competition, where active neuronal connections stay (maintained)
while inactive ones leave (eliminated). We further found that a cell adhesion molecule SIRP from
postsynaptic neurons stabilizes active synapses through its presynaptic receptor CD47, serving as a "Stay"
signal. To identify the signaling molecules that play critical roles in inactive axon elimination ("Go" signal), we
generated a new system in which neural activity and gene expression can be conditionally controlled in vivo,
using in utero electroporation. When neurotransmitter release is blocked in a subset of neurons in the cingulate
cortex, their callosal projections (the major connections between the cerebral hemispheres) are eliminated
during development. Using this system, we screened for signaling molecules that are upregulated in inactive
neurons right before their axons start to leave and identified the Ca2+-dependent tyrosine kinase Pyk2. Inactive
axons were not eliminated when a kinase-dead mutant of Pyk2 was expressed, indicating that Pyk2 activity is
necessary for inactive axons to leave. We further identified that a Pyk2-interacting kinase, JAK2, is also
necessary for inactive axon elimination. Consistently, Pyk2 and JAK2 are activated in inactive neurons. Finally,
we found that overexpression of Pyk2 or JAK2 induces axonal elimination even when axons are active. We
propose that the Pyk2-JAK2 pathway is the "Go" signal and serves as the determinant of axon refinement. To
further characterize this pathway and to understand how the "Go" and "Stay" pathways regulate activity-
dependent axon/synapse refinement, we propose to: Aim 1: Investigate the role of Pyk2 and JAK2 for
axon/synapse refinement in physiological conditions. Aim 2: Analyze the electrophysiological consequences of
Pyk2/JAK2 inactivation during synapse refinement using conditional KO mice. Aim 3: Examine whether the
Pyk2-JAK2 pathway provides cues for microglial clearance of inactive axons in vivo. Aim 4: Investigate the
interaction between the Stay (SIRP-CD47) and Go (Pyk2-JAK2) pathways in axon/synapse refinement in
vivo. Our project will molecularly delineate how neurons decide to establish functional synaptic connections in
the mammalian brain. Pyk2 and JAK2 are associated with various neuropsychiatric disorders. Many fo...

## Key facts

- **NIH application ID:** 9940894
- **Project number:** 5R01MH111647-04
- **Recipient organization:** BOSTON CHILDREN'S HOSPITAL
- **Principal Investigator:** Hisashi Umemori
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $652,027
- **Award type:** 5
- **Project period:** 2017-08-16 → 2022-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9940894

## Citation

> US National Institutes of Health, RePORTER application 9940894, How do neurons in the brain decide to refine their synaptic connections in vivo? (5R01MH111647-04). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/9940894. Licensed CC0.

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