Formation of functional synaptic connections is critical for proper functioning of the brain. After initial synaptic differentiation, synapses are maturated and stabilized by neural activity to establish appropriate synaptic connections. During maturation presynaptic boutons enlarge, more synaptic vesicles accumulate to the presynaptic terminal, the number of active zones and postsynaptic densities increases, and the shape of spines changes in response to synaptic activity. However, the molecular mechanisms underlying activity- dependent synapse maturation remain to be elucidated. Using the ability to cluster synaptic vesicles in cultured neurons as a bioassay, we have purified molecules that can organize presynaptic terminals from developing brains. This purification revealed two peaks of activity that induced synaptic vesicle clustering. One peak contains FGF22, and we have shown that FGFs promote differentiation of cerebellar, neuromuscular and hippocampal synapses1-3. The other peak, on which we focus here, contains the extracellular domain of signal regulatory protein 1 (SIRP1), a transmembrane immunoglobulin superfamily member4. SIRP1 is highly expressed in the hippocampus around the time of synapse maturation. It is localized in dendrites and concentrated at synapses. Interestingly, the extracellular domain of SIRP1 is cleaved and shed in response to cellular activation. The application of the extracellular domain of SIRP1 to cultured hippocampal neurons promotes synaptic vesicle clustering. Conditional SIRP1 knockout mice show defects in presynaptic maturation. From these preliminary results, we propose the following model for activity-dependent maturation of hippocampal synapses: After initial synapse formation by axon-dendrite contacts, neurotransmitter release from the presynaptic terminal induces the cleavage of postsynaptic SIRP1, and the shed ectodomain of SIRP1 in turn promotes the maturation of the presynaptic terminal. To test this hypothesis, we propose to:
7 Aim 1 : Determine whether ectodomain shedding is required for the presynaptic effect of SIRP1 7 Aim 2: Investigate the role of neural activity for SIRP1-dependent presynaptic maturation 7 Aim 3: Examine the importance of ectodomain shedding of SIRP1 for presynaptic maturation in vivo We will use molecular and cellular biological, biochemical, imaging and electrophysiological approaches. Through these studies we should understand the molecular mechanisms underlying functional synapse establishment in the hippocampus by neural activity. Many forms of neurological disorders including autism, schizophrenia, and Alzheimer's disease are associated with abnormal alterations of synapses in the hippocampus. Furthermore, a SIRP1 receptor, CD475 is implicated in learning, memory, Alzheimer's disease and schizophrenia6-9. Thus, our studies will also help design strategies to prevent and treat such neurological disorders. In future studies, we will use conditional SIRP1 knockout mice to investigate the in vivo role of SIRP1 and its ectodomain shedding in learning, memory formation and neurological disorders.

Public Health Relevance

To establish appropriate synaptic connections in the brain, synapses must be maturated by neural activity during development. However, the molecular mechanisms underlying activity-dependent synapse maturation are not known. Here, we will determine the role of activity-dependent cleavage of SIRP1, a synaptic cell adhesion molecule, in synapse maturation in the hippocampus, the structure known to be critical for long-term memory formation. Because defects in SIRP1 signaling are implicated in Alzheimer's disease and schizophrenia, our studies should yield novel insights into the pathophysiology and treatment of such devastating neurological disorders.

Agency
National Institute of Health (NIH)
Institute
National Institute of Mental Health (NIMH)
Type
Exploratory/Developmental Grants (R21)
Project #
5R21MH092614-02
Application #
8306730
Study Section
Synapses, Cytoskeleton and Trafficking Study Section (SYN)
Program Officer
Asanuma, Chiiko
Project Start
2011-08-01
Project End
2014-07-31
Budget Start
2012-08-01
Budget End
2014-07-31
Support Year
2
Fiscal Year
2012
Total Cost
$226,590
Indirect Cost
$76,590
Name
University of Michigan Ann Arbor
Department
Biochemistry
Type
Schools of Medicine
DUNS #
073133571
City
Ann Arbor
State
MI
Country
United States
Zip Code
48109
Williams, Aislinn J; Umemori, Hisashi (2014) The best-laid plans go oft awry: synaptogenic growth factor signaling in neuropsychiatric disease. Front Synaptic Neurosci 6:4
Toth, Anna B; Terauchi, Akiko; Zhang, Lily Y et al. (2013) Synapse maturation by activity-dependent ectodomain shedding of SIRP*. Nat Neurosci 16:1417-25