PirB regulates asymmetries in hippocampal circuitry

Left-right asymmetry is a fundamental feature of higher-order brain structure; however, the molecular basis of brain asymmetry remains unclear. We recently identified structural and functional asymmetries in mouse hippocampal circuitry that result from the asymmetrical distribution of two distinct populations of pyramidal cell synapses that differ in the density of the NMDA receptor subunit GluRε2 (also known as NR2B, GRIN2B or GluN2B). By examining the synaptic distribution of ε2 subunits, we previously found that β2-microglobulin-deficient mice, which lack cell surface expression of the vast majority of major histocompatibility complex class I (MHCI) proteins, do not exhibit circuit asymmetry. In the present study, we conducted electrophysiological and anatomical analyses on the hippocampal circuitry of mice with a knockout of the paired immunoglobulin-like receptor B (PirB), an MHCI receptor. As in β2-microglobulin-deficient mice, the PirB-deficient hippocampus lacked circuit asymmetries. This finding that MHCI loss-of-function mice and PirB knockout mice have identical phenotypes suggests that MHCI signals that produce hippocampal asymmetries are transduced through PirB. Our results provide evidence for a critical role of the MHCI/PirB signaling system in the generation of asymmetries in hippocampal circuitry

Laterality defects in hippocampal synapse morphology in PirB KO and <i>iv</i> mice.

<p>Postsynaptic density (PSD) area (A) and percentage of perforated synapses (B) were compared between left and right CA1 pyramidal cell synapses in the PirB KO and <i>iv</i> mice. No significant difference in these parameters between the left and right hippocampus was observed in either PirB KO or <i>iv</i> mice, whereas significant differences were found between PirB KO and <i>iv</i> hippocampi. Error bars represent s.e.m. An asterisk indicates <i>P</i> < 0.05; absence of an asterisk indicates <i>P</i> > 0.05.</p

Comparison of the concentration dependency of Ro 25–6981 inhibition of NMDA EPSCs in hippocampal synapses of PirB KO and <i>iv</i> mice.

<p>Schematic diagrams showing synaptic inputs onto the apical and basal dendrites of CA1 pyramidal neurons and the arrangement of electrodes. In hippocampal slices prepared from naïve mice, whole-cell recordings [Rec. (WC)] were made from CA1 pyramidal neurons. A stimulating electrode was placed in the stratum radiatum [Stim. (SR)] or the stratum oriens [Stim. (SO)] of the CA1 to activate apical or basal synapses, respectively. Sch, Schaffer collateral fibers; Com, commissural fibers. NMDA EPSCs were recorded at a holding potential of +10 mV. Relative amplitudes of NMDA EPSCs in the presence of several concentrations of Ro 25–6981 are expressed as percentages of control responses (mean ± SEM). Filled and open circles represent basal [iv (basal)] and apical [iv (apical)] synapses (<i>n</i> = 7 each), respectively, of CA1 pyramidal neurons in the <i>iv</i> mouse hippocampus. Open triangles represent CA1 apical synapses in the PirB KO hippocampus (<i>n</i> = 10 each).</p

Relationship between synaptic plasticity and stimulation frequency in hippocampal synapses of PirB KO and <i>iv</i> mice.

<p>(A) Schematic diagrams of the arrangement of electrodes for extracellular recording. Using hippocampal slices prepared from naïve mice, fEPSPs were recorded with an extracellular electrode [Rec. (field)] placed either in the stratum radiatum or stratum oriens of the CA1. To activate apical (Apical) or basal (Basal) dendritic synapses, a stimulating electrode was placed in the stratum radiatum [Stim. (SR)] or stratum oriens [Stim. (SO)], respectively. (B) Tetanic stimulation (100 Hz for 1 s, 3 trains, interval of 10 s) applied at time 0 (arrow) elicited LTPs of the fEPSP slope in hippocampal slices from both PirB KO and <i>iv</i> mice. Filled and open circles represent, respectively, basal [iv (basal), <i>n</i> = 7] and apical [iv (apical), <i>n</i> = 7] synapses of CA1 pyramidal neurons in the <i>iv</i> mouse hippocampus. Open triangles represent CA1 apical synapses in the PirB KO hippocampus (<i>n</i> = 10). Symbols and error bars represent means and SEM, respectively. The upper superimposed traces show representative fEPSPs recorded before (1) or 40 min after (2) tetanic stimulation. Scale bars: 1.0 mV (vertical) and 10 ms (horizontal). (C) Low-frequency stimulation (1 Hz for 15 min, thick bar) induced LTD in apical synapses of the <i>iv</i> mouse hippocampus (open circles, <i>n</i> = 7), but not in basal synapses of the <i>iv</i> mouse hippocampus (filled circles, <i>n</i> = 7) or in PirB KO synapses (open triangles, <i>n</i> = 10). The upper superimposed traces are representative fEPSPs recorded before (1) or 40 min after (2) low-frequency stimulation. Scale bars: 1.0 mV (vertical) and 10 ms (horizontal). (D) Stimulation frequency dependency of synaptic plasticity. Relative amplitudes of fEPSP slopes, estimated 40 min after tetanus, were plotted against stimulation frequency (mean ± SEM). Points at 0.1 Hz (test pulse frequency) indicate baseline values (horizontal dashed line). Symbols are the same as those in (B) and (C). *<i>P</i> < 0.05.</p

Hippocampal asymmetry and abnormalities in the <i>iv</i> and β2m KO mice.

<p>Left and right CA3 pyramidal neurons and their axons are colored red and blue, respectively. A postsynaptic CA1 pyramidal neuron is in the center, outlined in black, and it represents postsynaptic neurons in both left and right hemispheres. Closed and open circles represent ε2-dominant and ε2-non-dominant synapses, respectively. Apical, apical dendrites; Basal, basal dendrites; WT, wild-type; KO, knockout.</p