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 ß2-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 ß2-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.

Neuronal major histocompatibility complex class I molecules are implicated in the generation of asymmetries in hippocampal circuitry.

Left-right asymmetry is a fundamental feature of higher-order brain function; however, the molecular basis of brain asymmetry has remained unclear. We have recently demonstrated asymmetries in hippocampal circuitry resulting from the asymmetrical allocation of NMDA receptor (NMDAR) subunit GluR2 (NR2B) in pyramidal cell synapses. This asymmetrical allocation of 2 subunits affects the properties of NMDARs and generates two populations of synapses, '2-dominant' and '2-non-dominant' synapses, according to the hemispheric origin of presynaptic inputs and cell polarity of the postsynaptic neurone. To identify key regulators for generating asymmetries, we analysed the hippocampus of ß2-microglobulin (ß2m)-deficient mice lacking cell surface expression of major histocompatibility complex class I (MHCI). Although MHCI proteins are well known in the immune system, accumulating evidence indicates that MHCI proteins are expressed in the brain and are required for activity-dependent refinement of neuronal connections and normal synaptic plasticity. We found that ß2m proteins were localised in hippocampal synapses in wild-type mice. NMDA EPSCs in ß2m-deficient hippocampal synapses receiving inputs from both hemispheres showed similar sensitivity to Ro 25-6981, an 2 subunit-selective antagonist, with those in '2-dominant' synapses for both the apical and basal synapses of pyramidal neurones. The structural features of the ß2m-deficient synapse in addition to the relationship between the stimulation frequency and synaptic plasticity were also comparable to those of '2-dominant' synapses. These observations indicate that the ß2m-deficient hippocampus lacks '2-non-dominant' synapses and circuit asymmetries. Our findings provide evidence supporting a critical role of MHCI molecules for generating asymmetries in hippocampal circuitry.

Ethylene promotes the induction by auxin of the cortical microtubule randomization required for low-pH-induced root hair initiation in lettuce (Lactuca sativa L.) seedlings.

Transverse cortical microtubule (CMT) arrays in lettuce root epidermal cells randomize soon after a shift from pH 6.0 to pH 4.0, and this randomization is essential for root hair initiation. We investigated the hormonal regulation of CMT randomization. At pH 4.0, 1 micro M of the auxin competitive inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (PCIB), 0.1 micro M of the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) or 0.1 micro M of the ethylene action inhibitor Ag(+) suppressed CMT randomization and root hair initiation. At pH 6.0, addition of 0.1 micro M indole-3-acetic acid (IAA) or 1 micro M of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) induced CMT randomization and root hair initiation. Culturing with 0.1 micro M IAA plus 0.1 micro M AVG, or 1 micro M ACC plus 1 micro M PCIB also induced these phenomena. ACC (1 micro M) plus 100 micro M PCIB inhibited CMT randomization and root hair initiation, but 1 micro M AVG with 0.1 micro M Ag(+) and 0.1 micro M IAA induced them. These results suggest that auxin is essential for CMT randomization. As a higher concentration of PCIB was required to suppress CMT randomization when ACC was added, the greater amount of ethylene produced at pH 4.0 may promote the induction by auxin of CMT randomization in hair-forming cells.

Randomization of cortical microtubules in root epidermal cells induces root hair initiation in lettuce (Lactuca sativa L.) seedlings.

Root hair formation is induced when lettuce seedlings are transferred from liquid medium at pH 6.0 to fresh medium at pH 4.0. If seedlings are transferred to pH 6.0, no root hairs are formed. We investigated the role of microtubules in this low pH-induced root hair initiation in lettuce. At the hair-forming zone in root epidermal cells, microtubules were perpendicular to the longitudinal axis of the cell just after pre-culture. This arrangement became disordered as early as 5 min after transfer to pH 4.0, and became random by 30 min later. At pH 4.0, the randomization extended to the entire hair-forming zone of seedlings; at pH 6.0, however, randomization did not occur and transverse microtubules were maintained. When seedlings at pH 6.0 were treated with microtubule-depolymerizing drugs, root hairs were formed. In contrast, when a microtubule-stabilizing drug, taxol, was added to the medium, no root hairs formed, even at pH 4.0. These results suggest that the transverse cortical microtubules inhibit root hair formation, and that their destruction is necessary for initiation. Furthermore, the microfilament-disrupting drugs cytochalasin B and latrunculin B inhibited root hair initiation, suggesting that actin filaments are necessary for root hair initiation.

Isolation and characterization of the ACC synthase genes from lettuce (Lactuca sativa L.), and the involvement in low pH-induced root hair initiation.

Root hair formation is induced when lettuce seedlings are transferred from pH 6.0 to pH 4.0. Ethylene, auxin and light are essential to this process. To investigate the role of ethylene in root hair initiation, we isolated two 1-aminocyclopropane-1-carboxylic acid (ACC) synthase genes (Ls-ACS1 and Ls-ACS2). Seven motifs of known ACS proteins were highly conserved in Ls-ACS1 and Ls-ACS2. The Ls-ACS1 and Ls-ACS2 mRNA levels were constant at pH 6.0, which were lower than that in seedlings at pH 4.0. Ls-ACS1 and Ls-ACS2 transcripts accumulated at pH 4.0 and reached peak levels at 1 h and 30 min after acidification, respectively. Indole-3-acetic acid (IAA) induced the accumulation of both Ls-ACS1 and Ls-ACS2 transcripts, whereas ACC induced only Ls-ACS1 mRNA. These results suggest that acidification-induced auxin accumulations increase the Ls-ACS2 levels, which together with Ls-ACS2-induced ethylene raise the levels of Ls-ACS1. Furthermore, blue and white light gave the highest levels of both Ls-ACS1 mRNA and ethylene production. Darkness was less effective, and red light had an intermediate effect. The different light conditions had no effect on the levels of Ls-ACS2 mRNA. These observations support the involvement of Ls-ACS1 in the production of ethylene, which is crucial for root hair initiation.