ß3 integrin is dispensable for conditioned fear and hebbian forms of plasticity in the hippocampus.

Integrins play key roles in the developing and mature nervous system, from promoting neuronal process outgrowth to facilitating synaptic plasticity. Recently, in hippocampal pyramidal neurons, ß3 integrin (ITGß3) was shown to stabilise synaptic AMPA receptors (AMPARs) and to be required for homeostatic scaling of AMPARs elicited by chronic activity suppression. To probe the physiological function for ITGß3-dependent processes in the brain, we examined whether the loss of ITGß3 affected fear-related behaviours in mice. ITGß3-knockout (KO) mice showed normal conditioned fear responses that were similar to those of control wild-type mice. However, anxiety-like behaviour appeared substantially compromised and could be reversed to control levels by lentivirus-mediated re-expression of ITGß3 bilaterally in the ventral hippocampus. In hippocampal slices, the loss of ITGß3 activity did not compromise hebbian forms of plasticity--neither acute pharmacological disruption of ITGß3 ligand interactions nor genetic deletion of ITGß3 altered long-term potentiation (LTP) or long-term depression (LTD). Moreover, we did not detect any changes in short-term synaptic plasticity upon loss of ITGß3 activity. In contrast, acutely disrupting ITGß1-ligand interactions or genetic deletion of ITGß1 selectively interfered with LTP stabilisation whereas LTD remained unaltered. These findings indicate a lack of requirement for ITGß3 in the two robust forms of hippocampal long-term synaptic plasticity, LTP and LTD, and suggest differential roles for ITGß1 and ITGß3 in supporting hippocampal circuit functions.

Analysis of mRNAs that are enriched in the post-synaptic domain of the neuromuscular junction.

The identity of synaptically-enriched genes was investigated by comparing the abundance of various mRNAs in the synaptic and extra-synaptic regions of the same muscle fibers. The mRNAs for several known synaptic proteins were significantly elevated in the synaptic region when measured by real-time PCR. The synaptic mRNAs were then further analyzed using microarrays and real-time PCR to identify putative regulators of the neuromuscular junction (NMJ). MRF4 was the only member of the MyoD family that was concentrated at the mature NMJ, suggesting that it may have a unique role in the maintenance of post-synaptic specialization. Three potential regulators of the NMJ were identified and confirmed by real-time PCR: glia maturation factor gamma was concentrated at the NMJ whereas Unr protein and protein tyrosine phosphatase were repressed synaptically. The identification of synaptically-repressed genes may indicate that synaptic specialization is created by a combination of positive and negative signals.

Fibroblast growth factor-5 is expressed in Schwann cells and is not essential for motoneurone survival.

Fibroblast growth factor-5 (FGF-5) is a putative target-derived survival factor for motoneurones as it is concentrated in the synaptic portions of skeletal muscles and because it promotes the survival of embryonic motoneurones in vitro. A variety of experimental approaches have been used to examine this possibility. The expression of FGF-5 in the neuromuscular system was analysed using the reverse transcription-polymerase chain reaction (RT-PCR). Both splice variants of FGF-5 were detected in adult rat skeletal muscle, sciatic nerve, and spinal cord. The expression of FGF-5 in skeletal muscle was up-regulated after denervation. At first sight this appears to be consistent with FGF-5 being a target-derived factor. However, FGF-5 protein was detected in Schwann cells, macrophages, vascular smooth muscle and endothelial cells, but not in muscle fibres. The absence of FGF-5 in muscle fibres was confirmed by RT-PCR examination of isolated muscle fibres. Furthermore, FGF-5 protein was also not detected in denervated fibres, as would be expected for a neuronal survival factor. Denervation did however lead to up-regulation of FGF-5 in the Schwann cells of the distal nerve trunk. This may indicate that FGF-5 is either an autocrine regulator of Schwann cells or a Schwann cell-derived neurotrophic factor. The latter appears not to be the case for two reasons. First, the double-ligation technique was used to show that endogenous FGF-5 is not transported in motor axons. Second, stereological estimates of the number of motoneurones in an FGF-5 null mutant (Angora) mouse failed to reveal any loss of motoneurones. Collectively these experiments suggest that FGF-5 is not a physiological regulator of motoneurones, and therefore raise the possibility that it is an autocrine regulator of Schwann cells.