An NCAM mimetic, FGL, alters hippocampal cellular morphometry in young adult (4 month-old) rats.

The neural cell adhesion molecule, NCAM, is ubiquitously expressed within the CNS and has roles in development, cognition, neural plasticity and regulation of the immune system. NCAM is thus potentially an important pharmacological target for treatment of brain diseases. A cell adhesion mimetic FGL, a 15 amino-acid peptide derived from the second fibronectin type-III module of NCAM, has been shown to act as a neuroprotective agent in experimental disease and ageing models, restoring hippocampal/cognitive function and markedly alleviating deleterious changes in the CNS. However, the effects of FGL on the hippocampus of young healthy rats are unknown. The present study has examined the cellular neurobiological consequences of subcutaneous injections of FGL, on hippocampal cell morphometry in young (4 month-old) rats. We determined the effects of FGL on hippocampal volume, pyramidal neuron number/density (using unbiased quantitative stereology), and examined aspects of neurogenesis (using 2D morphometric analyses). FGL treatment reduced total volume of the dorsal hippocampus (associated with a decrease in total pyramidal neuron numbers in CA1 and CA3), and elevated the number of doublecortin immunolabeled neurons in the dentate gyrus, indicating a likely influence on neurogenesis in young healthy rats. These data indicate that FGL has a specific age dependent effect on the hippocampus, differing according to the development and maturity of the CNS.

Age-related changes in the hippocampus (loss of synaptophysin and glial-synaptic interaction) are modified by systemic treatment with an NCAM-derived peptide, FGL.

Altered synaptic morphology, progressive loss of synapses and glial (astrocyte and microglial) cell activation are considered as characteristic hallmarks of aging. Recent evidence suggests that there is a concomitant age-related decrease in expression of the presynaptic protein, synaptophysin, and the neuronal glycoprotein CD200, which, by interacting with its receptor, plays a role in maintaining microglia in a quiescent state. These age-related changes may be indicative of reduced neuroglial support of synapses. FG Loop (FGL) peptide synthesized from the second fibronectin type III module of neural cell adhesion molecule (NCAM), has previously been shown to attenuate age-related glial cell activation, and to 'restore' cognitive function in aged rats. The mechanisms by which FGL exerts these neuroprotective effects remain unclear, but could involve regulation of CD200, modifying glial-synaptic interactions (affecting neuroglial 'support' at synapses), or impacting directly on synaptic function. Light and electron microscopic (EM) analyses were undertaken to investigate whether systemic treatment with FGL (i) alters CD200, synaptophysin (presynaptic) and PSD-95 (postsynaptic) immunohistochemical expression levels, (ii) affects synaptic number, or (iii) exerts any effects on glial-synaptic interactions within young (4 month-old) and aged (22 month-old) rat hippocampus. Treatment with FGL attenuated the age-related loss of synaptophysin immunoreactivity (-ir) within CA3 and hilus (with no major effect on PSD-95-ir), and of CD200-ir specifically in the CA3 region. Ultrastructural morphometric analyses showed that FGL treatment (i) prevented age-related loss in astrocyte-synaptic contacts, (ii) reduced microglia-synaptic contacts in the CA3 stratum radiatum, but (iii) had no effect on the mean number of synapses in this region. These data suggest that FGL mediates its neuroprotective effects by regulating glial-synaptic interaction.

Age-Induced Loss of Mossy Fibre Synapses on CA3 Thorns in the CA3 Stratum Lucidum.

Advanced ageing is associated with hippocampal deterioration and mild cognitive decline. The hippocampal subregion CA3 stratum lucidum (CA3-SL) receives neuronal inputs from the giant mossy fibre boutons of the dentate gyrus, but relatively little is known about the integrity of this synaptic connection with ageing. Using serial electron microscopy and unbiased stereology, we examined age-related changes in mossy fibre synapses on CA3 thorny excrescences within the CA3-SL of young adults (4-month-old), middle-aged (12-month-old), and old-aged (28-month-old) Wistar rats. Our data show that while there is an increase in CA3 volume with ageing, there is a significant (40-45%) reduction in synaptic density within the CA3-SL of 12- and 28-month-old animals compared with 4-month-old animals. We also present preliminary data showing that the CA3 neuropil in advanced ageing was conspicuously full of lipofuscin and phagolysosome positive, activated microglial cellular processes, and altered perivascular pathology. These data suggest that synaptic density in the CA3-SL is significantly impaired in ageing, accompanied by underlying prominent ultrastructural glial and microvascular changes.

A neural cell adhesion molecule-derived peptide, FGL, attenuates glial cell activation in the aged hippocampus.

Neuroglial activation is a typical hallmark of ageing within the hippocampus, and correlates with age-related cognitive deficits. We have used quantitative immunohistochemistry and morphometric analyses to investigate whether systemic treatment with the Neural Cell Adhesion Molecule (NCAM)-derived peptide FG Loop (FGL) specifically alters neuroglial activation and population densities within the aged rat hippocampus (22 months of age). A series of 50 µm paraformaldehyde/acrolein-fixed sections taken throughout the dorsal hippocampus (5 animals per group) were immunostained to detect astrocytes (GFAP and S100ß) and microglial cells (CD11b/OX42 and MHCII/OX6), and analysed using computerised image analysis and optical segmentation (Image-Pro Plus, Media Cybernetics). FGL treatment reduced the density of CD11b+ and MHCII+ microglia in aged animals, concomitant with a reduction in immunoreactivity for these phenotypic markers. FGL treatment also markedly reduced GFAP immunoreactivity within all hippocampal subfields in aged animals, without exerting an appreciable effect on the density of S100ß+ cells. These results demonstrate that FGL can indeed regulate neuroglial activation and reduce microglial cell density in the aged hippocampus, and support its potential use as a therapeutic agent in age-related brain disorders.