PSA-NCAM in the developing and mature thalamus.

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is involved in several morphogenetic processes of the central nervous system. In the present study the expression of PSA-NCAM has been investigated in the rat thalamus during embryonic and postnatal development using light and electron microscopic immunocytochemical techniques. At all the examined ages, PSA-NCAM staining in the thalamus was mainly observed along neuronal plasmatic membranes and absent in astrocytes identified by labelling with cytoskeletal (vimentin and glial fibrillary acidic protein) and membrane (GABA transporter-3) markers. At embryonic day 14 the immunoreactivity was restricted to the dorsal thalamic mantle and to the region of reticular thalamic migration and subsequently it extended throughout the whole thalamic primordium. PSA-NCAM labelling remained intense and homogeneously distributed along perinatal period, but from P4 it began to decrease selectively, persisting throughout adulthood only in the reticular nucleus, ventral lateral geniculate nucleus and midline and intralaminar nuclei. The expression of this adhesion molecule differed in areas characterized by the presence of neurons containing distinct calcium binding proteins, as PSA-NCAM labelling was intense around calretinin-positive neurons, whereas it decreased in some calbindin-immunoreactive regions. These findings show evidence of a selective neuronal expression of PSA-NCAM in developing thalamus, supporting its suggested role in cell migration and synaptogenesis as it occurs in the cerebral cortex. In adulthood PSA-NCAM could instead be a marker of thalamic nuclei that retain a potential for synaptic plasticity.

Expression studies in gliomas and glial cells do not support a tumor suppressor role for LGI1.

Disruptions of LGI1 in glioblastoma (GBM) cell lines and LGI1 mutations in families with autosomal dominant epilepsy imply a role for LGI1 in glial cells as well as in neurons. Although we and others could not find LGI1 mutations in malignant gliomas, our initial studies appeared to support the idea that LGI1 is poorly expressed or absent in these tumors. Microarray data suggested that LGI1 could be involved in the control of matrix metalloproteinases, and we found that tumors derived from U87 glioblastoma cells overexpressing LGI1 were less aggressive than U87 control tumors. To our surprise, we observed that LGI1 expression after differentiation of murine neural stem cells was robust in neurons but negligible in glial cells, in agreement with immunohistochemistry studies on rodent brain. This observation could suggest that the variable levels of LGI1 expression in gliomas reflect the presence of neurons entrapped within the tumor. To test this hypothesis, we investigated LGI1 expression in parallel with expression of the neuronal marker NEF3 by real-time PCR on 30 malignant gliomas. Results showed a strong, positive correlation between the expression levels of these two genes (P < 0.0001). Thus, our data confirm that LGI1 is involved in cell-matrix interactions but suggest that its expression is not relevant in glial cells, implying that its role as a tumor suppressor in gliomas should be reconsidered.

Subpallial origin of a population of projecting pioneer neurons during corticogenesis.

Pyramidal neurons of the mammalian cerebral cortex are generated in the ventricular zone of the pallium whereas the subpallium provides the cortex with inhibitory interneurons. The marginal zone contains a subpial stream of migratory interneurons and two different classes of transient neurons, the pioneer neurons provided with corticofugal axons, and the reelin-expressing Cajal-Retzius cells. We found in cultured slices that the medial ganglionic eminence provides the reelin-negative pioneer neurons of the marginal zone. Pioneer neurons sent long projection axons that went through the cortical plate and reached the subplate and the lateral border of the lateral ganglionic eminence. In the cultured slices, pioneer neurons were functionally mature: they displayed a voltage-gated sodium current, expressed functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and showed gamma-aminobutyric acid type A (GABAA) postsynaptic events that were modulated by presynaptic AMPA receptors. Pioneer neurons expressed the adhesion molecules L1 and TAG-1; the latter has been reported to control tangential migrations to the neocortex [Denaxa, M., Chan, C.-H., Schachner, M., Parnavelas, J. & Karagogeos, D. (2001) Development (Cambridge, U.K.) 128, 4635-4644], and we show here that the pioneer neurons of the marginal zone are the cellular substrate of such a function. Finally, we show that, in early corticogenesis, reelin controls both the tangential migration of cortical interneurons toward the cortical plate and the tangential migration of pioneer neurons toward the marginal zone.

Substrates and routes of migration of early generated neurons in the developing rat thalamus.

We investigated the substrates supporting neuronal migration, and its routes, during early thalamic development in the rat. Neurons and axonal and glial fibres were identified in embryos with single and double immunohistochemistry; dynamic data were obtained with cell tracers in short-term organotypic cultured slices. The earliest thalamic neurons, originating from the ventricular neuroepithelium between embryonic days 13 and 15, include those of the reticular thalamic nucleus. At this developmental stage, calretinin, calbindin or gamma-aminobutyric acid immunostaining revealed both radially and nonradially orientated neurons in the region of reticular thalamic migration, between the dorsal and ventral thalamic primordia. In cultured slices, injections of fluorescent dyes in the neuroepithelium labelled neurons in a migratory stream along radial glia in the same zone. Some labelled fusiform cells departed from this radial trajectory along orthogonal routes within the dorsal thalamus. Confocal microscopy revealed nonradially orientated neurons in close apposition with a fibre system parallel to the lateral thalamic surface. These fibres expressed axonal markers, including the intermediate filament protein alpha-internexin and a polysialylated form of neuronal cell adhesion molecule. Active migration of nonradially orientated neurons along neuronal substrates was confirmed in living cultured slices. In addition, in vitro and ex vivo experiments revealed neurons migrating tangentially in association with glial fibres. These results provide novel evidence that: (i) early generated thalamic neurons follow nonradial routes in addition to glia-linked radial migration; and (ii), nonradially migrating thalamic neurons move along both glial and axonal substrates, which could represent a distinctive feature of thalamic development.