Characterization of factors regulating lamina-specific growth of thalamocortical axons

Abstract

During development, most thalamocortical axons extend through the deep layers to terminate in layer 4 of neocortex. To elucidate the molecular mechanisms that underlie the formation of layer-specific thalamocortical projections, axon outgrowth from embryonic rat thalamus onto postnatal neocortical slices which had been fixed chemically was used as an experimental model system. When the thalamic explant was juxtaposed to the lateral edge of fixed cortical slice, thalamic axons extended farther in the deep layers than the upper layers. Correspondingly, thalamic axons entering from the ventricular side extended farther than those from the pial side. In contrast, axons from cortical explants cultured next to fixed cortical slices tended to grow nearly as well in the upper as in the deep layers. Biochemical aspects of lamina-specific thalamic axon growth were studied by applying several enzymatic treatments to the cortical slices prior to culturing. Phosphatidylinositol phospholipase C treatment increased elongation of thalamic axons in the upper layers without influencing growth in the deep layers. Neither chondroitinase, heparitinase, nor neuraminidase treatment influenced the overall projection pattern, although neuraminidase slightly decreased axonal elongation in the deep layers. These findings suggest that glycosylphosphatidylinositol-linked molecules in the cortex may contribute to the laminar specificity of thalamocortical projections by suppressing thalamic axon growth in the upper cortical layers.

Inhibitory Mechanism by Polysialic Acid for Lamina-Specific Branch Formation of Thalamocortical Axons

“…The methods of fixation of cortical slices and enzymatic treatments have been described elsewhere (Yamamoto et al, 2000). In brief, the occipital cortex was dissected from P7 rat pups (Sprague Dawley) and then cut into 250-mthick slices.…”

“…The thalamic explant was placed at either the ventricular side or pial side of the fixed cortical slice. In the ventricular arrangement, the white matter or layer 6 was often removed from the fixed cortical slices to compensate for the fact that only a small number of axons reached layer 4 in the fixed slices (Yamamoto et al, 2000). The cultures were maintained at 37°C in an environment of humidified 95% air and 5% C O 2 .…”

“…Time lapse imaging of thalamocortical axons in this system has further revealed that the initial branches appear in the target layer and that branching and axonal stop behaviors do not always occur together (Yamamoto et al, 1997). These findings raise the possibility that branch formation is regulated by local cues, possibly membrane-associated molecules, which are independent of those governing axonal termination (Yamamoto et al, 2000).…”

“…As shown in Figure 4, endo N was found to completely remove PSA immunoreactivity, which is normally associated with NCAMs. Likewise, chondroitinase and PI-PLC treatments were found to reduce chondroitin sulfate and TAG-1 levels, respectively (Yamamoto et al, 2000).…”

Mechanisms Underlying Development of Visual Maps and Receptive Fields

“…A few molecules have been identified that regulate layer-specific connectivity in the retina, such as sidekicks and DSCAMS (Yamagata et al 2002, Yamagata & Sanes 2008 (Figure 2b). Also N-cadherin, in collaboration with neural activity, regulates layer specificity of geniculo-cortical projections (Poskanzer et al 2003, Uesaka et al 2007, Yamamoto et al 2000 (Figure 2b). No one has yet reported regarding the molecules that mediate layer-specific targeting of functionally defined visual processing streams, although such cues have been proposed to exist (Meissirel et al 1997;Kawasaki et al 2004).…”