Cytotactin and cytotactin-binding proteoglycan. An interactive pair of extracellular matrix proteins

Chondroitin sulfate proteoglycans in the developing cerebral cortex: the distribution of neurocan distinguishes forming afferent and efferent axonal pathways

The first thalamocortical axons to arrive in the developing cerebral cortex traverse a pathway that is separate from the adjacent intracortical pathway for early efferents, suggesting that different molecular signals guide their growth. We previously demonstrated that the intracortical pathway for thalamic axons is centered on the subplate (Bicknese et al. [1994] J. Neurosci. 14:3500-3510), which is rich in chondroitin sulfate proteoglycans (CSPGs; Sheppard et al. [1991] J. Neurosci. 11:3928-3942), whereas efferent axons cross the subplate to exit in a zone containing much less CSPG. To define the molecular composition of the subplate further, we used antibodies against CSPG core proteins and chondroitin sulfate disaccharides in an immunohistochemical analysis of their distribution in the developing neocortex of the rat. Immunolabeling for neurocan, a central nervous system-specific CSPG (Rauch et al. [1992] J. Biol. Chem. 267:19537-19547), and for chondroitin 6-sulfate and unsulfated chondroitin becomes prominent in the subplate before the arrival of thalamic afferents. Immunolabeling is initially sparse in the cortical plate but appears later in maturing cortical layers. A postnatal decline in immunolabeling occurs uniformly for most proteoglycans, but, in the somatosensory cortex, labeling for neurocan, phosphacan, and chondroitin 4- and 6-sulfate declines in the centers of the whisker barrels before the walls. In contrast to neurocan, immunolabeling for other proteoglycans is either uniformly distributed (syndecan-1, N-syndecan, 5F3, phosphacan, chondroitin 4-sulfate), restricted to axons (PGM1), distributed exclusively on nonneuronal elements (2D6, NG2, and CD44), or undetectable (9.2.27, aggrecan, decorin). Thus, neurocan is a candidate molecule for delineating the intracortical pathway of thalamocortical axons and distinguishing it from that of cortical efferents.

Tenascin is a cytoadhesive extracellular matrix component of the human hematopoietic microenvironment

Tenascin is a large extracellular matrix (ECM) glycoprotein found in restricted tissue locations in the adult organism. It is copiously synthesized in regenerative organs or regenerating tissues and by certain tumors. We have analyzed the expression of tenascin in human long term bone marrow cultures as well as in cryostat sections of native bone marrow and found it strongly expressed by the stromal cells of the microenvironment. Two different protein subunits of 280 and 220 kD were detected by immunoblotting. These two forms are derived most likely from two different mRNA splice variants of 6 and 8 kb detected by Northern blotting. The in vivo analysis of cryostat sections showed a codistribution with other ECM molecules such as fibronectin and collagen type III in the microenvironment surrounding the maturing hematopoietic cells. Using two independent cell adhesion assays tenascin could be shown to function as a cytoadhesive molecule for hematopoietic cells. These data suggest a direct involvement of tenascin in the retention of hematopoietic progenitor cells in the stroma.