Modulators of axonal growth and guidance at the brain midline with special reference to glial heparan sulfate proteoglycans

Bilaterally symmetric organisms need to exchange information between the left and right sides of their bodies to integrate sensory input and to coordinate motor control. Thus, an important choice point for developing axons is the Central Nervous System (CNS) midline. Crossing of this choice point is influenced by highly conserved, soluble or membrane-bound molecules such as the L1 subfamily, laminin, netrins, slits, semaphorins, Eph-receptors and ephrins, etc. Furthermore, there is much circumstantial evidence for a role of proteoglycans (PGs) or their glycosaminoglycan (GAG) moieties on axonal growth and guidance, most of which was derived from simplified models. A model of intermediate complexity is that of cocultures of young neurons and astroglial carpets (confluent cultures) obtained from medial and lateral sectors of the embryonic rodent midbrain soon after formation of its commissures. Neurite production in these cocultures reveals that, irrespective of the previous location of neurons in the midbrain, medial astrocytes exerted an inhibitory or non-permissive effect on neuritic growth that was correlated to a higher content of both heparan and chondroitin sulfates (HS and CS). Treatment with GAG lyases shows minor effects of CS and discloses a major inhibitory or non-permissive role for HS. The results are discussed in terms of available knowledge on the binding of HSPGs to interative proteins and underscore the importance of understanding glial polysaccharide arrays in addition to its protein complement for a better understanding of neuron-glial interactions

Heterogeneity of median and lateral midbrain radial glia and astrocytes

In the developing mammalian midbrain, radial glial cells are divided into median formations and lateral radial systems with differential properties including rate and timing of cell proliferation, expression of cytoskeletal and calcium-binding proteins, storage of glycogen and relations to afferent fiber systems. To test hypothesis that radial glial cells of median and lateral midbrain sectors and/or their derivatives are heterogeneous in their relations with local neurons, an in vitro system has been developed and has also been characterized in terms of extracellular matrix (ECM) components. Confluent astrocyte cultures, derived from median (M) or lateral (L) embryonic mouse midbrain sectors, were used as substrates for culturing dissociated cells from median (m) or lateral (l) sectors of embryonic midbrains. In spite of the morphological invariance of glial substrates at confluency, cells that were plated onto these substrates and that were immunoreactive for neuronal markers (MAP2, polysialylated N-CAM or betaIII tubulin) showed differences in the aggregation of somata and in the length, caliber and branching of neurites. These differences, which depend mostly on the sector of origin of astrocytes (L: permissive, M: non-permissive for neuronal growth), suggest that the substrates may differ in adhesiveness and/or their carrying of growth-promoting vs. growth-interfering molecules. Indeed, L and M cultures differ in laminin deposition patterns (L: fibrillar, M: punctate pattern). Furthermore, sulfated glycosaminoglycans (s-GAGs) isolated from the pericellular (P), intracellular (I) and extracellular (E) compartments of these sectoral cultures also showed correlations with the ability to support neurite growth. The total amount of s-GAGs in M cultures was twice that in L cultures and was particularly high in the P compartment, with about 3 times as much heparan sulfate (HS) and about 15 times as much chondroitin sulfate (CS) in this fraction of M than in the corresponding compartment of L glia. Our results indicate that cultured astrocytes have heterogeneous properties including different organizatio of their extracellular matrix that reflect the roles played by their parent radial glia in regions favorable to axonal growth or barrier regions of the developing brain.

Effects of the thyroid hormone (T3) on astrocytes [correction of effecs]

Thyroid hormones have profound effects on growth and development. In the brain L-3,5,3'- triiodothyronine (T3), the bioactive hormone, is involved with the harmonious development acting in neuronal and glial cell differentiation. T3 acts on the cells by interacting with nuclear receptors that can regulate the expression of several genes. Astrocytes also show receptors to the hormone. We reported herein data on the effects of T3 on astrocytes. We have verified that T3 has a morphological effect on cultured cortical astrocytes with rearrangement of GFAP filaments, and induces proliferation in the cultured cerebellar astrocytes of newborn rats. We discuss here the effects of T3 on astrocytes, considering the possibility that thyroid hormone prepares the astrocytes to interact with neurons.