No abstract

In the developing CNS alpha- and beta-dystroglycan are highly concentrated in the endfeet of radial neuroepithelial cells at the contact site to the basal lamina. We show that injection of anti-dystroglycan Fab fragments, knockdown of dystroglycan using RNAi, and overexpression of a dominant-negative dystroglycan protein by microelectroporation in neuroepithelial cells of the chick retina and optic tectum in vivo leads to the loss of their radial morphology, to hyperproliferation, to an increased number of postmitotic neurons, and to an altered distribution of several basally concentrated proteins. Moreover, these treatments also altered the oriented growth of axons from retinal ganglion cells and from tectal projection neurons. In contrast, expression of non-cleavable dystroglycan protein in neuroepithelial cells reduced their proliferation and their differentiation to postmitotic neurons. These results demonstrate that dystroglycan plays a key role in maintaining neuroepithelial cell morphology, and that interfering with dystroglycan function influences proliferation and differentiation of neuroepithelial cells. These data also suggest that an impaired dystroglycan function in neuroepithelial cells might be responsible for some of the severe brain abnormalities observed in certain forms of congenital muscular dystrophy.

Clustering or overexpression of the transmembrane form of the extracellular matrix proteoglycan agrin in neurons results in the formation of numerous highly motile filopodia-like processes extending from axons and dendrites. Here we show that similar processes can be induced by overexpression of transmembrane-agrin in several non-neuronal cell lines. Mapping of the process-inducing activity in neurons and non-neuronal cells demonstrates that the cytoplasmic part of transmembrane agrin is dispensable and that the extracellular region is necessary for process formation. Site-directed mutagenesis reveals an essential role for the loop between ␤-sheets 3 and 4 within the Kazal subdomain of the seventh follistatin-like domain of TM-agrin. An aspartic acid residue within this loop is critical for process formation. The seventh follistatin-like domain could be functionally replaced by the first and sixth but not by the eighth follistatin-like domain, demonstrating a functional redundancy among some follistatin-like domains of agrin. Moreover, a critical distance of the seventh follistatin-like domain to the plasma membrane appears to be required for process formation. These results demonstrate that different regions within the agrin protein are responsible for synapse formation at the neuromuscular junction and for process formation in central nervous system neurons and suggest a role for agrin's follistatin-like domains in the developing central nervous system.Agrin is a proteoglycan with a molecular mass of Ͼ500 kDa that is expressed in many tissues (1, 2). The function of agrin is best characterized in skeletal muscle where it is a key organizer during formation, maintenance, and regeneration of the neuromuscular junction (2-4). Accordingly, mice with an inactivation of the agrn gene die at birth due to non-functional neuromuscular junctions and consequent respiratory failure (5).Little is known about the role of agrin in tissues other than skeletal muscle, in particular in the central nervous system (for review see Refs. 1, 2, 6). Although neurons from mice with a targeted deletion of the agrn gene form synaptic specializations in vitro and in vivo (7,8), the acute suppression of agrin expression or function by antisense oligonucleotides or antibodies influences the formation and function of interneuronal synapses (9, 10). Likewise, brains of agrin-deficient mice, whose perinatal death was prevented by the re-expression of agrin in motor neurons, have a severely reduced number of pre-and postsynaptic specializations as well as functional deficits at excitatory synapses in the CNS 3 (11). Although these data are consistent with a role of agrin during CNS synaptogenesis, the precise function of agrin during CNS development remains unclear.Agrin has been cloned from several species, and the sequences are highly homologous. The agrin cDNAs predict a number of domains with similarity to other extracellular matrix proteins, including four EGF-like repeats and three domains with similarity to globular domain of the lamini...