Developmental pattern of plasminogen activator activity in chick optic lobe

uPA-uPAR molecular complex is involved in cell signaling during neuronal migration and neuritogenesis

BACKGROUND

In the development of the central nervous system (CNS), neuronal migration and neuritogenesis are crucial processes for establishing functional neural circuits. This relies on the regulation exerted by several signaling molecules, which play important roles in axonal growth and guidance. The urokinase-type plasminogen activator (uPA)-in association with its receptor-triggers extracellular matrix proteolysis and other cellular processes through the activation of intracellular signaling pathways. Even though the uPA-uPAR complex is well characterized in nonneuronal systems, little is known about its signaling role during CNS development.

RESULTS

In response to uPA, neuronal migration and neuritogenesis are promoted in a dose-dependent manner. After stimulation, uPAR interacts with α5- and β1-integrin subunits, which may constitute an αβ-heterodimer that acts as a uPA-uPAR coreceptor favoring the activation of multiple kinases. This interaction may be responsible for the uPA-promoted phosphorylation of focal adhesion kinase (FAK) and its relocation toward growth cones, triggering cytoskeletal reorganization which, in turn, induces morphological changes related to neuronal migration and neuritogenesis.

CONCLUSIONS

uPA has a key role during CNS development. In association with its receptor, it orchestrates both proteolytic and nonproteolytic events that govern the proper formation of neural networks.

Identification of ?A-like protocadherin expressed during chick development

The protocadherins are calcium-dependent cell adhesion molecules of the cadherin superfamily that have been described in numerous species. Although less well characterized than classical cadherins, the protocadherins are also thought to facilitate critical cell-cell interactions during embryonic development. We have cloned a novel protocadherin from the embryonic chick utilizing a monoclonal antibody produced against a peanut agglutinin-binding fraction of cultured chick limb tissue to screen a lambdaZAP cDNA expression library from the stage 25 limb. A 2.8 kb cDNA clone was obtained that encoded multiple cadherin-like ectodomains. Northern blotting revealed a single 4.6 kb RNA transcript that was highly enriched in the stage 43 chick brain. Utilization of 3' Rapid amplification of cDNA ends (RACE) identified the entire 2.4 kb reading frame. The chick protocadherin contained five cadherin-like extracellular repeats and a highly conserved cytoplasmic domain. Amino acid alignment of the extracellular domains revealed marked identity to the human gammaA protocadherin subfamily. In situ hybridization showed low levels of mRNA localization in several developing chick tissues, but stronger expression in the neural tube and dorsal root ganglia at stage 27. In the stage 43 chick brain, protocadherin mRNA was noted in discrete regions, particularly within the developing optic lobe. As for protocadherins described in other species, these results suggest that this novel gammaA-like protocadherin may also play a role in chick neural development.

Developmental pattern of plasminogen activator activity in chick brain hemispheres

Plasminogen activators play key roles in several developmental events. In previous works we demonstrated the existence of typical developmental patterns of protease activity in the chick optic lobe and cerebellum. The aim of this work is to study the temporal pattern of development of plasminogen activator activity in the brain hemispheres. Plasminogen activator activity was assayed in soluble fractions derived by ultracentrifugation from Triton X-100 treated membrane fractions by using a radial fibrinolytic assay. Employing different inhibitors and anti-plasminogen activators antibodies we showed that developing brain hemispheres express only one type of enzyme which corresponds to the urokinase-type. Other results indicate that the protease activity displays a temporal pattern which completely differs from those of general parameters of development. This suggests that the plasminogen activator activity is developmentally regulated and could display specific functions during particular stages of development.

Temporal pattern of plasminogen activator activity in the developing chick cerebellum

Plasminogen activators are considered to be involved in several developmental events. The present work aims at characterizing the developmental pattern of expression of plasminogen activators in the chick cerebellum. Soluble fractions derived by ultracentrifugation from Triton X-100 treated membrane fractions were used for determination of the enzyme activity with a radial fibrinolytic assay. By using specific inhibitors and different anti-plasminogen activators antibodies it is shown that only one type of the enzyme, the urokinase-type plasminogen activator, is expressed during the cerebellum ontogeny. Our results show the existence of a bimodal pattern of enzyme activity with two peaks that temporally coincide with the processes of massive neuronal migration, neurite outgrowth and synapse formation and plasticity. It is proposed that plasminogen activator could play a role in these developmental events and that its pattern of variability is developmentally regulated.