Intercellular communication in neuronal elaboration and circuit formation: a role for EphA signaling

Abstract

The mammalian cerebral cortex begins to take shape during embryonic development. The generation of cortical neurons is stereotyped: initial proliferation of cells is followed by differentiation of progeny into neurons and then the movement of these neurons to their final locations within the cerebral cortex, where they begin to elaborate into morphologically complex, differentiated neurons. However, not all cortical neurons are equal. Indeed, the diversity and complexity of neuronal shapes in the cerebral cortex stands as a testament to the intricacies of this natural system, as each morphological class of neurons is dedicated to a particular function. Importantly, cell shape is mirrored in specific cellular connectivity: particular classes of neurons pair selectively via synapses with specific partners, and thereby receive and send specific information. This thesis identifies and characterizes cell surface based signaling that guides neurons as they attain complex shapes and then directs the maturation of synapses and the function of cerebral cortical circuits.

Patterns of gene expression in the cerebral cortex suggest that two members of the Eph family of receptor tyrosine kinases, EphA4 and EphA7, interact with ephrin ligands to define the morphological, and therefore functional, characteristics of cortical neurons. First, the role of EphA4 and EphA7 in the elaboration of cortical neurons was examined. An in vitro approach was used: cortical neurons from wild type of Eph mutant mice were grown on patterned substrates and the sensitivity of axons and dendrites to ephrin ligand was determined. In this context, both axons and dendrites were responsive to Eph engagement; however, axon guidance relies upon EphA4, whereas dendrites use EphA7 to navigate the defined terrain. Nonetheless, both receptors use the Src and Tsc signaling cascades to translate extracellular signals into intracellular responses.

As cortical neurons mature, cellular protrusions, termed spines, form on dendritic shafts. Dendritic spines serve as the primary recipients of excitatory input in the cortex and represent a biophysical assembly well suited to maximize neuronal responses. During their genesis, spines are thought to arise as immature filopodia that mature over time, as the protrusion changes shape and as post-synaptic proteins become organized. Analyses of Golgi stained wild type and mutant cortex demonstrate that EphA4 and EphA7 impact the density and maturity of pyramidal cell dendritic spines, both as they form and when they are mature. In vitro, the absence of EphA7 reduces the density of mature synaptic sites, changes the frequency of detection of spontaneous neurotransmitter release, and delays the maturation of cortical circuitry. In parallel, gain-of-function of EphA4 results in more mature, over-active networks.

In summary, EphA4 and EphA7 impact the initial extension of cellular processes as neurons mature and guide the maturation of synaptic sites, thus coordinating both form and function of cortical neurons.