Process formation of the renal glomerular podocyte: is there common molecular machinery for processes of podocytes and neurons?

The renal glomerular podocyte exhibits a highly arborized morphology. In comparison with the neuron, which is the best studied process-bearing cell, the podocyte major processes share many cell biological characteristics with neuronal dendrites. Both podocytes and neurons develop microtubule-based thick processes with branching morphology and both have thin actin-based projections (i.e. podocyte foot processes and dendritic spines). Formation of podocyte processes and neuronal dendrites depends on the assembly of microtubules. Because the assembly of microtubules is regulated by phosphorylation of microtubule-associated proteins, inhibition of protein phosphatases abolishes and inhibition of protein kinases promotes process formation. Podocytes and dendrites also share the machinery of intracellular traffic of membranous vesicles, as well as cytoskeletal elements, which is indispensable for the elongation of these processes. Furthermore, these two cell types share expression of various molecules working for signal transduction, transmembranous transport and intercellular contacts. Such common gene expression implies a similar transcriptional regulation in these cells. Concerning the formation of podocyte foot processes and dendritic branches, actin filaments are thought to play a central role in orchestrating the function of various molecules and the regulation of actin assembly is necessary to establish and maintain such sophisticated cellular architecture. The molecular mechanism of foot process formation seems to include Rho family small GTP-binding proteins, which are known to be responsible for the establishment of dendritic branching morphology.