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.

Background: Podocytes, renal glomerular visceral epithelial cells, have two kinds of processes, namely major processes containing microtubules (MTs) and foot processes with actin filaments (AFs). The present study investigated how MTs are organized by the Rho-ROCK signal transduction pathway during process formation of podocytes. Method: After induction of differentiation, podocytes of the conditionally immortalized mouse cell line were treated with Y-27632, a specific inhibitor of ROCK, and exoenzyme C3, an inhibitor of RhoA, as well as with forskolin whose effects include inhibition of RhoA, in order to inhibit the Rho-ROCK pathway. Results: Inhibition of ROCK significantly enhanced the formation of thick processes containing MT bundles. Y-27632 promoted process formation even in the presence of latrunculin A which disrupts AFs, strongly suggesting that ROCK directly regulates MT assembly. Treatment with Y-27632 increased MT stability, and stabilized MTs preferentially localized in podocyte processes. Moreover, when treated with a combination of Y-27632 and forskolin, and with Y-27632 and C3 as well, podocytes developed not only MT-based thick processes but also AF-based thin projections. Conclusions: These data indicate a contribution of ROCK in MT organization to promote podocyte process formation, although it was originally thought to regulate AF assembly. AF-based thin projections seem to be induced mainly by inhibition of RhoA and ROCK. The present study reveals a significant role of the Rho-ROCK signal pathway in the reorganization of both MTs and AFs during process formation of podocytes.

Prosaposin is the precursor of four sphingolipid activator proteins (saposins A, B, C, and D) for lysosomal hydrolases and is abundant in the nervous system and muscle. In addition to its role as a precursor of saposins in lysosomes, intact prosaposin has neurotrophic effects in vivo or in vitro when supplied exogenously. We examined the distribution of prosaposin in the central and peripheral nervous systems and its intracellular distribution. Using a monospecific antisaposin D antibody that crossreacts with prosaposin but not with saposins A, B, or C, immunoblot experiments showed that both the central and peripheral nervous systems express unprocessed prosaposin and little saposin D. Using the antisaposin D antibodies, we demonstrated that prosaposin is abundant in almost all neurons of both the central and peripheral nervous systems, including autonomic nerves, as well as motor and sensory nerves. Immunoelectron microscopy using double staining with antisaposin D and anticathepsin D antibodies showed strong prosaposin immunoreactivity mainly in the lysosomal granules in the neurons in both the central and peripheral nervous systems. The expression of prosaposin mRNA, examined using in situ hybridization, was observed in these same neurons. Our results suggest that prosaposin is synthesized ubiquitously in neurons of both the central and peripheral nervous systems.

Cajal's initial glomeruli (IG) and Dogiel's pericellular nests (PCNs) were first described from methylene blue preparations of healthy animal tissues around the beginning of the last century. Since that time, although many reports have been published concerning these structures, few have focused on their development and phylogeny in healthy animals. The aim of this study was to examine the phylogenetic development of the sensory neurons in Cajal's IG (also called axonal glomeruli) and Dogiel's PCNs in the dorsal root ganglion (DRG) of the healthy adult frog, chick, rat, and rabbit. The three-dimensional architecture of the neurons was observed in ganglia by scanning electron microscopy after removal of the connective tissue. The neurons in the DRG of fish are known to be bipolar, but DRG neurons in the species examined here were found to be pseudounipolar, with single stem processes. The proportion of neurons having IG or PCNs increased with increasing phylogenetic complexity in the species examined here. Cajal's initial glomeruli, the convolution of the stem process near the parent cell body: In frogs, the ganglia were small and the neuronal stem processes were very short and straight. In chicks, the stem processes were longer; sometimes very long, tortuous processes were observed. However, no neurons with typical IG were observed in either species. Typical IG were observed in rats and rabbits; their occurrence was much more frequent in rabbits. Pseudounipolarization, i.e., the transition from bipolar to pseudounipolar neurons, is thought to save space, limit the length of neuronal processes, and reduce conduction time. However, an explanation of the evolutionary advantage of the IG, which is formed by the excessive prolongation of the stem process, remains elusive. The cytological and electrophysiological importance of IG has been discussed. Dogiel's pericellular nests (PCNs), which resemble balls of yarn made of thin unmyelinated nerve fibers around DRG neurons, have been observed in the DRG of rats and rabbits, but not in frogs or chicks. This interesting structure shows not only ontogenetic development in healthy animals but also phylogenetic development among species. The nerve fibers in the PCNs were less than 1.2 mum in diameter and had some varicosities. An immunohistochemical study using anti-tyrosine hydroxylase (TH) antibody revealed that some PCNs contain TH-positive nerve fibers and varicosities. Such TH-positive PCNs disappear after sympathectomy. These results suggest that the PCNs are made up of autonomic nerve fibers.

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