Neurosecretory cells without neurosecretion: evidence of an independently regulated trait of the cell phenotype

Abstract

During the past few decades, unexpected similarities have emerged between neurones and endocrine cells, especially in relation to secretion and secretory mechanisms. These two cell types are, in fact, recognized as having two parallel pathways of regulated secretion, the first sustained by synaptic-like vesicles (SLV), which predominate in most neurones where they are used for the release of classical, low-molecular weight neurotransmitters, and the second sustained by dense granules (DG) employed for the release of mixtures of compounds such as amines, proteins, peptides and nucleotides. The nature of the secretory products can also be identical in the two types of cell. For example, acetylcholine and GABA, considered until recently to be typical neurotransmitters, are also released by chromaffin and pancreatic â cells, respectively. The proteins of the chromogranin/secretogranin family, discovered in chromaffin and pituitary cells, are common to various types of neurone. Most importantly, the same integral membrane and soluble proteins known to govern the process of regulated exocytosis have been found to operate in both cell systems, including the SNAREs, the G proteins rab3A and B and the phosphoprotein rbSec1Ïmunc18 (S ollner et al. 1993;Jacobsson et al. 1994). Thus, in spite of their well-known structural and functional differences, the definition of 'neurosecretory' appears justified for both types of cell based on the common molecular properties of their secretory programme. In the present review, the ability of the cell to express this specific programme will be referred to as neurosecretion competence. Until recently, studies of neurosecretion competence were conducted primarily from the perspective of cellular differentiation. In various types of endocrine cells, the first traces of the specific hormone(s), accompanied by characteristic aspects of the phenotype, were observed to appear at distinct developmental stages and recognized to depend on the activity of multiple transcription factors expressed co-ordinately or in sequence during the course of differentiation (Groves et al. 1995;Anderson, 1997;Sosa-Pineda et al. 1997; St-Onge et al. 1997). However, whether the appearance of full neurosecretion competence depends exclusively on transcriptional events remains open to question. In fact (i) examples of divergence from the expected correlation between factor expression and phenotype acquisition have been reported (Schoenherr & Anderson, 1995;Scholl et al. 1996;Atouf et al. 1997), (ii) the

Enlargeosome, an Exocytic Vesicle Resistant to Nonionic Detergents, Undergoes Endocytosis via a Nonacidic Route

“…The clone of the rat pheochromocytoma PC12 cell line used here, PC12-27, is advantageous for the present studies: it is rich with enlargeosomes (Borgonovo et al, 2002); it maintains most of the molecular and structural properties typical of wt PC12, a neurosecretory line devoid of enlargeosomes, which can therefore be used for reference; it is specifically defective of the neurosecretion program, in particular it fails to express both types of neurosecretory organelles, synapticlike microvesicles (SLMVs) and dense granules (DGs), as well as the plasmalemma and soluble proteins instrumental to their exocytic discharge (Malosio et al, 1999;Grundschober et al, 2002); and it is very poor of lysosome exocytosis (Borgonovo et al, 2002). Therefore, the participation of lysosomes to the cell surface enlargement induced by stimulation is negligible.…”

“…In contrast, in two fibroblast lines (Chinese hamster ovary [CHO] and 3T3; Coorssen et al, 1996;Ninomiya et al, 1996) and in PC12-27 (Kasai et al, 1999), a neurosecretory clone defective of regulated secretion (Malosio et al, 1999;Grundschober et al, 2002), the first observations, made by electrophysiological capacitance/patchclamp assays, consisted in considerable (15-30%) expansions of the plasma membrane in response to large increases of the cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] i ), induced by photolysis of caged Ca 2ϩ compounds. In PC12-27, these responses start a few hundreds of milliseconds after stimulation and develop rapidly (t 1/2 Ͻ1 s), sustained by the tetanus toxin (TeTx)-insensitive exocytosis of small vesicles (diameter Ͻ0.1 m; Kasai et al, 1999).…”

Neurosecretion Competence

“…In contrast to another reported cell model (12), it cannot be reverted by stimulatory treatments or by changes in growth conditions. However, the NI phenotype defect can be rescued by fusion of the NI cells with WT cells (regardless of the species or phenotype) and following transfection with a WT PC12 cDNA library (11,13). These data strongly suggest neurosecretion incompetence to reflect the lack of expression of a specific program.…”

“…In previous studies carried out by Northern and Western blotting, the degree of heterogeneity of the various PC12 clones had been underestimated. Indeed, of 29 gene products investigated in a single NI and a single WT clone, all the 11 mRNAs/proteins directly related to neurosecretion were found down-regulated, whereas the others (related to membranes, cytoskeleton, signaling, and endocytosis) appeared in contrast invariant (8,11,13). Neurosecretion competence had been therefore envisaged as a highly specific program, expressed independently of the other PC12 traits.…”

“…In fact about 70% of their tested genes are similarly expressed. In previous studies, NI-27 cells had already been shown to express markers typical of the WT neurosecretory PC12 cells, such as tyrosine hydroxylase, neuronal-kinesin, synapsin I, neurofilament H, and voltageoperated Ca 2ϩ channels (8,11). This list can now be expanded based on the present analysis.…”