Spastin, a new AAA protein, is altered in the most frequent form of autosomal dominant spastic paraplegia

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is a genetically heterogeneous neurodegenerative disorder characterized by progressive spasticity of the lower limbs. Among the four loci causing AD-HSP identified so far, the SPG4 locus at chromosome 2p2-1p22 has been shown to account for 40-50% of all AD-HSP families. Using a positional cloning strategy based on obtaining sequence of the entire SPG4 interval, we identified a candidate gene encoding a new member of the AAA protein family, which we named spastin. Sequence analysis of this gene in seven SPG4-linked pedigrees revealed several DNA modifications, including missense, nonsense and splice-site mutations. Both SPG4 and its mouse orthologue were shown to be expressed early and ubiquitously in fetal and adult tissues. The sequence homologies and putative subcellular localization of spastin suggest that this ATPase is involved in the assembly or function of nuclear protein complexes.     

Dissociation of dopamine release in the nucleus accumbens from intracranial self-stimulation

Mesolimbic dopamine-releasing neurons appear to be important in the brain reward system. One behavioural paradigm that supports this hypothesis is intracranial self-stimulation (ICS), during which animals repeatedly press a lever to stimulate their own dopamine-releasing neurons electrically. Here we study dopamine release from dopamine terminals in the nucleus accumbens core and shell in the brain by using rapid-responding voltammetric microsensors during electrical stimulation of dopamine cell bodies in the ventral tegmental area/substantia nigra brain regions. In rats in which stimulating electrode placement failed to elicit dopamine release in the nucleus accumbens, ICS behaviour was not learned. In contrast, ICS was acquired when stimulus trains evoked extracellular dopamine in either the core or the shell of the nucleus accumbens. In animals that could learn ICS, experimenter-delivered stimulation always elicited dopamine release. In contrast, extracellular dopamine was rarely observed during ICS itself. Thus, although activation of mesolimbic dopamine-releasing neurons seems to be a necessary condition for ICS, evoked dopamine release is actually diminished during ICS. Dopamine may therefore be a neural substrate for novelty or reward expectation rather than reward itself.     

Coordination of agonist-induced Ca2+-signalling patterns by NAADP in pancreatic acinar cells

Many hormones and neurotransmitters evoke Ca2+ release from intracellular stores, often triggering agonist-specific signatures of intracellular Ca2+ concentration. Inositol trisphosphate (InsP3) and cyclic adenosine 5'-diphosphate-ribose (cADPR) are established Ca2+-mobilizing messengers that activate Ca2+ release through intracellular InsP3 and ryanodine receptors, respectively. However, in pancreatic acinar cells, neither messenger can explain the complex pattern of Ca2+ signals triggered by the secretory hormone cholecystokinin (CCK). We show here that the Ca2+-mobilizing molecule nicotinic acid adenine dinucleotide phosphate (NAADP), an endogenous metabolite of beta-NADP, triggers a Ca2+ response that varies from short-lasting Ca2+ spikes to a complex mixture of short-lasting (1-2s) and long-lasting (0.2-1 min) Ca2+ spikes. Cells were significantly more sensitive to NAADP than to either cADPR or InsP3, whereas higher concentrations of NAADP selectively inactivated CCK-evoked Ca2+ signals in pancreatic acinar cells, indicating that NAADP may function as an intracellular messenger in mammalian cells.     

Chaperone-like activity of the AAA domain of the yeast Yme1 AAA protease

The AAA domain, a conserved Walker-type ATPase module, is a feature of members of the AAA family of proteins, which are involved in many cellular processes, including vesicular transport, organelle biogenesis, microtubule rearrangement and protein degradation. The function of the AAA domain, however, has not been explained. Membrane-anchored AAA proteases of prokaryotic and eukaryotic cells comprise a subfamily of AAA proteins that have metal-dependent peptidase activity and mediate the degradation of non-assembled membrane proteins. Inactivation of an orthologue of this protease family in humans causes neurodegeneration in hereditary spastic paraplegia. Here we investigate the AAA domain of the yeast protein Yme1, a subunit of the iota-AAA protease located in the inner membrane of mitochondria. We show that Yme1 senses the folding state of solvent-exposed domains and specifically degrades unfolded membrane proteins. Substrate recognition and binding are mediated by the amino-terminal region of the AAA domain. The purified AAA domain of Yme1 binds unfolded polypeptides and suppresses their aggregation. Our results indicate that the AAA domain of Ymel has a chaperone-like activity and suggest that the AAA domains of other AAA proteins may have a similar function.