Corticotropin-releasing factor is a potent inhibitor of sexual receptivity in the female rat

Corticotropin-releasing factor (CRF), the recently characterized and synthesized 41-amino acid polypeptide isolated from ovine hypothalami, has been shown to be a potent stimulator of adenohypophyseal beta-endorphin and corticotropin (ACTH) secretion both in vitro and in vivo. In common with other regulatory peptides, CRF has also been demonstrated to possess extra-hypophysiotropic roles. Indeed, intracerebroventricularly (i.c.v.) administered CRF elicits several endocrine and behavioural responses compatible with the concept that this peptide could be a key signal in coordinating the organism's endocrine and behavioural responses to stressful and other adaptive stimuli. We now provide the first evidence for neurally placed CRF in the control of a specific hormone-dependent behavioural response and unequivocally demonstrate an extremely potent suppressive effect of CRF on sexual behaviour in the female rat when microinfused into the arcuate-ventromedial area of the hypothalamus (ARC-VMH) and the mesencephalic central grey (MCG).     

Structural basis of microtubule severing by the hereditary spastic paraplegia protein spastin

Spastin, the most common locus for mutations in hereditary spastic paraplegias, and katanin are related microtubule-severing AAA ATPases involved in constructing neuronal and non-centrosomal microtubule arrays and in segregating chromosomes. The mechanism by which spastin and katanin break and destabilize microtubules is unknown, in part owing to the lack of structural information on these enzymes. Here we report the X-ray crystal structure of the Drosophila spastin AAA domain and provide a model for the active spastin hexamer generated using small-angle X-ray scattering combined with atomic docking. The spastin hexamer forms a ring with a prominent central pore and six radiating arms that may dock onto the microtubule. Helices unique to the microtubule-severing AAA ATPases surround the entrances to the pore on either side of the ring, and three highly conserved loops line the pore lumen. Mutagenesis reveals essential roles for these structural elements in the severing reaction. Peptide and antibody inhibition experiments further show that spastin may dismantle microtubules by recognizing specific features in the carboxy-terminal tail of tubulin. Collectively, our data support a model in which spastin pulls the C terminus of tubulin through its central pore, generating a mechanical force that destabilizes tubulin-tubulin interactions within the microtubule lattice. Our work also provides insights into the structural defects in spastin that arise from mutations identified in hereditary spastic paraplegia patients.     

How kinesin waits between steps

Kinesin-1 (conventional kinesin) is a dimeric motor protein that carries cellular cargoes along microtubules by hydrolysing ATP and moving processively in 8-nm steps. The mechanism of processive motility involves the hand-over-hand motion of the two motor domains ('heads'), a process driven by a conformational change in the neck-linker domain of kinesin. However, the 'waiting conformation' of kinesin between steps remains controversial-some models propose that kinesin adopts a one-head-bound intermediate, whereas others suggest that both the kinesin heads are bound to adjacent tubulin subunits. Addressing this question has proved challenging, in part because of a lack of tools to measure structural states of the kinesin dimer as it moves along a microtubule. Here we develop two different single-molecule fluorescence resonance energy transfer (smFRET) sensors to detect whether kinesin is bound to its microtubule track by one or two heads. Our FRET results indicate that, while moving in the presence of saturating ATP, kinesin spends most of its time bound to the microtubule with both heads. However, when nucleotide binding becomes rate-limiting at low ATP concentrations, kinesin waits for ATP in a one-head-bound state and makes brief transitions to a two-head-bound intermediate as it walks along the microtubule. On the basis of these results, we suggest a model for how transitions in the ATPase cycle position the two kinesin heads and drive their hand-over-hand motion.     

Movement of microtubules by single kinesin molecules

Kinesin is a motor protein that uses energy derived from ATP hydrolysis to move organelles along microtubules. Using a new technique for measuring the movement produced in vitro by individual kinesin molecules, it is shown that a single kinesin molecule can move a microtubule for several micrometers. New information about the mechanism of force generation by kinesin is presented.     

Reciprocal changes in corticotropin-releasing factor (CRF)-like immunoreactivity and CRF receptors in cerebral cortex of Alzheimer's disease

Alzheimer's disease is a progressive degenerative disease of the nervous system characterized neuropathologically by the presence of senile plaques and neurofibrillary tangles in amygdala, hippocampus and neocortex. Dysfunction and death of basal forebrain cholinergic neurones projecting to forebrain targets are associated with marked decreases in cholinergic markers, including the activity of choline acetyltransferase (ChAT). Although cortical levels of somatostatin and somatostatin receptors are reduced in Alzheimer's, no consistent changes have been reported in other neuropeptide systems. We have now examined in control and Alzheimer's brain tissues pre- and postsynaptic markers of corticotropin-releasing factor (CRF), a hypothalamic peptide regulating pituitary-adrenocortical secretion which also seems to act as a neurotransmitter in the central nervous system (CNS). We have found that in Alzheimer's, the concentrations of CRF-like immunoreactivity (CRF-IR) are reduced and that there are reciprocal increases in CRF receptor binding in affected cortical areas. These changes are significantly correlated with decrements in ChAT activity. Our results strongly support a neurotransmitter role for CRF in brain and demonstrate, for the first time, a modulation of CNS CRF receptors associated with altered CRF content. These observations further suggest a possible role for CRF in the pathophysiology of the dementia. Future therapies directed at increasing CRF levels in brain may prove useful for treatment.     

Stimulation of food intake in rats by centrally administered hypothalamic growth hormone-releasing factor

Hypothalamic growth hormone-releasing factors (GRFs) have been purified recently from human pancreatic (hp) tumours and from rat hypothalamus (rh). GRF peptides have strong homology with peptides of the glucagon, vasoactive intestinal polypeptide and PHI-27 family. Aside from their potent actions on release of somatotropin, no other biological actions of GRFs have been reported. GRF has been localized in neurones bordering the ventromedial hypothalamic nucleus, a region associated frequently with experimental analysis of feeding behaviour. We now report that intracerebroventricularly (i.c.v.)-administered rhGRF and hpGRF(1-40) in doses of 0.2, 2.0 and 20.0 pmol, produced an increase in food intake in hungry rats. This effect seemed to be specific to GRF as i.c.v. injections of a structurally related but physiologically inactive peptide in the same doses had no effect on feeding. In addition, peripheral injections of rhGRF or growth hormone had no effect on food intake, suggesting that the present effects may be mediated centrally. Injections (i.c.v.) of rhGRF (0.2, 2.0 and 20.0 pmol) had no effect on general activity, suggesting that GRF does not produce nonspecific arousal.