Functions and effects of creatine in the central nervous system

Creatine kinase catalyses the reversible transphosphorylation of creatine by ATP. In the cell, creatine kinase isoenzymes are specifically localized at strategic sites of ATP consumption to efficiently regenerate ATP in situ via phosphocreatine or at sites of ATP generation to build-up a phosphocreatine pool. Accordingly, the creatine kinase/phosphocreatine system plays a key role in cellular energy buffering and energy transport, particularly in cells with high and fluctuating energy requirements like neurons. Creatine kinases are expressed in the adult and developing human brain and spinal cord, suggesting that the creatine kinase/phosphocreatine system plays a significant role in the central nervous system. Functional impairment of this system leads to a deterioration in energy metabolism, which is phenotypic for many neurodegenerative and age-related diseases. Exogenous creatine supplementation has been shown to reduce neuronal cell loss in experimental paradigms of acute and chronic neurological diseases. In line with these findings, first clinical trials have shown beneficial effects of therapeutic creatine supplementation. Furthermore, creatine was reported to promote differentiation of neuronal precursor cells that might be of importance for improving neuronal cell replacement strategies. Based on these observations there is growing interest on the effects and functions of this compound in the central nervous system. This review gives a short excursion into the basics of the creatine kinase/phosphocreatine system and aims at summarizing findings and concepts on the role of creatine kinase and creatine in the central nervous system with special emphasis on pathological conditions and the positive effects of creatine supplementation.

Interhemispheric balance of overt attention: a theta burst stimulation study

Interhemispheric imbalance is discussed as a pathophysiological mechanism in visuospatial neglect. It is suggested that after a lesion of the right hemisphere the mutual transcallosal inhibition is impaired, resulting in an increased activity of the left hemisphere. We investigated the interhemispheric balance of attention in healthy subjects by using a free visual exploration task and by interfering with the neural activity of the posterior parietal cortex (PPC) of either hemisphere using an inhibitory transcranial magnetic stimulation routine with theta burst stimulation (TBS). Subjects explored colour photographs of real-life scenes presented on a computer screen under four conditions: (i) without TBS; (ii) after TBS over the right PPC; (iii) after TBS over the left PPC; and (iv) after TBS over the right PPC and, after the first half of the task, over the left PPC. Eye movements were measured, and distribution of mean cumulative fixation duration over screen halves was analyzed. TBS over the right PPC resulted in a significant rightward shift of mean cumulative fixation duration of approximately 30 min. The shift could be reversed when a subsequent train of TBS was applied over the left PPC. However, left PPC stimulation alone had no significant effect on visual exploration behaviour. The present study shows that the effect of TBS on the PPC depends on which hemisphere is stimulated and on the state of the contralateral homologue area. These findings are in accordance with the predictions of the interhemispheric rivalry model in neglect.

Neglect-like visual exploration behaviour after theta burst transcranial magnetic stimulation of the right posterior parietal cortex

The right posterior parietal cortex (PPC) is critically involved in visual exploration behaviour, and damage to this area may lead to neglect of the left hemispace. We investigated whether neglect-like visual exploration behaviour could be induced in healthy subjects using theta burst repetitive transcranial magnetic stimulation (rTMS). To this end, one continuous train of theta burst rTMS was applied over the right PPC in 12 healthy subjects prior to a visual exploration task where colour photographs of real-life scenes were presented on a computer screen. In a control experiment, stimulation was also applied over the vertex. Eye movements were measured, and the distribution of visual fixations in the left and right halves of the screen was analysed. In comparison to the performance of 28 control subjects without stimulation, theta burst rTMS over the right PPC, but not the vertex, significantly decreased cumulative fixation duration in the left screen-half and significantly increased cumulative fixation duration in the right screen-half for a time period of 30 min. These results suggest that theta burst rTMS is a reliable method of inducing transient neglect-like visual exploration behaviour.

Linear and Piecewise Functions

This morning Dr. Battle will review basic concepts of linear functions and piecewise functions and how they can be used as models for real-world applications. She will also introduce techniques for using a spreadsheet to analyze data.

Serine residues 994 and 1023/25 are important for insulin receptor kinase inhibition by protein kinase C isoforms beta2 and theta

AIMS/HYPOTHESIS: Inhibition of the signalling function of the human insulin receptor (HIR) is one of the principle mechanisms which induce cellular insulin resistance. It is speculated that serine residues in the insulin receptor beta-subunit are involved in receptor inhibition either as inhibitory phosphorylation sites or as part of receptor domains which bind inhibitory proteins or tyrosine phosphatases. As reported earlier we prepared 16 serine to alanine point mutations of the HIR and found that serine to alanine mutants HIR-994 and HIR-1023/25 showed increased tyrosine autophosphorylation when expressed in human embryonic kidney (HEK) 293 cells. In this study we examined whether these mutant receptors have a different susceptibility to inhibition by serine kinases or an altered tyrosine kinase activity. METHODS: Tyrosine kinase assay and transfection studies. RESULTS: In an in vitro kinase assay using IRS-1 as a substrate we could detect a higher intrinsic tyrosine kinase activity of both receptor constructs. Additionally, a higher capacity to phosphorylate the adapter protein Shc in intact cells was seen. To test the inhibition by serine kinases, the receptor constructs were expressed in HEK 293 cells together with IRS-1 and protein kinase C isoforms beta2 and theta. Phorbol ester stimulation of these cells reduced wild-type receptor autophosphorylation to 58 % or 55 % of the insulin simulated state, respectively. This inhibitory effect was not observed with HIR-994 and HIR-1023/25, although all other tested HIR mutants showed similar inhibition induced by protein kinase C. CONCLUSION/INTERPRETATION: The data suggest that the HIR-domain which contains the serine residues 994 and 1023/25 is important for the inhibitory effect of protein kinase C isoforms beta2 and theta on insulin receptor autophosphorylation.

Specific roles of Rac1 and Rac2 in motile functions of HT1080 fibrosarcoma cells

Rho family proteins are constitutively activated in the highly invasive human fibrosarcoma HT1080 cells. We now investigated the specific roles of Rac1 and Rac2 in regulating morphology, F-actin organization, adhesion, migration, and chemotaxis of HT1080 cells. Downregulation of Rac1 using specific siRNA probes resulted in cell rounding, markedly decreased spreading, adhesion, and chemotaxis of HT1080 cells. 2D migration on laminin-coated surfaces in contrast was not markedly affected. Selective Rac2 depletion did not affect cell morphology, cell adhesion, and 2D migration, but significantly reduced chemotaxis. Downregulation of both Rac1 and Rac2 resulted in an even more marked reduction, but not complete abolishment, of chemotaxis indicating distinct as well as overlapping roles of both proteins in chemotaxis. Rac1 thus is selectively required for HT1080 cell spreading and adhesion whereas Rac1 and Rac2 are both required for efficient chemotaxis.

Functions of lipid raft membrane microdomains at the blood-brain barrier

The blood-brain barrier (BBB) is a highly specialized structural and functional component of the central nervous system that separates the circulating blood from the brain and spinal cord parenchyma. Brain endothelial cells (BECs) that primarily constitute the BBB are tightly interconnected by multiprotein complexes, the adherens junctions and the tight junctions, thereby creating a highly restrictive cellular barrier. Lipid-enriched membrane microdomain compartmentalization is an inherent property of BECs and allows for the apicobasal polarity of brain endothelium, temporal and spatial coordination of cell signaling events, and actin remodeling. In this manuscript, we review the role of membrane microdomains, in particular lipid rafts, in the BBB under physiological conditions and during leukocyte transmigration/diapedesis. Furthermore, we propose a classification of endothelial membrane microdomains based on their function, or at least on the function ascribed to the molecules included in such heterogeneous rafts: (1) rafts associated with interendothelial junctions and adhesion of BECs to basal lamina (scaffolding rafts); (2) rafts involved in immune cell adhesion and migration across brain endothelium (adhesion rafts); (3) rafts associated with transendothelial transport of nutrients and ions (transporter rafts).

One session of repeated parietal theta burst stimulation trains induces long-lasting improvement of visual neglect

BACKGROUND AND PURPOSE: Visual neglect is a frequent disability in stroke and adversely affects mobility, discharge destination, and length of hospital stay. It is assumed that its severity is enhanced by a released interhemispheric inhibition from the unaffected toward the affected hemisphere. Continuous theta burst transcranial magnetic stimulation (TBS) is a new inhibitory brain stimulation protocol which has the potential to induce behavioral effects outlasting stimulation. We aimed to test whether parietal TBS over the unaffected hemisphere can induce a long-lasting improvement of visual neglect by reducing the interhemispheric inhibition. METHODS: Eleven patients with left-sided visual neglect attributable to right hemispheric stroke were tested in a visual perception task. To evaluate the specificity of the TBS effect, 3 conditions were tested: 2 TBS trains over the left contralesional posterior parietal cortex, 2 trains of sham stimulation over the contralesional posterior parietal cortex, and a control condition without any intervention. To evaluate the lifetime of repeated trains of TBS in 1 session, 4 trains were applied over the contralesional posterior parietal cortex. RESULTS: Two TBS trains significantly increased the number of perceived left visual targets for up to 8 hours as compared to baseline. No significant improvement was found with sham stimulation or in the control condition without any intervention. The application of 4 TBS trains significantly increased the number of perceived left targets up to 32 hours. CONCLUSIONS: The new approach of repeating TBS at the same day may be promising for therapy of neglect.