Alkaline pH changes in the cerebellum of asymptomatic HIV-infected individuals

Pontocerebellar contribution to postural instability and psychomotor slowing in HIV infection without dementia

Postural instability occurs in HIV infection, but quantitative balance tests in conjunction with neuroimaging are lacking. We examined whether infratentorial brain tissue volume would be deficient in nondemented HIV-infected individuals and whether selective tissue deficits would be related to postural stability and psychomotor speed performance. The 123 participants included 28 men and 12 women with HIV infection without dementia or alcohol use disorders, and 40 men and 43 women without medical or psychiatric conditions. Participants completed quantitative balance testing, Digit Symbol test, and a test of finger movement speed and dexterity. An infratentorial brain region, supratentorial ventricular system, and corpus callosum were quantified with MRI-derived atlas-based parcellation, and together with archival DTI-derived fiber tracking of pontocerebellar and internal and external capsule fiber systems, brain measures were correlated with test performance. The tissue ratio of the infratentorium was ~3% smaller in the HIV than control group. The HIV group exhibited performance deficits in balancing on one foot, walking toe-to-heel, Digit Symbol substitution task, and time to complete all Digit Symbol grid boxes. Total infratentorial tissue ratio was a significant predictor of balance and Digit Symbol scores. Balance scores did not correlate significantly with ventricular volumes, callosal size, or internal or external capsule fiber integrity but did so with indices of pontocerebellar tract integrity. HIV-infected individuals specifically recruited to be without complications from alcohol use disorders had pontocerebellar tissue volume deficits with functional ramifications. Postural stability and psychomotor speed were impaired and attributable, at least in part, to compromised infratentorial brain systems.

Toxoplasma gondii lysine acetyltransferase GCN5-A functions in the cellular response to alkaline stress and expression of cyst genes

Parasitic protozoa such as the apicomplexan Toxoplasma gondii progress through their life cycle in response to stimuli in the environment or host organism. Very little is known about how proliferating tachyzoites reprogram their expressed genome in response to stresses that prompt development into latent bradyzoite cysts. We have previously linked histone acetylation with the expression of stage-specific genes, but the factors involved remain to be determined. We sought to determine if GCN5, which operates as a transcriptional co-activator by virtue of its histone acetyltransferase (HAT) activity, contributed to stress-induced changes in gene expression in Toxoplasma. In contrast to other lower eukaryotes, Toxoplasma has duplicated its GCN5 lysine acetyltransferase (KAT). Disruption of the gene encoding for TgGCN5-A in type I RH strain did not produce a severe phenotype under normal culture conditions, but here we show that the TgGCN5-A null mutant is deficient in responding to alkaline pH, a common stress used to induce bradyzoite differentiation in vitro. We performed a genome-wide analysis of the Toxoplasma transcriptional response to alkaline pH stress, finding that parasites deleted for TgGCN5-A fail to up-regulate 74% of the stress response genes that are induced 2-fold or more in wild-type. Using chromatin immunoprecipitation, we verify an enrichment of TgGCN5-A at the upstream regions of genes activated by alkaline pH exposure. The TgGCN5-A knockout is also incapable of up-regulating key marker genes expressed during development of the latent cyst form, and is impaired in its ability to recover from alkaline stress. Complementation of the TgGCN5-A knockout restores the expression of these stress-induced genes and reverses the stress recovery defect. These results establish TgGCN5-A as a major contributor to the alkaline stress response in RH strain Toxoplasma.

Protozoan parasites cause significant disease in humans and livestock, and many of our current therapies have serious side effects or are being rendered useless due to the development of drug resistance. These parasites typically have complex life cycles involving multiple hosts and some, like Toxoplasma gondii, have the ability to remain in the host for life as a latent tissue cyst. Toxoplasma is one of the most successful parasites on Earth because the ability to develop into a tissue cyst greatly facilitates transmission through carnivores. Cyst formation also is responsible for recrudescent infection in immunocompromised patients. The conversion of Toxoplasma from its replicating cell to the cyst is triggered by stress, but we have little understanding of how the parasite stress response functions. In this study, we identify the genes involved in Toxoplasma's response to alkaline stress, which is a known trigger of cyst development. We also establish that a lysine acetyltransferase enzyme called TgGCN5-A is required for type I RH strain Toxoplasma to respond normally to alkaline stress. Parasites lacking TgGCN5-A are no longer capable of activating genes induced during cyst formation triggered by alkaline pH.

Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

The maintenance of pH homeostasis in the CNS is of key importance for proper execution and regulation of neurotransmission, and deviations from this homeostasis are a crucial factor in the mechanism underlying a spectrum of pathological conditions. The first few sections of the review are devoted to the brain operating under normal conditions. The article commences with an overview of how extrinsic factors modelling the brain at work: neurotransmitters, depolarising stimuli (potassium and voltage changes) and cyclic nucleotides as major signal transducing vehicles affect pH in the CNS. Further, consequences of pH alterations on the major aspects of CNS function and metabolism are outlined. Next, the major cellular events involved in the transport, sequestration, metabolic production and buffering of protons that are common to all the mammalian cells, including the CNS cells. Since CNS function reflects tight interaction between astrocytes and neurons, the pH regulatory events pertinent to either cell type are discussed: overwhelming evidence implicates astrocytes as a key player in pH homeostasis in the brain. The different classes of membrane proteins involved in proton shuttling are listed and their mechanisms of action are given. These include: the Na+/H+ exchanger, different classes of bicarbonate transporters acting in a sodium-dependent- or -independent mode, monocarboxylic acid transporters and the vacuolar-type proton ATPase. A separate section is devoted to carbonic anhydrase, which is represented by multiple isoenzymes capable of pH buffering both in the cell interior and in the extracellular space. Next, impairment of pH regulation and compensatory responses occurring in brain affected by different pathologies: hypoxia/ischemia, epilepsy, hyperammonemic encephalopathies, cerebral tumours and HIV will be described. The review is limited to facts and plausible hypotheses pertaining to phenomena directly involved in pH regulation: changes in pH that accompany metabolic stress but have no distinct implications for the pH regulatory mechanisms are not dealt with. In most cases, the vast body of knowledge derived from in vitro studies remains to be verified in in vivo settings.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.