Akt activation by growth factors is a multiple-step process: the role of the PH domain

The protein kinase encoded by the Akt proto-oncogene is activated by phospholipid binding, membrane translocation and phosphorylation. To address the relative roles of these mechanisms of Akt activation, we have employed a combination of genetic and pharmacological approaches. Transient transfection of NIH3T3 cells with wild-type Akt, pleckstrin homology (PH) domain mutants, generated on the basis of a PH domain structural model, and phosphorylation site Akt mutants provided evidence for a model of Akt activation consisting of three sequential steps: (1) a PH domain-dependent, growth factor-independent step, marked by constitutive phosphorylation of threonine 450 (T450) and perhaps serine 124 (S124), that renders the protein responsive to subsequent activation events; (2) a growth factor-induced, PI3-K-dependent membrane-translocation step; and (3) a PI3-K-dependent step, characterized by phosphorylation at T308 and S473, that occurs in the cell membrane and is required for activation. When forced to translocate to the membrane, wild-type Akt and PH domain Akt mutants that are defective in the first step become constitutively active, suggesting that the purpose of this step is to prepare the protein for membrane translocation. Both growth factor stimulation and forced membrane translocation, however, failed to activate a T308A mutant. This, combined with the finding that T308D/S473D double mutant is constitutively active, suggests that the purpose of the three-step process of Akt activation is the phosphorylation of the protein at T308 and S473. The proposed model provides a framework for a comprehensive understanding of the temporal and spatial requirements for Akt activation by growth factors.     

Chicken dystrophy. The geometry of the transverse tubules

The pectoral muscle of chickens afflicted with muscular dystrophy, when examined with the electron microscope, contains a) numerous, often quite large vesicles with and without caveolar evaginations, b) tubules with caveolar evaginations, and c) tubular networks. We have demonstrated that all these structures are derivatives of the transverse tubles as revealed by tracer studies and freeze-fracture complementary replicas. The membranes of transverse tubular origin show a small number of intramembranous particles on both P and E faces with no complementary geometry. The membranes of the free sacrcoplasmic reticulum (SR) and the junctional SR of normal and dystrophic muscle appear identical in complementary freeze-fracture replicas. Vesicles that carry only a small number of particles on both E and P faces exposed by freeze-fracturing in isolated SR preparations can be taken as presumptive evidence and serve as a morphologic marker for transverse tubular origin of such vesicles when mitochondrial and lysosomal contamination has been excluded.     

Mechanical properties of intercellular contacts of small intestinal epithelium

It has been shown that when long-acting forces are applied, the intercellular contacts in the small intestine epithelium are destroyed at the values of these forces lower than the adhesion ones obtained at the application of short-term loads. Starting with some threshold value of the load viscose-elastic properties of intercellular contacts are shown up. Thus the threshold varies essentially for different cells of the population: 4 variations are observed. A removal of bivalent cations from the tissue significantly decreases the threshold value of the forces and decreases the viscosity while the excessive concentrations of bivalent cations increase the threshold value of forces, the membrane viscosity and contact components.     

Reversible dementia due to dural arteriovenous fistula: a simple surgical option

Dural AVMs can produce a wide variety of symptoms related to raised intracranial pressure, venous congestion, and cerebral ischaemia. We present a unique case of reversible dementia, due to venous hypertension. The cerebral ischaemia was caused by extensive bilateral arteriovenous malformations of the external carotid system, which drained into the superior sagittal and transverse sinuses and resulted in venous hypertension. Although partial occlusion by endovascular embolization and ligation procedures had some effect, only 'scalping and silastic implantation' gave satisfactory amelioration of symptoms.     

Some characteristics of the differential diagnosis of coniotuberculosis]

Biopsies taken from 132 patients during operations were studied histologically. The control material was taken from 54 corpses. The age of the patients and controls ranged from 5 to 68 years. Under investigation were pieces of tissue (0,5X0,2 cm) fixed in 12% neutral formalin. The nerve elements in the serous stage of all forms of panaritium had reactive changes, those in the purulent stage had destructive changes as well. The degree of the changes grown with age, from the serous stage to the purulent one, from unguinal panaritium to osteal panaritium. The least alterations were observed in the serous stage of unguinal panaritium and subcutaneous whitlow in patients from 15 to 25 years of age with the term of the disease 1-2 days. The greatest changes were found in severe forms of panaritium in people older than 50 years with the term of the disease 3-8 days.