Galina Semenova

PAK signalling during the development and progression of cancer

p21-activated kinases (PAKs) are positioned at the nexus of several oncogenic signalling pathways. Overexpression or mutational activation of PAK isoforms frequently occurs in various human tumours, and recent data suggest that excessive PAK activity drives many of the cellular processes that are the hallmarks of cancer. In this Review, we discuss the mechanisms of PAK activation in cancer, the key substrates that mediate the developmental and oncogenic effects of this family of kinases, and how small-molecule inhibitors of these enzymes might be best developed and deployed for the treatment of cancer.

Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities.

The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 degrees C for 20 min and to temperature 42 degrees C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 degrees C for 72h. The net photosynthetic rates (P(N)) and the fluorescence ratios F(v)/F(m) were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 degrees C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO(2) assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 degrees C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed.

Structural features of the salt glands of the leaf of Distichlis spicata 'Yensen 4a' (Poaceae)

The epidermal salt glands of the leaf of Distichlis spicata ‘Yensen 4a’ (Poaceae) have a direct contact with one or two water-storing parenchyma cells, which act as collecting cells. A vacuole occupying almost the whole volume of the collecting cell has a direct exit into the extracellular space (apoplast) through the invaginations of the parietal layer of the cytoplasm, which is interrupted in some areas so that the vacuolar-apoplastic continuum is separated only by a single thin membrane, which looks as a valve. On the basis of ultrastructural morphological data (two shapes of the extracellular channels, narrow and extended, are found in basal cells), the hypothesis on the mechanical nature of the salt pump in the basal cell of Distichlis leaf salt gland is proposed. According to the hypothesis, a driving force giving ordered motion to salt solution from the vacuole of the collecting cell through the basal cell of the salt gland to cap cell arises from the impulses of a mechanical compression–expansion of plasma membrane, which penetrates the basal cell in the form of extracellular channels. The acts of compression–expansion of these extracellular channels can be realized by numerous microtubules present in the basal cell cytoplasm.

The thylakoid membrane in a wide pH range

Summary The structural organisation of thylakoid systems of pea chloroplasts subjected to osmotic shock after incubation in a wide range of pH values (from 1.0 to 12.0) was studied. It was found that the percentage of thylakoid profiles stacked into granas is 86 % at pH 4.0, 72 % at pH 7.4 and 25 % at pH 10.0. In the pH range from 6.0 to 10.0 under the conditions of osmotic shock, a complete disordering of thylakoid systems, unstacking, and fragmentation of thylakoids are observed. Decreasing medium pH to 5.5-3.5 leads to a shortening of agranal thylakoids, the appearance of numerous lipid drops, and a shrinkage of thylakoid systems, which cease to respond to osmotic shock. Thylakoid systems remain structured, and the stacking in granas is retained. At pH levels from 3.0 to 1.0, thylakoid systems appear as spherical clusters of tightly stacked vesicles, whose structural organisation also does not change under osmotic shock. Increasing the pH of the medium to 11-12 leads to a complete unstacking of thylakoids and a swelling of single thylakoid sheets, which also do not respond to osmotic shock. A correlation between the release of acyl lipids from the thylakoid membrane and the loss of the native response of the membrane to osmotic shock is discussed.

PKM2 Enters the Morpheein Academy

In this issue of Molecular Cell, Gao et al. (2012) show that the glycolytic enzyme PKM2, in its dimeric form, possesses protein kinase activity and phosphorylates STAT3 in the nucleus, thereby driving expression of genes that promote transformation.

Light-induced reversible local fusions of thylakoid membranes in the presence of dibucaine or tetracaine.

The dynamics of structural changes in pea chloroplasts in the presence of 25-50 microM dibucaine or tetracaine has been examined using electron microscopy. The light-induced uptake of anesthetic cations by thylakoids is attended by the appearance of local fusions of stroma-exposed thylakoid membranes. The first membrane protrusions and interthylakoid contacts are observed after 4 s illumination and they become numerous by 10 s. As a result, a network of anastomoses is formed which is maintained during at least 10 min. These effects are reversible in the dark and can be reproduced several times. The formation of membrane fusions is inhibited by the addition of protonophore. It is supposed that the energy-dependent uptake of protonated anesthetics by thylakoids leads to an increase in positive surface charge and thus a lateral pressure on the inner side of the thylakoid membrane. The appearance of membrane protrusions (crinkles) having the positive curvature of their inner surface may be considered as a way of compensating for lateral pressure. Presumably, anastomoses result from the fusion of crinkles to adjacent thylakoids.

Effect of urea and distilled water on the structure of the thylakoid system

Summary On the basis of structural data, an attempt is made to substantiate the hypothesis that in a thylakoid membrane, there exists an integral continuous network of intramembrane, lateral protein-protein interactions. It is shown that the forces determining the extent of flattening of the thylakoid system and the forces responsible for the packing of thylakoids in granas act independently. These two discrete groups of bonds, lateral bonds of the external leaflet of the membrane and the bonds of intermembrane stacking, provide a peculiar configuration of the system of internal chloroplast membranes, namely, the granal packing and the extent of flattening of the thylakoid system.

The relationship between the transformation of thylakoid acyl lipids and the formation of tubular lipid aggregates visible on fracture faces

Summary The chemical transformation of thylakoid acyl lipids in isolated pea ( Pisum satium L.) chloroplasts is shown to be accompanied by the formation of near-thylakoid lipid drops, whose internal structure appears as tubular aggregates characteristic for the phase separation of non-bilayer forming lipids. This correlation is revealed for chloroplasts isolated at pH 4.6 and for chloroplasts stored for 10-12 days at + 5 °. In chloroplasts stored for 10 days at + 2 °, chemical changes are minimal, and lipid drops are absent. In chloroplasts isolated at pH 4.6, a decrease in the volume and an increase in density take place, due to a reduction in the length of agranal thylakoids and the free space between thylakoids.

Suppression of RAC1 -driven malignant melanoma by group A PAK inhibitors

Activating mutations in the RAC1 gene have recently been discovered as driver events in malignant melanoma. Expression of this gene is associated with melanocyte proliferation, and melanoma cells bearing this mutation are insensitive to BRAF inhibitors such as vemurafenib and dabrafenib, and also may evade immune surveillance due to enhanced expression of PD-L1. Activating mutations in RAC1 are of special interest, as small-molecule inhibitors for the RAC effector p21-activated kinase (PAK) are in late-stage clinical development and might impede oncogenic signaling from mutant RAC1. In this work, we explore the effects of PAK inhibition on RAC1P29S signaling in zebrafish embryonic development, in the proliferation, survival and motility of RAC1P29S-mutant human melanoma cells, and on tumor formation and progression from such cells in mice. We report that RAC1P29S evokes a Rasopathy-like phenotype on zebrafish development that can be blocked by inhibitors of PAK or MEK. We also found and that RAC1-mutant human melanoma cells are resistant to clinical inhibitors of BRAF but are uniquely sensitive to PAK inhibitors. These data suggest that suppressing the PAK pathway might be of therapeutic benefit in this type of melanoma.

Targeting group I p21-activated kinases to control malignant peripheral nerve sheath tumor growth and metastasis

Malignant peripheral nerve sheath tumors (MPNSTs) are devastating sarcomas for which no effective medical therapies are available. Over 50% of MPSNTs are associated with mutations in NF1 tumor suppressor gene, resulting in activation of Ras and its effectors, including the Raf/Mek/Erk and PI3K/Akt/mTORC1 signaling cascades, and also the WNT/β-catenin pathway. As Group I p21-activated kinases (Group I Paks, PAK1/2/3) have been shown to modulate Ras-driven oncogenesis, we asked if these enzymes might regulate signaling in MPNSTs. In this study we found a strong positive correlation between the activity of PAK1/2/3 and the stage of human MPNSTs. We determined that reducing Group I Pak activity diminished MPNST cell proliferation and motility, and that these effects were not accompanied by significant blockade of the Raf/Mek/Erk pathway, but rather by reductions in Akt and β-catenin activity. Using the small molecule PAK1/2/3 inhibitor Frax1036 and the MEK1/2 inhibitor PD0325901, we showed that the combination of these two agents synergistically inhibited MPNST cell growth in vitro and dramatically decreased local and metastatic MPNST growth in animal models. Taken together, these data provide new insights into MPNST signaling deregulation and suggest that co-targeting of PAK1/2/3 and MEK1/2 may be effective in the treatment of patients with MPNSTs.