Progesterone Stimulates the Activity of the Promoters of Peripheral Myelin Protein-22 and Protein Zero Genes in Schwann Cells

Abstract: To understand better the mechanisms by which progesterone (PROG) promotes myelination in the PNS, cultured rat Schwann cells were transiently transfected with reporter constructs in which luciferase expression was controlled by the promoter region of either the peripheral myelin protein-22 (PMP22) or the protein zero (P₀) genes. PROG stimulated the P₀ promoter and promoter 1, but not promoter 2, of PMP22. The effect of PROG was specific, as estradiol and testosterone only weakly activated promoters. Dose-response curves for stimulation of both promoter constructs by PROG were biphasic. RU486, a PROG antagonist, did not abolish the effect of PROG, but stimulated promoter activities by itself. In the human carcinoma cell line T47D expressing high levels of PROG receptor, PROG did not stimulate the P₀ and PMP22 promoters, whereas the promoter region of the mouse mammary tumor virus was fully activated. Thus, the activation by PROG of promoter activity of two peripheral myelin protein genes is Schwann-cell specific.     

Nogo-A expressed in Schwann cells impairs axonal regeneration after peripheral nerve injury

Injured axons in mammalian peripheral nerves often regenerate successfully over long distances, in contrast to axons in the brain and spinal cord (CNS). Neurite growth-inhibitory proteins, including the recently cloned membrane protein Nogo-A, are enriched in the CNS, in particular in myelin. Nogo-A is not detectable in peripheral nerve myelin. Using regulated transgenic expression of Nogo-A in peripheral nerve Schwann cells, we show that axonal regeneration and functional recovery are impaired after a sciatic nerve crush. Nogo-A thus overrides the growth-permissive and -promoting effects of the lesioned peripheral nerve, demonstrating its in vivo potency as an inhibitor of axonal regeneration.     

Single-unit responses selective for whole faces in the human amygdala

The human amygdala is critical for social cognition from faces, as borne out by impairments in recognizing facial emotion following amygdala lesions [1] and differential activation of the amygdala by faces [2-5]. Single-unit recordings in the primate amygdala have documented responses selective for faces, their identity, or emotional expression [6, 7], yet how the amygdala represents face information remains unknown. Does it encode specific features of faces that are particularly critical for recognizing emotions (such as the eyes), or does it encode the whole face, a level of representation that might be the proximal substrate for subsequent social cognition? We investigated this question by recording from over 200 single neurons in the amygdalae of seven neurosurgical patients with implanted depth electrodes [8]. We found that approximately half of all neurons responded to faces or parts of faces. Approximately 20% of all neurons responded selectively only to the whole face. Although responding most to whole faces, these neurons paradoxically responded more when only a small part of the face was shown compared to when almost the entire face was shown. We suggest that the human amygdala plays a predominant role in representing global information about faces, possibly achieved through inhibition between individual facial features.     

ARABIDOPSIS TRITHORAX1 dynamically regulates FLOWERING LOCUS C activation via histone 3 lysine 4 trimethylation

Trithorax function is essential for epigenetic maintenance of gene expression in animals, but little is known about trithorax homologs in plants. ARABIDOPSIS TRITHORAX1 (ATX1) was shown to be required for the expression of homeotic genes involved in flower organogenesis. Here, we report a novel function of ATX1, namely, the epigenetic regulation of the floral repressor FLOWERING LOCUS C (FLC). Downregulation of FLC accelerates the transition from vegetative to reproductive development in Arabidopsis thaliana. In the atx1 mutant, FLC levels are reduced and the FLC chromatin is depleted of trimethylated, but not dimethylated, histone 3 lysine 4, suggesting a specific trimethylation function of ATX1. In addition, we found that ATX1 directly binds the active FLC locus before flowering and that this interaction is released upon the transition to flowering. This dynamic process stands in contrast with the stable maintenance of homeotic gene expression mediated by trithorax group proteins in animals but resembles the dynamics of plant Polycomb group function.     

The single nuclear lamin of Caenorhabditis elegans forms in vitro stable intermediate filaments and paracrystals with a reduced axial periodicity

The lamins of the tunicate Ciona intestinalis and the nematode Caenorhabditis elegans show unusual sequence features when compared to the more than 35 metazoan lamin sequences currently known. We therefore analyzed the in vitro assembly of these two lamins by electron microscopy using chicken lamin B2 as a control. While lamin dimers usually appear as a rod carrying two globules at one end, these globules are absent from Ciona lamin, which lacks the central 105-residue region of the tail domain. The deletion of 14 residues or two heptads from the coiled coil rod domain of the single C.elegans lamin results in a 1.5-nm shortening of the dimer rod. Similarly, the paracrystals assembled from the C.elegans lamin exhibit a 3.1-nm reduction of the true axial repeat compared to that of chicken lamin B2 paracrystals. We speculate that the banding pattern in the C.elegans lamin paracrystals arises from a relative stagger between dimers and/or a positioning of the globular tail domain relative to the central rod that is distinct from that observed in chicken lamin B2 paracrystals. Here we show that a nuclear lamin can assemble in vitro into 10-nm intermediate filaments (IFs). C.elegans lamin in low ionic strength Tris-buffers at a pH of 7.2-7.4 provides a stable population of lamin IFs. Some implications of this filament formation are discussed.     

Self-assembly of human MxA GTPase into highly ordered dynamin-like oligomers

Human MxA protein is a member of the interferon-induced Mx protein family and an important component of the innate host defense against RNA viruses. The Mx family belongs to a superfamily of large GTPases that also includes the dynamins and the interferon-regulated guanylate-binding proteins. A common feature of these large GTPases is their ability to form high molecular weight oligomers. Here we determined the capacity of MxA to self-assemble into homo-oligomers in vitro. We show that recombinant MxA protein assembles into long filamentous structures with a diameter of about 20 nm at physiological salt concentration as demonstrated by sedimentation assays and electron microscopy. In the presence of guanosine nucleotides the filaments rearranged into rings and more compact helical arrays. Our data indicate that binding and hydrolysis of GTP induce conformational changes in MxA that may be essential for viral target recognition and antiviral activity.     

Sequence comparisons of intermediate filament chains: evidence of a unique functional/structural role for coiled-coil segment 1A and linker L1

A comprehensive analysis of the sequences of all types of intermediate filament chains has been undertaken with a particular emphasis on those of segment 1A and linker L1. This has been done to assess whether structural characteristics can be recognized in the sequences that would be consistent with the role of each region in the recently proposed "swinging head" hypothesis. The analyses show that linker L1 is the most flexible rod domain region, that it is the most elongated structure (on a per residue basis), and that it is the most variable region as regards sequence and length. Segment 1A has one of the two most highly conserved regions of sequence in the rod domain (the other being at the end of segment 2B), with seven particular residues conserved across all chain types. It also contains one of the very few potential interchain ionic interactions that could be conserved across all chain types. However, the aggregation of chains in segment 1A is specified less precisely overall by interchain ionic interactions than are the other coiled-coil segments. The apolar residue contents in positions a and d of the heptad substructure are the highest of any coiled-coil segment in the intermediate filament family. Segment 1A also displays an amino acid composition atypical of not only coiled-coil segments 1B and 2B, but indeed of two-stranded coiled coils in general. Nonetheless, molecular modeling based on the crystal structure of the monomeric 1A fragment from human vimentin shows that coiled-coil formation is plausible. The most extensive regions of apolar/aromatic residues lie at the C-terminal end of segment 2B in the helix termination motif and in segment 1A in and close to the helix initiation motif. The predicted stability of the individual alpha-helices in segment 1A is greater than in those comprising segments 1B and 2B, though potential intrachain ionic interactions are either lacking or are minimal in number. Analysis of the 1A sequence and those regions immediately N- and C-terminal to it has shown that the capping residues are near optimal close to the previously predicted ends, thus adding to the likely stability of the alpha-helical structure. However, a second terminating sequence is predicted in 1A (about 10 residues back from the C-terminus). This allows the possibility of some unwinding of the alpha-helical structure of 1A immediately adjacent to linker L1 when the head domains no longer stabilize the coiled-coil structure. All of these data are consistent with the concept of a flexible hinge at L1 and with the ability of the two alpha-helical coiled-coil strands to separate under appropriate conditions and partly unwind at their C-terminal ends to allow the head domains a greater degree of mobility, thus facilitating function.     

Visualizing nature at work from the nano to the macro scale

Visalizing the structure and dynamics of proteins, supramolecular assemblies, and cellular components are often key to our understanding of biological function. Here, we focus on the major approaches in imaging, analyzing, and processing biomedical data ranging from the atomic to the macro scale. Relevant biomedical applications at different length scales are chosen to illustrate and discuss the various aspects of data acquisition using multiple modalities including electron microscopy and scanning force microscopy. Moreover, powerful scientific software is presented for processing, analyzing, and visualizing heterogeneous data. Examples of using this software in the context of visualizing biological nano-machines are presented and discussed.