Context-specific functions of Notch in Drosophila blood cell progenitors.

Drosophila Larval hematopoiesis takes place at the lymph gland, where myeloid-like progenitors differentiate into Plasmatocytes and Crystal Cells, under regulation of conserved signaling pathways. It has been established that the Notch pathway plays a specific role in Crystal Cell differentiation and maintenance. In mammalian hematopoiesis, the Notch pathway has been proposed to fulfill broader functions, including Hematopoietic Stem Cell maintenance and cell fate decision in progenitors. In this work we describe different roles that Notch plays in the lymph gland. We show that Notch, activated by its ligand Serrate, expressed at the Posterior Signaling Center, is required to restrain Core Progenitor differentiation. We define a novel population of blood cell progenitors that we name Distal Progenitors, where Notch, activated by Serrate expressed in Lineage Specifying Cells at the Medullary Zone/Cortical Zone boundary, regulates a binary decision between Plasmatocyte and Crystal Cell fates. Thus, Notch plays context-specific functions in different blood cell progenitor populations of the Drosophila lymph gland.

Hydroxylation and translational adaptation to stress: some answers lie beyond the STOP codon.

Regulation of protein synthesis contributes to maintenance of homeostasis and adaptation to environmental changes. mRNA translation is controlled at various levels including initiation, elongation and termination, through post-transcriptional/translational modifications of components of the protein synthesis machinery. Recently, protein and RNA hydroxylation have emerged as important enzymatic modifications of tRNAs, elongation and termination factors, as well as ribosomal proteins. These modifications enable a correct STOP codon recognition, ensuring translational fidelity. Recent studies are starting to show that STOP codon read-through is related to the ability of the cell to cope with different types of stress, such as oxidative and chemical insults, while correlations between defects in hydroxylation of protein synthesis components and STOP codon read-through are beginning to emerge. In this review we will discuss our current knowledge of protein synthesis regulation through hydroxylation of components of the translation machinery, with special focus on STOP codon recognition. We speculate on the possibility that programmed STOP codon read-through, modulated by hydroxylation of components of the protein synthesis machinery, is part of a concerted cellular response to stress.

Slow axonal transport mechanisms move neurofilaments relentlessly in mouse optic axons.

Pulse-labeling studies of slow axonal transport in many kinds of axons (spinal motor, sensory ganglion, oculomotor, hypoglossal, and olfactory) have led to the inference that axonal transport mechanisms move neurofilaments (NFs) unidirectionally as a single continuous kinetic population with a diversity of individual transport rates. One study in mouse optic axons (Nixon, R. A., and K. B. Logvinenko. 1986. J. Cell Biol. 102:647-659) has given rise to the different suggestion that a significant and distinct population of NFs may be entirely stationary within axons. In mouse optic axons, there are relatively few NFs and the NF proteins are more lightly labeled than other slowly transported slow component b (SCb) proteins (which, however, move faster than the NFs); thus, in mouse optic axons, the radiolabel of some of these faster-moving SCb proteins may confuse NF protein analyses that use one dimensional (1-D) SDS-PAGE, which separates proteins by size only. To test this possibility, we used a 2-mm "window" (at 3-5 mm from the posterior of the eye) to compare NF kinetics obtained by 1-D SDS-PAGE and by the higher resolution two-dimensional (2-D) isoelectric focusing/SDS-PAGE, which separates proteins both by their net charge and by their size. We found that 1-D SDS-PAGE is insufficient for definitive NF kinetics in the mouse optic system. By contrast, 2-D SDS-PAGE provides essentially pure NF kinetics, and these indicate that in the NF-poor mouse optic axons, most NFs advance as they do in other, NF-rich axons. In mice, greater than 97% of the radiolabeled NFs were distributed in a unimodal wave that moved at a continuum of rates, between 3.0 and 0.3 mm/d, and less than 0.1% of the NF population traveled at the very slowest rates of less than 0.005 mm/d. These results are inconsistent with the proposal (Nixon and Logvinenko, 1986) that 32% of the transported NFs remain within optic axons in an entirely stationary state. As has been found in other axons, the axonal transport system of mouse optic axons moves NFs and other cytoskeletal elements relentlessly from the cell body to the axon tip.

Eyes transplanted to tadpole tails send axons rostrally in two spinal-cord tracts.

Axons from eyes transplanted to the tail in Xenopus larvae enter the caudal spinal cord and follow two adjacent tracts rostrally to the level of the cerebellum. When eyes are transplanted to the ear area, optic axons enter the hindbrain and follow the same tracts rostrally and caudally. These sensory pathways normally contain the embryonic sensory system of the Rohon-Beard axons and the descending and ascending tracts of nerve V. We propose that the transplanted optic axons have followed a continuous substrate sensory pathway normally shared by a number of different sensory tracts.

Preferred microtubules for vesicle transport in lobster axons.

The hypothesis that transported vesicles are preferentially associated with a subclass of microtubules has been tested in lobster axons. A cold block was used to collect moving vesicles in these axons; this treatment caused the vesicles to accumulate in files along some of the microtubules. Quantitative analysis of the number of vesicles associated with microtubule segments indicated that lobster axons have two distinct populations of microtubules--transport microtubules that are the preferred substrates for vesicle transport and architectural microtubules that contribute to axonal structure.

Determining the structural stability of UiO-67 with respect to time: a solid-state NMR investigation.

The stability of UiO-67 has been questioned for some time. We have used solid-state NMR to investigate the temporal stability of this MOF. Proper activation is necessary to achieve optimal surface area. However, even with proper activation, the long-term (30+ days) fate of UiO-67 is hydrolysis of the linker-metal bonds and, ultimately, pore collapse.

The Statistical Modeling of Aging and Risk of Transition Project: Data Collection and Harmonization Across 11 Longitudinal Cohort Studies of Aging, Cognition, and Dementia.

Longitudinal cognitive trajectories and other factors associated with mixed neuropathologies (such as Alzheimer's disease with co-occurring cerebrovascular disease) remain incompletely understood, despite being the rule and not the exception in older populations. The Statistical Modeling of Aging and Risk of Transition study (SMART) is a consortium of 11 different high-quality longitudinal studies of aging and cognition (N=11,541 participants) established for the purpose of characterizing risk and protective factors associated with subtypes of age-associated mixed neuropathologies (N=3,001 autopsies). While brain donation was not required for participation in all SMART cohorts, most achieved substantial autopsy rates (i.e., > 50%). Moreover, the studies comprising SMART have large numbers of participants who were followed from intact cognition and transitioned to cognitive impairment and dementia, as well as participants who remained cognitively intact until death. These data provide an exciting opportunity to apply sophisticated statistical methods, like Markov processes, that require large, well-characterized samples. Thus, SMART will serve as an important resource for the field of mixed dementia epidemiology and neuropathology.

Experimental increase of neurofilament transport rate: decreases in neurofilament number and in axon diameter.

In 2,5-hexanedione (2,5-HD)-induced axonal neuropathy, the rate of neurofilament (NF) transport increases in optic axons. To test the prediction that increases in the rate of polymer transport in any one locality of the axon lead directly to a decrease in the number of NF in that locality, NF and microtubules (MT) were quantitatively analyzed in axonal cross sections. In 2,5-HD axons the number of NF was 38% of that in control axons while the number of MT was not significantly changed; it appears that the drug treatment decreases NF number in the proximal axon regions, most directly through an increase in rate of NF transport. In those regions, the cross-sectional areas of the 2,5-HD-treated axons were 45% smaller than those of control axons; although the axons had shrunk in diameter, they retained their normal cylindrical shapes as measured by the index of circularity. Reduced internal expansive forces in the axon, working in conjunction with the normal external compressive forces, appear to reduce the radius of the axon. Quantitative analyses demonstrated that the average and the maximum lateral spacings between NF-NF, NF-MT, and MT-MT were all 30% larger in 2,5-HD-treated axons than in control axons. This suggests that polymers are relatively free to move laterally away from one another and to fill the available space within the axon. These observations are not consistent with models wherein 2,5-HD acts to crosslink the NF into an immobile network that can no longer advance within the axon. Instead, it appears more likely that 2,5-HD acts selectively on the interaction between some NF and the slow transport mechanism to increase the rate of NF transport.

Substrate pathways which guide growing axons in Xenopus embryos.

Optic axons from eyes transplanted to hindbrain and spinal cord (i.e., non-optic areas) in Xenopus embryos reproducibly follow a particular pair of continuous tracts, the external sensory tract and the internal sensory tract. The course of the external sensory tract may coincide with the course of the general somatic afferent tracts (i.e., the ascending and descending tracts of nerve V, the tract of Lissauer, and the Rohon-Beard cell tracts). The course of the internal sensory tract parallels that of the external sensory tract but runs more medially. Transplanted optic axons follow the same tracts regardless of where along the neural axis the axons have entered the CNS. What kind of developmental cues guide these optic axons? It is unlikely that the guidance cues are organized as spatial coordinates, rather it appears that the optic axons of these two tracts are guided along pre-existing substrate routes. We have called these routes substrate pathways. Substrate pathways may be a device normally used for organizing fiber tracts in the developing nervous system.

Substrate pathways demonstrated by transplanted Mauthner axons.

A substrate pathway is a set of aligned guidance cues. (Such cues may be either cells or molecules.) CNS substrate pathways can be demonstrated by transplanting axons to different starting locations. The stereotyped routes of transplanted axons will then demonstrate the locations of effective substrate pathways. To map CNS substrate pathways, Mauthner axons were transplanted to various unnatural locations along the CNS of Xenopus embryos. The routes of 24 experimental Mauthner axons were traced. Twenty-one of these axons grew along parts of a stereotyped route extending in the ventral marginal zone from the caudal diencephalon through the spinal cord. This ventral substrate pathway ran the length of the basal plate; thus, we call it a basal substrate pathway. One experimental Mauthner axon grew along the alar substrate pathway previously demonstrated by transplanted optic axons. The demonstrations of the alar and the basal substrate pathways suggest that during development a few long substrate pathways organize the overall layout of the long tracts of the CNS. We propose that the pattern of the earliest CNS substrate pathways is established in the neural plate and is topologically preserved as the neural plate rolls into a neural tube. This pattern may be manifest as the three-dimensional organization of the early marginal zones formed by the peripheral processes (the endfeet) of certain developing ependyma and radial glia. Subsequently, the detailed anatomy of the axon tracts and the specific terminations of individual axons are probably determined by other local chemical cues.

Comparative analysis of dendritic architecture of identified neurons using the Hausdorff distance metric.

Dendritic trees often are complex, three-dimensional structures. Comparative morphologic studies have not yet provided a reliable measure to analyze and compare the geometry of different dendritic trees. Therefore, it is important to develop quantitative methods for analyzing the three-dimensional geometry of these complex trees. The authors developed a comparison measure based on the Hausdorff distance for comparing quantitatively the three-dimensional structure of different neurons. This algorithm was implemented and incorporated into a new software package that the authors developed called NeuroComp. The authors tested this algorithm to study the variability in the three-dimensional structure of identified central neurons as well as measuring the structural differences between homologue neurons. They took advantage of the uniform dendritic morphology of identified interneurons of an insect, the giant interneurons of the cockroach. More specifically, after establishing a morphometric data base of these neurons, the authors found that the algorithm is a reliable tool for distinguishing between dendritic trees of different neurons, whereas conventional metric analysis often is inadequate. The authors propose to use this method as a quantitative tool for the investigation of the effects of various experimental paradigms on three-dimensional dendritic architecture.

Cognitive performance in surgically menopausal women on estrogen.

Estrogen replacement therapy (ERT) may help maintain normal cognitive function. Nondemented surgically menopausal women on ERT (n = 10) enrolled in a longitudinal aging study performed better than age- and education-matched control subjects (n = 25) on selected tests of verbal memory and constructional ability. These results suggest that ERT initiated soon after surgical menopause can have long-term neuroprotective effects in cognitively intact women.

Prevention and treatment of sciatic denervation disuse osteoporosis in the rat tibia with capacitively coupled electrical stimulation.

Osteoporosis in the sciatic-denervated rat tibia was both prevented and reversed with a capacitively coupled electrical field. In both the prevention of the development of osteoporosis and the reversal of a previously established osteoporosis, a statistically significant enhancement of wet weight, dry weight, ashed weight, ultimate strength, cortical area, cortical thickness, and a concomitant decrease in cortical porosity occurred in the stimulated, denervated tibiae of the experimental animals compared with the nonstimulated, denervated tibiae of the control animals. These effects exhibited dose-response characteristics. A 60 kHz symmetrical sinewave signal was effective in preventing osteoporosis at a range of 5-10 peak-to-peak, and it was effective in reversing osteoporosis at 10 V peak-to-peak. Reversal of a well-established osteoporosis in laboratory animals has not been reported previously. Continued investigation into the use of a capacitively coupled electrical field in the prevention and treatment of osteoporosis seems warranted from these studies.

Quantitative effects of NGF on the growth of embryonic frog axons.

Sparse, dissociated cultures of embryonic Xenopus CNS neurons were grown with and without NGF. Under both conditions the same number of neurons survived and extended neurites, and under both conditions the neurites moved at approximately the same overall rates and with the same degree of straightness. On the other hand, neurons in the NGF-supplemented cultures had more neurites and these neurites branched 64% more often. Detailed measurements showed that the axons elongated 44% faster in NGF and that this increase could be ascribed to a selective increase in the stepping rate of axonal elongation. These observations raise the possibility that NGF may selectively modulate the rate of movement of the core cytoskeleton of the axon.

Axonal branch shapes.

To distinguish the relative roles of the intrinsic and the extrinsic determinants of axonal branching shapes, a number of key branch parameters were measured under a variety of conditions. Branch shapes of frog and of chick axons were analyzed in tissue culture, and these in vitro patterns were compared with in vivo branch patterns of axons in tadpole tail fins. The shape of a branch junction can be characterized by the sizes of its branch angles. In all cases, branch junctions had only two branches (3 branch angles), and the bifurcation angle between them was usually the smallest. The shape of the branch junction was constant in a wide variety of environments, but the exact branch angles, as well as the numbers of branches per axon and the numbers of axons per neuron, could be modulated by changes in the substrate adhesivity.

Cytomatrix protein residence times differ significantly between the tract and the terminal segments of optic axons.

The window method of radiolabeled protein analysis was used to study the transport kinetics of axonally transported cytomatrix proteins as they move through segments of mouse optic axons. Three slow component b (SCb) proteins--actin, a 30 kDa protein, and clathrin--were radiolabeled in the eye and were followed for up to 119 days by quantitative one-dimensional gel electrophoresis. These proteins appeared first in the optic nerve, next in the tract, and last in the superior colliculus. All of the radiolabeled proteins had passed through the optic axons and had been effectively removed from the terminals by 119 days. Two different axonal segments ('windows') were examined in detail: a segment of the axon shaft region in the optic tract, and a segment of axon terminal region in the midbrain superior colliculus. The median transit times of the 3 proteins were 53-100% longer in the colliculus than in the tract, and the pulse transients (the total area under the transport curve in each window) were 180-350% larger in the colliculus than in the tract. These results indicate that at least certain cytomatrix and cytoskeletal proteins have longer residence times in the terminal regions than in the axon proper.

Slow component B protein kinetics in optic nerve and tract windows.

The transport kinetics of 3 radiolabeled slow component b (SCb) proteins (a 30 kDa protein, clathrin, and actin) were examined in the axons of mouse retinal ganglion cells. To view the transit of these proteins through the entire optic pathway between the eye and the target cells, we used two different windows: (1) a 2 mm segment from the optic nerve located 3-5 mm from the eye, and (2) a 2 mm segment from the optic tract located past the chiasm 6-8 mm from the eye. The radiolabeled proteins from these windows were separated by 1- and 2-dimensional SDS-PAGE, and the individual radiolabeled bands were quantified. Radiolabeled proteins entered and cleared the optic axons between 1 and 119 days post-labeling. All these proteins had broader transport waves in the more distal optic tract window than in the more proximal optic nerve window. The spreading of transport waves as they advance along the axon appears to be produced by a playing out of the natural heterogeneity of axonal transport rates within each population of labeled proteins. Our results confirm the proposals that clathrin and the 30 kDa protein are transported principally with SCb and that actin is transported both with SCb and with SCa. Although these proteins can be generally classified with SCb, their detailed kinetics differed (for example, their median transit times differed) and, in summary, their characteristic rates of movement can be ordered as: clathrin greater than 30 kDa protein greater than actin.(ABSTRACT TRUNCATED AT 250 WORDS)

The maximum rate of neurofilament transport in axons: a view of molecular transport mechanisms continuously engaged.

Neurofilaments (NFs) were radiolabeled in the optic systems of mice. The leading edge of the radiolabeled NF waveform was distinguished near the injection site (the eye) both by liquid scintillation spectroscopy and visually from fluorographs. The fastest NFs were found to be translocated at rates of between 72 and 144 mm/day. It appears that the continuous (maximal) operation of the slow axonal transport machinery can move polymers intra-axonally at rates one hundred times greater than those previously reported.

Microtubules have special physical associations with smooth endoplasmic reticula and mitochondria in axons.

Ultrastructural morphometry was used to document the non-random spatial distributions of organelles within the compact myelinated region of avian oculomotor axons. These regions contain large numbers of loosely packed neurofilaments (NFs) (241/microns 2) and only a relatively small number of microtubules (MTs) (4/microns 2), mitochondria (0.6/microns 2), and smooth endoplasmic reticulum (SER) (1.6/microns 2). Random co-occurrences between the relatively sparsely distributed MTs, mitochondria, and SER are probably infrequent in these axons. The actual co-occurrences of MTs, mitochondria, and SER with MTs were counted and compared to the co-occurrences expected in a random Poisson distribution. At long distances (200 nm), the co-occurrences were random. At shorter distances (40 nm and less), MTs were still randomly associated with other MTs. However, at these shorter distances, the spatial associations of mitochondria with MTs and of SER with MTs were not random; such preferential stable associations may be produced by specific MT associated cross-bridging proteins. In axons, MTs tend to be clustered together, giving the appearance of MT bundles. We propose that the MT-MT bundling is an indirect result of MT concentration along the continuous intra-axonal SER network, to which the MTs are apparently tied directly by dynamic molecular cross-bridges.

Neurofilaments assume a less random architecture at nodes and in other regions of axonal compression.

Neurofilament distributions were mathematically characterized in four chicken somatic motor axons at each of four histologically distinct regions: compact myelinated regions, compact myelinated regions associated with Schwann cell nuclei, Schmidt-Lanterman clefts, and nodes of Ranvier. Compact myelinated regions had the largest cross-sectional areas, the lowest neurofilament densities, and the most random neurofilament organizations--nodes of Ranvier had the smallest cross-sectional areas, the highest neurofilament densities, and the most ordered architectures. In these myelinated axons, the closest natural neurofilament spacing was 25 nm. Mathematical analyses of serial sections suggested that neurofilament interactions are sufficiently weak and transient to permit a full range of variation from random to ordered cytoskeletal architectures as the neurofilaments move longitudinally through the few micron span of the paranodal-nodal region of a single axon.