Quantification of microtubule stutters: dynamic instability behaviors that are strongly associated with catastrophe

Microtubules (MTs) are cytoskeletal fibers that undergo dynamic instability (DI), a remarkable process involving phases of growth and shortening separated by stochastic transitions called catastrophe and rescue. Dissecting DI mechanism(s) requires first characterizing and quantifying these dynamics, a subjective process that often ignores complexity in MT behavior. We present a Statistical Tool for Automated Dynamic Instability Analysis (STADIA) that identifies and quantifies not only growth and shortening, but also a category of intermediate behaviors that we term “stutters.” During stutters, the rate of MT length change tends to be smaller in magnitude than during typical growth or shortening phases. Quantifying stutters and other behaviors with STADIA demonstrates that stutters precede most catastrophes in our in vitro experiments and dimer-scale MT simulations, suggesting that stutters are mechanistically involved in catastrophes. Related to this idea, we show that the anticatastrophe factor CLASP2γ works by promoting the return of stuttering MTs to growth. STADIA enables more comprehensive and data-driven analysis of MT dynamics compared with previous methods. The treatment of stutters as distinct and quantifiable DI behaviors provides new opportunities for analyzing mechanisms of MT dynamics and their regulation by binding proteins.     

Functional analysis of trophoblast giant cells in parthenogenetic mouse embryos

Diploid mouse embryos containing only maternal DNA (parthenotes) fail, in part, because the inner cell mass does not induce the trophoblast to grow. In this study, we asked whether any of the defects in parthenotes may arise from alterations in trophoblast function. We examined the expression of genes important for normal trophoblast function and found several trophoblast genes that were expressed at normal levels in the primary trophoblast cells of parthenotes: E-cadherin, a cell adhesion molecule, was expressed normally in both the ICM and trophectoderm of parthenogenetic blastocysts and blastocyst outgrowths; the gene for Hxt, a basic helix-loop-helix factor that regulates trophoblast development, was expressed in both zygotic and parthenogenetic giant cells; placental lactogen-1, a hormone that is normally secreted by trophoblast giant cells, was expressed in most of both parthenogenetic and normal trophoblast cells; and the 92 kDa matrix metalloproteinase, gelatinase B, also known as MMP-9, was secreted at equivalent levels by both zygotic and parthenogenetic blastocyst outgrowths. However, once the outgrowths had developed, a subpopulation of trophoblast cells in parthenogenetic embryos had decreased DNA replication and significantly fewer nucleoli per nucleus than did zygotic embryos. Moreover, the parthenogenetic trophoblast cells growing out from blastocysts had a decreased viability in culture. These data suggest that, although parthenogenetic embryos are able to initiate primary trophoblast differentiation, the stability and continued differentiation of trophoblast giant cells may be abnormal. Our data support the hypothesis that the deficiency of secondary trophoblast giant cells may contribute to the decline of parthenogenetic embryos and suggest that the factors controlling this subset of trophoblast are distinct from those for primary trophoblast. Dev Genet 20:1–10, 1997. © 1997 Wiley-Liss, Inc.     

Interstitial 3-Methoxytyramine Reflects Striatal Dopamine Release: An In Vivo Microdialysis Study

Previous ex vivo studies have provided indirect evidence that the dopamine (DA) metabolite 3-methoxytyramine (3-MT) may be a useful index of DA release in vivo. In the present study, in vivo microdialysis was utilized to assess directly the relationship between extracellular DA and 3-MT in the striatum of rats following a variety of pharmacological manipulations. Apomorphine, a DA receptor agonist, produced a rapid, transient decrease in both DA and 3-MT. Conversely, the DA receptor antagonist haloperidol produced a concomitant increase in extracellular DA and 3-MT. Increases in DA and 3-MT were also noted following the administration of the DA uptake inhibitor, bupropion. Local application of tetrodotoxin resulted in the complete elimination of measurable amounts of DA and 3-MT in the dialysate, gamma-Butyrolactone also greatly decreased DA and 3-MT. Finally, d-amphetamine produced a large increase in DA and 3-MT in animals that had been treated previously with gamma-butyrolactone. The Pearson correlation coefficients for DA and 3-MT following these manipulations ranged from 0.87 to 0.97. These data indicate that interstitial 3-MT is an accurate index of DA release. However, when compared with previous ex vivo findings, the present results also suggest that changes in tissue concentrations of 3-MT may not reliably reflect DA release following certain pharmacological manipulations.     

Increased Axonal Ribosome Numbers Is an Early Event in the Pathogenesis of Amyotrophic Lateral Sclerosis

Myelinating glia cells support axon survival and functions through mechanisms independent of myelination, and their dysfunction leads to axonal degeneration in several diseases. In amyotrophic lateral sclerosis (ALS), spinal motor neurons undergo retrograde degeneration, and slowing of axonal transport is an early event that in ALS mutant mice occurs well before motor neuron degeneration. Interestingly, in familial forms of ALS, Schwann cells have been proposed to slow disease progression. We demonstrated previously that Schwann cells transfer polyribosomes to diseased and regenerating axons, a possible rescue mechanism for disease-induced reductions in axonal proteins. Here, we investigated whether elevated levels of axonal ribosomes are also found in ALS, by analysis of a superoxide dismutase 1 (SOD1)^G93A mouse model for human familial ALS and a patient suffering from sporadic ALS. In both cases, we found that the disorder was associated with an increase in the population of axonal ribosomes in myelinated axons. Importantly, in SOD1^G93A mice, the appearance of axonal ribosomes preceded the manifestation of behavioral symptoms, indicating that upregulation of axonal ribosomes occurs early in the pathogenesis of ALS. In line with our previous studies, electron microscopy analysis showed that Schwann cells might serve as a source of axonal ribosomes in the disease-compromised axons. The early appearance of axonal ribosomes indicates an involvement of Schwann cells early in ALS neuropathology, and may serve as an early marker for disease-affected axons, not only in ALS, but also for other central and peripheral neurodegenerative disorders.