Developmental plasticity of red blood cell homeostasis

Most human physiologic set points like body temperature are tightly regulated and show little variation between healthy individuals. Red blood cell (RBC) characteristics such as hematocrit and mean cell volume are stable within individuals but can vary by 20% from one healthy person to the next. The mechanisms for the majority of this inter‐individual variation are unknown and do not appear to involve common genetic variation. Here, we show that environmental conditions present during development, namely in utero iron availability, can exert long‐term influence on a set point related to the RBC life cycle. In a controlled study of rhesus monkeys and a retrospective study of humans, we use a mathematical model of in vivo RBC population dynamics to show that in utero iron deficiency is associated with a lowered threshold for RBC clearance and turnover. This in utero effect is plastic, persisting at least 2 years after birth and after the cessation of iron deficiency. Our study reports a rare instance of developmental plasticity in the human hematologic system and also shows how mathematical modeling can be used to identify cellular mechanisms involved in the adaptive control of homeostatic set points. Am. J. Hematol. 89:459–466, 2014. © 2014 Wiley Periodicals, Inc.     

Late onset muscle plasticity in the whisker pad of enucleated rats

Blindness leads to a major reorganization of neural pathways associated with touch. Because incoming somatosensory information influences motor output, it is plausible that motor plasticity occurs in the blind. In this work, we evaluated this issue at the peripheral level in enucleated rats. Whisker muscles in enucleated rats 160 days of age or older showed increased cytochrome oxidase activity, capillary density, motor plate size, and amplitude of evoked field potentials as compared with their control counterparts. Such differences were not observed at ages 10 and 60 days, the capillary density was the exception being greater in the enucleated rat at the latter age. Interestingly, there was a trend to increased neurotrophin-3 concentrations in the whisker pads of enucleated rats throughout postnatal development. Our results show that neonatal enucleation leads to late onset plasticity of the whisker's motor system.     

Prevalence and characteristics of disinhibition during bronchoscopy with midazolam

BackgroundDisinhibition is sometimes experienced during bronchoscopy with sedation. However, data on disinhibition during bronchoscopy are scarce. We examined the prevalence and characteristics of disinhibition during bronchoscopy with midazolam.MethodsThis retrospective study analyzed consecutive patients who underwent bronchoscopy between November 2019 and December 2020. The severity of disinhibition was defined as follows: mild, disinhibition sometimes requiring restraints by assistants; moderate, disinhibition always requiring restraints by assistants; and severe, disinhibition requiring antagonization of sedation by flumazenil to continue bronchoscopy.ResultsAmong 251 eligible patients who were sedated using midazolam, 36 (14.3%; 95% confidence interval [CI], 10.5%–19.2%), 42 (16.7%; 95% CI, 12.6%–21.8%), and 7 (2.8%; 95% CI, 1.4%–5.6%) experienced mild, moderate, and severe disinhibition, respectively. Depression (odds ratio [OR] 2.77; 95% CI, 1.20–6.41), endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) (OR 10.23; 95% CI, 1.02–103.01, referred to brushing/bronchial washing/observation), and increased administration of midazolam (OR 1.20; 95% CI, 1.02–1.42, per 1-mg increase) were independently associated with moderate-to-severe disinhibition. Patients experiencing moderate disinhibition reported significantly better scores for discomfort during bronchoscopy. Besides the maximum systolic and diastolic blood pressures during bronchoscopy, the changes in hemodynamic and respiratory statuses during bronchoscopy or complications did not significantly differ between patients experiencing moderate-to-severe disinhibition and those experiencing none-to-mild disinhibition.ConclusionsModerate-to-severe disinhibition occurred in 19.5% of patients during bronchoscopy with midazolam. We should focus on disinhibition when patients have depression or are planning to undergo EBUS-TBNA, and sparing the administration of midazolam might reduce the occurrence of disinhibition.Clinical trial registrationUMIN000038571     

Rapid enhancement of two-step wiring plasticity by estrogen and NMDA receptor activity

Cortical information storage requires combined changes in connectivity and synaptic strength between neurons, but the signaling mechanisms underlying this two-step wiring plasticity are unknown. Because acute 17β-estradiol (E2) modulates cortical memory, we examined its effects on spine morphogenesis, AMPA receptor trafficking, and GTPase signaling in cortical neurons. Acute E2 application resulted in a rapid, transient increase in spine density, accompanied by temporary formation of silent synapses through reduced surface GluR1. These rapid effects of E2 were dependent on a Rap/AF-6/ERK1/2 pathway. Intriguingly, NMDA receptor (NMDAR) activation after E2 treatment potentiated silent synapses and elevated spine density for as long as 24 h. Hence, we show that E2 transiently increases neuronal connectivity by inducing dynamic nascent spines that “sample” the surrounding neuropil and that subsequent NMDAR activity is sufficient to stabilize or “hold” E2-mediated effects. This work describes a form of two-step wiring plasticity relevant for cortical memory and identifies targets that may facilitate recovery from brain injuries.     

Fuel homeostasis during physical inactivity induced by bed rest

The consequences of physical inactivity on fuel homeostasis were evaluated during 7 days of head-down bed rest (HDBR), a model mimicking weightlessness. Eight men (32.4 +/- 1.9 yr; body mass index, 23.9 +/- 0.7 kg/m2) and eight women (27.9 +/- 0.9 yr; body mass index, 20.9 +/- 0.6 kg/m2) underwent an oral glucose tolerance test (OGTT; 1 g/kg) before and after HDBR. The glucose load was labeled with 13C and associated with D-[6,6-2H2] glucose infusion, indirect calorimetry, breath tests, and plasma measurements to determine the glucose turnover and biodisponibility, substrate oxidation, and endocrine responses. Body composition was assessed using H2(18)O dilution. In addition, hormones were measured in daily blood and 24-h urine samples. No change in body composition was noted. Daily fasting insulin increased during HDBR (men, 34%; women, 26%), as did the insulin to glucose ratio (men, 30%; women, 25%). The normetanephrine level dropped (men, 30%; women, 16%), but metanephrine was unchanged. During OGTTs, the insulin response was increased after HDBR (men, 47%; women, 67%), whereas plasma glucose levels were similar. Nonesterified fatty acids and beta-hydroxybutyrate levels were lower. Endogenous glucose production dropped (28%), and exogenous glucose oxidation increased (28%) only in men. Resting energy expenditure was unchanged, but nonproteic respiratory quotient increased (men, 10%; women, 14%). Basal levels of lipid oxidation dropped in both sexes (approximately 90%), but those of carbohydrate oxidation increased in men (40%); as did lipogenesis in women (570%). In response to OGTTs, lipid oxidation was 80% reduced in both sexes after HDBR, but carbohydrate oxidation increased (25%) in men. Lipogenesis occurred in men (304%) and women (74%), but the latter had higher absolute levels. Therefore, 7 days of HDBR resulted in 1) reduced sympathetic activity, 2) insulin resistance suggested at the muscle level in men and at both the muscle and liver levels in women, 3) no changes in glucose biodisponibility, suggesting no alterations in the gastrointestinal function, and 4) a shift toward carbohydrate oxidation in men and a net lipogenesis in women. Such results suggest gender differences in response to sedentary life style and warrant further analysis.     

Synaptic homeostasis and input selectivity follow from a calcium-dependent plasticity model

Modifications in the strengths of synapses are thought to underlie memory, learning, and development of cortical circuits. Many cellular mechanisms of synaptic plasticity have been investigated in which differential elevations of postsynaptic calcium concentrations play a key role in determining the direction and magnitude of synaptic changes. We have previously described a model of plasticity that uses calcium currents mediated by N-methyl-D-aspartate receptors as the associative signal for Hebbian learning. However, this model is not completely stable. Here, we propose a mechanism of stabilization through homeostatic regulation of intracellular calcium levels. With this model, synapses are stable and exhibit properties such as those observed in metaplasticity and synaptic scaling. In addition, the model displays synaptic competition, allowing structures to emerge in the synaptic space that reflect the statistical properties of the inputs. Therefore, the combination of a fast calcium-dependent learning and a slow stabilization mechanism can account for both the formation of selective receptive fields and the maintenance of neural circuits in a state of equilibrium.     

Synaptic plasticity by antidromic firing during hippocampal network oscillations

Learning and other cognitive tasks require integrating new experiences into context. In contrast to sensory-evoked synaptic plasticity, comparatively little is known of how synaptic plasticity may be regulated by intrinsic activity in the brain, much of which can involve nonclassical modes of neuronal firing and integration. Coherent high-frequency oscillations of electrical activity in CA1 hippocampal neurons [sharp-wave ripple complexes (SPW-Rs)] functionally couple neurons into transient ensembles. These oscillations occur during slow-wave sleep or at rest. Neurons that participate in SPW-Rs are distinguished from adjacent nonparticipating neurons by firing action potentials that are initiated ectopically in the distal region of axons and propagate antidromically to the cell body. This activity is facilitated by GABA_A-mediated depolarization of axons and electrotonic coupling. The possible effects of antidromic firing on synaptic strength are unknown. We find that facilitation of spontaneous SPW-Rs in hippocampal slices by increasing gap-junction coupling or by GABA_A-mediated axon depolarization resulted in a reduction of synaptic strength, and electrical stimulation of axons evoked a widespread, long-lasting synaptic depression. Unlike other forms of synaptic plasticity, this synaptic depression is not dependent upon synaptic input or glutamate receptor activation, but rather requires L-type calcium channel activation and functional gap junctions. Synaptic stimulation delivered after antidromic firing, which was otherwise too weak to induce synaptic potentiation, triggered a long-lasting increase in synaptic strength. Rescaling synaptic weights in subsets of neurons firing antidromically during SPW-Rs might contribute to memory consolidation by sharpening specificity of subsequent synaptic input and promoting incorporation of novel information.     

Neuronal plasticity and stressor toxicity during aging

Brain aging, Alzheimer disease and stroke share common elements of deficits in calcium regulation, declines in mitochondrial function, increases in generation of reactive oxygen species (ROS), accumulated damage from ROS and immune system dysfunction. The problem is to distinguish less significant side reactions, such as gray hair, from aspects of aging that contribute to disease. Toward establishing cause and effect relationships, a neuron cell culture system is described that allows comparisons with age under uniform environmental conditions. This neuron culture model indicates that susceptibility to death by apoptosis and consequences of the inflammatory response from β-amyloid are age-related and an inherent characteristic of the neurons. Further mechanistic investigations are possible. New therapeutic approaches are suggested that combine inhibition of calcium overloads (calcium channel blockers), reduced ROS damage (melatonin, N-acetyl-cysteine), and bolstered mitochondrial function and energy generation (creatine). Together with newly demonstrated capabilities for adult and aged neuron regeneration and multiplication, i.e. plasticity, these approaches offer new hope toward reversing age-related decrements and damage from neurodegenerative disease.     

Changes in the neuroendocrine control of energy homeostasis by adiposity signals during aging

Energy balance in mammals is modulated by peripheral signals that inform the brain about the magnitude of fat stores, the amount of food in the gastrointestinal tract, and the level of nutrients such as glucose in the circulation. Among these, insulin and leptin are considered adiposity signals involved in the long-term maintenance of fat stores. Here we review the mechanisms of action of leptin and insulin in the hypothalamus and how these mechanisms are altered during aging in rat models. Aged rats are characterized by increased fat mass, central leptin and insulin resistance, and hyperleptinemia. Leptin resistance is manifested by its failure to inhibit food intake, deplete fat stores, down regulate its own expression in adipose tissue, and increase energy expenditure. Moreover, leptin and insulin signaling are decreased in hypothalamus from aged rats. Calorie restriction and fasting studies provide controversial data on the cause-effect interrelationship between increased adiposity and development of central leptin resistance. Although in the absence of obesity leptin resistance seems to be a characteristic of aged animals, adiposity could either reinforce it or cause an early onset of this resistance. More studies are necessary to clarify the role of the hypothalamus in the development of age-associated obesity and insulin resistance.     

Glucose homeostasis during prolonged suppression of glucagon and insulin secretion by somatostatin.

Somatostatin was infused for 5-8 hr into five normal men and eleven normal, conscious dogs. This infusion resulted in a persistent decline in plasma glucagon (40-60%) and insulin (30-45%). Plasma gluccose fell 15-25% during the initial 1-2 hr, but subsequently rose to hyperglycemic levels (130-155 mg/100ml) by 3-6 hr, despite persistent hypoglucagonemia. Glucose production initially declined by 40-50%, but later rose to levels 15-20% above basal rates while peripheral glucose utilization fell to levels 20-30% below basal, thereby accounting for hyperglycemia. Infusion of exogenous insulin so as to restore plasma insulin to preinfusion values or cessation of the somatostatin infusion with restoration of endogenous insulin secretion resulted in a prompt reduction of plasma glucose to baseline values. Prevention of the initial somatostatin-induced hypoglycemic response by intravenous infusion of glucose failed to prevent the delayed hyperglycemia. We conclude that somatostatin caused only transient hypoglycemia in normal subjects and that hyperglycemia eventually developes as a consequence of insulin deficiency. These data indicate that basal glucagon secretion is not essential for the development of fasting hyperglycemia and support the conclusion that insulin deficiency rather than glucagon excess is the primary factor responsible for abnormal glucose homeostasis in the diabetic.     

Epigenetic-mediated decline in synaptic plasticity during aging

Cognitive decline observed in aging mammals is associated with decreased long-term synaptic plasticity, especially long-term potentiation (LTP). Recent work has uncovered a connection between LTP, histone acetylation, and brain-derived neurotrophic factor (BDNF)/neurotrophin receptor B (trkB) signaling. LTP, histone acetylation, and BDNF/trkB signaling decrease in old animals, Because an apparent positive feedback loop links these processes, treatment with histone deacetylase inhibitors or a trkB agonist restores LTP in the hippocampus of old animals. These results coupled with exciting work on histone methylation and life span in Caneorhabditis elegans suggest that epigenetic changes may play a significant role in aging. Such dysfunctional epigenetic pathways may provide novel targets for cognitive enhancing therapeutics.     

Synaptic disinhibition during maintenance of long-term potentiation in the CA1 hippocampal subfield.

Long-term potentiation (LTP) in the CA1 region of the hippocampus is widely believed to occur through a strengthening of efficacy of excitatory synapses between afferent fibers and pyramidal cells. An alternative mechanism of LTP, reduction of efficacy of synaptic inhibition, was examined in the present report. The present study demonstrates that the maintenance of LTP in the CA1 hippocampal subfield of guinea pigs is accompanied by impairment of type A gamma-aminobutyric acid (GABA) receptor function, particularly at apical dendritic sites of CA1 pyramidal cells. Enhanced excitability of GABAergic interneurons during LTP represents a strengthening of inhibitory efficacy. The net effect of opposite modifications of synaptic inhibition during LTP of CA1 pyramidal cells is an overall impairment of the strength of GABAergic inhibition, and disinhibition could contribute importantly to CA1 pyramidal cell LTP.     

Basophil effector function and homeostasis during helminth infection

Basophils are effector cells of the innate immune system that are associated with allergic inflammation and infections with helminth parasites. However, their development and in vivo functions are largely unknown. Here, we characterize basophil development, turnover, tissue localization, and effector function during infection with the helminth Nippostrongylus brasiliensis. Our results demonstrate that under homeostatic conditions basophils have a lifespan of about 60 hours. N brasiliensis-induced basophilia is caused by increased de novo production of basophils in the bone marrow. Basophils were found near the marginal zone in the red pulp of the spleen, in the lamina propria of the small intestine, and in the lung parenchyma. Activated basophils promoted systemic eosinophilia, were associated with differentiation of alternatively activated macrophages in the lung, and contributed to efficient worm expulsion, demonstrating that basophils play a crucial role as effector cells in type 2 immune responses.     

Rapid compaction during RNA folding

We have used small angle x-ray scattering and computer simulations with a coarse-grained model to provide a time-resolved picture of the global folding process of the Tetrahymena group I RNA over a time window of more than five orders of magnitude. A substantial phase of compaction is observed on the low millisecond timescale, and the overall compaction and global shape changes are largely complete within one second, earlier than any known tertiary contacts are formed. This finding indicates that the RNA forms a nonspecifically collapsed intermediate and then searches for its tertiary contacts within a highly restricted subset of conformational space. The collapsed intermediate early in folding of this RNA is grossly akin to molten globule intermediates in protein folding.     

Wnt-reporter expression pattern in the mouse intestine during homeostasis

Background

The canonical Wnt signaling pathway is a known regulator of cell proliferation during development and maintenance of the intestinal epithelium. Perturbations in this pathway lead to aberrant epithelial proliferation and intestinal cancer. In the mature intestine, proliferation is confined to the relatively quiescent stem cells and the rapidly cycling transient-amplifying cells in the intestinal crypts. Although the Wnt signal is believed to regulate all proliferating intestinal cells, surprisingly, this has not been thoroughly demonstrated. This important determination has implications on intestinal function, especially during epithelial expansion and regeneration, and warrants an extensive characterization of Wnt-activated cells.

Methods

To identify intestinal epithelial cells that actively receive a Wnt signal, we analyzed intestinal Wnt-reporter expression patterns in two different mouse lines using immunohistochemistry, enzymatic activity, in situ hybridization and qRT-PCR, then corroborated results with reporter-independent analyses. Wnt-receiving cells were further characterized for co-expression of proliferation markers, putative stem cell markers and cellular differentiation markers using an immunohistochemical approach. Finally, to demonstrate that Wnt-reporter mice have utility in detecting perturbations in intestinal Wnt signaling, the reporter response to gamma-irradiation was examined.

Results

Wnt-activated cells were primarily restricted to the base of the small intestinal and colonic crypts, and were highest in numbers in the proximal small intestine, decreasing in frequency in a gradient toward the large intestine. Interestingly, the majority of the Wnt-reporter-expressing cells did not overlap with the transient-amplifying cell population. Further, while Wnt-activated cells expressed the putative stem cell marker Musashi-1, they did not co-express DCAMKL-1 or cell differentiation markers. Finally, gamma-irradiation stimulated an increase in Wnt-activated intestinal crypt cells.

Conclusion

We show, for the first time, detailed characterization of the intestine from Wnt-reporter mice. Further, our data show that the majority of Wnt-receiving cells reside in the stem cell niche of the crypt base and do not extend into the proliferative transient-amplifying cell population. We also show that the Wnt-reporter mice can be used to detect changes in intestinal epithelial Wnt signaling upon physiologic injury. Our findings have an important impact on understanding the regulation of the intestinal stem cell hierarchy during homeostasis and in disease states.     

Are reperfusion-induced arrhythmias caused by disinhibition of an arrhythmogenic component of ischemia?

Isolated rat hearts were used to examine whether reperfusion-induced arrhythmias may be caused by washout of substances accumulating during ischemia. This was achieved by subjecting hearts to 10 min of regional ischemia and rendering them transiently inexcitable during the first 1.5 min of reperfusion. Transient inexcitability was induced by switching to cold solution (4 degrees C) shortly before reperfusion (-1.5 min). In controls (no hypothermia), the incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF) were 83% and 92%, respectively, during the first 1.5 min of reperfusion. Transient hypothermia caused inexcitability and asystole, impaired recovery of coronary flow and abolished VT and VF (all P less than 0.05). On subsequent rewarming to 37 degrees C, coronary flow and sinus rate recovered in all hearts. However, VT and VF occurred in only 58% and 25%, respectively (P less than 0.05). These values were similar to those of new episodes of VT and VF occurring in controls during the equivalent period. Therefore arrhythmias had been abolished during transient hypothermia, not merely delayed. The relative contributions of transient impairment of recovery of coronary flow and transient asystole to the antiarrhythmic effects were examined in a further 10 groups of hearts (n = 12/group) in which reperfusion conditions were transiently manipulated. We utilized combinations of hypothermia, ventricular pacing, acetylcholine (ACh) 55 microM (to cause asystole and impairment of recovery of coronary flow), and right atrial excision and left atrial pacing (to permit bradycardia to be transiently induced during reperfusion by temporarily switching off the pacemaker). The results indicated that transient hypothermia was antiarrhythmic as a result of a reduction of excitability, not because of bradycardia or impairment of recovery of flow. The data support the hypothesis that reperfusion unmasks (disinhibits) latent arrhythmogenic components of ischemia (particularly during the first 1.5 min of reperfusion) and that, by inducing inexcitability, transient hypothermia allows these substances to be washed out without their arrhythmogenic effects being manifested. The identities of the arrhythmogenic and antiarrhythmic substances remain to be determined; we suggest that cyclic AMP and potassium, respectively, are likely candidates.     

Environmental influences on cognitive and brain plasticity during aging

In the current article, we provide a critical review of the extant literature that has focused on environmental influences on cognitive and brain plasticity over the adult life span. The review includes both human epidemiological, and human and nonhuman cross-sectional and longitudinal research. We review a number of factors that have been suggested to reduce age-related cognitive decline including both formal and informal education, leisure pursuits, intellectual engagement, and expertise in different skill domains. We also examine the literature on cognitive and physical fitness training. We conclude with a discussion of the gaps in the literature and suggestions for future research.     

Dopamine-dependent synaptic plasticity in striatum during in vivo development

The neurotransmitters dopamine (DA) and glutamate in the striatum play key roles in movement and cognition, and they are implicated in disorders of the basal ganglia such as Parkinson's disease. Excitatory synapses in striatum undergo a form of developmental plasticity characterized by a decrease in glutamate release probability. Here we demonstrate that this form of synaptic plasticity is DA and DA D2 receptor dependent. Analysis of spontaneous synaptic responses indicates that a presynaptic mechanism involving inhibition of neurotransmitter release underlies the developmental plasticity. We suggest that a major role of DA in the striatum is to initiate mechanisms that regulate the efficacy of excitatory striatal synapses, producing a decrease in glutamate release.