Neural tube patterning by Krox20 and emergence of a respiratory control

Protein Phosphatase 2ACα Regulates ATR-Mediated Endogenous DNA Damage Response Against Microcephaly

Protein phosphatase 2A (PP2A) is an abundant heterotrimeric holoenzyme in eukaryotic cells coordinating with specific kinases to regulate spatial-temporal protein dephosphorylation in various biological processes. However, the function of PP2A in cortical neurogenesis remains largely unknown. Here, we report that neuronal-specific deletion of Pp2acα in mice displayed microcephaly, with significantly smaller brains and defective learning and memory ability. Mechanistically, neuronal Pp2acα deficiency resulted in elevated endogenous DNA damage and activation of ATR/CHK1 signaling. It was further induced by the loss of direct interaction between PP2AC and ATR as well as the function of PP2AC to dephosphorylate ATR. Importantly, ATR/CHK1 signaling dysregulation altered both the expression and activity of several critical downstream factors including P53, P21, Bcl2, and Bax, which led to decreased proliferation of cortical progenitor cells and increased apoptosis in developing cortical neurons. Taken together, our results indicate an essential function of PP2ACα in endogenous DNA damage response-mediated ATR signaling during neurogenesis, and defective PP2ACα in neurons contributes to microcephaly.

The effects of five types of tea solutions on epiboly process, neural and cardiovascular development, and locomotor capacity of zebrafish

The effects of teas on embryonic development are still known little. The objective of this study was to compare and analyze developmental effects of green tea, delicate flavor oolong tea, strong flavor oolong tea, black tea, and pu'er tea using zebrafish embryos. Embryos were exposed in tea solutions from one-cell stage; the morphology, locomotor capacity, and gene expression of embryos or larvae were analyzed. The results showed that either tea could decrease the length of body and the size of head and eyes. The effect of green tea had the most significant effects on morphology. Only green tea disturbed cell movement, epiboly, and nervous system development. All five tea solutions caused heart structure alternations and lowered heart rates, and effects caused by green tea were severe. Green tea inhibited the formation of dorsal aorta and segmental arteries and decreased the velocity and total movement distance of larvae. In conclusion, the toxicity of green tea to epiboly, neural and cardiovascular development, and locomotor capacity is more severe than that of other teas. Our study played a warning role for safety consumption of teas and provided references for further study of tea's physiological and pharmacological effects and biological activity.

The effects of copper oxide nanoparticles on dorsoventral patterning, convergent extension, and neural and cardiac development of zebrafish

Currently, nanoparticles are widely used in biomedicine and industry. CuO nanoparticles (CuO-NPs) are versatile materials in our daily life and their toxicity has drawn extensive attention. In this study, we concentrate on the effect of CuO-NPs on early zebrafish development. The results reveal that CuO-NPs can induce abnormal phenotypes of a smaller head and eyes and delayed epiboly. The gene expression pattern shows that CuO-NPs spatially narrow the expression of dorsal genes chordin and goosecoid and alter the expression of dlx3, ntl and hgg which are related to the cell migration of gastrulation. The decreased expression of pax2 and pax6 involved in neural differentiation was accordant with the decreased sizes of neural structures. Cmlc2 expression suggests that CuO-NPs prevented looping of the heart tube during cardiogenesis. Furthermore, quantitative RT-PCR results suggest that the CuO-NPs could increase the canonical Wnt signaling pathway to narrow the expression of chordin and goosecoid in dorsoventral patterning as well as decrease the transcription of Wnt5 and Wnt11 to result in slower, less directed movements and an abnormal cell shape. These findings indicated the CuO-NPs exert developmental toxicity. The present study evaluates the ecological and developmental toxicity, providing warnings about the application of CuO-NPs.

Transport protein evolution deduced from analysis of sequence, topology and structure

The vast majority of well studied transmembrane channels, secondary carriers, primary active transporters and group translocators are believed to have arisen vis intragenic duplication events from simple channel-forming peptides with just 1-3 transmembrane α-helical segments, found ubiquitously in nature. Only a few established channel-forming proteins appear to have evolved via other pathways. The proposed pathway for the evolutionary appearance of the five types of transport proteins involved intragenic duplication of transmembrane pore-forming peptide-encoding genes, giving rise to channel proteins. These gave rise to single protein secondary carriers which upon superimposition of addition protein domains and proteins, including energy-coupling proteins and extracytoplasmic receptors, gave rise to multidomain, multicomponent carriers, primary active transporters and group translocators. Some of the largest and best characterized superfamilies of these transmembrane transport proteins are discussed from topological and evolutionary standpoints.

Exploring the Effects of Different Types of Surfactants on Zebrafish Embryos and Larvae

Currently, surfactants are widely distributed in the environment. As organic pollutants, their toxicities have drawn extensive attention. In this study, the effects of anionic [sodium dodecyl sulphate (SDS) ], cationic [dodecyl dimethyl benzyl ammonium chloride (1227)] and non-ionic [fatty alcohol polyoxyethylene ether (AEO) ] surfactants on zebrafish larval behaviour were evaluated. Five behavioural parameters were recorded using a larval rest/wake assay, including rest total, number of rest bouts, rest bouts length, total activity and waking activity. The results revealed that 1227 and AEO at 1 μg/mL were toxic to larval locomotor activity and that SDS had no significant effects. Moreover, we tested the toxicities of the three surfactants in developing zebrafish embryos. AEO exposure resulted in smaller head size, smaller eye size and shorter body length relative to SDS and 1227. All three surfactants incurred concentration-dependent responses. Furthermore, in situ hybridisation indicated that smaller head size may be associated with a decreased expression of krox20. The altered expression of ntl demonstrated that the developmental retardation stemmed from inhibited cell migration and growth. These findings provide references for ecotoxicological assessments of different types of surfactants, and play a warning role in the application of surfactants.

[Motor behavior of human fetuses during the second trimester of gestation: a longitudinal ultrasound study]

INTRODUCTION

The aim of this research is to contribute to knowledge of the normal spontaneous motor behavior of the human fetus during the second trimester of pregnancy. This study focuses on five patterns of spontaneous fetal movement: startle (S), axo-rhizomelic rhythmia (ARR), axial stretching (AS), general movement (GM), and diaphragmatic contraction (DC).

METHODS

A cohort of 13 subjects was followed up using 2D obstetrical ultrasound images at 12, 16, 20, and 24 weeks of gestation. As inclusion criteria, neonatal neurological examination and general movements after eutocic delivery at term were normal in all of the subjects, and their neuromotor and cognitive development until the end of pre-school age were also normal.

RESULTS

All these five motor patterns are present at the beginning of the 2(nd) gestational trimester, but their quantitative and qualitative traits are diverse according to gestational ages. The phasic, isolated or rhythmically repeated movements, S and ARR, are prominent at 12 and 16 weeks of gestation, and then their presence gradually diminishes. By contrast, tonic and complex AS and GM movements increase their presence and quality at 20 and 24 weeks. RAR constitute a particular periodic motor pattern not described in previous literature. Moreover, the incidence of DC is progressive throughout the trimester, in clusters of 2-6 arrhythmic and irregular beats. Fetal heart rate increases during fetal motor active periods.

CONCLUSIONS

All five normal behavioral patterns observed in the ultrasounds reflect the progressive tuning of motor generators in human nervous system during mid-pregnancy.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

The histone demethylase Jarid1b ensures faithful mouse development by protecting developmental genes from aberrant H3K4me3

Embryonic development is tightly regulated by transcription factors and chromatin-associated proteins. H3K4me3 is associated with active transcription and H3K27me3 with gene repression, while the combination of both keeps genes required for development in a plastic state. Here we show that deletion of the H3K4me2/3 histone demethylase Jarid1b (Kdm5b/Plu1) results in major neonatal lethality due to respiratory failure. Jarid1b knockout embryos have several neural defects including disorganized cranial nerves, defects in eye development, and increased incidences of exencephaly. Moreover, in line with an overlap of Jarid1b and Polycomb target genes, Jarid1b knockout embryos display homeotic skeletal transformations typical for Polycomb mutants, supporting a functional interplay between Polycomb proteins and Jarid1b. To understand how Jarid1b regulates mouse development, we performed a genome-wide analysis of histone modifications, which demonstrated that normally inactive genes encoding developmental regulators acquire aberrant H3K4me3 during early embryogenesis in Jarid1b knockout embryos. H3K4me3 accumulates as embryonic development proceeds, leading to increased expression of neural master regulators like Pax6 and Otx2 in Jarid1b knockout brains. Taken together, these results suggest that Jarid1b regulates mouse development by protecting developmental genes from inappropriate acquisition of active histone modifications.

Exploring the toxicity of a bismuth-asparagine coordination polymer on the early development of zebrafish embryos

Nanoparticles are widely used in nanomedicine, raising concerns about their toxicity. In this study, the toxicity of bismuth-asparagine coordination polymer spheres (BACP-2) was assessed in zebrafish embryos. Injection of 1-4 cell stage embryos with BACP-2 resulted in smaller head size (particularly smaller eye size), shorter body length, and pericardial edemas. The severity and occurrence of the resulting phenotype were concentration-dependent. The expression of genes such as krox20, orthodenticle homeobox 2 (otx2), and cardiac myosin light chain-2 (cmlc2) indicates that the effects of BACP-2 on the head and heart were related to changes in gene expression patterns. A delay in epiboly was observed, and the expression levels of the no tail (ntl) gene indicated that the delay in epiboly resulted both from the effect of BACP-2 on cell migration during epiboly and from slow growth. These findings indicate that BACP-2 exhibits concentration-dependent developmental toxicity, providing insight into the nanotoxicity of bismuth derivatives, which must be rigorously evaluated with respect to toxicity before their application in nanomedicine.

Chronic hyperoxia and the development of the carotid body

Preterm infants often experience hyperoxia while receiving supplemental oxygen. Prolonged exposure to hyperoxia during development is associated with pathologies such as bronchopulmonary dysplasia and retinopathy of prematurity. Over the last 25 years, however, experiments with animal models have revealed that moderate exposures to hyperoxia (e.g., 30-60% O(2) for days to weeks) can also have profound effects on the developing respiratory control system that may lead to hypoventilation and diminished responses to acute hypoxia. This plasticity, which is generally inducible only during critical periods of development, has a complex time course that includes both transient and permanent respiratory deficits. Although the molecular mechanisms of hyperoxia-induced plasticity are only beginning to be elucidated, it is clear that many of the respiratory effects are linked to abnormal morphological and functional development of the carotid body, the principal site of arterial O(2) chemoreception for respiratory control. Specifically, developmental hyperoxia reduces carotid body size, decreases the number of chemoafferent neurons, and (at least transiently) diminishes the O(2) sensitivity of individual carotid body glomus cells. Recent evidence suggests that hyperoxia may also directly or indirectly impact development of the central neural control of breathing. Collectively, these findings emphasize the vulnerability of the developing respiratory control system to environmental perturbations.

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Irregular Breathing in Mice following Genetic Ablation of V2a Neurons

Neural networks called central pattern generators (CPGs) generate repetitive motor behaviors such as locomotion and breathing. Glutamatergic neurons are required for the generation and inhibitory neurons for the patterning of the motor activity associated with repetitive motor behaviors. In the mouse, glutamatergic V2a neurons coordinate the activity of left and right leg CPGs in the spinal cord enabling mice to generate an alternating gait. Here, we investigate the role of V2a neurons in the neural control of breathing, an essential repetitive motor behavior. We find that, following the ablation of V2a neurons, newborn mice breathe at a lower frequency. Recordings of respiratory activity in brainstem-spinal cord and respiratory slice preparations demonstrate that mice lacking V2a neurons are deficient in central respiratory rhythm generation. The absence of V2a neurons in the respiratory slice preparation can be compensated for by bath application of neurochemicals known to accelerate the breathing rhythm. In this slice preparation, V2a neurons exhibit a tonic firing pattern. The existence of direct connections between V2a neurons in the medial reticular formation and neurons of the pre-Bötzinger complex indicates that V2a neurons play a direct role in the function of the respiratory CPG in newborn mice. Thus, neurons of the embryonic V2a lineage appear to have been recruited to neural networks that control breathing and locomotion, two prominent CPG-driven, repetitive motor behaviors.

Brainstem respiratory oscillators develop independently of neuronal migration defects in the Wnt/PCP mouse mutant looptail

The proper development and maturation of neuronal circuits require precise migration of component neurons from their birthplace (germinal zone) to their final positions. Little is known about the effects of aberrant neuronal position on the functioning of organized neuronal groups, especially in mammals. Here, we investigated the formation and properties of brainstem respiratory neurons in looptail (Lp) mutant mice in which facial motor neurons closely apposed to some respiratory neurons fail to migrate due to loss of function of the Wnt/Planar Cell Polarity (PCP) protein Vangl2. Using calcium imaging and immunostaining on embryonic hindbrain preparations, we found that respiratory neurons constituting the embryonic parafacial oscillator (e-pF) settled at the ventral surface of the medulla in Vangl2^Lp/+ and Vangl2^Lp/Lp embryos despite the failure of tangential migration of its normally adjacent facial motor nucleus. Anatomically, the e-pF neurons were displaced medially in Lp/+ embryos and rostro-medially Lp/Lp embryos. Pharmacological treatments showed that the e-pF oscillator exhibited characteristic network properties in both Lp/+ and Lp/Lp embryos. Furthermore, using hindbrain slices, we found that the other respiratory oscillator, the preBötzinger complex, was also anatomically and functionally established in Lp mutants. Importantly, the displaced e-pF oscillator established functional connections with the preBötC oscillator in Lp/+ mutants. Our data highlight the robustness of the developmental processes that assemble the neuronal networks mediating an essential physiological function.

Xenomitochondrial mice: investigation into mitochondrial compensatory mechanisms

Xenomitochondrial mice, harboring evolutionarily divergent Mus terricolor mitochondrial DNA (mtDNA) on a Mus musculus domesticus nuclear background (B6NTac(129S6)-mt(M. terricolor)/Capt; line D7), were subjected to molecular and phenotypic analyses. No overt in vivo phenotype was identified in contrast to in vitro xenomitochondrial cybrid studies. Microarray analyses revealed differentially expressed genes in xenomitochondrial mice, though none were directly involved in mitochondrial function. qRT-PCR revealed upregulation of mt-Co2 in xenomitochondrial mice. These results illustrate that cellular compensatory mechanisms for mild mitochondrial dysfunction alter mtDNA gene expression at a proteomic and/or translational level. Understanding these mechanisms will facilitate the development of therapeutics for mitochondrial disorders.

Developmental basis of the rostro-caudal organization of the brainstem respiratory rhythm generator

The Hox genetic network plays a key role in the anteroposterior patterning of the rhombencephalon at pre- and early-segmental stages of development of the neural tube. In the mouse, it controls development of the entire brainstem respiratory neuronal network, including the pons, the parafacial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC). Inactivation of Krox20/Egr2 eliminates the pFRG activity, thereby causing life-threatening neonatal apnoeas alternating with respiration at low frequency. Another respiratory abnormality, the complete absence of breathing, is induced when neuronal synchronization fails to develop in the preBötC. The present paper summarizes data on a third type of respiratory deficits induced by altering Hox function at pontine levels. Inactivation of Hoxa2, the most rostrally expressed Hox gene in the hindbrain, disturbs embryonic development of the pons and alters neonatal inspiratory shaping without affecting respiratory frequency and apnoeas. The same result is obtained by the Phox2a(+/-) mutation modifying the number of petrosal chemoafferent neurons, by eliminating acetylcholinesterase and by altering Hox-dependent development of the pons with retinoic acid administration at embryonic day 7.5. In addition, embryos treated with retinoic acid provide a mouse model for hyperpnoeic episodic breathing, widely reported in pre-term neonates, young girls with Rett's syndrome, patients with Joubert syndrome and adults with Cheyne-Stokes respiration. We conclude that specific respiratory deficits in vivo are assignable to anteroposterior segments of the brainstem, suggesting that the adult respiratory neuronal network is functionally organized according to the rhombomeric, Hox-dependent segmentation of the brainstem in embryos.

Reciprocal co-regulation of EGR2 and MECP2 is disrupted in Rett syndrome and autism

Mutations in MECP2, encoding methyl-CpG-binding protein 2 (MeCP2), cause the neurodevelopmental disorder Rett syndrome (RTT). Although MECP2 mutations are rare in idiopathic autism, reduced MeCP2 levels are common in autism cortex. MeCP2 is critical for postnatal neuronal maturation and a modulator of activity-dependent genes such as Bdnf (brain-derived neurotropic factor) and JUNB. The activity-dependent early growth response gene 2 (EGR2), required for both early hindbrain development and mature neuronal function, has predicted binding sites in the promoters of several neurologically relevant genes including MECP2. Conversely, MeCP2 family members MBD1, MBD2 and MBD4 bind a methylated CpG island in an enhancer region located in EGR2 intron 1. This study was designed to test the hypothesis that MECP2 and EGR2 regulate each other’s expression during neuronal maturation in postnatal brain development. Chromatin immunoprecipitation analysis showed EGR2 binding to the MECP2 promoter and MeCP2 binding to the enhancer region in EGR2 intron 1. Reduction in EGR2 and MeCP2 levels in cultured human neuroblastoma cells by RNA interference reciprocally reduced expression of both EGR2 and MECP2 and their protein products. Consistent with a role of MeCP2 in enhancing EGR2, Mecp2-deficient mouse cortex samples showed significantly reduced EGR2 by quantitative immunofluorescence. Furthermore, MeCP2 and EGR2 show coordinately increased levels during postnatal development of both mouse and human cortex. In contrast to age-matched Controls, RTT and autism postmortem cortex samples showed significant reduction in EGR2. Together, these data support a role of dysregulation of an activity-dependent EGR2/MeCP2 pathway in RTT and autism.

Transcription factors and the genetic organization of brain stem respiratory neurons

Breathing is a genetically determined behavior generated by neurons in the brain stem. Transcription factors, in part, determine the basic developmental identity of neurons, but the relationships between these genes and the neural populations generating and modulating respiration are unclear. The diversity of brain stem populations has been proposed to result from a combinatorial code of transcription factor expression corresponding to the anterior-posterior (A-P) and dorsal-ventral (D-V) location of a neuron's birth. I provide a schematic of transcription factor coding identifying at least 15 genetically distinct D-V subdivisions of brain stem neurons that, combined with A-P patterning, may provide a genetic organization of the brain stem in general, with the eventual goal of describing respiratory populations in particular. Using a combination of fate mapping in transgenic mouse lines and immunohistochemistry, we confirm the parabrachial nuclei are derived from a subset of Atoh1 expression progenitor neurons. I hypothesize the Kölliker-Fuse nucleus can be uniquely defined in the neonate mouse by the coexpression of the transcription factor FoxP2 in Atoh1-derived neurons of rhombomere 1.

Development of vagal afferent projections circumflex to the obex in the embryonic chick brainstem visualized with voltage-sensitive dye recording

Using voltage-sensitive dye recording, we surveyed neural responses related to the vagus nerve in the embryonic chick brainstem. In our previous studies, we identified four vagus nerve-related response areas in the brainstem. On the stimulated side, they included (1) the nucleus of the tractus solitarius (NTS: the primary sensory nucleus) and (2) the dorsal motor nucleus of the vagus nerve (DMNV), whereas on the contralateral side, they corresponded to (3) the parabrachial nucleus (PBN: the second/higher-ordered nucleus) and (4) the medullary non-NTS region. In the present study, in addition to these areas, we identified another response area circumflex to the obex. The intensity of the optical signal in the response area was much smaller than that in the NTS/DMNV, and the spatio-temporal pattern could be discerned after signal averaging. The conduction rate to the response area was slower than that to the other four areas. Ontogenetically, the response area was distributed on the stimulated side at the 6-day embryonic stage, and it spread into the contralateral side in 7- and 8-day embryonic stages. These distribution patterns were consistent with projection patterns of vagal afferent fibers stained with a fluorescent tracer, suggesting that the response area included a primary sensory nucleus. In comparison with the functional development of the other four response areas, we traced the functional organization of vagus nerve-related nuclei in the embryonic brainstem.

Krox20 is down-regulated following triazole in vitro embryonic exposure: a polycompetitor-based assay

This study was conducted in order to analyse gene-expression alterations in rat embryos following exposure to triazoles, using an easy-handling approach. Triazole derivatives have been shown to alter the morphology of cranio-facial structures and to induce abnormalities in hindbrain patterning and neural crest cell migration. Specification of hindbrain segments is regulated by retinoic acid and the hox code. Krox20 was chosen as molecular marker for its specific distribution in the anterior neural tube. In fact, this zinc-finger protein is expressed in rhombomere 3 and 5. Mis-regulation of Krox20 levels have shown to induce severe alterations in the correct patterning of the rhomboencephalon and the derived structures. In order to analyse Krox20 mRNA levels in rat embryos exposed in vitro to the triazole derivative triadimefon, a semi-quantitative approach utilising the competitive RT-PCR was chosen. A lambda phage-based plasmid construct that could compete with target and internal standard gene at the same time during enzymatic reaction was generated. Results were confirmed by real-time RT-PCR analysis on the same samples. Our data show a down-regulation of Krox20 transcript levels after exposure to the triazole derivative, implying a key role of this molecule in the pathogenic pathway induced by triazole exposure.

Ontogeny of central rhythm generation in chicks and rodents

Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior-posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20(-/-), Hoxa1(-/-) and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.

Developmental plasticity of the hypoxic ventilatory response after perinatal hyperoxia and hypoxia

Both genetic and environmental factors influence the normal development of the respiratory control system. This review examines the role perinatal O2 plays in the development of normoxic breathing and the hypoxic ventilatory response in mammals. Hyperoxia and hypoxia elicit plasticity in respiratory control that is unique to development and may persist weeks to years after return to normoxia. Specifically, both hyperoxia and hypoxia during early postnatal development attenuate the adult hypoxic ventilatory response, but the underlying mechanisms for this plasticity differ. Hyperoxia attenuates the hypoxic ventilatory response through potentially life-long changes in carotid body function. Neonatal hypoxia appears to have short-term effects on carotid body function, but persistent changes in the hypoxic ventilatory response may instead reflect changes in respiratory mechanics or related neural pathways. Overall, it appears that a relatively narrow range of environmental O2 is consistent with "normal" postnatal respiratory control development, predisposing animals to potentially maladaptive plasticity in the face of disease or atypical environmental conditions.