Neuroanatomical distribution of angiotensin-II-like neuropeptide within the central nervous system of the crab Chasmagnathus; physiological changes triggered by water deprivation

Into the deep: Exploring the molecular mechanisms of hyperactive behaviour induced by three rare earth elements in early life-stages of the deep-sea scavenging amphipod Tmetonyx cicada (Lysianassidae)

With increasing socio-economic importance of the rare earth elements and yttrium (REY), Norway has laid out plans for REY mining, from land-based to deep-sea mining, thereby enhancing REY mobility in the marine ecosystem. Little is known about associated environmental consequences, especially in the deep ocean. We explored the toxicity and modes of action of a light (Nd), medium (Gd) and heavy (Yb) REY-Cl3 at four concentrations (3, 30, 300, and 3000 μg L-1) in the Arcto-boreal deep-sea amphipod Tmetonyx cicada. At the highest concentration, REY solubility was limited and increased with atomic weight (Nd < Gd < Yb). Lethal effects were practically restricted to this treatment, with the lighter elements being more acutely toxic than Yb (from ∼50 % mortality in the Gd-group at dissolved 689-504 μg L-1 to <20 % in the Yb-group at ca. 2000 μg L-1), which could be a function of bioavailability. All three REY induced hyperactivity at the low-medium concentrations. Delving into the transcriptome of T. cicada allowed us to determine a whole array of potential (neurotoxic) mechanisms underlying this behaviour. Gd induced the vastest response, affecting serotonin-synthesis; sphingolipid-synthesis; the renin-angiotensin system; mitochondrial and endoplasmic reticulum functioning (Gd, Nd); and lysosome integrity (Gd, Yb); as well as the expression of hemocyanin, potentially governing REY-uptake (Gd, Yb). While Nd and Yb shared only few pathways, suggesting a link between mode of action and atomic weight/radius, almost all discussed mechanisms imply the disruption of organismal Ca-homeostasis. Despite only fragmental genomic information available for crustaceans to date, our results provide novel insight into the toxicophysiology of REY in marine biota. The neurotoxic/behavioural effects in T. cicada at concentrations with potential environmental relevance warn about the possibility of bottom-up ecological consequences in mining exposed fjords and deep-sea ecosystems, calling for follow-up studies and regulatory measures prior to the onset of REY mining in Norway.

The role of the renin-angiotensin system (RAS) in salinity adaptation in Pacific white shrimp (Litopenaeus vannamei)

The renin-angiotensin system (RAS) is a hormonal system that plays an important role in the regulation of blood pressure and cardiovascular homeostasis in mammals. In fishes, the RAS pathway participates in osmoregulation and salinity adaptation. However, the role of the RAS pathway in invertebrates, particularly in crustaceans, remains unknown. In this study, four key genes of the RAS pathway (LV-ACE, LV-APN, LV-AT1R, and LV-RR) were cloned, characterized, and their expression levels were detected in the eyestalk, hepatopancreas, and muscle of Litopenaeus vannamei during long-term and short-term low salinity stress. The results showed that LV-ACE, LV-APN, LV-AT1R, and LV-RR encode 666, 936, 175, and 323 amino acids, respectively. Low salinity stress downregulated the expression levels of LV-ACE, LV-APN, LV-AT1R, and LV-RR in L. vannamei, indicating that the RAS pathway was suppressed under low salinity. Moreover, these genes play important roles in the regulation of drinking rate, controlling urine output, blood glucose, and blood pressure, indicating that their downregulation probably affected the homeostasis of shrimps. These findings provide novel insights into the mechanism of salinity adaptation in L. vannamei.

Contextual memory reactivation modulates Ca2+-activity network state in a mushroom body-like center of the crab N. granulata

High-order brain centers play key roles in sensory integration and cognition. In arthropods, much is known about the insect high-order centers that support associative memory processes, the mushroom bodies. The hypothesis that crustaceans possess structures equivalent to the mushroom bodies -traditionally called hemiellipsoid body- has been receiving neuroanatomical endorsement. The recent functional support is limited to the short term: in a structure of the true crab Neohelice granulata that has many insect-like mushroom bodies traits, the plastic learning changes express the context attribute of an associative memory trace. Here, we used in vivo calcium imaging to test whether neuronal activity in this structure is associated with memory reactivation in the long-term (i.e., 24 h after training). Long-term training effects were tested by presenting the training-context alone, a reminder known to trigger memory reconsolidation. We found similar spontaneous activity between trained and naïve animals. However, after training-context presentation, trained animals showed increased calcium events rate, suggesting that memory reactivation induced a change in the underlying physiological state of this center. Reflecting the change in the escape response observed in the paradigm, animals trained with a visual danger stimulus showed significantly lower calcium-evoked transients in the insect-like mushroom body. Protein synthesis inhibitor cycloheximide administered during consolidation prevented calcium mediated changes. Moreover, we found the presence of distinct calcium activity spatial patterns. Results suggest that intrinsic neurons of this crustacean mushroom body-like center are involved in contextual associative long-term memory processes.

More than one way to smell ashore - Evolution of the olfactory pathway in terrestrial malacostracan crustaceans

Crustaceans provide a fascinating opportunity for studying adaptations to a terrestrial lifestyle because within this group, the conquest of land has occurred at least ten times convergently. The evolutionary transition from water to land demands various morphological and physiological adaptations of tissues and organs including the sensory and nervous system. In this review, we aim to compare the brain architecture between selected terrestrial and closely related marine representatives of the crustacean taxa Amphipoda, Isopoda, Brachyura, and Anomala with an emphasis on the elements of the olfactory pathway including receptor molecules. Our comparison of neuroanatomical structures between terrestrial members and their close aquatic relatives suggests that during the convergent evolution of terrestrial life-styles, the elements of the olfactory pathway were subject to different morphological transformations. In terrestrial anomalans (Coenobitidae), the elements of the primary olfactory pathway (antennules and olfactory lobes) are in general considerably enlarged whereas they are smaller in terrestrial brachyurans compared to their aquatic relatives. Studies on the repertoire of receptor molecules in Coenobitidae do not point to specific terrestrial adaptations but suggest that perireceptor events - processes in the receptor environment before the stimuli bind - may play an important role for aerial olfaction in this group. In terrestrial members of amphipods (Amphipoda: Talitridae) as well as of isopods (Isopoda: Oniscidea), however, the antennules and olfactory sensilla (aesthetascs) are largely reduced and miniaturized. Consequently, their primary olfactory processing centers are suggested to have been lost during the evolution of a life on land. Nevertheless, in terrestrial Peracarida, the (second) antennae as well as their associated tritocerebral processing structures are presumed to compensate for this loss or rather considerable reduction of the (deutocerebral) primary olfactory pathway. We conclude that after the evolutionary transition from water to land, it is not trivial for arthropods to establish aerial olfaction. If we consider insects as an ingroup of Crustacea, then the Coenobitidae and Insecta may be seen as the most successful crustacean representatives in this respect.

Memory built in conjunction with a stressor is privileged: Reconsolidation-resistant memories in the crab Neohelice

The dynamics of memory processes are conserved throughout evolution, a feature based on the hypothesis of a common origin of the high-order memory centers in bilateral animals. Reconsolidation is just one example. The possibility to interfere with long-term memory expression during reconsolidation has been proposed as potentially useful in clinical application to treat traumatic memories. However, several pieces of evidence in rodents show that either robust fear memories or stressful events applied before acquisition promote reconsolidation-resistant memories, i.e., memories that are resistant to the interfering effect of drugs on memory reconsolidation. Conceivably, the generation of these reconsolidation-resistant fear memories also occurs in humans. Is the induction of reconsolidation-resistant memories part of the dynamics of memory processes conserved throughout evolution? In the semiterrestrial crab Neohelice granulata, memory reconsolidation is triggered by a short reminder without reinforcement. Here, we show that an increase in the salience of the aversive stimulus augmented the memory strength; nonetheless, the protein synthesis inhibitor cycloheximide still disrupted the reconsolidation process. However, crabs stressed by a water-deprivation episode before a strong training session built up a memory that was now reconsolidation-resistant. We tested whether these reconsolidation-resistant effects can be challenged by changing parametric conditions of memory-reminder sessions; multiple memory reactivations without reinforcement were not able to trigger the labilization-reconsolidation of this resistant memory. Overall, the present findings suggest that generation of reconsolidation-resistant memories can be another part of the dynamics of memory processes conserved throughout evolution that protects privileged information from change.

Context-dependent memory traces in the crab's mushroom bodies: Functional support for a common origin of high-order memory centers

The hypothesis of a common origin for the high-order memory centers in bilateral animals is based on the evidence that several key features, including gene expression and neuronal network patterns, are shared across several phyla. Central to this hypothesis is the assumption that the arthropods' higher order neuropils of the forebrain [the mushroom bodies (MBs) of insects and the hemiellipsoid bodies (HBs) of crustaceans] are homologous structures. However, even though involvement in memory processes has been repeatedly demonstrated for the MBs, direct proof of such a role in HBs is lacking. Here, through neuroanatomical and immunohistochemical analysis, we identified, in the crab Neohelice granulata, HBs that resemble the calyxless MBs found in several insects. Using in vivo calcium imaging, we revealed training-dependent changes in neuronal responses of vertical and medial lobes of the HBs. These changes were stimulus-specific, and, like in the hippocampus and MBs, the changes reflected the context attribute of the memory trace, which has been envisioned as an essential feature for the HBs. The present study constitutes functional evidence in favor of a role for the HBs in memory processes, and provides key physiological evidence supporting a common origin of the arthropods' high-order memory centers.

Neural correlates of expression-independent memories in the crab Neohelice

The neural correlates of memory have been usually examined considering that memory retrieval and memory expression are interchangeable concepts. However, our studies in the crab Neohelice (Chasmagnathus) granulata and in other memory models have shown that memory expression is not necessary for memory to be re-activated and become labile. In order to examine putative neural correlates of memory in the crab Neohelice, we contrast changes induced by training in both animal's behavior and neuronal responses in the medulla terminalis using in vivo Ca(2+) imaging. Disruption of long-term memory by the amnesic agents MK-801 or scopolamine (5μg/g) blocks the learning-induced changes in the Ca(2+) responses in the medulla terminalis. Conversely, treatments that lead to an unexpressed but persistent memory (weak training protocol or scopolamine 0.1μg/g) do not block these learning-induced neural changes. The present results reveal a set of changes in the neural activity induced by training that correlates with memory persistence but not with the probability of this memory to be expressed in the long-term. In addition, the study constitutes the first in vivo evidence in favor of a role of the medulla terminalis in learning and memory in crustaceans, and provides a physiological evidence indicating that memory persistence and the probability of memory to be expressed might involve separate components of memory traces.

Characterization of angiotensin I-converting enzyme from anterior gills of the mangrove crab Ucides cordatus

Angiotensin I-converting enzyme (ACE) is a well-known metallopeptidase that is found in vertebrates, invertebrates and bacteria. We isolated from the anterior gill of the crab Ucides cordatus an isoform of ACE, here named crab-ACE, which presented catalytic properties closely resembling to those of mammalian ACE. The enzyme was purified on Sepharose-lisinopril affinity chromatography to apparent homogeneity and a band of about 72 kDa could be visualized after silver staining and Western blotting. Assays performed with fluorescence resonance energy transfer (FRET) selective ACE substrates Abz-FRK(Dnp)P-OH, Abz-SDK(Dnp)P-OH and Abz-LFK(Dnp)-OH, allowed us to verify that crab-ACE has hydrolytic profile very similar to that of the ACE C-domain. In addition, we observed that crab-ACE can hydrolyze the ACE substrates, angiotensin I and bradykinin. The enzyme was strongly inhibited by the specific ACE inhibitor lisinopril (Ki of 1.26 nM). However, in contrast to other ACE isoforms, crab-ACE presented a very particular optimum pH, being the substrate Abz-FRK(Dnp)-P-OH hydrolyzed efficiently at pH 9.5. Other interesting characteristic of crab-ACE was that the maximum hydrolytic activity was reached at around 45°C. The description of an ACE isoform in Ucides cordatus is challenging and may contribute to a better understanding of the biochemical function of this enzyme in invertebrates.

Memory beyond expression

The idea that memories are not invariable after the consolidation process has led to new perspectives about several mnemonic processes. In this framework, we review our studies on the modulation of memory expression during reconsolidation. We propose that during both memory consolidation and reconsolidation, neuromodulators can determine the probability of the memory trace to guide behavior, i.e. they can either increase or decrease its behavioral expressibility without affecting the potential of persistent memories to be activated and become labile. Our hypothesis is based on the findings that positive modulation of memory expression during reconsolidation occurs even if memories are behaviorally unexpressed. This review discusses the original approach taken in the studies of the crab Neohelice (Chasmagnathus) granulata, which was then successfully applied to test the hypothesis in rodent fear memory. Data presented offers a new way of thinking about both weak trainings and experimental amnesia: memory retrieval can be dissociated from memory expression. Furthermore, the strategy presented here allowed us to show in human declarative memory that the periods in which long-term memory can be activated and become labile during reconsolidation exceeds the periods in which that memory is expressed, providing direct evidence that conscious access to memory is not needed for reconsolidation. Specific controls based on the constraints of reminders to trigger reconsolidation allow us to distinguish between obliterated and unexpressed but activated long-term memories after amnesic treatments, weak trainings and forgetting. In the hypothesis discussed, memory expressibility--the outcome of experience-dependent changes in the potential to behave--is considered as a flexible and modulable attribute of long-term memories. Expression seems to be just one of the possible fates of re-activated memories.

Food odor, visual danger stimulus, and retrieval of an aversive memory trigger heat shock protein HSP70 expression in the olfactory lobe of the crab Chasmagnathus granulatus

Although some of the neuronal substrates that support memory process have been shown in optic ganglia, the brain areas activated by memory process are still unknown in crustaceans. Heat shock proteins (HSPs) are synthesized in the CNS not only in response to traumas but also after changes in metabolic activity triggered by the processing of different types of sensory information. Indeed, the expression of citosolic/nuclear forms of HSP70 (HSC/HSP70) has been repeatedly used as a marker for increases in neural metabolic activity in several processes, including psychophysiological stress, fear conditioning, and spatial learning in vertebrates. Previously, we have shown that, in the crab Chasmagnathus, two different environmental challenges, water deprivation and heat shock, trigger a rise in the number of glomeruli of the olfactory lobes (OLs) expressing HSC/HSP70. In this study, we initially performed a morphometric analysis and identified a total of 154 glomeruli in each OL of Chasmagnathus. Here, we found that crabs exposed to food odor stimuli also showed a significant rise in the number of olfactory glomeruli expressing HSC/HSP70. In the crab Chasmagnathus, a powerful memory paradigm based on a change in its defensive strategy against a visual danger stimulus (VDS) has been extensively studied. Remarkably, the iterative presentation of a VDS caused an increase as well. This increase was triggered in animals visually stimulated using protocols that either build up a long-term memory or generate only short-term habituation. Besides, memory reactivation was sufficient to trigger the increase in HSC/HSP70 expression in the OL. Present and previous results strongly suggest that, directly or indirectly, an increase in arousal is a sufficient condition to bring about an increase in HSC/HSP70 expression in the OL of Chasmagnathus.