Circadian and Circatidal Rhythms of Protein Abundance in the Intertidal Mussel Mytilus californianus

The intertidal zone is a dynamic environment that fluctuates with the 12.4-h tidal and 24-h light/dark cycle to predictably alter food availability, temperature, air exposure, wave action, oxygen partial pressure, and osmotic conditions. Intertidal sessile bivalves exhibit behavioral or physiological changes to minimize the persistent challenges of fluctuating environmental conditions, such as adjusting gaping behavior and heart rate. At the cellular level, transcriptomic studies on mussels’ baseline circadian and circatidal rhythms have determined that the circadian rhythm is the dominant transcriptional rhythm. However, as proteins reflect the basic molecular phenotype of an organism and their abundance may differ greatly from that of mRNA, these methods could fail to detect important cyclical changes in the proteome that cope with the regular stress of tidal rhythms. For this study, we acclimated intertidal Mytilus californianus to circadian (12:12 h light/dark cycle) and circatidal (6:6 h tidal cycle) conditions in a tidal simulator and sampled gill tissue from mussels every 2 h for 48 h for proteomic analysis. Approximately 86% of the proteins that were detected exhibited rhythmicity over the time course. The circadian cycle primarily determined the cyclic abundance of energy metabolism proteins, pivoting around the transition to the nighttime high tide. The tidal cycle contributed to alterations in cytoskeletal components, ER protein processing and vesicular trafficking, extracellular matrix and immune proteins, and oxidative stress and chaperoning proteins. We also found evidence that post-translational modifications may be important for driving these rhythms, as acetylation and phosphorylation motifs were enriched in the rhythmic proteins and we identified rhythms in elements of methylation, mitochondrial peptide processing, and acylation. These dynamic changes in proteins across numerous functional categories indicate that the combination of circadian and tidal cycles drive complex cellular changes to coordinate processes in a changing environment. This variation clearly shows that differential expression studies and biomonitoring efforts cannot assume a static baseline of cellular conditions in intertidal mussels

Are Circadian cycles the dominant proteome rhythym in the intertidal mussel Mytilus californianus?

Mytilus californianus, also known as the California mussel, is a marine bivalve that is abundant along the West coast from Alaska to southern Baja California. They mainly reside in the upper-middle intertidal zone and cling to pier pilings and surf exposed rocks. They create multi-layered beds, which form a habitat for algae and many species of invertebrates. Intertidal mussels live in a naturally dynamic environment. It has previously been reported (Connor and Gracey, 2011) that the 24-hour circadian (day to night) rhythm of the intertidal mussel Mytilus californianus is primarily responsible for its rhythmic gene expression, as opposed to the 12.4-hour tidal cycles. Because tidal cycles challenge intertidal mussels through heat stress, salinity stress, hypoxia, and food availability, the dominance of the circadian cycle is surprising. However, transcriptomics may fail to detect up to half of the variation in the proteins that comprise the final functional phenotype of the organism. Using two-dimensional gel electrophoresis and mass spectrometry, we aimed to identify whether the proteome—the protein expression—of this organism also followed the same circadian rhythmic expression as its transcriptome

Ephrin-A5 Suppresses Neurotrophin Evoked Neuronal Motility, ERK Activation and Gene Expression

During brain development, growth cones respond to attractive and repulsive axon guidance cues. How growth cones integrate guidance instructions is poorly understood. Here, we demonstrate a link between BDNF (brain derived neurotrophic factor), promoting axonal branching and ephrin-A5, mediating axonal repulsion via Eph receptor tyrosine kinase activation. BDNF enhanced growth cone filopodial dynamics and neurite branching of primary neurons. We show that ephrin-A5 antagonized this BDNF-evoked neuronal motility. BDNF increased ERK phosphorylation (P-ERK) and nuclear ERK entry. Ephrin-A5 suppressed BDNF-induced ERK activity and might sequester P-ERK in the cytoplasm. Neurotrophins are well established stimulators of a neuronal immediate early gene (IEG) response. This is confirmed in this study by e.g. c-fos, Egr1 and Arc upregulation upon BDNF application. This BDNF-evoked IEG response required the transcription factor SRF (serum response factor). Notably, ephrin-A5 suppressed a BDNF-evoked neuronal IEG response, suggesting a role of Eph receptors in modulating gene expression. In opposite to IEGs, long-term ephrin-A5 application induced cytoskeletal gene expression of tropomyosin and actinin. To uncover specific Eph receptors mediating ephrin-As impact on neurotrophin signaling, EphA7 deficient mice were analyzed. In EphA7 deficient neurons alterations in growth cone morphology were observed. However, ephrin-A5 still counteracted neurotrophin signaling suggesting that EphA7 is not required for ephrin and BDNF crosstalk. In sum, our data suggest an interaction of ephrin-As and neurotrophin signaling pathways converging at ERK signaling and nuclear gene activity. As ephrins are involved in development and function of many organs, such modulation of receptor tyrosine kinase signaling and gene expression by Ephs might not be limited to the nervous system