Temporal changes in zooplankton indicators highlight a bottom-up process in the Bay of Marseille (NW Mediterranean Sea).

Sixteen years (2005-2020) of zooplankton monitoring in the Bay of Marseille (N-W Mediterranean Sea) are analyzed in relation to physical, meteorological, climatic and biotic data. Samples were collected every two weeks by a vertical haul (0-55 m) of a 200 µm plankton net. Different indices characterizing the mesozooplankton are compared: biomass dry weight of four size fractions between 200 and 2000 µm; abundances of the whole of the mesozooplankton and of 13 main taxonomic groups defined from plankton imagery; seasonal onset timing of each zooplankton group; and two other types of indices: the first characterized diversity based on abundance data, and the second was derived from zooplankton size spectra shape. The clearest pattern in the environmental compartment was an overall decreasing trend in nutrients, shifts in phytoplankton metrics (i.e. size structure and particulate organic matter), and changes in winter conditions (i.e. increasing temperatures, precipitation and NAO). Interannual patterns in the mesozooplankton community were: (i) a decrease of total abundance (ii) a decrease in biomass for the four size fractions, with an earlier decrease for the 1000-2000 µm size fraction (in 2008); (iii) a reduced dominance of copepods (calanoids and oithonoids) and a concomitant increase in abundance of other taxonomic groups (crustaceans, pteropods, chaetognaths, salps) which induced higher diversity; (iv) a first shift in size spectra towards smaller sizes in 2009, when the 1000-2000 µm size fraction biomass decreased, and a second shift towards larger sizes in 2013 along with increased diversity; and (iv) a later onset in the phenology for some zooplankton variables and earlier onset for salps. Concomitant changes in the phytoplankton compartment, winter environmental conditions, zooplankton community structure (in size and diversity) and zooplankton phenology marked by a shift in 2013 suggest bottom-up control of the pelagic ecosystem.

Isotopic and biochemical trophic markers reveal the complexity of interactions at the base of pelagic food webs (Mediterranean sea).

To gain insight into the impact of bottom-up changes in the plankton community on planktivorous fish in the context of the decline of small pelagic fisheries in the Northwestern Mediterranean Sea, we have conducted an extensive year-long study. The investigation combined biochemical analyses (proteins, carbohydrates, and lipids) with C and N stable isotope analyses (SIA) to simultaneously study phytoplankton, zooplankton, and eight planktivorous fish species (Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita, Sprattus sprattus, Cepola macrophthalma, Chromis chromis, Boops boops, and Spicara maena). This study is the first to analyze both stable isotope and biochemical compositions in coastal particulate organic matter (POM) size classes (0.7-2.7 µm, 2.7-20 µm, and 20-200 µm), zooplankton size classes (200-300 µm, 300-500 µm, 500-1000 µm, 1000-2000 µm, and >2000 µm), and taxonomic groups. We demonstrated that: (1) POM stable isotope compositions varied based on its spatial origin, the taxonomic composition of its biota, and its biochemical content; (2) δ15N values increased with zooplankton size classes and groups, indicating different trophic levels; (3) Phytoplankton exhibited a lipid-rich composition (∼55%), while zooplankton and fish muscles were protein-rich (∼61% and ∼66%, respectively). Bayesian stable isotope mixing models revealed that, on average: (1) POM from oceanic waters contributed the most to the POM in the bay (>51%), with a dominance of pico-POM (∼43%); (2) The 200-1000 µm zooplankton primarily consumed nano-POM, the 1000-2000 µm zooplankton mostly consumed micro-POM (∼64%), and the >2000 µm zooplankton also mostly consumed micro-POM; (3) Mesozooplankton (200-2000 µm) constituted the main portion (∼42%) of the diet for planktivorous fish species, while macrozooplankton organisms (>2000 µm) were the primary food resource (∼43%) for both B. boops and S. sprattus. Our study underscores the complexity of the pelagic food web and highlights the bottom-up transfer of organic matter from the smallest phytoplankton size fractions to planktivorous fish.

Nanoplanktonic diatoms are globally overlooked but play a role in spring blooms and carbon export.

Diatoms are one of the major primary producers in the ocean, responsible annually for ~20% of photosynthetically fixed CO2 on Earth. In oceanic models, they are typically represented as large (>20 µm) microphytoplankton. However, many diatoms belong to the nanophytoplankton (2-20 µm) and a few species even overlap with the picoplanktonic size-class (<2 µm). Due to their minute size and difficulty of detection they are poorly characterized. Here we describe a massive spring bloom of the smallest known diatom (Minidiscus) in the northwestern Mediterranean Sea. Analysis of Tara Oceans data, together with literature review, reveal a general oversight of the significance of these small diatoms at the global scale. We further evidence that they can reach the seafloor at high sinking rates, implying the need to revise our classical binary vision of pico- and nanoplanktonic cells fueling the microbial loop, while only microphytoplankton sustain secondary trophic levels and carbon export.

Toxicity and endocytosis of spinocerebellar ataxia type 6 polyglutamine domains: role of myosin IIb.

Spinocerebellar ataxia type 6 (SCA6) is a dominantly inherited neurodegenerative disease caused by a small expansion of CAG repeats in the sequence coding for the cytoplasmic C-terminal region of the Ca(v)2.1 subunit of P/Q-type calcium channels. We have tested the toxicity of mutated Ca(v)2.1 C-terminal domains expressed in the plasma membrane. In COS-7 cells, CD4-green fluorescent protein fused to Ca(v)2.1 C-terminal domains containing expanded 24 polyglutamine (Q) tracts displayed increased toxicity and stronger expression at the cell surface relative to 'normal' 12 Q tracts, partially because of reduced endocytosis. Glutathione S-transferase pull-down and proteomic analysis indicated that Ca(v)2.1 C-termini interact with the heavy and light chains of cerebellar myosin IIB, a molecular motor protein. This interaction was confirmed by coimmunoprecipitation from rat cerebellum and COS-7 cells and shown to be direct by binding of in vitro-translated (35)S-myosin IIB heavy chain. In COS-7 cells, incremented polyglutamine tract length increased the interaction with myosin IIB. Furthermore, the myosin II inhibitor blebbistatin reversed the effects of polyglutamine expansion on plasma membrane expression. Our findings suggest a key role of myosin IIB in promoting accumulation of mutant Ca(v)2.1Ct at the plasma membrane and suggest that this gain of function might contribute to the pathogenesis of SCA6.

Multiple determinants in voltage-dependent P/Q calcium channels control their retention in the endoplasmic reticulum.

Surface expression level of voltage-dependent calcium channels is tightly controlled in neurons to avoid the resulting cell toxicity generally associated with excessive calcium entry. Cell surface expression of high voltage-activated calcium channels requires the association of the pore-forming subunit, Cavalpha, with the auxiliary subunit, Cavbeta. In the absence of this auxiliary subunit, Cavalpha is retained in the endoplasmic reticulum (ER) through mechanisms that are still poorly understood. Here, we have investigated, by a quantitative method based on the use of CD8 alpha chimeras, the molecular determinants of Cavalpha2.1 that are responsible for the retention, in the absence of auxiliary subunits, of P/Q calcium channels in the ER (referred to here as 'ER retention'). This study demonstrates that the I-II loop of Cavalpha2.1 contains multiple ER-retention determinants beside the beta subunit association domain. In addition, the I-II loop is not the sole domain of calcium channel retention as two regions identified for their ability to interact with the I-II loop, the N- and C-termini of Cavalpha2.1, also produce ER retention. It is also not an obligatory determinant as, similarly to low-threshold calcium channels, the I-II loop of Cavalpha1.1 does not produce ER retention in COS7 cells. The data presented here suggests that ER retention is suppressed by sequential molecular events that include: (i). a correct folding of Cavalpha in order to mask several internal ER-retention determinants and (ii). the association of other proteins, including the Cavbeta subunit, to suppress the remaining ER-retention determinants.

Trafficking of L-type calcium channels mediated by the postsynaptic scaffolding protein AKAP79.

Accurate calcium signaling requires spatial and temporal coordination of voltage-gated calcium channels (VGCCs) and a variety of signal transduction proteins. Accordingly, regulation of L-type VGCCs involves the assembly of complexes that include the channel subunits, protein kinase A (PKA), protein kinase A anchoring proteins (AKAPs), and beta2-adrenergic receptors, although the molecular details underlying these interactions remain enigmatic. We show here, by combining extracellular epitope splicing into the channel pore-forming subunit and immunoassays with whole cell and single channel electrophysiological recordings, that AKAP79 directly regulates cell surface expression of L-type calcium channels independently of PKA. This regulation involves a short polyproline sequence contained specifically within the II-III cytoplasmic loop of the channel. Thus we propose a novel mechanism whereby AKAP79 and L-type VGCCs function as components of a biosynthetic mechanism that favors membrane incorporation of organized molecular complexes in a manner that is independent of PKA phosphorylation events.

The interaction between the I-II loop and the III-IV loop of Cav2.1 contributes to voltage-dependent inactivation in a beta -dependent manner.

We have investigated the molecular mechanisms whereby the I-II loop controls voltage-dependent inactivation in P/Q calcium channels. We demonstrate that the I-II loop is localized in a central position to control calcium channel activity through the interaction with several cytoplasmic sequences; including the III-IV loop. Several experiments reveal the crucial role of the interaction between the I-II loop and the III-IV loop in channel inactivation. First, point mutations of two amino acid residues of the I-II loop of Ca(v)2.1 (Arg-387 or Glu-388) facilitate voltage-dependent inactivation. Second, overexpression of the III-IV loop, or injection of a peptide derived from this loop, produces a similar inactivation behavior than the mutated channels. Third, the III-IV peptide has no effect on channels mutated in the I-II loop. Thus, both point mutations and overexpression of the III-IV loop appear to act similarly on inactivation, by competing off the native interaction between the I-II and the III-IV loops of Ca(v)2.1. As they are known to affect inactivation, we also analyzed the effects of beta subunits on these interactions. In experiments in which the beta(4) subunit is co-expressed, the III-IV peptide is no longer able to regulate channel inactivation. We conclude that (i) the contribution of the I-II loop to inactivation is partly mediated by an interaction with the III-IV loop and (ii) the beta subunits partially control inactivation by modifying this interaction. These data provide novel insights into the mechanisms whereby the beta subunit, the I-II loop, and the III-IV loop altogether can contribute to regulate inactivation in high voltage-activated calcium channels.