Inducing prion protein shedding as a neuroprotective and regenerative approach in pathological conditions of the brain:from theory to facts

In the last decades,the role of the prion protein(PrP) in neurodegenerative diseases has been intensively investigated,initially in prion diseases of humans(e.g., Creutzfeldt-J akob disease) and animals(e.g.,scrapie in sheep,chronic wasting disease in deer and elk,or "mad cow disease" in cattle).Templated misfolding of physiological cellular prion protein(PrPC) into an aggregation-prone isoform(termed PrP "Scrapie"(PrPSc)),self-re plication and spreading of the latter inside the brain and to peripheral tissues,and the associated formation of infectious proteopathic seeds(termed "prions")are among the essential pathogenic mechanisms underlying this group of fatal and transmissible spongiform encephalopathies.Late r,key roles of the correctly folded PrPCwere identified in more common human brain diseases(such as Alzheimer s disease or Parkinson’s disease) associated with the misfolding and/or accumulation of other proteins(such as amyloid-β,tau or α-synuclein,respectively).PrPChas also been linked with n euro protective and regenerative functions,for instance in hypoxic/ischemic conditions such as stroke.However,despite a mixed "bouquet" of suggested functions,our understanding of pathological and,especially,physiological roles played by PrPCin the brain and beyond is ce rtainly incomplete.Interactions with various other proteins at the cell surfa ce or within intracellular compartments may account for the functional diversity linked with PrPC.Moreover,conserved endogenous proteolytic processing of PrPCgenerates seve ral defined PrPCfragments,possibly holding intrinsic functions in physiological and pathological conditions,thus making the "true and complete biology" of this protein more complicated to be elucidated.Here,we focus on one of those released PrPCfragments,namely shed PrP(sPrP),generated by a membrane-proximate ADAM10-mediated cleavage event at the cell surfa ce.Similar to other soluble PrP fragments(such as the N1 fragment representing PrP’s released N-terminal tail upon the major α-cleavage event)or expe rimentally employed recombinant PrP,sPrP is being suggested to act n euro protective in Alzheimer’s disease and other protein misfolding diseases.Seve ral lines of evidence on extracellular PrPC(fragments) suggest that induction of PrPCrelease co uld be a future therapeutic option in various brain disorders.Our recent identification of a substrate-specific approach to stimulate the shedding by ADAM 10,based on ligands binding to cell surface PrPC,may further set the stage for research into this direction.

Recent advances in plant membrane-bound transcription factor research: Emphasis on intracellular movement

Transcription factors constitute numerous signal transduction networks and play a central role in gene expression regulation. Recent studies have shown that a limited portion of transcription factors are anchored in the cellular membrane, storing as dormant forms. Upon exposure to environmental and developmental cues, these transcription factors are released from the membrane and translocated to the nucleus, where they regulate associated target genes. As this process skips both transcriptional and translational regulations, it guarantees prompt response to external and internal signals. Membrane- bound transcription factors （MTFs） undergo several unique steps that are not involved in the action of canonical nuclear transcription factors： proteolytic processing and intracellular movement. Recently, alternative splicing has also emerged as a mechanism to liberate MTFs from the cellular membranes, establishing an additional activation scheme independent of proteolytic processing. Multiple layers of MTF regulation add complexity to transcriptional regulatory scheme and ensure elaborate action of MTFs. In this review, we provide an overview of recent findings on MTFs in plants and highlight the molecular mechanisms underlying MTF liberation from cellular membranes with an emphasis on intracellular movement.

Cleavage of the Carboxyl-Terminus of LEACS2, a Tomato 1-Aminocyclopropane-l-Carboxylic Acid Synthase Isomer, by a 64-kDa Tomato Metalloprotease Produces a Truncated but Active Enzyme

1-Aminocyclopropane-1-carboxylic acid （ACC） synthase （ACS） is the principal enzyme in phytohormone ethylene biosynthesis. Previous studies have shown that the hypervariable C-terminus of ACS is proteolytically processed in vivo. However, the protease responsible for this has not yet been identified. In the present study, we investigated the processing of the 55-kDa full-length tomato ACS （LeACS2） into 52-, 50- and 49-kDa truncated isoforms in ripening tomato （Lycopersicon esculentum Mill. cv. Cooperation 903） fruit using the sodium dodecyl sulfate-boiling method. Meanwhile, an LeACS2-processing protease was purified via multi-step column chromatography from tomato fruit. Subsequent biochemical analysis of the 64-kDa purified protease revealed that it is a metalloprotease active at multiple cleavage sites within the hypervariable C-terminus of LeACS2. N-terminal sequencing and matrix-assisted laser desorption/ionization time-of-flight analysis indicated that the LeACS2-processing metalloprotease cleaves at the C-terminal sites Lys^438, Glu^447, Lys^448, Asn^456, Ser^460, Ser^462, Lys^463, and Leu^474, but does not cleave the N- terminus of LeACS2. Four C-terminus-deleted （26-50 amino acids） LeACS2 fusion proteins were overproduced and subjected to proteolysis by this metalloprotease to identify the multiple cleavage sites located on the N-terminal side of the phosphorylation site Ser^460. The results indisputably confirmed the presence of cleavage sites within the region between the α-helix domain （H14） and Ser^460 for this metalloprotease. Furthermore, the resulting C-terminally truncated LeACS2 isoforms were active enzymatically. Because this protease could produce LeACS2 isoforms in vitro similar to those detected in vivo, it is proposed that this metalloprotease may be involved in the proteolysis of LeACS2 in vivo.