Patent Publication Number: US-2022220164-A1

Title: A genetically encoded protein synthesis inhibitor

Description:
The present invention relates to an expression system for a genetically encoded protein synthesis inhibitor containing RNA N-glycosidase activity split into two components. The expression system can be combined with genetic targeting systems to achieve cell- and/or tissue-type-specific and/or temporally-specific control of protein synthesis in a host, particularly in a mammalian host. 
     BACKGROUND 
     Proteins are the main functional unit within cells that control biological processes. In all cells, protein synthesis is used to respond to extra- and/or intracellular cues to remodel cellular function. In the brain protein synthesis is crucial for some forms and specific temporal phases of synaptic plasticity 1 . Previous studies probing the role of protein synthesis have used chemical inhibitors, often common antibiotics, which are effective but lack functional and cell type specificity. 
     Ribosome-inactivating proteins (RIPs) are known genetically encoded protein synthesis inhibitors 2,3 . They form a family of well-characterized toxins, which specifically and irreversibly inhibit protein synthesis in eukaryotic cells by enzymatically altering the 28S rRNA of the large 60S ribosomal subunit leading to the abolition of the interaction between the large 60S ribosomal subunit and translation elongation factor 2. RIPs act at low doses because their catalytic activities allow complete inactivation of ribosomes and protein synthesis at a less-than-equimolar ratio to their substrate. RIPs display rRNA N-glycosidase activity (EC 3.2.2.22) and depurinate 28S rRNA by cleaving the bond between adenine and ribose in the sarcin-ricin loop of the molecule, thus preventing regroupment of translation elongation factors in subsequent protein synthesis. 
     A large number of RIPs have been isolated from various species of plants and bacteria with varying degrees of toxicity. These RIPs have been subdivided into several categories. Class 1 RIPs are monomeric proteins of approximately 30 kDa which possess RNA N-glycosidase enzymatic activity. In contrast, class 2 RIPs are composed of an A-chain with RNA N-glycosidase activity wherein said A-chain is associated to one or several B-chains of approximately 35 kDa. The B-chain is a lectin-like peptide that has strong affinity for sugar moieties displayed in the surface of cells and helps promote translocation through the plasma membrane. Class 3 RIPs (or sometimes called atypical class 1 RIPs) are monomeric proteins and synthesized as an inactive precursor (ProRIP) that is proteolytically converted into a functional toxin which is a complex of two subunits each of which alone lacks RNA N-glycosidase activity. 
     The use of RIPs as protein synthesis inhibitors for molecular-biological, physiological or pharmaceutical studies, however, has been hampered, since their administration to eukaryotic cells leads to an irreversible inhibition of protein synthesis. 
     Thus, it is an object of the present invention to overcome these disadvantages and provide a genetically encoded protein synthesis inhibitor which allows temporal and/or spatial control of protein synthesis in a eukaryotic host. 
     SUMMARY OF THE INVENTION 
     The present inventors have developed an expression system for a genetically encoded protein synthesis inhibitor, which is a modified RIP. The experiments were carried out using an atypical class 1 RIP from maize ( Zea mays ). Direct expression of the inactive precursor (ProRIP) encoded by a single nucleic acid sequence in cultured hippocampal neurons resulted in a complete shutdown of protein synthesis in a majority of the cells. Expression of the RIP α-chain and the RIP β-chain encoded by separate nucleic acid sequences, however, provided spatial and temporal control of protein synthesis inhibition. Surprisingly, the cells even exhibited functional recovery of protein synthesis capabilities. 
     A first aspect of the present invention relates to an expression system for a genetically encoded protein synthesis inhibitor having RNA N-glycosidase activity, comprising
         (a) a first nucleic acid sequence encoding a first component of the protein synthesis inhibitor in operative linkage with a first expression control sequence,   and   (b) a second nucleic acid sequence encoding a second component of the protein synthesis inhibitor in operative linkage with a second expression control sequence,       

     wherein the expression system is adapted for expressing the first nucleic acid sequence and the second nucleic acid sequence separate from each other, wherein the first component and the second component together form a complex having RNA N-glycosidase activity, and wherein the first component alone and the second component alone lack RNA N-glycosidase activity. 
     A further aspect of the present invention relates to a host, e.g. a cell or a non-human organism, comprising an expression system as described above. 
     Still a further aspect of the present invention is a method of inhibiting protein synthesis in a host, comprising the steps: 
     (i) introducing an expression system as described above into the host, and 
     (ii) expressing in the host a functional protein synthesis inhibitor encoded by the expression system whereby protein synthesis is inhibited in the host or in a part thereof. 
     Still a further aspect of the present invention is a kit for providing expression of a genetically encoded protein synthesis inhibitor having N-glycosidase activity in a host, comprising: 
     (a) a first nucleic acid sequence encoding a first component of the protein synthesis inhibitor in operative linkage with a first inducible expression control sequence, 
     (b) a second nucleic acid molecule encoding a second component of the protein synthesis inhibitor in operative linkage with a second inducible expression control sequence, wherein the first nucleic acid sequence and the second nucleic acid sequence are provided for expression separate from each other, 
     wherein the first component and the second component together form a complex having RNA N-glycosidase activity, and 
     wherein the first component alone and the second component alone lack RNA N-glycosidase activity, 
     (c) optionally a cell- and/or tissue-type specific expression control sequence in operative linkage with a third nucleic acid sequence responsible for restricting expression of (a) and (b) to a targeted cell and/or tissue, and 
     (d) optionally means for inducing expression of the first nucleic acid sequence (a) and expression of the second nucleic acid sequence (b). 
     The expression system or host as described above can be used in basic research, e.g. for the cell- and/or tissue-type specific inhibition of protein synthesis and/or for reversible inhibition of protein synthesis in a eukaryotic host, particularly in a mammalian host. Further, the expression system and the host can be used in screening procedures or in medicine, e.g. in the treatment of cancer or in the treatment of neurological disorders. 
     DETAILED DESCRIPTION 
     The present invention particularly relates to the following items, which are part of the description:
         1. An expression system for a genetically encoded protein synthesis inhibitor having RNA N-glycosidase activity, comprising
           (a) a first nucleic acid sequence encoding a first component of the protein synthesis inhibitor in operative linkage with a first expression control sequence, and   (b) a second nucleic acid sequence encoding a second component of the protein synthesis inhibitor in operative linkage with a second expression control sequence,
               wherein the expression system is adapted for expressing the first nucleic acid sequence and the second nucleic acid sequence separate from each other,   wherein the first component and the second component together form a complex having RNA N-glycosidase activity, and   wherein the first component alone and the second component alone lack RNA N-glycosidase activity.   
               
           2. The system of item 1 wherein the first component and the second component form a functional enzyme or a part thereof wherein the functional enzyme exerts its RNA N-glycosidase activity by depurinating an adenine on the sarcin-ricin loop of the 28S rRNA located on the large 60S subunit of a eukaryotic ribosome.   3. The system of item 1 or 2,
           wherein the first component comprises an α-chain from a class 3 ribosome-inactivating protein (RIP), particularly an α-chain from a Panicoideae RIP, more particularly a maize RIP α-chain, or a functional variant thereof,   and/or   wherein the second component comprises a β-chain from a class 3 ribosome-inactivating protein (RIP), particularly a β-chain from a Panicoideae RIP, more particularly a maize RIP β-chain, or a functional variant thereof.   
           4. The system of any one of items 1-3,
           wherein the first nucleic acid sequence and the second nucleic acid sequence have different transcriptional starting points.   
           5. The system of any one of items 1-4,
           wherein (i) the first expression control sequence and the second expression control sequence are separate from each other, or   wherein (ii) the first expression control sequence and the second expression control sequence are combined in a single bi-directional expression control sequence.   
           6. The system of any one of items 1-5,
           wherein the first component comprises an amino acid sequence selected from:   (a) the amino acid sequence shown in SEQ ID NO. 1;   (b) an amino acid sequence having an identity of at least 50%, 60%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% to the amino acid sequence of (a); and   (c) an amino acid sequence which is a functional variant of the amino acid sequence of (a) and/or (b).   
           7. The system of any one of items 1-6,
           wherein the second component comprises an amino acid sequence selected from:   (a) the amino acid sequence shown in SEQ ID NO. 2;   (b) an amino acid sequence having an identity of at least 50%, 60%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% to the amino acid sequence of (a); and   (c) an amino acid sequence which is a functional variant of the amino acid sequence of (a) and/or (b).   
           8. The system of any one of items 1-7,
           wherein the first expression control sequence and/or the second expression control sequence are inducible expression control sequences.   
           9. The system of item 8,
           wherein the first and/or second expression control sequences are inducible by temperature shift, by light, by addition or removal of one or more chemical substances (e.g. of synthetic or biological origin) or by any combination thereof.   
           10. The system of item 8 or 9,
           wherein the inducible expression control sequences are chosen from tetracycline-controlled expression control sequences, hormone-controlled expression control sequences, light-controlled expression control sequences, and/or heat-shock expression control sequences.   
           11. The system of any one of items 1-10, further comprising:
           (c) a third nucleic acid sequence encoding a regulator, e.g. an activator for the first and/or second expression control sequence in operative linkage with a third expression control sequence.   
           12. The system of item 11,
           wherein the third expression control sequence is a cell- and/or tissue-type specific expression control sequence, in particular a cell- and/or tissue-type specific expression control sequence inducible by an external stimulus.   
           13. The system of any one of items 11-12,
           wherein the third expression control sequence is an expression control sequence specific for cells from brain tissue or specific for cells forming pre-cancerous or cancerous tissue.   
           14. The system of any one of items 1-13,
           which is inducible by a site-specific recombinase, e.g. a Cre recombinase.   
           15. The system of any one of items 1-14,
           which is a viral expression system.   
           16. The system of any one of items 1-14,
           which is a non-viral expression system, e.g. encoded on at least one plasmid vector.   
           17. The system of item 15 or 16,
           which is adapted for direct extrachromosomal expression and/or for chromosomal integration and expression.   
           18. The system of any one of items 1-17,
           which provides spatial and/or temporal expression control, particularly spatial and temporal expression control for the protein synthesis inhibitor.   
           19. A host comprising an expression system of any one of items 1-18.   20. The host of item 19,
           which is transfected, transformed or transduced with the expression system.   
           21. The host of item 19 or 20,
           which is an isolated cell, a cell preparation, a cell culture, an organoid, a tissue or an organ, particularly of mammalian origin, and more particularly of human origin.   
           22. The host of item 19 or 20,
           which is a non-human organism, particularly of mammalian origin.   
           23. The host of any one of items 19-22,
           wherein the expression system provides spatial and/or temporal expression control, particularly spatial and temporal expression control for the protein synthesis inhibitor in the host.   
           24. The host of any one of items 19-23,
           wherein the expression system provides a reversible inhibition of protein synthesis in the host.   
           25. A method of inhibiting protein synthesis in a host, comprising the steps:
           (i) introducing an expression system of any one of items 1-17 into the host, and   (ii) expressing in the host a functional protein synthesis inhibitor encoded by the expression system whereby protein synthesis is inhibited in the host or in a part thereof.   
           26. The method of item 25,
           wherein expression of the functional protein synthesis inhibitor is under spatial and/or temporal control, particularly under spatial and temporal control.   
           27. The method of item 25 or 26,
           wherein expression of the functional protein synthesis inhibitor is a cell- and/or tissue-specific expression.   
           28. The method of any one of items 25-27,
           wherein expression of the functional protein synthesis inhibitor is reversible.   
           29. The method of any one of items 25-28,
           wherein expression of the functional protein synthesis inhibitor is under conditions providing a subsequent recovery of protein synthesis in the host.   
           30. An expression system of any one of items 1-18 for use in medicine.   31. A host of any one of items 19-24 for use in medicine.   32. The expression system of item 30 or the host of item 31 for the use in the treatment of cancer or in the treatment of neurological disorders, e.g. for the treatment of Fragile X syndrome.   33. Use of an expression system of any one of items 1-18, or a host of any one of items 18-23 for a cell- and/or tissue-type specific inhibition of protein synthesis and/or for a reversible inhibition of protein synthesis in a eukaryotic host, particularly in a mammalian host.   34. Use of an expression system of any one of items 1-18, or a host of any one of items 18-23 for a screening procedure.   35. The use of item 31 for drug screening, e.g. for screening for agents against neurological disorders or for agents against cancer.   36. A kit for providing expression of a genetically encoded protein synthesis inhibitor having N-glycosidase activity in a host, comprising:
           (a) a first nucleic acid sequence encoding a first component of the protein synthesis inhibitor in operative linkage with a first inducible expression control sequence,   (b) a second nucleic acid molecule encoding a second component of the protein synthesis inhibitor in operative linkage with a second inducible expression control sequence,
               wherein the first nucleic acid sequence and the second nucleic acid sequence are provided for expression separate from each other,   wherein the first component and the second component together form a complex having RNA N-glycosidase activity, and   wherein the first component alone and the second component alone lack RNA N-glycosidase activity,   
               (c) optionally a cell- and/or tissue-type specific expression control sequence in operative linkage with a third nucleic acid sequence responsible for restricting expression of (a) and (b) to a targeted cell and/or tissue, and   (d) optionally means for inducing expression of the first nucleic acid sequence (a) and expression of the second nucleic acid sequence (b).   
               

     The present invention relates to a system for expressing a genetically encoded protein synthesis inhibitor (gePSI) having RNA N-glycosidase activity in a host, particularly in a eukaryotic host. The expression system can be any suitable type of eukaryotic expression system, e.g. a viral expression system or a non-viral expression system, e.g. a plasmid-based expression system. The expression system may be adapted for direct extrachromosomal expression and/or for chromosomal integration and chromosomal expression. The expression system may be suitable for providing expression in any type of eukaryotic host, e.g. an animal host such as a vertebrate host, particularly a mammalian host, or an insect host, a plant host or a fungal host, preferably a mammalian host. 
     The expression system comprises a plurality of nucleic acid sequences, which may be single- or double stranded nucleic acid, e.g. DNA and/or RNA sequences. In particular, the expression system comprises a first nucleic acid sequence encoding a first component of a protein synthesis inhibitor in operative linkage with a first expression control sequence, a second nucleic acid sequence encoding a second component of a protein synthesis inhibitor in operative linkage with a second expression control sequence, and optionally a third nucleic acid sequence encoding a regulator, e.g. an activator, for the first and/or second expression control sequence, particularly an activator for both the first and second expression control sequence in operative linkage with a third expression control sequence. The codon usage in the protein-coding sequences may be adapted, e.g. optimized for expression in the respective host. The expression control sequences typically are adapted for expression in the respective host and may comprise promoter and transcription regulation sequence elements, particularly inducible transcription regulation elements and/or cell- and/or tissue-type-specific transcription regulation elements. In addition to the above sequences, the expression system may comprise further sequences, e.g. propagation, replication and/or integration sequences and/or selection marker sequences. 
     The nucleic acid sequences constituting the expression system may be present on a single expression vehicle, e.g. a viral or non-viral vector, or on a plurality of two or more expression vehicles, e.g. viral or non-viral vectors. For example, the expression system may comprise at least two separate expression vehicles, wherein the first nucleic acid sequence and the second nucleic acid sequence are located on a first vehicle, e.g. a first plasmid, and the third nucleic acid sequence is located on a second vehicle, e.g. a second plasmid. 
     The expression system is adapted for separate expression of the first nucleic acid sequence and the second nucleic acid sequence. This results in the direct production of two separate expression products, i.e. without any previous proteolytic cleavage. Thus, the direct expression products, i.e. the polypeptides encoded by the first or second nucleic acid sequence, respectively, are not covalently linked to each other. The first polypeptide is a first component of a protein synthesis inhibitor and the second polypeptide is a second component of a protein synthesis inhibitor, wherein the first component and the second component together form a complex, in particular a dimeric complex having RNA N-glycosidase activity. The first component alone and the second component alone, however, are devoid of RNA N-glycosidase activity, e.g. assessed by measuring the translational activity of transfected cells. Neuronal cells transfected with either the first or second component show normal levels of translation as shown by metabolic labeling (see Example 1,  FIG. 2 b - e   ). 
     In certain embodiments, the first nucleic acid sequence and/or the second nucleic acid sequence may comprise sequence portions encoding heterologous protein domains resulting in the expression of a fusion polypeptide comprising the first component of the protein synthesis inhibitor fused to a heterologous polypeptide domain and/or the second component of the protein synthesis inhibitor fused to a heterologous polypeptide domain. Heterologous polypeptide domains, if present, may be fused to the N- and/or C-terminus of the first component or the second component, respectively. 
     In certain embodiments, the first nucleic acid sequence and the second nucleic acid sequence have different transcriptional starting points. In certain embodiments, the first expression control sequence and the second expression control sequence are separate from each other, i.e. the first expression control sequence does not control the expression of the second nucleic acid molecule and the second expression control sequence does not control the expression of the first nucleic acid sequence. In certain embodiments, the first expression control sequence and the second expression control sequence may be combined in a single bi-directional expression control sequence. 
     The first component and the second component of the protein synthesis inhibitor may form a functional enzyme or a functional part thereof, wherein the functional enzyme or functional part thereof has RNA N-glycosidase activity resulting in an inhibition of eukaryotic protein synthesis. In certain embodiments, the functional enzyme exerts its RNA N-glycosidase activity by depurinating an adenine in the sarcin-ricin loop of the 28S rRNA located on the large 60S subunit of a eukaryotic ribosome. In certain embodiments, the inhibition of the protein synthesis is reversible, i.e. the protein synthesis is only inhibited for a certain period of time, and subsequently starts again. 
     In certain embodiments, the protein synthesis inhibitor of the present invention is a polypeptide complex comprising a first component and a second component non-covalently bound to each other wherein the first and second component together have RNA N-glycosidase activity and wherein the individual components lack RNA N-glycosidase activity. In particular, some of the amino acids constituting the catalytically active center of the protein synthesis inhibitor are present on the first component and some of the amino acids constituting the catalytically active center of the protein synthesis inhibitor are present on the second component. 
     The first component and the second component may be derived from class 1 RIPs, class 2 RIPs or class 3 RIPs. In certain embodiments, the protein synthesis inhibitor is a modified class 1 RIP. Native class 1 RIPs are monomeric proteins. According to the present invention, a class 1 RIP is split into two components wherein the first component alone and the second component alone lack activity, and wherein the first component and the second component together form a non-covalent complex having RNA N-glycosidase activity. In certain embodiments, the protein synthesis inhibitor of the present invention is a modified class 2 RIP. Native class 2 RIPs can consist of multiple subunits, wherein one subunit (referred to as chain A) is the subunit with RNA N-glycosidase activity (similar to a class 1 RIP), and the other chain(s) (referred to as chain(s) B) are subunit(s) that are associated with chain A and promote translocation through the plasma membrane. According to the present invention, the chain A of a class 2 RIP is split into two components wherein the first component alone and the second component alone lack RNA N-glycosidase activity and wherein the first component and the second component together form a non-covalent complex having RNA N-glycosidase activity. 
     In particular embodiments, the protein synthesis inhibitor of the present invention is a modified class 3 RIP. Native class 3 RIPs (or sometimes called atypical class 1 RIPs) are monomeric proteins encoded by a single nucleic acid sequence. They are synthesized as an inactive precursor that is proteolytically converted into a functional toxin which is a complex of two subunits (referred to as α-chain and β-chain) each of which alone lacks RNA N-glycosidase activity. According to the present invention, the nucleic acid sequence encoding the class 3 RIP is split into two separate sequences which are separately expressed, e.g. by operative linkage to separate expression control sequences. Examples of such class 3 RIPs are, e.g., maize ( Zea mays ) or other members of the subfamily Panicoideae such as  Z. mays parviglumis, Z. luxurians, Z. mays mexicana, T. dactyloides , or  S. bicolor  as described in the literature 5 . Thus, in certain embodiments, the first component comprises an α-chain from a Panicoideae RIP, particularly a maize RIP α-chain, or a functional variant thereof, and/or the second component comprises a β-chain from a Panicoideae RIP, particularly a maize RIP β-chain, or a functional variant thereof. The term “functional variant” in this context relates to modified α- and β-chains which have RNA N-glycosidase activity (e.g. assessed by measuring the translational activity of transfected cells with metabolic labeling) wherein non-essential amino acids, i.e. amino acids or amino acid stretches which are not essential for the catalytic activity are deleted and/or substituted, and/or wherein heterologous amino acids or amino acid stretches are inserted. In certain embodiments, a functional variant has at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% RNA N-glycosidase activity of a protein synthesis inhibitor which is a complex of the α-chain shown in SEQ ID. NO. 1 and of the β-chain shown in SEQ ID. NO. 2. 
     In certain embodiments, the first component comprises an amino acid sequence selected from:
         (d) the amino acid sequence shown in SEQ ID NO. 1;   (e) an amino acid sequence having an identity of at least 50%, 60%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% to the amino acid sequence of (a); and   (f) an amino acid sequence which is a functional variant of the amino acid sequence of (a) and/or (b).       

     In certain embodiments, the second component comprises an amino acid sequence selected from:
         (d) the amino acid sequence shown in SEQ ID NO. 2;   (e) an amino acid sequence having an identity of at least 50%, 60%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% to the amino acid sequence of (a); and   (f) an amino acid sequence which is a functional variant of the amino acid sequence of (a) and/or (b).       

     SEQ ID NO. 1 shows the amino acid sequence of a maize RIP α-chain as used by the present inventors. SEQ ID NO. 2 shows the amino acid sequence of a maize RIP β-chain as used by the present inventors. 
     Amino acid identity may be determined by standard methods, e.g. the BLAST algorithm as described in the literature (Altschul et al., J. Mol. Biol. 215, 1990, 403-410). 
     The expression system of the invention provides expression of a protein synthesis inhibitor in a host, particularly in a eukaryotic host, e.g. an animal host such as a vertebrate, particularly mammalian, or insect host, a plant host or a fungal host. Thus, a further aspect of the invention relates to a host comprising the expression system as described above. Introduction of the expression system into the host may be carried out by conventional methods of nucleic acid transfer. For example, the host may be transfected, transformed or transduced with the expression system which may comprise a single expression vehicle or a plurality of different expression vehicles. The nucleic acid sequences containing the constituents of the expression system may be present as extrachromosomal elements and/or may be integrated into the host chromosome. 
     In certain embodiments, the host may be an isolated cell, a cell preparation, a cell culture, an organoid, a tissue or an organ, particularly of mammalian origin, and more particularly of human origin. In certain embodiments, the host may be a non-human organism, particularly a mammalian organism, e.g. a rodent such as a mouse, rat, hamster etc., an insect such as  Drosophila , a fish such as zebrafish. 
     In certain embodiments, the expression system provides spatial and/or temporal expression control, particularly spatial and temporal expression control for the genetically encoded protein synthesis inhibitor in a host. The term “spatial control” means that expression of the protein synthesis inhibitor takes place in specific parts, e.g. cell types, tissue types and/or organs of the host, whereas in other parts of the host substantially no expression takes place. Spatial control can particularly be achieved by a cell- or tissue-type specific expression control sequence, e.g. promotor. Preferably, at least one of the expression control sequences contained in an expression system according to the invention is cell-type specific. Most preferably, the third expression control sequence is a cell-type specific expression control sequence, e.g. promoter. Alternatively or additionally, the first and/or second expression control sequence may be cell-type specific. The term “temporal control” means that expression of the protein synthesis inhibitor in the host takes place during a certain, e.g. predetermined time interval, whereas outside this time interval substantially no expression takes place. Temporal control can particularly be achieved by an expression control sequence inducible with a chemical substance that is applied externally. Preferably, at least one of the expression control sequences contained in an expression system according to the invention is inducible. For example, the first and/or second expression control sequence may be inducible. Preferably, both types of expression control, spatial and temporal, are combined. 
     Spatial and temporal specificity can be provided, for example, by means of a first expression control sequence (e.g. promoter) which is cell-type specific in combination with a second expression control sequence (e.g. promoter) which is inducible. 
     According to a preferred embodiment, spatial and temporal specificity is provided by means of first and second expression control sequences (e.g. promotors) which are inducible, for example by external stimuli (e.g. doxycycline) and the product of a third cell-type specific expression element (i.e. a third nucleic acid sequence encoding a regulator, e.g. an activator protein). The third expression element consists of a cell-type specific expression control sequence (e.g. promoter) driving the production of the activator protein. 
     The separation of the protein synthesis inhibitor into several individually inactive components and the subdivision of the expression system by using separate expression control sequences, each of which can be cell type-specific and preferably inducible, minimizes the probability of an undesired activity of the protein synthesis inhibitor occurring. Especially when using inducible expression control sequences, the split approach counters the intrinsic leakiness common to inducible expression systems. 
     A functional protein synthesis inhibitor can only be formed if the first and second nucleic acid sequences and, if present, the third nucleic acid sequence are actually expressed. This can be precisely regulated by spatial and temporal control of the different expression control sequences involved. 
     In certain embodiments, a host is selected which comprises at least two different types of cells or tissues, thereby allowing a spatial expression control of the protein synthesis inhibitor. 
     In certain embodiments, the first expression control sequence and/or the second expression control sequence are inducible expression control sequences. The term “inducible expression control sequence” particularly means an expression control sequence, which can be induced or activated by external stimuli. For example, this term includes expression control sequences which are inducible by a temperature shift, e.g. increase of temperature, by light, e.g. by radiating the host with light of a specific wavelength, by addition or removal of one or more chemical substances, e.g. of synthetic or biological origin, or by any combination thereof. Inducible expression control sequences are well known in the art. For example, the inducible expression control sequences may be chosen from tetracycline-controlled expression control sequences, hormone-controlled expression control sequences, e.g. an estrogen-receptor fusion system 21 , light-controlled expression control sequences, and/or heat-shock expression control sequences 22 . A specific embodiment is the commercially available tetracycline-controlled expression system TetOn3G or any variant thereof, e.g. a light-controlled system such as PA-TetOn 23 . 
     In certain embodiments, the expression system comprises a further genetic element, i.e. a third nucleic acid sequence encoding a regulator, e.g. an activator, for the first and/or second expression control sequence in operative linkage with a third expression control sequence. In these embodiments, the third expression control sequence may be a cell- and/or tissue-type specific expression control sequence, e.g. an expression control sequence specific for cells from brain, or for cells from organs or glands such as heart, liver, lung, kidney, stomach, duodenum, colon, pancreas, thyroid, ovary, or cervix or an expression control sequence specific for cells forming pre-cancerous or cancerous tissue. 
     In still further embodiments, the expression system is inducible by a site-specific recombinase, e.g. a Cre recombinase, and comprises recognition sites for said recombinase providing an induction of gene expression for the first, second and/or third nucleic acid sequence in the presence of a suitable recombinase. For example, the expression system may be provided with FLEX-switches 24  to make it compatible with a Cre recombinase containing host. 
     In certain embodiments, the expression system provides a reversible inhibition of protein synthesis in the host, i.e. the expression system may be controlled in a way that expression of the first and the second nucleic acid sequence encoding the first and the second component of the protein synthesis inhibitor, respectively, may be turned on and off again when providing appropriate external stimuli to the first and/or second expression control sequence. External stimuli can include a chemical substance (e.g. doxycycline) that is applied externally to provide temporal control. In certain embodiments, the expression in the host is turned off and then turned on for a predetermined period of time, e.g. about at least about 1 h to about 1 week, e.g. about 2 h to about 48 h, depending from the effect which should be achieved. In certain embodiments, the inhibition of protein synthesis occurs under conditions providing a subsequent recovery of protein synthesis in the host, i.e. after turning off the expression system the host can recover and resume protein synthesis. 
     In a particularly preferred embodiment, the expression system is regulated by two components: a further genetic element encoding a regulator for the first and/or second expression control sequence under control of a cell- and/or tissue-type specific expression control sequence (to provide spatial control) and a chemical substance (e.g. doxycycline) that is applied externally (to provide temporal control). The combination of a split-inhibitor with this particular inducible setup has the following advantages: 
     i) The cell-type specific expression control sequence, e.g. promoter, (spatial control) is only required to be specific for the targeted cell-type. Strong expression or inducibility are not required. 
     ii) The system allows for a strong inducibility and expression of the inhibitor, regardless of the targeted cell-type. 
     iii) A basal activity of the inducible promoter in the non-induced state can lead to unwanted intoxication, which can become problematic for highly effective/toxic proteins (e.g. RIPs). The separate expression of the RIP chains (α- &amp; β-chains) in the present system solves this issue. Leakage will now produce an inactive part of the RIP (α- or β-chain), that needs to find a suitable (also leaked) counterpart. This ensures a tightly regulated expression, where the protein synthesis in the target cell-population is completely unaffected until the external stimulus (e.g. doxycycline) is added. 
     Still a further aspect of the present invention relates to a method of inhibiting protein synthesis in a host, wherein the expression system as described above is introduced into the host, e.g. by transfection, transformation or transduction and the functional protein synthesis inhibitor encoded by the expression system, i.e. the non-covalent polypeptide complex comprising the first component and the second component as described above is expressed, whereby protein synthesis is inhibited in the host or in a part thereof. As indicated above, expression of the functional protein synthesis inhibitor is particularly under spatial and/or temporal control, or particularly under spatial and temporal control. Further, expression of the functional protein synthesis inhibitor may be a cell- and/or tissue-specific expression, i.e. expression of the functional protein synthesis inhibitor takes place only in a part, i.e. specific cells and/or tissues of the host. In certain embodiments, expression of the functional protein synthesis inhibitor is under conditions providing a recovery of protein synthesis after turning off expression of the protein synthesis inhibitor. 
     The expression system and the host of the present invention can be used in several fields, e.g. in medicine, basic research, medicine or in drug screening. 
     The expression system of the invention is useful for applications in basic research allowing cell- and/or tissue-type-specific inhibition of protein synthesis and providing targeted insight into function control and maintenance in complex tissues, organelles, or cell organoid cultures. This can be readily achieved with the expression system of the present invention, but is impossible with known chemical inhibitors (e.g. anisomycin, cycloheximide). They are potent and work fast, but are unspecific to the cell-types they inhibit and also have off-target effects 25 . This poses limitations for their use in tissue/organelles that comprise many different cell-types. For example, new cellular subtypes have been identified in brain regions 26-28  and other regions of the body 29 , highlighting the heterogeneity of the cellular network. Inhibiting protein synthesis in one specific cell-type, leaving the remaining network intact allows targeted insight into the necessity of proteome remodeling in a particular cell type. This can be readily achieved with the use of the invention, but is impossible with universal chemical inhibitors that cannot be spatially controlled. 
     The expression system of the invention is also useful for applications in medicine. A hallmark of cancer cells is a dysregulation of the transcriptional machinery evoked by an overexpression of transcription factors and their constitutive activity 30 . This characteristic can be exploited to specifically target the protein synthesis inhibitor of the invention to cancer cells by making its expression dependent on these cancer-specific and/or cancer-enriched genes 31  and/or transcription factors (e.g. promoters for survivin 32 , probasin 33,34 , or hTERT 35 ). The inhibitor can then be used to either ‘kill’ these cells via a prolonged protein synthesis blockade and/or weaken their metastatic activity and work in synergy with available drug treatments. A key to the success of these genetic methods is the precise targeting of the therapeutic gene to cancer cells with no/minor impact on the surrounding healthy tissue. Due to its small size and broad compatibility with genetic targeting tools, the expression system of the invention provides great potential in this regard. The delivery mechanism (e.g. virus) can be tailored to the specific cancer/tissue type and advanced controlled induction systems, e.g. light-controlled induction systems, can further increase specificity not only to the targeted cancer cells but to the region where the inhibitor is synthesized. This is in particular interesting for the treatment of cancer types where cancer cells can be irradiated, e.g. skin cancer. Another way to restrict expression of the inhibitor to cancerous cells, which can be applied in synergy with the above mentioned methods, is the use of specific codons in the sequence that are favored by cancer cells 36 . 
     In addition, a common drug-design principle is to distinguish normal and cancer cells via the presence of cancer specific proteins (e.g. receptors) on the cell surface. Using an inhibitor-mediated approach works, in contrast, from inside the cell as the specificity of the system is conferred by the specific expression profile of the cancer cells. This makes this tool more flexible/dynamic (resistant to mutations) compared to systems that only probe the surface, by detecting extracellular cues. 
     Further fields of application are neurological disorders, e.g. Fragile X syndrome (FXS) which is a prominent inherited form of intellectual disability (ID) and autism spectrum disorder (ASD), and has a strong behavioral impact on patients 37 . Due to its frequency and severity there is a great need for therapeutic approaches to help alleviate the accompanying symptoms. The FXS itself is caused by a mutation in the FMR1 gene. The trinucleotide CGG is repeated &gt;200 times within the 5′UTR (untranslated region) of the gene, silencing its expression 38 . As a result the gene product fragile X mental retardation 1 protein (FMRP) is not produced anymore. FMRP is an mRNA binding protein and plays a key role in gene expression regulating (usually repressing) the translation of many mRNAs involved in the development and maintenance of synaptic structures 38 . As a regulator FMRP has been shown to repress the translation of synaptic mRNAs (e.g. Shank1, Arc or PSD-95)  40-42 , but its absence also leads to a general increase in translation 43 . Without FMRP present an uncontrolled over-production of proteins brings along an imbalance that needs to be countered by therapeutic means. 
     There are different therapeutic strategies aiming at targets that are upstream of translational regulation 44-45 , but also efforts using chemical protein synthesis inhibitors to shift translation back into the physiological range 46 . Here the expression system of the invention poses a promising alternative/basis for the development of a genetic therapy. The chemical inhibitors lack cell-type specificity and as such our genetic approach would potentially allow for a more specific targeting to affected cells (e.g. neurons) with less side effects. The present invention provides another advantage as the protein-sequence can be modified to modulate the toxicity to dial in the level of inhibition that is needed to return to a normal translational homeostasis. In addition to FXS there is also emerging data about other ASDs that involve dysregulated, increased translation 47,48 . 
     Finally, the present invention provides a kit for providing expression of a genetically encoded protein synthesis inhibitor in a suitable host. 
     In the following, the invention is explained in more detail by the figures and examples as described below. 
    
    
     
       FIGURE LEGENDS 
         FIG. 1 . Design and function of a genetically encoded protein inhibitor (gePSI) 
       a-c, The working principle. a, the gePSI renders ribosomes inoperative by depurinating adenine-4324 on the sarcin/ricin loop of the 28s rRNA. b, The inducible expression control system TetOn enables the temporal and cell type-specific expression of gePSI upon doxycycline (dox) administration. In the presence of dox the cell type-specific expression of the gePSI is enabled, shutting off protein synthesis. c, Schematic depicition of the transfected plasmids: (i) ProRIP and (ii) the gePSI (see also  FIG. 2 a   ). Addition of dox together with expression of the Tet-On 3G protein leads to expression of either construct. 
       d-e, Representative fluorescence images of hippocampal neurons transfected with the ProRIP (d) or the gePSI (e). Nascent protein signal is shown in red, anti-MAP2 immunostaining to visualize neuronal morphology is shown in blue. GFP (green) or dashed white outlines indicate transfected neurons. The following plasmids were co-transfected in these experiments: pCMV-Tet3G+pTRE3G-ProRIP+pCMV-AcGFP or pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP. Scale bar=20 μm. 
       f-g, Analysis of (d-e). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=239, 46, 65; mean=1, 0.34, 0.19; sd=0.31, 0.29, 0.06; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test (f). From left to right: n=369, 82, 86; mean=1, 1.1, 0.21; sd=0.34, 0.39, 0.1; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test (g). 
         FIG. 2 . gePSI expression element controls 
       a, Scheme of plasmids used in this study. A plasmid carrying the Tet3G transactivator was co-transfected with plasmids driving the expression of the synthetic ProRIP (1), the gePSI (2) or the separate gePSI chains (3-4) under the control of an inducible pTRE3G or pTRE3G-Bi promoter. 
       b-c, Representative fluorescent images show hippocampal neurons co-transfected with pCMV-Tet3G+pCMV-AcGFP+pTRE3G-Bi-α-Chain (b) or pTRE3G-Bi-β-Chain (c). Nascent protein signal in red, anti-MAP2 immunostaining in blue and GFP in green (white outlines). Scale bar=20 μm. 
       d-e, Quantification of gePSI—alpha-Chain (b) and —beta-Chain (c) with (4 h) and without induction of their expression via dox. Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=66, 93; mean=1.11, 0.96; sd=0.37, 0.48; ns p&gt;0.05; Mann-Whitney U test (d). From left to right: n=95, 84; mean=1.13, 0.98; sd=0.38, 0.46; ns p&gt;0.05; Mann-Whitney U test (e). 
         FIG. 3 . Detection of gePSI action using FUNCAT and comparison with pharmacological protein synthesis inhibition 
       a, Representative fluorescent images of neurons 4 h after induction (4 h dox) or mock-induction (no dox) of gePSI expression in culture. The translational state of cells was assessed by an alternative metabolic labeling approach using fluorescent non-canonical amino acid tagging (FUNCAT) with the methionine analogue L-azido homoalanine (AHA/4 mM for 1.5 h). AHA incorporation was visualized via ClickChemistry (AHA, red), neuron shape is represented with MAP2 (blue) and gePSI-construct bearing neurons additionally express GFP constitutively (green/white outlines). Scale bar=20 μm. b, Quantification of (a). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=39, 44; mean=1, 0.15; sd=0.39, 0.07; p ****&lt;0.0001; Mann-Whitney U test. 
       c-d, Comparison of gePSI induction effect to anisomycin treatment. c, Representative fluorescent images of hippocampal neurons 4 h after induction (4 h dox), mock-induction (no dox) or 1 h anisomycin treatment. Nascent protein (Puro) in red, anti-MAP2 immunostaining in blue and GFP in green to indicate transfected neurons (white outlines). Scale bar=20 μm. d, Quantification of (c). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean intensity of the dox minus control. From left to right: n=23, 32, 31; mean=1, 0.14, 0.11; sd=0.28, 0.05, 0.03; p ****&lt;0.0001, ns p&gt;0.05; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. The following plasmids were co-transfected in these experiments: pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP. 
         FIG. 4 . gePSI expressing cells do not exhibit compromised cell health relative to control cells 
       a, Representative images of hippocampal neuron cultures treated without (0 h) or with 10 mM H 2 O 2  for 1 h or 24 h. Cell death was visualized using the propidium iodide (PI) exclusion assay (red) and nucleoli were stained with the Hoechst dye (blue). Scale bar=20 μm. 
       b, Quantification of (a). Each dot represents the PI signal from one neuron normalized to the mean PI intensity of untreated neurons. From left to right: n=83, 107, 96; mean=1, 29.80, 67.32; sd=6.81, 35.43, 19.83; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
       c, Scheme (top) and representative images (bottom) of gePSI- or GFP- (ctrl) transfected primary hippocampal neurons (white outlines) that were continuously induced with 100 ng/ml dox for 1,2,4 or 6 d (dox was refreshed after 3 d). PI signal in red and Hoechst stain in blue. The following plasmids were co-transfected in these experiments: pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP or pCMV-AcGFP. Scale bar=20 μm. 
       d, Analysis of (c). Each dot represents the PI signal intensity from one transfected neuron normalized to the mean intensity of the dox minus control. From left to right: n=46, 38, 36, 39, 32, 46; mean=1, 1.58, 1.03, 2.32, 5.03, 1.22; sd=0.56, 1.64, 0.43, 4.23, 12.54, 0.63; ** p&lt;0.01, ns p&gt;0.05; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
         FIG. 5 . Temporally—resolved protein synthesis inhibition 
       a-b, On-kinetics of gePSI in neurons. a, Schematic (top) and analysis (bottom) of nascent protein signal after 0, 0.5, 1, 2 and 4 h of dox incubation; protein synthesis was measured for the last 10 min of each dox incubation period. Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=19, 20, 18, 20, 19; mean=1.03, 0.97, 0.9, 0.2, 0.18; sd=0.27, 0.27, 0.39, 0.17, 0.04; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. In these experiments and those shown below (b-d), the following plasmids were cotransfected: pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP. b, Schematic (top) and analysis (bottom) of nascent protein signal after 0, 0.5, 1 and 2 h of dox incubation, followed by a chase without dox up to 4 h. Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=59, 50, 41, 45; mean=1.1, 0.34, 0.3, 0.23; sd =0.3, 0.28, 0.25, 0.15; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
       c-d, Reversibility of gePSI-mediated protein synthesis inhibition. c, Schematic (top) and representative images of hippocampal neurons transfected with gePSI (bottom). Nascent protein signal is shown in red. GFP (green) or dashed white outlines indicate transfected neurons. Images represent the time points indicated in the schematic. Scale bar=20 μm. d, Analysis of (c). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=115, 121, 115, 115, 100; mean=1.1, 0.26, 0.57, 0.86, 1; sd=0.32, 0.23, 0.48, 0.54, 0.43; ns p&gt;0.05, ** p&lt;0.01, *** p&lt;0.001, **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
         FIG. 6 . Extended or enhanced gePSI expression in neurons that exhibit retarded protein synthesis recovery 
       a, Quantification of nascent protein signal after induction of gePSI transfected hippocampal neurons for 4 h using dox concentrations ranging from 1 to 1000 ng/ml. Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. In these experiments and those shown below (b-e), the following plasmids were co-transfected: pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP. From left to right: n=32, 35, 38, 47, 34; mean=1.11, 1, 0.27, 0.23, 0.2; sd=0.3, 0.4, 0.2, 0.11, 0.06; p ****&lt;0.0001, ns p&gt;0.05; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. The following plasmids were co-transfected: pCMV-Tet3G+pTRE3G-Bi-gePSI-pCMV-AcGFP. 
       b, Representative images of hippocampal neurons transfected with gePSI prior to the addition of dox (no dox) or after a 4 h incubation with dox (4 h dox), as well as examples of neurons that exhibited recovered protein synthesis levels (recovery-yes) or not (recovery-no) 24 h after the dox washout. Nascent protein signal is shown in red, anti-MAP2 immunostaining to visualize neuronal morphology is shown in blue and gePSI mRNA in white. Dashed white outlines indicate transfected neurons. Scale bar=20 μm. 
       c, Quantification of (b). Each dot represents the mRNA signal from one neuron normalized to the mean mRNA signal of all neurons in the no dox control. From left to right: n=40, 34, 48, 26; mean=1, 11.3, 3.4, 0.95; sd=0.38, 4.56. 1.55, 0.54; **** p&lt;0.0001, *** p&lt;0.001, ns p&gt;0.05; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
       d, Representative images showing TUNEL staining in neurons before the induction of gePSI, 4 h post induction (4 h dox), 24 h (4 h dox+24 h chase) and 48 h (4 h dox+48 h chase) after dox washout and positive control. Arrowhead highlights background signal caused by lipofectamine transfection. Scale bar=5 μm. 
       e, Analysis of (d). TUNEL intensities were normalized to the mean TUNEL intensity of the positive control. From left to right: n=60, 67, 58, 49, 92; mean=0.4, 0.32, 0.27, 0.31, 1; sd=0.12, 0.12, 0.16, 0.13, 0.59; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
         FIG. 7 . Cell type-specific protein synthesis inhibition 
       a-f, Cell type-specific expression of gePSI in hippocampal cell cultures (containing both neurons and glial cells) or HeLa cells. Specificity was achieved using a Camk2a (a) or a GFAP (b) promoter to restrict transactivator expression in excitatory hippocampal neurons or astrocytes, respectively. The universal mammalian cytomegalovirus (CMV) promoter was used in HeLa cultures (e). Nascent protein signal in red and GFP in green. White outlines indicate transfected cells. The correct cell type for the heterogonous neuronal cultures is verified via FISH (cyan). HeLa cultures were homogenous and representative transfected HeLa cells are visualized with outlines (white dotted line) in the nascent-protein-channel. Scale bars=20 μm. c, d, f, The nascent protein signal in transfected cells was normalized to the respective signal from neighboring, untransfected cells. Each dot represents the mean intensity from one cell. HeLa cells treated with dox are shown relative to the dox minus control. From left to right: n=21, 20; mean=1.01, 0.14; sd=0.26, 0.07; **** p&lt;0.0001; Mann-Whitney U test (c). n=35, 38; mean=0.89, 0.37; sd=0.37, 0.36; **** p&lt;0.0001; Mann-Whitney U test (d). From left to right: n=288, 270; mean=0.84, 0.47; sd=0.55, 0.36; **** p&lt;0.0001; Mann-Whitney U test (f). The following plasmids were co-transfected: pCamk2a-Tet3G or pGFAP-Tet3G or pCMV-Tet3G+pTRE3G-Bi-gePSI-pCamk2a-AcGFP or pTRE3G-Bi-gePSI-pGFAP-AcGFP or pTRE3G-Bi-gePSI-pCMV-AcGFP. 
         FIG. 8 . Cre-dependent expression of the gePSI 
       a, Scheme of the Cre-dependent gePSI-plasmid using the Cre recombinase for the cell type-specific expression of the gePSI. 
       b, Representative fluorescence images of hippocampal neurons transfected with the floxed Tet3G, and the gePSI with or without Cre recombinase. Nascent protein signal in red, anti-HA immunostaining in “fire” look-up to visualize Cre expression and GFP in green (white outlines) to indicate transfected neurons. Scale bar=20 μm. 
       c, Quantification of (b). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=57, 77, 65; mean=1.13, 1.04, 0.29; sd=0.32, 0.32, 0.23; **** p&lt;0.0001; Kruskal-Wallis test followed by Dunn&#39;s multiple comparison test. 
       d, Representative fluorescence images of hippocampal neurons transfected with the floxed Tet3G, gePSI and Cre recombinase under the Camk2a promoter. Nascent protein signal in red, FISH against Camk2a mRNA in cyan and GFP in green. Scale bar=20 μm. 
       e, Quantification of (d). Each dot represents the nascent protein signal intensity from one neuron normalized to the mean nascent protein signal of neighboring, untransfected neurons. From left to right: n=65, 42, 31; mean=1.1, 0.15; sd=0.3, 0.04; p ****&lt;0.0001; two-tailed, unpaired t-test. The following plasmids were cotransfected: pCAG-Cre-IRES2-eGFP+pCMV-FLEX (inverted)-Tet3g-3xHA-FLEX (inverted)+pTRE3G-Bi-gePSI; pCamk2a-Cre+pCMV-FLEX (inverted)-Tet3g-3xHA-FLEX (inverted)+pTRE3G-Bi-gePSI+pCamk2a-AcGFP. 
         FIG. 9 . gePSI induction does not perturb neuronal responses but prevents spine structural plasticity. 
       a, Representative images of stimulated neurons without (no dox) and with (4 h dox) gePSI induction. Nascent protein signal in red, anti-MAP2 in blue and GCaMP6 in green. The arrowheads indicate the 2p glutamate uncaging spots. The following plasmids were co-transfected in these experiments: pCMV-Tet3G+pTRE3G-BigePSI+pCMV-GCaMP6s (a-c). Scale bar=20 μm. 
       b, Magnification of the uncaging spots in (a). Images show calcium signal 100 ms before, during (stim), and 100/500 ms after 2P glutamate uncaging. Scale bar=2 μm. 
       c, Analysis of calcium signal after gePSI induction (4 h dox, red) and without gePSI induction (no dox, blue). Plotted is the mean change in calcium signal to 30× 2P uncaging events (delivered at 1 Hz) compared to the baseline calcium signal before stimulation. For analysis, the 5 frames following each uncaging event were averaged and the mean of the 30 events were compared between treatments. n=10, 12; ns p&gt;0.05; Mann-Whitney U test. 
       d, Schematic (top) of experiment and group analysis (bottom) of spine-head volumes after local glutamate uncaging. gePSI expression without dox induction (blue, solid line, n=21 spines from 6 cells), dox administration with the isolated expression of Tet3G (blue, dashed line, n=14 spines from 4 cells) or gePSI induction (red, solid line, n=15 spines from 4 cells). Depicted are means±SEM. gePSI w/dox compared to gePSI w/o dox: p&lt;0.0005; gePSI w/o dox compared to Tet3G w/dox: p=0.5; One way ANOVA, Tukey test for mean comparisons. The following plasmids were co-transfected in these experiments: pCMV-Tet3G+pTRE3G-Bi-gePSI+pCMVGCaMP6s+pCMV-PSD-95-mCherry (d-e). e, Representative images of stimulated spines (dashed white outline) with (4 h dox) and without (no dox) gePSI induction; directly before (0 min) and 25/55 min after spine stimulation. The dot indicates the 2p glutamate uncaging spot. Scale bar=2 μm. 
         FIG. 10 . Schematic depiction of plasmid pTRE3G-Bi-gePSI-pCMV-AcGFP. 
       The α-chain and the β-chain of maize RIP are encoded by separate nucleic acid molecules in operative linkage with the tetracycline-inducible bi-directional TREG3 BI promoter. The vector backbone is derived from pAAV-cDNA6-V5His (Vector Biolabs). 
         FIG. 11 . Schematic depiction of plasmid pCamK2a-TetOn3G. 
       The nucleic acid sequence encoding the transactivator TetOn3G is in operative linkage with the tissue-specific CamKa2 promoter. The vector backbone is derived from pAAV-cDNA6-V5His (Vector Biolabs). 
         FIG. 12 . First part of the sequence of ProRIP from maize. 
       Nucleic acid sequence and amino acid sequence (triple letter code; SEQ ID NO: 3, amino acids 1-146) are shown. ProRIP is indicated in blue/light blue, the α-chain is indicated in light green, active site residues are indicated in dark green. 
         FIG. 13 . Second part of the sequence of ProRIP from maize. 
       Nucleic acid sequence and amino acid sequence (triple letter code; SEQ ID NO: 3, amino acids 147-268) are shown. ProRIP is indicated in blue/light blue, the β-chain is indicated in light green, active site residues are indicated in dark green. 
         FIG. 14 . Amino acid sequence alignment. 
       Amino acid sequence alignment (single letter code) between MOD (gj 25422082; SEQ ID NO: 4) and RIP superfamily member (Cdd:pfam00161; SEQ ID NO: 5). Blue indicates different sequence, red indicates same sequence, active site residues are shown by green rectangles. 
         FIG. 15 . Amino acid sequence alignment. 
       Amino acid sequence alignment (single letter code) between MOD (Query; SEQ ID NOs: 9, 11 and 13, respectively) and different RIP family members (upper: ricin-like, SEQ ID NO: 10; middle: trichosanthin, SEQ ID NO: 12; lower: gelonin, SEQ ID NO: 14). Blue box indicates sequence similarity, red box indicates conserved active site residues. 
     
    
    
     EXAMPLE 1 
     Development of a Genetically Encoded Protein Synthesis Inhibitor (gePSI) Based on a Ribosome Inactivating Protein (RIP) from Maize 
     Example 1 and  FIGS. 1-9  are taken from a manuscript submitted and accepted by nature Methods for publication as a ‘Brief Communication’ in 2019. It is scheduled for publication under the title ‘A genetically encodable cell type specific protein synthesis inhibitor’ by the authors ‘Maximilian Heumüller, Caspar Glock, Vidhya Rangaraju, Anne Biever, and Erin M. Schuman’. 
     1. Results 
     We developed a fully genetically encoded protein synthesis inhibitor (gePSI) that allows one to control protein synthesis in a temporal and spatial manner. 
     To develop a gePSI we considered the class of bacterial and plant toxins from the Shiga and Ricin families that are known as “ribosome inactivating proteins” (RIPs) 2,3 ; most RIPs effect a complete shutdown of protein synthesis by depurinating adenine-4324 on the sarcin/ricin-loop of the 28S rRNA4 ( FIG. 1 a   ). 
     Specifically, we used an atypical class-1 RIP from maize ( Zea mays ) 5,6 . The full-length maize RIP contains two catalytically active chains as well as inactivation regions, which are proteolytically eliminated during RIP activation to increase enzyme potency. In order to induce the expression of various gePSI candidates in hippocampal cultures we used the TetOn3G system in which doxycycline (dox) introduction enables expression ( FIG. 1 b, c    and 2a). 
     We measured the efficacy of different candidate gePSIs by metabolic labeling of transfected cultured hippocampal neurons with a brief pulse of puromycin, followed by detection of nascent protein in situ 7 . The synthetic ProRIP molecule contains the active chains as well as a single internal inactivation region ( FIG. 1 c   , (i)). The mere co-transfection of ProRIP (pCMV-Tet3G+pTRE3G-ProRIP+pCMVAcGFP) into neurons in the absence of dox induction resulted in complete shutdown of protein synthesis in a majority of the cells ( FIG. 1 d, f   ). 
     Thus, we physically separated the two chains (labelled “alpha and beta”,  FIG. 1 c   ) of the ProRIP. We co-transfected neurons with a plasmid containing both chains driven by a bi-directional dox-inducible promoter as well as the activator plasmid ( FIG. 1 c   , (ii);  FIG. 2 a   ). Using this strategy, protein synthesis levels in transfected but uninduced neurons (“no dox”) were indistinguishable from control, untransfected neurons ( FIG. 1 e, g   ). In contrast, a 4 h induction resulted in a dramatic and uniform reduction of protein synthesis in transfected, but not in neighboring untransfected, neurons ( FIG. 1 e, g   ). 
     The inhibition of protein synthesis required the combined expression of the alpha and beta chains, as expression of either chain alone did not inhibit protein synthesis ( FIG. 2 b - e   ). The gePSI-induced inhibition of protein synthesis was also detected with an alternative metabolic labeling approach that uses non-canonical amino acids 8,9  ( FIG. 3 a, b   ) and was indistinguishable from that observed following treatment with the chemical protein synthesis inhibitor anisomycin ( FIG. 3 c, d   ). Moreover, expression of the gePSI did not promote cell death even when continuously expressed for 6 days ( FIG. 4 a - d   ), indicating that it can be used to perturb protein synthesis without obvious adverse effects on cellular health. 
     The time scales over which protein synthesis influences cellular and neuronal plasticity range from minutes to days 1,10  To determine the temporal precision of the protein synthesis inhibition driven by the gePSI, we varied the duration of the expression induction by dox from 0.5 to 4 h ( FIG. 5 a   ). In the last 10 min of the dox induction we measured protein synthesis, as before. Inducing the expression of the gePSI for less than 1 h did not result in a consistent inhibition of protein synthesis in hippocampal neurons ( FIG. 5 a   ). Inducing for 2 or 4 h, however, resulted in a consistent and complete inhibition of neuronal protein synthesis ( FIG. 5 a   ). The observed requirement for at least 2 h of induction could represent the time needed to achieve adequate levels of the gePSI mRNA or the time needed for the gePSI protein to express. To distinguish between these possibilities, we induced with dox for variable durations but then delayed the measurement of protein synthesis to a uniform end point of 4 h following the initiation of induction ( FIG. 5 b   ). This experiment revealed that even a 30 min induction of the gePSI transcription was sufficient to significantly reduce protein synthesis measured 3.5 h later ( FIG. 5 b   ) although the inhibition of protein synthesis exhibited even greater reliability with longer induction periods ( FIG. 5 b   ). 
     In many in vitro and in vivo experiments, plasticity is induced by a brief external event that results in a defined period of protein synthesis that is required for the plasticity expression 11,12  To determine whether we could effect a temporally discrete period of protein synthesis inhibition that later exhibits recovery, we first optimized the dox concentration ( FIG. 6 a   ), induced the gePSI and then examined protein synthesis in neurons at 4, 24, 48 and 72 h ( FIG. 5 c, d   ). As before, protein synthesis was inhibited following 4 h of gePSI induction ( FIG. 5 c, d   ). Probing 24 h later, however, revealed a significant recovery of protein synthesis levels in a large proportion of neurons, which increased over time to almost 90% recovery after 72 h. In order to address why some neurons did not recover we quantified gePSI expression using fluorescence in situ hybridization, reasoning that enhanced persistence or levels of gePSI expression might lead to prolonged recovery times ( FIG. 6 b, c   ). Indeed, we found that neurons, which did not show recovery after 24 h exhibited higher gePSI mRNA levels when compared to fully recovered neurons. We noted that those neurons that exhibited retarded recovery within the time-course measured, however, did not exhibit enhanced apoptosis when probed 24/48 h after a 4 h gePSI induction ( FIG. 6 d, e   ). These data indicate that the gePSI can transiently inhibit protein synthesis in cells for a defined period of time; these cells then exhibit functional recovery of their protein synthesis capabilities. 
     The ideal gePSI should exhibit spatial (e.g. tissue- and/or cell-type-specific) as well as temporal control. An exemplary implementation of a cell type-specific gePSI involves the use of a cell type-specific promoter to drive gePSI expression ( FIG. 1 b   ), for example in Camk2a-positive ( FIG. 7 a, c   ), GFAP-positive ( FIG. 7 b, d   ) cells or other cell lines ( FIG. 7 e - f   ). To maximize flexibility, we combined the platform with a Cre-inducible system ( FIG. 8 a   ). To include temporal control, we floxed an HA (epitope)-tagged Tet3G (activator) element to enable Cre-induction, either via an external construct, or via introduction to a transgenic animal line. The Cre-dependent expression of Tet3G together with the addition of dox provides cell type-specificity and temporal control, respectively. We transfected neurons with the floxed Tet3G, the gePSI and a Cre recombinase plasmid. Only those cells that expressed Cre and were treated with dox exhibited significantly reduced protein synthesis ( FIG. 8 b, c   ). These data indicate that the gePSI can be used to bring about a temporally discrete episode of protein synthesis inhibition in an identified cell type ( FIG. 8 d, e   ). 
     Although some short-term forms of synaptic plasticity rely exclusively on covalent modifications of pre-existing proteins, many forms of plasticity, both synaptic and behavioral, require protein synthesis 1,13 . To test the function of the gePSI during plasticity we first examined the compatibility of gePSI expression with synaptic transmission. We expressed the Ca 2+ -indicator GCaMP6s in cultured hippocampal neurons and determined their ability to respond to extracellular stimulation, using two-photon (2P) uncaging of caged glutamate ( FIG. 9 a - c   ). gePSI-expressing neurons were competent to respond to single stimuli of individual dendritic spines as well as trains of stimulation ( FIG. 9 a - c   ). Using 2P glutamate uncaging, we induced a form of plasticity that is known to require protein synthesis 12  ( FIG. 9 d   ). This plasticity results in an enlargement of the stimulated spine, which is associated with enhanced synaptic responsiveness. Single spine stimulation (0.5 Hz) in neurons transfected with the gePSI in the absence of dox exhibited a rapid and persistent (˜ for up to 55 min) increase in spine-head volume ( FIG. 9 d, e   ). Similarly, neurons that expressed Tet3G but not the gePSI also exhibited the stimulation-induced plasticity. However, neurons that expressed the gePSI in the presence of dox exhibited only a small increase in spine-head volume immediately after stimulation that decayed back to baseline values within 10 min or so, indicating a requirement for protein synthesis in the stimulated neuron. 
     Here we have developed a genetically encoded protein synthesis inhibitor that can be induced to effectively, rapidly (e.g. within a few hours), and reversibly shutdown protein synthesis in targeted cells; neighboring cells are unaffected. By judicious use of both Cre-targeting and temporal induction, neurons that project to specific areas could be targeted for inhibition in vivo to probe the importance of different cell populations in learning and memory. In addition, the cell type-specific inhibition we achieve suggests that the gePSI can be used clinically to target identifiable cell populations, e.g. rapidly proliferating cells, to abbreviate cancer states. 
     2. Material and Methods 
     Cell Culture 
     Dissociated rat hippocampal neurons were prepared from P0-1 day-old rat pups as previously described 14 . Neurons were plated onto poly-d-lysine coated glass-bottom Petri dishes (MatTek) and cultured in neuronal growth medium (NGM, Neurobasal-A supplemented with B27 and GlutaMAX). The cultures were maintained in a humidified incubator at 37° C. and 5% CO 2 . HeLa cells were maintained in Dulbecco&#39;s modified Eagle&#39;s medium (DMEM, Invitrogen) supplemented with 10% FCS at 37° C. in a 5% CO 2  atmosphere. Once cells reached a confluency of 70% they were passaged. Only cells that were passaged less than 10 times were used for experiments. 
     Transient Transfection of Hippocampal Neurons or HeLa Cells 
     Primary hippocampal neurons were transfected at DIV 11 using CombiMag following the manufacturer&#39;s recommendations. Experiments were performed at least 12 h post-transfection to guarantee sufficient expression of the constructs. HeLa cells, at 50% confluency, were transfected with Lipofectamin 2000 according to the manufacturer&#39;s guidelines. The inducible expression system TetOn3G was purchased from Takara Bio—Clontech (#631337). Additional plasmids used in this study were gifted by P. C. Shaw, C. Hanus, S. Sternson 15  (Addgene #18925), J. M. Wilson (Addgene #105558) and A. Chenn 16  (Addgene #26646). Cloning was performed using Gibson Cloning. 
     Dual, triple or quadruple transfections were performed with the following plasmids: 
     pCMV-TetOn3G+pTRE3G-Bi-ProRIP/α-Chain/β-Chain+pCMV-AcGFP-N1 pCMV-TetOn3G+pTRE3G-Bi-gePSI-pCMV-AcGFP+pCamK2a-TetOn3G+pTRE3G-Bi-gePSI-pCamK2a-AcGFP pGFAP-TetOn3G+pTRE3G-Bi-gePSI-pGFAP-AcGFP pCMV-FLEX (inverted)-TetOn3G-3xHA-FLEX (inverted)+pTRE3G-Bi-gePSI-pCMV-AcGFP+pCAG-Cre-IRES2-eGFP (Addgene #26646) pCMV-FLEX (inverted)-TetOn3G-3xHA-FLEX (inverted)+pTRE3G-Bi-gePSI+pCamK2a-Cre (Addgene #105558)+pCamKa2-AcGFP 
     In each of the transfections, a 1:4 ratio between the activator plasmid (TetOn3G) and the responder plasmid (pTRE3G-Bi) was used. The β-chain of gePSI is C-terminally fused to a murine ornithine decarboxylase (ODC36) degron. 
     A schematic depiction of plasmid pTRE3G-Bi-gePSI-pCMV-AcGFP is shown in  FIG. 10 . A schematic depiction of plasmid pCamK2a-TetOn3G is shown in  FIG. 11 . 
     Doxycycline Hyclate (Dox) Treatment 
     Doxycycline hyclate (Sigma-Aldrich/Merck, D9891) was dissolved in H 2 O to yield a 1 mg/ml stock solution. Dox was added to the growth medium of transfected cells to a final working concentration of 1 μg/ml to induce gePSI expression (unless otherwise stated). For the protein synthesis recovery experiments 10 ng/ml was used, neurons were washed three times with NGM and replaced with their growth medium that had been saved prior to dox treatment. 
     Fluorescent Non-Canonical Amino Acid Tagging (FUNCAT) 
     FUNCAT was performed as previously described 8,9  with the following modification: a biotin-alkyne (Acetylene-PEG4-Biotin, Jena Bioscience) was used as a tag in the copper-catalyzed [3+2] azide-alkyne cycloaddition (CuAAC) click reaction. Hippocampal neurons were incubated in methionine-free Neurobasal-A medium (custom-made by Life Technologies) supplemented with 4 mM AHA (prepared as described in 17 ) for 1.5 h. Cells were washed two times with PBS-MC (1× PBS, pH 7.4, 1 mM MgCl 2 , 0.1 mM CaCl 2 ) and fixed for 20 min in PFA-sucrose (4% paraformaldehyde, 4% sucrose in PBS-MC) at room temperature. Cells were washed with 1× PBS, permeabilized with 0.5% Triton X-100 in 1× PBS, pH 7.4, for 15 min and blocked with blocking buffer (4% goat serum in 1× PBS) for 1 h. Cells were equilibrated to optimal click conditions by washes with 1× PBS, pH 7.8. After the click reaction, cells were incubated with primary antibodies (anti-MAP2 (SySy, 188004, 1:1000), anti-biotin (Cell Signaling, 5597, 1:2000) in blocking buffer overnight at 4° C. Secondary antibodies were incubated in blocking buffer for 45 min at room temperature. 
     Immunocytochemistry and Puromycylation 
     Nascent proteins were labeled by incubating cells with puromycin (stock solution 50 mg/ml in H 2 O; Sigma-Aldrich/Merck, P8833) at a final concentration of 3 μM for 10 min in full medium at 37° C. in a humidified atmosphere with 5% CO 2 . Cells were then rinsed in 1× PBS and fixed for 20 min in PFA (4% paraformaldehyde in 1× PBS), permeabilized with 0.5% Triton X-100 in 1× PBS supplemented with 4% goat serum, for 15 min and blocked with blocking buffer (4% goat serum in 1× PBS) for 1 h. Primary antibodies (anti-MAP2 (SySy, 188004, 1:1000), anti-puromycin (Kerafast, EQ0001, 1:2000)) were incubated in blocking buffer overnight at 4° C. Secondary antibodies (ThermoFisher, A11030, 1:1000; Jackson I R, 106-475-003, DyLight 405, 1:500) were incubated in blocking buffer for 45 min at room temperature. 
     High-Resolution In Situ Hybridization 
     In situ hybridization was performed using the Quantigene ViewRNA ISH Cell Assay for Fluorescence with probes targeting the coding sequence of Camk2a (VC6-11639), GFAP (VC6-11478) or gePSI (α-Chain: VPWCWCK (type 6); β-Chain: VPXGPWH (type 1)). The manufacturer&#39;s protocol was applied with the following modifications: Cells were fixed for 20 min at room temperature using a 4% paraformaldehyde solution (4% paraformaldehyde, 5.4% glucose, 0.01 M sodium meta-periodate, in lysine-phosphate buffer). The Proteinase K treatment was omitted in order to preserve the integrity of the cells. After completing the in situ hybridization, cells were washed with 1× PBS and incubated in blocking buffer (4% goat serum in 1× PBS) for 1 hr. Nascent proteins and neuronal morphology was stained as described above. 
     Cell Health Assays 
     To assess cell health two assays were used: 
     The propidium iodide (PI) exclusion assay was performed by incubating cells in full medium with 1 μl/ml PI (stock solution 1 mg/ml in H 2 O; invitrogen/Thermo Fisher Scientific, P1304MP) and 20 μM Hoechst 33342 (stock solution 20 mg/ml in H 2 O; Sigma-Aldrich/Merck, B2261) for 10 min at 37° C. in a humidified atmosphere with 5% CO 2 . Cells where then transferred microscope (pre-heated chamber at 37° C.) for live imaging. For the positive control, cells were treated with a final concentration of 10 mM H 2 O 2  (stock solution 27% H 2 O 2 ; Alfa Aesar, L13235) for 1 h or 24 h in full medium prior to PI/Hoechst staining. 
     The apoptosis assay ‘Click-iT™ TUNEL Alexa Fluor™ 647 Imaging Assay’ from ThermoFisher Scientific was used according to the manufacturer&#39;s protocol. For the positive control, cells were treated with DNAse I solution for 30 min after fixation as indicated in the protocol. 
     Imaging and Image Analysis 
     Fluorescence imaging was performed with a LSM780 or LSM880 laser scanning confocal microscope (Zeiss) with ×20 air objective (Plan Apochromat 20×/0.8 M27) or ×40 oil objective (Plan Apochromat 40×/1.4 oil DIC M27) with appropriate excitation laser lines and spectral detection windows. Images were acquired in 16-bit mode as z-stacks, covering the entire thickness of the cell, with 1024×1024- or 2048×2048-pixel image resolution. Laser power and detector gain were adjusted to ensure a good dynamic range, without saturating pixels. Imaging conditions were held constant within an experiment. For image analysis the raw images were split into single-channel images and maximum-intensity projections were created (Zeiss-Zen Black). To quantify the nascent protein signal for individual cells the cell body (soma for neurons) was manually traced to create a mask using the MAP2- and GFPchannel, the traced masks were then used in the ‘metabolic labeling’-channel and the mean intensity within the mask was measured (NIH-ImageJ). In order to correct for technical deviations (e.g. small differences in puro incubation times), the puro-signal of transfected cells was normalized to the mean puro-signal of a maximum of 10 untransfected, neighboring cells. 
     All live imaging experiments were performed at a temperature of 37° C. in modified E4 imaging buffer containing (in mM) 120 NaCl, 3 KCl, 10 HEPES (buffered to pH 7.4), 4 CaCl 2  (lacking MgCl 2 ) and 10 Glucose. Live cell imaging was performed using an inverted spinning disk confocal microscope (3i imaging systems; model CSU-X1) using the Slidebook 5.5 software. For glutamate uncaging experiments, images were acquired with a Plan-Apochromat 63×/1.4 Oil DIC objective at laser powers 1.1 mW (488 nm) and 0.8 mW (561 nm) using an Evolve 512 camera (Photometrics). 488 nm excitation and 525/30 nm emission filters were used for GCaMP6s; 561 nm excitation and 617/73 nm emission filters were used for PSD95-mCherry fluorescence. Images were analyzed using ImageJ, unless specified otherwise. OriginPro 2017 was used for data analysis, statistical testing and plotting graphs. 
     Uncaging and Spine-Head Volume Measurements 
     Neurons were transfected with GCaMP6s, PSD95-mCherry, pCMV Tet3G and pTRE3G-Bi-gePSI plasmid constructs. The absence of the pTRE3G-Bi-gePSI construct was used as control. Neurons were either treated with dox for 4 hours (for gePSI-induction) prior to the imaging, or untreated (control). Both gePSI-induced and untreated control neurons were identified by changes in GCaMP6s fluorescence corresponding to calcium transients in dendrites and spines. PSD95-mCherry fluorescence was used to identify spines for glutamate uncaging. Before glutamate uncaging, neuronal media was replaced with 1 μM TTX (Citrate salt, 2 mM stock made in water), 50 μM Forskolin (Tocris Bioscience, 100 mM stock made in DMSO), 2 mM 4-Methoxy-7-nitroindolinyl-caged-L-glutamate (MNI caged glutamate) (Tocris Bioscience, 100 mM stock made in E4 buffer) in modified E4 buffer. Glutamate uncaging was performed using a multiphoton-laser 720 nm (Chameleon, Coherent) and a Pockels cell (Conoptics) for controlling the uncaging pulses. Spines that were at least 50 μm away from the cell body were chosen for uncaging experiments. To test a spine&#39;s response to an uncaging pulse, an uncaging spot (˜2 μm 2 ) close to a spine head was selected and two to three uncaging pulses at 10 ms pulse duration per pixel and 2.5 mW power were given and the spine was checked for spine-specific calcium transients. For synaptic transmission experiments, an uncaging protocol of 30 uncaging pulses at 1 Hz with 10 ms pulse duration per pixel was used. For plasticity experiments, an uncaging protocol of 60 uncaging pulses at 0.5 Hz with 10 ms pulse duration per pixel, at 2.5 mW power was used. Images were acquired every 10 min for up to 55 min. For analysis of spine morphology, we used a custom-written Matlab script. Ten images from each time point were averaged and a line crossing the center of the spine-head was drawn. The fluorescence intensity measured along the line was fit to a Gaussian to obtain the full width at half maximum—defined as the spinehead diameter 18 . Spine-head diameter was converted to spine-head volume, under the assumption that the spine-head is spherical. The uncertainty of the spine-head diameter measurement was determined from standard error of the mean and was converted to the uncertainty of the spine-head volume by standard error propagation 19 . 
     Sample Size Choice and Statistics 
     Sample size choice was determined as follows: within one replicate the maximum number of culture dishes was limited to 12 to ensure consistent experimental procedure for all dishes. The experimenter selected the transfected cells in the GFPchannel blind to the ‘metabolic-labeling’-channel (not blind to the condition) and all or at least 20 transfected cells in a dish were imaged. Unless otherwise stated, all experiments were repeated with dishes originating from two independent cell preparations. Within one experiment at least 2 dishes were used per condition. The experiments in supplementary  FIG. 2-4  were performed with multiple dishes originating from a single neuronal preparation (prepared from ˜10 P0-1 rat pups). The “n” in the figure legends depict the number of cells imaged per condition. For quantification and testing of statistical significance, data are displayed as combined graphs from multiple experiments. For all data sets, the Pearson omnibus normality test was applied to test for normal distribution. If normality was proven, Student&#39;s, unpaired t-test (for two groups) was applied. For non-normally distributed data sets, the Mann-Whitney U test (for two groups) or Kruskal-Wallis test was applied (for three or more groups). An F-test was applied to compare variances before Student&#39;s t-test. For multiple-comparison analysis, Dunn&#39;s test of multiple comparisons was applied. 
     Example 2 
     General Concept of gePSI Development Based on Active Conservation in Ribosome-Inactivating Proteins 
     We examined the structure of the maize RIP used in Example 1 to evaluate the similarity to other members of the RIP family. In a structure-function study on maize RIP in 2007 6 , certain similarities between the active sites of members of the RIP family were found. The active site residues are substantially conserved throughout the RIP family and spread over the entire sequence of the protein. 
     Thus, based on the results obtained in Example 1, it is likely to develop an expression system for other members of the RIP family, e.g. class 1 and class 2 RIPs by splitting the nucleic acid sequence encoding a monomeric RIP protein with N-glycosidase activity or a monomeric RIP subunit with N-glycosidase activity into two separate nucleic acid sequences such that some of the active site residues are encoded by each of the two separate nucleic acid sequences. 
       FIGS. 12 and 13  show the sequence of the maize RIP precursor ProRIP (blue &amp; light blue) with α-/β-chain regions highlighted (light green). The amino acid sequence of the ProRIP is shown in SEQ ID NO: 3. The amino acids that make up the catalytically active site in the final toxin are highlighted in dark green (Tyr74, Tyr110, Glu187, Arg190 and Trp221; taken from Mak et al. (2007) 6 ). 
     These active site residues are at least partially conserved throughout most or all RIPs and thus can be identified by amino acid sequence alignments although their positions within the individual family members may be slightly different. 
     We carried out a BLAST (Basic Local Alignment Search Tool) alignment of the amino acid sequence designated “MOD” shown in SEQ ID NO: 4 (that is an α-chain and β-chain of maize RIP depicted as single chain) to identify regions of local similarity between sequences. Doing so we can determine the sequence similarity between different RIP family members but we can also compare the maize RIP sequence against a ‘superfamily’ member ( FIG. 14 ) that can be seen as a consensus sequence from many RIP proteins 20  shown in SEQ ID NO: 5. The sequence of MOD (both chains) shares ˜40% identity with the averaged ‘superfamily RIP’. 
     A further BLAST search of portions of the MOD sequence (SEQ ID NO: 9, 11 and 13) against portions of specific RIPs ricin-like (SEQ ID NO: 6/SEQ ID NO: 10), trichosanthin (SEQ ID NO: 7/SEQ ID NO: 12) or gelonin (SEQ ID NO: 8/SEQ ID NO: 14) provides lower overall similarities of ˜25%, ˜21% and ˜24% ( FIG. 15 ). The active site residues however are conserved and may be identified although their positions within the different proteins can vary. 
     Despite being variable in their amino acid sequences, all RIPs have N-glycosidase activity on a common target. They target the 28S rRNA on the large 60S ribosomal subunit and cleave off (depurinate) the adenine located on sarcin-ricin loop of that particular rRNA 2 . Without this functional loop, ribosomes cannot bind specific elongation factors anymore and thus are functionally inactive. By splitting a nucleic acid sequence encoding a RIP such that active site residues are present on each of the two parts, represents a general concept of an expression system for a gePSI (analogous to the expression system of Example 1). 
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