[Safety and efficacy of LK565--a new polymer ultrasound contrast medium]

Metabolic engineering of microorganisms for the production of L-arginine and its derivatives

L-arginine (ARG) is an important amino acid for both medicinal and industrial applications. For almost six decades, the research has been going on for its improved industrial level production using different microorganisms. While the initial approaches involved random mutagenesis for increased tolerance to ARG and consequently higher ARG titer, it is laborious and often leads to unwanted phenotypes, such as retarded growth. Discovery of L-glutamate (GLU) overproducing strains and using them as base strains for ARG production led to improved ARG production titer. Continued effort to unveil molecular mechanisms led to the accumulation of detailed knowledge on amino acid metabolism, which has contributed to better understanding of ARG biosynthesis and its regulation. Moreover, systems metabolic engineering now enables scientists and engineers to efficiently construct genetically defined microorganisms for ARG overproduction in a more rational and system-wide manner. Despite such effort, ARG biosynthesis is still not fully understood and many of the genes in the pathway are mislabeled. Here, we review the major metabolic pathways and its regulation involved in ARG biosynthesis in different prokaryotes including recent discoveries. Also, various strategies for metabolic engineering of bacteria for the overproduction of ARG are described. Furthermore, metabolic engineering approaches for producing ARG derivatives such as L-ornithine (ORN), putrescine and cyanophycin are described. ORN is used in medical applications, while putrescine can be used as a bio-based precursor for the synthesis of nylon-4,6 and nylon-4,10. Cyanophycin is also an important compound for the production of polyaspartate, another important bio-based polymer. Strategies outlined here will serve as a general guideline for rationally designing of cell-factories for overproduction of ARG and related compounds that are industrially valuable.

Isolation of cyanophycin from tobacco and potato plants with constitutive plastidic cphATe gene expression

A chimeric cyanophycin synthetase gene composed of the cphATe coding region from the cyanobacterium Thermosynechococcus elongatus BP-1, the constitutive 35S promoter and the plastid targeting sequence of the integral photosystem II protein PsbY was transferred to the tobacco variety Petit Havanna SRI and the commercial potato starch production variety Albatros. The resulting constitutive expression of cyanophycin synthetase leads to polymer contents in potato leaf chloroplasts of up to 35 mg/g dry weight and in tuber amyloplasts of up to 9 mg/g dry weight. Both transgenic tobacco and potato were used for the development of isolation methods applicable for large-scale extraction of the polymer. Two different procedures were developed which yielded polymer samples of 80 and 90% purity, respectively.

Gd-DTPA-loaded PLGA microbubbles as both ultrasound contrast agent and MRI contrast agent--a feasibility research

OBJECTIVE

To prepare the Gd-DTPA-loaded poly (lactic-co-glycolic acid) (PLGA) microbubbles for both ultrasound imaging and MRI imaging.

METHODS

Biodegradable high-molecular-weight poly (lactic-co-glycolic acid) (PLGA) was used as an encapsulating vehicle to fabricate a contrast agent composed of gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) and fluorocarbon-filled microbubbles by using double emulsion method. The morphology, size, and Gd loading capacity of PLGA microparticles were assessed. The efficacy of the contrast agent for ultrasound and magnetic resonance (MR) imaging was evaluated by in vivo imaging of hepatic vessels and the liver parenchyma of rabbit following injection of the contrast agent. Additionally, the liver and kidney functions were examined before and at 7, 14, 30 days after injection.

RESULTS

Electron microscopy showed Gd loaded PLGA microbubbles were spherical in shape. The mean diameter were 1.47 +/- 0.38 microm and the Gd loading was 25 +/- 2 microg/mg. Right after the injection of the PLGA microbubbles, the ultrasound echo intensity of hepatic vessels and the liver parenchyma was significantly enhanced. Moreover, enhanced signal intensity of the liver parenchyma on MR images was observed for approximately 30 min after injection of the contrast agent. Neither marked changes in the liver and kidney functions nor other severe complications were detected during the follow-up period.

CONCLUSION

Gd-DTPA-loaded PLGA microbubbles could enhance both ultrasound imaging and MRI imaging.

Tuber-specific cphA expression to enhance cyanophycin production in potatoes

The production of biodegradable polymers that can be used to substitute petrochemical compounds in commercial products in transgenic plants is an important challenge for plant biotechnology. Nevertheless, it is often accompanied by reduced plant fitness. To decrease the phenotypic abnormalities of the sprout and to increase polymer production, we restricted cyanophycin accumulation to the potato tubers by using the cyanophycin synthetase gene (cphA(Te)) from Thermosynechococcus elongatus BP-1, which is under the control of the tuber-specific class 1 promoter (B33). Tuber-specific cytosolic (pB33-cphA(Te)) as well as tuber-specific plastidic (pB33-PsbY-cphA(Te)) expression resulted in significant polymer accumulation solely in the tubers. In plants transformed with pB33-cphA(Te), both cyanophycin synthetase and cyanophycin were detected in the cytoplasm leading to an increase up to 2.3% cyanophycin of dry weight and resulting in small and deformed tubers. In B33-PsbY-cphA(Te) tubers, cyanophycin synthetase and cyanophycin were exclusively found in amyloplasts leading to a cyanophycin accumulation up to 7.5% of dry weight. These tubers were normal in size, some clones showed reduced tuber yield and sometimes exhibited brown sunken staining starting at tubers navel. During a storage period over of 32 weeks of one selected clone, the cyanophycin content was stable in B33-PsbY-cphA(Te) tubers but the stress symptoms increased. However, all tubers were able to germinate. Nitrogen fertilization in the greenhouse led not to an increased cyanophycin yield, slightly reduced protein content, decreased starch content, and changes in the amounts of bound and free arginine and aspartate, as compared with control tubers were observed.

Plastid targeting strategies for cyanophycin synthetase to achieve high-level polymer accumulation in Nicotiana tabacum

The production of biodegradable polymers in transgenic plants is an important challenge in plant biotechnology; nevertheless, it is often accompanied by reduced plant fitness. In order to decrease the phenotypic abnormalities caused by cytosolic production of the biodegradable polymer cyanophycin, and to increase polymer accumulation, four translocation pathway signal sequences for import into chloroplasts were individually fused to the coding region of the cyanophycin synthetase gene (cphA(Te)) of Thermosynechococcus elongatus BP-1, resulting in the constructs pRieske-cphA(Te), pCP24-cphA(Te), pFNR-cphA(Te) and pPsbY-cphA(Te). These constructs were expressed in Nicotiana tabacum var. Petit Havana SRI under the control of the constitutive cauliflower mosaic virus (CaMV) 35S promoter. Three of the four constructs led to polymer production. However, only the construct pPsbY-cphA(Te) led to cyanophycin accumulation exclusively in chloroplasts. In plants transformed with the pCP24-cphA(Te) and pFNR-cphA(Te) constructs, water-soluble and water-insoluble forms of cyanophycin were only located in the cytoplasm, which resulted in phenotypic changes similar to those observed in plants transformed with constructs lacking a targeting sequence. The plants transformed with pPsbY-cphA(Te) produced predominantly the water-insoluble form of cyanophycin. The polymer accumulated to up to 1.7% of dry matter in primary (T(0)) transformants. Specific T(2) plants produced 6.8% of dry weight as cyanophycin, which is more than five-fold higher than the previously published value. Although all lines tested were fertile, the progeny of the highest cyanophycin-producing line showed reduced seed production compared with control plants.

Production of cyanophycin, a suitable source for the biodegradable polymer polyaspartate, in transgenic plants

The production of biodegradable polymers in transgenic plants in order to replace petrochemical compounds is an important challenge for plant biotechnology. Polyaspartate, a biodegradable substitute for polycarboxylates, is the backbone of the cyanobacterial storage material cyanophycin. Cyanophycin, a copolymer of l-aspartic acid and l-arginine, is produced via non-ribosomal polypeptide biosynthesis by the enzyme cyanophycin synthetase. A gene from Thermosynechococcus elongatus BP-1 encoding cyanophycin synthetase has been expressed constitutively in tobacco and potato. The presence of the transgene-encoded messenger RNA (mRNA) correlated with changes in leaf morphology and decelerated growth. Such transgenic plants were found to produce up to 1.1% dry weight of a polymer with cyanophycin-like properties. Aggregated material, able to bind a specific cyanophycin antibody, was detected in the cytoplasm and the nucleus of the transgenic plants.

Immunological evaluation of the new stable ultrasound contrast agent LK565: a phase one clinical trial

Background

Ultrasound contrast agents (UCAs) allow the enhancement of vascular definition, thereby providing more diagnostic information. LK565 is a new second-generation UCA based on synthetic polymers of aspartic acid which is eliminated from the blood stream via phagocytosis. LK565 forms very stable air-filled microspheres and is capable of repeated passage through the pulmonary capillary bed after peripheral intravenous injection. This characteristic allows examination of the cardiac function or extracardiac vessel abnormalities up to 15 minutes.

Methods

A phase one clinical study was conducted on 15 healthy volunteers to identify the development of an undesirable immune response. Phagocytosis capacity, TNF-α secretion, and MHC class II upregulation of monocytes was monitored, as well as microsphere specific antibody development (IgM, IgG). Furthermore, the kinetics of the activation surface markers CD69, CD25, CD71, and CD11b on leukocytes were analyzed.

Results

Due to LK565-metabolism the administration of the UCA led to saturation of phagocytes which was reversible after 24 hrs. Compared to positive controls neither significant TNF-α elevation, neither MHC class II and activation surface markers upregulation, nor specific antibody development was detectable.

Conclusion

The administration of LK565 provides a comfortable duration of signal enhancement, esp. in echocardiography, without causing a major activation cascade or triggering an adaptive immune response. To minimize the risk of undesirable adverse events such as anaphylactoid reactions, immunological studies should be included in clinical trials for new UCAs. The use of LK565 as another new ultrasound contrast agent should be encouraged as a safe means to provide additional diagnostic information.