Cloning of genes related to polyamine metabolism has enabled the generation of genetically modified mice and rats overproducing or devoid of proteins encoded by these genes. Our first transgenic mice overexpressing ODC (ornithine decarboxylase) were generated in 1991 and, thereafter, most genes involved in polyamine metabolism have been used for overproduction of the respective proteins, either ubiquitously or in a tissue-specific fashion in transgenic animals. Phenotypic characterization of these animals has revealed a multitude of changes, many of which could not have been predicted based on the previous knowledge of the polyamine requirements and functions. Animals that overexpress the genes encoding the inducible key enzymes of biosynthesis and catabolism, ODC and SSAT (spermidine/spermine N1-acetyltransferase) respectively, appear to possess the most pleiotropic phenotypes. Mice overexpressing ODC have particularly been used as cancer research models. Transgenic mice and rats with enhanced polyamine catabolism have revealed an association of rapidly depleted polyamine pools and accelerated metabolic cycle with development of acute pancreatitis and a fatless phenotype respectively. The latter phenotype with improved glucose tolerance and insulin sensitivity is useful in uncovering the mechanisms that lead to the opposite phenotype in humans, Type 2 diabetes. Disruption of the ODC or AdoMetDC [AdoMet (S-adenosylmethionine) decarboxylase] gene is not compatible with mouse embryogenesis, whereas mice with a disrupted SSAT gene are viable and show no harmful phenotypic changes, except insulin resistance at a late age. Ultimately, the mice with genetically altered polyamine metabolism can be used to develop targeted means to treat human disease conditions that they relevantly model.
The development of sophisticated gene-transfer techniques to manipulate the mammalian genome has enabled the study of the expression and function of individual genes in a whole-body context. With these techniques, the importance of any protein, structural or metabolism-related, can be subjected to evaluation in genetically modified animals. Based on the knowledge acquired, a considerable number of disease models mimicking various human pathologies have been generated.
The natural polyamines are critical for normal cellular growth and differentiation, and the tissue levels of individual polyamines are maintained optimal via complex regulatory mechanisms. Although these levels can be transiently manipulated by physiological stimuli or with inhibitors or inducers of polyamine metabolic enzymes, genetic manipulation is a method offering a versatile means to affect polyamine homoeostasis in different tissues and for an extended period, even throughout life. The animals with genetically altered polyamine synthesis or catabolism not only help to define the physiological importance of the polyamines in the whole body, but they also offer means to develop treatment modalities for pathophysiological conditions that result from distorted polyamine homoeostasis.
As polyamine synthesis is needed to maintain proliferation, and elevated polyamine levels are often associated with accelerated growth and cancer, it is not surprising that the first transgenic mice with genetically engineered polyamine metabolism were animals overexpressing the ODC (ornithine decarboxylase) gene encoding the key regulatory enzyme in polyamine biosynthesis. Depending on the extent and type of overexpression, somewhat controversial results have initially been reported. For instance, life-long ubiquitous overexpression of the ODC gene under its own promoter was, to our surprise, not associated with generally enhanced tumorigenesis in any tissue of the transgenic mice in comparison with wild-type littermates . The main factor determining this phenotype is most likely the fact that putrescine was not converted further into spermidine and spermine in the tissues of these animals , indicating that powerful regulatory mechanisms prevent overaccumulation of the higher polyamines to potentially harmful levels. In contrast, hair-follicle-targeted expression of truncated ODC cDNA resulted in prominent dermal ODC activity, polyamine accumulation, hair loss and spontaneous formation of malignant tumours in skin . Interestingly, the first visible characteristic of mice overexpressing ubiquitously the genomic construct encoding the key catabolic enzyme, SSAT (spermidine/spermine N1-acetyltransferase), was their hairlessness, which was likewise related to accumulation of putrescine in the skin . It is thus suggested that the overaccumulation of putrescine, either via de novo synthesis or via back conversion from spermidine and spermine, is a critical factor in the development of skin neoplasms.
In general, the altered phenotype of the transgenic mice may result from the perturbation of polyamine homoeostasis, from the impact of the accelerated polyamine metabolic cycle on other metabolic pathways, or from both.
Polyamine metabolism as target for genetic engineering
Figure 1 depicts the steps of the polyamine metabolic pathway and shows the enzymes that are the targets of overexpression or gene disruption. References for each transgenic line can be found in Table 1, albeit the list does not provide full coverage of characterization of the lines. The genes and cDNAs encoding the key enzymes of biosynthesis, the inducible ODC and AdoMetDC (S-adenosylmethionine decarboxylase), or catabolism, the inducible SSAT, have been used to generate both transgenic mice and rats. All of these genes have also been disrupted in knockout mice devoid of the respective enzyme proteins. Mice overexpressing Az (antizyme), a protein inhibitor of ODC, likewise serve as models possessing decreased ODC activity in targeted tissues. SpmS (spermine synthase)-overexpressing mice are phenotypically normal and viable , whereas knockout of this gene has not been achieved in mice. However, embryonic stem cells with a disrupted SpmS gene have been established and studied . Existence of a mutated mouse strain with an X-chromosomal deletion that affects SpmS and a phosphate-regulating gene Phex  compensates for the lack of gene-disrupted mice generated via targeted homologous recombination within the SpmS gene. The catabolic enzymes PAO (polyamine oxidase), catalysing the oxidation of acetylated spermidine and spermine, and SMO (spermine oxidase), catalysing the conversion of spermine directly into spermidine, appear to be the only enzymes immediately involved in polyamine metabolism, the overexpression of which in transgenic animals has not so far been reported.
Aspects and approaches in the production of rodent lines with genetically engineered polyamine metabolism
Methods of genetic modification
Mice or rats with transgene overexpression are usually produced via a conventional method of pronuclear injection of the transgene construct into one-cell embryos. The drawback of this method is the inability to control the site(s) of transgene integration and the copy number integrated. The random aberrant chromosomal environment may greatly influence the expression by either inducing or silencing it, and hence transgene copy number does not necessarily correlate with the level of expression. For gene inactivation, constructs with deleterious mutations disrupting the function of the genes are incorporated into embryonic stem cells via homologous recombination. These cells are used to produce chimaeric mice and eventually mice completely devoid of the functional gene. In this method, the site of manipulation is controlled and all the other genes remain intact. The lack of appropriate rat embryonic stem cells has unfortunately limited the use of this method to mice.
When we first started to use transgene techniques we chose genomic DNA, namely ODC and later AdoMetDC, SpdS (spermidine synthase) and SSAT genes under their own promoters, to work with. The idea was to have all of the regulatory elements in the constructs so that the expression of the transgenes would be regulated in a similar fashion to the expression of the endogenous genes. Such an approach would serve best the study of regulation of gene expression under different conditions in vivo. Others decided to use cDNAs of the same genes instead, with the obvious aim to achieve maximum overexpression, controlled by a strong and tissue-specific promoter and free of any potential silencer elements. The latter approach would serve the purpose of studying the consequences of robust overexpression in a given tissue. As an additional feature, inducibility of the promoter allows the follow-up of consequences of transgene induction at a desired time, whereas constitutive expression affects the animal throughout its life. As seen in Table 1, ubiquitous expression has been achieved with the genomic constructs, and targeted expression to selected tissues, such as skin, heart, pancreas and liver, has been achieved with the aid of tissue-specific promoters.
Genetic background of mice
Embryo production and quality is to some extent dependent on the mouse strain used. Depending on the preference of each transgene facility, transgenic lines coming from them may originally be generated and subsequently maintained in various mouse strains, which makes the direct comparison of transgenic lines difficult or questionable. It has also been noticed that phenotypic traits may be lost, or others acquired, upon backcrossing the mice to a different background from that originally used. C57Bl/6 is the most convenient mouse strain in terms of being the same as the background of the most knockout mice available. It should be pointed out here that, although we use embryos from a hybrid background (BalbC×DBA2) as recipients of transgenes and the initial characterization of transgenic mice has usually been carried out in the same background, the majority of phenotypic changes have been preserved upon later backcrossing the mice into a C57Bl/6 background. The importance of genetic background is also exemplified in the consequences of skin-targeted expression of Az: tumour incidence in response to two-stage carcinogenesis is decreased in a carcinogenesis resistant C57Bl/6J strain as compared with the sensitive DBA/2J strain, although both carry the same transgene in the same locus .
Despite the fact that success in generation of transgenic founders does not automatically guarantee expression of the transgenes and appearance of such a phenotype that is worth investigating, a great many transgenic mice seem to exhibit a noteworthy phenotype, often much more pleiotropic than expected at the time of construct design. When characterizing transgenic animals, it is initially necessary to compare two or more lines carrying the same transgene to exclude the potential effect of the integration site on the expression and the subsequent phenotype. In rare cases, transgene(s) may integrate within another gene causing a specific phenotype owing to inactivation of that gene and not exclusively to the expression of the transgene.
Table 1 lists most of the genetically modified rodents with altered polyamine metabolism that have been reported so far. The lines listed are singly transgenic lines, some of which show markedly changed phenotypes, whereas others carry no or very mild phenotypic changes in comparison with wild-type animals. The lack of a distinct phenotype in general may result from low transgene expression or some compensatory physiological responses. Because of the latter, no distinct phenotype may be observed, despite the transgene activity-driven alterations in polyamine homoeostasis. Comparison of phenotypes between individual mouse lines that overproduce the same enzyme may sometimes be confusing owing to controversial phenotypic characteristics. The phenotypic differences are obviously attributable to the differing regulatory elements in gene constructs, dissimilar levels of expression in a given tissue and the different genetic background of mice used. Fortunately, the observed phenotype in many transgenic mouse or rat lines may mimic some human diseases closely enough so that the animals can be used as models to study the pathogenetic processes of these diseases and the role of polyamines in them. The phenotypic changes in mouse and rat lines listed in Table 1 have been described in detail in extensive reviews [9–11].
Crossbreeding of genetically modified rodent lines together is a way to broaden the impact of genetic alterations and to study the modulatory effect of impaired polyamine homoeostasis on other models, especially those designed for carcinogenesis studies. For instance, doubly transgenic mice carrying both a K6-ODC transgene and a v-Ha-ras transgene developed spontaneous skin carcinomas, although either line alone did not , indicating the role for activated ODC and elevated polyamines in tumorigenesis. This conclusion is further supported by subsequent work showing that inhibition of ODC with DFMO (difluoromethylornithine) caused a regression of evolved tumours in K6-ODC/Ras mice . Overexpression of Az in the skin of doubly transgenic Az/MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] mice delayed tumorigenesis in comparison with MEK mice alone, leading to the same conclusion about the importance of polyamines in tumorigenesis . Yet in another example, haplosufficiency for ODC delayed lymphomagenesis in immunoglobulin enhancer (Emu)-myc-transgenic mice crossed with heterozygously ODC-deficient mice . In a recent study, deafness and sensitivity of Gy mice (carrying the SpmS deletion) to DFMO treatment could be reversed by crossing these mice with SpmS-overexpressing mice , emphasizing the importance of maintaining sufficient spermine levels in the inner ear.
Diseases associated with altered polyamine metabolism
Diseases associated with changes in polyamine levels
Central nervous system abnormalities
Our studies suggest an important beneficial role for putrescine in the brain of mice overexpressing either the ODC or SSAT gene under its own promoter. By using these transgenic animals, we have been able to show convincingly that putrescine accumulation elevates seizure threshold [17,18] and protects against ischaemia/reperfusion damage , but leads to impaired spatial learning [17,20]. These changes can be speculated to result from the role of putrescine as an antagonist of the NMDA (N-methyl-d-aspartate) receptor. Hence, the elevated putrescine level causes a partial blockade of the receptor. Hypoactivity in both sexes and spatial learning impairment in female SSAT-transgenic mice may also be associated with their altered hormone metabolism, leading to elevated circulating levels of adrenocorticotropin and corticosterone, and decreased levels of testosterone, thyroid-stimulating hormone and thyroxine . Such hormonal changes can be attributable to the activation of the hypothalamic–pituitary axis that has been indicated in depression, behavioural abnormalities and learning disabilities.
Kidney and liver injury
Although putrescine is believed to protect the brain against injury, its function in other tissues is not similar. For instance, activation of SSAT and consequent putrescine accumulation is associated with liver and kidney ischaemia/reperfusion injuries in wild-type animals. When SSAT-knockout mice were subjected to occlusion and reperfusion protocols, they were significantly protected against injuries . The results clearly suggest that lack of SSAT-mediated polyamine back-conversion is beneficial to these tissues. Whether the protection in SSAT-knockout animals is rather associated with lack of polyamine catabolism as such or with the maintenance of the levels of the higher polyamines is not clear. Renal spermidine and spermine levels were similar in both wild-type and knockout mice in response to the ischaemia/reperfusion procedure, whereas their hepatic levels were decreased substantially in the wild-type mice and slightly in SSAT-knockout mice . These results indicate that the mechanism of tissue damage is different in the two tissues, albeit it appears to be mediated by SSAT in both. Eventually, the by-products of polyamine catabolism produced in PAO-catalysed reactions may play a significant role in tissue injuries.
We observed that partial hepatectomy activated polyamine catabolism and caused a failure to initiate liver regeneration in SSAT-transgenic rats when compared with non-transgenic littermates . The transgenic rats showed accumulation of hepatic putrescine with a simultaneous profound decrease in spermidine and spermine contents. Apparently, putrescine alone was not sufficient to support liver regeneration. Pretreatment of the transgenic rats with α-methylspermidine prior to partial hepatectomy compensated for the loss of natural spermidine and restored liver regeneration, indicating that the regenerative process is indeed dependent on the presence of the higher polyamines . Thus the methylated polyamine analogues may have potential in the treatment of liver traumas involving activated polyamine catabolism.
The overaccumulation of putrescine is a critical factor in tumorigenesis, as already discussed above. Loss of hair after the first hair cycle has been shown in mice overexpressing ODC driven by the keratin promoter , or by SSAT driven by its own promoter  or by a metallothionein promoter . The common denominator in these mice is the excessive accumulation of putrescine in their skin. Lan et al.  used a model in which ODC was driven by an involucrin promoter and, moreover, could be induced by virtue of oestrogen receptor ligand-binding protein fused to ODC. Induction of the fusion transgene with 4-hydroxytamoxifen resulted in putrescine accumulation and stimulated keratinocyte activation. Vascularization with increased expression of differentiation markers similar to those seen in wound healing was similarly observed in the transgenic skin .
The hairless phenotype of SSAT-transgenic mice is associated with early degeneration of hair follicles evident at the first follicle regression phase, catagen . Dermal cysts and epidermal utriculi thereafter replace the normal hair follicles. Doubly transgenic mice, overexpressing both ODC and SSAT, exhibited an even higher accumulation of putrescine and more severe skin abnormalities than either singly transgenic line alone, again emphasizing the role of putrescine in this phenotype. In addition, inhibition of putrescine biosynthesis led to alleviation of the cutaneous changes and regrowth of hair . Analysis of epidermal differentiation markers revealed that keratinocyte differentiation was clearly impaired in the transgenic mice, as well as in organotypic cultures of keratinocytes overexpressing SSAT . These results demonstrate indisputably that polyamines, putrescine in particular, regulate the differentiation of keratinocytes.
Dramatic consequences following rapid depletion of spermidine and spermine are evident in transgenic animals in which the SSAT transgene is inducible by zinc through a metallothionein promoter and mainly expressed in pancreas and liver. Such rats develop acute necrotizing pancreatitis within 24 h after induction of transgene expression . Contrary to conditions in brain and skin, where putrescine plays a central role, development of pancreatitis is primarily caused by depletion of spermidine and spermine in the transgenic animals. The pancreas is the richest source of spermidine in the body, and development of dramatic polyamine deficiency clearly precedes the onset of tissue damage. The strongest evidence in support of the role of polyamines in maintenance of pancreatic integrity comes from findings that metabolically relatively stable methylated polyamine analogues that fulfil the physiological roles of the natural polyamines prevent tissue damage caused by trypsinogen activation, an early step in acute pancreatitis, and protect animals from mortality that is associated with severe pancreatitis [22,29]. The potential contribution of hydrogen peroxide and reactive aldehyde products of the PAO-catalysed reaction to tissue damage in the transgenic model could be ruled out, as inhibition of PAO did not alleviate pancreatitis at all . This model shows all of the general hallmarks of severe acute pancreatitis and could therefore be considered as a relevant model to study the pathogenesis of this disease. Activated polyamine catabolism leading to polyamine depletion has similarly been observed in standard experimental models of pancreatitis, induced by caerulein or l-arginine , or by taurodeoxycholate . Activation of polyamine catabolism appears to be a common phenomenon in the pathogenesis of pancreatitis induced by different stimuli, although it may not be an equally important factor in each case. The final outcome is a result of all of the pathways activated after a given stimulus.
Diseases associated with an accelerated polyamine metabolic cycle
Links to Type 2 diabetes
A thorough investigation of the mechanisms leading to the fatless phenotype in SSAT-transgenic mice has introduced a novel concept of polyamine catabolism-related phenotypic changes in white adipose tissue. Fat tissue of these mice exhibited an highly increased putrescine pool, but the levels of other polyamines were only moderately depleted [31,32]. The increased activities of SSAT and the biosynthetic enzymes could be taken as evidence for an accelerated polyamine metabolic flux in these animals . Indeed, this could be deduced from the depleted pools of ATP  and acetyl-CoA , both being immediately used in two reactions per each metabolic cycle depicted in Figure 2. The driving forces of this cycle thus are: (i) elevated SSAT activity, forced by transgene expression; (ii) activated ODC contributing to putrescine accumulation; and (iii) AdoMetDC activation facilitated by abundant putrescine and leading to adequate supply of dcAdoMet (decarboxylated AdoMet) for spermidine and spermine synthesis. The direct metabolic consequences of accelerated flux are: (i) excessive consumption of acetyl-CoA in SSAT-catalysed reactions; (ii) excessive consumption of ATP in the formation of AdoMet; and (iii) increased production of hydrogen peroxide and reactive aldehydes in PAO or SMO-catalysed reactions (Figure 2).
The downstream consequences of accelerated flux in a whole animal, such as seen in SSAT-transgenic mice, are far-reaching. Tissues exhibiting accelerated flux may suffer from energy shortage as such. Furthermore, rates of other metabolic pathways may be affected. In white adipose tissue, the elevated AMP/ATP ratio appears to induce AMPK (AMP-activated protein kinase) which in turn activates PGC-1α [PPARγ (peroxisome-proliferator-activated receptor γ) co-activator 1α] and reduces formation of malonyl-CoA from acetyl-CoA by inhibiting acetyl-CoA carboxylase. Excessive consumption of acetyl-CoA in polyamine flux as such leads to its limited availability and thus partly also results in depletion of malonyl-CoA. As malonyl-CoA is a substrate of fatty acid biosynthesis and negatively regulates fatty acid oxidation, its depletion is manifested as decreased fatty acid synthesis and increased fatty acid oxidation . PGC-1α is an important regulator of energy metabolism: it stimulates oxidative phosphorylation, thermogenesis, mitochondrial biogenesis, uncoupling, fatty acid oxidation and glucose transport. Most of these parameters were found to be increased in the fat tissue of SSAT-transgenic mice , as outlined in Figure 3. The fact that exogenously administered putrescine did not induce a similar expression profile in wild-type mice or glucose uptake in 3T3-L1 adipocytes ruled out the mechanisms that an elevated putrescine pool alone caused the striking phenotype of SSAT mice. As the metabolic profile of patients with Type 2 diabetes is totally opposite to that in SSAT-transgenic mice, the latter can be considered as a reverse model of the disease. Interestingly, and in support of our view, SSAT-deficient mice show an opposite metabolic profile when compared with SSAT mice. These mice gain fat and develop insulin resistance upon aging [32,33].
The rate of the metabolic cycle can be regulated by interfering with the enzymatic reactions involved in it. The flux in white adipose tissue was slowed down by inhibition of the de novo synthesis of putrescine by DFMO, which restored fat accumulation and the altered gene expression profiles . Overexpressing ODC together with SSAT can also accelerate the cycle further. Doubly transgenic mice with ODC and SSAT overexpression governed by a metallothionein promoter were shown to exhibit enhanced flux in liver . Since the same mice had more dramatic skin abnormalities , the effect of metabolic changes, such as depletion of ATP, and excessive production of toxic side-products cannot be excluded as factors contributing to the skin phenotype.
Differential modulation of tumorigenesis by SSAT expression can be seen in two experimental approaches. In one approach, SSAT-transgenic mice were crossed with TRAMP (transgenic prostate adenocarcinoma model) mice. It appeared that tumour outgrowth and progression were suppressed by enhanced SSAT activity in doubly transgenic mice when compared with TRAMP mice . The fact that the levels of the higher polyamine pools were minimally changed in the prostate of TRAMP/SSAT mice and the activities of the biosynthetic enzymes were increased, speaks for the mechanism that an enhanced polyamine cycle is the determinant of the outcome. This conclusion is also supported by the finding that, in the doubly transgenic mice, the prostatic levels of acetyl-CoA were significantly decreased in comparison with wild-type or TRAMP mice . In another approach, SSAT-transgenic and knockout mice were crossed with intestinal tumorigenesis model ApcMin/+ (MIN) mice. Unexpectedly, SSAT-overexpressing mice crossed with MIN mice developed more intestinal adenomas than MIN mice alone, whereas SSAT-knockout mice crossed with MIN mice developed fewer . Again, despite overexpression of SSAT in the former hybrid, spermidine and spermine levels were not greatly affected in normal intestinal or colonic tissue or in tumours therein, but the biosynthetic enzymes were activated as an indication of an accelerated polyamine cycle. The opposite was observed in the hybrid lacking SSAT activity. In light of these findings, one may conclude that the differential effect of enhanced polyamine cycle on tumorigenesis in various tissues may depend on such determinants as accumulation of flux-related metabolites, sensitivity of cells to them, and the overall metabolic environment .
Reproductive organs appear to be very sensitive to disturbed polyamine metabolism. Extremely high testicular overexpression of transgene-derived ODC and consequent overaccumulation of putrescine rendered a male founder mouse infertile, and also less pronounced overexpression in ODC-transgenic mice reduced reproductive performance of males due to disturbed spermatogenesis  as a result from altered spermatogonial DNA synthesis . In comparison with ODC-transgenic males, the males overexpressing SSAT were completely fertile. This difference is most probably explained by a very moderate expression of the SSAT transgene and modest accumulation of putrescine in the testes of SSAT-transgenic males . In contrast, the SSAT-overexpressing females developed ovarian hypofunction and uterine hypoplasia leading to infertility at a very young fertile age . Interestingly, the expression levels of numerous genes were different in the uterus and ovary of transgenic females when compared with those in their non-transgenic littermates . The exact roles of the differentially expressed genes in the observed phenotypic changes have not been clarified, but the findings tend to indicate that polyamines play an important role in controlling molecular mechanisms of reproductive tract development and function. In this context, it should again be borne to mind that the adult transgenic females had practically no visceral fat in their body. Therefore the possible effect of fatlessness in the hormonal regulation of reproduction in these mice cannot be ruled out.
Tissue-specific response to activated polyamine catabolism
Based on our findings with regard to the phenotypic changes in SSAT-transgenic mice (as described above), we can conclude that changes leading to pathogenesis in different tissues of these mice include distinctly disturbed homoeostasis, i.e. altered polyamine pattern, or virtually unaltered spermidine and spermine pools, but presumably enhanced flux or both. Activation of both biosynthetic enzymes, ODC and AdoMetDC, has been observed in all tissues studied, but the extent varies from tissue to tissue. This fact suggests the tissue-specific contribution as a driving force to the metabolic cycle. Among the tissues studied, some show clear-cut depletion of the higher polyamines (Table 2, categories A and B) to which the phenotypic changes may be primarily attributable. Other tissues with largely unaltered levels of the higher polyamines may be more prone to putrescine- and/or flux-mediated changes (Table 2, categories C and D).
Internal organs such as liver, pancreas, spleen, heart and kidney in adult SSAT-transgenic females and heart in young males were found to be enlarged . Organomegaly is most likely a compensatory response to activated polyamine catabolism-caused stress. It is thus not surprising that activated polyamine catabolism in mice is associated with reduced life expectancy as compared with wild-type mice, and that the lifespan of doubly transgenic mice with ODC and SSAT overexpression is even more reduced .
The examples of the uses of transgenic and knockout models described in the present chapter show that these animals are relevant in the studies of polyamine functions, but also as models of pathological conditions and some specific human diseases. The exact mechanisms involved in these conditions still remain elusive in many cases. For generation of more sophisticated transgenic disease models, well-designed constructs with tissue-specific and preferably inducible or conditional expression/knockout are needed. Such models are currently being developed in our laboratory to target altered transgene expression more precisely. For instance, SSAT is being expressed exclusively in the pancreas by virtue of a conditionally activated elastase promoter or in adipose tissue by virtue of an adipose-tissue-specific fatty-acid-binding protein (aP2) gene promoter.
From a technical point of view, better methods of genetic engineering may replace the conventional pronuclear microinjection and knockout technology. A modern and much more efficient way of transgenesis is to use lentiviral vectors to carry transgenes into embryos via viral transduction. Gene silencing with the aid of short-interfering RNAs is a method of choice to study the loss-of-function, and it is also applicable to rats.
• ODC or SSAT overexpression gives rise to a hairless phenotype that appears to be at least partially related to putrescine accumulation
• Induction of SSAT overexpression leads to delayed liver regeneration and development of acute pancreatitis following rapid depletion of the higher polyamines in SSAT-overexpressing rodents
• SSAT overexpression in fat tissue is associated with changes in glucose and energy metabolism that are regulated by accelerated polyamine metabolic flux, rather than by depleted polyamine pools
• Male reproduction is disturbed by ODC overexpression, whereas female reproduction is sensitive to SSAT overexpression
• SSAT expression modulates tumorigenesis differentially depending on the tissue
- © The Authors Journal compilation © 2009 Biochemical Society