A method for mass harvesting islets (Brockmann bodies) from teleost fish

Ancestral genomic duplication of the insulin gene in tilapia: An analysis of possible implications for clinical islet xenotransplantation using donor islets from transgenic tilapia expressing a humanized insulin gene

Tilapia, a teleost fish, have multiple large anatomically discrete islets which are easy to harvest, and when transplanted into diabetic murine recipients, provide normoglycemia and mammalian-like glucose tolerance profiles. Tilapia insulin differs structurally from human insulin which could preclude their use as islet donors for xenotransplantation. Therefore, we produced transgenic tilapia with islets expressing a humanized insulin gene. It is now known that fish genomes may possess an ancestral duplication and so tilapia may have a second insulin gene. Therefore, we cloned, sequenced, and characterized the tilapia insulin 2 transcript and found that its expression is negligible in islets, is not islet-specific, and would not likely need to be silenced in our transgenic fish.

A review of piscine islet xenotransplantation using wild-type tilapia donors and the production of transgenic tilapia expressing a "humanized" tilapia insulin

Most islet xenotransplantation laboratories have focused on porcine islets, which are both costly and difficult to isolate. Teleost (bony) fish, such as tilapia, possess macroscopically visible distinct islet organs called Brockmann bodies which can be inexpensively harvested. When transplanted into diabetic nude mice, tilapia islets maintain long-term normoglycemia and provide human-like glucose tolerance profiles. Like porcine islets, when transplanted into euthymic mice, they are rejected in a CD4 T-cell-dependent manner. However, unlike pigs, tilapia are so phylogenetically primitive that their cells do not express α(1,3)Gal and, because tilapia are highly evolved to live in warm stagnant waters nearly devoid of dissolved oxygen, their islet cells are exceedingly resistant to hypoxia, making them ideal for transplantation within encapsulation devices. Encapsulation, especially when combined with co-stimulatory blockade, markedly prolongs tilapia islet xenograft survival in small animal recipients, and a collaborator has shown function in diabetic cynomolgus monkeys. In anticipation of preclinical xenotransplantation studies, we have extensively characterized tilapia islets (morphology, embryologic development, cell biology, peptides, etc.) and their regulation of glucose homeostasis. Because tilapia insulin differs structurally from human insulin by 17 amino acids, we have produced transgenic tilapia whose islets stably express physiological levels of humanized insulin and have now bred these to homozygosity. These transgenic fish can serve as a platform for further development into a cell therapy product for diabetes.

Encapsulated piscine (tilapia) islets for diabetes therapy: studies in diabetic NOD and NOD-SCID mice

BACKGROUND

Our goal was to improve islet transplantation as a therapy for patients with type I diabetes mellitus. Because human donor islets are scarce, we are studying islet xenografts in the diabetic NOD mouse model. We hypothesize that optimal xenoislet survival will be achieved by the combination of donor islet immunoisolation with recipient immunosuppression. We and others have studied adult and neonatal porcine islets as sources of tissue for microencapsulated islet xenografts, but we believe it is also advantageous to consider using islets from fish, which can be raised in large numbers relatively quickly and economically. Therefore, in this study, we have evaluated the function of microencapsulated xenogeneic piscine (tilapia) islets transplanted intraperitoneally (IP) in NOD mice in the presence of CD4(+) T-cell depletion and/or costimulatory blockade.

METHODS

Spontaneously diabetic NOD mice or streptozotocin (STZ)-diabetic NOD-SCID mice were transplanted IP with microencapsulated tilapia islets. Recipient immunosuppression included anti-CD4 mAb, CTLA4-Ig, anti-CD80 mAb, anti-CD86 mAb, or anti-CD154 mAb, alone or in combination. Graft function was evaluated by blood glucose (BG) levels, intravenous (IV) and oral glucose tolerance tests (GTTs), histologic and immunohistochemical analyses of grafts, and flow cytometric analysis of peritoneal cells.

RESULTS

Encapsulated tilapia islets normalized random BG levels for up to 210 days in NOD-SCID mice. In diabetic NOD mice, encapsulated tilapia islets were rejected on day 11 ± 4 with a peritoneal infiltrate of macrophages, eosinophils, B cells, occasional neutrophils, but few T cells. Immunohistochemical staining demonstrated the presence of murine IgG on tilapia islets within capsules of rejecting, non-immunosuppressed mice, as well as murine IgG-positive lymphocytes in the layer of host cells surrounding those capsules. These findings suggested that our barium (Ba)-gelled alginate capsules are permeable to IgG and that anti-piscine antibodies may be involved in the rejection of encapsulated tilapia islets in untreated mice. No single immunosuppressive agent prolonged encapsulated tilapia islet survival in NOD mice, but the combination of CTLA4-Ig plus anti-CD154 mAb extended tilapia islet graft survival until rejection at 119 ± 20 days and inhibited host cell recruitment to the peritoneal cavity. Triple treatment with CTLA4-Ig, anti-CD154 mAb, and anti-CD4 mAb allowed graft survival for 157 ± 35 days with little evidence of a host cellular reaction. IV and oral glucose tolerance tests (GTTs) of recipients with functioning xenografts demonstrated remarkably normal metabolic function.

CONCLUSIONS

We conclude that microencapsulated tilapia islets can survive long term with excellent metabolic control in diabetic mice given targeted immunosuppression, suggesting that cross-species physiological incompatibility may not compromise the applicability of this novel approach for future clinical applications. We predict that an improved microcapsule that prevents the entrance of IgG will enhance tilapia islet survival in this model, possibly allowing the application of this technique with limited or no immunosuppression.

Regulation of insulin gene expression and insulin production in Nile tilapia (Oreochromis niloticus)

Compared to mammals, little is known about insulin gene expression in fish. Using transient transfection experiments and mammalian insulinoma cell lines we demonstrate that transcription of the Nile tilapia (Oreochromis niloticus) insulin gene is (a) regulated in a beta-cell-specific manner; and (b) not sensitive to the glucose stimulations. Deletion analysis of the 1575 bp 5' insulin gene flanking sequence revealed that cooperative interactions between regulatory elements within the proximal (-1 to -396 bp) and the distal (-396 bp to -1575 bp) promoter regions were necessary for induction of the beta-cell-specific transcription. Effects of glucose and arginine on endogenous insulin secretion, translation, and transcription in isolated tilapia Brockmann bodies were determined using Northern hybridization, Western analysis, and quantitative RT-PCR. Similar to the regulation of mammalian insulin, we found that increases of glucose (1-70 mM) and arginine (0.4-25 mM) induced insulin secretion. However, transcription of the insulin gene was activated only by extremely high concentrations of glucose and arginine added simultaneously. When stimulated for 24 h with low concentrations of both inducers or with either of them added separately, tilapia beta-cells were able to replenish secreted insulin and to maintain insulin stores at a constant level without elevations of the insulin mRNA levels. Since the basal level of insulin mRNA was approximately 3.7-fold higher in tilapia beta-cells than it is in mammalian beta-cells, insulin production in tilapia cells probably relies on an enlarged intracellular insulin mRNA pool and does not require the transcriptional activation of the insulin gene.

Insulin expression in the brain and pituitary cells of tilapia (Oreochromis niloticus)

While the presence of immunoreactive insulin in the central nervous system of many vertebrate species is well known, the origin of brain insulin is still debated. In this study, we applied RT-PCR, quantitative RT-PCR (qRT-PCR), and Northern hybridization to examine expression of the insulin gene in different tissues of an adult teleost fish, the Nile Tilapia (Oreochromis niloticus). We found that the insulin gene is transcribed at a high level in Brockmann bodies (pancreatic islet organs) and at a low level in the brain and pituitary gland. In the brain, insulin transcripts were detected in all areas by qRT-PCR and in situ hybridization. The highest level of insulin mRNA was found in the hypothalamus. The level of insulin transcription in the pituitary gland was 6-fold higher than that in the brain and 4.6-fold higher than that in the hypothalamus. Furthermore, insulin mRNA and immunoreactive insulin-like protein was detected in the pituitary gland using in situ hybridization, immunohistochemistry, and Western blot analysis. Our results indicate that in adult tilapia insulin expression is not restricted to the endocrine pancreatic cells, but also occurs in endocrine cells of the pituitary gland and in the neuronal cells of the brain, suggesting that the brain/pituitary gland might represent extrapancreatic origin of insulin production.

Things we have learned from tilapia islet xenotransplantation

An islet xenotransplantation model has been developed using tilapia (Oreochromis niloticus) as the donors. Studies using this model for the treatment of experimental type 1 diabetes in mice have produced promising results including the maintenance of long-term normoglycemia and mammalian-like glucose tolerance profiles in islet graft recipients. Islet encapsulation has also provided a promising method for the prevention of graft rejection, and strains of transgenic tilapia expressing a [desThrB30] human insulin molecule have been produced. In addition to studying islet transplantation for the treatment of type 1 diabetes, these studies have also produced insights into piscine glucose homeostasis. Studies demonstrating the glucose responsiveness of tilapia islets are described. In addition, work performed by our group and by others pertaining to presence and nature of piscine glucose transporters is reviewed. Finally, studies addressing some of the broader challenges of islet xenotransplantation are discussed with particular attention paid to the post-transplantation fate of the various islet cell populations and the proteins they produce.

Effect of glucose toxicity on intraportal tilapia islet xenotransplantation in nude mice

BACKGROUND

Discordant xenogeneic islets transplanted intraportally into athymic nude rats experience primary non-function and are rapidly destroyed. Recently, it has been reported that adult porcine islets transplanted intraportally into nude mice are also rapidly destroyed and that this constitutes a new model for instant blood-mediated inflammatory reaction (IBMIR).

METHODS

Tilapia (fish) islets were harvested, mechanically broken into mammalian islet-sized fragments, cultured for 48 h, and transplanted via the portal vein into athymic or euthymic mice.

RESULTS

There were several groups of recipient mice. Streptozotocin-diabetic nude mice received 400 islets via the portal vein (n = 12). Recipients were killed when hyperglycemic (>200 mg/dl); livers and native pancreases were examined histologically. Mean graft survival time, based on function, was 5.4 +/- 1.2 days; at autopsy, histology showed occasional viable islets. We also performed a group of transplants in non-diabetic nude mice (n = 6) and then killed the recipients 2 or 4 weeks later; all had abundant viable, well-granulated islet grafts based on histology. Therefore, the intraportal environs in nude mice are not incompatible with discordant fish islets; rather, it appears as if hyperglycemia adversely affects the intraportal islet grafts (i.e. ''glucose toxicity''). To test this hypothesis, transplants were performed into non-diabetic nude mice and allowed to engraft for either 3 days (n = 6) or 10 days (n = 8) prior to injection of streptozotocin (200 to 220 mg/kg i.v.) to destroy the beta-cells in the recipients' native islets (n.b. tilapia islets are exceedingly resistant to streptozotocin); these recipients were followed for 28 days post-transplantation (or until hyperglycemic) and then killed for histology. Mean graft function exceeded 25 days for both groups and viable well-granulated, tilapia islets grafts were readily identified in all recipients; in all but one, the native pancreases were markedly beta-cell depleted -- confirming that normoglycemia was due to functional fish islet xenografts.

CONCLUSIONS

Our results suggest that ''glucose toxicity'' plays a role in the immediate demise of intraportal tilapia islet xenografts.

Production of transgenic tilapia with Brockmann bodies secreting [desThrB30] human insulin

BACKGROUND

Tilapia are commercially important tropical fish which, like many teleosts, have anatomically discrete islet organs called Brockmann bodies. When transplanted into diabetic nude mice, tilapia islets provide long-term normoglycemia and mammalian-like glucose tolerance profiles.

METHODS

Using site-directed mutagenesis and linker ligation we have "humanized" the tilapia insulin gene so that it codes for [desThrB30] human insulin while maintaining the tilapia regulatory sequences. Following microinjection into fertilized eggs, we screened DNA isolated from whole fry shortly after hatching by PCR. Positive fish were grown to sexual maturity and mated to wild-types and positive Fl's were further characterized.

RESULTS

Human insulin was detected in both serum and in the clusters of beta cells scattered throughout the Brockmann bodies. Surrounding non-beta cells as well as other tissues were negative indicating beta cell specific expression. Purification and sequencing of both A-and B-chains verified that the insulin was properly processed and humanized.

CONCLUSIONS

After extensive characterization, transgenic tilapia could become a suitable, inexpensive source of islet tissue that can be easily mass-produced for clinical islet xenotransplantation. Because tilapia islets are exceedingly resistant to hypoxia by mammalian standards, transgenic tilapia islets should be ideal for xenotransplantation using immunoisolation techniques.

Tilapia islet grafts are highly alloxan-resistant

We have previously shown that dose-response studies performed in streptozotocin (STZ)-diabetic nude mouse recipients bearing established, functioning islet xenografts can be used to directly compare in vivo STZ-sensitivity between donor species and that tilapia (fish) islet grafts are exceedingly STZ-resistant. Using this method, we tested whether tilapia islets are sensitive to alloxan. Tilapia or rat islets were transplanted under the renal capsules of STZ-diabetic nude mice. Recipients with normal glucose tolerance tests (GTTs) on day 30-35 were injected with increasing i.v. doses of alloxan and blood glucose levels were followed for 5-7 days and then GTTs were repeated. Next, mice were killed and their grafts/native pancreata examined histologically (including insulin stains). Control nude mice were also injected with increasing i.v. doses of alloxan. Based upon non-fasting blood glucose levels, GTT, and graft histology, the following observations were made: (1) Tilapia islet xenografts were uniformly resistant to i.v. doses of 75 mg/kg (n=3), 150 mg/kg (n=4), and 300 mg/kg (n=3). (2) Rat islet recipients became uniformly severely diabetic after alloxan i.v. doses of 50-70 mg/kg (n=6) (i.e., equivalent to the dosage needed to induce diabetes in rats). (3) Control nude mice were severely diabetic at doses of 75 mg/kg (4/5) and 150 mg/kg (n=3/3). Alloxan dose-response studies were also performed in tilapia. Interestingly, tilapia appeared more sensitive than tilapia islet grafts. Although 75 mg/kg i.v. had little effect in tilapia, higher doses caused severe beta cell necrosis, diabetes, and systemic damage; however, this seeming discrepancy can be explained as tilapia have about one-quarter of the blood volume of mice (i.e., as a percentage of body weight) and so the actual concentration in the blood was about 4-fold higher at each dose. We conclude that tilapia beta cells are highly resistant to the beta cell toxin alloxan.

Evidence that tilapia islets do not express alpha-(1,3)gal: implications for islet xenotransplantation

BACKGROUND

Cell therapy for diabetes using teleost fish islet tissue has emerged as an intriguing alternative to the use of islet tissue from mammalian pancreases. The islet tissue, called Brockman bodies (BBs), is anatomically distinct from the pancreatic exocrine tissue and can be easily identified and isolated. Islets harvested from Nile tilapia (Oreochromis niloticus), when transplanted into streptozotocin-diabetic nude mice, produce long-term normoglycemia and achieve mammalian-like glucose tolerance profiles. We asked whether tilapia express the alpha-(1,3)gal epitope, the immunodominant target of human xenogeneic responses.

METHODS AND RESULTS

Immunostaining with the alpha-(1,3)gal-specific IB4 lectin on tilapia BB, liver, heart, spleen, and head kidney was negative, as was staining with murine anti-alpha-gal-specific monoclonal antibodies. Absence of alpha-gal-specific binding of IB4 or murine anti-gal mAbs to dispersed BBs was confirmed by fluorescent-activated cell sorter analysis. Tilapia BB cell membranes failed to reduce binding of anti-alpha-(1,3)gal-specific mAb in an enzyme-linked immunosorbent assay (ELISA) inhibition assay, while porcine and murine tissue lysates did. Tilapia BB cell lysates were shown to be devoid of alpha-1,3 galactosyltransferase activity by ELISA. Transplantation of tilapia BBs into diabetic alpha-gal knockout (gal KO) mice was not associated with accelerated xenograft rejection when compared with wild type control recipients (mean survival time 6.5 days vs. 7.2 days). Tilapia BBs failed to induce a rise in anti-gal IgG and IgM titers in gal KO mice, while the transplant of wild type mouse islets into gal KO mice caused a significant rise in anti-gal IgG and IgM antibodies.

CONCLUSIONS

We conclude that tilapia BBs are devoid of alpha-gal expression, and may offer an alternative to swine as a donor species for islet xenotransplantation.

Piscine Islet Xenotransplantation

Tilapia, a teleost fish species with large anatomically discrete islet organs (Brockmann bodies; BBs) that can be easily harvested without expensive and fickle islet isolation procedures, make an excellent donor species for experimental islet xenotransplantation research. When transplanted into streptozotocin-diabetic nude or severe combined immunodeficient mice, BBs provide long-term normoglycemia and mammalian-like glucose tolerance profiles. However, when transplanted into euthymic recipients, the mechanism of islet xenograft rejection appears very similar to that of islets from "large animal" donor species such as the very popular fetal/neonatal porcine islet cell clusters (ICCs). Tilapia islets are more versatile than ICCs and can be transplanted (1) into the renal subcapsular space, the cryptorchid or noncryptorchid testis, or intraportally as neovascularized cell transplants; (2) as directly vascularized organ transplants; or (3) intraperitoneally after microencapsulation. Unlike the popular porcine ICCs, BBs function immediately after transplantation; thus, their rejection can be assessed on the basis of loss of function as well as other parameters. We have also shown that transplantation of tilapia BBs into nude mice can be used to study the possible implications of cross-species physiological incompatibilities in xenotransplantation. Unfortunately, tilapia BBs might be unsuitable for clinical islet xenotransplantation because tilapia insulin differs from human insulin by 17 amino acids and, thus, would be immunogenic and less biologically active in humans. Therefore, we have produced transgenic tilapia that express a "humanized" tilapia insulin gene. Future improvements on these transgenic fish may allow tilapia to play an important role in clinical islet xenotransplantation.

Clinical islet transplantation

Type 1 diabetes affects over 1 million persons in the United States, with over 30,000 new cases diagnosed annually. Transplantation of new insulin-producing b cells, in the form of the whole pancreas or isolated islets, has been shown to ameliorate the disease by eliminating the need for exogenous insulin and normalizing glycosylated hemoglobin levels. Islet transplants are a particularly attractive form of therapy because they are a minimally invasive procedure and are more likely to be scaled-up to treat the large numbers of people affected by diabetes. Currently, only a handful of programs have been successful in the endeavor. Nevertheless, the early clinical experience strongly demonstrates that islet transplantation is an effective treatment strategy in select patients with type 1 diabetes. To scale up this therapy and use it earlier in the disease and for more people, the shortage of suitable donor tissue must be solved and the requirement of lifelong immunosuppression must be minimized.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Streptozotocin dose-response curve in tilapia, a glucose-responsive teleost fish

Streptozotocin (STZ) causes beta cell necrosis and insulin-dependent diabetes in many species. The specificity of this beta cell toxin relates to its structure as an alkylating agent with an attached glucose moiety. STZ uptake by rodent beta cells appears to be via the GLUT-2 glucose transporter. Teleost fish, in general, are severely glucose intolerant. The effects of STZ were examined in tilapia, a teleost fish with highly glucose-responsive islets. Fasted tilapia were given 0, 100, 150, 200, 250, 300, or 350 mg/kg STZ iv. Plasma glucose levels were followed for 72 h and the fish autopsied. Histological sections of islets were stained by immunoperoxidase for tilapia insulin. Severe hyperglycemia was seen in 20, 80, and 100% of fish receiving 250, 300, and 350 mg/kg doses; however, sections of islets showed only partial degranulation with no evidence of beta cell necrosis. Another group of fish receiving the highest dose were followed longer to determine whether beta cell necrosis and permanent hyperglycemia ensued. All fish died or were killed within 9 days because of severe hepatic failure characterized by hepatic necrosis, jaundice, and ascites; islet morphology was relatively normal suggesting, even in a glucose-sensitive species, that fish islets either do not take up STZ or are highly resistant to its "diabetogenic" effects. Tilapia may thus be a useful model to elucidate mechanisms of action of STZ. Furthermore, STZ may provide important insights into differences in glucose uptake and metabolism by mammalian and piscine beta cells.

Immunocytochemical characterization of the pancreatic islet cells of the Nile Tilapia (Oreochromis niloticus)

The cellular composition and topography of the pancreatic islet of Oreochromis niloticus, now known to be a donor source for islet xenotransplantation studies, were characterized. Whole tilapia islets were harvested using an enzymatic method and then further digested into single-cell preparations. Cell cytospin preparations of islet cells and paraffin sections of whole islets were stained using antisera against tilapia insulin, human glucagon, salmon somatostatin-25 (SST-25), human somatostatin-14 (SST-14), and salmon peptide tyrosine-tyrosine (PYY) using the immunoperoxidase method. Cell counts, performed on cytospin preparations using a Quantimet 570 computerized image analysis system, revealed that O. niloticus islets contained 78% endocrine cells and 22% immunonegative cells (i. e., mainly nucleated erythrocytes and rare tissue eosinophils). The proportions of immunopositive endocrine cell types were: 42.3% insulin immunopositive cells, 11.5% glucagon immunopositive cells, 23.1% SST-25 immunopositive cells, 21.8% SST-14 immunopositive cells, and 1.3% PYY immunopositive cells. Islet cell topography was evaluated using histologic sections of whole endocrine pancreata including large, medium, and small islets. Round to polygonal insulin immunopositive cells with round central nuclei were distributed in clusters throughout both the principal and the smaller islets. Elongate SST-14 immunopositive cells were closely associated with the clusters of insulin immunopositive cells; both were surrounded by SST-25 immunopositive cells, which were similar in shape to the insulin immunopositive cells. There were elongate glucagon immunopositive cells throughout the islets, whereas the PYY immunopositive cells were restricted to the periphery and to channels of fibrovascular connective tissue penetrating the islets.

GLUT-4 Deficiency and severe peripheral resistance to insulin in the teleost fish tilapia

Teleost fish, in general, are glucose intolerant; this trait has been attributed to piscine islets secreting insulin primary in response to amino acid secretogogues rather than glucose. However, pancreatic islet from the teleost fish tilapia, when transplanted into diabetic nude mice, were glucose responsive even though tilapia were severely glucose intolerant. This suggested a strong peripheral resistance to the glucostatic effects of insulin. Using Western blotting with polyclonal antibodies as well as Northern analysis for mRNA, tilapia tissues were found to be devoid of GLUT-4, the insulin-sensitive glucose transporter responsible for the hypoglycemic effect of insulin in mammals. The absence of GLUT-4 in peripheral tissues may explain why tilapia, and possibly other teleost fish, are severely glucose intolerant. This suggests that tilapia islets have evolved along mammalian lines to be glucose sensitive while tilapia peripheral tissue have diverged widely. Using the same methods, tilapia were found to have a very limited tissue distribution of the insulin-independent glucose transporter, GLUT-1, which is responsible for basal glucose transport in mammalian cells. It is suggested that tilapia provide a naturally occurring GLUT-4 knockout model.

Functional comparison of mouse, rat, and fish islet grafts transplanted into diabetic nude mice

Equal volumes of teleost fish (tilapia), Lewis rat, or CD-1 mouse islets were transplanted under the kidney capsules of streptozotocin-diabetic athymic nude mice. Nonfasting blood glucose levels were monitored in recipient mice over a period of 30 days. Mean nonfasting blood glucose levels in recipients of tilapia (n = 7), rat (n = 8), and murine (n = 8) islets were 78.8, 77.0, and 115 mg/dl, respectively. Mean blood glucose levels were significantly higher in recipients of murine islets than in recipients of fish and rat islets. After Day 30, intraperitoneal glucose tolerance tests were performed on recipient mice. Mean fasted blood glucose levels in mouse, rat, and fish islet recipients were 113.3, 89.8, and 72.7 mg/dl, respectively. All three groups of recipient mice had similar glucose tolerance profiles with mean glucose disappearance rates (K values) between 4.3 and 5.7. Tilapia islet grafts resulted in a significantly lower baseline for blood glucose values than either rat or mouse islet grafts.