Substrate specificity of porcine renin: P1', P1, and P3 residues of renin substrates are crucial for activity

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

Clinical and molecular correlation defines activity of physiological pathways in life-sustaining kidney xenotransplantation

Porcine kidney xenotransplantation is accelerating towards clinical translation. However, despite the demonstrated ability of porcine kidneys to remove metabolic waste products, questions remain about their ability to faithfully recapitulate renal endocrine functions after transplantation. Here we analyze xenograft growth and function of two kidney dependent endocrine pathways in seventeen cynomolgus macaques after kidney xenotransplantation from gene edited Yucatan minipigs. Xenograft growth, the renin-angiotensinogen aldosterone-system, and the calcium-vitamin D-parathyroid hormone axis are assessed using clinical chemistries data, renin activity and beta-C-terminal-telopeptide assays, kidney graft RNA-sequencing and serial ultrasonography. We demonstrate that xenografts transplanted from minipigs show only modest growth and do not substantially contribute to recipient RAAS pathway activity. However, parathyroid hormone-independent hypercalcemia and hypophosphatemia are observed, suggesting a need for close monitoring and timely intervention during human testing. Further study of these phenotypes is warranted in designing prospective clinical trials.

Renin-angiotensin-aldosterone system function in the pig-to-baboon kidney xenotransplantation model

After pig-to-baboon kidney transplantation, episodes of hypovolemia and hypotension from an unexplained mechanism have been reported. This study evaluated the renin-angiotensin-aldosterone system post-kidney xenotransplantation. Kidneys from genetically-engineered pigs were transplanted into 5 immunosuppressed baboons after the excision of the native kidneys. Immunosuppressive therapy was based on the blockade of the CD40/CD154 costimulation pathway. Plasma renin, angiotensinogen (AGT), angiotensin II (Ang II), aldosterone levels, and urine osmolality and electrolytes were measured in healthy pigs, healthy nonimmunosuppressed baboons, and immunosuppressed baboons with life-supporting pig kidney grafts. After pig kidney transplantation, plasma renin and Ang II levels were not significantly different, although Ang II trended lower, even though plasma AGT and potassium were increased. Plasma aldosterone levels were unchanged. Urine osmolality and sodium concentration were decreased. Even in the presence of increasing AGT and potassium levels, lower plasma Ang II concentrations may be because of reduced, albeit not absent, the reactivity of pig renin to cleave baboon AGT, suggesting an impaired response of the renin-angiotensin-aldosterone system to hypovolemic and hypotensive episodes. The maintenance of aldosterone may be protective. The reduced urine osmolality and sodium concentration reflect the decreased ability of the pig kidney to concentrate urine. These considerations should not prohibit successful clinical pig kidney xenotransplantation.

The science of xenotransplantation for nephrologists

PURPOSE OF REVIEW

The field of xenotransplantation has seen remarkable progress since its inception with recent preclinical trials in human recipients pushing kidney xenotransplantation one-step closer to clinical reality. In this review, we update practicing clinicians on recent advances in kidney xenotransplantation given the proximity of clinical trials in humans.

RECENT FINDINGS

Early studies in the field established the physiologic basis of xenotransplantation and suggested that the pig kidney will support human physiology. Genetic engineering of source pigs has greatly reduced the immunogenicity of kidney grafts, and studies in nonhuman primates have demonstrated the viability of kidney xenotransplants for months after transplantation. Finally, a recent study in a novel preclinical human model demonstrated that key findings in NHP experiments are generalizable to humans, namely, the absence of hyperacute rejection.

SUMMARY

Overall, it appears that critical physiologic, immunologic and technical barriers to implementation of clinical trials in humans have been overcome.

Generation of functional chimeric kidney containing exogenous progenitor-derived stroma and nephron via a conditional empty niche

Generation of new kidneys can be useful in various research fields, including organ transplantation. However, generating renal stroma, an important component tissue for structural support, endocrine function, and kidney development, remains difficult. Organ generation using an animal developmental niche can provide an appropriate in vivo environment for renal stroma differentiation. Here, we generate rat renal stroma with endocrine capacity by removing mouse stromal progenitor cells (SPCs) from the host developmental niche and transplanting rat SPCs. Furthermore, we develop a method to replace both nephron progenitor cells (NPCs) and SPCs, called the interspecies dual replacement of the progenitor (i-DROP) system, and successfully generate functional chimeric kidneys containing rat nephrons and stroma. This method can generate renal tissue from progenitors and reduce xenotransplant rejection. Moreover, it is a safe method, as donor cells do not stray into nontarget organs, thus accelerating research on stem cells, chimeras, and xenotransplantation.

Physiological aspects of pig kidney xenotransplantation and implications for management following transplant

Successful organ transplantation between species is now possible, using genetic modifications. This article aims to provide a comprehensive overview of the differences and similarities in kidney function between humans, primates, and pigs, in preparation for pig-allograft to human xenotransplantation. The kidney, as the principal defender of body homeostasis, acts as a sensor, effector, and regulator of physiologic feedback systems. Considerations are made for anticipated effects on each system when a pig kidney is placed into a human recipient. Discussion topics include anatomy, global kidney function, sodium and water handling, kidney hormone production and response to circulating hormones, acid-base balance, and calcium and phosphorus handling. Based on available data, pig kidneys are anticipated to be compatible with human physiology, despite a few barriers.

Physiologic Aspects of Pig Kidney Transplantation in Nonhuman Primates

Xenotransplantation can provide a solution to the current shortage of human organs for patients with terminal renal failure. The increasing availability of genetically engineered pigs, effective immunosuppressive therapy, and antiinflammatory therapy help to protect pig tissues from the primate immune response and can correct molecular incompatibilities. Life-supporting pig kidney xenografts have survived in NHP for more than 6 mo in the absence of markers of consumptive coagulopathy. However, few reports have focused on the physiologic aspects of life-supporting pig kidney xenografts. We have reviewed the literature regarding pig kidney xenotransplantation in NHP. The available data indicate (1) normal serum creatinine, (2) normal serum electrolytes, except for a trend toward increased calcium levels and a transient rise in phosphate followed by a fall to slightly subnormal values, (3) minimal or modest proteinuria without hypoalbuminemia (suggesting that previous reports of proteinuria likely were due to a low-grade immune response rather than physiologic incompatibilities), (4) possible discrepancies between pig erythropoietin and the primate erythropoietin receptor, and (5) significant early increase in kidney graft size, which might result from persistent effects of pig growth hormone. Further study is required regarding identification and investigation of physiologic incompatibilities. However, current evidence suggests that, in the absence of an immune response, a transplanted pig kidney likely would satisfactorily support a human patient.

TAT-BoNT/A(₁₋₄₄₈), a novel fusion protein as a therapeutic agent: analysis of transcutaneous delivery and enzyme activity

Botulinum neurotoxin type A (BoNT/A) has been used as an injectable therapeutic agent for the treatment of some abnormal muscle contractions. In this study, TAT(47-57) peptide, a cell-penetrating peptide, was fused with the catalytic domain of BoNT/A for therapeutic purposes. HeLa and BE(2)-C cell lines were treated separately with purified TAT-BoNT/A(1-448) recombinant protein, and transduction of protein was analyzed by western blotting. Also, transcutaneous delivery through mouse skin surface was evaluated by immunohistochemistry. The in vitro catalytic activity of TAT-BoNT/A(1-448) was evaluated by HPLC. The presence of recombinant protein was detected in both of the cell lines as well as mouse skin cryosections after 60 and 120 min of incubation. The concentration of intracellular proteins was increased over time. HPLC analysis showed that this fusion protein has a biological activity 1.5 times as much as the full-length BoNT/A(1-448) protein. TAT-BoNT/A(1-448) fusion protein is biologically active and can transmit through living cells in vitro and in vivo successfully and more effectively compared with BoNT/A(1-448) protein as control.

The His-Pro-Phe motif of angiotensinogen is a crucial determinant of the substrate specificity of renin

The amino acid sequence His-Pro-Phe as N-terminal residues 6–8 of the natural renin substrate, angiotensinogen, is conserved among species. We investigated whether this His-Pro-Phe motif functions as the determinant of the substrate specificity of renin. Mutant angiotensinogens in which the Ile-His-Pro-Phe-His-Leu sequence at positions 5–10 of wild-type angiotensinogen was replaced by either His-Pro-Phe-His-Leu-Leu or Ala-Ile-His-Pro-Phe-His were cleaved by renin at the C-terminal side of residues 9 and 11, respectively, while wild-type angiotensinogen was cleaved at residue 10. A triple Ala substitution for the His-Pro-Phe motif of angiotensinogen prevented its cleavage by renin. In contrast, triple Ala substitution for residues 9–11, including the natural site of cleavage by renin, allowed cleavage between the two Ala residues at positions 10 and 11. Furthermore, the 33-residue C-terminal peptide of human megsin, which carries a naturally occurring His-Pro-Phe sequence, was cleaved by renin at the C-terminal side of the His-Pro-Phe-Leu-Phe sequence. These results indicate that the His-Pro-Phe motif of angiotensinogen is a crucial determinant of the substrate specificity of renin. By binding to a corresponding pocket on renin, the His-Pro-Phe motif may act as a molecular anchor to recruit the scissile peptide bond to a favorable site for catalysis.

Selected physiologic compatibilities and incompatibilities between human and porcine organ systems

The shortage of donor organs is a major barrier to clinical organ transplantation. Although xenotransplantation is considered one of the alternatives to human organ transplantation, there are immunologic and physiologic incompatibilities between humans and pigs. With the exception of coagulation, the major potential physiologic incompatibilities relating to function of the kidney, heart, liver, lungs, pancreatic islets, and hormones are reviewed. Some of these physiologic differences can be overcome by producing genetically altered pigs to improve compatibility with humans. The possibility of producing such pigs for organ transplantation is considered.

Substrate specificity of the hepatitis C virus serine protease NS3

The substrate specificity of a purified protein encompassing the hepatitis C virus NS3 serine protease domain was investigated by introducing systematic modifications, including non-natural amino acids, into substrate peptides derived from the NS4A/NS4B cleavage site. Kinetic parameters were determined in the absence and presence of a peptide mimicking the protease co-factor NS4A (Pep4A). Based on this study we draw the following conclusions: (i) the NS3 protease domain has an absolute requirement for a small residue in the P1 position of substrates, thereby confirming previous modelling predictions. (ii) Optimization of the P1 binding site occupancy primarily influences transition state binding, whereas the occupancy of distal binding sites is a determinant for both ground state and transition state binding. (iii) Optimized contacts at distal binding sites may contribute synergistically to cleavage efficiency.

Endoproteinase activity of type A botulinum neurotoxin: substrate requirements and activation by serum albumin

Type A botulinum neurotoxin, a zinc-dependent endoproteinase that selectively cleaves the neuronal protein SNAP-25, can also cleave relatively short peptides. We found that bovine and other serum albumins stimulated the type A-catalyzed hydrolysis of synthetic peptide substrates, through a direct effect on the kinetic constants of the reaction. Furthermore, with bovine serum albumin in the assays, the optimum substrate size was 16 residues (11 on the amino-terminal side of the cleavage site and 5 on the carboxy-terminal side). To further investigate the catalytic requirements of the neurotoxin, peptides were synthesized with various amino acid substitutions at the P5 through P5' substrate sites. Changes at all of these locations affected values for both kcat and K(m). Substitutions at the P2, P1', and P2' sites had more pronounced effects on hydrolysis rates than did substitutions at the P1 site. Enzyme-substrate interactions at the P3' threonine probably involved the side-chain methyl group rather than the hydroxyl group. Replacing the P2' alanine with leucine eliminated detectable hydrolysis, but not binding, since this peptide was an inhibitor. A negatively charged residue was preferred at P5, but not at P4. The data indicate that type A botulinum neurotoxin has an extended substrate recognition region and a requirement for arginine as the P1' residue.