Loss of CD26/DPP-4 activity has been described in both rat and murine models. A naturally occurring inactivating mutation has been described in a strain of Fischer 344 rats, whereas targeted inactivation of the CD26 gene produced a knockout phenotype in mice. Dpp4-/- mice, and incretin receptor knockout mice have proven useful for delineating the mechanism(s) of action of DPP-4 inhibitors

Inactivation of the CD26 gene through homologous recombination produces surprisingly normal mice. Despite the putative importance of DPP-4/CD26 for immune function, CD26-/- mice develop normally with no major immune phenotype reported in the absence of immunological challenge. Glycemic excursion is significantly reduced however, in association with increased levels of glucose-stimulated insulin and decreased degradation of both GLP-1 and GIP. This mouse defines an essential role for DPP-4 in the control of incretin degradation and glucose homeostasis. See Enhanced insulin secretion and improved glucose tolerance in mice lacking CD26. Proc Natl Acad Sci U S A 2000 97(12):6874-6879

The DPP-4 knockout mouse has been a useful model for defining essential peptide substrates whose levels are perturbed (increased or decreased) following genetic inactivation of DPP-4. Analysis of peptide/protein profiles using "peptidomics" demonstrated several peptides whose levels are elevated in kidney of DPP-4-/- mice, including diazepam binding inhibitor (92-105), meprin-b (21-41), sorbitol dehydrogenase (25-40), and elongation factors 1a (281-291) and elongation factor 2 (848-858). Conversely, several peptides were increased in DPP-4+/+ kidney, specifically DBI(94-103), Meprin-b (25-41), histidine-triad nucleotide-binding protein (42-63), vimentin(440-460), and mitochondrial 28s ribosomal protein S36(87-101). The validity of this data was illustrated by parallel analyses in WT mice treated with the DPP-4 inhibitor vildagliptin. These findings illustrate the potential for mass spectrometry-based peptide profiling for understanding the essential role of DPP-4 in peptide metabolism. See Peptidase substrates via global peptide profiling Nature Chemical Biology Published online: 16 November 2008; | doi:10.1038/nchembio.126

Several gut peptides and DPP-4 substrates, including GIP, PYY, and GLP-2, regulate bone resorption and/or bone formation. Although GLP-1 is not known to directly regulate skeletal homeostasis, Chizumi Yamada and colleagues have shown that the Glp1r-/- mouse exhibits a number of defects in bone density, including cortical osteopenia and enhanced bone fragility, likely attributable to increased bone resorption. Moreover, the actions of GLP-1R agonists appear to be indirect, possibly associated with reduced GLP-1R-dependent calcitonin production by rodent thyroid C cells. Whether treatment with GLP-1R agonists will modify bone density or strength has not been determined in humans. See The Murine Glp1r is essential for control of bone resorption Endocrinology 2008 Feb;149(2):574-9.

Intriguingly, as many of these hormones, specifically GLP-1, GIP, PYY, and GLP-2, are all DPP-4 substrates, changes, specifically reduction of DPP-4 activity, may result in potential perturbation in bone biology perhaps through indirect modulation of active peptide hormone activity. As patients with type 2 diabetes exhibit increased fracture rates due to a reduction in bone quality, Kyle and colleagues examined multiple parameters of bone strength and bone quality in Dpp4+/+ vs Dpp4-/- mice, with or without estrogen deficiency induced by ovariectomy (OVX). Kyle also studied the same parameters in wildtype mice placed on a high fat diet, with or without OVX, and treated with the DPP-4 inhibitor sitagliptin, or the thiazolidinedione, pioglitazone. The degree of glucose control achieved with the two different anti-diabetic agents was comparable for sitagliptin vs. pioglitazone, as assessed by glucose tolerance and HbA1c, however pioglitazone-treated mice exhibited enhanced insulin sensitivity, significant weight gain and fat accumulation. Consistent with previous reports, the TZD pioglitazone reduced bone mineral density, and bone mineral volume, and decreased bone quality as demonstrated by significant adverse changes in vertebral compression parameters. Pioglitazone also reduced the mineral apposition rate and increased bone marrow adiposity. In contrast, sitagliptin treatment of WT mice was largely neutral on multiple parameters of bone mineral density and bone quality, however modest increases in bone mineral density, trabecular bone mineralization profiles and trabecular architecture were observed with sitagliptin. Similarly, Dpp4-/- mice exhibited very little perturbation in bone mineral density or quality, however both sitagliptin-treated or Dpp4-/- mice did experience further reductions in BMD and bone quality following OVX. See Kyle, K.A., Willett, T.L., Baggio, L.L., Drucker, D. J., Grynpas, M. Differential Effects of PPAR-g Activation vs. Chemical or Genetic reduction of DPP-4 Activity on Bone Quality in Mice Endocrinology 2011

The DPP-4 knockout mouse appears relatively resistant to the development of glucose intolerance and diabetes following several months of high fat feeding. The mouse failed to become obese on a high fat diet, exhibits reduced food intake together with enhanced metabolic energy expenditure, and was comparatively resistant to diabetogenic doses of streptozotocin. The complete explanation for all of these findings is not known, but may be related in part to enhanced levels of incretins such as GLP-1 and GIP. See Mice lacking dipeptidyl peptidase IV are protected against obesity and insulin resistance. Proc Natl Acad Sci U S A. 2003 May 27; 100(11) :6825-30

Original studies of glucose homeostasis in the Fischer DPP-4 mutant rat by the Vancouver group (The enteroinsular axis in dipeptidyl peptidase IV-negative rats. Metabolism. 1996 Nov;45(11):1335-41)  had not shown evidence for improved glucose tolerance nor enhanced levels of GLP-1. A subsequent study from Nagakura and colleagues (Improved Glucose Tolerance via Enhanced Glucose-Dependent Insulin Secretion in Dipeptidyl Peptidase IV-Deficient Fischer Rats. Biochem Biophys Res Commun. 2001 Jun 8;284(2):501-6) characterized the enteroinsular axis in a Japanese strain of the DPP-4 mutant rat (F344/DuCrj). These investigators find enhanced glucose tolerance, increased insulin, and enhanced levels of glucose-stimulated GLP-1, consistent with the findings from the DPP-4-knockout mouse  Enhanced insulin secretion and improved glucose tolerance in mice lacking CD26. Proc Natl Acad Sci U S A 2000 97(12):6874-6879). The reasons for the different results obtained by the Japanese versus the Vancouver groups are not clear, but may be due to different experimental methodology, and a slightly different strain of the DPP-4-mutant rat. 

Consistent with findings in the study of CD26-/- mice following exposure to a high fat diet, Fischer344 mutant rats fed a high fat diet for 7 weeks were resistant to the development of obesity, and exhibited reduced food intake, decreased blood glucose, improved insulin sensitivity and increased levels of plasma GLP-1 as described in Improvement of high fat-diet-induced insulin resistance in dipeptidyl peptidase IV-deficient Fischer rats. Life Sci. 2002 May 31; 71(2):227-38

Similarly, in a subsequent study of immune function in CD26-/- mice, Yan and colleagues examined lymphocyte subpopulations, and the response to pokeweed mitogen (PWM). Several changes were observed in the cytokine response to PWM. Furthermore, serum levels of total IgG, IgG1, IgG2a and IgE, were lower in sera of CD26(-/-) mice following PWM and they detected reduced IL-4, IL-2 and delayed IFN-gamma production in the CD26-/- mice after PWM challenge. The authors conclude that CD26 contributes to the regulation of development, maturation and migration of CD4(+) T, NK and NKT cells, cytokine secretion, T cell-dependent antibody production and immunoglobulin isotype switching of β cells, as outlined in  Deficiency of CD26 results in a change of cytokine and immunoglobulin secretion after stimulation by pokeweed mitogen. Eur J Immunol. 2003 Jun; 33(6): 1519-27.

Subsequent analysis of the immune response in CD26-/- mice provides direct evidence for an essential role of CD26 in the control of Th1 immune responses, via TGF-b-1-dependent regulation of inflammation. CD26-/- mice exhibit diminished production of TGF-beta-1 in response to antigenic stimulation, and markedly enhanced severity of experimental inflammation, IL-2, and TNF-a following antigenic stimulation. See TGF-beta1-Mediated Control of Central Nervous System Inflammation and Autoimmunity through the Inhibitory Receptor CD26. J Immunol. 2007 Apr 1;178(7):4632-40

A link between CD26 expression/activity and levels of the proinflammatory chemokine stromal cell-derived factor-1 (SDF-1) was established in studies of experimental arthritis in mice, and observational studies in human subjects with rheumatoid arthritis. CD26-/- mice exhibited comparatively greater degrees of experimental joint inflammation following induction of antigen-induced arthritis, but no major differences in the basal humoral or cellular immune response were detected in CD26-/- vs. control mice with AIA. However T cell activation led to increased interferon-g production in lymph node supernatants from CD26-/- mice. The cleavage of the chemokine SDF-1 was modulated by CD26 activity both in vitro, and in vivo. Intriguingly, the expression of the receptor for SDF-1, CXCR-4, was increased in CD26-/- joint tissue. These findings raise the possibility that CD26 expression may modify the activity of experimental inflammation in part through changing the levels of chemokines with inflammatory activity. See Circulating CD26 Is Negatively Associated with Inflammation in Human and Experimental Arthritis. Am J Pathol. 2005 Feb;166(2):433-442

CD26/DPP-4 also appears to be a critical determinant of the metabolism of substance P. CD26-/- mice exhibit higher levels of circulating substance P (~2-fold) and exhibited short latencies to nociceptive stimuli such as hot plate or tail pinch testing. These findings were reversed following administration of a SP receptor antagonist. Similarly, administration of DPP-4 inhibitors reduced latencies in DPP-4 knockout mice, whereas administration of the DPP-4 enzyme reversed the phenotype and increased latencies in DPP-4 knockout mice. Hence these findings implicate an essential biological role for DPP-4 in control of substance P biology. See CD26 modulates nociception in mice via its dipeptidyl-peptidase IV activity. Behav Brain Res. 2005 Sep 7; [Epub ahead of print]

Original analyses of immune function and T cells from the CD26/DPP-4 mutant Fischer 344 rat In vitro immune responsiveness of rats lacking active dipeptidylpeptidase IV. Cell Immunol. 1994 Oct 15;158(2):269-80 or the CD26/DPP-4 knockout mouse did not  reveal major defects in lym p hocyte activation or immune function. 

However, the Fischer 344 mutant rat does exhibit defects in T cell subsets in response to immunization, or in the setting of experimental asthma. The number of CD4(+) T lymphocytes was markedly reduced compared to wild-type F344 and the decrease in T cell recruitment  in  CD26-deficient rats was associated with significantly reduced OVA-specific IgE-titres. See CD26 (dipeptidyl-peptidase IV)-dependent recruitment of T cells in a rat asthma model. Clin Exp Immunol. 2005 Jan;139(1):17-24

F344 rats also exhibit increased paratracheal edema following captopril administration whereas co-administration of the neurokinin receptor antagonist Spantide markedly attenuated the extent of edema formation as outlined in Dipeptidyl peptidase IV deficiency increases susceptibility to angiotensin-converting enzyme inhibitor-induced peritracheal edema. J Allergy Clin Immunol. 2007 May 24; [Epub ahead of print]

Is peptide YY highly sensitive to DPP-4? Aaboe and colleagues administered sitagliptin for 12 weeks to subjects with T2DM already treated with metformin. Peptide hormone levels were assessed following a MTT and a hyperglycemic clamp with arginine. There was no effect of sitagliptin on levels of intact GLP-2. In contrast,sitagliptintherapy reduced total PYY and PYY(3- 36), and increased PYY(1- 36) and intact GLP-1 and GIP. Hence, ambient meal-stimulated levels of PYY are highly sensitive to DPP-4, and reveal that PYY is an essential physiological substrate of DPP-4. See Twelve weeks treatment with the DPP-4 inhibitor, sitagliptin, prevents degradation of peptide YY and improves glucose and non-glucose induced insulin secretion in patients with type 2 diabetes mellitus Diabetes Obes Metab. 2010 Apr;12(4):323-33.

The importance of NPY as a DPP-4 substrate has been examined in several preclinical studies. Although DPP-4 inhibition alone does not appear to cause hypertension in human subjects, in select models, infusion of NPY in the presence of sitagliptin led to the potentiation of the renovascular responses to angiotensin II in the kidneys of SHR rats as described in Sitagliptin augments sympathetic enhancement of the renovascular effects of angiotensin II in genetic hypertension. Hypertension. 2008 Jun;51(6):1637-42. Similarly, inhibition of DPP-4 activity with P32/98 potentiated the hypertensive effects of NPY in SHR rats trated concomitantly with other blood presure lowering agents such as captopril or hydralazine Effects of dipeptidyl peptidase iv inhibition on arterial blood pressure. Clin Exp Pharmacol Physiol. 2008 Jan;35(1):29-34

NPY has also been implicated in the pathophysiology of inflammation-association edema, as DPP-4-inhibition potentiates the effects of inflammation-associated edema in the rat paw following Con-A admnisitration Effect of neuropeptide Y on inflammatory paw edema in the rat: involvement of peripheral NPY Y1 and Y5 receptors and interaction with dipeptidyl-peptidase IV (CD26). J Neuroimmunol. 2002 Aug;129(1-2):35-42.