Transcriptomic analysis of diabetic kidney disease and neuropathy in mouse models of type 1 and type 2 diabetes

Diabetic kidney disease (DKD) and diabetic peripheral neuropathy (DPN) are common complications of type 1 (T1D) and type 2 (T2D) diabetes. However, the mechanisms underlying pathogenesis of these complications are unclear. In this study, we optimized a streptozotocin-induced db/+ murine model of T1D and compared it to our established db/db T2D mouse model of the same C57BLKS/J background. Glomeruli and sciatic nerve transcriptomic data from T1D and T2D mice were analyzed by self-organizing map and differential gene expression analysis. Consistent with prior literature, pathways related to immune function and inflammation were dysregulated in both complications in T1D and T2D mice. Gene-level analysis identified a high degree of concordance in shared differentially expressed genes (DEGs) in both complications and across diabetes type when using mice from the same cohort and genetic background. As we have previously shown a low concordance of shared DEGs in DPN when using mice from different cohorts and genetic backgrounds, this suggests that genetic background may influence diabetic complications. Collectively, these findings support the role of inflammation and indicate that genetic background is important in complications of both T1D and T2D.

Interleukin-10 Reduces Neurogenic Inflammation and Pain Behavior in a Mouse Model of Type 2 Diabetes

Purpose

Neurogenic inflammation is a major component of chronic neuropathic pain. Previously, we established the db/db mouse as an animal model of painful diabetic neuropathy (PDN) of type 2 diabetes. In the current study, we investigate the roles of interleukin (IL)-10, an anti-inflammatory cytokine, in the development of neurogenic inflammation and pain behavior in db/db mouse.

Materials and Methods

We first studied IL-10 expression in lumbar dorsal root ganglion (LDRG) neurons of db/db mice using immunohistochemistry, immunoblots, and reverse transcription polymerase chain reaction during the period of pain behavior (from 8 to 16 wk of age). To determine if the reduced IL-10 expression mediates the mechanical allodynia in db/db mice, we administered recombinant mouse IL-10 or saline (control) intraperitoneally to control db/+ and db/db mice starting at 8 wk of age. IL-10 treatment was repeated every other day for 2 wk until the mice reached 10 wk of age.

Results

During the period of pain behavior, reduction of IL-10 protein and gene expression was detected in LDRG of db/db mice. Treatment with recombinant IL-10, from 8 to 10 wk of age, alleviates pain behaviors in db/db mice without affecting other diabetic parameters. In parallel, IL-10 treatment reduced the upregulation of nerve growth factor (NGF), inducible nitric oxide synthase (iNOS), tumor necrosis factor (TNF)-α, and high-affinity NGF receptor (Trk A) in LDRG, as well as the numbers of iNOS-positive Langerhans cells and CD-68-positive dermal dendritic cells in the hind-foot-pad skin in db/db mice.

Conclusion

Our findings suggest that the reduction in neuronal IL-10 increases inflammatory phenomena, ultimately contributing to PDN. These results suggest that the dysregulation of cytokine-mediated inflammation contributes to the development of PDN in db/db mice. Targeting this pathophysiologic mechanism could be an effective approach for treating PDN in type 2 diabetes.

Juvenile murine models of prediabetes and type 2 diabetes develop neuropathy

Peripheral neuropathy (neuropathy) is a common complication of obesity and type 2 diabetes in children and adolescents. To model this complication in mice, 5-week-old male C57BL/6J mice were fed a high-fat diet to induce diet-induced obesity (DIO), a model of prediabetes, and a cohort of these animals was injected with low-dose streptozotocin (STZ) at 12 weeks of age to induce hyperglycemia and type 2 diabetes. Neuropathy assessments at 16, 24 and 36 weeks demonstrated that DIO and DIO-STZ mice displayed decreased motor and sensory nerve conduction velocities as early as 16 weeks, hypoalgesia by 24 weeks and cutaneous nerve fiber loss by 36 weeks, relative to control mice fed a standard diet. Interestingly, neuropathy severity was similar in DIO and DIO-STZ mice at all time points despite significantly higher fasting glucose levels in the DIO-STZ mice. These mouse models provide critical tools to better understand the underlying pathogenesis of prediabetic and diabetic neuropathy from youth to adulthood, and support the idea that hyperglycemia alone does not drive early neuropathy.

This article has an associated First Person interview with the first author of the paper.

Summary: The mouse models described in this paper provide critical tools to better understand the underlying pathogenesis of prediabetic and diabetic neuropathy from youth to adulthood.

Cytokine-mediated inflammation mediates painful neuropathy from metabolic syndrome

Painful neuropathy (PN) is a prevalent condition in patients with metabolic syndrome (MetS). However, the pathogenic mechanisms of metabolic syndrome-associated painful neuropathy (MetSPN) remain unclear. In the current study, high-fat-fed mice (HF mice) were used to study MetSPN. HF mice developed MetS phenotypes, including increased body weight, elevated plasma cholesterol levels, and insulin resistance in comparison with control-fat-fed (CF) mice. Subsequently, HF mice developed mechanical allodynia and thermal hyperalgesia in hind paws after 8 wk of diet treatment. These pain behaviors coincided with increased densities of nociceptive epidermal nerve fibers and inflammatory cells such as Langerhans cells and macrophages in hind paw skin. To study the effect of MetS on profiles of cytokine expression in HF mice, we used a multiplex cytokine assay to study the protein expression of 12 pro-inflammatory and anti-inflammatory cytokines in dorsal root ganglion and serum samples. This method detected the elevated levels of proinflammatory cytokines, including tumor necrosis factor (TNF)-α, and interleukin (IL)-6, IL-1β as well as reduced anti-inflammatory IL-10 in lumbar dorsal root ganglia (LDRG) of HF mice. Intraperitoneal administration of IL-10 reduced the upregulation of pro-inflammatory cytokines and alleviated pain behaviors in HF mice without affecting MetS phenotypes. Our findings suggested targeting HF-induced cytokine dysregulation could be an effective strategy for treating MetSPN.

The Metabolic Syndrome and Microvascular Complications in a Murine Model of Type 2 Diabetes

To define the components of the metabolic syndrome that contribute to diabetic polyneuropathy (DPN) in type 2 diabetes mellitus (T2DM), we treated the BKS db/db mouse, an established murine model of T2DM and the metabolic syndrome, with the thiazolidinedione class drug pioglitazone. Pioglitazone treatment of BKS db/db mice produced a significant weight gain, restored glycemic control, and normalized measures of serum oxidative stress and triglycerides but had no effect on LDLs or total cholesterol. Moreover, although pioglitazone treatment normalized renal function, it had no effect on measures of large myelinated nerve fibers, specifically sural or sciatic nerve conduction velocities, but significantly improved measures of small unmyelinated nerve fiber architecture and function. Analyses of gene expression arrays of large myelinated sciatic nerves from pioglitazone-treated animals revealed an unanticipated increase in genes related to adipogenesis, adipokine signaling, and lipoprotein signaling, which likely contributed to the blunted therapeutic response. Similar analyses of dorsal root ganglion neurons revealed a salutary effect of pioglitazone on pathways related to defense and cytokine production. These data suggest differential susceptibility of small and large nerve fibers to specific metabolic impairments associated with T2DM and provide the basis for discussion of new treatment paradigms for individuals with T2DM and DPN.

The role of oxidative stress in nervous system aging

While oxidative stress is implicated in aging, the impact of oxidative stress on aging in the peripheral nervous system is not well understood. To determine a potential mechanism for age-related deficits in the peripheral nervous system, we examined both functional and morphological changes and utilized microarray technology to compare normal aging in wild-type mice to effects in copper/zinc superoxide dismutase-deficient (Sod1(-/-)) mice, a mouse model of increased oxidative stress. Sod1(-/-) mice exhibit a peripheral neuropathy phenotype with normal sensory nerve function and deficits in motor nerve function. Our data indicate that a decrease in the synthesis of cholesterol, which is vital to myelin formation, correlates with the structural deficits in axons, myelin, and the cell body of motor neurons in the Sod1(+/+) mice at 30 months and the Sod1(-/-) mice at 20 months compared with mice at 2 months. Collectively, we have demonstrated that the functional and morphological changes within the peripheral nervous system in our model of increased oxidative stress are manifested earlier and resemble the deficits observed during normal aging.

Increased axonal regeneration and swellings in intraepidermal nerve fibers characterize painful phenotypes of diabetic neuropathy

We examined changes in intraepidermal nerve fibers (IENFs) to differentiate patients with diabetic neuropathy (DN) and diabetic neuropathic pain (DN-P) from those with DN without pain (DN-NOP). Punch skin biopsies were collected from the proximal thigh (PT) and distal leg (DL) of normal subjects, patients with type 2 diabetes without evidence of DN (DM), or DN-P and DN-NOP patients. Protein gene product 9.5-positive (PGP+) immunohistochemistry was used to quantify total IENF, and growth-associated protein 43 (GAP43) for regenerating IENF. Compared to normal subjects and patients with type 2 diabetes without evidence of DN, both DN-P and DN-NOP have reduced PGP+ IENF densities in DL and PT. Although GAP43+ IENF densities were also reduced in DL for both DN-P and DN-NOP, the GAP43+ IENF densities in PT of DN-P remained at the control levels. Higher GAP43/PGP ratios were detected in DN-P compared to DN-NOP in the DL and PT. In parallel, increased numbers of axonal swellings per PGP+ fiber (axonal swelling/PGP) were detected in DN-P compared to normal subjects, patients with type 2 diabetes without evidence of DN, and DN-NOP in the DL. These axonal swellings were positive for tropomyosin-receptor-kinase A and substance P, suggesting that they are associated with nociception.

PERSPECTIVE

Among patients with DN, the ratios of GAP43/PGP and axonal swelling/PGP are likely to differentiate painful from painless phenotypes.

Neurogenic factor-induced Langerhans cell activation in diabetic mice with mechanical allodynia

Background

Langerhans cells (LCs) are antigen-presenting dendritic cells located in the skin. It has been reported that LC activation is associated with painful diabetic neuropathy (PDN); however, the mechanism of LC activation is still unclear.

Methods

The db/db mouse, a rodent model of PDN, was used to study the roles of LCs in the development of PDN in type 2 diabetes. Hind foot pads from db/db and control db/+ mice from 5 to 24 weeks of age (encompassing the period of mechanical allodynia development and its abatement) were collected and processed for immunohistochemistry studies. LCs were identified with immunohistochemistry using an antibody against CD207 (Langerin). The intraepidermal nerve fibers and subepidermal nerve plexus were identified by immunohistochemistry of protein gene product 9.5 (PGP 9.5) and tropomyosin-receptor kinase (Trk) A, the high affinity nerve growth factor receptor.

Results

CD207-positive LCs increased in the db/db mouse during the period of mechanical allodynia, from 8 to 10 weeks of age, in both the epidermis and subepidermal plexus. At 16 weeks of age, when mechanical allodynia diminishes, LC populations were reduced in the epidermis and subepidermal plexus. Epidermal LCs (ELCs) were positive for Trk A. Subepidermal LCs (SLCs) were positive for CD68, suggesting that they are immature LCs. Additionally, these SLCs were positive for the receptor of advanced glycation end products (RAGE) and were in direct contact with TNF-α-positive nerve fibers in the subepidermal nerve plexus during the period of mechanical allodynia. Intrathecal administration of SB203580, a p38 kinase inhibitor, significantly reduced mechanical allodynia, TNF-α expression in the subepidermal plexus, and increased both ELC and SLC populations during the period of mechanical allodynia.

Conclusions

Our data support the hypothesis that increased LC populations in PDN are activated by p38-dependent neurogenic factors and may be involved in the pathogenesis of PDN.

Three-dimensional imaging of nociceptive intraepidermal nerve fibers in human skin biopsies

A punch biopsy of the skin is commonly used to quantify intraepidermal nerve fiber densities (IENFD) for the diagnosis of peripheral polyneuropathy (1,2). At present, it is common practice to collect 3 mm skin biopsies from the distal leg (DL) and the proximal thigh (PT) for the evaluation of length-dependent polyneuropathies (3). However, due to the multidirectional nature of IENFs, it is challenging to examine overlapping nerve structures through the analysis of two-dimensional (2D) imaging. Alternatively, three-dimensional (3D) imaging could provide a better solution for this dilemma. In the current report, we present methods for applying 3D imaging to study painful neuropathy (PN). In order to identify IENFs, skin samples are processed for immunofluorescent analysis of protein gene product 9.5 (PGP), a pan neuronal marker. At present, it is standard practice to diagnose small fiber neuropathies using IENFD determined by PGP immunohistochemistry using brightfield microscopy (4). In the current study, we applied double immunofluorescent analysis to identify total IENFD, using PGP, and nociceptive IENF, through the use of antibodies that recognize tropomyosin-receptor-kinase A (Trk A), the high affinity receptor for nerve growth factor (5). The advantages of co-staining IENF with PGP and Trk A antibodies benefits the study of PN by clearly staining PGP-positive, nociceptive fibers. These fluorescent signals can be quantified to determine nociceptive IENFD and morphological changes of IENF associated with PN. The fluorescent images are acquired by confocal microscopy and processed for 3D analysis. 3D-imaging provides rotational abilities to further analyze morphological changes associated with PN. Taken together, fluorescent co-staining, confocal imaging, and 3D analysis clearly benefit the study of PN.

Neuron-astrocyte signaling network in spinal cord dorsal horn mediates painful neuropathy of type 2 diabetes

Activation of the neuronal-glial network in the spinal cord dorsal horn (SCDH) mediates various chronic painful conditions. We studied spinal neuronal-astrocyte signaling interactions involved in the maintenance of painful diabetic neuropathy (PDN) in type 2 diabetes. We used the db/db mouse, an animal model for PDN of type 2 diabetes, which develops mechanical allodynia from 6 to 12 wk of age. In this study, enhanced substance P expression was detected in the presynaptic sensory fibers innervating lamina I-III in the lumbar SCDH (LSCDH) of the db/db mouse at 10 wk of age. This phenomenon is associated with enhanced spinal ERK1/2 phosphorylation in projection sensory neurons and regional astrocyte activation. In addition, peak phosphorylation of the NR1 subunit of N-methyl-D-aspartate receptor (NMDAR), along with upregulation of neuronal and inducible nitric oxide synthase (nNOS and iNOS) expression were detected in diabetic mice. Expression of nNOS and iNOS was detected in both interneurons and astrocytes in lamina I-III of the LSCDH. Treatment with MK801, an NMDAR inhibitor, inhibited mechanical allodynia, ERK1/2 phosphorylation, and nNOS and iNOS upregulation in diabetic mice. MK801 also reduced astrocytosis and glial acidic fibrillary protein upregulation in db/db mice. In addition, N(G)-nitro-L-arginine methyl ester (L-NAME), a nonspecific NOS inhibitor, had similar effects on NMDAR signaling and NOS expression. These results suggest that nitric oxide from surrounding interneurons and astrocytes interacts with NMDAR-dependent signaling in the projection neurons of the SCDH during the maintenance of PDN.

Nerve growth factor/p38 signaling increases intraepidermal nerve fiber densities in painful neuropathy of type 2 diabetes

Painful diabetic neuropathy (PDN) is a common, yet devastating complication of type 2 diabetes. At this time, there is no objective test for diagnosing PDN. In the current study, we measured the peptidergic intraepidermal nerve fiber densities (IENFD) from hind paws of the db/db mouse, an animal model for type 2 diabetes, during the period of mechanical allodynia from 6 to 12 weeks of age. Intraepidermal nerve fibers (IENF) of the hind footpads were identified by protein gene product (PGP) 9.5 immunohistochemistry. The peptidergic IENF were determined by double immunofluorescence using anti-PGP9.5 and antibodies against tropomyosin-receptor-kinase (Trk) A. We observed a significant increase in PGP9.5-positive IENFD at 8 and 10 weeks of age. Similarly, Trk A-positive peptidergic IENF, which also express substance P and calcitonin gene related peptide in db/db mice, were observed to be elevated from 1.5 to 2 fold over controls. This upregulation ended at 16 weeks of age, in accordance with the reduction of mechanical allodynia. Anti-NGF treatment significantly inhibited the upregulation of peptidergic IENFD during the period of mechanical allodynia, suggesting that increased neurotrophism may mediate this phenomenon. In addition, SB203580, an inhibitor of p38, blocked the increase in peptidergic IENFD in db/db mice. The current results suggest that peptidergic IENFD could be a potential diagnostic indicator for PDN in type 2 diabetes. Furthermore, the inhibition of NGF-p38 signaling could be a potential therapeutic strategy for treating this painful condition.

p38 mediates mechanical allodynia in a mouse model of type 2 diabetes

BACKGROUND

Painful Diabetic Neuropathy (PDN) affects more than 25% of patients with type 2 diabetes; however, the pathogenesis remains unclear due to lack of knowledge of the molecular mechanisms leading to PDN. In our current study, we use an animal model of type 2 diabetes in order to understand the roles of p38 in PDN. Previously, we have demonstrated that the C57BLK db/db (db/db) mouse, a model of type 2 diabetes that carries the loss-of-function leptin receptor mutant, develops mechanical allodynia in the hind paws during the early stage (6-12 wk of age) of diabetes. Using this timeline of PDN, we can investigate the signaling mechanisms underlying mechanical allodynia in the db/db mouse.

RESULTS

We studied the role of p38 in lumbar dorsal root ganglia (LDRG) during the development of mechanical allodynia in db/db mice. p38 phosphorylation was detected by immunoblots at the early stage of mechanical allodynia in LDRG of diabetic mice. Phosphorylated p38 (pp38) immunoreactivity was detected mostly in the small- to medium-sized LDRG neurons during the time period of mechanical allodynia. Treatment with an antibody against nerve growth factor (NGF) significantly inhibited p38 phosphorylation in LDRG of diabetic mice. In addition, we detected higher levels of inflammatory mediators, including cyclooxygenase (COX) 2, inducible nitric oxide synthases (iNOS), and tumor necrosis factor (TNF)-alpha in LDRG neurons of db/db mice compared to non-diabetic db+ mice. Intrathecal delivery of SB203580, a p38 inhibitor, significantly inhibited the development of mechanical allodynia and the upregulation of COX2, iNOS and TNF-alpha.

CONCLUSIONS

Our findings suggest that NGF activated-p38 phosphorylation mediates mechanical allodynia in the db/db mouse by upregulation of multiple inflammatory mediators in LDRG.