Aldose reductase activity and myo-inositol levels in sciatic nerve and dorsal root ganglia of the diabetic mutant mouse [C57/BL/Ks (db/db)]

Novel transgenic mouse models develop retinal changes associated with early diabetic retinopathy similar to those observed in rats with diabetes mellitus

Retinal capillary pericyte degeneration has been linked to aldose reductase (AR) activity in diabetic retinopathy (DR). Since the development of DR in mice and rats has been reported to differ and that this may be linked to differences in retinal sorbitol levels, we have established new murine models of early onset diabetes mellitus as tools for investigating the role of AR in DR. Transgenic diabetic mouse models were developed by crossbreeding diabetic C57BL/6-Ins2(Akita)/J (AK) with transgenic C57BL mice expressing green fluorescent protein (GFP), human aldose reductase (hAR) or both in vascular tissues containing smooth muscle actin-α (SMAA). Changes in retinal sorbitol levels were determined by HPLC while changes of growth factors and signaling were investigated by Western Blots. Retinal vascular changes were quantitatively analyzed on elastase-digestion flat mounts. Results show that sorbitol levels were higher in neural retinas of diabetic AK-SMAA-GFP-hAR compared to AK-SMAA-GFP mice. AK-SMAA-GFP-hAR mice showed induction of the retinal growth factors VEGF, IGF-1, bFGF and TGFβ, as well as signaling changes in P-Akt, P-SAPK/JNK, and P-44/42 MAPK. Increased loss of nuclei per capillary length and a significant increase in the percentage of acellular capillaries presented in 18 week old AK-SMAA-GFP-hAR mice. These changes are similar to those observed in streptozotocin-induced diabetic rats. Retinal changes in both mice and rats were prevented by inhibition of AR. These studies confirm that the increased expression of AR in mice results in the development of retinal changes associated with the early stages of DR that are similar to those observed in rats.

Impaired adenosine monophosphate-activated protein kinase signalling in dorsal root ganglia neurons is linked to mitochondrial dysfunction and peripheral neuropathy in diabetes

Mitochondrial dysfunction occurs in sensory neurons and may contribute to distal axonopathy in animal models of diabetic neuropathy. The adenosine monophosphate-activated protein kinase and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signalling axis senses the metabolic demands of cells and regulates mitochondrial function. Studies in muscle, liver and cardiac tissues have shown that the activity of adenosine monophosphate-activated protein kinase and PGC-1α is decreased under hyperglycaemia. In this study, we tested the hypothesis that deficits in adenosine monophosphate-activated protein kinase/PGC-1α signalling in sensory neurons underlie impaired axonal plasticity, suboptimal mitochondrial function and development of neuropathy in rodent models of type 1 and type 2 diabetes. Phosphorylation and expression of adenosine monophosphate-activated protein kinase/PGC-1α and mitochondrial respiratory chain complex proteins were downregulated in dorsal root ganglia of both streptozotocin-diabetic rats and db/db mice. Adenoviral-mediated manipulation of endogenous adenosine monophosphate-activated protein kinase activity using mutant proteins modulated neurotrophin-directed neurite outgrowth in cultures of sensory neurons derived from adult rats. Addition of resveratrol to cultures of sensory neurons derived from rats after 3-5 months of streptozotocin-induced diabetes, significantly elevated adenosine monophosphate-activated protein kinase levels, enhanced neurite outgrowth and normalized mitochondrial inner membrane polarization in axons. The bioenergetics profile (maximal oxygen consumption rate, coupling efficiency, respiratory control ratio and spare respiratory capacity) was aberrant in cultured sensory neurons from streptozotocin-diabetic rats and was corrected by resveratrol treatment. Finally, resveratrol treatment for the last 2 months of a 5-month period of diabetes reversed thermal hypoalgesia and attenuated foot skin intraepidermal nerve fibre loss and reduced myelinated fibre mean axonal calibre in streptozotocin-diabetic rats. These data suggest that the development of distal axonopathy in diabetic neuropathy is linked to nutrient excess and mitochondrial dysfunction via defective signalling of the adenosine monophosphate-activated protein kinase/PGC-1α pathway.

Mitochondrial stress and the pathogenesis of diabetic neuropathy

Diabetic neuropathy is a major complication of diabetes that affects the sensory and autonomic nervous systems and leads to significant morbidity and impact on quality of life of patients. Mitochondrial stress has been proposed as a major mediator of neurodegeneration in diabetes. This review briefly summarizes the nature of sensory and autonomic nerve dysfunction and presents these findings in the context of diabetes-induced nerve degeneration mediated by alterations in mitochondrial ultrastructure, physiology and trafficking. Diabetes-induced dysfunction in calcium homeostasis is discussed at length and causative associations with sub-optimal mitochondrial physiology are developed. It is clear that across a range of complications of diabetes that mitochondrial physiology is impaired, in general a reduction in electron transport chain capability is apparent. This abnormal activity may predispose mitochondria to generate elevated reactive oxygen species (ROS), although experimental proof remains lacking, but more importantly will deleteriously alter the bioenergetic status of neurons. It is proposed that the next five years of research should focus on identifying changes in mitochondrial phenotype and associated cellular impact, identifying sources of ROS in neurons and analyzing mitochondrial trafficking under diabetic conditions.

Expression of hexokinase isoforms in the dorsal root ganglion of the adult rat and effect of experimental diabetes

The effect of streptozotocin (STZ)-induced diabetes on expression and activity of hexokinase, the first enzyme and rate-limiting step in glycolysis, was studied in sensory neurons of lumbar dorsal root ganglia (DRG). The DRG and sciatic nerve of adult rats expressed the hexokinase I isoform only. Immunofluorescent staining of lumbar DRG demonstrated that small-medium neurons and satellite cells exhibited high levels of expression of hexokinase I. Large, mainly proprioceptive neurons, had very low or negative staining for hexokinase I. Intracellular localization and biochemical studies on intact DRG from adult rats and cultured adult rat sensory neurons revealed that hexokinase I was almost exclusively found in the mitochondrial compartment. Duration of STZ-diabetes of 6 or 12 weeks diminished hexokinase activity by 28% and 30%, respectively, in lumbar DRG compared with age matched controls (P<0.05). Quantitative Western blotting showed no effect of diabetes on hexokinase I protein expression in homogenates or mitochondrial preparations from DRG. Immunofluorescent staining for hexokinase I showed no diabetes-dependent change in small-medium neuron expression in DRG, however, large neurons became positive for hexokinase I (P<0.05). Such complex effects of diabetes on hexokinase I expression in the DRG may be due to glucose-driven up-regulation of expression or the result of impaired axonal transport and perikaryal accumulation in the large neuron sub-population. Because hexokinase is the rate-limiting enzyme of glycolysis these results imply that metabolic flux through the glycolytic pathway is reduced in diabetes. This finding, therefore, questions the role of high glucose-induced metabolic flux as a key driving force in reactive oxygen species generation by mitochondria.

The leptin-deficient (ob/ob) mouse: a new animal model of peripheral neuropathy of type 2 diabetes and obesity

Whereas functional, metabolic, neurotrophic, and morphological abnormalities of peripheral diabetic neuropathy (PDN) have been extensively explored in streptozotocin-induced diabetic rats and mice (models of type 1 diabetes), insufficient information is available on manifestations and pathogenetic mechanisms of PDN in type 2 diabetic models. The latter could constitute a problem for clinical trial design because the vast majority of subjects with diabetes have type 2 (non-insulin dependent) diabetes. This study was aimed at characterization of PDN in leptin-deficient (ob/ob) mice, a model of type 2 diabetes with relatively mild hyperglycemia and obesity. ob/ob mice ( approximately 11 weeks old) clearly developed manifest sciatic motor nerve conduction velocity (MNCV) and hind-limb digital sensory nerve conduction velocity (SNCV) deficits, thermal hypoalgesia, tactile allodynia, and a remarkable ( approximately 78%) loss of intraepidermal nerve fibers. They also had increased sorbitol pathway activity in the sciatic nerve and increased nitrotyrosine and poly(ADP-ribose) immunofluorescence in the sciatic nerve, spinal cord, and dorsal root ganglion (DRG). Aldose reductase inhibition with fidarestat (16 mg . kg(-1) . d(-1)), administered to ob/ob mice for 6 weeks starting from 5 weeks of age, was associated with preservation of normal MNCV and SNCV and alleviation of thermal hypoalgesia and intraepidermal nerve fiber loss but not tactile allodynia. Sciatic nerve nitrotyrosine immunofluorescence and the number of poly(ADP-ribose)-positive nuclei in sciatic nerve, spinal cord, and DRGs of fidarestat-treated ob/ob mice did not differ from those in nondiabetic controls. In conclusion, the leptin-deficient ob/ob mouse is a new animal model that develops both large motor and sensory fiber and small sensory fiber PDN and responds to pathogenetic treatment. The results support the role for increased aldose reductase activity in functional and structural changes of PDN in type 2 diabetes.

Administration of an aldose reductase inhibitor, ONO-2235, to streptozotocin-diabetic mice restores reductions of DRG neuronal attachment to extracellular matrix proteins in vitro

Attachments of cultured dorsal root ganglion (DRG) neurons to the extracellular matrix (ECM) proteins (type I and IV collagens, laminin and fibronectin) and the adhesion ligand arginine-glycine-aspartic acid (RGD) were impaired in mice 2 weeks after the induction of diabetes by streptozotocin (STZ). However, administration of the aldose reductase inhibitor, ONO-2235, to the STZ-diabetic mice for 1 week restored DRG neuronal attachment to the ECM proteins and RGD to a level close to normal mice. These results suggest that activation of the aldose reductase and subsequent metabolic disorders in diabetic animals may play an important role in detrimental alterations of the neuronal cell-surface receptors for the ECM proteins.

A freeze-fracture study of the perineurium in galactose neuropathy: morphological changes associated with endoneurial oedema

Feeding rats with galactose as 40% of their diet results in peripheral nerve oedema related to the intrafascicular accumulation of galactitol and sodium. In this study, associated changes in the perineurium were examined by the freeze-fracture replication technique. Perineurial cells are linked by tight junctions (zonulae occludentes). In normal animals these are made up of anastomosing strands organized in a belt-like arrangement along the margins of continuous cells. The majority of the tight junctions in the galactose-fed animals displayed structural abnormalities. These ranged from slight separation of the strands to fragmentation and dispersal, with looping of isolated strands. Some of the tight junctions contained large dilated compartments within the junctional network. Short lengths of intramembranous particles, probably representing assembly or disassembly of tight junctional strands, were also observed. The membranes of perineurial cells normally possess numerous openings of caveolae. A quantitative assessment showed that the mean density of these caveolae openings was increased in the galactose-fed rats as compared with controls. The alterations in the tight junctions resemble those that have been produced experimentally in epithelia by subjecting them to abnormal osmotic gradients. They also resemble those seen in human diabetic neuropathy in which osmotic disturbances involving the perineurium have been considered to occur. If the alterations involve the inner layers of the perineurium, they are likely to impair its barrier function. The increased number of caveolae openings in galactose neuropathy may represent a reaction to the endoneurial oedema and indicate that the pinocytotic-like vesicles have a transport function.