Endoneurial sodium accumulation in galactosemic rat nerves

Is diabetes the risk factor for poor neurological recovery after cervical spine surgery? A review of the literature

The poor prognosis of cervical spine surgery is mainly manifested as poor neurological recovery and the presence of new upper extremity dysfunction that promotes significant psychological and physiological burdens on patients. Many factors influence the prognosis of cervical spine surgery, including the age of patients, the time and mode of surgery, and the surgical technique used. However, in clinical studies, it has been observed that patients with diabetes have a higher probability of poor prognosis after surgery. Therefore, we review the pathophysiology of diabetic neuropathies and discuss its impact on cervical nerve system function, especially in cervical nerve roots and upper limb peripheral nerve conduction.

Current and Future Treatments for Classic Galactosemia

Type I (classic) galactosemia, galactose 1-phosphate uridylyltransferase (GALT)-deficiency is a hereditary disorder of galactose metabolism. The current therapeutic standard of care, a galactose-restricted diet, is effective in treating neonatal complications but is inadequate in preventing burdensome complications. The development of several animal models of classic galactosemia that (partly) mimic the biochemical and clinical phenotypes and the resolution of the crystal structure of GALT have provided important insights; however, precise pathophysiology remains to be elucidated. Novel therapeutic approaches currently being explored focus on several of the pathogenic factors that have been described, aiming to (i) restore GALT activity, (ii) influence the cascade of events and (iii) address the clinical picture. This review attempts to provide an overview on the latest advancements in therapy approaches.

Therapies for galactosemia: a patent landscape

Galactosemia is the inherited inability to metabolise galactose. The most common results from a lack of galactose 1-phosphate uridylyltransferase activity. The current treatment, removal of galactose from the diet, is inadequate and often fails to prevent long-term complications. Since 2015, three patents have been filed describing novel therapies. These are: the use of aldose reductase inhibitors to reduce cataracts and, possibly, other symptoms; salubrinal to stimulate cellular stress responses; mRNA therapy to increase cellular galactose 1-phosphate uridylyltransferase activity. The viability of all three is supported by academic studies. The potential and drawbacks of all three are discussed and evaluated.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Homeostatic regulation of the endoneurial microenvironment during development, aging and in response to trauma, disease and toxic insult

The endoneurial microenvironment, delimited by the endothelium of endoneurial vessels and a multi-layered ensheathing perineurium, is a specialized milieu intérieur within which axons, associated Schwann cells and other resident cells of peripheral nerves function. The endothelium and perineurium restricts as well as regulates exchange of material between the endoneurial microenvironment and the surrounding extracellular space and thus is more appropriately described as a blood-nerve interface (BNI) rather than a blood-nerve barrier (BNB). Input to and output from the endoneurial microenvironment occurs via blood-nerve exchange and convective endoneurial fluid flow driven by a proximo-distal hydrostatic pressure gradient. The independent regulation of the endothelial and perineurial components of the BNI during development, aging and in response to trauma is consistent with homeostatic regulation of the endoneurial microenvironment. Pathophysiological alterations of the endoneurium in experimental allergic neuritis (EAN), and diabetic and lead neuropathy are considered to be perturbations of endoneurial homeostasis. The interactions of Schwann cells, axons, macrophages, and mast cells via cell-cell and cell-matrix signaling regulate the permeability of this interface. A greater knowledge of the dynamic nature of tight junctions and the factors that induce and/or modulate these key elements of the BNI will increase our understanding of peripheral nerve disorders as well as stimulate the development of therapeutic strategies to treat these disorders.

Nerve conduction velocity, laser Doppler flow, and axonal caliber in galactose and streptozotocin diabetes

In an initial study, the effects of galactose intoxication on nerve laser Doppler blood flow (NLDF) and nerve conduction velocity (NCV) were assessed after 1-16 weeks of galactose feeding in pentobarbital-anesthetized rats. NLDF was not significantly changed at any time point. NCV was significantly reduced after 16, but not 1 or 4, weeks of galactose feeding. In a second study, NLDF was not significantly changed by 4 weeks of galactose intoxication, but streptozotocin-diabetic NLDF was significantly reduced compared to both control (P<0.001) and galactose-intoxicated rats (P<0.05). Compared to control animals, sciatic motor NCV was significantly (P<0.001) reduced in the galactose group, while sciatic and saphenous sensory NCVs were not significantly changed. In the streptozotocin-diabetic rats, motor and sensory NCVs were all significantly reduced (P<0.001). In contrast to the NCV findings, mean caliber of myelinated axons in both the saphenous and sciatic nerves was reduced in galactose-intoxicated, but not streptozotocin-diabetic rats. The observed sequence of changes associated with these two models of diabetic neuropathy is not consistent with the proposed roles of ischemia and axonal dwindling in the reported nerve conduction deficits.

Decreased endoneurial fluid electrolytes in normal rat sciatic nerve after aldose reductase inhibition

The role of the enzyme aldose reductase in nerve homeostasis was examined by treating rats with an aldose reductase inhibitor. Female Sprague-Dawley rats were treated with Ponalrestat (25 mg/kg/day) or with excipient alone for 4 to 12 weeks before examining electrophysiologic function, endoneurial fluid electrolyte concentrations, nerve polyol levels, water content and (Na+,K+)-ATPase activity. Sorbitol, the product of glucose metabolism by aldose reductase, was detected in all nerves from control animals, whereas it was below detection limits in 7 of 11 nerves from Ponalrestat-treated rats. Ponalrestat treatment reduced endoneurial fluid sodium and chloride concentrations by 25% and 37%, respectively (both P < 0.001). No differences in nerve water content, conduction velocity, or ATPase activities were detected. These data, and previous studies demonstrating that increased flux through aldose reductase causes the accumulation of endoneurial electrolytes, suggest a role for this enzyme in modulation of the endoneurial microenvironment. However, short-term inhibition of aldose reductase does not appear to affect nerve function. Thus, our findings do not elicit concerns regarding the use of aldose reductase inhibitors in the treatment of clinical diabetic neuropathy.

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.

Fine-structural localization of aldose reductase and ouabain-sensitive, K(+)-dependent p-nitro-phenylphosphatase in rat peripheral nerve

Aldose reductase was visualized by light and electron microscopy using a goat anti-rat antibody with immunoperoxidase and immunogold, respectively. Ouabain-sensitive, K(+)-dependent, p-nitro-phenylphosphatase, a component of (Na+, K+)-ATPase, was localized at the electron microscopic level by enzyme histochemistry using p-nitro-phenylphosphate as substrate. In peripheral nerve, spinal ganglia and roots, the Schwann cell of myelinated fibers was the principal site of aldose reductase localization. Immunostaining was intense in the paranodal region and the Schmidt-Lanterman clefts as well as in cytoplasm of the terminal expansions of paranodal myelin lamellae and the nodal microvilli. Schwann cell cytoplasm of unmyelinated fibers were faintly labelled. Endoneurial vessel endothelia, pericytes and perineurium failed to bind appreciable amounts of aldose reductase antibody. However, mast cell granules bound antibody strongly. In contrast, p-nitro-phenylphosphatase reaction product was detected in the nodal axolemma, terminal loops of Schwann cell cytoplasm and the innermost layer of perineurial cells. In endothelial cells, reaction product was localized on either the luminal or abluminal, or on both luminal and abluminal plasmalemma. Endothelial vesicular profiles were often loaded with reaction product. Occasional staining of myelin and axonal organelles was noted. Mast cells lacked reaction product.

Disruption of cellular elements and water in neurotoxicity: studies using electron probe X-ray microanalysis

Regulation of elements and water in nerve cells is a complex, multifaceted process which appears to be vulnerable to neurotoxic events. However, much of our knowledge concerning the potential role of elements in nerve cell injury is limited by the relatively gross level of corresponding analyses. If we are to confirm and understand the proposed role, more precise and detailed information is needed. As indicated in this commentary, research employing electron probe microanalysis and digital X-ray imaging has begun to provide this necessary information. Recent EPMA studies of nerve and glial cells in the peripheral and central nervous systems have shown that each cell type and their corresponding morphologic compartments exhibit unique distributions of elements and water. The use of microprobe analysis has allowed us to document precisely how elements and water redistribute in morphological compartments of damaged nerve cells. Accumulating evidence from EPMA studies suggests that, rather than being an epiphenomenon, intracellular changes in diffusible elements might mediate the functional and structural consequences of neurotoxic insult. It is also evident from this research that elements other than Ca might play a pertinent role in the injury response and that changes in intraneuronal elemental composition might develop according to a specific temporal pattern, e.g., transection-induced sequential alterations in axonal K, Na, Cl, and Ca. Therefore, rather than conducting end-point studies, longitudinal investigations are necessary to define the sequential pattern of elemental perturbation associated with a given neurotoxic event. Such research can also help identify the role of individual elements in the injury response. Future microprobe studies should be combined with measurements of ion levels (e.g., using fura-2 or ion selective electrodes) to provide a comprehensive and dynamic view of elemental deregulation. In addition, parallel biochemical studies should be performed to determine mechanisms of elemental disruption and possible biochemical and metabolic consequences of this disruption. Although evidence presented in this commentary suggests that each type of neurotoxic event produces a characteristic pattern of decompartmentalization, further work is necessary to confirm this possibility. Finally, based on a presumed involvement of elements in nerve injury, efforts are currently underway in several laboratories to develop appropriate pharmacological therapies for certain chemical- and trauma-induced neuropathological conditions (Dretchen et al., 1986; El-Fawal et al., 1989; Beattie et al., 1989).(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the blood-nerve barrier in experimental nerve edema

Nerve edema is a common response to the nerve injury seen in many peripheral neuropathies and is an important component of Wallerian degeneration. However, independent pathologic effects of nerve edema that aggravate or induce nerve injury extend the role of edema beyond that of an epiphenomenon of injury. New insights into the mechanism and impact of nerve edema come largely from animal models. In the following review, we discuss the cause and consequences of nerve edema with particular reference to endoneurial fluid pressure and its relevance to the nerve microenvironment. Experimental models of nerve edema include conditions with increased vascular permeability such as lead poisoning, experimental allergic neuritis, and murine globoid leukodystrophy. Increased perineurial permeability induced by local anesthetics and neurolytic drugs can also induce nerve edema sufficient to increase endoneurial fluid pressure. Both perineurial and vascular permeability are increased after damage induced by crush, freeze, or laser injury. One of the most important forms of nerve edema is induced by external compression; the significance of this change is that edema has local compressive effects that persist after the external pressure has been relaxed. Nerve edema and increased endoneurial fluid pressure also occur in conditions in which vascular permeability appears to be unchanged such as experimental diabetic neuropathy and in hexachlorophene intoxication. In both of these conditions, reduced nerve blood flow has been demonstrated in rats and is viewed as a consequence of increased endoneurial fluid pressure. Whatever its mechanism, endoneurial edema has important structural and functional consequences for nerve fibers. A clear understanding of the underlying pathology of the nerve microenvironment may provide useful insights into treatment of clinical neuropathies.

Aldose reductase in the etiology of diabetic complications. 3. Neuropathy

Aldose reductase has been shown to be present in both autonomic and somatic nerves. Activation of this enzyme and the polyol pathway has been demonstrated in diabetic animal models to cause a range of biochemical, functional, and structural consequences that include the accumulation of sorbitol and fructose; axoglial dysjunction; paranodal demyelination; abnormalities in axonal transport, blood flow, and vascular permeability; and resistance to ischemic transmission of action potentials. These data provide an insight into the range of processes that if activated may either singly or in combination result in altered patterns of nerve function and structural alterations in diabetic neuropathy. In animal models of diabetes, it has been shown that inhibition of aldose reductase can modify these diabetes-induced changes. It is hoped that the results of large-scale controlled trials will provide clinical evidence to support these data.

Dose-dependence of endoneurial fluid sodium and chloride accumulation in galactose intoxication

Endoneurial edema in galactose neuropathy was studied in a colony of Sprague-Dawley rats fed diets containing 0%, 10%, 20% or 40% D-galactose for approx. 200 days. Endoneurial fluid was analyzed by X-ray microanalysis for electrolyte concentration, by microgravimetry of whole nerve segments for water content, by measurement of endoneurial fluid pressure and by morphometry in transverse sections of nerve. Galactose intoxication resulted in dose-dependent increases in endoneurial fluid sodium and chloride that were directly associated with increases in nerve water content and endoneurial fluid pressure. The presence of edema and its dose-dependence was also confirmed by morphometric analysis of sciatic nerves at the light microscopic level. The data demonstrate that electrolyte-induced osmotic imbalances in endoneurial fluid are dependent on the amount of galactose ingested and suggest that the dose-related accumulation of sodium and chloride in endoneurial fluid contributes substantially to the pathogenesis of galactose neuropathy.

Perineurial sodium-potassium-ATPase activity in streptozotocin-diabetic rats

Perineurial sodium-potassium-ATPase activity was estimated in streptozotocin-diabetic rats and compared with that in control animals. Total ATPase activity was found to be reduced by 34% and ouabain-sensitive ATPase activity by 53%. This finding is significant in relation to the putative role of the perineurium as a metabolically active perifascicular diffusion barrier that regulates the composition of the endoneurial fluid, as is its possible relevance to the occurrence of endoneurial edema in diabetes.

Treatment with an aldose reductase inhibitor can reduce the susceptibility of fast axonal transport following nerve compression in the streptozotocin-diabetic rat

The effect of treatment with an aldose reductase inhibitor on the susceptibility of peripheral nerves to compression was studied in rats made diabetic by the injection of streptozotocin (50 mg.kg-1). The response to nerve compression was determined in untreated diabetic rats after 22 days of diabetes and compared with the response in two similar groups of diabetic rats which had been treated with the aldose reductase inhibitor 'Statil' (ICI 128436; 25 mg.kg-1.day-1 orally) either from the induction of diabetes or for 7 days prior to nerve compression. Two groups of non-diabetic rats were treated with 'Statil' for either 22 days or 7 days to act as controls. Inhibition of fast axonally transported proteins was induced by local compression of the sciatic nerves 4 h after application of 3H-leucine to the motor neurone cell bodies in the spinal cord. The inhibition of fast axonal transport was quantified by calculation of a transport block ratio. Compression at 30 mmHg for 3 h induced a significantly greater (p less than 0.05) inhibition of axonal transport at the site of compression in nerves of untreated diabetic rats (transport block ratio 0.96 +/- 0.24, n = 8) than in nerves of control rats treated with the aldose reductase inhibitor for either the shorter time of 7 days (0.71 +/- 0.17, n = 10) or the longer time of 22 days (0.69 +/- 0.08, n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Edema and increased endoneurial sodium in galactose neuropathy. Reversal with an aldose reductase inhibitor

Galactose neuropathy was produced in rats by feeding a diet containing 30% D-galactose. After 12 weeks of galactose ingestion, all rats developed bilateral cataracts, polydypsia and polyuria. These galactose-intoxicated animals were divided into two groups that both continued with the galactose diet: animals that were treated with the aldose reductase inhibitor, ICI 128,436, for 4-6 weeks, and a control group of animals that received just excipient. At the end of the study, endoneurial fluid pressures, nerve water contents and endoneurial fluid electrolyte concentrations were determined from sciatic nerves of treated and untreated animals. The extent of neuropathy in each animal was evaluated by light microscopy. Treatment of galactose-intoxicated rats with ICI 128,436 restored to normal levels the elevated endoneurial sodium concentration, increased water content and interstitial fluid pressure characteristic of galactose neuropathy. These results, obtained with an agent that blocks the sorbitol pathway, associate elevated sodium with an osmotic force contributing to edema and increased endoneurial fluid pressure in galactose neuropathy and suggest that endoneurial sodium levels are linked to blood-sugar concentration.