Apoptosis In The Sc

Despite the abundance of SC in the peripheral nerve, less is known about SC than DRG apoptosis in the diabetic PNS. Several lines of evidence support morphological changes of apoptosis in SC in vitro, in models of diabetic neuropathy, and in human diabetic neuropathy (22,110). SC obtained from the dorsal root of diabetic animals exhibit chromatin clumping and disruption of the myelin surrounding atrophic axons (Fig. 3) (110). Schwann-like satellite cells from corresponding diabetic DRG show severe chromatin clumping, and perikaryeal vacuolation consistent with Mt ballooning and disruption of the internal Mt cristae structure. Similar observations have been made in vitro using the chromatin stain bisbenzamide. Under high glucose conditions, SC nuclei break apart into brightly staining apoptotic clusters indicative of chromatin condensation and nuclear fragmentation, changes that are confirmed with TUNEL and propidium iodide staining. Furthermore, high glucose conditions promote cleavage of caspase-3 and -7. The antiapoptotic BCL family protein, BCL-xL is expressed in normal SC, but is not significantly increased or decreased under high glucose conditions. However, overexpression of BCL-xL protects SC from apoptosis in vitro (110).

Changes in animal models of diabetes are reproduced in human SC. In human sural nerve SCs from patients with moderately severe diabetic neuropathy, several key changes consistent with SC apoptosis are observed in some, but not all SC. These include: nuclear chromatin condensation, shrinkage of the SC cytosol, swelling and disruption of the Mt and of the rough endoplasmic reticulum, disruption of the normal Mt cristae structure, and formation of cytoplasmic vacuoles (21,22). Eventually, there is loss of the SC nucleus and cytosol with preservation of the plasma membrane and supporting collagen, forming ghost cells. These changes are consistent with single cell deletion observed in apoptosis, but rarely in necrosis. No inflammatory infiltrates are observed in the affected nerves, such as might be seen with necrosis of the peripheral nerve. One potential cause for glucose-induced apoptosis is by generation of AGEs. In SC treated with different iso-forms of AGE, a change in the A¥M leading to depolarization and SC apoptosis was induced by AGE-2 and -3, but not aGe-1. Apoptosis was ameliorated by the antioxidant a-lipoic acid and by inhibition of p38 signaling. In addition, AGE-2 and -3 significantly

Fig. 3. Electron micrograph of SC from control and STZ-treated diabetic rats. Control animals (A,B) and STZ-treated rats made diabetic for 1 month (C-E). Control SC, showing normal diffuse chromatin staining in the nucleus (N), and normal axon (A) with intact myelin lamellae showing little or no myelin splitting. (A) Satellite cells (S), which are Schwann-like cells, from a control animal showing normal diffuse chromatin staining in the nucleus (N), and cytoplasm. The satellite cells lie adjacent to DRG neurons (Nu) that show normal cytoplasmic components. (B) In the dorsal root, from diabetic animals, there is clumping of the chromatin (Ch) in the SC (S), atrophy of axons (A), and disruption of myelin surrounding the axons. (C) In Satellite cells (S) from a diabetic dorsal root ganglion, there is severe chromatin clumping (Ch), shrinkage of the perikaryeon, and prominent vacuolation. An atrophic axon (A) is seen, nestled between two Schwann-like satellite cells, adjacent to DRG neurons (Nu), which also show evidence of perikaryeal vacuolation. (E) End stage changes in a diabetic Schwann cell (S). There is nuclear chromatin clumping and fragmentation (Ch) coupled with prominent vacuolation (V) resulting from ballooning of mitochondria and disruption of their cristae. Bars in each panel indicate magnification. (Published with permission from ref. 110.)

Fig. 3. Electron micrograph of SC from control and STZ-treated diabetic rats. Control animals (A,B) and STZ-treated rats made diabetic for 1 month (C-E). Control SC, showing normal diffuse chromatin staining in the nucleus (N), and normal axon (A) with intact myelin lamellae showing little or no myelin splitting. (A) Satellite cells (S), which are Schwann-like cells, from a control animal showing normal diffuse chromatin staining in the nucleus (N), and cytoplasm. The satellite cells lie adjacent to DRG neurons (Nu) that show normal cytoplasmic components. (B) In the dorsal root, from diabetic animals, there is clumping of the chromatin (Ch) in the SC (S), atrophy of axons (A), and disruption of myelin surrounding the axons. (C) In Satellite cells (S) from a diabetic dorsal root ganglion, there is severe chromatin clumping (Ch), shrinkage of the perikaryeon, and prominent vacuolation. An atrophic axon (A) is seen, nestled between two Schwann-like satellite cells, adjacent to DRG neurons (Nu), which also show evidence of perikaryeal vacuolation. (E) End stage changes in a diabetic Schwann cell (S). There is nuclear chromatin clumping and fragmentation (Ch) coupled with prominent vacuolation (V) resulting from ballooning of mitochondria and disruption of their cristae. Bars in each panel indicate magnification. (Published with permission from ref. 110.)

suppressed the SC replication rate, enhanced the release of TNF-a and IL-ip, and activated nuclear factor-kB (111).

In contrast, certain growth factors can reduce SC apoptosis. Growth factors such as NGF might also serve as antioxidants and this function might contribute to their role as possible therapeutic entities in diabetic neuropathy (112-114). NGF in physiological concentrations is able to reduce SC MMD (22). In contrast, pretreatment with 50 ^g/mL p75 neurotrophin receptor (p75 NTR) functional blocking antibody blocks the effect of NGF on the ATM. High glucose induces dose-dependent cleavage of caspases in SC. NGF also reduces apoptosis in SC measured using caspase-3 or TUNEL. This reduction in apoptosis is reversed by pretreatment with p75 NTR function blocking antibody (22). These results indicate that NGF mediates some of its Mt stabilizing effects and antiapoptotic effects through the p75 NTR. IGF-I is also protective to SC in culture at higher concentrations (10 nM) that active the IGF-I receptor but not the insulin receptor (110). In contrast, PI3K, but not MAP/MEK kinase inhibitors block the protective effect of IGF-I. These findings are consistent with those in DRG neurons where IGF-I protects SC from apoptosis through PI3K signaling intermediates. Interestingly, the addition of IGF-I, in either normal glucose or high glucose, has no effect on BCL-xL expression in native SC or BCL-xL trans-fected SC suggesting a possible non-BCL-dependent mechanism in preventing apop-tosis with glucose stress.

Diabetes Sustenance

Diabetes Sustenance

Get All The Support And Guidance You Need To Be A Success At Dealing With Diabetes The Healthy Way. This Book Is One Of The Most Valuable Resources In The World When It Comes To Learning How Nutritional Supplements Can Control Sugar Levels.

Get My Free Ebook


Post a comment