Molecular mechanisms in the initiation phase of Wallerian degeneration

Axonal degeneration is an early hallmark of nerve injury and many neurodegenerative diseases. The discovery of the Wallerian degeneration slow mutant mouse, in which axonal degeneration is delayed, revealed that Wallerian degeneration is an active progress and thereby illuminated the mechanisms underlying axonal degeneration. Nicotinamide mononucleotide adenylyltransferase 2 and sterile alpha and armadillo motif-containing protein 1 play essential roles in the maintenance of axon integrity by regulating the level of nicotinamide adenine dinucleotide, which seems to be the key molecule involved in the maintenance of axonal health. However, the function of nicotinamide mononucleotide remains debatable, and we discuss two apparently conflicting roles of nicotinamide mononucleotide in Wallerian degeneration. In this article, we focus on the roles of these molecules in the initiation phase of Wallerian degeneration to improve our understanding of the mechanisms underpinning this phenomenon.

Short-term diabetic hyperglycemia suppresses celiac ganglia neurotransmission, thereby impairing sympathetically mediated glucagon responses

Short-term hyperglycemia suppresses superior cervical ganglia neurotransmission. If this ganglionic dysfunction also occurs in the islet sympathetic pathway, sympathetically mediated glucagon responses could be impaired. Our objectives were 1) to test for a suppressive effect of 7 days of streptozotocin (STZ) diabetes on celiac ganglia (CG) activation and on neurotransmitter and glucagon responses to preganglionic nerve stimulation, 2) to isolate the defect in the islet sympathetic pathway to the CG itself, and 3) to test for a protective effect of the WLD(S) mutation. We injected saline or nicotine in nondiabetic and STZ-diabetic rats and measured fos mRNA levels in whole CG. We electrically stimulated the preganglionic or postganglionic nerve trunk of the CG in nondiabetic and STZ-diabetic rats and measured portal venous norepinephrine and glucagon responses. We repeated the nicotine and preganglionic nerve stimulation studies in nondiabetic and STZ-diabetic WLD(S) rats. In STZ-diabetic rats, the CG fos response to nicotine was suppressed, and the norepinephrine and glucagon responses to preganglionic nerve stimulation were impaired. In contrast, the norepinephrine and glucagon responses to postganglionic nerve stimulation were normal. The CG fos response to nicotine, and the norepinephrine and glucagon responses to preganglionic nerve stimulation, were normal in STZ-diabetic WLD(S) rats. In conclusion, short-term hyperglycemia's suppressive effect on nicotinic acetylcholine receptors of the CG impairs sympathetically mediated glucagon responses. WLD(S) rats are protected from this dysfunction. The implication is that this CG dysfunction may contribute to the impaired glucagon response to insulin-induced hypoglycemia seen early in type 1 diabetes.

Axonal transport plays a crucial role in mediating the axon-protective effects of NmNAT

Deficits in axonal transport are thought to contribute to the pathology of many neurodegenerative diseases. Expressing the slow Wallerian degeneration protein (Wld(S)) or related nicotinamide mononucleotide adenyltransferases (NmNATs) protects axons against damage from a broad range of insults, but the ability of these proteins to protect against inhibition of axonal transport has received little attention. We set out to determine whether these proteins can protect the axons of cultured hippocampal neurons from damage due to hydrogen peroxide or oxygen-glucose deprivation (OGD) and, in particular, whether they can reduce the damage that these agents cause to the axonal transport machinery. Exposure to these insults inhibited the axonal transport of both mitochondria and of the vesicles that carry axonal membrane proteins; this inhibition occurred hours before the first signs of axonal degeneration. Expressing a cytoplasmically targeted version of NmNAT1 (cytNmNAT1) protected the axons against both insults. It also reduced the inhibition of transport when cells were exposed to hydrogen peroxide and enhanced the recovery of transport following both insults. The protective effects of cytNmNAT1 depend on mitochondrial transport. When mitochondrial transport was inhibited, cytNmNAT1 was unable to protect axons against either insult. The protective effects of mitochondrially targeted NmNAT also were blocked by inhibiting mitochondrial transport. These results establish that NmNAT robustly protects the axonal transport system following exposure to OGD and reactive oxygen species and may offer similar protection in other disease models. Understanding how NmNAT protects the axonal transport system may lead to new strategies for neuroprotection in neurodegenerative diseases.

Protection of mouse retinal ganglion cell axons and soma from glaucomatous and ischemic injury by cytoplasmic overexpression of Nmnat1

PURPOSE

The Wlds mutation affords protection of retinal ganglion cell (RGC) axons in retinal ischemia and in inducible and hereditary preclinical models of glaucoma. We undertook the present study to determine whether the Nmnat1 portion of the chimeric protein provides axonal and somatic protection of RGCs in models of ischemia and glaucoma, particularly when localized to nonnuclear regions of the cell.

METHODS

The survival and integrity of RGC axons and soma from transgenic mice with confirmed cytoplasmic overexpression of Nmnat1 in retina and optic nerve (cytNmnat1-Tg mice) were examined in the retina and postlaminar optic nerve 4 days following acute retinal ischemia, and 3 weeks following the chronic elevation of intraocular pressure.

RESULTS

Ischemia- and glaucoma-induced disruptions of proximal segments of RGC axons that comprise the nerve fiber layer in wild-type mice were both robustly abrogated in cytNmnat1-Tg mice. More distal portions of RGC axons within the optic nerve were also protected from glaucomatous disruption in the transgenic mice. In both disease models, Nmnat1 overexpression in extranuclear locations significantly enhanced the survival of RGC soma.

CONCLUSIONS

Overexpression of Nmnat1 in the cytoplasm and axons of RGCs robustly protected against both ischemic and glaucomatous loss of RGC axonal integrity, as well as loss of RGC soma. These findings reflect the more pan-cellular protection of CNS neurons that is realized by cytoplasmic Nmnat1 expression, and thus provide a therapeutic strategy for protecting against retinal neurodegeneration, and perhaps other CNS neurodegenerative diseases as well.