Kinesin-1 and degenerative changes in optic nerve axons in NMDA-induced neurotoxicity

BDNF improves axon transportation and rescues visual function in a rodent model of acute elevation of intraocular pressure

Optic neuropathies lead to blindness; the common pathology is the degeneration of axons of the retinal ganglion cells. In this study, we used a rat model of retinal ischemia-reperfusion and a one-time intravitreal brain-derived neurotrophic factor (BDNF) injection; then we examined axon transportation function, continuity, physical presence of axons in different part of the optic nerve, and the expression level of proteins involved in axon transportation. We found that in the disease model, axon transportation was the most severely affected, followed by axon continuity, then the number of axons in the distal and proximal optic nerve. BDNF treatment relieved all reductions and significantly restored function. The molecular changes were more minor, probably due to massive gliosis of the optic nerve, so interpretation of protein expression data should be done with some caution. The process in this acute model resembles a fast-forward of changes in the chronic model of glaucoma. Therefore, impairment in axon transportation appears to be a common early process underlying different optic neuropathies. This research on effective intervention can be used to develop interventions for all optic neuropathies targeting axon transportation.

Impairment of Axonal Transport in Diabetes: Focus on the Putative Mechanisms Underlying Peripheral and Central Neuropathies

Diabetes mellitus is a chronic disease with numerous complications that severely impact on the quality of life of patients. Different neuropathies may arise as complications associated with the nervous system, both peripherally and at the central level. The mechanisms behind these neuronal complications are far from being clarified, but axonal transport impairment, a vital process for neuronal physiology, has been described in the context of experimental diabetes. Alterations in neuronal cytoskeleton and motor proteins, deficits in ATP supply or neuroinflammation, as processes that disturb the effective transport of cargoes along the axon, were reported as putative causes of axonal impairment, ultimately leading to axonal degeneration. The main goal of the present review is to reunite the main studies in the literature exploring diabetes-induced alterations likely involved in axonal transport deficits, and call the attention for the uttermost importance of further exploring the field. Understanding the mechanisms underlying neuronal deficits in diabetes is crucial for the development of new therapeutic strategies to prevent neuronal degeneration in diabetes and related neuropathies.

Interaction of complement system and microglia activation in retina and optic nerve in a NMDA damage model

It is known that intravitreally injected N-methyl-d-aspartate (NMDA) leads to fast retina and optic nerve degeneration and can directly activate microglia. Here, we analyzed the relevance for microglia related degenerating factors, the proteins of the complement system, at a late stage in the NMDA damage model. Therefore, different doses of NMDA (0 (PBS), 20, 40, 80 nmol) were intravitreally injected in rat eyes. Proliferative and activated microglia/macrophages (MG/Mϕ) were found in retina and optic nerve 2 weeks after NMDA injection. All three complement pathway proteins were activated in retinas after 40 and 80 nmol NMDA treatment. 80 nmol NMDA injection also lead to more numerous depositions of complement factors C3 and membrane attack complex (MAC) in retina and MAC in optic nerve. Additionally, more MAC⁺ depositions were detected in optic nerves of the 40 nmol NMDA group. In this NMDA model, the retina is first affected followed by optic nerve damage. However, we found initiating complement processes in the retina, while more deposits of the terminal complex were present 2 weeks after NMDA injection in the optic nerve. The complement system can be activated in waves and possibly a second wave is still on-going in the retina, while the first activation wave is in the final phase in the optic nerve. Only the damaged tissues showed microglia activation as well as proliferation and an increase of complement proteins. Interestingly, the microglia/macrophages (MG/Mϕ) in this model were closely connected with the inductors of the classical and lectin pathway, but not with the alternative pathway. However, all three initiating complement pathways were upregulated in the retina. The alternative pathway seems to be triggered by other mechanisms in this NMDA model. Our study showed an ongoing interaction of microglia and complement proteins in a late stage of a degenerative process.

Concentration-Dependent Inner Retina Layer Damage and Optic Nerve Degeneration in a NMDA Model

The intravitreal injection of N-methyl-D-aspartate (NMDA), a glutamate analogue, is an established model for fast retinal ganglion cell (RGC) degeneration. Yet, NMDA does not cause specific RGC damage. Now, the effects on the whole retina were analyzed. Additionally, the related effects for the structure and apoptotic levels of the optic nerve were investigated. Therefore, different NMDA concentrations were intravitreally injected in rats (20, 40, or 80 nmol NMDA or PBS). At days 3 and 14, Brn-3a⁺ RGCs were degenerated. A damage of calretinin⁺ amacrine cells was also recognized at day 14. Only a slight damage was observed in regard to PKCα⁺ bipolar cells, while rhodopsin⁺ photoreceptors remained intact. A long-lasting retinal microglia response was observed from day 3 up to day 14. Furthermore, a partial degeneration of the optic nerve was noted. At day 3, the SMI-32⁺ neurofilaments were just slightly affected, whereas the neurofilament structure was further degenerated at day 14. However, the luxol fast blue (LFB)-stained myelin structure remained intact from day 3 up to day 14. Interestingly, apoptotic mechanisms, like FasL and Fas co-localization as well as caspase 3 activation, were restricted to the optic nerve of the highest NMDA group at this late stage of degeneration. The degeneration of the optic nerve is probably only a side effect of neuronal degeneration of the inner retinal layers. The intact myelin structure might form a barrier against the direct influence of NMDA. In conclusion, this model is very suitable to test therapeutic agents, but it is important to analyze all inner retina layers and the optic nerve to determine their efficacy in this model more precisely.

JIP3 Activates Kinesin-1 Motility to Promote Axon Elongation

Kinesin-1 is a molecular motor responsible for cargo transport along microtubules and plays critical roles in polarized cells, such as neurons. Kinesin-1 can function as a dimer of two kinesin heavy chains (KHC), which harbor the motor domain, or as a tetramer in combination with two accessory light chains (KLC). To ensure proper cargo distribution, kinesin-1 activity is precisely regulated. Both KLC and KHC subunits bind cargoes or regulatory proteins to engage the motor for movement along microtubules. We previously showed that the scaffolding protein JIP3 interacts directly with KHC in addition to its interaction with KLC and positively regulates dimeric KHC motility. Here we determined the stoichiometry of JIP3-KHC complexes and observed approximately four JIP3 molecules binding per KHC dimer. We then determined whether JIP3 activates tetrameric kinesin-1 motility. Using an in vitro motility assay, we show that JIP3 binding to KLC engages kinesin-1 with microtubules and that JIP3 binding to KHC promotes kinesin-1 motility along microtubules. We tested the in vivo relevance of these findings using axon elongation as a model for kinesin-1-dependent cellular function. We demonstrate that JIP3 binding to KHC, but not KLC, is essential for axon elongation in hippocampal neurons as well as axon regeneration in sensory neurons. These findings reveal that JIP3 regulation of kinesin-1 motility is critical for axon elongation and regeneration.

Diabetes induces changes in KIF1A, KIF5B and dynein distribution in the rat retina: implications for axonal transport

Diabetic retinopathy is a leading cause of vision loss and blindness. Disruption of axonal transport is associated with many neurodegenerative diseases and might also play a role in diabetes-associated disorders affecting nervous system. We investigated the impact of type 1 diabetes (2 and 8 weeks duration) on KIF1A, KIF5B and dynein motor proteins in the retina. Additionally, since hyperglycemia is considered the main trigger of diabetic complications, we investigated whether prolonged exposure to elevated glucose could affect the content and distribution of motor proteins in retinal cultures. The immunoreactivity of motor proteins was evaluated by immunohistochemistry in retinal sections and by immunoblotting in total retinal extracts from streptozotocin-induced diabetic and age-matched control animals. Primary retinal cultures were exposed to high glucose (30 mM) or mannitol (osmotic control; 24.5 mM plus 5.5 mM glucose), for seven days. Diabetes decreased the content of KIF1A at 8 weeks of diabetes as well as KIF1A immunoreactivity in the majority of retinal layers, except for the photoreceptor and outer nuclear layer. Changes in KIF5B immunoreactivity were also detected by immunohistochemistry in the retina at 8 weeks of diabetes, being increased at the photoreceptor and outer nuclear layer, and decreased in the ganglion cell layer. Regarding dynein immunoreactivity there was an increase in the ganglion cell layer after 8 weeks of diabetes. No changes were detected in retinal cultures. These alterations suggest that axonal transport may be impaired under diabetes, which might contribute to early signs of neural dysfunction in the retina of diabetic patients and animal models.

Molecular mechanisms of retinal ganglion cell degeneration in glaucoma and future prospects for cell body and axonal protection

Glaucoma, which affects more than 70 million people worldwide, is a heterogeneous group of disorders with a resultant common denominator; optic neuropathy, eventually leading to irreversible blindness. The clinical manifestations of primary open-angle glaucoma (POAG), the most common subtype of glaucoma, include excavation of the optic disc and progressive loss of visual field. Axonal degeneration of retinal ganglion cells (RGCs) and apoptotic death of their cell bodies are observed in glaucoma, in which the reduction of intraocular pressure (IOP) is known to slow progression of the disease. A pattern of localized retinal nerve fiber layer (RNFL) defects in glaucoma patients indicates that axonal degeneration may precede RGC body death in this condition. The mechanisms of degeneration of neuronal cell bodies and their axons may differ. In this review, we addressed the molecular mechanisms of cell body death and axonal degeneration in glaucoma and proposed axonal protection in addition to cell body protection. The concept of axonal protection may become a new therapeutic strategy to prevent further axonal degeneration or revive dying axons in patients with preperimetric glaucoma. Further study will be needed to clarify whether the combination therapy of axonal protection and cell body protection will have greater protective effects in early or progressive glaucomatous optic neuropathy (GON).

Involvement of endoplasmic reticulum stress in optic nerve degeneration following N-methyl-D-aspartate-induced retinal damage in mice

We evaluated time-dependent optic nerve degeneration and the role of endoplasmic reticulum (ER) stress in this process following retinal ganglion cell death in mice. Retinal damage was induced by intravitreal injection of N-methyl-D-aspartate (NMDA). Neurofilament heavy (NFH)- and phosphorylated NFH (pNFH)-positive axons were time-dependently decreased in optic nerves at 1, 3, 7, 14, and 28 days after NMDA injection. Expression of glial fibrillary acidic protein (GFAP)-positive astroglial cells and ionized calcium-binding adaptor molecule 1 (Iba1)-positive microglial cells showed a significant increase in the optic nerve at 7, 14, and 28 days after NMDA injection. In contrast, expression of myelin basic protein (MBP)-positive oligodendrocytes showed a significant decrease in the optic nerve at 7, 14, and 28 days after NMDA injection. In quantitative RT-PCR analysis, expressions of glucose-regulated protein 78 (Grp78)/BiP, Grp94, Calreticulin, C/EBP homologous protein (Chop), and the ER degradation enhancer mannosidase alpha-like 1 (Edem1) genes were increased in the optic nerve at 14 days after NMDA injection. In addition, the Grp94 gene was increased at 7 days after NMDA injection, and the Edem1 gene was increased at 3, 7, and 28 days after NMDA injection. GRP78 and CHOP proteins were colocalized with MBP in the optic nerve after NMDA injection. These findings suggest that the axonal degeneration is dramatic until 7 days after NMDA injection and that glial cells may play some role in the degeneration of the optic nerve. Furthermore, ER stress may play a pivotal role in the decrease of MBP-positive oligodendrocytes after NMDA-induced retinal damage.

Disruption of NMDA receptors in oligodendroglial lineage cells does not alter their susceptibility to experimental autoimmune encephalomyelitis or their normal development

Pharmacological studies have suggested that oligodendroglial NMDA glutamate receptors (NMDARs) mediate white matter injury in a variety of CNS diseases, including multiple sclerosis (MS). We tested this hypothesis in experimental autoimmune encephalomyelitis (EAE), a model of human MS, by timed conditional disruption of oligodendroglial NR1, an essential subunit of functional NMDARs, using an inducible proteolipid protein (Plp) promoter-driven Cre-loxP recombination system. We found that selective ablation of oligodendroglial NR1 did not alter the clinical severity of EAE elicited in C57BL/6 mice by immunization with myelin oligodendrocyte glycoprotein peptide 35-55 (MOG-peptide), nor were there significant differences between the oligodendroglial NR1 KO and non-KO mice in numbers of axons lost in spinal cord dorsal funiculi or severity of spinal cord demyelination. Similarly, constitutive deletion of NR3A, a modulatory subunit of oligodendroglial NMDARs, did not alter the course of MOG-peptide EAE. Furthermore, conditional and constitutive ablation of NR1 in neonatal oligodendrocyte progenitor cells did not interrupt their normal maturation and differentiation. Collectively, our data suggest that oligodendroglial lineage NMDARs are neither required for timely postnatal development of the oligodendroglial lineage, nor significant participants in the pathophysiology of MOG-peptide EAE.