In Vivo Imaging of Secondary Neurodegeneration Associated With Phosphatidylserine Externalization Along Axotomized Axons.

Purpose: Axonal degeneration in acute and chronic disorders is well-characterized, comprising retrograde (proximal) and Wallerian (distal) degeneration, but the mechanism of propagation remains less understood. Methods: Laser injury with a diode-pumped solid-state 532 nm laser was used to axotomize retinal ganglion cell axons. We used confocal in vivo imaging to demonstrate that phosphatidylserine externalization is a biomarker of early axonal degeneration after selective intraretinal axotomy. Results: Quantitative dynamic analysis revealed that the rate of axonal degeneration was fastest within 40 minutes, then decreased exponentially afterwards. Axonal degeneration was constrained within the same axotomized axonal bundles. Remarkably, axon degeneration arising from the site of injury induced a secondary degeneration of distal normal axons. Conclusions: Axonal degeneration in vivo is a progressive process associated with phosphatidylserine externalization, which can propagate not only along the axon but to adjacent uninjured axons. This finding has implications for acute and chronic neurodegenerative disorders associated with axonal injury.

Pivotal roles for membrane phospholipids in axonal degeneration.

Membrane phospholipids are critical components of several signaling pathways. Maintained in a variety of asymmetric distributions, their trafficking across the membrane can be induced by intra-, extra-, and intercellular events. A familiar example is the externalization of phosphatidylserine from the inner leaflet to the outer leaflet in apoptosis, inducing phagocytosis of the soma. Recently, it has been recognized that phospholipids in the axonal membrane may be a signal for axonal degeneration, regeneration, or other processes. This review focuses on key recent developments and areas for ongoing investigations. KEY FACTS: Phosphatidylserine externalization propagates along an axon after axonal injury and is delayed in the Wallerian degeneration slow (WldS) mutant. The ATP8A2 flippase mutant has spontaneous axonal degeneration. Microdomains of axonal degeneration in spheroid bodies have differential externalization of phosphatidylserine and phosphatidylethanolamine. Phospholipid trafficking could represent a mechanism for coordinated axonal degeneration and elimination, i.e. axoptosis, analogous to apoptosis of the cell body.

Axonal degeneration induces distinct patterns of phosphatidylserine and phosphatidylethanolamine externalization.

Axonal degeneration is a common feature of multiple neurodegenerative diseases, yet the mechanisms underlying its various manifestations are incompletely understood. We previously demonstrated that axonal degeneration is associated with externalization of phosphatidylserine (PS), which precedes morphological evidence of degeneration, is redox-sensitive, and is delayed in Wallerian degeneration slow (WldS) mutant animals. Phosphatidylethanolamine (PE) is the other major membrane phospholipid in the inner leaflet of the cell membrane, and given that PS signals apoptosis, phagocytosis, and degeneration, we hypothesized that PS and PE membrane dynamics play distinct roles in axonal degeneration. To test this hypothesis, axonal degeneration was induced with calcium ionophores in postnatal rat retinal ganglion cells, and PS- and PE-specific fluorescent probes used to measure their externalization over time. In untreated cells, cell-surface PS was prominent in the cell body alone. Elevation of intracellular calcium with calcium ionophores resulted in significantly increased levels of PS externalization in the cell body, axon, and axon growth cone. Unlike PS, cell-surface PE was diffusely distributed in untreated cells, with comparable levels across the soma, axons, and axon terminals. After exposure to calcium ionophores, PE externalization significantly increased in the cell body and axon. Elevated intracellular calcium also resulted in the formation of axonal blebs which exclusively contained externalized PS, but not PE. Together, these results indicated distinct patterns of externalized PS and PE in normal and degenerating neurons, suggesting a differential role for these phospholipids in transducing neuronal injury.

Cobalamin-Associated Superoxide Scavenging in Neuronal Cells Is a Potential Mechanism for Vitamin B12-Deprivation Optic Neuropathy.

Chronic deficiency of vitamin B12 is the only nutritional deficiency definitively proved to cause optic neuropathy and loss of vision. The mechanism by which this occurs is unknown. Optic neuropathies are associated with death of retinal ganglion cells (RGCs), neurons that project their axons along the optic nerve to the brain. Injury to RGC axons causes a burst of intracellular superoxide, which then signals RGC apoptosis. Vitamin B12 (cobalamin) was recently shown to be a superoxide scavenger, with a rate constant similar to superoxide dismutase. Given that vitamin B12 deficiency causes an optic neuropathy through unknown mechanisms and that it is a potent superoxide scavenger, we tested whether cobalamin, a vitamin B12 vitamer, would be neuroprotective in vitro and in vivo. We found that cobalamin scavenged superoxide in neuronal cells in vitro treated with the reduction-oxidation cycling agent menadione. In vivo confocal scanning laser ophthalmoscopy demonstrated that optic nerve transection in Long-Evans rats increased superoxide levels in RGCs. The RGC superoxide burst was significantly reduced by intravitreal cobalamin and resulted in increased RGC survival. These data demonstrate that cobalamin may function as an endogenous neuroprotectant for RGCs through a superoxide-associated mechanism.

Neuroprotection in Glaucoma: Animal Models and Clinical Trials.

Glaucoma is a progressive neurodegenerative disease that frequently results in irreversible blindness. Glaucoma causes death of retinal ganglion cells (RGCs) and their axons in the optic nerve, resulting in visual field deficits and eventual loss of visual acuity. Glaucoma is a complex optic neuropathy, and a successful strategy for its treatment requires not only better management of known risk factors such as elevated intraocular pressure and the development of improved tools for detecting RGC injury but also treatments that address this injury (i.e., neuroprotection). Experimental models of glaucoma provide insight into the cellular and molecular mechanisms of glaucomatous optic neuropathy and aid the development of neuroprotective therapies.

Axonal Degeneration in Retinal Ganglion Cells Is Associated with a Membrane Polarity-Sensitive Redox Process.

Axonal degeneration is a pathophysiological mechanism common to several neurodegenerative diseases. The slow Wallerian degeneration (WldS) mutation, which results in reduced axonal degeneration in the central and peripheral nervous systems, has provided insight into a redox-dependent mechanism by which axons undergo self-destruction. We studied early molecular events in axonal degeneration with single-axon laser axotomy and time-lapse imaging, monitoring the initial changes in transected axons of purified retinal ganglion cells (RGCs) from wild-type and WldS rat retinas using a polarity-sensitive annexin-based biosensor (annexin B12-Cys101,Cys260-N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) ethylenediamine). Transected axons demonstrated a rapid and progressive change in membrane phospholipid polarity, manifested as phosphatidylserine externalization, which was significantly delayed and propagated more slowly in axotomized WldS RGCs compared with wild-type axons. Delivery of bis(3-propionic acid methyl ester)phenylphosphine borane complex, a cell-permeable intracellular disulfide-reducing drug, slowed the onset and velocity of phosphatidylserine externalization in wild-type axons significantly, replicating the WldS phenotype, whereas extracellular redox modulation reversed the WldS phenotype. These findings are consistent with an intra-axonal redox mechanism for axonal degeneration associated with the initiation and propagation of phosphatidylserine externalization after axotomy.SIGNIFICANCE STATEMENT Axonal degeneration is a neuronal process independent of somal apoptosis, the propagation of which is unclear. We combined single-cell laser axotomy with time-lapse imaging to study the dynamics of phosphatidylserine externalization immediately after axonal injury in purified retinal ganglion cells. The extension of phosphatidylserine externalization was slowed and delayed in Wallerian degeneration slow (WldS) axons and this phenotype could be reproduced by intra-axonal disulfide reduction in wild-type axons and reversed by extra-axonal reduction in WldS axons. These results are consistent with a redox mechanism for propagation of membrane polarity asymmetry in axonal degeneration.

Ultrastructural maturation of human bone marrow mesenchymal stem cells-derived cardiomyocytes under alternative induction of 5-azacytidine.

Adult cardiomyocytes lack the ability to proliferate and are unable to repair damaged heart tissue, therefore differentiation of stem cells to cardiomyocytes represents an exceptional opportunity to study cardiomyocytes in vitro and potentially provides a valuable source for replacing damaged tissue. However, characteristic maturity of the in vitro differentiated cardiomyocytes and methods to achieve it are yet to be optimized. In this study, differentiation of human bone marrow-mesenchymal stem cells (hBM-MSCs) into cardiomyocytes is accomplished and the process investigated ultrastructurally. The hBM-MSCs were alternatively treated with 5 µM of 5-azacytidine (5-aza) for 8 weeks resulting in differentiation to cardiomyocytes. Expressions of cardiomyocyte-specific genes [cardiac α-actinin, cardiac ß-myosin heavy chain (MHC) and connexin-43] and proteins (cardiac α-actinin, cardiac troponin and connexin-43) were confirmed in a time-dependent manner from the first to the fifth weeks post-induction. Ultrastructural maturation of hBM-MSCs-derived cardiomyocyte (MSCs-CM) corresponded with increase in number and organization of myofilaments in cells over time. Starting from week five, organized myofibrils along with developing sarcomeres were detectable. Later on, MSCs-CM were characterized by the presence of sarcoplasmic reticulum, T-tubules and diads as cardiomyocytes connected to each other by intercalated disc-like structures. Here, we showed the potential of hBM-MSCs as a source for the production of cardiomyocytes and confirmed mature ultrastructural characteristics of these cells using our alternative incubation method.

Acetylcholinesterase inhibition promotes retinal vasoprotection and increases ocular blood flow in experimental glaucoma.

PURPOSE: A clear correlation between vascular deficits and retinal ganglion cell (RGC) loss in glaucoma has not yet been established. The question arose as to whether there is loss of inner retinal vessels following intraocular pressure (IOP) increase and, if so, whether it occurs prior to, concomitantly with, or after RGC death. We also sought to establish whether galantamine, an acetylcholinesterase inhibitor that promotes RGC survival, can protect the retinal microvasculature and enhance blood flow in experimental glaucoma. METHODS: Ocular hypertension was induced in Brown Norway rats by injection of hypertonic saline into an episcleral vein. Retinas were processed for simultaneous visualization of the retinal microvasculature and RGCs in glaucomatous and control eyes. Retinal blood flow was examined by quantitative autoradiography using N-isopropyl-p-[(14)C]-iodoamphetamine. Vascular reactivity was further assessed using an in vitro retinal microvasculature preparation. RESULTS: Substantial loss of retinal capillaries was observed after induction of ocular hypertension. The onset of both microvasculature and RGC loss occurred early and proceeded at a similar rate for at least 5 weeks after the initial damage. Systemic administration of galantamine preserved microvasculature density and improved retinal blood flow in glaucomatous retinas. The vasoactive effects of galantamine on retinal microvessels occurred through activation of muscarinic acetylcholine receptors both in vitro and in vivo. CONCLUSIONS: The onset and progression of microvessel and RGC loss are concomitant in experimental glaucoma, suggesting a tight codependence between these cellular compartments. Early interventions aimed to protect the retinal microvasculature and improve blood supply are likely to be beneficial for the treatment of glaucoma.

The molecular basis of retinal ganglion cell death in glaucoma.

Glaucoma is a group of diseases characterized by progressive optic nerve degeneration that results in visual field loss and irreversible blindness. A crucial element in the pathophysiology of all forms of glaucoma is the death of retinal ganglion cells (RGCs), a population of CNS neurons with their soma in the inner retina and axons in the optic nerve. Strategies that delay or halt RGC loss have been recognized as potentially beneficial to preserve vision in glaucoma; however, the success of these approaches depends on an in-depth understanding of the mechanisms that lead to RGC dysfunction and death. In recent years, there has been an exponential increase in valuable information regarding the molecular basis of RGC death stemming from animal models of acute and chronic optic nerve injury as well as experimental glaucoma. The emerging landscape is complex and points at a variety of molecular signals - acting alone or in cooperation - to promote RGC death. These include: axonal transport failure, neurotrophic factor deprivation, toxic pro-neurotrophins, activation of intrinsic and extrinsic apoptotic signals, mitochondrial dysfunction, excitotoxic damage, oxidative stress, misbehaving reactive glia and loss of synaptic connectivity. Collectively, this body of work has considerably updated and expanded our view of how RGCs might die in glaucoma and has revealed novel, potential targets for neuroprotection.

A cell-permeable phosphine-borane complex delays retinal ganglion cell death after axonal injury through activation of the pro-survival extracellular signal-regulated kinases 1/2 pathway.

The reactive oxygen species (ROS) superoxide has been recognized as a critical signal triggering retinal ganglion cell (RGC) death after axonal injury. Although the downstream targets of superoxide are unknown, chemical reduction of oxidized sulfhydryls has been shown to be neuroprotective for injured RGCs. On the basis of this, we developed novel phosphine-borane complex compounds that are cell permeable and highly stable. Here, we report that our lead compound, bis (3-propionic acid methyl ester) phenylphosphine borane complex 1 (PB1) promotes RGC survival in rat models of optic nerve axotomy and in experimental glaucoma. PB1-mediated RGC neuroprotection did not correlate with inhibition of stress-activated protein kinase signaling, including apoptosis stimulating kinase 1 (ASK1), c-jun NH2-terminal kinase (JNK) or p38. Instead, PB1 led to a striking increase in retinal BDNF levels and downstream activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) pathway. Pharmacological inhibition of ERK1/2 entirely blocked RGC neuroprotection induced by PB1. We conclude that PB1 protects damaged RGCs through activation of pro-survival signals. These data support a potential cross-talk between redox homeostasis and neurotrophin-related pathways leading to RGC survival after axonal injury.

Maintenance of axo-oligodendroglial paranodal junctions requires DCC and netrin-1.

Paranodal axoglial junctions are essential for the segregation of myelinated axons into distinct domains and efficient conduction of action potentials. Here, we show that netrin-1 and deleted in colorectal cancer (DCC) are enriched at the paranode in CNS myelin. We then address whether netrin-1 signaling influences paranodal adhesion between oligodendrocytes and axons. In the absence of netrin-1 or DCC function, oligodendroglial paranodes initially develop and mature normally but later become disorganized. Lack of DCC or netrin-1 resulted in detachment of paranodal loops from the axonal surface and the disappearance of transverse bands. Furthermore, the domain organization of myelin is compromised in the absence of netrin-1 signaling: K+ channels inappropriately invade the paranodal region, and the normally restricted paranodal distribution of Caspr expands longitudinally along the axon. Our findings identify an essential role for netrin-1 and DCC regulating the maintenance of axoglial junctions.