Axonal transport of the mitochondria-specific lipid, diphosphatidylglycerol, in the rat visual system

Ixabepilone-induced mitochondria and sensory axon loss in breast cancer patients

Background

We sought to define the clinical and ultrastructure effects of ixabepilone (Ix), a microtubule-stabilizing chemotherapy agent on cutaneous sensory nerves and to investigate a potential mitochondrial toxicity mechanism.

Methods

Ten breast cancer patients receiving Ix underwent total neuropathy score clinical (TNSc) assessment, distal leg skin biopsies at cycle (Cy) 3 (80–90 mg/m²), Cy5 (160–190 mg/m²), and Cy7 (>200 mg/m²) and were compared to 5 controls. Skin blocks were processed for EM and ultrastructural morphometry of Remak axons done.

Results

At baseline, Ix-treated subjects had higher TNSc values (4.5 ± 0.8 vs. 0.0 ± 0.0), greater percentage of empty (denervated) Schwann cells (29% vs. 12%), altered axonal diameter (422.9 ± 17 vs. 354.9 ± 14.8 nm, P = 0.01), and axon profiles without mitochondria tended to increase compared to control subjects (71% vs. 70%). With increasing cumulative Ix exposure, an increase in TNSc values (Cy3: 5.4 ± 1.2, Cy7: 10 ± 4, P < 0.001), empty Schwann cells (39% by Cy7), and dilated axons (in nm, Cy3: 506.3 ± 22.1, Cy5: 534.8 ± 33, Cy7: 527.8 ± 24.4; P < 0.001) was observed. In addition, axon profiles without mitochondria (Cy3:74%, Cy7:78%) and mitochondria with abnormal morphology (grade 3 or 4) increased from 24% to 79%. Schwann cells with atypical mitochondria and perineuronal macrophage infiltration in dermis were noted.

Interpretation

This study provides functional and structural evidence that Ix exposure induces a dose-dependent toxicity on small sensory fibers with an increase in TNSc scores and progressive axonal loss. Mitochondria appear to bear the cumulative toxic effect and chemotherapy-induced toxicity can be monitored through serial skin biopsy-based analysis.

In vivo chromium-enhanced MRI of the retina

Chromium (Cr) has been used histologically to stabilize lipid fractions in the retina and is suggested to enhance oxidizable lipids in brain MRI. This study explored the feasibility, sensitivity, and specificity of in vivo chromium-enhanced MRI of retinal lipids by determining its spatiotemporal profiles and toxic effect after intravitreal Cr(VI) injection to normal adult rats. One day after 3 μL Cr(VI) administration at 1-100 mM, the retina exhibited a dose-dependent increase in T1-weighted hyperintensity until 50 mM. Time-dependently, significant T1-weighted hyperintensity persisted up to 2 weeks after 10 mM Cr(VI) administration. Three-dimensional chromium-enhanced MRI of ex vivo normal eyes at isotropic 50-μm resolution showed at least five alternating bands across retinal layers, with the outermost layer being the brightest. This agreed with histology indicating alternating lipid contents with the highest level in the photoreceptor layer of the outer retina. Although Cr(VI) reduction may induce oxidative stress and depolymerize microtubules, manganese-enhanced MRI after chromium-enhanced MRI showed a dose-dependent effect of Cr toxicity on manganese uptake and axonal transport along the visual pathway. These results potentiated future longitudinal chromium-enhanced MRI studies on retinal lipid metabolism upon further optimization of Cr doses with visual cell viability.

Mitochondrial dysfunction in distal axons contributes to human immunodeficiency virus sensory neuropathy

OBJECTIVE

Accumulation of mitochondrial DNA (mtDNA) damage has been associated with aging and abnormal oxidative metabolism. We hypothesized that in human immunodeficiency virus-associated sensory neuropathy (HIV-SN), damaged mtDNA accumulates in distal nerve segments, and that a spatial pattern of mitochondrial dysfunction contributes to the distal degeneration of sensory nerve fibers.

METHODS

We measured levels of common deletion mutations in mtDNA and expression levels of mitochondrial respiratory chain complexes of matched proximal and distal nerve specimens from patients with and without HIV-SN. In mitochondria isolated from peripheral nerves of simian immunodeficiency virus (SIV)-infected macaques, a model of HIV-SN, we measured mitochondrial function and generation of reactive oxygen species.

RESULTS

We identified increased levels of mtDNA common deletion mutation in postmortem sural nerves of patients with HIV-SN as compared to uninfected patients or HIV patients without sensory neuropathy. Furthermore, we found that common deletion mutation in mtDNA was more prevalent in distal sural nerves compared to dorsal root ganglia. In a primate model of HIV-SN, freshly isolated mitochondria from sural nerves of macaques infected with a neurovirulent strain of SIV showed impaired mitochondrial function compared to mitochondria from proximal nerve segments.

INTERPRETATION

Our findings suggest that mtDNA damage accumulates in distal mitochondria of long axons, especially in patients with HIV-SN, and that this may lead to reduced mitochondrial function in distal nerves relative to proximal segments. Although our findings are based on HIV-SN, if confirmed in other neuropathies, these observations could explain the length-dependent nature of most axonal peripheral neuropathies.

Mitochondrial transport and docking in axons

Proper transport and distribution of mitochondria in axons and at synapses are critical for the normal physiology of neurons. Mitochondria in axons display distinct motility patterns and undergo saltatory and bidirectional movement, where mitochondria frequently stop, start moving again, and change direction. While approximately one-third of axonal mitochondria are mobile in mature neurons, a large proportion remains stationary. Their net movement is significantly influenced by recruitment to stationary or motile states. In response to the diverse physiological states of axons and synapses, the mitochondrial balance between motile and stationary phases is a possible target of regulation by intracellular signals and synaptic activity. Efficient control of mitochondrial retention (docking) at particular stations, where energy production and calcium homeostasis capacity are highly demanded, is likely essential for neuronal development and function. In this review, we introduce the molecular and cellular mechanisms underlying the complex mobility patterns of axonal mitochondria and discuss how motor adaptor complexes and docking machinery contribute to mitochondrial transport and distribution in axons and at synapses. In addition, we briefly discuss the physiological evidence how axonal mitochondrial mobility impacts synaptic function.

Axonal transport of membranous and nonmembranous cargoes: a unified perspective

Membranous and nonmembranous cargoes are transported along axons in the fast and slow components of axonal transport, respectively. Recent observations on the movement of cytoskeletal polymers in axons suggest that slow axonal transport is generated by fast motors and that the slow rate is due to rapid movements interrupted by prolonged pauses. This supports a unified perspective for fast and slow axonal transport based on rapid movements of diverse cargo structures that differ in the proportion of the time that they spend moving. A Flash feature (http://www.jcb.org/cgi/content/full/jcb.200212017/DC1) accompanies this Mini-Review.

Theoretical analysis of lipid transport in sciatic nerve

We modify our previous mathematical model of axonal transport to analyze data on the fast transport of lipids in rat sciatic nerve given in Toews et al. (J. Neurochem. 40, 555-562 (1983)). The theoretical model accounts well for the shapes of the profiles of phosphatidylcholine, phosphatidylethanolamine, cholesterol and diphosphatidylglycerol. The parameters obtained support the qualitative conclusions of Toews et al. and provide quantitative estimates of the underlying processes, e.g., rates of vesicle and mitochondria translocation, rate constants for association and dissociation between vesicles, kinesin and microtubules, rates of deposition and rates of loss of each class of lipid from the nerve by leakage or via removal by the retrograde transport system. The analysis suggests that two classes of vesicles moving at different speeds may be involved in the transport of phosphatidylcholine and phosphatidylethanolamine.

Axoplasmic transport of mitochondria in cultured dorsal root ganglion cells

The movements of individual mitochondria in cultured mouse dorsal root ganglion cells were directly observed by using fluorescent staining with rhodamine 123 in combination with video microscopic techniques. This gives greater spatial and temporal resolution and much higher specificity than possible by conventional methods. The instantaneous velocities were 0.55 +/- 0.11 microns/s anterograde and 0.60 +/- 0.10 microns/s retrograde. Movement of the mitochondria was in fits and starts, and some reversed direction. The number of mitochondria moving retrogradely was 1.5-1.9 times greater than the number moving anterogradely. The average length of mitochondria moving retrogradely was 2.8 microns and of mitochondria moving anterogradely was 4.1 microns. These results suggest that mitochondria increase their numbers by division in the nerve fiber terminal.

Acrylamide-induced increases in deposition of axonally transported glycoproteins in rat sciatic nerve

The axonal transport of proteins, glycoproteins, and gangliosides in sensory neurons of the sciatic nerve was examined in adult rats exposed to acrylamide via intraperitoneal injection (40 mg/kg of body weight/day for nine consecutive days). The L5 dorsal root ganglion was injected with either [35S]methionine to label proteins or [3H]glucosamine to label, more specifically, glycoproteins and gangliosides. At times ranging from 2 to 6 h later, the sciatic nerve and injected ganglion were excised and radioactivity in consecutive 5-mm segments determined. In both control and acrylamide-treated animals, outflow profiles of [35S]methionine-labeled proteins showed a well defined crest which moved down the nerve at a rate of approximately 340 mm/day. Similar outflow profiles and transport rates were seen for [3H]glucosamine-labeled glycoproteins in control animals. However, in animals treated with acrylamide, the crest of transported labeled glycoprotein was severely attenuated as it moved down the nerve. This finding suggests that in acrylamide-treated animals, axonally transported glycoproteins were preferentially transferred (unloaded or exchanged against unlabeled molecules) from the transport vector to stationary axonal structures. We also examined the clearance of axonally transported glycoproteins distal to a ligature on the nerve. The observed impairment of clearance in acrylamide-treated animals relative to controls is supportive of the above hypothesis. Acrylamide may directly affect the mechanism by which axonally transported material is unloaded from the transport vector. Alternatively, the increased rate of unloading might reflect an acrylamide-induced increase in the demand for axonally transported material.

Organelle dynamics in lobster axons: anterograde, retrograde and stationary mitochondria

Mitochondria in isolated motor axons from the walking legs of lobster were observed with differential interference contrast optics and video microscopic techniques. Movements of the mitochondria were analyzed in time-lapse videotape records. The mean velocity of transport in the retrograde direction (1.33 +/- 0.64 micron/s) was greater than the mean velocity of transport in the anterograde direction (0.72 +/- 0.26 micron/s). The mean lengths of the mitochondria moving in the retrograde and anterograde directions were only slightly different (6.9 microns and 5.5 microns, respectively). No correlation was found between mitochondrial length and average velocity or reciprocal velocity. The instantaneous velocities of mitochondria were distributed over a range of approximately 3 micron/s; both the anterograde and retrograde distributions contained a small proportion of values whose sign was opposite to the modal value. The variation in instantaneous velocity took place at frequencies close to 0.1 Hz. Some mitochondria displayed longitudinally oriented oscillatory movements of a similar low frequency. While the movement of most mitochondria was parallel to the axis of the axon, transverse deviations and complex circular paths were sometimes observed. Some mitochondria reversed their orientation and continued in the same direction, so that the end which had been the leading end became the trailing end. Many mitochondria immediately beneath the plasma membrane were stationary and adhered strongly to the plasma membrane when the axoplasmic structure was disrupted. In electron micrographs, fine strands connected peripheral mitochondria and the plasma membrane. These strands may anchor the stationary mitochondria to the plasma membrane.

Axonal transport characteristics of gangliosides in sensory axons of rat sciatic nerve

The distribution of axonally transported gangliosides and glycoproteins along the sciatic nerve was examined from 3 h to 4 weeks following injection of[3H]glucosamine into the fifth lumbar dorsal root ganglion of adult rats. Incorporation of labeled precursor into these glycoconjugates reached a maximal level in the ganglion within 6 h. Outflow patterns of radioactivity for glycoproteins showed a well-defined crest with a transport rate of approximately 330 mm/day. In contrast, the crest of transported gangliosides was continuously attenuated, implying a significant deposition along the axon, and an alternative method of calculating velocity was required. Analysis of accumulation of labeled material at double ligatures demonstrated both anterograde and retrograde transport of glycoproteins and gangliosides and allowed for the calculation of an anterograde transport rate of about 270 mm/day for each. Additional evidence of ganglioside transport is provided in that the TLC pattern of transported radioactive gangliosides accumulating at a ligature is significantly different from the pattern seen in the dorsal root ganglion or following intraneural administration of the labeled precursor. These data indicate that gangliosides are transported at the same rapid rate as glycoproteins but are subject to a more extensive exchange with stationary material than are glycoproteins.

The role of phosphatidylserine decarboxylase in brain phospholipid metabolism

In brain, phosphatidylethanolamine can be synthesized from free ethanolamine either by a pathway involving the formation of CDP-ethanolamine and its transfer to diglyceride, or by base-exchange of ethanolamine with existing phospholipids. Although de novo synthesis from serine has also been demonstrated, the metabolic pathway involved is not known. The enzyme phosphatidylserine decarboxylase appears to be involved in the synthesis of much of the phosphatidylethanolamine in liver, but the significance of this route in brain has been challenged. Our in vitro studies demonstrate the existence of phosphatidylserine decarboxylase activity in rat brain and characterize some of its properties. This enzyme is localized in the mitochondrial fraction, whereas the enzymes involved in base-exchange and the cytidine pathway are localized to microsomal membranes. Parallel in vivo studies showed that after the intracranial injection of L-[G-3H]serine, the specific activity of phosphatidylserine was greater in the microsomal fractions than in the mitochondrial fraction, whereas the opposite was true for phosphatidylethanolamine. When L-[U-14C]serine and [1-3H]ethanolamine were simultaneously injected, the 14C/3H ratio in mitochondrial phosphatidylethanolamine was 10 times that in microsomal phosphatidylethanolamine. The results demonstrate that serine is incorporated into the base moiety of phosphatidylethanolamine primarily through the decarboxylation of phosphatidylserine in brain mitochondria. A minimal value of 7% for the contribution of phosphatidylserine decarboxylase to whole-brain phosphatidylethanolamine synthesis can be estimated from the in vivo data.

Bidirectional axonal transport of thallium in frog sciatic nerve

Bidirectional axonal transport of radioactivity was demonstrated in frog sciatic nerve in vitro and in vivo after local application of the thallium isotope 204Tl+ to the nerve. The transport rate was similar in both directions and about 30 min/day at 18 degrees C. The transport was depressed by 2,4-DNP, low-temperature and vinblastine. A somewhat larger amount of radioactivity was transported in the retrograde than in the anterograde direction. The possibility that K+ is in part replaced by Tl+, which is transported bound to organelles, e.g. mitochondria, will be considered. Thallium might be a useful tool for future studies of axonal transport.

Differences in the kinetics of axonal transport for individual lipid classes in rat sciatic nerve

Lipid precursors ([2-3H]glycerol for phospholipids and [3H]acetate for cholesterol) were injected into the L-5 dorsal root ganglion of adult rats. At various times, animals were killed, the ganglion and consecutive 5-mm segments of sciatic nerve were dissected, and lipids were extracted and analyzed by TLC. Individual lipid classes exhibited markedly different transport patterns. The crest of radioactive phosphatidylcholine moved as a sharply defined front at about 300 mm/day, with a relatively flat plateau behind the moving crest. Although some radioactive phosphatidylethanolamine also moved at the same rate, the crest was continually attenuated as it moved so that a gradient of radioactive phosphatidylethanolamine along the axon was maintained for several days. Transported diphosphatidylglycerol exhibited a defined crest, as did phosphatidylcholine, but moved at about half the rate. Labeled cholesterol was transported at a rapid rate similar to that for phosphatidylcholine and phosphatidylethanolamine, but like phosphatidylethanolamine, the initial moving crest of radioactivity was continually attenuated. Relative to the phospholipids, cholesterol showed a more prolonged period of accumulation in the axons and was more metabolically stable. We propose that most labeled phosphatidylcholine, phosphatidylethanolamine, and cholesterol is transported in similar (or the same) rapidly moving membranous particles. Once incorporated into these particles, molecules of phosphatidylcholine tend to maintain associated with them during transport. In contrast, molecules of phosphatidylethanolamine and cholesterol in these transported particles exchange extensively with unlabeled molecules in stationary axonal structures. Diphosphatidylglycerol, localized in a specialized organelle, the mitochondrion, is transported at a slower rate than other phospholipids, and does not exchange with other structures.