Adaptation of a Commercial NAD+ Quantification Kit to Assay the Base-Exchange Activity and Substrate Preferences of SARM1

Here, we report an adapted protocol using the Promega NAD/NADH-Glo™ Assay kit. The assay normally allows quantification of trace amounts of both oxidized and reduced forms of nicotinamide adenine dinucleotide (NAD) by enzymatic cycling, but we now show that the NAD analog 3-acetylpyridine adenine dinucleotide (AcPyrAD) also acts as a substrate for this enzyme-cycling assay. In fact, AcPyrAD generates amplification signals of a larger amplitude than those obtained with NAD. We exploited this finding to devise and validate a novel method for assaying the base-exchange activity of SARM1 in reactions containing NAD and an excess of the free base 3-acetylpyridine (AcPyr), where the product is AcPyrAD. We then used this assay to study competition between AcPyr and other free bases to rank the preference of SARM1 for different base-exchange substrates, identifying isoquinoline as a highly effect substrate that completely outcompetes even AcPyr. This has significant advantages over traditional HPLC methods for assaying SARM1 base exchange as it is rapid, sensitive, cost-effective, and easily scalable. This could represent a useful tool given current interest in the role of SARM1 base exchange in programmed axon death and related human disorders. It may also be applicable to other multifunctional NAD glycohydrolases (EC 3.2.2.6) that possess similar base-exchange activity.

NMNAT2 supports vesicular glycolysis via NAD homeostasis to fuel fast axonal transport

BACKGROUND

Bioenergetic maladaptations and axonopathy are often found in the early stages of neurodegeneration. Nicotinamide adenine dinucleotide (NAD), an essential cofactor for energy metabolism, is mainly synthesized by Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) in CNS neurons. NMNAT2 mRNA levels are reduced in the brains of Alzheimer's, Parkinson's, and Huntington's disease. Here we addressed whether NMNAT2 is required for axonal health of cortical glutamatergic neurons, whose long-projecting axons are often vulnerable in neurodegenerative conditions. We also tested if NMNAT2 maintains axonal health by ensuring axonal ATP levels for axonal transport, critical for axonal function.

METHODS

We generated mouse and cultured neuron models to determine the impact of NMNAT2 loss from cortical glutamatergic neurons on axonal transport, energetic metabolism, and morphological integrity. In addition, we determined if exogenous NAD supplementation or inhibiting a NAD hydrolase, sterile alpha and TIR motif-containing protein 1 (SARM1), prevented axonal deficits caused by NMNAT2 loss. This study used a combination of techniques, including genetics, molecular biology, immunohistochemistry, biochemistry, fluorescent time-lapse imaging, live imaging with optical sensors, and anti-sense oligos.

RESULTS

We provide in vivo evidence that NMNAT2 in glutamatergic neurons is required for axonal survival. Using in vivo and in vitro studies, we demonstrate that NMNAT2 maintains the NAD-redox potential to provide "on-board" ATP via glycolysis to vesicular cargos in distal axons. Exogenous NAD+ supplementation to NMNAT2 KO neurons restores glycolysis and resumes fast axonal transport. Finally, we demonstrate both in vitro and in vivo that reducing the activity of SARM1, an NAD degradation enzyme, can reduce axonal transport deficits and suppress axon degeneration in NMNAT2 KO neurons.

CONCLUSION

NMNAT2 ensures axonal health by maintaining NAD redox potential in distal axons to ensure efficient vesicular glycolysis required for fast axonal transport.

NMN: The NAD precursor at the intersection between axon degeneration and anti-ageing therapies

The past 20 years of research on axon degeneration has revealed fine details on how NAD biology controls axonal survival. Extensive data demonstrate that the NAD precursor NMN binds to and activates the pro-degenerative enzyme SARM1, so a failure to convert sufficient NMN into NAD leads to toxic NMN accumulation and axon degeneration. This involvement of NMN brings the axon degeneration field to an unexpected overlap with research into ageing and extending healthy lifespan. A decline in NAD levels throughout life, at least in some tissues, is believed to contribute to age-related functional decay and boosting NAD production with supplementation of NMN or other NAD precursors has gained attention as a potential anti-ageing therapy. Recent years have witnessed an influx of NMN-based products and related molecules on the market, sold as food supplements, with many people taking these supplements daily. While several clinical trials are ongoing to check the safety profiles and efficacy of NAD precursors, sufficient data to back their therapeutic use are still lacking. Here, we discuss NMN supplementation, SARM1 and anti-ageing strategies, with an important question in mind: considering that NMN accumulation can lead to axon degeneration, how is this compatible with its beneficial effect in ageing and are there circumstances in which NMN supplementation could become harmful?

NMNAT2 supports vesicular glycolysis via NAD homeostasis to fuel fast axonal transport

Background

Bioenergetic maladaptations and axonopathy are often found in the early stages of neurodegeneration. Nicotinamide adenine dinucleotide (NAD), an essential cofactor for energy metabolism, is mainly synthesized by Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) in CNS neurons. NMNAT2 mRNA levels are reduced in the brains of Alzheimer's, Parkinson's, and Huntington's disease. Here we addressed whether NMNAT2 is required for axonal health of cortical glutamatergic neurons, whose long-projecting axons are often vulnerable in neurodegenerative conditions. We also tested if NMNAT2 maintains axonal health by ensuring axonal ATP levels for axonal transport, critical for axonal function.

Methods

We generated mouse and cultured neuron models to determine the impact of NMNAT2 loss from cortical glutamatergic neurons on axonal transport, energetic metabolism, and morphological integrity. In addition, we determined if exogenous NAD supplementation or inhibiting a NAD hydrolase, sterile alpha and TIR motif-containing protein 1 (SARM1), prevented axonal deficits caused by NMNAT2 loss. This study used a combination of genetics, molecular biology, immunohistochemistry, biochemistry, fluorescent time-lapse imaging, live imaging with optical sensors, and anti-sense oligos.

Results

We provide in vivo evidence that NMNAT2 in glutamatergic neurons is required for axonal survival. Using in vivo and in vitro studies, we demonstrate that NMNAT2 maintains the NAD-redox potential to provide "on-board" ATP via glycolysis to vesicular cargos in distal axons. Exogenous NAD+ supplementation to NMNAT2 KO neurons restores glycolysis and resumes fast axonal transport. Finally, we demonstrate both in vitro and in vivo that reducing the activity of SARM1, an NAD degradation enzyme, can reduce axonal transport deficits and suppress axon degeneration in NMNAT2 KO neurons.

Conclusion

NMNAT2 ensures axonal health by maintaining NAD redox potential in distal axons to ensure efficient vesicular glycolysis required for fast axonal transport.

SARM1 is a multi-functional NAD(P)ase with prominent base exchange activity, all regulated bymultiple physiologically relevant NAD metabolites

SARM1 is an NAD(P) glycohydrolase and TLR adapter with an essential, prodegenerative role in programmed axon death (Wallerian degeneration). Like other NAD(P)ases, it catalyzes multiple reactions that need to be fully investigated. Here, we compare these multiple activities for recombinant human SARM1, human CD38, and Aplysia californica ADP ribosyl cyclase. SARM1 has the highest transglycosidation (base exchange) activity at neutral pH and with some bases this dominates NAD(P) hydrolysis and cyclization. All SARM1 activities, including base exchange at neutral pH, are activated by an increased NMN:NAD ratio, at physiological levels of both metabolites. SARM1 base exchange occurs also in DRG neurons and is thus a very likely physiological source of calcium-mobilizing agent NaADP. Finally, we identify regulation by free pyridines, NADP, and nicotinic acid riboside (NaR) on SARM1, all of therapeutic interest. Understanding which specific SARM1 function(s) is responsible for axon degeneration is essential for its targeting in disease.

•

Base exchange is a prominent, and sometimes completely dominant, SARM1 activity

•

Physiologically relevant NMN:NAD ratios may regulate all of SARM1's multiple activities

•

Physiological NADP may inhibit SARM1 more potently than NAD and via a distinct site

•

NaR and VR both selectively inhibit SARM1 and are thus possible effectors or drug leads

The NAD+ precursor NMN activates dSarm to trigger axon degeneration in Drosophila

Axon degeneration contributes to the disruption of neuronal circuit function in diseased and injured nervous systems. Severed axons degenerate following the activation of an evolutionarily conserved signaling pathway, which culminates in the activation of SARM1 in mammals to execute the pathological depletion of the metabolite NAD⁺. SARM1 NADase activity is activated by the NAD⁺ precursor nicotinamide mononucleotide (NMN). In mammals, keeping NMN levels low potently preserves axons after injury. However, it remains unclear whether NMN is also a key mediator of axon degeneration and dSarm activation in flies. Here, we demonstrate that lowering NMN levels in Drosophila through the expression of a newly generated prokaryotic NMN-Deamidase (NMN-D) preserves severed axons for months and keeps them circuit-integrated for weeks. NMN-D alters the NAD⁺ metabolic flux by lowering NMN, while NAD⁺ remains unchanged in vivo. Increased NMN synthesis by the expression of mouse nicotinamide phosphoribosyltransferase (mNAMPT) leads to faster axon degeneration after injury. We also show that NMN-induced activation of dSarm mediates axon degeneration in vivo. Finally, NMN-D delays neurodegeneration caused by loss of the sole NMN-consuming and NAD⁺-synthesizing enzyme dNmnat. Our results reveal a critical role for NMN in neurodegeneration in the fly, which extends beyond axonal injury. The potent neuroprotection by reducing NMN levels is similar to the interference with other essential mediators of axon degeneration in Drosophila.

Neurotoxin-mediated potent activation of the axon degeneration regulator SARM1

Axon loss underlies symptom onset and progression in many neurodegenerative disorders. Axon degeneration in injury and disease is promoted by activation of the NAD-consuming enzyme SARM1. Here, we report a novel activator of SARM1, a metabolite of the pesticide and neurotoxin vacor. Removal of SARM1 completely rescues mouse neurons from vacor-induced neuron and axon death in vitro and in vivo. We present the crystal structure of the Drosophila SARM1 regulatory domain complexed with this activator, the vacor metabolite VMN, which as the most potent activator yet known is likely to support drug development for human SARM1 and NMNAT2 disorders. This study indicates the mechanism of neurotoxicity and pesticide action by vacor, raises important questions about other pyridines in wider use today, provides important new tools for drug discovery, and demonstrates that removing SARM1 can robustly block programmed axon death induced by toxicity as well as genetic mutation.

Enrichment of SARM1 alleles encoding variants with constitutively hyperactive NADase in patients with ALS and other motor nerve disorders

SARM1, a protein with critical NADase activity, is a central executioner in a conserved programme of axon degeneration. We report seven rare missense or in-frame microdeletion human SARM1 variant alleles in patients with amyotrophic lateral sclerosis (ALS) or other motor nerve disorders that alter the SARM1 auto-inhibitory ARM domain and constitutively hyperactivate SARM1 NADase activity. The constitutive NADase activity of these seven variants is similar to that of SARM1 lacking the entire ARM domain and greatly exceeds the activity of wild-type SARM1, even in the presence of nicotinamide mononucleotide (NMN), its physiological activator. This rise in constitutive activity alone is enough to promote neuronal degeneration in response to otherwise non-harmful, mild stress. Importantly, these strong gain-of-function alleles are completely patient-specific in the cohorts studied and show a highly significant association with disease at the single gene level. These findings of disease-associated coding variants that alter SARM1 function build on previously reported genome-wide significant association with ALS for a neighbouring, more common SARM1 intragenic single nucleotide polymorphism (SNP) to support a contributory role of SARM1 in these disorders. A broad phenotypic heterogeneity and variable age-of-onset of disease among patients with these alleles also raises intriguing questions about the pathogenic mechanism of hyperactive SARM1 variants.

Novel HDAC6 Inhibitors Increase Tubulin Acetylation and Rescue Axonal Transport of Mitochondria in a Model of Charcot-Marie-Tooth Type 2F

Disruption of axonal transport causes a number of rare, inherited axonopathies and is heavily implicated in a wide range of more common neurodegenerative disorders, many of them age-related. Acetylation of α-tubulin is one important regulatory mechanism, influencing microtubule stability and motor protein attachment. Of several strategies so far used to enhance axonal transport, increasing microtubule acetylation through inhibition of the deacetylase enzyme histone deacetylase 6 (HDAC6) has been one of the most effective. Several inhibitors have been developed and tested in animal and cellular models, but better drug candidates are still needed. Here we report the development and characterization of two highly potent HDAC6 inhibitors, which show low toxicity, promising pharmacokinetic properties, and enhance microtubule acetylation in the nanomolar range. We demonstrate their capacity to rescue axonal transport of mitochondria in a primary neuronal culture model of the inherited axonopathy Charcot-Marie-Tooth Type 2F, caused by a dominantly acting mutation in heat shock protein beta 1.

Mitochondrial impairment activates the Wallerian pathway through depletion of NMNAT2 leading to SARM1-dependent axon degeneration

Wallerian degeneration of physically injured axons involves a well-defined molecular pathway linking loss of axonal survival factor NMNAT2 to activation of pro-degenerative protein SARM1. Manipulating the pathway through these proteins led to the identification of non-axotomy insults causing axon degeneration by a Wallerian-like mechanism, including several involving mitochondrial impairment. Mitochondrial dysfunction is heavily implicated in Parkinson’s disease, Charcot-Marie-Tooth disease, hereditary spastic paraplegia and other axonal disorders. However, whether and how mitochondrial impairment activates Wallerian degeneration has remained unclear. Here, we show that disruption of mitochondrial membrane potential leads to axonal NMNAT2 depletion in mouse sympathetic neurons, increasing the substrate-to-product ratio (NMN/NAD) of this NAD-synthesising enzyme, a metabolic fingerprint of Wallerian degeneration. The mechanism appears to involve both impaired NMNAT2 synthesis and reduced axonal transport. Expression of WLD^S and Sarm1 deletion both protect axons after mitochondrial uncoupling. Blocking the pathway also confers neuroprotection and increases the lifespan of flies with Pink1 loss-of-function mutation, which causes severe mitochondrial defects. These data indicate that mitochondrial impairment replicates all the major steps of Wallerian degeneration, placing it upstream of NMNAT2 loss, with the potential to contribute to axon pathology in mitochondrial disorders.

NAD+ cleavage activity by animal and plant TIR domains in cell death pathways

SARM1 (sterile alpha and TIR motif containing 1) is responsible for depletion of nicotinamide adenine dinucleotide in its oxidized form (NAD⁺) during Wallerian degeneration associated with neuropathies. Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors recognize pathogen effector proteins and trigger localized cell death to restrict pathogen infection. Both processes depend on closely related Toll/interleukin-1 receptor (TIR) domains in these proteins, which, as we show, feature self-association-dependent NAD⁺ cleavage activity associated with cell death signaling. We further show that SARM1 SAM (sterile alpha motif) domains form an octamer essential for axon degeneration that contributes to TIR domain enzymatic activity. The crystal structures of ribose and NADP⁺ (the oxidized form of nicotinamide adenine dinucleotide phosphate) complexes of SARM1 and plant NLR RUN1 TIR domains, respectively, reveal a conserved substrate binding site. NAD⁺ cleavage by TIR domains is therefore a conserved feature of animal and plant cell death signaling pathways.

Low levels of NMNAT2 compromise axon development and survival

Nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) is an endogenous axon maintenance factor that preserves axon health by blocking Wallerian-like axon degeneration. Mice lacking NMNAT2 die at birth with severe axon defects in both the peripheral nervous system and central nervous system so the complete absence of NMNAT2 in humans is likely to be similarly harmful but probably rare. However, there is evidence of widespread natural variation in human NMNAT2 mRNA expression so it is important to establish whether reduced levels of NMNAT2 have consequences that impact health. While mice that express reduced levels of NMNAT2, either those heterozygous for a silenced Nmnat2 allele or compound heterozygous for one silenced and one partially silenced Nmnat2 allele, remain overtly normal into old age, we now report that Nmnat2 compound heterozygote mice present with early and age-dependent peripheral nerve axon defects. Compound heterozygote mice already have reduced numbers of myelinated sensory axons at 1.5 months and lose more axons, likely motor axons, between 18 and 24 months and, crucially, these changes correlate with early temperature insensitivity and a later-onset decline in motor performance. Slower neurite outgrowth and increased sensitivity to axonal stress are also evident in primary cultures of Nmnat2 compound heterozygote superior cervical ganglion neurons. These data reveal that reducing NMNAT2 levels below a particular threshold compromises the development of peripheral axons and increases their vulnerability to stresses. We discuss the implications for human neurological phenotypes where axons are longer and have to be maintained over a much longer lifespan.

Severe biallelic loss-of-function mutations in nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) in two fetuses with fetal akinesia deformation sequence

The three nicotinamide mononucleotide adenylyltransferase (NMNAT) family members synthesize the electron carrier nicotinamide adenine dinucleotide (NAD⁺) and are essential for cellular metabolism. In mammalian axons, NMNAT activity appears to be required for axon survival and is predominantly provided by NMNAT2. NMNAT2 has recently been shown to also function as a chaperone to aid in the refolding of misfolded proteins. Nmnat2 deficiency in mice, or in its ortholog dNmnat in Drosophila, results in axon outgrowth and survival defects. Peripheral nerve axons in NMNAT2-deficient mice fail to extend and innervate targets, and skeletal muscle is severely underdeveloped. In addition, removing NMNAT2 from established axons initiates axon death by Wallerian degeneration. We report here on two stillborn siblings with fetal akinesia deformation sequence (FADS), severely reduced skeletal muscle mass and hydrops fetalis. Clinical exome sequencing identified compound heterozygous NMNAT2 variant alleles in both cases. Both protein variants are incapable of supporting axon survival in mouse primary neuron cultures when overexpressed. In vitro assays demonstrate altered protein stability and/or defects in NAD⁺ synthesis and chaperone functions. Thus, both patient NMNAT2 alleles are null or severely hypo-morphic. These data indicate a previously unknown role for NMNAT2 in human neurological development and provide the first direct molecular evidence to support the involvement of Wallerian degeneration in a human axonal disorder. SIGNIFICANCE: Nicotinamide Mononucleotide Adenylyltransferase 2 (NMNAT2) both synthesizes the electron carrier Nicotinamide Adenine Dinucleotide (NAD⁺) and acts a protein chaperone. NMNAT2 has emerged as a major neuron survival factor. Overexpression of NMNAT2 protects neurons from Wallerian degeneration after injury and declining levels of NMNAT2 have been implicated in neurodegeneration. While the role of NMNAT2 in neurodegeneration has been extensively studied, the role of NMNAT2 in human development remains unclear. In this work, we present the first human variants in NMNAT2 identified in two fetuses with severe skeletal muscle hypoplasia and fetal akinesia. Functional studies in vitro showed that the mutations impair both NMNAT2 NAD⁺ synthase and chaperone functions. This work identifies the critical role of NMNAT2 in human development.

Homozygous NMNAT2 mutation in sisters with polyneuropathy and erythromelalgia

We identified a homozygous missense mutation in the gene encoding NAD synthesizing enzyme NMNAT2 in two siblings with childhood onset polyneuropathy with erythromelalgia. No additional homozygotes for this rare allele, which leads to amino acid substitution T94M, were present among the unaffected relatives tested or in the 60,000 exomes of the ExAC database. For axons to survive, axonal NMNAT2 activity has to be maintained above a threshold level but the T94M mutation confers a partial loss of function both in the ability of NMNAT2 to support axon survival and in its enzymatic properties. Electrophysiological tests and histological analysis of sural nerve biopsies in the patients were consistent with loss of distal sensory and motor axons. Thus, it is likely that NMNAT2 mutation causes this pain and axon loss phenotype making this the first disorder associated with mutation of a key regulator of Wallerian-like axon degeneration in humans. This supports indications from numerous animal studies that the Wallerian degeneration pathway is important in human disease and raises important questions about which other human phenotypes could be linked to this gene.

Mislocalization of neuronal tau in the absence of tangle pathology in phosphomutant tau knockin mice

Hyperphosphorylation and fibrillar aggregation of the microtubule-associated protein tau are key features of Alzheimer's disease and other tauopathies. To investigate the involvement of tau phosphorylation in the pathological process, we generated a pair of complementary phosphomutant tau knockin mouse lines. One exclusively expresses phosphomimetic tau with 18 glutamate substitutions at serine and/or threonine residues in the proline-rich and first microtubule-binding domains to model hyperphosphorylation, whereas its phosphodefective counterpart has matched alanine substitutions. Consistent with expected effects of genuine phosphorylation, association of the phosphomimetic tau with microtubules and neuronal membranes is severely disrupted in vivo, whereas the phosphodefective mutations have more limited or no effect. Surprisingly, however, age-related mislocalization of tau is evident in both lines, although redistribution appears more widespread and more pronounced in the phosphomimetic tau knockin. Despite these changes, we found no biochemical or immunohistological evidence of pathological tau aggregation in mice of either line up to at least 2 years of age. These findings raise important questions about the role of tau phosphorylation in driving pathology in human tauopathies.

Reduced number of axonal mitochondria and tau hypophosphorylation in mouse P301L tau knockin neurons

Expression of the frontotemporal dementia-related tau mutation, P301L, at physiological levels in adult mouse brain (KI-P301L mice) results in overt hypophosphorylation of tau and age-dependent alterations in axonal mitochondrial transport in peripheral nerves. To determine the effects of P301L tau expression in the central nervous system, we examined the kinetics of mitochondrial axonal transport and tau phosphorylation in primary cortical neurons from P301L knock-in (KI-P301L) mice. We observed a significant 50% reduction in the number of mitochondria in the axons of cortical neurons cultured from KI-P301L mice compared to wild-type neurons. Expression of murine P301L tau did not change the speed, direction of travel or likelihood of movement of mitochondria. Notably, the angle that defines the orientation of the mitochondria in the axon, and the volume of individual moving mitochondria, were significantly increased in neurons expressing P301L tau. We found that murine tau phosphorylation in KI-P301L mouse neurons was diminished and the ability of P301L tau to bind to microtubules was also reduced compared to tau in wild-type neurons. The P301L mutation did not influence the ability of murine tau to associate with membranes in cortical neurons or in adult mouse brain. We conclude that P301L tau is associated with mitochondrial changes and causes an early reduction in murine tau phosphorylation in neurons coupled with impaired microtubule binding of tau. These results support the association of mutant tau with detrimental effects on mitochondria and will be of significance for the pathogenesis of tauopathies.

•

Expression of P301L tau reduces the number of axonal mitochondria in mice.

•

Motile mitochondria exhibit increased volume in axons of neurons with P301L tau.

•

P301L tau expressed in knockin mice is hypophosphorylated.

•

The P301L tau mutation impairs microtubule binding but does not affect tau membrane localization.

Absence of SARM1 rescues development and survival of NMNAT2-deficient axons

SARM1 function and nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) loss both promote axon degeneration, but their relative relationship in the process is unknown. Here, we show that NMNAT2 loss and resultant changes to NMNAT metabolites occur in injured SARM1-deficient axons despite their delayed degeneration and that axon degeneration specifically induced by NMNAT2 depletion requires SARM1. Strikingly, SARM1 deficiency also corrects axon outgrowth in mice lacking NMNAT2, independently of NMNAT metabolites, preventing perinatal lethality. Furthermore, NAMPT inhibition partially restores outgrowth of NMNAT2-deficient axons, suggesting that the NMNAT substrate, NMN, contributes to this phenotype. NMNAT2-depletion-dependent degeneration of established axons and restricted extension of developing axons are thus both SARM1 dependent, and SARM1 acts either downstream of NMNAT2 loss and NMN accumulation in a linear pathway or in a parallel branch of a convergent pathway. Understanding the pathway will help establish relationships with other modulators of axon survival and facilitate the development of effective therapies for axonopathies.

Axonal trafficking of NMNAT2 and its roles in axon growth and survival in vivo

The NAD-synthesizing enzyme NMNAT2 is critical for axon survival in primary culture and its depletion may contribute to axon degeneration in a variety of neurodegenerative disorders. Here we discuss several recent reports from our laboratory that establish a critical role for NMNAT2 in axon growth in vivo in mice and shed light on the delivery and turnover of this survival factor in axons. In the absence of NMNAT2, axons fail to extend more than a short distance beyond the cell body during embryonic development, implying a requirement for NMNAT2 in axon maintenance even during development. Furthermore, we highlight findings regarding the bidirectional trafficking of NMNAT2 in axons on a vesicle population that undergoes fast axonal transport in primary culture neurites and in mouse sciatic nerve axons in vivo. Surprisingly, loss of vesicle association boosts the axon protective capacity of NMNAT2, an effect that is at least partially mediated by a longer protein half-life of cytosolic NMNAT2 variants. Analysis of wild-type and variant NMNAT2 in mouse sciatic nerves and Drosophila olfactory receptor neuron axons supports the existence of a similar mechanism in vivo, highlighting the potential for regulation of NMNAT2 stability and turnover as a mechanism to modulate axon degeneration in vivo.

MEK inhibitor U0126 reverses protection of axons from Wallerian degeneration independently of MEK-ERK signaling

Wallerian degeneration is delayed when sufficient levels of proteins with NMNAT activity are maintained within axons after injury. This has been proposed to form the basis of 'slow Wallerian degeneration' (Wld^S), a neuroprotective phenotype conferred by an aberrant fusion protein, Wld^S. Proteasome inhibition also delays Wallerian degeneration, although much less robustly, with stabilization of NMNAT2 likely to play a key role in this mechanism. The pan-MEK inhibitor U0126 has previously been shown to reverse the axon-protective effects of proteasome inhibition, suggesting that MEK-ERK signaling plays a role in delayed Wallerian degeneration, in addition to its established role in promoting neuronal survival. Here we show that whilst U0126 can also reverse Wld^S-mediated axon protection, more specific inhibitors of MEK1/2 and MEK5, PD184352 and BIX02189, have no significant effect on the delay to Wallerian degeneration in either situation, whether used alone or in combination. This suggests that an off-target effect of U0126 is responsible for reversion of the axon protective effects of Wld^S expression or proteasome inhibition, rather than inhibition of MEK1/2-ERK1/2 or MEK5-ERK5 signaling. Importantly, this off-target effect does not appear to result in alterations in the stabilities of either Wld^S or NMNAT2.

Subcellular localization determines the stability and axon protective capacity of axon survival factor Nmnat2

Modulation of the subcellular localization of the endogenous axon survival factor Nmnat2 boosts its axon protective capacity, suggesting a novel approach to delaying axon degeneration in neurodegenerative disease.

Axons require a constant supply of the labile axon survival factor Nmnat2 from their cell bodies to avoid spontaneous axon degeneration. Here we investigate the mechanism of fast axonal transport of Nmnat2 and its site of action for axon maintenance. Using dual-colour live-cell imaging of axonal transport in SCG primary culture neurons, we find that Nmnat2 is bidirectionally trafficked in axons together with markers of the trans-Golgi network and synaptic vesicles. In contrast, there is little co-migration with mitochondria, lysosomes, and active zone precursor vesicles. Residues encoded by the small, centrally located exon 6 are necessary and sufficient for stable membrane association and vesicular axonal transport of Nmnat2. Within this sequence, a double cysteine palmitoylation motif shared with GAP43 and surrounding basic residues are all required for efficient palmitoylation and stable association with axonal transport vesicles. Interestingly, however, disrupting this membrane association increases the ability of axonally localized Nmnat2 to preserve transected neurites in primary culture, while re-targeting the strongly protective cytosolic mutants back to membranes abolishes this increase. Larger deletions within the central domain including exon 6 further enhance Nmnat2 axon protective capacity to levels that exceed that of the slow Wallerian degeneration protein, Wld^S. The mechanism underlying the increase in axon protection appears to involve an increased half-life of the cytosolic forms, suggesting a role for palmitoylation and membrane attachment in Nmnat2 turnover. We conclude that Nmnat2 activity supports axon survival through a site of action distinct from Nmnat2 transport vesicles and that protein stability, a key determinant of axon protection, is enhanced by mutations that disrupt palmitoylation and dissociate Nmnat2 from these vesicles.

Neurons are polarized cells that rely on bidirectional transport to deliver thousands of cargos between the cell body and the most distal ends of their axons. One cargo that is of particular importance is the NAD-synthesising enzyme Nmnat2. This surprisingly unstable protein is produced in the cell body and its constant supply into axons is required to keep them alive. If this supply is interrupted, Nmnat2 levels in the distal axon drop below a critical threshold, leading to axon degeneration. The rapid turnover of Nmnat2 contributes critically to the time course of axon degeneration. If its half-life could be extended, axons may be able to survive transient interruptions of its supply. In this study, we find that disruption of Nmnat2 localization to axonal transport vesicles increases both its half-life and its capacity to protect injured neurites. Specifically, association of Nmnat2 with transport vesicles reduces it stability by making it vulnerable to ubiquitination and proteasome-mediated degradation. These findings suggest that modulation of the subcellular localization of Nmnat2 on transport vesicles could serve as a potential avenue for therapeutic treatment of axon degeneration.

Nicotinamide mononucleotide adenylyltransferase 2 (Nmnat2) regulates axon integrity in the mouse embryo

Using transposon-mediated gene-trap mutagenesis, we have generated a novel mouse mutant termed Blad (Bloated Bladder). Homozygous mutant mice die perinatally showing a greatly distended bladder, underdeveloped diaphragm and a reduction in total skeletal muscle mass. Wild type and heterozygote mice appear normal. Using PCR, we identified a transposon insertion site in the first intron of Nmnat2 (Nicotinamide mononucleotide adenyltransferase 2). Nmnat2 is expressed predominantly in the brain and nervous system and has been linked to the survival of axons. Expression of this gene is undetectable in Nmnat2^blad/blad mutants. Examination of the brains of E18.5 Nmnat2^blad/blad mutant embryos did not reveal any obvious morphological changes. In contrast, E18.5 Nmnat2^blad/blad homozygotes showed an approximate 60% reduction of spinal motoneurons in the lumbar region and a more than 80% reduction in the sensory neurons of the dorsal root ganglion (DRG). In addition, facial motoneuron numbers were severely reduced, and there was virtually a complete absence of axons in the hind limb. Our observations suggest that during embryogenesis, Nmnat2 plays an important role in axonal growth or maintenance. It appears that in the absence of Nmnat2, major target organs and tissues (e.g., muscle) are not functionally innervated resulting in perinatal lethality. In addition, neither Nmnat1 nor 3 can compensate for the loss of Nmnat2. Whilst there have been recent suggestions that Nmnat2 may be an endogenous modulator of axon integrity, this work represents the first in vivo study demonstrating that Nmnat2 is involved in axon development or survival in a mammal.

The MEK-ERK pathway negatively regulates bim expression through the 3' UTR in sympathetic neurons

Background

Apoptosis plays a critical role during neuronal development and disease. Developing sympathetic neurons depend on nerve growth factor (NGF) for survival during the late embryonic and early postnatal period and die by apoptosis in its absence. The proapoptotic BH3-only protein Bim increases in level after NGF withdrawal and is required for NGF withdrawal-induced death. The regulation of Bim expression in neurons is complex and this study describes a new mechanism by which an NGF-activated signalling pathway regulates bim gene expression in sympathetic neurons.

Results

We report that U0126, an inhibitor of the prosurvival MEK-ERK pathway, increases bim mRNA levels in sympathetic neurons in the presence of NGF. We find that this effect is independent of PI3-K-Akt and JNK-c-Jun signalling and is not mediated by the promoter, first exon or first intron of the bim gene. By performing 3' RACE and microinjection experiments with a new bim-LUC+3'UTR reporter construct, we show that U0126 increases bim expression via the bim 3' UTR. We demonstrate that this effect does not involve a change in bim mRNA stability and by using PD184352, a specific MEK1/2-ERK1/2 inhibitor, we show that this mechanism involves the MEK1/2-ERK1/2 pathway. Finally, we demonstrate that inhibition of MEK/ERK signalling independently reduces cell survival in NGF-treated sympathetic neurons.

Conclusions

These results suggest that in sympathetic neurons, MEK-ERK signalling negatively regulates bim expression via the 3' UTR and that this regulation is likely to be at the level of transcription. This data provides further insight into the different mechanisms by which survival signalling pathways regulate bim expression in neurons.

Endogenous Nmnat2 is an essential survival factor for maintenance of healthy axons

We conclude that endogenous Nmnat2 prevents spontaneous degeneration of healthy axons and propose that, when present, the more long-lived, functionally related WldS protein substitutes for Nmnat2 loss after axon injury. Endogenous Nmnat2 represents an exciting new therapeutic target for axonal disorders.

The molecular triggers for axon degeneration remain unknown. We identify endogenous Nmnat2 as a labile axon survival factor whose constant replenishment by anterograde axonal transport is a limiting factor for axon survival. Specific depletion of Nmnat2 is sufficient to induce Wallerian-like degeneration of uninjured axons which endogenous Nmnat1 and Nmnat3 cannot prevent. Nmnat2 is by far the most labile Nmnat isoform and is depleted in distal stumps of injured neurites before Wallerian degeneration begins. Nmnat2 turnover is equally rapid in injured Wld^S neurites, despite delayed neurite degeneration, showing it is not a consequence of degeneration and also that Wld^S does not stabilize Nmnat2. Depletion of Nmnat2 below a threshold level is necessary for axon degeneration since exogenous Nmnat2 can protect injured neurites when expressed at high enough levels to overcome its short half-life. Furthermore, proteasome inhibition slows both Nmnat2 turnover and neurite degeneration. We conclude that endogenous Nmnat2 prevents spontaneous degeneration of healthy axons and propose that, when present, the more long-lived, functionally related Wld^S protein substitutes for Nmnat2 loss after axon injury. Endogenous Nmnat2 represents an exciting new therapeutic target for axonal disorders.

In a normally functioning neuron, the cell body supplies the axon with materials needed to keep it healthy. This complex logistical activity breaks down completely after injury and often becomes compromised in neurodegenerative diseases, leading to degeneration of the isolated axon. Whilst there are probably many important cargoes delivered from the cell body that isolated axons cannot exist without indefinitely, proteins that are short-lived will be depleted first, so loss of these proteins is likely to act as a trigger for degeneration. Using clues from a mutant mouse whose axons are protected from such degeneration, we have identified delivery of Nmnat2, a protein with an important enzyme activity, as a limiting factor in axon survival. Importantly, Nmnat2 is very labile and its levels decline rapidly in injured axons before they start to degenerate. Even uninjured axons degenerate in a similar way without it. These properties are consistent with loss of Nmnat2 being a natural stimulus for axon degeneration, and it might therefore be a suitable target for therapeutic intervention.

Knockdown mortality, repellency, and residual effects of insecticides for control of adult Bactericera cockerelli (Hemiptera: Psyllidae)

The psyllid Bactericera cockerelli (Sulc) (Hemiptera: Psyllidae) is the vector of a bacterial pathogen causing zebra chip (ZC) disease in potato, Solanum tuberosum L. (Solanaceae). Recently, ZC has become important to potato growers and the potato chipping industry in the southwestern United States, Mexico, Central America, and New Zealand. In the current study, we evaluated the knockdown effect of 12 insecticides against adult B. cockerelli, and we examined the effects of treating potato leaves: top side only, bottom side only, or both sides. Within 48 h of exposure, abamectin showed the highest and most consistent rate of adult B. cockerelli mortality and a dosage response to three dosages within labeled field rates. Choice tests conducted in petri dishes, involving untreated potato leaves and leaves treated with abamectin, revealed that abamectin did not affect host preference of adult B. cockerelli. A residual test under field conditions revealed that, although highly effective immediately after application, abamectin-effect was nonsignificantly different from control treatments 24 and 48 h after field application. Higher adult B. cockerelli mortality was recorded from potato plants treated with ground application compared with aerial 48 h after application. Our results indicated that abamectin has a high knockdown effect on adult B. cockerelli and that it can be used effectively in insecticide rotation programs against adult B. cockerelli and ZC control in potatoes.

The proapoptotic dp5 gene is a direct target of the MLK-JNK-c-Jun pathway in sympathetic neurons

The death of sympathetic neurons after nerve growth factor (NGF) withdrawal requires de novo gene expression. Dp5 was one of the first NGF withdrawal-induced genes to be identified and it encodes a proapoptotic BH3-only member of the Bcl-2 family. To study how dp5 transcription is regulated by NGF withdrawal we cloned the regulatory regions of the rat dp5 gene and constructed a series of dp5-luciferase reporter plasmids. In microinjection experiments with sympathetic neurons we found that three regions of dp5 contribute to its induction after NGF withdrawal: the promoter, a conserved region in the single intron, and sequences in the 3′ untranslated region of the dp5 mRNA. A construct containing all three regions is efficiently activated by NGF withdrawal and, like the endogenous dp5, its induction requires mixed-lineage kinase (MLK) and c-Jun N-terminal kinase (JNK) activity. JNKs phosphorylate the AP-1 transcription factor c-Jun, and thereby increase its activity. We identified a conserved ATF site in the dp5 promoter that binds c-Jun and ATF2, which is critical for dp5 promoter induction after NGF withdrawal. These results suggest that part of the mechanism by which the MLK-JNK-c-Jun pathway promotes neuronal apoptosis is by activating the transcription of the dp5 gene.

Wld S protein requires Nmnat activity and a short N-terminal sequence to protect axons in mice

The slow Wallerian degeneration (Wld^S) protein protects injured axons from degeneration. This unusual chimeric protein fuses a 70–amino acid N-terminal sequence from the Ube4b multiubiquitination factor with the nicotinamide adenine dinucleotide–synthesizing enzyme nicotinamide mononucleotide adenylyl transferase 1. The requirement for these components and the mechanism of Wld^S-mediated neuroprotection remain highly controversial. The Ube4b domain is necessary for the protective phenotype in mice, but precisely which sequence is essential and why are unclear. Binding to the AAA adenosine triphosphatase valosin-containing protein (VCP)/p97 is the only known biochemical property of the Ube4b domain. Using an in vivo approach, we show that removing the VCP-binding sequence abolishes axon protection. Replacing the Wld^S VCP-binding domain with an alternative ataxin-3–derived VCP-binding sequence restores its protective function. Enzyme-dead Wld^S is unable to delay Wallerian degeneration in mice. Thus, neither domain is effective without the function of the other. Wld^S requires both of its components to protect axons from degeneration.

Abeta, tau and ApoE4 in Alzheimer's disease: the axonal connection

Mutations in amyloid precursor protein (APP), tau and apolipoprotein E4 (ApoE4) lead to Alzheimer's disease (AD) or related pathologies. Pathogenesis and interactions between these pathways have been studied in mouse models. Here, we highlight the fact that axons are important sites of cellular pathology in each pathway and propose that pathway convergence at the molecular level might occur in axons. Recent developments suggest that axonal transport of APP influences beta-amyloid deposition and that tau regulates axonal transport. ApoE4 influences both axonal tau phosphorylation and amyloid-induced neurite pathology. Thus, a better understanding of axonal events in AD might help connect the pathogenic mechanisms of beta-amyloid, ApoE4 and tau, indicating the most important steps for therapeutic targeting.

Evidence for increased complexity in the regulation of Bim expression in sympathetic neurons: involvement of novel transcriptional and translational mechanisms

The BH3-only protein Bim is induced following NGF deprivation in developing sympathetic neurons and contributes to their death by apoptosis. The regulation of Bim activity is complex, and involves both transcriptional and posttranslational mechanisms. We have previously shown that both the FOXO subfamily of Forkhead transcription factors and the JNK/c-Jun pathway contribute to the transcriptional induction of Bim expression and subsequent apoptosis of sympathetic neurons following NGF deprivation. Bim activity can also be modulated by JNK-mediated phosphorylation after NGF deprivation in these cells. Here, we provide evidence for additional complexity in the transcriptional and translational control of Bim expression. We show that the first intron of the bim gene contains elements with silencer and enhancer properties that can modulate the basal activity and NGF deprivation-induced activity of the previously characterized bim promoter. Surprisingly, we find that some of the elements responsible for these effects are linked to two novel, alternative promoters located towards the 3' end of the intron that have minimal, or no activity in sympathetic neurons. Finally, we provide evidence that Bim expression is reduced in sympathetic neurons by the presence of an upstream open reading frame in the 5' leader of bim transcripts.

FOXO transcription factors directly activate bim gene expression and promote apoptosis in sympathetic neurons

Developing sympathetic neurons die by apoptosis when deprived of NGF. BIM, a BH3-only member of the BCL-2 family, is induced after NGF withdrawal in these cells and contributes to NGF withdrawal-induced death. Here, we have investigated the involvement of the Forkhead box, class O (FOXO) subfamily of Forkhead transcription factors in the regulation of BIM expression by NGF. We find that overexpression of FOXO transcription factors induces BIM expression and promotes death of sympathetic neurons in a BIM-dependent manner. In addition, we find that FKHRL1 (FOXO3a) directly activates the bim promoter via two conserved FOXO binding sites and that mutation of these sites abolishes bim promoter activation after NGF withdrawal. Finally, we show that FOXO activity contributes to the NGF deprivation-induced death of sympathetic neurons.

One INK4 gene and no ARF at the Fugu equivalent of the human INK4A/ARF/INK4B tumour suppressor locus

The INK4A/ARF/INK4B locus, conserved in mammals, encodes three polypeptides that regulate cell proliferation via the pRb and p53 tumour suppressor pathways. The locus is mutated in many cancers. The related, tandemly-linked INK4A and INK4B genes encode the p16(INK4A) and p15(INK4B) members of the INK4 family of cyclin-dependent kinase inhibitors which block phosphorylation of pRb, whereas the third product, ARF, derived from an alternative reading frame of INK4A, regulates p53 activity. We assessed the status of this unusual locus in the puffer fish, Fugu rubripes, and identified two INK4 genes using degenerate PCR and hybridization analyses. Sequence conservation and conservation of synteny between human and Fugu predict one gene to be an INK4A or INK4B homologue and the other an INK4D homologue. Analysis of the Fugu INK4A/B gene and the surrounding 40-kb of genomic DNA did not reveal the presence of any ARF-encoding potential or another related INK4 gene. We conclude that the gene duplication event that generated adjacent INK4A and INK4B genes and the association of ARF with the ancestral INK4A gene occurred after the divergence of the lineage leading to mammals from fish. Thus, unlike mammals, the fish p53 and pRb tumour suppressor pathways are not regulated by a single locus.

Are human milk long-chain polyunsaturated fatty acids related to visual and neural development in breast-fed term infants?

OBJECTIVE

To determine whether docosahexaenoic acid (DHA) is related to visual and neural development in term breast-fed infants.

DESIGN

A prospective study of 83 infants who were exclusively breast-fed for at least 3 months. We determined red blood cell and plasma fatty acids at 2 months, visual acuity at 2, 4, 6, and 12 months, speech perception and an object search task at 9 months, Bayley's mental development index and psychomotor development index at 6 and 12 months, and novelty pReference at 6 and 9 months.

RESULTS

The infant red blood cell phosphatidylethanolamine DHA was significantly related to visual acuity at 2 months of age (r = 0.32, P =.01) and 12 months of age (r = 0.30, P =.03). The ability to discriminate nonnative retroflex and phonetic contrasts at 9 months of age was related to the plasma phospholipid DHA (r = 0.48, P <.02) and red blood cell phosphatidylethanolamine DHA (r = 0.26, P =.02) at 2 months of age after adjusting for covariates.

CONCLUSION

DHA may influence the development of visual acuity and neural pathways associated with the developmental progression of language acquisition in term breast-fed infants. The extent to which our results can be attributed solely to DHA from maternal sources through breast milk or in gestation or other confounding factors remains to be determined.

Extensive gene order differences within regions of conserved synteny between the Fugu and human genomes: implications for chromosomal evolution and the cloning of disease genes

The suitability of the Fugu genome to facilitate the identification of candidate human disease genes using comparative positional cloning is dependent upon the extent to which synteny and gene order are conserved between the two species. We have cloned seven Fugu genes which are closely linked to Surfeit genes in two regions of the Fugu genome and have mapped and ordered their human homologues both by PCR analysis of the Genebridge 4 panel of radiation hybrids and by fluorescence in situ hybridization. All seven human genes map to a 3 Mb region of chromosome band 9q34.1, approximately 2-4 Mb proximal to the human Surfeit genes. Although both Fugu regions are syntenic with human chromosome band 9q34, the relative order of the genes differs greatly in the two species. Indeed, some of the genes that are adjacent in the Fugu genome are separated by at least 2-4 Mb in the human genome. This suggests that intra-chromosomal rearrangements, most probably inversions, have been common during the 900 million years of divergent evolution separating Fugu and human. The utility of Fugu to facilitate human disease gene identification by comparative positional cloning is questioned in light of these results.

Evolution of U24 and U36 snoRNAs encoded within introns of vertebrate rpL7a gene homologs: unique features of mammalian U36 variants

U24 and U36 are members of the box C/D-containing group of antisense snoRNAs which possess long (9-21 nucleotide) conserved stretches of sequence complementarity to 18S and 28S rRNA and act as guides for the site-specific ribose methylation of rRNA. Both U24 and two variants of U36 are encoded within introns of the human and chicken rpL7a genes. We now report that an additional U36 variant is encoded within intron 4 of the human rpL7a gene and that murine homologs of the three human U36 variants are encoded within the same adjacent introns (4, 5, and 6) of the mouse rpL7a gene. We also show that, like that of the chicken, the Fugu rubripes rpL7a gene possesses only two U36-like sequences within introns 4 and 5. Whereas the two U36 variants in chicken and Fugu possess stretches of complementarity to both 18S and 28S rRNAs, it is noted that only one mammalian variant (U36b) possesses both. Unusually, the stretch of complementarity to 18S rRNA in the mammalian U36a variants and the stretch of complementarity to 28S rRNA in the mammalian U36c variants are not present, appearing to have diverged extensively from their consensus sequence. Additionally, the mammalian U36 variants show a unique heterogeneity in their potential to form a terminal stembox structure predicted for many other box C/D-containing antisense snoRNAs. Finally, the Saccharomyces cerevisiae small nuclear RNA, snR47, is shown to be homologous to the vertebrate U36 snoRNA.

The comparative genomic structure and sequence of the surfeit gene homologs in the puffer fish Fugu rubripes and their association with CpG-rich islands

The puffer fish Fugu rubripes (Fugu) has a compact genome approximately one-seventh the size of man, mainly owing to small intron size and the presence of few dispersed repetitive DNA elements, which greatly facilitates the study of its genes at the genomic level. It has been shown previously that, whereas the Surfeit genes are tightly clustered at a single locus in mammals and birds, the genes are found at three separate loci in the Fugu genome. Here, Fugu gene homologs of all six Surfeit genes (Surf-1 to Surf-6) have been cloned and sequenced, and their gene structure has been compared with that of their mammalian and avian homologs. The predicted protein products of each gene are well conserved between vertebrate species, and in most cases their gene structures are identical to their mammalian and avian homologs except for the Fugu Surf-6 gene, which was found to lack an intron present in the mouse gene. In addition, we have identified conserved regulatory elements at the 5' and 3' ends of the Surf-3/rpL7a gene by comparison with the mammalian and chicken Surf-3/rpL7a gene homologs, including the presence of a polypyrimidine tract at the extreme 5' end of this ribosomal protein gene. The Fugu Surfeit gene homologs appear to be associated with CpG-rich islands, like the Surfeit genes in higher vertebrates, but these Fugu CpG islands are similar to the nonclassical islands characteristic of other fish species. Our observations support the use of the Fugu genome to study vertebrate gene structure, to predict the structure of mammalian genes, and to identify vertebrate regulatory elements. [The sequence data described in this paper have been submitted to the data library under accession nos. Y15170 (Surf-2, Surf-4), Y15171 (Surf-3, Surf-1, Surf-6), and Y15172 (Surf-5.)]

Prevalence and severity of morbidity among Gujarati Asian elders: a controlled comparison

Asian elders make relatively heavy use of health services: this may be due to higher levels of morbidity, but controlled comparisons have not been carried out. A comparison of the prevalence and severity of chronic diseases and use of health services of Asian and indigenous elders was made. A sample of 59 Gujarati Asians of mean age 62.9 years and 59 indigenous subjects of mean age 63.9 years of whom 42% (25 in each group) were female drawn from a general practice was studied. Asian subjects had a higher prevalence of diagnosed diseases, with the exception of chronic obstructive airways disease, but lower risk of falls and urinary incontinence. Asian subjects had higher life satisfaction scores and lower prevalence of depressed mood. Asian women were more likely to have had contact with primary care services. Both Asian men and women had more frequent hospital admissions, but similar levels of out-patient attendance. Body mass index, blood pressure and shoulder joint range of movement were similar for both Asians and the indigenous population. Asian subjects had significantly lower peak expiratory flow rates and hand grip strength. Asian elders have a higher risk of chronic diseases, but the impact of disease (indicated by life satisfaction, mood, and common disabilities) is less than among the indigenous population. Lower peak expiratory flow rates and grip strength among Asian elders are of concern since they may lead to premature arrival at age-related thresholds of physical capacity essential for independence in activities of daily living.

Intimacy and terminal care

Four cases are summarized in which the general practitioner is involved in the terminal care of one partner of a stable marital relationship. The need to conceptualize the expectations of the patient, the family and the doctor in terminal care is stressed. An attempt is made to illustrate how the quality of the pre-existing sexual relationship, the dying person's own sexuality, and ultimately the capacity for physical expression of intimacy in the marriage profoundly influence choices in terminal care and the quality of dying.