Selective neuronal vulnerability in neurodegenerative diseases: from stressor thresholds to degeneration

Life-or-death decisions upon axonal damage

In this issue of Neuron, Hu et al. (2012) report that upon axonal damage, CHOP and XBP1 unfolded protein response pathways are not recruited equally and have opposite effects on neuronal survival. XBP1 pathway boosting may represent a valuable neuroprotective strategy.

Reduced axonal transport and increased excitotoxic retinal ganglion cell degeneration in mice transgenic for human mutant P301S tau

The effects of tau hyperphosphorylation and aggregation on axonal transport were investigated in the optic nerve of mice transgenic for human mutant P301S tau. Transport was examined using cholera toxin B tracing. Retrograde transport was reduced in transgenic mice at 3 and 5 months of age, when compared to C57/Bl6 control mice. Anterograde axonal transport was also reduced in 3-month-old transgenic mice. Mild excitotoxic injury of retinal ganglion cells resulted in greater nerve cell loss in retinas from 3- and 5-month old P301S transgenic mice, when compared to controls. In conjunction with the detection of abnormal tau in the optic nerve in human and experimental glaucoma, the present findings suggest that tau hyperphosphorylation and aggregation may constitute targets for neuroprotective therapies in glaucoma as well as tauopathies.

Shared resistance to aging and ALS in neuromuscular junctions of specific muscles

Normal aging and neurodegenerative diseases both lead to structural and functional alterations in synapses. Comparison of synapses that are generally similar but respond differently to insults could provide the basis for discovering mechanisms that underlie susceptibility or resistance to damage. Here, we analyzed skeletal neuromuscular junctions (NMJs) in 16 mouse muscles to seek such differences. We find that muscles respond in one of three ways to aging. In some, including most limb and trunk muscles, age-related alterations to NMJs are progressive and extensive during the second postnatal year. NMJs in other muscles, such as extraocular muscles, are strikingly resistant to change. A third set of muscles, including several muscles of facial expression and the external anal sphinter, succumb to aging but not until the third postnatal year. We asked whether susceptible and resistant muscles differed in rostrocaudal or proximodistal position, source of innervation, motor unit size, or fiber type composition. Of these factors, muscle innervation by brainstem motor neurons correlated best with resistance to age-related decline. Finally, we compared synaptic alterations in normally aging muscles to those in a mouse model of amyotrophic lateral sclerosis (ALS). Patterns of resistance and susceptibility were strikingly correlated in the two conditions. Moreover, damage to NMJs in aged muscles correlated with altered expression and distribution of CRMP4a and TDP-43, which are both altered in motor neurons affected by ALS. Together, these results reveal novel structural, regional and molecular parallels between aging and ALS.

A network diffusion model of disease progression in dementia

Patterns of dementia are known to fall into dissociated but dispersed brain networks, suggesting that the disease is transmitted along neuronal pathways rather than by proximity. This view is supported by neuropathological evidence for "prion-like" transsynaptic transmission of disease agents like misfolded tau and beta amyloid. We mathematically model this transmission by a diffusive mechanism mediated by the brain's connectivity network obtained from tractography of 14 healthy-brain MRIs. Subsequent graph theoretic analysis provides a fully quantitative, testable, predictive model of dementia. Specifically, we predict spatially distinct "persistent modes," which, we found, recapitulate known patterns of dementia and match recent reports of selectively vulnerable dissociated brain networks. Model predictions also closely match T1-weighted MRI volumetrics of 18 Alzheimer's and 18 frontotemporal dementia subjects. Prevalence rates predicted by the model strongly agree with published data. This work has many important implications, including dimensionality reduction, differential diagnosis, and especially prediction of future atrophy using baseline MRI morphometrics.

Predicting regional neurodegeneration from the healthy brain functional connectome

Neurodegenerative diseases target large-scale neural networks. Four competing mechanistic hypotheses have been proposed to explain network-based disease patterning: nodal stress, transneuronal spread, trophic failure, and shared vulnerability. Here, we used task-free fMRI to derive the healthy intrinsic connectivity patterns seeded by brain regions vulnerable to any of five distinct neurodegenerative diseases. These data enabled us to investigate how intrinsic connectivity in health predicts region-by-region vulnerability to disease. For each illness, specific regions emerged as critical network "epicenters" whose normal connectivity profiles most resembled the disease-associated atrophy pattern. Graph theoretical analyses in healthy subjects revealed that regions with higher total connectional flow and, more consistently, shorter functional paths to the epicenters, showed greater disease-related vulnerability. These findings best fit a transneuronal spread model of network-based vulnerability. Molecular pathological approaches may help clarify what makes each epicenter vulnerable to its targeting disease and how toxic protein species travel between networked brain structures.

Alternative oxidase rescues mitochondria-mediated dopaminergic cell loss in Drosophila

Mitochondrial dysfunction is commonly observed in degenerative disorders, including Alzheimer's and Parkinson's disease that are characterized by the progressive and selective loss of neuronal subpopulations. It is currently unclear, however, whether mitochondrial dysfunction is primary or secondary to other pathogenic processes that eventually lead to age-related neurodegeneration. Here we establish an in vivo Drosophila model of mitochondrial dysfunction by downregulating the catalytic subunit of mitochondrial DNA (mtDNA) polymerase in cholinergic, serotonergic and dopaminergic neurons. The resulting flies are characterized by lowered respiratory chain activity, premature aging, age-related motor deficits as well as adult onset, progressive and cell-type-specific, dopaminergic neurodegeneration. Using this model, we find that associated lethality can be partially rescued by targeting PINK1/parkin signaling or Drp1, both of which have been implicated in mitochondrial dynamics and Parkinson's disease. Bypassing mitochondrial complex III/IV deficiencies with Alternative oxidase (AOX), however, fully restores ATP levels and prevents dopaminergic neurodegeneration. In contrast, ATP levels and neurodegeneration are not rescued when mitochondrial complex I deficiencies are bypassed with NADH-Q oxidoreductase. Our results demonstrate that mtDNA-mediated mitochondrial dysfunction can cause age-related and cell-type-specific neurodegeneration which AOX is able to alleviate and indicate that AOX or its surrogates may prove useful as a therapeutic tool for limiting respiratory chain deficiencies caused by mtDNA decline in healthy aging and neurodegenerative disease.

Mutant TDP-43 and FUS cause age-dependent paralysis and neurodegeneration in C. elegans

Mutations in the DNA/RNA binding proteins TDP-43 and FUS are associated with Amyotrophic Lateral Sclerosis and Frontotemporal Lobar Degeneration. Intracellular accumulations of wild type TDP-43 and FUS are observed in a growing number of late-onset diseases suggesting that TDP-43 and FUS proteinopathies may contribute to multiple neurodegenerative diseases. To better understand the mechanisms of TDP-43 and FUS toxicity we have created transgenic Caenorhabditis elegans strains that express full-length, untagged human TDP-43 and FUS in the worm's GABAergic motor neurons. Transgenic worms expressing mutant TDP-43 and FUS display adult-onset, age-dependent loss of motility, progressive paralysis and neuronal degeneration that is distinct from wild type alleles. Additionally, mutant TDP-43 and FUS proteins are highly insoluble while wild type proteins remain soluble suggesting that protein misfolding may contribute to toxicity. Populations of mutant TDP-43 and FUS transgenics grown on solid media become paralyzed over 7 to 12 days. We have developed a liquid culture assay where the paralysis phenotype evolves over several hours. We introduce C. elegans transgenics for mutant TDP-43 and FUS motor neuron toxicity that may be used for rapid genetic and pharmacological suppressor screening.

Links between electrophysiological and molecular pathology of amyotrophic lateral sclerosis

Multiple deficits have been described in amyotrophic lateral sclerosis (ALS), from the first changes in normal functioning of the motoneurons and glia to the eventual loss of spinal and cortical motoneurons. In this review, current results, including changes in size, and electrical properties of motoneurons, glutamate excitotoxicity, calcium buffering, deficits in mitochondrial and cellular transport, impediments to proteostasis which lead to stress of the endoplasmic reticulum (ER), and glial contributions to motoneuronal vulnerability are recapitulated. Results are mainly drawn from the mutant SOD1 mouse model of ALS, and emphasis is placed on early changes that precede the onset of symptoms and the interplay between molecular and electrical processes.

New strategies in neuroprotection and neurorepair

There are currently few clinical strategies in place, which provide effective neuroprotection and repair, despite an intense international effort over the past decades. One possible explanation for this is that a deeper understanding is required of how endogenous mechanisms act to confer neuroprotection. This mini-review reports the proceedings of a recent workshop "Neuroprotection and Neurorepair: New Strategies" (Iguazu Falls, Misiones, Argentina, April 11-13, 2011, Satellite Symposium of the V Neurotoxicity Society Meeting, 2011) in which four areas of active research were identified to have the potential to generate new insights into this field. Topics discussed were (i) metallothionein and other multipotent neuroprotective molecules; (ii) oxidative stress and their signal mediated pathways in neuroregeneration; (iii) neurotoxins in glial cells, and (iv) drugs of abuse with neuroprotective effects.