Cytoskeletal changes during development and aging in the cortex of neurofilament light protein knockout mice

Ozone modified hypothalamic signaling enhancing thermogenesis in the TDP-43A315T transgenic model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS), a devastating progressive neurodegenerative disease, has no effective treatment. Recent evidence supports a strong metabolic component in ALS pathogenesis. Indeed, metabolic abnormalities in ALS correlate to disease susceptibility and progression, raising additional therapeutic targets against ALS. Ozone (O₃), a natural bioactive molecule, has been shown to elicit beneficial effects to reduce metabolic disturbances and improved motor behavior in TDP-43^A315T mice. However, it is fundamental to determine the mechanism through which O₃ acts in ALS. To characterize the association between O₃ exposure and disease-associated weight loss in ALS, we assessed the mRNA and protein expression profile of molecular pathways with a main role in the regulation of the metabolic homeostasis on the hypothalamus and the brown adipose tissue (BAT) at the disease end-stage, in TDP-43^A315T mice compared to age-matched WT littermates. In addition, the impact of O₃ exposure on the faecal bacterial community diversity, by Illumina sequencing, and on the neuromuscular junctions (NMJs), by confocal imaging, were analysed. Our findings suggest the effectiveness of O₃ exposure to induce metabolic effects in the hypothalamus and BAT of TDP-43^A315T mice and could be a new complementary non-pharmacological approach for ALS therapy.

Neurofilament Light Regulates Axon Caliber, Synaptic Activity, and Organelle Trafficking in Cultured Human Motor Neurons

Neurofilament light (NFL) is one of the proteins forming multimeric neuron-specific intermediate filaments, neurofilaments, which fill the axonal cytoplasm, establish caliber growth, and provide structural support. Dominant missense mutations and recessive nonsense mutations in the neurofilament light gene (NEFL) are among the causes of Charcot-Marie-Tooth (CMT) neuropathy, which affects the peripheral nerves with the longest axons. We previously demonstrated that a neuropathy-causing homozygous nonsense mutation in NEFL led to the absence of NFL in patient-specific neurons. To understand the disease-causing mechanisms, we investigate here the functional effects of NFL loss in human motor neurons differentiated from induced pluripotent stem cells (iPSC). We used genome editing to generate NEFL knockouts and compared them to patient-specific nonsense mutants and isogenic controls. iPSC lacking NFL differentiated efficiently into motor neurons with normal axon growth and regrowth after mechanical axotomy and contained neurofilaments. Electrophysiological analysis revealed that motor neurons without NFL fired spontaneous and evoked action potentials with similar characteristics as controls. However, we found that, in the absence of NFL, human motor neurons 1) had reduced axonal caliber, 2) the amplitude of miniature excitatory postsynaptic currents (mEPSC) was decreased, 3) neurofilament heavy (NFH) levels were reduced and no compensatory increases in other filament subunits were observed, and 4) the movement of mitochondria and to a lesser extent lysosomes was increased. Our findings elaborate the functional roles of NFL in human motor neurons. NFL is not only a structural protein forming neurofilaments and filling the axonal cytoplasm, but our study supports the role of NFL in the regulation of synaptic transmission and organelle trafficking. To rescue the NFL deficiency in the patient-specific nonsense mutant motor neurons, we used three drugs, amlexanox, ataluren (PTC-124), and gentamicin to induce translational read-through or inhibit nonsense-mediated decay. However, the drugs failed to increase the amount of NFL protein to detectable levels and were toxic to iPSC-derived motor neurons.

TDP-43 mislocalization drives neurofilament changes in a novel model of TDP-43 proteinopathy

Mislocalization of the TAR DNA-binding protein 43 (TDP-43) from the nucleus to the cytoplasm is a common feature of neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). The downstream in vivo cellular effects of this mislocalization are not well understood. To investigate the impact of mislocalized TDP-43 on neuronal cell bodies, axons and axonal terminals, we utilized the mouse visual system to create a new model of TDP-43 proteinopathy. Mouse (C57BL/6J) retinal ganglion cells (RGCs) were transduced with GFP-tagged human wildtype TDP-43 (hTDP-WT-GFP) and human TDP-43 with a mutation in the nuclear localization sequence (hTDP-ΔNLS-GFP), to cause TDP-43 mislocalization, with ∼60% transduction efficiency achieved. Expression of both hTDP-WT-GFP and hTDP-ΔNLS-GFP resulted in changes to neurofilament expression, with cytoplasmic TDP-43 being associated with significantly (p<0.05) increased neurofilament heavy expression in the cell soma, and both forms of altered TDP-43 leading to significantly (p<0.05) decreased numbers of neurofilament-positive axons within the optic nerve. Alterations to neurofilament proteins were associated with significantly (p<0.05) increased microglial density in the optic nerve and retina. Furthermore expression of hTDP-WT-GFP was associated with a significant (p<0.05) increase in pre-synaptic input into RGCs in the retina. The current study has developed a new model allowing detailed examination of alterations to TDP-43 and will contribute to the knowledge of TDP-43-mediated neuronal alterations and degeneration.

Iron is increased in the brains of ageing mice lacking the neurofilament light gene

There has been strong interest in the role of metals in neurodegeneration, and how ageing may predispose the brain to related diseases such as Alzheimer’s disease. Recent work has also highlighted a potential interaction between different metal species and various components of the cytoskeletal network in the brain, which themselves have a reported role in age-related degenerative disease and other neurological disorders. Neurofilaments are one such class of intermediate filament protein that have a demonstrated capacity to bind and utilise cation species. In this study, we investigated the consequences of altering the neurofilamentous network on metal ion homeostasis by examining neurofilament light (NFL) gene knockout mice, relative to wildtype control animals, at adulthood (5 months of age) and advanced age (22 months). Inductively coupled plasma mass spectroscopy demonstrated that the concentrations of iron (Fe), copper (Cu) and zinc (Zn) varied across brain regions and peripheral nerve samples. Zn and Fe showed statistically significant interactions between genotype and age, as well as between genotype and region, and Cu demonstrated a genotype and region interaction. The most substantial difference between genotypes was found in Fe in the older animals, where, across many regions examined, there was elevated Fe in the NFL knockout mice. This data indicates a potential relationship between the neurofilamentous cytoskeleton and the processing and/or storage of Fe through ageing.

Neurofilament light interaction with GluN1 modulates neurotransmission and schizophrenia-associated behaviors

Neurofilament (NFL) proteins have recently been found to play unique roles in synapses. NFL is known to interact with the GluN1 subunit of N-methyl-D-aspartic acid (NMDAR) and be reduced in schizophrenia though functional consequences are unknown. Here we investigated whether the interaction of NFL with GluN1 modulates synaptic transmission and schizophrenia-associated behaviors. The interaction of NFL with GluN1 was assessed by means of molecular, pharmacological, electrophysiological, magnetic resonance spectroscopy (MRS), and schizophrenia-associated behavior analyses. NFL deficits cause an NMDAR hypofunction phenotype including abnormal hippocampal function, as seen in schizophrenia. NFL-/- deletion in mice reduces dendritic spines and GluN1 protein levels, elevates ubiquitin-dependent turnover of GluN1 and hippocampal glutamate measured by MRS, and depresses hippocampal long-term potentiation. NMDAR-related behaviors are also impaired, including pup retrieval, spatial and social memory, prepulse inhibition, night-time activity, and response to NMDAR antagonist, whereas motor deficits are minimal. Importantly, partially lowering NFL in NFL+/- mice to levels seen regionally in schizophrenia, induced similar but milder NMDAR-related synaptic and behavioral deficits. Our findings support an emerging view that central nervous system neurofilament subunits including NFL in the present report, serve distinctive, critical roles in synapses relevant to neuropsychiatric diseases.

Disruption of leptin signalling in a mouse model of Alzheimer's disease

Disruption of leptin signalling has been implicated as playing a role in the development of Alzheimer's disease (AD). Leptin has previously been shown to be affected by amyloid-beta (Aβ)-related signalling; however, pathways that link leptin to the disease pathogenesis have not been determined. To characterize the association between increasing age-dependent Aβ levels with leptin signalling and the vulnerable brain regions in AD, we assessed the mRNA and protein expression profile of leptin and leptin receptor (Ob-Rb) at 9 and 18-month-age in APP/PS1 mice. Immunohistochemical labelling demonstrated that leptin and Ob-Rb proteins were localised to neocortical and hippocampal neurons in APP/PS1 and wildtype (WT) mice. Neuronal leptin and Ob-Rb immunolabelling was more prominent in the neocortex of both groups at 9 month of age, while, at 18 months, labelling was reduced in the hippocampus of APP/PS1 mice relative to WT. Immunoblotting analysis demonstrated decreased hippocampal leptin levels, concomitantly with an increased Ob-Rb levels, in APP/PS1 mice compared with WT controls at 18 month of age. While no leptin mRNA was found in either of the groups analysed, Ob-Rb mRNA was significantly decreased in the hippocampus of APP/PS1 mice at both ages analysed. In addition, a significant decreased protein kinase B (Akt) activity concomitantly with an upregulation of suppressor of cytokine signaling-3 (SOCS3) and protein-tyrosine phosphatase 1B (PTP1B) transcripts was present. Thus, these results collectively indicate alterations of leptin signalling in the hippocampus of APP/PS1 mice, providing novel insights about the pathways that could link aberrant leptin signaling to the pathological changes of AD.

Combination treatment with leptin and pioglitazone in a mouse model of Alzheimer's disease

INTRODUCTION

Combination therapy approaches may be necessary to address the many facets of pathologic change in the brain in Alzheimer's disease (AD). The drugs leptin and pioglitazone have previously been shown individually to have neuroprotective and anti-inflammatory actions, respectively, in animal models.

METHODS

We studied the impact of combined leptin and pioglitazone treatment in 6-month-old APP/PS1 (APPswe/PSEN1dE9) transgenic AD mouse model.

RESULTS

We report that an acute 2-week treatment with combined leptin and pioglitazone resulted in a reduction of spatial memory deficits (Y maze) and brain β-amyloid levels (soluble β-amyloid and amyloid plaque burden) relative to vehicle-treated animals. Combination treatment was also associated with amelioration in plaque-associated neuritic pathology and synapse loss, and also a significantly reduced neocortical glial response.

DISCUSSION

Combination therapy with leptin and pioglitazone ameliorates pathologic changes in APP/PS1 mice and may represent a potential treatment approach for AD.

C9ORF72 expression and cellular localization over mouse development

INTRODUCTION

A majority of familial frontotemporal lobar dementia and amyotrophic lateral sclerosis cases are associated with a large repeat expansion in a non-coding region of the C9ORF72 gene. Currently, little is known about the normal function and the expression pattern of the C9ORF72 protein. The aims of this study were to characterize the expression pattern and cellular localization of the three reported mouse isoforms of C9orf72, over a developmental time-course in primary cultured cortical neurons and brain tissue from C57BL/6 mice.

RESULTS

We demonstrated that the different isoforms of C9ORF72 at the mRNA and protein level undergo alterations in expression during development and into adulthood. Cellular fractionation and immunofluorescence demonstrated that levels of nuclear and cytoplasmic expression of isoforms changed significantly over the time course. Additionally, immunofluorescence studies showed C9ORF72 labeling as puncta throughout neurons, extending beyond the microtubule cytoskeleton into actin-rich structures such as filopodia and growth cones. Finally, synaptosome preparations demonstrated the presence of C9ORF72 isoform 1 in synaptic-rich fractions from adult mouse brain.

CONCLUSION

In summary, the presence of C9ORF72 as puncta and within synaptic-rich fractions may indicate involvement at the synapse and differential expression of isoforms in nuclei and cytoplasm may suggest distinct roles for the isoforms. Determining the physiological role of C9ORF72 protein may help to determine the role it plays in disease.

Identification of Site-Specific Stroke Biomarker Candidates by Laser Capture Microdissection and Labeled Reference Peptide

The search to date for accurate protein biomarkers in acute ischemic stroke has taken into consideration the stage and/or the size of infarction, but has not accounted for the site of stroke. In the present study, multiple reaction monitoring using labeled reference peptide (LRP) following laser capture microdissection (LCM) is used to identify site-specific protein biomarker candidates. In middle cerebral artery occlusion (MCAO) rat models, both intact and infarcted brain tissue was collected by LCM, followed by on-film digestion and semi-quantification using triple-quadrupole mass spectrometry. Thirty-four unique peptides were detected for the verification of 12 proteins in both tissue homogenates and LCM-captured samples. Six insoluble proteins, including neurofilament light polypeptide (NEFL), alpha-internexin (INA), microtubule-associated protein 2 (MAP2), myelin basic protein (MBP), myelin proteolipid protein (PLP) and 2′,3′-cyclic-nucleotide 3′-phosphodiesterase (CNP), were found to be site-specific. Soluble proteins, such as neuron-specific enolase (NSE) and ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1), and some insoluble proteins, including neurofilament heavy polypeptide (NEFH), glial fibrillary acidic protein (GFAP), microtubule-associated protein tau (MAPT) and tubulin β-3 chain (TUBB3), were found to be evenly distributed in the brain. Therefore, we conclude that some insoluble protein biomarkers for stroke are site-specific, and would make excellent candidates for the design and analysis of relevant clinical studies in the future.

The influence of aging on the number of neurons and levels of non-phosporylated neurofilament proteins in the central auditory system of rats

In the present study, an unbiased stereological method was used to determine the number of all neurons in Nissl stained sections of the inferior colliculus (IC), medial geniculate body (MGB), and auditory cortex (AC) in rats (strains Long Evans and Fischer 344) and their changes with aging. In addition, using the optical fractionator and western blot technique, we also evaluated the number of SMI-32-immunoreactive (-ir) neurons and levels of non-phosphorylated neurofilament proteins in the IC, MGB, AC, and visual cortex of young and old rats of the two strains. The SMI-32 positive neuronal population comprises about 10% of all neurons in the rat IC, MGB, and AC and represents a prevalent population of large neurons with highly myelinated and projecting processes. In both Long Evans and Fischer 344 rats, the total number of neurons in the IC was roughly similar to that in the AC. With aging, we found a rather mild and statistically non-significant decline in the total number of neurons in all three analyzed auditory regions in both rat strains. In contrast to this, the absolute number of SMI-32-ir neurons in both Long Evans and Fischer 344 rats significantly decreased with aging in all the examined structures. The western blot technique also revealed a significant age-related decline in the levels of non-phosphorylated neurofilaments in the auditory brain structures, 30–35%. Our results demonstrate that presbycusis in rats is not likely to be primarily associated with changes in the total number of neurons. On the other hand, the pronounced age-related decline in the number of neurons containing non-phosphorylated neurofilaments as well as their protein levels in the central auditory system may contribute to age-related deterioration of hearing function.

Neurofilament dynamics and involvement in neurological disorders

Neurons are extremely polarised cells in which the cytoskeleton, composed of microtubules, microfilaments and neurofilaments, plays a crucial role in maintaining structure and function. Neurofilaments, the 10-nm intermediate filaments of neurons, provide structure and mechanoresistance but also provide a scaffolding for the organization of the nucleus and organelles such as mitochondria and ER. Disruption of neurofilament organization and expression or metabolism of neurofilament proteins is characteristic of certain neurological syndromes including Amyotrophic Lateral Sclerosis, Charcot-Marie-Tooth sensorimotor neuropathies and Giant Axonal Neuropathy. Microfluorometric live imaging techniques have been instrumental in revealing the dynamics of neurofilament assembly and transport and their functions in organizing intracellular organelle networks. The insolubility of neurofilament proteins has limited identifying interactors by conventional biochemical techniques but yeast two-hybrid experiments have revealed new roles for oligomeric, nonfilamentous structures including vesicular trafficking. Although having long half-lives, new evidence points to degradation of subunits by the ubiquitin-proteasome system as a mechanism of normal turnover. Although certain E3-ligases ubiquitinating neurofilament proteins have been identified, the overall process of neurofilament degradation is not well understood. We review these mechanisms of neurofilament homeostasis and abnormalities in motor neuron and peripheral nerve disorders. Much remains to discover about the disruption of processes that leads to their pathological aggregation and accumulation and the relevance to pathogenesis. Understanding these mechanisms is crucial for identifying novel therapeutic strategies.