In two parallel, separate studies, Joe El Khoury and we (a group led by Bruce Lamb and including Sungho Lee, Nick Varvel, and myself) crossed CX3CR1 KOs to APP-PS1 mice (using distinct APP-PS1 models, ours from Matthias Jucker; El Khoury’s from Dave Borchelt) and monitored amyloid deposition. Our results were entirely concordant (using slightly different methods of analysis): there was a strong, gene dosage-dependent decrease in amyloid deposition in the CX3CR1 KO mice. This decrease was not associated with evident change in APP expression, nor in processing. Further, there were fewer microglia associated with each core plaque in the CX3CR1 KOs. The hypothesis was that CX3CR1 KO microglia are more efficient at amyloid phagocytosis, therefore clearing more with fewer cells. Since then, Bruce’s lab has in vitro data to support this hypothesis. These findings (obtained independently by our lab and that of El Khoury) are neither concordant nor discordant with those from Herms et al: their assessment of insoluble Aβ appears to show a non-significant reduction in the KO mice...  Read more

In two parallel, separate studies, Joe El Khoury and we (a group led by Bruce Lamb and including Sungho Lee, Nick Varvel, and myself) crossed CX3CR1 KOs to APP-PS1 mice (using distinct APP-PS1 models, ours from Matthias Jucker; El Khoury’s from Dave Borchelt) and monitored amyloid deposition. Our results were entirely concordant (using slightly different methods of analysis): there was a strong, gene dosage-dependent decrease in amyloid deposition in the CX3CR1 KO mice. This decrease was not associated with evident change in APP expression, nor in processing. Further, there were fewer microglia associated with each core plaque in the CX3CR1 KOs. The hypothesis was that CX3CR1 KO microglia are more efficient at amyloid phagocytosis, therefore clearing more with fewer cells. Since then, Bruce’s lab has in vitro data to support this hypothesis. These findings (obtained independently by our lab and that of El Khoury) are neither concordant nor discordant with those from Herms et al: their assessment of insoluble Aβ appears to show a non-significant reduction in the KO mice (Fig 1I), although the small number of animals assessed might preclude statistical significance.

In another study from Bruce’s group (Kiran Bhaskar), CX3CR1 KO mice (crossed with mice ‘humanized’ for tau [hTau mice]) showed worse tau pathology, dependent on IL1 and p38MAP kinase and resulting in cognitive impairment.

In response, then, to the key question about microglia in general and CX3CR1 in particular, it appears that the altered microglial reaction in CX3CR1 KO mice is a double-edged sword, producing better amyloid phagocytosis and worse tau pathology.

Combining the two aspects of AD pathology (in the triple-Tg) and focusing on a novel assay for neuron loss (monitoring with two-photon imaging), Herms et al. showed benefit related to absence of CX3CR1. Their work represents (to our knowledge) the first evidence for neuron loss in the triple-Tg AD model, and one which would not be observed using stereology (1.8 percent of neurons within one month). It remains uncertain why a uniform 1.8 percent neuron loss would not, however, be recognized if it persisted for six to 12 months. The authors’ hypothesis that microglial activation precedes neuron loss and therefore is causative needs further study: injury to neurons activates microglia, and it can clearly be seen in Fig. 1 (compare day 0 in 1c and 1e) that the +/- microglia are already activated. This conclusion becomes even more solid when one considers that the -/- microglia have two copies of GFP, while the +/- microglia have one and, if imaged similarly, would appear smaller and less prominent.

In summary, Herms et al. have shown neuronal cell loss in an AD model using two-photon imaging, and have provided evidence that microglial CX3CR1 is involved, somehow, in that process. The relationship to amyloid deposition or toxicity, or to tau pathology, needs to be studied further at the mechanistic level.

View all comments by Richard Ransohoff

The recent report from the Herms group offers new insight into the enigmatic relationship between microglia and AD pathobiology. The authors have focused on whether fractalkine receptor on microglial cells participates in neuronal loss using Frank LaFerla’s 3xTg-AD model. The novelty in this paper is really twofold: demonstration of in vivo neuronal loss in real-time, and new biology showing the role of microglial fractalkine receptor (CX3CR1) in mediating this neuronal death. The authors should be commended for taking such an elegant approach, utilizing two-photon intravital imaging. It is interesting that these authors observe neuronal loss within two weeks in fractalkine receptor-sufficient 3xTg-AD mice. This report comes on the heels of another recent Nature Neuroscience paper from Mathias Jücker’s group, where those authors used a ganciclovir cd11b suicide gene approach to destroy microglia in a transgenic APP/PS1 mouse model of AD for two to four weeks. Surprisingly, those authors did not detect altered cerebral amyloidosis or amyloid-associated neuritic dystrophy in AD...  Read more

The recent report from the Herms group offers new insight into the enigmatic relationship between microglia and AD pathobiology. The authors have focused on whether fractalkine receptor on microglial cells participates in neuronal loss using Frank LaFerla’s 3xTg-AD model. The novelty in this paper is really twofold: demonstration of in vivo neuronal loss in real-time, and new biology showing the role of microglial fractalkine receptor (CX3CR1) in mediating this neuronal death. The authors should be commended for taking such an elegant approach, utilizing two-photon intravital imaging. It is interesting that these authors observe neuronal loss within two weeks in fractalkine receptor-sufficient 3xTg-AD mice. This report comes on the heels of another recent Nature Neuroscience paper from Mathias Jücker’s group, where those authors used a ganciclovir cd11b suicide gene approach to destroy microglia in a transgenic APP/PS1 mouse model of AD for two to four weeks. Surprisingly, those authors did not detect altered cerebral amyloidosis or amyloid-associated neuritic dystrophy in AD model mice that were microglia-deficient. When taking the Jücker report together with this present work, one wonders whether there are not AD mouse model-specific effects of microglia. Of course, the only way to answer such a question would be to reproduce both sets of findings in other AD animal models.

I’d like to comment on the present authors’ data showing that fractalkine receptor-sufficient microglia increase in velocity when moving toward the neurons that are marked for death prior to the actual neuronal loss. Perhaps one of the more penetrating questions is, Are microglia initiating neuronal loss or acting at a point downstream, but still on the pathway to, neuronal death? I am sure that we will continue to grapple with this and other questions that have been prompted by this interesting work.

View all comments by Terrence Town

This paper by Fuhrmann et al. shows elegant two-photon imaging of neurons and microglia in the 3x transgenic model. It is certainly a technical tour de force. The most striking result is that there is neuron loss in this model, which has not been previously described. The numbers are low, however, tens of neurons per cubic millimeter of cortex per month, which is probably much less than 1 percent of the total (YFP-positive neurons are only a subset of neurons). Is this too few to come to a definitive conclusion about the role of CX3CR1 in neurodegeneration? Perhaps, but the best way to address this would be a more low-tech approach in a model with significant neuronal loss. Nonetheless, addressing the role of microglia in the AD brain is important, and these results are certainly intriguing.

View all comments by Brian Bacskai

This article elegantly shows the strength of transcriptome analysis for the rapid discovery of a new drug. In this study, the authors identified the therapeutic potential of modulating CD40L in ALS using an animal model.

Through transcriptome analysis, this group identified the upregulation of CD40L-related pathway in three tissues that are all relevant to motor neuron degeneration: muscle, spinal cord, and sciatic nerve. This signaling pathway related to CD40L activation became more prominent as the disease progressed; this finding justifiably led the investigators to test its implication to therapy. The therapeutic potential was tested in mSOD1 mice, and anti-CD40L was found to be effective with respect to both disease onset and progression. The authors compared the results to those observed in inflammatory diseases and, based on Mac-1 expression and T cell activation, suggested that the therapy acts in the animal model of mSOD1 as anti-inflammatory treatment; such a conclusion should be taken with caution, and more so when it comes to clinical translation.

CD40L was...  Read more

This article elegantly shows the strength of transcriptome analysis for the rapid discovery of a new drug. In this study, the authors identified the therapeutic potential of modulating CD40L in ALS using an animal model.

Through transcriptome analysis, this group identified the upregulation of CD40L-related pathway in three tissues that are all relevant to motor neuron degeneration: muscle, spinal cord, and sciatic nerve. This signaling pathway related to CD40L activation became more prominent as the disease progressed; this finding justifiably led the investigators to test its implication to therapy. The therapeutic potential was tested in mSOD1 mice, and anti-CD40L was found to be effective with respect to both disease onset and progression. The authors compared the results to those observed in inflammatory diseases and, based on Mac-1 expression and T cell activation, suggested that the therapy acts in the animal model of mSOD1 as anti-inflammatory treatment; such a conclusion should be taken with caution, and more so when it comes to clinical translation.

CD40L was originally described on T lymphocytes; its expression has since been detected on a wide variety of cells, including platelets, mast cells, macrophages, basophils, NK cells, B lymphocytes, as well as non-hematopoietic macrophages. Primarily, in its bound form, CD40L serves as a self-controlling, co-stimulatory molecule; thus, it acts as a mechanism of prevention of unnecessary lymphocyte activation and works at multiple levels. CD40L allows full immune cell activation, prevents anergy or apoptosis, induces differentiation to effector or memory status, sustains cell proliferation, and allows cell-cell crosstalk and cooperation. Therefore, neutralizing CD40L might lead to different effects at different stages of the disease. Moreover, its mechanism of action may be critically affected by the dosing, resulting in an effect suggestive of Dr. Jekyll and Mr. Hyde. This situation is very much reminiscent of minocycline in ALS, which showed similar efficacy in animal models of mSOD1 and failed in human trials. The case of minocycline might represent a general phenomenon with respect to the use of anti-inflammatory therapies in ALS. Such therapies are beneficial in inflammatory diseases such as multiple sclerosis, and, in their relevant animal model, experimental autoimmune encephalomyelitis (EAE). As opposed to ALS, these diseases are inflammatory in their etiology, whereas ALS is characterized by local inflammation, but is not considered an inflammatory disease. Moreover, elevated CD40L in mSOD1 mice might represent beneficial attempts to cope with the disease that are not sufficiently controlled. Therefore, blockage of CD40L may have a beneficial phase/effect/outcome at certain disease stages, but not in a blanket way. Thus, targeting a co-stimulatory molecule as a therapeutic approach may interrupt essential beneficial immune responses in addition to targeting the disease process.

View all comments by Michal Schwartz

Against the backdrop of sometimes disappointing results from genomewide association studies of the transcriptome (GWAS-T), the work by Lincecum and colleagues represents a triumph for this approach. The authors applied transcriptome analysis to the high-copy SOD1 transgenic mouse model of ALS. Importantly, they thoroughly investigated central and peripheral tissues from SOD1 mice at timepoints prior to, during, and after disease onset. Their GWAS-T results pointed to co-stimulatory immune and inflammatory molecules as being centrally associated with ALS-like pathology in this system, and they utilized a sophisticated statistical algorithm to arrive at the CD40-CD40L interaction as a candidate treatment target. They then treated SOD1 mice with a neutralizing CD40L antibody and found benefit by virtually any index of ALS-like disease: the biologic therapy improved body weight maintenance and survival, reduced inflammatory lesions, decreased motor neuron loss, and attenuated expression of immune co-stimulatory genes.

I read this work with enthusiasm and excitement, because over...  Read more

Against the backdrop of sometimes disappointing results from genomewide association studies of the transcriptome (GWAS-T), the work by Lincecum and colleagues represents a triumph for this approach. The authors applied transcriptome analysis to the high-copy SOD1 transgenic mouse model of ALS. Importantly, they thoroughly investigated central and peripheral tissues from SOD1 mice at timepoints prior to, during, and after disease onset. Their GWAS-T results pointed to co-stimulatory immune and inflammatory molecules as being centrally associated with ALS-like pathology in this system, and they utilized a sophisticated statistical algorithm to arrive at the CD40-CD40L interaction as a candidate treatment target. They then treated SOD1 mice with a neutralizing CD40L antibody and found benefit by virtually any index of ALS-like disease: the biologic therapy improved body weight maintenance and survival, reduced inflammatory lesions, decreased motor neuron loss, and attenuated expression of immune co-stimulatory genes.

I read this work with enthusiasm and excitement, because over a decade ago we demonstrated that pharmacologic or genetic blockade of CD40-CD40L interaction mitigated AD-like pathology in transgenic mouse models of the disease. This included reduction of: abnormal tau proteins, cerebral amyloidosis, brain inflammation including gliosis, and behavioral impairment (Tan et al., 1999; Tan et al., 2002). At that time, many in the field of AD research were unwilling to accept that the immune system played any role in the pathogenesis of AD, let alone that immune molecules could be targeted for AD treatment. It is terribly exciting that these authors have extended the concepts that we were exploring vis-à-vis CD40-CD40L in AD to another key neurodegenerative disease: ALS. I hope that the authors are able to successfully translate their findings to the clinical syndrome.

References:
Tan J, Town T, Paris D, Mori T, Suo Z, Crawford F, Mattson MP, Flavell RA, Mullan M. Microglial activation resulting from CD40-CD40L interaction after beta-amyloid stimulation. Science. 1999 Dec 17;286(5448):2352-5. Abstract

Tan J, Town T, Crawford F, Mori T, DelleDonne A, Crescentini R, Obregon D, Flavell RA, Mullan MJ. Role of CD40 ligand in amyloidosis in transgenic Alzheimer's mice. Nat Neurosci. 2002 Dec;5(12):1288-93. Abstract

View all comments by Terrence Town