Autofluorescence: A Potential Biomarker for White Matter Injury in MS?

Autofluorescence may serve as a biomarker of prior and ongoing white matter injury in multiple sclerosis (MS), which may be “highly relevant to the progressive phase of MS” and other age-related diseases of the brain, according to new, proof-of-concept findings published in Multiple Sclerosis Journal.

The authors of the study arrived at their conclusion after using broadband, spectral confocal microscopy to determine whether the autofluorescent signatures in autopsied experimental and clinical tissue was biologically meaningful. Standard histopathology was used to determine the severity and chronicity of tissue injury.

The results of the study indicated that despite robust remyelination, autofluorescent deposits had accumulated in regions of prior pathology in the months following demyelination induced by a toxin called cuprizone. The authors of the study found that autofluorescent deposits were visibly confined to white matter, proportional to the severity of lesions, and exhibited differential fluorescence as time progressed. They also observe similar features in autopsied MS tissue.

Neurology Learning Network discussed these findings and their implications further with study author Andrew Caprariello, PhD, from the Department of Clinical Neurosciences and Hotchkiss Brain Institute at the University of Calgary’s Cumming School of Medicine in Canada.

Neurology Learning Network: In your study, you and your colleagues noted that the balance of tissue injury and repair determines outcomes of chronic neurological disorders like MS. Could you elaborate on this?

Dr Caprariello: This study was really borne out of a fascination with the pathogenesis of MS, particularly its progressive stages. We aim to understand why the disease tends to progress, despite a potent decrease in inflammatory attacks due to advances in disease-modifying therapies, and whether there are tools that could illuminate the biological underpinnings of the early stages of neurodegeneration. For a long time, we did not appreciate that progressive MS and the neurodegeneration that underpins it likely begin much earlier than disease diagnosis. This prompted us to investigate whether there are ways that we can more closely monitor when and what subtle changes are happening in people living with MS so that we could potentially begin to predict how early intervention might be needed and how outcomes of progression could be altered.

NLN: The results of your study indicated that autofluorescence spectroscopy illuminates prior and ongoing white matter injury. What are the implications of this finding?

Dr Caprariello: This study is essentially the scientific equivalent of turning lemons into lemonade. Autofluorescence occurs naturally in proteins, oxidized lipids, and other biological molecules that have their own fluorescence. Whether the body accrues injury or ages in a healthy manner with no known disease, this autofluorescence accumulates. As such, it has historically been viewed as an impediment to the study of tissues and how disease affects these tissues. However, my colleagues and I noticed in a mouse model of progressive MS that there was a progressive and localized over-accumulation of autofluorescence, and we wondered whether it could be used to inform the underlying health of tissues. Subsequently, we deployed a very sensitive spectral confocal microscope, which allowed us to view the entire visible wavelength of light in these tissues and perform much more careful analysis of the tissue components as they transition from a healthy state to a diseased state. A number of interesting features emerged, particularly when compared with standard histopathological techniques.

NLN: What other future research is needed in this area?

Dr Caprariello: As any scientist might say, the sample size could always be bigger. However, access to MS tissue can be challenging, so we were admittedly underpowered in this study. Nevertheless, the results were quite clear that, as lesions age and become increasingly chronic, there are different autofluorescent signatures that can be tracked and used to determine the severity as well as the chronicity of affected tissue. In future research, chronic lesions will need to be compared with acute, relatively new lesions. Our study showed that acute and a mix of acute and chronic lesions were associated with a predictable progression of autofluorescence, but future research will need to address the appearance of chronic lesions in autofluorescent microscopy, since these are the hallmark of progressive MS.

We also observed in our study was that there was an accumulation of autofluorescence not only in white matter, but also in gray matter. Interestingly, in white matter, autofluorescence accumulated primarily in microglial cells/macrophages, whereas in gray matter, autofluorescence accumulated in neurons. There are interesting theories about how the white and gray matter evolve with MS and how they may contribute to or underpin the progressive phase of MS. Our sensitive technique illuminates divergent mechanisms of cellular pathology in various sub-compartments of affected brains. This will be an interesting area of future research.

Another area of future research would involve examining the gray matter, where we observed the accumulation of autofluorescence in neurons, because this might be a more direct mechanism for the degeneration of neurons. In addition, since we know that autofluorescence accumulates with healthy aging, another area for future research would involve investigating how the autofluorescence spectral signatures in any brain compartment vary with aging and age-related disorders.

NLN: What key takeaways about this topic do you hope to leave with neurologists and related clinicians who treat multiple sclerosis?

Dr Caprariello: I have several key takeaways. First, autofluorescence is far from an impediment to histopathological analysis. In fact, it is a very sensitive biomarker of cumulative injury to the brain and could really impact how we consider and study disease, as well as how we may be able to intervene much earlier to alter the outcomes of these disabling diseases. Second, spectral microscopy adds a label-free element to an evolving classification system of MS lesions. In 2017, there was a seminal classification study that looked at the relative states of macrophages and oligodendrocyte myelin in lesions and used those 2 markers as a proxy for determining the relative age and chronicity in staging these lesions. I see our technique as another piece of the puzzle in the elusive quest to discover the causative mechanisms of tissue destruction in MS. Lastly, autofluorescence has the potential for broad applicability beyond MS to healthy aging, as well as diseases that manifest later in life, such as Alzheimer disease and Parkinson disease.

—Christina Vogt

Reference:
Morgan ML, Kaushik DL, Stys PK, Caprariello AV. Autofluorescence spectroscopy as a proxy for chronic white matter pathology. Mult Scler J. Published online August 11, 2020. doi:10.1177/1352458520948221