Redefining the Aging Process

This post highlights the approach and findings of a new research article published in Disease Models and Mechanisms (DMM). This feature was written by J. Brucker Nourse Jr. as part of a graduate level seminar at The University of Alabama (taught by DMM Editorial Board member, Prof. Guy Caldwell) on current topics related to use of animal and cellular model systems in studies of human disease.

J. Brucker Nourse Jr.
Department of Biological Sciences, The University of Alabama, Tuscaloosa, AL, USA

Commentary on:

Spurrier1,2, Shukla1, McLinden3, Johnson1, Giniger1.“Altered expression of the Cdk5 activator-like protein, Cdk5α, causes neurodegeneration in part by accelerating the rate of aging.” Disease Models & Mechanisms. no.11 (2018): 1-14.

1. National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland

2. The John Hopkins University/National Institutes of Health Graduate Partnership Program, National Institutes of Health, Bethesda, Maryland

3. National Institute on Aging, National Institutes of Health, Bethesda, Maryland

 
Developments in modern medicine have allowed humans to reach life expectancies that surpass prior generations (Mills et al., 2016). The tradeoff for increased longevity in most populations has led to greater occurrences of neurodegenerative diseases (NDD), such as Parkinson’s disease (PD), Alzheimer’s disease (AD), and Amyotrophic Lateral Sclerosis (ALS). One commonality among NDD is that age is the greatest risk factor (Savica et al., 2013; Nho et al., 2016; Valdez et al., 2012). The bridge between age and NDD incidence remains a mystery, which is why investigation into this link is necessary in order to identify potential therapeutic targets. The majority of NDD cases are associated with individuals over the age of 50. However, there are rare early-onset diagnoses of NDD suggesting that the physiological age of a person may supersede their chronological age, in terms of disease development.

Cyclin-dependent kinase 5 (Cdk5) and its regulation have become a key area of interest for researchers, as the activation of Cdk5 can result in the hyper-phosphorylation of tau and increased amyloid beta aggregation in the brains of AD patients (Wilkanie et al., 2016). Additionally, Cdk5 and one of its activators, p35, were recently found in the Lewy Bodies of the brains of PD patients (Wilkanie et al., 2016). Numerous studies have also demonstrated that abnormalities in Cdk5 expression, both increases and decreases, are associated with multiple types of NDD (Wilkanie et al., 2016). Cdk5 obviously merits researchers’ exploration. A recent article by Spurrier et al. (Spurrier et al., 2018) sought out to observe how manipulating Cdk5α, the Drosophila p35 homologue, would impact NDD and aging in vivo.

Understanding the impacts of alterations to the expression levels of Cdk5α in vivo is a crucial stepping stone into uncovering its role in the aging process. Spurrier and colleagues measured the number of GFP-expressing gamma motor neurons present in the mushroom body (MB) of Drosophila that either had wild-type (WT), overexpression (OE), or knockout (KO) levels of Cdk5α in the motor neurons. Wild-type flies showed a steady, gradual decline of gamma motor neurons over time; however, the Cdk5α-OE and Cdk5α-KO flies had a steep decline with significance appearing at 30- and 45-days old (figure 1B in Spurrier et al., 2018). The associated loss of motor neurons with fluctuations in Cdk5α expression was validated through a motor function assay. Flies with either Cdk5α-OE or Cdk5α-KO demonstrated significant defects in climbing ability from 10- to 45-days old (figure 1D in Spurrier et al., 2018). Flies were also subjected to a lifespan assay that resulted in Cdk5α-OE and Cdk5α-KO flies having a decrease in longevity compared to wild-type animals (figure 1C in Spurrier et al., 2018). It is noteworthy that Cdk5α-OE yielded more severe phenotypes in the neuronal loss and lifespan assays. Collectively, these findings suggest that dysregulation of Cdk5α is involved in accelerating the aging process.

In order to confirm that Cdk5α-induced neuronal loss occurs in a degenerative manner, Spurrier and colleagues measured neurodegenerative phenotypes in Cdk5α-OE and Cdk5α-KO flies. Dysregulation in autophagy is implicated in the pathogenesis of NDD (Menzies et al., 2015). To follow autophagy in vivo, Spurrier et al. used the two autophagy markers: autophagy-related protein 8 (Atg8), a required component of the autophagosomal membrane, and Ref(2)P, a homologue of nucleoporin p62. Wild-type Drosophila steadily expressed both autophagic proteins, with a slight increase in protein levels as the animals aged (figures 2A-D in Spurrier et al., 2018). Cdk5α-KO flies had a significant increase in autophagic markers; however, Cdk5α-OE flies showed a greater fold change (figures 2A-D in Spurrier et al., 2018). Together, these findings suggest a dysregulation of autophagy. This observation corroborates with previous studies that suggest an increase in autophagy protects against NDD (Menzies et al., 2015). The researchers’ observation of an impairment in autophagy fits the narrative that Cdk5 is involved in the pathogenesis of NDD.

In addition to autophagy, researchers used the flies’ sensitivity to oxidative stress to measure aging (Uttara et al., 2009). Flies of different ages, ranging between 3- and 45-days old, were treated with hydrogen peroxide (H2O2) and paraquat, then tested for viability. Cdk5α-OE and Cdk5α-KO flies were significantly less tolerant for oxidative stress when compared to wild-type for both treatments (figures 6A-B in Spurrier et al., 2018). Young Cdk5α-OE and Cdk5α-KO flies had a stress tolerance that was more similar to older wild-type flies. This further supports the notion that modified activation of Cdk5 fast-tracks the aging process.

A major gap in the field of neuropathology is understanding how aging enhances disease susceptibility. Spurrier and colleagues have approached this question by developing an elegant method to identify the physiological age of flies. They compared the expression levels of the genes typically involved in basic biological processes (e.g. metabolism, mitochondrial homeostasis, immunity) in wild-type flies as they aged to establish a standard curve of gene expression, adjusted over time. They then compared the expression levels of these genes to flies with either Cdk5α-OE or Cdk5α-KO genotypes. Both increasing and abolishing expression of Cdk5α led to accelerated physiological aging (figures 3B, E & 5D-E in Spurrier et al., 2018). Spurrier et al. also revealed an overlap of genes that were affected by Cdk5α levels (figures 4B, D in Spurrier et al., 2018). Even though the expression of Cdk5α is limited to neurons, genes affected by the alteration of its expression showed an accelerated aging trend in both the head and thorax of Drosophila (figures 5D-E in Spurrier et al., 2018). The researchers noted that the results from the thorax were unexpected, but this demonstrates that neuronal Cdk5α can cause systemic changes that alter the lifespan of the organism.

Together, the in vivo assays and bioinformatics analyses in Drosophila suggest that altering the activation of Cdk5 by manipulating Cdk5α expression can lead to accelerated aging and enhanced neurodegenerative phenotypes. While both the overexpression and null mutations were deleterious, the Cdk5α-OE flies presented exaggerated phenotypes in the majority of the assays, which is consistent with previous findings (Wilkanie et al., 2016). Spurrier and colleagues have superbly devised an unbiased way to determine physiological aging through genome-wide expression profiling. Establishing physiological age is a tremendous tool that can be implemented in future NDD studies to validate that physiological aging supersedes chronological aging. The next question to address in this field is identifying potential genetic targets and exogenous factors that manipulate the physiological aging process. The aging-related genes in the present study offer a potential starting point to provide a fuller picture of the aging process and its link to the genesis of diseases.

References

1. Menzies, Fiona M., Angeleen Fleming, and David C. Rubinsztein. “Compromised autophagy and neurodegenerative diseases.” Nature Reviews Neuroscience 16, no. 6 (2015): 345.

2. Mills, Kathryn F., Shohei Yoshida, Liana R. Stein, Alessia Grozio, Shunsuke Kubota, Yo Sasaki, Philip Redpath et al. “Long-term administration of nicotinamide mononucleotide mitigates age-associated physiological decline in mice.” Cell Metabolism 24, no. 6 (2016): 795-806.

3. Nho, Kwangsik, Andrew J. Saykin, and Peter T. Nelson. “Hippocampal sclerosis of aging, a common Alzheimer’s disease ‘Mimic’: risk genotypes are associated with brain atrophy outside the temporal lobe.” Journal of Alzheimer’s Disease 52, no. 1 (2016): 373-383.

4. Savica, Rodolfo, Brandon R. Grossardt, James H. Bower, Bradley F. Boeve, J. Eric Ahlskog, and Walter A. Rocca. “Incidence of dementia with Lewy bodies and Parkinson disease dementia.” JAMA Neurology 70, no. 11 (2013): 1396-1402.

5. Spurrier, Shukla, McLinden, Johnson, Giniger. “Altered expression of the Cdk5 activator-like protein, Cdk5α, causes neurodegeneration in part by accelerating the rate of aging.” Disease Models & Mechanisms. no.11 (2018): 1-14.

6. Uttara, Bayani, Ajay V. Singh, Paolo Zamboni, and R. T. Mahajan. “Oxidative stress and neurodegenerative diseases: a review of upstream and downstream antioxidant therapeutic options.” Current Neuropharmacology 7, no. 1 (2009): 65-74.

7. Valdez, Gregorio, Juan C. Tapia, Jeff W. Lichtman, Michael A. Fox, and Joshua R. Sanes. “Shared resistance to aging and ALS in neuromuscular junctions of specific muscles.” PloS One 7, no. 4 (2012): e34640.

8. Wilkaniec, Anna, Grzegorz A. Czapski, and Agata Adamczyk. “Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses.” Journal of Neurochemistry 136, no. 2 (2016): 222-233.

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