Neurite Sprouting and Synapse Deterioration in the Aging Caenorhabditis elegans Nervous System

Abstract

C. elegans is a powerful model for analysis of the conserved mechanisms that modulate healthy aging. In the aging nematode nervous system, neuronal death and/or detectable loss of processes are not readily apparent, but because dendrite restructuring and loss of synaptic integrity are hypothesized to contribute to human brain decline and dysfunction, we combined fluorescence microscopy and electron microscopy (EM) to screen at high resolution for nervous system changes. We report two major components of morphological change in the aging C. elegans nervous system: 1) accumulation of novel outgrowths from specific neurons, and 2) physical decline in synaptic integrity. Novel outgrowth phenotypes, including branching from the main dendrite or new growth from somata, appear at a high frequency in some aging neurons, but not all. Mitochondria are often associated with age-associated branch sites. Lowered insulin signaling confers some maintenance of ALM and PLM neuron structural integrity into old age, and both DAF-16/FOXO and heat shock factor transcription factor HSF-1 exert neuroprotective functions. hsf-1 can act cell autonomously in this capacity. EM evaluation in synapse-rich regions reveals a striking decline in synaptic vesicle numbers and a dimunition of presynaptic density size. Interestingly, old animals that maintain locomotory prowess exhibit less synaptic decline than same-age decrepit animals, suggesting that synaptic integrity correlates with locomotory healthspan. Our data reveal similarities between the aging C. elegans nervous system and mammalian brain, suggesting conserved neuronal responses to age. Dissection of neuronal aging mechanisms in C. elegans may thus influence the development of brain healthspan-extending therapies.

Directional Trans-Synaptic Labeling of Specific Neuronal Connections in Live Animals

“…Consistent with this observation, the average synapse size significantly increased in 3-day-old adults and, with less statistical certainty, in 6-and 9-day-old adults ( Figure 4G). On the other hand, the total synaptic area remained constant with a trend for decreasing total area in aging animals, in line with electron microscopic studies (Toth et al 2012) (Figure 4H). These results suggest an expansion of connectivity during early adulthood, followed by a consolidation phase and eventual decline.…”

A Novel MitoTimer Reporter Gene for Mitochondrial Content, Structure, Stress, and Damage in Vivo

“…To validate these findings in vivo, we generated transgenic C. elegans with touch neuron-specific expression under control of the neuron-specific promoter mec-4 and observed green and red mitochondria in the soma and distributed through the sensory neuron process (Fig. 1d), which is similar to mitochondrial targeted standard GFP as reported previously (33). We also generated UAS-MitoTimer transgenic flies and crossed with Mef2-Gal4 transgenic flies to generate Mef2-Gal4ϾUAS-MitoTimer flies with muscle expression (34).…”

Guanine Nucleotide Exchange Factor OSG-1 Confers Functional Aging via Dysregulated Rho Signaling in Caenorhabditis elegans Neurons

“…We conclude that OSG-1 does not contribute to the regulation of life span in C. elegans. The nervous system of C. elegans undergoes several agedependent morphological and functional changes including cytoskeletal disorganization, axon beading, defasciculation, deteriorations in synaptic transmission, and neurite branching (Pan et al 2011;Tank et al 2011;Toth et al 2012;Liu et al 2013), and the results of this study add new insight by implicating Rho signaling in neuronal aging. In particular, the actin cytoskeleton appears to be one of the targets of OSG-1/RHO-1 as evidenced by the fact that knockdown of ARX-3 and ARX-5 alone or with RHO-1 or with OSG-1 ameliorated the loss of GFP signal in the ASER neuron in a nonadditive fashion.…”