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Retrograde tracing from fast and slow twitch muscles of the lower extremity revealed that aggregates form preferentially in MN cell bodies attached to fast twitch muscle.
Understanding the pathophysiology of this condition seems crucial for therapeutic design, yet few electrophysiological studies in actively degenerating animal models have been reported.
These models may also lack the diversity of MN physiology present in the mature spinal cord.
Here we studied a transgenic strain of ALS mice, G85R SOD1YFP (16), that develops motor symptoms by ∼3 mo of age, associated with progressive accumulation of aggregates in MN cell bodies (from ∼1 mo of age), attended by MN cell loss. We first developed, using Ch AT- EGFP mice that express GFP fluorescence in MNs (17), an acute slice preparation of adult mouse spinal cord that yielded healthy MNs in animals up to and beyond 6 mo of age, readily visualized by their fluorescence, enabling whole cell patch-clamp recordings when coupled with differential interference contrast (DIC) imaging.
1) and, along with DIC, allowed whole cell patch recordings.
At all ages up to 6 mo, lumbar MNs had stable resting membrane potentials between −60 and −79 m V and exhibited spontaneous excitatory postsynaptic potentials (EPSPs; Fig. During recording, cells could be filled with biocytin (contained in the patch pipette internal solution), allowing for later detection, after fixation of the slice, with fluorescent Alexa 594-coupled streptavidin to reveal the size and anatomy of the neuron, in particular revealing long dendritic arborizations, sometimes greater than 1 mm in length in the medio-lateral direction of the slice (Fig.
From these studies, however, there does not appear to be a clear consensus on the changes that occur in MNs before and during the development of symptoms (7).
For example, whereas research on the neuromuscular junction has revealed preferential denervation of fast twitch (type IIb) muscle fibers (8–10), the relationship of this selective susceptibility at the muscle level to pathophysiologic change in the spinal cord is not clear.
These results begin to describe an order of the pathophysiologic events in ALS. Inherited forms of ALS, accounting for ∼10% of cases, potentially inform about disease mechanisms, including: protein folding and quality control [e.g., mutant superoxide dismutase 1 (SOD1), ubiquilin2, and VCP]; RNA binding proteins (e.g., TDP43, FUS, and HNRNPA1); or a DNA expansion (C9ORF72 hexanucleotide expansion).
The clinical courses of the various heritable forms and the 90% of cases that are considered sporadic are not distinct, however, reflecting a potentially shared progressive loss of MNs and motor circuit dysfunction (4).
In a G85R SOD1YFP transgenic mouse model of ALS, which becomes paralyzed by 5–6 mo, where MN cell bodies are fluorescent, enabling the same type of recording from spinal cord tissue slices, we observed that all four MN subtypes were present at 2 mo of age.
At 4 mo, by which time substantial neuronal SOD1YFP aggregation and cell loss has occurred and symptoms have developed, one of the fast firing subtypes that innvervates fast twitch muscle was lost.
For the ex vivo alternative, slice physiology is challenging because most mouse models develop disease after 1 mo of age, a time when spinal cord tissue becomes more sensitive to ischemia (11, 12), making isolation of viable slices difficult.