Radiolabeling motoneuron proteins in the isolated frog spinal cord preparation

The role of the cytoskeleton in cell body enlargement, increased nuclear eccentricity and chromatolysis in axotomized spinal motor neurons

BACKGROUND

When spinal motor axons are injured, the nucleolus, nucleus and cell body of the injured cell transiently increase in size, the nucleus becomes more eccentrically placed, and the organization of polyribosomes into Nissl bodies is temporarily disrupted. The mechanisms for these classical morphological responses to axotomy have not been satisfactorily explained.

RESULTS

In this study we address the role of the cell body cytoskeleton in these structural changes. We show that the cytoskeleton of uninjured lumbar motor neuron cell bodies maintains nucleolar, nuclear and cell body size and nuclear position. When isolated, the relatively insoluble cell body cytoskeleton contains Nissl bodies and lipofuscin granules. After axotomy, protein labeling increases markedly and the cytoskeleton enlarges, increasing nucleolar, nuclear and cell body size, as well as nuclear eccentricity. Nearly all of the protein mass that accumulates in the cell body after axotomy appears to be added to the cytoskeleton.

CONCLUSION

We conclude that axotomy causes the conjugate enlargement of the nucleolus, nucleus and cell body and increases nuclear eccentricity in spinal motor neurons by adding protein to the cytoskeleton. The change in nuclear position, we propose, occurs when cytoskeletal elements of the axon cannot enter the shortened axon and "dam up" between the nucleus and axon hillock. As a consequence, we suggest that Nissl body-free axonal cytoskeleton accumulates between the nucleus and axon, displaces Nissl body-containing cytoskeleton, and produces central chromatolysis in that region of the cell.

Magnitude, laterality, and uniformity of swelling in axotomized spinal motoneurons: lack of evidence for influence by the distal stump

Injury to frog lumbar motor axons produces a coordinated, allometric enlargement of the nucleolus, nucleus, and cell body of the injured neuron. The mechanisms by which swelling is initiated and sustained are not known. In this study, we have sought evidence for a role of the severed distal stump in the magnitude, laterality, and uniformity of the swelling response in frog spinal motoneurons. We find that swelling of motoneuron nucleoli, nuclei, and perikarya after unilateral spinal nerve transection is exclusively ipsilateral and uniform among motoneurons of different sizes. Removal of the severed distal stump does not affect the magnitude, unilaterality, or uniformity of the swelling responses. Thus, the distal stump appears to play no role in initiating swelling following spinal nerve transection.