Date of Award
Medical Doctor (MD)
Elizabeth A. Jonas
Short term synaptic plasticity is required for working memory and executive functions, which become impaired in neurodegenerative diseases such as dementia. A form of short-term plasticity that can be induced by repeated stimulation of neurons is called post-tetanic potentiation (PTP). One way in which PTP occurs is through an increase in the number of vesicles at the active zone, the specialized region of the presynaptic terminal where synaptic transmission occurs. The calyx of Held is a large, rapidly firing synapse located in the auditory brainstem that is involved in sound localization. We have previously shown that the dense layer of filamentous actin (F-actin) in the calyx of Held is organized by the potassium channel Kv3.3 and facilitates endocytosis of vesicles, enabling sustained synaptic transmission. OBJECTIVES: To determine whether tetanic stimulation of the calyx of Held synapse would result in: (1) the reorganization of the F-actin cytoskeleton, allowing for the accumulation of additional vesicles at the active zone; and (2) changes in mitochondrial structure and location with respect to active zones. We used three-dimensional super-resolution microscopy to visualize and reconstruct the rat calyx of Held synapse with and without stimulation and used transmission electron microscopy (TEM) to visualize the structure of presynaptic mitochondria and active zones in these synapses. Repeated stimulation resulted in contraction of the F-actin cytoskeleton near synaptic vesicles, with the mean diameter of filamentous actin structures decreasing by 10.12% following stimulation (p=0.0491). There was no difference in the average number of docked vesicles, but we did observe a 41.31% increase in the average number of reluctant vesicles that clustered near the active zone without docking (p=0.0409). The fraction of docked vesicles that clustered together in groups of four or more decreased from 0.3858 without stimulation to 0.2159 following stimulation (p=0.0364), indicating a net gain of docked vesicles at weaker release sites and a loss of docked vesicles at stronger ones. F-actin also moved away from zones of vesicle clustering, with the fraction of overlap between F-actin and the synaptic vesicle marker VGlut1 decreasing from 0.1797 to 0.1200 (p=0.0120). Stimulation also induced a 57.49% increase in mitochondrial matrix electron density compared to mitochondria in synapses that were not stimulated. PTP in the calyx of Held synapse involves dynamic remodeling of the actin cytoskeleton, an increase in the number of vesicles clustered at active zones, and a targeted increase in vesicle docking at weaker release sites. Mitochondria undergo structural changes indicative of an altered metabolic state. These findings contribute to the emerging link between the structural dynamics of the cytoskeleton, mitochondrial metabolism, synaptic function, and neurodegenerative disease.
Subramanian, Shobana, "Presynaptic Plasticity In A Mammalian Synapse Involves Actin Remodeling And Vesicle Recruitment" (2022). Yale Medicine Thesis Digital Library. 4125.