Effect of amyloids on the vesicular machinery: implications for somatic neurotransmission

Abstract

Certain neurodegenerative diseases are thought to be initiated by the aggregation of amyloidogenic proteins. However, the mechanism underlying toxicity remains obscure. Most of the suggested mechanisms are generic in nature and do not directly explain the neuron-type specific lesions observed in many of these diseases. Some recent reports suggest that the toxic aggregates impair the synaptic vesicular machinery. This may lead to an understanding of the neuron-type specificity observed in these diseases. A disruption of the vesicular machinery can also be deleterious for extra-synaptic, especially somatic, neurotransmission (common in serotonergic and dopaminergic systems which are specifically affected in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively), though this relationship has remained unexplored. In this review, we discuss amyloid-induced damage to the neurotransmitter vesicular machinery, with an eye on the possible implications for somatic exocytosis. We argue that the larger size of the system, and the availability of multi-photon microscopy techniques for directly visualizing monoamines, make the somatic exocytosis machinery a more tractable model for understanding the effect of amyloids on all types of vesicular neurotransmission. Indeed, exploring this neglected connection may not just be important, it may be a more fruitful route for understanding AD and PD.

Keywords: Alzheimer's; Parkinson's; monoamine; multiphoton microscopy; neurodegeneration; protein aggregation.

Figure 1.

A schematic depicting the putative effects of amyloid aggregates on the different parts and processes in a neuron. The boxes marked red constitute the effects on vesicular machinery, the focus of this review. Transporters: VMAT, vesicular monoamine; SERT, serotonin; NET, norepinephrine; GLUT, glutamate; DAT, dopamine.

Figure 2.

Membrane interaction and internalization of amyloid β_1–40 (Aβ₄₀) incubated with RN46A cells. (a) Binding of Aβ₄₀ oligomers to cell membranes of serotonergic neuron-derived cell line RN46A. ((i),(ii)) Transmission and ((iii),(iv)) confocal sections of two sets of cells with similar initial cell densities (excitation: 543 nm, emission: 550–700 nm) at time zero. ((v),(vi)) The same cells after 30 min of incubation with (v) buffer and (vi) 250 nM solution of rhodamine-labelled Aβ₄₀(R-Aβ₄₀). (b) Internalization of Aβ₄₀ oligomers upon longer R-Aβ₄₀ exposure: ((i),(ii)) confocal sections of two sets of cells with similar initial cell densities (excitation: 543 nm, emission: 550–700 nm) at time zero. ((iii),(iv)). The same cells after 120 min of incubation with (iii) buffer and (iv) 250 nM solution of R-Aβ_40. (c) Aggregation of Aβ₄₀ as seen in cultured neuronal cell membranes: a maximum entropy method based model-free data analysis (using the MEMFCS algorithm [109]) of diffusion of R-Aβ₄₀ on RN46A cell membranes incubated with 1 µM Aβ₄₀ solution for 1 h, determined by FCS. Diffusion time is proportional to size. Methods have been described elsewhere [111,112]. (d) Colocalization of serotonin and vesicles labelled with FM1–43. Quasi-simultaneous imaging of (i) serotonin (5-HT) using three-photon excitation and (ii) FM1–43 (a vesicle labelling dye), using one photon excitation in cultured serotonergic neurons depolarized by high K⁺ exocytosis buffer. In both the images, white arrows show colocalized vesicular structures, and red arrows mark spots visible in the serotonin channel only, most probably vesicles which were not recycled. (Adapted from Sarkar et al. [50].)