A pathoconnectome of early neurodegeneration: Network changes in retinal degeneration

Abstract

Connectomics has demonstrated that synaptic networks and their topologies are precise and directly correlate with physiology and behavior. The next extension of connectomics is pathoconnectomics: to map neural network synaptology and circuit topologies corrupted by neurological disease in order to identify robust targets for therapeutics. In this report, we characterize a pathoconnectome of early retinal degeneration. This pathoconnectome was generated using serial section transmission electron microscopy to achieve an ultrastructural connectome with 2.18nm/px resolution for accurate identification of all chemical and gap junctional synapses. We observe aberrant connectivity in the rod-network pathway and novel synaptic connections deriving from neurite sprouting. These observations reveal principles of neuron responses to the loss of network components and can be extended to other neurodegenerative diseases.

Keywords: Abnormal ribbon morphology; Connectomics; GABAergic amacrine cells; Neurodegeneration; Pathoconnectomics; Retinal degeneration; Retinitis pigmentosa; Rod pathway.

Figure 1

Overview of RPC1. (A) Vertical section through tissue directly adjacent to the tissue processed for the RPC1 volume. (B) 3D composite volume of RPC1. Pseudocolored sections illustrate the locations of CMP sections for small molecules indicated in 1C. (C) Overlaid CMP sections from RPC1 on their adjacent TEM sections.

Figure 2:

Simple diagram of bipolar cell network. Solid arrows indicate glutamatergic synapses. Open arrows indicate inhibitory synapses (GABA or glycine). Solid circles indicate ionotropic glutamate receptors, while mGluR6 is indicated by the double open circles. Gap junctions are indicated by zig-zag lines.

Figure 3:

RodBC dendritic contacts. (A) Representative rod photoreceptor synapse of RodBC 822 postsynaptic to rod. (B) Representative cone photoreceptor synapse between a cone photoreceptor and RodBC 1223. (C) 3D rendering of cone photoreceptor 23084 main terminal and 2 neurite processes. (D) Representative photoreceptor synapse between a cone neurite ribbon and RodBC 822 (E) Representative photoreceptor synapses between an indeterminate photoreceptor and RodBC 822. Black arrowheads indicate ribbon densities, while white arrowheads indicate associated post-synaptic densities. Viking Annotation and Pseudocolored Images Scale bars 500nm. Higher Magnification Scale Bars 250nm

Figure 4:

Spherical Ribbons in RodBCs of RPC1. (A) Example normal ribbon made by RodBCs. (B) 2 example spherical ribbons. (C) The hollow conformation of spherical synapses. Scale bars: 100nm

Figure 5:

RodBC gap junctions with Aii GACs (A) 3D rendering of all 16 RodBCs from RPC1. (B) RodBC axonal arbor fields in RPC1. (C) Convex Hulls of all RodBCs in RPC1 (D-E) Gap junctions between RodBCs and Aii GACs. Left panel is 3D rendering with inset higher magnification image of specific gap junction annotations. White boxed image corresponds to white box in adjacent left panel. * indicate ribbons and # indicates gap junctions arising in same varicosity. 100nm scale images are 25k recaptures of gap junction structures.

Figure 6:

Ascending process of GABAergic amacrine cells. (A) CMP overlay of GABA section 565 on its neighboring TEM section. Intensity of red color indicates higher levels of GABA contained within the cell. (B) 3D rendering of the 4 γACs with ascending processes in RPC1. (C) Synapse between γAC 2627 and OFF-BC 1231 in the OPL. White box indicates region of synapse shown on the right in increasing magnification. Black arrowhead indicates the presynaptic vesicle cloud. (D) Synapse between γAC 993 and RodBC 1242 in the OPL. White box indicates region of synapse shown on the right in increasing magnification. Black arrowhead indicates the presynaptic vesicle cloud. (E) Synapse between γAC 997 and HzC 38325 in the OPL. White box indicates region of synapse shown on the right in increasing magnification. Black arrowhead indicates the presynaptic vesicle cloud. Scale bars: 250nm (Viking annotated) 100nm (smaller inset).

Figure 7:

Altered network found in RPC1. (A) Phase 0 (Healthy) bipolar cell network. (B) Phase 1 corrupted network. Solid arrows indicate glutamatergic synapses. Open arrows indicate inhibitory synapses (GABA or glycine). Solid circles indicate ionotropic glutamate receptors, while mGluR6 is indicated by the double open circles. Gap junctions are indicated by zig-zag lines. Red lines indicate aberrant connectivities observed in the RPC1 volume.