Dynamics of ER morphology

The healthy brain needs fully functional intracellular organelles, which are distributed in even the smallest parts of the nerve cells.  Activation of synaptic receptors during normal brain function triggers a number of processes that are essential for information processing. The majority of these signaling pathways converge on the endoplasmic reticulum (ER), which is of particular importance for basic neuronal function: neurotransmission and cell survival.

In vivo two-photon image of cortical neurons expressing enhanced green fluorescent protein targeted to ER lumen (EGFP-ER).

The key roles of ER in neurons are biosynthesis, Ca2+ store and signaling. The neuronal ER is functionally and structurally heterogeneous, and forms the single largest continuous organelle that extends from the nuclear envelope to subset of dendritic spines, and through the axon to presynaptic terminals. This organization allows long-distance trafficking of proteins in secretory pathway and cytosol-independent tunneling of ions between distal neuronal compartments. Intraluminal diffusion and equilibration of proteins and ions preserves the efficiency of protein folding machinery, facilitates neurotransmission and different forms of synaptic plasticity. In addition, the neuronal ER is an excitable organelle that conveys synaptically evoked regenerative Ca2+ waves that spread in dendrites, and can invade nuclear envelope. Often, the neuronal ER is conceptualized as a signal integrating organelle that couples spatially and temporally separated events in the cell i.e. synaptic activation with gene transcription. Thus, the alteration of ER continuity, even if transient would affect fundamental neuronal functions.
Yet, transgression from the continuous to discontinuous ER, in contrast to non-neuronal cells, is linked in neurons to apoptotic or necrotic cell death and the neuronal ER structure-function interplay is poorly understood. Furthermore, there is reportedly no longitudinal description of ER structural dynamics in the living brain.

Here, using two-photon imaging we investigate the ER morphology in intact brains in anesthetized mice. We developed real-time quantification of ER morphology dynamics and perform functional assessments of ER continuity in vivo. This is performed simultaneously with intracellular Ca2+ imaging and electrophysiological recordings of brain neuronal activity.

We describe an unusual property of ER rapidly responding to synaptic activation, with important implications to brain function in vivo. We characterize neuronal ER dynamics in physiology and in conditions that typically accompany ischemic stroke or brain trauma. Using different pharmacological approaches we determine the major signaling pathway that is involved in regulation of ER structural dynamics and suggest a new target for novel neuroprotective strategies.

We expect to present the results in 2017.