Muscle A-Kinase Anchoring Protein-α is an Injury-Specific Signaling Scaffold Required for Neurotrophic- and Cyclic Adenosine Monophosphate-Mediated Survival

Abstract

Neurotrophic factor and cAMP-dependent signaling promote the survival and neurite outgrowth of retinal ganglion cells (RGCs) after injury. However, the mechanisms conferring neuroprotection and neuroregeneration downstream to these signals are unclear. We now reveal that the scaffold protein muscle A-kinase anchoring protein-α (mAKAPα) is required for the survival and axon growth of cultured primary RGCs. Although genetic deletion of mAKAPα early in prenatal RGC development did not affect RGC survival into adulthood, nor promoted the death of RGCs in the uninjured adult retina, loss of mAKAPα in the adult increased RGC death after optic nerve crush. Importantly, mAKAPα was required for the neuroprotective effects of brain-derived neurotrophic factor and cyclic adenosine-monophosphate (cAMP) after injury. These results identify mAKAPα as a scaffold for signaling in the stressed neuron that is required for RGC neuroprotection after optic nerve injury.

Regulation of Neuronal Survival and Axon Growth by a Perinuclear cAMP Compartment

“…Confocal images were acquired with a confocal laser scanning microscope (Zeiss 880) and a ϫ10 magnification lens. The imaging and quantification were performed in a masked fashion as previously described (Wang et al, 2015). Briefly, the retinas were divided into four quadrants, and one digital micrograph was taken from a fixed distance from the periphery of each of the four fields.…”

“…We have demonstrated that mAKAP␣ expression is required for neurite growth in vitro and neuronal survival in vivo (Wang et al, 2015), but how its perinuclear localization affects these processes has not been explored. mAKAP is localized to the nuclear envelope via protein-protein interactions with KASH domain-containing, transmembrane isoforms of nesprin-1 that heterodimerize via their spectrin-repeat domains with mAKAP (Pare et al, 2005a).…”

“…The morphology of the neuron lends itself to compartmentalized signaling, given the extraordinarily large distances that often exist between axons, dendrites, and the soma, as well as because of the physical constraints upon diffusion conferred by the geometry of structures such as dendritic spines (Terenzio et al, 2017). For example, cAMP is an important second messenger for the development of nervous system connectivity, neuronal metabolism (Averaimo and Nicol, 2014;Stiles et al, 2014), growth cone guidance, activity-dependent neuronal survival and axon extension in vitro (Goldberg et al, 2002b), as well as neuroprotection and axonal regeneration in vivo (Wang et al, 2015). Despite being a watersoluble inherently diffusible second messenger, cAMP is subject to extensive compartmentation, especially with regard to the regulation of its canonical effector protein kinase A (PKA; Wild and Dell'Acqua, 2018).…”

“…mAKAP (AKAP6; Fig. 1A) is a modular scaffold protein localized to the nuclear envelope, which is expressed as a 250 kDa alternatively-spliced ␣-isoform in hippocampal neurons and retinal ganglion cells (RGCs) and a 230 kDa ␤-isoform in cardiac and skeletal myocytes (Michel et al, 2005;Pare et al, 2005a;Wang et al, 2015). mAKAP was initially identified as a PKA scaffold (Kapiloff et al, 1999) and was later found to bind both type 2 and type 5 adenylyl cyclase and the cAMP-specific PDE isoform PDE4D3, thereby providing the potential infrastructure for entirely local cAMP regulation within a perinuclear compartment (Dodge et al, 2001;Kapiloff et al, 2009).…”

“…Additional research has revealed that mAKAP orchestrates large multimolecular signalosomes (Ͼ20 binding partners identified) that transduce not only cAMP, but also calcium, phospholipid, mitogen-activated protein kinase, and hypoxic signaling (Passariello et al, 2015). Recently, we discovered that mAKAP␣ expression is required for RGC survival and neurite growth in vitro and in vivo (Wang et al, 2015). The mechanisms by which mAKAP␣ contributes to neuroprotection and neurite extension, however, remain unknown, including whether mAKAP␣dependent cAMP signaling is relevant to these processes.…”