Muscle A kinase anchoring protein mAKAP also known as

Muscle A-kinase anchoring protein (mAKAP; also known as AKAP6) is a 255kDa scaffold protein localized to the nuclear envelope of neurons and striated myocytes and that binds PKA as well as a large number of signaling enzymes implicated in stress responses (Passariello et al., 2015). mAKAP also binds the cAMP target Epac1, adenylyl cyclases (types II and V), and the cAMP-specific phosphodiesterase 4D3. Because these enzymes participate in negative and positive feedback, cAMP levels are predicted to be tightly regulated around mAKAP “signalosomes,” providing local control over the phosphorylation of relevant PKA substrates. Differential PKA localization by scaffolds like mAKAP is likely to be especially important in cell types like neurons where subcellular domains, including cell bodies, ionomycin and dendrites, are often separated across great distances. In neurons, other AKAPs such as AKAP79 function in synaptic signaling (Weisenhaus et al., 2010), but it is unknown whether any PKA scaffolds have a role in neuronal survival or axon growth. Neurons express the longer α isoform of mAKAP, mAKAPα, which contains an amino-terminal sequence with additional binding sites, e.g. for 3-phosphoinositide-dependent protein kinase-1 (PDK1) (Michel et al., 2005). The function of neuronal mAKAPα has not been characterized, albeit deletion of the exon encoding the mAKAPα-specific N-terminus resulted in a failure-to-thrive phenotype (Michel et al., 2005). Besides binding enzymes related to cAMP signaling, mAKAPα also binds in neurons the mitogen-activated protein kinases MEK5 and ERK5 that are known to be important for neuronal survival in response to neurotrophins (Michel et al., 2005, Dodge-Kafka et al., 2005, Wang et al., 2006, Watson et al., 2001). Given that mAKAPα can coordinate cAMP and ERK5 signaling, we hypothesized that mAKAPα might be involved in survival or axon growth signaling in neurons, which require both of these signaling pathways to optimally promote these functions in RGCs(Goldberg et al., 2002). We now present evidence that mAKAPα is expressed in RGCs but discovered in fact that mAKAPα does not regulate RGC survival during normal development or in the absence of injury, but rather mediates stress-specific signaling of survival and axon growth in vitro and survival in vivo after optic nerve injury.
Materials & Methods
Results
Discussion The central observation of our study is that mAKAPα is required for pro-survival signaling in RGCs after optic nerve injury in the adult, but not during normal development or in uninjured RGCs. This difference implies that divergent signaling pathways control neuronal survival during development and following injury. These results are reminiscent of those recently found following mAKAPβ knock-out in the cardiac myocyte wherein mAKAPβ was dispensable in the developing heart, but required for the induction of pathological cardiac remodeling following pressure overload or catecholamine infusion (Kritzer et al., 2014a). Taken together, these data suggest that mAKAP signalosomes play critical roles in the regulation of intracellular signaling pathways invoked in disease or injury. Interruption of target-derived neurotrophic support is a key factor diminishing RGC survival following axon injury (Yip and So, 2000, Shen et al., 1999, Moore and Goldberg, 2010). Following optic nerve crush, trophic survival signaling can be induced by exogenous BNDF or cAMP (Shen et al., 1999), but here we find that this signaling was effective in promoting survival only in RGCs expressing mAKAPα. We found endogenous mAKAPα expression dropped significantly in RGCs after optic nerve crush, contemporaneously with the induction of RGC death. Complete loss of mAKAPα via gene deletion further exacerbated RGC loss. In addition, any enhancement of RGC survival by exogenous cAMP and/or BDNF after injury depended upon the residual (50–60%) mAKAPα normally expressed after optic nerve crush-mediated protein down-regulation. mAKAPα overexpression was also deleterious to cultured RGCs, confirming that overexpression of binding proteins for critical signaling proteins are often disruptive to signaling pathways (Scott et al., 2013). Together our data imply that the organization of perinuclear signalosomes by the mAKAPα scaffold is limiting for the transduction of cAMP and neurotrophic factor signaling required for RGC survival following trauma or insult.