Huntingtin phosphorylation on serine 421 is significantly reduced in the striatum and by polyglutamine expansion in vivo

Abstract

Huntington disease (HD) results from polyglutamine expansion in the huntingtin protein (htt). Despite the widespread tissue expression pattern of htt, neuronal loss is highly selective to medium spiny neurons of the striatum. Huntingtin is phosphorylated on serine-421 (S421) by the pro-survival signaling protein kinase Akt (PKB) and this has been previously shown to be protective against the toxicity of polyglutamine-expanded htt in cell culture. Using an antibody specific for htt phosphorylated on S421, we now demonstrate that htt phosphorylation is present at significant levels under normal physiological conditions in human and mouse brain. Furthermore, htt phosphorylation shows a regional distribution with the highest levels in the cerebellum, less in the cortex, and least in the striatum. In cell cultures and in YAC transgenic mice, the endogenous phosphorylation of polyglutamine-expanded htt is significantly reduced relative to wild-type htt. The presence and pattern of significant htt phosphorylation in the brain indicates that this dynamic post-translational modification is important for the regulation of htt and may contribute to the selective neurodegeneration seen in HD.

PH domain leucine-rich repeat protein phosphatase 1 contributes to maintain the activation of the PI3K/Akt pro-survival pathway in Huntington's disease striatum

“…12 However, Akt activation status in 6-month-old YAC128 mice and in 6-and 10-week-old R6/2 mice was previously shown to be unchanged. 36 Our observation that the activation of the Akt pathway in the presence of mhtt is a brain region-dependent effect explains why that study failed to find changes in pAkt (Ser473) levels as active Akt was measured in whole brain lysates. 36 Akt activation has been largely shown as one of the main mechanisms to prevent neuronal death during injury.…”

“…36 Our observation that the activation of the Akt pathway in the presence of mhtt is a brain region-dependent effect explains why that study failed to find changes in pAkt (Ser473) levels as active Akt was measured in whole brain lysates. 36 Akt activation has been largely shown as one of the main mechanisms to prevent neuronal death during injury. 8 Accordingly, Akt prevents neuronal death induced by mhtt, 14 and increasing Akt expression has beneficial effects in Drosophila models of HD.…”

Insulin and IGF-1 improve mitochondrial function in a PI-3K/Akt-dependent manner and reduce mitochondrial generation of reactive oxygen species in Huntington’s disease knock-in striatal cells

“…Akt activation is an early pro-survival striatal response in knock-in Hdh Q111 mice and STHdh Q111/Q111 cells [28]; importantly, activation of IGF-1/Akt pathway caused Htt phosphorylation at Ser421, decreasing mHtt nuclear inclusions and mHtt toxicity [27] and regulated anterograde and retrograde transport defects in HD cortical neurons [78]. Another study demonstrated that p-(Ser473)Akt was unchanged in YAC128 and in R6/2 HD mice, but the levels of p-(Ser421)Htt were decreased in the striatum of YAC128 mice and in cells expressing mHtt [79]. In contrast, insulin receptor substrate (IRS)-2, which activates PI-3K/Akt and mTOR cascade, promoted mitochondrial dysfunction and oxidative stress in R6/2 mice; however, in the same study, the authors showed that IRS-2 protein levels were unchanged in the striatum of patients with grade II HD [80].…”

Phosphorylation of Huntingtin by Cyclin-Dependent Kinase 5 Is Induced by DNA Damage and Regulates Wild-Type and Mutant Huntingtin Toxicity in Neurons

“…Htt is itself a phosphoprotein with a toxicity when mutated that is regulated by phosphorylation (Humbert et al, 2002;Rangone et al, 2004;Luo et al, 2005;Warby et al, 2005;Pardo et al, 2006;Schilling et al, 2006). Cdk5 phosphorylates htt at S434 and regulates its proteolysis (Luo et al, 2005).…”

“…In disease, polyQ expansion in htt leads to a reduction of BDNF transcription and transport, thereby reducing neurotrophic support (Zuccato et al, 2001;Gauthier et al, 2004). Arguments for a key role of the protein context in pathogenesis also come from the observations that cleavage and posttranslational modifications regulate the toxicity of polyQ-htt (Humbert et al, 2002; (Humbert et al, 2002;Warby et al, 2005;Pardo et al, 2006).…”