Serines 13 and 16 are critical determinants of full-length human mutant huntingtin induced disease pathogenesis in HD mice

Abstract

The N-terminal 17 amino acids of huntingtin (NT17) can be phosphorylated on serines 13 and 16; however, the significance of these modifications in Huntington's disease pathogenesis remains unknown. In this study, we developed BAC transgenic mice expressing full-length mutant huntingtin (fl-mhtt) with serines 13 and 16 mutated to either aspartate (phosphomimetic or SD) or alanine (phosphoresistant or SA). Both mutant proteins preserve the essential function of huntingtin in rescuing knockout mouse phenotypes. However, fl-mhtt-induced disease pathogenesis, including motor and psychiatric-like behavioral deficits, mhtt aggregation, and selective neurodegeneration are abolished in SD but preserved in SA mice. Moreover, modification of these serines in expanded repeat huntingtin peptides modulates aggregation and amyloid fibril formation in vitro. Together, our findings demonstrate that serines 13 and 16 are critical determinants of fl-mhtt-induced disease pathogenesis in vivo, supporting the targeting of huntingtin NT17 domain and its modifications in HD therapy.

Figure 1. Generation and biochemical characterization of the phosphomimetic (SD) and phosphoresistant (SA) BACHD transgenic mice

(A) A schematic representation of the S13,16A and S13,16D mutations within the NT17 domain in the BACHD construct (Gray et al., 2008). (B) A representative Western blot of cortical protein extracts made from one month old BACHD, BACHD-L, SA, SD-B, SD-C, and SD-C heterozygous (het) mice. The blot was probed with polyQ-specific 1C2 antibody followed by anti-tubulin antibody for loading controls. (C) Quantification of the fl-mhtt expression levels in various HD transgenic mouse lines. Densitometry values of individual bands are based on three 1C2 Western blots using independent cortical extracts. The input in each lane is normalized using the anti-α-tubulin control. The results of three independent experiments are expressed as means±SEM. One-way ANOVA analysis reveals that there are significant differences between the fl-mhtt expression levels in BACHD, BACHD-L, SA, SD-C, SD-B, and SD-C (het) mice (F(4,11)=22.090, p=0.000). The Post hoc test (LSD) indicates that the BACHD-L fl-mhtt expression level is significantly lower than the fl-mhtt expression level in the BACHD (p=0.000), SA (p=0.001), SD-C (p=0.017), and SD-B (p=0.010) mice, but does not differ from the SD-C (het) mice (p=0.340).

Figure 2. Mutant Huntingtin with S13 and S16 Mutations Preserve Normal Htt Function During Development and in Adult Cortical Neurons

(A) Heterozygous SD-B, SD-C and SA transgenes were bred in two successive generations with murine Hdh heterozygous knockout mice(Zeitlin et al., 1995) to generate the rescue mice (i.e. fl-mhtt transgene rescue the embryonic lethality of htt null mice). Since the homozygous Hdh null mice are embryonic lethal (ibid), the expected ratio of rescue mice among the live mice born is 1 out of 7. The rescue mice for SD-B, SD-C and SA were born with such Mendelian ratio. (B and C) Western blot analyses with mAb2166 (Htt) and 1C2 to confirm the rescue mice only express fl-mhtt and lack endogenous murine htt. (D–G) SD fl-mhtt retains the essential adult neuronal function of htt in vivo. Nissl stained coronal brain sections of 12-month old SD-B rescue mice (F and G) and WT littermates (D and E) at two different magnifications. No evidence of late-onset cortical neurodegeneration, as previously reported in the forebrain-specific htt null mice (Dragtsis et al, 2000), was detected in these rescue mice. Scale bars = 50 μm.

Figure 3. The SA but not the SD mice demonstrate motor and psychiatric-like behavioral deficits

A mixed model two-way ANOVA design was used to examine the Rotarod performance in a cohort of SA, SD-B, SD-C mice at 2, 6 and 12 months of ages. (A) SA mice exhibit significant Rotarod deficits starting at 2 months of age and persisting at 6 and 12 month ages (* p < 0.0001, Student t test). (B and C) Two independent lines of SD mice (SD-C and SD-B homozygotes) did not exhibit any Rotarod deficits compared to their respective WT littermate controls at 2, 6 and 12 months. (D) Comparison of 6 month age BACHD-L, SA, SD-B and SD-C lines Rotarod performance. SD-B and SD-C mice did not show any significant impairment in their 6-month Rotarod performance compared to wildtype controls. On the other hand, both BACHD-L mice and SA mice exhibited significant impairment compared to WT, SD-B and SD-C mice (one-way ANOVA with Fisher’s LSD post hoc test, p < 0.05, details see main text). (E) The SA mice (p=0.0002, Student’s t test) but not SD mice demonstrate significantly enhanced anxiety in light/dark box exploration test.

Figure 4. The SA but not the SD mice elicit selective neurodegeneration at 12-month age

(A) Significant forebrain weight loss was detected in 12-month SA mice compared to their WT littermate controls (p=0.0064, Student’s t test), but not in SD-B or SD-C mice compared to their respective controls. (B) In all three genotypes, SA, SD-B and SD-C, no significant cerebellar atrophy was detected (p>0.05, Student’s t test). (C) Unbiased stereology reveals significant cortical volume loss in SA mice compared with WT littermates (p=0.0030, Student’s t test), but not SD-B mice (p=0.9025, Student’s t test). (D) Unbiased stereology reveals significant striatal volume loss in SA mice compared with their WT littermates (p=0.0336, Student’s t test). SD-B mice do not exhibit significant striatal volume loss compared to their WT controls (p=0.8635, Student’s t test).

Figure 5. SD mutations but not SA mutations prevent the accumulation of mhtt polyQ aggregate in the brain

Mutant huntingtin aggregates in HD patient brains and existing HD mouse brains can be readily detected by immunostaining with 4H7H7, a monoclonal antibody selectively recognizes the expanded polyQ epitope (see Figures S3, S6, S7, and Supplementary Results). Using the same antibody, we can readily detect mhtt aggregates accumulated in the cortical layers 2–3, layers 5–6 and striata of 12 month old BACHD and SA mice but not WT littermate controls brains. Such staining are absent in the 2-month old SA mice (Figure S8). However, in the 12-month old SD-B and SD-C brains, 4H7H7 stained mhtt aggregates are absent in all brain regions including cortical layers 2–3, layers 5–6 and striata. Scale bar = 50 μm.

Figure 6. In vitro aggregation of exon-1 peptides and a schematic model on serines 13 and 16 as a critical molecular switch of HD pathogenesis

(A). HPLC based sedimentation assays indicate differences in the aggregation of WT:10.8uM (●, R² = 0.9832, S.E. = 6.7), SA:10.9uM (◆, R² = 0.9843, S.E. = 6.3) and SD:10uM (▲, R² = 0.9863, S.E. = 3.2) NT17Q₃₇P₁₀K₂ peptides. The experiments were done in duplicate (for WT) and triplicate (SA and SD mutants) and the data was fit using Sigma Plot 10.0 software to a single exponential three-parameter decay function. (B–H). Electron microscope images on aggregates harvested after 96 hrs aggregation time. WT (B) and SA mutant (C) peptide aggregates are morphologically equivalent. SD mutant peptide aggregates (D–H) include none of these mature amyloids, but rather are composed of a mixture of alternative morphologies associated with aborted and/or alternative aggregation pathways. (I) A schematic model of how serines 13 and 16 act as a molecular switch to dichotomize fl-mhtt induced disease pathogenesis. Depending on the molecular state of S13 and S16, fl-mhtt may be involved in two distinct molecular pathways. In the toxic mhtt pathway, when S13 and S16 are not phosphorylated, a state mimicked by SA mutations, fl-mhtt may mediate its toxicity via interaction with yet undefined protein X, or it may be cleaved into toxic mhtt polyQ fragments which may misfold and form toxic monomers, oligomers, aggregates and mature amyloid fibrils. Moreover, these mhtt species may be difficult to be cleared by the lysosome or proteosome (Thompson et al., in press). Overall, the unphosphorylated or SA form of fl-mhtt can elicit HD pathogenesis in vivo. In contrast, phosphorylation of S13 and S16, modeled by SD mutations, may induce biophysical changes to alter its polyQ-induced protein misfolding and/or protein-protein interactions (e.g. interacting with protein Y instead of protein X), and slow the kinetics of htt aggregation and impair the formation of mature amyloid fibrils, thereby enhancing the clearance of mhtt species. Overall, our study provides the proof-of-principle that molecular targeting of S13 and S16 (i.e. SD mutations) may convert fl-mhtt into a functional but non-toxic form in vivo, and hence represents an attractive novel target for HD therapy.