Biologic that disrupts PDE11A4 homodimerization in hippocampus CA1 reverses age-related cognitive decline of social memories in mice

Age-related abnormalities in phosphodiesterase 11A (PDE11A), which degrades 3',5'-cAMP/cGMP and is enriched in the ventral hippocampus (VHIPP), drive age-related cognitive decline (ARCD) of social memories. Age-related PDE11A4 ectopically accumulates within the membrane compartment and in filamentous structures termed ghost axons. Previous studies show that expressing an isolated PDE11A4-GAF-B binding domain disrupts homodimerization and reverses aging-like PDE11A4 accumulations in vitro. Here, we show that in vivo lentiviral expression of the isolated PDE11A4-GAFB domain in hippocampal CA1 of aged mice reverses age-related PDE11A4 accumulations and ARCD of social transmission of food preference memory (STFP). It also improves 7-day remote long-term memory for social odor recognition without affecting non-social odor recognition. In vitro studies show that disrupting homodimerization does not alter the catalytic activity of PDE11A4 but may reverse age-related decreases in cGMP by relocating PDE11A4 from a cGMP-rich to a cAMP-rich pool independently of other intramolecular relocation signals (PDE11A4-pS162). Altogether, these data suggest that a biologic designed to disrupt PDE11A4 homodimerization may hold therapeutic potential for age-related PDE11A4 proteinopathies.

Conserved age-related increases in hippocampal PDE11A4 cause unexpected proteinopathies and cognitive decline of social associative memories

In humans, associative memories are more susceptible to age-related cognitive decline (ARCD) than are recognition memories. Reduced cAMP/cGMP signaling in the hippocampus may contribute to ARCD. Here, we found that both aging and traumatic brain injury-associated dementia increased the expression of the cAMP/cGMP-degrading enzyme phosphodiesterase 11A (PDE11A) in the human hippocampus. Further, age-related increases in hippocampal PDE11A4 mRNA and protein were conserved in mice, as was the increased vulnerability of associative versus recognition memories to ARCD. Interestingly, mouse PDE11A4 protein in the aged ventral hippocampus (VHIPP) ectopically accumulated in the membrane fraction and filamentous structures we term "ghost axons." These age-related increases in expression were driven by reduced exoribonuclease-mediated degradation of PDE11A mRNA and increased PDE11A4-pS117/pS124, the latter of which also drove the punctate accumulation of PDE11A4. In contrast, PDE11A4-pS162 caused dispersal. Importantly, preventing age-related increases in PDE11 expression via genetic deletion protected mice from ARCD of short-term and remote long-term associative memory (aLTM) in the social transmission of food preference assay, albeit at the expense of recent aLTM. Further, mimicking age-related overexpression of PDE11A4 in CA1 of old KO mice caused aging-like impairments in CREB function and remote social-but not non-social-LTMs. RNA sequencing and phosphoproteomic analyses of VHIPP identified cGMP-PKG-as opposed to cAMP-PKA-as well as circadian entrainment, glutamatergic/cholinergic synapses, calcium signaling, oxytocin, and retrograde endocannabinoid signaling as mechanisms by which PDE11A deletion protects against ARCD. Together, these data suggest that PDE11A4 proteinopathies acutely impair signaling in the aged brain and contribute to ARCD of social memories.

PDE11A negatively regulates lithium responsivity

Lithium responsivity in patients with bipolar disorder has been genetically associated with Phosphodiesterase 11A (PDE11A), and lithium decreases PDE11A mRNA in induced pluripotent stem cell-derived hippocampal neurons originating from lithium-responsive patients. PDE11 is an enzyme uniquely enriched in the hippocampus that breaks down cyclic AMP and cyclic GMP. Here we determined whether decreasing PDE11A expression is sufficient to increase lithium responsivity in mice. In dorsal hippocampus and ventral hippocampus (VHIPP), lithium-responsive C57BL/6J and 129S6/SvEvTac mice show decreased PDE11A4 protein expression relative to lithium-unresponsive BALB/cJ mice. In VHIPP, C57BL/6J mice also show differences in PDE11A4 compartmentalization relative to BALB/cJ mice. In contrast, neither PDE2A nor PDE10A expression differ among the strains. The compartment-specific differences in PDE11A4 protein expression are explained by a coding single-nucleotide polymorphism (SNP) at amino acid 499, which falls within the GAF-B homodimerization domain. Relative to the BALB/cJ 499T, the C57BL/6J 499A decreases PDE11A4 homodimerization, which removes PDE11A4 from the membrane. Consistent with the observation that lower PDE11A4 expression correlates with better lithium responsiveness, we found that Pde11a knockout mice (KO) given 0.4% lithium chow for 3+ weeks exhibit greater lithium responsivity relative to wild-type (WT) littermates in tail suspension, an antidepressant-predictive assay, and amphetamine hyperlocomotion, an anti-manic predictive assay. Reduced PDE11A4 expression may represent a lithium-sensitive pathophysiology, because both C57BL/6J and Pde11a KO mice show increased expression of the pro-inflammatory cytokine interleukin-6 (IL-6) relative to BALB/cJ and PDE11A WT mice, respectively. Our finding that PDE11A4 negatively regulates lithium responsivity in mice suggests that the PDE11A SNPs identified in patients may be functionally relevant.