PKMζ-PKCι/λ double-knockout demonstrates atypical PKC is crucial for the persistence of hippocampal LTP and spatial memory.

PKMζ is a persistently active atypical PKC (aPKC) isoform thought to maintain late-phase long-term potentiation (late-LTP) and long-term memory. PKMζ-knockout mice, however, still exhibit hippocampal LTP and spatial memory while lacking neocortical LTP, questioning whether this kinase is fundamental to enduring synaptic potentiation and memory. Tsokas et al. (2016) suggested that the other aPKC, PKCι/λ, may compensate for PKMζ during maintenance in the hippocampus of PKMζ-null mice. In wild-type mice, PKCι/λ drives early-LTP and short-term memory, whereas in PKCι/λ-knockout mice, PKMζ compensates by supporting both early- and late-phase processes. Here, we show that PKCι/λ is persistently upregulated during maintenance in two mouse models: PKMζ-conditional knockout mice, and double-knockout mice carrying both conditional deletion of PKCι/λ and constitutive loss of PKMζ. Because PKCι/λ-gene excision is inducible in the double-knockout line, we could characterize the persistent increase of PKCι/λ in late-LTP prior to its deletion. To examine PKCι/λ function, we induced its deletion in the hippocampus. Whereas mutual compensation preserves LTP when either PKCι/λ or PKMζ alone is knocked out, double-knockout of both PKCι/λ and PKMζ eliminates late-LTP. Double-knockout also abolishes spatial long-term memory without affecting short-term memory. Thus, when PKMζ is absent, PKCι/λ persists to maintain hippocampal late-LTP and long-term memory.