Pin1 is a molecular switch that binds and isomerizes phosphorylated Ser/Thr-Pro containing proteins, thereby regulating their biological activity [20
]. The function of Pin1, in neurons in general and at synapses in particular, is largely unknown. Here, we show that Pin1, by regulating translation and inhibiting PKMζ production, is involved in LTP and synaptic plasticity.
Pin1 abundance is increased during cell division and in continuously dividing tumor cell lines and is an independent risk factor for increased tumor grade and metastatic potential [48
]. Pin1 abundance has been implicated in the pathogenesis Alzheimers disease, possibly by mislocalization to hyperphosphorylated tau [22
]. Compared to wild type animals, accelerated apoptotic neurodegeneration occurs in Pin−/−
mice, likely through misregulation of Bim-EL, a BH3 containing, proapoptotic member of the Bcl-2 family [20
]. These effects of Pin1 depend on its cytoplasmic or nuclear location; here, we show that Pin1 is highly abundant and constitutively active in post-synaptic terminals of rodent brain where, under basal conditions, it suppresses protein synthesis. These results suggest that Pin1 may also antagonize neurodegeneration by decreasing the synthesis of potentially toxic proteins such as the transcription factor E-26-like protein 1 (Elk-1), whose dendritic translation induced neuronal apoptosis [49
Under basal conditions, 4E-BP is partially phosphorylated [52
] which is associated with maximal affinity for eIF4E and suppression of translation [17
]. Glutamate activates p70-S6K, PKC and mTOR [50
] all of which phosphorylate 4E-BP at multiple sites [17
]. Akt does not directly act on 4E-BP although it is required for 4E-BP inhibition after growth factor mediated signaling [18
]. Therefore, we propose that under basal conditions in dendrites, active Pin1 binds to and isomerizes minimally (hypo) phosphorylated 4E-BP1, preventing additional phosphorylation and facilitating its inhibitory interactions with eIF4E. Pin1 isomerization inhibits hyperphosphorylation of tau by restricting its access to protein phosphatase 2A (PP2A), and neurons from Pin1−/−
mice accumulate hyperphosphorylated proteins [35
]. Glutamatergic signaling rapidly increases dendritic translation by inducing phosphorylation of Pin1 at Ser16
and 4E-BP at multiple sites including Thr37
] which inactivate Pin1 and 4E-BP, respectively. Despite glutamatergic signaling, we did not observe a change in the degree of interaction between Pin1 and 4E-BP. Similar events were observed between Pin1 and heterogenous nuclear ribonucleoprotein (hnRNP D) in activated immune cells [24
]. Because PKMζ-mediated Ser16
phosphorylation of Pin1 clearly prevents Pin1 interactions with its targets, these results suggest Pin1 PPIase activity may be inactivated through additional mechanisms or that distinct pools of differentially modified Pin1 exist in dendritic compartments.
Classical PKCs indirectly cause the dephosphorylation of 4E-BP1/2 [54
] and consequently suppress translation. After eIF4E dissociates from 4E-BP, it is activated by PKC-mediated phosphorylation [56
]. It is also possible that Pin1 isomerizes hypophosphorylated eIF4E, enhancing its binding to hypophosphorylated 4E-BP1/2 and suppressing translation. Either model is consistent with the increase in basal translation and insensitivity to glutamate activation of translation found in SN derived from Pin1−/−
mice. Because Pin1 has not been implicated in translational control in other cell types, these functional attributes may be unique to neurons.
PKMζ, an atypical PKC, is transcribed independently under the control of an internal promoter within the PKCζ gene [11
], and its mRNA is subsequently transported to dendrites and dendritic spines [57
]. We show that PKCζ/PKMζ, known regulators of synaptic plasticity, influence Pin1 function. Consistent with a direct regulatory role, Pin1 activity was increased in SN treated with myristoylated-PKMζ inhibitor peptides and Pin1 activity was directly inhibited by PKMζ in vitro
, which has previously been shown to inactivate Pin1 substrate binding activity, is one likely site of Pin1 modification by PKMζ [47
In addition to regulating Pin1, PKCζ/PKMζ promote protein synthesis in SN. Specific blockade of PKCζ/PKMζ reduced both basal and Glu-induced translation in WT and Pin1 KO preparations. These results suggest that PKCζ/PKMζ function independently and downstream from Pin1 to maintain dendritic translation after glutamatergic signaling. These results may also explain how PKCζ/PKMζ blockade can markedly and quickly reverse the maintenance phase of L-LTP, as well as memory persistence [7
]. The classical view that protein synthesis is not required for the maintenance of L-LTP [58
] is at odds with this interpretation. However, memory can last a lifetime, whereas the turnover of dendritic proteins is usually on the order of several hours, indicating that on-going protein synthesis must be involved [60
]. Thus, the physiologic significance of this previously unrecognized function of PKMζ is yet to be fully resolved.
PKCζ/PKMζ has been implicated in several cellular signaling cascades, but little is known about their specific substrates. Our data identify Pin1 as a PKCζ/PKMζ substrate and possibly 4E-BP or eIF4E. We propose that Pin1 regulates LTP through bidirectional interactions with PKMζ, whereby GluR-mediated signaling decreases Pin1 activity, leading to an increase in PKMζ abundance. PKMζ maintains translation and suppresses Pin1 through phosphorylation on Ser16. Thus, PKMζ likely contributes to the maintenance of L-LTP through the induction of dendritic translation. Pin1, as an upstream modulator of PKMζ abundance, plays an integral role in the maintenance of L-LTP.