Inhibition of mitogen-activated protein kinase and stimulation of Akt kinase signaling pathways: Two approaches with therapeutic potential in the treatment of neurodegenerative disease

Abstract

The neurodegenerative diseases of adulthood, including Alzheimer's disease (AD) and Parkinson's disease (PD), pose an enormous and growing public health burden. Although effective symptomatic treatments exist for PD, and, to a lesser extent, for AD, there is no therapy for these disorders which will forestall their progression. With the rise of the concept of programmed cell death (PCD) came the realization that even in the absence of complete knowledge of proximate causes neuroprotection may nevertheless be possible by targeting the pathways of PCD. One set of signaling pathways that have been implicated in cell death are the mitogen-activated protein kinase (MAPK) pathways. The possibility of blocking these pathways and thereby providing neuroprotection has recently been put to the test in a clinical trial of a mixed lineage kinase inhibitor in the treatment of PD. Unfortunately, this trial failed to demonstrate a protective effect. Based on considerations related to the implementation of the trial, it would be premature to conclude that inhibition of MAPK signaling is a failed strategy. In spite of these negative results, the MAPK and related kinase pathways retain their importance as potential targets in PD. In relation to pathogenesis, the discovery of mutations in the mixed lineage kinase (MLK)-like kinase leucine-rich repeat kinase 2 (LRRK2) suggests a role for these kinases in regulating the viability of dopamine neurons. In relation to treatment, the survival signaling kinase Akt has been demonstrated in vivo to mediate striking neurotrophic and antiapoptotic effects. Thus, it is likely that therapeutic targets related to these kinase signaling pathways will emerge.

Figure 1. The mitogen-activated protein kinase (MAPK) signaling pathways

MAPK signaling pathways are organized in a three tier structure. JNK, which mediates phosphorylation and activation of c-jun, is a MAPK in the first tier. Also in this group of kinases are the p38 MAPKs and the ERKs (not shown). The MAPKs are activated by phosphorylation of Tyr and Thr residues by the MAPK kinases in the second tier. In this tier, MKK4 and MKK7, in particular, mediate activation of JNK. Upstream to the MAPKKs, in the third tier, several families of kinases have been reported to activate JNKs (see the text). Among these kinases, the MLKs have been implicated in neuron death.

Figure 2. The protein domain structures of MLK3 and DLK

MLK3 and DLK provide representative examples of the domain structures of the MLKs. Common to all MLKs is the kinase domain (green), which mediates phosphorylation, and the leucine zipper domain (blue), which mediates protein-protein dimerization. Exemplified here by MLK3, the MLKs1–4 contain an N-terminal Src-homology-3 (SH3) domain (brown), which may be involved in autoregulatory mechanisms. MLKs1–4 also contain a Cdc42/rac-interactive binding (CRIB) motif (red) that is believed to mediate activation by Cdc42.

Figure 3. The domain structure of the AGC kinases

The protein isoforms of Akt, and other AGC kinases, have a central kinase domain, and a C-terminal hydrophobic domain. Abbreviations: PH: pleckstrin homology domain, HM: hydrophobic motif, SGK1: serum and glucocorticoid induced protein kinase, PDK1: 3-phosphoinsitide-dependent kinase 1. Adapted from (Hanada et al., 2004), with permission.

Figure 4. Akt signaling pathways

The physiologic pathways for the activation of Akt, as described in the text, are shown on the left side of the Figure. On the right side are shown the pathways for constitutive activation of Akt following N-terminal incorporation of a myristoylation signal (yellow oval). The myristoylation signal is targeted by N-myristoyltransferase (NMT) for the post-translational transfer of myristate onto the N-terminal glycine of the modified protein. Myristic acid is a C14 saturated fatty acid which targets the protein to the inner surface of the cell membrane, where it is phosphorylated, even in the absence of PI3K activation. This myristoylated form of Akt was used in our in vivo studies on SN dopamine neurons, described in the text (Ries et al., 2006).

Figure 5. Anti-apoptotic mechanisms of Akt

Akt has been described to inhibit apoptosis at multiple levels: signaling pathways, transcription factors, and, in the intrinsic pathway of PCD, at pre-mitochondrial, mitochondrial, and post-mitochondrial levels. Detailed mechanisms for some of these pathways are presented in the text. For more complete reviews, see (Datta et al., 1999; Brunet et al., 2001; Downward, 2004).

Figure 6. Effect of AAV Myr-Akt on SN dopaminergic neurons in a 22 month old mouse

Aged mice were injected into the substantia nigra with either AAV GFP or AAV Myr-Akt and sacrificed for morphologic analysis 7 weeks later. Myr-Akt induced a 31% increase in the total volume of the SN as compared to the contralateral non-injected SN. There was also an increase in the size of individual TH-positive neurons. Myr-Akt also induced a vigorous sprouting response in aged mice, with a 37% increase in striatal TH optical density values on the AAV Myr-Akt injected side in comparison to contralateral control and both striata of AAV GFP injected mice (not shown).