p21 activated kinases (PAKs) are members of a family of enzymes. They serve as targets for the small GTP binding proteins CDC42 and Rac and have been implicated in a wide range of biological activities.
P21-activated kinases (PAKs), members of a growing family of serine/threonine kinases, are present in all eukaryotes. They participate in a great variety of cellular processes, including control of the cell cycle, dynamics of the cytoskeleton, regulation of apoptosis, and control of transcription. PAK kinase activity is regulated by members of the Rho family of GTPases, specifically Cdc42 and Rac. These GTPases bind to PAK kinases solely in their active forms. PAKs are kept m the "off" state by interacting with an N-terminal autoreguiatory fragment that forces the kinase to adopt an inactive fold and prevents the phosphorylation of threonine 423, necessary for activation. GTP-bound Cdc42 or Raccan interactwith partofdie autoreguiatory region,prying it away from the kinase and enabling autophosphorylation and structural rearrangement of the kinase domain into an active fold.
PAK1, the best-characterized member of the PAK family, is a serine/threonine protein kinase and the main downstream effector of the Rho GTPase Cdc42 and Rac. Over the years, the mammalian PAK family has grown to include five more members. On the basis of biochemical and structural features, the PAK family of higher eukaryotes is subdivided into two classes: group 1 includes PAKs 1-3, and group 2 is comprised of PAKs 4-6. the members of group 1 PAKs share higher degree of homology than group 2 PAKs. Although the latter have also been termed PAKs, group 2 PAKs differ strikingly in their structural organization and regulation from group 1 PAKs. In addition, each group has unique cellular functions.
Role of group 1 PAKs in the central nervous system
PAKs plays an important role in controlling numerous steps during brain development, including neuronal polarization, neurite outgrowth, axon guidance, and dendrite initiation, outgrowth and branching.
1. Neuronal polarity
Neurons are highly polarized cells and contain two distinct types of processes, a single axon and several dendrites. These two compartments acquire specific characteristics that allow neurons to receive, process, and transmit information. Neuronal polarization is crucial for correct migration and subsequent specification of axons and dendrites in order to establish a complex and organized network. Many experiments using cultured embryonic hippocampal neurons have revealed that differentiation of these neurons can be separated into five stages based on a series of morphological changes they undergo. Shortly after plating, the neurons resemble flat epithelial cells with a central nucleus and small protrusion veils and a few spikes (stage 1). These truncated protrusions have growth cones at their tips, and rapidly develop into several immature short neurites, which are morphologically indistinguishable from each other (stage 2). Subsequently, one neurite grows at a rapid rate, immediately breaks the initial morphological symmetry, and establishes the polarity (stage 3). A few days later, the neurons continue to extend their neurites (stage 4). Approximately seven days after plating, neurons form synaptic contacts through dendritic spines and axon terminals (stage 5).
Several studies have demonstrated PAK-regulated cell polarity in different cell types and species. In recent years, compelling evidence has suggested that PAK1 plays an important role in regulating neuronal polarity. The loss and gain-of-function approach reveals that PAK1 is essential for the specification of an axon and dendrites in hippocampal neurons. Uniform hyperactivation of PAK1 at the membrane of all neurites or loss of PAK1 expression disrupts both neuronal morphology and the specification of an axon and dendrites. Moreover, neurite outgrowth is suppressed after PAK1 inhibition. Together, these data indicate that the localization of active PAK1 is pivotal for neuronal polarization. The distribution and levels of active PAK1 change during the neuronal polarization. In stage 1 and 2 neurons, active PAK1 is not polarized and is primarily located in perinuclear regions. In the early stage 3, active PAK1 is preferentially enriched in one neurite which subsequently undergoes rapid growth and polarity establishment. At the same time, active PAK1 is down-regulated in the other neurites which subsequently form dendrites. This axonal specific accumulation of active PAK1 appears to be temporary. Once the axon specification is achieved and the neurons reach the stage 4 or 5 boundary, the active PAK1 is localized in dendritic spines.
2. Neurite outgrowth
Nervous system function is dependent upon a complex neuronal network, which requires adequate neuronal connections and remodeling. The extension of neurites toward the proper targets is one of the bases for the establishment of the patterning and specificity of these connections between neurons. Neurite outgrowth is thus an enormously important process during neuronal development. The Rho-GTPase Cdc42 and Rac have been implicated as critical mediators required for neurite initiation and outgrowth. PAK1, a main effector downstream of Cdc42 and Rac, has been shown to play an essential role in these processes. For example, plasma membrane targeting of PAK1 induces neurites outgrowth following NGF stimulation in the neuronal cell line PC12. The membrane localization is critical since PAK1 is not sufficient to induce neurite outgrowth when cytoplasmically expressed. Althrough this neurite induction is largely independent of the kinase activity of PAK1, it does require structural elements present in both the N- and C-terminus. In a recent study, loss of PAK1 expression using an shRNA approach in hippocampal neurons causes a pronounced global inhibition of neurite outgrowth.
3. Neuronal migration
Advances have been made revealing the important role of PAK kinases in controlling neuronal migration both in vitro and in vivo. PAK1 acts as an upstream regulator of several factors affecting the organization and dynamics of cytoskeleton rearrangement during cell migration. Mounting evidence supports a pivotal role of LIMK and cofilin in the developing brain, especially for neuronal migration. PAK to LIMK to cofilin has been shown to signal together in controlling this process. PAK1 activated by GTP-bound Cdc42 or Rac transphosphorylates LIMK, and increases LIMK-mediated phosphorylation and inactivation of the actin-regulatory protein cofilin, resulting in decreased depolymerizationof F-actin. A PAK1 specific inhibitor blocks LIMK induced cytoskeletal change. The involvement of PAK1 in neuronal migration has also been demonstrated in migrating chicken embryonic cerebellum, where the expression of PAK increases the spreading of the leading process.
Zhao, Z. (2010). Role of p21 activated kinases in the neuroendocrine actions of estrogen (Doctoral dissertation, Northwestern University).