Plk5, a polo box domain-only protein with specific roles in neuron differentiation and glioblastoma suppression.

Polo was originally identified in Drosophila as a mutant with mitotic and meiotic defects in the microtubule spindle (51). This mutation was later mapped to a serine-threonine protein kinase specifically concentrated in dividing cells (32). Polo-like kinases (Plks) are found in all eukaryotes and are characterized by a highly conserved kinase domain and one or two C-terminal polo box domains (PBDs). These include the Plk1 subfamily, containing Drosophila polo and mammalian Plk1; the SAK subfamily, containing Drosophila SAK and mammalian Plk4, both of which are widely distributed across the eukaryotes; and the Plk2 subfamily, containing vertebrate Plk2 and Plk3 and also including homologs from echinoderms (4, 8, 10, 35). The PBD participates in subcellular localization and partner interaction (16, 54). The founding member of the family, Plk1, localizes to the cytoplasm and centrosomes in interphase and concentrates to the kinetochores and the cytokinesis bridge during cell division. This protein has major functions in centrosome maturation, mitotic entry, and cytokinesis (4, 8, 40, 53). The other members of the family are less studied. Plk4 (Sak) is a critical regulator required for centriole duplication both in Drosophila and in mammals (9, 21). Plk2 (also known as Snk) localizes to the centrosome and may also participate in centrosome biology and S-phase checkpoints (37). Plk3 (Fnk or Prk) activity peaks in G1 and localizes to the nucleolus in interphase. This protein may function in S-phase entry (64), and it is activated in response to replicative stress and genotoxic insults leading to apoptosis in a p53-dependent manner (56, 61, 62). The three Plk subfamilies have distinct functions and operate in multiple cell types. Their expression is regulated differentially in cells and tissues and in response to several cellular processes and stimuli (59). Whereas Plk1 and Plk4 are found only in dividing cells, Plk2 and Plk3 are also expressed in neurons and other, nondividing differentiated cells (47, 55). Plk2 and perhaps Plk3 seem to have a crucial function in modulating synaptic plasticity in neuronal dendritic spines through the phosphorylation of the spindle-associated protein SPAR (3, 46). Plk2 also phosphorylates the neuronal alpha-synuclein, a phospho-protein accumulated in several neurological diseases, such as Parkinson's and dementia with Lewy bodies (23). Given the critical roles of Plk1 during cell division and its frequent overexpression in human tumors (15), it has been considered a cancer therapeutic target (34, 50). Several small-molecule Plk1 inhibitors that compete with ATP at the kinase domain are in clinical trials (39), and efforts are ongoing to identify and validate small molecules that bind to the PBD to inhibit the interaction of Plk1 with its partners (39, 42). These efforts, however, must take into account that inhibition of Plk2, Plk3, or Plk4 may lead to tumor development, as these less-known Plks may function as tumor suppressors in specific cell types (28, 39, 52, 63). The mammalian genome contains a fifth member of the Plk family, Plk5, initially described as a putative protein encoded by a pseudogene (12) and recently linked to DNA damage (2). Although mouse cells express a full-length Plk5 similar in size to Plk2 or Plk3, human cells express a shorter PLK5 form in which the kinase domain is disrupted due to a stop codon in exon 6, which is followed by an in-frame ATG codon immediately downstream, in the boundary between exons 6 and 7. However, both the murine (long) and human (short) forms display similar cellular functions, and the kinase domain of the murine protein seems to be inactive in kinase assays. Plk5 is specifically expressed in the eye and the brain as well as the ovary in the mouse. Interference with Plk5 expression in PC12 cells and primary hippocampus neuroblasts results in reduced axon growth and neurite formation upon stimulation with neuronal growth factors, suggesting a role for Plk5 in the proper formation of neurite processes. Accordingly, the human PLK5 (hPLK5) protein is detected in the cytoplasm of neurons and glia. Interestingly, PLK5 is dramatically downregulated in human brain tumors, and its expression is inactivated by hypermethylation of the PLK5 promoter region. Reexpression of PLK5 in glioblastoma multiforme (GBM) cells results in the induction of apoptosis, suggesting a therapeutic potential of this protein.