DYRK Quick Guide

DYRK protein kinases

Ulf Soppa and Walter Becker*

What are DYRKs?

 The designation DYRK (for ‘dual-specificity tyrosine (Y) phosphorylation-regulated kinase’) highlights the peculiar biochemical characteristics of this protein kinase family. The hallmark of dual-specificity protein kinases is their ability to phosphorylate aromatic (tyrosine) as well as aliphatic (serine and threonine) amino acid residues.

But what is the point of dual specificity?

Like other functionally related kinases, kinases of the DYRK family need to be activated before they can phosphorylate their substrates. To attain full catalytic activity, DYRKs require phosphorylation at the second tyrosine residue of a conserved YxY motif in their activation loop. In contrast to mitogen-activated protein kinases (MAPKs), where activation depends on phosphorylation of a TxY motif by upstream kinases, DYRKs activate themselves during translation by intramolecular (cis) autophosphorylation (Figure 1A). The mature and active kinase then phosphorylates substrates on serine and threonine residues in a consensus motif that has a proline residue carboxy-terminal to the target site (RX1-2S/TP). In contrast to former assumptions, mammalian DYRK1A (at least) retains tyrosine kinase activity as a mature protein and is able to autophosphorylate tyrosines outside the activation loop.

How were the DYRKs discovered?

Several DYRK-related kinases were identified as developmental regulators in genetically tractable organisms. The founding member of the DYRK family is Yak1, which turned up in a screen for negative growth regulators in baker’s yeast (Saccharomyces cerevisiae). Upregulation of Yak1 arrests cell proliferation and stimulates the cellular stress response to promote cell survival. In fission yeast (Schizosaccharomyces pombe), a gradient of the DYRK homologue kinase Pom1 controls cell polarity and cytokinesis. Furthermore, overexpression of YakA causes cell[1]cycle arrest and promotes transition from the unicellular to a multicellular phase in the social amoeba Dictyostelium discoideum. With respect to multicellular organisms, a screen of Drosophila mutants identified a Yak1/DYRK1-related kinase that was termed minibrain (mnb): mnb mutant fl ies have a reduced brain size due to abnormal neuroblast proliferation.

What about mammalian DYRKs?

The five mammalian DYRK isoforms are phylogenetically categorized as class I DYRKs (DYRK1A and DYRK1B) and class II DYRKs (DYRK2, DYRK3 and DYRK4). The high degree of sequence similarity with DYRKs in lower eukaryotes suggests that the functional properties of mammalian DYRKs are conserved. Indeed, they also act as negative regulators of the cell cycle. DYRK1A in particular regulates the length of G1 phase in various cell types and modulates the decision between cell-cycle re-entry and cell-cycle exit (Figure 1B). Thus, DYRK1A seems to be a master regulator of cell proliferation and quiescence or differentiation. DYRK1B appears to have similar functions in a more limited spectrum of cell types. For instance, DYRK1B is upregulated when myoblasts differentiate and is involved in muscle development. Furthermore, overexpression or activation of DYRK1B in cancer or during the course of a stress response can drive cell cycle arrest to allow repair and/or cell survival. Among class II DYRKs, DYRK2 is also a negative regulator of S-phase entry and was found to be underexpressed in tumors. DYRK3 plays a role in erythropoiesis, where it reduces cell cycling and expansion of early stage erythroblasts. Although the function of DYRK4 remains obscure, it is obvious that cell-cycle regulation is a major function of mammalian DYRKs.

Do DYRKs have functions other than cell-cycle control?

Absolutely! As the best characterized member of the family, DYRK1A has been virtually overloaded with diverse functions in chromatin remodeling, transcriptional control, alternative mRNA splicing, homocysteine metabolism, circadian timekeeping, cardiomyocyte hypertrophy, electrophysiological properties and synaptic function in neurons, which is by no means a comprehensive list. Transgenic mouse strains with an extra copy of the DYRK1A gene as well as mice with a heterozygous loss-of-function allele of DYRK1A have provided insights into the roles of DYRK1A in neuronal development, synaptic transmission and neurodegeneration. Although some of the proposed functions require further confirmation, DYRK1A is undoubtedly a pleiotropic kinase with important roles in many different cellular processes.

How do DYRKs fulfi l these functions?

DYRKs alter the turnover of target proteins by phosphorylating residues that inhibit or activate protein degradation. For example, DYRK1A and DYRK1B phosphorylate the cell-cycle regulators p27Kip1 and cyclin D1 at sites that affect their protein turnover during G1 phase (Figure 1B). Besides direct phosphorylation, DYRKs also operate as priming kinases for subsequent phosphorylation of targets by glycogen synthase kinase 3 (GSK3), as exemplified in the case of the c-Myconcoprotein and the microtubule[1]associated protein tau. DYRKs, in particular DYRK1A, regulate gene expression on multiple levels, including direct effects on transcription factors, effects on histone modification and the phosphorylation of the carboxy-terminal domain of RNA polymerase II. The protein kinase activity appears to be essential for most functions of DYRKs, although the non-catalytic domains can act as scaffolds for signaling complexes.

Are DYRKs clinically relevant?

DYRK1A attracts vast attention due to its association with genetic diseases. Because of its localization on chromosome 21, the DYRK1A gene is 1.5-fold overexpressed in trisomy 21. Increased activity of DYRK1A has been correlated with abnormal brain development, cognitive disabilities and an early onset of Alzheimer’s disease in individuals with Down syndrome. In particular, overexpression of DYRK1A impairs neurodevelopment by deregulating cell-cycle progression and differentiation of neuronal precursors. Hyperphosphorylation of tau and other Alzheimer-related neuronal target proteins by DYRK1A potentially contributes to early neurodegeneration in Down syndrome as well as to Alzheimer’s disease fin the normal population (Figure 1C). The proposed role in Down syndrome implies that DYRK1A is a highly dosage-sensitive gene. In support of this assumption, heterozygous disruption of the DYRK1A gene causes a rare syndrome called MRD7 (mental retardation, autosomal dominant 7, OMIM entry #614104). These patients are characterized by microcephaly, severe mental retardation, autistic behavior and seizures. Moreover, de novo mutations of DYRK1A have been identified in families with cases of sporadic autism spectrum disorder. Taken together, the clinical evidence strongly supports the idea that DYRK1A has a key role in brain development. DYRK1B has received attention as a potential oncogenic driver, because the DYRK1B gene is overexpressed in certain cancer types. New interest was recently sparked by the discovery that a gain-of-function mutation of the DYRK1B gene is causative for a familial form of the metabolic syndrome.

Might some DYRKs serve as drug targets?

The increased activity of DYRK1A in Down syndrome is in principle accessible to pharmacological intervention with kinase inhibitors. However, the delicate dose-dependency observed for the phenotypic effects of the DYRK1A gene counsels caution, at least regarding prenatal therapy. The accumulating evidence for a role of DYRK1A in Alzheimer’s disease has further boosted the search for potent and specifi c inhibitors of DYRK1A. Beyond that, DYRK1A and DYRK1B are considered as therapeutic targets in cancer therapy, because they can drive cell-cycle exit and quiescence of tumour cells. Hence, DYRK1 inhibitors might promote cell-cycle re-entry of quiescent tumour cells and thereby increase their susceptibility to chemotherapy or radiotherapy.

Where can I find out more?

Aranda, S., Laguna, A., and de la Luna, S. (2011). DYRK family of protein kinases: evolutionary relationships, biochemical properties, and functional roles. FASEB J. 25, 449–462. Becker, W. (2012). Emerging role of DYRK family protein kinases as regulators of protein stability in cell cycle control. Cell Cycle 11, 3389–3394. Hu, J., Deng, H., Friedman, E.A. (2013). Ovarian cancer cells, not normal cells, are damaged by Mirk/Dyrk1B kinase inhibition. Int. J. Cancer 132, 2258–2269. Soppa, U., Schumacher, J., Florencio Ortiz, V., Pasqualon, T., Tejedor, F.J., and Becker, W. (2014). The Down syndrome-related protein kinase DYRK1A phosphorylates p27(Kip1) and Cyclin D1 and induces cell cycle exit and neuronal differentiation. Cell Cycle 13, 2084–2100. Institute of Pharmacology and Toxicology, Medical Faculty of RWTH Aachen University, Wendlingweg 2, 52074 Aachen, Germany *E-mail: wbecker@ukaachen.de