Gene name - minibrain
Cytological map position - 16F1--16F2
Function - serine/threonine kinase
Keywords - optic lobes, brain
Symbol - mnb
FlyBase ID: FBgn0259168
Genetic map position - 1-58.2
Classification - minibrain family
Cellular location - potentially nuclear
|Recent literature||Shaikh, M. N., Gutierrez-Avino, F., Colonques, J., Ceron, J., Hammerle, B. and Tejedor, F. J. (2016). Minibrain drives the Dacapo dependent cell cycle exit of neurons in the Drosophila brain by promoting asense and prospero expression. Development [Epub ahead of print]. PubMed ID: 27510975
A key issue in neurodevelopment is to understand how precursor cells decide to stop dividing and commence their terminal differentiation at the correct time and place. This study shows that minibrain (mnb), the Drosophila ortholog of the Down syndrome candidate gene MNB/DYRK1A, is transiently expressed in newborn neuronal precursors known as ganglion cells (GCs). Mnb promotes the cell cycle exit of GCs through a dual mechanism that regulates the expression of the cyclin-dependent kinase inhibitor Dacapo, the homolog of vertebrate p27kip1. On the one hand, Mnb upregulates the expression of the proneural transcription factor (TF) Asense, which promotes Dacapo expression. On the other, Mnb induces the expression of Prospero, a homeodomain TF that in turn inhibits the expression of Deadpan, a pan-neural TF that represses dacapo. In addition to its effects on Asense and Prospero, Mnb also promotes the expression of the neuronal-specific RNA regulator Elav, strongly suggesting that Mnb facilitates neuronal differentiation. These actions of Mnb ensure the precise timing of neuronal birth, coupling the mechanisms that regulate neurogenesis, cell cycle control and terminal differentiation of neurons.
In an extensive screen for Drosophila mutants with altered brain structure, a mutant, minibrain, was identified with a markedly reduced brain volume. The brains of adult mutant flies show no gross alterations in neuronal architecture. Major size reductions are seen in the optic lobes (50%-70%), most markedly in the lobula complex and in the central brain (40%-50%). The central brain hemispheres are reduced mainly in their ventral to dorsal and, respectively, anterior and posterior extensions. Axon bundles that project from the lobula complex to the lateral protocerebrum (optic stalk) are visibly thinner in such mutants (Tejedor, 1995).
A chronology of optic lobe development follows, before briefly describing the role of Minibrain in optic lobe cell proliferation.
A major part of the adult insect protocerebrum is the optic lobe, consisting of the first, second and third optic ganglia (known as the lamina, medulla and lobula/lobula plate, respectively). The optic lobes of the adult brain are derived from neuroblasts organized during the third instar larvae into two columnar epithelia: the inner proliferation center (ipc) and the outer proliferation center (opc).
First instar: A total of 30 to 40 precursor cells of the optic anlagen are found superficially in the lateral cell body layer of each brain hemisphere at the time of hatching (the transition from embryonic development to the first larval stage). These cells differ from the remaining cells of the hemisphere due to their somewhat smaller size. Within the early first instar, labelled nuclei appear in these smaller cells. The cells become larger, ellipsoid and epithelially arranged. From the second half of the first instar onwards two different epithelia can be distinguished; these develop into the opc and ipc.
Second instar: No obvious pattern of labeling is detectable until the end of the second instar, when the production of postmitotic neurons begins. At the end of the second instar, there are about 700 neuroblasts in the opc and about 400 neuroblasts in the ipc. Subsequently, a proliferation zone is formed at the medial edge of the opc. Mitotic figures and some small ganglion mother cells can be distinguished adjacent to the neuroblasts. The number of cells produced by this proliferation zone amounts to approximately 40,000, consisting of the medulla, the outermost cell layer of the optic lobe (Hofbauer, 1991).
Third instar: During the middle of the third instar, a second proliferation zone (still part of the opc) develops at the lateral rim of the opc. This zone consists of neuroblasts that have become separated from the main anlage by a deep furrow. Ganglion mother cells and ganglion cells are produced at the inner edge of this crescent. However, this zone yields many fewer cells than found in the medulla anlagen (about 6000 at 25 hours after puparium formation). These cells form the lamina (Hofbauer, 1991). Since mitotically active lamina precursor cells, which normally produce lamina neurons, are absent in mutants that lack retinal innervation, it is concluded that the arrival of photoreceptor axons in the larval brain initiates cell division to produce lamina neurons (Selleck, 1991). For more information about the effect of retinal innervation, see the development of the lamina visual center of the brain.
Two different populations of ganglion cells originate from the lateral proliferation zone of the ipc. Most of the cells differentiate to become the cell body layer of the lobula plate. The other cells participate in the formation of the inner medulla neuropil. About 15000 cells are generated from the lateral proliferation zone. An additional small dorsal proliferation zone develops from the ipc and these cells become part of the lobula cell body layer (Hofbauer, 1991).
Ganglion cells begin to grow axons shortly after their final mitosis: corresponding to the gradient of cell proliferation, there is also a gradient of differentiation in each developing neuropil. The axons of the lamina cells grow centrally, forming a fine fiber sheet at the inner margin of the lamina cell body area at right angles to the medulla neuropil. When differentiation of the lamina starts during the middle of the third instar, the youngest part of the lamina neuropil is in contact with the youngest, most lateral part of the medulla neuropil. The cells of the lobula complex send their axons centrally and form a neuropil opposite and almost parallel to the medulla neuropil. Lobula and lobula plate will develop from this fiber mass. The first fibers connecting the different visual neuropils appear very early, at a time when only the minor part of the ganglion cells are postmitotic. Photoreceptor cell axons start growing into the brain during the middle of the third instar, beginning at the posterior edge of the developing eye and progressing towards the anterior. At the same time, the proliferation of lamina ganglion cell progenitors begins and the newly generated lamina cells become connected with newly ingrowing photoreceptor cells (Hofbauer, 1991).
When the development of optic lobes in minibrain mutants is compared with that of wild type, it is found that the outer proliferation centers (opcs) in mutants attain an irregular structure. The opcs are distinguishable in third instar larvae, appearing in tightly packed, ribbon-like layers of cells, demarcated from the hemisphere. The proliferating neuroblasts appear as a very regular dome-like pattern in wild-type brain. One thick and two thin ribbons of bromodeoxuridine (BrdU) labeled cells (indicating cells that have gone through the DNA synthetic S phase) can be distinguished in the distal brain hemispheres. This regular spatial distribution of BrdU labeled neuroblasts is disturbed in the opcs of mnb mutants. In strong alleles, this ribbon of opc neuroblasts is condensed, and the BrdU-labelled nuclei are not distributed evenly. The regular structure of the thin ribbons disappears, resulting in a scattered distribution of labeled nuclei in these areas. It is important to note that in most cases the altered opc structure does not apparently change the number of labeled cells in comparison with wild type. Nevertheless, abnormally large cells with dark nuclei (probably degenerating cells) are frequently seen near to neuroblast-like cells. In comparison with wild type, mutants are missing a clear demarcation between neuroblasts and gangion cell bodies. In contrast to mutant opcs, no detectable structural alteration is seen in mutant ipcs. From observations of pupal brains, it is concluded that the minibrain phenotype is primarily caused by the inability of mutants to generate a sufficient number of optic lobe and central brain neurons during postembryonic development (Tejedor, 1995).
Bases in 5' UTR - 2030
Exons - 10
Bases in 3' UTR - 940 for 3' UTR of exon 10
The Mnb protein kinases share extensive sequence similarity with kinases involved in the regulation of cell division. This includes three domains similar to shaggy kinases. The proteins have a distinct amino-terminal domain, a kinase core domain, and a characteristic but variable C-terminal. The amino-terminal domain harbors a potential nuclear translocation signal. The core domain extends approximately from amino acid 97 to 425. Several amino acids residues are conserved in all serine-threonine kinases. The Mnb protein kinases share the most extensive sequence similarity with YAK1 protein kinase, with a sequence identity of 33.7%. The C-terminal domains of Minibrain related kinases contains a so-called GAS sequence, rich in glycine, alanine, and serine. The alignment of Shaggy and Minibrain kinases shows that both proteins have an overall identity of 28.8% over 420 amino acids (Tejedor, 1995).
date revised: 7 Sept 97
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