In contrast to cyclin D1 nulls (cD1 -/-), mice without cyclin D2 (cD2 -/-) lack cerebellar stellate interneurons; the reason for this is unknown. In the present study in cortex, we found a disproportionate loss of parvalbumin (PV) interneurons in cD2 -/-mice. This selective reduction in PV subtypes was associated with reduced frequency of GABA-mediated inhibitory postsynaptic currents in pyramidal neurons, as measured by voltage-clamp recordings, and increased cortical sharp activity in the EEGs of awake-behaving cD2 -/-mice. Cell cycle regulation was examined in the medial ganglionic eminence (MGE), the major source of PV interneurons in mouse brain, and differences between cD2 -/-and cD1 -/-suggested that cD2 promotes subventricular zone (SVZ) divisions, exerting a stronger inhibitory influence on the p27 Cdk-inhibitor (Cdkn1b) to delay cell cycle exit of progenitors. We propose that cD2 promotes transit-amplifying divisions in the SVZ and that these ensure proper output of at least a subset of PV interneurons.

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S phase entry, I.e. start of DNA replication, Is a crucial step in proliferation. Inhibition of S phase entry correlates with Inbibition of hypusine formation, an event affecting only the eukaryotie Initiation factor $A (elF4A), Its hypuslnecontalning sequence wus postulated to authorize polysomal utilization of specific trauscripts for proteins necessary to enable DNA replication. Using ndmodne to reversibly suppress the hyposinc-forming deoxyhypusyi hydmxylase (E.C. 1.1¢99.29) in cells while differentially displaying their polysomal versus non-polysemal mRNA popolations, we report the detection and cla~!fleation of several mRNA species that indeed disappear from and reappear at !lo1-ysomes in con~rt with inhibition and d_!s!nhibition, respectively, of hypusine formation. Based on initial sequence data, two trauslationally controlled enzymes, both critical for proliferation, are identified us candicate wodncts of such mRNAs, methionine adenosyitrausferuse (E.C 2.$.1.6) and cytochrome-c oxiduse 0gC 1.9.3.1) subunit 1. The existence of such potative hyposinedependent messenger nnclde adds (/O, mas) prevides the basis for a proposal on their molecular function in onset of multiplication.Key words: Post-translational modification; eIF-5A; Hypusine; Cell cycle control; Translational control; mRNA I, IntroductionThe unique amino acid hypusine [~-(4-amino-2(R).hydroxybutyl)-t.-lysine] is formed post-translationally by the sequential action of deoxyhypusyl synthase (DOHS) and deoxyhypusyl hydroxylase (DOHH) on a single lysyl residue in only one cellular protein, the eukaryotic initiation factor 5A (elF-SA) [I-3]. The hypusine-containing domain shows remarkable interspecies conservation, and a sequence of 12 amino acids containing the hypusine residue has been strictly conserved throughout eukaryotic evolution, from fungi to plants, insects, and mammals [3]. Hypusine formation, which has not been demonstrated in prokaryotes [2], is an absolute requirement for growth of $. cerevesiae [4], and a number of *Correspondin8 authors, Fax: (I) (212) 746 0300 and Fax: (I) (212) 746 6382, respectively.'For Stephen Biko (December 18, 1944.-September 12, 1977.reports have established correlations between hypusine formation and the proliferation of eukaryotic cells (see [2,3]). However, the exact physiological function of this presumed translation initiatic~n factor is unknown, and at present it even is unclear whether elF-SA functions at the pretranslational and/ or the translational level [3].Deoxyhypusyl hydroxylase (E.C. 1.14.99.29), which catalyses the final step of hypusine biosynthesis, is apparently related [5,6] to other protein hydroxy]ating enzymes such as prolyl 4-hydroxylase (E.C 1.14.11.2) [7] and is effectively inhibited by mimosine (3-(N-(3-hydroxypyridin-4-one))-2[S]-aminopropionic acid), both in vitro and in culture [3]. This coincides with proliferative arrest in the late GI phase ofthe cell cycle [3,[8][9][10][11]. Fast recovery ofcellular DOHH activity occurs upon mimo,,;ine removal, an effect parallele...