ZPR1 is essential for survival and is required for localization of the survival motor neurons (SMN) protein to Cajal bodies

Spinal muscular atrophy (SMA) is an autosomal recessive disease in humans that is characterized by degeneration of the α-motor neurons of the spinal cord anterior horn. Infants born with SMA exhibit progressive muscle atrophy and paralysis that leads to respiratory failure and early death. Severe forms of SMA are caused by mutation of the survival motor neurons 1 (SMN1) gene (11, 24, 28, 51). Two copies of the human SMN gene (SMN1 and SMN2) that are located in an inverted repeat on chromosome 5q13 have been identified. These two genes are very similar, but transcripts from the SMN2 gene undergo alternative splicing due to a translationally silent nucleotide difference (C → T, codon 280) in exon 7 (31, 36). In severe forms of SMA, the SMN1 gene is deleted and the SMN2 gene predominantly expresses a truncated SMN protein that lacks sequences derived from exon 7. The expression of low levels of full-length SMN protein causes motor neuron degeneration and SMA. SMN has been implicated in the growth, development, and survival of spinal cord motor neurons (34, 42, 44). Biochemical analysis demonstrates that SMN plays an essential role in the assembly and maturation of spliceosomal small nuclear ribonucleoproteins (snRNPs) (35, 43, 53). After transcription, the Sm class of snRNAs (U1, U2, U4, and U5) are exported to the cytoplasm, where they are assembled with seven Sm proteins (SmB/B', SmD1 to SmD3, SmE, SmF, and SmG) to form Sm-core. The SMN protein complex is required for the specific assembly of Sm-core complexes on U snRNAs. This is mediated by SMN interactions with the U snRNA (52) and with the Arg/Gly-rich COOH tails of SmB, SmD1, and SmD3 (9). High-affinity interactions with SMN require that these Sm proteins be modified to contain symmetrical dimethyl-arginine (4, 10). SMN plays a second role in the maturation of snRNPs following the assembly of snRNAs with Sm proteins. The Sm-core undergoes hypermethylation to form the 2′,2′,7′-trimethylguanosine (TMG) 5′-cap that is required for the recruitment of import receptors necessary for the translocation of snRNPs into the nucleus (50). Trimethylguanosine synthase 1 (TSG1), the enzyme that is responsible for the formation of the TMG 5′-cap, interacts with SMN (37). The hypermethylated 5′-cap of U snRNA recruits snurportin 1 to the snRNP complex (21) and snurportin 1 is able to bind both SMN (39) and importin β (21) to facilitate nuclear import of mature snRNP complexes. Recent studies have demonstrated that the zinc finger protein ZPR1 represents a new component of SMN complexes (16). ZPR1 is part of a cytoplasmic snRNP complex that contains SMN, Sm proteins, U snRNA, snurportin 1, and importin β (39). The binding partner of ZPR1 in the SMN complex has not yet been identified, but it has been established that ZPR1 does not directly bind SMN (16). ZPR1 colocalizes with SMN in the nucleus, where both proteins accumulate in gems and Cajal bodies. It is likely that the binding of ZPR1 to SMN complexes is significant because SMN mutations that are associated with SMA disease disrupt the association of ZPR1 with SMN complexes (16). Furthermore, it is established that SMA patients express low levels of ZPR1 (20). The purpose of this study was to examine the role of ZPR1 in mouse development. The effect of ZPR1 deficiency was investigated by disruption of the Zpr1 gene by using homologous recombination and also by Zpr1 gene silencing using RNA interference. We report that ZPR1 deficiency caused reduced growth and increased apoptosis. The effects of ZPR1 deficiency were associated with defects in the subcellular localization of snRNPs.