Passage
Alzheimer disease (AD) is a progressive condition affecting memory and cognition that can be pathologically characterized by the presence of neurofibrillary tangles (NFTs) . NFTs, aggregates of hyperphosphorylated tau proteins that have dissociated from microtubules, are believed to contribute to neuronal degradation and subsequent AD symptoms. In general, ribosomes initiate translation of tau mRNA by binding the 7-methylguanosine (m7Gpp) cap present at the end of the 5′ untranslated region (UTR) of the mRNA molecule. However, researchers believe that tau protein translation initiation can occur without recognition of the m7Gpp cap. This alternate mechanism, referred to as cap-independent translation, indicates that ribosomes are recruited by internal ribosome entry sites (IRES) generally located in the 5′ UTR of the mRNA sequence.To determine the initiation mechanism of tau mRNA translation, neuroblastoma cells of AD mice were transfected with two different DNA constructs (Figure 1) . Construct 1 contained the 5′ UTR of tau mRNA isoforms (v1 and v2) extracted from cells displaying NFTs. Construct 2 included the 5′ UTR of the β-globulin gene, which normally codes for a plasma protein. The luciferase reporter genes, Renillla and Photinus, were also included in the two constructs.
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Figure 1 DNA constructs containing β-globulin or tau gene used to transfect neuroblastoma cellsLuciferase gene expression from the constructs was monitored in the absence and presence of a synthetic m7Gpp cap analog (Figure 2) . Translation of Renillla luciferase mRNA depended on ribosomal binding to the m7Gpp cap of the mRNA, but translation of Photinus luciferase required ribosome recruitment by an IRES in the 5′ UTR of the adjacent gene.
Figure 2 Luciferase activity in neuroblastoma cells exposed to m7Gpp cap analogIn a second study, researchers used siRNA to target initiation factor eIF4E, a peptide translated in a cap-dependent manner that facilitates the binding of ribosomes to the m7Gpp cap. Expression of factor eIF4E, elongation factor eEF2K, and tau v2 protein was assessed in siRNA-treated cells (Figure 3) .
Figure 3 Western blot of eIF4E, eEF2K, and tau v2 extracted from non-AD cells treated with eIF4E siRNA
Adapted from Veo BL, Krushel LA. Translation initiation of the human tau mRNA through an internal ribosomal entry site. J Alzheimers Dis. 2009; 16(2) :271-5.
-The data in Figure 2 suggest that ribosomes interact with the:

Question

Passage
Alzheimer disease (AD) is a progressive condition affecting memory and cognition that can be pathologically characterized by the presence of neurofibrillary tangles (NFTs) . NFTs, aggregates of hyperphosphorylated tau proteins that have dissociated from microtubules, are believed to contribute to neuronal degradation and subsequent AD symptoms. In general, ribosomes initiate translation of tau mRNA by binding the 7-methylguanosine (m7Gpp) cap present at the end of the 5′ untranslated region (UTR) of the mRNA molecule. However, researchers believe that tau protein translation initiation can occur without recognition of the m7Gpp cap. This alternate mechanism, referred to as cap-independent translation, indicates that ribosomes are recruited by internal ribosome entry sites (IRES) generally located in the 5′ UTR of the mRNA sequence.To determine the initiation mechanism of tau mRNA translation, neuroblastoma cells of AD mice were transfected with two different DNA constructs (Figure 1) . Construct 1 contained the 5′ UTR of tau mRNA isoforms (v1 and v2) extracted from cells displaying NFTs. Construct 2 included the 5′ UTR of the β-globulin gene, which normally codes for a plasma protein. The luciferase reporter genes, Renillla and Photinus, were also included in the two constructs.
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Figure 1 DNA constructs containing β-globulin or tau gene used to transfect neuroblastoma cellsLuciferase gene expression from the constructs was monitored in the absence and presence of a synthetic m7Gpp cap analog (Figure 2) . Translation of Renillla luciferase mRNA depended on ribosomal binding to the m7Gpp cap of the mRNA, but translation of Photinus luciferase required ribosome recruitment by an IRES in the 5′ UTR of the adjacent gene.

Figure 2 Luciferase activity in neuroblastoma cells exposed to m7Gpp cap analogIn a second study, researchers used siRNA to target initiation factor eIF4E, a peptide translated in a cap-dependent manner that facilitates the binding of ribosomes to the m7Gpp cap. Expression of factor eIF4E, elongation factor eEF2K, and tau v2 protein was assessed in siRNA-treated cells (Figure 3) .

Figure 3 Western blot of eIF4E, eEF2K, and tau v2 extracted from non-AD cells treated with eIF4E siRNA
Adapted from Veo BL, Krushel LA. Translation initiation of the human tau mRNA through an internal ribosomal entry site. J Alzheimers Dis. 2009; 16(2) :271-5.
-The data in Figure 2 suggest that ribosomes interact with the:

Quizplus · Accepted Answer

11ecba2d_0f6c_8f6f_a3bf_2be0fc644cc7_MD0008_00 After eukaryotic pre-mRNA is produced, it is converted to mature mRNA to prevent degradation in the cytoplasm: The 5′ end of the mRNA is "capped" with m7Gpp. A poly-A tail consisting of adenosine nucleotides is added to the 3′ end of mRNA. In general, ribosomes initiate cap-dependent translation by binding the 5′ cap of eukaryotic mRNA. However, the passage introduces a cap-independent process in which ribosomes initiate translation by binding IRES within the 5′ UTR of mRNA. Figure 2 illustrates how increasing concentration of an m7Gpp cap analog affects the luciferase activity of the proteins translated from the mRNA of Constructs 1 (tau 5′ UTR) and 2 (β-globulin 5′ UTR). Paragraph 3 states that Renilla is translated only through a cap-dependent process. At low concentrations of m7Gpp cap analog, ribosomes bind to the 5′ cap on Constructs 1 and 2(Choice B), and Renilla is translated from both constructs. (Note that the ribosome then dissociates by reading the stop codon in the Renilla mRNA.) However, Renilla protein expression decreases with increasing concentrations of cap analog because the cap analog outcompetes the 5′ caps on mRNA transcripts, blocking the ribosomal binding site. Based on the passage, Photinus is translated only via a cap-independent process. Figure 2 shows that Photinus expression is initially high and rises further despite increasing concentrations of cap analog, indicating that ribosomes bind the IRES within the tau 5′ UTR of Construct 1 to enable cap-independent translation of Photinus. (Note that the ribosome then dissociates by reading the stop codon in the Photinus mRNA.) However, Photinus expression remains near zero for Construct 2 because β-globulin lacks IRES in its 5′ UTR for the ribosomes to bind (Choices A and C). Educational objective: Eukaryotic translation initiation begins when ribosomes bind the m7Gpp cap at the 5′ end of the mRNA sequence; however, cap-independent processes do exist.

Passage Alzheimer Disease (AD) Is a Progressive Condition Affecting Memory and and Cognition