Parkin uses the UPS to ship off dysfunctional mitochondria

Parkin is a ubiquitin E3 ligase that is implicated in familial Parkinson disease (PD). Previous studies have established its role in mitophagy, a pathway whereby dysfunctional mitochondria are targeted for autophagic degradation. We recently reported that a major function of Parkin in dysfunctional mitochondria is to activate the ubiquitin-proteasome system (UPS) for proteolysis of multiple outer membrane proteins, and that such activation of the UPS is a critical step in Parkin-mediated mitophagy. Here, we discuss the possible roles of the UPS in mitophagy and the pathogenesis of PD

Transitions from AFDC to SSI Prior to Welfare Reform – Policy Brief

The Supplemental Security Income (SSI) and Temporary Assistance for Needy Families (TANF) programs serve overlapping target groups. SSI serves adults and children with disabilities from low-income families, while TANF serves low-income families with children. Consequently, policy changes in one program can affect the other. The target group for Aid to Families with Dependent Children (AFDC), TANF’s predecessor, also overlapped with SSI’s target group. Many have anticipated that the replacement of AFDC with TANF in August 1996 would eventually increase SSI participation as TANF recipients with disabilities sought SSI benefits to avoid TANF work requirements and time limits

Impacts of Expanding Health Care Coverage on the Employment and Earnings of Participants in the SSI Work Incentive Program - Policy Brief

While people with disabilities often say that a loss of public health insurance is a deterrent to work, it is rare to find situations in which they might actually exhibit such a behavioral response to a change in access. Expansions in the income threshold for SSI work incentives program (Sections 1619(a) and (b)) provide an opportunity to observe such a response. Section 1619(b) allows SSI recipients to maintain Medicaid eligibility even if their income is above the level that makes them ineligible for SSI payments. If earnings increase beyond the 1619(b) threshold, however, the person loses their SSI and Medicaid eligibility. Section 1619(b) income thresholds vary significantly across states and over time. Stapleton and Tucker (2000) use the variation in Section 1619(b) income thresholds to examine the employment, earnings and program participation patterns of SSI recipients who have incomes near the threshold level for their state. They find strong evidence that many SSI recipients restrain their earnings to stay below the 1619(b) threshold. It is important to note, however, that the findings only provide evidence on the behavior of a small portion of the population with disabilities (i.e., SSI recipients who work). Nonetheless, this evidence seems to provide strong empirical support for the hypothesis that lack of access to health insurance is an important work disincentive for people with disabilities. They also find that 1619(b) participation varies significantly from month to month. Consequently, cross-sectional estimates on the share of SSI recipients participating in 1619(b) significantly understate the share of SSI recipients who ever participate. These findings are consistent with previous findings that cross-sectional estimates of employment tend to understate multi-period employment patterns for the broader population with disabilities

Emerging functions of mammalian mitochondrial fusion and fission

Mitochondria provide a myriad of services to the cell, including energy production, calcium buffering and regulation of apoptosis. How these diverse functions are coordinated among the hundreds of mitochondria in a given cell is largely unknown, but is probably dependent on the dynamic nature of mitochondria. In this review, we explore the latest developments in mitochondrial dynamics in mammals. These studies indicate that mitofusins and OPA1 are essential for mitochondrial fusion, whereas Fis1 and Drp1 are essential for mitochondrial fission. The overall morphology of the mitochondrial population depends on the relative activities of these two sets of proteins. In addition to the regulation of mitochondrial shape, these molecules also play important roles in cell and tissue physiology. Perturbation of mitochondrial fusion results in defects in mitochondrial membrane potential and respiration, poor cell growth and increased susceptibility to cell death. These cellular observations may explain why mitochondrial fusion is essential for embryonic development. Two inherited neuropathies, Charcot–Marie–Tooth type 2A and autosomal dominant optic atrophy, are caused by mutations in mitofusin 2 and OPA1, suggesting that proper regulation of mitochondrial dynamics is particularly vital to neurons. Mitochondrial fission accompanies several types of apoptotic cell death and appears important for progression of the apoptotic pathway. These studies provide insight into how mitochondria communicate with one another to coordinate mitochondrial function and morphology

The Prefusogenic Intermediate of HIV-1 gp41 Contains Exposed C-peptide Regions

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is composed of a complex between the surface subunit gp120, which binds to cellular receptors, and the transmembrane subunit gp41. Upon activation of the envelope glycoprotein by cellular receptors, gp41 undergoes conformational changes that mediate fusion of the viral and cellular membranes. Prior to formation of a fusogenic "trimer-of-hairpins" structure, gp41 transiently adopts a prefusogenic conformation whose structural features are poorly understood. An important approach toward understanding structural conformations of gp41 during HIV-1 entry has been to analyze the structural targets of gp41 inhibitors. We have constructed epitope-tagged versions of 5-Helix, a designed protein that binds to the C-peptide region of gp41 and inhibits HIV-1 membrane fusion. Using these 5-Helix variants, we examined which conformation of gp41 is the target of 5-Helix. We find that although 5-Helix binds poorly to native gp41, it binds strongly to gp41 activated by interaction of the envelope protein with either soluble CD4 or membrane-bound cellular receptors. This preferential interaction with activated gp41 results in the accumulation of 5-Helix on the surface of activated cells. These results strongly suggest that the gp41 prefusogenic intermediate is the target of 5-Helix and that this intermediate has a remarkably "open" structure, with exposed C-peptide regions. These results provide important structural information about this intermediate that should facilitate the development of HIV-1 entry inhibitors and may lead to new vaccine strategies

Complementation between mouse Mfn1 and Mfn2 protects mitochondrial fusion defects caused by CMT2A disease mutations

Mfn2, an oligomeric mitochondrial protein important for mitochondrial fusion, is mutated in Charcot-Marie-Tooth disease (CMT) type 2A, a peripheral neuropathy characterized by axonal degeneration. In addition to homooligomeric complexes, Mfn2 also associates with Mfn1, but the functional significance of such heterooligomeric complexes is unknown. Also unknown is why Mfn2 mutations in CMT2A lead to cell type–specific defects given the widespread expression of Mfn2. In this study, we show that homooligomeric complexes formed by many Mfn2 disease mutants are nonfunctional for mitochondrial fusion. However, wild-type Mfn1 complements mutant Mfn2 through the formation of heterooligomeric complexes, including complexes that form in trans between mitochondria. Wild-type Mfn2 cannot complement the disease alleles. Our results highlight the functional importance of Mfn1–Mfn2 heterooligomeric complexes and the close interplay between the two mitofusins in the control of mitochondrial fusion. Furthermore, they suggest that tissues with low Mfn1 expression are vulnerable in CMT2A and that methods to increase Mfn1 expression in the peripheral nervous system would benefit CMT2A patients

Domain interactions within Fzo1 oligomers are essential for mitochondrial fusion

Mitofusins are conserved GTPases essential for the fusion of mitochondria. These mitochondrial outer membrane proteins contain a GTPase domain and two or three regions with hydrophobic heptad repeats, but little is known about how these domains interact to mediate mitochondrial fusion. To address this issue, we have analyzed the yeast mitofusin Fzo1p and find that mutation of any of the three heptad repeat regions (HRN, HR1, and HR2) leads to a null allele. Specific pairs of null alleles show robust complementation, indicating that functional domains need not exist on the same molecule. Biochemical analysis indicates that this complementation is due to Fzo1p oligomerization mediated by multiple domain interactions. Moreover, we find that two non-overlapping protein fragments, one consisting of HRN/GTPase and the other consisting of HR1/HR2, can form a complex that reconstitutes Fzo1p fusion activity. Each of the null alleles disrupts the interaction of these two fragments, suggesting that we have identified a key interaction involving the GTPase domain and heptad repeats essential for fusion

The Fusion Activity of HIV-1 gp41 Depends on Interhelical Interactions

Infection by human immunodeficiency virus type I requires the fusogenic activity of gp41, the transmembrane subunit of the viral envelope protein. Crystallographic studies have revealed that fusion-active gp41 is a "trimer-of-hairpins" in which three central N-terminal helices form a trimeric coiled coil surrounded by three antiparallel C-terminal helices. This structure is stabilized primarily by hydrophobic, interhelical interactions, and several critical contacts are made between residues that form a deep cavity in the N-terminal trimer and the C-helix residues that pack into this cavity. In addition, the trimer-of-hairpins structure has an extensive network of hydrogen bonds within a conserved glutamine-rich layer of poorly understood function. Formation of the trimer-of-hairpins structure is thought to directly force the viral and target membranes together, resulting in membrane fusion and viral entry. We test this hypothesis by constructing four series of gp41 mutants with disrupted interactions between the N- and C-helices. Notably, in the three series containing mutations within the cavity, gp41 activity correlates well with the stability of the N-C interhelical interaction. In contrast, a fourth series of mutants involving the glutamine layer residue Gln-653 show fusion defects even though the stability of the hairpin is close to wild-type. These results provide evidence that gp41 hairpin stability is critical for mediating fusion and suggest a novel role for the glutamine layer in gp41 function

Disruption of fusion results in mitochondrial heterogeneity and dysfunction

Mitochondria undergo continual cycles of fusion and fission, and the balance of these opposing processes regulates mitochondrial morphology. Paradoxically, cells invest many resources to maintain tubular mitochondrial morphology, when reducing both fusion and fission simultaneously achieves the same end. This observation suggests a requirement for mitochondrial fusion, beyond maintenance of organelle morphology. Here, we show that cells with targeted null mutations in Mfn1 or Mfn2 retained low levels of mitochondrial fusion and escaped major cellular dysfunction. Analysis of these mutant cells showed that both homotypic and heterotypic interactions of Mfns are capable of fusion. In contrast, cells lacking both Mfn1 and Mfn2 completely lacked mitochondrial fusion and showed severe cellular defects, including poor cell growth, widespread heterogeneity of mitochondrial membrane potential, and decreased cellular respiration. Disruption of OPA1 by RNAi also blocked all mitochondrial fusion and resulted in similar cellular defects. These defects in Mfn-null or OPA1-RNAi mammalian cells were corrected upon restoration of mitochondrial fusion, unlike the irreversible defects found in fzo yeast. In contrast, fragmentation of mitochondria, without severe loss of fusion, did not result in such cellular defects. Our results showed that key cellular functions decline as mitochondrial fusion is progressively abrogated