m-AAA proteases, mitochondrial calcium homeostasis and neurodegeneration

Abstract

The function of mitochondria depends on ubiquitously expressed and evolutionary conserved m-AAA proteases in the inner membrane. These ATP-dependent peptidases form hexameric complexes built up of homologous subunits. AFG3L2 subunits assemble either into homo-oligomeric isoenzymes or with SPG7 (paraplegin) subunits into hetero-oligomeric proteolytic complexes. Mutations in AFG3L2 are associated with dominant spinocerebellar ataxia (SCA28) characterized by the loss of Purkinje cells, whereas mutations in SPG7 cause a recessive form of hereditary spastic paraplegia (HSP7) with motor neurons of the cortico-spinal tract being predominantly affected. Pleiotropic functions have been assigned to m-AAA proteases, which act as quality control and regulatory enzymes in mitochondria. Loss of m-AAA proteases affects mitochondrial protein synthesis and respiration and leads to mitochondrial fragmentation and deficiencies in the axonal transport of mitochondria. Moreover m-AAA proteases regulate the assembly of the mitochondrial calcium uniporter (MCU) complex. Impaired degradation of the MCU subunit EMRE in AFG3L2-deficient mitochondria results in the formation of deregulated MCU complexes, increased mitochondrial calcium uptake and increased vulnerability of neurons for calcium-induced cell death. A reduction of calcium influx into the cytosol of Purkinje cells rescues ataxia in an AFG3L2-deficient mouse model. In this review, we discuss the relationship between the m-AAA protease and mitochondrial calcium homeostasis and its relevance for neurodegeneration and describe a novel mouse model lacking MCU specifically in Purkinje cells. Our results pledge for a novel view on m-AAA proteases that integrates their pleiotropic functions in mitochondria to explain the pathogenesis of associated neurodegenerative disorders.

Figure 1

Functions of mitochondrial proteases. Mitochondrial proteases execute multiple functions that are crucial for mitochondrial function. Besides protein quality surveillance, many proteases control a variety of processes within mitochondria (shown in boxes) by either proteolytic processing or by mediating the rapid turnover of regulatory proteins.

Figure 2

The m-AAA protease regulates MCU complex assembly by mediating the degradation of non-assembled EMRE subunits. An EMRE-MICU assembly intermediate binds to MCU subunits to form a gated MCU complex, while the m-AAA protease degrades non-assembled EMRE subunits. In the absence of the m-AAA protease, excess EMRE can bind either to MICU to form gated MCU complexes or can interact directly with MCU resulting in the formation of constitutively open MCU complexes lacking MICU regulatory proteins. This increases the vulnerability of cells to Ca²⁺ overload and Ca²⁺-induced cell death.

Figure 3

The deletion of Mcu does not prevent the degeneration of Purkinje cells lacking AFG3L2. Afg3l2^fl/fl and Mcu^fl/fl were bred with transgenic mice-expressing Cre recombinase under the control of the L7 promoter specifically in Purkinje cells. Cerebelli of 6-week-old offsprings of the indicated phenotypes were stained with calbindin to visualize cerebellar Purkinje cells, IBA1 to mark activated microglia and GFAP for reactive astrocytes (as described in ref.). The experiments are in agreement with the national ethical guidelines for studies in animals (84-02.04.2015.A402). GCL, granule cells layer; PCL, Purkinje cell layer; ML, molecular cell layer. A representative picture is shown for each condition. Scale bar, 100 μm.