Desmin

Desmin is a type III intermediate filament found near the Z line in sarcomeres. It was first purified in 1977, the gene was characterized in 1989, and the first knock-out mouse was created in 1996. Desmin is only expressed in vertebrates, however homologous proteins are found in many organisms. It is a 52kD protein that is a subunit of intermediate filaments in skeletal muscle tissue, smooth muscle tissue, and cardiac muscle tissue.

Suggested Functions
The function of desmin has been deduced through studies in knockout mice, however the underlying mechanism of its action is not known. These possibilities may be the result of interactions with other proteins and not desmin itself. More research needs to be done on how desmin's expression and interactions in the muscle cell in order to determine its exact function.

Desmin is one of the earliest protein markers for muscle tissue in embryogenesis as it is detected in the somites of myoblasts. Although it is present early in the development of muscle cells it is expressed at low levels and increases as the cell nears terminal differentiation the muscle cell matures only desmin is present. A similar protein, vimentin, is present in higher amounts during embryogenesis while desmin is present in higher amounts after differentiation. This suggests that there may be some interaction between the two in determining muscle cell differentiation. However desmin knockout mice develop normally and only experience defects later in life. Since desmin is expressed at a low level during differentiation another protein may be able to compensate for desmin's function early in development but not later on.

Desmin is also important in muscle cell architecture and structure since it connects many components of the cytoplasm. The sarcomere is a component of muscle cells composed of filaments and myosin motor proteins which allow the cell to contract. Desmin forms a scaffold around the Z-disk of the sarcomere and connects the Z-disk to the subsarcolemmal cytoskeleton (the cytoplasmic part of the muscle cell plasma membrane). It links the myofibrils laterally by connecting the Z-disks. Through its connection to the sarcomere Desmin connects the contractile apparatus to the cell nucleus, mitochondria, and post-synaptic areas of motor endplates. These connections maintain the structural and mechanical integrity of the cell during contraction while also helping in force transmission and longitudinal load bearing. There is some evidence that desmin may also connect the sarcomere to the extracellular matrix (ECM) through desmosomes which could be important in signalling between the ECM and the sarcomere which could regulate muscle contraction and movement.

Finally, desmin may be important in mitochondria function. When desmin is not functioning properly there is improper mitochondrial distribution, number, morphology and function. Since desmin links the mitochondria to the sarcomere it may transmit information about contractions and energy need and through this regulate the aerobic respiration rate of the muscle cell.

Knockout Phenotype
When the gene for desmin is knocked out it is no longer able to function properly. Mice with the desmin knockout gene develop normally and are fertile, however soon after birth they begin to show defects in skeletal, smooth and cardiac muscle; in particular the diaphragm and heart are affected. The mice without desmin are weaker and fatigue more easily than wild type mice and the muscle fibers are more likely to be damaged during contraction, presumably because the desmin is responsible for keeping the muslce fibers aligned. Mice without desmin also have impaired mitochondrial function.

Associated Diseases
Desmin Related Myopathy (DRM or Desminopathy) is a subgroup of the myofibrillar myopathy diseases and is the result of a mutation in the gene that codes for desmin which prevents it from forming protein filaments, instead forming aggregates of desmin and other proteins throughout the cell. The sarcomeres become misaligned and result in the disorganization of muscle fibers. This mutation also results in muscle cell death by apoptosis and necrosis. The muscle cell may also be disorganized because the aggregates may interrupt other filament structures and/or normal cellular function. Desminopathies are very rare diseases and only 60 patients have been diagnosed with so far, however this number probably does not accurately represent the population due to frequent mis or under diagnosis. Common symptoms of the disease are weakness and atrophy in the distal muscles of the lower limbs which progresses to the hands and arms, then to the trunk, neck and face. Respiratory impairment often follows. There are three major types of inheritance for this disease: Autosomal dominant, autosomal recessive and de novo. The most severe form is autosomal recessive and it also has the earliest onset. It usually involves all three muscle tissues and leads to cardiac and respiratory failure as well as intestinal obstruction. Autosomal Dominant inheritance shows a later onset and slower progression. It usually involves only one or two of the muscle tissues. De novo diseases occur when a new mutation arises in the person that was not inherited through either parent. This form has a wide range of symptoms and varies depending on the mutation made. There is currently no cure for the disease but treatments to help the symptoms are available.

Structure
There are three major domains to this protein: a conserved alpha helix rod, a variable non alpha helix head, and a carboxy-terminal tail. Desmin, as all intermediate filaments, shows no polarity when assmebled. The rod domain consists of 308 amino acids with parallel alpha helical coiled coil dimers and three linkers to disrupt it. The rod connects to the head domain. The head domain 84 amino acids with many arginine, serine, and aromatic residues and is important in filament assembly and dimer-dimer interactions. The tail domain is responsible for the integration of filaments and interaction with proteins and organelles.